Tunneling Spectroscopy by Level Matching in the Spin Rotating Frame.

In this thesis it is reported how the level-matching NMR in the spin rotating frame can be used to establish the tunneling-state manifold of a methyl group. The energy levels are identified from the spectrum of Zeeman-tunneling level matching resonances where fast CH_3 group population transfers take place. The three -pulse sequence (ABC) used for observing such resonances consists of a spin-locking sequence (AB) followed in ~500 mus by an r.f. pulse C with a comb of pi/2 pulses preceding it. With this new experimental method both the Zeeman to tunneling polarization transfer (during the pulse B) and the reverse transfer (during the pulse C) are detected. The proton Zeeman energy splitting homega_ {rm Z} is varied by changing, point by point, the magnitude of the effective field in the proton nuclear spin rotating frame from a few Gauss to 50 Gauss. The tilt of the effective field in the rotating frame can be set at any angle, from Theta=90^circ (normal rotating frame) to Theta=45 ^circ (45^circ tilted rotating frame). Since dipole-dipole interaction is dependent on tilt, changing the tilt allows us to manipulate the dipole-dipole interaction. In this way the resolution is improved (to better than 10 KHz). In addition the symmetry of the transitions can be identified. This tunneling spectroscopy is limited to tunneling splittings less than ~ 800 KHz. Tunneling spectra of strongly hindered CH _3 torsional oscillator pairs (of methylmalonic acid, dimethyl sulfide, propionic acid and hexane) are reported. The level matching resonances in all these materials were detected at omega_{rm Z}=nomega_{rm T} with n = 1/4, 1/3, 1/2, 2/3 and 1. In one case n = 2 spectral peak was observed. Two noninteracting CH_3 particles manifold, composed of AA, AE and EE states, explains the observed multi-quanta transitions driven by intra- and/or inter-methyl group dipole-dipole interactions to first or second order. Level matching resonances at omega_{rm Z}=2omega _{rm T}/3 and omega _{rm Z}=2omega_{ rm T}, which indicate that two methyl groups simultaneously undergo a symmetry breaking transition, were observed in the Theta=54.7^circ tilted rotating frame. The dependencies of level -matching transitions on Zeeman-tunneling mixing time, tilt angle and temperature are reported. By adding a train of C pulses, "comb-C", to the ABC sequence it is possible to monitor consecutive polarization transfers from tunneling to Zeeman states, leading to a saturation of tunneling states. The dependence of the saturation rate on the Zeeman splitting also yields a resonance peak at the effective magnetic field which satisfies a level-matching condition. The experimental results for propionic acid are reported. The ABC method is used to study the torsion-torsion interaction of methyl groups with low hindering potential as well. The model compound is acetylacetone which has tunneling frequencies in the GHz range. In the rotating -frame level-matching experiment transitions between methyl states, which are split due to the methyl-methyl torsional interaction, are observed. The level matching resonances detected at Delta=nomega_{ rm T}, n=1/4, 1/3, 1/2 and 1, are interpreted in terms of three-weakly-coupled -methyl groups. hDelta is the torsion -torsion E state splitting which was found to be 27.2 +/- 0.5 G. Dependencies of these level matching transition peaks on the main field strength, mixing time and temperature are also reported. (Abstract shortened by UMI.).