Spectroscopic determination of interconversion rates among three nuclear spin isomers of methane in crystalline II

Abstract

We measured infrared absorption spectra of crystalline II of CH4 and succeeded in detecting a prominent Q(2) peak in the ν3 vibrational region by rapid cooling after annealing as well as previously reported rovibrational and librational-vibrational peaks. The integral intensities of the R(0), R(1), and Q(2) peaks were found to show biexponential dependence on time. This clearly demonstrates the interconversion among the three nuclear-spin isomers occupying low-lying rotational levels. The two relaxation rates obtained by biexponential fitting were (0.48, 2.3), (1.1, 4.1), (2.3, 5.1), and (3.4, 15.3) in units of inverse hour (h−1) at 5.2, 6.0, 6.5, and 7.0 K, respectively.

Infrared Spectroscopic Study of Solid Methane: Nuclear Spin Conversion of Stable and Metastable Phases

“…Recently, Sugimoto et al 13 suggested that the use of eq 1 to fit the change of the peak area vs time might not be entirely applicable for a system of more than two spin isomers. Using a much faster cooling from 30 to 5 K to prepare their samples than they used in previous work, they found that their data in the 5−7 K range can be fit using…”

“…Recently, Sugimoto et al suggested that the use of eq to fit the change of the peak area vs time might not be entirely applicable for a system of more than two spin isomers. Using a much faster cooling from 30 to 5 K to prepare their samples than they used in previous work, they found that their data in the 5–7 K range can be fit using We found that biexponential fitting is only necessary for isotherms of R (1) peak deposited at 30 K but not for R (0), since the fit with one exponential is excellent (see Figure ).…”

“…In solid methane, this measurement is a bit challenging because some energy levels are close and the transition can be rapid. As pointed out by Sugimoto et al, the expression for the rate eq might not capture the change of intensity, especially on a short time scale and for the R (1) band. Based on a simple model taking into account the occupation of the lowest rotational levels, they proposed to fit data in the 5–7 K range with a double exponential where X stands for the R (0) transition.…”

“…Molecules with two or more than two hydrogen atoms, such as H 2 , H 2 CO, H 2 O, NH 3 , and CH 4 , can undergo a change of the nuclear spin, or nuclear spin conversion (NSC), while keeping the same parity of the wave function. , The case of CH 4 is particularly interesting because of the presence of three nuclear spin isomers; furthermore, the distribution of populations in the isomers can give clues to the origin and processing of CH 4 ice in comets . The nuclear spin conversion has not been observed in the gas phase and can be studied in laboratory time scales only in ices, whether in solid CH 4 or CH 4 embedded in rare-gas , or para-hydrogen matrices. , …”

“…In addition to these intramolecular magnetic interactions, the J = 2 and J = 3 levels are degenerate with respect to I (outlined inside the gray box). Adapted from refs , .…”