Probing Reactivity of Gold Atoms with Acetylene and Ethylene with VUV Photoionization Mass Spectrometry and Ab Initio Studies.

Reaction of gold atoms with acetylene and ethylene in a laser ablation source produces a number of gold-containing species. Their photoionization efficiency (PIE) curves are measured using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. Their structures are assigned by comparing the experimental ionization energies and PIE curves to those of potential isomers calculated at the CAM-B3LYP/aug-cc-pVTZ level. For smaller molecules, the contribution of ionization to excited electronic states of the cation is also included using photoionization cross sections calculated using ePolyScat. Reaction with acetylene produces adducts Au(C2H2) and Au(C2H2)2, as well as HAu(C4H2). Reaction with ethylene leads to adducts Au(C2H4), Au(C2H4)2, an adduct with a gold dimer, Au2(C2H4), as well as the gold hydrides AuH, HAu(C2H4), and HAu(C4H4). [Au,C4,H7] is also observed, and it likely corresponds to a gold alkyl, H2C═C(H)-Au(C2H4). Reactions leading to production of odd-hydrogen species are endothermic and are likely due to translationally or electronically excited gold atoms. These measurements provide the first directly measured ionization energy for gold hydride, IE(AuH) = 10.25 ± 0.05 eV. Combining this value with the dissociation energy of AuH+ gives a dissociation energy D0(AuH) = 3.15 ± 0.12 eV. Several other ionization energies are measured: IE(Au2(C2H4)) = 8.42 ± 0.05 eV, IE(HAu(C2H4)) = 9.35 ± 0.05 eV, IE(HAu(C4H2)) = 8.8 ± 0.1 eV, and IE(HAu(C4H4)) = 8.8 ± 0.1 eV

Vibrational Spectroscopy of Intermediates in Benzene-to-Pheno Conversion by FeO+

Gas-phase FeO+ can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C6H5]+ insertion intermediate and Fe+(C6H5OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe+ with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O–H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe+(C6H5OH), the O–H stretch is observed at 3598 cm−1. Photodissociation primarily produces Fe+ + C6H5OH; Fe+(C6H4) + H2O is also observed. IRMPD of [HO-Fe-C6H5]+ mainly produces FeOH+ + C6H5 and the O–H stretch spectrum consists of a peak at ∼3700 cm−1 with a shoulder at ∼3670 cm−1. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO+ + C6H6 reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products

Vibrational Spectroscopy of Intermediates of C-H 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-CH3]+ insertion intermediate and Fe+(CH3OH) 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-C6H5] + mainly produces FeOH+ + C6H5 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+(CH3OH) 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

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

Probing Reactivity of Gold Atoms with Acetylene and Ethylene with VUV Photoionization Mass Spectrometry and Ab Initio Studies.

Reaction of gold atoms with acetylene and ethylene in a laser ablation source produces a number of gold-containing species. Their photoionization efficiency (PIE) curves are measured using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. Their structures are assigned by comparing the experimental ionization energies and PIE curves to those of potential isomers calculated at the CAM-B3LYP/aug-cc-pVTZ level. For smaller molecules, the contribution of ionization to excited electronic states of the cation is also included using photoionization cross sections calculated using ePolyScat. Reaction with acetylene produces adducts Au(C2H2) and Au(C2H2)2, as well as HAu(C4H2). Reaction with ethylene leads to adducts Au(C2H4), Au(C2H4)2, an adduct with a gold dimer, Au2(C2H4), as well as the gold hydrides AuH, HAu(C2H4), and HAu(C4H4). [Au,C4,H7] is also observed, and it likely corresponds to a gold alkyl, H2C═C(H)-Au(C2H4). Reactions leading to production of odd-hydrogen species are endothermic and are likely due to translationally or electronically excited gold atoms. These measurements provide the first directly measured ionization energy for gold hydride, IE(AuH) = 10.25 ± 0.05 eV. Combining this value with the dissociation energy of AuH+ gives a dissociation energy D0(AuH) = 3.15 ± 0.12 eV. Several other ionization energies are measured: IE(Au2(C2H4)) = 8.42 ± 0.05 eV, IE(HAu(C2H4)) = 9.35 ± 0.05 eV, IE(HAu(C4H2)) = 8.8 ± 0.1 eV, and IE(HAu(C4H4)) = 8.8 ± 0.1 eV

Dissociation Energy and Electronic and Vibrational Spectroscopy of Co⁺(H₂O) and Its Isotopomers

The electronic spectra of Co⁺(H₂O), Co⁺(HOD), and Co⁺(D₂O) have been measured from 13 500 to 18 400 cm^–1 using photodissociation spectroscopy. Transitions to four excited electronic states with vibrational and partially resolved rotational structure are observed. Each electronic transition has an extended progression in the metal–ligand stretch, v₃, and the absolute vibrational quantum numbering is assigned by comparing isotopic shifts between Co⁺(H₂¹⁶O) and Co⁺(H₂¹⁸O). For the low-lying excited electronic states, the first observed transition is to v₃′ = 1. This allows the Co⁺–(H₂O) binding energy to be determined as <i>D</i>₀(0 K)­(Co⁺–H₂O) = 13730 ± 90 cm^–1 (164.2 ± 1.1 kJ/mol). The photodissociation spectrum shows a well-resolved <i>K</i>_<i>a</i> band structure due to rotation about the Co–O axis. This permits determination of the spin rotation constants ϵ_<i>aa</i>^″ = −6 cm^–1 and ϵ_<i>aa</i>^′ = 4 cm^–1. However, the <i>K</i>_<i>a</i> rotational structure depends on v₃′. These perturbations in the spectrum make the rotational constants unreliable. From the nuclear spin statistics of the rotational structure, the ground state is assigned as ³B₁. The electronic transitions observed are from the Co⁺(H₂O) ground state, which correlates to the cobalt ion’s ³F, 3d⁸ ground state, to excited states which correlate to the ³F, 3d⁷4s and ³P, 3d⁸ excited states of Co⁺. These excited states of Co⁺ interact less strongly with water than the ground state. As a result, the excited states are less tightly bound and have longer metal–ligand bonds. Calculations at the CCSD­(T)/aug-cc-pVTZ level also predict that binding to Co⁺ increases the H–O–H angle in water from 104.1° to 106.8°, as the metal removes electron density from the oxygen lone pairs. The O–H stretching frequencies of the ground electronic state of Co⁺(H₂O) and Co⁺(HOD) have been measured by combining IR excitation with visible photodissociation in a double resonance experiment. In Co⁺(H₂O) the O–H symmetric stretch is ν₁″ = 3609.7 ± 1 cm^–1. The antisymmetric stretch is ν₅″ = 3679.5 ± 2 cm^–1. These values are 47 and 76 cm^–1, respectively, lower than those in bare H₂O. In Co⁺(HOD) the O–H stretch is observed at 3650 cm^–1, a red shift of 57 cm^–1 relative to bare HOD