Monitoring structural changes of liquids frozen in nanopores.

Less dense packing of molecules in frozen liquids confined to cylindrical glass pores was observed to depend on pore size. This conclusion was derived by monitoring the rotational tunneling of methyl protons, which reside on studied molecules, with nuclear magnetic resonance. For example, the tunneling frequency of dimethyl sulfide and propionic acid at 10 K was observed to be larger in pores than in bulk. This is interpreted as being due to a decrease in the hindering potential. In another type of tunneling spectrum which is due to methyl-methyl interaction, observed in acetyl acetone at 10 K, the splitting decreases as the pores become smaller. It is demonstrated that in both types of materials the shifts of the methyl tunneling splittings in pores are the result of the reduced intermolecular interaction in the pore core region. This in turn indicates that the unit cell size of liquids frozen in nanopores is slightly increased. The increase is largest in smallest pores.

Tunneling spectroscopy from magnetization evolution in a tilted rotating frame of nuclear spins.

The time evolution of the proton Zeeman magnetization in the rotating frame at the magic angle theta(M) = cos(-1)(1/3) is calculated for an isolated tunneling methyl group and its Fourier transform is given. The calculation compares well with the experimental spectra of CH(3)CD(2)I and methylmalonic acid. It is shown that Fourier transform spectroscopy of the magnetization evolution in a tilted RF frame represents an excellent alternative to the analogous experiment performed at exact resonance, resulting in improved resolution and a much better signal-to-noise ratio.

Design of a high-power NMR probe for low-temperature studies.

A low-temperature, high-power NMR probe head design is described which eliminates the problem of electric arc discharge commonly experienced during radiofrequency pulse cycling in a helium environment. A polychlorotrifluoroethylene (Kel-F) coil former, fitted with a solenoid coil, is heat-shrunk onto stainless-steel flanges and spot-welded inside a stainless-steel probe head assembly connected to a hollow coaxial transmission-line probe shaft. By this means, the sample coil and all high-voltage elements can effectively be isolated in a vacuum, while at the same time permitting good thermal contact between the sample and cryogenic gas. This design was used in NMR studies in the 4.6 K < or = T < or = 77 K temperature range for RF pulse durations < or = 50 ms (and longer for low RF amplitudes) and amplitudes up to approximately 60 G.

Spectroscopy from proton spin magnetization evolution in their rotating frame: A study of small tunneling splitting.

The time evolution of proton Zeeman magnetization in the rotating frame at exact resonance, omega = omega(0), is evaluated for an isolated tunneling methyl group CH(3). The Fourier transform of this evolution in time is calculated and both its real and imaginary components are presented. It is shown that the real component does not depend significantly on the strength of the preparation pulse when the tunneling splitting of the methyl rotator ground state is less than 100 kHz. It is also found that the imaginary component of the transform is inversely proportional to the strength of the preparation RF pulse. This is a consequence of the partial dephasing of proton spins during the preparation pulse. The results of the calculation compare well with the experimental spectra of CH(3)CD(2)I.