Abstract
The rotational spectrum of the CHClF2-H2O weakly bound dimer has been measured using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy in the 5-18 GHz range. The structure of the complex has been determined by analysis of the moments of inertia of five isotopologues of the dimer. The primary interaction between the two monomers is a weak C-H center dot center dot center dot O contact (R-H center dot center dot center dot O = 2.332(3) angstrom) with a C-Cl center dot center dot center dot H-O contact also present (R-Cl center dot center dot center dot H = 2.749(13) angstrom). The observed structure is in reasonable agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level, although these predict a Cl center dot center dot center dot H distance that is significantly longer than the experimental results indicate. The rotational transitions of all isotopologues containing H2O or D2O were doubled, with relative intensities of the observed transitions consistent with an internal rotation of the water molecule leading to exchange of equivalent hydrogen atoms. Fitting the upper and lower components of the transitions using an effective Hamiltonian with the ERHAM program has yielded an energy difference between the tunneling states of 16.0(4) GHz, resulting in an estimate of the barrier to internal rotation of 195(5) cm(-1) (to be compared with an ab initio estimate of similar to 117 cm(-1)). The binding energy of the complex is estimated to be similar to 5.5(2) kJ/mol (similar to 460 cm(-1)) from a pseudo-diatomic approximation and assumption of a Lennard-Jones intermolecular potential. (C) 2011 Elsevier Inc. All rights reserved.