Sensitivity-Enhanced Quantitative ¹³C NMR Spectroscopy via Cancellation of ¹J_CH Dependence in DEPT Polarization Transfers

Abstract

A scheme has been developed to eliminate virtually all signal intensity dependence on 1JCH in polarization transfers between 1H and 13C nuclei, reducing differences in signal intensity to only 1.5% over the entire natural 1JCH range. The scheme relies on the summation of time-domain data acquired with four suitably selected Delta delays so that the J dependence is essentially canceled in the final, signal-averaged free-induction decay. These Delta delays have been incorporated into the DEPT pulse sequence to create sensitivity-enhanced experiments for collecting quantitative 13C{1H} spectra. Four experiments, each with unique read pulse angles, give quantitative spectra with 200-300% more sensitivity than conventional 13C spectra acquired with inverse-gated 1H decoupling. The experiments are ideal for recording spectra with improved quantitative information or for substantially reducing the long acquisition times indicative of quantitative 13C experiments. The ability of the experiments to provide quantitative spectra was demonstrated with a simple ethylbenzene solution, however, they can easily be adapted to various applications for analysis of complex mixtures.

Accordion‐optimized DEPT experiments

“…This represents a total experimental time around 7 and 9 min. In comparison, the QDEPT scheme [6] would require 133 scans (about 7 min) to obtain SNR of at least 10 for the four selected resonances. However, the possibility of establishing carbon multiplicity is lost when using the QDEPT scheme, which is in essence the main attribute of DEPT experiments.…”

“…[30] The number of scans required for obtaining an SNR of at least 10 for all resonances is also indicated, given that the SNR values shown have been obtained using 144 scans (the QDEPT scheme must be recorded using multiples of 48 scans). [6] ACCORDEPT spectra have been sampled using 16 been added for comparison purposes. This spectrum exhibits aliphatic resonances that are significantly attenuated, as a result of the non-optimal accordion optimization.…”

“…[6,7] Actually, the QDEPT experiment has been precisely designed to cancel the signal intensity dependence on 1 J CH in polarization transfers between 1 H and 13 C and could therefore be applied to address the problem of weak responses. However, the possibility of establishing carbon multiplicity is lost, and hence the QDEPT scheme does not appear very useful for routine measurements excepted when quantitative data are needed.…”

Stereoselective Nickel‐Catalyzed Cycloisomerization of 1,6‐Dienes

“…(E)-Diethyl allylcrotylmalonate (26), carrying a methyl instead of an aryl group, resulted in a non-selective cycloisomerization with [NiA C…”

Conformational Changes in Small Ligands Upon Tetanus Toxin Binding

“…Data were collected with 900 pulse widths, or alternatively, quantitative 1 3 C1IH data sets were recorded with the sensitivity-enhanced Q-DEPT pulse sequence. 29 Recorded data were multiplied by an exponential window function with a linebroadening factor of 2 Hz before Fourier transformation into spectra and manual phase correction into pure absorption mode.…”