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NMR versus molecular modelling: menthone and isomenthone
Author(s) -
Smith William B.,
Amezcua Carlos
Publication year - 1998
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199806)36:13<s3::aid-omr276>3.0.co;2-6
Subject(s) - conformational isomerism , chemistry , isopropyl , electronegativity , computational chemistry , ab initio , menthone , proton nmr , crystallography , thermodynamics , stereochemistry , organic chemistry , molecule , menthol , physics
trans ‐2‐Isopropyl‐5‐methylcyclohexanone (menthone) may be considered as locked in the diequatorial conformer. However, the cis isomer (isomenthone) may exist in two major conformations with the isopropyl group either equatorial or axial. No measure of the equilibrium mix has been determined previously. Proton NMR allows such a determination at room temperature via the vicinal couplings between protons on carbons 5 and 6. Combined with modelling results, the calculated value with the isopropyl axial was 79%, of the total in good agreement with an earlier prediction. Two different conformer searching programs were employed to determine the various conformer populations. It was found that molecular mechanics estimates of the Boltzmann distributions varied considerably from the experimental results. These differences were not due to substituent electronegativity effects or to hydrogen bonding with the deuterochloroform solvent. Semiempirical PM3 and low‐level ab initio calculations of the conformer energies also failed to account adequately for the conformer populations. DFT calculations (Becke3LYP/6–31G*) with zero‐point energy corrections gave results most consistent with the experimentally determined values (67.5% axial isopropyl conformer). © 1988 John Wiley & Sons, Ltd.