Infrared and Raman spectra, conformational stability, ab initio calculations and vibrational assignment of 2-fluorobutane

Abstract

The infrared spectra (3500–50 cm −1) of gas and solid and the Raman spectrum (3500–50 cm −1) of liquid 2-fluorobutane, CH 3CHFCH 2CH 3, have been recorded. Variable temperature studies over the range −105 to −150 °C of the infrared spectra (3500–400 cm −1) of the sample dissolved in liquid krypton have also been recorded. By utilizing the relative intensities of six conformer pairs each for both Me- trans/F- trans and Me- trans/H- trans, the Me- trans conformer is found to be the lowest energy form with an enthalpy difference to the F- trans conformer of 102±10 cm −1 (1.21±0.12 kJ mol −1) whereas the H- trans conformer is the highest energy form with an enthalpy difference of 208±21 cm −1 (2.49±0.25 kJ mol −1) higher than the Me- trans form. At ambient temperature, it is estimated that there is 50±2% of the Me- trans form, 31±1% of the F- trans form, and 19±1% of the H- trans conformer present. Equilibrium geometries and total energies of the three conformers have been determined by ab initio calculations with full electron correlation by the perturbation method to second order using a number of basis sets. A complete vibrational assignment is proposed for the Me- trans conformer and many of the fundamentals have been identified for the other two forms based on the force constants, relative infrared and Raman intensities, and depolarization ratios obtained from MP2/6-31G(d) ab initio calculations. The spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.