The carbon-13 chemical shifts and the analysis of the relaxation times T1 and long range 13C− 1H coupling constants of quinoline and of 1-(X-quinolyl)ethyl acetate derivatives

Abstract

The 13C chemical shifts and the 13C− 1H coupling constants of quinoline (1-(X-quinolyl)ethyl acetate derivatives (where X=−CH(OAc)CH 3 substituted at positions 2,4,5–8) are reported. Substituent chemical shift (SCS) effects for the ethyl acetate group are additive at all positions. A substantial upfield shift of 4.5 and 4.8 ppm was observed at C-4 and C-5, arising from the peri interaction of 5- and 4-ethyl acetate substituents respectively. A vicinal ( peri) 3 J CCCH coupling constant of approximately 5 Hz is observed between both C 5−H 4 and C 4−H 5. Carbon-13 relaxation times ( T 1) and nuclear Overhauser enhancements (η) have been measured for quinoline and its derivatives, and the contributions of dipolar, T 1 DD, and spin rotation, T 1 SR, relaxation have been determined. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism whenever protons are bound directly to the carbons under investigation. Non-protonated ring carbons are relaxed by both DD and SR mechanisms. Anisotropic motion has an easily observable effect on the DD contribution to T 1, and can form the basis for spectral assignments, as in 1-phenylethyl acetate. Long-range 13C− 1H coupling constants were observed both between ring carbons and between ring carbons with ring side-chain hydrogens. These results have been used for the structure determination of the title compounds.