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Mass-selected clusters of the para-dichlorobenzene cation (p-C6H4Cl2+, pDCB+) are characterized by vibrational and electronic photodissociation spectroscopy in a molecular beam expansion. Infrared (IR) spectra of pDCB+–Ln with L = Ar (n = 1–7) and L = H2O (n = 1) and (pDCB)2+–Arn with n ≤ 2 are recorded in the C–H stretch range in the 2B2g ground electronic state of pDCB+. The IR spectra with L = Ar display essentially no shift as a function of the cluster size and approximate to high accuracy the hitherto unknown IR spectra of pDCB+ and its (pDCB)2+ dimer. In addition to the two IR active C–H stretch fundamentals at 3095 and 3107 cm−1, several intense combination bands are detected and assigned in the 2800–3100 cm−1 range for pDCB+. Comparison with the spectrum of neutral pDCB demonstrates structural changes upon ionization, which are consistent with the results of quantum chemical calculations performed at the B3LYP/6-311++G** level. Analysis of the photofragmentation branching ratios for pDCB+–Arn yields a binding energy of ∼550 cm−1 for the roughly equivalent π-bonded Ar ligands. The IR spectrum of pDCB+–H2O displays broader transitions with only minor redshifts (∼5–15 cm−1) due to the stronger intermolecular interaction. The C–H stretch spectrum is consistent with a π-bonded charge-dipole configuration. The IR spectrum of (pDCB)2+, as inferred from the spectrum of the Ar-tagged species, is rather different from those of pDCB+ and pDCB, indicative of a charge-resonance type interaction in this homodimer cation. The electronic spectra of the B 2B3u ← X 2B2g transition near 500 nm are reported for pDCB+–Ar, pDCB+–N2, and pDCB+–H2O. Only small shifts of the electronic origin transition are observed upon complexation, indicating little change in the intermolecular interaction energy upon electronic excitation. This conclusion is also supported by the modest excitation of unresolved intermolecular modes, suggesting that also the geometry changes in these clusters are small upon B ← X excitation. |
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