Collect. Czech. Chem. Commun. 2001, 66, 99-108

Experimental and Theoretical Studies of Rotational Barriers in Aceto-, N-Methylaceto- and N-Phenylacetohydroxamic Acid

David A. Browna,*, Laurence P. Cuffea, Geraldine M. Fitzpatricka, Noel J. Fitzpatricka, William K. Glassa and Kara M. Herlihyb

a Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
b Biacore Inc., Piscataway, NJ 08854, U.S.A.


Experimental and theoretical calculations for the E and Z forms of aceto-, N-methylaceto- and N-phenylacetohydroxamic acid are reported. The experimental method was NMR spectroscopy, while the computational methods included Hartree-Fock, Møller-Plesset and density functional theory calculations, with and without solvation, using either the Onsager or Tomasi's PCM method. In all calculations zero point energy corrections were included. The computed results when compared with the experimental ones show that, irrespective of the method used, the differences in the rotational barriers, ∆(E-TS) and ∆(Z-TS), are slight and below the 3 kcal mol-1 limit of the theoretical methods. In general the results using the PCM method were worse than the ones obtained from gas phase calculations or using the Onsager method, even though the PCM method is computationally most expensive. The calculations show, using either the Hartree-Fock or the B3LYP approach, that considering solvation using the Onsager method improves agreement with the experiment results. The calculated barrier heights, excluding the PCM method, agree broadly with the experimental results. Thus using the Onsager approach or gas phase calculations adequate results for barrier heights, but not for relative differences, were obtained.

Keywords: Solvent effects; Hydroxamic acids; Rotational barriers; Hindered rotation; Atropisomerism; Ab initio calculations; NMR spectroscopy.

References: 50 live references.