Heo Jiyoung, Kim Nam Joon
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
J Phys Chem A. 2007 Sep 13;111(36):8857-63. doi: 10.1021/jp0734124. Epub 2007 Aug 21.
The nucleophilic addition reactions of water and ammonia molecules toward the C5-C6 double bond of thymine radical cations were investigated using density functional theory. We predicted that the nucleophilic addition favored the C5-site of thymine radical cations, in contrast to the previous experimental observations in bulk solution where the addition product to the C6-site was dominant. Considering the molecular orbital factors, we estimated the relative reactivity of the C5- and C6-sites of thymine radical cations for the nucleophilic addition of ammonia. We found that the C5 was more reactive than the C6 for the small-size clusters of Thy1(NH3)n+, n = 0-2, in the gas phase and even in aqueous solution, though the difference in the reactivity between the two sites became smaller as the number of ammonia molecules increased. This variation of the reactivity was attributed to the electron density redistribution within the thymine radical cations induced by the ammonia molecules as a nucleophile. We suggest that the dominance of the C6-addition product in bulk solution is mainly due to the higher stability of the C6-addition product by solvation, rather than to the higher reactivity of the C6-site for the nucleophilic addition.
利用密度泛函理论研究了水分子和氨分子对胸腺嘧啶自由基阳离子C5-C6双键的亲核加成反应。我们预测亲核加成反应有利于胸腺嘧啶自由基阳离子的C5位点，这与之前在本体溶液中的实验观察结果相反，在本体溶液中，C6位点的加成产物占主导地位。考虑到分子轨道因素，我们估计了胸腺嘧啶自由基阳离子C5和C6位点对氨亲核加成的相对反应活性。我们发现，对于气相甚至水溶液中Thy1(NH3)n+（n = 0-2）的小尺寸簇，C5比C6更具反应活性，尽管随着氨分子数量的增加，两个位点之间的反应活性差异变小。这种反应活性的变化归因于作为亲核试剂的氨分子引起的胸腺嘧啶自由基阳离子内的电子密度重新分布。我们认为，本体溶液中C6加成产物的主导地位主要是由于C6加成产物通过溶剂化具有更高的稳定性，而不是由于C6位点对亲核加成具有更高的反应活性。
