Vibrational predissociation spectroscopy of the (H2O)(6-21)- clusters in the OH stretching region: evolution of the excess electron-binding signature into the intermediate cluster size regime

We report vibrational predissociation spectra of the (H2O)n- cluster ions in the OH stretching region to determine whether the spectral signature of the electron-binding motif identified in the smaller clusters [Hammer et al. Science 306, 675 (2004)] continues to be important in the intermediate size regime (n = 7-21). This signature consists of a redshifted doublet that dominates the OH stretching region, and has been traced primarily to the excitation of a single water molecule residing in a double H-bond acceptor (AA) binding site, oriented with both of its H atoms pointing toward the excess electron cloud. Strong absorption near the characteristic AA doublet is found to persist in the spectra of the larger clusters, but the pattern evolves into a broadened triplet around n = 11. A single free OH feature associated with dangling hydrogen atoms on the cluster surface is observed to emerge for n > or = 15, in sharp contrast to the multiplet pattern of unbonded OH stretches displayed by the H+(H2O)n clusters throughout the n = 2-29 range. We also explore the vibration-electronic coupling associated with normal-mode displacements of the AA molecule that most strongly interact with the excess electron. Specifically, electronic structure calculations on the hexamer anion indicate that displacement along the -OH2 symmetric stretching mode dramatically distorts the excess electron cloud, thus accounting for the anomalously large oscillator strength of the AA water stretching vibrations. We also discuss these vibronic interactions in the context of a possible relaxation mechanism for the excited electronic states involving the excess electron.     

Identification of two distinct electron binding motifs in the anionic water clusters: a vibrational spectroscopic study of the (H2O)6- isomers

Photoelectron spectroscopy of the water cluster anions, (H2O)n-, has revealed that several isomeric forms are present for most sizes, and here, we use vibrational spectroscopy to address the structure of the (H2O)6- isomer that more weakly binds the extra electron. To overcome the severe line broadening that occurs in the OH stretching region of this isomer caused by fast electron autodetachment, we concentrate on the low-energy bending modes of the perdeutero isotopomer. Sharp spectroscopic signatures are recovered for two isomers using argon predissociation spectroscopy, and the resulting bands are heavily overlapped. To extract their independent contributions to the observed spectra, we exploit the substantial dependence of their relative populations on the number of attached argon atoms in the (D2O)6-.Ar(m) clusters, determined by photoelectron spectroscopy. The vibrational spectra of each isomer can then be isolated by spectral subtraction, which is implemented with a covariance mapping approach. The resulting band patterns establish that the more weakly binding isomer does not display the characteristic electron-binding motif common to the more strongly bound isomer class. Whereas the strongly binding isomer features a single water molecule pointing toward the excess electron cloud with both of its hydrogen atoms, the spectrum of the more weakly binding isomer suggests a structure where the electron is bound by a number of dangling OH groups corresponding to water molecules in acceptor-donor binding sites.     

Infrared spectrum and structural assignment of the water trimer anion

The bending vibrational spectrum of the perdeutero isotopomer of the water trimer anion has been measured and compared with spectra calculated using the MP2, CCSD, and Becke3LYP electronic structure methods. Due to its low electron binding energy (approximately 150 meV), only the OD bending region of the IR spectrum of (D2O)3(-) is accessible experimentally, with electron ejection dominating at higher photon energies. The calculated spectrum of the isomer having three water molecules arranged in a chain agrees best with the experimental spectrum. In the chain isomer, the excess electron is bound to the terminal water monomer with two dangling OH groups. This is consistent with the electron binding mechanism established previously for the (H2O)n(-) (n = 2, 4-6) anions.     

Excess electron and lithium atom solvation in water clusters at finite temperature: an ab initio molecular dynamics study of the structural, spectral, and dynamical behavior of (H2O)6- and Li(H2O)6

The roles of hydrogen bonds in the solvation of an excess electron and a lithium atom in water hexamer cluster at 150 K have been studied by means of ab initio molecular dynamics simulations. It is found that the hydrogen bonded structures of (H(2)O)(6)(-) and Li(H(2)O)(6) clusters are very different from each other and they dynamically evolve from one conformer to other along their simulation trajectories. The populations of the single acceptor, double acceptor, and free type water molecules are found to be significantly high unlike that in pure water clusters. Free hydrogens of these type of water molecules primarily capture the unbound electron density in these clusters. It is found that the binding motifs of the free electron evolve with time and the vertical detachment energy of (H(2)O)(6)(-) and vertical ionization energy of Li(H(2)O)(6) also change with time. Assignments of the observed peaks in vibrational power spectra are done, and we found direct correlations between the time-averaged population of water molecules in different hydrogen bonding states and the spectral features. The dynamical aspects of these clusters have also been studied through calculations of time correlations of instantaneous stretch frequencies of OH modes which are obtained from the simulation trajectories through a time series analysis.     

Exploring the correlation between network structure and electron binding energy in the (H(2)O)(7)(-) cluster through isomer-photoselected vibrational predissociation spectroscopy and ab initio calculations: addressing complexity beyond types I-III

We report a combined photoelectron and vibrational spectroscopy study of the (H(2)O)(7)(-) cluster anions in order to correlate structural changes with the observed differences in electron binding energies of the various isomers. Photoelectron spectra of the (H(2)O)(7)(-) . Ar(m) clusters are obtained over the range of m=0-10. These spectra reveal the formation of a new isomer (I') for m>5, the electron binding energy of which is about 0.15 eV higher than that of the type I form previously reported to be the highest binding energy species [Coe et al., J. Chem. Phys. 92, 3980 (1990)]. Isomer-selective vibrational predissociation spectra are obtained using both the Ar dependence of the isomer distribution and photochemical depopulation of the more weakly (electron) binding isomers. The likely structures of the isomers at play are identified with the aid of electronic structure calculations, and the electron binding energies, as well as harmonic vibrational spectra, are calculated for 28 low-lying forms for comparison with the experimental results. The HOH bending spectrum of the low binding type II form is dominated by a band that is moderately redshifted relative to the bending origin of the bare water molecule. Calculations trace this feature primarily to the bending vibration localized on a water molecule in which a dangling H atom points toward the electron cloud. Both higher binding forms (I and I') display the characteristic patterns in the bending and OH stretching regions signaling electron attachment primarily to a water molecule in an AA binding site, a persistent motif found in non-isomer-selective spectra of the clusters up to (H(2)O)(50)(-).     

Theoretical infrared spectrum of the ethanol hexamer

The hydrogen bond network of ethanol clusters is among the most complex hydrogen bond networks of molecular clusters. One of the reasons of its complexity arises from the number of possible ethanol monomers (there are three isoenergetic isomers of the ethanol monomer). This leads to difficulties in the exploration of potential energy surfaces (PESs) of ethanol clusters. In this work, we have explored the PES of the ethanol hexamer at the MP2/aug-cc-pVDZ level of theory. We have provided structures and their relative stability at 0 K and for temperatures ranging from 20 to 400 K in the gas phase. These structures are used to compute the theoretical infrared (IR) spectrum of the ethanol hexamer at the MP2/aug-cc-pVDZ level of theory. As a result, 98 different structures have been investigated, and six isomers are reported to be the most isoenergetically stable structures of the ethanol hexamer. These isomers are folded cyclic structures in which the stability is enhanced by the implication of CH⋯O interactions. Our investigations show that the PES of the ethanol hexamer is very flat, yielding several isoenergetic structures. Furthermore, we have noted that several isomers contribute to the population of the ethanol hexamer at high temperatures. As far as the IR spectroscopic study is concerned, we have found that the IR spectra of the most stable structures are in good agreement with the experiment. Considering this agreement, these structures are used to assign the experimental peaks in the CH-stretching region. We concluded that the stability of the structures of the ethanol hexamer is related both to OH⋯O hydrogen bonds and CH⋯O interactions. Overall, we have found that the IR spectrum of the ethanol hexamer, calculated from the contribution of all the possible stable structures weighted by their probability, excellently reproduce the experimental spectrum of the ethanol hexamer.     

Quantum Drude oscillator model for describing the interaction of excess electrons with water clusters: an application to (H2O)13(-)

Cluster anions for which the excess electron occupies an extended nonvalence orbital can be described by use of a model Hamiltonian employing quantum Drude oscillators to represent the polarizable charge distributions of the monomers. In this work, a Drude model for water cluster anions is described and used to investigate the (H2O)13(-) cluster. Several low-energy isomers are characterized, and the finite-temperature properties of the cluster are investigated by means of parallel tempering Monte Carlo simulations. Two structural motifs, one with double-acceptor water monomers and the other with four-membered rings of double-acceptor single-donor monomers with four free OH groups pointed in the same direction, are found to lead to large (approximately > eV) electron binding energies. The distributions of the computed vertical detachment energies qualitatively reproduce the experimentally measured photoelectron spectrum, and our simulations indicate that both of the main peaks in the measured spectrum derive from several isomers.     

Microscopic solvation of a lithium atom in water-ammonia mixed clusters: solvent coordination and electron localization in presence of a counterion

The microsolvation structures and energetics of water-ammonia mixed clusters containing a lithium atom, i.e., Li(H(2)O)(n)(NH(3)), n = 1-5, are investigated by means of ab initio theoretical calculations. Several structural aspects such as the solvent coordination to the metal ion and binding motifs of the free valence electron of the metal are investigated. We also study the energetics aspects such as the dependence of vertical ionization energies on the cluster size, and all these structural and energetics aspects are compared to the corresponding results of previously studied anionic water-ammonia clusters without a metal ion. It is found that the Li-O and Li-N interactions play a very important role in stabilizing the lithium-water-ammonia clusters, and the presence of these metal ion-solvent interactions also affect the characteristics of electron solvation in these clusters. This is seen from the spatial distribution of the singly occupied molecular orbital (SOMO) which holds the ejected valence electron of the Li atom. For very small clusters, SOMO electron density is found to exist mainly at the vicinity of the Li atom, whereas for larger clusters, it is distributed outside the first solvation shell. The free dangling hydrogens of water and ammonia molecules are involved in capturing the SOMO electron density. In some of the conformers, OH{e}HO and OH{e}HN types of interactions are found to be present. The presence of the metal ion at the center of the cluster ensures that the ejected electron is solvated at a surface state only, whereas both surface and interiorlike states were found for the free electron in the corresponding anionic clusters without a metal ion. The vertical ionization energies of the present clusters are found to be higher than the vertical detachment energies of the corresponding anionic clusters which signify a relatively stronger binding of the free electron in the presence of the positive metal counterion. The shifts in different vibrational frequencies are also calculated for the larger clusters, and the results are discussed for some of the selective modes of water and ammonia molecules that are directly influenced by the location and hydrogen bonding state of these molecules in the clusters.     

一种室温离子液体与水相互作用的密度泛函理论研究

采用量子化学计算中的密度泛函理论(DFT),在B3LYP/6-31+G(d,p)计算水平上研究了离子液体1-乙基-3-甲基咪唑四氟硼酸盐([Emim]BF4)及其与水分子形成的复合物的稳定构型和相互作用能;经振动频率分析得到了[Emim]BF4及其与水的复合物的红外光谱.计算结果表明,相对于水分子与阳离子的作用而言,水分子与阴离子的作用对离子液体结构的影响更大.与此同时,实验测得的[Emim]BF4的红外光谱与计算结果吻合.     

Density functional theory study of the structures and stabilities of CuO\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{-}$\end{document} and CuO3

Hybrid density functional theory calculations on the structures, vibrational frequencies, electron binding and dissociation energies, and bonding properties of CuO$_{3}^{-}$ and CuO₃ species have been carried out. Stable isomers containing an O₃ subunit and composed of O₂ bound to CuO have been located on the potential energy hypersurfaces of CuO$_{3}^{-}$ and CuO₃. The isomers formed by O₂ bonded to CuO in side‐on and end‐on coordination are more stable than those containing an O₃ subunit. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 162–168, 2001     

Ab initio investigation of the lower-energy candidate structures for (H<Subscript>2</Subscript>O)<Subscript>10</Subscript><Superscript>+</Superscript> water cluster

Low-lying structures of water cationic clusters and the compounds with the OH radical have become a hot topic in recent years. We here investigate the cluster \( {\left({\mathrm{H}}_2\mathrm{O}\right)}_{10}^{+} \) and calculate its ideal structures by the quantum chemical calculation together with the particle swarm optimization method. We analyzed the properties of the obtained lower-energy isomers of \( {\left({\mathrm{H}}_2\mathrm{O}\right)}_{10}^{+} \). Their energies are further re-optimized and demonstrated at three different methods with two basis sets. Based on our numerical calculations, a new cage-like structure of \( {\left({\mathrm{H}}_2\mathrm{O}\right)}_{10}^{+} \) with the lowest energy is obtained at MP2/aug-cc-pVDZ level. Our results showed the comparison of energy order at different conditions and demonstrated the influence of temperature on the relative Gibbs energy and IR spectra. Moreover, we also contained the molecule orbitals to discuss the stability of these representative isomers.     

Interfacial structures of acidic and basic aqueous solutions

Phase-sensitive sum-frequency vibrational spectroscopy was used to study water/vapor interfaces of HCl, HI, and NaOH solutions. The measured imaginary part of the surface spectral responses provided direct characterization of OH stretch vibrations and information about net polar orientations of water species contributing to different regions of the spectrum. We found clear evidence that hydronium ions prefer to emerge at interfaces. Their OH stretches contribute to the "ice-like" band in the spectrum. Their charges create a positive surface field that tends to reorient water molecules more loosely bonded to the topmost water layer with oxygen toward the interface, and thus enhances significantly the "liquid-like" band in the spectrum. Iodine ions in solution also like to appear at the interface and alter the positive surface field by forming a narrow double-charge layer with hydronium ions. In NaOH solution, the observed weak change of the "liquid-like" band and disappearance of the "ice-like" band in the spectrum indicates that OH(-) ions must also have excess at the interface. How they are incorporated in the interfacial water structure is, however, not clear.     

Electron binding motifs of (H2O)n- clusters

It is has been established that the excess electrons in small (i.e., n < or = 7) (H2O)n- clusters are bound in the dipole field of the neutral cluster and, thus, exist as surface states. However, the motifs for the binding of an excess electron to larger water clusters remain the subject of considerable debate. The prevailing view is that electrostatic interactions with the "free" OH bonds of the cluster dominate the binding of the excess electron in both small and large clusters. In the present study, a quantum Drude model is used to study selected (H2O)n- clusters in the n = 12-24 size range with the goal of elucidating different possible binding motifs. In addition to the known surface and cavity states, we identify a new binding motif, where the excess electron permeates the hydrogen-bonding network. It is found that electrostatic interactions dominate the binding of the excess electron only for isomers with large dipole moments, whereas in isomers without large dipole moments polarization and correlation effects dominate. Remarkably, for the network-permeating states, the excess electron binds even in the absence of electrostatic interactions.     

Product Vibrational State Distributions of F+CH3OH Reaction on Full-Dimensional Accurate Potential Energy Surface

The hydrogen abstraction reaction of methanol with fluorine atoms can produce HF and CH\begin{document}$ _3 $\end{document}O or CH\begin{document}$ _2 $\end{document}OH radicals, which are important in the environment, combustion, radiation, and interstellar chemistry. In this work, the dynamics of this typical reaction is investigated by the quasi-classical trajectory method based on a recently developed globally accurate full-dimensional potential energy surface. Particularly, the vibrational state distributions of the polyatomic products CH\begin{document}$ _3 $\end{document}O and CH\begin{document}$ _2 $\end{document}OH are determined by using the normal mode analysis method. It is found that CH\begin{document}$ _3 $\end{document}O and CH\begin{document}$ _2 $\end{document}OH are dominantly populated in the ground state when the reactants are at the ground ro-vibrational state. The OH stretching mode, torsional mode, H\begin{document}$ _2 $\end{document}CO out-of-plane bending mode and their combination bands in the CH\begin{document}$ _2 $\end{document}OH product can be effectively excited once the OH stretching mode of the reactant CH\begin{document}$ _3 $\end{document}OH is excited to the first vibrationally excited state. Most of the available energy flows into the HF vibrational energy and the translational energy in both channels, while the radical products, CH\begin{document}$ _3 $\end{document}O or CH\begin{document}$ _2 $\end{document}OH, receive a small amount of energy, consistent with experiment, which is an indication of its spectator nature.     

Crystallographic characterization of the structure of the endohedral fullerene [Er2@C82 isomer I] with C(s) cage symmetry and multiple sites for erbium along a band of ten contiguous hexagons

The structure of one of the three previously separated isomers of {Er2@C82} has been determined through a single-crystal X-ray structure determination of the noncovalent adduct, {Er2@C82 Isomer I}.{CoII(OEP)}.1.4(C6H6).0.3(CHCl3). The C82 cage is identified specificlly as the Cs(82:6) isomer (one of nine possible isolated pentagon isomers) from the crystallographic data. The carbon atoms of the C82 cage were individually identified and refined with only a constraint that required the two halves of the cage to possess similar bond lengths. Although the carbon cage is well ordered at 113 K, the erbium atoms are disordered. The electron density within the cage of {Er2@C82 Isomer I} has been modeled with two major sites with occupancies of 0.35 and 21 other individual erbium sites with occupancies ranging from 0.138 to 0.011. These erbium sites all reside near the walls of the fullerence and cluster near a band of ten contiguous hexagons that encircles the carbon cage. Since two other isomers of C82 (C3v(82:8) and C2v(82:9)) have a similar band of ten contiguous hexagons, it is tempting to speculate that the other two known isomers of {Er2@C82} have these cage structures.     

Comprehensive analysis of the hydrogen bond network morphology and OH stretching vibrations in protonated methanol-water mixed clusters, H(+)(MeOH)1(H2O)n (n = 1-8)

Density functional theory (DFT) calculations of protonated methanol-water mixed clusters, H (+)(MeOH) 1(H 2O) n ( n = 1-8), were extensively carried out to analyze the hydrogen bond structures of the clusters. Various structural isomers were energy optimized, and their relative energies with zero point energy corrections and temperature dependence of the free energies were examined. Coexistence of different morphological isomers was suggested. Infrared spectra were simulated on the basis of the optimized structures. The infrared spectra were also experimentally measured for n = 3-9 in the OH stretching vibrational region. The observed broad bands in the hydrogen-bonded OH stretch region were assigned in comparison with the simulations. From the DFT calculations, the preferential proton location was also investigated. Clear correlations between the excess proton location and the cluster morphology were found.     

A first principles molecular dynamics study of excess electron and lithium atom solvation in water-ammonia mixed clusters: structural, spectral, and dynamical behaviors of [(H2O)5NH3]- and Li(H2O)5NH3 at finite temperature

First principles molecular dynamics simulations are carried out to investigate the solvation of an excess electron and a lithium atom in mixed water-ammonia cluster (H(2)O)(5)NH(3) at a finite temperature of 150 K. Both [(H(2)O)(5)NH(3)](-) and Li(H(2)O)(5)NH(3) clusters are seen to display substantial hydrogen bond dynamics due to thermal motion leading to many different isomeric structures. Also, the structures of these two clusters are found to be very different from each other and also very different from the corresponding neutral cluster without any excess electron or the metal atom. Spontaneous ionization of Li atom occurs in the case of Li(H(2)O)(5)NH(3). The spatial distribution of the singly occupied molecular orbital shows where and how the excess (or free) electron is primarily localized in these clusters. The populations of single acceptor (A), double acceptor (AA), and free (NIL) type water and ammonia molecules are found to be significantly high. The dangling hydrogens of these type of water or ammonia molecules are found to primarily capture the free electron. It is also found that the free electron binding motifs evolve with time due to thermal fluctuations and the vertical detachment energy of [(H(2)O)(5)NH(3)](-) and vertical ionization energy of Li(H(2)O)(5)NH(3) also change with time along the simulation trajectories. Assignments of the observed peaks in the vibrational power spectra are done and we found a one to one correlation between the time-averaged populations of water and ammonia molecules at different H-bonding sites with the various peaks of power spectra. The frequency-time correlation functions of OH stretch vibrational frequencies of these clusters are also calculated and their decay profiles are analyzed in terms of the dynamics of hydrogen bonded and dangling OH modes. It is found that the hydrogen bond lifetimes in these clusters are almost five to six times longer than that of pure liquid water at room temperature.     

Structure and formation of gaseous [C4H5O]+ Ions. II–Ions of the type HCC?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}(OH)(CH3)

Loss of an alkyl group X? from acetylenic alcohols HC?C? CX(OH)(CH₃) and gas phase protonation of HC?C? CO? CH₃ are both shown to yield stable HC?C? \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}(OH)(CH₃) ions. Ions of this structure are unique among all other [C₄H₅O]⁺ isomers by having m/z 43 [C₂H₃O]⁺ as base peak in both the metastable ion and collisional activation spectra. It is concluded that the composite metastable peak for formation of m/z 43 corresponds to two distinct reaction profiles which lead to the same product ion, CH₃\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}?O, and neutral, HC?CH. It is further shown that the [C₄H₅O]⁺ ions from related alcohols (like HC?C? CH(OH)(CH₃)) which have an α-H atom available for isomerization into energy rich allenyl type molecular ions, consist of a second stable structure, H₂C?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}? C(OH)?CH₂.     

Dissociative electron attachment to the hydrogen-bound OH in water dimer through the lowest anionic Feshbach resonance

The lowest energy Feshbach resonance state of the water dimer anion is computationally studied as the hydrogen-bonded OH moiety is stretched from its equilibrium position toward the hydrogen bond acceptor. The purpose is to treat a simple model system to gain insight into how hydrogen bonding may affect dissociative electron attachment to water in condensed phases. In the case of a water monomer anion, the analogous potential surface is known to be repulsive, leading directly to dissociation of H(-). In contrast, in the dimer anion, a barrier is found to dissociation of the hydrogen-bonded OH moiety such that the migrating hydrogen can be held near the Franck-Condon region in a quasibound vibrational state for a time long compared to the OH vibrational period. This behavior is found both for the case of an icelike dimer structure and for a substantial majority of liquidlike dimer structures. These findings raise the possibility that due to effects of hydrogen bonding, a molecule-centered anionic entity that is metastable both to electron detachment and to bond dissociation may live long enough to be considered as a species in the radiolysis of condensed water phases.     

Products and mechanism for the OH radical initiated oxidation of acrylonitrile,methacrylonitrile, and allylcyanide in the presence of NO

The photooxidation of acrylonitrile, methacylonitrile, and allylcyanide in the presence of NO was studied in parts per million concentration using the long-path Fourier transform IR spectroscopic method. The stoichiometry of the OH radical initiated oxidation of methacrylonitrile was established as \documentclass{article}\pagestyle{empty}\begin{document}$ \left( {{\rm OH}} \right) + {\rm CH}_{\rm 2} = {\rm C}\left( {{\rm CH}_{\rm 3} } \right){\rm CN + 2NO + 2O}_{\rm 2} \mathop {\hbox to 20pt{\rightarrowfill}}\limits^{1.0} {\rm HCHO + CH}_{\rm 3} {\rm COCN + 2NO}_{{\rm 2}} + \left( {{\rm OH}} \right) $\end{document}. The yield of HCHO for acrylonitrile and allylcyanide was found to be ca. 100 and 80%, and the stoichiometric reactions were assessed to proceed, \documentclass{article}\pagestyle{empty}\begin{document}$ \left( {{\rm OH}} \right) + {\rm CH}_{\rm 2} = {\rm CHCN + 2NO + 2O}_{\rm 2} \mathop {\hbox to 20pt{\rightarrowfill}}\limits^{1.0} {\rm HCHO + HCOCN + 2NO}_{\rm 2} + \left( {{\rm OH}} \right) $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \left( {{\rm OH}} \right) + {\rm CH}_{\rm 2} = {\rm CHCH}_{\rm 2} {\rm CN + 2NO + 2O}_{\rm 2} \mathop {\hbox to 20pt{\rightarrowfill}}\limits^{0.8} {\rm HCHO + HCOCH}{\rm 2} {\rm CN + 2NO}_{\rm 2} + \left( {{\rm OH}} \right) $\end{document}, respectively. These results revealed that the reaction mechanism for these unsaturated organic cyanides are analogous to that of olefins.