FTIR adsorption studies of H2O and CH3OH in the isostructural H-SSZ-13 and H-SAPO-34: formation of H-bonded adducts and protonated clusters

The acidity of the isostructural H-SSZ-13 and H-SAPO-34 has been investigated by transmission FTIR spectroscopy using H2O and CH3OH as molecular probes. Interactions between the zeolitic samples and the probe molecules led to perturbations and proton transfers directly related to the acidity of the materials. The entire set of acidic sites in H-SSZ-13 interacts with H2O and CH3OH to give H-bonded adducts or protonated species. H3O+ is not formed in appreciable amounts upon H2O adsorption on H-SSZ-13, but at high coverages H2O generates clusters that have a proton affinity sufficiently high to abstract protons from the zeolite framework. Parallel experiments carried out for H-SAPO-34 showed that the H2O clusters abstract protons from Br?nsted sites only to a minor extent. Moving to CH3OH, even if it has a higher proton affinity than H2O and should expectingly experience an easier protonation, proton transfer is totally absent in H-SAPO-34 under our set of conditions. The clear evidence of methanol protonation in H-SSZ-13 definitely states the strong acidic character of this material. When irreversibly adsorbed CH3OH is present in H-SSZ-13, an appreciable amount of (CH3)2O is formed upon heating to 573 K. Compared to its SAPO analogue, the present set of data indisputably points to H-SSZ-13 as the strongest Br?nsted acidic material.     

A DFT study of the mechanism of Ni superoxide dismutase (NiSOD): role of the active site cysteine-6 residue in the oxidative half-reaction

In the present DFT study, the catalytic mechanism of H2O2 formation in the oxidative half-reaction of NiSOD, E-Ni(II) + O2- + 2H+ --> E-Ni(III) + H2O2, has been investigated. The main objective of this study is to investigate the source of two protons required in this half-reaction. The proposed mechanism consists of two steps: superoxide coordination and H2O2 formation. The effect of protonation of Cys6 and the proton donating roles of side chains (S) and backbones (B) of His1, Asp3, Cys6, and Tyr9 residues in these two steps have been studied in detail. For protonated Cys6, superoxide binding generates a Ni(III)-O2H species in a process that is exothermic by 17.4 kcal/mol (in protein environment using the continuum model). From the Ni(III)-O2H species, H2O2 formation occurs through a proton donation by His1 via Tyr9, which relative to the resting position of the enzyme is exothermic by 4.9 kcal/mol. In this pathway, a proton donating role of His1 residue is proposed. However, for unprotonated Cys6, a Ni(II)-O2- species is generated in a process that is exothermic by 11.3 kcal/mol. From the Ni(II)-O2- species, the only feasible pathway for H2O2 formation is through donation of protons by the Tyr9(S)-Asp3(S) pair. The results discussed in this study elucidate the role of the active site residues in the catalytic cycle and provide intricate details of the complex functioning of this enzyme.     

用密度泛函方法研究质子化苯基丙酮-水团簇

The structure and growth trend of the protonated acetophenone-water clusters have been investigated using the DFT-B3LYP method combined with the standard 6-31+G(d,p) basis set. In order to obtain more accurate single-point energy the B3LYP/6-311++G(3df,2p) method was adapted. The results show that the formation of H+C8H8O-H2O is a barrierless reaction process and the equilibrium distance between the proton and the O atom in C8H8O molecule is 1.015 A. For H+C8H8O-(H2O)n(n=1,2,3) clusters, the proton lies between the acetophenone molecule C8H8O and the water molecule H2O. The distance between the proton and the O atom of the C8H8O molecule increased from n=1 to n=3; C8H8O-H+-H2O can be regarded as an solvation shell. For H+C8H8O (H2O)n (n=4,5,6,7,8) clusters, the proton lies between the two H2O molecules forming a H5O2+ structure, C8H8O-H5O2+ is an important structure, which the other H2O molecules will attack from different sides.     

Nuclear magnetic resonance parameters for methane molecule trapped in clathrate hydrates

The calculations of the nuclear shielding and spin-spin coupling constants were carried out for two models of clathrate hydrates, 5(12) and 5(12)6(8), using the density functional theory three-parameter Becke-Lee-Yang-Parr method with the basis set aug-cc-pVDZ (optimization) and HuzIII-su3 (NMR parameters). Particular attention has been devoted to evaluate the influence of a geometrical arrangement, the effect of long-range interactions on the NMR shielding of methane molecule, and to predict whether (13)C and (1)H chemical shifts can distinguish between guests in two clathrate hydrates cages. The correlation of the changes in the (17)O shielding constants depend strongly on the hydrogen-bonding topology. The intermolecular hydrogen-bond transmitted (1h)J(OH) spin-spin coupling constants are substantial. The increase of their values is connected with the elongation of the intramolecular O-H bond and the shortening of the intermolecular O···H distance. These data suggests that hydrogen bonds between double donor-single acceptor (DDA)-type water molecules acting as a proton acceptor from single donor-double acceptor (DAA)-type water molecules are stronger than ones formed by DAA-type water molecules acting as an acceptor for a DDA water proton. These state-of-the-art calculations confirmed the earlier experimental findings of the cage-dependency of (13)C chemical shift of methane.     

Interactions of metal ions with water: ab initio molecular orbital studies of structure, vibrational frequencies, charge distributions, bonding enthalpies, and deprotonation enthalpies. 2. Monohydroxides

The formation and properties of a wide range of metal ion monohydroxides, M(n)(+)[OH(-)], where n = 1 and 2, have been studied by ab initio molecular orbital calculations at the MP2(FULL)/6-311++G**//MP2(FULL)/6-311++G** and CCSD(T)(FULL)/6-311++G**//MP2(FULL)/6-311++G** computational levels. The ions M(n)()(+) are from groups 1A, 2A, 3A, and 4A in the second, third, and fourth periods of the Periodic Table and from the first transition series. Geometrical parameters, vibrational frequencies, atomic charge distributions, orbital occupancies, and bonding enthalpies are reported. The M(n)(+)-O distances are shorter in the hydroxides than in the corresponding hydrates (published previously as Part 1, Inorg. Chem. 1998, 37, 4421-4431) due to a greater electrostatic interaction in the hydroxides. The natural bond orbitals for most of the first-row transition metal ion hydroxides do not contain a formal metal-oxygen bonding orbital; nevertheless the atomic charge distributions show that for both n = 1 and 2 a significant amount of electron density is consistently transferred from the hydroxide ion to the bound metal ion. Deprotonation enthalpies for the hydrates have been evaluated according to the simple dissociation process, M(n)(+)[OH(2)] --> M(n)(+)[OH(-)] + H(+), and also via proton transfer to another water molecule, M(n)(+)[OH(2)] + H(2)O --> M(n)(+)[OH(-)] + H(3)O(+). The drastic reduction in these deprotonation enthalpies as H(2)O molecules are sequentially bonded in the first coordination shell of the metal ion (amounting to 71, 64, 85, and 91 kcal/mol for the bonding of six water molecules to Mg(2+), Ca(2+), Mn(2+), and Zn(2+), respectively) is found to be due to the greater decrease in the bonding enthalpies for the hydroxides relative to the hydrates. Proton transfer to bases other than water, for example side chain groups of certain amino acids, could more than offset the decrease in deprotonation energy due to the filling of the first coordination shell. Linear relationships have been found between the pK(a) values for ionization of the Mg(2+), Ca(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) aquo ions, and Delta for the bonding of the first water molecule, for the bonding of the hydroxide ion, and for proton dissociation from the monohydrate. Similar relationships have also been found between the pK(a) values and the reciprocal of the M-O bond lengths in both the monohydrates and hydroxides. Thus the ionization of metal hydrates in water echoes the properties of the monomeric species M(n)(+)[OH(2)].     

Gas phase hydration and deprotonation of the cyclic C3H3+ cation. Solvation by acetonitrile, and comparison with the benzene radical cation

The binding energies of the first 5 H2O molecules to c-C3H3+ were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH+...X hydrogen bonds. The ion solvation with the more polar CH3CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C3H3+(H2O)4 cluster consists of a cyclic water tetramer interacting with the c-C3H3+ ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C3H3+ ion is deprotonated by 3 or more H2O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H2O)nH+ clusters. The kinetics of the associative proton transfer (APT) reaction C3H3+ + nH2O --> (H2O)nH+ + C3H2* exhibits an unusually steep negative temperature coefficient of k = cT(-63+/-4) (or activation energy of -37 +/- 1 kcal mol(-1)). The behavior of the C3H3+/water system is exactly analogous to the benzene+*/water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments.     

Photodissociation of diiodomethane in acetonitrile solution and fragment recombination into iso-diiodomethane studied with ab initio molecular dynamics simulations

Series of hydrates of the most stable glycine-H+/2H2+ in the gas phase are presented at the B3LYP level. The results show that only the amino hydrogens and hydroxyl hydrogens can be monohydrated for the glycine-H+, and the amino hydrogens are preferred. The H6(O4) of glycine-2H2+ is the best site for a water molecule to attach, i.e., the corresponding hydrate is the most stable one among its isomers. Calculations reveal that the binding energies of hydrated hydrogens decrease relative to their counterparts in the isolated glycine-H+/2H2+ complexes and they are positive values and without proton transfer except those of monohydrated glycine-2H2+ complexes with the combination modes of H3O+...(glycine-H+). The complex H3O+...(glycine-H+) is formed by the combination of a H2O molecule and one hydroxyl-site proton of glycine-2H2+, and with the proton transfer to H2O. Here the interaction between the proton of H3O+ and the glycine-H+ mainly depends on an electronic one instead of an initial covalent one of the isolated glycine-2H2+. The generation of the bond between the H3O+ and the glycine-H+ makes the energy of the complex higher than the energy sum of its two separated species (or two reactants of the complex), just like the case of M+...(glycine-H+) bond (M = Li,Na). The observation can explain satisfactorily why the combinations of both a proton and an alkali ion or two alkali ions to a glycine molecule can make the corresponding complex hold reservation energy bond(s), while the combination of two protons and a glycine in our previous work cannot [H. Ai et al., J. Chem. Phys. 117, 7593 (2002)]. For the glycine-2H2+, monohydration at the any site of its amino hydrogens can make the binding strength of any other neighboring proton (hydrogens) stronger relative to its counterpart in the isolated glycine-2H2+. Further hydration, especially at the site of either of hydroxyl hydrogens, would disfavor the reservation energy of the system.     

Is proton cationization promoted by polyatomic primary ion bombardment during time-of-flight secondary ion mass spectrometry analysis of frozen aqueous solutions?

Ion bombardment of pure water ice by Au+ monoatomic and Au3 + and C60 + polyatomic projectiles results in the emission of two series of water cluster ions-(H2O)n + and (H2O)nH+-with n ranging from 1 to >40. The cluster ion yields are very significantly higher under polyatomic ion bombardment than when using an Au+ primary ion. The yield of the protonated water species (H2O)nH+ is found to be enhanced by increasing ion fluence. C60 + bombardment results in a very dramatic increase in the (H2O)nH+ yield and decrease in the yield of (H2O)n +. Au3 + also significantly increased the yield of protonated species relative to the non-protonated but to a lesser extent than C60 +. Bombardment by Au+ also increased the yield of protonated species but to a very much smaller extent. The hypothesis that the protonated species may enhance the yield of [M+H]+ from solute molecules in solution has been investigated using two amino acids, alanine and arginine, and a nucleic base, adenine. The data suggest that the protons produced by the sputtering of water ice are depleted in the presence of these solutes and concurrently the yields of solute-related [M+H]+ and immonium secondary ions are greatly enhanced. These yield enhancements are analysed in the light of other possible contributors such as increased rates of sputtering under polyatomic beams and increased secondary ion yields as a consequence of solute dispersion. It is concluded that enhanced proton attachment is occurring in polyatomic sputtered frozen aqueous solutions.     

Apparent stoichiometry of water in proton hydration and proton dehydration reactions in CH3CN/H2O solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH(+) + n(H(2)O) = X + (H(2)O)(n)H(+) was studied for strong mineral acids (negatively charged X, X = ClO(4)ˉ, Clˉ, Brˉ, Iˉ, CF(3)SO(3)ˉ) and for strong cationic acids (uncharged X, X = R*NH(2), H(2)O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H(+)(H(2)O)(2), remain almost unchanged at large n values up to at least 10 M of water.     

Eigen versus Zundel complexes in HCl-water mixtures

There is an ongoing debate on the nature of hydration of the hydrogen ion, H+ in solution, and the extent to which Eigen or Zundel complexes occur. Here, our previous neutron diffraction data on a solution of 1:9 HCl in water are reanalyzed using a new starting hypothesis for the Monte Carlo simulation of the data. Either bare H+ ions, all H3O+ ions, or all H5O2 + ions are allowed in the simulation box together with the water and chlorine ions. All three simulations give a satisfactory fit to the experimental data. From the simulation with simple H+ ions, it is found that all H+ ions form one strong and very short hydrogen bond with water molecules and that on average 75% of them also engage in a second, slightly longer hydrogen bond. This result can be interpreted alternatively either in terms of the formation of a high percentage of asymmetric Zundel complexes or in terms of the formation of distorted H3O+ ions, which in turn form two or three hydrogen bonds, respectively, with neighboring molecules opposite their unbonded hydrogen sites (thus forming Eigen complexes). Therefore the new analysis is not inconsistent with our previous conclusion that the solution consists primarily of Eigen complexes, but does highlight the difficulty of making a clear distinction between Eigen and Zundel complexes due to the continuous random network of hydrogen bonds formed between water and hydrated protons. The role of hydrogen ion to chloride counterion contacts is also discussed in these solutions.     

Mechanistic study of proton transfer and HD exchange in ice films at low temperatures (100-140 K)

We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.     

Simulation of proton migration pathways in phenolsulfonic acid-based membranes by B3LYP/6-31G** DFT calculations

Geometrical and energetic characteristics of clusters simulating crystal hydrates of phenolsulfonic acid (PSA) and its complexes with poly(vinyl alcohol) (PVA) and the pathways of proton transport therein were determined using the DFT (B3LYP) theory with the 6-31G** basis set. In the formation of proton-conducting PVA—PSA-based membranes, it is energetically favorable to have at least one water molecule in close proximity of the SO₃H fragment. In water-free media, the proton migration along the SO₃H group is hindered by a barrier of 30–34 kcal mol⁻¹. In the presense of water, the proton conductivity follows the relay mechanism with the activation barrier of 5–8 kcal mol⁻¹, which is close to the experimetally observed barrier of 4–6 kcal mol⁻¹. Thus, the relay mechanism of proton transfer in a sulfonic acid—water complex is energetically the most favorable. The most energetically favorable isomer is the one with the PSA and PVA fragments H-bonded through a water molecule. The deficiency of water causes the PVA OH protons to be involved in hydrogen bonding as well. The role of PVA is to align the acid molecules and participate in the relay proton transfer. Introduction of an aldehyde into the membrane results in significant improvement of its physical properties. The aldehyde reacts with the hydroxyl groups of PVA. At high humidity, one may expect little effect of the degree of cross-linking on the proton mobility.     

HCl hydrates as model systems for protonated water

Ab initio molecular dynamics simulations are presented of vibrational dynamics and spectra of crystal HCl hydrates. Depending on the composition, the hydrates include distinct protonated water forms, which in their equilibrium structures approximate either the Eigen ion H3O+(H2O)3 (in the hexahydrate) or the Zundel H2O...H+...OH2 ion (in the di- and trihydrate). Thus, the hydrates offer the opportunity to study spectra and dynamics of distinct species of protonated water trapped in a semirigid solvating environment. The experimentally measured spectra are reproduced quite well by BLYP/DZVP-level calculations employing Fourier transform of the system dipole. The large overall width (800-1000 cm-1) of structured proton bands reflects a broad range of solvating environments generated by crystal vibrations. The aqueous HCl solution was also examined in search of an objective criterion for separating the contributions of "Zundel-like" and "Eigen-like" protonated forms. It is suggested that no such criterion exists since distributions of proton-related structural properties appear continuous and unimodal. Dipole derivatives with respect to OH and O...H+ stretches in water and protonated water were also investigated to advance the understanding of the corresponding IR intensities. The effects of H bonding and solvation on the intensities were analyzed with the help of the Wannier centers' representation of electron density.     

Effects of confinement on small water clusters structure and proton transport

Analyses of the structure of two to four water molecule clusters confined between two benzene and between two naphthalene molecules have been performed using ab initio methods. The water clusters tend to maximize the number of hydrogen bonds via formation of a cyclic network. The oxygen atoms locate approximately in the middle of the confined geometry, and the dipole vectors arrange either parallel or pointing to the surfaces. Energy barriers for proton transfer calculated for H3O+-(H2O) complexes in the same confined geometries suggest that there is a specific range of confinement that helps to lower the energy barriers of the proton transfer. When the walls are too close to each other, at a separation of 4 A, the energy barriers are extremely high. Confinement does not lower the barrier energies of proton transfer when the H3O+-(H2O) complexes are located further from each of the surfaces by more than approximately 8 A.     

Bis-Alkylureido Imidazole Artificial Water Channels

Nature creates aquaporins to effectively transport water, rejecting all ions including protons. Aquaporins (AQPs) has brought inspiration for the development of Artificial Water Channels (AWCs). Imidazole-quartet (I-quartet) was the first AWC that enabled to self-assemble a tubular backbone for rapid water and proton permeation with total ion rejection. Here, we report the discovery of bis-alkylureido imidazole compounds, which outperform the I-quartets by exhibiting ≈3 times higher net and single channel permeabilities (10⁷ H₂O/s/channel) and a ≈2–3 times lower proton conductance. The higher water conductance regime is associated to the high partition of more hydrophobic bis-alkylureido channels in the membrane and to their pore sizes, experiencing larger fluctuations, leading to an increase in the number of water molecules in the channel, with decreasing H-bonding connectivity. This new class of AWCs will open new pathways toward scalable membranes with enhanced water transport performances.     

氢键链质子接受能力对质子转移反应影响的理论研究

采用密度泛函B3LYP方法,6-311+g(d,p)基组,对甲酸与质子性溶剂分子形成的HCOOH-S_1-S_2(S_1和S_2分别为H_2O和NHF2)复合物在气相时发生的基态三质子转移反应过程进行了理论研究.4个甲酸复合物HCOOH-H_2O-H_2O,HCOOH-NHF2-NHF2,HCOOH-H_2O-NHF2及HCOOH-NHF2-H_2O中发生的三质子转移反应都是以异步协同质子迁移方式进行的.甲酸复合物中的氢键链组成和连接方式对基态三质子转移反应能垒有显著影响.HCOOH-S_1-S_2复合物中氢键链的质子接受能力可以表示为a×β1+b×β2(a+b=1).当a=0.45,b=0.55时,HCOOH-S_1-S_2中氢键链的质子接受能力和HCOOH-S_1-S_2复合物中的质子转移反应能垒成线性关系.氢键链的质子接收能力越强,反应能垒越低.     

Hydration of the bisulfate ion: atmospheric implications

Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO(4)(-)(H(2)O)(n=1-6) and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO(4)(-)/H(2)O system to the neutral H(2)SO(4)/H(2)O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO(4)(-)/H(2)O and H(2)SO(4)/H(2)O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO(4)(-)(H(2)O)(n=1-5) is favorable at 298.15 K, and that of HSO(4)(-)(H(2)O)(n=1-6) is favorable for T < 273.15 K. The HSO(4)(-) ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.     

Helical water chain mediated proton conductivity in homochiral metal-organic frameworks with unprecedented zeolitic unh-topology

Four new homochiral metal-organic framework (MOF) isomers, [Zn(l-L(Cl))(Cl)](H(2)O)(2) (1), [Zn(l-L(Br))(Br)](H(2)O)(2) (2), [Zn(d-L(Cl))(Cl)](H(2)O)(2) (3), and [Zn(d-L(Br))(Br)](H(2)O)(2) (4) [L = 3-methyl-2-(pyridin-4-ylmethylamino)butanoic acid], have been synthesized by using a derivative of L-/D-valine and Zn(CH(3)COO)(2)·2H(2)O. A three-periodic lattice with a parallel 1D helical channel was formed along the crystallographic c-axis. Molecular rearrangement results in an unprecedented zeolitic unh-topology in 1-4. In each case, two lattice water molecules (one H-bonded to halogen atoms) form a secondary helical continuous water chain inside the molecular helix. MOFs 1 and 2 shows different water adsorption properties and hence different water affinity. The arrangement of water molecules inside the channel was monitored by variable-temperature single-crystal X-ray diffraction, which indicated that MOF 1 has a higher water holding capacity than MOF 2. In MOF 1, water escapes at 80 °C, while in 2 the same happens at a much lower temperature (～40 °C). All the MOFs reported here shows reversible crystallization by readily reabsorbing moisture. In MOFs 1 and 2, the frameworks are stable after solvent removal, which is confirmed by a single-crystal to single-crystal transformation. MOFs 1 and 3 show high proton conductivity of 4.45 × 10(-5) and 4.42 × 10(-5) S cm(-1), respectively, while 2 and 4 shows zero proton conductivity. The above result is attributed to the fact that MOF 1 has a higher water holding capacity than MOF 2.     

Ultrafast proton transfer of three novel quinone cyanine photoacids

Steady-state and time-resolved emission techniques were used to study the photoprotolytic properties of three recently synthesized strong quinone cyanine photoacids (QCy7 and its sulfonated derivatives). The rate coefficient of the excited-state proton transfer (ESPT), k(PT), of the three dyes is roughly 1.5 × 10(12) s(-1), a high value that is comparable to the solvation dynamics rate of large polar organic molecules in H(2)O and D(2)O. It is twice as fast as the proton transfer rate between two adjacent water molecules in liquid water. We found that, as expected, two of the sulfonated photoacids geminately recombines with the proton at an elevated rate. The accelerated geminate recombination process of the sulfonated derivatives is different from a simple diffusion process of protons. The ESPT rate coefficient of these molecules is the highest recorded thus far.     

Clusters of hydrated methane sulfonic acid CH3SO3H.(H2O)n (n = 1-5): a theoretical study

Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H.(H2O)n (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA.(H2O)3, the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (>10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA.(H2O)5, the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA.(H2O)n clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.