Ab initio studies of structural features not easily amenable to experiment. 42. Molecular geometries and conformational analysis of methylbutanoate
- 1 February 1985
- journal article
- research article
- Published by Wiley in Journal of Computational Chemistry
- Vol. 6 (1) , 61-67
- https://doi.org/10.1002/jcc.540060110
Abstract
Conformational energy profiles were calculated for τ1, the CCCO torsion, and τ2, the CCCC torsion, of methyl butanoate, using Pulay's ab initio gradient procedure at the 4‐21G level with geometry optimization at each point. In addition, the structures of seven conformations were fully relaxed, including the energy minima (τ1, τ2) = (0, −60), (0, 180), (120, 180), (120, −60), and the maxima (0, 0), (180, 180), and (60, −60). The calculated geometries confirm the previously formulated rule that, in saturated hydrocarbons, a CH bond trans to a CC bond (CHs) is consistently shorter than a CH bond (CHa) trans to another CH bond. Specifically, for XC(α) ( O)C(β)C(γ)C(δ) systems, the following rules can be formulated, incorporating results from previous studies of butanal, butanoic acid, and 2‐pentanone: (1) C(δ)Hs < C(δ)Ha in all the conformers in which the δ‐methyl group is remote from the ester group; whereas, in all the conformers in which nonbonded interactions are possible between the C(δ)‐methyl and the ester groups, the bonding pattern is affected by a CH ⃛OC interaction. (2) In the most stable conformers, (0, 60), C(β)Ha < C(β)Hs, and C(γ)Ha < C(γ)Hs, regardless of X. (3) The average CC bonds in the τ2 = 180° conformers are consistently shorter than those with τ2 = 60° (compared at τ1 constant). In the most stable conformations (τ1 = 0°, τ2 = 60° or 180°), the bonding sequence is consistently C(α)C(β) < C(β)C(γ) < C(γ)C(δ); whereas, when τ1 = 120°, C(α)C(β) < C(β)C(γ) > C(γ)C(δ).Keywords
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