Interpretation of NMR Spectra. I. A2BXx and A2BKk Spectra

Difficulties encountered in analyzing NMR spectra are greatly multiplied as the number of interacting nuclei increases. The concept of effective internal chemical shifts or effective Larmor frequencies is developed here to simplify the study of complex NMR spectra in which a group of magnetically equivalent nuclei (Xx) is greatly shifted from a strongly coupled group. This concept permits decomposition of a complex NMR spectrum into two or more simpler groups of transitions called subspectra. These subspectra are then analyzed by previously developed methods. It is proved that this decomposition is independent of the nature of the strongly coupled group. A discussion is given of the change in appearance of spectra as the shift of the Xx group from the strongly coupled group is decreased and small deviations from the ``X'' approximation appear. The Xx group is relabeled Kk (x=k) when such deviations appear. The form of this breakdown depends only on x (or k) and is given for general x (or k). Explicit expressions for the A2BXx system are tabulated. Detailed analyses are presented for the following series of A2BXx or A2BKk spectra: 2,6-dichlorofluorobenzene (A2BX); 1-chloro−2,3-butadiene (A2BX2 or A2BK2); methylallene (A2BX3 or A2BK3).