A comparative theoretical study of the hydrogen bonding between cyanide or isocyanide hydrides and water

MO and DFT calculations predict a lowering of about 30% of the energy gap between the stable and metastable pair HCN/HNC of cyanides when the two isomers are linked by H-bonds to water H2O and its first polymers. The value of about 11 kcal/mol obtained both at the CCSD(T) and PW91PW91-DFT levels does vary hardly beyond the dimer. MP2 overrates the absolute magnitude of the gap, which precludes its exclusive use as a reference for the quality of the DFT functionals. The analysis of the -O…H- bonding in these complexes in terms of energy, geometry, electron distribution and vibration frequencies is consistent with a local picture where the role of the acidity of the hydrogen donor is dominating.