Products of the addition of water molecules to Al3O3- clusters: structure, bonding, and electron binding energies in Al3O4H2-, Al3O5H4-, Al3O4H2, and Al3O5H4

Two stable products of reactions of water molecules with the Al3O3- cluster, Al3O4H2- and Al3O5H4-, are studied with electronic structure calculations. There are several minima with similar energies for both anions and the corresponding molecules. Dissociative absorption of a water molecule to produce an anionic cluster with hydroxide ions is thermodynamically favored over the formation of Al3O3-(H2O)n complexes. Vertical electron detachment energies of Al3O4H2- and Al3O5H4- calculated with ab initio electron propagator methods provide a quantitative interpretation of recent anion photoelectron spectra. Contrasts and similarities in these spectra may be explained in terms of the Dyson orbitals associated with each transition energy.     

Interaction of water, methanol, and ammonia with AlxOy-: a comparative theoretical study of Al5O4- versus Al3O3-

The chemical reactions of water, methanol, and ammonia with Al5O4- have been studied using electronic structure calculations. The chemistry of Al5O4- with these molecules is different from that of Al3O3-. While Al3O3- dissociatively adsorbs two water molecules (and methanol), Al5O4- reacts with only one. In addition, Al5O4- does not show any reaction with ammonia while recent experimental and theoretical studies suggest that Al3O3- chemisorbs ammonia. These apparent differences in their chemical reactivity have been explained based on the thermodynamic stability of the corresponding reaction products and kinetic barriers associated with their formation.     

Addition of NH3 to Al3O3-

Recent computational studies on the addition of ammonia (NH3) to the Al3O3- cluster anion [A. Guevara-Garcia, A. Martinez, and J. V. Ortiz, J. Chem. Phys. 122, 214309 (2005)] have motivated experimental and additional computational studies, reported here. Al3O3- is observed to react with a single NH3 molecule to form the Al3O3NH3- ion in mass spectrometric studies. This is in contrast to similarly performed studies with water, in which the Al3O5H4- product was highly favored. However, the anion PE spectrum of the ammoniated species is very similar to that of Al3O4H2-. The adiabatic electron affinity of Al3O3NH3 is determined to be 2.35(5) eV. Based on comparison between the spectra and calculated electron affinities, it appears that NH3 adds dissociatively to Al3O3-, suggesting that the time for the Al3O3-NH3 complex to either overcome or tunnel through the barrier to proton transfer (which is higher for NH3 than for water) is short relative to the time for collisional cooling in the experiment.     

Calculation of electron detachment energies for water cluster anions: an appraisal of electronic structure methods, with application to (H2O)20- AND (H2O)24-

We present benchmark calculations of vertical electron detachment energies (VDEs) for various conformers of (H2O)n-, using both wave function and density functional methods, in sequences of increasingly diffuse Gaussian basis sets. For small clusters (n < or = 6), a systematic examination of VDE convergence reveals that it is possible to converge this quantity to within approximately 0.01 eV of the complete-basis limit, using a highly diffuse but otherwise economical Pople-style basis set of double-zeta quality, with 28 atom-centered basis functions per water molecule. Floating-center basis functions can be useful but are not required to obtain accurate VDEs. Second-order M?ller-Plesset perturbation (MP2) theory suffices to obtain VDEs that are within 0.05 eV of the results from both experiment and coupled-cluster theory, and which always err toward underbinding the extra electron. In contrast to these consistent predictions, VDEs calculated using density functional theory (DFT) vary widely, according to the fraction of Hartree-Fock exchange in a given functional. Common functionals such as BLYP and B3LYP overestimate the VDE by 0.2-0.5 eV, whereas a variant of Becke's "half and half" functional is much closer to coupled-cluster predictions. Exploratory calculations for (H2O)20- and (H2O)24- cast considerable doubt on earlier calculations that were used to assign the photoelectron spectra of these species to particular cluster isomers.     

Electronically excited states of water clusters of 7-azaindole: structures, relative energies, and electronic nature of the excited states

The geometries of 1H-7-azaindole and the 1H-7-azaindole(H(2)O)(1-2) complexes and the respective 7H tautomers in their ground and two lowest electronically excited pi-pi(*) singlet states have been optimized by using the second-order approximated coupled cluster model within the resolution-of-the-identity approximation. Based on these optimized structures, adiabatic excitation spectra were computed by using the combined density functional theory/multireference configuration interaction method. Special attention was paid to comparison of the orientation of transition dipole moments and excited state permanent dipole moments, which can be determined accurately with rotationally resolved electronic Stark spectroscopy. The electronic nature of the lowest excited state is shown to change from L(b) to L(a) upon water complexation.     

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.     

Electric field effects on water clusters (n = 3-5): systematic ab initio study of structures, energetics, and transition states

The structures, energetics, and transition states of water clusters (trimer to pentamer, n = 3-5) are investigated as a function of electric field by using ab initio calculations. With an increasing strength of the field, the most stable cyclic structures of trimer, tetramer, and pentamer open up to align their dipole moments along the direction of the field. For the lower strength (below 0.3 V/angstroms) of the electric field, the dipole moment of each water monomer is along the same direction with the field, while it retains the cyclic structure. For the higher strength of the field, to have a higher dipole moment for the cluster along the field direction, each cyclic structure opens up to form a linear chain or "water wire." We have investigated the transition state structures between the cyclic and linear forms for the field strengths of 0.3-0.4 V/angstroms where both cyclic and linear forms are energetically comparable.     

Multireference configuration interaction studies on higher valence and Rydberg states of OClO, ionization potentials, and electron detachment energies

MRCI results are reported for the vertical excitation energies (VEE) and oscillator strengths f of doublet states of OClO up to 11 eV, including 3b(1) → 4s, 4p, 3d, 5s, 5p, 4d, and most 1a(2), 8a(1), 5b(2) → 4s and 4p Rydberg states. The lowest Rydberg states 3b(1) → 4s and 3b(1) → 4p(x) have mixed valence-Rydberg character. The observed spectral bands were reassigned to include valence states which have generally higher oscillator strengths. The well-known valence state 1(2)A(2) has a VEE of 3.63 eV, and a relatively high f of 0.042. Overall, the calculated oscillator strengths are in good agreement with measured values. The lowest quartet state, 1(4)B(2), lies at 6.95 eV. Quartet Rydberg states start with 1a(2) → 4s at 9.28 eV. According to calculated vertical ionization potentials (VIP) of OClO, the second VIP at 12.59 eV is reassigned from 1(3)B(1) to 1(3)B(2) (ionization from 1a(2), rather than 8a(1)), and the third VIP at 12.63 eV from 1(1)B(1) to 1(3)B(1) (ionization from 8a(1)). Vertical electron detachment energies of OClO(-) have been calculated up to 8.9 eV. There is good agreement with experimental values.     

Studies in hydrogen bonding: Association within small clusters of water,methanol, and ammonia molecules using Jorgensen's intermolecular pair potentials

The geometries of the dimer, trimer, and tetramer hydrogen-bonded clusters of water, methanol, and ammonia molecules have been derived using previously published intermolecular pair potentials containing constants optimized from ab initio calculations. The lowest energy forms for the dimers of all three types of molecules have an open structure, while the trimers and tetramers have cyclic structures. The results are compared with those previously described using another empirical potential, EPEN .     

Sequential addition of H2O, CH3OH, and NH3 to Al3O3-: a theoretical study

Photoelectron spectra of two species, Al3O3(H2O)2- and Al3O3(CH3OH)2-, that are produced by the addition of two water or methanol molecules to Al3O3- are interpreted with density-functional geometry optimizations and electron propagator calculations of vertical electron detachment energies. In both cases, there is only one isomer that is responsible for the observed spectral features. A high barrier to the addition of a second molecule may impede the formation of Al3O3N2H6- clusters in an analogous experiment with NH3.     

Tautomeric forms of azolide anions: vertical electron detachment energies and Dyson orbitals

Several decouplings of the electron propagator, including the relatively new P3+ approximation for the self-energy, have been used to calculate vertical electron detachment energies of tautomeric forms of closed-shell, pentagonal, aromatic anions in which ring carbons without bonds to hydrogens appear. This study extends previous work in which the most stable forms of anionic, five-member rings with one to five nitrogens were considered. Whereas the lowest electron detachment energies sometimes are assigned by Koopmans's theorem results to pi orbital vacancies, electron propagator calculations always obtain sigma orbital vacancies for the ground states of the doublet radicals. Higher electron detachment energies that correspond to excited doublets with pi vacancies also are presented. The predicted transition energies are in good agreement with low-intensity peaks in recent anion photoelectron spectra that have been assigned to less stable, tautomeric forms of these anions.     

Benchmark calculations on the electron detachment energies of MO3* and M2O6* (M = Cr, Mo, W)

Neutral and anionic molecules of the monomers and dimers of the group VIB transition metal oxides (MO3 and M2O6) were studied with density functional theory (DFT) and coupled cluster CCSD(T) theory. Franck-Condon simulations of the photoelectron spectra were carried out for the transition from the ground state of the anion to that of the neutral molecule. Molecular structures from the DFT and CCSD(T) methods are compared. Electron detachment energies reported in the literature were evaluated. The calculated adiabatic and vertical electron detachment energies (ADEs and VDEs) were compared with the experimental results. CCSD(T) gives results within 0.12 eV for the ADEs. CCSD(T) predicts VDEs that are in error by as much as 0.3 eV for M = Cr. DFT hybrid functionals were found to give poor results for the ADEs and VDEs for M = Cr due to the substantial amount of multireference character in the wavefunction, whereas the pure DFT functionals give superior results. For M = Mo and W, excellent agreement was found for both CCSD(T) and many DFT fucntionals. The BP86 functional yields the best overall results for the VDEs of all the metal oxide clusters considered. Heats of formation calculated at the CCSD(T) level extrapolated to the complete basis set limit are also in good agreement with available experimental data.     

The influence of electron correlation on reaction energies. The dimerization energies of BH3 and LiH

Results of rigorous computations employing extended Gaussian-type basis sets are reported for BH₃, B₂H₆, LiH, and Li₂H₂ in their respective equilibrium geometries. The dimerization energy of BH₃ is calculated as −20.7 kcal/mol within the Hartree-Fock approximation and as −36.6 kcal/mol if electron correlation is included. The corresponding results for the dimerization of LiH are −47.3 kcal/mol and −48.3 kcal/mol. Partitioning of the correlation energy contributions allows to attribute the effect of electron correlation to the increase of next neighbour bond interactions on the dimerization of BH₃ and LiH. The difficulties of accurate computations of reaction energies are discussed in detail.     

Energetics, transition states, and intrinsic reaction coordinates for reactions associated with O(3P) processing of hydrocarbon materials

Electronic structure calculations based on multiconfiguration wave functions are used to investigate a set of archetypal reactions relevant to O(3P) processing of hydrocarbon molecules and surfaces. These include O(3P) reactions with methane and ethane to give OH plus methyl or ethyl radicals, O(3P) + ethane to give CH3O + CH3, and secondary reactions of the OH product radical with ethane and the ethyl radical. Geometry optimization is carried out with CASSCF/cc-pVTZ for all reactions, and with CASPT2/cc-pVTZ for O(3P) + methane/ethane. Single-point energy corrections are applied with CASPT2, CASPT3, and MRCI + Q with the cc-pVTZ and cc-pVQZ basis sets, and the energies extrapolated to the complete basis set limit (CBL). Where comparison of computed barriers and energies of reaction with experiment is possible, the agreement is good to excellent. The best agreement (within experimental error) is found for MRCI + Q/CBL applied to O(3P) + methane. For the other reactions, CASPT2/CBL and MRCI + Q/CBL predictions differ from experiment by 1-5 kcal/mol for 0 K enthalpies of reaction, and are within 1 kcal/mol of the best-estimate experimental range of 0 K barriers for O(3P) + ethane and OH + ethane. The accuracy of MRCI + Q/CBL is limited mainly by the quality of the active space. CASPT2/CBL barriers are consistently lower than MRCI + Q/CBL barriers with identical reference spaces.     

Development of a 3-body:many-body integrated fragmentation method for weakly bound clusters and application to water clusters (H2O)(n = 3-10, 16, 17)

A 3-body:many-body integrated quantum mechanical (QM) fragmentation method for non-covalent clusters is introduced within the ONIOM formalism. The technique captures all 1-, 2-, and 3-body interactions with a high-level electronic structure method, while a less demanding low-level method is employed to recover 4-body and higher-order interactions. When systematically applied to 40 low-lying (H(2)O)(n) isomers ranging in size from n = 3 to 10, the CCSD(T):MP2 3-body:many-body fragmentation scheme deviates from the full CCSD(T) interaction energy by no more than 0.07 kcal mol(-1) (or <0.01 kcal mol(-1) per water). The errors for this QM:QM method increase only slightly for various low-lying isomers of (H(2)O)(16) and (H(2)O)(17) (always within 0.13 kcal mol(-1) of the recently reported canonical CCSD(T)/aug-cc-pVTZ energies). The 3-body:many-body CCSD(T):MP2 procedure is also very efficient because the CCSD(T) computations only need to be performed on subsets of the cluster containing 1, 2, or 3 monomers, which in the current context means the largest CCSD(T) calculations are for 3 water molecules, regardless of the cluster size.     

Theoretical study of stable structures and photoelectron spectra of mass-selected Al12Cs-, Al11Cs2-, and Al10Cs3- clusters

The geometric and electronic structures of the ground and low-lying states for the Al(12)Cs(-), Al(11)Cs(2) (-), and Al(10)Cs(3) (-) clusters were examined using the density functional theory. Semi-icosahedral structures of the Al(12)Cs(-) and Al(11)Cs(2) (-) clusters were found as the ground state. The most stable structure of the Al(10)Cs(3) (-) cluster is a distorted icosahedron structure. The vertical detachment energy of these clusters and the anion photoelectron spectra (PES) were compared. The peaks of the anion PES were assigned on the basis of the shell model. The single peak of 3.1-3.2 or 2.5-2.7 eV for the Al(12)Cs(-) or Al(11)Cs(2) (-) cluster, respectively, is observed due to the electron detachment from the 2p or 1f or 1f+2p shells. Two large peaks of 2.1 eV and 3.1-3.3 eV correspond to the electron detachments from the 1f+2p and 2p, and 1d+1f shells, respectively. It was found that a second peak appears with the hybridization of the 1d and 1f shells due to the distortion from the icosahedral structure in the Al(10)Cs(3) (-) cluster.