(DeltaH degrees ), determined at 157 K in independent studies at two laboratories, were found to be (kcal/mol) 1.76 +/- 0.10 (Me), 1.54 +/- 0.12 (Et), and 1.40 +/- 0.15 (i-Pr); the corresponding conformational entropies (DeltaS degrees, eu) were 0.2 +/- 0.2 (Me), 1.3 +/- 0.8 (Et), and 3.5 +/- 0.9 (i-Pr). Computational studies at the QCISD level gave satisfactory agreement with the experimental results, but
甲基-(1),乙基-(2)和
异丙基环己烷(3)的构象焓(DeltaH度),熵(DeltaS度)和自由能(-DeltaG度)已通过实验和计算方法进行了重新研究。描述了一种新颖的实验方法来评估高度偏向的构象平衡,该方法避免了直接测量大的轴向/赤道异构体比率以确定平衡常数的需要:C(2,6)的自然丰度(13)C信号烷基
环己烷的赤道异构体的共振可以用作内部参考,并且该轴向峰异构体的能带面积与轴向异构体中富集的(13)C核的能带面积之比在对统计差异和不同的(13)进行校正后得出K监视信号的C含量。在两个实验室的独立研究中确定的实验构象焓(DeltaH度)为(kcal / mol)1.76 +/- 0.10(Me),1.54 +/- 0.12(Et)和1.40 + /-0.15(i-Pr);相应的构象熵(DeltaS度,eu)为0.2 +/- 0.2(Me),1.3 +/- 0.8(Et)和3.5 +/-