Two-bond Coupling Constants Research Articles

Angular dependence of two-bond coupling constant 2 J PNC in the NN-dimethylaminophosphines

Low-temperature carbon-13 n.m.r. spectra of NN-dimethylaminophosphines show the two-bond coupling constant 2JPNC to be large and positive when the CN bond is cis with respect to the phosphorus lone pair, whilst it becomes small and negative in the trans-conformation.

Nuclear Magnetic Resonance Spectroscopy. A Stereospecific 3 J CF Coupling in the Low-Temperature 13 C Nmr Spectrum of 1,1-Difluorocyclohexane

The proton-decoupled (13)C nmr spectrum of 1,1-difluorocyclohexane has been examined at room temperature and at -90 degrees C. There are only minor changes in the one-bond and two-bond carbon-fluorine scalar coupling constants at the lower temperature; however, the triplet observed for C-3 ((3)J(CF) = 4.7 Hz) collapses to a doublet ((3)J(CF) = 9.5 Hz) at -90 degrees C. It is proposed that only the equatorial fluorine is coupled with the C-3 carbon as the result of operation of a back-lobe orbital interaction.

13C-C-H spin-spin coupling constants involving the formyl proton in aldehydes

The two-bond 13C-C-H coupling constants involving the formyl proton of about 40 aldehydes were measured in natural abundance. The sign of the coupling constant was found, by a decoupling technique, to be the same as the directly bonded 13C-II coupling constant. On the basis of molecular orbital theory, the two-bond coupling constants of acetaldehyde, acrolein and propargyl aldehyde were evaluated. From the calculations it was found that the main contribution to the coupling constant comes from the delocalization of the lone-pair electrons of the carbonyl oxygen atom, and the low excitation energy of these electrons. The substituent effects on, and the temperature dependence of, the two-bond coupling constant were qualitatively elucidated along these lines.