Halocarbon Matrices Research Articles

ChemInform Abstract: Radical Cations of cis- and trans-1,3-Di- and 1,3,5- Trimethylcyclohexanes. Matrix Influence on Two Nearly Degenerate SOMOs

Radical cations of cis and trans isomers of 1,3-di- and 1,3,5-tri-methylcyclohexane, stabilized in various γ-irradiated solute–halocarbon matrices at 77 K and above, have been investigated by means of EPR spectroscopy. The hfs constants have been compared with those calculated using semiempirical methods, INDO (spin density) and MNDO (geometry optimizations). The SOMOs of the methyl-substituted cyclohexane cations are all similar to either of the orbitals, ag or bg, following a Jahn–Teller split of the eg orbital (HOMO) of cyclohexane. The selection of SOMO depends on matrix and isomer. The trans isomers have one axial methyl group and the cations were found to take an ag-like SOMO in all matrices used, with large spin density on the equatorial hydrogen attached to the axially substituted ring-carbon. Using CF3CCl3 as matrix the EPR signal due to the trans-1,3,5-trimethylcyclohexane cation contains (in addition) a spectrum of the bg-like SOMO superimposed. The cation of cis-1,3-dimethylcyclohexane shows contributions from both SOMOs in the matrices CF3-cC6F11, CF2ClCFCl2 and CF3CCl3, but exclusively a bg-like SOMO in cC6F12. A bg-like SOMO is observed for the cis-1,3,5-trimethylcyclohexane cation in all matrices. The abundance ratios, [ag]/[bg], for the cases where the matrix-stabilized cation shows contribution from both bg- and ag-like SOMOs simultaneously, are found to be ca. 1/4 from line shape simulations. No prominent temperature dependence of this ratio is detected.

Formation and structure of dimer radical cations of fluorinated benzenes in solid matrices

Structures of dimeric radical cations of fluorinated benzenes (C6H5F+ and 1,4-C6H4F2+) formed by γ-irradiation in solid halocarbon matrices were investigated by means of EPR and DFT calculations. For both the dimer cations, sandwich type structures, where the two benzene rings are symmetrically overlapped, were suggested, and the 19F-hyperfine coupling constants of the dimeric cations were found to be smaller than half that of the corresponding monomer cations. This was qualitatively explained in terms of the anti-bonding feature of the C–C bond between the two benzene rings and the C–F bond. DFT calculations performed for the monomer and dimer cations successfully account for this phenomenon. Anisotropic 19F hyperfine coupling (b) was used as a good indicator for evaluating the distance between the two benzene rings. The formation of the dimer cations strongly depends on the matrices and the concentration of the solutes, suggesting that molecular diffusion in the solid systems is an important factor in the dimerization.

EPR spectra and structure of bis-chlorinated dimethyl ether radical cation, CH2ClOCH2Cl+

The EPR spectrum of the CH2ClOCH2Cl+ radical cation, generated by ionizing radiation in a halocarbon matrix at 77 K, consists of a major septet due to two equivalent Cl nuclei and an additional triplet due to two protons in the terminal CH2Cl groups. This assignment was confirmed using CD2ClOCD2Cl+. Theoretical calculations were performed to obtain the optimized geometries and theoretical hyperfine coupling constants (hfcc) for CH2ClOCH2Cl+ using density functional theory (DFT). The results suggested Structures I (with Cs symmetry) and II (with C2 symmetry), where the two Cl, two C and one oxygen atoms make a five-membered ring and the unpaired electron is largely and equally shared by the two Cl nuclei. A reasonable magnitude in the proton coupling was obtained only for Structure I, having an energy lower than that of Structure II. The results were compared with those in our previous study on Cl(CH2)3Cl+, giving similar EPR spectra. For Cl(CH2)3Cl+, a geometry corresponding to Structure II was suggested from the proton coupling calculated by the CNDO/2 method. In this study, we reexamined the calculation by the DFT method and reached the conclusion that Cl(CH2)3Cl+ preferred a structure corresponding to Structure I.

An ESR and ab-initio MO study of πCH2– n0 orbital crossover in cycloalkylketone radical cations

The cyclobutanone and cyclopentanone radical cations were generated in a halocarbon matrix by ionizing radiation at 77 K. The ESR spectrum of a cyclobutanone radical cation consisted of a triple-quintet hf structure with ca. 0.9 mT (2H) and 2.4 mT (4H) splittings. Whereas, both cyclopentanone and 2,2,5,5-tetradeuteriocyclopentanone radical cations gave an identical triplet with 1.5 mT (2H) splitting. Based on the 1 H hf splittings, it was concluded that the SOMO in cycloalkylketone radical cations was changed from a π CH2 oribital to a n 0 (and σ C–C) orbital (i.e., π CH2– n 0 orbital crossover) when the ring size increased from a four-member to a five-member.

Structure of dimeric radical cations of benzene and toluene in halocarbon matrices: an EPR, ENDOR and MO study

The structure of dimeric cations of benzene and toluene formed in X-irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf couplings due to twelve equivalent protons and the isotropic coupling (aiso) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with ∣A‖∣<∣A⊥∣ were obtained in CF3CCl3 even at 50 K. The rhombic dipolar coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH3C6H5)2+, ENDOR transitions of the CH3 and H(4,4′) protons were observed. The isotropic hf coupling of the CH3 protons deviated even more strongly from the half value, being rather close to one-third of the value of the monomer. The hf coupling of the H(4,4′) protons was almost half the coupling of the monomer. It was suggested that the anomalous hf couplings of the CH3 protons were due to the interaction between the two rings through the σ-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH3 protons hf coupling in the (CH3C6H5)2+ were successfully evaluated.

Effect of matrix and substituent on the electronic structure of trapped benzene radical cations

The structure and dynamics of the radical cations produced from benzene, monodeuterated benzene and toluene in various low-temperature matrices were characterized by EPR and ENDOR spectroscopy. It was found that the nature of the matrix had a dramatic effect on the EPR spectra of benzene cation. Rigid structures corresponding to the 2B1g and 2B2g states are revealed in solid argon and halocarbon (CFCl3) matrices, respectively, whereas only dynamically averaged patterns are observed in other hosts used (krypton, xenon, sulfur hexafluoride). Deuterium monosubstitution has no appreciable effect on the cation structure observed in argon and halocarbon matrices, which implies matrix control of the preferred electronic state. In contrast, the toluene radical cation exhibits only a 2B2g-like structure both in argon and in CFCl3 matrices, that is, the internal structural effect strongly predominates over environment effects in this case. The results are discussed in qualitative terms taking into consideration the matrix and substituent effects on the charge distribution in benzene cation.

ENDOR and ESR Studies of Radical Cations of Methyl-Substituted Benzene in Halocarbon Matrices

The radical cations of methyl-substituted benzene toluene, p-xylene, o-xylene, m-xylene, and their deuterated analogues generated in CFCl3 and CF3CCl3 matrices by X-ray irradiation at 77 K were investigated by ESR and high-resolution ENDOR spectroscopy. The ESR and ENDOR spectra in these radical cations are dominated by large axially symmetric hyperfine splitting due to methyl-group protons. The anisotropic hyperfine coupling constants (hfcc) of methyl protons and ring protons in radical cations of toluene and p-xylene were measured accurately by ENDOR spectroscopy. In the case of deuterated p-xylene and o-xylene radical cations in CF3CCl3 matrix, the hyperfine coupling constants are scaled by their magnetogyric ratio (γH/γD = 6.51). Theoretical calculations of the isotropic and dipolar hyperfine coupling constants in these radical cations are found to be in good agreement with experimental results. The experimental and theoretical calculations supported the stabilization of the 2B2g type of state for the...

Hydrogen molecule detachment in irradiated 1,2-dimethyl cyclohexane: stereo-selective reaction

Abstract An EPR study was carried out for 1,2-dimethylcyclohexane (1,2-DMCH) irradiated by ionizing radiation at 77 K in the presence of SF 6 and cyclopropane ( c C 3 H 6 ). In addition to the 1,2-dimethylcyclohexyl radical, the 1,2-dimethylcyclohexene cation radical was observed which was formed by a selective hydrogen detachment at the C1 and C2 positions of 1,2-dimethylcyclohexane cation radical (1,2-DMCH + ) in the 1,2-DMCH matrix. The reaction shows a cis - trans stereo-selectivity and proceeded only for the cis -isomer. Furthermore, the 1,2-DMCH + generated in the halocarbon matrix was found to be converted into the 1,2-DMCHe + by photo-illumination at λ ≧350 nm.

Asymmetrically distorted structure of radical cations of ethylsilanes, studied by electron spin resonance spectroscopy, ab initio and density functional theories

ESR spectra for the radical cations of SiEt2H2, SiEt3H, SiEt4 and SiEt2MeH, generated in halocarbon matrices by ionizing radiation at 77 K, consist of a doublet with a coupling constant of ca. 6.3 mT to a 1H nucleus. Accumulation of our data for the cations with C–Si–C bonds suggests that the cations adopt asymmetrically distorted structures with an unpaired electron largely in one of the elongated Si–C bonds and that the 1H nucleus in the “trans’' position to the bond results in the large hyperfine (hf) splitting observed. Calculations at abinitio theory and density functional theory (DFT) levels strongly support this conclusion. The reversible spectral changes, observed for SiEt2H2+ and SiEt2MeH+ in CF2BrCF2Br matrices, have been analyzed with a three-site jumping model for the internal rotation of the methyl group having the 1H nucleus assigned and a two-site jumping model for a selective bond length alternation between two adjacent Si–C bonds.

ENDOR and EPR studies of benzene radical cations in halocarbon matrices: the static Jahn–Teller distortion of the monomer and geometry of the dimer cation

ENDOR investigations of the benzene radical cation in a CFCl 3 matrix at 30 K gave evidence for a static Jahn–Teller distortion with the unpaired electron density predominantly in the 2b 2g orbital. In this distortion the equivalence of the six protons is lost, with the major spin densities on H 1 and H 4. From ENDOR measurements the isotropic and dipolar couplings were accurately measured ( a iso=−25.31 MHz (H 1 and H 4) and a iso=−5.43 MHz (H 2, H 3, H 5 and H 6)) which agree with those reported earlier from EPR measurements and theoretical calculations. (C 6H 6) 2 +., formed by warming in CFCl 3 at 140 K and initially present in CF 3CCl 3, was characterised with ENDOR and assigned to a π-electron complex. The intermolecular distance was estimated from the measured dipolar hyperfine coupling to be in the range 2–3 Å.

Model Calculations of Matrix Effects on the Conversion of Propene Radical Cations into Allyl Radicals in Halocarbon Matrices.

A comparative study has been undertaken of the conversion of propene radical cations into allyl radicals in the gas phase and in CCl4 and CF4 matrices, The semiempirical UHF/PM3 method, complemented with ab initio PMP2/3-21G and PMP2/6-31G(d, p) calculations, is employed to model this process, which is observed to occur through an ion-molecule reaction by a proton transfer from the propene radical cation to a neutral propene molecule. Single molecules of CCl4 and CF4 are used to model the influence of the corresponding matrices, following two different pathways depending on which proton is transferred. It is found that the reaction in the gas phase occurs without activation energy. In contrast, a barrier is found on the potential energy curve of the reaction in the CCl4 matrix. The strongest interactions are found for CCl4, resulting in a 6.9 kcal mol(-1) activation energy (2.6 kcal mol(-1) for the second pathway). The reaction in CF4 has an intermediate profile with an activation energy of only 2.0 kcal mol(-1) (no barrier for the second pathway). The interactions between the migrating proton and the matrix molecule are interpreted in terms of the basicity of the matrix.

Electron spin relaxation of some alkane cation radicals as an indicator of dynamics between the distorted structures

ESR saturation phenomenon was observed for various alkane cation radicals derived from cyclohexane, propane and norbornane in frozen halocarbon matrices at 4 K. In addition to spectral diffusion by spin-spin interaction between the radicals, which is a common mechanism of ESR relaxation in the solid state, isotropic hyperfine modulation by the dynamics between the equivalent distorted structures was found to be very important for those centers which show Jahn-Teller (JT) or pseudo-JT effects. The effect of annealing of the sample at 77 K is also discussed.

ChemInform Abstract: Theoretical Study of Reactions of the 1‐Butene Radical Cation in Frozen Halocarbon Matrices

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Theoretical study of reactions of the 1-butene radical cation in frozen halocarbon matrixes

The thermal addition reaction between the 1-butene radical cation and an ethene molecule, and the photochemical isomerization reaction converting the 1-butene cation into the radical cation of cis-2-butene, observed in low-temperature halocarbon matrices, have been studied by means of semiempirical PM3 and ab initio UHF, PUHF, MP2, and PMP2 methods. For the addition reaction, a stable primary addition complex is isolated on the potential surface governing the reaction as well as a transition-state structure with a hydrogen atom half-way transferred

An ESR and MO study of the butatriene and tetramethylbutatriene radical cations

Abstract The radical cations of butatriene (BT +• ) and tetramethylbutatriene (TMBT +• ) have been stabilized in low-temperature halocarbon matrices and studied by ESR spectroscopy. The spectra of BT +• and TMBT +• exhibit isotropic hyperfine splittings of 8.3 G (4H) and 13.0 G (12H), respectively. The skew angles of BT +• (25°) and TMBT +• (50°) are estimated by comparing the experimental hyperfine splittings with those obtained by semiempirical MO calculations.

Molecular motion of the morpholin-1-yl radical in CF 2ClCFCl 2 as studied by ESR. Use of residual anisotropy of powder spectra to extract dynamics

The dynamics of the deprotonated neutral morpholin-1-yl radical, trapped in halocarbon matrix, CF 2ClCFCl 2, has been studied utilizing electron spin resonance (ESR) spectroscopy. The experimental lineshapes of the radical exhibit an alterating linewidth effect in the temperature range 105–144 K. The major changes of the ESR lineshape were governed by the averaging of the nitrogen hyperfine anisotropy while no exchange of the isotropic hyperfine coupling constants was observed. Geometrical parameters specifying the restricted anisotropic rotation of the whole radical trapped in the matrix could be extracted. Two methods for simulating anisotropic exchange broadened ESR spectra, the secular and non-perturbative, were utilized to investigate the dynamics of the radical. A surprisingly simple “three-site” jump model with a barrier of ≈ 3.6 kcal/mol can be applied in the simulation of the experimental spectra. It has been shown that the secular method cannot reproduce the exchange broadened ESR spectra of systems with large hyperfine anisotropy undergoing large internal reorganisation.

Electronic structure, methyl group reorientation and reactions of radical cations of 1,2,4-trimethylcyclohexanes: an EPR study

Two isomeric radical cations of 1,2,4-Me3-cyclohexane, r-1,t-2, t-4-Me3-c-C6+ and r-1, t-2, c-4-Me3-c-C6+, have been studied at cryogenic temperatures adopting the halocarbon matrix technique. Both isomer cations show the same EPR hyperfine (hf) pattern with a triplet of triplets with a1H= 60 G (2 H) and a2H= 32 G (2 H) at 4.2 K. The hf parameters coincide with those of the cis and trans isomers of the 1,2-Me2-c-C6 cation, implying a similar electronic ground state structure in which the main part of the unpaired electron resides in the C-1–C-2–σ-bond. The EPR spectra show a reversible temperature change between ca. 77 and 180 K. The temperature-dependent spectra can be explained by a dynamic effect in which the methyl groups on C-1 and C-2 rotate. Adopting a three-site jump model the rate constants were evaluated to be ca. 5.0 × 106 s–1(100 K) to ca. 5.0 × 107 s–1(180 K) with an activation energy of 1.1 kcal mol.–1 This low activation energy is compared to methyl rotation in radical cations of propane+ and butane+, and is discussed in terms of the unpaired electron distribution within the framework of carbon atoms. Furthermore, it is shown that secondary and tertiary alkyl radicals are predominantly formed by thermal reactions of r-1, t-2, t-4-Me3-c-C6+ and r-1, t-2, c-4-Me3-c-C6+, respectively, in the CF3–c-C6F11 matrix.

ESR Spectra of Some Allene Radicals in Low-Temperature Matrices

Abstract ESR spectra of the radicals of allene (1,2-propadiene) derivatives generated by 60Co γ-ray irradiation in halocarbon matrices have been observed at low temperatures. The geometrical structures of the radicals have been determined from the experimental ESR spectra and with the aid of semiempirical MO calculations invoking the UHF-AM1 and UHF-MNDO methods. After exposure of the allene derivatives to 60Co γ-ray in some halocarbon matrices at 77 K, the temperature raising gave rise to neutral radicals such as allylic and/or deprotonation-type radicals; these thermal reactions have been investigated in detail.

ESR Spectra of Para-Substituted Alkyl- and Alkenylbenzene Radical Cations in Halocarbon Matrices

Abstract The ESR spectra of several para-substituted alkyl- and alkenylbenzene radical cations generated by 60Co γ-ray irradiation in halocarbon matrices at 77 K have been studied with the aid of semiempirical MO calculations. It has been found that the ethyl conformation of 1,4-diethylbenzene radical cation (1,4-DEB+. ) in CCl2FCClF2 is largely different from that in CCl3CF3. Further the ethyl and n-propyl groups in 4-ethyltoluene and 4-n-propyltoluene radical cations in CCl2FCClF2 take different conformations from that of 1,4-DEB +.. The conformations of the alkylbenzene radical cations depend strongly both on the matrices and on the substituent groups. A peculiar radiolytical reaction of 1,4-diisopropylbenzene to a deprotonation-type neutral radical in CCl2FCClF2 has been found, similarly to the case of photolysis.

ChemInform Abstract: ESR and INDO‐MO Studies of 2‐Methylbutane, 3‐Methylpentane and 3‐ Methylhexane Cations Produced in Several Halocarbon Matrices γ‐ Irradiated at 77 K.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.