The combined use of ab initio molecular orbital theory and matrix isolation infrared spectroscopy in the study of molecular interactions

The infrared spectra of a linear (C_s) and a cyclic (C_i) water dimer and a linear (C_s) and a cyclic (C_2h) ammonia dimer have been calculated, at the second order level of MØller-Plesset perturbation theory, using the 6-31G^** basis set. The calculated spectra have been compared with the spectra of water and ammonia isolated in nitrogen and argon matrices. In the case of water, theory predicts the linear to be the more stable isomer, and the observed spectrum can only be interpreted in terms of the linear structure. For ammonia, while the experimentally determined spectrum fits the calculated spectrum of the linear dimer fairly closely, the prediction of which is the more stable structure is still ambiguous.