Rovibrational transitions and nuclear spin conversion of methane in parahydrogen crystals

Abstract

Solid parahydrogen is an excellent matrix for matrix-isolation spectroscopy because of its high spectral resolution. Here we describe the rovibrational structure and nuclear spin conversion of CH4 embedded in parahydrogen crystals studied by infrared absorption spectroscopy. The vibration–rotation absorptions of CH4 exhibit time-dependent intensity changes at 4.8 K. These changes are interpreted to be a result of the I=1→I=2 nuclear spin conversion that accompanies the J=1→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 due to the orthohydrogen molecules mixes the nuclear spin states, which accelerates the conversion.

Nuclear spin conversion of methane in solid parahydrogen

“…our previous paper, 21 we have analyzed the temporal changes of the absorption peaks in terms of the mole fraction c͑t͒ which is defined as 2.…”

“…The rotational anomaly has been observed also in solid environment for various H containing molecules such as H 2 , CH 4 , H 2 O, NH 3 , CH 3 , CH 3 OH, and CH 3 F. [17][18][19][20][21][22][23][24][25][26][27] In condensed phases, the conversion process is accelerated by the existence of phonons. The acceleration of the nuclear spin conversion by phonons makes the conversion rates in a time scale of 1 -10 5 s in cryomatrices despite the weakness of the intrinsic magnetic interactions.…”

“…In a series of papers, we have demonstrated that molecules in solid parahydrogen exhibit well-resolved rotational structures in vibrational transitions, 21,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] which have been completely analyzed using crystal field theory. 38,43 The rovibrational transitions of molecules in solid parahydrogen show linewidths as narrow as 100 MHz with no sign of inhomogeneous broadening.…”

“…35,38 These rotationally resolved infrared spectra are sharp enough to distinguish nuclear spin isomers of some molecules such as CH 4 , 30 H 2 O, 39,44 and CH 3 OH. 27 In a previous paper, 21 we have investigated the nuclear spin conversion between the J =1, I = 1 and J =0, I = 2 states of CH 4 in solid hydrogen containing orthohydrogen concentrations between 0.01% and 75% and found that the conversion rate increases as the orthohydrogen concentration increases, but the rate becomes constant below 2% of orthohydrogen concentration. At a higher orthohydrogen concentration, the magnetic moment of the I = 1 nuclear spin of orthohydrogen accelerates the nuclear spin conversion of CH 4 .…”

Nuclear spin conversion of water inside fullerene cages detected by low-temperature nuclear magnetic resonance

“…The seminal case is that of H 2 , where the nuclear spin singlet state (I = 0) correlates with even rotational quantum numbers J (para-H 2 ), while the nuclear spin triplet states (I = 1) correlate with odd values of J (ortho-H 2 ). 1 Spin isomerism is also exhibited by many other small symmetrical molecules, such as water, [2][3][4][5][6][7][8][9][10][11][12][13][14] ethene, [15][16][17] and methane, [18][19][20][21] as well as freely rotating parts of a molecule, such as methyl (-CH 3 ) groups with low rotational barriers. [22][23][24][25] In many cases, the process of spin-isomer conversion, which requires a change in nuclear spin symmetry, is considerably slower than the spatial and nuclear spin transitions between quantum states of the same spin isomer.…”