Infrared multiple-photon dissociation spectroscopy of group II metal complexes with salicylate

Ion trap tandem mass spectrometry with collision‐induced dissociation, and the combination of infrared multiple‐photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations, were used to characterize singly charged, 1:1 complexes of Ca²⁺, Sr²⁺ and Ba²⁺ with salicylate. For each metal‐salicylate complex, the CID pathways are: (a) elimination of CO₂ and (b) formation of [MOH]⁺ where M = Ca²⁺, Sr²⁺ or Ba²⁺. DFT calculations predict three minima for the cation‐salicylate complexes which differ in the mode of metal binding. In the first, the metal ion is coordinated by O atoms of the (neutral) phenol and carboxylate groups of salicylate. In the second, the cation is coordinated by phenoxide and (neutral) carboxylic acid groups. The third mode involves coordination by the carboxylate group alone. The infrared spectrum for the metal‐salicylate complexes contains a number of absorptions between 1000 and 1650 cm^–1, and the best correlation between theoretical and experimental spectra is found for the structure that features coordination of the metal ion by phenoxide and the carbonyl O of the carboxylic acid group, consistent with the calculated energies for the respective species. Copyright © 2011 John Wiley & Sons, Ltd.     

Infrared multiple photon dissociation spectroscopy of trapped ions

This tutorial review presents the technique of infrared multiple-photon dissociation (IRMPD) spectroscopy of mass-selected trapped ions. This requires coupling of a tunable infrared laser with mass spectrometry instrumentation. IRMPD spectroscopy has recently blossomed due to the emergence of widely tunable free electron lasers, as well as on-going developments of benchtop lasers. The merits of different trapping approaches in mass spectrometry are discussed in the light of photodissociation experiments. This tutorial discusses current capabilities, as well as limitations of the technique.     

Infrared multiphoton dissociation spectroscopy of gas-phase mass-selected hydrocarbon-Fe+ complexes

Infrared spectra in the mid-infrared region (800-1600 cm(-1)) of highly unsaturated Fe(+)-hydrocarbon complexes isolated in the gas phase are presented. These organometallic complexes were selectively prepared by ion-molecule reactions in a Fourier transform ion cycloton mass spectrometer (FTICR-MS). The infrared multiphoton dissociation (IRMPD) technique has been employed using the free electron laser facility CLIO (Orsay, France) to record the infrared spectra of the mass selected complexes. The experimental IRMPD spectra present the main features of the corresponding IR absorption spectra calculated ab initio. As predicted by these calculations, the experimental spectra of three selectively prepared isomers of Fe+(butene) present differences in the 800-1100 cm(-1) range. On the basis of the comparison with calculated IR spectra, the IRMPD spectrum of Fe(butadiene)(+) suggests that the ligand presents the s-trans isomeric form. This study further confirms the potentialities of IRMPD spectroscopy for the structural characterization of organometallic ionic highly reactive intermediates in the gas phase. In conjunction with soft ionization techniques such as electrospray, this opens the door to the gas-phase characterization of reactive intermediates associated with condensed phase catalysts.     

Infrared multiple photon dissociation spectroscopy of potassiated proline

The structure of proline in [proline + K]+ has been investigated in the gas phase using high level DFT and MP2 calculations and infrared photo dissociation spectroscopy with a free electron laser (FELIX). The respective FELIX spectrum of [proline + K]+ matches convincingly the calculated spectra of two structurally closely related and nearly iso-energetic zwitterionic salt bridge (SB) structures. An additional unresolved band at approximately 1725 cm(-1) matching with the characteristic CO stretching mode of charge solvation (CS) structures points toward the presence of a minor population of these conformers of proline in [proline + K]+. However, theory predicts a significant energy gap of 18.9 kJ mol(-1) (B3LYP/6-311++G(2d,2p)) or 15.6 kJ mol(-1) (MP2) between the lowest CS conformer of proline and the clearly favored SB structure.     

Infrared spectroscopy of discrete uranyl anion complexes

The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO22+) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO2A(S)n]+, where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n = 0-3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO2A]+) were as low or lower than measurements for [UO2]2+ ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30-40 cm(-1) higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO2A]+ produced [UO2AS]+, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO2AS]+ were in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO2AS]+ species having different solvent neutrals, values decreased with increasing neutral nucleophilicity.     

Infrared action spectroscopy and dissociation dynamics of the HOOO radical

The HOOO radical has long been postulated to be an important intermediate in atmospherically relevant reactions and was recently deemed a significant sink for OH radicals in the tropopause region. In the present experiments, HOOO radicals are generated in a pulsed supersonic expansion by the association of O(2) and photolytically generated OH radicals, and the spectral signature and vibrational predissociation dynamics are investigated via IR action spectroscopy, an IR-UV double resonance technique. Rotationally resolved IR action spectra are obtained for trans-HOOO in the fundamental (nu(OH)) and overtone (2nu(OH)) OH stretching regions at 3569.30 and 6974.18 cm(-1), respectively. The IR spectra exhibit homogeneous line broadening, characteristic of a approximately 26-ps lifetime, which is attributed to intramolecular vibrational redistribution and/or predissociation to OH and O2 products. In addition, an unstructured feature is observed in both the OH fundamental and overtone regions of HOOO, which is likely due to cis-HOOO. The nascent OH X(2)Pi, v = 0 or v = 1, products following vibrational predissociation of HOOO, nu(OH) or 2nu(OH), respectively, have been investigated using saturated laser-induced fluorescence measurements. A distinct preference for population of Pi(A') Lambda-doublets in OH was observed and is indicative of a planar dissociation of trans-HOOO in which the symmetry of the bonding orbital is maintained.     

Infrared multiphoton and collision-induced dissociation studies of some gaseous alkylamine ions

Collision-induced dissociation and infrared multiphoton dissociation of ions formed in di- and tri-ethylamine, di- and tri-n-propylamine, and di-isopropylamine were investigated by Fourier-transform ion-cyclotron resonance mass spectrometry. Molecular ions of all amines except di-n-propylamine produced similar fragment ions when subjected to either dissociation technique. The initial fragmentation involved C_αC_β bond cleavage, loss of an alkyl radical, and formation of an immonium ions. Subsequent fragmentations of the immonium ions produced by both dissociation mechanisms involved McLafferty-type rearrangements and loss of alkenes. The molecular ion of di-n-propylamine fragmented by a different mechanism when subjected to infrared irradiation. Protonated molecules of di- and tri-n-propylamine yielded C₃H₆ and an ammonium ion upon infrared multiphoton dissociation, while protonated molecules of the other amines did not dissociate when this technique was applied. In contrast, collision-induced dissociation produced fragmentation for all protonated molecules. Explanation of the different fragmentations observed for the two dissociation techniques is given in terms of a mechanism involving a tight transition state for protonated di- and tri-n-propylamine dissociation.     

Infrared multiple photon dissociation spectroscopy of sodium and potassium chlorate anions

The structures of gas‐phase, metal chlorate anions with the formula [M(ClO₃)₂]^?, M = Na and K, were determined using tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Structural assignments for both anions are based on comparisons of the experimental vibrational spectra for the two species with those predicted by density functional theory (DFT) and involve conformations that feature either bidentate or tridentate coordination of the cation by chlorate. Our results strongly suggest that a structure in which both chlorate anions are bidentate ligands is preferred for [Na(ClO₃)₂]^?. However, for [K(ClO₃)₂]^? the best agreement between experimental and theoretical spectra is obtained from a composite of predicted spectra for which the chlorate anions are either both bidentate or both tridentate ligands. In general, we find that the overall accuracy of DFT calculations for prediction of IR spectra is dependent on both functional and basis set, with best agreement achieved using frequencies generated at the B3LYP/6‐311+g(3df) level of theory. Copyright © 2009 John Wiley & Sons, Ltd.     

Infrared multiple photon dissociation action spectroscopy and computational studies of mass-selected gas-phase fluorescein and 2',7'-dichlorofluorescein ions

Fluorescein (FL) and its derivative 2',7'-dichlorofluoroescein (DCF) are well-known fluorescent dyes used in many biological and biochemical applications. Although extensive studies have been carried out to investigate their chemical and photophysical properties in different solvent media, little is known about their intrinsic behaviors in the gas phase. Here, infrared multiple photon dissociation (IRMPD) action spectra are reported for the three charged prototropic forms of FL and DCF and compared with computed IR spectra from electronic structure calculations. In each case, the measured spectra show good agreement with the calculated spectra of the lowest energy computed conformer. Moreover, the major bands of the monoanion IRMPD spectra show striking similarities to those of the dianions and are quite different from those of the cations. These experimental results clearly indicate that the gaseous monoanions are predominantly deprotonated on the xanthene chromophore, rather than the benzoate deprotonation site favored in solution. Investigations such as this, which provide a better understanding of intrinsic properties of ionic dyes, forms a baseline from which to elucidate solvent effects and will aid the rational design of dyes possessing desirable fluorescence properties.     

Infrared spectroscopy of HCN-salt complexes formed in liquid-helium nanodroplets

Rotationally resolved infrared spectra are reported for the binary complexes of HCN and LiF, LiCl, NaF, and NaCl, formed in helium nanodroplets. Stark spectroscopy is used to determine the dipole moments for these complexes. Ab initio calculations are also reported for these complexes, revealing the existence of several different isomers of these binary systems. In the frequency region examined in this experimental study we only observe one of these, corresponding to the salt binding to the nitrogen end of the HCN molecule. The experimental rotational constants, dipole moments, and vibrational frequency shifts are all compared with the results from ab initio calculations for this isomer.     

Infrared spectroscopy of arginine cation complexes: direct observation of gas-phase zwitterions

The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.     

Infrared spectroscopy of copper-resveratrol complexes: a joint experimental and theoretical study

Infrared multiple-photon dissociation spectroscopy has been used to record vibrational spectra of charged copper-resveratrol complexes in the 3500-3700 cm(-1) and 1100-1900 cm(-1) regions. Minimum energy structures have been determined by density functional theory calculations using plane waves and pseudopotentials. In particular, the copper(I)-resveratrol complex presents a tetra-coordinated metal bound with two carbon atoms of the alkenyl moiety and two closest carbons of the adjoining resorcinol ring. For these geometries vibrational spectra have been calculated by using linear response theory. The good agreement between experimental and calculated IR spectra for the selected species confirms the overall reliability of the proposed geometries.     

Rotational spectroscopy of jet-cooled molecular ions and ion complexes

Rotational spectra of the molecular ions HOCO⁺ and HOCS⁺, and the ion complexes, D₃⁺Ar and sym-D₂H⁺Ar were observed in a supersonic-jet expansion by using a Fabry-Perot type Fourier-transform microwave spectrometer cooperated with a pulsed discharge nozzle. Ion-formation efficiency for HOCS⁺ relative to the parent molecule under applied conditions was estimated to be ≈ 10⁻⁴. Tunneling splitting in the lowest rotational transition of D₃⁺-Ar was not resolved within the experimental linewidth of ≈ 100 kHz.     

Two-photon dissociation spectroscopy of state-selected HCl+ and DCl+ ions.

HCl+ and DCl+ ions were formed via the R(1) pump line of the f3delta2(v'=0)<--sigma+(v'=0) REMPI process. For these ions, the two-photon dissociation spectroscopy, resonance-enhanced via the A2sigma+(v')<--pi3/2(v'=0) transition, was investigated for various intermediate states of HCl+ (v'=4,5,6) and DCl+ (v'=6,7,8,9). From the analysis of the data, spectroscopic parameters of the X and the A states were derived (including the lambda-doubling in the X state and the spin-rotation coupling in the A state). Some of the parameters deviate considerably from literature data. The spectra provide clear evidence that the REMPI process employed for forming the ions has a very high rotational selectivity.     

Infrared multiphoton dissociation spectroscopy of cationized serine: effects of alkali-metal cation size on gas-phase conformation

The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li+(Ser) and Na+(Ser) are similar and relatively simple, whereas Cs+(Ser) includes distinctive new IR bands, and K+(Ser) and Rb+(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Li+ and Na+, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1[N,CO,OH]. For the cesiated complex, a band corresponding to a zwitterionic structure, ZW[CO2-], is clearly visible. K+(Ser) and Rb+(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3[COOH], in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally.     

Infrared multiphoton dissociation spectroscopy of cationized threonine: effects of alkali-metal cation size on gas-phase conformation

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li+(Thr) and Na+(Thr) are similar and relatively simple, whereas K+(Thr), Rb+(Thr), and Cs+(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1[N,CO,OH], binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K+(Thr), Rb+(Thr), and Cs+(Thr) exhibit evidence of the charge-solvated complex, M3[COOH], in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZW[CO2-], is also present, particularly for the Cs+ complex. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M+(Ser) complexes, although there are subtle distinctions that are discussed.