Sign determinations and INDO-MO calculations of 13C–15N and 15N–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.