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Keywords:

  • cyclobutanes;
  • conformations;
  • 1H NMR;
  • 4JHH couplings;
  • theoretical calculation

Abstract

The complete analysis of the complex 1H NMR spectra of some monosubstituted cyclobutanes was achieved to give all the 1H chemical shifts and nJHH (n = 2, 3 and 4) coupling constants in these molecules. The substituent chemical shifts of the substituents in the cyclobutane ring differ significantly from those in acyclic systems. For example, the OH and the NH2 groups in cyclobutanol and cyclobutylamine produce a large shielding of the hydrogens of the opposite CH2 group of the ring compared with little effect on the comparable methylene protons of butane. These effects and the other 1H shifts in the cyclobutanes were modelled successfully in the CHARGE program. The RMS error (calculated vs observed shifts) for the 34 1H shifts recorded was 0.053 ppm. The conformational equilibrium in these compounds between the axial and the equatorial conformers was obtained by comparing the observed and the calculated 4JHH couplings. These couplings in cyclobutanes, in contrast to the corresponding 3JHH couplings, show a pronounced orientation dependence; 4Jeq–eq is ca 5 Hz and 4Jax–axca 0 Hz. The couplings in the individual conformers were calculated at the B3LYP/EPR-III level. The conformer energy differences ΔGax–eq vary from 1.1 kcal mol−1 for OH to 0.2 kcal mol−1 for the CH2OH substituent. The values of the conformer energy differences are compared with the previous IR data and the corresponding theoretical values from molecular mechanics (MM) and DFT theory. Generally, good agreement is observed although both the MM and the DFT calculations deviate significantly from the observed values for some substituents. Copyright © 2010 John Wiley & Sons, Ltd.