Cation vibrational spectra of pyrimidine and its van der Waals complexes with Ar and N2 by ZEKE photoelectron spectroscopy

Cation vibrational spectra of pyrimidine as well as the pyrimidine-Ar, -Ar-2 and -N-2 van der Waals (vdW) complexes have been obtained by means of ZEKE (zero kinetic energy) photoelectron spectroscopy for the first time. Four totally symmetric fundamental vibrations (nu(1), nu(6a), nu(9a), nu(12)) and one non-totally symmetric vibration (nu(6b)) of the pyrimidine cation have been assigned by using the respective five SI vibrational levels as intermediate resonant states. Ab initio calculations (UHF/6-31G* MP2/6-31G*, CASSCF/6-31G* and B3LYP/6-31G*) were also carried out for the vibrational frequencies of the pyrimidine cation to compare with experimental values. The calculated frequencies of the 16b and 6b modes obtained from the UHF and CASSCF calculations deviate from the experimental values, but are in good agreement with the experimental values in the B3LYP calculations under C-2 nu symmetry. Four additional vibrational levels (nu(8a), nu(16a), nu(16b), nu(19a)) found from the ZEKE spectra have been assigned on the basis of the calculated frequencies. The I, values of the complexes were found to be lowered from the I-a value of bare pyrimidine (I-a = 75 261 +/- 6 cm(-1)) by 261 cm(-1) for the Ar complex, by 516 cm(-1) for the Ar-2 complex, and by 95 cm(-1) for the N-2 complex. In the ZEKE spectrum of pyrimidine-Ar, three low-frequency vibrational modes (b(x), b(y), s(z)) were also observed at 36, 40, and 50 cm(-1), respectively. The change in structure along with the D-0 <-- S-1 transitions was estimated from the Franck-Condon analysis of the relative band intensities for the pyrimidine-Ar complex. Some results obtained from MP2/3-21+G** ab initio calculations are also presented for the pyrimidine-Ar complex. (C) 1998 Elsevier Science B.V. All rights reserved.