Multiplicity editing including quaternary carbons: improved performance for the 13C-DEPTQ pulse sequence

An improved version of the DEPTQ experiment yielding the signal and multiplicity information for all carbon types including the signals of quaternary carbons is proposed. It encompasses all the known advantages of the basic DEPT experiment. In comparison to the original version, signals of the sensitivity-limiting quaternary carbons are markedly increased: the initial 13C pulse may be adjusted to the Ernst angle, the NOE build-up period is prolonged by the split relaxation delay and a partial recovery of signal losses due to instrumental imperfections is achieved by the incorporation of composite adiabatic 13C refocussing pulses. Furthermore, pure absorption lineshapes for all carbon types are obtained with only one single scan. These attributes make this experiment attractive for 13C analysis of small molecules (including spectral editing), particularly in high-throughput analysis laboratories.     

The DEPTQ+ Experiment: Leveling the DEPT Signal Intensities and Clean Spectral Editing for Determining CHn Multiplicities

We propose a new ¹³C DEPTQ⁺ NMR experiment, based on the improved DEPTQ experiment, which is designed to unequivocally identify all carbon multiplicities (Cq, CH, CH₂, and CH₃) in two experiments. Compared to this improved DEPTQ experiment, the DEPTQ⁺ is shorter and the different evolution delays are designed as spin echoes, which can be tuned to different ¹J_CH values; this is especially valuable when a large range of ¹J_CH coupling constants is to be expected. These modifications allow (i) a mutual leveling of the DEPT signal intensities, (ii) a reduction in J cross-talk in the Cq/CH spectrum, and (iii) more consistent and cleaner CH₂/CH₃ edited spectra. The new DEPTQ⁺ is expected to be attractive for fast ¹³C analysis of small-to medium sized molecules, especially in high-throughput laboratories. With concentrated samples and/or by exploiting the high sensitivity of cryogenically cooled ¹³C NMR probeheads, the efficacy of such investigations may be improved, as it is possible to unequivocally identify all carbon multiplicities, with only one scan, for each of the two independent DEPTQ⁺ experiments and without loss of quality.     

CRAPT: an improved version of APT with compensation for variations in JCH

A modified version of the attached proton test (APT) sequence for ¹³C spectral editing, which we call CRisis‐APT (CRAPT), is developed and tested on representative organic compounds. CRAPT incorporates ¹³C compensation for refocusing inefficiency with synchronized inversion sweeps (CRISIS) pulses in combination with ¹H broadband inversion pulses to give improved compensation for variations in ¹J_CH along with improved refocusing efficiency. It is shown that CRAPT gives edited ¹³C spectra with only small losses in sensitivity (between 8% and 15% for strychnine, 1 , menthol, 2 , cholecalciferol, 3 , and isotachysterol, 4 ), compared with basic ¹³C spectra obtained on the same compounds. CRAPT also gives significantly better signal/noise than DEPTQ for nonprotonated carbons. Therefore, we conclude that CRAPT is an improvement over APT or DEPTQ or a combination of DEPT135 with a full ¹³C spectrum for routine ¹³C spectral editing of organic compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Fast 13C-NMR Spectral Editing for Determining CHn Multiplicities

With an improved version of the ¹³C DEPTQ NMR experiment all carbon multiplicities (C_q, CH, CH₂ und CH₃) can be identified unequivocally and most conveniently in two experiments and at best with only one scan each. This simplifies the analysis of ¹³C-NMR data on a routine level in general. With higher sample amounts and/or exploiting the high sensitivity of cryogenically cooled ¹³C probeheads the efficiency of such investigations may be improved. This makes this method attractive for fast ¹³C analysis of small-to-medium sized molecules in high-throughput laboratories.