Optimized Quantitative DEPT and Quantitative POMMIE Experiments for ~(13)C NMR

摘要

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 5percent.