False AA'X Spin-Spin Coupling Systems in 13C NMR: Examples Involving Phosphorus and a 20-Year-Old Mystery in Off-Resonance Decoupling

Abstract

It is common in both NMR textbooks and journal articles to loosely describe certain ABX spin systems as AA′X spin systems (1)(2)(3)(4)(5). For purposes of illustration, consider the 13 CH 2 spin system of maleic anhydride shown in Figure 1. One of the AB pair of hydrogen atoms is attached to a 13 C atom, the X-nucleus in the spin system, but the other is attached to a 12 C atom, the NMR-inactive isotopomer of the Xnucleus, and so the symmetry of the molecule is broken. Rigorously, the different isotopes split the chemical shifts of the "false" AA′ pair, and the properly designated AB hydrogen atoms are chemically as well as magnetically inequivalent.In practice, however, the chemical shifts normally are indistinguishable, and many of those same textbooks and journal articles point out that while rigorously the system is ABX, for all practical purposes it can be analyzed as AA′X (1,6). The danger of all of this is that when the chemical shifts are not the same, one may fail to recognize that fact owing to the assumption that the isotopomer shift is always insignificant. Results and Discussion P ExamplesA number of authors have noted that diphosphines and transition metal complexes with two phosphine ligands represent interesting test cases of such false AA′X systems (6-8), as shown for diphos (1) in Figure 2. Here the AA′ nuclei are the 31 P atoms, and the X-nucleus is the lone 13 C atom anywhere in this molecule. Detailed descriptions of the expected appearance of AA′X systems have been published (3,4,6), and except for the few cases to be described below, the AA′X description suffices for diphosphorus compounds (2, 6, 7). Representative multiplets from 1 and 2 are shown in Figure 3 for illustration. A common feature of AA′X spectra is that each exhibits a peak at the center of a 3-or 5-line multiplet (for J AA′ ≠ 0); the relative heights of the lines are dependent on the relative values of J AA′ , J AX , and J A′X . The separation of lines 1 and 3 (Fig. 3a,b) or 2 and 4 ( Fig. 3c) is equal to |J PC + J P′C |. The coupling constant J PP′ (if relatively large) can be obtained from the outer "wings" at approximately ± J PP from the central peak (Fig. 3c), or (if relatively small) from matching the height of the central line with that in simulations of the observed spectrum (Fig. 3a). These multiplets are examples of "virtual coupling" (9), because even though the longer range coupling constant J P′C ≈ 0, lines due to the second phosphorus atom are present in the 13 C NMR spectrum caused by the strong coupling of the phosphorus atoms-hence the second phosphorus is "virtually" coupled to carbon. It is important to recognize, especially in the case of the deceptively simple "triplet", that the coupling constants cannot be extracted on the basis of familiar first-order analyses.In contrast to the AA′X patterns seen above, the multiplets due to the ipso and bridging carbon atoms of diphos exhibit split central lines in the 100.6 MHz 13 C NMR spectrum (Figure 4a,b). The spectra are readily modeled if one drops the AA′X ...

Platinum Complexes of 5,6‐Dihydroacenaphtho[5,6‐cd]‐1,2‐dichalcogenoles

“…The 31 P{ 1 H} NMR spectra of complex 6 displays the expected pattern for the AA` nuclei of an AA`X spin system; [26] in addition to the established single resonance accompanied by satellites for 1 J( 31 P, 195 Pt) coupling, two sets of 77 Se satellites are clearly present, with 2 J( 31 P, 77 Se) values of 47 Hz and 56 Hz corresponding to the cis and trans couplings. 1 J( 31 P A , 31 P A`) coupling constants are notoriously small for AA`X spin systems and were not observed in the spectra of 2, 4 and 6.…”

“…[26] The AA` nuclei correspond to the two 31 P atoms which align with either a cis or trans configuration with respect to the low abundant 77 Se NMR active isotope, the X-nucleus in the spin system ( Figure 1). The presence of the NMR-inactive isotopomer of the X-nucleus at the second Se coordinating site thus breaks the symmetry of the molecule and ensures the two 31 P environments are magnetically inequivalent.…”

“…Central naphthalene ring torsion angles C: (6)- (5)- (10)- (1) (1) C: (4)- (5)- (10)- (9) 179.55 (1) 178.69 (1) In comparison, the 31 P{ 1 H} NMR spectra of complexes 2 and 6 display the expected pattern for the AA` nuclei of an AA`X spin system. [26] In addition to the established single resonances accompanied by platinum satellites [2 δ = 20.19 31 P A`) coupling constants are notoriously small for AA`X spin systems [27] and were not observed in the 31 P{ 1 H} NMR spectra of complexes 2, 4 and 6. The partial solubility of the complexes also made obtaining 77 Se NMR spectra with reasonable signal-to-noise ratios for these complexes problematic.…”

“…These can be analysed as AA`X if the satellites for 31 P/ 77 Pt coupling are disregarded. [26] The AA` nuclei correspond to the two 31 P atoms which align with either a cis or trans configuration with respect to the X-nucleus in the spin system (the 77 Se NMR active isotope), making them magnetically inequivalent. The 31 P{ 1 H} NMR spectra of complex 6 displays the expected pattern for the AA` nuclei of an AA`X spin system; [26] 31 P A`) coupling constants are notoriously small for AA`X spin systems and were not observed in the spectra of 2, 4 and 6.…”

“…Whilst steric effects dominate the configuration of substituents around phosphorus, weak interactions between the functional groups of neighbouring phosphorus atoms can influence the structure of the complex. In complexes 1 and 2, two neighbouring phenyl rings attached to independent P atoms adopt a parallel alignment similar to the face-to-face offset arrangement (Figure 7), [31] with non-bonded distances between interacting centroids (cg (25)(26)(27)(28)(29)(30)···cg (31)(32)(33)(34)(35)(36): 1 3.765(1)Å; 2 3.764(1)Å) within the range for typical π···π stacking (3.3-3.8 Å). [31] Similarly, complexes 5 and 6 exhibit weak CH···π type non-conventional hydrogen bond interactions [32] between neighbouring methyl and phenyl moieties of adjacent phosphorus groups (5 C(14)-H(14C)···cg (23-28) Unsurprisingly, cleavage of the short E-E covalent bond and subsequent coordination to platinum in complexes 1, 2, 5 and 6 causes steric strain to accumulate between the bulky chalcogen atoms.…”