Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany.
Chemphyschem. 2012 Apr 10;13(5):1297-301. doi: 10.1002/cphc.201100932. Epub 2012 Feb 29.
We report on the microwave spectrum of ethyl valerate, C(4)H(9)-COO-C(2)H(5), observed by molecular beam Fourier transform microwave spectroscopy (MB-FTMW). Highly accurate rotational and centrifugal distortion constants of the two most abundant conformers were determined. Their structure was investigated by comparison of the experimental rotational constants with those obtained by quantum chemical calculations. MP2/ 6-311++G(d,p) calculations failed to give a proper description of the observed conformers in the supersonic jet. Therefore, also other basis sets were tried using Hartree-Fock, second-order Møller-Plesset (MP2), and B3LYP methods to predict the observed gas-phase structures of the molecule. The quantum chemical results guided the conformer assignment of the rotational constants, obtained from the microwave experiment. One of the two observed conformers has C(s) symmetry, while the most abundant conformer has C(1) symmetry. The main conformer possesses a strong spectrum with high intensities. Additionally, harmonic frequency calculations at different levels of theory where carried out and a low lying vibration of the entire n-butyl group against the rest of the molecule was identified. The symbiotic interplay of microwave gas-phase investigations and quantum chemical calculations becomes evident in our results.
我们报告了通过分子束傅里叶变换微波光谱（MB-FTMW）观测到的正丁酸乙酯（C(4)H(9)-COO-C(2)H(5)）的微波光谱。确定了两种最丰富构象体的高精度旋转和离心扭曲常数。通过将实验旋转常数与量子化学计算得到的常数进行比较，研究了它们的结构。MP2/6-311++G(d,p) 计算未能正确描述超音速射流中观察到的构象体。因此，还使用 Hartree-Fock、二阶 Møller-Plesset（MP2）和 B3LYP 方法尝试了其他基组，以预测分子的观察到的气相结构。量子化学结果指导了从微波实验获得的旋转常数的构象分配。两个观察到的构象体之一具有 C(s) 对称性，而最丰富的构象体具有 C(1) 对称性。主要构象体具有强烈的光谱，强度很高。此外，还进行了不同理论水平的谐频计算，并确定了整个正丁基基团相对于分子其余部分的低能振动。微波气相研究和量子化学计算的共生相互作用在我们的结果中显而易见。
