School of Chemistry, Monash University, Box 23, Clayton, Victoria 3800, Australia.
J Chem Phys. 2012 Aug 14;137(6):064306. doi: 10.1063/1.4742061.
The microwave spectra of the two conformers each, of the 1H and 3H tautomers of 4-vinylimidazole, have been measured in the 48-72 GHz spectral region. The 4-vinylimidazole was generated in situ by the facile decarboxylation of urocanic acid at its vaporization temperature of 220 °C. The recognition of this reaction casts doubt on the reliability of a previous published spectroscopic study apparently mistakenly thought to be of uncontaminated vaporized urocanic acid, a natural product of great interest in skin cancer etiology. Quantum chemical theoretical predictions of the structures of each of ten possible conformers∕tautomers of urocanic acid and four of 4-vinylimidazole were performed at the ab initio MP2∕cc-pVTZ level, with vibrational predictions at the B3LYP∕cc-pVTZ and M062X∕cc-pVTZ levels. The predicted values of rotational constants for all the urocanic acid species were found to be quite inconsistent with those of the four observed spectra. For the 4-vinylimidazole isomers, the calculated relative energies suggested that all four species would have substantial equilibrium mole fractions at 220 °C. The isomers were identified by matching the observed and calculated rotational constants. The resulting assignment was found to be consistent with the predicted and observed (14)N nuclear quadrupole hyperfine multiplet patterns for a suitable rotational transition, and with the observed versus empirically calculated inertial defects. With one exception, the predicted structures were found to be planar. Resembling the case of 1-vinylimidazole, where one conformer is nonplanar, one isomer of 4-vinylimidazole was found to be quasiplanar. This seems to belong to a class of spontaneous symmetry-breaking observed in the molecular structure of some otherwise planar vinyl aromatic compounds.
已在 48-72GHz 光谱区域测量了 4-乙烯基咪唑的 1H 和 3H 互变异构体的两种构象各自的微波光谱。4-乙烯基咪唑是通过乌洛托品在其 220°C 的汽化温度下的简便脱羧作用原位生成的。该反应的识别使人们对先前发表的光谱研究的可靠性产生了怀疑，该研究显然被误认为是未受污染的汽化乌洛托品的光谱研究，而乌洛托品是皮肤癌病因学中非常感兴趣的天然产物。乌洛托品的十个可能构象/互变异构体和四个 4-乙烯基咪唑的结构的量子化学理论预测是在从头算 MP2∕cc-pVTZ 水平上进行的，并用 B3LYP∕cc-pVTZ 和 M062X∕cc-pVTZ 水平进行了振动预测。发现所有乌洛托品物种的预测旋转常数值与四个观察到的光谱非常不一致。对于 4-乙烯基咪唑异构体，计算出的相对能量表明，在 220°C 时，所有四种物质都将具有相当大的平衡摩尔分数。通过将观察到的和计算出的旋转常数进行匹配来识别异构体。所得分配与合适的旋转跃迁的预测和观察到的（14）N 核四极超精细多谱线图案以及观察到的与经验计算的惯性缺陷一致。除一个例外，预测的结构被发现是平面的。类似于 1-乙烯基咪唑的情况，其中一个构象是非平面的，发现 4-乙烯基咪唑的一个异构体是准平面的。这似乎属于一类在某些其他平面乙烯基芳烃化合物的分子结构中观察到的自发对称破缺。
