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Spectra and structure of small ring compounds. Part LV –Conformational stability of bromocyclobutane from Raman and far‐infrared spectra of the gas
Author(s) -
Durig J. R.,
Little T. S.,
Lee Min Joo
Publication year - 1989
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.1250201202
Subject(s) - conformational isomerism , raman spectroscopy , chemistry , enthalpy , spectral line , infrared , potential energy , ring (chemistry) , molecule , infrared spectroscopy , analytical chemistry (journal) , atomic physics , thermodynamics , physics , optics , organic chemistry , chromatography , astronomy
Abstract The Raman (3200 to 10 cm −1 ) and far‐infrared (375 to 80 cm −1 ) spectra of gaseous bromocyclobutane, c‐C 4 H 7 Br, have been recorded. A series of Q ‐branches observed in both of these spectra, beginning at 149 cm −1 with successive transitions falling to lower frequencies, have been assigned to the ring‐puckering vibrations of both the low‐energy equatorial and high‐energy axial conformers. These data have been fit to an asymmetric potential function of the form V (cm −1 ) = (6.58 ± 0.21) × 10 5 X 4 − (3.27 ± 0.04) × 10 4 X 2 × (4.59 ± 0.19) × 10 4 X 3 with a reduced mass of 214 amu. Utilizing this potential, the difference between the puckering angles for the two conformers is calculated to be 6°, with the equatorial conformer having the larger value of 20°. Also from this potential function, the energy difference between the equatorial and axial forms is found to be 361 ± 20 cm −1 (1.03 kcal mol −1 ), with a barrier of 636 ± 20 cm −1 (1.82 kcal mol −1 ) to interconversion. Experimental values for the enthalpy difference between the two conformers have been determined for both the liquid (297 ± 27 cm −1 ) and gas (∼ 350 cm −1 ) from relative intensities of a pair of Raman lines over 90 and 58 °C temperature ranges respectively. These results are compared to the corresponding quantities for some similar molecules.