14 research outputs found
Infrared multiple photon dissociation spectroscopy of protonated histidine and 4-phenyl imidazole
pre-printThe gas-phase structures of protonated histidine (His) and the side-chain model, protonated 4-phenyl imidazole (PhIm), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by the free electron laser FELIX. To identify the structures present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level of theory. Relative energies of various conformers are provided by single point energy calculations carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. On the basis of these experiments and calculations, the IRMPD action spectrum for H+(His) is characterized by a mixture of [Nπ,Nα] and [Nπ,CO] conformers, with the former dominating. These conformers have the protonated nitrogen atom of imidazole adjacent to the side-chain (Nπ) hydrogen bonding to the backbone amino nitrogen (Nα) and to the backbone carbonyl oxygen, respectively. Comparison of the present results to recent IRMPD studies of protonated histamine, the radical His°+ cation, (HisArg),H22+(HisArg), and M+(His), where M+ = Li+, Na+, K+, Rb+, and Cs+, allows evaluation of the vibrational motions associated with the observed bands
Methane activation by cobalt cluster cations, Con+ (n=2-16): reaction mechanisms and thermochemistry of cluster-CHx (x=0-3) complexes
Journal ArticleThe kinetic energy dependences of the reactions of Con + (n=2-16) with CD4 are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. The main products are hydride formation, ConD+, dehydrogenation to form ConCD2 +, and double dehydrogenation yielding ConC+
Infrared multiple photon dissociation spectroscopy of cationized cysteine: effects of metal cation size on gas-phase conformation
ManuscriptAbstract The gas-phase structures of cationized cysteine (Cys) including complexes with Li+, Na+, K+, Rb+, and Cs+, as well as protonated Cys, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311G(d,p) (H+, Li+, Na+, and K+ complexes) and B3LYP/HW*/6-311G(d,p) (Rb+ and Cs+ complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential was used on the metals. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkalimetal cations, Li+(Cys) and Na+(Cys), is a charge-solvated, tridentate structure where the metal cation binds to the amine and carbonyl groups of the amino acid backbone and the sulfur atom of the side chain, [N,CO,S], in agreement with the predicted ground states of these complexes. For the larger alkali metal cation complexes, K+(Cys), Rb+(Cys), and Cs+(Cys), the spectra have very similar spectral features that are considerably more complex than the IRMPD spectra of Li+(Cys) and Na+(Cys). For these complexes, the bidentate [COOH] conformer, in which the metal cation binds to both oxygens of the carboxylic acid group, is a dominant contributor, although features associated with the tridentate [N,CO,S] conformer remain and those for the zwitterionic [CO2−] conformer are also clearly present. Theoretical results for Rb+(Cys) and Cs+(Cys) indicate that both [COOH] and [N,CO,S] conformers are low-energy structures. For H+(Cys), the IRMPD action spectrum is reproduced by [N,CO] conformers, in which the protonated amine group hydrogen bonds to the carbonyl oxygen atom and the sulfur atom of the amino acid side chain. Several low-energy [N,CO] conformers that differ only in their side-chain orientations are found and therefore have very similar predicted spectra
Direct Determination of the Ionization Energies of PtC, PtO, and PtO2 with VUVRadiation
Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements providethe first directly measured ionization energy for PtC, IE(PtC) = 9.45 +- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +- 0.1 eV and IE(PtO2) = 11.35 +- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +- 0.07 eV, D0(Pt-O)= 4.30 +- 0.12 eV, and D0(OPt-O) = 4.41 +- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules Delta H0 f,0(PtC(g)) = 701 +- 7 kJ/mol, Delta H0f,0(PtO(g)) = 396 +- 12 kJ/mol, and Delta H0f,0(PtO2(g)) = 218 +- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides
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Photofragment studies of carbon-oxygen and carbon-hydrogen bond activation by transition metal cations
Gas-phase transition metal cations can activate C-C and C-H bonds in hydrocarbons at room temperature. They can even activate C-O bonds in CO2. They thus serve as a relatively simple model for C-C and C-H activating catalysts. A better understanding of the crucial steps and a complete characterization of possible intermediates in these reactions is desirable in order to improve catalysts. Brief introductions to gas-phase transition metal chemistry and to the experimental techniques employed in these studies are given in Chapter 1. Chapter 2 describes the details of the apparatus, laser systems and data acquisition. In Chapters 3 and 4, we investigate the spectroscopy and photodissociation dynamics of V+(OCO). Electronic spectra of gas-phase V +(OCO) are measured in the near-infrared and the visible using photofragment spectroscopy. The visible band shows clearly resolved vibrational progressions in the metal-ligand stretch and rock, and in the OCO symmetric stretch and bend. We measure the OCO antisymmetric stretch frequencies in the ground and excited state of V+(OCO) using vibrationally mediated photodissociation (VMP). Photodissociation produces V+ + CO2 (non-reactive pathway) and VO+ + CO (reactive pathway). One-photon dissociation studies confirm mode selectivity and they are extended to higher energy. The effect of OCO antisymmetric stretch vibrations on reactivity is investigated using VMP. Exciting the antisymmetric stretch leads to a three-fold increase in the reactive VO+ channel, and combination bands involving the antisymmetric stretch all show similar enhanced reactivity. Electronic structure calculations were performed to characterize the dissociation pathways and excited electronic states of V+(OCO). In addition, spin-orbit coupling of quintet states to triplet states was calculated and used to compute intersystem crossing rates, which reproduce many of the observed mode-selective trends. The V+-OCO stretch and OCO antisymmetric stretch appear to enhance reactivity by increasing the intersystem crossing rate. In Chapter 5, photodissociation spectra of Fe+(CH 4)3 and Fe+(CH4)4 entrance channel complexes in the C-H stretching region are discussed. Vibrational spectroscopy reveals that bonding in these complexes is not merely electrostatic, but includes significant covalency, which weakens the C-H bonds and significantly lowers the C-H stretching frequencies. In addition, to get structural information from the experiments, we calculated the geometries and vibrational spectra of low-energy isomers
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Direct Determination of the Ionization Energies of PtC, PtO, and PtO2 with VUV Radiation
Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +- 0.1 eV and IE(PtO2) = 11.35 +- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +- 0.07 eV, D0(Pt-O)= 4.30 +- 0.12 eV, and D0(OPt-O) = 4.41 +- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules Delta H0 f,0(PtC(g)) = 701 +- 7 kJ/mol, Delta H0f,0(PtO(g)) = 396 +- 12 kJ/mol, and Delta H0f,0(PtO2(g)) = 218 +- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides
Direct Determination of the Ionization Energies of PtC, PtO, and PtO2 with VUV Radiation
Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +- 0.1 eV and IE(PtO2) = 11.35 +- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +- 0.07 eV, D0(Pt-O)= 4.30 +- 0.12 eV, and D0(OPt-O) = 4.41 +- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules Delta H0 f,0(PtC(g)) = 701 +- 7 kJ/mol, Delta H0f,0(PtO(g)) = 396 +- 12 kJ/mol, and Delta H0f,0(PtO2(g)) = 218 +- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides
Bond energies of ThO(+) and ThC(+): A guided ion beam and quantum chemical investigation of the reactions of thorium cation with O2 and CO
Kinetic energy dependent reactions of Th(+) with O2 and CO are studied using a guided ion beam tandem mass spectrometer. The formation of ThO(+) in the reaction of Th(+) with O2 is observed to be exothermic and barrierless with a reaction efficiency at low energies of k/kLGS = 1.21 ± 0.24 similar to the efficiency observed in ion cyclotron resonance experiments. Formation of ThO(+) and ThC(+) in the reaction of Th(+) with CO is endothermic in both cases. The kinetic energy dependent cross sections for formation of these product ions were evaluated to determine 0 K bond dissociation energies (BDEs) of D0(Th(+)-O) = 8.57 ± 0.14 eV and D0(Th(+)-C) = 4.82 ± 0.29 eV. The present value of D0 (Th(+)-O) is within experimental uncertainty of previously reported experimental values, whereas this is the first report of D0 (Th(+)-C). Both BDEs are observed to be larger than those of their transition metal congeners, TiL(+), ZrL(+), and HfL(+) (L = O and C), believed to be a result of lanthanide contraction. Additionally, the reactions were explored by quantum chemical calculations, including a full Feller-Peterson-Dixon composite approach with correlation contributions up to coupled-cluster singles and doubles with iterative triples and quadruples (CCSDTQ) for ThC, ThC(+), ThO, and ThO(+), as well as more approximate CCSD with perturbative (triples) [CCSD(T)] calculations where a semi-empirical model was used to estimate spin-orbit energy contributions. Finally, the ThO(+) BDE is compared to other actinide (An) oxide cation BDEs and a simple model utilizing An(+) promotion energies to the reactive state is used to estimate AnO(+) and AnC(+) BDEs. For AnO(+), this model yields predictions that are typically within experimental uncertainty and performs better than density functional theory calculations presented previously