Ab initio computation of the UV resonance Raman intensity pattern of aqueous imidazole

The UV resonance Raman (RR) spectrum of aqueous imidazole was modeled by ev aluating the gradients of the resonant pi --> pi* excited state, using vari ous levels of theory. To take account of H-bonding effects, two water molec ules were included in the model, one accepting an H-bond from and the other donating an H-bond to imidazole. The ground-state geometry and force field of the ImH . 2H(2)O complex was computed via SQM-DFT theory, with the B3LY P functional, yielding good agreement with experimental vibrational wavenum bers for the in-plane imidazole modes. Evaluation of excited-state gradient s with the semiempirical ZINDO method, which has previously been applied su ccessfully to metalloporphyrin RR spectra, failed to reproduce the imidazol e UVRR spectrum. However, the ab initio CIS method gave fairly good results . Agreement with the experimental intensity pattern improved when minimal S TO-3G or 3-21G* basis sets were upgraded to 6-31G*, but further refinement, to 6-31G**, did not alter the pattern, and addition of diffuse functions a ctually degraded the spectral quality. Likewise, more elaborate levels of a b initio theory, TDDFT and CASSCF(4,4), gave poorer agreement with experime nt then CIS. Thus optimum modeling of the UVRR spectrum was achieved with t he popular CIS/6-31G* methodology. Application of the Kramers-Kronig transf orm frequency correction to the omega (2)(k) approximation ('short-time lim it') altered the intensity distribution only slightly. Copyright (C) 2001 J ohn Wiley & Sons, Ltd.