Cooperative and diminutive hydrogen bonding in Y...HCN...HCN and NCH...Y...HCN trimers (Y=BF,CO,N2).

A computational study of the cooperative effect of hydrogen bonding in Y...HCN...HCN and its diminutive effect in NCH...Y...HCN (Y=BF,CO,N2) linear complexes relative to the Y...HCN dimer was undertaken at the MP2/6-311++G(2d,2p) level of theory. It was found that the additional hydrogen bond in Y...HCN...HCN leads to an enhanced Y...HCN dissociation energy, extended H-C bond length, and larger redshift of the H-C stretch relative to Y...HCN, while opposite features are observed in NCH...Y...HCN. The cooperativity is diminished as the hardness of the Y atom directly bonded to the HCN molecule increases. A particularly interesting result is that the small bond contraction and blueshift associated with the H-C bond in BF...HCN is converted to a small bond extension and redshift on the formation of the BF...HCN...HCN trimer.