Hydrogen-bonded complexes involving sulfur bases are found to be quite different from the analogous oxygen complexes, both experimentally and in theoretical calculations. In general, hydrogen bonds to sulfur not only are weaker than those to oxygen but also show a marked preference for a more “perpendicular” direction of approach to the donor atom. Ab initio calculations at the MP2/6-311++G(d,p) level on the complexes of hydrogen fluoride with H2O, H2S, H2CO, and H2CS reproduce these differences, as does a search of structures in the Cambridge Crystallographic Data Base. We show that the Laplacian of the charge density ∇2ρ predicts a qualitatively correct structure for all the systems considered, but gives poor quantitative predictions of hydrogen-bonding geometries. An analysis based upon Bader's atoms-in-molecules theory rationalizes the differences between sulfur and oxygen hydrogen bonds. A treatment of the hydrogen bond which explicitly considers the contributions of atomic multipoles to the electrostatic energy has more success than ∇2ρ in predicting H bond directionality. Hydrogen bond formation to oxygen is driven by charge−charge interactions, whereas with sulfur the stabilization arises principally from the interaction of the charge on the acidic hydrogen with the dipole and quadrupoles of sulfur.

Hartree–Fock/6-31G** calculations on the benzenoid hydrocarbons benzene, naphthalene, phenanthrene, anthracene, pyrene, tetracene, triphenylene, chrysene, perylene, and coronene are used to investigate the link between aromaticity and the electron distribution. Topological charge density analysis is used, concentrating on the electron distribution ρ (and its Hessian) at bond and ring critical points. With regard to the bond critical point data, it is shown that ρc, [Formula: see text]ρc, and the bond "ellipticity" ε are closely correlated with the bond lengths so, as aromaticity indicators, they have little to add over and above existing indices based on structure. However, the same properties evaluated at the ring critical points in the total density, and also at the equivalent stationary points in the π and σ densities, correlate closely with two different aromaticity indices (one based on structure, the other on magnetic properties), the curvature of ρ perpendicular to the ring plane giving (marginally) the best results. Hence a ring critical point (RCP) index is proposed as a way of quantifying aromaticity, based directly on the electron distribution. Keywords: quantum chemistry, electron density, aromaticity, aromaticity index, HOMA, NICS.

A precisely additive scheme for describing proton sponge basicity is presented as the sum of the proton affinity of an appropriate reference monoamine, the strain released on protonation, and the energy of the intramolecular hydrogen bond formed on protonation. This approach is then tested at the B3-LYP/6-31+G**//HF/6-31G** level on six diamine proton sponges (including two novel compounds) that are polycyclic aromatic hydrocarbon derivatives. A key result is that the loss of destabilizing strain energy on protonation is seldom an important contribution to enhanced basicity, and in some cases an increase in strain energy can actually take place which acts to lower the basicity. The scheme is further tested and discussed in the context of other types of proton sponge, including a bridgehead (bicyclic) diamine, a tricyclic tetraamine, and a "resonance-stabilized" vinamidine proton sponge. Linear relationships found between basicity, hydrogen-bond energy, and structural parameters of the free bases and protonated cations offer the possibility of estimating basicity purely from structure.

The proton affinities of the cis and trans conformers of 2,2′-bipyridine have been determined at the MP2/ 6-31G**//HF/6-31G** level of theory. The neutral molecule and its protonated cation are both shown to possess stable cis and trans conformers: the barriers for cis/trans interconversion, and the roles that electrostatic interactions and π-conjugation play in these barriers, are analyzed. In addition, the barriers to rotation through planar structures are reported. The structures, the effect of electron correlation on the energetics, and the ground state charge distributions are discussed (i) with respect to biphenyl and 2,2′-bipyrimidine and (ii) in the context of dinitrogen proton sponges. With regard to (i) and cis f trans interconversion, it appears that biphenyl, 2,2′-bipyridine, and 2,2′-bipyrimidine have almost identical conformers and rotational energetics. With regard to (ii), these results indicate that SCF estimates of proton affinities and proton transfer barriers in such compounds are overestimated (by 1-2% and ∼100%, respectively), and that barriers to rotation are underestimated by some 20-30%.

Hybrid density functional calculations are employed to explore the relationships between covalent bond order, as defined using the atomic overlap matrix (AOM) formalisms developed by Cioslowski, Ángyán and others, and the parameters derived from a topological analysis of the electron density. Relationships are obtained for the specific cases of C—C, C—N, C—O, C—P and C—S bonds. The simple Pauling bond order–bond length relationship describes the data reasonably well in most cases, but the correlations show considerable scatter. Although no single parameter acts as a unified descriptor of bond order for all types of bond, in each case it is possible to find a model which describes the bond order data significantly better than the Pauling model based on bond length. The relationships presented can therefore be utilized to estimate rapidly the covalent part of the bond order from a topological analysis of the charge distribution for very large systems where the AOM‐based methods can become impractical to apply, and for charge density distributions which have been obtained from experiment (e.g. elastic x‐ray scattering). Copyright © 2003 John Wiley & Sons, Ltd.