Optimized Quantitative DEPT and Quantitative POMMIE Experiments for 13C NMR

Abstract

In quantitative analysis, inverse gated (1)H decoupled (13)C NMR provides higher resolution than (1)H NMR. However, due to the lower sensitivity and longer relaxation time, (13)C NMR experiment takes much longer time to obtain a spectrum with adequate signal-to-noise ratio. The sensitivity can be enhanced with DEPT and INEPT approaches by transferring polarization from (1)H (I) to (13)C (S), but since the enhancements depend on coupling constants ( (1) J SI) and spin systems (SI, SI 2, SI 3), the enhancements for different spin systems are not uniform and quantitative analyses are seriously affected. To overcome these problems, Henderson proposed a quantitative DEPT (Q-DEPT) method by cycling selected read pulse angles and polarization-transfer delays (Henderson, T. J. J. Am. Chem. Soc. 2004, 126, 3682-3683), and satisfactory results for SI system are achieved. However, the optimization is incomplete for the SI 2 and SI 3 systems. Here, we present an improved version of Q-DEPT (Q-DEPT (+)) and a quantitative POMMIE (Q-POMMIE) where the cyclic delays and read pulse phases are applied. The improved methods prove to be suitable for all spin systems over a large J-coupling range (90-230 Hz), and the (13)C signals are nearly equally enhanced with standard deviation less than 5%.