Enthalpy difference between conformations of normal alkanes: Raman spectroscopy study of n-pentane and n-butane.

Conformation equilibrium in normal pentane (C(5)H(12)) was studied by the low-temperature gas-phase Raman spectroscopy method. A special retroreflecting multipass cell was constructed. Gas-phase (vapor) spectra were recorded in the temperature region of -130.3 to -23.1 degrees C and in the spectral range below 500 cm(-1). The peaks of trans-trans (399.0 cm(-1)), trans-gauche (328.9 cm(-1)), and gauche(+)-gauche(+) (267.1 cm(-1)) conformers (rotamers) of n-pentane were assigned using quantum chemistry data (MP2 and B3LYP methods with aug-cc-pVTZ basis set). The contour of each line was deconvoluted using ab initio data to obtain precise total integral intensity. The intensities at different temperatures were used to evaluate the enthalpy (energy) difference between trans-gauche and trans-trans (DeltaH(tg) = 618 +/- 6 cal/mol), and gauche(+)-gauche(+) and trans-trans (DeltaH(g+g+) = 940 +/- 20 cal/mol) conformers. Normal butane (n-butane) measurements under similar experimental conditions have been taken to understand the chain length influence. The C(4)H(10) enthalpy difference value has been evaluated (DeltaH(g) = 660 +/- 22 cal/mol). The results are compared with published experimental and theoretical data. The data presented here can be used as a reference for quantum chemistry calculations of conformation equilibrium in n-butane and n-pentane.