Linear and cyclic chains of HCN and HNC. Electronic interaction energies and NMR J-couplings.

P. F. PROVASI; G. A. AUCAR; M. SANCHEZ; I. ALKORTA; J. ELGUERO; S. P. A. SAUER

Mülheim an der Ruhr - Germany

Conferencia; W. Thiel's Group Seminars; 2005

Max-Planck-Institut für Kohlenforschung

A theoretical study of linear and cyclic clusters of (HCN)n and (HNC)n (up to n = 10) has been carried out at DFT and MP2 level of approximation methods. The cooperativity effects on both the electronic energy and NMR indirect nuclear spin-spin coupling constants J of the linear complexes (HCN)n and (HNC)n (n = 1-6) are discussed. We find strong correlations in the patterns of properties such as the interaction energy, hydrogen bond distances, and spin-spin coupling constants for both series of compounds. The intramolecular spin-spin couplings are with two exceptions dominated by the Fermi contact (FC) mechanism, while the FC term is the only nonvanishing contribution for the intermolecular couplings. The latter do not follow the Dirac vector model and are important only between nearest neighbors.The transition states linking the cyclic clusters show high energetic barriers that prevent the spontaneous transformation of the high-energy clusters, (HNC)n, into the low-energy ones, (HCN)n. The geometrical results have been analyzed using a Steiner- Limbach plot. The electron density and its Laplacian at the bond critical points correlate with the corresponding bond distances by means of two exponential functions, one for the open shell and another for the closed shell cases.