Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany.
J Phys Chem A. 2011 Jan 20;115(2):118-22. doi: 10.1021/jp102747f. Epub 2010 Dec 16.
We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.
我们通过分子束傅里叶变换微波光谱法观察了异戊酸乙酯的微波光谱。确定了最丰富构象的旋转和离心畸变常数。通过将实验旋转常数与从头算方法获得的旋转常数进行比较，研究了其结构。在第一步中，在 MP2/6-311++G** 理论水平上计算了各种构象的旋转常数。令人惊讶的是，与实验结果没有一致的地方。因此，我们得出结论，在异戊酸乙酯的情况下，需要更先进的量子化学方法来获得可靠的分子几何形状。在 MP3/6-311++G**、MP4/6-311++G** 和 CCSD/6-311++G** 水平上进行的从头算计算以及使用 B3LYP/6-311++G** 方法的密度泛函理论计算，对旋转常数给出了相似的结果，但与在 MP2/6-311++G** 水平上的结果明显不同。使用这些更先进的方法，最低能量构象的旋转常数与从微波光谱中获得的旋转常数非常吻合。
