Proportions of species observed in jet spectroscopy vibrational energy effects: Histamine tautomers and conformers
Proportions of species observed in jet spectroscopy vibrational energy effects: Histamine tautomers and conformers
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DOI：
10.1021/ja980560m
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发表时间：
1998-10-21
影响因子：
15
通讯作者：
Brown, RD
中科院分区：
文献类型：
--
作者：
Godfrey, PD;Brown, RD
已经发现可以理解的几个不同的互变异构体和构象异构体中观察到的毫米波喷射光谱组胺通过使用从头算能量计算在MP216-311++G（d，p）水平，包括估计的热自由能的相对丰度。构象松弛，以前发现是重要的，蚂蚁几个分子的可比的复杂性，已被发现有一个显着的作用，在确定的相对丰度的组胺物种可检测的冷却射流，本例表明，这种现象预计将影响一般的多构象射流光谱的解释。包括自由能修正的必要性也值得注意。组胺互变异构体的热平衡在气相中的毫米波光谱仪内喷射膨胀之前已被实验证实，通过比较丰度测量与高表面积和低表面积的入口系统。因此，使用相对丰度来表示相对自由能是有效的。所提出的从头计算产生光谱常数（例如，平面矩），其加强了在喷射光谱中检测到的四种物质的识别。特别是，确认了丰度最低的物质（3 G-Ic）的鉴别。在MP216-311++G（d，p）水平上计算的氮原子四极耦合常数与实验结果的一致性明显好于以前报道的计算结果，值得注意的是，它们不需要N-14核四极矩的标度。
It has been found possible to understand the observed relative abundances of the several different tautomers and conformers observed in the millimeter-wave jet spectroscopy of histamine by using ab initio energy calculations at the MP216-311++G(d,p) level and including an estimate of thermal free energies. Conformational relaxation, previously found to be import-ant for several molecules of comparable complexity, has been found to have a significant role in determining the relative abundances of histamine species detectable in the cooled jet, the present example being indicative that this phenomenon is expected to influence generally the interpretation of multiconformational jet spectra. The need to include free-energy corrections is also noteworthy. The thermal equilibration of histamine tautomers in the gas phase within the millimeter-wave spectrometer prior to jet expansion has been confirmed experimentally by comparing abundances measured with high surface area and low surface area inlet systems. Thus, it is valid to use relative abundances to indicate relative free energies. The presented ab initio calculations yield spectroscopic constants (e.g., planar moments) that tighten the identification of the four species detected in jet spectroscopy. In particular, the identity of the least abundant species (3 G-Ic) is confirmed. Nitrogen quadrupole coupling constants computed at the MP216-311++G(d,p) level are in noticeably better agreement with experiment than previously reported calculations, it being noteworthy that they require no scaling for the N-14 nuclear quadrupole moment.