A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Abstract:The term "hydrogen bond" has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X-HؒؒؒY hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

While the tetrahedral face of methane has an electron rich centre and can act as a hydrogen bond acceptor, substitution of one of its hydrogens with some electron withdrawing group (such as -F/OH) can make the opposite face electron deficient. Electrostatic potential calculations confirm this and high level quantum calculations show interactions between the positive face of methanol/methyl fluoride and electron rich centers of other molecules such as H2O. Analysis of the wave functions of atoms in molecules shows the presence of an unusual C···Y interaction, which could be called 'carbon bonding'. NBO analysis and vibrational frequency shifts confirm the presence of this interaction. Given the properties of alkyl groups bonded to electronegative elements in biological molecules, such interactions could play a significant role, which is yet to be recognized. This and similar interactions could give an enthalpic contribution to what is called the 'hydrophobic interactions'.

The low J (2 to 7) rotational spectrum of a symmetric-top benzene dimer has been obtained with a Balle/Flygare Fourier transform microwave spectrometer. Each transition is a symmetrical quartet with two J- and K-dependent tunneling splittings of 30 to 400 kHz. Rotational constants B̄, DJ, and DJK were determined to be 427.76(2) MHz, 7.2(3) kHz, and 0.869(5) MHz. The dimer is T-shaped with a benzene c.m. to c.m. distance of 4.96 Å.

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Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted benzenes. It shows that the oxygen is at the a axis of the dimer, well outside (0.48 Å) the hydrogens. However, the C2 axis of the H2O is not coincident with the a axis but is at an angle β of 37° to it, rotated so that the two hydrogens are equivalent. The sixfold axis of the benzene corresponds to the a axis, there is little or no tilt (γ) of the benzene. The c.m. (C6H6) to c.m. (H2O) distance R is 3.329 Å. The closely spaced hyperfine structure from the proton–proton magnetic dipole interaction and the deuterium quadrupole interaction was resolved for several dimers and transitions, principally J=0→1 and 1→2. The results demonstrate effective nuclear equivalence in dimers with H2O and D2O. Also, the symmetries found for their nuclear spin functions correlate with the lowest rotational levels of free water, the m=0 state with 000 and m=1 with 101 and 111. For the m=1, K=0 transitions of C6H6–H2O the correlation is with 111 and for the K=±1, with 101. These assignments are reversed for C6H6–D2O.

One hundred complexes have been investigated exhibiting D-X···A interactions, where X = H, Cl or Li and DX is the 'X bond' donor and A is the acceptor. The optimized structures of all these complexes have been used to propose a generalized 'Legon-Millen rule' for the angular geometry in all these interactions. A detailed Atoms in Molecules (AIM) theoretical analysis confirms an important conclusion, known in the literature: there is a strong correlation between the electron density at the XA bond critical point (BCP) and the interaction energy for all these interactions. In addition, we show that extrapolation of the fitted line leads to the ionic bond for Li-bonding (electrostatic) while for hydrogen and chlorine bonding, it leads to the covalent bond. Further, we observe a strong correlation between the change in electron density at the D-X BCP and that at the X···A BCP, suggesting conservation of the bond order. The correlation found between penetration and electron density at BCP can be very useful for crystal structure analysis, which relies on arbitrary van der Waals radii for estimating penetration. Various criteria proposed for shared- and closed-shell interactions based on electron density topology have been tested for H/Cl/Li bonded complexes. Finally, using the natural bond orbital (NBO) analysis it is shown that the D-X bond weakens upon X bond formation, whether it is ionic (DLi) or covalent (DH/DCl) and the respective indices such as ionicity or covalent bond order decrease. Clearly, one can think of conservation of bond order that includes ionic and covalent contributions to both D-X and X···A bonds, for not only X = H/Cl/Li investigated here but also any atom involved in intermolecular bonding.

The infrared chemiluminescence from the HF elimination reactions of CF3H, CF3CH3, CZHSF, CzFSH, n-C3F7H, and i-C,F7H has been used to assign the vibrational and rotational distributions of HF. The chemically activated fluoroalkane molecules were formed by H atom recombination with the appropriate fluoroalkyl radicals, which were generated by reactions of H atoms with the fluoroalkyl iodide precursor molecules. The HF vibrational distributions decline monotonically with increasing energy. The mean HF vibrational energy is larger than the statistical expectation, and 2 5 3 5 % of the potential energy of the exit channel is specifically released as HF vibrational energy. The HF(u) rotational excitation is modest, and (EK(HF)) seems to be equal to or less than the statistical expectation. The HF(o,J) distributions are used to discuss the dynamics of these HF elimination reactions. The energy disposal pattern from the HF elimination reaction from CF3H is compared to the vibrational energy distributions of HC1 from the CF,HCI, CFH,CI, and CFHCI, molecules that were generated by secondary reactions in the F + CFHzCI, CH3CI, and CH2C12 systems. In general, three-centered reactions of halomethanes release a larger fraction of the potential energy as (Ev(HX)) than do four-centered reactions of haloethanes.