Ab initio embedded cluster study of the excitation spectrum and Stokes shifts of Bi3+-doped Y2O3

The ab initio embedded cluster method coupled with correlated spin-orbit calculations has been used to interpret the excitation spectrum of a Bi(3+)-doped yttria crystal. Our results indicate that the Bi(3+) impurity can absorb light over a wider energy range in the C(2) site than in the S(6) site. Even if the computed absorption energies seem to be about 0.4 eV too high with respect to the experimental peaks for both sites, it is noteworthy that the embedded cluster model renders 93% of the large crystal redshift, about 6 eV. The determination of the geometry relaxation of the first shell of oxygen neighbors upon electronic excitation shows that the Stokes shift is smaller in the S(6) site than in the C(2) site. Combining all these results confirms the assignment of the violet emission to the S(6) site and that of the green emission to the C(2) site, as proposed by Boulon [J. Phys. (Paris) 32, 333 (1971)]. In addition, the nature of the metastable states which lie below the emitting ones and are responsible for the temperature dependence of the fluorescence lifetimes is discussed.     

Ab initio study on the reaction of Y+ + NH3 → Y+ NH + H2

The reaction of Y⁺ + NH₃ → Y⁺ NH + H₂ was theoretically investigated by ab initio MO methods. Two possible pathways (1–1 H₂ loss and 1–2 H₂ loss) on the singlet potential energy surface and reaction mechanism were examined and discussed. The singlet and triplet PESs of this reaction system were compared to confirm the correctness of spin conservation concepts. © 1996 John Wiley & Sons, Inc.     

Ab initio study of the mechanism of the atmospheric reaction: NO2 + O3 --> NO3 + O2

The atmospheric reaction NO2 + O3 --> NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single-reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5-6.1 kcal mol(-1), in reasonable agreement with experimental data. The calculated reaction enthalpy is -24.6 kcal mol(-1) and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 x 10(-18) cm(3) molecule(-1) s(-1). Both results can be regarded also as accurate predictions of the methodology employed in this article.     

Ab initio theoretical study of luminescence properties of Pr3+-doped Lu2O3

Ab initio embedded cluster calculations have been performed on $\hbox{Pr}^{3+}$ -doped $\hbox{Lu}_2\hbox{O}_3$ , in order to investigate the mechanism responsible for the highly efficient $^3P_0\rightarrow^1D_2$ non-radiative relaxation experimentally observed. $(\hbox{PrO}_6)^{9-}$ embedded clusters representing the C ₂ and S ₆ substitutional sites of Pr³⁺:Lu₂O₃ have been studied using wave function-based methods. It is found that an outward relaxation of the first coordination sphere around the impurity takes place upon doping. At the relaxed geometry of the lowest spin triplet 4f5d state, all the 4f5d states lie much higher in energy than all 4f ² states (except the ¹S multiplet). This result is in opposition to the interpretation of intersystem crossing through a low-lying 4f5d excited state of Pr³⁺ as the mechanism for the fast non-radiative ³ P ₀????¹ D ₂ relaxation proposed in the literature. Absorption to the lowest spin triplet 4f5d state is calculated to be around 4,800?cm^?1 higher for the C ₂ site than for the S ₆ site, supporting the assignment of bands in the excitation spectrum previously reported.     

Electron transfer reactivity of O2+O system in low-spin coupling: Ab Initio study at electron correlation level

The electron transfer reactivity of the O₂+O system in low-spin coupling is studied at the second-order unrestricted Møller–Plesset (full)/6-311+G* basis set level by using different transition state structures. The properties and stabilities of the encounter complexes are compared for the five selected coupling structures: two T type, collinear, parallel, and crossing. The activation barriers and the coupling matrix elements are also calculated. The results indicate that the structures of the encounter complexes directly affect the electron transfer mechanism and rate. These encounter complexes are structurally unstable, the contact distances between the acceptor O₂ and the donor O are generally large, the interaction is weak, and the structures are floppy. The electronic transmission factor for the reacting system, O₂+O, is less than unity; thus, the electron transfer reaction is nonadiabatic in nature. Analysis of the dependence of relevant kinetic parameters on various influencing factors has shown that the effect of the solvent medium on the coupling matrix element is small but that on the electron transfer rate is very large. Among the five selected transition state structures, the electron transfer is more likely to take place via T₁-type and P-type structures. In the low-spin coupling the favorable electronic states for two reacting species are ¹∑(O₂) and X²Π_g(O) instead of X³∑(O₂) and X²π_g(O), which are favorable for the high-spin (quartet state) coupling mechanism. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 989–998, 1999     

Bi3+-Er3+ and Bi3+-Yb3+ Codoped Cs2AgInCl6 Double Perovskite Near-Infrared Emitters

Bi³⁺ and lanthanide ions have been codoped in metal oxides as optical sensitizers and emitters. But such codoping is not known in typical semiconductors such as Si, GaAs, and CdSe. Metal halide perovskite with coordination number 6 provides an opportunity to codope Bi³⁺ and lanthanide ions. Codoping of Bi³⁺ and Ln³⁺ (Ln=Er and Yb) in Cs₂AgInCl₆ double perovskite is presented. Bi³⁺-Er³⁺ codoped Cs₂AgInCl₆ shows Er³⁺ f-electron emission at 1540 nm (suitable for low-loss optical communication). Bi³⁺ codoping decreases the excitation (absorption) energy, such that the samples can be excited with ca. 370 nm light. At that excitation, Bi³⁺-Er³⁺ codoped Cs₂AgInCl₆ shows ca. 45 times higher emission intensity compared to the Er³⁺ doped Cs₂AgInCl₆. Similar results are also observed in Bi³⁺-Yb³⁺ codoped sample emitting at 994 nm. A combination of temperature-dependent (5.7 K to 423 K) photoluminescence and calculations is used to understand the optical sensitization and emission processes.     

Ab initio study of the reaction of CHO+ with H2O and NH3

An MP4(full,SDTQ)/6-311++G(d,p)//MP2(full)/6-311++G(d,p) ab initio study was performed of the reactions of formyl and isoformyl cations with H₂O and NH₃, which play an important role in flame and interstellar chemistries. Two different confluent channels were located leading to CO+H₃O⁺/NH. The first one corresponds to the approach of the neutral molecule to the carbon atom of the cations. The second one leads to the direct proton transfer from the cations to the neutrals. At 900 K the separate products CO+H₃O⁺/NH are the most stable species along the Gibbs energy profiles for the processes. For the reaction with H₂O the reaction channel leading to HC(OH) (protonated formic acid) is disfavored with respect to the two CO+H₃O⁺ channels in agreement with the experimental evidence that H₃O⁺ is the major ion observed in hydrocarbon flames. According to our calculations, NH+H₂O are considerably more stable in Gibbs energy than NH₃+H₃O⁺;NH will predominate in the reaction zone when ammonia is added to CH₄+Ar diffusion flame, as experimentally observed. At 100 K the most stable structures are the intermediate complexes CO…HOH/HNH. Particularly the CO…HOH complex has a lifetime large enough to be detected and, therefore, could play a certain role in interstellar chemistry. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1432–1443, 1999     

Detailed interpretation of the 5f-6d absorption spectrum of U3+ in Cs2NaYCl6 and high pressure effects based on an ab initio simulation

The 5f-->6d(t(2g)) absorption spectrum of U(3+)-doped Cs(2)NaYCl(6) is simulated with a quantum chemical ab initio embedded-cluster approach applied to U(3+) substitutional defects of O(h) local symmetry. The first-principles results help to provide a detailed interpretation of the very rich experimental absorption spectrum of this material between 14 000 and 25 000 cm(-1). Also, the effects of high pressures up to 26 kbars on the absorption spectrum are predicted, the most relevant feature being a redshift of around 21 cm(-1)/kbar, which is the fingerprint of a bond length shortening upon 5f-->6d(t(2g)) excitation.     

Ab initio energies and product branching ratios for the O+C3H6 reaction

Intermediate and transition-state energies have been calculated for the O+C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O-C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300-3000 K temperature and 0.001-760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H+CH3CHCHO is favored over the entire temperature range, although the competing H2CO+CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol(-1), largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p相似文献   

Ab initio study of the F + CH3NHNH2 reaction mechanism

The F + CH(3)NHNH(2) reaction mechanism is studied based on ab initio quantum chemistry methods as follows: the minimum energy paths (MEPs) are computed at the UMP2/6-311++G(d,p) level; the geometries, harmonic vibrational frequencies, and energies of all stationary points are predicted at the same level of theory; further, the energies of stationary points and the points along the MEPs are refined by UCCSD(T)/6-311++g(3df,2p). The ab initio study shows that, when the F atom approaches CH(3)NHNH(2), the heavy atoms, namely N and C atoms, are the favorable combining points. For the two N atoms, two prereaction complexes with C(s) symmetry are generated and there exists seven possible subsequent reaction routes, of which routes 1, 2, 5, and 7 are the main channels. Routes 1, 2, and 5 are associated with HF elimination, with H from the amino group or imido group, and route 7 involves the N-N bond break. Routes 3 and 6 with relation to HF elimination with H from methyl, and route 4 involved the C-N bond break, are all energetically disfavored. For the C atom, the attack of F results in the break of the C-N bond and the products are CH(3)F + NHNH(2). This route is very competitive.     

Ab initio study of the inversion barrier in NF3+

Ab initio Hartree–Fock, Møller–Plesset perturbation theory (MP 2), and quadratic configuration interaction, using single and double substitutions (QCISD ), calculations were carried out for the NF₃⁺ ion. Optimized structures were examined at the various levels of theory. Calculation of the inversion barrier height shows the importance of optimizing the geometry at the post-Hartree–Fock level and the inclusion of polarization functions. The best calculated inversion barrier was 13.3 kcal/mol, compared to an experimental value of 17.3 kcal/mol. The dissociation transition state was computed to determine the well depth of the NF₃⁺ ion and its stability toward dissociation. The computed well depth was 28 and 48 kcal/mol at the SCF and MP 2 levels, respectively. © 1994 John Wiley & Sons, Inc.     

PH~3...H~2O体系分子间相互作用的从头计算MO研究

在MP2/6-311++G(3d,3p)水平,对PH~3...H~2O体系可能存在的氢键复合物进行了全自由度能量梯度优化,发现PH~3...H~2O体系存在三个能量极小结构A,B和C。其中结构A和B以H~2O作为质子授体,结构C以PH~3作为质子授体,结构A较结构B和C分别稳定6.52kJ/mol和8.18kJ/mol。结构A具有C~s对称性,其结构中P原子和O原子间距离为354.78nm,键角OHP为171.35ⅲ,属于接近于直线的传统型氢键结构。进一步在高级电子相关校正的MP4SDTQ下,用6-311++G(3df,3pd)基加上键函数{3s3p2d1f},通过BSSE校正,精确计算了结构A的结合能为-10.84kJ/mol。     

Ab initio potential energy surfaces of the ion-molecule reaction: C2H2 + O+

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double zeta) and cc-pVTZ (triple zeta) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction, C2H2(1Sigmag+) + O+(4S), for which collision experiment has been performed by Chiu et al. [J. Chem. Phys. 109, 5300 (1998)]. The minor low-energy process leading to the weak spin-forbidden product C2H2+ (2Piu) + O(1D) has been studied previously and will not be discussed here. The major pathways to form charge-transfer (CT) products, C2H2+ (2Piu) + O(3P) (CT1) and C2H2+ (4A2) + O(3P) (CT2), and the covalently bound intermediates are investigated. The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Int1 between CT2 products. This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states. As the C-O distance becomes shorter beyond the above intermediate, the C1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol. Although this TS connects diabatically to the covalent intermediate Int2, there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions. These findings are consistent with the experiment, in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies, respectively.     

Ab initio QM/MM dynamics of H3O+ in water

A molecular dynamics (MD) simulation based on a combined ab initio quantum mechanics/molecular mechanics (QM/MM) method has been performed to investigate the solvation structure and dynamics of H3O+ in water. The QM region is a sphere around the central H3O+ ion, and contains about 6-8 water molecules. It is treated at the Hartree-Fock (HF) level, while the rest of the system is described by means of classical pair potentials. The Eigen complex (H9O4+) is found to be the most prevalent species in the aqueous solution, partly due to the selection scheme of the center of the QM region. The QM/MM results show that the Eigen complex frequently converts back and forth into the Zundel (H5O2+) structure. Besides the three nearest-neighbor water molecules directly hydrogen-bonded to H3O+, other neighbor waters, such as a fourth water molecule which interacts preferentially with the oxygen atom of the hydronium ion, are found occasionally near the ion. Analyses of the water exchange processes and the mean residence times of water molecules in the ion's hydration shell indicate that such next-nearest neighbor water molecules participate in the rearrangement of the hydrogen bond network during fluctuative formation of the Zundel ion and, thus, contribute to the Grotthuss transport of the proton.     

Quantum chemical study of 4f-->5d excitations of trivalent lanthanide ions doped in the cubic elpasolite Cs2NaYCl6. Ce3+ to Tb3+

Wave-function-based ab initio calculations on the lowest states of the 4f(n),4f(n-1)5d(t2g)1, and 4f(n-1)5d(e(g))1 configurations of (LnCl6)3- clusters (Ln=Ce to Tb) embedded in the cubic elpasolite Cs2NaYCl6 have been performed, in an attempt to contribute to a comprehensive understanding of the 4f-->5d excitations of lanthanide ions in crystals. Reliable data are provided on the changes of bond lengths and breathing mode vibrational frequencies upon 4f-->5d(t2g) and 4f-->5d(e(g)) excitations, as well as on minimum-to-minimum and vertical absorption and emission transitions, and on the Stokes shifts. The available experimental data are discussed and predictions are made. The stabilization of the 4f-->5d(baricenter) excitation of the doped ions with respect to the 4f-->5d excitations of the free ions, which is a key variable for the understanding of these excitations in solid hosts, is analyzed and found to be due, in two-thirds, to dynamic ligand correlation effects and, in one-third, to orbital relaxation, charge transfer, and covalency effects present in a mean-field approximation.     

Oxygen ionic transport in Bi2O3-based oxides: II. The Bi2O3–ZrO2–Y2O3 and Bi2O3–Nb2O5–Ho2O3 solid solutions

Electrical conductivity, fluorite-type cubic unit cell volume and thermal expansion of the (Bi_{1− x} Nb _x )_{1− y} Ho _y O_1.5+δ (x=0.05 and 0.08; y=0.10−0.15) and (Bi_{1− x} Zr _x )_{1− y} Y _y O_1.5+δ (x=0.05 and 0.07; y=0.15) solid solutions have been found to decrease regularly with increasing dopant content. Annealing at temperatures below 900 K leads to a phase decomposition and to a sharp decrease in conductivity of the ceramics. Oxygen ion transference numbers have been determined by the e.m.f. method and by Faradaic efficiency measurement to exceed 0.9. A new technique of studying Faradaic efficiency has been proposed and verified using (Bi_0.95Zr_0.05)_0.85Y_0.15O_1.5+δ and Zr_0.90Y_0.10O_1.95 ceramic samples. Received: 31 October 1997 / Accepted: 18 December 1997     

自由基-分子反应:F+Propene(CH2CHCH3)的从头算研究

The radical-molecule reaction F+propene (CH2CHCH3) was studied in detail by using the Becke's three parameter Lee-Yang-Parr-B3LYP/6-311G(d,p) and coupled cluster with single, double, and triple excitationsCCSD(T)/6-311+G(2d,2p). It is shown that F+propene reaction mainly occurs through complex-formation mechanism: F attacks the double bond of propene leading to the formation of complex 1 and complex 2. As the two radical complexes are metastable, they can quickly dissociate to H+C3H5F, CH3+C2H3F and HF+C3H5. Based on the ab initio calculations, the CH3+C2H3F is the main channel, and the H elimination and HF forming channels also provide some contribution to products. The calculated values are in good agreement with the recently reported experimental results.