中文摘要：

采用密度泛函理论（DFT）-B3LYP方法在6-31G＊基组水平上，对芳香性氨基酸分子体系（Phe，[Phe-H]-，PheH＋，Tyr,[Tyr-H]-,TyrH＋，Trp，[Trp-H]-和TrpH＋）进行结构优化．在优化所得构型的基础上，利用含时密度泛函理论（TDDFT）．B3LYP在6．31G＊基组上计算了它们的激发态性质，并结合态求和方法研究了它们在二次谐波过程中的二阶极化率值．同时讨论了二次谐波非线性光学响应的起源及其产生变化的原因．计算结果表明，相对于中性的氨基酸分子，去质子化和质子化后的氨基酸分子的二阶极化率值都有明显的增加，且符合规律Phe〈PheH＋〈[Phe-H]-和Tyr〈TyrH＋〈[Tyr-H]-通过对它们电极化起源的分析，我们得到对于中性氨基酸分子，侧链芳香环上的π→π＊ 跃迁对二阶极化率起主要贡献：对去质子化和质子化后的氨基酸分子，吲哚环上的π→π＊ 电荷跃迁和α碳原子相连的氨基和羧基基团内电荷跃迁对二阶极化率起到同样重要的贡献．

英文摘要：

The geometrical structures of a series of neutral, protonated, and deprotonated aromatic amino acids （Phe, [Phe-H]-, PheH＋, Tyr, [Tyr--H]-, TyrH＋, Trp, [Trp--H]-, and TrpH＇） were optimized using density functional theory （DFT）-B3LYP with a 6-31G＊ basis set. Based on the optimized structures, the excited state properties were studied using time-dependent DFT at the B3LYP/6-31G＊ level. We calculated the second-order polarizabilities for second harmonic generation with the sum-over-states method. We examined the origins of the nonlinear optical responses and determined the cause for the variation in the second-order polarizabilities. Our calculations show that the second-order polarizabilities for protonated, and deprotonated aromatic amino acids are much higher than those for the neutral aromatic amino acids, with the order Phe〈PheH＊〈[Phe--H]- and Tyr〈TyrH＋〈[Tyr-H]-. By analyzing their electronic origins, we find that charge transitions in the side chains （benzene, phenol, and indole） make the main contributions to the second-order polarizability for neutral aromatic amino acids. For protonated and deprotonated aromatic amino acids,π→π＊ charge transfers within indole rings, and charge transfers within amino groups and the carboxyl groups attached to alpha-carbon atoms make almost identical contributions to the second-order polarizability.