Infrared Spectroscopy of Chloromethyl Radical in Solid Parahydrogen

Article pubs.acs.org/JPCA

Infrared Spectroscopy of Chloromethyl Radical in Solid Parahydrogen and Its Nuclear Spin Conversion Yuki Miyamoto,†,§ Masaaki Tsubouchi,†,∥ and Takamasa Momose*,†,‡ †

Department of Chemistry, and ‡Department of Physics and Astronomy, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada ABSTRACT: We present high-resolution infrared absorption spectra of chloromethyl radical produced by in situ UV photolysis of chloroiodomethane isolated in solid parahydrogen. The radicals were stable over a few days in solid parahydrogen kept at 3.6 K. Analysis of the rotation−vibration spectra revealed that the radical exhibited quantized one-dimensional rotational motion about the C−Cl bond, while the ortho and para nuclear spin species were still clearly distinguishable in the spectra. Temporal change of the spectra indicated that the nuclear spin conversion between the ortho and para nuclear spin species of the radical in solid parahydrogen occurred in a time scale of a few hours at 3.6 K. It was also found that the nuclear spin conversion became significantly slower in a higher concentration of chloroiodomethane.

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INTRODUCTION Molecules with symmetrically equivalent nuclei of nonzero nuclear spin can exist in different nuclear spin states called nuclear spin modifications.1 Conversion between different nuclear spin states is, in general, extremely slow due to weak magnetic interactions between nuclear spins.2 Since the conversion rate could be even longer than the time scale of the evolution of molecular clouds, the population ratio in different nuclear spin states has been considered as one of the important quantities in astrochemistry. The ratio has been observed for several molecules and has been used for the analysis of physical and chemical evolutions of molecular clouds and interstellar objects.3−6 However, relatively fast spin conversion rates have been reported for some systems. An accidental near degeneracy of rotational energy levels was found to accelerate the conversion between different nuclear spin species.7−9 It is also known that the spin conversion becomes fast in the cryogenic condensed phase even if no accidental near degeneracy of rotational levels exists. In previous papers, we have reported that the nuclear spin conversion of methane in solid parahydrgen (p-H2) at 5 K is on the order of several hours.10,11 Solid p-H2 is an ideal cryomatrix for the study of nuclear spin conversion because the rotational motion of doped molecules is fully quantized with minimal perturbation from the p-H2 environment.12−20 The nuclear spin conversion of various other molecules in solid p-H2 including CH3F,21 CH3OH,20 acetylacetone,22 as well as H223 has been studied, and their conversion rate was found to be widely distributed in the range of 10−105 s. The reasoning for the distribution of the conversion rates in condensed phase has not been understood well yet, and therefore further studies will need to be conducted. Here, we report the production of chloromethyl radical (CH2Cl) in solid p-H2, and the observation of its nuclear spin © 2013 American Chemical Society

conversion by high-resolution Fourier-transform infrared (FTIR) spectroscopy. The chloromethyl radical has two symmetrically equivalent hydrogen atoms, and therefore the ortho and para spin modifications exist. One of the purposes of the present study is to investigate whether radicals that possess nonzero electron spin angular momentum show a faster spin conversion rate. High-resolution microwave (MW) and infrared spectroscopy in the gas phase24−26 revealed that the radical has a planar structure with the ground electronic state of 2B1, i.e., the unpaired electron is in the p orbital distributed perpendicularly to the molecular plane. The proton hyperfine structure analysis of these high-resolution spectra showed that non-negligible electron spin density exists on two hydrogen atoms.24 In addition, the spin-rotation interaction constants of CH2Cl are 3 times larger than those of CH2F.26 In singlet molecules, the nuclear spin conversion is supposed to be induced by the mixing of different nuclear spin states via nuclear spin-rotation and the nuclear spin-nuclear spin interactions. In radicals, additional mixing between different nuclear spin states is expected through the electron spinrotation and hyperfine interactions, which may drastically accelerate the spin conversion rate. It is important to determine the spin conversion rate of radicals for further understanding of the spin conversion mechanism, which was the motivation of the present study. In this study, the radical was produced by in situ UV photolysis of chloroiodomethane (CH2ClI) isolated in solid pSpecial Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 10, 2012 Revised: March 18, 2013 Published: March 18, 2013 9510

dx.doi.org/10.1021/jp312122p | J. Phys. Chem. A 2013, 117, 9510−9517

The Journal of Physical Chemistry A

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H2. One of the salient features of solid p-H2 is its weak (or diminished) cage effect due to the quantum nature of solid pH2.18,27−30 Because of the weak cage effect, in situ UV photolysis results in a complete separation of photodissociated fragments followed by the isolation of radicals in solid p-H2. Here, we report that the photolysis of CH2ClI by an appropriate UV wavelength resulted in the production and isolation of the CH2Cl in solid p-H2. Analysis of spectral structure in high-resolution infrared spectra indicated that the produced CH2Cl radical possessed one-dimensional rotation about the C−Cl bond in solid p-H2, but the ortho and para nuclear spin species were still clearly distinguished in observed spectra. Temporal change in the spectral intensities allowed us to determine the nuclear spin conversion rate of CH2Cl in solid p-H2.

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Figure 1. Infrared absorption of CH2ClI and CH2Cl in solid p-H2 at 3.6 K. (a) Infrared absorption spectrum of CH2ClI. The sample was annealed at 5 K before the spectrum was taken in order to purify the crystal to the hexagonal-close-packed structure. (b) Infrared absorption spectrum taken after UV photolysis by a Xe lamp with a UV-D36A filter. New peaks are assigned to the absorption of CH2Cl as indicated in the spectrum. (c) Infrared absorption spectrum taken after reannealing at 5 K for 10 min. In all traces a−c, the absorption in the region of 1160−1200 cm−1 is multiplied by a factor of 0.5. The spectra in the region of 780−850 cm−1 were noisy compared with other spectral regions because of the absorption of BaF2 windows/substrate.

EXPERIMENTAL METHODS Parahydrogen was prepared by passing normal hydrogen gas through a ferric hydroxide oxide-based ortho−para catalyst (Aldrich, catalyst grade, 30−50 mesh) kept at 13.8 K in a closed cycle refrigerator (Daikin, Cryokelvin UV204SCL).13,15,31 The concentration of o-H2 impurities in the converted gas was 100 ppm or less, which was confirmed by the infrared absorption spectrum of CH3F in solid p-H2.31,32 The converted gas was temporarily stored in a stainless steel tank at room temperature. The precursor molecule, chloroiodomethane (CH2ClI, Aldrich 97% purity, natural abundances: 35Cl 75.78%, 37Cl: 24.22%,) was premixed with the p-H2 gas with a concentration of 10 ppm. A thin film of solid p-H2 sample was grown by spraying the premixed gas onto a BaF2 substrate, which was kept at 3.6 K by a closed cycle refrigerator (Sumitomo, RDK-408D-1W). The flow rate of the gas was 3 mmol min−1.14 After the crystal growth, the sample was annealed at 5 K for 20 min in order to purify the crystal structure to hexagonal-close-packed (hcp).33 The sample was then irradiated by a 450 W Xe lamp (Ushio, UXL-451) combined with a Toshiba UV-D36A glass filter for UV photolysis. The filter transmitted UV photons between 310 and 420 nm only. Infrared spectra were recorded by an FTIR spectrometer (Bruker IFS 120HR) with a resolution of 0.015 cm−1. The temperature of the sample was kept at 3.6 K ± 0.05 K all the time, which was regulated by a temperature controller (Cryogenic Control Systems, CryoCon 34) with a calibrated silicon diode (Lake Shore, Model DT-470-CU-13) temperature sensor.

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792 cm−1, 1188 cm−1, 2995−2997 cm−1, and 3060−3062 cm−1 in trace a correspond to the absorption of CH2ClI in an hcp lattice of solid p-H2. After the UV irradiation, the absorption of CH2ClI disappeared almost completely, while new absorption peaks were observed at 824−833 cm−1, 1393 cm−1, and 3048− 3052 cm−1 as shown in trace b. Referring to the previous studies,26,37−39 these peaks are assigned to the ν3 (CCl stretch), ν2 (CH2 scissors), and ν1 (CH2 symmetric-stretch) vibrational bands of CH2Cl, respectively. No other new peaks were observed after the UV irradiation, which indicates that the photodissociation CH 2ClI → CH 2Cl + I

(1)

is the sole process caused by the UV (λ > 310 nm) irradiation in solid p-H2. The successful in situ photodissociation is a result of the weak (or diminished) cage effect in solid p-H2.18,27−30 The CH2Cl radicals were stable in solid p-H2 at 3.6 K for a few days. This is reasonable since the reaction CH2Cl + H2 → CH3Cl + H is approximately 4.5 kcal mol−1 (= 1600 cm−1) endothermic.40 The reaction between the CH2Cl radical and surrounding H2 molecule does not proceed even via tunneling. The barrier for this reaction is estimated to be about 16.5 kcal mol−1 (5800 cm−1),40,41 which is too high to overcome thermally at 3.6 K. The observed absorption of the CH2Cl radical showed multiple peaks in each absorption band. It was found that the spectral structure of the CH2Cl radical changed upon annealing at 5 K as shown in Figure 1c. These changes are more clearly seen in traces a and b of Figure 2. Figure 2a was taken immediately after the UV irradiation, while Figure 2b was recorded after the sample was kept at 5 K for 10 min and quickly cooled to 3.6 K. The ν1 band at 3048−3052 cm−1 consisted of several lines just after the UV irradiation. These peaks are classified into two sets: four sharp peaks at the lower frequency side, and three broad peaks at the higher frequency side with fine structures.

RESULTS

Chloroiodomethane (CH2ClI) has the lowest electronic absorption in the wavelength range of 225 nm −325 nm with a maximum at 271 nm, known as the A-band.34,35 Photodissociation dynamics of CH2ClI into I and CH2Cl in the Aband has been investigated at various excitation wavelengths using a resonance enhanced multiphoton ionization spectroscopy.35 The observed quantum yield as a function of excitation energy follows almost the same trend as that of methyl iodide. The present photolysis of CH2ClI in solid p-H2 was done by irradiating the sample with UV photons whose wavelength were longer than 310 nm in order to avoid any secondary photodissociation of the CH2Cl radical.36 An approximately 20-h irradiation by the Xe lamp resulted in photodissociation of most of the CH2ClI molecules in solid p-H2. Figure 1 shows the infrared absorption (a) before and (b) after the UV irradiation, respectively. The absorption peaks at 9511



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radical is classified into two sets of peaks, whose behaviors upon reannealing were opposite. Since no new peaks were observed after the reannealing, these two sets are both attributable to the CH2Cl radicals, which interconverted into each other by reannealing and leaving the sample at 3.6 K. It is known that the annealing of solid p-H2 at 5 K purifies the crystal lattice to the hcp structure.33 Therefore, the four sharp peaks that became intense after the reannealing are attributable to the CH2Cl radicals trapped in an hcp lattice of solid p-H2. The three broad peaks must be due to the CH2Cl radicals trapped in slightly different environment of solid p-H2 or a complex with impurities such as o-H2,32,42 a photofragmented I atom,43 or a parent CH2ClI molecule. The latter possibility of any complex formation, however, can be excluded as described below. Yoshioka et al. reported multiple peaks in the infrared spectrum of CH3F in solid p-H2 with high o-H2 concentrations. These multiple peaks were assigned to the (CH3F)(o-H2)n clusters with n = 1−12.31,32,42 In the photodissociation of CH3I molecules, we have observed the formation of a complex with CH3 and a photofragmented I atom in solid p-H2, which showed extra peaks in the spin−orbit transition of I atoms.43 These extra peaks due to the complexes were more prominent at higher concentration of CH3I molecules. These reports suggest that the broad peaks in the ν1 band of CH2Cl could be due to the formation of clusters in solid p-H2. In order to check such a possibility, we observed CH2Cl radicals produced in solid p-H2 containing high concentration of either o-H2 or CH2ClI. The left panel of Figure 3 shows

Figure 2. Temporal change of the infrared absorption of CH2Cl in solid p-H2 at 3.6 K. (a) Just after UV irradiation. (b) After reannealing at 5 K for 10 min. The spectrum was taken at 3.6 K immediately after the reannealing. (c) After leaving the sample at 3.6 K for 225 min. (d) After leaving the sample for 1395 min. (e) Reannealing at 5 K for 10 min after the measurement of (d). The sample was continuously exposed to the weak IR light of the FTIR spectrometer between the measurements of trace (a) and trace (e).

The latter broad peaks disappeared after reannealing the sample for 10 min at 5 K, while the former sharp peaks became intense as seen in Figure 2b. The three broad lines, however, recovered slightly after a few hours as shown in traces c and d in Figure 2. These changes were reversible. By reannealing the sample at 5 K, the broad peaks disappeared again, and the sharp peaks recovered their intensities almost completely as shown in Figure 2e. The same behavior was observed in both the ν3 band at 830 cm−1 and the ν2 band at 1393 cm−1. Peaks at 824.71 cm−1 and 830.78 cm−1 in the ν3 band and peaks at 1392.53 cm−1 and 1392.60 cm−1 in the ν2 band remained after the reannealing, while peaks at 826.71 cm−1, 832.70 cm−1, and 1392.86 cm−1 disappeared after the reannealing. No other peaks were observed in the spectral region between 800 cm−1 and 4000 cm−1 at any time. It is also noted that the relative intensity among the four sharp peaks in the ν1 band at 3048 cm−1 changed over time. Just after the UV irradiation, the peaks at 3048.41 cm−1 and 3048.94 cm−1 were stronger than the peaks at 3048.22 cm−1 and 3048.72 cm−1. Upon reannealing at 5 K, the peaks at 3048.22 cm−1 and 3048.72 cm−1 became stronger than those at 3048.41 cm−1 and 3048.94 cm−1. The latter two peaks became gradually intense after a while, as seen in traces c and d of Figure 2. These spectral changes were associated with the nuclear spin conversion between ortho and para spin modifications of the radical. Detailed analysis of the conversion is discussed in the next section.

Figure 3. (left) Spectral behavior of the ν1 band of CH2Cl in a crystal with 10% o-H2. (right) A crystal with 200 ppm CH2ClI. Traces a−d are the same as those in Figure 2.

spectral behaviors of the ν1 band of CH2Cl radical produced in solid p-H2 containing 10% of o-H2, while the right panel shows those of a sample containing 200 pm of CH2ClI. The sample conditions are the same as those in Figure 2 for traces a−d. Besides the fine structures attributable to the nuclear spin conversion (see later section), no significant differences were observed for these samples compared to the dilute sample shown in Figure 2. If the broad peaks were due to the clusters with o-H2 or I atom, new peaks or intensity changes of the broad peaks should have been observed. No new peaks nor

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DISCUSSIONS Assignments of CH2Cl Absorption. As described in the previous section, the absorption of the ν1 band of the CH2Cl 9512



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top molecule. Two equivalent hydrogen atoms of the radical constitute two nuclear spin modifications: para with I = 0, and ortho with I = 1. Since the ground electronic state is 2B1, the Pauli exclusion principle requires that the rotational states with even Ka are coupled with the I = 1 nuclear spin state, while those with odd Ka are coupled with the I = 0 nuclear spin state. Here, Ka is the projection of the rotational angular momentum N onto the molecular a-axis, i.e., the axis along the C−Cl bond. At very low temperatures, only the lowest rotational level of each nuclear spin modification is expected to be occupied, i.e., the N = 0, Ka = 0, I = 1 state for the ortho species and the N = 1, Ka = 1, I = 0 state for the para species. The ortho species is lower in energy than the para species at very low temperatures. When the nuclear spin conversion takes place between these two spin modifications in a cryogenic matrix, the absorption intensity of the para species are expected to become gradually weaker, while that of the ortho species becomes stronger. Therefore, the peaks in Figure 2 that became gradually strong by leaving the sample at 3.6 K after the reannealing are assigned to the absorption of the ortho species, while those that became weak are to the para species. By taking into account the natural abundance of Cl isotopes (35Cl 75.78%, 37Cl: 24.22%,), the peaks at 3048.94 cm−1 and 3,048.72 cm−1 in the ν1 band are assigned to the absorption of ortho and para species of CH235Cl, respectively, and the peaks at 3048.41 cm−1 and 3048.22 cm−1 are assigned to the absorption of the ortho and para species of CH237Cl, respectively. Since the band at 3048 cm−1 corresponds to the ν1 symmetric CH2 stretching, the transition is an a-type transition with the ΔKa = 0 selection rule. In the gas phase, strong P- and R-branch structures were observed with relatively weak Qbranch structure for the ν1 band of the radical.26 Therefore, we expect one absorption peak for the ortho species and two absorption peaks, at least, for the para species at low temperatures. Contrary to this, we only observed one absorption peak for each spin isomers. The missing P- and Rbranch structures in the para species indicates that rotations along the b- and c-axes that are perpendicular to the C−Cl bond are quenched in solid p-H2. The quenching of the rotational motion along the b- and c-axes is reasonable because of the bulky molecular structure of the radical relative to the lattice size of solid p-H2. Anisotropic potentials caused by the interaction from the surrounding p-H2 lattice must be strong enough to quench the rotational motion about the b- and caxes, but weak enough to leave the rotation about the a-axis quantized. A similar quenching of the rotational motion, a single-axis rotation, was reported for CH3F in solid p-H2.21 When only the a-axis rotation exists, the rotational Hamiltonian becomes a simple form, Ĥ = (A/ℏ2)J2â , where Jâ = −iℏ ∂/∂χ with χ being the rotational angle about the a-axis, and A = ℏ2/(2Ia) with Ia being the moment of inertia about the a-axis. The eigenfunction for this Hamiltonian is ⟨χ|Ka⟩ = e−iKaχ with the energy of EKa = AK2a . Only Ka is a good quantum number to specify the rotational states of the one-dimensional rotor. Molecular group theory sufficient for the discussion of the nuclear spin statistics of the present one-dimensional rotor is the permutation group consisting of only the identity E and the permutation (12), where 1 and 2 are the numerical labels of the two hydrogen atoms. Based on the Pauli exclusion principle, it is derived for the single-axis rotation that the I = 0 para species occupies the Ka = 1 state, and the I = 1 ortho species occupies

intensity changes in these concentrated samples indicate that the three broad lines are not due to the cluster formation with impurities. We have found previously from the analysis of CH4 in solid p-H2 that the hcp lattice is a stable crystal structure of solid pH2. A less stable face-centered-cubic (fcc) lattice may form just after the deposition, but such a structure can be removed by an annealing process at 5 K. Therefore, the sharp peaks remained after the annealing at 5 K (traces b in Figure 2 and Figure 3) are mostly due to CH2Cl in the hcp lattice of solid p-H2, while the broad peaks that disappeared after the annealing at 5 K are due to CH2Cl trapped in slightly different environment than the hcp lattice. The fact that the broad peaks recovered gradually in a time scale of a day may indicate that the hcp lattice configuration may not be a completely stable environment for CH2Cl. It may be the case for large molecules such as CH2Cl because intermolecular interactions between a trapped molecule and H2 are stronger and more anisotropic than interactions between two H2 molecules. In any case, the energy difference between the two sites must be smaller than a few cm−1, and the energy barrier between the two sites is also very low, such that they interconvert into each other upon annealing, or by just leaving the sample for a day. Table 1 lists all the observed wavenumbers with their assignments. Quantum numbers listed in Table 1 are discussed in the next subsection. Rotational Motion. MW spectroscopy in the gas phase revealed that the CH2Cl radical has a planar structure with the rotational constants of A = 9.15 cm−1, B = 0.53 cm−1, and C = 0.50 cm−1 for the 35Cl species.24 It is a nearly prolate symmetric Table 1. Observed Transition Frequencies in Units of cm−1 and Their Assignments p-H2 before UV

after UV

791.6 1167.1 1188.1 1188.4 1188.6 2995.6 2997.0 3060.8 3062.3 824.71 826.71 830.78 832.70 1392.53 1392.60 1392.86 3048.22 3048.41 3048.72 3048.94 3050.01 3050.13 3050.75 3050.93 3051.53 a

Assignments CH2ClI p-H2 CH2ClI CH2ClI CH2ClI CH2ClI CH2ClI CH2ClI CH2ClI CH237Cl CH237Cla CH235Cl CH235Cla CH2Cl CH2Cl CH2Cla CH237Cl CH237Cl CH235Cl CH235Cl CH2Cla CH2Cla CH2Cla CH2Cla CH2Cla

ν9 U0(0) ν3 ν3 ν3 ν1 ν1 ν7 ν7 ν3 ν3 ν3 ν3 ν2, Ka ν2, Ka ν2 ν1, Ka ν1, Ka ν1, Ka ν1, Ka ν1, Ka ν1, Ka ν1, Ka ν1, Ka ν1

CH2 rock

=1 =0 = = = = = = = =

1 0 1 0 1 0 1 0

CH2 wag. CH2 wag. CH2 wag. CH2 s-stretch CH2 s-stretch CH2 a-stretch CH2 a-stretch CCl stretch CCl stretch CCl stretch CCl stretch CH2 scissors CH2 scissors CH2 scissors CH2 s-stretch CH2 s-stretch CH2 s-stretch CH2 s-stretch CH2 s-stretch CH2 s-stretch CH2 s-stretch CH2 s-stretch CH2 s-stretch

The CH2Cl radical trapped in a different trapping site. 9513



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Table 2. Vibrational Frequencies of CH2Cl in Units of cm−1 gasa

p-H2

a

band origin

CH235Cl

ν1 ν2 ν3

3048.94 1392.60 830.78

Reference .

b

37

Arb,c

CH2 Cl

CH235Cl

CH237Cl

3048.41 1392.60 824.71

3055.0760

3053.978

CH235Cl

CH237Cl

Krc

Xec

1391 826

1391 820

827

829

References 38 and 37. cReference 39.

the Ka = 0 state at very low temperatures. The same ΔKa = 0 selection rule is applied to the transition of the ν1 symmetric CH2 stretching band as in the case of standard threedimensional rotations. The transition frequency for the Ka = 0, I = 1 state is ν̃Ka = 0 = ν̃0, with ν̃0 being the band origin of the ν1 vibrational transition, while that for the Ka = 1, I = 0 state is ν̃Ka = 1 = ν̃0 + A′ − A″, where A″ and A′ are the rotational constant of the ground and excited vibrational states, respectively. Therefore, the separation between these two transitions becomes Δν̃ = ν̃Ka = 1 − ν̃Ka = 0 = A′ − A″. The rotational constants in the ground and excited states in the gas phase were reported as A″ = 9.150 cm−1 and A′ = 9.041 cm−1 for CH235Cl, and A″ = 9.150 cm−1 and A′ = 9.013 cm−1 for CH237Cl.24,26 Thus, the separations Δν̃ are expected to be −0.11 cm−1 and −0.14 cm−1 for CH235Cl and CH237Cl, respectively. These values coincide well with the separations of −0.22 cm−1 and −0.19 cm−1 between the two observed peaks corresponding to the absorption of CH235Cl and CH237Cl, respectively, in solid p-H2. The ν3 band at 830 cm−1 exhibited two peaks after the annealing, but the intensity of these two peaks did not show any clear temporal change as shown in Figure 2. Therefore, the peak at 830.78 cm−1 is attributable to CH235Cl and the peak at 824.71 cm−1 to CH237Cl . No splitting due to the Ka = 0 and Ka = 1 transitions was observed, which indicates that the rotational constant A′ in the ν3 excited state is almost the same as that in the ground state. This is reasonable since the ν3 band is the CCl stretching band whose A constant does not change much by the vibrational excitation. The ν2 band at 1392 cm−1 exhibited two peaks after the annealing. These two peaks show a temporal change associated with the nuclear spin conversion. Therefore, these two peaks are assigned to the Ka = 0, I = 1 and Ka = 1, I = 0 transitions for the high and low frequency peaks, respectively. No splitting due to the isotopes of Cl was detected in the ν2 band. The assignment is reasonable because the band origin frequency for the CH2 scissor motion is expected to be almost the same between two Cl isotopes. The frequencies of the Ka = 0 transition can be considered as the band origin of each vibrational transition. For the ν1 excitation, we obtained ν1(CH235Cl) = 3048.9 cm−1 and ν1(CH237Cl) = 3048.4 cm−1. The matrix shifts relative to the gas phase values are 6.2 cm−1 and 5.5 cm−1, respectively. The frequencies of the band origin of each vibrational mode are summarized in Table 2. Nuclear Spin Conversion. The temporal change in the intensities of the four sharp peaks of the ν1 band seen in Figure 2b,c,d is associated with the nuclear spin conversion from the Ka = 1, I = 0 state to the Ka = 0, I = 1 state. As in our previous study,10,11 we have analyzed the conversion kinetics using the mole fraction c(t) of the Ka = 0, I = 1 state, which is obtained by

c(t ) =

I0(t ) I0(t ) + βI1(t )

(2)

where I0(t) and I1(t) are the integrated intensity of the Ka = 0 transition and the Ka = 1 transition at time t, respectively, and β is the ratio of the transition probabilities between the Ka = 1 and the Ka = 0 transition. The coefficient β was determined such that the sum of the integrated intensities IT(t) = I0(t) + βI1(t) + γIX(t) remains constant at any time, t. Here, IX(t) is the total integrated intensity of the band corresponding to the different trapping sites, i.e., the three broad peaks at the higher frequency side of the ν1 band, and γ is the ratio of the transition probability of these broad peaks to the probability of the Ka = 0 transition. The parameter γ was also determined by the condition that the sum IT(t) is constant at any time. The constancy of the sum IT(t) is required to satisfy the law of mass conservation of CH2Cl based on the assignment described in the previous section. Figure 4 shows the integrated intensities of I0(t), I1(t), and IX(t) for the ν1 band of CH235Cl. By the nonlinear least-squares

Figure 4. Plots of the integrated intensity of the ν1 band of CH235Cl: ○, the Ka = 0 transition, I0(t); ●, the Ka = 1 transition, I1(t); +, the transitions of CH2Cl in a different trapping site, IX(t). The sum IT(t) = I0(t) + βI1(t) + γIX(t) is shown by ◇. The integrated intensity of the U0(0) transition of solid p-H2 is also shown by left-facing triangles, which is scaled to fit in the graph. The time when the sample was cooled down to 3.6 K after the reannealing at 5 K is set to the origin of the time, t = 0.

fitting of the parameters β and γ with respect to the observed intensities, we obtained the values of β = 0.15 ± 0.05 and γ = 0.24 ± 0.07 as the weighted average of three independent experiments. The sum IT(t) calculated with β = 0.15 and γ = 0.24 for each data point in Figure 4 is also shown in the figure. The sum IT(t) is reasonably constant over 1000 min. The integrated intensity of the U0(0) transition of solid p-H2 is also shown in Figure 4 in order to indicate that there was no significant evaporation of the crystal nor loss of the sample over 1000 min at 3.6 K. 9514



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the case of CH3F in solid p-H2,21 in which nearby o-H2 increased the conversion rate of CH3F. We further noticed that the rate of the nuclear spin conversion became significantly slower in a sample with higher CH2ClI concentration. Trace b of Figure 3 shows that the peaks at 3048.72 cm−1 and 3048.22 cm−1 were still stronger than the peaks at 3048.41 cm−1 and 3048.94 cm−1 after the reannealing at 5 K. Furthermore, the intensity ratio between the transitions of Ka = 1 and Ka = 0 stayed almost the same over 1400 min as seen in Figure 3b,c,d. Figure 6 shows the change of the mole fraction of c(t) (open

Figure 5 shows the plot of the mole fraction c(t) calculated by eq 2. The open squares show the mole fraction of the Ka = 0

Figure 5. Plots of the mole fraction c(t) (open square) and 1 − c(t) (open circle). The solid curves are the fitting curves based on eq 3.

state given in eq 2. The open circles show the mole fraction of the Ka = 1 state, which is equal to 1 − c(t). The change of the mole fraction of the Ka = 0 state was found to obey the firstorder equation c(t ) = [c0 − c∞] exp( −kt ) + c∞

(3)

Figure 6. Plot of the mole fraction c(t) (open square) and 1 − c(t) (open circle) in a sample with 200 ppm of CH2ClI.

where c0 and c∞ are the mole fraction at time t = 0 and t = ∞, respectively. The parameter k is the rate of the nuclear spin conversion from the I = 0 state to the I = 1 state of CH2Cl in solid p-H2. From the analysis of all the data, we obtained k = 0.0070 ± 0.0009 min−1, c∞ = 0.924 ± 0.002, and c0 = 0.851 ± 0.007 for CH235Cl. Due to the weakness of the observed transition, we were not able to determine the conversion rate for CH237Cl with enough accuracy, but the conversion rate of CH237Cl is roughly the same as that of CH235Cl. From the obtained c∞ value, we estimated the energy separation between the Ka = 0, I = 1 state and the Ka = 1, I = 0 state to be 5.2 cm−1, by assuming the Boltzmann distribution at 3.6 K, and the ratio of the degeneracy factor of 3/2 for Ka = 0/ Ka = 1. This energy separation is expected to be equal to the rotational constant A″ in the ground state in solid p-H2 if the radical exhibits a complete single axis rotation. The obtained value of 5.2 cm−1 is 56% of the gas phase A″ constant.26 The large reduction in the A″ constant in solid p-H2 may indicate that the rotation along the a-axis may also be strongly hindered due to anisotropic interaction from the lattice. Previously, we obtained the conversion rate of CH4 in solid p-H2 as 0.0026 min−1 at 3.4 K.11 Lee et al. have determined the conversion rate of the E−A species of the internal rotation of CH3OH in solid p-H2 as 0.018 h−1 = 0.0003 min−1 at 3.5 K.20 The conversion rate of CH3F was 0.13 h−1 = 0.002 min−1 at high o-H2 concentration, and 0.022 h−1 = 0.00037 min−1 at low o-H2 concentration at 3.3 K.21 The conversion rate of CH2Cl determined in the present work was the fastest among these, but the order was about the same. The present result indicates that the nonzero electron spin angular momentum may accelerate the nuclear spin conversion through electron spinrotation interaction and the hyperfine interactions, but not so significantly in CH2Cl. We also observed the nuclear spin conversion of CH2Cl in a sample with 10% o-H2 concentration. The obtained conversion rate was 0.0046 ± 0.0005 min−1, which was slightly slower than the rate of CH2Cl in pure solid p-H2. This trend is opposite to

square) and 1−c(t) (open circle) of this sample. The mole fraction c(t) seems to gradually increase, but the rate of the increase is significantly slower than that in Figure 5. The 200 ppm CH2ClI sample was irradiated by UV radiation for the same period of time (20 h) as that of the dilute sample. Infrared intensities indicated that the concentration of the CH2Cl radical was almost the same as that of the 10 ppm CH2ClI sample, while the precursor CH2ClI molecules remained significantly in the 200 ppm sample. Since no changes in the spectral feature of the CH2Cl radical were observed between the dilute sample (Figure 2) and the concentrated sample (Figure 3), it is not likely that any (CH2Cl)(CH2ClI)N clusters were formed. Proximity, however, of the precursor CH2ClI molecules seems to make the nuclear spin conversion at least 1 order of magnitude slower. At this moment, we do not have any concrete explanation on why the conversion rate was slow in the concentrated sample. Further studies with different systems are needed in order to understand the effect of impurities on the process of nuclear spin conversion in the condensed phase.

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CONCLUSIONS The CH2Cl radical isolated in solid p-H2 was investigated by high-resolution infrared spectroscopy, and its physical and chemical dynamics are discussed. The radical showed single axis rotation about the molecular a-axis. The band origins of the radical were shifted by a few cm−1 toward the lower frequency compared with the gas phase values. The nuclear spin conversion between the I = 0 and I = 1 states was observed, and the determined conversion rate was slightly faster than other molecules, but the order of the rate was about the same. The nonzero electron spin angular momentum of the radical may slightly enhance the nuclear spin conversion rate, but would not drastically change it. On the contrary, the existence of proximity CH2ClI molecule(s) made the nuclear spin conversion at least 1 order of magnitude slower than that in a 9515



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dilute sample. The existence of proximity impurity affected the conversion rate drastically. There still remains a question on the effect of impurity molecules to the nuclear spin conversion rate in condensed phase.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses §

Graduate School of Natural Science and Technology, Okayama University, Tsushima-naka 3-1-1 Kita-ku Okayama 700−8530, Japan. ∥ Kansai Photon Science Institute, Japan Atomic Energy Agency, Kizugawa, Kyoto 619-0215, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by a National Science and Engineering Research Discovery Grant in Canada, and a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture and Sports of Japan.

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Infrared Spectroscopy of Chloromethyl Radical in Solid Parahydrogen

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