Janet E. Del Bene

Extensive theoretical studies of the hydrogen‐bonded complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2

The structures and binding energies of the complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2 have been examined using much higher levels of theory than has been previously applied to these systems. These methods including large basis sets and full optimization of structures with the effects of electron correlation included, are known to give single bond energies to an accuracy of about 2 kcal mol−1 and are found in this study to give excellent agreement with the extensive experimental data available for the hydrogen fluoride and water dimers. The Cs openform of ammonia dimer remains a very shallow minimum energy structure at these levels, in agreement with previous theoretical results but seemingly in disagreement with experiment. The theoretical enthalpy of association of H5O+2 is found to be −35.0 kcal mol−1, in slight disagreement with the most recent experimental results, but in accord with earlier ones, which suggests that these experiments should be reexamined. The enthalpy of association...

Theoretical Study of Open Chain Dimers and Trimers Containing CH3OH and H2O

Ab initio minimal basis LCAOSCF molecular orbital calculations have been performed on open chain dimers and trimers containing methanol and water. The equilibrium structures and energies of the dimers are determined and compared. The rigidity of the hydrogen bond in each dimer is discussed in terms of calculated intermolecular force constants. The energies of seven open chain trimers are presented and analyzed. It is found that the energies of all trimers deviate from additivity, indicating the existence of a cooperative effect in hydrogen bonding in these systems.

Coupled-cluster calculations of the excitation energies of benzene and the azabenzenes

A series of equation-of-motion coupled-cluster (EOM-CC) calculations of the vertical excitation energies of benzene, pyridine, pyrazine, pyrimidine, pyridazine, symmetric triazine, and symmetric tetrazine have been performed. Single and double excitations have been included fully, and a noniterative approximation has been used to estimate triple excitation effects [the EOM-CCSD(T) method]. The basis set contains polarization functions and has reasonable diffuseness. Comparison is made with experimental data and second-order perturbation theory complete active space (CASPT2) theoretical data. The average EOM-CCSD(T) error for π→π* transitions is 0.11 eV and the error for n→π* transitions is 0.15 eV. Based on these small errors, several uncertain assignments for pyrazine and pyrimidine are substantiated.

Do coupling constants and chemical shifts provide evidence for the existence of resonance-assisted hydrogen bonds?

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17O–17O and 15N–15N spin–spin coupling constants (2h J O–O and 2h J N–N) for ten neutral structures which may exhibit intramolecular O–H–O or N–H–N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin–spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O–O distance and stronger hydrogen bond in the former. However, when the O–O or N–N distances are forced to be the same in corresponding saturated and unsaturated structures, the coupling constants are similar. There is a significant increase in coupling constants when the intramolecular hydrogen bond (IMHB) changes from asymmetric to symmetric, due to much shorter O–O or N–N distances. Symmetrization effects also significantly affect the value of the proton chemical shift. For hydrogen-bonded conformers the trends in chemical shifts and coupling constants are similar. A detailed analysis of the NMR properties of oxygen-containing systems leads to the conclusion that neither the coupling constants nor the proton chemical shifts provide any evidence for the existence of RAHBs. Furthermore, the strength of the O–H···O IMHBs, investigated through the use of appropriate homodesmotic reactions, indicates that the enhanced stability of the IMHB in the unsaturated compound is associated with the σ skeleton of the molecule that allows the oxygen atoms to be in closer proximity than in the saturated analogue.

Hydrogen bonds between first‐row hydrides and acetylene

The structures and association energies of the complexes of NH3, H2O, and HF with acetylene have been determined using Hartree–Fock and Mo/ller–Plesset theories. HF is found to act as a proton donor to acetylene, while H2O and NH3 act as proton acceptors.

Basis set and correlation effects on computed hydrogen bond energies of the dimers (AHn)2: AHn=NH3, OH2, and FH

Basis set expansion and correlation effects on computed hydrogen bond energies of the dimers (AHn)2, AHn=NH3, OH2, and FH, have been evaluated. The addition of diffuse functions is the single most important enhancement of split‐valence plus polarization basis sets, leading to a significant lowering of hydrogen bond energies at all levels of theory. In general, basis set enhancement effects do not appear to be additive. The correlation energy contribution increases the stabilization energies of these neutral hydrogen bonded complexes, with the second order Mo/ller–Plesset term being the dominant term.

What a difference a decade makes: progress in ab initio studies of the hydrogen bond

Abstract This article provides a summary of our studies of hydrogen-bonded complexes during the decade of the 90s. These studies began with systematic investigations of the methodological dependence of the computed structures and binding energies of these complexes. The MP2/6-31+G(d,p) level of theory was identified as the minimum level required to obtain reliable structures, while reliable energetics required larger polarized split-valence basis sets that include diffuse functions. While the experimental frequency shift of the A–H stretching band upon formation of an A–H–B hydrogen bond could also be reproduced at MP2/6-31+G(d,p) for a variety of hydrogen-bonded complexes, significant discrepancies were observed for others, including complexes of HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines. Resolving these discrepancies became the primary focus of our work, and redefined our research efforts. We solved a model two-dimensional nuclear Schrodinger equation to obtain anharmonic dimer- and proton-stretching frequencies, modeled matrix effects with external electric fields, and characterized hydrogen bond types as traditional, proton-shared, and ion-pair. We were able to resolve the observed discrepancies between theory and experiment, and explain the rather disparate effects of matrices on the IR spectra of closely related complexes. We also initiated studies of the NMR properties of the chemical shift of the hydrogen-bonded proton, and the A–B spin–spin coupling constant across the A–H–B hydrogen bond. We demonstrated the dominance of the Fermi-contact term for determining coupling constants in complexes with N–H–N, N–H–O, O–H–O, and Cl–H–N hydrogen bonds, and the distance dependence of this term. We also showed that the IR anharmonic proton-stretching frequency and the NMR spin–spin coupling constant are spectroscopic fingerprints of hydrogen bond type, which provide information about intermolecular distances in hydrogen-bonded complexes.

Vibrational spectroscopy of the hydrogen bond: An ab initio quantum-chemical perspective

The hydrogen bond has long been recognized as an important type of intermolecular interaction. Its infrared (IR) spectroscopic signature is the shift to lower frequency and the increase in intensity of the A-H stretching band upon formation of the A-H…B hydrogen bond. Ab initio calculations carried out with an appropriate wavefunction model and basis set, and using the harmonic approximation, can reasonably reproduce the shift of the A-H stretching band upon hydrogen bonding, if the equilibrium structure exists in a relatively deep potential well on the surface, so that both the V=0 and the V=1 vibrational states of the proton-stretching mode are confined within this well. However, if the equilibrium structure is found in a region of the surface which is broad and relatively flat, or if a second region of the surface can be accessed in either the V=0 or the V=1 vibrational state of the proton-stretching mode, then the harmonic approximation fails to describe the anharmonicity inherent in the surface. For ...

Molecular orbital theory of the hydrogen bond. PI electrons as proton acceptors

Abstract Ab initio SCF calculations with minimal STO-3G and slightly extended 4-31G basis sets have been performed on dimers having HF and H 2 O as proton donors, and the π electrons of C 2 H 2 , C 2 H 4 , HCN, and H 2 CO as proton acceptors. The equilibrium dimers with C 2 H 2 and C 2 H 4 have relatively weak hydrogen bonds which form preferentially at or near the midpoint of the C–C π bond. However, distortions of these dimers which move the hydrogen bonded proton away from the midpoint are associated with only a small amount of destabilization. With HCN and H 2 CO as proton acceptors, pi hydrogen bonding is most favorable at the negative end of the π bonds. However, these pi dimers are not equilibrium structures on the intermolecular potential surfaces, but relax to structures in which hydrogen bond formation occurs through a lone pair of electrons.

Hydrogen bonds between hydrogen halides and unsaturated hydrocarbons

Abstract Ab initio molecular orbital theory is used to study geometrics and energies of hydrogen-bonded complexes between hydrogen fluoride, hydrogen chloride (as proton donors) and acetylene, ethylene (as proton acceptors). Symmetrical T-shaped structures are found to be equilibrium structures for all four complexes. The strengths of the hydrogen bonds are found to be less than for conventional hydrogen bonds involving lone pairs of electrons.