Semi-empirical calculations of chemical shifts in partially paratropic anions

Abstract

A theoretical study is presented of the ring-current and excess-charge effects on the chemical shifts of the conjugate bases of fluorene, 9,10-dihydroanthracene, xanthene, thioxanthene, dibenzo[a,d]cycloheptene and dibenz-[b,f]azepine. Several approaches (SCF methods, coupled and uncoupled perturbation theories and ring-current models) have been compared.The use of coupled perturbation theory is necessary for those systems which exhibit paramagnetic ring currents. The chemical shifts are satisfactorily described if the PPP method and the so-called LA I approximation are invoked, and the various contributions are parametrized properly.

Metal stabilized carbanions. Kinetic acidity and1H n.m.r. spectrum of the π-(tricarbonylchromium)-5H-dibenzo[a,d]cycloheptenyl anion

“…Though the A,-J values have been calculated from chemical shifts recorded in two different solvents, they are still reliable since the solvent effect between liquid NH3 and ethers is negligible. 5 The complexation causes a downfield shift of all proton resonances, except that of 4-H, to different extents. In fact, the largest downfield shift, as measured by the difference between the baricentres (i.e.…”

“…~ Tts electronic structure exhi bits a marked paratropiccharacter as inferred by the fact that the range of the proton chemical shifts lies at considerably higher fields than that observed for analogous cyclic (4n i 2 ) ~ 'aromatic' anions which conversely exhibit a net diatropism. 5 In Table 1 , the IH n.m.r. parameters of the free and complexed DBCHT-anions together with the complexation effect on chemical shifts, A,_ f , are reported.…”