Ab initio calculation and multiphoton ionization studies of pyrimidine-(methanol)n clusters

The multiphoton ionization of the hydrogen-bonded clusters C4H4N2-(CH3OH)(n) was studied using a time-of-flight mass spectrometer at the wavelengths of 355 and 532 nm. At both wavelengths, a series of C4H4N2-(CH3OH)(n)H+ ions were obtained. The clusters were also studied by ab initio calculations at B3LYP/6-31G**, MP2/6-31G**, and B3LYP/6-311+G(2df.2p) levels. The equilibrium geometries of both neutral and ionic C4H4N2-(CH3OH)(n) (n = 1, 2) clusters and the dissociation channels and dissociaton energies of the ionic clusters are presented. The results show that when C4H4N2-CH3OH is vertically ionized, C4H4N2H+ and CH3O are the dominant products via a fast proton-trans fer reaction. A high energy barrier makes another channel corresponding to the production of C4H4N2H+ and CH2OH disfavored. The dominant dissociation products of C4H4N2-(CH3OH)(2)(+) should be protonated ions C4H4N2-(CH3OH)H+. In C4H4N2-(CH3OH)H+, the proton prefers to link with the N atom of pyrimidine.