IR+vacuum ultraviolet (118 nm) nonresonant ionization spectroscopy of methanol monomers and clusters: neutral cluster distribution and size-specific detection of the OH stretch vibrations

Small methanol clusters are formed by expanding a mixture of methanol vapor seeded in helium and are detected using vacuum UV (vuv) (118 nm) single-photon ionization/linear time-of-flight mass spectrometer (TOFMS). Protonated cluster ions, (CH3OH)(n-1)H+ (n=2-8), formed through intracluster ion-molecule reactions following ionization, essentially correlate to the neutral clusters, (CH3OH)n, in the present study using 118 nm light as the ionization source. Both experimental and Born-Haber calculational results clarify that not enough excess energy is released into protonated cluster ions to initiate further fragmentation in the time scale appropriate for linear TOFMS. Size-specific spectra for (CH3OH)n (n=4 to 8) clusters in the OH stretch fundamental region are recorded by IR+vuv (118 nm) nonresonant ion-dip spectroscopy through the detection chain of IR multiphoton predissociation and subsequent vuv single-photon ionization. The general structures and gross features of these cluster spectra are consistent with previous theoretical calculations. The lowest-energy peak contributed to each cluster spectrum is redshifted with increasing cluster size from n=4 to 8, and limits near approximately 3220 cm(-1) in the heptamer and octamer. Moreover, IR+vuv nonresonant ionization detected spectroscopy is employed to study the OH stretch first overtone of the methanol monomer. The rotational temperature of the clusters is estimated to be at least 50 K based on the simulation of the monomer rotational envelope under clustering conditions.     

Resonance Raman spectrum of the solvated electron in methanol: simulation within a cluster model

The microsolvation of the CH(3)OH(2) hypervalent radical in methanol clusters has been investigated by density functional theory. It is shown that the CH(3)OH(2) radical spontaneously decomposes within methanol clusters into protonated methanol and a localized solvated electron cloud. The geometric and electronic structures of these clusters as well as their vibrational frequencies have been characterized. Resonance Raman intensities, associated with the s --> p transition of the unpaired electron, have been estimated for CH(3)OH(2)M(n) (M = CH(3)OH, n = 1-3) clusters. It is shown that with increasing cluster size the simulated spectra converge toward the resonance Raman spectrum of the solvated electron in methanol measured recently by Tauber and Mathies (J. Am. Chem. Soc. 2004, 126, 3414). The results suggest that CH(3)OH(2)M(n) clusters are useful finite-size model systems for the computational investigation of the spectroscopic properties of the solvated electron in liquid methanol.     

Dynamics of electron solvation in I(-)(CH3OH)n clusters (4 ≤ n ≤ 11)

The dynamics of electron solvation following excitation of the charge-transfer-to-solvent precursor state in iodide-doped methanol clusters, I(-)(CH(3)OH)(n = 4-11), are studied with time-resolved photoelectron imaging. This excitation produces a I···(CH(3)OH)(n)(-) cluster that is unstable with respect to electron autodetachment and whose autodetachment lifetime increases monotonically from ~800 fs to 85 ps as n increases from 4 to 11. The vertical detachment energy (VDE) and width of the excited state feature in the photoelectron spectrum show complex time dependence during the lifetime of this state. The VDE decreases over the first 100-400 fs, then rises exponentially to a maximum with a ~1 ps time constant, and finally decreases by as much as 180 meV with timescales of 3-20 ps. The early dynamics are associated with electron transfer from the iodide to the methanol cluster, while the longer-time changes in VDE are attributed to solvent reordering, possibly in conjunction with ejection of neutral iodine from the cluster. Changes in the observed width of the spectrum largely follow those of the VDEs; the dynamics of both are attributed to the major rearrangement of the solvent cluster during relaxation. The relaxation dynamics are interpreted as a reorientation of at least one methanol molecule and the disruption and formation of the solvent network in order to accommodate the excess charge.     

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.     

Analysis of localization sites for an excess electron in neutral methanol clusters using approximate pseudopotential quantum-mechanical calculations

We have used a recently developed electron-methanol molecule pseudopotential in approximate quantum mechanical calculations to evaluate and statistically analyze the physical properties of an excess electron in the field of equilibrated neutral methanol clusters ((CH(3)OH)(n), n=50-500). The methanol clusters were generated in classical molecular dynamics simulations at nominal 100 and 200 K temperatures. Topological analysis of the neutral clusters indicates that methyl groups cover the surface of the clusters almost exclusively, while the associated hydroxyl groups point inside. Since the initial neutral clusters are lacking polarity on the surface and compact inside, the excess electron can barely attach to these structures. Nevertheless, most of the investigated cluster configurations do support weakly stabilized cluster anion states. We find that similarly to water clusters, the pre-existing instantaneous dipole moment of the neutral clusters binds the electron. The localizing electrons occupy diffuse, weakly bound surface states that largely engulf the cluster although their centers are located outside the cluster molecular frame. The initial localization of the excess electron is reflected in its larger radius compared to water due to the lack of free OH hydrogens on the cluster surface. The stabilization of the excess electron increases, while the radius decreases monotonically as the clusters grow in size. Stable, interior bound states of the excess electron are not observed to form neither in finite size methanol clusters nor in the equilibrium bulk.     

Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomers and clusters: new experimental results

We present new observations of the infrared (IR) spectrum of neutral methanol and neutral and protonated methanol clusters employing IR plus vacuum ultraviolet (vuv) spectroscopic techniques. The tunable IR light covers the energy ranges of 2500-4500 cm(-1) and 5000-7500 cm(-1). The CH and OH fundamental stretch modes, the OH overtone mode, and combination bands are identified in the vibrational spectrum of supersonic expansion cooled methanol (2500-7500 cm(-1)). Cluster size selected IR plus vuv nonresonant infrared ion-dip infrared spectra of neutral methanol clusters, (CH(3)OH)(n) (n=2,[ellipsis (horizontal)],8), demonstrate that the methanol dimer has free and bonded OH stretch features, while clusters larger than the dimer display only hydrogen bonded OH stretch features. CH stretch mode spectra do not change with cluster size. These results suggest that all clusters larger than the dimer have a cyclic structure with OH groups involved in hydrogen bonding. CH groups are apparently not part of this cyclic binding network. Studies of protonated methanol cluster ions (CH(3)OH)(n)H(+) n=1,[ellipsis (horizontal)],7 are performed by size selected vuv plus IR photodissociation spectroscopy in the OH and CH stretch regions. Energies of the free and hydrogen bonded OH stretches exhibit blueshifts with increasing n, and these two modes converge to approximately 3670 and 3400 cm(-1) at cluster size n=7, respectively.     

Electron solvation dynamics in I-(NH3)n clusters

Femtosecond photoelectron spectroscopy (FPES) is used to monitor the dynamics associated with the excitation of the charge-transfer-to-solvent (CTTS) precursor states in I-(NH3)n = 4-15 clusters. The FPE spectra imply that the weakly bound excess electron in the excited state undergoes partial solvation via solvent rearrangement on a time scale of 0.5-2 ps, and this partially solvated state decays by electron emission on a 10-50 ps time scale. Both the extent of solvation and the lifetimes increase gradually with cluster size, in contrast to the more abrupt size-dependent effects previously observed in I-(H2O)n clusters.     

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.     

Effects of excitation energy on the autodetachment lifetimes of small iodide-doped ROH clusters (R═H-, CH3-, CH3CH(2)-)

The effect of excitation energy on the lifetimes of the charge-transfer-to-solvent (CTTS) states of small (4 ≤ n ≤ 10) iodide-doped water and alcohol clusters was explored using femtosecond time-resolved photoelectron imaging. Excitation of the CTTS state at wavelengths ranging from 272 to 238 nm leads to the formation of the I···(ROH)(n)(-) (R═H-, CH(3)-, and CH(3)CH(2)-) species, which can be thought of as a vibrationally excited bare solvent cluster anion perturbed by an iodine atom. Autodetachment lifetimes for alcohol-containing clusters range from 1 to 71 ps, while water clusters survive for hundreds of ps in this size range. Autodetachment lifetimes were observed to decrease significantly with increasing excitation energy for a particular number and type of solvent molecules. The application of Klots' model for thermionic emission from clusters to I(-)(H(2)O)(5) and I(-)(CH(3)OH)(7) qualitatively reproduces experimental trends and reveals a high sensitivity to energy parametrization while remaining relatively insensitive to the number of vibrational modes. Experimental and computational results therefore suggest that the rate of electron emission is primarily determined by the energetics of the cluster system rather than by details of molecular structure.     

Vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters

In this work, we report on the vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH(+) (n = 1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH)n(H2O)H(+) (n = 2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH(+), (CH3OH)2(+), (CH3OH)nH(+) (n = 1-9), and (CH3OH)n(H2O)H(+) (n = 2-9) as a function of photon energy. With an increase in the water content in the molecular beam, there is an enhancement of photoionization intensity for the methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.     

Quantum-classical simulation of electron localization in negatively charged methanol clusters

A series of quantum molecular dynamics simulations have been performed to investigate the energetic, structural, dynamic, and spectroscopic properties of methanol cluster anions, [(CH(3)OH)(n)](-), (n = 50-500). Consistent with the inference from photo-electron imaging experiments, we find two main localization modes of the excess electron in equilibrated methanol clusters at ～200 K. The two different localization patterns have strikingly different physical properties, consistent with experimental observations, and are manifest in comparable cluster sizes to those observed. Smaller clusters (n ≤ 128) tend to localize the electron in very weakly bound, diffuse electronic states on the surface of the cluster, while in larger ones the electron is stabilized in solvent cavities, in compact interior-bound states. The interior states exhibit properties that largely resemble and smoothly extrapolate to those simulated for a solvated electron in bulk methanol. The surface electronic states of methanol cluster anions are significantly more weakly bound than the surface states of the anionic water clusters. The key source of the difference is the lack of stabilizing free hydroxyl groups on a relaxed methanol cluster surface. We also provide a mechanistic picture that illustrates the essential role of the interactions of the excess electron with the hydroxyl groups in the dynamic process of the transition of the electron from surface-bound states to interior-bound states.     

Structures of [(CO2)n(CH3OH)m](-) (n = 1-4, m = 1, 2) cluster anions

The infrared photodissociation spectra of [(CO 2) n (CH 3OH) m ] (-) ( n = 1-4, m = 1, 2) are measured in the 2700-3700 cm (-1) range. The observed spectra consist of an intense broad band characteristic of hydrogen-bonded OH stretching vibrations at approximately 3300 cm (-1) and congested vibrational bands around 2900 cm (-1). No photofragment signal is observed for [(CO 2) 1,2(CH 3OH) 1] (-) in the spectral range studied. Ab initio calculations are performed at the MP2/6-311++G** level to obtain structural information such as optimized structures, stabilization energies, and vibrational frequencies of [(CO 2) n (CH 3OH) m ] (-). Comparison between the experimental and the theoretical results reveals the structural properties of [(CO 2) n (CH 3OH) m ] (-): (1) the incorporated CH 3OH interacts directly with either CO 2 (-) or C 2O 4 (-) core by forming an O-HO linkage; (2) the introduction of CH 3OH promotes charge localization in the clusters via the hydrogen-bond formation, resulting in the predominance of CO 2 (-).(CH 3OH) m (CO 2) n-1 isomeric forms over C 2O 4 (-).(CH 3OH) m (CO 2) n-2 ; (3) the hydroxyl group of CH 3OH provides an additional solvation cite for neutral CO 2 molecules.     

Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined

Low-temperature fast-atom bombardment was found to be an excellent method for generating large protonated methanol clusters, (CH(3)OH)(n)H(+) (n = 2 to 15). Metastable dissociations of these clusters, involving elimination of one methanol molecule, were studied using mass-analyzed ion kinetic energy spectra (MIKES). From metastable peak profiles kinetic energy release (KER) distributions were obtained, even for clusters as large as (CH(3)OH)(15)H(+). The results were analyzed by a simple thermal model, by the finite heat bath theory (FHBT) and by the RRKM-based MassKinetics algorithm. The KER distribution was shown to correspond to a three-dimensional translational energy distribution, implying statistical energy partitioning in the transition state. The mean KER values and transition state temperatures were found to increase with cluster size, reaching 25 meV and approximately 210 K for large clusters (n = 10).     

Dehydrogenation of methanol by vanadium oxide and hydroxide cluster cations in the gas phase

Bare vanadium oxide and hydroxide cluster cations, V(m)O(n)+ and V(m)O(n-1) (OH)+ (m = 1-4, n = 1-10), generated by electrospray ionization, were investigated with respect to their reactivity toward methanol using mass spectrometric techniques. Several reaction channels were observed, such as abstraction of a hydrogen atom, a methyl radical, or a hydroxymethyl radical, elimination of methane, and adduct formation. Moreover, dehydrogenation of methanol to generate formaldehyde was found to occur via four different pathways. Formaldehyde was released as a free molecule either upon transfer of two hydrogen atoms to the cluster or upon transfer of an oxygen atom from the cluster to the neutral alcohol concomitant with elimination of water. Further, formaldehyde was attached to V(m)O(n)+ upon loss of H2 or neutral water to produce the cation V(m)O(n)(OCH(2))+ or V(m)O(n-1) (OCH(2))+, respectively. A reactivity screening revealed that only high-valent vanadium oxide clusters are reactive with respect to H2 uptake, oxygen transfer, and elimination of H2O, whereas smaller and low-valent cluster cations are capable of dehydrogenating methanol via elimination of H2. For comparison, the reactivity of methanol with the corresponding hydroxide cluster ions, V(m)O(n-1) (OH)+, was studied also, for which dominant pathways lead to both condensation and association products, i.e., generation of the ions V(m)O(n-1) (OCH(3))+ and V(m)O(n-1) (OH)(CH(3)OH)+, respectively.     

Reactions of Laser Ablation-magnesium Plasma with Methanol Clusters

IntroductionReactions of metal ions with neutral molecules orclusters produce a variety of metal complex ions andother new series of cluster ions including cations andanions.The laser ablation-molecular beam(LA-MB)method has marked its relevance in the st…     

Electronic and structural investigations of gold clusters doped with copper: Aun-1Cu- (n=13-19)

We have obtained the ground state and the equilibrium geometries of Au(n) (-) and Au(n-1)Cu(-) in the size range of n=13-19. We have used first principles density functional theory within plane wave and Gaussian basis set methods. For each of the cluster we have obtained at least 100 distinct isomers. The anions of gold clusters undergo two structural transformations, the first one from flat cage to hollow cage and the second one from hollow cage to pyramidal structure. The Cu doped clusters do not show any flat cage structures as the ground state. The copper doped systems evolve from a general 3D structure to hollow cage with Cu trapped inside the cage at n=16 and then to pyramidal structure at n=19. The introduction of copper atom enhances the binding energy per atom as compared to gold cluster anions.     

Cooperativity of hydrogen-bonded networks in 7-azaindole(CH3OH)n (n=2,3) clusters evidenced by IR-UV ion-dip spectroscopy and natural bond orbital analysis

IR-UV ion-dip spectra of the 7-azaindole (7AI)(CH(3)OH)(n) (n=1-3) clusters have been measured in the hydrogen-bonded NH and OH stretching regions to investigate the stable structures of 7AI(CH(3)OH)(n) (n=1-3) in the S(0) state and the cooperativity of the H-bonding interactions in the H-bonded networks. The comparison of the IR-UV ion-dip spectra with IR spectra obtained by quantum chemistry calculations shows that 7AI(CH(3)OH)(n) (n=1-3) have cyclic H-bonded structures, where the NH group and the heteroaromatic N atom of 7AI act as the proton donor and proton acceptor, respectively. The H-bonded OH stretch fundamental of 7AI(CH(3)OH)(2) is remarkably redshifted from the corresponding fundamental of (CH(3)OH)(2) by 286 cm(-1), which is an experimental manifestation of the cooperativity in H-bonding interaction. Similarly, two localized OH fundamentals of 7AI(CH(3)OH)(3) also exhibit large redshifts. The cooperativity of 7AI(CH(3)OH)(n) (n=2,3) is successfully explained by the donor-acceptor electron delocalization interactions between the lone-pair orbital in the proton acceptor and the antibonding orbital in the proton donor in natural bond orbital (NBO) analyses.     

Undissociated versus dissociated structures for water clusters and ammonia-water clusters: (H2O)n and NH3(H2O)n-1 (n = 5, 8, 9, 21). Theoretical study

To understand the autoionization of pure water and the solvation of ammonia in water, we investigated the undissociated and dissociated (ion-pair) structures of (H2O) n and NH3(H2O)n-1 (n = 5, 8, 9, 21) using density functional theory (DFT) and second order Moller-Plesset perturbation theory (MP2). The stability, thermodynamic properties, and infrared spectra were also studied. The dissociated (ion-pair) form of the clusters tends to favor the solvent-separated ion-pair of H3O+/NH4+ and OH-. As for the NH3(H2O)20 cluster, the undissociated structure has the internal conformation, in contrast to the surface conformation for the (H2O)21 cluster, whereas the dissociated structure of NH3(H2O)20 has the surface conformation. As the cluster size of (H2O)n/NH3(H2O)n-1 increases, the difference in standard free energy between undissociated and dissociated (ion-pair) clusters is asymptotically well corroborated with the experimental free energy change at infinite dilution of H3O+/NH4+ and OH-. The predicted NH and OH stretching frequencies of the undissociated and dissociated (ion-pair) clusters are discussed.     

Charge-transfer-to-solvent-driven dissolution dynamics of I- (H2O)2-5 upon excitation: excited-state ab initio molecular dynamics simulations

In contrast to the extensive theoretical investigation of the solvation phenomena, the dissolution phenomena have hardly been investigated theoretically. Upon the excitation of hydrated halides, which are important substances in atmospheric chemistry, an excess electron transfers from the anionic precursor (halide anion) to the solvent and is stabilized by the water cluster. This results in the dissociation of hydrated halides into halide radicals and electron-water clusters. Here we demonstrate the charge-transfer-to-solvent (CTTS)-driven femtosecond-scale dissolution dynamics for I-(H2O)n=2-5 clusters using excited state (ES) ab initio molecular dynamics (AIMD) simulations employing the complete-active-space self-consistent-field (CASSCF) method. This study shows that after the iodine radical is released from I-(H2O)n=2-5, a simple population decay is observed for small clusters (2 相似文献   

Infrared absorption of methanol clusters (CH3OH)n with n = 2-6 recorded with a time-of-flight mass spectrometer using infrared depletion and vacuum-ultraviolet ionization

We investigated IR spectra in the CH- and OH-stretching regions of size-selected methanol clusters, (CH(3)OH)(n) with n = 2-6, in a pulsed supersonic jet by using the IR-VUV (vacuum-ultraviolet) ionization technique. VUV emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation or excitation before ionization. The variations of intensity of protonated methanol cluster ions (CH(3)OH)(n)H(+) and CH(3)OH(+) and (CH(3)OH)(2)(+) were monitored as the IR laser light was tuned across the range 2650-3750 cm(-1). Careful processing of these action spectra based on photoionization efficiencies and the production and loss of each cluster due to photodissociation yielded IR spectra of the size-selected clusters. Spectra of methanol clusters in the OH region have been extensively investigated; our results are consistent with previous reports, except that the band near 3675 cm(-1) is identified as being associated with the proton acceptor of (CH(3)OH)(2). Spectra in the CH region are new. In the region 2800-3050 cm(-1), bands near 2845, 2956, and 3007 cm(-1) for CH(3)OH split into 2823, 2849, 2934, 2955, 2984, and 3006 cm(-1) for (CH(3)OH)(2) that correspond to proton donor and proton acceptor, indicating that the methanol dimer has a preferred open-chain structure. In contrast, for (CH(3)OH)(3), the splitting diminishes and the bands near 2837, 2954, and 2987 cm(-1) become narrower, indicating a preferred cyclic structure. Anharmonic vibrational wavenumbers predicted for the methanol open-chain dimer and the cyclic trimer with the B3LYP∕VPT2∕ANO1 level of theory are consistent with experimental results. For the tetramer and pentamer, the spectral pattern similar to that of the trimer but with greater widths was observed, indicating that the most stable structures are also cyclic.