Matrix isolation nuclear magnetic resonance studies of methyl group spin–rotation coupling

Molecular Zeeman interactions in NMR spectra of methyl groups

Methyl groups in organic solids generally behave as uniaxial quantum rotors. The existing NMR theory appears to be complete, capable of describing even the finest details of the temperature-dependent spectra of such objects. However, the once re-ported temperature effects in the carbon spectra of the 13C-labeled methyl group in a single crystal of acetylsalicylic acid have still not been explained. As the temperature decreases, in the quartet corresponding to the rapid motional averaging regime, the inner lines first begin to broaden but then narrow again, so that at 6 K a pattern similar to that at room temperature was observed. In the present work, these puzzling effects are explained quantitatively by invoking the molecular Zeeman (MZ) interaction. Like the spin-rotation (SR) interaction long known to occur in methyl groups, it engages the magnetic moments generated by their torsional motions. How-ever, it has not been considered in NMR spectroscopy until now. This is a surprising situation because in the magnetic fields currently used in NMR spectroscopy, the MZ interaction is orders of magnitude stronger than the (magnetic field independent) SR effects.

Experimental boundaries of the quantum rotor induced polarization (QRIP) in liquid state NMR

The Haupt-effect is a rather seldom used hyperpolarization method. It is based on the interdependence between nuclear spin states and rotational states of nearly free rotating methyl groups having C3 symmetry. A sudden change in temperature from 4.2 K to room temperature by fast dissolution yields considerably enhanced (13)C and (1)H resonance signals. This phenomenon is now termed quantum rotor induced polarization. More than 40 substances have been studied by this approach in order to identify them as polarizable by the 'Haupt-effect in the liquid state'. Influencing factors have been analyzed systematically. It could be concluded that substances having a high tunneling frequency, which is due to a small and narrow potential barrier, are most likely to feature quantum rotor induced polarization-enhanced signals.

Transfer of the Haupt-hyperpolarization to neighbor spins

The NMR hyperpolarization observed for freely rotating methyl groups by exerting a temperature jump from 4.2 K to 298 K can be transferred to spins which have a spin, spin coupling with the carbon of the methyl group. First, a spin echo sequence readjusts the primary up/down signals to an in-phase multiplet. This in-phase magnetization is then decoupled and transferred by a simple COSY step using one scan. The polarization factors at the neighbor spins are about 50 by comparing their signal-to-noise ratio with the signal strength after full relaxation.

Unexpected multiplet patterns induced by the Haupt-effect

An NMR polarization up to a factor of 100 compared to the room temperature signal of a fully equilibrated sample and up/down multiplets are observed when 4-methyl-pyridine or toluene are taken rapidly from liquid helium temperatures to room temperature by dissolving in acetone-d6. These findings result from the inherent coupling between rotational and nuclear spin states in methyl groups which can act as quantum rotors. The temperature jump causes changes in rotational and spin energy level population due to symmetry rules that is known as the Haupt-effect.

Tunneling-induced spin alignment at low and zero field

The transfer of rotational to spin angular momentum of CH3 groups according to the Haupt effect is shown to be independent of magnetic field strength, including zero field. Haupt enhanced pulsed nuclear resonance signals of gamma-picoline have been observed at fields below 50 mT with a sensitivity enhancement of more than 3 orders of magnitude over thermally polarized experiments.