Takahisa Ichikawa

ESR evidence of a distorted 2Ag electronic structure for the methylcyclohexane radical cation

Abstract The electronic structure of the methylcyclohexane (Me-cC 6 ) radical cation has been investigated by ESR in a perfluoromethylcyclohexane matrix at 4 K. The ESR spectrum of Me-cC 6 + consists of a triplet of doublets with signals at ≈ 74 G (2H) and ≈ 34 G (1H). With the aid of two selectively deuteriated methylcyclohexanes, Me- d 3 -cC 6 and Me-cC 6 -3,3,5,5- d 4 , the triplet and doublet are assigned to two equatorial hydrogens at the 2 and 5 positions, and to one of the methyl hydrogens in the trans position with respect to a certain elongated bond (C 6 C 1 ), respectively. The electronic structure appears to be a distorted A g state which originates from the 2 A g state of the cyclohexane ring.

Structure and reaction of methylcyclohexane radical cation: ESR studies

Abstract The radical cation of methylcyclohexane (Me-cC6) is detected by ESR after γ-irradiation at 77 K of a solid solution of the parent molecules in perfluoromethylcyclohexane (CF3-cC6F11) matrix. The isotropic 1H hyperfine (hf) splittings observed are identified with the help of a selectively deuteriated compound and INDO MO calculations: a1 = 20.1 G (2H), a2 = 42.7 G (2H), and a3 = 48.3 G (2H) for the two gauche protons of the methyl group, the equatorial methylene protons at the C2 and C6 positions, and those at the C3 and C5 positions, respectively, in a chair form with the methyl group at an equatorial position. The unpaired electron in Me-cC6+ occupies an a″ orbital in Cs symmetry. On warming, the radical cation undergoes deprotonation mainly at either the C3 (or C5) or C4 or both.

Structure of radical cations of some methyl-substituted alkanes: ESR and INDO MO studies

Abstract Origins of the H 1s spin densities on the trans CH protons were reinvestigated for the radical cations of some methyl-branched small alkanes based on ESR results and INDO MO calculations. It was concluded that the delocalization of spin densities could not be ignored to explain the large and non-equivalent proton hf couplings of the radical cations. The conformations of 2-methylbutane +• and 3-methylpentane +• trapped in CClF 2 CCl 2 F were found to be different from the previously reported ones.

Radical cations of cyclohexanes alkyl-substituted on one carbon : an ESR study of the Jahn-Teller distorted HOMO of cyclohexane

Cation radicals of cyclohexanes alkyl-substituted on one carbon have been stabilized in perfluoromethylcyclohexane and other halocarbon matrices at 4.2 K and studied by means of ESR spectroscopy. It was found that all have an electronic ground state resembling the 2Ag state of the cyclohexane cation, one of the possible states following a Jahn–Teller distortion of the D3d cyclohexane chair structure. The cations can be classified into two groups depending on the substituted alkyl group. To the first group belong the cations with a methyl group or a primary carbon (ethyl, n-propyl or isobutyl group) attached to the ring. The disubstituted cyclohexane cations of 1,1-dimethylcyclohexane and 1-methyl-1-ethylcyclohexane were also found to have a similar structure. The ESR spectra are characterized by a 1:2:1 three-line pattern with the hyperfine. (hf) splitting due to two magnetically equivalent equatorial ring hydrogens. The magnitude of the splitting was found to depend on the size and number of substituents, ranging from 74 G (methylcyclohexane˙+) to 55 G (isobutylcyclohexane˙+). An additional doublet, 17–34 G, due to a hydrogen on the substituent could be detected in certain cases. Such hydrogens are axial with one of the elongated C–C bonds in the ring structure which contains a relatively large fraction of the unpaired electron. It follows that the substituents are located asymmetrically with respect to an ag-like SOMO in the ring. In the second group a secondary or tertiary carbon connects the substituent to the ring, such as an isopropyl or tert-butyl group. The largest hf splittings are ca. 30 G in magnitude, due to certain hydrogens on the substituent which are axial with respect to the cyclohexyl bond. It follows that an ag-like SOMO in the ring here is symmetrically arranged with respect to the position of the substituent. Hyperconjugation is the dominating mechanism for the spin transfer in all cations reported in this study.

On the alkyl radical formation from alkane cations in halocarbons: results for various alkyl-substituted cyclohexanes

Abstract Alkyl radical formation in halocarbon matrices containing the parent radical cation has been studied for various alkyl-substituted cyclohexanes. Contrary to the reaction mechanism suggested earlier, no direct relationship was found between the locations of high hydrogen spin density in the cation and the site of the π-type alkyl radicals which are formed. Among the cis and trans isomers of di- and tri-methylcyclohexanes, tertiary alkyl radicals were found to be always formed most preferentially whenever the corresponding solute molecule has at least one axial methyl group on the chair conformation. The other isomers gave secondary alkyl radicals with the site in the ring structure.

A preliminary report on radical yield alternation in even and odd linear alkanes irradiated at 77 K: ESR evidence

Abstract It has been found that the yields of trapped alkyl radicals in linear alkanes irradiated at 77 K exhibit an alternation depending upon whether the carbon number is even or odd. The alternation becomes more prominent in a series of higher alkanes starting from n-C11H24 to n-C19H40, for which the crystal structures as well as the ESR spectral features are known to exhibit a regular alternation. The results suggest that the radiation chemical events are affected by the molecular packing, that is, the crystal structures at the chain end and/or the interchain structures.

Radiation-induced polymerization of 2,3-dimethylbutadiene in thiourea clathrates

Abstract It has been found that addition of 2,3-dimethylbutane (DBA) to thiourea clathrates containing 2,3-dimethylbutadiene (DBE) induces the production of oligomers of DBE. The gas chromatogram of dimers of DBE showed three separate peaks. They are attributed to a linear dimer (2,3,6,7-tetramethyl-2,6-octadiene) and cyclic dimers (1,2,4-trimethyl-4-isopropylcyclohexene-1 and 1,2,4-trimethyl-4-isopropenylcyclohexane-1). Yields of the dimers as functions of contents of DBA and dose of gamma irradiation are examined.

Free radicals produced in selectively deuterated 3-methylhexanes gamma-irradiated at 77 K as studied by ESR

Abstract 3-Methylhexane-3d 1 yields mainly the two kinds of secondary radicals formed by C-H bond fission on positions 2 and 5 and the yields of both radicals are equal. The ESR spectra of 3-methylhexane-h 16 and 3-methyl-d 3 -hexane can also be explained by formation of the secondary radicals on positions 2 and 5. The tertiary radical produced by C-H bond rupture on position 3 in 3-methyl-d 3 -hexane is estimated as low as 20% to the total radicals trapped. 3-Methylhexane-2,2,4,4,5,5-d 6 and 3-methylhexane-2,2,3,4,4-d 5 produce the tertiary radical on position 3 and the secondary radical on position 5, respectively, indicating the isotope effect in the bond fission. 3-Methyl-2,2,4,4-d 4 yields the tertiary radical (75%) on position 3 and the secondary radical (25%) on position 5. Both n-pentane-h 12 and n-pentane-2,2,4,4-d 4 are found to yield the secondary radicals on position 2. Discussion is given for isotope effects in bond fission of alkanes.