Optimized Quantitative DEPT and Quantitative POMMIE Experiments for 13C NMR

Abstract

In quantitative analysis, inverse gated ¹H decoupled ¹³C NMR provides higher resolution than ¹H NMR. However, due to the lower sensitivity and longer relaxation time, ¹³C 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 ¹H (I) to ¹³C (S), but since the enhancements depend on coupling constants (¹J_SI) and spin systems (SI, SI₂, SI₃), 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 SI₂ and SI₃ 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 ¹³C signals are nearly equally enhanced with standard deviation less than 5%.