Rotor Assisted Population Transfer (RAPT) is a solid-state NMR method for enhancing the sensitivity of the central transition spectrum of half-integer spin quadrupolar nuclei. The method is used to enhance the sensitivity of any solid-state NMR experiment on quadrupolar nuclei that draws polarization from the central transition. The approach enhances the central transition polarization by performing a selective saturation of the (unobserved) satellite transitions.
The selective saturation is obtained by applying a train of Frequency-Switched Gaussian pulses (FSG-RAPT)
With all satellites saturated the enhancement obtained with RAPT should increases by I + 1/2, where I is the nuclear spin. For example, the observed enhancements, shown below for nuclei with increasing nuclear spin, I, do indeed increase with nuclear spin, but fall a slightly short of the expected enhancement.
A measurement of the enhancement as a function of RAPT offset frequency can be used to measure quadrupolar coupling constants, Cq, as shown below. This approach combines the sensitivity advantage of the central transition with the Cq measurement precision advantage of the satellites.
In (a) is the simulated Gaussian RAPT profiles for 87Rb with Cq values as indicated and ηq=0.21. (b) Simulated RAPT profiles for 87Rb with ηq values as indicated and Cq=3.2 MHz. Solid dots in (b) are the experimental 87Rb RAPT enhancement profile for RbClO4. The RAPT preparation consisted of 20 Gaussian (σ=2.855 μsec, τp=14.5μsec.) pulses alternating in offset frequency with a rf field strength of 72 kHz.
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.
Below is the Multi-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.
Shown below are multi-RAPT spectra for nuclei with increasing nuclear spint.
The dependence of the FSG-RAPT enhancement on offset frequency for nuclei with different quadrupolar coupling constants was exploited to design the (π/2)CT-RAPT scheme below for the selective excitation of nuclei with large quadrupolar couplings.
In (a) below is the RAPT profiles for the different 87Rb sites as indicated. In (b) is the Bloch decay (black) and π/2 - RAPT spectra (green) of a mixture of rubidium salts. In (c) is the population manipulation in the π/2 - RAPT pulse scheme.