1H nuclear magnetic resonance and X-ray study of dihedral bond angles in the gauche O·CH2·CH2·O moiety of cyclic 7,8-dihydrodibenzo[f,h][1,4]dioxecin

Abstract

The ¹H n.m.r. spectra of the CH₂·CH₂ groups in two ortho, ortho′, ·CH₂·X·CH₂·CH₂·X·CH₂· bridged biphenyl derivatives (I)(X =–O–) and (II)(X = MeN) have been analysed. A set of four coupling constants has been derived: ³J_AA′, ³J_BB′, ³J_AB′(=³J_A′B), and ²J_AB. The ³J_HH values were found to be only slightly temperature- and solvent-dependent.

These coupling constants were used for a study of the geometry of the ·X·CH₂·CH₂·X· moiety of these molecules in solution. Dihedral bond angles were calculated by use of various known ³J_HH/ϕ relationships and the results compared with those obtained from an X-ray study of the ·O·CH₂·CH₂·O bridged derivative (I).

Crystals of (I) are orthorhombic, space group Aba2, with Z= 4 in a unit cell of dimensions a= 11.871 (1), b= 13.882(2), c= 7.875(2)Å. The structure was determined by direct methods and refined by least-squares techniques to R 0.033 for 410 observed reflections. The molecules have C₂ symmetry. Dihedral bond angles between the coupled proton pairs are 31 (4), 77(6), and 176(8)°.

The ³J_HH/ϕ relationship predicts successfully the deviation from the idealized value of 60° for the gauche-oriented proton pairs in (I).