Singlet NMR in a case of high molecular symmetry

Abstract

Spin-1/2 pairs support nuclear singlet and triplet states. The mean population difference between the singlet and triplet manifolds is termed singlet order. Under suitable circumstances, nuclear singlet order is highly resistant to several relaxation mechanisms, displaying a decay time constant TS, which may greatly exceed the time constant T1 for the equilibration of nuclear magnetization. We explore the nuclear singlet relaxation of an isotopolog of squarate in a high-pH aqueous solution. The 1,3–13C2-isotopolog of squarate exists as a minority species of 1–13C2-squarate. This isotopolog has a high degree of molecular symmetry. 18O-enrichment is used to generate secondary isotope shifts of the 13C resonances, providing access to 13C2 double-quantum coherence and 13C2 singlet order. The 13C signals from the 1,2–13C2 and 1,3–13C2 species are selected using geometric double-quantum filtration, and the double-quantum coherence is converted to singlet order and back again using customized double-quantum-to-singlet (DQ2S) and singlet-to-double-quantum (S2DQ) pulse sequences. We report 13C2 singlet lifetime measurements for 1,3–13C2-squarate in a high-pH aqueous solution, in a high magnetic field.