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首页> 外文期刊>Physical chemistry chemical physics: PCCP >Conformational preference and cationic structure of 2-methylpyrazine by VUV-MATI spectroscopy and natural bond orbital analysis
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Conformational preference and cationic structure of 2-methylpyrazine by VUV-MATI spectroscopy and natural bond orbital analysis

机译：VUV-MATI光谱和天然键分析2-甲基吡嗪的构象偏好和阳离子结构

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摘要

Alkylpyrazines, which are well-known as aromatic substances and traditional medicines, are interesting molecular systems, and their methyl conformations result in unique structural and dynamical properties. We explored the conformational preference of the methyl group and the highest occupied molecular orbitals (HOMOs) of 2-methylpyrazine and its cation by utilizing high-resolution one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy and natural bond orbital analysis to understand the relevant molecular activities. The measured VUV-MATI spectrum of 2-methylpyrazine revealed its adiabatic ionization energy and the vibrational frequencies of its cation. From the 0-0 band in the MATI spectrum under the zero-field limit, the accurate adiabatic ionization energy was determined as 9.0439 +/- 0.0006 eV (72 944 +/- 5 cm(-1)), which is lower than that of pyrazine. The peaks observed in the spectrum were unambiguously assigned based on vibrational frequencies and Franck-Condon factors from quantum chemical calculations for individual totally symmetric transitions between the S-0 and D-0 states using the simple one-photon dipole selection rules. The most convincing molecular structure of the 2-methylpyrazine cation was determined by Franck-Condon fit spectral simulations. Upon removal of an electron from the non-bonding orbital (HOMO) on the para nitrogen atoms, a significant structural change takes place along the vibrational motion associated with ring distortion by contraction of the N-N distance, resulting in prominent overtones and combination bands. In addition, the methyl substitution of pyrazine lowered the adiabatic ionization energy and the methyl group preferred the anti-configuration with respect to the pyrazine moiety in the D-0 state, resulting in a frozen internal rotation regardless of ionization.
机译：作为芳族物质和传统药物的烷基吡嗪是有趣的分子系统，它们的甲基构象导致独特的结构和动态性质。我们通过利用高分辨率单光子真空紫外分析阈值电离（Vuv-mati）光谱和天然键轨道，探讨了2-甲基吡嗪的构象优选2-甲基吡嗪和其阳离子的偏好和其阳离子分析了解相关分子活动。 2-甲基吡嗪的测量Vuv-Mati光谱显示其绝热电离能和阳离子的振动频率。从零场限制下的Mati光谱中的0-0波段，精确的绝热电离能量确定为9.0439 +/- 0.0006eV（72 944 +/- 5cm（-1）），低于此吡嗪。在频谱中观察到的峰值是基于使用简单的单光子偶极选择规则的S-0和D-0状态之间的各个对称转换的量子化学计算的振动频率和Franck-Condon因子来毫不含糊地分配。通过Franck-Condon适合光谱仿真测定2-甲基吡嗪阳离子的最令人信服的分子结构。在从氮原子上的非粘结轨道（HOMO）上除去电子时，通过N-N距离的收缩，沿着与环变形相关的振动运动进行显着的结构变化，从而产生突出的溢出和组合带。此外，吡嗪的甲基取代降低了绝热电离能量，并且甲基优选在D-0状态下相对于吡嗪部分的抗构型，导致冷冻内部旋转，无论电离如何。

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