Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)n (n = 3, 4) and mixed hydrogen fluoride/water (HF)n(H2O)n (n = 1, 2, 4) clusters

Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n, with n = 3, 4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n = 1, 2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the M?ller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.     

Vibrational predissociation spectrum of the carbamate radical anion, C(5)H(5)N-CO(2)(-), generated by reaction of pyridine with (CO(2))(m)(-)

We report the vibrational predissociation spectrum of C(5)H(5)N-CO(2)(-), a radical anion which is closely related to the key intermediates postulated to control activation of CO(2) in photoelectrocatalysis with pyridine (Py). The anion is prepared by the reaction of Py vapor with (CO(2))(m)(-) clusters carried out in an ionized, supersonic entrainment ion source. Comparison with the results of harmonic frequency calculations establishes that this species is a covalently bound molecular anion derived from the corresponding carbamate, C(5)H(5)N-CO(2)(-) (H(+)). These results confirm the structural assignment inferred in an earlier analysis of the cluster distributions and photoelectron spectra of the mixed Py(m)(CO(2))(n)(-) complexes [J. Chem. Phys. 2000, 113 (2), 596-601]. The spectra of the (CO(2))(m)(-) (m = 5 and 7) clusters are presented for the first time in the lower energy range (1000-2400 cm(-1)), which reveal several of the fundamental modes that had only been characterized previously by their overtones and combination bands. Comparison of these new spectra with those displayed by Py(CO(2))(n)(-) suggests that a small fraction of the Py(CO(2))(n)(-) ions are trapped entrance channel reaction intermediates in which the charge remains localized on the (CO(2))(m)(-) part of the cluster.     

Theoretical studies on photoelectron and IR spectral properties of Br2.-(H2O)n clusters

We report vertical detachment energy (VDE) and IR spectra of Br2.-.(H2O)n clusters (n=1-8) based on first principles electronic structure calculations. Cluster structures and IR spectra are calculated at Becke's half-and-half hybrid exchange-correlation functional (BHHLYP) with a triple split valence basis function, 6-311++G(d,p). VDE for the hydrated clusters is calculated based on second order Moller-Plesset perturbation (MP2) theory with the same set of basis function. On full geometry optimization, it is observed that conformers having interwater hydrogen bonding among solvent water molecules are more stable than the structures having double or single hydrogen bonded structures between the anionic solute, Br2.-, and solvent water molecules. Moreover, a conformer having cyclic interwater hydrogen bonded network is predicted to be more stable for each size hydrated cluster. It is also noticed that up to four solvent H2O units can reside around the solute in a cyclic interwater hydrogen bonded network. The excess electron in these hydrated clusters is localized over the solute atoms. Weighted average VDE is calculated for each size (n) cluster based on statistical population of the conformers at 150 K. A linear relationship is obtained for VDE versus (n+3)(-1/3) and bulk VDE of Br2.- aqueous solution is calculated as 10.01 eV at MP2 level of theory. BHHLYP density functional is seen to make a systematic overestimation in VDE values by approximately 0.5 eV compared to MP2 data in all the hydrated clusters. It is observed that hydration increases VDE of bromine dimer anion system by approximately 6.4 eV. Calculated IR spectra show that the formation of Br2.--water clusters induces large shifts from the normal O-H stretching bands of isolated water keeping bending modes rather insensitive. Hydrated clusters, Br2.-.(H2O)n, show characteristic sharp features of O-H stretching bands of water in the small size clusters.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.     

Low-temperature SIMS mass spectra of diethyl ether

For the first time a secondary ion mass spectrum of diethyl ether was obtained at low temperature. The spectrum recording became possible by carefully selecting the range of experimental conditions for the production of a cluster-type spectrum. This range is specified by the threshold for spectrum appearance above the melting temperature of the frozen sample and a fairly short time span of existence of the liquid estimated as only a few minutes. The latter necessitates rather rapid spectrum detection. In practice, about 1 min was available for recording of the cluster-type spectra. The secondary emission mass spectrum of diethyl ether appeared to be rich in peaks: along with abundant protonated clusters M(n).H(+) (n = 1-12), unusually intense [M(n) - H](+) and weaker M(n) (+.) peaks were present accompanied by several sets of fragmented clusters, [M(n) - 15](+), [M(n) - 29](+), [M(n) - 27](+), [M(n) - 45](+), and monohydrates, M(n).H(2)O.H(+). The analysis of all the peaks showed that the pattern of fragment clusters is qualitatively similar to the pattern of fragmentation of the diethyl ether molecular ion under high-energy electron impact. The general features of the behaviour of diethyl ether under low-temperature mass spectrometric conditions were similar to those observed earlier for some other organic solvents.     

Experimental and theoretical studies on the complexes of [Pb(m)-pyridyl]- (m = 1-4)

The pyridyl-lead complexes [Pb(m)-C5H4N](-) (m = 1-4), which are produced from the reactions between lead clusters formed by laser ablation and the pyridine molecules seeded in argon carrier gas, are studied by photoelectron (PE) spectra and density functional theory. The adiabatic electron affinity (EA) of [Pb(m)C5H4N](-) is obtained from PE spectra at photon energies of 308 and 193 nm. Theoretical calculation is carried out to elucidate their structures and bonding modes. A variety of geometries for the isomers are optimized to search for the lowest-energy geometry. By comparing the theoretical results, including the EA and simulated density of state spectra, with the experimental determination, the lowest-energy structures for each species are obtained. The following analysis of the molecular orbital composition provides the evidence that the pyridyl binds on lead clusters through the Pb-C sigma bond. Moreover, there is an apparent spin-state transition from triplet state toward singlet state for the ground-state structure of [Pb(m)C5H4N](-) with an increase of lead cluster.     

Microsolvation of the dicyanamide anion: [N(CN)(2)(-)](H(2)O)n (n = 0-12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.     

Ab initio potential and dipole moment surfaces for water. II. Local-monomer calculations of the infrared spectra of water clusters

We employ recent flexible ab initio potential energy and dipole surfaces [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)] to the calculation of IR spectra of the intramolecular modes of water clusters. We use a quantum approach that begins with a partitioned normal-mode analysis of perturbed monomers, and then obtains solutions of the corresponding Schro?dinger equations for the fully coupled intramolecular modes of each perturbed monomer. For water clusters, these modes are the two stretches and the bend. This approach is tested against benchmark calculations for the water dimer and trimer and then applied to the water clusters (H(2)O)(n) for n = 6-10 and n = 20. Comparisons of the spectra are made with previous ab initio harmonic and empirical potential calculations and available experiments.     

Eigen and Zundel forms of small protonated water clusters: structures and infrared spectra

The spectral properties of protonated water clusters, especially the difference between Eigen (H3O+) and Zundel (H5O2+) conformers and the difference between their unhydrated and dominant hydrated forms are investigated with the first principles molecular dynamics simulations as well as with the high level ab initio calculations. The vibrational modes of the excess proton in H3O+ are sensitive to the hydration, while those in H5O2+ are sensitive to the messenger atom such as Ar (which was assumed to be weakly bound to the water cluster during acquisitions of experimental spectra). The spectral feature around approximately 2700 cm-1 (experimental value: 2665 cm-1) for the Eigen moiety appears when H3O+ is hydrated. This feature corresponds to the hydrating water interacting with H3O+, so it cannot appear in the Eigen core. Thus, H3O+ alone would be somewhat different from the Eigen forms in water. For the Zundel form (in particular, H5O2+), there have been some differences in spectral features among different experiments as well as between experiments and theory. When an Ar messenger atom is introduced at a specific temperature corresponding to the experimental condition, the calculated vibrational spectra for H5O2+.Ar are in good agreement with the experimental infrared spectra showing the characteristic Zundel frequency at approximately 1770 cm-1. Thus, the effect of hydration, messenger atom Ar, and temperature are crucial to elucidating the nature of vibrational spectra of Eigen and Zundel forms and to assigning the vibrational modes of small protonated water clusters.     

Dissociation chemistry of hydrogen halides in water

To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and M?ller-Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H(2)O)(n) and X(-)(H(3)O)(+)(H(2)O)(n-1) are calculated. The dissociated structures of HF and HCl can be formed for n>/=4, while those of HBr and HI can be formed for n>/=3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for n>/=4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X-H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules.     

[Co(C~5H~5)~2]~n.[M(dmit)~2](M=Ni,Pd;n=0,1,2)型配合物的合成及表征

二茂金属[M'(C~5H~5)~2]^1^+的盐与(NBu~4)~n[M(dmit)~2](M=Ni, Pd; N=1,2)反应, 当M'=Fe, Ni; n=1时, 分别得到了导电配合物[Ni(dmit)~2]和[Pd(dmit)~2]; 当M'=Co, n=1,2时, 分别得到的是电荷几乎不转移的4个盐[Co(C~5H~5)~2]~n[Ni(dmit)~2]和[Co(C~5H~5)~2]~n[Pd(dmit)~2]。用ESCA、Raman谱及循环伏安图讨论了上述化合物形成时的电荷转移量。尽管[M(dmit)~2]的室温电导率相当大, 但其电导率随温度的变化曲线表明它们属于半导体。EHCO能带计算给出[Ni(dmit)~2]的能隙0.112eV, 与实测的电导活化能相当接近。     

CuCl3]- and [CuCl4]2- hydrates in concentrated aqueous solution: a density functional theory and ab initio study

In this work, structures and thermodynamic properties of [CuCl(3)](-) and [CuCl(4)](2-) hydrates in aqueous solution were investigated using density functional theory and ab initio methods. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) structures were both taken into account. Our calculations suggest that [CuCl(3)(H(2)O)(n)](-) clusters might favor a four-coordinated CIP structure with a water molecule coordinating with the copper atom in the equatorial position for n = 3 and 4 in aqueous solution, whereas the four-coordinated SSIP structure with one chloride atom dissociated becomes more stable as n increases to 5. For the [CuCl(4)](2-) cluster, the four-coordinated tetrahedron structure is more stable than the square-planar one, whereas for [CuCl(4)(H(2)O)(n)](2-) (n ≥ 1) clusters, it seems that four-coordinated SSIP structures are slightly more favorable than CIP structures. Our calculations suggest that Cu(2+) perhaps prefers a coordination number of 4 in CuCl(2) aqueous solution with high Cl(-) concentrations. In addition, natural bond orbital (NBO) calculations suggest that there is obvious charge transfer (CT) between copper and chloride atoms in [CuCl(x)](2-x) (x = 1-4) clusters. However, compared with that in the [CuCl(2)](0) cluster, the CT between the copper and chloride atoms in [CuCl(3)](-) and [CuCl(4)](2-) clusters becomes negligible as the number of attached redundant Cl(-) ions increases. This implies that the coordination ability of Cl(-) is greatly weakened for [CuCl(3)](-) and [CuCl(4)](2-) clusters. Electronic absorption spectra of these different hydrates were obtained using long-range-corrected time-dependent density functional theory. The calculated electronic transition bands of the four-coordinated CIP conformer of [CuCl(3)(H(2)O)(n)](-) for n = 3 and 4 are coincident with the absorption of [CuCl(3)](-)(aq) species (～284 and 384 nm) resolved from UV spectra obtained in CuCl(2) (ca. 10(-4) mol·kg(-1)) + LiCl (>10 mol·kg(-1)) solutions, whereas the calculated bands of [CuCl(3)(H(2)O)(n)](-) in their most stable configurations are not when n = 0 - 2 or n > 4, which means that the species [CuCl(3)](-)(aq) exists in those CuCl(2) aqueous solutions in which the water activity is neither too low nor too high. The calculated bands of [CuCl(4)(H(2)O)(n)](2-) clusters correspond to the absorption spectra (～270 and 370 nm) derived from UV measurements only when n = 0, which suggests that [CuCl(4)](2-)(aq) species probably exist in environments in which the water activity is quite low.     

Microhydration shell structure in Cl2*-.nH2O clusters: A theoretical study

We present the results of a detailed study on structure and electronic properties of hydrated cluster Cl2*-.nH2O (n = 1-7) based on a nonlocal density functional, namely, Becke's [J. Chem. Phys. 98, 1372 (1993)] half and half hybrid exchange-correlation functional with a split valence 6-311++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with various possible initial guess structures without any symmetry restriction. Several minimum energy structures (conformers) are predicted with a small difference in total energy. There is a competition between the binding of solvent H2O units with Cl2*- dimer radical anion directly through ion-molecule interaction and forming interwater hydrogen-bonding network in Cl2*-.nH2O (n > or = 2) hydrated cluster. Structure having interwater H-bonded network is more stable over the structure where H2O units are connected to the solute dimer radical anion Cl2*- rather independently either by single or double H bonding in a particular size (n) of hydrated cluster Cl2*-.nH2O. At the maximum four solvent H2O units reside in interwater H-bonding network present in these hydrated clusters. It is observed that up to six H2O units are independently linked to the anion having four double H bondings and two single H bondings suggesting the primary hydration number of Cl2*- to be 6. In all these clusters, the odd electron is found to be mostly localized over the two Cl atoms and these two atoms are bound by a three-electron hemibond. Calculated interaction (between solute and different water clusters) and vertical detachment energy profiles show saturation at n = 6 in the hydrated cluster Cl2*-.nH2O (n = 1-7). However, calculated solvation energy increases with the increase in number of solvent H2O molecules in the cluster. Interaction energy varies linearly with vertical detachment energy for the hydrated clusters Cl2*-.nH2O (n < or = 6). Calculation of the vibration frequencies show that the formation of Cl2*(-)-water clusters induces significant shifts from the normal stretching modes of isolated water. A clear difference in the pattern of IR spectra is observed in the O-H stretching region of water from hexa- to heptahydrated cluster.     

Vibrations and thermodynamics of clusters of polycyclic aromatic hydrocarbon molecules: the role of internal modes

The vibrational spectra of clusters of coronene molecules are theoretically calculated using a mixed quantum/classical scheme, each molecule being described by a tight-binding Hamiltonian, the intermolecular forces being provided by explicit Lennard-Jones and point charge sites. The normal modes of vibrations are shown to exhibit significant variations upon clustering. In particular, for large clusters intra- and intermolecular modes tend to mix and fill the mid-infrared range. We also calculate the heat capacity of the (C24H12)8 cluster as a function of temperature, emphasizing the isomerizations that take place during melting. Quantum delocalization effects, as obtained from the Pitzer-Gwinn semiclassical approximation, are important enough to wash out all signatures of the structural transitions on the caloric curve. On the basis of a simple two-state model we estimate that clusters containing about 300 molecules are required for melting to be detected on the caloric curve.     

Evolution of the electronic properties of Snn- clusters (n=4-45) and the semiconductor-to-metal transition

The electronic structure of Sn(n) (-) clusters (n=4-45) was examined using photoelectron spectroscopy at photon energies of 6.424 eV (193 nm) and 4.661 eV (266 nm) to probe the semiconductor-to-metal transition. Well resolved photoelectron spectra were obtained for small Sn(n) (-) clusters (n< or =25), whereas more congested spectra were observed with increasing cluster size. A distinct energy gap was observed in the photoelectron spectra of Sn(n) (-) clusters with n< or =41, suggesting the semiconductor nature of small neutral tin clusters. For Sn(n) (-) clusters with n> or =42, the photoelectron spectra became continuous and no well-defined energy gap was observed, indicating the onset of metallic behavior for the large Sn(n) clusters. The photoelectron spectra thus revealed a distinct semiconductor-to-metal transition for Sn(n) clusters at n=42. The spectra of small Sn(n) (-) clusters (n< or =13) were also compared with those of the corresponding Si(n) (-) and Ge(n) (-) clusters, and similarities were found between the spectra of Sn(n) (-) and those of Ge(n) (-) in this size range, except for Sn(12) (-), which led to the discovery of stannaspherene (the icosahedral Sn(12) (2-)) previously [L. F. Cui et al., J. Am. Chem. Soc. 128, 8391 (2006)].     

Cluster-based copper(II) coordination polymers with azido bridges and chiral magnets

Three homochiral layered complexes, [Cu3(R-chea)2(N3)6]n (1), [Cu3(S-chea)2(N3)6]n (2) (chea = 1-cyclohexylethylamine) and [Cu3(S-phpa)2(N3)6]n (3) (phpa = 1-phenylpropylamine), and three novel cluster-based coordination polymers, [Cu6(1,2-pn)4(N3)12]n (4) (1,2-pn = 1,2-diaminopropane), [[Cu8(en)4(N3)16] x H2O]n (5) (en = ethylenediamine) and [Cu6(N-Ipren)2(N3)12]n (6) (N-Ipren = N-isopropylethylenediamine), have been synthesized by the self-assembly reactions of Cu(NO3)2 x 3H2O, NaN3 and small organic amine ligands. Their crystal structures are determined by single-crystal X-ray diffraction. Complexes are composed of neutral 2D brick wall networks with only end-on azido bridges. Complexes and are 3D coordination polymers featuring copper-azido clusters and [Cu(diamine)2]2+ units which are linked by the azido bridges. Complex is a 3D coordination framework based on the hexanuclear copper(II) clusters [Cu6(N3)12(N-Ipren)2]. Magnetic studies show that complexes are interesting chiral ferromagnets with the magnetic transition temperature at ca. 5.0 K. Complexes and show ferromagnetic coupling in the copper-azido cluster units and antiferromagnetic interaction between neighboring units, while complex shows ferromagnetic ordering at 3.2 K.     

Structure and stability of (TiO2)n, (SiO2)n, and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] clusters

Structures, energetics, and vibrational spectra are investigated for small pure (TiO(2))(n), (SiO(2))(n), and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] oxide clusters by density functional theory (DFT). The BP86/ATZP level of theory is employed to obtain constitutional isomers of the oxide clusters. In accordance with previous studies, our calculations show three-dimensional compact structures are preferred for pure (TiO(2))(n) with oxo-stabilized higher hexavalent states, and linear chain structures are favored for pure (SiO(2))(n) with tetravalent states. However, the herein theoretically first reported mixed Ti(m)Si(n-m)O(2n) oxide clusters prefer either three-dimensional compact or linear chain structures depending upon the stoichiometry of the compound. Vibrational analysis of the important modes of some highly stable structures is provided. Coupled-cluster single and double excitation (with triples) [CCSD(T)] computed energy gaps for the TiO(2) dimers compare well with results from previous study. Excitation energies are computed by use of time-dependent (TD) DFT and equation-of-motion coupled-cluster calculations with singles and doubles (EOM-CCSD) for the most stable isomers.     

A three-dimensional ferromagnet based on linked copper-azido clusters

Two novel three-dimensional coordination polymers [Cu(6)(N(3))(12)(N-Eten)(2)](n) (1) (N-Eten=N-ethylethylenediamine) and {[Cu(9)(N(3))(18)(1,2-pn)(4)].H(2)O}(n) (2) (1,2-pn=1,2-diaminopropane) have been synthesized by the self-assembly reactions of Cu(NO(3))(2).3H(2)O, NaN(3) and small diamine ligands. Their molecular structures were determined by single-crystal X-ray diffraction. Complex 1 is composed of a neutral 3D coordination framework based on unprecedented hexanuclear copper(ii) clusters which features three types of bridging modes for azide (mu-1,1, mu-1,3 and mu-1,1,3). Complex 2 is a novel 3D coordination polymer featuring octanuclear copper-azido clusters and [Cu(diamine)(2)](2+) units which are linked by azido bridges. Magnetic studies for complex show ferromagnetic ordering at 3.5 K, where the azido bridges mediate ferromagnetic coupling between adjacent Cu(II) ions. The magnetic data for 1 were fitted to a uniform hexanuclear copper model which yielded g=2.21, J=6.26 cm(-1), zJ'=0.39 cm(-1). Complex 2 shows ferromagnetic coupling in the octanuclear unit and antiferromagnetic interaction between neighboring units.     

Infrared spectroscopy of small sodium-doped water clusters: interaction with the solvated electron

The measured vibrational OH-stretch spectra of size-selected Na(H2O)n clusters for n=8, 10, 16, and 20 are compared with first-principle calculations, which account for the interaction of the sodium cation, the electron, and the water molecules with the hydrogen-bonded network. The calculated harmonic frequencies are corrected by comparing similar results obtained for pure water clusters with experiment. The experimental spectra are dominated by intensity peaks between 3350 and 3550 cm(-1), which result from the interaction of the H atoms with the delocalized electron cloud. The calculations, which are all based upon the average spectra of the four lowest-energy isomers, indicate that most of the peaks at the lower end of this range (3217 cm(-1) for n=8) originate from the interaction of one H atom with the electron distribution in a configuration with a single hydrogen-bonding acceptor. Those at the upper end (3563 cm(-1) for n=8) come from similar interactions with two acceptors. The doublets, which arise from the interaction of both H atoms with the electron, appear in the red-shifted part of the spectrum. They are with 3369/3443 cm(-1) quite pronounced for n=8 but slowly vanish for the larger clusters where they mix with the other spectral interactions of the hydrogen-bonded network, namely, the fingerprints of the free, the double, and the single donor OH positions known from pure water cluster spectroscopy. For all investigated sizes, the electron is sitting at the surface of the clusters.     

Microhydration of NO3(-): a theoretical study on structure, stability and IR spectra

A systematic study on the structure and stability of nitrate anion hydrated clusters, NO3(-) x n H2O (n = 1-8) are carried out by applying first principle electronic structure methods. Several possible initial structures are considered for each size cluster to locate equilibrium geometry by applying a correlated hybrid density functional with 6-311++G(d,p) basis function. Three different types of arrangements, namely, symmetrical double hydrogen bonding, single hydrogen bonding and inter-water hydrogen bonding are obtained in these hydrated clusters. A structure having inter-water hydrogen bonding is more stable compared to other arrangements. Surface structures are predicted to be more stable over interior structures. Up to five solvent H2O molecules can stay around solute NO3(-) anion in structures having an inter-water hydrogen-bonded cyclic network. A linear correlation is obtained for weighted average solvent stabilization energy with the size (n) of the hydrated cluster. Distinctly different shifts of IR bands are observed in these hydrated clusters for different kinds of bonding environments of O-H and N=O stretching modes compared to isolated H2O and NO3(-) anion. Weighted average IR spectra are calculated on the basis of statistical population of individual configurations of each size cluster at 150 K.