THE PROBLEM OF THE EXISTENCE OF C(AR)-H...N INTRAMOLECULAR HYDROGEN-BONDS IN A FAMILY OF 9-AZAPHENYL-9H-CARBAZOLES

In order to demonstrate the possibility of intramolecular hydrogen bonds (IMHB) between an aromatic C-H and a pyridine nitrogen atom, the family of 9-arylcarbazoles [Ar = phenyl (1), 2-pyridyl (2) or 2-pyrimidinyl (3)], their 9-aryl-1 -azacarbazole analogues [Ar = phenyl (4), 2-pyridyl (5) or 2-pyrimidinyl (6)] and 9-phenyl-1,8-diazacarbazole (7), have been synthesized and the X-ray structure of all of them, except 4, has been determined. The unit cell parameters are as follows. For 1: a = 14.526(5) angstrom, b = 10.985(1) angstrom, c = 18.094(2) angstrom, beta = 113.45(2)-degrees, V = 2649(1) angstrom3, Z = 8, P2(1)/n. For 2: a = 14.625(4) angstrom, b = 11.685(6) angstrom, c = 16.636(1) angstrom, beta = 114.07(4)-degrees, V = 2596(3) angstrom3, Z = 8, P2(1)/n. For 3: a = 25.76(2) angstrom, b = 5.200(6) angstrom, c = 19.47(1) angstrom, beta = 112.07(5)-degrees, V = 2417(4) angstrom3, Z = 8, C2/c. For 5: a = 12.825(2) angstrom, b = 12.336(4) angstrom, c = 16.468(6) angstrom, beta = 11 1.41(3)-degrees, V = 2426(2) angstrom3, Z = 8, P2(1)/n. For 6: a = 10.682(3) angstrom, b = 29.033(9) angstrom. c = 7.524(3) angstrom, beta = 90.46(3)-degrees, V = 2333(1) angstrom3, Z = 8, P2(1)/c. For 7: a = 10.895(2) angstrom, b = 13.407(4) angstrom, c = 17.552 (6) angstrom, beta = 1 07.81(2)-degrees, V = 2441(2) angstrom3 , Z = 8, P2(1)/c. The data were obtained at 293 K except for compound 7 for which the experimental temperature was 200 K. All compounds except 3 exist in the crystal in two different conformations. The potential curves corresponding to, the rotation about the N(9)-C(aryl) bond of nine compounds (1-7 plus the non-synthesized 2-pyridyl- and 2-pyrimidinyl-1,8-diaza analogues) have been calculated. The crystallographic torsion angles are near the zone of minimum energies according to AM1 calculations, save in the case of compound 1 (9-phenylcarbazole) where one of the independent molecules has a torsion angle of 78.4-degrees, much larger than calculated. The calculated barrier through the planar state, DELTAE0, contains much information and appears to be related to the H-1 and C-13 chemical shifts of some selected molecules. Compound 3 [9-(2-pyrimidinyl)-9H-carbazole], with a torsion angle of 7.4(4)-degrees, is the clearest representative of the existence of C-H ... N IMHBs.