High-Resolution EPR and Quantum Effects on CH3, CH2D, CHD2, and CD3 Radicals under Argon Matrix Isolation Conditions

Astrochemically Relevant Radicals and Radical-Molecule Complexes: A New Insight from Matrix Isolation

The reactive open-shell species play a very important role in the radiation-induced molecular evolution occurring in the cold areas of space and presumably leading to the formation of biologically relevant molecules. This review presents an insight into the mechanism of such processes coming from matrix isolation studies with a main focus on the experimental and theoretical studies performed in the author's laboratory during the past decade. The radicals and radical cations produced from astrochemically relevant molecules were characterized by Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopy. Small organic radicals containing C, O, and N atoms are considered in view of their possible role in the formation of complex organic molecules (COMs) in space, and a comparison with earlier results is given. In addition, the radical-molecule complexes generated from isolated intermolecular complexes in matrices are discussed in connection with their model significance as the building blocks for COMs formed under the conditions of extremely restricted molecular mobility at cryogenic temperatures.

Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120 K when subjected to a sub-bandgap (405 nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100 K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

An EPR study on the radiolysis of isolated ethanol molecules in solid argon and xenon: matrix control of radiation-induced generation of radicals in cryogenic media

This paper addresses the basic question of the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures with possible implications for astrochemical issues. The radicals produced by X-ray irradiation of isolated ethanol molecules (C₂H₅OH and CH₃CD₂OH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH₃˙) and formyl (HCO˙) radicals resulting from the C-C bond cleavage were mainly produced in the case of solid argon, which was attributed to the significant role of "hot" ionic fragmentation and inefficient energy dissipation to medium. In contrast, irradiation in xenon results in the predominant formation of α-hydroxyethyl radicals (CH₃˙CHOH or CH₃˙CDOH(D) in the cases of C₂H₅OH and CH₃CD₂OH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CH₃CD₂O˙) radicals due to O-H bond cleavage, which convert to the α-hydroxyethyl radicals due to isomerization occurring at 7 K. The α-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH₃˙CHX species, and exhibit free rotation in solid xenon only at ca. 65 K.

Formation and Evolution of H2C3O+• Radical Cations: A Computational and Matrix Isolation Study

The family of isomeric H₂C₃O^+• radical cations is of great interest for physical organic chemistry and chemistry occurring in extraterrestrial media. In this work, we have experimentally examined a unique synthetic route to the generation of H₂C₃O^+• from the C₂H₂···CO intermolecular complex and also considered the relative stability and monomolecular transformations of the H₂C₃O^+• isomers through high-level ab initio calculations. The structures, energetics, harmonic frequencies, hyperfine coupling constants, and isomerization pathways for several of the most important H₂C₃O^+• isomers were calculated at the UCCSD(T) level of theory. The complementary FTIR and EPR studies in argon matrices at 5 K have demonstrated that the ionized C₂H₂···CO complex transforms into the E-HCCHCO^+• isomer, and this latter species is supposed to be the key intermediate in further chemical transformations, providing a remarkable piece of evidence for kinetic control in low-temperature chemistry. Photolysis of this species at λ = 410-465 nm results in its transformation to the thermodynamically most stable H₂CCCO^+• isomer. Possible implications of the results and potentiality of the proposed synthetic strategy to the preparation of highly reactive organic radical cations are discussed.

Photoinduced Electron Transfer in a Radical SAM Enzyme Generates an S-Adenosylmethionine Derived Methyl Radical

Radical SAM (RS) enzymes use S-adenosyl-l-methionine (SAM) and a [4Fe-4S] cluster to initiate a broad spectrum of radical transformations throughout all kingdoms of life. We report here that low-temperature photoinduced electron transfer from the [4Fe-4S]¹⁺ cluster to bound SAM in the active site of the hydrogenase maturase RS enzyme, HydG, results in specific homolytic cleavage of the S-CH₃ bond of SAM, rather than the S-C5' bond as in the enzyme-catalyzed (thermal) HydG reaction. This result is in stark contrast to a recent report in which photoinduced ET in the RS enzyme pyruvate formate-lyase activating enzyme cleaved the S-C5' bond to generate a 5'-deoxyadenosyl radical, and provides the first direct evidence for homolytic S-CH₃ bond cleavage in a RS enzyme. Photoinduced ET in HydG generates a trapped ^•CH₃ radical, as well as a small population of an organometallic species with an Fe-CH₃ bond, denoted Ω_M. The ^•CH₃ radical is surprisingly found to exhibit rotational diffusion in the HydG active site at temperatures as low as 40 K, and is rapidly quenched: whereas 5'-dAdo^• is stable indefinitely at 77 K, ^•CH₃ quenches with a half-time of ∼2 min at this temperature. The rapid quenching and rotational/translational freedom of ^•CH₃ shows that enzymes would be unable to harness this radical as a regio- and stereospecific H atom abstractor during catalysis, in contrast to the exquisite control achieved with the enzymatically generated 5'-dAdo^•.

Quantum Impurity Rotator in a Matrix of Quantum Rotors: Electron Paramagnetic Resonance Dynamics of CH3 in Solid CD4 Matrix

The rotational dynamics and the geometry of a light and flexible impurity molecule like methyl, matrix isolated in van der Waals solid, are supposed to be sensitive to the host molecule dynamics and order alterations of the matrix. In addition, the location of the impurity and its interaction with the matrix molecules is of prime importance. Large energy gaps between rotation levels of quantum rotators allow precise investigation of temperature-assisted quantum tunneling effects. The molecular rotation of methyl radicals isolated in the deuterated solid methane isotopomer, CD₄, was investigated both by experimental and theoretical electron paramagnetic resonance (EPR) methods. The reduction of the quantum rotation frequency evident from the EPR spectrum of methyl radical at liquid-He temperatures was explained and connected to the irregular ratio of the central doublet to the outer quartet hf transitions. The involvement of temperature in the alteration of methyl symmetry between the C₃ and D₃ groups and the molecular host-host and guest-host interaction strengths were also examined by constructing temperature profiles of the rotation correlation times in the three phases of solid methane. The present study proves the deep impact that a van der Waals matrix may have on the geometry and the rotation levels of a substitutionally trapped quantum impurity rotor, effects that are yet very little known. This close correlation between dynamics of an impurity particle and the matrix molecules has great potential in developing sensitive physicochemical probes for van der Waals solids.

Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K

ESR spectra and electron spin relaxation of nitroxide radical in 4-oxo-TEMPO-d16-(15)N in propylene glycol were studied at X-band in the temperature range 10-295K. The spin-lattice relaxation in the liquid viscous state determined from the resonance line shape is governed by three mechanisms occurring during isotropic molecular reorientations. In the glassy state below 200K the spin-lattice relaxation, phase relaxation and electron spin echo envelope modulations (ESEEM) were studied by pulse spin echo technique using 2-pulse and 3-pulse induced signals. Electron spin-lattice relaxation is governed by a single non-phonon relaxation process produced by localized oscillators of energy 76cm(-1). Electron spin dephasing is dominated by a molecular motion producing a resonance-type peak in the temperature dependence of the dephasing rate around 120K. The origin of the peak is discussed and a simple method for the peak shape analysis is proposed, which gives the activation energy of a thermally activated motion Ea=7.8kJ/mol and correlation time τ0=10(-8)s. The spin echo amplitude is strongly modulated and FT spectrum contains a doublet of lines centered around the (2)D nuclei Zeeman frequency. The splitting into the doublet is discussed as due to a weak hyperfine coupling of nitroxide unpaired electron with deuterium of reorienting CD3 groups.

Electron Paramagnetic Resonance Spectroscopic Study on Nonequilibrium Reaction Pathways in the Photolysis of Solid Nitromethane (CH3NO2) and D3-Nitromethane (CD3NO2)

Thin films of nitromethane (CH3NO2) along with its isotopically labeled counterpart D3-nitromethane (CD3NO2) were photolyzed at discrete wavelength between 266 nm (4.7 eV) and 121 nm (10.2 eV) to explore the underlying mechanisms involved in the decomposition of model compounds of energetic materials in the condensed phase at 5 K. The chemical modifications of the ices were traced in situ via electron paramagnetic resonance, thus focusing on the detection of (hitherto elusive) reaction intermediates and products with unpaired electrons. These studies revealed the formation of two carbon-centered radicals [methyl (CH3), nitromethyl (CH2NO2)], one oxygen-centered radical [methoxy (CH3O)], two nitrogen-centered radicals [nitrogen monoxide (NO), nitrogen dioxide (NO2)], as well as atomic hydrogen (H). The decomposition products of these channels and the carbon-centered nitromethyl (CH2NO2) radical in particular represent crucial reaction intermediates leading via sequential molecular mass growth processes in the exposed nitromethane samples to complex organic molecules as predicted previously by dynamics calculations. The detection of the nitromethyl (CH2NO2) radical along with atomic hydrogen (H) demonstrated the existence of a high-energy decomposition pathway, which is closed under collisionless conditions in the gas phase.

Rotation Dynamics Do Not Determine the Unexpected Isotropy of Methyl Radical EPR Spectra

A simple first-principles electronic structure computation, further qc (quantum chemistry) computation, of the methyl radical gives three equal hf (hyperfine) couplings for the three protons with the unpaired electron. The corresponding dipolar tensors were notably rhombic and had different orientations and regular magnitude components, as they should, but what the overall A-tensor was seen by the electron spin is a different story! The final g = (2.002993, 2.002993, 2.002231) tensor and the hf coupling results obtained in vacuum, at the B3LYP/EPRIII level of theory clearly indicate that in particular the above A = (-65.19, -65.19, 62.54) MHz tensor was axial to a first approximation without considering any rotational dynamics for the CH3. This approximation was not applicable, however, for the trifluoromethyl CF3 radical, a heavier and nonplanar rotor with very anisotropic hf coupling, used here for comparison. Finally, a derivation is presented explaining why there is actually no need for the CH3 radicals to consider additional rotational dynamics in order for the electron to obtain an axially symmetric hf (hyperfine) tensor by considering the simultaneous dipolar couplings of the three protons. An additional consequence is an almost isotropic A-tensor for the electron spin of the CH3 radical. To the best of our knowledge, this point has not been discussed in the literature before. The unexpected isotropy of the EPR parameters of CH3 was solely attributed to the rotational dynamics and was not clearly separated from the overall symmetry of the species. The present theoretical results allowed a first explanation of the "forbidden" satellite lines in the CH3 EPR spectrum. The satellites are a fingerprint of the radical rotation, helping thus in distinguishing the CH3 reorientation from quantum rotation at very low temperatures.

Isolation of the CH3˙ rotor in a thermally stable inert matrix: first characterization of the gradual transition from classical to quantum behaviour at low temperatures

By isolating the CH₃˙ rotor in a stable clathrate host the gradual transition from classical to quantum behavior was observed.

Anomalous EPR intensity distribution of the methyl radical quartet adsorbed on the surface of porous materials. Comparison with solid gas matrix isolation

The two inner lines of the EPR quartet of methyl radicals trapped in cryogenic gas matrices are superpositions of the inner transitions of an A-proton-spin quartet and an E-proton-spin doublet. Their intensity relative to the outer lines provides information on the population of the methyl-rotation quantum states. The above intensity ratio for the CH3 in solids is a challenging problem of the quantum dynamics and statistical thermodynamics. The influence of the quantum-mechanical/inertial rotation on the intensity distribution of the hf components of methyl radical on the surface of porous materials, e.g., silica gel, is investigated by EPR line shape simulations and compared with spectra of the radical isolated in the bulk of solid gas samples. The experimental part of this study includes the first in literature EPR observation of methyl radical in the bulk of N2O solid and provides new essential information on CH3 in CO2 and Ar matrices, thus, covering both strongly hindered and almost free rotation of the radical. We verify the observation of nonrotating methyl radicals in a N2O matrix, discovered earlier in cold CO2, give a thorough account of their EPR characteristics, and explore their formation at the inner surface of porous materials. Combination of a classical spin-Hamiltonian with employment of quantum effects due to nuclear spin-rotation coupling and the radical symmetry were used to interpret the experimental spectral observations. The cause of experimentally found unexpected contribution of the excited degenerate E-doublets to the EPR spectrum down to 4.2 K and A/E transition amplitude ratios sometimes as high as ca. 1:8 at liquid-N2 temperature is sought. The validity of Bose-Einstein quantum (BEq-) statistics of the spin rotation states in addition to the classical Maxwell-Boltzmann (Boltzmann) statistics was also assessed against experimental population A/E-ratio data. The BEq-statistics were not previously applied to similar systems. Furthermore, detailed consideration of the laboratory/free space rotational degeneracy and the planarity of methyl radical was also included in this work.

Rotation of methyl radicals in a solid krypton matrix

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH(3)) in a solid krypton matrix at 17-31 K temperature range. The radicals were produced by dissociating methane by plasma bursts generated by a focused 193 nm excimer laser radiation during the krypton gas condensation on the substrate. The ESR spectrum exhibits only isotropic features at the temperature range examined, and the intensity ratio between the symmetric (A) and antisymmetric (E) spin state lines exhibits weaker temperature dependence than in a solid argon matrix. However, the general appearance of the methyl radical spectrum depends strongly on temperature due to the pronounced temperature dependency of the E state linewidths. The rotational energy level populations are analyzed based on the static crystal field model, pseudorotating cage model, and quantum chemical calculations for an axially symmetric, planar rotor. Crystal field strength parameter values of -140 cm(-1) in Ar and -240 cm(-1) in Kr match most closely the experimentally observed rotational energy level shifts from the gas phase value. In the alternative model, considering the lattice atom movement in a pseudorotating cage, the effective lowering of the rotational constants B and C to 80%-90% leads to similar effects.

Inertial rotation and matrix interaction effects on the EPR spectra of methyl radicals isolated in 'inert' cryogenic matrices

The CW-EPR lineshapes of methyl and small methyl-like radicals trapped in noble gas matrices at liquid He temperatures are substantially different from the expected classical EPR spectra. At low temperatures they show small or negligible anisotropy in studies using different experimental techniques and have a temperature dependence that differs from systems whose motional dynamics is diffusion controlled. At liquid He temperatures, before the Boltzmann statistics take over in the classical high temperature realm, the spectral intensities are dominated by quantum statistics. These properties, which were obtained experimentally at temperatures about 5 K and lower, and up to about 20 K, can be attributed to quantum effects of inertial rotary motion and its coupling to the nuclear spin of the radical. Methyl-like radicals have nuclear-exchange symmetry and contain the lightest possible isotopes, protons, and deuterons. In the ideal case of absent radical-matrix interaction, the methyl rotation about the central heavier carbon atom guaranties minimal moments of inertia. However, the theoretical interpretation of the above effects and other related quantum effects, as well as recognition of the important physics which lead to them, is not a simple matter. The literature accumulated on the subject over the years is successful but contains several unresolved questions. Recently obtained spectra of methyl radicals in Kr, N(2) and CO matrices, which are less inert than the smaller noble gas Ar, were shown to exhibit greater, but certainly slight, overall anisotropic spectral features while in earlier experimental studies the anisotropy was practically absent. Even gases of smaller radii such as Ne and H(2) at liquid He temperatures show interesting differences as hosts of methyl radicals compared to Ar. Investigation of other possible causes of this difference, not excluding the experimentally controlled ones related to the sample preparation and the MW power saturation of the CW-EPR measurement, were conducted in this work.

Electron spin resonance spectroscopy of molecules in large precessional motion: a case of H6(+) and H4D2(+) in solid parahydrogen

We have measured electron spin resonance (ESR) spectra of H6+ and H4D2(+) ions produced in gamma-ray irradiated solid parahydrogen. Anisotropic hyperfine-coupling constants for H6(+) and H4D2(+) determined by the analysis of ESR lines at 4.2K were -0.06 and -0.12 mT, respectively, which were opposite in sign to and much smaller than theoretical results of 1.17-1.25 mT. Although no change was observed in H6(+), the constant for H4D2(+) increased to be 1.17 mT at 1.7 K, which is very close to the theoretical value. We concluded that H6+ both at 4.2 and 1.7 K and H4D2(+) at 4.2K should be in a large precessional motion with the angle of 57-59 degrees, but the precession of H4D2(+) is stopped at 1.7 K.

Rotation of methyl radicals in a solid argon matrix

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in a solid argon matrix at 14-35 K temperatures. The radicals were produced by dissociating methane by plasma bursts generated either by a focused 193 nm laser radiation or a radio frequency discharge device during the gas condensation on the substrate. The ESR spectrum exhibits axial symmetry at the lowest temperature and is ascribed to ground state molecules with symmetric total nuclear spin function I=3/2. The hyperfine anisotropy (Aparallel)-Aperpendicular) was found to be -0.01 mT, whereas that of the g value was 2.5x10(-5). The anisotropy is observed for the first time in Ar and is manifested by the splitting of the low-field transition. Elevation of temperature leads reversibly to the appearance of excited state contribution having antisymmetric I=1/2. As a function of the sample temperature, the relative intensities of symmetric and antisymmetric spin states corresponding to ground and excited rotor states, respectively, proton hyperfine and electron g-tensor components, and spin-lattice relaxation rates were determined by a numerical fitting procedure. The experimental observations were interpreted in terms of a free rotation about the C3 axis and a thermal activation of the C2-type rotations above 15 K. The ground and excited rotational state energy levels were found to be separated by 11.2 cm-1 and to exhibit significantly different spin-lattice coupling. A crystal field model has been applied to evaluate the energy levels of the hindered rotor in the matrix, and crystal field parameter varepsilon4=-200 cm-1, corresponding to a 60 cm-1 effective potential barrier for rotation of the C3 axis, was obtained.

High-resolution ESR study of the H...CH3, H...CHD2, D...CH2D, and D...CD3 radical pairs in solid argon

High-resolution electron spin resonance (ESR) spectra of radical pairs of a hydrogen atom that coupled with a methyl radical (H...CH3, H...CHD2, D...CH2D, and D...CD3) were observed for X-ray irradiated solid argon containing selectively deuterium-labeled methanes, CH4, CH2D2, and CD4, at 4.2 K. The double-quartet 1H-hyperfine (hf) splittings of ca. 26 and 1.16 mT at the Deltam(s) = +/-1 and Deltam(s) = +/-2 transitions, which are one-half of the isotropic 1H-hf splittings of an isolated H-atom and a CH3 radical, were attributed to the H...CH3 pair. The 1H-hf splittings at the Deltam(s) = +/-1 transition were further split by the fine structure (fs) due to the electron dipole-dipole coupling. Because of the high-resolution spectra, three different sets of the fs splitting, d, are clearly resolved in the spectra of both the H...CH3 and the D...CD3 pairs. The separation distance (inter-spin distance), R, between the H-atom and the CH3 radical being in pairs was evaluated from the d values based on a point-dipole interaction model. For the case of the H...CH3 pair, the observed d values of 4.2, 4.9, and 5.1 mT yield the respective separations, R = 0.87, 0.83, and 0.82 nm, to probe the trapping site of the pair in an Ar crystalline lattice (fcc). For the pair with R = 0.87 nm, for example, we propose that the CH3 radical occupies a substitutional site and the counter H-atom occupies either the interstitial tetrahedral sites directed away from the CH3 radicals by a distance of 0.87 nm or the interstitial octahedral sites by a distance of 0.88 nm. When a mixture of CH4 and CD4 in a solid Ar matrix was irradiated, only two different radical pairs, H...CH3 and D...CD3, were observed. This result clearly demonstrates that the hydrogen atom and methyl radicals, which undergo a pairwise trapping, can originate from the same methane molecule.

EPR and DFT study on the stabilization of radiation-generated methyl radicals in dehydrated Na-A zeolite

Electron paramagnetic resonance (EPR) spectroscopy was applied to study paramagnetic species stabilized in Na-A zeolite exposed to gaseous methane and gamma-irradiated at 77 K. Two types of EPR spectra were recorded during thermal annealing of zeolite up to room temperature. Owing to the results for the zeolite exposed to (13)CH(4) the multiplet observed at 110 K was assigned to a (.-)CH(3)...Na(+) complex. After decay of the multiplet, the isotropic quartet of methyl radical was recorded in the temperature range of 170-280 K. On the basis of the EPR parameters it is postulated that (.-)CH(3) radicals in this temperature region are able to freely rotate inside the zeolite cage. The structures of the (.-)CH(3)...Na(+) adsorption complex and respective hyperfine coupling constants were calculated by applying DFT quantum chemical methods. Two different models were applied to represent the zeolite framework: the 6T structure of one six-membered ring and the 3T cluster. The hyperfine coupling constants calculated for the (.-)CH(3)...Na(+) adsorption complex for both applied models show very good agreement with those obtained experimentally.

Magnetic resonance in systems with equivalent spin-1/2 nuclides. Part 1

Electron paramagnetic resonance (EPR) spectra of S=1/2 systems XL(n) with n equivalent nuclei having spin I=1/2 have been simulated for microwave frequencies in the L-, X-, and W-bands. It has been shown that for n>2 nuclei, the EPR spectra have a more complicated form than anticipated from the usual oversimplified analysis, which predicts n+1 lines with intensity ratios given by the coefficients of the binomial expansion. For the XL(n) system with n=3, the EPR spectra in fact consist of six lines. The exact solution of the spin-hamiltonian for this case has been obtained, which gives four levels in zero magnetic field. For n>2 systems, the degeneracy of the energy levels cannot be completely removed by the Zeeman electronic and nuclear interactions. For n>4, certain spin states cannot occur, consistent with the (generalized) Pauli exclusion principle. Discussion of the underlying theory, invoking exchange degeneracy and the appropriate permutation group theory, is included in some detail. Analogous considerations hold for NMR spectroscopy of non-radicals.