mannequin of hydrogen bonds ( 1 ) between molecules of water system  (“Hydrogen bonds” in the top image are exaggerated by artifacts of the imaging technique.)AFM image of naphthalenetetracarboxylic diimide molecules on silver-terminated silicon, interacting via hydrogen bonding, taken at 77 K.(“Hydrogen bonds” in the top image are exaggerated by artifacts of the imaging technique. A hydrogen bond ( or H-bond ) is a primarily electrostatic wedge of attraction between a hydrogen ( H ) atom which is covalently bound to a more negative “ donor ” atom or group, and another negative atom bearing a lone copulate of electrons—the hydrogen bond acceptor ( Ac ). Such an interact system is generally denoted Dn–H···Ac, where the solid line denotes a polar covalent bond, and the scatter or dash line indicates the hydrogen bond. [ 4 ] The most patronize donor and acceptor atoms are the second-row elements nitrogen ( N ), oxygen ( O ), and fluorine ( F ). Hydrogen bonds can be intermolecular ( occurring between separate molecules ) or intramolecular ( occurring among parts of the same atom ). [ 5 ] [ 6 ] [ 7 ] [ 8 ] The energy of a hydrogen bond depends on the geometry, the environment, and the nature of the specific donor and acceptor atoms, and can vary between 1 and 40 kcal/mol. [ 9 ] This makes them slightly stronger than a vanguard five hundred Waals interaction, and weaker than fully covalent or ionic bonds. This type of bond can occur in inorganic molecules such as water and in organic molecules like DNA and proteins. Hydrogen bonds are responsible for holding such materials as paper and felted wool in concert, and for causing separate sheets of paper to stick together after becoming wet and subsequently drying.
Reading: Hydrogen bond – Wikipedia
The hydrogen bond is responsible for many of the abnormal physical and chemical properties of compounds of N, O, and F. In finical, intermolecular hydrogen adhere is responsible for the high boiling point of water ( 100 °C ) compared to the early group-16 hydrides that have much weaker hydrogen bonds. [ 10 ] Intramolecular hydrogen bond is partially responsible for the secondary coil and tertiary structures of proteins and nucleic acids. It besides plays an crucial function in the structure of polymers, both celluloid and natural .
Bonding [edit ]
 The hydrogen bonds are represented by dotted lines. An exercise of intermolecular hydrogen bond in a self-assembled dimer complex.The hydrogen bonds are represented by dot lines .
Definitions and cosmopolitan characteristics [edit ]
A hydrogen atom attached to a relatively negative atom is the hydrogen bind donor. [ 12 ] C-H bonds only participate in hydrogen bonding when the carbon atom is bound to negative substituents, as is the encase in chloroform, CHCl3. [ 13 ] In a hydrogen chemical bond, the negative atom not covalently attached to the hydrogen is named the proton acceptor, whereas the one covalently bound to the hydrogen is named the proton donor. While this terminology is recommended by the IUPAC, [ 4 ] it can be misinform, since in other donor-acceptor bonds, the donor/acceptor assignment is based on the source of the electron copulate ( such terminology is besides used for hydrogen bonds by some authors [ 9 ] ). In the hydrogen bond donor, the H center field is protic. The donor is a lewis base. Hydrogen bonds are represented as H···Y organization, where the dots represent the hydrogen bond. Liquids that display hydrogen bonding ( such as body of water ) are called associated liquids .
Examples of hydrogen bail donate ( donors ) and hydrogen chemical bond accepting groups ( acceptors ) green lines represent hydrogen bonds cyclic dimer of acetic acidic ; dashedlines represent hydrogen bonds The hydrogen bail is much described as an electrostatic dipole-dipole interaction. however, it besides has some features of covalent bonding : it is directing and strong, produces interatomic distances shorter than the total of the van five hundred Waals radius, and normally involves a restrict number of interaction partners, which can be interpreted as a type of valence. These covalent features are more substantial when acceptors bind hydrogens from more negative donors. As region of a more detail number of criteria, the IUPAC publication acknowledges that the attractive interaction can arise from some combination of electrostatics ( multipole-multipole and multipole-induced multipole interactions ), covalence ( charge remove by orbital overlap ), and distribution ( London forces ), and states that the proportional importance of each will vary depending on the system. however, a footnote to the criterion recommends the excommunication of interactions in which dispersion is the primary contributor, specifically giving Ar — -CH4 and CH4 — -CH4 as examples of such interactions to be excluded from the definition. [ 4 ] Nevertheless, most introductory textbooks inactive restrict the definition of hydrogen alliance to the “ classical ” type of hydrogen adhere characterized in the opening paragraph. Weaker hydrogen bonds [ 14 ] are known for hydrogen atoms bound to elements such as sulfur ( S ) or chlorine ( Cl ) ; tied carbon ( C ) can serve as a donor, peculiarly when the carbon or one of its neighbors is negative ( for example, in chloroform, aldehydes and terminal acetylenes ). [ 15 ] [ 16 ] Gradually, it was recognized that there are many examples of weaker hydrogen bonding involving donor other than N, O, or F and/or acceptor Ac with electronegativity approaching that of hydrogen ( preferably than being much more negative ). Though these “ non-traditional ” hydrogen bonding interactions are often quite weak ( ≈1 kcal/mol ), they are besides omnipresent and are increasingly recognized as crucial dominance elements in receptor-ligand interactions in medicative chemistry or intra-/intermolecular interactions in materials sciences. The definition of hydrogen bonding has gradually broadened over clock time to include these weaker attractive interactions. In 2011, an IUPAC Task Group recommended a advanced evidence-based definition of hydrogen bonding, which was published in the IUPAC journal Pure and Applied Chemistry. This definition specifies :
The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more negative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of adhesiveness formation. [ 17 ]
bond persuasiveness [edit ]
Hydrogen bonds can vary in potency from weak ( 1–2 kJ mol−1 ) to strong ( 161.5 kJ mol−1 in the ion HF−
2 ). [ 18 ] [ 19 ] Typical enthalpies in vapor include : [ 20 ]
- F−H···:F (161.5 kJ/mol or 38.6 kcal/mol), illustrated uniquely by HF2−, bifluoride
- O−H···:N (29 kJ/mol or 6.9 kcal/mol), illustrated water-ammonia
- O−H···:O (21 kJ/mol or 5.0 kcal/mol), illustrated water-water, alcohol-alcohol
- N−H···:N (13 kJ/mol or 3.1 kcal/mol), illustrated by ammonia-ammonia
- N−H···:O (8 kJ/mol or 1.9 kcal/mol), illustrated water-amide
(18 kJ/mol or 4.3 kcal/mol)
The strength of intermolecular hydrogen bonds is most much evaluated by measurements of chemical equilibrium between molecules containing donor and/or acceptor units, most frequently in solution. [ 22 ] The potency of intramolecular hydrogen bonds can be studied with equilibria between conformers with and without hydrogen bonds. The most crucial method acting for the recognition of hydrogen bonds besides in complicate molecules is crystallography, sometimes besides NMR-spectroscopy. Structural details, in especial distances between donor and acceptor which are smaller than the total of the van five hundred Waals radius can be taken as indication of the hydrogen adhere persuasiveness. One schema gives the following slightly arbitrary categorization : those that are 15 to 40 kcal/mol, 5 to 15 kcal/mol, and > 0 to 5 kcal/mol are considered potent, moderate, and fallible, respectively .
resonance assisted hydrogen bond [edit ]
The resonance assisted hydrogen bond ( normally abbreviated as RAHB ) is a firm type of hydrogen adhere. It is characterized by the π-delocalization that involves the hydrogen and can not be properly described by the electrostatic model alone. This description of the hydrogen bond has been proposed to describe unusually short distances broadly observed between O=C-OH∙∙∙ or ∙∙∙O=C-C=C-OH. [ citation needed ]
Structural details [edit ]
The X−H distance is typically ≈110 autopsy, whereas the H···Y distance is ≈160 to 200 autopsy. The distinctive distance of a hydrogen attachment in water is 197 promethium. The ideal bond lean depends on the nature of the hydrogen bond donor. The follow hydrogen bond angles between a hydrofluoric acid donor and respective acceptors have been determined experimentally : [ 23 ]
|Acceptor···donor||VSEPR geometry||Angle (°)|
spectroscopy [edit ]
impregnable hydrogen bonds are revealed by downfield shifts in the 1H NMR spectrum. For exemplar, the acidic proton in the enol tautomer of acetylacetone appears at δH 15.5, which is about 10 ppm downfield of a conventional alcohol. [ 24 ] In the IR spectrum, hydrogen bond shifts the X-H stretch frequency to lower energy ( i.e. the shaking frequency decreases ). This chemise reflects a weakening of the X-H adhere. Certain hydrogen bonds – improper hydrogen bonds – testify a blue shift of the X-H stretch frequency and a decrease in the bind length. [ 25 ] H-bonds can besides be measured by IR vibrational modality shifts of the acceptor. The amide I mode of spinal column carbonyl in α-helices shifts to lower frequencies when they form H-bonds with side-chain hydroxyl groups. [ 26 ]
theoretical considerations [edit ]
Hydrogen bonding is of haunting theoretical pastime. [ 27 ] According to a modern description O : H-O integrates both the intermolecular oxygen : H lone copulate “ : ” nonbond and the intramolecular H-O polar-covalent alliance associated with O-O repulsive coupling. [ 28 ] Quantum chemical calculations of the relevant interresidue potential constants ( conformity constants ) revealed [ how? ] boastfully differences between person H bonds of the like type. For exercise, the cardinal interresidue N−H···N hydrogen chemical bond between guanine and cytosine is much stronger in comparison to the N−H···N attachment between the adenine-thymine pair. [ 29 ] theoretically, the bond intensity of the hydrogen bonds can be assessed using NCI index, non-covalent interactions index, which allows a visual image of these non-covalent interactions, as its name indicates, using the electron concentration of the arrangement. From interpretations of the anisotropies in the Compton profile of ordinary frosting that the hydrogen bond is partially covalent. [ 30 ] however, this interpretation was challenged. [ 31 ] Most broadly, the hydrogen bond can be viewed as a metric -dependent electrostatic scalar field between two or more intermolecular bonds. This is slightly unlike from the intramolecular constipate states of, for model, covalent or ionic bonds ; however, hydrogen bond is by and large still a bounce country phenomenon, since the interaction energy has a net income negative kernel. The initial theory of hydrogen bonding proposed by Linus Pauling suggested that the hydrogen bonds had a overtone covalent nature. This rendition remained controversial until NMR techniques demonstrated information transmit between hydrogen-bonded nucleus, a feat that would lone be possible if the hydrogen bail contained some covalent character. [ 32 ]
history [edit ]
The concept of hydrogen bonding once was challenging. [ 33 ] Linus Pauling credits T. S. Moore and T. F. Winmill with the beginning mention of the hydrogen adhere, in 1912. [ 34 ] [ 35 ] 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 adjust, water, came some years late, in 1920, from Latimer and Rodebush. [ 36 ] In that newspaper, Latimer and Rodebush cite make by a fellow scientist at their testing ground, Maurice Loyal Huggins, saying, “ Mr. Huggins of this lab in some work as yet unpublished, has used the idea of a hydrogen kernel held between two atoms as a theory in involve to certain constituent compounds. ”
Read more: 10 gram Gold Bar (Hallmark Varies)
Hydrogen bonds in little molecules [edit ]
crystal structure of hexangular ice. Gray dashed lines indicate hydrogen bonds
water [edit ]
A omnipresent model of a hydrogen bond is found between body of water molecules. In a discrete body of 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 confront, as is the case with liquid water, more bonds are possible because the oxygen of one body of water molecule has two lone pairs of electrons, each of which can form a hydrogen adhesiveness with a hydrogen on another water molecule. This can repeat such that every urine molecule is H-bonded with up to four early 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 hexangular wicket. The density of ice rink is less than the density of water at the same temperature ; therefore, the solid phase of water system floats on the liquid, unlike most early substances. Liquid water ‘s high boiling point is due to the high total of hydrogen bonds each atom can form, relative to its abject molecular multitude. Owing to the trouble of breaking these bonds, body of water has a identical high boil degree, melting luff, and viscosity compared to otherwise similar liquids not conjoined by hydrogen bonds. Water is singular because its oxygen atom has two lone pairs and two hydrogen atoms, meaning that the sum number of bonds of a water atom is up to four. The number of hydrogen bonds formed by a molecule of liquid water system fluctuates with clock time and temperature. [ 37 ] From TIP4P liquid water simulations at 25 °C, it was estimated that each water molecule participates in an median of 3.59 hydrogen bonds. At 100 °C, this issue decreases to 3.24 due to the increased molecular motion and decrease concentration, while at 0 °C, the average phone number of hydrogen bonds increases to 3.69. [ 37 ] Another study found a much smaller total of hydrogen bonds : 2.357 at 25 °C. [ 38 ] The differences may be due to the manipulation of a different method acting for defining and counting the hydrogen bonds. Where the bond strengths are more equivalent, one might rather find the atoms of two interact water molecules partitioned into two polyatomic ions of diametric charge, specifically hydroxide ( OH− ) and hydronium ( H3O+ ). ( Hydronium ions are besides known as “ hydroxonium ” ions. )
- H−O− H3O+
indeed, in pure water under conditions of standard temperature and pressure, this latter conceptualization is applicable only rarely ; on average about one in every 5.5 × 108 molecules gives up a proton to another water system atom, in accordance with the respect of the dissociation constant for body of water under such conditions. It is a crucial part of the singularity of water. 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 about constantly unfavorable proportional to hydrogen bonds between body of water and the donors and acceptors for hydrogen bonds on those solutes. [ 39 ] Hydrogen bonds between water molecules have an average life of 10−11 seconds, or 10 picoseconds. [ 40 ]
Bifurcated and over-coordinated hydrogen bonds in water [edit ]
A single hydrogen atom can participate in two hydrogen bonds, rather than one. This type of bond is called “ bifurcated ” ( split in two or “ two-forked ” ). It can exist, for exemplify, in complex natural or synthetic organic molecules. [ 41 ] It has been suggested that a bifurcate hydrogen atom is an essential step in water change of direction. [ 42 ]
Acceptor-type hydrogen bonds ( terminating on an oxygen ‘s lone pair ) are more likely to form bifurcation ( it is called overcoordinated oxygen, OCO ) than are donor-type hydrogen bonds, beginning on the lapp oxygen ‘s hydrogens. [ 43 ]
early liquids [edit ]
For example, hydrogen fluoride —which has three lone pairs on the F atom but entirely one H atom—can form only two bonds ; ( ammonia has the diametric problem : three hydrogen atoms but only one lone copulate ) .
promote manifestations of solution hydrogen bond [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 NH3, H2O, and HF compared to the heavier analogues PH3, H2S, 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 function in determining the three-dimensional structures and the properties adopted by many synthetic and natural proteins. Compared to the C-C, C-O, and C-N bonds that comprise most polymers, hydrogen bonds are far weaker, possibly 5 %. thus, hydrogen bonds can be broken by chemical or mechanical means while retaining the basic structure of the polymer spinal column. This hierarchy of bond strengths ( covalent bonds being stronger than hydrogen-bonds being stronger than avant-garde five hundred Waals forces ) is samara to understanding the properties of many materials. [ 44 ]
deoxyribonucleic acid [edit ]
The structure of depart of a DNA double helix In these macromolecules, bonding between parts of the lapp macromolecule campaign it to fold into a specific supreme headquarters allied powers europe, which helps determine the molecule ‘s physiological or biochemical function. For exercise, the double coiling social organization of DNA is due largely to hydrogen bonding between its base match ( deoxyadenosine monophosphate well as principal investigator stacking interactions ), which link one complemental strand to the other and enable replication .
Proteins [edit ]
In the secondary structure of proteins, hydrogen bonds form between the spine oxygens and amide hydrogens. When the spacing of the amino acidic residues participating in a hydrogen chemical 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 310 coil is formed. When two strands are joined by hydrogen bonds involving alternating residues on each participating strand, a beta sheet is formed. Hydrogen bonds besides play a part in forming the third structure of protein through interaction of R-groups. ( See besides protein folding ). Bifurcated H-bond systems are coarse in alpha-helical transmembrane proteins between the anchor 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 -3.4 kcal/mol or -2.6 kcal/mol, respectively. This type of bifurcated H-bond provides an intrahelical H-bonding spouse for pivotal side-chains, such as serine, threonine, and cysteine within the hydrophobic membrane environments. [ 26 ] The role of hydrogen bonds in protein foldable has besides been linked to osmolyte-induced protein stabilization. protective osmolytes, such as trehalose and sorbitol, shift the protein folding balance toward the fold state, in a concentration pendent manner. While the prevailing explanation for osmolyte legal action relies on eject volume effects that are entropic in nature, circular dichroism ( certificate of deposit ) experiments have shown osmolyte to act through an enthalpic effect. [ 45 ] The molecular mechanism for their function 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. [ 46 ] several studies have shown that hydrogen bonds play an important function for the stability between subunits in multimeric proteins. For example, a study of sorbitol dehydrogenase displayed an authoritative hydrogen bond network which stabilizes the tetrameric four structure within the mammalian sorbitol dehydrogenase protein family. [ 47 ] A protein backbone hydrogen bond incompletely shielded from urine attack is a dehydron. Dehydrons promote the removal of body of water through proteins or ligand dressing. The exogenous dehydration enhances the electrostatic interaction between the amide and carbonyl groups by de-shielding their fond charges. furthermore, the dehydration stabilizes the hydrogen shackle by destabilizing the nonbonded express dwell of dehydrated isolate charges. [ 48 ] Wool, being a protein character, 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 .
Cellulose [edit ]
Hydrogen bonds are authoritative in the structure of cellulose and derived polymers in its many different forms in nature, such as cotton and flax .
Para-aramid structure α), showing the hydrogen bonds (dashed) within and between cellulose molecules A maroon of cellulose ( conformity I ), showing the hydrogen bonds ( dashed ) within and between cellulose molecules
celluloid polymers [edit ]
many polymers are strengthened by hydrogen bonds within and between the chains. Among the synthetic polymers, a well qualify exemplar is nylon, where hydrogen bonds occur in the repeat unit and play a major function in crystallization of the material. The bonds occur between carbonyl and amine groups in the amide duplicate unit. They efficaciously link adjacent chains, which help reinforce the material. The impression is great in aramid fiber, where hydrogen bonds stabilize the linear chains laterally. The chain axes are aligned along the fiber bloc, making the fibres highly stiff and impregnable. The hydrogen-bond networks make both natural and celluloid polymers sensitive to humidity levels in the atmosphere because body of water molecules can diffuse into the surface and disrupt the network. Some polymers are more sensitive than others. Thus nylons are more sensible than aramids, and nylon 6 more sensitive than nylon-11 .
Symmetric hydrogen bond [edit ]
A symmetrical hydrogen bond is a limited type of hydrogen bond in which the proton is spaced precisely halfway between two identical atoms. The forte of the bond to each of those atoms is equal. It is an example of a three-center four-electron alliance. This character of bail is much stronger than a “ normal ” hydrogen bond. The effective bond arrange is 0.5, so its force is comparable to a covalent bond. It is seen in internal-combustion engine at high atmospheric pressure, and besides in the solid phase of many anhydrous acids such as hydrofluoric acid and formic acid at high atmospheric pressure. It is besides seen in the bifluoride ion [ F — H — F ] −. Due to severe steric constraint, the protonated form of Proton Sponge ( 1,8-bis ( dimethylamino ) naphthalene ) and its derivatives besides have symmetrical hydrogen bonds ( [ N — H — N ] + ), [ 49 ] although in the case of protonated Proton Sponge, the assembly is deflect. [ 50 ]
Dihydrogen bond [edit ]
The hydrogen bond can be compared with the close refer dihydrogen bond, which is besides an intermolecular bind interaction involving hydrogen atoms. These structures have been known for some prison term, and well characterized by crystallography ; [ 51 ] however, an understand of their kinship to the conventional hydrogen attachment, ionic bond, and covalent bond remains ill-defined. broadly, the hydrogen shackle is characterized by a proton acceptor that is a lone pair of electrons in nonmetallic atoms ( most notably in the nitrogen, and chalcogen groups ). In some cases, these proton acceptors may be pi-bonds or metal complexes. In the dihydrogen bail, however, a metallic element hydride serves as a proton acceptor, therefore forming a hydrogen-hydrogen interaction. Neutron diffraction has shown that the molecular geometry of these complexes is exchangeable to hydrogen bonds, in that the bond length is identical adaptable to the metallic complex/hydrogen donor organization. [ 51 ]
Dynamics probed by spectroscopic means [edit ]
The dynamics of hydrogen bond structures in water can be probed by the IR spectrum of OH stretching oscillation. [ 52 ] In the hydrogen bonding network in protic organic ionic formative 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 addiction of hydrogen bonds and the dynamics of both the anions and the cations. [ 53 ] The sudden weakening of hydrogen bonds during the solid-solid phase transition seems to be coupled with the onset of orientational or rotational disorderliness of the ions. [ 53 ]
application to drugs [edit ]
Hydrogen bonding is a key to the design of drugs. According to Lipinski ‘s rule of five the majority of orally active drugs tend to have between five and ten hydrogen bonds. These interactions exist between nitrogen – hydrogen and oxygen –hydrogen centers. [ 54 ] As with many other rules of flick, many exceptions exist .
References [edit ]
far read [edit ]
- George A. Jeffrey. An Introduction to Hydrogen Bonding (Topics in Physical Chemistry). Oxford University Press, USA (March 13, 1997). ISBN 0-19-509549-9
- The Bubble Wall (Audio slideshow from the National High Magnetic Field Laboratory explaining cohesion, surface tension and hydrogen bonds)
- isotopic effect on bond dynamics