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

Abstract

Abstract Cation vibrational spectra of pyrimidine as well as the pyrimidine–Ar, –Ar2 and –N2 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 (ν1, ν6a, ν9a, ν12) and one non-totally symmetric vibration (ν6b) of the pyrimidine cation have been assigned by using the respective five S1 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 C2v symmetry. Four additional vibrational levels (ν8a, ν16a, ν16b, ν19a) found from the ZEKE spectra have been assigned on the basis of the calculated frequencies. The Ia values of the complexes were found to be lowered from the Ia value of bare pyrimidine (Ia=75 261±6 cm−1) by 261 cm−1 for the Ar complex, by 516 cm−1 for the Ar2 complex, and by 95 cm−1 for the N2 complex. In the ZEKE spectrum of pyrimidine–Ar, three low-frequency vibrational modes (bx, by, sz) were also observed at 36, 40, and 50 cm−1, respectively. The change in structure along with the D0←S1 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.