15min:
NUCLEAR SPIN MODIFICATION OF METHANE IN PARAHYDROGEN CRYSTALS.

MIZUHO FUSHITANI, MASAAKI MIKI AND TAKAMASA MOMOSE, Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, JAPAN.

Solid parahydrogen is an excellent matrix for matrix-isolation spectroscopy because of its high spectral resolution. Here, we report the nuclear spin symmetry selection rule of a chemical reaction CH 2 + H2 rightarrow CH 4 and the nuclear spin conversion of CH4 embedded in parahydrogen crystals studied by infrared absorption spectroscopy. Due to the nuclear spin modification, not only the J=0 rotational level but also the J=1 level of CH4 is populated even if the temperature is lowered sufficiently. Since conversion among different nuclear spin states is very slow, population ratio between the J=1 and J=0 levels observed just after the crystal growth reflects the ratio of A and F nuclear spin states before the cooling. The vibration-rotation absorption of CH4 in parahydrogen crystal grown from a premixed gas of methane and hydrogen molecules shows the population ratio of approximately (J=1):(J=0)=2:1. On the other hand, CH4 produced by the photochemical reaction of CH 3 + H2 + 193 nm rightarrow CH 2 + H + H2 rightarrow CH 4 + H was found to have four times larger population in the J=1 rotational level than in the J=0 level just after the reaction. The former reflects the ratio of the A and F nuclear spin states being 5:9 at room temperature, while the latter is due to a nuclear spin selection rule of the reaction.

In addition, the vibration-rotation absorptions of CH4 exhibit time-dependent intensity changes at 4.8K. These changes are interpreted to be a result of the I=1 (F) rightarrow I=2 (A) nuclear spin conversion which accompanies the J=1 rightarrow J=0 rotational relaxation. The half-lifetime of the upper J=1 rotational state is unchanged by the addition of up to 2% orthohydrogen molecules, but decreases with more than 10% orthohydrogen molecules. The increase of the decay rate at higher orthohydrogen concentration indicates that the magnetic field gradient across CH4 caused by orthohydrogen molecules mixes the nuclear spin states which accelerate the conversion.