Infrared multiple photon dissociation spectroscopy of protonated histidine and 4-phenyl imidazole

The 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, H+(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

Infrared Multiple Photon Dissociation Spectroscopy of Cationized Histidine: Effects of Metal Cation Size on Gas-Phase Conformation

The gas phase structures of cationized histidine (His), including complexes with Li+, Na+, Rb+, and Cs+, 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/6-311+G(d,p) (Li+, Na+, and K+ complexes) and B3LYP/HW*/6-311+G(d,p) (Rb+ and Cs+ complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were 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 only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li+(His) and Na+(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO, N alpha,N-1], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K+(His), Rb+(His), and Cs+(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li+(His) and Na+(His). For these complexes, the bidentate [CO,N-1] conformer in:which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N-alpha,N-1] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb+(His) and Cs+(His) indicate that both [CO,N-1] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers

Infrared multiple photon dissociation spectroscopy of protonated histidine and 4-phenyl imidazole

The 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, H+(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

Infrared multiple photon dissociation spectroscopy of cationized histidine: effects of metal cation size on gas-phase conformation

The gas phase structures of cationized histidine (His), including complexes with Li+, Na+, K+, Rb+, and Cs+, 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/6-311+G(d,p) (Li+, Na+, and K+ complexes) and B3LYP/HW*/6-311+G(d,p) (Rb+ and Cs+ complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were 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 only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li+(His) and Na+(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,Nα,N1], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K+(His), Rb+(His), and Cs+(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li+(His) and Na+(His). For these complexes, the bidentate [CO,N1] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,Nα,N1] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb+(His) and Cs+(His) indicate that both [CO,N1] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers

Infrared multiple photon dissociation spectroscopy of cationized cysteine: Effects of metal cation size on gas-phase conformation

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 alkali metal 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

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