维生素C为酸性己糖衍生物, 有L-型(抗坏血酸(AA))和D-型(脱氢抗坏血酸(DHA))两种异构体, DHA是AA的第一个稳定氧化产物, 是AA的可逆氧化形式, 因此, 对AA的任何性质或度量的讨论都将涉及同一体系中DHA的性质。 紫外光谱是电子跃迁难易程度和几率的直观体现, 理论计算方法与分子模型的构建不合理, 都将导致对维生素C的最大吸收峰产生误判, 从而无法准确的表征维生素C的激发性质。 因此, 为准确探究维生素C的抗氧化机理, 在液相环境中, 基于密度泛函理论(DFT)和含时密度泛函(TD-DFT)理论, 分别采用pbepbe/6-311++g(2d, 2p)方法和B3LYP/6-311++g(2d, 2p)方法, 计算并分析了维生素C的抗坏血酸和脱氢抗坏血酸分子的结构、 紫外光谱及电子激发特征。 结果表明: pbepbe/6-311++g(2d, 2p)是计算AA紫外吸收光谱更精确的方法; DHA比AA的环状结构发生了显著的平面扭曲。 紫外光谱分析可知, 基态跃迁到S1, S2, S3, S4, S14和S18激发态为AA产生紫外光谱的主要原因, AA位于200.171 5 nm处的吸收峰包含 n→ π^*, n→ σ^*电子跃迁, 266.9248 nm处的吸收峰包含 n→ π^*和 π→ π^*的跃迁。 基态跃迁到S6, S9, S12, S13, S15, S16, S17, S19和S20激发态为DHA产生紫外光谱的主要原因, DHA的最强吸收峰位于181.024 8 nm处, 具有 n→ σ^*和 n→ π^*的跃迁特征, 231.346 39 nm处微弱的吸收峰指认为 n→ π^*跃迁, 282.466 8 nm处的吸收峰主要对应 n→ π^*的跃迁; 通过空穴-电子分布及其衍生量的分析, 可定性地对AA吸收峰起主要作用的7个激发态的特征及对DHA吸收峰起主要作用的9个激发态的特征进行详细的指认。 其中对AA紫外光谱起主要贡献的S4, S13和S14激发态与对DHA紫外光谱起主要贡献的S6, S9, S17和S20激发态电荷转移较明显, 空穴的质心中心和电子质心的中心分离较明显, 可以指认为电荷转移激发, 而其他激发态的电子与空穴分离程度很低, 指认为局域激发。

Vitamin C is an acidic hexose derivative, which has two isomers of L-type (ascorbic acid (AA)) and D-type (dehydroascorbic acid (DHA)). DHA is the first stable oxidation product of AA and is the reversible oxidized form of AA. Therefore, any discussion of the nature and measurement of AA will involve the nature of DHA in the same system. The ultraviolet spectrum is a visual representation of how easy and difficult the electron transition is. Unreasonable theoretical calculation method and molecular model construction will lead to misjudgment of the maximum absorption peak of vitamin C, and thus can not accurately characterize the excitation properties of vitamin C. In order to accurately explore the antioxidant mechanism of vitamin C, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the molecular structure, ultraviolet spectrum and electron excitation characteristics of ascorbic acid (AA) and dehydroascorbic acid (DHA) of vitamin C were calculated and analyzed at the level of pbepbe/6-311++g(2d,2p) and B3LYP/6-311++g(2d,2p) in liquid phase environment in this paper. The results showed that the pbepbe/6-311++g(2d, 2p) method is the more accurate method to calculate the ultraviolet absorption spectrum of AA. Compared with AA, the ring structure of DHA has a significant plane distortion than AA. According to the analysis of the spectral contribution shows that the ground state transition to S1, S2, S3, S4, S14, S18 excited state is the main reason for AA ultraviolet spectrum, the absorption peak of AA at 200.171 5 nm contains the electronic excitations of n→ π^* and n→ σ^* electronic transitions, the absorption peak at 266.924 8 nm contains n→ π^* and π→ π^* transitions. The reason for the ultraviolet spectrum of DHA is mainly due to the ground state transition to S6, S9, S12, S13, S15, S16, S17, S19, S20 excited state, the strongest absorption peak of DHA is located at 181.024 8 nm, which has the transition characteristics of n→ σ^* and n→ π^*. The weak absorption peak at 231.346 39 nm refers to the n→ π^* transition, and the absorption peak at 282.466 8 nm mainly corresponds to the n→ π^* transition. By analysing the hole-electron distribution and its derivatives, it is possible to qualitatively identify the characteristics of the 7 excited states that play a major role in the AA absorption peak and the 9 excited states that make major contributions in the DHA absorption peak. Among them, the S4, S13, S14 excited states that make major contributions to the AA ultraviolet spectrum and the S6, S9, S17, S20 excited states that make major contributions to the DHA ultraviolet spectrum have obvious charge transfer, the centroid center of the hole and the center of the electron centroid are separated, which can be referred to as the charge transfer excitation, the separation of electrons and holes in other excited states is very small, which can be referred to as local excitation.