Raji Chem World

Thursday, May 29, 2008

NMR Solvent Peaks

NMR Solvent Peaks You can know here, What are solvent used for NMR analyses and their chemical shift values for proton and C13. http://organicgandee.googlepages.com/nmrsolvenpeaks

Tuesday, May 20, 2008

Preparing a Scientific Presentation

Preparing a Scientific Presentation

Identify your audience

Look upon the presentation as a dialogue with audience not a monologue. Acceptance of the speaker by the audience is key. Try to find out beforehand who might be in attendance.

Structure your material

Don't exceed the allotted time; as a rule of thumb keep presentation to 80% of allotted time. Be able to summarize content of presentation in two or three well-constructed sentences Tell 'em what you're gonna tell 'em, then tell 'em, then tell 'em what you've told 'em. In 45 minute talk: 15 minutes for introduction, 25 minutes for presentation, 5 minutes to summarize and conclude. A well-prepared abstract, an organized set of well-chosen viewgraphs, a concise `cheat-sheet,' and an outline (perhaps displayed in the corner of every viewgraph) should all help to keep you on track during your seminar.

Know your stuff

The decision not to speak is sometimes more beneficial to a person's reputation than a lecture devoid of data. Accurate, complete, well-phrased descriptions of scientific information portray speaker as a knowledgeable, reliable source of information. In contrast, glib, inaccurate statements that appear open to multiple interpretations gradually elicit skepticism and distrust. Critical examination of the information is indispensable.

Rehearse

Always rehearse a presentation. Prepare for each seminar for every individual occasion de novo, always with the specific audience in mind. Prepare-- then relax To give a good presentation it is essential to be relaxed. To ease into the presentation, a nervous speaker should write down a few opening sentences on a sheet of paper and read them out in verbatim in as natural and controlled a voice as possible at the beginning of the presentation up to the first viewgraph, making sure not to speak too fast. Dress for success Dressing up for a scientific presentation conveys two important messages: respect for your audience and willingness to conform.

The Structure of a Scientific Presentation

The title: information in a nutshell

Context and perspective: Zooming in Zooming in is the only effective method to put a presentation in perspective. The presentation must start with the description of an important general principle, then gradually focus in from there onto the experimental (or theoretical) model that the speaker wishes to describe. Advantages of zooming in: (1) it emphasizes to the audience that the work to be described bears a relevance to an important scientific principle rather than being an insignificant, isolated contribution; (2) zooming in defines the intellectual borders of the presentation. When putting a presentation in a historical perspective, always give appropriate credit to contribution of others in the field.

Telling a story

There is a distinct difference between summarizing a collection of facts and telling an exciting and interesting story. A clear thinker separates the central, relevant issues from merely supportive peripheral information and will not allow the direct line of thought to be interrupted by sidetracks. A story should have one focus and convey a single major message. To construct the plot for a scientific story, it is often useful to phrase the basic idea underlying the talk as a question.

Mainstream and Sidetracks.

There are three simple rules for preventing the loss of momentum as results of sidetracks. Keep the number of sidetracks to a minimum and use only those that are absolutely essential. Keep the excursion from the mainstream as brief as possible, providing the minimal amount of ancillary information that is absolutely crucial for a full appreciation of the presentation's mainstream. Always make clear where the sidetrack starts and when it is complete, return to the same point of the mainstream.

Formulation and Argumentation

The lecture must proceed as a logical unfolding of information. During the presentation facts must be enumerated in sequential steps, each step firmly founded on the previous one. Remember that your labyrinth of knowledge, with its familiar shortcuts, alternate routes and interconnections is unfamiliar to the audience listening to your story for the first time. To communicate effectively, avoid the use of hyperbole and jargon whenever possible! Speech reflects our thought processes, and an imprecise speaker is often an unfocused thinker. You should carefully analyze the often fuzzy borders that separate experimental evidence from speculation. The care with which this intellectual process is performed is reflected in the manner in which you formulate your sentences. By recognizing the limits of your experiment (or theory or explanation) and clearly defining the conditions under which your conclusions are valid, you gain the respect and credibility of your audience.

The conclusion: brief and to the point

Zooming out can be a valuable tool near the end of a presentation, when you remind the audience once again that the data relates back to major scientific principle with which you begin. The conclusion should be firm and decisive. The conclusion of the presentation is its most important moment. It provides the take-home message, often the only thing that will be remembered. It determines the final impression and impact that you will make on your audience. The conclusion should always be reduced to a concise statement, preferably shown as text or a simple diagram on the overhead. The conclusion should consist of a simple major statement, with not more than two or three connotations, if these are absolutely essential. The conclusion should very clearly demarcate the end of the lecture. The most important rule for a scientific presentation is to finish on time and on a clear and resonant note.

Overhead Transparencies

The three most important points for transparencies (and slides): Clean Simple Necessary to the story line Transparencies should illustrate a single point and, like the presentation itself, have only one focus. Complex data delivered during a seminar cannot be fully appreciated unless the speaker separates them into a series of simplified constituents. Try to avoid showing tables. Most audiences find equations intimidating and are likely to "tune out" as soon as one appears on the screen. [jww: If you must use an equation, define the science of all symbols before the equation.] Some general rules Lettering on viewgraphs and slides can never be too big. Uniformity of style throughout the presentation accentuates and underscores the flow and coherence of the talk. Rules for figures Graphs should contain clearly labelled axes. The less busy a figure appears, the more justice it does to the information it attempts to communicate. Delete all information from the figures that is irrelevant to the presentation. A figure can be enhanced by a line or two concisely describing the conclusions to be drawn from the figure. If the same figure has to be used more than once, use a duplicate rather than disrupt the momentum of the presentation by having to hunt for the early viewgraph. Poster presentation: the young scientist's debut performance Consider a poster primarily as an opportunity for exchange of ideas and dialogue, rather than merely a forum for data presentation. The poster should be aesthetic and clean. Simplicity above all. The poster should tell a story. Include only material relevant to the story line. Choose brief and informative title. In upper left hand corner, provide concise introduction that indicates why work presented is important within context of a major scientific principle. Describe approach in an engaging, condensed style without excessive detail. Organize presentation of data in a logical, coherent sequence. In lower right hand corner, state small number of well-phrased conclusions and a major, concise summary statement. Remember that it is not the number of people who come to view your poster, but the quality of interactions with them that determines its success. Last points Carefully double-check sequence and orientation before presentation; even more important if you are using slides. Anything out of the ordinary usually gives a presentation that special memorable touch, setting it apart from others.

Delivery

Voice Control and Eye Contact Effective use of the voice, eye contact, posture, gestures, and enthusiasm distinguish a routine presentation from a memorable one. The characteristics of delivery in terms of voice control can be separated into several interrelated properties: sound, volume, speed, and rhythm. Articulation and eye contact are the two most important components of voice presentation. Take the time to articulate every work of each sentence clearly, while maintaining eye contact with your audience. As with written text, the end of the sentence designates the "stress" position. It is here the audience expects to be provided with the most important information. Nervous, hurried speech often leads to inaccurate articulation. Take your time and do not speak faster than your normal conversational speed. Monotony is the greatest enemy of a scientific presentation. Plain silence is preferable to mere noise. Slowing down is a remedy for 90 percent of most speakers' problems. Looking straight at members of the audience establishes the notion that you are talking to them, not just in front of them. Foreign speakers who have severe language problems giving a scientific presentation should: Rehearse and practice the presentation often, preferably with a friend who is a native English speaker, and almost learn it by heart. Structure your viewgraphs in such a way that the images are able to convey most of the story by themselves, even if you are hard to understand. Posture and gestures. Stand straight up. Do not be stationary--change positions occasionally and move around the podium/platform. Avoid distracting mannerisms like swinging the pointer aimlessly around. Speaking with a hand in your pocket looks sloppy and unattractive. Gestures can underscore spoken language. Enthusiasm: the indispensable ingredient. Genuine enthusiasm accounts for 90 percent of a speaker's success. Answering questions. The speaker should attempt to control the crowd, permitting question as the speaker's convenience. By making it politely clear that the audience should not interrupt, the speaker will discourage impulsive ad hoc questions and can focus on the presentations. In doing so, you will also establish control and authority. [jww. This is tricky than it seems since an inexperienced speaker may have omitted an essential piece of information the audience need to understand the talk. A clarifying question promptly answered may save such a situation.] Always answer questions briefly and to the point. It is in many cases advantageous to repeat the question before answering it. In addition, repeating the question gives you the chance to rephrase the question. It is always a good idea to be polite and gracious. The most important advice to remember is, communicate with your audience and convey enthusiasm about your work.

Summary

Three devices can put a presentation in the desired perspective. Indicate the scope of the presentation by an informative title. "Zoom in" to the topic during the introductory segment of the presentation and "zoom out" near its end. Decide on the underlying question that the presentation seeks to address; then divide that question into a hierarchically organized array of subquestions, and develop the presentations as a series of answers to these questions. The mainstream of the presentation should address a single focus issue, tuned to the interests of the audience. Sidetracks from this mainstream should be brief and should always return to the same point in the mainstream where they started. Omit information not directly relevant to the focus of the presentation, and avoid backtracking. The statements constituting the mainstream of the presentation should delineate a clear, logical line of thought. Formulate explanations of scientific concepts and experimental (or theoretical) methodology unambiguously, without professional jargon. The presentation should end with a clearly formulated, concise conclusion. When the take-home message has been delivered, stop.

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Thursday, May 15, 2008

Hydrogen: the essentials Brief description: hydrogen is the lightest element. It is by far the most abundant element in the universe and makes up about about 90% of the universe by weight. Hydrogen as water (H2O) is absolutely essential to life and it is present in all organic compounds. Hydrogen is the lightest gas. Hydrogen gas was used in lighter-than-air balloons for transport but is far too dangerous because of the fire risk (Hindenburg). It burns in air to form only water as waste product and if hydrogen could be made on sufficient scale from other than fossil fuels then there might be a possibility of a hydrogen economy. Note that while normally shown at the top of the Group 1 elements in the periodic table, the term "alkaline metal" refers only to Group 1 elements from lithium onwards. Table: basic information about and classifications of hydrogen. Name: Hydrogen Symbol: H Atomic number: 1 Atomic weight: 1.00794 (7) [see notes g m r] Standard state: gas at 298 K CAS Registry ID: 1333-74-0 Group in periodic table: 1 Group name: (none) Period in periodic table: 1 Block in periodic table: s-block Colour: colourless Classification: Non-metallic The lifting agent for the ill fated Hindenberg ballooon was hydrogen rather than the safer helium. The image below is the scene probably in a way you have not seen it before. This is a "ray-traced" image reproduced with the permission of Johannes Ewers, the artist, who won first place with this image in the March/April 1999 Internet Raytracing Competition. For details of ray-tracing you can't beat the POV-Ray site. Isolation Isolation: in the laboratory, small amounts of hydrogen gas may be made by the reaction of calcium hydride with water. CaH2 + 2H2O → Ca(OH)2 + 2H2 This is quite efficient in the sense that 50% of the hydrogen produced comes from water. Another very convenient laboratory scale experiment follows Boyle's early synthesis, the reaction of iron filings with dilute sulphuric acid. Fe + H2SO4 → FeSO4 + H2 There are many industrial methods for the production of hydrogen and that used will depend upon local factors such as the quantity required and the raw materials to hand. Two processes in use involve heating coke with steam in the water gas shift reaction or hydrocarbons such as methane with steam. CH4 + H2O (1100°C) → CO + 3H2 C(coke) + H2O (1000°C) → CO + H2 In both these cases, further hydrogen may be made by passing the CO and steam over hot (400°C) iron oxide or cobalt oxide. CO + H2O → CO2 + H2

Wednesday, May 14, 2008

Chemistry Song

Dear Chemist,

Here you find the chemistry Song about Grignard!!

http://organicgandee.googlepages.com/chemistrysong

Gandee Chemistry World

http://rajigandee.blogspot.com/

http://in.groups.yahoo.com/group/chemistry_learners/

Tuesday, April 8, 2008

CHEMISTRY JOURNALS

Friday, February 29, 2008

Hydrogen Bond in Water molecule

Water Hydrogen Bonding Hydrogen bonding occurs when an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one, so that it may be considered to be acting as a bond between them [99].h Typically this occurs where the partially positively charged hydrogen atom lies between partially negatively charged oxygen and nitrogen atoms, but is also found elsewhere, such as between fluorine atoms in HF2- and between water and the smaller halide ions F-, Cl- and Br- (for example, HO-H····Br-, [178, 1190]; the strength of hydrogen bonding reducing as the halide radius increases), and to a much smaller extent to I- [190] and even xenon [941 ]. Even very weak C-H····OH2 hydrogen bonds (~ 4 kJ mol-1) are being increasingly recognized [1293 ]. In theoretical studies, strong hydrogen bonds even occur to the hydrogen atoms in metal hydrides (for example, LiH····HF; [217]). Hydrogen bond strength In water the hydrogen atom is covalently attached to the oxygen of a water molecule (492.2148 kJ mol-1 [350]) but has (optimally) an additional attraction (about 23.3 kJ mol-1 a1 [168]; almost 5 x the average thermal collision fluctuation at 25°C)a2 to a neighboring oxygen atom of another water molecule that is far greater than any included van der Waals interactioni . Water's hydrogen bonding holds water molecules up to about 15% closer than if than if water was a simple liquid with just van der Waals interactions. However, as hydrogen bonding is directional it restricts the number of neighboring water molecules to about four rather than the larger number found in simple liquids (for example, xenon atoms have twelve nearest neighbors in the liquid state. Formation of hydrogen bonds between water molecules gives rise to large, but mostly compensating, energetic changes in enthalpy (becoming more negative) and entropy (becoming less positive). Both changes are particularly large, based by per-mass or per-volume basis, due to the small size of the water molecule. This enthalpy-entropy compensation is almost complete, however, with the consequence that very small imposed enthalpic or entropic effects may exert a considerable influence on aqueous systems. It is possible that hydrogen bonds between para-H2O, possessing no ground state spin, are stronger and last longer than hydrogen bonds between orth-H2O [1150 ]. The hydrogen bond in water is part (about 90%) electrostatic and part (about 10%) covalent [96]d and may be approximated by bonds made up of covalent HO-H····OH2, ionic HOδ--Hδ+····Oδ-H2, and long-bonded covalent HO-··H––O+H2 parts with HO-H····OH2 being very much more in evidence than HO-··H––O+H2, where there would be expected to be much extra non-bonded repulsion. Hydrogen bonding effects all the molecular orbitals even including the inner O1s (1a1) orbital which is bound 318 kJ mol-1 (3.3 eV) less strongly in a tetrahedrally hydrogen bonded bulk liquid phase compared to the gas phase [1227 ]. X-ray spectroscopic probing indicates that the electron transitions between molecular orbitals (changing with the local hydrogen bonding topology) with differing such contributions may shift on a time scale of less than a femtosecond. Contributing to the strength of water's hydrogen bonding are nuclear quantum effects (zero point vibrational energy) which bias the length of the O-H covalent bond longer than its 'equilibrium' position length (as the shorter HO-H····OH2 hydrogen bonds are stronger), so also increasing the average dipole moment [554 ]. On forming the hydrogen bond, the donor hydrogen atom stretches away from its oxygen atom and the acceptor lone-pair stretches away from its oxygen atom and towards the donor hydrogen atom [585 ], both oxygen atoms being pulled towards each other. An important feature of the hydrogen bond is that it possesses direction; by convention this direction is that of the shorter O-H ( " type="#_x0000_t75">) covalent bond (the O-H hydrogen atom being donated to the O-atom acceptor atom on another H2O molecule). In 1H-NMR studies, the chemical shift of the proton involved in the hydrogen bond moves about 0.01 ppm K-1 upfield to lower frequency (plus about 5.5 ppm further upfield to vapor at 100°C); that is, becomes more shielded with reducing strength of hydrogen bonding [222] as the temperature is raised; a similar effect may be seen in water's 17O NMR, moving about 0.05 ppm K-1 upfield plus 36-38 ppm further upfield to vapor at 100°C.b Increased extent of hydrogen bonding within clusters results in a similar effect; that is, higher NMR chemical shifts with greater cooperativity [436 ]. The bond strength depends on its length and angle, with the strongest hydrogen bonding in water existing in the short linear proton-centered H5O2+ ion at about 120 kJ mol-1. However, small deviations from linearity in the bond angle (up to 20°) possibly have a relatively minor effect [100]. The dependency on bond length is very important and has been shown to exponentially decay with distance [101]. Some researchers consider the hydrogen bond to be broken c if the bond length is greater than 3.10 Å or the bond angle less than 146° [173],c2 although ab initio calculations indicate that most of the bonding energy still remains and more bent but shorter bonds may be relatively strong; for example, one of the hydrogen bonds in ice-four (143°). Similarly O····H-O interaction energies below 10 kJ mol-1 have been taken as indicative of broken hydrogen bonds although they are almost 50% as strong as 'perfect' hydrogen bonds and there is no reason to presuppose that it is solely the hydrogen bond that has been affected with no contributions from other interactions. Also, the strength of bonding must depend on the orientation and positions of the other bonded and non-bonded atoms and 'lone pair' electrons [525 ]. There is a trade-off between the covalent and hydrogen bond strengths; the stronger is the H····O bond, the weaker the O-H covalent bond, and the shorter the O····O distance. The weakening of the O-H covalent bond gives rise to a good indicator of hydrogen bonding energy; the fractional increase in its length determined by the increasing strength of the hydrogen bonding [217]; for example, when the pressure is substantially increased (~ GPa) the remaining hydrogen bonds (H····O) are forced shorter [655 ] causing the O-H covalent bonds to be elongated. Hydrogen bond strength can be affected by electromagnetic and magnetic effects. Dissociation is a rare event, occurring only twice a day that is, only once for every 1016 times the hydrogen bond breaks. Hydrogen bond cooperativity When a hydrogen bond forms between two water molecules, the redistribution of electrons changes the ability for further hydrogen bonding. The water molecule donating the hydrogen atom has increased electron density in its 'lone pair' region [577 ], which encourages hydrogen bond acceptance, and the accepting water molecule has reduced electron density centered on its hydrogen atoms and its remaining 'lone pair' region [577], which encourages further donation but discourages further acceptance of hydrogen bonds. This electron redistribution thus results in both the cooperativity (e.g. accepting one hydrogen bond encourages the donation of another) and anticooperativity (for example, accepting one hydrogen bond discourages acceptance of another) in hydrogen bond formation in water networks. Cooperative hydrogen bonding increases the O-H bond length whilst causing a 20-fold greater reduction in the H····O and O····O distances [436]. The increase in bond length has been correlated with the hydrogen bond strength and resultant O-H stretch vibrations [1318 ]. Thus O····O distances within clusters are likely to be shorter than those at the periphery, in agreement with the icosahedral cluster model. If the hydrogen bond is substantially bent then it follows that the bond strength is weaker. The main criteria to determine the strength of hydrogen bonds are their (relatively inaccurately determined) intermolecular distances and the (more precise) wavenumbers of their stretching vibrational modes and those of the donor hydrogen covalent bond.e Any factors, such as polarization, that reduces the hydrogen bond length, is expected to increase its covalency. There is still some dispute over the size of this covalency,d however any covalency will increase the network stability relative to purely electrostatic effects. The hydrogen bond in water dimers is sufficiently strong to result in the dimers persisting within the gas state at significant concentrations (for example, ~0.1% H2O at 25°C and 85% humidity) to contribute significantly to the absorption of sunlight and atmospheric reaction kinetics [266]. The molecular orbitals involved in the hydrogen bonding between two water molecules (50 KB) and five water molecules (29 KB) in a cyclic pentamer are given on other pages. Although the hydrogen atoms are often shown along lines connecting the oxygen atoms, this is now thought to be indicative of time-averaged direction only and unlikely to be found to a significant extent even in ice. Liquid water consists of a mixture of short, straight and strong hydrogen bonds and long, weak and bent hydrogen bonds with many intermediate between these extremes. Short hydrogen bonds in water are strongly correlated with them being straighter [1083 ]. Proton magnetic shielding studies give the following average parameters for the instantaneous structure of liquid water at 4°C; non-linearity, distances and variance; all increasing with temperature [458 ]. Note that the two water molecules below are not restricted to perpendicular planes and only a small proportion of hydrogen bonds are likely to have this averaged structure. The hydrogen bond length of water varies with temperature and pressure. As the covalent bond lengths vary much less with temperature and pressure, most of the densification of ice 1h due to reduced temperature or increased pressure must be due to reduction in the hydrogen bond length. This hydrogen bond length variation can be shown from the changes in volume of ice 1h [818 ]. As hydrogen bond strength depends almost linearly on its length (shorter length giving stronger hydrogen bonding), it also depends almost linearly (outside extreme values) on the temperature and pressure [818]. The latest molecular parameters for water are given elsewhere. At 0 K the O····O distance in ice Ih is 2.75 Å. The energy of a linear hydrogen bond depends on the orientation of the water molecules relative to the hydrogen bond.j Note that in liquid water, the instantaneous hydrogen bonded arrangement of most molecules is not as symmetrical as shown here. In particular, the positioning of the water molecules donating hydrogen bonds to the accepting positions on a water molecule (that is, the water molecules behind in the diagram above, labeled 'd') are likely to be less tetrahedrally placed, due to the lack of substantial tetrahedrally positioned 'lone pair' electrons, than those water molecules that are being donated to from that water molecule (that is, the water molecules top and front in the diagram above, labeled 'a' [1224 ]. Also, the arrangement may well consist of one pair of more tetrahedrally arranged strong hydrogen bonds (one donor and one acceptor) with the remaining hydrogen bond pair (one donor and one acceptor) being either about 6 kJ mol-1 weaker [573 ], less tetrahedrally arranged [373, 396 ] or bifurcated [573]; perhaps mainly due to the anticooperativity effects mentioned below. Such a division of water into higher (4-linked) and lower (2-linked) hydrogen bond coordinated water has been shown by modelling [1349 ]. X-ray absorption spectroscopy confirms that, at room temperature, 80% of the molecules of liquid water have one (cooperatively strengthened) strong hydrogen bonded O-H group and one non-, or only weakly, bonded O-H group at any instant (sub-femtosecond averaged and such as may occur in pentagonally hydrogen bonded clusters), the remaining 20% of the molecules being made up of four-hydrogen-bonded tetrahedrally coordinated clusters [613 ]. There is much debate as to whether such structuring represents the more time-averaged structure, which is understood by some to be basically tetrahedral [1024 ]. g Liquid water contains by far the densest hydrogen bonding of any solvent with almost as many hydrogen bonds as there are covalent bonds. These hydrogen bonds can rapidly rearrange in response to changing conditions and environments (for example, solutes). The hydrogen bonding patterns are random in water (and ice Ih); for any water molecule chosen at random, there is equal probability (50%) that the four hydrogen bonds (that is, the two hydrogen donors and the two hydrogen acceptors) are located at any of the four sites around the oxygen. Water molecules surrounded by four hydrogen bonds tend to clump together, forming clusters, for both statistical [11] and energetic reasons. Hydrogen bonded chains (that is, O-H····O-H····O) are cooperative [379 ]; the breakage of the first bond is the hardest, then the next one is weakened, and so on (see the cyclic water pentamer). Thus unzipping may occur with complex macromolecules held together by hydrogen bonding, for example, nucleic acids. Such cooperativity is a fundamental property of liquid water where hydrogen bonds are up to 250% stronger than the single hydrogen bond in the dimer [77]. A strong base at the end of a chain may strengthen the bonding further. The cooperative nature of the hydrogen bond means that acting as an acceptor strengthens the water molecule acting as a donor [76]. However, there is an anticooperative aspect in so far as acting as a donor weakens the capability to act as another donor, for example, O····H-O-H····O [77]. It is clear therefore that a water molecule with two hydrogen bonds where it acts as both donor and acceptor is somewhat stabilized relative to one where it is either the donor or acceptor of two. This is the reason why it is suspected that the first two hydrogen bonds (donor and acceptor) give rise to the strongest hydrogen bonds [79]. An interesting way of describing the cooperative/anticooperative nature of the water dimer hydrogen bond is to use the nomenclature d'a'DAd''a'' where DA represents the donor-acceptor nature of the hydrogen bond, the d'a' represents the remaining donor-acceptor status of the donating water molecule and d''a'' represents the remaining donor-acceptor status of the accepting water molecule [852 ]. Individually, the most energetically favored donating water molecules have the structures 02D, 12D, 01D and 11D with 00D and 10D disfavored whereas the most energetically favored accepting water molecules have the structures A20, A21, A10 and A11 with A00 and A01 disfavored. Cations may induce strong cooperative hydrogen-bonding around them due to the polarization of water O-H by cation-lone pair interactions (Cation+····O-H····O-H). Luck et al [78] introduced a cooperativity factor for this effect, which varied as the Hofmeister series from K+ (1.08) to Zn2+ (2.5). Total hydrogen bonding around ions may be disrupted however as if the electron pair acceptance increases (for example, in water around cations) so the electron pair donating power of these water molecules is reduced; with opposite effects in the hydration water around anions. These changes in the relative hydration ability of salt solutions are responsible for the swelling and deswelling behavior of hydrophilic polymer gels [317]. The substantial cooperative strengthening of hydrogen bond in water is dependent on long range interactions [98]. Breaking one bond generally weakensf those around whereas making one bond generally strengthens those around and this, therefore, encourages larger clusters, for the same average bond density. The hydrogen-bonded cluster size in water at 0°C has been estimated to be 400 [77]. Weakly hydrogen-bonding surface restricts the hydrogen-bonding potential of adjacent water so that these make fewer and weaker hydrogen bonds. As hydrogen bonds strengthen each other in a cooperative manner, such weak bonding also persists over several layers and may cause locally changed solvation. Conversely, strong hydrogen bonding will be evident at distance. The weakening of hydrogen bonds, from about 23 kJ mol-1 to about 17 kJ mol-1, is observed when many bonds are broken at superheating temperatures (> 100°C) so reducing the cooperativity [173]. The breakage of these bonds is not only due to the more energetic conditions at high temperature but also results from a related reduction in the hydrogen bond donating ability by about 10% for each 100°C increase [218]. The loss of these hydrogen bonds results in a small increase in the hydrogen bond accepting ability of water, due possibly to increased accessibility [218]. Every hydrogen bond formed increases the hydrogen bond status of two water molecules and every hydrogen bond broken reduces the hydrogen bond status of two water molecules. The network is essentially complete at ambient temperatures; that is, (almost) all molecules are linked by at least one unbroken hydrogen bonded pathway. Hydrogen bond lifetimes are 1 - 20 ps [255] whereas broken bond lifetimes are about 0.1 ps with the proportion of 'dangling' hydrogen bonds persisting for longer than a picosecond being insignificant [849 ]. Broken bonds are basically unstable [849] and will probably reform to give same hydrogen bond (as shown by the slow ortho-water/para-water equilibrium process [410 ]), particularly if the other three hydrogen bonds are in place; hydrogen bond breakage being more dependent on the local structuring rather than the instantaneous hydrogen bond strength [833 ]. If not, breakage usually leads to rotation around one of the remaining hydrogen bond(s) [673 ] and not to translation away, as the resultant 'free' hydroxyl group and 'lone pair' are both quite reactive. Also important, if seldom recognized, is the possibility of the hydrogen bond breaking, as evidenced by physical techniques such as IR, Raman or NMR and caused by loss of hydrogen bond 'covalency' due to electron rearrangement, without any angular change in the O-H····O atomic positions. Thus, clusters may persist for much longer times [329] than common interpretation of data from these methods indicates. Evidence for this may be drawn from the high degree of hydrogen bond breakage seen in the IR spectrum of ice [699 ], where the clustering is taken as lasting essentially forever. Rearranging hydrogen bonds The molecular orbitals of water indicate that the two 'lone pairs' of electrons do not give distinct directed electron density in isolated molecules, with tetrahedral nature of water's hydrogen bonding due to four-coordination involving two donor and two acceptor hydrogen bonds. However trigonal (approximately planar) hydrogen bonding is also possible with two donor and one acceptor hydrogen bonds associated with individual water molecules. The lack of substantial tetrahedrally positioned 'lone pair' electrons may ease this process, at a cost of one hydrogen bond energy. Also the acceptor hydrogen bond in three coordinated but tetrahedral arrangements (two donor and one acceptor hydrogen bonds with one vacant acceptor site) can slide through a planar arrangement to the vacant tetrahedral site without breaking. This flexibility in the hydrogen bonding topology facilitates hydrogen-bonding rearrangements. Bifurcated hydrogen bonds Bifurcated hydrogen bonds (where both hydrogen atoms from one water molecule are hydrogen bonding to the same other water molecule, or one hydrogen atom simultaneously forms hydrogen bonds to two other water molecules) have just under half the strength of a normal hydrogen bond (per half the bifurcated bond) and present a low-energy route for hydrogen-bonding rearrangements [255]. They allow the constant randomization of the hydrogen bonding within the network. However, it should be noted that they require the breakage of two hydrogen bonds; one hydrogen bond to form the bifurcated arrangement and another to make way for a different hydrogen bond to form. Any necessary rotation may also involve bending or stretching other hydrogen bonds. Bifurcation of hydrogen bonds cannot cause their net breakage and only occur when a broken hydrogen bond releases a lone pair to accept the incoming hydrogen bond donor [1135 ]. Trifurcated hydrogen bonds (where one hydrogen atom simultaneously forms hydrogen bonds to three other water molecules, forming a tetrahedral face) may also form but only have about one sixth the strength of a normal hydrogen bond per third of the bifurcated bond [573], require free lone pairs on all three bound water molecules and the rest of local cluster must also be poorly hydrogen bonded. Information transfer Hydrogen bonding carries information about solutes and surfaces over significant distances in liquid water. The effect is synergistic, directive and extensive. Thus, in the diagram opposite, strong hydrogen-bonding in molecule (1), caused by solutes or surfaces, will be transmitted to molecules 2 and 3, then to 5 and 6 and then as combined power to 8. The effect is reinforced by additional polarization effects and the resonant intermolecular transfer of O-H vibrational energy, mediated by dipole-dipole interactions and the hydrogen bonds [142]. Reorientation of one molecule induces corresponding motions in the neighbors. Thus solute molecules can 'sense' (for example, effect each others solubility) each other at distances of several nanometers and surfaces may have effects extending to tens of nanometers. This long range correlation of molecular orientation has recently been confirmed using hyper-Rayleigh light scattering [152] and is a reason for the high dielectric constant of water and the consequential reduction in this dielectric constant as the temperature is raised and the number of hydrogen bonds is reduced [239]. Where water molecules are next to flat hydrophobic surfaces, and unable to form extensive clathrate structuring, some hydrogen bonds must be broken and the water molecules will tend to change orientation, from one hydrogen bond directed orthogonally away from the surface (as in clathrates) to one hydrogen bond directed orthogonally towards the surface, in order to minimize the energy requirement. Also the water molecules tend to collapse into their shallow energy minima due to increased non-bonded interactions. Although there may be a consequentially increased density in the first water layer, the second and subsequent water shells compensate by forming stronger hydrogen bonds and a less dense structure. Consequences of this include differential solvation properties affecting surface absorption. Hydrogen bonding rearrangement offers a low energy pathway for the transfer of hydrogen atoms during tautomerism, in a way similar to Grotthuss mechanism for hydrogen ion transport. Shown opposite is adenine tautomerism that can give rise to Adenine - Cytosine (mutation producing) pairing, which uses the rare tautomer on the left. Footnotes a1 This is the energy (ΔH) required for breaking and completely separating the bond, and should equal about half the enthalpy of vaporization. On the same basis ΔS = 37 J deg-1 mol-1 [168]. (Lower enthalpies for the hydrogen bond have been reported [1369], varying between ~6-23 kJ mol-1, with entropies ~29-46 J deg-1 mol-1, depending on the assumptions made ). Just breaking the hydrogen bond in liquid water leaving the molecules essentially in the same position requires only about 25% of this energy; recently estimated at 6.3 kJ mol-1 [690 ]. If the hydrogen bond energy is determined from the excess heat capacity of the liquid over that of steam (assuming that this excess heat capacity is attributable to the breaking of the bonds) ΔH = 9.80 kJ mol-1 [274]. A number of estimates give the equivalent ΔG at about 2 kJ mol-1 at 25°C [344]; however from the equilibrium content of hydrogen bonds (1.7 mol-1) it is -5.7 kJ mol-1. The hydrogen bonding in ice Ih is about 3 kJ mol-1 stronger than liquid water (= 28 kJ mol-1 at 0 K, from lattice energy including non-bonded interactions) and evidenced by an about 4 pm longer, and hence weaker, O-H covalent bond. Hydrogen bonds in D2O are more linear, shorter [554] and stronger than in H2O and those in T2O are expected to be stronger still. Thus given the choice, hydrogen bonds form with the preference O-T····O > O-D····O > O-H····O. [Back] a2 The average molecular linear translational energy is RT/2. The average collision energy is RT (2.479 kJ mol-1). 2% of collisions have energy greater than the energy required to break the bonds (9.80 kJ mol-1, [274]) as determined by excess heat capacity. [Back] b Unfortunately this is difficult to use as a tool, however, due to the averaging of the shift and the complexity of the system. The spin-lattice relaxation times (T1, ~3.6 s, 25°C) of the water protons is also a function of the hydrogen bonding, being shorter for stronger bonding. The effect of solutes, however, shows the chemical shift and spin-lattice relaxation time are not correlated, as solutes may reduce the extent of hydrogen bonding at the same time as increasing its strength [281]. [Back] c Whether a hydrogen bond is considered broken or just stretched and/or bent should be defined by its strength but, as the isolated bond strength may be difficult to determine, this often remains a matter of definition based on distances and angles. An arrangement with strained geometry is very unlikely to last long. It may, however, occur during the breakage, formation or partner-switching (that is, bifurcation) of a hydrogen bond or arise transiently, due to thermal effects or other molecular interactions, in a long-lived hydrogen bond. The lifetime of a hydrogen bond (if more than 10-13 s) presents another measure of hydrogen bond formation but this also suffers from uncertainties in the definition of its geometry. [Back] c2 Other workers use more generous parameters; for example, in [848 ], the hydrogen bond length must be less than 3.50 Å and the bond angle greater than 120°. The importance of choosing a correct definition for the hydrogen bonds has been examined [1240 ]. The simple distance criterion of 2.50 Å for the H····O distance was found very useful and cheapest in computational terms whereas methods based on energy proved poor. Adding further criteria, such as the bond angles, proved of marginal use [1240]. Using simulations, it has been proposed that purely geometric and energetic definitions are inaccurate as they may overestimate the connectivity and lifetime of hydrogen bonds and cannot distinguish improper relative orientations [1335 ]. Such overestimates may, however, be balanced by underestimates due to the cut-off parameters. [Back] d There is still some controversy surrounding this partial covalency with both for (for example, [411 ] gives the 3a1 orbital as most responsible for the hydrogen bonding via orbital mixing), against (for example, [437 ] favors 'antibonding' rather than bonding due to the charge transfer) and neutral [438 ] in the recent literature. If the water hydrogen bond is considered within the context of the complete range of molecular hydrogen bonding then it appears most probable that it is not solely electrostatic [447 ]; indeed the continuous transformation of ice VII to ice X would seem to indicate a continuity of electron sharing between water molecules. Although N-H····N and N-H····O hydrogen bonds are known to be weaker than the O-H····O hydrogen bonds in water, there is clear evidence for the bonds' covalent natures from NMR. In nucleic acids, inter-nucleotide N-H····N coupling (2JNN, using 15N nuclei) confirms some covalent nature in the N-H····N hydrogen bond [779 ]. Also, 3-bond NMR (3JNC) splitting has been found through peptide N-H····O=C hydrogen bonds in proteins, confirming some covalent nature in the N-H····O hydrogen bond [780 ]. [Back] e The O-H vibrational frequency does not follow the O····O hydrogen bond length exactly due to dispersion of the hydrogen bond O-H····O angle [439]. [Back] f However note that some hydrogen bonds may distort a hydrogen bonded cluster such that when such a bond breaks the detached cluster may form a more optimum tetrahedrally bonded arrangement with lower energy and thus reclaiming some or most of the energy lost by bond breakage. [Back] g The interpretation of the structure of water in terms of strands and rings of doubly-linked hydrogen-bonded molecules [613] was not confirmed by a Compton scattering study [1083] where the data was consistent with 3.9 hydrogen bonds (Roo≤3.2Â) around each water molecule, and has been disputed by another X-ray absorption spectroscopic study [690a], which presents a case for the 'non-, or only weakly, bonded O-H groups' to form the majority of O-H groups present and that these groups are more strongly bonded. Also, Bowron challenges the above interpretation (that is, [613]) in the Discussion included in [746 ] and a Raman study supports the fully tetrahedrally hydrogen bonded model [875 ]. This dispute was thought to have been resolved by an ab initio molecular dynamics study [832 ] that shows 170 fs fluctuations of 2.2-fold strength between the two donor hydrogen bonds from each water molecule whilst the overall geometric connectivity is retained, in line with the hypothesis first presented above. However this study [832] has attracted serious criticism [1159 ], leaving its conclusions seemingly unproven. Recent ab initio calculations of the x-ray cross section of liquid water shows only 20% broken hydrogen bonds are present [1059 ] and a novel force field for water, developed from first principles, gives 3.8 shared tetrahedrally coordinated hydrogen bonds per water molecule [1189 ]. Also, an ab initio quantum mechanical/molecular mechanics molecular dynamics simulation study shows that although the time averaged hydrogen bonding is about four shared hydrogen bonds per water molecule, the instantaneous value is significantly lower at about 2.8 shared hydrogen bonds per water molecule [922 ]. Tetrahedrally-coordinated water seems most accepted at the present time, but it is clear that a mixture of a minority of higher (4-linked) and a majority of lower (2-linked) hydrogen bond coordinated water can be fitted equally well with the experimental data [1350 ]. [Back] h The hydrogen bond in water was first suggested by Latimer and Rodebush in 1920 [789 ]. [Back] i The van der Waals attraction has been estimated as high as about 5.5 kJ mol-1 [548 ] based on isoelectronic molecules at optimal separation, but is likely to be repulsive within a hydrogen bond due to the close contact (see for example, [736]). Separating the hydrogen bond components, as below, helps our understanding, although in reality these components are combined. Attraction/repulsion ++ electrostatic attraction long range interaction (< 30 Å) based on point charges, or on dipoles plus quadrupoles, and so on. They may be considered as varying with distance-1. ++ polarization attraction due to net attractive effects between charges and electron clouds (< 8 Å), which may increase cooperatively dependent on the local environment. They may be considered as varying with distance-4. This net attractive effect may contain a small repulsive element due to slightly increased electron cloud overlap. + covalency attraction highly directional and increases on hydrogen bonded cyclic cluster formation. It is very dependent on the spatial arrangement of the molecules within the local environment (< 6 Å) + dispersive attraction interaction (< 6 Å) due to coordinated effects of neighboring electron clouds. They may be considered as varying with distance-6. -- electron repulsion very short range interaction (< 4 Å) due to electron cloud overlap. They may be considered as varying with distance-12. [Back] j In an unstrained tetrahedral network (such as ice Ih) only the six structures below can arise with no structures at intermediate angles. The hydrogen bond energy depends particularly on the angle of rotation around the hydrogen bond, as below, due to the interaction between the molecular dipoles. Note that the hydrogen bonds in the structure pairs (a) and (e), and (b) and (d) have identical energies. In ice Ih with no net dipole moment, the configurations with extreme cis/trans ratios have 56.3% cis (i.e. a+e+f) or 64.7% trans (that is, b+c+d) but the calculated difference in energies was only 0.12% (0.06 kJ mol-1) [858 ]; much lower than the expected (several kJ mol-1) difference in energy between trans and cis structures c and f. As a, c and e involve protons in hydrogen bonds parallel to the c-axis, their increased strength relative to b, d and f may be causative to the (0.3%) shortened c-axis in the ice Ih unit cell.

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