An ab initio Study of Butyrolactone and 3-Hydroxybutyrolactone. Semiempirical Calculations on Proton Coupling Constants in These Molecules

Abstract

Optimization with the 3-21G basis set of the geometry of the envelope conformation of butyrolactone leads to a structure with an out-of-plane angle of 29.7° for the flap. Similar optimization of the twisted-ring conformation leads to a structure lying only 0.2 kJ mol^-1 higher in energy and having a twist angle of 18.7°. These structures differ mainly in a vertical translation of C3 and C4 by c. 0.14 Ǻ with respect to the C2-C1-O₂ plane. The ring may be twisted or bent at low energy- cost, and the barrier to inversion via the planar-ring (C_S) structure is only 4.7 kJ mol^-1. It is suggested that the envelope conformation is but one of many transient structures in a rapid ring-inversion process. For 3-hydroxybutyrolactone, the optimized twisted-ring conformation with twist angle 22.0° lies 0.3 kJ mol^-1 below the optimized envelope conformation with out-of-plane angle 33.7° again these structures differ mainly in a vertical translation of C 3 and C 4 by c. 0.15 Ǻ with respect to the C 2-C 1-O₂ plane. Moderate ring twisting and bending are rather more costly than in butyrolactone itself, and the hydroxy group clearly occupies the pseudo-axial position on C 3. Calculated bond lengths and angles for both molecules are mostly within the corresponding experimental ranges for a series of γ-lactones. Proton- proton spin coupling constants calculated by the FINITE/INDO method for the optimized 3-hydroxybutyrolactone structures are in moderate-to-good agreement with experimental values, and agreement is improved by application of a thermal population-averaging procedure. Thermally averaged values for butyrolactone are also presented.