Abstract:

Quantitative NMR experiments are an essential part of NMR analysis, which play a critical role in component analysis and compound structure identification. Carbon atoms form the framework of organic compounds, and ¹³C NMR has unique advantages in organic analysis due to its wide chemical shift range, narrow spectral peaks, and broadband decoupling capability. However, the low natural abundance, low gyromagnetic ratio, and long longitudinal relaxation time of ¹³C nuclei hinder its wider application in quantitative experiments. In our previous work, we proposed the Q-DEPT⁺ pulse sequence and designed a double loop of pulse flip angle and polarization transfer time, which allows for uniform sensitivity enhancement for the three types of carbon nuclei, CH, CH₂, and CH₃, within a wide ¹J_CH range, making it suitable for quantitative ¹³C NMR. In this study, we further optimized the polarization transfer time and read pulse width of the Q-DEPT⁺ experiment by using a genetic algorithm, and replaced the 180° hard pulse in the ¹³C channel with a G5 composite pulse that compensates for the frequency offset effect. The optimized pulse sequence was named Q-DEPT ⁺⁺. Quantitative experiments were performed on cholesterol acetate in CDCl₃ by using the reverse-gated decoupling pulse sequence (zgig), Q-DEPT⁺, and Q-DEPT⁺⁺ respectively, and the quantification accuracy and sensitivity of the three pulse sequences were compared. The results showed that Q-DEPT⁺⁺ has obvious improvement in both quantification accuracy and sensitivity.

Key words: liquid-state NMR, quantitative NMR, ¹³C NMR, DEPT, genetic algorithm, sensitivity enhancement