13C-13C spin-spin coupling constants in six-membered ring azaaromatic compounds

Abstract

The 13C-{1H} NMR spectra of monomethyl substituted diazines, protonated at the picoline and 2-methylopyrimidine nitrogen atoms, have been analyzed, along with that of pyridine N-oxide and its 2-methyl derivatives protonated at the oxygen atom. Direct and vicinal 13C-13C spin-spin coupling constants (SSCC) have been measured. It was found that the 13C-13C SSCC in diazines follow additivity rules which are based on consideration of the number and mutual orientation or distribution of nitrogen atoms in the ring. It has also been demonstrated that increased direct 1JCC values involving methyl group carbon atoms in α-positions to nitrogen atoms in the aromatic ring are due to effects associated with unshared electron pairs.