Infrared spectroscopy of rovibrational transitions of methyl radicals (CH3, CD3) in solid parahydrogen
Graphical abstract
The ν3 and ν4 vibrational spectra of CD3 isolated in solid parahydrogen. The observed rotational structure indicates that the radical is freely rotating at a substitutional site of an hcp lattice.
Highlights
► CD3 and CH3 in solid H2 were observed by high-resolution FTIR for the first time. ► The rotational constants of CD3 is only a few % smaller than the gas phase values. ► Crystal field parameters indicate significant quantum effects in interactions.
Introduction
Molecular symmetry and group theory play a key role in analyzing high-resolution spectra of molecules in the gas phase, whose details are described in a textbook by Bunker and Jensen [1]. Space is isotropic in the gas phase under field-free condition, so that the rotational states of molecules are classified only by the symmetry of molecules. Mixings of different rotational states of molecules occur under anisotropic external fields, resulting in additional fine splittings in high-resolution spectra. The symmetries of the space and molecules have to be taken into account simultaneously in order to classify the rotational states of molecules under external fields. Such a theory is called the extended group theory [2], [3], [4].
In a series of papers [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], we have demonstrated that molecules isolated in solid parahydrgen can exhibit quantized rotational states, and that the spectral fine structures can be analyzed quantitatively by extended group theory. Small molecules, such as water [11] and methane [5], [9], [10], rotate almost freely at a single substitutional site of the hexagonal close packed (hcp) lattice of solid parahydrogen. The ro-vibration spectra of molecules in solid parahydrogen show linewidths as narrow as 100 MHz (≃0.003 cm−1) [16], which is sharp enough to resolve fine structures due to the anisotropic electrostatic field around the molecules. The narrow linewidths are due in part to the weak intermolecular interaction between hydrogen molecules and the large lattice constant of solid parahydrogen [14], [15]. The analysis of ro-vibrational spectra of CH4 and CD4 in solid parahydrogen revealed that the rotational constants of methane in solid parahydrogen were about 10% smaller than those in the gas phase [5], [9].
In this paper, we studied ro-vibrational transitions of CH3 and CD3 in solid parahydrogen and analyzed rotational fine structures by the crystal field theory based on the extended group theory. Infrared spectra of the methyl radicals in solid Ar have been extensively studied by Jacox et al. [17], [18] and Snelson [19]. They found that the rotational levels of the radicals did not undergo major perturbations in the Ar matrix environment. Recently, the rotational motion of CH3 in an Ar matrix was studied in detail by EPR spectroscopy [20].
Section snippets
Experiments
The methyl radicals, CH3 and CD3, were produced by in situ photolysis of methyl iodides in solid parahydrogen. Details of our experimental setups were described in previous papers [21], [22], [23], [24]. Parahydrogen gas containing less than 0.05% orthohydrogen [25] was mixed with a small amount (∼10 ppm) of methyl iodide (CH3I or CD3I). The mixed gas was continuously fed into a copper optical cell, at about 8.5 K, to grow a transparent crystal. The optical cell was 3.0 cm in length and 1.7 cm in
Observed spectra
Fig. 1 shows the infrared absorption spectra of CH3 in solid parahydrogen at 4.5 K. The upper and lower traces show the spectra of the doubly degenerate stretching (ν3) transition and the doubly degenerate bending (ν4) transition, respectively. The spectra showed a simple doublet in each transition, whose peak wavenumbers and polarizations relative to the crystal axis are listed in Table 1. The assignments in Table 1 will be discussed in later sections. No temporal changes in the intensities
Theory
Analysis of the rotation–vibration spectra of molecules in crystals is complicated due to the existence of the space symmetry. Here, the theory relevant to the analysis of the spectra of CH3 and CD3 in solid parahydrogen is discussed briefly. More details can be found in our previous papers [3], [4]. Each radical is assumed to occupy a single substitutional site of an hcp lattice, as shown in Fig. 3. It is also assumed that radicals are well separated from the counter iodine atom and therefore
Rotational constants and Coriolis parameters
The rotational constants of CD3 in solid parahydrogen were found to be about 4% smaller than those in the gas phase for both B and C constants in all states. The reduction is smaller than that of methane in solid parahydrogen of about 10% [6], [9].
The reduction of the rotational constants corresponds to the increase of the effective moment of inertia due to the perturbation from the surrounding environment. Similar increases of the moment of inertia have been reported for molecules embedded in
Conclusions
This study constitutes the first report of a rotationally resolved and completely assigned rovibrational spectrum for a radical rotating in the solid phase. The crystal field theory was adapted for the analysis of the spectra of CD3 molecules trapped in hcp D3h symmetry sites in solid parahydrogen. The model reproduced all the qualitative features of these spectra in both the ν3 and ν4 states. The rotational constants of the CD3 radical were found to be only a few percent smaller than those in
Acknowledgments
The study was partially supported by Grant-in-aid for Scientific Research of the Ministry of Education, Science, Culture, and Sports of Japan, and a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant.
References (45)
- et al.
Vib. Spectrosc.
(2004) J. Mol. Spectrosc.
(1977)- et al.
Chem. Phys. Lett.
(2002) - et al.
Molecular Symmetry and Spectroscopy
(1998) - et al.
J. Chem. Phys.
(1973) J. Chem. Phys.
(1997)- et al.
J. Chem. Phys.
(1997) - et al.
J. Chem. Phys.
(1997) - et al.
Bull. Chem. Soc. Jpn.
(1998) - et al.
J. Chem. Phys.
(1998)
J. Chem. Phys.
J. Chem. Phys.
J. Chem. Phys.
J. Chem. Phys.
J. Chem. Phys.
Solid Hydrogen
Theory of the Properties od Solid H2, HD, D2
Hydrogen Properties for Fusion Energy
Phys. Rev. Lett.
J. Chem. Phys.
J. Phys. Chem.
J. Chem. Phys.
J. Chem. Phys.
Cited by (11)
New Theoretical Infrared Line List for the Methyl Radical with Accurate Vibrational Band Origins from High-Level Ab Initio Calculations
2022, Journal of Physical Chemistry AAn ab initio study of the ground and excited electronic states of the methyl radical
2016, Physical Chemistry Chemical PhysicsFormation of N<inf>3</inf>, CH<inf>3</inf>, HCN, and HNC from the far-uv photolysis of CH<inf>4</inf> in nitrogen ice
2015, Astrophysical Journal, Supplement SeriesUV photochemistry of benzene and cyclohexadienyl radical in solid parahydrogen
2015, Journal of Physical Chemistry AInfrared spectroscopy and 193 nm photochemistry of methylamine isolated in solid parahydrogen
2015, Journal of Physical Chemistry A
- 1
Present address: RIKEN ASI Terahertz Imaging and Sensing Team, 519-1399 Aramakiaoba, Aoba, Sendai 980-0845, Japan.
- 2
Present address: Department of Chemistry, Graduate School of Science, Nagoya University, Chikusaku, Nagoya 464-8602, Japan.