Sign determinations and INDO-MO calculations of13C–15N and15N–15N spin–spin coupling constants of 3-methyl- and 3,6-dimethyl-pyridazines

Abstract

13 C–15N Nuclear spin-coupling constants, nJ(C–N), of 3-methyl- and 3,6-dimethyl-pyridazines and one-bond 15N–15N spin-coupling constant of 3-methylpyridazine have been observed with the intention of investigating the influence of the two nitrogen lone pairs on the magnitudes and signs of nJ(C–N) and 1J(N–N). Signs of nJ(C–N) were determined by comparing the splittings of each carbon atom in the 3-methylpyridazine with those in the 3,6-dimethylpyridazine or by the single-frequency off-resonance proton-decoupling technique. The 1J(C–N) values were found to be large and positive (+6.0 and +6.4 Hz), in marked contrast to the case of pyridine (+0.6 Hz). Although C–C–N two-bond coupling constants in the pyridazine ring (+1.3 Hz) are roughly similar to the corresponding coupling in pyridine (+2.5 Hz), C–N–N two-bond coupling constants were also large and positive (+4.8 Hz). INDO-MO calculations of nJ(C–N) reproduced well not only the observed trends of the differences between 3-methylpyridazine and pyridine but also the absolute magnitude and the sign of each spin-coupling constant. The magnitude of 1J(N–N) was also found to be large (22.1 Hz), and its sign was concluded to be negative on the basis of selective 15N decoupling experiments.