Infrared spectroscopic investigation of nuclear spin conversion in solid CH4

Abstract

Infrared spectra of solid CH4 were studied in the ν3 and ν4 vibrational regions. The phase I crystal around 30 K showed broad absorption bands, whereas the phase II crystal at 6.9-10.3 K exhibited splitting of these bands after annealing above 20 K. The split peaks were assigned to the librating and almost freely rotating molecules in phase II on the basis of the peak spacings and time evolution of the peak intensities. From the quantitative analysis of the temporal changes of the R(0) and R(1) peak intensities, the relaxation rates of the numbers of molecules with J = 0 (I = 2) and J = 1 (I = 1) were determined in the temperature range of 6.9-10.3 K. We fitted the function resulting from a combination of direct and indirect relaxation processes mediated by phonons to the temperature dependence of these rates and obtained the activation energies of the indirect process: C ≃ 36 K. Since this value is higher than the energies of perturbed J = 2 states relative to the J = 1 state, we argue that the nuclear spin conversion through the J = 3 state also takes place.

Infrared spectroscopy of water clusters isolated in methane matrices: Effects of isotope substitution and annealing

“…3(a) to the Q(1), R(0), and R(1) rovibrational transitions of the O h species, respectively, based on a previous spectroscopic study of solid CH 4 . 20 The P(1)-branch feature is likely to be hidden in the broad absorption band centered around 3009 cm −1 due to the librating species. In Fig.…”

“…In the presence of a trace of water monomers and clusters, the absorption features due to the O h molecules are less distinct than those in the pure solid. 20 Therefore, the existence of the solutes is likely to prevent solid CH 4 from crystallizing enough. This idea also agrees with the larger extent of the band split at the smaller solute-concentration, which is shown in Fig.…”

“…In a previous study, a CH 4 film deposited on a gold substrate at 7 K showed a broad featureless ν 3 band attributed to amorphous CH 4 just after the condensation, and annealing above 20 K caused the split of the ν 3 band characteristic of Phase II crystalline CH 4 . 20 Thus, the structure of solid CH 4 is monitored by infrared spectroscopy and is controlled by annealing.…”

First Synthesis and Characterization of CH4@C60

“…The endohedral fullerenes H 2 @C 60 and H 2 O@C 60 are exceptional platforms for the study of nuclear spin isomerism, in which only certain combinations of nuclear spin states and molecular rotational states are allowed by the Pauli principle. We are particularly interested in CH 4 @C 60 , since spin isomerism is also exhibited by methane, which exists as three nuclear spin isomers with the J =0 rotational state having nuclear spin I =2, the J =1 rotational state having nuclear spin I =1, and the J =2 rotational state having nuclear spin states I =0 and I =1 . Methane is one of the largest possible guests for C 60 and herein, we report conditions for optimized CH 4 encapsulation by 3 and the first successful closure sequence to reform the pristine C 60 cage.…”

First Synthesis and Characterization of CH4@C60

“…Thee ndohedral fullerenes H 2 @C 60 and H 2 O@C 60 are exceptional platforms for the study of nuclear spin isomer- ism, [24,[31][32][33] in which only certain combinations of nuclear spin states and molecular rotational states are allowed by the Pauli principle.W ea re particularly interested in CH 4 @C 60 ,s ince spin isomerism is also exhibited by methane,w hich exists as three nuclear spin isomers with the J = 0r otational state having nuclear spin I = 2, the J = 1r otational state having nuclear spin I = 1, and the J = 2r otational state having nuclear spin states I = 0and I = 1. [34,35] Methane is one of the largest possible guests for C 60 [36] and herein, we report conditions for optimized CH 4 encapsulation by 3 and the first successful closure sequence to reform the pristine C 60 cage.Our work constitutes the first synthesis of CH 4 @C 60 and raises the exciting prospect of accessing other endohedral fullerenes,A @C 60 ,i nw hich the endohedral species is a" large" guest molecule;i ncluding A = O 2 ,N 2 ,C O, NO, NH 3 ,C H 3 OH, CH 2 O, and CO 2 ,a sw ell as the atoms Ar and Kr. CH 4 @C 60 was prepared according to the procedures shown in Scheme 1.…”