The Molecular Structure of 2,5-Dihydrofuran from the Analysis of the Nematic Liquid Crystal Nuclear Magnetic Resonance Spectrum

Abstract

The nematic phase n.m.r. spectrum of partially oriented 2,5-dihydrofuran has been analyzed. Corrections to the structure for ring puckering and methylene rocking motions, according to the model due to Malloy, give improved agreement between experimental and calculated direct dipole-dipole coupling constants.Le spectre r.m.n. en phase nkmatique du dihydro-2,s furanne partiellement orient6 a CtC CtudiC. Introduction In four-membered ring compounds the out-ofplane vibrational motion leads to a puckering of the ring. The potential function for this vibration usually has a double minimum, and if the lowest vibrational state lies below the hump the molecule is nonplanar in the ground state. Five-membered rings have two out-of-plane vibrations which can interact to give pseudo-rotation unless there is a double bond in the ring in which case the torsional motion about this bond is at much higher energy and the remaining low frequency out-of-plane motion reduces to a one dimensional puckering. Information about ring puckering motions can be obtained from microwave spectroscopy from the dependence of the moments of inertia on the vibrational quantum number and/or from the actual vibrational absorption spectrum in the far i.r. region. The potential function arises from two contributions: a quartic term, originally predicted by Bell (l), due to ring strain which tends to keep the ring in a planar conformation, and a quadratic term due to torsion about bonds tending to force a nonplanar conformation to avoid eclipsing between adjacent methylene groups. The expression used for the potential function is of the form

Proton nuclear magnetic resonance spectra of cyclic monoenes: hydrocarbons, ketones, heterocycles, and benzo derivatives

“…432 The activation parameters derived from the A2 -AB exchange are Ea = 14.9 ± 0.4 kcal/mol, log A = 14. (191,198) and Benzocycloalkanedlones (194)(195)(196) Compound earlier proton NMR work, the other parameters differ considerably, a fact that was attributed to systematic errors of unknown origin.…”

“…The spectrum of 1,1-difluorobenzocyclopropene (198) has also been analyzed completely.513 Here the relative chemical shift ( , ) is much larger than In the hydrocarbon, and the analysis of the spectrum, performed for the proton AA'BB' spin system with 19F decoupling as well as for the full six-spin system, presented no problem. Relative signs of H,F coupling constants were determined by spin-tickling experiments.…”

Nematic phase nuclear magnetic resonance spectra and vibrationally averaged structures of cyclopropyl chloride, bromide, and cyanide

“…Small differences in structure and motional constants are evident but these are within ex~erimental error. The error limits quoted in these tables are derived from the experimental errors in the Dij values (26).…”

Nematic phase nuclear magnetic resonance studies of the molecular structures of cyclobutanone and methylenecyclobutane

“…In the first case the fit is worse than for the rigid planar case. The preferred distance values are given in column 111 of Table 3 where the error limits are calculated probable errors based upon the errors in the measured direct dipole coupling constants (13). The nmr data can be combined with the microwave results to give a more complete structure.…”