GEOMETRIC STRUCTURE AND VIBRATIONAL-SPECTRUM OF TETRAHYDROFURAN

Ab initio calculations for the symmetric C2, C(s), and C2v forms and for two unsymmetric C1 forms of tetrahydrofuran were performed both at the SCF level and accounting for electron correlation by full second-order Moller-Plesset perturbative treatment. The standard STO-3G, 6-31G, and 6-31G** bases and a 6-31G* basis with different exponents for polarization functions on oxygen and carbons were applied to a complete optimization of the geometrical parameters within each given molecular symmetry. The most reliable computations have given the C2 conformation on the absolute energy minimum, the C(s) structure on a transition state, only 0.3 kcal mol-1 higher, and the C2v form on an energy maximum, 4.7 kcal mol-1 high. While bond lengths appear nearly conformation-independent, angles and dihedrals show a marked flexibility in the delicate balance between bond-angle strain and ring-angle torsion that governs the interconversion between the different forms. The puckering amplitude q is seen to be nearly constant, at 39.5 pm, for both the C2 and C(s) structures. The vibrational spectrum was reinvestigated by recording the infrared spectrum of the vapor and the infrared and Raman spectra of solutions in apolar solvents and of the liquid and the solid at different temperatures. The spectrum of the solid between 30 and 120 K was investigated for the first time by neutron inelastic scattering experiments. With the guide of the computed spectrum as predicted at the MP2/6-31G* level of theory, all the normal modes have been identified and assigned in terms of symmetry coordinates. Experimental evidence strongly suggests that in solid tetrahydrofuran the pseudorotational motion becomes a large-amplitude ring-deformation vibration with a fundamental frequency of about 140 cm-1.