The gauche-trans Energy Difference of n-Butane from a Doppler Limited Investigation of the 740 cm- 1 CH2-Rocking Region

Abstract

Abstract Tunable diode laser spectra of n-butane gas were recorded in the 740 cm -1 CH2-rocking region using path lengths of typically 300 m, pressures below 1 mbar and temperatures between 170 and 300 K. The spectra are complicated by the overlap of the two trans and gauche rotameric fundamentals, and by a strong absorption background probably caused by the addition of hot-band transitions involving excitation of one or more quanta of the torsion vibrations around the central C - C bond. Analysis was aided by Fourier transform recordings in medium and high resolution and lead to the assignment of rovibrational features close to the band centers at v0 (trans) = (733.5797 ±0.0015) cm -1 and v0(gauche) = (747.5116 ± 0.0018) cm-1 of the trans and gauche rotamers, respectively, and to medium accurate spectroscopic parameters. A statistical evaluation of the temperature dependence of the relative absorption intensities of near-by trans and gauche transitions resulted in the trans-gauche rotameric energy difference of ΔH = (246 ± 18) cm -1 or (2.94 + 0.21) kJ mol-1 . This value is lower than those from previous determinations but still indicates an interesting decrease of the gauche-trans energy difference in going from gas to the liquid state.