Spin-1 Double Quantum Spectroscopy in the Isotropic Phase: INADEQUATE Spectra of 2H,2H and 6Li,6Li AX Spin Systems

Abstract

A density matrix treatment is presented which describes the main features of INADEQUATE experiments for spin-1 AX spin systems. It is shown that apart from two-spin double quantum peaks, which carry correlation information, one-spin double quantum peaks are to be expected. In the latter, homoculear spin-spin coupling is amplified in the F, domain where the signal splitting amounts to 45. The theoretical predictions are verified for the 'H,'H AX system of 3,3'-[D,]norcamphor. In addition, experimental results for a 6Li,6Li A,X, or A A X X spin system are presented and the influence of spin-lattice relaxation on the relative intensity of the one-spin double quantum peaks for 6Li,6Li experiments is demonstrated. KEY WORDS NMR; 'H NMR; 6Li NMR; spin-1 nuclei; double quantum spectroscopy; INADEQUATE; coupling constants Double quantum spectroscopy (DQS) has been a popular NMR experiment in connection with molecular structure elucidation. In particular, proton and carbon DQS,'.' the latter termed INADEQUATE for the natural abundance situation, have found much application. We have shown recently,"' with specific reference to 'H and 6Li, that multiple quantum spectroscopy in the isotropic phase is invaluable in visualizing spin connectivity even when nuclei with spin > 1/2 are involved, in particular because indirect spin-spk couplings for these nuclei are often not resolved. This is especially true for the spin-1 group nuclei (2H, 6Li, 14N).In this work, we explored in detail some properties of the double quantum INADEQUATE spectra of spin-1 nuclei. In connection with spin-1 isotopes, two different varieties of double quantum coherences (DQC) are to be distinguished, uiz. one-spin and two-spin DQC4*' (Scheme 1, dotted and dashed lines, respectively). Considering a two-spin-1 AX system, the standard double quantum preparation sequencegives rise to the following density matrix for cp = x, starting from thermal equilibrium :* Authors to whom correspondence should be addressed. It is worth noting that the sequence leads to one-spin DQC that is in phase quadrature with two-spin DQC, e.g. one-spin DQC of phase T x and two-spin DQC of phase +_ y, respectively, in Eqns (2) and (3). From the above, it is clear that the Preparation of two-spin DQC is optimum for z = 1/(45), whereas the preparation of the one-spin DQC is optimal at z = 1/ (25). In reality, of course, the process of coherent transfer in these systems is a severely damped oscillation.

Carbanion Chemistry

“…A detailed treatment of the INADEQUATE spectra of coupled spin 1 nuclei (e.g., 2 H, 6 Li, 14 N) was added subsequently. 78 …”

Alkyllithium Mixed Aggregates: Dynamic Behavior and Comprehensive Analysis of NMR2J7Li–7LiSpin–Spin Coupling

“…However, the scalar 2 J Li–Li coupling constant is too small to be resolved on the lithium 1D NMR spectra . In the case of the 6 Li nucleus, indirect two-dimensional NMR methods were then used to identify a coupling between two nonequivalent lithium atoms [ 6 Li– 6 Li COSYLr, 6 Li– 6 Li INADEQUATE], but they could not provide a resolved measurement of the coupling.…”