Search

Anharmonicity and matrix effects play important roles in determining the proton-stretching frequencies in hydrogen-bonded complexes of HCl and HBr with NH3 and N(CH3)3. These effects have been investigated through ab initio calculations carried out at MP2/aug‘-cc-pVDZ for complexes with HCl and at MP2/6-31+G(d,p) for complexes with HBr. The potential surfaces of these complexes are very anharmonic, since the region surrounding the global minimum may be very broad and relatively flat, or a second region of the surface, displaced from the global minimum, can be accessed in either the ground (v = 0) or the first excited (v = 1) state of the proton-stretching mode. As a result, two-dimensional anharmonic frequencies, particularly for the proton-stretching vibration, can be dramatically different from the corresponding harmonic frequencies. Moreover, the zero-point energy contribution to binding enthalpies based on harmonic vibrational frequencies can be significantly overestimated in some complexes. To model ...

The nuclear vibrational wave function and zero-point vibrational energy of CH5(+) are calculated using quantum diffusion Monte Carlo techniques on an interpolated potential energy surface constructed from CCSD(T)/aug'-cc-pVTZ ab initio data. From this multidimensional wave function, the vibrationally averaged rotational constants and radial distribution functions for atom-atom distances within the molecule are constructed. It is found that the distributions of all 10 H-H distances are bimodal and identical. The radial distribution functions obtained for the five C-H distances are also identical, but unimodal. The three rotational constants were found to be 3.78, 3.80, and 3.83 cm(-1). These values indicate that the ground state of CH5(+) is significantly more symmetric than its global minimum energy structure. We conclude that the zero-point motion of CH5(+) renders all five protons equivalent in the ground state and precludes the assignment of a unique structure to the molecule.