Anharmonicities and Isotopic Effects in the Vibrational Spectra of X−·H2O, ·HDO, and ·D2O [X = Cl, Br, and I] Binary Complexes
Vibrational predissociation spectra of the argon-tagged halide monohydrates, X−•H2O•Ar (X = Cl, Br, or I), are recorded from 800 to 3800 cm−1 by monitoring the loss of the argon atom. We use this set of spectra to investigate how the spectral signatures of the hydrogen-bonding and large-amplitude hindered rotations of the water molecule are affected by incremental substitution of the hydrogen atoms by deuterium. All six vibrational modes of the X−•H2O complexes are assigned through fundamental transitions, overtones, or combination bands. To complement the experimental study, harmonic and reduced-dimensional calculations of the vibrational spectra are performed based on the MP2/aug-cc-pVTZ level of theory and basis set. Comparison of these results with those from the converged six-dimensional calculations of Rheinecker and Bowman [ J. Chem. Phys. 2006,125, 133206.] show good agreement, with differences smaller than 30 cm−1. The simpler method has the advantage that it can be readily extended to the heavier halides and was found to accurately recover the wide range of behaviors displayed by this series, including the onset of tunneling between equivalent minima arising from the asymmetrical (single ionic hydrogen-bonded) equilibrium structures of the complexes.
How the Shape of an H-Bonded Network Controls Proton-Coupled Water Activation in HONO Formation
Many chemical reactions in atmospheric aerosols and bulk aqueous environments are influenced by the surrounding solvation shell, but the precise molecular interactions underlying such effects have rarely been elucidated. We exploited recent advances in isomer-specific cluster vibrational spectroscopy to explore the fundamental relation between the hydrogen (H)–bonding arrangement of a set of ion-solvating water molecules and the chemical activity of this ensemble. We find that the extent to which the nitrosonium ion (NO+)and water form nitrous acid (HONO) and a hydrated proton cluster in the critical trihydrate depends sensitively on the geometrical arrangement of the water molecules in the network. Theoretical analysis of these data details the role of the water network in promoting charge delocalization.
Vibrational predissociation spectra of the Ar-tagged [CH4 · H3O+] binary complex: spectroscopic signature of hydrogen bonding to an alkane
Vibrational predissociation spectra of the Ar-tagged [H3O+ ⋅ X], X = CH4, CD4, N2, and Ar complexes are analysed to explore the hydrogen bonding acceptor properties of an alkane. The observed red shift in the OH stretching transition of the donor is found to be significantly smaller than anticipated by the previously reported trend in this value with the proton affinity of the acceptor [Science 316, 249 (2007)] 1. Specifically, the alkane-induced red shift of the OH stretching frequency is less than that caused by the conventional proton acceptor, N2, even though the latter is a weaker base than methane. The origin of this effect is discussed in the context of the structures of the complexes and the molecular rearrangements required for complete proton transfer to hydrocarbons as opposed to the situation in conventional H-bond acceptors.
Structural characterization of (C2H2)1–6+ cluster ions by vibrational predissociation spectroscopy
Vibrational predissociation spectra are reported for the cationic acetylene clusters,(C2H2)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 Ark+ with C2H2 to create C2H2+⋅Arm clusters, which were then converted into larger members of the (C2H2)n+⋅Ar series by sequential addition of acetylene molecules. The (C2H2)2+⋅Ar spectrum indicates that this species is predominantly present as the cyclobutadiene cation. Although mobility measurements on the electron-impact-generated (C2H2)3+ ion indicated that it primarily occurs as the benzene cation, [nbsp;P. O. Momoh, J. Am. Chem. Soc. 128, 12408 (2006) ] photofragmentation of (C2H2)3+⋅Ar in the C–H stretching region is dominated by the loss of C2H2 in addition to the weakly bound Ar atom. This suggests that the dominant n = 3species formed by sequential addition of C2H2 is based on a covalently bound C4H4+core ion. Interestingly, the spectrum of this core C4H4+ species is different from that found for the cyclobutadiene cation, displaying instead a new band pattern that is retained in the higher (C2H2)3–6+ clusters. Multiple isomers are clearly involved, as yet anotherpattern of bands is recovered when the (C2H2)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(C2H2)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 boundC4H4+ core. There are absorptions, however, which are consistent with a minor contribution from (C2H2)n+ clusters based on the benzene cation.
Structural Evolution of the [(CO2)n(H2O)]− Cluster Anions: Quantifying the Effect of Hydration on the Excess Charge Accommodation Motif
The [(CO2)n(H2O)]− cluster anions are studied using infrared photodissociation (IPD) spectroscopy in the 2800−3800 cm−1 range. The observed IPD spectra display a drastic change in the vibrational band features at n = 4, indicating a sharp discontinuity in the structural evolution of the monohydrated cluster anions. The n = 2 and 3 spectra are composed of a series of sharp bands around 3600 cm−1, which are assignable to the stretching vibrations of H2O bound to C2O4− in a double ionic hydrogen-bonding (DIHB) configuration, as was previously discussed (J. Chem. Phys. 2005, 122, 094303). In the n ≥ 4 spectrum, a pair of intense bands additionally appears at ≈3300 cm−1. With the aid of ab initio calculations at the MP2/6-31+G* level, the 3300 cm−1 bands are assigned to the bending overtone and the hydrogen-bonded OH vibration of H2O bound to CO2− via a single O−H•••O linkage. Thus, the structures of [(CO2)n(H2O)]− evolve with cluster size such that DIHB to C2O4− is favored in the smaller clusters with n = 2 and 3 whereas CO2− is preferentially stabilized via the formation of a single ionic hydrogen-bonding (SIHB) configuration in the larger clusters with n ≥ 4.
Generating Spectra from Ground-State Wave Functions: Unraveling Anharmonic Effects in the OH−·H2O Vibrational Predissociation Spectrum
An approach is described for calculating anharmonic spectra for polyatomic molecules using only the ground-state probability amplitude. The underlying theory is based on properties of harmonic oscillator wave functions and is tested for Morse oscillators with a range of anharmonicities. More extensive tests are performed with H3O2−, using the potential and dipole surfaces of Bowman and co-workers [J. Am. Chem. Soc. 2004, 126, 5042]. The resulting energies are compared to earlier studies that employed the same potential surface, and the agreement is shown to be very good. The vibrational spectra are calculated for both H3O2− and D3O2−. In the case of H3O2−, comparisons are made with a previously reported experimental spectrum below 2000 cm− 1. We also report the spectrum of H3O2− from 2400−4500 cm−1, which extends 500 cm−1 above the region reported earlier, revealing several new bands. As the only fundamentals in this spectral region involve the OH stretches, the spectrum is surprisingly rich. On the basis of comparisons of the experimental and calculated spectra, assignments are proposed for several of the features in this spectral region.
Vibrational predissociation spectra of the On–, n = 3–10, 12 clusters: Even–odd alternation in the core ion
We report vibrational predissociation spectra of the O−n, n = 3–10, 12 cluster ions in the 700–2400 cm−1 range. The odd numbered clusters are consistent with their identiﬁcation as O−3·(O2)n. The even numbered clusters are based on the O−4 core ion, where the ﬁrst two O2 molecules add to O−4 in locations that individually break the symmetry of the core ion, but together restore this symmetry. Beyond O−4·(O2)2, subsequent O2 attachment yields bands close to that of neutral O2, indicating that the special character of the O−4·(O2)2 cluster is retained.
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 H3O+ and the creation of a hydroxyl radical. We trap the nascent [H3O+••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 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 H5O2+, H3O2−, and F−•H2O complexes.
Experiment and theory in harmony
Mark A. Johnson at Yale University discusses how the two sides of physical chemistry have necessarily developed together, and looks at how their synergy dictates the direction of contemporary research.
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 NO2−•H2O 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 NO2−•H2O•Arm parent ion cluster prior to vibrational excitation and then isolate the NO2−•H2O•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 NO2−•H2O•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.
Photoelectron imaging study of vibrationally mediated electron autodetachment in the type I isomer of the water hexamer anion
We exploit the high collection efficiency of negative ion photoelectron imaging for low energy electrons to monitor the energy and angular distributions of the photoelectrons arising from vibrational excitation of the water hexamer anion in the vicinity of the OH stretching fundamentals. Photoelectrons from the low electron binding energy isomer (type II) appear as a smoothly varying outer ring with the anisotropic angular distribution expected for direct photodetachment. The higher binding isomer (type I) yields very slow electrons that are strongly modulated through the OH stretching resonances, which are discussed in the context of a statistical (IVR-based) ejection mechanism.
Vibrationally Induced Proton Transfer in F−(H2O) and F−(D2O)
Vibrational predissociation spectra of the F−(H2O)•Ar and F−(D2O)•Ar complexes are observed over a range of 600 to 3800 cm−1, which include bands attributed to the fundamentals as well as the first two overtones of the vibrations primarily associated with the shared hydrogen. This information allows us to characterize both the extended potential surface confining the anionic H-bonded hydrogen and the degree to which this motion is coupled to the motions of other atoms in the complex. We analyze these new data with reduced dimensional treatments using explicit potential energy and electric dipole moment surfaces. The often employed one-dimensional treatment with fixed OF distance does not even qualitatively account for the observed isotope dependent level structures, but a simple extension to two dimensions, corresponding to the OF distance and the shared proton position, accurately recovers the observed spectra. The resulting two-dimensional wave functions are used to evaluate the extent of proton transfer in each vibrational level. The main conclusion of this work is that vibrational excitation of the shared proton can be regarded as optically driven, intracluster proton transfer.
Intermolecular Proton Binding in the Presence of a Large Electric Dipole: Ar-Tagged Vibrational Predissociation Spectroscopy of the CH3CN·H+·OH2 and CH3CN·D+·OD2 Complexes
We report Ar-predissociation vibrational spectra of the binary proton-bound hydrates of acetonitrile (AN), AN•H+•OH2 and AN•D+•OD2, in the 600−3800 cm−1 energy range. This complex was specifically chosen to explore the nature of the intermolecular proton bond when there is a large difference between the electric dipole moments of the two tethered molecules. Sharp, isotope-dependent bands in the vicinity of 1000 cm−1 are traced to AN•H+•OH2 vibrations involving the parallel displacement of the shared proton along the heavy atom axis, νsp(). These transitions lie much lower in energy than anticipated by a recently reported empirical trend which found the νsp() fundamentals to be strongly correlated with the difference in proton affinities (ΔPA) between the two tethered molecules (Roscioli et al., Science, 2007, 316, 249). The different behavior of the AN•H+•OH2complex is discussed in the context of the recent theoretical prediction (Fridgen, J. Phys. Chem A., 2006, 110, 6122) that a large disparity in dipole moments would lead to such a deviation from the reported (ΔPA) trend.
Isolating the spectra of cluster ion isomers using Ar-“tag” -mediated IR-IR double resonance within the vibrational manifolds: Application to NO2−⋅H2O
We demonstrate a method for isolating the vibrational predissociation spectra of different structural isomers of mass-selected cluster ions based on a population-labeling double resonance scheme. This involves a variation on the “ion dip” approach and is carried out with three stages of mass selection in order to separate the fragment ion signals arising from a fixed-frequency population-monitoring laser and those generated by a scanned laser that removes population of species resonant in the course of the scan. We demonstrate the method on the Ar-tagged NO2−⋅H2O cluster, where we identify the spectral patterns arising from two isomers. One of these structures features accommodation of the water molecule in a double H-bond arrangement, while in the other, H2O attaches in a single ionic H-bond motif where the nominally free OH group is oriented toward the N atom of NO2−. Transitions derived from both the NO2− and H2O constituents are observed for both isomers, allowing us to gauge the distortions suffered by both the ion and solvent molecules in the different hydration arrangements.
Argon cluster-mediated isolation and vibrational spectra of peroxy and nominally D3h isomers of CO3−and NO3−
Vibrational predissociation spectra are reported for two isomeric forms of the gas-phase ions, CO3− and NO3−. The peroxy forms, (OOCO− and OONO−) were isolated using an Ar-mediated synthetic scheme involving exchange of CO and NO for the more weakly bound Ar ligands in O2−⋅Arm clusters, while the forms based on a central heteroatom (CO3− and NO3−) were generated by electron impact on CO2 and HNO3 vapor. The simple two-band spectrum of OOCO− indicates that it is best described as the O2−⋅CO ion-molecule complex, whereas the covalently bound CO3− form yields a much more complicated vibrational spectrum with bands extending out to 4000 cm−1. In contrast, the NO3− ion yields a simple spectrum with only one transition as expected for the antisymmetric NO stretching fundamental of a species with D3h structure. The spectrum of the peroxynitrite isomer, OONO−, displays intermediate complexity that can be largely understood in the context of fundamentals associated with its cis and trans structures previously characterized in an Ar matrix.