Low Symmetry in Molecules with Heavy Peripheral Atoms. The Gas-Phase Structure of Perfluoro(methylcyclohexane), C6F11CF3

详细信息    查看全文

• 作者：Graeme R. Kafka ; Sarah L. Masters ; Derek A. Wann ; Heather E. Robertson ; David W. H. Rankin
• 刊名：Journal of Physical Chemistry A
• 出版年：2010
• 出版时间：October 21, 2010
• 年：2010
• 卷：114
• 期：41
• 页码：11022-11026
• 全文大小：117K
• 年卷期：v.114,no.41(October 21, 2010)
• ISSN：1520-5215

文摘

When refining structures using gas electron diffraction (GED) data, assumptions are often made in order to reduce the number of required geometrical parameters. Where these relate to light, peripheral atoms there is little effect on the refined heavy-atom structure, which is well defined by the GED data. However, this is not the case when heavier atoms are involved. We have determined the gas-phase structure of perfluoro(methylcyclohexane), C₆F₁₁CF₃, using three different refinement methods and have shown that our new method, which makes use of both MP2 and molecular mechanics (MM) calculations to restrain the peripheral-atom geometry, gives a realistic structure without the need for damaging constraints. Only the conformer with the CF₃ group in an equatorial position was considered, as ab initio calculations showed this to be 25 kJ mol⁻¹ lower in energy than the axial conformer. Refinements combining both high-level and low-level calculations to give constraints were superior both to those based only on molecular mechanics and to those in which assumptions about the geometry were imposed.