Protonation sites in pyrimidine and pyrimidinamines in the gas phase
Protonation sites in pyrimidine and pyrimidinamines in the gas phase
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DOI：
10.1021/ja9634785
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发表时间：
1997-03-05
影响因子：
15
通讯作者：
Turecek, F
中科院分区：
文献类型：
--
作者：
Nguyen, VQ;Turecek, F
通过中和-再电离质谱法和MP2/6 - 311G(2d,p)从头算计算，阐明了气态嘧啶（1）、2 - 嘧啶胺（2）和4 - 嘧啶胺（3）的质子化位点。嘧啶被NH₄⁺和t - C₄H₉⁺质子化时，仅在氮原子上进行，其质子亲和能（PA）经计算为879 kJ/mol，与实验数据高度吻合。1中C - 2和C - 5位置的PA分别为573和645 kJ/mol。2被r - C₄H₉⁺在N - 1和/或N - 3以及胺基处以2∶1的统计比例质子化，这与计算得到的PA分别为909和864 kJ/mol相符。3在N - 1、N - 3和胺基处被质子化，其PA分别为943、905和835 kJ/mol。2和3中C - 5位置的碱性较弱，PA分别为807和781 kJ/mol，未被所用的气相酸进攻。气态1、2和3的热力学质子亲和能遵循它们在溶液中的碱性顺序。讨论了取代基电子效应对局部质子亲和能的影响。通过MP2/6 - 311G(2d,p)和B3LYP/6 - 311G(2d,p)发现，由环质子化的1、2和3的互变异构体衍生的嘧啶鎓自由基在其平衡几何构型以及通过垂直电子转移形成时是稳定的。发现由N - 1和N - 3质子化的3衍生的嘧啶鎓自由基具有不寻常的动力学稳定性。
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 NH4+ and t-C4H9+ 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 r-C4H9+ 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.