Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland.
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.
Molecules. 2023 Feb 10;28(4):1692. doi: 10.3390/molecules28041692.
The orientation and magnitude of the molecular electric dipole moment are key properties relevant to topics ranging from the nature of intermolecular interactions to the quantitative analysis of complex gas-phase mixtures, such as chemistry in astrophysical environments. Stark effect measurements on rotational spectra have been the method of choice for isolated molecules but have become less common with the practical disappearance of Stark modulation spectrometers. Their role has been taken over by supersonic expansion measurements within a Fabry-Perot resonator cavity, which introduces specific technical problems that need to be overcome. Several of the adopted solutions are described and compared. Presently, we report precise electric dipole moment determinations for the two most stable conformers of the selected molecules of confirmed or potential astrophysical relevance: -propanol, -butanol, and -butyl cyanide. All dipole moment components have been precisely determined at supersonic expansion conditions by employing specially designed Stark electrodes and a computer program for fitting the measured Stark shifts, inclusive of cases with resolved nuclear quadrupole hyperfine structure. The experimental values are compared with suitable quantum chemistry computations. It is found that, among the tested levels of computation, vibrationally averaged dipole moments are the closest to the observation and the molecular values are, as in the lighter molecules in the series, largely determined by the hydroxyl or nitrile groups.
分子电偶极矩的取向和大小是与从分子间相互作用的本质到复杂气相混合物（如天体物理环境中的化学）的定量分析等广泛主题相关的关键性质。旋转光谱的斯塔克效应测量一直是分离分子的首选方法，但随着斯塔克调制光谱仪的实际消失，这种方法变得不那么常见了。它们的作用已经被超声速膨胀测量在法布里-珀罗谐振腔中取代，这引入了需要克服的特定技术问题。本文描述并比较了几种采用的解决方案。目前，我们报告了对两个最稳定构象体的精确电偶极矩测定，这两个构象体是选定的具有确定或潜在天体物理相关性的分子：-丙醇、-丁醇和-丁腈。通过使用专门设计的斯塔克电极和用于拟合测量的斯塔克位移的计算机程序，包括具有分辨核四极超精细结构的情况，在超声速膨胀条件下精确地确定了所有偶极矩分量。实验值与合适的量子化学计算进行了比较。结果发现，在所测试的计算水平中，振动平均偶极矩与观测值最接近，并且分子值与系列中较轻的分子一样，主要由羟基或腈基决定。
