间甲基苯甲醚分子旋转异构体的质量分辨阈值电离光谱

间甲基苯甲醚分子有顺式和反式两个转动异构体.利用单光共振双光子电离技术和质量分辨阈值电离技术,研究了间甲基苯甲醚分子顺反异构体的基态到第一电子激发态(S1←S0)的跃迁和阈值电离.得到顺式、反式间甲基苯甲醚分子S1态的激发能(E1)分别为(36049±2)和(36117±2)cm-1,绝热电离能(Ip)分别为(64859±5)和(65110±5)cm-1.结合从头算法和密度泛函理论的量子化学计算,解释了顺式、反式间甲基苯甲醚分子E1和Ip存在差异的原因,并且对S1态和离子基态D0态出现的谱峰进行了标识.间甲基苯甲醚分子顺反异构体在S1态和D0态的活性振动主要是甲基转动、面内环的运动和与取代基相关的弯曲振动.间甲基苯甲醚分子的S1态振动光谱、D0态离子光谱以及理论计算均表明这两个转动异构体在D0态的几何构型与S1态的中性几何构型相比有较大改变,取代基与取代基、取代基与苯环间的相互作用强度高低次序为:S0S1D0.

Rotamers of m-Methylanisole Studied by Mass-Analyzed Threshold Ionization Spectroscopy

The S₁←S₀ electronic transition and threshold ionization of the cis and trans rotamers of m-methylanisole were investigated by using one- color resonant two- photon ionization and mass- analyzed threshold ionization techniques. The first electronic excitation energies (E₁) of the cis and trans rotamers were determined to be (36049±2) and (36117±2) cm^-1, while the adiabatic ionization energies (I_p) were (64859 ± 5) and (65110 ± 5) cm^-1, respectively. The results of ab initio and density functional theory calculations provide a satisfactory interpretation for our experimental findings concerning the difference in the transitional energies of the cis and trans rotamers and assist in assigning the vibronic and cation spectra obtained in the present study. The observed active vibrations of both rotamers in the S₁ and D₀ states primarily consist of methyl torsion, in- plane ring deformation, and substituent- sensitive bending modes. Both experimental and theoretical results show that, for both cis and trans isomers, the geometry of the cation in the D0 state is somewhat different from that of the neutral species in the S₁ state. In addition, the strengths of both the through- space substituent- substituent and substituent- ring interactions were found to follow the order: S₀10.