Ab Initio Calculation and Multiphoton Ionization Studies of Pyrimidine−(Methanol)n Clusters

The multiphoton ionization of the hydrogen-bonded clusters C 4 H 4 N 2 −(CH 3 OH) n was studied using a time-of-flight mass spectrometer at the wavelengths of 355 and 532 nm. At both wavelengths, a series of C 4 H 4 N 2 −(CH 3 OH) 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 C 4 H 4 N 2 −(CH 3 OH) n ( n = 1, 2) clusters and the dissociation channels and dissociaton energies of the ionic clusters are presented. The results show that when C 4 H 4 N 2 −CH 3 OH is vertically ionized, C 4 H 4 N 2 H + and CH 3 O are the dominant products via a fast proton-transfer reaction. A high energy barrier makes another channel corresponding to the production of C 4 H 4 N 2 H + and CH 2 OH disfavored. The dominant dissociation products of C 4 H 4 N 2 −(CH 3 OH) 2 + should be protonated ions C 4 H 4 N 2 −(CH 3 OH)H + . In C 4 H 4 N 2 −(CH 3 OH)H +, the proton prefers to link with the N atom of pyrimidine.