Improvement in the cross-polarization NMR experiment for suppression of rigid protonated carbons

Abstract

.4 very useful NMR experiment for solids simply requires the use of a calculated delay for the decoupling pulse (1). For a proper choice of the decoupler delay TD, singly or doubly protonated carbons can usually be suppressed from the carbon13 NMR spectrum. Owing to their usually rapid motion at room temperatures, triply protonated carbons (methyl groups) are usually not suppressed as easily. This experiment has received various titles such as “dipolar dephasing” or “interrupted decoupler.” It has been used by various researchers to simplify complicated carbon13 spectra of biological molecules (2, 3), to estimate carbon types in fuels (4), and to substantiate chemical reaction mechanisms (5). One problem with this experiment is that the decoupler delay time TD introduces an additional linear phase error in the observed NMR spectrum if the signal digitizer is triggered at the end of the delay. Alternately, if the digitization is begun prior to the delay, the additional linear phase error can be avoided at the expense of possibly recording (1) the would-be suppressed carbon signals and (2) an rf transient when the decoupler pulse is activated. If a chemical-shift refocusing pulse is added to the rare-spin pulse sequence at the center of the decoupler delay TD, then the digitization can begin at the end of the delay without the undesired linear phase error. The pulse sequence is depicted in Fig. 1. Also shown are the appropriate rf rotating-frame phases for a 4-cycle phase alternated quadrature signal averaging. The rare-spin FID is inverted by alternating the spin-lock direction of the abundant spin (X, x), (6, 7). The quadrature outputs of the real (R) and imaginary (I) channels of the digitizer are added/subtracted from the even (E) and odd (0) memory locations as described in Fig. 1. Carbon13 measurements were made on a commercially available (Aldrich) powder of pdi-tert-butylbenzene (PDTBB). The proton T, of this compound was measured as approximately 0.3 sec. Solids spectra were recorded with an IBM Instruments, Inc. 100 MHz Solids Accessory and a Bruker Instruments, Inc. WPlOOSY console. The carbon-l 3 spectra shown in Fig. 2 were recorded at 25.18 MHz using approximately 300 mg of the powder, a 4.8 kHz MAS rotor speed, a cross-polarization contact time of 2 msec, a spectral width of 15 kHz, a filter width of 19 kHz, a repetition time of 3 set, and 48 signal averages. The matched Hartmann-Hahn and decoupling rf fields were 42 kHz. The width of the ?r refocusing pulse on the carbon13 channel was 12