Structure, Energetics, and Vibrational Spectra of H-Bonded Systems. Dimers and Trimers of HF and HCl

The title complexes are studied by ab initio methods using double-ζ basis sets augmented by diffuse sp and two sets of polarization functions. The binding energies decrease in the order (HF)2 \u3e HF…HCl \u3e (HCl)2, all of which form pseudolinear H bonds. While not important for (HF)2, correlation plays an increasingly larger role as each HF is replaced by HCl, contributing 23 of the total binding energy of (HCl)2. (HF)3 and (HCl)3 are both C3h cyclic and exhibit cooperativity, particularly (HF)3; correlation appears to be negligible in the three-body nonadditive interaction energies. The red shift of the HF stretch in (HF)3 is nearly twice as great as in the dimer while little enhancement is observed in the HCl analogues. The other two observable bands in the trimers are of fairly high intensity and correspond to an out-of-plane and in-plane bend. The intermolecular stretching modes in the dimers and trimers are of very low intensity. A clear relationship is noted between the intermolecular frequencies and the strength of the interaction