Mizuho Fushitani

Chemical reactions in quantum crystals

Solid parahydrogen, known as a quantum crystal, is a novel matrix for the study of physical and chemical processes of molecules at low temperatures by high-resolution infrared spectroscopy. Ro-vibrational motion of molecules embedded in solid parahydrogen is well quantized on account of the weak interaction in the crystal. In addition, molecules are almost free from the cage effect because of the softness of the quantum crystal, so that a variety of chemical reactions could be observed for molecules in solid parahydrogen at liquid He temperatures. In this article, we survey our recent studies on photodissociation of methyl radicals and subsequent reactions in solid parahydrogen, and discuss (1) the nuclear spin selection rule in chemical reactions and (2) pure tunnelling reactions obtained from the analysis of the present system. Contents PAGE 1.  Introduction 534 2.  Solid parahydrogen as a matrix 535 3.  Experiments 536 4.  Photochemistry of methyl iodide 537 5.  Nuclear spin selection rule in chemical reactions 542 6.  Tunnelling chemical reactions 546 7.  Conclusion 550 Acknowledgement 550 References 550

Nuclear spin conversion of methane in solid parahydrogen

The nuclear spin conversion of CH(4) and CD(4) isolated in solid parahydrogen was investigated by high resolution Fourier transform infrared spectroscopy. From the analysis of the temporal changes of rovibrational absorption spectra, the nuclear spin conversion rates associated with the rotational relaxation from the J=1 state to the J=0 state for both species were determined at temperatures between 1 and 6 K. The conversion rate of CD(4) was found to be 2-100 times faster than that of CH(4) in this temperature range. The faster conversion in CD(4) is attributed to the quadrupole interaction of D atoms in CD(4), while the conversion in CH(4) takes place mainly through the nuclear spin-nuclear spin interaction. The conversion rates depend on crystal temperature strongly above 3.5 K for CH(4) and above 2 K for CD(4), while the rates were almost constant below these temperatures. The temperature dependence indicates that the one-phonon process is dominant at low temperatures, while two-phonon processes become important at higher temperatures as a cause of the nuclear spin conversion.

Photoinduced reactions of methyl radical in solid parahydrogen

Photolysis of methyl iodide in solid parahydrogen (p-H2) at about 5 K is studied with ultraviolet light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical, whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed vibrational absorption. It is concluded that methyl radical in the ground state does not react with p-H2 molecules appreciably but that the radical in the electronic excited state of B(2A1′), accessible by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH2 a(1A1) and a hydrogen atom (2S) and that the singlet methylene reacts with a p-H2 molecule to give methane.

Tunneling chemical reactions in solid parahydrogen: Direct measurement of the rate constants of R+H2→RH+H (R=CD3, CD2H, CDH2, CH3) at 5 K

Tunneling chemical reactions between deuterated methyl radicals and the hydrogen molecule in a parahydrogen crystal have been studied by Fourier transform infrared spectroscopy. The tunneling rates of the reactions R + H2 --> RH + H (R = CD3,CD2H,CDH2) in the vibrational ground state were determined directly from the temporal change in the intensity of the rovibrational absorption bands of the reactants and products in each reaction in solid parahydrogen observed at 5 K. The tunneling rate of each reaction was found to differ definitely depending upon the degree of deuteration in the methyl radicals. The tunneling rates were determined to be 3.3 x 10(-6) s(-1), 2.0 x 10(-6) s(-1), and 1.0 x 10(-6) s(-1) for the systems of CD3, CD2H, and CDH2, respectively. Conversely, the tunneling reaction between a CH3 radical and the hydrogen molecule did not proceed within a weeks time. The upper limit of the tunneling rate of the reaction of the CH3 radical was estimated to be 8 x 10(-8) s(-1).

A study on diffusion of H atoms in solid parahydrogen

Diffusion of hydrogen atoms in solid parahydrogen was investigated using high-resolution infrared spectroscopy. Hydrogen atoms were produced as by-products of a photoinduced reaction of nitric oxides embedded in solid parahydrogen. The diffusion of the hydrogen atoms is mainly terminated by the reaction H+NO→HNO. The diffusion rate determined from the increase of the intensity of rotation–vibration transitions of HNO molecules was found to be two orders of magnitude larger than that determined by the self-recombination reaction of H+H→H2 in pure parahydrogen crystals.

Nuclear spin selection rule in the photochemical reaction of CH3 in solid parahydrogen

Photolysis of a methyl radical CH3 in solid parahydrogen produces a methane molecule CH4 via the reaction between an intermediate singlet methylene 1CH2 and a parahydrogen molecule H2. Conservation of nuclear spin during the reaction has been investigated by the intensity distribution of the rotation-vibration spectrum of methane produced by the reaction. It was found that the population of each nuclear spin state of methane just after the reaction was different from that of the statistical ratio, which indicates that a nuclear spin selection rule does exist in the reaction. However, the observed population was significantly different from the theoretically predicted ratio. The discrepancy between the experiment and the theory may indicate a breakdown of the nuclear spin conservation during the reaction, if the reaction mechanism in solid parahydrogen is the same as in the gas phase.

The 2P1/2 ← 2P3/2 transition of the iodine atom photoproduced from alkyl iodides in solid parahydrogen: detection of new absorptions

Abstract Alkyl iodides were photolyzed in solid parahydrogen at about 5 K. The first photolysis with 253.7 nm photons yielded two products by reactions, RI +hν (253.7 nm )→ R free + I free and ( R – I ), the latter being a complex between the radical R and the iodine atom I . Subsequent photolysis with 193 nm photons activated the radical moiety in both products to induce a reaction, R + H 2 +hν (193 nm )→ RH + H , which gave the products RH free and (RH– I ). The radical R and the alkane RH were characterized by the mid-IR absorption while the iodine atoms in I free , and ( R – I ) and (RH– I ) were identified by the near IR absorption of the magnetic dipole transition of the atom.

Correlation between Nuclear Spin Ratio of Cyclic C3H2 and Chemical Evolution in TMC-1 Cores

The ortho-to-para nuclear spin ratio of cyclopropenylidene (cyclic C3H2) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines (J = 212-101, 312-303, 321-312, and 330-321) and four para lines (J = 202-111, 211-202, 220-211, and 322-313) of cyclic C3H2 in six cores in TMC-1. A statistical equilibrium analysis of the observed intensities revealed apparent correlation between the ortho-to-para ratio of cyclic C3H2 and chemical evolution of the molecular cloud; the ortho-to-para ratio in younger cores is less than 2, while that in older cores is close to the statistical ratio of 3. A simple chemical model calculation indicates that the correlation is ascribed to the conservation of nuclear spin angular momenta in chemical reactions relevant to the production, destruction, and reproduction of cyclic C3H2. Since cyclic C3H2 has been observed in a variety of sources, the ortho-to-para ratio of cyclic C3H2 may be a useful probe of chemical evolution of molecular clouds.

Stimulated Stokes downconversion in liquid and solid parahydrogen

We report the results of our preliminary investigations into the suitability of condensed-phase parahydrogen as a Raman-shifting medium for infrared cavity ringdown laser absorption spectroscopy. We have observed the conversion of ∼10-ns pulses of 532-nm radiation into first-, second-, and third-order vibrational Stokes radiation in bulk liquid and solid parahydrogen after a single 11-cm pass. Unexpectedly, we find that liquid H2 yields more efficient conversion than solid H2 with certain focal geometries, and that in the case of the solid, a collimated or loosely focused pump geometry is more efficient than a tight focus.

Quantum property of solid hydrogen as revealed by high-resolution laser spectroscopy

Pure vibrational overtone transitions of solid parahydrogen are studied using high-resolution laser spectroscopy. Extremely narrow spectral linewidth (∼20 MHz) allows us to observe rich spectral structure that originates in subtle intermolecular interactions in the crystal. It is found that anisotropy of the distribution of zero-point lattice vibration of hydrogen molecules perturbs the energy levels of the vibrationally excited states significantly. A large amplitude of zero-point lattice vibration, an intrinsic propoerty of quantum solids, is directly observed from the present high-resolution spectroscopy. The first observation of a pure vibrational overtone transition of solid orthodeuterium is also discussed.