CN Bond Lengths Research Articles

The effect of hydrogen bonding on structural parameters ii. an ab initio study of the monohydrated formamide molecule

Four hydrogen-bonded formamide-water complexes have been studied by ab initio calculations, two where the amino group acts as a donor and two where the carbonyl oxygen is an acceptor. The results indicate that the effect on the conjugated NCO fragment depends on both the type and the energy of the hydrogen bond formed. Although, in all cases the formation of a hydrogen bond leads to increased conjugation, expressed as a shortening of the CN bond and a corresponding lengthening of the CO bond, there is a significant difference in the effect of the two types of hydrogen bonds. This difference may be explained by changes in the electron populations. In two of the complexes the effect of varying the hydrogen bond length has been studied in some detail. It is found that the effect on the conjugated system depends on the length of the hydrogen bond, and analytical expressions have been found for the variations of the CO and CN bond lengths with changes in the hydrogen bond length. Potential functions for the N-H β O and O-H β O hydrogen bonds have also been derived.

A comparative ab initio study of some molecular systems based on hydrazine and formamide

A series of ab initio calculations has been performed on formamide and its hydrogen bonded dimer, s-diformylhydrazine, s-dimethylhydrazine and hydrazine. All systems were assumed planar. The CO, CN and NN bond lengths were optimized in all cases, in order to study the changes of these bonds with variations in the environment. Gross atomic populations and force constants were also considered, and in some calculations the effect of basis enlargements was studied. Significant changes were found in the formamide unit upon dimerization via hydrogen bonds or a NN bond. And, also, a systematic variation was found for the NN bond by going from hydrazine via dimethylhydrazine to diformylhydrazine. The results of this study indicate that the effect of substitution within the framework used is well simulated by ab initio calculations.

A theoretical study of the chemisty of Furan, Phrrole, Benzofuran, Indole, Dibenzofuran, and Carbazole

It is shown that it is possible, within the framework of the simple H�ckel molecular- orbital method, to account fully for all observations on electrophilic substitution in furan, pyrrole, benzofuran, indole, dibenzofuran, and carbazole, in terms of the π-electron distributions in these compounds. The values of coulomb parameters required to obtain this correlation are physically reasonable and in particular are in agreement with values found independently by a variable electronegativity self-consistent field calculation on pyrrole. The previously puzzling difference in orientation of substitution in benzofuran and indole is accounted for in the present study in terms of a difference in the auxiliary inductive effects of the two hetero-atoms. It seems possible to attribute this difference ultimately to differences in the CO and CN bond lengths and to differences in the effective nuclear charges for 2pπ atomic orbitals on the two hetero-atoms. Attention is drawn to an " isoprotonic principle " for estimating the value of the primary coulomb parameter of a hetero-atom. (It is probably a special case of a more general principle-that the primary coulomb parameter is predominantly determined by the charge on the core and scarcely depends upon the nature of the nucleus at the centre of the core.) A spurious H�ckel molecular-orbital model for the oxygen heterocycles is noted. It accounts for the chemistry of the heterocycles but represents a physically unsatisfactory view of the electronic properties of oxygen in these compounds.