Proton-polarization transfer enhancement of a heteronuclear spin multiplet with preservation of phase coherency and relative component intensities

Abstract

The DEPT pulse sequence (π/2)(H,y)−(2J)−1−π(H), (π/2)(C,x)−(2J)−1 −ϑ(H,x)π(C)−(2J)−1−(acquire 13C) is analyzed theoretically for a variable ϑ pulse for three spin systems: CH, CH2, and CH3. It is shown that the pulse train produces an enhanced distortion-free 13C signal which has the following characteristics: (a) there is phase coherency within and between the components of the 13C multiplets; (b) the enhancements vary with ϑ as (γH/γC)sin ϑ for CH, (γH/γC)sin 2ϑ for CH2, and (3γH/4γC) (sin ϑ+sin 3ϑ) for CH3. Experimental evidence is provided for these predictions. An important application of the DEPT pulse train is for the generation of both individual proton-coupled and proton-decoupled 13C methine (CH), methylene (CH2), and methyl (CH3) subspectra. This can be readily achieved by forming suitable combinations of DEPT spectra determined at ϑ = (π/4), (π/2), and (3π/4). Such spectral editing is less sensitive to variations in J values than the INEPT pulse sequence. Signal enhancement for 195Pt and 29Si NMR signals are also demonstrated using the DEPT sequence. The only disadvantage of this pulse train compared with the INEPT sequence appears to be its greater sensitivity to spin relaxation, a consequence of its time span.