New DEFT sequences for the acquisition of one-dimensional carbon NMR spectra of small unlabelled molecules.
Résumé
The acquisition time and quality of 1D (13)C{(1)H} spectra can be improved substantially by using a modified driven equilibrium Fourier transform (DEFT) sequence, which is specifically designed to compensate for the effects of B(1) inhomogeneity, pulse miscalibration and frequency offsets. The new sequence, called uniform driven equilibrium Fourier transform (UDEFT), returns the carbon magnetization with a high accuracy along its equilibrium position after each transient is complete. Thus, the sequence allows the use of relaxation delays (RD), which are much shorter than the carbon T1 of the molecule, thereby speeding up the acquisition process of 1D (13)C{(1)H} spectra. To achieve this level of performance, UDEFT employs a refocusing element constituted by a composite adiabatic carbon pulse surrounded by two 90 degrees carbon pulses whose phases are designed to compensate for 90 degrees pulse miscalibrations in an MLEV manner (90 degrees (+x) - tau(FID)- 180(+y)(Adia)- tau- 90 degrees (+x) - 180 degrees (+x)(Adia)). A version of the UDEFT sequence allows recording 1D (13)C{(1)H} spectra devoid of heteronuclear NOE by using a matched adiabatic (1)H decoupling scheme where an even number of 180 degrees adiabatic pulses is applied during the UDEFT module. Spectra of a solution of 300 mM camphor that contains some carbon nuclei with very long T1 relaxation times (90 s and 78 s) were acquired with 128 scans in 10 min using a 5 s relaxation delay.