Protonation Sites in Pyrimidine and Pyrimidinamines in the Gas Phase

Protonation sites in gaseous pyrimidine ( 1 ), 2-pyrimidinamine ( 2 ), and 4-pyrimidinamine ( 3 ) were elucidated by neutralization−reionization mass spectrometry and MP2/6-311G(2d,p) ab initio calculations. Pyrimidine was protonated with NH 4 + and t -C 4 H 9 + exclusively at the nitrogen atoms whose proton affinity (PA) was calculated at 879 kJ mol - 1 in excellent agreement with the experimental datum. The C-2 and C-5 positions in 1 had PA = 573 and 645 kJ mol - 1, respectively. 2 was protonated with t -C 4 H 9 + at N-1 and/or N-3 and the amine group in a statistical 2:1 ratio, consistent with the calculated PA = 909 and 864 kJ mol - 1, respectively. 3 was protonated at N-1, N-3, and the amine group, which had PA = 943, 905, and 835 kJ mol - 1, respectively. The C-5 positions in 2 and 3 were less basic, PA = 807 and 781 kJ mol - 1, respectively, and were not attacked by the gas-phase acids used. The thermodynamic proton affinities of gaseous 1, 2, and 3 followed the order of their basicities in solution. The substituent electronic effects on the topical proton affinities are discussed. Pyrimidinium radicals derived from the tautomers of ring-protonated 1, 2, and 3 were found by MP2/6-311G(2d,p) and B3LYP/6-311G(2d,p) to be stable in their equilibrium geometries and upon formation by vertical electron transfer. An unusual kinetic stability was found for pyrimidinium radicals derived from N-1- and N-3-protonated 3 .