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

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: 3JAA′, 3JBB′, 3JAB′(=3JA′B), and 2JAB. The 3JHH 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 3JHH/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)A. 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 3JHH/ϕ relationship predicts successfully the deviation from the idealized value of 60° for the gauche-oriented proton pairs in (I).