Molecular complexes. Part 12. Dimeric toluene, torsional vibrations, dipoles and isomeric complexes in H-1 NMR studies of weak arene complexes. Temperature dependence of CH signals

The refined (AUS concept, CCl4, external ref) H-1 NMR method provided assoc iation constants K and approximate complex shifts IK for stacking complexes of aromatic hydrocarbons D with A when A is 4-nitrobenzaldehyde (1) or is related to 1. D covers the benzene ring of A but torsionally vibrating subs tituents in A influence topology and K. In the absence of D, vibrations of A can be slowed by low temperatures making A signals go downfield in accord with an increased planarity of A. Vibration of CHO (a dipole) in complexes of 1 with benzene B or toluene T is made non-symmetric by interactions wit h the quadrupole of D. The large naphthalene (N) hinders vibrations thus en hancing the contact interface by a more planar 1 whose intramolecular deshi elding of protons is increased providing small IK values. Aldehyde 1 forms complexes both with T and with its stacking dimer T-2 since IK values for 1 -T are significantly greater than for 1-B. Complexing with T2 is not found when the molecular dimensions of A allow a dipole-dipole interaction with T as in face-to-face complexes of 1-ethyltheobromine dagger or of 1-chloro-2 ,4-dinitrobenzene (7). Different protons give slightly different values of K for 7-T and this points to an isomeric edge-on complex with both T and T- 2 where K for the two edge-on protons (shortest distance to D) is the sum o f the binary K and the small K for the the ternary (7 + T-2) complex. Compo und 7 and 1,3,5-trimethylbenzene show some of the effect described for 1-N.