NMR in the Advanced Inorganic Lab. II. Introduction to Quadrupolar Coupling: What to Do When the “n + 1” Rule and Pascal’s Triangle Fail

As an introduction to quadrupolar effects in NMR spectroscopy, students use low field ( ¹H, 60 MHz), benchtop  ¹³C NMR spectroscopy to contrast the spin–spin coupling behavior of  ¹³C to the dipolar  ¹H and quadrupolar 2D in familiar solvents C(H/D)Cl₃, C(H/D)₂Cl₂, and (C(H/D)₃)₂CO. To explain the observations, students are introduced to the spin magnetic quantum number, I, and its use in predicting first order coupling patterns. To explain the multiplets, both the number of lines and the relative intensities, students must expand their understanding of coupling beyond the mnemonics “n + 1” and Pascal’s triangle for binomial expansion and instead utilize the nuclear spin magnetic quantum number, I, for the quadrupolar nucleus 2D. The exercise is a useful prelude to the introduction of NMR spectroscopy involving quadrupolar nuclei (e.g.,  ¹⁴N (I = 1);  ¹¹B (I = 3/2)) in upper-level inorganic chemistry courses or as a fundamental description of coupling in physical chemistry courses. An advantage of the low field spectrum ( ¹³C, 15 MHz) is the observation of magnetic nonequivalence of the two methyl groups in (CH₃)₂CO in which the  ¹³C resonance for the methyl group is readily observed as a quartet of quartets.

Reference

T. Leon Venable, Journal of Chemical Education, DOI: 10.1021/acs.jchemed.3c01291