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spectroscopy structural details in particular distances between donor and acceptor which are smaller than the sum of the van der waals radii can be taken as indication of the hydrogen bond strength one scheme gives the following somewhat arbitrary classification those that are 63 to 167 kj mol 15 to 40 kcal mol 21 to 63 kj mol 5 to 15 kcal mol and 0 to 21 kj mol 0 to 5 kcal mol are considered strong moderate and weak respectively 20 hydrogen bonds involving c h bonds are both very rare and weak 24 resonance assisted hydrogen bond edit the resonance assisted hydrogen bond commonly abbreviated as rahb is a strong type of hydrogen bond it is characterized by the π delocalization that involves the hydrogen atom and cannot be properly described by the electrostatic model alone this description of the hydrogen bond has been proposed to describe unusually short distances generally observed between o c oh or o c c c oh 25 structural details edit the x h distance is typically 110 pm whereas the h y distance is 160 to 200 pm the typical length of a hydrogen bond in water is 197 pm the ideal bond angle depends on the nature of the hydrogen bond donor the following hydrogen bond angles between a hydrofluoric acid donor and various acceptors have been determined experimentally 26 acceptor donor vsepr geometry angle hcn hf c n h angle linear 180 h 2 co hf c o h angle trigonal planar 120 h 2 o hf h o h angle pyramidal 46 h 2 s hf h s h angle pyramidal 89 so 2 hf s o h angle trigonal 145 spectroscopy edit strong hydrogen bonds are revealed by downfield shifts in the 1 h nmr spectrum for example the acidic proton in the enol tautomer of acetylacetone appears at δ h displaystyle delta _ text h 15 5 which is about 10 ppm downfield of a conventional alcohol 27 in the ir spectrum hydrogen bonding shifts the x h stretching frequency to lower energy i e the vibration frequency decreases this shift reflects a weakening of the x h bond certain hydrogen bonds improper hydrogen bonds show a blue shift of the x h stretching frequency and a decrease in the bond length 28 h bonds can also be measured by ir vibrational mode shifts of the acceptor the amide i mode of backbone carbonyls in α helices shifts to lower frequencies when they form h bonds with side chain hydroxyl groups 29 the dynamics of hydrogen bond structures in water can be probed by this oh stretching vibration 30 in the hydrogen bonding network in protic organic ionic plastic crystals poipcs which are a type of phase change material exhibiting solid solid phase transitions prior to melting variable temperature infrared spectroscopy can reveal the temperature dependence of hydrogen bonds and the dynamics of both the anions and the cations 31 the sudden weakening of hydrogen bonds during the solid solid phase transition seems to be coupled with the onset of orientational or rotational disorder of the ions 31 theoretical considerations edit hydrogen bonding is of persistent theoretical interest 32 according to a modern description o h o integrates both the intermolecular o h lone pair nonbond and the intramolecular h o polar covalent bond associated with o o repulsive coupling 33 quantum chemical calculations of the relevant interresidue potential constants compliance constants revealed how large differences between individual h bonds of the same type for example the central interresidue n h n hydrogen bond between guanine and cytosine is much stronger in comparison to the n h n bond between the adenine thymine pair 34 theoretically the bond strength of the hydrogen bonds can be assessed using nci index non covalent interactions index which allows a visualization of these non covalent interactions as its name indicates using the electron density of the system citation needed interpretations of the anisotropies in the compton profile of ordinary ice claim that the hydrogen bond is partly covalent 35 however this interpretation was challenged 36 and subsequently clarified 37 most generally the hydrogen bond can be viewed as a metric dependent electrostatic scalar field between two or more intermolecular bonds this is slightly different from the intramolecular bound states of for example covalent or ionic bonds however hydrogen bonding is generally still a bound state phenomenon since the interaction energy has a net negative sum the initial theory of hydrogen bonding proposed by linus pauling suggested that the hydrogen bonds had a partial covalent nature this interpretation remained controversial until nmr techniques demonstrated information transfer between hydrogen bonded nuclei a feat that would only be possible if the hydrogen bond contained some covalent character 38 history edit the concept of hydrogen bonding once was challenging 39 linus pauling credits t s moore and t f winmill with the first mention of the hydrogen bond in 1912 40 41 moore and winmill used the hydrogen bond to account for the fact that trimethylammonium hydroxide is a weaker base than tetramethylammonium hydroxide the description of hydrogen bonding in its better known setting water came some years later in 1920 from latimer and rodebush 42 in that paper latimer and rodebush cited the work of a fellow scientist at their laboratory maurice loyal huggins saying mr huggins of this laboratory in some work as yet unpublished has used the idea of a hydrogen kernel held between two atoms as a theory in regard to certain organic compounds hydrogen bonds in small molecules edit crystal structure of hexagonal ice gray dashed lines indicate hydrogen bonds structure of nickel bis dimethylglyoximate which features two linear hydrogen bonds water edit an ubiquitous example of a hydrogen bond is found between water molecules in a discrete water molecule there are two hydrogen atoms and one oxygen atom the simplest case is a pair of water molecules with one hydrogen bond between them which is called the water dimer and is often used as a model system when more molecules are present as is the case with liquid water more bonds are possible because the oxygen of one water molecule has two lone pairs of electrons each of which can form a hydrogen bond with a hydrogen on another water molecule this can repeat such that every water molecule is h bonded with up to four other molecules as shown in the figure two through its two lone pairs and two through its two hydrogen atoms hydrogen bonding strongly affects the crystal structure of ice helping to create an open hexagonal lattice the density of ice is less than the density of water at the same temperature thus the solid phase of water floats on the liquid unlike most other substances citation needed liquid water s high boiling point is due to the high number of hydrogen bonds each molecule can form relative to its low molecular mass owing to the difficulty of breaking these bonds water has a very high boiling point melting point and viscosity compared to otherwise similar liquids not conjoined by hydrogen bonds water is unique because its oxygen atom has two lone pairs and two hydrogen atoms meaning that the total number of bonds of a water molecule is up to four 43 the number of hydrogen bonds formed by a molecule of liquid water fluctuates with time and temperature 44 from tip4p liquid water simulations at 25 c it was estimated that each water molecule participates in an average of 3 59 hydrogen bonds at 100 c this number decreases to 3 24 due to the increased molecular motion and decreased density while at 0 c the average number of hydrogen bonds increases to 3 69 44 another study found a much smaller number of hydrogen bonds 2 357 at 25 c 45 defining and counting the hydrogen bonds is not straightforward however because water may form hydrogen bonds with solute proton donors and acceptors it may competitively inhibit the formation of solute intermolecular or intramolecular hydrogen bonds consequently hydrogen bonds between or within solute molecules dissolved in water are almost always unfavorable relative to hydrogen bonds between water and the donors and acceptors for hydrogen bonds on those solutes 46 hydrogen bonds between water molecules have an average lifetime of 10 11 seconds or 10 picoseconds 47 bifurcated and over coordinated hydrogen bonds in water edit a single hydrogen atom can participate in two hydrogen bonds this type of bonding is called bifurcated split in two or two forked it can exist for instance in complex organic molecules 48 it has been suggested that a bifurcated hydrogen atom is an essential step in water reorientation 49 acceptor type hydrogen bonds terminating on an oxygen s lone pairs are more likely to form bifurcation it is called overcoordinated oxygen oco than are donor type hydrogen bonds beginning on the same oxygen atom s hydrogen atoms 50 other liquids edit for example hydrogen fluoride which has three lone pairs on the f atom but only one h atom can form only two bonds ammonia has the opposite problem three hydrogen atoms but only one lone pair h f h f h f displaystyle ce h f h f h f further manifestations of solvent hydrogen bonding edit increase in the melting point boiling point solubility and viscosity of many compounds can be explained by the concept of hydrogen bonding negative azeotropy of mixtures of hf and water the fact that ice is less dense than liquid water is due to a crystal structure stabilized by hydrogen bonds dramatically higher boiling points of nh 3 h 2 o and hf compared to the heavier analogues ph 3 h 2 s and hcl where hydrogen bonding is absent viscosity of anhydrous phosphoric acid and of glycerol dimer formation in carboxylic acids and hexamer formation in hydrogen fluoride which occur even in the gas phase resulting in gross deviations from the ideal gas law pentamer formation of water and alcohols in apolar solvents hydrogen bonds in polymers edit hydrogen bonding plays an important role in determining the three dimensional structures and the properties adopted by many proteins compared to the c c c o and c n bonds that comprise most polymers hydrogen bonds are far weaker perhaps 5 as strong thus hydrogen bonds can be broken by chemical or mechanical means while retaining the basic structure of the polymer backbone this hierarchy of bond strengths covalent bonds being stronger than hydrogen bonds being stronger than van der waals forces is relevant in the properties of many materials 51 dna edit the structure of part of a dna double helix hydrogen bonding between guanine and cytosine one of two types of base pairs in dna in these macromolecules bonding between parts of the same macromolecule cause it to fold into a specific shape which helps determine the molecule s physiological or biochemical role for example the double helical structure of dna is due largely to hydrogen bonding between its base pairs as well as pi stacking interactions which link one complementary strand to the other and enable replication citation needed proteins edit in the secondary structure of proteins hydrogen bonds form between the backbone oxygens and amide hydrogens when the spacing of the amino acid residues participating in a hydrogen bond occurs regularly between positions i and i 4 an alpha helix is formed when the spacing is less between positions i and i 3 then a 3 10 helix is formed when two strands are joined by hydrogen bonds involving alternating residues on each participating strand a beta sheet is formed hydrogen bonds also play a part in forming the tertiary structure of protein through interaction of r groups see also protein folding citation needed bifurcated h bond systems are common in alpha helical transmembrane proteins between the backbone amide c o of residue i as the h bond acceptor and two h bond donors from residue i 4 the backbone amide n h and a side chain hydroxyl or thiol h the energy preference of the bifurcated h bond hydroxyl or thiol system is 14 or 167 kj mol 3 4 or 40 kcal mol respectively this type of bifurcated h bond provides an intrahelical h bonding partner for polar side chains such as serine threonine and cysteine within the hydrophobic membrane environments 29 the role of hydrogen bonds in protein folding has also been linked to osmolyte induced protein stabilization protective osmolytes such as trehalose and sorbitol shift the protein folding equilibrium toward the folded state in a concentration dependent manner while the prevalent explanation for osmolyte action relies on excluded volume effects that are entropic in nature circular dichroism cd experiments have shown osmolyte to act through an enthalpic effect 52 the molecular mechanism for their role in protein stabilization is still not well established though several mechanisms have been proposed computer molecular dynamics simulations suggest that osmolytes stabilize proteins by modifying the hydrogen bonds in the protein hydration layer 53 several studies have shown that hydrogen bonds play an important role for the stability between subunits in multimeric proteins for example a study of sorbitol dehydrogenase displayed an important hydrogen bonding network which stabilizes the tetrameric quaternary structure within the mammalian sorbitol dehydrogenase protein family 54 a protein backbone hydrogen bond incompletely shielded from water attack is a dehydron dehydrons promote the removal of water through proteins or ligand binding the exogenous dehydration enhances the electrostatic interaction between the amide and carbonyl groups by de shielding their partial charges furthermore the dehydration stabilizes the hydrogen bond by destabilizing the nonbonded state consisting of dehydrated isolated charges 55 wool being a protein fibre is held together by hydrogen bonds causing wool to recoil when stretched however washing at high temperatures can permanently break the hydrogen bonds and a garment may permanently lose its shape 56 other polymers edit para aramid structure a strand of cellulose conformation i α showing the hydrogen bonds dashed within and between cellulose molecules the properties of many polymers are affected by hydrogen bonds within and or between the chains prominent examples include cellulose and its derived fibers such as cotton and flax in nylon hydrogen bonds between carbonyl and the amide n h effectively link adjacent chains which gives the material mechanical strength hydrogen bonds also affect the aramid fibre where hydrogen bonds stabilize the linear chains laterally the chain axes are aligned along the fibre axis making the fibres extremely stiff and strong hydrogen bond networks make both polymers sensitive to humidity levels in the atmosphere because water molecules can diffuse into the surface and disrupt the network some polymers are more sensitive than others thus nylons are more sensitive than aramids and nylon 6 more sensitive than nylon 11 citation needed symmetric hydrogen bond edit a symmetric hydrogen bond is a special type of hydrogen bond in which the proton is spaced exactly halfway between two ...
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