Liang Yue, Zhang Yichi, Fábri Csaba, Ma Jiarui, Zhao Jianbao, Billinghurst Brant, Chen Ziqiu
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China.
Phys Chem Chem Phys. 2021 Apr 14;23(14):8549-8556. doi: 10.1039/d1cp00188d. Epub 2021 Mar 26.
A discrepancy between theoretical and experimental values of the rotational barrier in benzaldehyde has been observed, which was attributed to inaccurate experimental results in part. Here, we report results on the -CHO torsion of benzaldehyde (CHCHO) based on a high resolution spectroscopic investigation in the far-infrared range in an effort to remove the experimental ambiguity. The rotationally-resolved vibrational spectra were measured with an unapodized resolution of 0.00064 cm using synchrotron-based Fourier transform infrared (FTIR) spectroscopy at the Canadian Light Source. The torsional fundamental ν = 109.415429(20) cm was unambiguously assigned via rovibrational analysis, followed by the tentative assignment of the first (2ν-ν) and second (3ν- 2ν) hot bands at 107.58 cm and 105.61 cm, respectively, by comparison of the observed Q branch structures at high resolution with simulation based on a previous microwave study. This assignment is different from any previous low resolution infrared studies in which the intensity patterns were misleading. The key result of the assignment of the first three transitions allowed the determination of the barrier to internal rotation of (hc)1533.6 cm (4.38 kcal mol). When compared with calculated results from vibrational second-order perturbation theory (VPT2) and the quasiadiabatic channel reaction path Hamiltonian (RPH) approach, the experimental value is still too low and this suggests that the discrepancy between theory and experiment remains despite the best experimental efforts.
已观察到苯甲醛中旋转势垒的理论值与实验值之间存在差异，部分原因是实验结果不准确。在此，我们报告基于远红外范围内高分辨率光谱研究的苯甲醛（CHCHO）-CHO扭转结果，以消除实验上的不确定性。使用加拿大光源处基于同步加速器的傅里叶变换红外（FTIR）光谱，以0.00064 cm的未变迹分辨率测量了旋转分辨振动光谱。通过振转分析明确确定了扭转基频ν = 109.415429(20) cm，随后通过将高分辨率下观察到的Q支结构与基于先前微波研究的模拟结果进行比较，分别初步确定了第一（2ν-ν）和第二（3ν- 2ν）热谱带位于107.58 cm和105.61 cm处。这一归属与以往任何低分辨率红外研究不同，在以往研究中强度模式具有误导性。对前三个跃迁的归属的关键结果使得能够确定内旋转势垒为（hc）1533.6 cm（4.38 kcal mol）。与振动二阶微扰理论（VPT2）和准绝热通道反应路径哈密顿量（RPH）方法的计算结果相比，实验值仍然过低，这表明尽管进行了最佳的实验努力，理论与实验之间的差异仍然存在。
