Predissociation Spectroscopy Research Articles

Probing Ionophore Selectivity in Argon-Tagged Hydrated Alkali Metal Ion–Crown Ether Systems

Crown ethers are an important family of compounds that are closely related to naturally occurring ionophores. Thus, crown ethers are useful in modeling the size-selective behavior of ionophores. Using a combination of infrared predissociation spectroscopy and density functional theory calculations, we have investigated M(+)(18-crown-6 ether)(H(2)O)(1-4) Ar complexes, where M = Li, Na, K, Rb and Cs in the gas phase. The argon-tagging technique was used to lower the internal energies (effective temperatures ~100 K), yielding well-resolved spectra in the OH stretching region for systems containing up to three waters. Spectral changes were monitored as both the size of the ion and degree of hydration were varied. While there is not a particular spectroscopic signature of gas-phase selectivity reported in this work, the unique role that K(+) plays in the systems studied, as a "bridge" between the smaller and larger alkali metal ions, is consistent with the well-known special affinity for K(+) by 18-crown-6 ether in the aqueous phase.

Quantum vibrational analysis and infrared spectra of microhydrated sodium ions using an ab initio potential

We present a full-dimensional potential energy surface and a dipole moment surface (DMS) for hydrated sodium ion. These surfaces are based on an n-body expansion for both the potential energy and the dipole moment, truncated at the two-body level for the H(2)O-Na(+) interaction and also for the DMS. The water-water interaction is truncated at the three-body level. The new full-dimensional two-body H(2)O-Na(+) potential is a fit to roughly 20,000 coupled-cluster single double (triple)/aug-cc-pVTZ energies. Properties of this two-body potential and the potential describing (H(2)O)(n)Na(+) clusters, with n up to 4 are given. We then report anharmonic, coupled vibrational calculations with the "local-monomer model" to obtain infrared spectra and also 0 K radial distribution functions for these clusters. Some comparisons are made with the recent infrared predissociation spectroscopy experiments of Miller and Lisy [J. Am. Chem. Soc. 130, 15381 (2008).].

Assigning Predissociation Infrared Spectra of Microsolvated Hydronium Cations H3O+⋅(H2)n (n=0, 1, 2, 3) by Ab Initio Molecular Dynamics

Messenger predissociation spectroscopy is an important experimental method to obtain vibrational spectra of molecular ions or complexes such as protonated water clusters H(+)⋅(H(2)O)(n) in the gas phase. However, the molecular properties and thus the linear infrared spectra may be modified upon microsolvation with typical messengers such as H(2) molecules or noble gas atoms. Employing ab initio molecular dynamics for the H(2)-microsolvated hydronium ion, we investigate these effects explicitly as a function of an increasing number of messengers up to filling the first microsolvation shell, that is, for H(3)O(+)⋅(H(2))(n) (n=0, 1, 2, 3). We find that microsolvation with H(2) lowers the inversion barrier of the hydronium core, which governs the inversion tunnel splitting due to umbrella motion, and thus accelerates the inversion dynamics. By comparison to experiment a comprehensive band assignment for the O-H stretching region is given, and thereby the observed blueshift of stretching bands with increasing n is explained. Furthermore, detailed analyses reveal intricate intra- and intermolecular anharmonic mode couplings induced by the messengers, which yield a rich vibrational dynamics in these, at first glance, simple systems. Finally, the virtues but also the shortcomings of the ab initio molecular dynamics approach to vibrational spectroscopy are discussed.

Structures and vibrational spectroscopy of partially reduced gas-phase cerium oxide clusters

This work demonstrates that the most stable structures of even small gas-phase aggregates of cerium oxide with 2-5 cerium atoms show structural motifs reminiscent of the bulk ceria. This is different from main group and transition metal oxide clusters, which often display structural features that are distinctly different from the bulk structure. The structures of Ce(2)O(2)(+), Ce(3)O(4)(+), and (CeO(2))(m)CeO(+) clusters (m = 0-4) are unambiguously determined by a combination of global structure optimizations at the density functional theory level and infrared vibrational predissociation spectroscopy of the cluster-rare gas atom complexes. The structures of Ce(2)O(2)(+) and Ce(2)O(3)(+) exhibit a Ce-O-Ce-O four-membered ring with characteristic absorptions between 430 and 680 cm(-1). Larger clusters have common structural features containing fused Ce-O-Ce-O four-membered rings which lead to intense absorption bands at around 500 and 650 cm(-1). Clusters containing a terminal Ce=O bond show a characteristic absorption band between 800 and 840 cm(-1). For some cluster sizes multiple isomers are observed. Their individual infrared signatures are identified by tuning their relative population through the choice of He, Ne or Ar messenger atoms. The present results allow us to benchmark different density functionals which yield different degrees of localization of unpaired electrons in Ce 4f states.

Infrared spectroscopy of Li +(CH 4) n, n = 1–9, clusters

We report the infrared spectra of Li +(CH 4) n , n = 1–9, clusters in the C−H stretching region using vibrational predissociation spectroscopy. Vibrational frequencies and geometries were calculated at the B3LYP/6-31+G ∗ level with zero-point corrected binding energies at the MP2/6-31+G ∗ level. Experiments and calculations indicated a first solvent shell size of four CH 4. Second solvent shell ligands were identified by smaller frequency shifts and changes in relative intensity, suggesting a reduced influence of Li + beyond the first shell. Perturbation of the CH 4 vibrational modes appears to be dominated by the Li +⋯CH 4 electrostatic interaction, while CH 4⋯CH 4 interactions between first and second shell molecules have no measurable effect.

Vibrational predissociation spectroscopy of the H 2-tagged mono- and dicarboxylate anions of dodecanedioic acid

Vibrational predissociation spectroscopy of the HOOC(CH 2) 10COO − and −OOC(CH 2) 10COO − anions is carried out by predissociation of weakly bound H 2 molecules. The HOOC(CH 2) 10COO − (H 2) 2 and −OOC(CH 2) 10COO − (H 2) 10 cluster ions are formed by H 2 attachment to the electrospray-generated bare ions in an ion trap cooled to below 20 K using a closed cycle helium cryostat. The photofragmentation behavior indicates that the H 2 binding energy is about 600 cm −1, which is similar in strength to that found in Ar-tagged ions. The spectra indicate that the monoanion adopts a cyclic structure through the formation of an asymmetrical, internal anionic H-bond.

Anharmonic RRKM Calculation for the Dissociation of (H2O)2H+ and Its Deuterated Species (D2O)2D+

Investigations on the dissociation kinetics of hydrated protonium ions, (H(2)O)(2)H(+) and their deuterated species (D(2)O)(2)D(+), are reported based on the harmonic and anharmonic oscillator model using the transition state theory and ab initio calculations. We find that the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+) exhibits a distinct threshold behavior due to the existence of activation energies. Moreover, the deviation between the harmonic and anharmonic dissociation rate constants becomes larger in the high energy or temperature range, with the rate constants becoming unreasonably large under the harmonic oscillator model. The isotope effect is found to become more distinct but only in the case of the anharmonic oscillator model. These results show that the anharmonic Rice-Ramsperger-Kassel-Marcus (RRKM) theory can provide a reasonably good description for the dissociation of (H(2)O)(2)H(+) and (D(2)O)(2)D(+). Furthermore, a theoretical model to demonstrate the principle of vibrational predissociation spectroscopy (VPS) is established from the viewpoint of RRKM theory and applied in determining the experimental conditions and understanding the role of the dissociation rate constant k(E) played in the VPS experiment, using (H(2)O)(2)H(+) and (D(2)O)(2)D(+) as examples.

ChemInform Abstract: Infrared Spectra of H+(H2O)5-8 Clusters: Evidence for Symmetric Proton Hydration.

Protonated water clusters, H+(H2O)n (n = 5−8), from a supersonic expansion have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The experimental spectra were obtained at an estimated cluster temperature of 170 ± 20 K. Recorded absorption bands at the frequency range of 2700−3900 cm-1 are attributed to the free- and hydrogen-bonded-OH stretches of the ion core and the surrounding solvent molecules. Ab initio calculations, performed at the B3LYP/6-31+G* level, indicate that geometries of the H+(H2O)5-8 isomers are close in energy, with the excess proton either localized on a single water molecule, yielding H3O+(H2O)n-1, or equally shared by two molecules, yielding H5O2+(H2O)n-2. Systematic comparison of the experimental and computed spectra provides compelling evidence for both cases. The unique proton-transfer intermediate H5O2+(H2O)4 was identified, for the first time, by its characteristic bonded-OH stretching absorptions at 3178 cm-1. The existence of five-membe...

Exploring Gas-Phase Ion−Ionophore Interactions: Infrared Spectroscopy of Argon-Tagged Alkali Ion-Crown Ether Complexes

Argon-tagged alkali metal ion-crown ether complexes were generated in the gas phase and investigated using a combination of infrared predissociation (IRPD) spectroscopy, density functional, and symmetry-adapted perturbation theory (SAPT). The IRPD spectra of M(+)(12-crown-4 ether)Ar and M(+)(18-crown-6 ether)Ar, where M = Li, Na, K, Rb, and Cs, were collected in the CH stretching region, using the argon-messenger technique. The gas-phase neutral vibrations for each crown ether shift to higher frequency when complexed to an alkali metal ion. Similarities in the experimental IRPD spectra of Li(+)(12-crown-4 ether)Ar and Li(+)(18-crown-6 ether)Ar indicate that the binding of Li(+) is similar for both crown ethers. In the 12-crown-4 ether systems, there are trackable changes with ion size in the IRPD spectra until the point is reached where the ion is too large to bind to the interior of the macrocycle. Starting with Na(+), the 18-crown-6 spectra vary only slightly as the ion size is increased. The overall profiles of the IRPD spectra indicate that a similar configuration of the complexes is adopted for the ions larger than Na(+). The calculated SAPT interaction energies mirror the trends exhibited by the IRPD spectra and provide interesting insights on the roles of the different interaction energy components in binding the cation to the crown-ether.

Infrared Spectroscopy of Ionophore-Model Systems: Hydrated Alkali Metal Ion 18-Crown-6 Ether Complexes

We report our efforts to study host-guest complexes in the gas phase using a combination of cluster spectroscopy and density functional theory. Mass-selected M(+)(18-crown-6 ether)(H(2)O)(1-4) complexes for the alkali metal ion series were probed using infrared predissociation (IRPD) spectroscopy in the OH stretching region. As the degree of hydration is increased, the IRPD spectra undergo significant changes as the strong 18c6...M(+) interaction weakens and allows H(2)O...H(2)O hydrogen-bonding interactions to compete. The size of the ion is important in determining when this transition occurs. For the smaller ions, Li(+) and Na(+), the 18c6...M(+) interaction proves to be more resilient and is still dominant with two and three waters present. The potassium cation, with its optimum size match with the 18-cown-6 ether cavity, serves as a bridge between the larger and smaller alkali metal ions. In particular, we found a structure for K(+)(18-crown-6 ether)(H(2)O)(2) that appears to be a building block for K(+)(18-crown-6 ether)(H(2)O)(3) complexes and is also believed to be present in Rb(+)(18-crown-6 ether)(H(2)O)(2,3) and Cs(+)(18-crown-6 ether)(H(2)O)(2,3). With four waters present, we were unable to spectrally resolve features associated with individual water molecules due to broad hydrogen bonding. However, results for Cs(+)(18-crown-6 ether)(H(2)O)(4) suggest that H(2)O...H(2)O hydrogen bonding has become the dominant interaction present at this size.

Structural characterization of (C2H2)1–6+ cluster ions by vibrational predissociation spectroscopy

Vibrational predissociation spectra are reported for the cationic acetylene clusters, (C(2)H(2))(n) (+), n=1-6, in the region of the C-H stretching fundamentals. For n=1 and 2, predissociation could only be observed for the Ar-tagged clusters. These were prepared by charge-transfer collisions of Ar(k) (+) with C(2)H(2) to create C(2)H(2) (+)Ar(m) clusters, which were then converted into larger members of the (C(2)H(2))(n) (+)Ar series by sequential addition of acetylene molecules. The (C(2)H(2))(2) (+)Ar spectrum indicates that this species is predominantly present as the cyclobutadiene cation. Although mobility measurements on the electron-impact-generated (C(2)H(2))(3) (+) ion indicated that it primarily occurs as the benzene cation, [P. O. Momoh, J. Am. Chem. Soc. 128, 12408 (2006)] photofragmentation of (C(2)H(2))(3) (+)Ar in the C-H stretching region is dominated by the loss of C(2)H(2) in addition to the weakly bound Ar atom. This suggests that the dominant n=3 species formed by sequential addition of C(2)H(2) is based on a covalently bound C(4)H(4) (+) core ion. Interestingly, the spectrum of this core C(4)H(4) (+) species is different from that found for the cyclobutadiene cation, displaying instead a new band pattern that is retained in the higher (C(2)H(2))(3-6) (+) clusters. Multiple isomers are clearly involved, as yet another pattern of bands is recovered when the (C(2)H(2))(3) (+)Ar action spectrum is recorded in the (minor) Ar loss fragmentation channel. One of these features does appear in the location of the single band characteristic of the Ar-tagged benzene cation reported earlier [Phys. Chem. Chem. Phys. 4, 24 (2002)], supporting a scenario where the benzene cation is one of the isomers present. We then compare the Ar predissociation results with (C(2)H(2))(n) (+) spectra obtained when the ions are prepared by electron impact ionization of neutral acetylene clusters. The photofragmentation behavior and vibrational spectra indicate that the dominant species formed in this way also occur with a covalently bound C(4)H(4) (+) core. There are absorptions, however, which are consistent with a minor contribution from (C(2)H(2))(n) (+) clusters based on the benzene cation.

Infrared Spectroscopy of Multiply Charged Metal Ions: Methanol-Solvated Divalent Manganese 18-Crown-6 Ether Systems

We have developed an electrosonic spray ionization source to successfully generate divalent Mn(2+)(18-crown-6)(CH(3)OH)(1-3) complexes in the gas phase. These complexes have been investigated using infrared predissociation spectroscopy in both the CH and OH stretching regions and with density functional theory calculations. To resolve complications from overlapping bands in the CH stretching region due to CH(3)OH and 18-crown-6 CH stretching modes along with strongly perturbed OH stretching modes, we have used d(1) and d(4) methanol substitution. For n = 1, the Mn(2+)...18-crown-6 geometry is highly distorted from its gas-phase neutral configuration in order to maximize favorable electrostatic interactions between the 18-crown-6 macrocyclic oxygens and Mn(2+). For n = 2 and 3, CH(3)OH...CH(3)OH and CH(3)OH...18-crown-6 interactions compete with the Mn(2+)...18-crown-6 interaction, as evidenced by intense hydrogen-bonded OH stretching modes shifted over 500 cm(-1) to lower frequency.

Charge and Temperature Dependence of Biomolecule Conformations: K+Tryptamine(H2O)n=0−1Arm=0−1 Cluster Ions

The impact of temperature and charge on the conformation of tryptamine (Tryp) is examined in the gas phase by infrared laser-vibrational predissociation spectroscopy in the 2800-3800 cm(-1) region. Previous studies of neutral Tryp(H(2)O)(n) clusters showed preferential stabilization of specific tryptamine conformers through hydrogen bonding. When complexed with the biologically significant potassium ion, the only conformers found to form under these experimental conditions are built on hitherto unobserved neutral Tryp conformers. The electrostatic interaction between K(+), the tryptamine NH(2) lone pair, and the indole ring in K(+)(Tryp) favors the formation of these new conformers. The observed K(+)(Tryp)(H(2)O) conformers vary significantly from the previously reported neutral Tryp(H(2)O) structure. Using the argon tagging method, we show how variations in temperature impact the observed infrared spectra, demonstrating that different conformers are populated under the different experimental conditions. In addition, the presence of a high-energy conformer of K(+)(Tryp)(H(2)O), trapped by the argon evaporative cooling process, was identified. Exploring the conformational landscape of hydrated cluster ions bearing flexible biomolecules is now possible.

Infrared consequence spectroscopy of gaseous protonated and metal ion cationized complexes

In this article, the new and exciting techniques of infrared consequence spectroscopy (sometimes called action spectroscopy) of gaseous ions are reviewed. These techniques include vibrational predissociation spectroscopy and infrared multiple photon dissociation spectroscopy and they typically complement one another in the systems studied and the information gained. In recent years infrared consequence spectroscopy has provided long-awaited direct evidence into the structures of gaseous ions from organometallic species to strong ionic hydrogen bonded structures to large biomolecules. Much is being learned with respect to the structures of ions without their stabilizing solvent which can be used to better understand the effect of solvent on their structures. This review mainly covers the topics with which the author has been directly involved in research: structures of proton-bound dimers, protonated amino acids and DNA bases, amino acid and DNA bases bound to metal ions and, more recently, solvated ionic complexes. It is hoped that this review reveals the impact that infrared consequence spectroscopy has had on the field of gaseous ion chemistry.

Infrared spectroscopy of gas-phase hydrated K +:18-crown-6 complexes: Evidence for high energy conformer trapping using the argon tagging method

We report our findings on the K +(18-crown-6 ether)(H 2O) 1Ar 1–4 system using gas-phase infrared predissociation (IRPD) spectroscopy. With the argon tagging technique, we have been able to observe two different conformers, including a conformer that features a bidentate H 2O actively competing with K + for the preferred binding site inside the 18-crown-6 cavity. The detection of this conformer in our experiment was surprising since density functional theory (DFT) calculations predict it to be 55.6 kJ mol −1 higher in energy than the lower energy, traditional (K + bound)-type conformer. We have been able to selectively probe each conformer by varying both the loss channel monitored and the number of solvating argons. The bidentate conformer was present in the IRPD spectra only when monitoring the action spectrum leading to the loss of all ligands. This suggests that the bidentate conformer undergoes rearrangement following photoexcitation to the much more stable bare K +(18-crown-6) complex. We explored the barrier to such rearrangement using D 2O substitution in the experiment and with DFT calculations.

Spectroscopic Study of the Ion−Radical H-Bond in H4O2+

The primary event in the ionization of water involves rapid proton transfer, leading to charge localization on H(3)O(+) and the creation of a hydroxyl radical. We trap the nascent [H(3)O(+).(*)OH] exit channel intermediate in the bimolecular reaction by Ar-mediated ionization of the neutral water dimer and characterize the nature of this ion-radical complex using vibrational predissociation spectroscopy of the Ar-tagged species. The resulting bands involving the displacement of the bridging proton are broad and appear as a strong triplet centered around 2000 cm(-1). The observed band pattern is analyzed with theoretical calculations to identify the origin of the anhamonic effects evident in the spectrum. In the course of this work, expressions were derived for treating the coupling terms within a sinc-DVR. Although this level of treatment did not reveal the assignment of the triplet structure, its characteristic approximately 100 cm(-1) spacing suggests activity involving the frustrated rotation of the hydroxyl radical upon excitation of the bridging-proton vibration parallel to the heavy atom axis. The behavior of this system is considered in the context of that reported previously for the related H(5)O(2)(+), H(3)O(2)(-), and F(-).H(2)O complexes.

Vibrationally Induced Interconversion of H-Bonded NO2−·H2O Isomers within NO2−·H2O·Arm Clusters Using IR−IR Pump−Probe through the OH and NO Stretching Vibrations

We introduce a method based on sequential application of vibrational predissociation spectroscopy to explore the high-amplitude rearrangements available in a small H-bonded complex that is vibrationally excited within a larger Ar cluster. The weakly bound Ar atoms play the role of a solvent in mediating the energy content of the embedded system, ultimately quenching it into local minima through evaporation. We demonstrate the approach on the NO(2)(-) x H(2)O binary hydrate, which is known to occur in two nearly isoenergetic isomeric forms. The scheme involves three stages of mass separation to select a particular NO(2)(-) x H(2)O x Ar(m) parent ion cluster prior to vibrational excitation and then isolate the NO(2)(-) x H(2)O x Ar fragment ions for interrogation using resonant vibrational predissociation with a second infrared laser. The initial vibrational excitation selectively energizes one of the isomers through one of its characteristic resonances while the predissociation spectrum of the NO(2)(-) x H(2)O x Ar fragment encodes the distribution of isomers present after Ar evaporation. Isomerization from the front- to backside form is found to occur upon excitation of the NO stretch near 1200 cm(-1); although the reverse reaction is not observed upon excitation of the NO stretch, it is observed upon excitation of the higher-energy OH stretching fundamental near 3000 cm(-1). We discuss these observations in the context of the calculated isomerization energetics, which focus on the minimum energy structures for the isomers as well as the transition states for their interconversion.

Vibrational spectroscopy of size-selected neutral and cationic clusters combined with vacuum-ultraviolet one-photon ionization detection

Various vibrational spectroscopic techniques combined with vacuum-ultraviolet one-photon ionization mass spectrometry have recently been developed for jet-cooled molecules and clusters. These techniques open general applications of size-selected infrared and also Raman spectroscopies to neutral clusters by overcoming difficulties in the conventional vibrational spectroscopic methods for jet-cooled clusters. Their spectroscopic principles have been demonstrated by investigations for neutral clusters of simple protic molecules without an ultraviolet chromophore, which is necessary for the conventional vibrational spectroscopic methods of gas phase clusters based on the fluorescence or multiphoton ionization detection. In addition, the vacuum-ultraviolet photoionization detection scheme enables us to perform infrared predissociation spectroscopy of unstable cluster cations such as unprotonated cluster cations of protic molecules. In this Perspective, the spectroscopic principles and their applications for neutral and cationic clusters are reviewed.

Identification of Conical Structures in Small Aluminum Oxide Clusters: Infrared Spectroscopy of (Al2O3)1−4(AlO)+

The vibrational spectroscopy of the electronically closed-shell (Al 2O 3) n (AlO) (+) cations with n = 1-4 is studied in the 530-1200 cm (-1) range by infrared predissociation spectroscopy of the corresponding ion-He atom complexes in combination with quantum chemical calculations. In all cases we find, assisted by a genetic algorithm, global minimum structures that differ considerably from those derived from known modifications of bulk alumina. The n = 1 and n = 4 clusters exhibit an exceptionally stable conical structure of C 3 v symmetry, whereas for n = 2 and n = 3, multiple isomers of lower symmetry and similar energy may contribute to the recorded spectra. A blue shift of the highest energy absorption band is observed with increasing cluster size and attributed to a shortening of Al-O bonds in the larger clusters. This intense band is assigned to vibrational modes localized on the rim of the conical structures for n = 1 and n = 4 and may aid in identifying similar, highly symmetric structures in larger ions.

Infrared predissociation spectroscopy of cluster cations of protic molecules, (NH3)n+, n=2–4 and (CH3OH)n+, n=2,3

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH(4) (+)...NH(2) or CH(3)OH(2) (+)...OCH(3), is formed. Theoretical calculations at the MP2 and B3LYP levels support the formation of the proton-transferred type structures for the cluster cations, and indicate that they are formed by proton-transfer following the photoionization of the neutral clusters.