THE SYN ROTATIONAL BARRIER IN BUTANE

The syn barrier for rotation about the central bond in butane has been determined via ab initio theoretical techniques, with various basis sets up to and including triple-ζ with two sets of polarization functions of carbon, and with varying amounts of electron correlation up to and including the coupled cluster with single and double excitations (CCSD) level of theory. After including zero-point and thermal corrections, these studies give the best available ΔH*298 value for this quantity at present as 4.89 kcal/mol. Previous theoretically determined syn-anti energy differences that disagreed with spectroscopically and molecular mechanics derived values are shown to be too large because of inadequate basis sets. © 1990, American Chemical Society. All rights reserved.