The utility of infrared spectroscopy for quantitative conformational analysis at a single temperature M.P. Freitas a , C.F. Tormena a , R. Rittner a, *, R.J. Abraham b a Physical Organic Chemistry Laboratory, Instituto de Quı ´mica, UNICAMP, Cx. Postal 6154, 13083-862 Campinas, SP, Brazil b Chemistry Department, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK Received 22 October 2002; accepted 11 November 2002 Abstract The carbonyl stretching vibration of 2-bromocyclohexanone (1) has been measured in a variety of solvents. It is shown that its component intensities are not only dependent on the populations of the axial and equatorial conformers, but are also dependent on the molar absorptivities (o ) which are specific for each conformer in each solvent. In CCl 4 , the axial and equatorial conformers have o values of 417 and 818 l mol 1 cm 1 , respectively, while in CH 3 CN solution, the values were 664 and 293 l mol 1 cm 1 . These results are supported by results of theoretical calculations of frequencies, which gave an intensity of 223.8 kM mol 1 (1782 cm 1 ) for the axial and 174.4 kM mol 1 (1802 cm 1 ) for the equatorial conformer, indicating that the axial conformer presents a larger molar absorptivity than the equatorial one in the vapor phase. Moreover, the results presented here clearly demonstrate that although infrared spectroscopy at a single temperature can be an important auxiliary technique for conformational analysis, it must not be used to quantify conformational preferences of a molecule if the absorption molar coefficients for each conformer are not known or not amenable to experimental determination. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectroscopy; Conformational analysis; Molar absorption coefficients; 2-Bromocyclohexanone 1. Introduction Conformational isomerism has been widely studied by nuclear magnetic resonance and infra- red spectroscopies, by analyzing the conforma- tional preferences in different solvents or temperatures [1  /7]. Several conformational studies utilizing infrared spectroscopy have been performed, but there is no general agreement between the IR results and the ones from other techniques, like NMR spectro- scopy. An illustrative example is 2-bromocyclo- hexanone [2,4,8]. While NMR studies for 2- bromocyclohexanone utilizing chemical shifts or coupling constants show percentages of axial conformation which vary from 30% in DMSO to 89% in C 6 H 12 , the IR results do not present the same pattern [9], with the axial population remain- ing almost constant when changing solvents. * Corresponding author. Fax:  /55-19-3788-3023/3150. E-mail address: rittner@iqm.unicamp.br (R. Rittner). Spectrochimica Acta Part A 59 (2003) 1783  /1789 www.elsevier.com/locate/saa 1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(02)00436-5