Outer-Valence Green's Function Study of Cycloalkane and Cycloalkyl−Alkane Compounds

Far-Ultraviolet Spectroscopy and Quantum Chemical Calculation Studies of the Conformational Dependence on the Electronic Structure and Transitions of Cyclohexane, Methyl and Dimethyl Cyclohexane, and Decalin; Effects of Axial Substitutions on the Electronic Transitions

Far-ultraviolet (FUV) spectra were measured for cyclohexane, methyl cyclohexane, six isomers of dimethyl cyclohexane, and cis- and trans-decalin. Attenuated total reflection-FUV (ATR-FUV) spectroscopy, which we originally proposed, provides systematic information about the excitation states of saturated organic molecules and the hyperconjugation of σ bonds. The FUV spectra of cyclohexane and methyl cyclohexane in neat liquids showed a band with central wavelengths near 155 and 162 nm. The simulation spectrum of cyclohexane calculated by time-dependent density-functional theory (TD-DFT) (CAM-B3LYP/aug-cc-pVTZ) gives two bands at 146 and 152 nm owing to the transition from HOMO-2 to Rydberg 3pz (Tb) and those from HOMO and HOMO-1 to Rydberg 3px/3py (Ta), respectively. The simulation spectrum of methyl cyclohexane with the equatorial substituent has peaks at approximately the same positions as cyclohexane. The calculated molar absorption coefficient is larger than that of cyclohexane, estimating the observed FUV spectra very well. The FUV spectra of dimethyl cyclohexane with two methyl substituents at the equatorial positions (trans-1,2-, cis-1,3-, and trans-1,4-) and trans-decalin had similar features to those of cyclohexane and methylcyclohexane. The TD-DFT calculations revealed that the shoulders at the shorter- and longer-wavelength sides of the band center of dimethyl cyclohexane (with methyl substituents at equatorial positions) and trans-decalin are assigned to Tb and Ta, respectively. In the case of dimethyl cyclohexane with one methyl substituent in the axial position (cis-1,2-, trans-1,3-, and cis-1,4-) and cis-decalin, the band caused by Tb decreased compared to those of the other compounds. The decrease in intensity and the longer-wavelength shift of the Tb band for dimethyl cyclohexane (with one methyl group at the axial position) and cis-decalin revealed that the band on the longer-wavelength side was assigned to the overlap band of Ta and Tb. The reason for such a large spectral alternation for the axial substitution may be the increase in the orbital energy of HOMO-2, which has its electron density concentrated at the axial C-H bond. Regarding the effect of the hyperconjugation of C-C and C-H σ orbitals, the second perturbation energies of the interaction between C_α-H_ax and C_β-H_ax were estimated for molecules by natural bond orbital (NBO) analysis. There is a correlation between the orbital energies of HOMO-2 and the changes in vicinal interaction by axial substitution.

The band 12 issue in the electron momentum spectra of norbornane: a comparison with additional Green's Function calculations and ultraviolet photoemission measurements

In continuation of a recent study of the electronic structure of norbornane [J. Chem. Phys., 2004, 121, 10525] by means of electron momentum spectroscopy (EMS), we present Green's Function calculations of the ionization spectrum of this compound at the ADC(3) level using basis sets of varying quality, along with accurate evaluations at the CCSD(T) level of the vertical (26.5 eV) and adiabatic (22.1 eV) double ionization thresholds under C(2v) symmetry. The obtained results are compared with newly recorded ultraviolet photoemission spectra (UPS), up to binding energies of 40 eV. The theoretical predictions are entirely consistent with experiment and indicate that, in a vertical depiction of ionization, shake-up states at binding energies larger than approximately 26.5 eV tend to decay via emission of a second electron in the continuum. A band of s-type symmetry that has been previously seen at approximately 25 eV in the electron impact ionization spectra of norbornane is entirely missing in the UPS measurements and theoretical ADC(3) spectra. With regard to these results and to the time scales characterizing electron-electron interactions in EMS (10(-17) s) as compared with that (10(-13) s) of photon-electron interactions in UPS, and considering the p-type symmetry of the electron momentum distributions for the nearest 1b(1) and 1b(2) orbitals, this additional band can certainly not be due to adiabatic double ionization processes starting from the ground electronic state of norbornane, or to exceptionally strong vibronic coupling interactions between cationic states derived from ionization of the latter orbitals. It is therefore tentatively ascribed to autoionization processes via electronically excited and possibly dissociating states.