Ab initio studies on photochemical reactions of Al atoms with H2 molecules

Potential energy surfaces are computed for the five lowest electronic states of the Al + H₂ system in its symmetric nuclear arrangement. Mechanisms of photochemical reactions of Al atoms with H₂ molecules are proposed, based on the calculated potential energy surfaces. The insertion reaction of the ground-state Al atom into the H₂ molecule is difficult under normal conditions. However, photoexcited Al atoms are capable of reacting with H₂ molecules along different pathways. The results obtained are consistent with experimental findings. The potential energy profiles of the dissociation reaction, AlH₂ → AlH + H, are traced by employing the UMP2 energy gradient method. Photocexcited Al atoms react with H₂ molecules along the 2 ²A₁ state pathway, and the AlH₂(²Σ_g⁺) formed dissociates easily into AlH(¹Σ) and H(²S). The dissociation reaction of ground-state AlH₂ is difficult.     

Ab initio molecular orbital study of potential energy surface for the H2NO(B1)→NO(Π)+H2 reaction

The mechanism of the H₂NO(²B₁)→NO(²Π)+H₂ reaction has been examined using ab initio molecular orbital methods. Ground-state and first-excited-state potential surfaces were plotted at the FOCI/cc-pVTZ level of theory as functions of two appropriate internal degrees of freedom. A conical intersection was found on the C_s pathway that is symmetric with respect to the plane perpendicular to the molecular plane of C_2v H₂NO(²B₁). It is therefore considered that trajectories that start from H₂NO(²B₁) towards the product region detour around the conical intersection, pass through the neighborhood of the transition state that is located at the saddle point on the C_s pathway, and finally reach the products, NO(²Π)+H₂. Thus we can explain the mechanism of the H₂NO(²B₁)→NO(²Π)+H₂ reaction, which has remained unclear to date.     

Structural and spectroscopic effects of vibronic coupling in the C2F radical

The lowest three (1²A^′,2²A^′,1²A^′′) potential energy surfaces of the C₂F radical have been studied at the ab initio level, using Multi Reference Configuration Interaction techniques. For linear geometries, the three surfaces correlate with a ²Π and a ²Σ⁺ state, which are very close in energy. Only the X²A^′ ground state was found to be bent, with R_CC=1.271 Å; R_CF=1.276 Å; CCF=165°, and a barrier to linearity of 275 cm⁻¹. The spin–rovibronic energy levels up to J=7/2 have been calculated using a recently developed method [Carter et al., Mol. Phys. 98 (2000) 1967]. Almost all of the resulting levels arise from a strong mixture of two out of three electronic states and their assignment is intrinsically ambiguous. A partial characterization, based on the shape of the vibronic wavefunctions, has been made.     

On the photodissociation of ozone in the range of 5–9 eV

The photoabsorption spectrum of ozone in the UV range (5–9 eV) is calculated from a short-time wave packet propagation using six potential energy surfaces obtained from ab initio electronic structure calculations. It is shown that the (unnamed) band around 7 eV, which is immediately adjacent to the intense Hartley band, is primarily due to excitation of three electronic states: 5 ¹A′ (3 ¹A₁), 6 ¹A′ (4 ¹A₁), and 4 ¹A″ (2 ¹B₁). Excitation of the state 8 ¹A′ (¹B₂) leads to a broad and intense band starting around 8 eV with a maximum near 9.1 eV. In full accord with the recent experimental study of Brouard et al. [M. Brouard, R. Cireasa, A.P. Clark, G.C. Groenenboom, G. Hancock, S.J. Horrocks, F. Quadrini, G.A.D. Ritchie, C. Vallance, J. Chem. Phys. 125 (2006) 133308], the excitation at 193 nm (6.42 eV) involves at least two states (5 ¹A′ and 4 ¹A″) different from the state excited in the Hartley band (3 ¹A′). The dynamics along the dissociation path is discussed in terms of one-dimensional potential curves. Several avoided crossings among the excited ¹A′ as well as the ¹A″ states point to a complicated fragmentation process. Although a quantitative analysis of branching ratios is not possible on the basis of the present calculations, we surmise, that in addition to and O(¹D) + O₂(¹Δ_g), the next higher spin-allowed channel, , also is likely to be a major product channel, in agreement with experimental observations.     

HS+HO2气相反应机理及主通道速率常数的理论研究

采用CCSD(T)/6-311++G(3df, 2pd)//B3LYP/6-311+G(2df, 2p)双水平计算方法构建了HO₂+HS反应体系的单、三重态反应势能面,并对该反应主通道的速率常数进行了研究。研究结果表明,标题反应经历了八条反应通道,其中三重态反应通道R1是标题反应主通道。此通道包含路径Path 1 (R → ³IM1 → ³TS1 → P1(³O₂+H₂S))和Path 1a (R → ³IM1a → ³TS1a → P1(³O₂+H₂S))两条路径。利用经典过渡态理论(TST)与变分过渡态理论(CVT)并结合小曲率隧道效应模型(SCT),分别计算了主路径Path 1和Path 1a在200-800 K温度范围内的速率常数k^TST、k^CVT和k^CVT/SCT,在此温度区间内路径Path 1和Path 1a具有负温度系数效应。速率常数计算结果显示,对主路径Path 1和Path 1a而言,变分效应在计算温度段内有一定影响,与此同时量子力学隧道效应在低温段有显著影响。路径Path 1和Path 1a的CVT/SCT速率常数的三参数表达式分别为k₁^CVT/SCT(200-800 K) = 1.54×10^-5T^-2.70exp(1154/T) cm³ ·molecule^-1·s^-1和k_1a^CVT/SCT(200-800 K) = 5.82×10^-8T^-1.84exp(1388/T) cm³·molecule^-1·s^-1。     

Geometry relaxation effects in the 1 Bu and 2 Ag states of trans-1,3-butadiene

MCSCF and MRCI calculations on the first three singlet states of trans-1,3-butadiene are presented. Flexible basis sets were applied and full geometry optimization was carried out at the MCSCF level for planar and selected non-planar structures including twisting and pyramidalization of terminal CH₂-groups. Geometry relaxations in and excitation energies to 1 ¹B_u and 2 ¹A_g states are discussed in detail. For planar structures the covalent 2 ¹A_g state is lower in energy than the 1 ¹B_u state. If non-planar geometry relaxations are allowed, the lowest lying non-planar excited singlet state turns out to be ionic with one terminal CH₂ group rotated by 90°. Limitations of the current investigations due to restrictions in the MRCI treatment and because of incomplete scanning of excited state surfaces are pointed out.     

Ab initio study of the electronic spectrum of dichlorocarbene CCl2

The equilibrium geometries, excitation energies, force constants and vibrational frequencies for seven low-lying electronic states X ¹A₁, ¹B₁, ³B₁, ¹A₂, ³A₂, ¹B₂ and ³B₂ of dichlorocarbene CCl₂ have been calculated at the MRSDCI level with a double-zeta plus polarization basis set. Our calculated equilibrium geometry for the X ¹A₁ state, excitation energy for X ¹A₁ → ¹B₁ and vibrational frequencies for the X ¹A₁ and ¹B₁ states are in good agreement with experimental data. The electronic transition dipole moments, oscillator strengths for the ¹B₁ → X ¹A₁ and ¹B₂ → X ¹A₁ transitions, radiative lifetimes for the ¹B₁ and ¹B₁ states are calculated using MRSDCI wavefunctions, predicting results in reasonable agreement with experiment.     

A theoretical study on the ionization of H2S with analysis of vibrational structure of the photoelectron spectra

Ab initio calculations were performed to study the molecular structures and the vibrational levels of the low-lying ionic states (²B₁, ²A₁ and ²B₂) of hydrogen sulphide. The equilibrium molecular structure and the vibrational analysis of these states are presented. The normal vibrational calculations at the RHF level and more extensive calculations at the SDCI level using the explicit vibrational Hamiltonians were used for the vibrational analysis. The theoretical ionization intensity curves including the vibrational structures of these ionic states are also presented and compared with the photoelectron spectrum. The results show that the global shape of the potential energy surface has to be taken into consideration in order to analyze the ²A₁ and ²B₂ states. A new assignment of the photoelectron spectra of H₂S is proposed.     

Theoretical study on reaction mechanism of the vinyl radical with nitrogen atom

The complex triplet potential energy surface of the C₂H₃N system is investigated at the UB3LYP and CCSD(T) (single-point) levels in order to explore the possible reaction mechanism of C₂H₃ radical with N(⁴S). Eleven minimum isomers and 18 transition states are located. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are obtained. Starting from the energy-rich reactant C₂H₃+N(⁴S), the first step is the attack of the N atom on the C atom having one H atom attached in C₂H₃ radical and form the intermediate C₂H₃N(1). The associated intermediate 1 can lead to product P₁ CH₂CN+H and P₂ ³CH₂+³HCN by the cleavage of C–H bond and C–C bond, respectively. The most favorable pathway for the C₂H₃+N(⁴S) reaction is the channel leading to P₁, which is preferred to that of P₂ due to the comparative lower energy barrier. The formation of P₃ ³C₂H₂+³NH through hydrogen-abstraction mechanism is also feasible, especially at high temperature. The other pathways are less competitive comparatively.     

The interstellar gas-phase formation of CO2 – Assisted or not by water molecules?

Using state of the art methods of quantum chemistry, potential energy surfaces for the formation of and CO₂ (³B₂) from CO + O (¹D) and CO + O (³P), respectively, have been studied. At the MRSDCI level, we show that the formation of from O (³P) is strongly connected with the height of the barrier localized on the CO + O (³P) entrance channel. At the CCSD(T) level with a large basis set we calculate this barrier to be 5.9 kcal/mol. Consequently, we confirm that the gas-phase formation of CO₂ in interstellar molecular clouds is inefficient. To mimic the formation of CO₂, through the Eley–Rideal mechanism, on the water ice surfaces of interstellar grains, we have extended our study to consider the formation of CO₂ in the presence of water molecules. We show, using density functional and CCSD(T) methods, that the barrier located on the CO + O (³P) reaction entrance channel is hardly affected by the presence of water molecules. We therefore suggest that CO₂ formation, through the Eley–Rideal mechanism, on the water ice surfaces of interstellar grains, should be inefficient too.     

Laser spectroscopy on the A0-X1Σ transition of T1I

The first excited electronic state of TII was investigated by laser excitation and laser fluorescence spectroscopy. Almost completely resolved rotational fine structure was observed with the help of a collimated molecular beam and a single-mode cw dye laser. From the derived Dunham parameters Y_lk a RKR potential for the excited state is calculated. The unusual shape of the potential energy curve can be understood as originating from an avoided crossing of the ionic ground-state potential and a non-bonding covalent state of the atomic asymptote (6p) ²P_1/2(TI)+(5p)^{5 2}P_3/2(I).     

Theoretical calculational studies on the mechanism of thermal dissociations for RN3 (R=CH3, CH3CH2, (CH3)2CH, (CH3)3C)

Mechanisms of RN₃ (R=CH₃, CH₃CH₂, (CH₃)₂CH, (CH₃)₃C) dissociations are proposed based on CAS, MP2 and B3LYP methods. The energy gaps between the ground-state reactants RN₃ and the intersystem crossing (ISC) points are only a little lower than respective potential energy barriers of the spin-allowed reactions, ¹RN₃ → ¹RN + ¹N₂. The ISC point, therefore, is considered as a transition state of the spin-forbidden reactions, ¹RN₃ → ³RN + ¹N₂. The methods of IRC and topological analysis of electron density are used to explain and predict the thermal dissociation pathways of the reactions studied.     

Classical trajectory calculation for ion-pair formation in K+O2 collisions

Three-dimensional trajectory surface hopping calculations were performed on two diabatic energy surfaces. The covalent surface describes the K(²S) + O₂(³Σ⁻_g) state and the ionic surface K⁺(¹S) + O⁻₂(²Π_g). Transitions from one surface to another were computed through the Landau—Zener model. At small deflection angles, the energy loss distribution exhibits two peaks, as observed, due to O⁻₂ in its electronic ground state and to vibrationally excited O⁻₂.     

Dissociation reactions of the vinyl radical in the AA″ state

In the present theoretical work we have explored mechanisms of dissociation reactions of the vinyl radical in the A²A″ state (C₂H₃ (A²A″)) and examined possible pathways for nonadiabatic dissociation of C₂H₃ (A²A″) into C₂H₂ (X¹Σ_g⁺). In the calculations we used the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with the cc-pVDZ and cc-pVTZ basis sets. Mechanisms for the following three dissociation channels of C₂H₃ in the A²A″ state were explored: (1) C₂H₃ (A²A″) → C₂H₂ (trans, ³A_u) + H, (2) C₂H₃ (A²A″) → C₂H₂ (cis, ³A₂) + H, and (3) C₂H₃ (A²A″) → H₂CC (³A₂) + H. The CASSCF and CASPT2 potential energy curve calculations for the C₂H₃ (A²A″) dissociation channels (1)–(3) indicate that there is neither transition state nor intermediate for each of the channels. At the CASPT2//CASSCF/cc-pVTZ level, the dissociation energies for channels (1)–(3) are predicted to be 84.3, 91.1, and 86.9 kcal/mol, respectively. For a recently observed nonadiabatic dissociation of C₂H₃ (A²A″) into C₂H₂ (X¹Σ_g⁺) + H [J. Chem. Phys. 111 (1999) 3783], two previously suggested internal conversion (IC) pathways were examined based on our CASSCF and CASPT2 calculations. Our preliminary CASSCF and CASPT2 calculations indicate that the assumed IC pathway via the twisted C₂H₃ (A²A^′) structure might be feasible. The CASSCF/cc-pVTZ geometry optimization and frequency analysis calculations were performed for the four C_2v bridge structures in the ²B₂, ²A₂, ²B₁, and ²A₁ states along the pathways of the 1²A^′ (X²A^′), 1²A″ (A²A″), 2²A″, and 2²A^′ states of C₂H₃, respectively, and the CASPT2//CASSCF/cc-pVTZ energetic results indicate that the assumed IC pathway, via a C_2v (²A₂) structure and then ²A₂/²A₁ surface crossing, be not feasible since at their excitation wavelengths (327.4 and 366.2 nm) the C_2v (²A₂) structure could not be accessed.     

Reaction dynamics of the Ca(D2, PJ) + CH3I → CaI + CH3 system: chemiluminescence, energy disposal and product polarization

The Ca(¹D₂, ³P_J) + CH₃ → CaI(A,B) + CH₃ reactions system has been studied by measuring its chemiluminescence under beam-gas conditions. Absolute values of the state-to-state reaction cross-sections were determined at low collision energy . In addition, the electronic branching ratio and product energy disposal have been determined for each metastable reaction. The major changed observed in the chemiluminescence when comparing the Ca(¹D₂) reaction versus that of Ca(³P_J) is the total yield associated with the former reaction. To the best of our spectral resolution neither the electronic branching ratio e.g. CaI(A)/CaI(B) nor the internal CaI energy disposal change significantly as the metastable Ca(¹D₂)/Ca(³P_J) ratio is varied. In spite of the fact that the Ca(³P_J) reaction is less exoergic, the CaI product appears with a higher fraction of internal energy than that of Ca(¹D₂) reaction. Thus, the fraction of the total energy appearing in CaI internal energy amounts to 57.5% in the Ca(³P_J) reaction while it is 19.3% only for the Ca(¹D₂) reaction. This difference is discussed in the light of a distinct mechanism associated with the attack of the excited Ca atom into the C---I bond. No significant chemiluminescence yield was found for the energetically open CaCH^*₃ channels.

The product chemiluminescence polarization was also measured as a function of the metastable concentration. A significant degree of polarization was found depending upon the specific electronic excitation. The analysis of the polarization emission associated to the parallel CaI(X ²Σ⁺ ← B ²Σ⁺) emission led into a strong polarization of the product rotational angular momentum. The comparison of the product rotational alignment for the kinematically identical Ca(¹D₂, ³P_J, ¹P₁) + CH₃ → CaI^* (B₂Σ⁺) + CH₃ reaction system showed that the CaI rotational polarization diminishes in the ³P_J → ¹D₂ → ¹P₁ sequence, e.g. as the reaction exothermicity increases. In addition the degree of polarization associated with other emission bands as for example CaI(X ²Σ⁺ ← A ²Π_1/2) indicates the presence of a parallel transition which was been interpreted as mixing of Hund's case (a) and (c) appropriate for this heavy CaI diatom produced with a high rotational excitation.     

An NMR study of the conformational flexibility of phenyl acetate dissolved in a nematic liquid crystalline solvent

The ¹H, ²H and ¹³C NMR spectra of phenyl acetate, phenyl acetate-[¹³CO] and phenyl acetate-[C²H₃] dissolved in a nematic liquid crystalline solvent have been analysed to yield dipolar coupling, D_ij. These have been interpreted using the additive potential model to provide information on the molecular conformation, resulting in three possible shapes for V(φ), the potential energy for rotation about the ring-oxygen bond. A comparison with the results of molecular orbital calculations leads to the conclusion that a potential with a minimum at 54.4° ± 0.1° is the most probable.     

气相中CrO2+和H2反应的理论研究

用密度泛函UB3LYP/6-311++G(3df, 3pdpd)//6-311G(2dd, p)方法计算研究了在二重态和四重态两个势能面上的气相反应：CrO₂⁺ + H₂→CrO⁺+ H₂O. 对影响反应机理和反应速率的势能面交叉进行了讨论, 并运用Hammond 假设和Yoshizawa 等的内禀反应坐标(IRC)单点垂直激发计算的方法找出了势能面交叉点(crossing point (CP)). 运用碎片分子轨道(fragment molecular orbital(FMO))理论, 对初始复合物2IM1和4IM1的轨道相关进行了分析, 解释了CrO₂⁺活化H—H σ键及H₂迁移的机理.     

The lively chemistry of the di-μ-methylene-bis(pentamethylcyclopentadienyl-rhodium) complexes, analogues of surface reactions in heterogeneous catalysis

The chemistry of the di-μ-methylene-bis(pentamethylcyclopentadienyl-rhodium) complexes is reviewed. The complex [{(η⁵-C₅Me₅)RhCl₂}₂] (1a) reacted with MeLi to give, after oxidative work-up, blood-red cis-[{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(Me)₂], 2. This has the two rhodiums in the +4 oxidation state (d⁵), and linked by a metal-metal bond (2.620 Å). Trans-2 was formed on isomerisation of cis-2 in the presence of Lewis acids, or by direct reaction of 1a with Al₂Me₆, followed by dehydrogenation with acetone. The Rh-methyls in [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(Me)₂] were readily replaced under acidic conditions (HX) to give [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(X)₂] (X = Cl, Br or I); these latter complexes reacted with a variety of RMgX to give [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(R)₂] (R = alkyl, Ph, vinyl, etc.). Trans-2 also reacted with HBF₄ in the presence of L to give first [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(Me)(L)]⁺ and then [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(L)₂]²⁺ (L = MeCN, CO, etc.). The {(η⁵-C₅Me₅)Rh(μ-CH₂)}₂ core is rather kinetically inert and also forms a variety of complexes with oxy-ligands, both cis-, e.g. [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(μ-OAc)]⁺ and trans-, such as [(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(H₂O)₂]²⁺. The complexes [{(η⁵-C₅Me₅)Rh(μ-CH₂)}₂(R)L]⁺ (R = Me or aryl) in the presence of CO, or [{(η⁵-C₄Me₅)Rh(μ-CH₂)}₂(R)₂] (R = Me, Ph or CO₂Me) in the presence of mild oxidants, readily yield the C---C---C coupled products RCH=CH₂. The mechanisms of these couplings have been elucidated by detailed labelling studies: they are more complex than expected, but allow direct analogies to be drawn to C---C couplints that occur during Fischer-Tropsch reactions on rhodium surfaces.     

Two-centre model potential energy calculations for the Σ and Π states of Li2, Na2, K2, Rb2 and Cs2

Two-centre model potential calculations have been carried out for the ²Σ⁺_g,u and ²Π_g,u states of Li⁺₂, Na⁺₂, K⁺₂, Rb⁺₂ and Cs⁺₂. Comparison with other model potential calculations suggests that reliable potential curves have been obtained. The results indicate the usefulness of calculating diatomic energies by the method proposed.