Helical C(2) structure of perfluoropentane and the C(2v) structure of perfluoropropane.
Helical C(2) structure of perfluoropentane and the C(2v) structure of perfluoropropane.
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全氟戊烷的螺旋C(2)结构和全氟丙烷的C(2v)结构。
DOI：
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发表时间：
2010
影响因子：
2.9
通讯作者：
M. Onda
中科院分区：
文献类型：
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作者：
J. Fournier;R. Bohn;J. Montgomery;M. Onda
饱和烃具有完全交错键和180度二面角的结构。用氟取代氢会导致180度的轻微偏移，从而产生螺旋结构。对纤维的X射线衍射研究和对全氟烷烃的计算研究估计，从反位开始的二面角约为17度。用脉冲射流傅里叶变换微波光谱仪观测了全氟戊烷及其三种（13）C同位素的转动光谱，并进行了归属。母种的转动常数分别为A 990.6394（3）MHz、B 314.0002（1）MHz和C 304.3703（1）MHz。确定一个精确的二面角一直是具有挑战性的，因为螺旋扭曲已被证明是相当敏感的结构输入和约束。一系列包含各种模型约束和Kraitchman分析的r（0）结构给出了13-19度的扭转角范围。一个客观的方法，它只假设整体C（2）对称性，是从从头算模型的主坐标的比例的平方根的观察到的二阶矩计算的二阶矩。计算模型的比例结构在不同的理论水平上精确地再现了母二阶矩和（13）C二阶矩，给出了17 +/- 1度的二面角。全氟丙烷的微波光谱也被观察到并分配。转动常数分别为A 1678.5982（9）MHz、B 900.1968（10）MHz和C 955.3216（11）MHz。与较长的全氟烷烃不同，全氟丙烷具有非螺旋的C（2 v）结构。计算结果与实验结果非常吻合。
Saturated hydrocarbons have structures with completely staggered bonds and dihedral angles of 180 degrees . Substituting hydrogen by fluorine results in a slight shift from 180 degrees , giving rise to a helical structure. X-ray diffraction studies on fibers and computational studies on perfluoroalkanes estimate a dihedral angle of about 17 degrees from the trans position. The rotational spectra of perfluoropentane and its three (13)C isotopomers have been observed and assigned using a pulsed-jet Fourier transform microwave spectrometer. The rotational constants for the parent species are A 990.6394(3) MHz, B 314.0002(1) MHz, and C 304.3703(1) MHz, respectively. The determination of an exact dihedral angle has been challenging, as the helical twist has proven to be quite sensitive to the structural inputs and constraints. A series of r(0) structures incorporating various model constraints and a Kraitchman analysis gives a range of 13-19 degrees for the torsional angle. An objective approach, which only assumes overall C(2) symmetry, is to scale the principal coordinates from ab initio models by the square root of the ratio of the observed second moments to the computed second moments. The scaled structures of computed models at various levels of theory reproduce the parent second moments exactly and the (13)C second moments very well, giving a dihedral angle of 17 +/- 1 degrees from trans. The microwave spectrum of perfluoropropane has also been observed and assigned. The rotational constants are A 1678.5982(9) MHz, B 900.1968(10) MHz, and C 955.3216(11) MHz, respectively. Unlike longer perfluoroalkanes, perfluoropropane has a nonhelical, C(2v) structure. Computations are in excellent agreement with experimental results.