CH•••O Interaction Lowers Hydrogen Transfer Barrier to Keto-Enol Tautomerization of β-Cyclohexanedione: Combined Infrared Spectroscopic and Electronic Structure Calculation Study
CH•••O Interaction Lowers Hydrogen Transfer Barrier to Keto-Enol Tautomerization of β-Cyclohexanedione: Combined Infrared Spectroscopic and Electronic Structure Calculation Study
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DOI：
10.1021/jp2108736
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发表时间：
2012-04-19
影响因子：
2.9
通讯作者：
Chakraborty, Tapas
中科院分区：
文献类型：
--
作者：
Bandyopadhyay, Biman;Pandey, Prasenjit;Chakraborty, Tapas
本文研究了β-环己二酮(β-CHD)在Ar基质中的分子缔合和酮-烯醇互变异构化，以及在冷(8K)KBrR窗口沉积纯β-CHD蒸气制备的固体薄膜中的分子缔合和酮烯醇互变异构化反应。红外光谱表明，在低压蒸气和Ar基质中，分子完全以二酮互变的形式存在。β-CHD在Ar基质中的酮烯醇异构体在8-28K温度范围内发生酮烯醇互变异构化，而在纯的双酮互变异构体薄膜中，只有在165K以上的温度下才开始转化。观察到的阈值似乎与分子间模式的激发有关，而在高温下记录的红外光谱显示振动带宽变窄，这与薄膜中分子的重新取向有关。在B3LYP/6-311++G**和MP2/6-311++G**水平上的电子结构计算表明，基质隔离二聚体没有发生互变异构化，这与电子结构计算预测的势垒是一致的。过渡态计算预测，CH中心点O相互作用对降低互变异构化势垒有显著作用，从裸分子的60kcal/mol以上降至二聚体的35-45kcal/mol。
Molecular association and keto-enol tautomerization of beta-cyclohexanedione (beta-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure beta-CHD vapor on a cold (8 K) KBr window. Infrared spectra reveal that, in low-pressure vapor and argon matrix, the molecules are exclusively in diketo tautomeric form. The CH center dot center dot center dot O hydrogen bonded dimers of the diketo tautomer are produced by annealing the matrix at 28 K. No indication is found for keto-enol tautomerization of beta-CHD in dimeric complexes in argon matrix within the temperature range of 8-28 K. On the, other hand, in thin film of pure diketo tautomer, the conversion initiates only when the film is heated at temperatures above 165 K. The observed threshold appears to be associated with excitation of the intermolecular modes, and the IR spectra recorded at high temperatures display narrowing of vibrational bandwidths, which has been associated with reorientations of the molecules in the film. The nonoccurrence of tautomerization of the matrix isolated dimer is consistent with the barrier predicted by electronic structure calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The transition state calculation predicts that CH center dot center dot center dot O interaction has a dramatic effect on lowering of the tautomerization barrier, from more than 60 kcal/mol for the bare molecule to similar to 35-45 kcal/mol for dimers.