Quadrupole coupling constants in linear (HCN)n clusters: Theoretical and experimental evidence for cooperativity effects in C–H⋅⋅⋅N hydrogen bonding

Ab initio quadrupole coupling constants χ(D), χ(14N) for deuterium and nitrogen nuclei are obtained for the sequence of linear (HCN)n clusters (up to n=6) at the RHF/6-31+G* level, complementing previous studies of n-dependent ‘‘cooperative’’ effects in these C–H...N hydrogen-bonded species. For the dimer and trimer, the theoretical values are compared with experimental equilibrium values of Gutowsky and co-workers and found to successfully reproduce both the magnitudes and patterns of measured shifts. For larger (HCN)n clusters, the n-dependent trends in χ(D), χ(14N) appear to be correlated with cooperativity effects found previously (binding energies, geometrical parameters, dipole moments, ir frequencies and intensities). This suggests that quadrupole coupling measurements can provide a useful probe of cooperative H-bonding phenomena in gaseous and condensed media.