
%0 Journal Article
%A Riggs, NV 
%T An <i>ab initio</i> Study of Butyrolactone and 3-Hydroxybutyrolactone. Semiempirical Calculations on Proton Coupling Constants in These Molecules
%J Australian Journal of Chemistry
%D 1985/11/01
%V 38
%N 11
%P 1575-1583
%K 
%R https://doi.org/10.1071/CH9851575
%U https://www.publish.csiro.au/paper/CH9851575 
%X 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&#176; 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&#176;. These structures differ mainly in a vertical translation of C3 and C4 by c. 0.14 &#506; 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 (CS) 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&#176; lies 0.3 kJ mol-1 below the optimized envelope conformation with out-of-plane angle 33.7&#176; again these structures differ mainly in a vertical translation of C 3 and C 4 by c. 0.15 &#506; 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 &#947;-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. 
