Rosado Mário T S, Jesus António J Lopes, Reva Igor D, Fausto Rui, Redinha José S
Department of Chemistry, University of Coimbra, 3004-535, Coimbra, Portugal.
J Phys Chem A. 2009 Jul 2;113(26):7499-507. doi: 10.1021/jp900771g.
The complete conformational space of monomeric 1,3-butanediol has been characterized theoretically, and 73 unique stable conformers were found at the MP2/6-311++G(d,p) level. These were classified into nine families whose members share the same heavy atom backbone configurations and differ in the hydrogen atom orientations. The first and third most populated backbone families are governed by the formation of an intramolecular hydrogen bond; however, the second precludes this type of interaction and was frequently overlooked in previous studies. Its stability is determined by the relatively high entropy of its main conformers. The hydrogen bonding of four of the most important conformers was characterized by means of atoms in molecules (AIM, also known as QTAIM) and natural bond orbital (NBO) analyses. Using appropriate isodesmic reactions, hydrogen bonding energy stabilizations of 12-14 kJ mol(-1) have been found. Experimentally, monomeric molecules of 1,3-butanediol were isolated in low-temperature inert matrixes, and their infrared spectra were analyzed from the viewpoint of the conformational distribution. All the relevant transition states for the conformational interconversion reaction paths were characterized at the same level of theory to interpret the conformational cooling dynamics observed in the low-temperature matrixes. The energy barriers for rotation of the OH groups were calculated to be very low (<3 kJ mol(-1)). These barriers were overcome in the experiments at 10 K (Ar matrix), in the process of matrix deposition, and population within each family was reduced to the most stable conformers. Further increase in the substrate temperature (up to 40 K, Xe matrix) resulted in conformational cooling where the medium-height barriers (approximately 13 kJ mol(-1)) could be surmounted and all conformational population converted to the ground conformational state. Remarkably, this state turned to consist of two forms of the most stable hydrogen bonded family, which were predicted by calculations to be accidentally degenerated and were found in the annealed matrix in equal amounts. All of these experimentally observed conformational cooling processes were analyzed and supported by full agreement with the theoretical calculations.
理论上已对单体1,3 - 丁二醇的完整构象空间进行了表征，在MP2/6 - 311++G(d,p)水平上发现了73个独特的稳定构象体。这些构象体被分为九个家族，其成员具有相同的重原子骨架构型，仅氢原子取向不同。占据数量第一和第三的骨架家族受分子内氢键形成的支配；然而，第二个家族排除了这种类型的相互作用，并且在先前的研究中经常被忽视。其稳定性由其主要构象体相对较高的熵决定。通过分子中的原子（AIM，也称为QTAIM）和自然键轨道（NBO）分析对四个最重要构象体的氢键进行了表征。使用适当的等键反应，发现氢键能量稳定化值为12 - 14 kJ mol⁻¹。实验上，1,3 - 丁二醇的单体分子在低温惰性基质中被分离出来，并从构象分布的角度对其红外光谱进行了分析。在相同理论水平上对构象互变反应路径的所有相关过渡态进行了表征，以解释在低温基质中观察到的构象冷却动力学。计算得出OH基团旋转的能垒非常低（<3 kJ mol⁻¹）。在10 K（氩基质）的实验中，在基质沉积过程中这些能垒被克服，每个家族内的构象体数量减少到最稳定的构象体。底物温度进一步升高（高达40 K，氙基质）导致构象冷却，其中中等高度的能垒（约13 kJ mol⁻¹）可以被跨越，所有构象体数量都转化为基态构象状态。值得注意的是，这种状态由最稳定的氢键家族的两种形式组成，计算预测这两种形式是偶然简并的，并且在退火基质中以相等的量被发现。所有这些实验观察到的构象冷却过程都经过了分析，并与理论计算完全一致地得到了支持。
