Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, USA.
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS UMR 7583, Université Paris-Est Créteil, Université de Paris, Institute Pierre Simon Laplace, 61 avenue du Général de Gaulle, Créteil, France.
J Chem Phys. 2020 Feb 21;152(7):074301. doi: 10.1063/1.5142401.
The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V potential barriers of about 22 cm and 584-588 cm for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations.
在喷射冷却条件下，从 8GHz 到 18GHz 记录的 4-甲基苯乙酮的傅里叶变换微波光谱揭示了由环甲基的低旋转势垒引起的大隧道分裂，以及由乙酰甲基的高扭转势垒引起的小分裂。大分裂尤其难以建模，而小分裂由于 120kHz 的分辨率限制而难以分析。两个甲基基团的内部旋转组合导致每个旋转跃迁分裂成五个扭转物种，这些物种使用经过修改的 XIAM 代码和新开发的 ntop 代码进行解析和拟合，均方根偏差接近测量精度，为环和乙酰甲基基团的 V 势能垒分别提供了约 22cm 和 584-588cm 的估计值。通过分别使用奇数幂次运算符拟合五个扭转物种，有助于进行分配。在光谱中只能识别出一种构象，其中所有重原子都位于对称平面上，这与使用量子化学计算进行构象分析的结果一致。
