Intracluster Proton Transfer Reaction Research Articles

Water Network Shape-Dependence of Local Interactions with the Microhydrated -NO2- and -CO2- Anionic Head Groups by Cold Ion Vibrational Spectroscopy.

We report the structural evolutions of water networks and solvatochromic response of the CH3NO2- radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CH3NO2-·(H2O)n=1-6 clusters. The OH stretching bands evolve with a surprisingly large discontinuity at n = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO2-·(H2O)n=5-7 (R = CD3CD2), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators. Electronic structure calculations trace this behavior to the formation of a "book" isomer of the water hexamer that adopts a configuration in which one of the water molecules resides in an acceptor-acceptor-donor (AAD) (A = acceptor, D = donor) H-bonding site. Excitation of the bound OH in the AAD site explores the local network topology best suited to stabilize an incipient -XO2H-OH-(H2O)2 intracluster proton-transfer reaction. These systems thus provide particularly clear examples where the network shape controls the potential energy landscape that governs water network-mediated, intracluster proton transfer. The CH stretching bands of the CH3NO2-·(H2O)n=1-6 clusters also exhibit strong solvatochromic shifts, but in this case, they smoothly blue-shift with increasing hydration with no discontinuity at n = 6. This behavior is analyzed in the context of the solute-ion polarizability response and partial charge transfer to the water networks.

Proton transfer from water to aromatic N-heterocyclic anions from DFT-MD simulations

The phenomenon of proton transfer from water to six N-heterocyclic anions and free energy landscapes of this process are studied using both electronic structure calculations and first principles molecular metadynamics simulations. Our investigation involves microhydrated and aqueous phase interaction of water with six aromatic heterocyclic anions relevant to chemistry and biology: imidazolide, pyrrolide, benzimidazolide, 2-cyanopyrrolide, indolide, and indazolide. The basic structures of all these heterocyclic anions differ by substituted functional groups as well as fused rings. We study the proton transfer reaction and the minimum number of required water molecules for the reaction in hydrated microclusters. We find out that at least four water molecules are necessary for hydrated clusters to facilitate the intracluster proton transfer reaction from water to anions except for pyrrolide, for which this magic number is 3. To obtain the reaction free energy and activation barrier of the proton transfer process in an aqueous solution, the metadynamics method based first principles molecular dynamics simulations were performed. The complete proton transfer was observed in aqueous solutions for all the anions. The water molecule directly involved in proton transfer becomes acidic due to the cooperative effect of neighboring water molecules. From the metadynamics simulation, we obtain the values of activation barrier for the proton transfer processes from neutral water to anions, and the highest activation barrier is obtained for benzimidazolide, whereas the lowest activation barrier is obtained for pyrrolide. The structures and free energy profiles of the process for all the anions are discussed, and a comparative outlook of the study is presented here.

Tracking the Structural Evolution of 4-Aminobenzoic Acid in the Transition from Solution to the Gas Phase.

Here, cryogenic ion mobility-mass spectrometry (cryo-IM-MS) is used to investigate intracluster proton transfer reactions of 4-aminobenzoic acid during the transition from solution to the gas phase. Previous studies have shown that protonation of the amine group of 4-aminobenzoic acid (4-ABAH+) is favored in solution (N-protomer), whereas protonation of the carboxylic acid group is favored in the gas phase (O-protomer). Results from cryo-IM-MS (80 K) studies of hydrated 4-ABAH+ ions, 4-ABAH+(H2O)n, are interpreted as evidence that the proton transfer reaction occurs through a water bridge at n = 6 connecting the -NH3+ and -COOH groups, that is, a Grotthuss mechanism. The weak binding energy of water molecules imposes limits for obtaining first-principles collisional cross sections (CCSs) of hydrated ions; consequently, candidate structures for 4-ABAH+(H2O)0-6 ions are derived by correlating experimental arrival-time distributions to theoretically determined CCSs. To our knowledge, these are the first first-principles determinations of CCS for hydrated ions. Apolar cosolvents, particularly acetonitrile, have been postulated to inhibit proton transfer by blocking the Grotthuss mechanism, but our data suggest that acetonitrile simply stabilizes the ammonium ion.

Size-Restricted Proton Transfer within Toluene-Methanol Cluster Ions

To understand the interaction between toluene and methanol, the chemical reactivity of [(C6H5CH3)(CH3OH) n=1-7](+) cluster ions has been investigated via tandem quadrupole mass spectrometry and through calculations. Collision Induced Dissociation (CID) experiments show that the dissociated intracluster proton transfer reaction from the toluene cation to methanol clusters, forming protonated methanol clusters, only occurs for n = 2-4. For n = 5-7, CID spectra reveal that these larger clusters have to sequentially lose methanol monomers until they reach n = 4 to initiate the deprotonation of the toluene cation. Metastable decay data indicate that for n = 3 and n = 4 (CH3OH)3H(+) is the preferred fragment ion. The calculational results reveal that both the gross proton affinity of the methanol subcluster and the structure of the cluster itself play an important role in driving this proton transfer reaction. When n = 3, the cooperative effect of the methanols in the subcluster provides the most important contribution to allow the intracluster proton transfer reaction to occur with little or no energy barrier. As n >or= 4, the methanol subcluster is able to form ring structures to stabilize the cluster structures so that direct proton transfer is not a favored process. The preferred reaction product, the (CH3OH)3H(+) cluster ion, indicates that this size-restricted reaction is driven by both the proton affinity and the enhanced stability of the resulting product.

Electronic spectroscopy of benzene–water cluster cations, [C 6H 6–(H 2O) n] + ( n = 1–4): spectroscopic evidence for phenyl radical formation through size-dependent intracluster proton transfer reactions

Electronic spectra of benzene–(water) n cluster cations ([Bz–(H 2O) n ] +, n = 1–4) in the visible and near ultraviolet regions were measured. The spectral feature of [Bz–(H 2O) 1–3] + is similar to that of bare Bz +, showing that the electronic structure of the benzene cation is preserved in the cluster cations up to n = 3. At n = 4, however, a remarkable decrease of the absorption intensity takes place, indicating an essential change of the electronic structure of the benzene moiety. This result is consistent with the size dependent intracluster proton transfer reaction which has been suggested by the previous infrared spectroscopic study.

Surface impact induced fragmentation and charging of neat and mixed clusters of SO 2 and H 2O

Cluster impact induced charge separation was studied with neat and mixed clusters of SO 2 and H 2O impinging at hyperthermal velocities on a SiO x target held at different temperatures. At 750 K, impact of SO 2 clusters produces positively and negatively charged cluster fragments of the form [(SO 2) n −K,Na] + and [(SO 2) m −SO 2] − which is explained in terms of the known cluster-impact induced pickup of alkali surface adsorbates. The high sensitivity of this channel promises applications for the analysis of alkali surface contaminations. The dominant fragments observed during the impact of binary SO 2/H 2O clusters at the same target temperature were [(H 2O) n −H] + and [(SO 2) p (H 2O) q −HSO 3] −, which indicates an intracluster proton transfer reaction. Similar fragments were also detected as minor spectral progressions during the surface impact of neat SO 2 clusters at a target temperature of 380 K. This is interpreted in terms of an impact-induced pickup of water surface adsorbates into the neat sulfur dioxide cluster during the surface collision. Upon impact of neat water clusters, fragment ions of the form [(H 2O) n −H] + and [(H 2O) q −OH] − are observed, which can be explained in terms of the autoprotolysis reaction in bulk water.

Multiphoton ionization and density functional studies of pyrimidine–(water)n clusters

The multiphoton ionization of pyrimidine–(water)n clusters at conditions of supersonic expansion is studied using a time-of-flight mass spectrometer at the wavelengths of 355 and 532 nm. At both wavelengths, a series of protonated C4H4N2–(H2O)nH+ cluster ions are obtained. The production of these protonated cluster ions requires an intracluster proton transfer reaction. The protonated products are also suggested to relate to the excitation or ionization of water molecules. Ab initio calculations show that the dissociation of C4H4N2–(H2O)n+ cluster ions prefer to produce protonated ions. Two ways of producing protonated cluster ions are discussed. The reaction mechanism of intracluster proton transfer and the geometric structures of pyrimidine–(water)n clusters were depicted. The proton pulling effect and the atomic charge changes in the protonated cluster ions are also discussed.

Multiphoton lonization and Ab lnitio Calculation of C<sub>4</sub>H<sub>5</sub>N-(NH<sub>3</sub>)<sub>n</sub> Hydrogen-bond Clusters

Multiphoton ionization of binary clusters (C4H5N) -NH3 at 355 and 532 nm have been investigated using a TOF mass spectrometer, The experimenal results showed that normal cluster ions and protonated cluster ions were produced at both laser wavelengths. The protonated products came from the intracluster proton transfer reaction. The existence of NH3 in the clusters increased the ionization cross sections of the clusters, The higher ionization efficiency at 355 nm results from the (2 + 1) resonance multiphoton ionization of the pyrrole molecule. A peculiar low abundance of (C4H5N)- (NH3)(2)(+) was observed, ab initio calculations indicated that when ammonia number n greater than or equal to 3, two hydrogen bonds between ammnia chain and pyrrole ring can be formed, and the cluster become more stable. The channel ratio for the formation of protonated and unprotonated products increases with the increase of n, in agreement with the trend of proton affinity of (C4H5N) - (NH3)(n),

Multiphoton Ionization and ab Initio Calculation Studies of the Hydrogen-Bonded Clusters C4H5N−(H2O)n

The multiphoton ionization of the hydrogen-bonded clusters C4H5N−(H2O)n was studied using a time-of-flight mass spectrometer at the laser wavelengths of 355 and 532 nm. At both wavelengths, a series of C4H5N−(H2O)n+ and the protonated C4H5N−(H2O)nH+ were obtained. The two-photon resonance ionization processes at 355 nm make this wavelength produce obviously more abundant ions of pyrrole and the clusters than 532 nm. Ab initio calculations show that in the protonated products, the proton prefers to link with the α-C of pyrrole rather than with the N atom. The production of the protonated products requires an intracluster proton transfer reaction. The protonated products obtained at 532 nm are suggested to arise from an intracluster penning ionization or a charge transfer process. The abnormally higher intensities of C4H4N−(H2O)n+ (n ≥ 1) than those of C4H4N+ are attributed to the stabilization effects of the cluster formation on the dissociation products C4H4N+ of the pyrrole molecule.

Proton transfer mechanism in the ionic methanol dimer

Intracluster proton transfer reaction in the methanol ionic dimer has been re-investigated using VUV ionization with synchrotron radiation. The energetic thresholds of the proton transfer issued from the methyl group or the hydroxyl group have been obtained. These results are discussed in view of previous calculations and experiments

Concerted Reactions of Charge Transfer and Covalent Bond Formation in Ionized Alkylbenzene−Isobutene Clusters. Copolymerization of Styrene−Isobutene and α-Methylstyrene−Isobutene Clusters

Mixed aromatic−isobutene clusters (Am.In, where A = toluene, p-xylene, mesitylene, 1,2,4-trimethylbenzene, styrene, and α-methylstyrene) have been ionized by resonant two-photon ionization in the vicinity of the aromatic's transition, and by two-photon ionization using 248 nm and 193 nm photons. Intracluster reactions leading to the formation of isobutene dimer cation C8H16•+ are observed following the two photon ionization of the binary clusters only when the aromatic precursor has an ionization potential greater than or equal to that of the isobutene dimer. The observation of this process in the gas phase and within clusters suggests that a similar initiation mechanism may take place in solution following the photoionization of an appropriate aromatic initiator. Evidence has been presented that points to the successive covalent additions of isobutene molecules on styrene and α-methylstyrene radical cations within the binary clusters. The intracluster reactions appear to yield two intermediate isomers. One isomer is consistent with an acyclic 1,4-radical cation which can initiate further polymerization via cationic or radical propagation depending on the nature of the available monomers in the cluster. The second isomer has probably a cyclic structure in which the ionic and the radical sites are interacting, and this gives rise to a stable product which manifests itself in the appearance of an enhanced ion intensity for the styrene−isobutene or the α-methylstyrene−isobutene radical cation. The effect of water on the cluster copolymerization has been examined. Intracluster proton transfer reactions within the (α-methylstyrene)(isobutene)(water)n species with n ≥ 3 producing protonated water clusters have been observed. The results are consistent with the distonic structure of the α-methylstyrene−isobutene radical cation which can initiate either cationic or radical propagation. These cluster studies present good model systems to examine the early stages of copolymerization and the reactivity ratios of different monomers.

Size Dependence of Intracluster Proton Transfer of Phenol−(H2O)n (n = 1−4) Cations

The intracluster proton transfer reaction occurring in the hydrogen-bonded cluster cations of phenol (C6H5OH) with water has been studied with trapped ion photodissociation (TIP) spectroscopy. Electronic spectra of [C6H5OH−(H2O)n]+ (n = 1−4) were observed in the region 380−400 nm, which reveals the chromophore of the cluster ions. It was found that the cluster ions of n ≤ 2 have the phenol ion (C6H5OH+) chromophore, while the n ≥ 3 cluster ions exhibit the phenoxy radical (C6H5O) chromophore, demonstrating that n = 3 is the critical size for intracluster proton transfer. Chromophore switching, occurring in the cluster ions of phenol with other solvent molecules as well as water, was discussed on the basis of the proton affinity of the solvent species. It was concluded that the intracluster proton transfer is a “push and pull” reaction determined only by the difference in the proton affinity between the phenoxy radical and the solvent molecules.

Observation of unprotonated ammonia cluster ions generated via multiphoton ionization mass spectrometry

We have studied ammonia cluster ions utilizing multiphoton ionization and a reflection time-of-flight mass spectrometer. The observed intensity distributions of {NH3}+n and {NH3}+n/{NH3}n−1H+ indicate that the formation of protonated ammonia cluster ions is due to a fast intracluster proton transfer reaction following ionization, similar to that which occurs in single photon ionization.

Photodissociation dynamics of water containing clusters. I. Kr⋅H2O+

The mass selected Kr⋅H2O+ cluster is photodissociated in the range 514 to 357 nm using lines from an argon ion laser. Product branching ratios are measured and shown to be a strong function of photon wavelength; Kr+/H2O products dominate at 357 nm (90%) but are equal in intensity to H2O+/Kr products at 514 nm. A small KrH+/OH product is observed at all wavelengths (∼5%), representing the first observation of a photoinduced, intracluster proton transfer reaction. The total cross section is estimated to be ∼2×10−19 cm2 at 514 nm. Laser polarization studies indicated the Kr+/H2O products come from direct accessing of a repulsive upper state (intracluster charge–transfer reaction). Both Kr+(2P3/2) and Kr+(2P1/2) spin–orbit states are formed, but their branching ratio is very strongly dependent on wavelength: 100% Kr+(2P3/2) at 514 nm, 100% Kr+(2P1/2) at 357 nm, and variable amounts of each in between. Analysis of the kinetic energy distribution of Kr+/H2O products indicates H2O is strongly rotationally excited (0.18 to 0.23 eV). This fact, coupled with analysis from an impulsive model for Kr+–H2O dissociation suggests the Kr atom is above (or below) the H2O+ plane in the Kr⋅H2O+ ground state, situated closer to the O end of the molecule. Further analysis of the Kr+/H2O kinetic energy distribution yields the binding energy D00(Kr–H2O+) =0.33± 0.1 eV. Polarization studies indicate H2O+/Kr products arise from a bound upper state. Phase space theory modeling of the kinetic energy distribution indicates the H2O+ product is formed with ∼1.3 eV internal energy. Two models are discussed, one that suggests H2O+(Ã 2A1) is formed and a second that suggests H2O+ is the chromophore, internally converts to vibrationally hot H2O+(X̃ 2B1) and slowly leaks vibrational energy to the c

Photoionization of water clusters at 11.83 eV: Observation of unprotonated cluster ions (H2O)+n (2≤n≤10)

Unprotonated cluster ions (H2O)+n (2≤n≤10) and (Ar)m(H2O)+n (1≤m≤3; 2≤n≤7) are detected in the mass spectra, for the first time, in addition to normally observed protonated ions (H2O)n−1H+ when supersonic cluster beams of water–argon mixtures (P≳2 atm) are photoionized with the vacuum-UV resonance lines at 11.83 and 11.62 eV. The protonated water cluster ions (H2O)n−1H+ are produced by ionization of neutral water clusters (H2O)n followed by intracluster proton-transfer reactions, whereas the unprotonated cluster ions (H2O)+n are generated via the photoionization of water–argon binary clusters: (Ar)m(H2O)n+hν →(H2O)+n+mAr+e−. The excess energies on ionization can be randomized within the [(Ar)m(H2O)+n]*vip clusters (‘‘intracluster excess energy dissipation’’) and finally converted to the decompositions of argon atoms, giving rise to the stable (H2O)+n and various (Ar)k(H2O)+n ions. The (H2O)+n ions have the distinct potential energy minima along the proton transfer reaction coordinates.