Infrared spectra of the protonated neurotransmitter histamine: competition between imidazolium and ammonium isomers in the gas phase

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The infrared (IR) spectrum of protonated histamine (histamineH+) was recorded in the 575–1900 cm−1 fingerprint range by means of IR multiple photon dissociation (IRMPD) spectroscopy. The IRMPD spectrum of mass-selected histamineH+ ions was obtained in a Fourier transform ion cyclotron resonance mass spectrometer coupled to an electrospray ionization source and an IR free electron laser. A variety of isomers were identified and characterized by quantum chemical calculations at the B3LYP and MP2 levels of theory using the cc-pVDZ basis set. The low-energy isomers are derived from various favourable protonation sites—all of which are N atoms—and different orientations of the ethylamine side chain with respect to the heterocyclic imidazole ring. The measured IRMPD spectrum was monitored in the NH3 loss channel and exhibits 14 bands in the investigated spectral range, which were assigned to vibrational transitions of the most stable isomer, denoted A. This imidazolium-type isomer A with protonation at the imidazole ring and gauche conformation of the ethylamine side chain is significantly stabilized by an intramolecular ionic Nπ–H+⋯Nα hydrogen bond to the ethylamino group. The slightly less stable ammonium-type isomer B with protonation at the ethylamino group is only a few kJ mol−1 higher in energy and may also provide a minor contribution to the observed IRMPD spectrum. Isomer B is derived from A by simple proton transfer from imidazole to the ethylamino group along the intramolecular Nπ–H+⋯Nα hydrogen bond via a low barrier, which is calculated to be of the order of 5–15 kJ mol−1. Significantly, the most stable structure of isolated histamineH+ differs from that in the condensed phase by both the protonation site and the conformation of the side chain, emphasizing the important effects of solvation on the structure and function of this neurotransmitter. The effects of protonation on the geometric and electronic structure of histamine are evaluated by comparing the calculated properties of isomer A with those of the most stable structure of neutral histamine A(n).EC/FP7/226716/EU/European Light Sources Activities - Synchrotrons and Free Electron Lasers/ELIS

Structure and reactivity of solvated transition metal ions and complexes

Within this thesis a series of molecular species has been studied, with focus on hydrogen bonded species and on (solvated) transition metal complexes. Experimental techniques such as FT-ICR-MS and IRMPD were combined with ab initio calculations for the determination of structure and reactivity of the aforementioned types of systems. On the basis of high level electronic structure calculations of neutral water clusters (H2O)n with n = 17-21 a transitional size regime has been determined, where a structural stabilization between all-surface and interior configurations alternates with the addition or removal of a single water molecule. Electronic structure calculations suggested that for n = 17 and 19 the interior configuration would be energetically more stable than the all-surface one. The gas phase infrared spectrum of the singly hydrated ammonium ion, NH4+(H2O), had previously been recorded by photodissociation spectroscopy of mass selected ions and interpreted by means of ab initio calculations. The present work provides additional information on the shape of the potential energy curves of NH4+(H2O) along the N-H distance on MP2/aug-cc-pVDZ level of theory yielding an anharmonic potential shape. Calculation of potential energy curves of the O-H mode of the intramolecular hydrogen bond of various dicarboxylic acids (oxalic to adipic acid) revealed that the shapes of the potentials directly correlate to the size of the system and the resulting ring strain The shape of the potential is also influenced by the charge of the system. Calculation of anharmonic frequencies based on the VPT2 approach lead to reasonable results in all systems with narrow potentials. IRMPD spectra of complexes in the gas phase have been recorded for a series of cationic vanadium oxide complexes when reacted with acetonitrile, methanol and ethanol. The experimental spectra are compared to calculated absorption spectra. The systematic DFT study identifies potential candidates for reductive nitrile coupling in cationic transition metal acetonitrile complexes. On the basis of the calculations, the formation of metallacyclic structures in group 3 through 7 complexes can be ruled out. Solvation of the transition metal cation by five acetonitrile ligands leads to a reductive nitrile coupling reaction in three types of complexes, namely those containing either niobium, tantalum or tungsten.Struktur und Reaktivität solvatisierter Übergangsmetallionen und -komplex

Structure and reactivity of solvated transition metal ions and complexes

Within this thesis a series of molecular species has been studied, with focus on hydrogen bonded species and on (solvated) transition metal complexes. Experimental techniques such as FT-ICR-MS and IRMPD were combined with ab initio calculations for the determination of structure and reactivity of the aforementioned types of systems. On the basis of high level electronic structure calculations of neutral water clusters (H2O)n with n = 17-21 a transitional size regime has been determined, where a structural stabilization between all-surface and interior configurations alternates with the addition or removal of a single water molecule. Electronic structure calculations suggested that for n = 17 and 19 the interior configuration would be energetically more stable than the all-surface one. The gas phase infrared spectrum of the singly hydrated ammonium ion, NH4+(H2O), had previously been recorded by photodissociation spectroscopy of mass selected ions and interpreted by means of ab initio calculations. The present work provides additional information on the shape of the potential energy curves of NH4+(H2O) along the N-H distance on MP2/aug-cc-pVDZ level of theory yielding an anharmonic potential shape. Calculation of potential energy curves of the O-H mode of the intramolecular hydrogen bond of various dicarboxylic acids (oxalic to adipic acid) revealed that the shapes of the potentials directly correlate to the size of the system and the resulting ring strain The shape of the potential is also influenced by the charge of the system. Calculation of anharmonic frequencies based on the VPT2 approach lead to reasonable results in all systems with narrow potentials. IRMPD spectra of complexes in the gas phase have been recorded for a series of cationic vanadium oxide complexes when reacted with acetonitrile, methanol and ethanol. The experimental spectra are compared to calculated absorption spectra. The systematic DFT study identifies potential candidates for reductive nitrile coupling in cationic transition metal acetonitrile complexes. On the basis of the calculations, the formation of metallacyclic structures in group 3 through 7 complexes can be ruled out. Solvation of the transition metal cation by five acetonitrile ligands leads to a reductive nitrile coupling reaction in three types of complexes, namely those containing either niobium, tantalum or tungsten.Struktur und Reaktivität solvatisierter Übergangsmetallionen und -komplex

IR spectroscopy of isolated metal-organic complexes of biocatalytic interest: Evidence for coordination number four for Zn2+(imidazole)(4)

The characterization of the interactions of Zn2+ ions with imidazole ligands is vital for understanding the function of a plethora of zinc enzymes at the molecular level. The infrared multiple photon dissociation (IRMPD) spectrum of mass selected Zn2+(imidazole)(4) cations, [ZnIm(4)](2+), was obtained in the 670-1840 cm(-1) fingerprint range by coupling the infrared free electron laser (IR-FEL) at the Centre Laser Infrarouge d'Orsay (CLIO) with a quadrupole ion trap mass spectrometer equipped with an electrospray ionization (ESI) source. The experimental efforts are complemented by quantum chemical calculations for [ZnIm(n)](2+) with n = 1-4 at the B3LYP level using basis sets ranging from cc-pVDZ to aug-cc-pVTZ. By comparison with calculated linear absorption spectra, the transitions observed in the IRMPD spectrum are assigned to vibrational modes of the imidazole ligands. In combination, the experimental data and the calculations provide detailed information about structure, metal-ligand bonding, charge distribution, and binding energy of the [ZnIm(4)](2+) complex in the gas phase. The superior abundance of the n = 4 complex of [ZnIm(n)](2+) in the mass spectra of the ESI source is indicative of the preferred coordination number CN = 4 for Zn2+ interacting with imidazole ligands in both the gas and the liquid phase. Comparison of the IR spectra of [ZnIm(n)](2+) with that of bare Im reveals the impact of the strong Zn2+-Im interaction on the electronic, geometric, and vibrational structure of the aromatic ligands upon sequential filling of the first coordination shell. Comparison with the structural and vibrational properties of the imidazole cation demonstrates that the metal-to-ligand charge transfer in [ZnIm(n)](2+) is dominated by sigma donation, whereas contributions from pi donation are minor. (C) 2011 Elsevier B.V. All rights reserved

Structure of Zirconocene Complexes Relevant for Olefin Catalysis: Infrared Fingerprint of the Zr(C5H5)(2)(OH)(CH3CN)(+) Cation in the Gas Phase

Cationic zirconocene complexes are active species in Ziegler-Natta catalysis for olefin polymerization. structure and metal-ligand bond strength strongly influence their activity. In the present work, the multiphoton dissociation (IRMPD) spectrum of mass selected Zr(C5H5)(2)(OH)(CH3CN)(+) cations was in the 300-1500 cm(-1) fingerprint range by coupling a Fourier-transform ion cyclotron resonance mass spectrometer equipped with an electrospray ionization (ESI) Source and the infrared free electron (IR-FEL) at the Centre Laser Infrarouge d'Orsay (CLIO). The experimental efforts are complemented quantum chemical Calculations at the MP2 and B3LYP levels using the 6-311G* basis set. assignments of transitions observed in the IRMPD spectra to modes of the Zr-O-H, C5H5, and CH3CN moieties are based oil comparison to calculated linear absorption spectra. Both the experimental data and Calculations provide unprecedented information about structure, metal-ligand bonding, charge and binding energy of the complex

Vibrational Spectra and Structures of Neutral Si_<i>m</i>C_<i>n</i> Clusters (<i>m</i> + <i>n</i> = 6): Sequential Doping of Silicon Clusters with Carbon Atoms

Vibrational spectra of mixed silicon carbide clusters Si_<i>m</i>C_<i>n</i> with <i>m</i> + <i>n</i> = 6 in the gas phase are obtained by resonant infrared–vacuum-ultraviolet two-color ionization (IR–UV2CI for <i>n</i> ≤ 2) and density functional theory (DFT) calculations. Si_<i>m</i>C_<i>n</i> clusters are produced in a laser vaporization source, in which the silicon plasma reacts with methane. Subsequently, they are irradiated with tunable IR light from an IR free electron laser before they are ionized with UV photons from an F₂ laser. Resonant absorption of one or more IR photons leads to an enhanced ionization efficiency for Si_<i>m</i>C_<i>n</i> and provides the size-specific IR spectra. IR spectra measured for Si₆, Si₅C, and Si₄C₂ are assigned to their most stable isomers by comparison with calculated linear absorption spectra. The preferred Si_<i>m</i>C_<i>n</i> structures with <i>m</i> + <i>n</i> = 6 illustrate the systematic transition from chain-like geometries for bare C₆ to three-dimensional structures for bare Si₆. In contrast to bulk SiC, carbon atom segregation is observed already for the smallest <i>n</i> (<i>n</i> = 2)

Chiral Transformation in Protonated and Deprotonated Adipic Acids through Multistep Internal Proton Transfer

Protonated and deprotonated adipic acids (PAA: HOOC-(CH(2))(4)-COOH(2)(+) and DAA: HOOC-(CH(2))(4)-COO(-)) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H(2)O center dot center dot center dot H center dot center dot center dot OH(2))(+) Zundel-like symmetric hydrogen bonding, whereas that of DAA has H(3)O(+) Eigen-like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel-like ions for PAA and Eigen-like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH center dot center dot center dot O short hydrogen-bond stretching peaks are predicted in the range of 1000-1700 cm(-1) in the Car-Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen-bond peaks. The O-H-O stretching peaks in the range of 1800-2700 cm(-1) become insignificant above around 150 K and are almost washed out at about 300 K.close0