High-resolution nuclear magnetic resonance spectra of phenanthrenes—V: Dibenzo[c,g]phenanthrene and benzo[g, h, i]perylene

Abstract Complete iterative analyses have been carried out for the 60 MHz 1 H NMR spectra of dibenzo[ c, g ]phenanthrene (I), or 3,4,5,6-dibenzophenanthrene, in solutions of carbon disulphide, [ 2 H 6 ]benzene, [ 2 H 5 ]pyridine and dimethylsulphoxide, and for the 100 MHz spectrum of benzo[ g,h,i ]perylene (II), or 1,12-benzoperylene, in carbon disulphide; 60 MHz spectra of I in perfluorobenzene, acetone and cyclo hexane are also reported. Spin-tickling experiments at 100 MHz helped in the assignment of H-2 upfield of H-3 in I and double resonance confirmed long-range coupling of the low-field H-1 to H-5; further broadening of H-1 is attributed to weak long-range coupling to H-6 rather than to intramolecular relaxation involving H-14. Assignment of the H-5,6 AB system and the H-7 singlet in I was helped by measurements in different solvents; these also showed that v 2 and v 3 in the ABCD system differ least in carbon disulphide solution, and that H-1 moves downfield in benzene solution. Coupling constants, J ortho for unhindered protons in I and II may be predicted from Huckel molecular-orbital (HMO) π-bond orders via empirical equations, while C-C-H bond distortions can account for the high values of overcrowded protons. HMO calculations give fairly good agreement for chemical shifts of unhindered protons H-3 to H-7 of the planar II. An empirical parameter, φ, representing accessibility of a proton to disc-shaped molecules (with allowance for the differential effect on the reference), is lower for H-4,5 than for H-6,7 in I and suggests increased distortion of C-6,7. In [ 2 H 5 ]pyridine solution, all protons in I suffer downfield shifts, and in dimethylsulphoxide only the protected H-1 in I escapes large downfield shifts.