title: A 1H NMR and theoretical investigation of the conformations of some monosubstituted cyclobutanes.
abstract: The complete analysis of the complex (1)H NMR spectra of some monosubstituted cyclobutanes was achieved to give all the (1)H chemical shifts and (n)J(HH) (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 NH(2) groups in cyclobutanol and cyclobutylamine produce a large shielding of the hydrogens of the opposite CH(2) group of the ring compared with little effect on the comparable methylene protons of butane. These effects and the other (1)H shifts in the cyclobutanes were modelled successfully in the CHARGE program. The RMS error (calculated vs observed shifts) for the 34 (1)H 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 (4)J(HH) couplings. These couplings in cyclobutanes, in contrast to the corresponding (3)J(HH) couplings, show a pronounced orientation dependence; (4)J(eq-eq) is ca 5 Hz and (4)J(ax-ax) ca 0 Hz. The couplings in the individual conformers were calculated at the B3LYP/EPR-III level. The conformer energy differences ΔG(ax-eq) vary from 1.1 kcal mol(-1) for OH to 0.2 kcal mol(-1) for the CH(2)OH 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.
authors.affiliation: Chemistry Department, University of Liverpool, Crown St, Liverpool L69 7ZD, UK. rabraham@liv.ac.uk
authors.forename: Raymond J
authors.initials: RJ
authors.lastname: Abraham
authors.forename: Paul
authors.initials: P
authors.lastname: Leonard
authors.forename: Cláudio F
authors.initials: CF
authors.lastname: Tormena
mesh_terms: D003503:Cyclobutanes / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D011522:Protons; D012015:Reference Standards
chemical_list: D003503:Cyclobutanes; D011522:Protons