J. Am. Chem. SOC. zyxwvu 1988, 110,6701-6705 6701 far as we know (for intermolecular phenomena, see ref zyxwvuts 12, for example). Molecular Design. Finally, we call the readers' attention to the molecular-design-oriented aspect of this work. In principle, in intramolecular D-A type molecules, relative spatial and sym- metry relationships between (among) D and A type groups can be designed and realized much more easily than in intermolecular cases. Moreover, this is also the case for the energetics. For example, by choosing the combination of D and A type groups rather freely it is easy intramolecularly to locate CT bands well separated from local excitation bands so as to make assessments of CT transition moments experimentally with reasonable accu- racy. From these standpoints, such D-A type molecules zyxwvut as treated here are quite attractive and fundamentally important for model studies of CT phenomena. Conclusions The electronic and orbital structure of DCB and TMDCB has been discussed from various standpoints, the group orbital concept being used as the common basis throughout the discussion. It has been established that the first LU-PMOS of DCB and TMDCB are almost completely localized on the MN-like moieties from the RP data. The first s-s electronic transition of each compound is an intramolecular CT transition mainly associated with the electron migration from the CHD-like moiety b2 HO- zyxwvu T-GO to the MN-like moiety az LU-*-GO and provides a typical example of an unusual type of CT transition, a C T transition between the orbitals C T interaction forbidden in the ground state. The mixing coefficients of the component BGOs in the HOMOs of DCB and TMDCB have been evaluated from the PE spectral data and the EA spectral band intensity data, independently from and consistently with each other. The consistency of the mixing coefficients evaluated from the two different standpoints strongly supports the validity of the used models and substantiates the intensity-borrowing model of the first band of each compound from the local allowed ?T*V electronic excitation of the A type group. This conclusion has further been supported by a C M type model consideration for the CT transition moments. This is the first experimental substantiation of intensity borrowing by intramo- (12) (a) Yakushi, K.; Iguchi, M.; Kuroda, H. Bull. Chem. Soc. Jpn. 1979, 52, 3180. (b) Amano, T.; Kuroda, H.; Akamatsu, H. Ibid. 1969, 42, 671. lecular CT bands from local excitations. The CT character of the first band of DCB is enhanced by tetramethylation, and this is mainly ascribed to the destabilization of the basis HO-a-GO of the D type group relative to that of the A type one, therefore, to the enhanced localization of the HOMO onto the D type group by the tetramethylation of DCB. The integration of PE spec- troscopy, cyclic voltammetry, EA spectroscopy, and quantum theoretical approaches is very powerful for the multiphased study of molecular electronic structure, in particular, when combined with well molecular-designed model systems as demonstrated in this work. Experimental and Computational Methods Materials. 2,3-Dicyanobarrelene (DCB)13 and its 5,6,7,8-tetramethyl derivative (TMDCB)*J3 were synthesized according to the methods de- scribed in the literature indicated. Measurements. The gas-phase He I photoelectron spectra of DCB and TMDCB were measured with the instrument described f~rmerly."~'~ The sample inlet and target chamber systems were heated and kept at 121 OC during the measurements. The reduction potentials were measured with a Yanaco polarographic analyzer P-1100, Pt wires being used as working and counter electrodes. The solutions contained 0.1 M Et,NCIO, supporting electrolyte. The potentials were determined under nitrogen atmosphere at room temper- ature relative to a saturated calomel electrode (SCE) connected with a salt bridge. The electronic absorption spectra were measured on a Hitachi 340 recording spectrophotometer. Computational Details. The CNDO type SCF-MO-CI calculations of DCB and TMDCB were carried out on a FACOM M380 computer at The Institute of Physical and Chemical Research with the method described formerly.' All the singly excited configurations between 10 HOMOs and 10 LUMOs were taken into account in the CI calculations. The molecular geometrical structural parameters assumed in the calcu- lations are based on the X-ray diffraction analysis data for TMDCB.16 Registry No. DCB, 18341-68-9; TMDCB, 19004-90-1. (13) Ciganek, E. Te?ruhedron zyxw Le??. 1967, 3321. (14) Kobayashi, T.; Kubota, T.; Ezumi, K. J. Am. Chem. Soc. 1983,105, (15) Kobayashi, T.; Yokota, K.; Nagakura, S. J. Electron Spcrrosc. Relur. (16) Nakagawa, A.; Tanaka, J.; Yamochi, H.; Nakasuji, K.; Murata, I., 2172. Phenom. 1973,2, 449. to be submitted for publication. Isomerization of the Dimethyl Sulfoxide Radical Cation and the Possible Analogies to the Neutral Species Lars Carhen* and Helge Egsgaard Contribution from the Chemistry Department, Rim National Laboratory, DK-4000 Roskilde, Denmark. Received March 28, 1988 Abstract: The interconvertibility of the dimethyl sulfoxide (DMSO) radical cation, its proton tautomeric aci-DMSO, and methyl methanesulfenateisomers, respectively, has been been investigated by application of collision activation mass spectrometry and metastable ion studies. The isomerizationreactions are discussed zyxwvuts based on isotopic labeling studies as well as thermodynamical considerations. The interconvertibility of the radical cations of DMSO and aci-DMSO as well as the isomerization of the methyl methanesulfenate radical cation into the radical cation of DMSO could be concluded, whereas no unambiguous evidence concerning the possible operation of the DMSO to methyl methanesulfenateisomerization was obtained. Low-pressure gas-phase pyrolysis studies have been applied to elucidate the possible analogies to the corresponding neutral species. In connection with our continuing interest in isomerization reactions in the gas phase, we recently reported a study on the isomerizations of the nitromethane radical cation in the gas phase' as a part of our search for the neutral aci-nitromethane. In order (1) Egsgaard, H.; Carlsen, L.; Elbel, S. Ber. Bunrenges. Phys. Chem. 1986, to disclose the possible existence of such aci tautomers we are currently studying systems containing "CH,=X=O" moieties (X = N, P, S), which potentially are able to rearrange into the corresponding aci tautomers "H2C=X-OH". These studies involve neutral species as well as radical cations. The present paper describes our studies of the dimethyl sulfoxide system, involving three different structures, which are dimethyl 90, 369-374. 0002-7863/88/1510-6701$01.50/0 0 1988 American Chemical Society