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Vibrational spectroscopy of intermediates of carbon-hydrogen bond activation by transition metal oxide cations
Direct, efficient oxidation of alkanes is a long-standing goal of catalysis. Gas phase FeO+ can convert methane to methanol and benzene to phenol under thermal conditions. Two key intermediates of these reactions are the [HO-Fe-R]+ insertion intermediate and Fe+(ROH) (R=CH3 or C6H5) exit channel complex. This work describes measurements of the vibrational spectra of these intermediates and electronic structure theory calculations of the potential energy surfaces for the reactions. They help to characterize the mechanism for these reactions. Chapter 1 describes previous studies of methane-to-methanol and benzene-to-phenol conversion by gas-phase transition metal oxide cations. Spectra of gas-phase reaction intermediates are obtained using photofragment spectroscopy, in which absorption of a photon leads to bond breaking. Utuilizing this technique to measure vibrational spectra is challenging, due to the low photon energies involved. Techniques used to measure vibrational spectra of ions - argon tagging, infrared multiple photon dissociation (IRMPD), vibrationally mediated photodissociation (VMP) and infrared laser assisted photodissociation spectroscopy (IRLAPS) are also detailed in chapter 1. The photofragment spectrometer and laser systems used in these studies are described in chapter 2, as is a multi-pass mirror arrangement which I implemented. This greatly improved the quality of vibrational spectra, particularly those measured using IRMPD. Chapter 3 describes studies of the O-H and C-H stretching vibrations of two intermediates of the FeO+ + CH4 reaction. These intermediates are selectively formed by reaction of laser ablated Fe + with specific organic precursors and are cooled in a supersonic expansion. Vibrations of the sextet and quartet states of the [HO-Fe-CH 3]+ insertion intermediate and Fe+(CH 3OH) exit channel complex are measured by IRMPD and Ar-tagging. Studies of the O-H stretching vibrations of the [HO-Fe-C6H5] + and Fe+(C6H5OH) intermediates of the FeO+ + C6H6 reaction are discussed in chapter 4. For Fe+(C6H5OH), the O-H stretch is observed at 3598 cm-1. Photodissociation primarily produces Fe+ + C6H5OH. IRMPD of [HO-Fe-C 6H5] + mainly produces FeOH+ + C6H 5 and the O-H stretch spectrum consists of a peak at ~3700 cm -1 with a shoulder at ~3670 cm-1. Chapter 5 compares three techniques - IRMPD, argon-tagging, and IRLAPS - in the quality of the measured vibrational spectra of Ag+(CH 3OH) ions produced under identical conditions. The sharpest spectrum is obtained using IRLAPS. The O-H stretch is observed at 3660 cm -1. Monitoring loss of argon from Ag+(CH3OH)(Ar) gives a slightly broader peak, with no significant shift. The vibrational spectrum obtained using IRMPD is shifted to 3635 cm-1, is substantially broader, and is asymmetrical, tailing to the red.
Altinay, Gokhan, "Vibrational spectroscopy of intermediates of carbon-hydrogen bond activation by transition metal oxide cations" (2010). Doctoral Dissertations Available from Proquest. AAI3427491.