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Issue 5, 2001
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Infrared and Raman spectra, conformational stability, ab initio calculations of structure and vibrational assignment of 5-fluoropent-2-yne

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Abstract

The infrared spectra (3500–50 cm−1) of the gas and solid and the Raman spectra (3500–50 cm−1) of the liquid and solid were recorded for 5-fluoropent-2-yne, CH3–C[triple bond, length half m-dash]C–CH2CH2F. Variable temperature studies of the infrared spectrum (3500–400 cm−1) of 5-fluoropent-2-yne dissolved in liquid krypton and xenon were also recorded. Utilizing antigauche conformer pairs, the enthalpy difference was determined as 272 ± 11 cm−1 (3.25 ± 0.13 kJ mol−1) and 297 ± 31 cm−1 (3.55 ± 0.37 kJ mol−1) from the xenon and krypton solutions, respectively, with the anti rotamer the more stable form. Equilibrium geometries and energies of the two conformers have been determined by ab initio and hybrid density functional theory (DFT) methods using a number of basis sets. A vibrational assignment is proposed based on the force constants, relative intensities, depolarization ratios from the ab initio and DFT calculations and on vibrational–rotational band contours obtained using the calculated equilibrium geometries. From calculated energies it is shown that the CH3 group exhibits almost completely free rotation, which is in agreement with the observation of sub-band structure in the degenerate methyl vibrations from which values of the Coriolis coupling constant, ζ, were determined. The results are discussed and compared with the corresponding quantities for some similar molecules.

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Supplementary files

Article information


Submitted
25 Sep 2000
Accepted
30 Nov 2000
First published
12 Feb 2001

Phys. Chem. Chem. Phys., 2001,3, 776-785
Article type
Paper

Infrared and Raman spectra, conformational stability, ab initio calculations of structure and vibrational assignment of 5-fluoropent-2-yne

S. Bell, X. Zhu, G. A. Guirgis and J. R. Durig, Phys. Chem. Chem. Phys., 2001, 3, 776
DOI: 10.1039/B007765H

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