Radical cations of cis- and trans-1,3-di- and 1,3,5-trimethylcyclohexanes. Matrix influence on two nearly degenerate SOMOs

Abstract

Radical cations of cis and trans isomers of 1,3-di-and 1,3,5-tri-methyIcyclohexane, stabilized in various y-irradiated solute-halocarbon matrices at 77 K and above, have been investigated b y means of EPR spectroscopy. The hfs constants have been compared w i t h those calculated using semiempirical methods, I NDO (spin density) and MN DO (geometry optimizations). The SOMOs of the methyl-substituted cyclohexane cations are all similar t o either of the orbitals, a, or b, , following a Jahn-Teller split of the e, 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 a,-like SOMO in all matrices used, with large spin density o n the equatorial hydrogen attached t o the axially substituted ring-carbon. Using CF,CCI, as matrix the EPR signal due to the trans-l,3,5-trimethylcyclohexane cation contains (in addition) a spectrum of the b,-like SOMO superimposed. The cation of cis-I ,3-dimethylcyclohexane shows contributions from both SOMOs in the matrices CF,-cC,F,,, CF,CICFCI, and CF3CCI3, but exclusively a b,-like SOMO in cC6F12. A b,-like SOMO is observed for the cis-l,3,5-trimethylcyclohexane cation in all matrices. The abundance ratios, [a,]/[b,],for the cases where the matrix-stabilized cation shows contribution from both b,and a,-like SOMOs simultaneously, are found t o be ca. 1/4 from line shape simulations. No prominent temperature dependence of this ratio is detected. 'A" ( C,) A'(C,) 'A" (C,) 'A'(C,) ' A' ( C J 'A'K,) ' A" (C,) 'A"(C,)36 (2 H); 29 (2 H) 93 (2 H); 12 (2 H ) 38 (2 H); 29 (2 H) 107 (1 H); 64 (1 H); 35 ( I H) * 1 cal = 4.184 J.

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.…”

“…It has been established that cations of cyclic alkanes can exist with different ground states; recent studies of the cis and trans decalin cations [24,25], silacycloalkane [20,21] and certain isomers of 1,3-dimethyl-and 1,3,5-trimethyl-cyclohexane cation [18] revealed the coexistence of two different ground states in several cases with the relative populations determined by the matrix and temperature. This means that the energy difference between electronic ground states is comparable with the solvation energy, or, the product kBT (kB: Boltzmann constant, T: temperature), depending on if the choice of electronic ground state is caused by the different solvation energy for different sites in the matrix or thermal population of identically solvated cations, respectively.…”

“…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]. …”