Single crystal neutron diffraction analysis (15 K) and ab initio molecular orbital calculations for ethylene carbonate

Abstract

The crystal structure of ethylene carbonate, (1,3-dioxolan-2-one), C 3H 4O 3, has been refined from single-crystal neutron diffraction data at 15 K. The crystal data are C2/ c, Z = 4, molecular symmetry C 2, a = 8.817 (2), b = 6.243 (2), c = 6.644 (2) Å, β = 99.31 (1) °. The dioxolan ring has the twist (T) conformation with Cremer-Pople puckering parameters q = 0.2484 (3) Å, φ = 270 °. When corrected for thermal motion, the bond lengths are C-C 1.522 A, C-O 1.457 and 1.342 Å, CO 1.203 Å, C-H 1.091 and 1.097 Å. The O-C-C-O torsion angle is 24.8°. Ab initio molecular orbital calculations for the isolated molecule, at rest, using GAUSSIAN-82 with HF/3-21G, 6-31G and 6-31G * bases, also predicted the twist conformation. The 3-21G basis gave the best agreement for the puckering parameter ( q = 0.215 Å). The difference between calculated and thermally corrected experimental bond lengths ranged between 0.05 Å (C-O, 6-31G *) and 0.006 Å (C-C, 6-31G *), depending on the basis set used. A calculation of the potential energy surface at HF/3-21G indicates that the transition state is planar and therefore the molecule is not a pseudorotor. A small difference in the methylene C-H bond lengths, which is both observed and predicted by theory, is interpreted as evidence for “lone-pair hyperconjugation” between the nearly axial C-H σ * antibonding orbital and the carbonate π bonding orbital.