-
[show abstract]
[hide abstract]
ABSTRACT: LaO(2) was produced in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and ab initio electronic structure calculations. The calculations included density functional theory, second-order perturbation theory, coupled cluster theory, and complete active space self-consistent field methods. The adiabatic ionization energy of the molecule and vibrational frequencies of the molecule and its cation were measured accurately for the first time from the MATI spectrum. Numerous ionization processes of lanthanum dioxide, peroxide, and superoxide were considered; the (3)B(2) ← (4)B(2) electronic transition of the dioxide was assigned upon comparison with the observed spectrum. The ionization energy and O-La-O bending frequency of the (4)B(2) neutral state are 4.9760 (6) eV and 92 cm(-1), respectively. The La-O stretching and O-La-O bending frequencies of the (3)B(2) cationic state are 656 and 122 cm(-1), respectively. The (4)B(2) state is formed by two electron transfer from lanthanum to oxygen atoms, and the (3)B(2) state is produced by the further removal of a lanthanum 6s-based electron.
The Journal of chemical physics 07/2012; 137(3):034307. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Anion photoelectron spectroscopy and quantum chemical calculations at the density functional theory (DFT), coupled cluster theory (CCSD(T)), and complete active space self-consistent field (CASSCF) theory levels are employed to study the reduced transition metal oxide clusters M(4)O(10)(-) (M = Cr, W) and their neutrals. Photoelectron spectra are obtained at 193 and 157 nm photon energies, revealing very different electronic structures for the Cr versus W oxide clusters. The electron affinity and HOMO-LUMO gap are measured to be 3.68 ± 0.05 and 0.7 eV, respectively, for the Cr(4)O(10) neutral cluster, as compared to 4.41 ± 0.04 and 1.3 eV for W(4)O(10). A comprehensive search is performed to determine the ground-state structures for M(4)O(10) and M(4)O(10)(-), in terms of geometry and electronic states by carefully examining the calculated relative energies at the DFT, CCSD(T), and CASSCF levels. The ground states of Cr(4)O(10) and Cr(4)O(10)(-) have tetrahedral structures similar to that of P(4)O(10) with the anion having a lower symmetry due to a Jahn-Teller distortion. The ground states of W(4)O(10) and W(4)O(10)(-) have butterfly shape structures, featuring two fused five-member rings with a metal-metal multiple bond between the central metal atoms. The much stronger WW bonding than the CrCr bonding is found to be the primary cause for the different ground state structures of the reduced Cr(4)O(10)(0/-) versus W(4)O(10)(0/-) oxide clusters. The photoelectron spectra are assigned by comparing the experimental and theoretical adiabatic and vertical electron detachment energies, further confirming the determination of the ground electronic states of M(4)O(10) and M(4)O(10)(-). The time-dependent DFT method is used to calculate the excitation energies of M(4)O(10). The TD-DFT results in combination with the self-consistently calculated vertical detachment energies for some of the excited states at the DFT and CCSD(T) levels are used to assign the higher energy bands. Accurate clustering energies and heats of formation of M(4)O(10) are calculated and used to calculate accurate reaction energies for the reduction of M(4)O(12) to M(4)O(10) by CH(3)OH, as well as for the oxidation of M(4)O(10) to M(4)O(12) by O(2). The performance of the DFT method with the B3LYP and BP86 functionals in the calculations of the relative energies, electron detachment energies, and excitation energies are evaluated, and the BP86 functional is found to give superior results for most of these energetic properties.
The Journal of Physical Chemistry A 05/2012; 116(21):5256-71. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Monobenzene complexes of yttrium (Y), lanthanum (La), and lutetium (Lu), M(C(6)H(6)) (M = Y, La, and Lu), were prepared in a laser-vaporization supersonic molecular beam source and studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and ab initio calculations. The calculations included the second-order perturbation, the coupled cluster with single, double, and perturbative triple excitation, and the complete active space self-consistent field methods. Adiabatic ionization energies and metal-benzene stretching frequencies of these complexes were measured for the first time from the ZEKE spectra. Electronic states of the neutral and ion complexes and benzene ring deformation were determined by combining the spectroscopic measurements with the theoretical calculations. The ionization energies of M(C(6)H(6)) are 5.0908 (6), 4.5651 (6), and 5.5106 (6) eV, and the metal-ligand stretching frequencies of [M(C(6)H(6))](+) are 328, 295, and 270 cm(-1) for M = Y, La, and Lu, respectively. The ground states of M(C(6)H(6)) and [M(C(6)H(6))](+) are (2)A(1) and (1)A(1), respectively, and their molecular structures are in C(2v) point group with a bent benzene ring. The deformation of the benzene ring upon metal coordination is caused by the pseudo Jahn-Teller interaction of (1(2)E(2)+1(2)A(1)+2(2)E(2)) e(2) at C(6v) symmetry. In addition, the study shows that spectroscopic behaviors of Y(C(6)H(6)) and La(C(6)H(6)) are similar to each other, but different from that of Lu(C(6)H(6)).
The Journal of chemical physics 04/2012; 136(13):134310. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A robust metadata database called the Collaborative Chemistry Database Tool (CCDBT) for massive amounts of computational chemistry raw data has been designed and implemented. It performs data synchronization and simultaneously extracts the metadata. Computational chemistry data in various formats from different computing sources, software packages, and users can be parsed into uniform metadata for storage in a MySQL database. Parsing is performed by a parsing pyramid, including parsers written for different levels of data types and sets created by the parser loader after loading parser engines and configurations.
Journal of molecular graphics & modelling 04/2012; 34:67-75. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Infrared spectra of three new thorium oxide species have been obtained in argon and neon matrixes. All of the products are experimentally characterized using isotopic oxygen samples with the aid of electronic structure calculations. Ground state thorium atoms react with O(2) to form the ThO(2) molecules, which can dimerize to give Th(2)O(4) products. Th(2)O(4) is predicted to have nonplanar C(2h) symmetry for its closed shell singlet ground state. The rhombus-shaped Th(2)O(2) molecule in the (1)A(g) (D(2h)) ground state is also observed and its formation is proposed via the reaction of Th(2) with O(2). In addition, electron capture of neutral thorium dioxide results in the formation of the ThO(2)(-) anion. It is predicted to have a doublet ground state with a geometry similar to that of the neutral ThO(2) molecule. Electronic structure calculations on the unobserved Th(2)O and Th(2)O(3) molecules are also provided.
The Journal of Physical Chemistry A 12/2011; 115(50):14407-16. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The surface chemistry of the hafnium-based atomic layer deposition precursor, tetrakis(ethylmethylamido) hafnium (TEMAH), on hydrogen-terminated Si(100) was studied for comparison with the previously reported tetrakis(dimethylamido) hafnium (TDMAH) results by in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and in vacuo X-ray photoelectron spectroscopy (XPS). The adsorption and reaction experiments were investigated at substrate temperatures between 25 and 400 °C. Gas-phase products were detected by in situ transmission FTIR. Density functional theory (DFT) calculations were performed for gas-phase decomposition as well as the adsorption and reaction of TEMAH onto a hydrogen-terminated model of a Si(100) surface. The formation of cyclo species containing alkyl C on Hf formed by a intramolecular insertion reaction and Hf–H species formed by β-hydride elimination on the surface were identified by XPS at temperatures > 270 °C and a partial pressure of about 0.02 Torr for the former and ATR-FTIR at 100 °C and 0.1 Torr, for the latter. The latter was confirmed by a TEMAH/D2O reaction experiment, supporting the previous observations of Hf–H species for TDMAH. The observation of an alkyl C on Hf as a proposed cyclo species is sensitive to experimental conditions, such as residence time, pressure, and temperature. Decompositions at higher pressures had an increased reaction rate. Transmission FTIR detected the formation of the gas-phase products ethylmethylamine and methylethyleneimine or ethylmethyleneimine. DFT calculations predicted the interface between the adsorbed molecules and H-Si(100) to involve Hf–Si, Hf–N–Si, and/or Hf–N–C–Si bonds. The formation of bridged Si–N interfacial bonds provides a thermodynamically allowable way to generate these surface species and is supported by the observation of a N 1s XPS peak associated with Si–N.
09/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The stability, electronic structure, and thermochemical properties of the pure Li(n) and boron-doped Li(n)B (n = 1-8) clusters in both neutral and cationic states are studied using electronic structure methods. The global equilibrium structures are established, and their heats of formation are evaluated using the G3B3 and CCSD(T)/CBS methods based on the density functional theory geometries. Theoretical adiabatic ionization energies (IE(a)) for the Li(n) clusters are in good agreement with experiment: Li(2) (G3B3, 5.21 eV; CCSD(T), 5.14 eV; expt, 5.1127 ± 0.0003 eV), Li(3) (4.16, 4.11, 4.08 ± 0.10), Li(4) (4.76, 4.68, 4.70 ± 0.05), Li(5) (4.11, 4.06, 4.02 ± 0.10), Li(6) (4.46, 4.32, 4.20 ± 0.10), Li(7) (4.07, 3.99, 3.94 ± 0.10), and Li(8) (4.49, 4.31, 4.16 ± 0.10). The Li(4) experimental IE(a) has been revised on the basis of the Franck-Condon simulations. Species Li(5)B, Li(6)B(+), Li(7)B, and Li(8)B(+) exhibit high stability as compared to their neighbors, which can be understood by considering the magic numbers of the phenomenological shell model (PSM).
The Journal of Physical Chemistry A 06/2011; 115(26):7673-86. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Heats of formation, adiabatic and diabatic bond dissociation energies (BDEs) of the model Schrock-type metal complexes M(NH)(CRR′)(OH)2 (M = Cr, Mo, W; CRR′ = CH2, CHF, CF2) and MO2(OH)2 compounds, and Brønsted acidities and fluoride affinities for the M(NH)(CH2)(OH)2 transition metal complexes are predicted using high level CCSD(T) calculations. The metallacycle intermediates formed by reaction of C2H4 with M(NH)(CH2)(OH)2 and MO2(OH)2 are investigated at the same level of theory. Additional corrections were added to the complete basis set limit to obtain near chemical accuracy (±1 kcal/mol). A comparison between adiabatic and diabatic BDEs is made and provides an explanation of trends in the BDEs. Electronegative groups bonded on the carbenic carbon lead to less stable Schrock-type complexes as the adiabatic BDEs of M═CF2 and M═CHF bonds are much lower than the M═CH2 bonds. The Cr compounds have smaller BDEs than the W or Mo complexes and should be less stable. Different M(NH)(OH)2(C3H6) and MO(OH)2(OC2H4) metallacycle intermediates are investigated, and the lowest-energy metallacycles have a square pyramidal geometry. The results show that consideration of the singlet–triplet splitting in the carbene in the initial catalyst as well as in the metal product formed by the retro [2 + 2] cycloaddition is a critical component in the design of an effective olefin metathesis catalyst in terms of the parent catalyst and the groups being transferred.
06/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Density functional theory (DFT) and coupled cluster theory (CCSD(T)) were used to study the group 6 metal (M = Cr, Mo, W) hydroxides: MO3−m(OH)2m (m = 1−3), M2O6−m(OH)2m (m = 1−5), M3O9−m(OH)2m (m = 1, 2), and M4O11(OH)2. The calculations were done up to the complete basis set (CBS) limit for the CCSD(T) method. Molecular structures of many low-energy conformers/isomers were located. Brønsted acidities in the gas phase and pKa values in aqueous solution were predicted for MO3−m(OH)2m (m = 1−3) and MnO3n−1(OH)2 (n = 2−4). In addition, Brønsted basicities and Lewis acidities (fluoride affinities) were predicted for MO3−m(OH)2m (m = 1−3) as well as the metal oxide clusters MnO3n (n = 1−3). The metal hydroxides were predicted to be strong Brønsted acids and weak to modest Brønsted bases and Lewis acids. The pKa values can have values as negative as −31. Potential energy surfaces for the hydrolysis of the MnO3n (n = 1−4) clusters were calculated. Heats of formation of the metal hydroxides were predicted from the calculated reaction energies, and the agreement with the limited available experimental data is good. The first hydrolysis step leading to the formation of MnO3n−1(OH)2 was predicted to be exothermic, with the exothermicity becoming less negative as n increases and essentially converged at n = 3. Reaction rate constants for the hydrogen transfer steps were calculated using transition state theory and RRKM theory. Further hydrolysis of MnO3n−1(OH)2 tends to be endothermic especially for M = Cr. Fifty-five DFT exchange-correlation functionals were benchmarked for the calculations of the reaction energies, complexation energies, and reaction barriers by comparing to our CCSD(T) results. Overall, the DFT results for the potential energy surfaces are semiquantitatively correct, but no single functional works for all processes and all three metals. Among the functionals benchmarked, the wB97, wB97X, B1B95, B97-1, mPW1LYP, and X3LYP functionals have the best performance. Linear correlations between the calculated reaction barrier and reaction energy for hydrogen transfer reactions to the terminal ═O atom and to the bridge O atom were found to be quite different, indicating their different reaction properties. The calculated Lewis acidity (fluoride affinity) was found to best correlate with the calculated adsorption energy, the dissociative adsorption energy, and the reaction barrier for hydrogen transfer reactions to the terminal ═O atom.
03/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The adsorption and reaction of tetrakis(dimethylamido)hafnium (TDMAH) on hydrogen terminated Si(100) were studied by using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), transmission IR, and quadrupole mass spectrometry (Q-MS). Surface and gas phase reactions were investigated at temperatures between 25 and 300 °C. Density functional theory (DFT) calculations benchmarked by coupled cluster calculations on small models were performed for gas phase decomposition via intramolecular insertion and β-hydride elimination as well as the adsorption and reaction of TDMAH onto a hydrogen terminated Si(100) surface. N−Si and CH2−Si bonds due to reactions on the Si windows were observed in transmission IR, while N−Ge and CH2−Ge bonds on a Ge internal reflectance element (IRE) were observed by ATR-FTIR at 25 and 100 °C. Also observed were the formation of Hf−H bonds and three-member-ring species on the Si surface; the former was confirmed by a control D2O exchange reaction experiment. Both transmission IR and Q-MS indicated the presence of decomposition products dimethylamine (DMA) and N-methyl methyleneimine (MMI). The calculated bond dissociation energies (BDE) at the CCSD(T)/CBS level roughly follow the order of Hf−O > Hf−N > N−H, C−H, Si−N > Si−H, Si−C > N−C, Hf−H > Hf−Si, and the BDEs of the same chemical bond can vary substantially in different molecules. The interface is predicted by DFT calculations to involve Hf−Si, Hf−N−Si, and/or HfNC-Si bonds. TDMAH decomposition products, such as MMI, can form a C−Si or N−Si bond with the silicon surface. The combined experimental and theoretical results suggest that insertion and β-hydride elimination reactions can occur during bidentate chemisorption on the H−Si(100) surface by forming N−Si bonds.
07/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: The hydrolysis of titanium tetrachloride (TiCl(4)) to produce titanium dioxide (TiO(2)) nanoparticles has been studied to provide insight into the mechanism for forming these nanoparticles. We provide calculations of the potential energy surfaces, the thermochemistry of the intermediates, and the reaction paths for the initial steps in the hydrolysis of TiCl(4). We assess the role of the titanium oxychlorides (Ti(x)O(y)Cl(z); x = 2-4, y = 1, 3-6, and z = 2, 4, 6) and their viable reaction paths. Using transition-state theory and RRKM theory, we predicted rate constants including the effect of tunneling. Heats of formation at 0 and 298 K are predicted for TiCl(4), TiCl(3)OH, TiOCl(2), TiOClOH, TiCl(2)(OH)(2), TiCl(OH)(3), Ti(OH)(4), and TiO(2) using the CCSD(T) method with correlation consistent basis sets extrapolated to the complete basis set limit and compared with the available experimental data. Clustering energies and heats of formation are calculated for neutral clusters. The calculated heats of formation were used to study condensation reactions that eliminate HCl or H(2)O. The reaction energy is substantially endothermic if more than two HCl molecules are eliminated. The results show that the mechanisms leading to formation of TiO(2) nanoparticles and larger ones are complicated and will have a strong dependence on the experimental conditions.
The Journal of Physical Chemistry A 07/2010; 114(28):7561-70. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The hydrolysis of titanium tetrachloride (TiCl4) to produce titanium dioxide (TiO2) nanoparticles has been studied to provide insight into the mechanism for forming these nanoparticles. We provide calculations of the potential energy surfaces, the thermochemistry of the intermediates, and the reaction paths for the initial steps in the hydrolysis of TiCl4. We assess the role of the titanium oxychlorides (TixOyClz; x = 2−4, y = 1, 3−6, and z = 2, 4, 6) and their viable reaction paths. Using transition-state theory and RRKM theory, we predicted rate constants including the effect of tunneling. Heats of formation at 0 and 298 K are predicted for TiCl4, TiCl3OH, TiOCl2, TiOClOH, TiCl2(OH)2, TiCl(OH)3, Ti(OH)4, and TiO2 using the CCSD(T) method with correlation consistent basis sets extrapolated to the complete basis set limit and compared with the available experimental data. Clustering energies and heats of formation are calculated for neutral clusters. The calculated heats of formation were used to study condensation reactions that eliminate HCl or H2O. The reaction energy is substantially endothermic if more than two HCl molecules are eliminated. The results show that the mechanisms leading to formation of TiO2 nanoparticles and larger ones are complicated and will have a strong dependence on the experimental conditions.
06/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Geometry parameters, frequencies, heats of formation, and bond dissociation energies are predicted for simple alkali metal compounds (hydrides, chlorides, fluorides, hydroxides and oxides) of Li, Na, and K from coupled cluster theory [CCSD(T)] calculations including core-valence correlation with the aug-cc-pwCVnZ basis set (n = D, T, Q, and 5). To accurately calculate the heats of formation, the following additional correction were included: scalar relativistic effects, atomic spin-orbit effects, and vibrational zero-point energies. For calibration purposes, the properties of some of the lithium compounds were predicted with iterative triple and quadruple excitations via CCSDT and CCSDTQ. The calculated geometry parameters, frequencies, heats of formation, and bond dissociation energies were compared with all available experimental measurements and are in excellent agreement with high-quality experimental data. High-level calculations are required to correctly predict that K(2)O is linear and that the ground state of KO is (2)Sigma(+), not (2)Pi, as in LiO and NaO. This reliable and consistent set of calculated thermodynamic data is appropriate for use in combustion and atmospheric simulations.
The Journal of Physical Chemistry A 03/2010; 114(12):4272-81. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The group IVB transition-metal dioxide clusters and their anions, (MO(2))(n) and (MO(2))(n)(-) (M = Zr, Hf; n = 1-4), are studied with coupled cluster (CCSD(T)) theory and density functional theory (DFT). Similar to the results for M = Ti, these oxide clusters have a number of low-lying isomeric structures, which can make it difficult to predict the ground electronic state especially for the anion. Electron affinities for the low-lying structures are calculated and compared with those for M = Ti. Electron affinities of these clusters depend strongly on the cluster structures. Anion photoelectron spectra are calculated for the monomer and dimer and demonstrate the possibility for structural identification at a spectral line width of <or=0.05 eV. Electron excitation energies from the low-lying states to the singlet and triplet excited states are calculated self-consistently, as well as by the time-dependent DFT and equation-of-motion coupled cluster (EOM-CCSD) methods. The calculated excitation energies are compared to the band energies of bulk oxides, indicating that the excitation energy is not yet converged for n = 4 for these clusters. The excitation energies of the low-lying isomeric clusters are less than the bulk metal oxide band gaps and suggest that these clusters could be useful photocatalysts with a visible light source.
The Journal of Physical Chemistry A 02/2010; 114(7):2665-83. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the third row transition metal hexafluorides. The electron affinities, heats of formation, first (MF(6) --> MF(5) + F) and average M-F bond dissociation energies, and fluoride affinities of MF(6) (MF(6) + F(-) --> MF(7)(-)) and MF(5) (MF(5) + F(-) --> MF(6)(-)) were calculated. The electron affinities which are a direct measure for the oxidizer strength increase monotonically from WF(6) to AuF(6), with PtF(6) and AuF(6) being extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values, and are as follows: WF(6) (3.15 eV), ReF(6) (4.58 eV), OsF(6) (5.92 eV), IrF(6) (5.99 eV), PtF(6) (7.09 eV), and AuF(6) (8.20 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated, and only three gave satisfactory results. The corresponding pentafluorides are extremely strong Lewis acids, with OsF(5), IrF(5), PtF(5), and AuF(5) significantly exceeding the acidity of SbF(5). The optimized geometries of the corresponding MF(7)(-) anions for W through Ir are classical MF(7)(-) anions with M-F bonds; however, for PtF(7)(-) and AuF(7)(-) non-classical anions were found with a very weak external F-F bond between an MF(6)(-) fragment and a fluorine atom. These two anions are text book examples for "superhalogens" and can serve as F atom sources under very mild conditions, explaining the ability of PtF(6) to convert NF(3) to NF(4)(+), ClF(5) to ClF(6)(+), and Xe to XeF(+) and why Bartlett failed to observe XePtF(6) as the reaction product of the PtF(6)/Xe reaction.
Inorganic Chemistry 02/2010; 49(3):1056-70. · 4.60 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the third row transition metal hexafluorides. The electron affinities, heats of formation, first (MF6 → MF5 + F) and average M−F bond dissociation energies, and fluoride affinities of MF6 (MF6 + F− → MF7−) and MF5 (MF5 + F− → MF6−) were calculated. The electron affinities which are a direct measure for the oxidizer strength increase monotonically from WF6 to AuF6, with PtF6 and AuF6 being extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values, and are as follows: WF6 (3.15 eV), ReF6 (4.58 eV), OsF6 (5.92 eV), IrF6 (5.99 eV), PtF6 (7.09 eV), and AuF6 (8.20 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated, and only three gave satisfactory results. The corresponding pentafluorides are extremely strong Lewis acids, with OsF5, IrF5, PtF5, and AuF5 significantly exceeding the acidity of SbF5. The optimized geometries of the corresponding MF7− anions for W through Ir are classical MF7− anions with M−F bonds; however, for PtF7− and AuF7− non-classical anions were found with a very weak external F−F bond between an MF6− fragment and a fluorine atom. These two anions are text book examples for “superhalogens” and can serve as F atom sources under very mild conditions, explaining the ability of PtF6 to convert NF3 to NF4+, ClF5 to ClF6+, and Xe to XeF+ and why Bartlett failed to observe XePtF6 as the reaction product of the PtF6/Xe reaction.
01/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We report a comparative study of reduced transition metal oxide clusters, M(3)O(8)(-) (M = Cr, W) anions and their neutrals, via anion photoelectron spectroscopy (PES) and density functional theory (DFT) and molecular orbital theory (CCSD(T)) calculations. Well-resolved PES spectra are obtained for M(3)O(8)(-) (M = Cr, W) at 193 and 157 nm photon energies. Different PES spectra are observed for M = Cr versus M = W. Extensive DFT and CCSD(T) calculations are performed to locate the ground and low-lying excited states for the neutrals and anions. The ground states of Cr(3)O(8) and Cr(3)O(8)(-) are predicted to be the (3)B(2) and (4)B(2) states of a C(2v) structure, respectively, revealing ferromagnetic spin coupling for Cr 3d electrons. In contrast, the ground states of W(3)O(8) and W(3)O(8)(-) are predicted to be the (1)A' state (C(s) symmetry) and the (2)A(1) state (C(2v) symmetry), respectively, showing metal-metal d-d bonding in the anion. The current cluster geometries are in qualitative agreement with prior DFT studies at the PBE level for M = Cr and the B3LYP level for M = W. The BP86 and PW91 functionals significantly outperform the B3LYP functional for the Cr species, in terms of relative energies, electron detachment energies, and electronic excitation energies, whereas the B3LYP functional is better for the W species. Accurate heats of formation for the ground states of M(3)O(8) are calculated from the clustering energies and the heats of formation of MO(2) and MO(3). The energetics have been used to predict redox reaction thermochemistry.
The Journal of Physical Chemistry A 09/2009; 113(42):11273-88. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Interspin distances between the Ti(3+) ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7'-apo-7'-(4-carboxyphenyl)-beta-carotene (coordinated to Ti(3+)), canthaxanthin (formed as a random distribution of isomers), and beta-ionone (model for a short-chain polyene) to Ti(3+) framework sites were determined using the pulsed EPR relaxation enhancement method. To estimate the electron transfer distances, the temperature dependence of relaxation rates was analyzed in both siliceous and metal-substituted siliceous materials. The phase memory times, T(M), of the carotenoid radicals were determined from the best fits of two-pulse ESEEM curves. The spin-lattice relaxation times, T(1), of the Ti(3+) ion were obtained from the inversion recovery experiment with echo detection on a logarithmic time scale in the temperature range of 10-150 K. The relaxation enhancement for the carotenoid radicals in TiMCM-41 as compared to that in MCM-41 is consistent with an interaction between the radical and the fast relaxing Ti(3+) ion. For canthaxanthin and beta-ionone, a dramatic effect on the carotenoid relaxation rate, 1/T(M), occurs at 125 and 40 K, respectively, whereas for carboxy-beta-carotene 1/T(M) increases monotonically with increasing temperature. The interspin distances for canthaxanthin and beta-ionone were estimated from the 1/T(M) - 1/T(M0) difference, which corresponds to the Ti(3+) contribution at the temperature where the maximum enhancement in the relaxation rate occurs. Determination of the interspin distances is based on calculations of the dipolar interaction, taking into consideration the unpaired spin density distribution along the 20-carbon polyene chain, which makes it possible to obtain a fit over a wider temperature interval. A distribution of the interspin distances between the carotenoid radical and the Ti(3+) ion was obtained with the best fit at approximately 10 A for canthaxanthin and beta-ionone and approximately 9 A for 7'-apo-7'-(4-carboxyphenyl)-beta-carotene with an estimated error of +/-3 A. The interspin distances do not depend on 1/T(M) - 1/T(M0) for carboxy-beta-carotene which shows no prominent peak in the relaxation rate over the temperature range measured.
The Journal of Physical Chemistry B 07/2009; 113(25):8704-16. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Total atomization energies (TAEs) and normalized clustering energies (NCEs) of group IVB (MO(2))(n) (M = Ti, Zr, Hf) and VIB (MO(3))(n) (M = Cr, Mo, W) transition metal oxide clusters up to n = 4 were calculated at the coupled cluster [CCSD(T)] and density functional theory (DFT) levels. For all the clusters studied, the TAEs calculated at the CCSD(T) level were found to be strongly basis set dependent, whereas the NCEs were significantly less basis set dependent. Here we further develop an efficient strategy for calculating accurate thermodynamic properties of large clusters based on those of the cluster unit and the NCEs. The calculated TAEs, NCEs, and heats of formations for these clusters were compared with available experimental data. We also benchmarked the performance of popular DFT exchange-correlation functionals for the calculations of the TAEs and NCEs. The performance of many DFT functionals for the calculation of the TAEs strongly depends on the choice of the electronic state for the transition metal atom. Hybrid functionals were found to generally outperform pure functionals in the calculation of NCEs, and the PBE1PBE functional has the best performance with average deviations of approximately 1 kcal/mol for the dimers and approximately 2 kcal/mol for the trimers and tetramers. The benchmarked functionals all display gradual degradation in performance with increasing cluster size.
The Journal of Physical Chemistry A 07/2009; 113(27):7861-77. · 2.95 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Following complexation with certain dendrimer functionalities, metal ions can be reduced to zerovalent metal nanoparticles via UV irradiation and with the dendrimer oxidized to a radical cation. EPR-silent Zn(II) ions can serve as the oxidizing agent, enabling the nature of the dendrimer radical cation to be examined. Spectral simulations and quantum chemical calculations were carried out to elucidate the nature of the free radicals. Spectral simulations in conjunction with electronic structure calculations suggest that the electron spin density is localized on the central N−C−C−N core structure and delocalized over the N and C atoms in the core. The radical cations of model structures with the ethylenediamine (EDA) core and that of the G0-NH2 polyamidoamine (PAMAM) were found to have a weak central one-electron C−C bond. The description of the molecular structure of the cation falls between the limit of two iminium-type ions with a charge of +0.5 e on each (1/2+R2N = CH21/2•) interacting by a one-electron C−C bond and the other limit of a 1/2+1/2•NR2−CH2−CH2−NR21/2+1/2• structure with a spin of 1/2 and a charge of 1/2 on each N. For EDA, our calculated ionization energies and heats of formation at the coupled cluster (CCSD(T)) level are in good agreement with available experimental data. The ionization energy of the G0-NH2 PAMAM was found to be substantially lower than that of EDA. The reduction in the ionization energies for the dendrimers and other effects such as metal−ligand interaction and solvation contribute to the reduction of metal cations by dendrimers with UV irradiation. Similar experiments with the G0-NH2 poly(propylene imine) (PPI) did not produce metal nanoparticles, indicating these effects are not as favorable as those for G0-NH2 PAMAM.
04/2009;
