We report high resolution vibrational spectra in the HBr (2560 cm(-1)) and DBr (1840 cm(-1)) stretching regions for Br-HBr and Br-DBr entrance channel complexes isolated in solid parahydrogen (pH2). The Br-HBr complexes are generated by synthesizing solid pH2 crystals doped with trace amounts of HBr/Br2 mixtures followed by 355 nm in situ photodissociation of Br2 to form Br atoms. After photolysis is complete, the solid is warmed from 2 to 4.3 K resulting in the irreversible formation of Br-HBr complexes. The large 36.63 cm(-1) HBr monomer-to-complex induced vibrational shift to lower energy measured in these studies is consistent with the linear Br-HBr hydrogen bonded structure predicted from theory. The 0.02 cm(-1) Br-HBr absorption linewidths indicate a 1 ns vibrational excited state lifetime for these entrance channel complexes in solid pH2.

The reactions between ferrocene (Cp2Fe) (2a) and ozone (O3) were studied using low-temperature matrix-isolation techniques coupled with theoretical density functional theory (DFT) calculations. Co-deposition of Ar/Cp2Fe and Ar/O3 gas mixtures onto a cryogenically cooled CsI window produced a dark-green charge-transfer complex, Cp2Fe-O3, that photodecomposed upon red (λ ≥ 600 nm) and infrared (λ ≥ 1000 nm) irradiation but was stable to green or blue irradiation. Products of photodecomposition were characterized by FT-IR, oxygen-18 labeling, and DFT calculations using the B3LYP functionals and the 6-311G++(d,2p) basis set. Likely, photochemical products included four structures having the molecular formula C10H10FeO, identified by DFT calculations based on their calculated infrared spectra and (18)O isotope shifts. Each of these calculated molecules had one intact and fully coordinated η(5)-C5H5 cyclopentadienyl (Cp) ring and (1) an η(5)-C5H5O cyclic ether (pyran ring) (2b), (2) an η(4)-C5H5O linear aldehyde (2c), (3) a bidentate cyclic aldehyde with a seven-membered ring including the iron atom (2d), or (4) an Fe-O bond and an η(2)-C5H5 (Cp) ring (2e). No conclusive evidence for a gas-phase thermal reaction between ferrocene and ozone was observed under the conditions of these experiments. However, strong evidence for a surface-catalyzed thermal reaction was observed in merged-jet experiments wherein the gases were premixed before deposition. Surface-catalyzed ferrocene-ozone reaction products included a thin film of Fe2O3 observed on the walls of the merged tube as well as cyclopentadiene (C5H6), cyclopentadienone (C5H4O), and further oxidation products observed in the matrix. Possible mechanisms for both the photochemical and the thermal reactions are discussed.

The reactions of ozone with ferrocene (cp2Fe) and with n-butylferrocene (n-butyl cp2Fe) were studied using matrix isolation, UV-vis spectroscopy, and theoretical calculations. The codeposition of cp2Fe with O3 and of n-butyl cp2Fe with O3 into an argon matrix led to the production of 1:1 charge-transfer complexes with absorptions at 765 and 815 nm, respectively. These absorptions contribute to the green matrix color observed upon initial deposition. The charge-transfer complexes underwent photochemical reactions upon irradiation with red light (λ ≥ 600 nm). Theoretical UV-vis spectra of the charge-transfer complexes and photochemical products were calculated using TD-DFT at the B3LYP/6-311G++(d,2p) level of theory. The calculated UV-vis spectra were in good agreement with the experimental results. MO analysis of these long-wavelength transitions showed them to be n→ π* on the ozone subunit in the complex and indicated that the formation of the charge-transfer complex between ozone and cp2Fe or n-butyl cp2Fe affects how readily the π* orbital on O3 is populated when red light (λ ≥ 600 nm) is absorbed. 1:1 complexes of cp2Fe and n-butyl cp2Fe with O2 were also observed experimentally and calculated theoretically. These results support and enhance previous infrared studies of the mechanism of photooxidation of ferrocene by ozone, a reaction that has considerable significance for the formation of iron oxide thin films for a range of applications.

The ozonolysis reactions of 1,3- and 1,4-cyclohexadiene have been studied using a combination of matrix isolation, infrared spectroscopy, and theoretical calculations. Experimental and theoretical results demonstrate that these reactions predominantly do not follow the long-accepted Criegee mechanism. Rather, the reaction of O3 with 1,4-cyclohexadiene leads to the essentially barrierless formation of benzene, C6H6, and H2O3. These two species are then trapped in the same argon matrix cage and weakly interact to form a molecular complex. There is also evidence for the formation of a small amount of the primary ozonide as a minor product, formed through a transition state that is slightly higher in energy. The reaction of O3 with 1,3-cyclohexadiene follows two pathways, one of which is the Criegee mechanism through a low energy transition state leading to formation of the primary ozonide. In addition, with a similar barrier, ozone abstracts a single hydrogen from C5 while adding to C1, forming a hydroperoxy intermediate. This study presents two of the rare cases in which the Criegee mechanism is not the dominant pathway for the ozonolysis of an alkene as well as the first evidence for dehydrogenation of an alkene by ozone.

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