Durig James R, Zheng Chao, Gounev Todor K, Herrebout Wouter A, van der Veken Benjamin J
Department of Chemistry, University of Missouri-Kansas City, 64110, USA.
J Phys Chem A. 2006 May 4;110(17):5674-84. doi: 10.1021/jp057305q.
Variable temperature (-55 to -145 degrees C) studies of the infrared spectra (3500 to 100 cm(-1)) of ethylamine, CH(3)CH(2)NH(2), dissolved in liquid krypton and/or xenon have been recorded. From these data, the enthalpy differences have been determined to be 54 +/- 4 cm(-1) (0.65 +/- 0.05 kJ/mol), with the trans conformer (methyl group relative to the lone pair of electrons on nitrogen) being the more stable form. It is estimated that there is 61 +/- 1% of the doubly degenerate gauche form present at ambient temperature. The conformational energetics have been calculated with the Møller-Plesset perturbation method to the second order (MP2(full)) and the fourth order (MP4(SDTQ)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. Basis sets with diffuse functions lead to incorrect prediction of the conformational stability. On the basis of the frequencies of the torsional transitions along with the determined experimental enthalpy difference and gauche dihedral angle, the potential function governing conformational interchange has been obtained, and the determined Fourier cosine coefficients are V(1) = -207 +/- 48, V(2) = 320 +/- 67, V(3) = 1072 +/- 25, V(4) = 55 +/- 11, and V(5) = -96 +/- 28 cm(-1), with a trans-to-gauche barrier of 1286 cm(-1), and a gauche-to-gauche barrier of 715 cm(-1). The 3-fold methyl rotational barriers have been determined to be 1241 +/- 4 and 1281 +/- 10 cm(-1) for the gauche and trans conformers, respectively. By utilizing the previously reported microwave rotational constants combined with the structural parameters predicted at the MP2(full)/6-311+ G(d,p) level, adjusted r(0) structural parameters have been obtained. A complete vibrational assignment is given for the trans conformer, which is supported by normal coordinate calculations utilizing scaled force constants from ab initio B3LYP/6-311++G(3df,3pd) calculations. Proposed assignments are also made for the fundamentals of the gauche conformer. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for similar molecules.
已记录了溶解在液态氪和/或氙中的乙胺（CH₃CH₂NH₂）在可变温度（-55至-145摄氏度）下的红外光谱（3500至100厘米⁻¹）。根据这些数据，确定焓差为54±4厘米⁻¹（0.65±0.05千焦/摩尔），其中反式构象体（甲基相对于氮上的孤对电子）是更稳定的形式。据估计，在环境温度下存在61±1%的双重简并gauche形式。使用二阶（MP2(full)）和四阶（MP4(SDTQ)）的Møller-Plesset微扰方法以及利用各种基组的B3LYP方法通过密度泛函理论计算了构象能量学。具有弥散函数的基组会导致对构象稳定性的错误预测。根据扭转跃迁的频率以及确定的实验焓差和gauche二面角，获得了控制构象互换的势函数，确定的傅里叶余弦系数为V(1)= -207±48，V(2)= 320±67，V(3)= 1072±25，V(4)= 55±11，V(5)= -96±28厘米⁻¹，反式到gauche的势垒为1286厘米⁻¹，gauche到gauche的势垒为715厘米⁻¹。gauche和反式构象体的3重甲基旋转势垒分别确定为1241±4和1281±10厘米⁻¹。通过利用先前报道的微波旋转常数并结合在MP2(full)/6 - 311 + G(d,p)水平预测的结构参数，获得了调整后的r(0)结构参数。给出了反式构象体的完整振动归属，这得到了利用从头算B3LYP/6 - 311++G(3df,3pd)计算的标度力常数进行的简正坐标计算的支持。还对gauche构象体的基频进行了拟议的归属。讨论了这些光谱和理论研究的结果，并与类似分子的相应结果进行了比较。
