Internal vibrations of a molecule consisting of rigid segments. I. Non-interacting internal vibrations.
Internal vibrations of a molecule consisting of rigid segments. I. Non-interacting internal vibrations.
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由刚性链段组成的分子的内部振动。
DOI：
10.1107/s0108767392005245
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发表时间：
1993
期刊：
影响因子：
--
通讯作者：
Craven,BM
中科院分区：
文献类型：
--
作者：
He,XM;Craven,BM
对于分子晶体，提出了一个程序来解释实验确定的原子均方各向异性位移参数（ADP）的整体分子振动与内部振动的假设，该分子由一组连接的刚性段。使用分段体的可变内部坐标来描述内部振动（分子扭转或弯曲模式）。在这份由两部分组成的报告的第一篇中，假设作为零阶近似，关于成对段之间的连接轴的内部振动彼此不相关，并且与整体分子刚体振动不相关。作为一阶近似，每个内部振动可以与外部振动相关的可能性也被考虑。这种方法的一个重要特征是要求内部振动在任何时候对分子的总动量的贡献为零，因此内部坐标必须与外部坐标正交。此外，每个内部振动都涉及分子中所有原子的运动。所得的内部振动参数是不变的参考段的选择。用此方法分析了六种小分子（均三硝基苯、腺苷、四氰基醌二甲烷、苯甲酰胺、α-氰基乙酸酰肼和N-乙酰基-L-色氨酸甲酰胺）的晶体结构测定结果。因此，计算了分子的键长和角度的振动校正以及频率和力常数（具有e.s.d.’s）用于每个内部扭转或弯曲振动。与其他用于描述内部振动的模型相比，总ADP如何在内部和整体分子振动之间分配存在差异。
For molecular crystals, a procedure is proposed for interpreting experimentally determined atomic mean square anisotropic displacement parameters (ADPs) in terms of the overall molecular vibration together with internal vibrations with the assumption that the molecule consists of a set of linked rigid segments. The internal librations (molecular torsional or bending modes) are described using the variable internal coordinates of the segmented body. In paper I of this two-part report, it is assumed as a zero-order approximation that the internal vibrations about the linkage axes between pairs of segments are uncorrelated with each other and with the overall molecular rigid-body vibrations. As a first-order approximation, the possibility that each internal vibration can be correlated with the external vibrations is also considered. An important feature of this approach is that the internal librations are required to give zero contribution to the overall momentum of the molecule at all times, so the internal coordinates must be orthogonai to the external ones. Also, each of the internal librations involves the motion of all atoms in the molecule. The resulting internal vibrational parameters are invariant to the choice of reference segment. With this procedure, the experimental ADPs obtained from crystal structure determinations involving six small molecules (sym-trinitrobenzene, adenosine, tetracyanoquinodimethane, benzamide, α-cyanoacetic acid hydrazide and N-acetyl-l-tryptophan methylamide) have been analyzed. As a consequence, vibrational corrections to the bond lengths and angles of the molecule are calculated as well as the frequencies and force constants (with e.s.d.'s) for each internal torsional or bending vibration. Compared with other models used for describing internal vibrations, there are differences in how the total ADP is partitioned between the internal and overall molecular vibrations.