A complete active space self-consistent field (CASSCF) ab initio study of phenol-N-2: the properties of a weak hydrogen-bonded system in its S-1 excited state

The hydroxy-bound isomer of the phenol-N-2 van der Waals complex has been studied, together with the phenol monomer, at the CAS(8,7)/6-31G* and CAS(8,7)/cc-pVDZ levels of theory in the S-1 electronically excited state as well as the neutral and cation ground states. Calculated geometries, excitation energies, harmonic vibrational frequencies and rotational constants were in good agreement with experiment, especially when using the Dunning cc-pVDZ basis set, although the greatest deviation from observed values predictably occurred for the S-1 excited state. In particular, where rotational constants calculated for phenol were reproduced to within a few megahertz of experiment for the S-0 and S-1 states, for phenol-N-2, rotational fits to the REMPI spectrum showed the CASSCF calculations to have underestimated rotational constants, particularly the A and A' values, by about 7%. Binding energies for the complex compare very well to experimental values for S-0 and D-0 but were underestimated by about 20% for the S-1 state. Vibrational analyses of inter- and intramolecular modes showed good agreement with literature for the neutral and ionic ground states of phenol-N-2. However, for the S-1 excited state, although good agreement was achieved for the a' symmetry modes, those of a symmetry showed significant errors. The results obtained for the electronically excited state demonstrate that calculations at this level provide a good description of the S-1 state and can genuinely be a useful aid to assignment of REMPI/LIF excitation spectra of van der Waals complexes of intermediate bond strength.