Molecular mechanics assessment of the configurational statistics of polyoxyethylene
Molecular mechanics assessment of the configurational statistics of polyoxyethylene
复制标题
聚氧乙烯构型统计的分子力学评估
DOI：
10.1002/jcc.540050306
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
R. Gandour
中科院分区：
文献类型：
--
作者：
D. Baldwin;W. Mattice;R. Gandour
先前的研究人员表明，未受扰动的长聚氧乙烯链的构型依赖物理性质的统计机械平均值对 C-C 键旋转的左反式能量差很敏感。理论与实验之间的一致性只能通过将该能量显着调整为远离半经验构象能量计算预测的值来获得。目前的工作检验了 MM2 在评估未受干扰的长聚氧乙烯链的构象特性方面是否成功。进行计算，确定 CH3OCH2CH2OCH2CH2OCH2CH2OCH3 中所示键的旋转异构体及其能量。这些能量用于分配在具有对称三重相互依赖旋转的旋转异构状态链的构型分配函数中使用的统计权重。采用常规生成矩阵方案来评估均方无扰动端到端距离、均方无扰动偶极矩及其温度系数。与之前采用的计算方案相反，MM2被发现可以提供绕CC旋转的gauche-trans能量差的估计，这与未受扰动的聚合物的已知尺寸和偶极矩一致。 MM2 还为旋转异构态处理所需的大多数其他参数提供了良好的估计。一个值得注意的例外是围绕 C-O 键旋转的左反式能量差。 MM2 高估了这种能量差异。
Previous investigators have shown that statistical mechanical averages for configuration‐dependent physical properties of long unperturbed polyoxyethylene chains are sensitive to the gauche–trans energy difference for rotation about CC bonds. Agreement between theory and experiment could be obtained only by significant adjustment of this energy away from values predicted by semiempirical conformational energy computations. The present work examines the success of MM2 in evaluating conformational properties of long unperturbed polyoxyethylene chains. Calculations are performed which identify the rotational isomers, and their energies, for the indicated bonds in CH3OCH2CH2OCH2CH2OCH2CH2OCH3. These energies are used to assign statistical weights utilized in the configuration partition function for a rotational isomeric state chain with symmetric threefold interdependent rotations. The customary generator matrix scheme is employed to evaluate the mean‐square unperturbed end‐to‐end distance, mean‐square unperturbed dipole moment, and their temperature coefficients. Contrary to computational schemes employed previously, MM2 is found to provide an estimate of the gauche–trans energy difference for rotation about CC which is in harmony with the known dimensions and dipole moments of the unperturbed polymer. MM2 also provides good estimates for most of the other parameters required in the rotational isomeric state treatment. A notable exception is provided by the gauche–trans energy difference for rotation about the CO bond. This energy difference is overestimated by MM2.