AH⋯π hydrogen bonding to acetylene and benzene: The role of intramolecular coupling

Hydrogen-bonded complexes of acetylene and benzene with a series of triatomic molecules (HOF, HCN, HNC, HNO, HNS, and HSN), with trihalomethanes HCX3, (X = F, Cl, and Br) and with acetylene are investigated at MP2/6-311++G(2d,2p) and MP2/aug-cc-pVTZ levels. It is shown that the appearance of red- or blue-shifts of AH stretching frequencies in these intermolecular complexes is directly related to the sign of the intramolecular coupling between AH and vicinal AX bonds, with or without hydrogen bonding. Therefore, intramolecular scans and conventional normal coordinate analysis on isolated donors and acceptors are the appropriate tools to discern between the two cases, prior to calculations on complexes.