ИСТИНА

2D INADEQUATE is a widely used pulse sequence to reveal the homonuclear through-bond connectivites in molecules. The introduction of the refocused INADEQUATE synchronized with MAS has expended the application of this technique to solid materials[1]. In contrast to classical INADEQUATE sequence, the refocused one allow obtaining in-phase doublet signals in 2D spectra and possesses much higher sensitivity for solids. In 1995 INADEQUTE-CR (composite refocusing), another modification of the INADEQUATE technique with improved resolution and sensitivity was suggested[2]. This modification suppresses one component (left or right) of each J-doublet signal and adds its intensity to the residual one. In 1999 possible application of this technique for solids was shown [3], however, no any 2D INADEQUATE-CR MAS NMR experiments has been published yet. Here we report the application of 2D 29Si phase-sensitive INADEQUATE-CR pulse sequence for pure-silica zeolite BEA under MAS conditions. AVANCE-II 400WB Bruker NMR spectrometer and 4 mm double-channel MAS probe were used for the experiments. The pure-silica BEA zeolite enriched with 25% 29Si was synthesized by hydrothermal synthesis in fluoride media[4]. 1D 29Si MAS NMR spectrum of calcined Si-BEA material contains nine signals. Although the number of signals coincides with number of T-position in BEA structure, the intensities of individual signals do not correlate with the distribution of atoms in T-positions and no direct assignment of the signals in 29Si MAS NMR spectrum can be performed . The application of 2D refocused INADEQUATE technique does not solve the attribution problem as spectrum of 29Si-BEA contains a big amount of overlapping J-doublet signals. In the case of INADEQUATE-CR, the situation is completely different. Due to the transformation of doublets to singlets the remarkable resolution with FWHM around 7 and 14 Hz in SQ and DQ dimensions, correspondingly, has been achieved. The presence of two well-resolved subsystems of signals has been clearly identified. INADEQUATE-CR and refocused INADEQUATE has demonstrated the similar sensitivity. It can be explained by longer duration of INADEQUATE-CR sequence than refocused one. The possible sensitivity enhancement from CR-scheme is eliminating by effect of relaxation. [1] A. Lesage, M. Bardet, L. Emsley, J. Am. Chem. Soc., 1999, 121, 10987. [2] N. C. Nielsen, H. Thøgersen, O. W. Sørensen, J. Am. Chem. Sос., 1995, 117, 11365 [3] R. Verel, J. D. van Beek, B. H. Meier, J. Magn. Reson., 1999, 140, 300. [4] M. A. Camblor, A. Corma and S. Valencia, Chem. Commun., 1996, 236