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 evaluating the gradients of the resonant pi --> pi* excited state, using various levels of theory. To take account of H-bonding effects, two water molecules 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 B3LYP functional, yielding good agreement with experimental vibrational wavenumbers for the in-plane imidazole modes. Evaluation of excited-state gradients with the semiempirical ZINDO method, which has previously been applied successfully to metalloporphyrin RR spectra, failed to reproduce the imidazole UVRR spectrum. However, the ab initio CIS method gave fairly good results. Agreement with the experimental intensity pattern improved when minimal STO-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 actually degraded the spectral quality. Likewise, more elaborate levels of ab initio theory, TDDFT and CASSCF(4,4), gave poorer agreement with experiment then CIS. Thus optimum modeling of the UVRR spectrum was achieved with the popular CIS/6-31G* methodology. Application of the Kramers-Kronig transform frequency correction to the omega (2)(k) approximation ('short-time limit') altered the intensity distribution only slightly. Copyright (C) 2001 John Wiley & Sons, Ltd.