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首页> 外文期刊>Chirality: The pharmacological, biological, and chemical consequences of molecular asymmetry >Unusual CD couplet pattern observed for the pi* <- n transition of enantiopure (Z)-8-methoxy-4-cyclooctenone: An experimental and theoretical study by electronic and vibrational circular dichroism spectroscopy and density functional theory calculatio
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Unusual CD couplet pattern observed for the pi* <- n transition of enantiopure (Z)-8-methoxy-4-cyclooctenone: An experimental and theoretical study by electronic and vibrational circular dichroism spectroscopy and density functional theory calculatio

机译：对映体（Z）-8-甲氧基-4-环辛烯酮的pi * <-n跃迁观察到的异常CD偶联模式：通过电子和振动圆二色谱和密度泛函理论计算的实验和理论研究

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摘要

Ultraviolet absorption (UV) and electronic circular dichroism (ECD) spectra of enantiopure (Z)-8-methoxy-4-cyclooctenone (MCO) were measured in hexane to give a normal single UV absorption band at 298 nm, which is assigned to the carbonyl's pi*<- n transition. Unexpectedly, the ECD spectrum exhibited an apparent couplet pattern with vibrational fine structures. Obviously, the conventional CD exciton coupling mechanism cannot be applied to this bisignate CD signal observed for single-chromophoric MCO. Variable temperature-ECD and vibrational circular dichroism (VCD) spectral measurements, simultaneous UV and ECD spectral band resolution, and density functional theory (DFT) calculations of energy and structure revealed that this apparent CD couplet originates from a rather complicated spectral overlap of more than three conformers of MCO, two of which exhibit mirror-imaged ECD spectra at appreciably deviated wavelengths. In the simultaneous band-resolution analysis, the observed UV and ECD spectra were best fitted to four overlapping bands. Two major conformers were identified by comparing the experimental IR and VCD spectra with the simulated ones, and the other two by comparing the observed UV and ECD spectra with the theoretical ones obtained by time-dependent DFT calculations. It was shown that the combined use of experimental ECD and VCD spectra and theoretical DFT calculations can give a reasonable interpretation for the Cotton effects of the conformationally flexible molecule MCO.
机译：在己烷中测量对映纯（Z）-8-甲氧基-4-环辛烯酮（MCO）的紫外吸收（UV）和电子圆二色性（ECD）光谱，得出在298 nm处的正常单个UV吸收带，该谱带指定为羰基的pi * <-n过渡。出乎意料的是，ECD谱表现出具有振动精细结构的明显的耦合模式。显然，常规的CD激子耦合机制不能应用于这种针对单发色MCO的双信号CD信号。可变温度ECD和振动圆二色性（VCD）光谱测量，同时的UV和ECD光谱分辨率以及能量和结构的密度泛函理论（DFT）计算表明，这种明显的CD耦合源于一个非常复杂的光谱重叠，其重叠范围大于MCO的三个构象异构体，其中两个在明显偏离的波长下表现出镜像的ECD光谱。在同时频带分辨率分析中，观察到的UV和ECD光谱最适合四个重叠频带。通过将实验的IR和VCD光谱与模拟的光谱进行比较，确定了两个主要的构象异构体，另外两个通过将观察到的UV和ECD光谱与通过时变DFT计算获得的理论光谱进行了比较。结果表明，结合使用实验性ECD和VCD光谱以及理论DFT计算可以对构象柔性分子MCO的Cotton效应给出合理的解释。

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