SHARING OF ELECTRONS IN MOLECULES - CHARACTERIZATION OF HYDROGEN-BONDS

The behaviors of the electrons in binary complexes formed from HF, H2O , and NH3 as well as in the complex formed between HF and CH4 are anal yzed at the single electron level in order to characterize the nature of intermolecular bonding in dimers that may contain hydrogen bonds. T he analysis uses the quantitative measures of the degree to which a si ngle electron is shared between two points zeta and zeta', the sharing index I(zeta; zeta'), and the underlying sharing amplitude [zeta; zet a'], as well as the derived quantities, bond indices, self-sharing ind ices, delocalization indices, and volume-point sharing indices. The bo nd indices, together with the delocalization indices and the self-shar ing indices, give a gross dissection of the sharing of an electron bet ween the volumes associated with atoms and between molecules. These in dices give a preliminary indication of the variety of the strengths of sharing in the complexes, but they do not necessarily distinguish bet ween bonding and antibonding sharing. It is the sharing amplitude whic h indicates the wide variety of types of intermolecular sharing in the complexes, the sharing ranging from covalent to antibonding in the co mplexes studied. By comparing the sharing in two conformations of the complexes between two given molecules, such as the complexes HOH-NH3 a nd H2NH-OH2 involving water and ammonia, the difference between the sh aring in complexes in which the bonding is, according to the acid-base nature of the moieties, conducive to proton transfer (the natural ord er) and in which the bonding is not conducive to proton transfer (the unnatural order) can be determined. In general the intermolecular shar ing is found to be larger in the conformation conducive to proton tran sfer, with the sharing amplitude with the fixed point on the bridge pr oton clearly indicating that in the natural conformation a covalent bo nd (although weak in some cases) is formed to the proton acceptor in c ontrast to the unnatural conformation in which the sharing amplitude i ndicates a non- or antibonded behavior to the proton acceptor. The ele ctron shared from the proton acceptor is also found to be more delocal ized over the bridge proton and the proton donor in the natural confor mation than in the unnatural conformation. We propose that the term '' hydrogen bond'' should be restricted to those bonds which involve a br idge proton between the moieties forming the complex and for which the sharing amplitude from the bridge proton to the proton acceptor has t he characteristics of a covalent bond. We suggest that the other compl exes, which are locally stable, should not be considered to be hydroge n-bonded, The weakest covalent intermolecular sharing is found in FH-F H while the strongest is in FH-NH3, the value of the intermolecular sh aring in the latter being almost the same as between the fluorine and the bridge proton in the complex. The weakest antibonding tin terms of sharing) is found in H3CH-FH, and the strongest antibonding in the NH 3 dimers.