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It has been demonstrated that sufficient numbers of mass-selected cations can be directly deposited into growing inert gas matrices at low temperatures for spectroscopic analysis. However, the mechanism that must exist to maintain electrical neutrality in the matrix is not understood. We report here the direct observation of counterions in matrices formed during the codeposition of mass-selected cations and neon. The addition of carbon dioxide to the neon matrix gas during deposition of a mass-selected cation beam results in detectable quantities of CO2 •-. The direct measurement of negative charges, as positive ions impinge on a cold target, provides insights into the mechanism through which charge neutrality is maintained in this experiment. Negative charge formation is strongly dependent on the temperature of the metallic surfaces surrounding the matrix window and the presence of condensable gases on these surfaces.

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Resonance Raman spectra of two p-methoxybenzyltrialkylsilanes (alkyl = methyl and ethyl) have been obtained both as their neutral charge-transfer complexes with tetracyanoethylene in steady-state cw experiments and as their radical cations via two-color pump−probe transient measurements. The ground-state charge-transfer resonant spectra exhibit intensity predominantly in phenyl-localized modes, suggesting that vertical excitation to the contact ion-pair state involves little participation of the bond that is known to undergo subsequent nucleophile-assisted cleavage in the separated radical cation. Quantitative modeling of the absolute cross sections for the methyl compound is used to determine the mode-specific reorganization energies accompanying vertical electron transfer. Transient spectra of the relaxed radical cations show more than 20 resonance-enhanced modes, several of which have significant contributions from the C−Si stretching coordinate based on frequency shifts between the natural abundance and benzyl 13C labeled methyl compounds. These modes with significant benzyl C−Si stretching character are considerably lower in frequency in the radical cation than in the neutral, indicating weakening of this bond upon oxidation. The experimental frequencies are reproduced quite well by density functional theory calculations at the B3LYP/6-31g(d,p) level which give a C−Si bond length increase of 0.10 Å upon oxidation.

Vibrational spectroscopic characterization of neon matrices in which mass-selected CS2 •+ and CS•+ were deposited reveals absorptions due to (ν3) CS2 •+ (1207.1 cm-1) and (ν3) CS2 •- (1159.4 cm-1). The results are compared to previous CO2 •+ studies from this laboratory [Godbout et al. J. Phys. Chem. 1996, 100, 2892]. We also report controlled annealing studies of the matrices in which clustering of ionic species with neutral molecules is observed.

Sufficient quantities of mass-selected cations have been isolated in inert matrices for vibrational spectroscopic observation for the first time. When 15-25-nA beams of CF3+ from CF3C1, CFsBr, or CF3H are co-deposited for 10-25 h with neon or argon at 5 K, the antisymmetric stretching vibration (u3) of the cation can be observed by FTIR spectroscopy. The mechanism by which the matrix maintains the necessary approximate neutrality is not certain; however, both positive and negative charges are detected when the matrices containing massselected cations are allowed to warm.

Resonance Raman spectra of the radical cations of phenylcyclopropane and trans-1-phenyl-2-methylcyclopropane are reported. A near-UV pump pulse excites a photosensitizer which oxidizes the species of interest, and a visible probe pulse delayed by 35 ns obtains the spectrum of the radical ion. The transient Raman spectra of the phenylcyclopropane radical cations show nine or ten enhanced modes for which assignments are suggested based on density functional theory (DFT) results, previously published calculations on the resonant excited state and comparison between the unsubstituted and methyl-substituted compounds. The transient spectra are consistent with the large effect of methyl substitution on the geometry of the radical cation predicted by DFT. The resonance Raman spectrum of the electron donor-acceptor complex between phenylcyclopropane and tetracyanoethylene is also obtained on resonance with the visible charge-transfer absorption band, but the spectra are very weak and only a few resonance enhanced lines are observed. These results are compared with previously published data on the p-methoxybenzyltrimethylsilane charge-transfer complex and radical cation.

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