Effect of Sustaining Pulses on Lifetime of Singlet States

Singlet states in NMR are discovered earlier and they show greater lifetime as compare to normal spin lattice (T1) and spin-spin (T2) relaxation time constants. Singlet states have to be sustained by the application of pulses which serve to remove the chemical shift while retaining the coupling. This is usually achieved by applying a continuous wave (CW) radiofrequency field to remove the chemical shift difference between the two spins. A windowed alternative (multiple π pulses with larger delay than pulse length for sustaining the long lived states-WDW) is considered here because of the power deposition in the sample, which leads to deleterious effects on the sample, would be lesser than the CW sequence. We have studied the efficiency of various composite pulse sequences, developed for heteronuclear decoupling or isotropic mixing, for sustaining singlet states. CW, WDW, WALTZ-16, FLOPSY-8 and MLEV-16 sustaining pulses were used for sustaining the long-lived states or singlet states in two spin systems, one with weak J- coupling, dibromothiophene (DBT), and another with strong J- coupling, citric acid. On comparing experimental and simulated data we inferred that WALTZ-16 sustaining pulse is working well (compared for TLLS values at on resonance and off resonance) for off-resonance locking of the LLS, whereas CW and WDW locking pulses are not suitable for off resonance locking as LLS decays at faster rate on increasing offset. One could use WDW pulse for sustaining long lived states instead of CW.