Anti, Ortho, and Gauche Conformers of Perfluoro-n-butane: Matrix-Isolation IR Spectra and Calculations

Abstract

Nitrogen matrix-isolation IR spectra have been obtained for each of the three conformers of n-C4F10 (gauche, ortho, and anti) by trapping a hot conformer mixture on a cold CsI window and subsequent matrix annealing and spectral differencing. They were assigned by comparison with results of HF/6-31G* calculations, and the nature of the normal modes has been analyzed using the total energy distribution procedure. At the fully optimized MP2/6-31G* (frozen core) level, the CCCC dihedral angles and relative energies (kcal/mol) are 54.2 degrees and 0.68 (gauche), 94.8 degrees and 1.63 (ortho), and 165.5 degrees and 0 (anti). Single-point MP2/6-311G* (frozen core) relative energies at these geometries are 0.85, 2.12, and 0 kcal/mol, respectively. Only a minute amount of the ortho conformer is trapped in nitrogen matrix, and none in other matrices that were tried. A variation of the relative intensities of IR peaks of the gauche and anti conformers as a function of the temperature of the gas before deposition yields an ''average'' Delta H value of about 0.9 kcal/mol, with the anti conformer more stable. The temperature range covered was too small to reveal the expected bilinear nature of the van't Hoff plot.