Donetsk National Technical University, 58 Artema Strasse, 83000 Donetsk, Ukraine.
J Phys Chem B. 2012 Feb 23;116(7):2173-82. doi: 10.1021/jp211913p. Epub 2012 Feb 9.
In the frameworks of the quantum-chemical semiempirical PM3 method, the thermodynamic parameters of trans-isomers of unsaturated carboxylic acids at the air/water interface were studied. Systems with 18-26 carbon atoms in the alkyl chain and different positions of the double bond are considered. Using quantum-chemical semiempirical PM3 method enthalpy, Gibbs' energy of monomers' formation from the elementary compounds and absolute entropy of trans-unsaturated carboxylic acids are calculated. It has been shown that thermodynamic parameters mentioned above for isomers with the same number of carbon atoms in the hydrocarbon chain but different position of double bond are practically the same within the margin of error. For dimers, trimers, and tetramers of the four trans-unsaturated carboxylic acids, the thermodynamic parameters of clusterization were calculated. It is shown that the position of double bond does not significantly affect the values of thermodynamic parameters of formation and clusterization of carboxylic acids with equal alkyl chain lengths. The only exception is the case that the double bond is in the ω-position (extremely distanced from the carboxylic group). In this case, the number of intermolecular interactions between alkyl chains is changed. Spontaneous clusterization of trans- in the standard conditions is possible for molecules that possess more than 16-17 carbon atoms in the alkyl chain. These threshold values exceed the corresponding values that were calculated previously using the quantum-chemical PM3 method for saturated carboxylic acids (12-13 carbon atoms in the alkyl chain) and are a little bit smaller than the corresponding parameters for cis-unsaturated carboxylic acids (18-19 carbon atoms). These values agree with experimental parameters. Also, the calculated structural parameters of trans-unsaturated carboxylic acids' monolayer for the unit cell with a = 6.98 Å, b = 8.30 Å, and for the molecular tilt angle with 64.95° agree with the experimental parameters.
在量子化学半经验 PM3 方法的框架内，研究了不饱和羧酸反式异构体在空气/水界面的热力学参数。考虑了烷基链中具有 18-26 个碳原子和不同双键位置的系统。使用量子化学半经验 PM3 方法计算了单体从基本化合物形成的焓、吉布斯自由能以及反式不饱和羧酸的绝对熵。结果表明，对于具有相同碳原子数的异构体，在误差范围内，上述热力学参数在具有不同位置的双键的异构体中实际上是相同的。对于四种反式不饱和羧酸的二聚体、三聚体和四聚体，计算了聚集的热力学参数。结果表明，双键的位置不会显著影响具有相同烷基链长度的羧酸的形成和聚集的热力学参数值。唯一的例外是双键处于ω 位（离羧基基团非常远）的情况。在这种情况下，烷基链之间的分子间相互作用的数量发生了变化。在标准条件下，对于具有超过 16-17 个碳原子的烷基链的分子，反式可以自发聚集。这些阈值超过了使用量子化学 PM3 方法先前为饱和羧酸（烷基链中 12-13 个碳原子）计算的相应值，并且略小于顺式不饱和羧酸（18-19 个碳原子）的相应参数。这些值与实验参数一致。此外，对于具有 a = 6.98 Å、b = 8.30 Å 的单位晶胞和分子倾斜角为 64.95°的反式不饱和羧酸单层的计算结构参数与实验参数一致。
