Subramanian Prasad

Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium. We describe a new RAPT scheme that uses the remaining polarization of the satellites to obtain a further enhancement of the central transition by performing RAPT-enhanced experiments multiple times before waiting for re-equilibration of the spin system. For 27Al (I=5/2) in albite we obtain a multiple RAPT enhancement of 3.02, a 48% increase over single RAPT. For 93Nb (I=9/2) in NaNbO(3) we obtain a multiple RAPT enhancement of 5.76, an 89% increase over single RAPT. We also describe a data processing procedure for obtaining the maximum possible signal-to-noise ratio.

Selective suppression and excitation of solid-state NMR resonances based on quadrupole coupling constants.

The dependence of the (Rotor Assisted Population Transfer) RAPT enhancement on offset frequency for nuclei experiencing different quadrupolar couplings has been exploited to design two new spectral editing schemes, pi/2-RAPT and RAPT-pi-RAPT, for the selective excitation or suppression, respectively, of nuclei with large quadrupolar couplings. Both approaches are demonstrated on the 87 Rb spectrum of Rb(2)SO(4), which contains two resonances with C(q) values of 2.6 and 5.3 MHz. The conditions for optimal selectivity are discussed. Combining pi/2-RAPT with the RIACT MQ-MAS experiment it is also demonstrated how a pure absorption mode triple quantum MQ-MAS spectrum devoid of narrow resonances can be obtained.

Mixed domain models for the distribution of aluminum in high silica zeolite SSZ-13.

High silica zeolite SSZ-13 with Si/Al ratios varying from 11 to 17 was characterized by aluminum-27 and silicon-29 NMR spectroscopy. Aluminum-27 MAS and MQMAS NMR data indicated that in addition to tetrahedral aluminum sites, a fraction of aluminum sites are present in distorted tetrahedral environments. Although in samples of SSZ-13 having high Si/Al ratios all aluminum atoms are expected to be isolated, silicon-29 NMR spectra revealed that in addition to isolated aluminum atoms (Si(1Al)), non-isolated aluminum atoms (Si(2Al)) exist in the crystals. To model these contributions of the various aluminum atoms, a mixed-domain distribution was developed, using double-six membered rings (D6R) as the basic building units of SSZ-13. A combination of different ideal domains, one containing isolated and the other with non-isolated aluminum sites, has been found to describe the experimental silicon-29 NMR data.

Ion distribution in copper exchanged zeolites by using Si-29 spin lattice relaxation analysis.

Transition metal-containing zeolites, particularly those with smaller pore size, have found extensive application in the selective catalytic reduction (SCR) of environmental pollutants containing nitrogen oxides. We report these zeolites have dramatically faster silicon-29 (Si-29) spin lattice relaxation times (T1) compared to their sodium-containing counterparts. Paramagnetic doping allows one to acquire Si-29 MAS spectra in the order of tens of seconds without significantly affecting the spectral resolution. Moreover, relaxation times depend on the method of preparation and the next-nearest neighbor silicon Qn(mAl) sites, where n=4 and m=0-4, respectively. A clear trend is noted between the effectiveness of Cu exchange and the Si-29 NMR relaxation times. It is anticipated that the availability of this tool, and the enhanced understanding of the nature of the active sites, will provide the means for designing improved SCR catalysts.