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

Abstract

isomeric radical cations of 1,2,4-Me,-cyclohexane, r-1 ,t-2,t-4-Me,-c-C6+ and r-I ,t-2,c-4-Me3c-C6+, have been studied at cryogenic temperatures adopting the halocarbon matrix technique. Both isomer cations show the same EPR hyperfine ( h f ) pattern with a triplet of triplets with alH = 6 0 G ( 2 H) and azH = 32 G (2 H) at 4.2 K. The hf parameters coincide with those of the cis and trans isomers of the 1,2-Me,-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-2-0-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 t o be ca. 5.0 x l o 6 s-I (100 K) to ca. 5.0 x lo7 s-1 (180 K) with an activation energy of 1.1 kcal moI.-l This low activation energy is compared t o 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-Me,-c-C6+ and r-1 ,t-2,c-4-Me3-c-C6+, respectively, in the CF,-c-C,F,, matrix.

A study of the dynamic equilibrium between symmetrical and distorted 1,2,3-trimethylcyclohexane radical cations

“…The two hydrogens causing the hf triplet are located on two methyl groups, CI' and C2' with their C-H bonds being coaxial to the C1-C2 bond of the ring in which large part of the unpaired electron spin is located. This is in agreement with the known radical cation structures of a variety of methylsub-stituted cyclohexanes [12][13][14][15][16][17][18][19]. Note the possibility of forming an asymmetrical structure being the mirror image by localizing the unpaired electron in the C2-C3 bond of the two symmetrical isomers and with the assignment of hf splittings due to the symmetry-related hydrogens.…”

“…Previously we have elucidated the electronic structures of cations of a variety of alkyl-substituted derivatives, and particularly for the tases with alkyl-groups in two positions, such as cis and trans isomers of 1,1-, 1,2-, 1,3-and 1,4-substitution [12][13][14][15][16][17][18][19]. The 1,3-substituted cations showed the coexistente of two different ground states [18], implying that the energy difference between the electronic states in cations of this and related structures is small.…”

“…A symmetrical structure was justified by the fact that the temperature effect is observed only for the symmet¡ isomers. The hfs constants of the symmetrical geometry were found by assuming a structure with resemblance to related cation structures [12][13][14][15][16][17][18][19], refined in the simulations by being treated as parameters.…”

“…100 and 150 K there is an additional reversible effect of the ESR line-shapes of all three cation isomers due to the onset of methyl group rotation [19]. This dynamical process will not be discussed further in this study.…”

“…Cyclic alkanes with degenerate HOMO (Highest Occupied Molecular Orbital) have attracted attention due to the possibility of having Jahn-Teller distorted ground states [6][7][8][9][10][11]. A degenerate HOMO can also be split by introducing substituents, which has encouraged us to investigate a variety of alkyl-substituted cyclohexane cations [12][13][14][15][16][17][18][19].…”