Measurement of long-range heteronuclear coupling constants using the peak intensity in classical 1D HMBC spectra

In this contribution, we show that the magnitude of heteronuclear long-range coupling constants can be directly extracted from the classical 1D HMBC spectra, as all multiplet lines of a cross-peak always and exclusively vanish for the condition Δ = k/ⁿ J_CH . To the best of our knowledge, this feature of the classical HMBC has not yet been noticed and exploited. This condition holds true, irrespective of the magnitude and numbers of additional active and passive homonuclear ⁿ J_HH' couplings. Alternatively, the ⁿ J_CH value may also be evaluated by fitting the peak's intensity in the individual spectra to its simple sin(πⁿ J_CH Δ)exp(-Δ/T_2eff ) dependence. Compared to the previously proposed J-HMBC sequences that also use the variation of the cross-peak's intensity for extracting the coupling constants, the classical HMBC pulse sequence is significantly more sensitive.