电子效应调控丁烯二腈光分子开关的CASSCF和MS-CASPT2理论研究

采用多组态CASSCF方法和MS-CASPT2方法研究了丁烯二腈中性分子及阳离子和阴离子的顺-反异构化机理.结果表明,中性分子和离子态的光顺-反异构化反应经历不同的非绝热跃迁途径:中性丁烯二腈受光激发至S1态后,需克服一个不低于19.7 k J/mol的能垒才有机会到达基态和激发态间的圆锥交叉(S_1/S_0-CI),随后经非辐射跃迁回到基态,S_1/S_0-CI在结构上偏离C=C双键旋转路径,且能量较高,因此会降低旋转速度,阻碍旋转的单向性;丁烯二腈阳离子和阴离子自由基的D_0态和D_1态旋转势能面在90°处相交,优化的D_1/D_0-CI与D_1态中间体的结构和能量均相近,因此从D1态经由D_1/D_0-C_I无辐射跃迁到D_0态的过程无势垒,在此过程中C=C旋转方向性得到最大限度的保持.研究结果证实了电子诱导不仅能降低基态热旋转势垒,而且能够调控光旋转的非绝热跃迁机理.     

On the location of conical intersections in CH2BrCl using MS-CASPT2 methods

Multiconfigurational second-order perturbation theory has been employed to calculate two-dimensional potential energy surfaces for the lowest low-lying singlet electronic states of CH2BrCl as a function of the two carbon-halogen bonds. The photochemistry of the system is controlled by a nonadiabatic crossing occurring between the A and B bands, attributed to the b1A' and c1A' states, which are found almost degenerate and forming a near-degeneracy line of almost equidistant C-Br and C-Cl bonds. A crossing point in the near-degeneracy line is identified as a conical intersection in this reduced two-dimensional space. The positions of the conical intersection located at CASSCF, single-state (SS)-CASPT2, and multistate (MS)-CASPT2 levels of theory are compared, also paying attention to the nonorthogonality problem of perturbative approaches. To validate the presence of the conical intersection versus an avoided crossing, the geometrical phase effect has been checked using the multiconfigurational MS-CASPT2 wave function.     

Excited-State Proton Transfer and Decay in Hydrogen-Bonded Oxazole System:MS-CASPT2//CASSCF Study

Herein we have employed high-level multi-reference CASSCF and MS-CASPT2 electronic structure methods to systematically study the photochemical mechanism of intramolecularly hydrogen-bonded 2-(2'-hydroxyphenyl)-4-methyloxazole. At the CASSCF level, we have optimized minima, conical intersections, minimum-energy reaction paths relevant to the excited-state intramolecular proton transfer (ESIPT), rotation, photoisomerization, and the excited-state deactivation pathways. The energies of all structures and paths are refined by the MS-CASPT2 method. On the basis of the present results, we found that the ESIPT process in a conformer with the OH…N hydrogen bond is essentially barrierless process; whereas, the ESIPT process is inhibited in the other conformer with the OH…O hydrogen bond. The central single-bond rotation of the S₁ enol species is energetically unfavorable due to a large barrier. In addition, the excited-state deactivation of the S₁ keto species, as a result of the ultrafast ESIPT, is very efficient because of the existence of two easily-approached keto S₁/S₀ conical intersections. In stark contrast to the S₁ keto species, the decay of the S₁ enol species is almostly blocked. The present theoretical study contributes valuable knowledge to the understanding of photochemistry of similar intramolecularly hydrogen-bonded molecular and biological systems.     

A MS-CASPT2 study of the low-lying electronic excited states of CH2BrCl

The low-lying singlet excited states of CH₂BrCl have been calculated using multiconfigurational CASSCF, second-order perturbation theory CASPT2 and its multistate extension MS-CASPT2. The CASSCF method shows spurious valence–Rydberg mixing and a wrong order of states. Inclusion of dynamical correlation by single root CASPT2 lowers dramatically the energy of the valences states but does not lead to a complete separation between valence and Rydberg states. This situation is improved by the MS-CASPT2 calculations, which gives two valence states for both A^′ and A″ symmetries below the lowest Rydberg state, corresponding to n(Br)→σ^*(C–Br) and n(Cl)→σ^*(C–Cl) transitions at 6.1 eV (203 nm) and 7.2 eV (173 nm), and being repulsive along C–Br and C–Cl coordinates.     

Excited-State Double Proton Transfer of 1, 8-Dihydroxy-2-Naphthaldehyde: a MS-CASPT2//CASSCF Study

Excited-state double proton transfer (ESDPT) is a controversial issue which has long been plagued with theoretical and experimental communities. Herein, we took 1, 8-dihydroxy-2-naphthaldehyde (DHNA) as a prototype and used combined complete active space self-consistent field (CASSCF) and multi-state complete active-space second-order perturbation (MS-CASPT2) methods to investigate ESDPT and excited-state deactivation pathways of DHNA. Three different tautomer minima of S1-ENOL, S1-KETO-1, and S1-KETO-2 and two crucial conical intersections of S1S0-KETO-1 and S1S0-KETO-2 in and between the S₀ and S₁ states were obtained. S1-KETO-1 and S1-KETO-2 should take responsibility for experimentally observing dual-emission bands. In addition, two-dimensional potential energy surfaces (2D-PESs) and linear interpolated internal coordinate paths connecting relevant structures were calculated at the MS-CASPT2//CASSCF level and confirmed a stepwise ESDPT mechanism. Specifically, the first proton transfer from S1-ENOL to S1-KETO-1 is barrierless, whereas the second one from S1-KETO-1 to S1-KETO-2 demands a barrier of ca. 6.0 kcal/mol. The linear interpolated internal coordinate path connecting S1-KETO-1 (S1-KETO-2) and S1S0-KETO-1 (S1S0-KETO-2) is uphill with a barrier of ca. 12.0 kcal/mol, which will trap DHNA in the S₁ state while therefore enabling dual-emission bands. On the other hand, the S₁/S₀ conical intersections would also prompt the S₁ system to decay to the S₀ state, which could be to certain extent suppressed by locking the rotation of the C5$-$C8$-$C9$-$O10 dihedral angle. These mechanistic insights are not only helpful for understanding ESDPT but also useful for designing novel molecular materials with excellent photoluminescent performances.     

Insights into the Thermal and Photochemical Reaction Mechanisms of Azidoacetonitrile. Spectroscopic and MS-CASPT2 Calculations

This work studies the photochemical and thermal decompositions of azidoacetonitrile (N₃CH₂CN) from both the experimental and theoretical points of view. The data of the photochemical experiments are taken from the literature, while the thermal decomposition have been carried out by us. In addition, we have performed ab initio calculations of the multiconfigurational type [complete active space self-consistent field (CASSCF) and the multistate multireference perturbation theory (MS-CASPT2)]. It is found that the first step of both type of decompositions is N₂ elimination and formation of closed shell singlet nitrene. Afterwards, the nitrene tends to rapidly rearrange into formimidoyl cyanide (HNCHCN). As both reactions progress, the imine isomerizes into formimidoyl isocyanide (HNCHNC). The photoisomerization of the imine takes places thorough a conical intersection, while the same reaction on the ground electronic state occurs via a conventional transition state. The last step of the global reaction is decomposition of the imines into HCN and CNH. In photochemical conditions, the conjunction of the imines and its dissociation products (HCN and CNH) yields adenine     

Ab initio investigation on the structures and spectra of the firefly luciferin

The ground state (S₀) geometry of the firefly luciferin (LH2) was optimized by both DFT B3LYP and CASSCF methods. The vertical excitation energies (T _v) of three low-lying states (S₁, S₂, and S₃) were calculated by TD-DFT B3LYP//CASSCF method. The S₁ geometry was optimized by CASSCF method. Its T _v and the transition energy (T _e) were calculated by MS-CASPT2//CASSCF method. Both the TD-DFT and MS-CASPT2 calculated S₁ state T _v values agree with the experimental one. The IPEA shift greatly affects the MS-CASPT2 calculated T _v values. Some important excited states of LH2 and oxyluciferin (oxyLH2) are charge-transfer states and have more than one dominant configuration, so for deeply researching the firefly bioluminescence, the multireference calculations are desired. Supported by the National Natural Science Foundation of China (Grant No. 20673012) and the Major State Basic Research Development Programs (Grant No. 2004CB719903)     

Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes

The hydrocarbons 1‐cyclopentylidene‐1a,9b‐dihydro‐1H ‐cyclopropa[l ]phenanthrene and 1‐cyclobutylidene‐1a,9b‐dihydro‐1H ‐cyclopropa[l ]phenanthrene undergo photolysis in solution at ambient temperature to produce cyclohexyne and cyclopentyne, respectively. These strained cycloalkynes, formed via the putative cycloalkylidenecarbenes, were intercepted as Diels–Alder adducts. Calculations at the CCSD(T)/cc‐pVTZ//B3LYP/6‐31+G* level of theory show that singlet cyclopentylidenecarbene has to overcome a barrier of 9.1 kcal mol⁻¹ to rearrange into cyclohexyne (with ΔE for ring expansion=−15.1 kcal mol⁻¹). By contrast, cyclobutylidenecarbene only needs to surmount a barrier of 1.6 kcal mol⁻¹ to rearrange into cyclopentyne (with ΔE for ring expansion=−6.2 kcal mol⁻¹).     

An Ab Initio Study on the Mechanism of the Atmospheric Reaction NH2+O3→H2NO+O2

The atmospheric reaction NH₂+O₃→H₂NO+O₂ has been investigated theoretically by using MP2, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. At the MP2 level of theory, the hypersurface of the potential energy (HPES) shows a two step reaction mechanism. Therefore, the mechanism proceeds along two transition states (TS1 and TS2), separated by an intermediate designated as Int. However, when the single‐reference higher correlated QCISD and the multiconfigurational CASSCF methodologies have been employed, the minimum structure Int and TS2 are not found on the HPES, which thus confirms a direct reaction mechanism. Single‐reference high correlated and multiconfigurational methods consistently predict the barrier height of the reaction to be within the range of 3.9 to 6.6 kcal mol^?1, which is somewhat higher than the experimental value. ¹ The calculated reaction enthalpy is ?67.7 kcal mol^?1.     

Ab initio investigation of the diels-alder reaction between 2H-phosphole and phosphaethene: A model for phosphole dimerization

All four possible Diels-Alder reactions between 2H-phosphole and phosphaethene were examined at various theoretical levels, including HF, MP4SDQ, CCSD(T), and CASSCF. MP2/6-31G^* geometry optimizations could not be employed since the potential energy surface is qualitatively incorrect at this level of theory, due to the inherent underestimation of the activation energies (ameliorated at higher-order MP or coupled-cluster levels). Solvent effects were examined employing the Onsager, polarized continuum, and isodensity and surface polarized continuum models. At MP4SDQ/6-31G^*//HF/6-31G^* these reactions are exothermic by 34–38 kcal mol⁻¹ and have very low activation energies, 5–7 kcal mol⁻¹. The P P/C C regioisomer products are lower in energy than the C P isomers and, within each pair, the exo isomer is lower in energy. At low computational levels the smallest activation energy is for the reaction leading to the C P endo product. Larger basis sets, electron correlation, and solvent favor the transition state leading to the experimentally observed P P/C P endo isomer. The dimerization of phosphole is, therefore, kinetically controlled. Based on geometric and electron density analysis, the reactions are concerted and synchronous. © 1997 by John Wiley & Sons, Inc.     

Organoboron compounds: XXIX. A comparison of pπ-pπ, bonding in alkoxy and alkylthiopiperidinophenylboranes

The results of a VT ¹³C NMR study of alkoxy and alkylthio derivatives of 2- and 3-methylpiperidinophenylboranes demonstrate that the p_π-p_π bonding between boron and oxygen is some 3 kcal mol⁻¹ stronger than that between sulphur and boron.     

Theoretical Investigations on the Agostic Interactions of the Molybdenum and Manganese Complexes

The essential participation of agostic interactions in C−H bond activation, cyclometallation and other catalytic processes has been widely observed. To quantitatively evaluate the Mo−H−C agostic interaction in the Mo β/γ- agostomers [CpMo(CO)₂(PiPr₃)]⁺ ( Mo , 1 and Mo , 2 ) and the Mn−H−C agostic interaction in the Mn α/ϵ-agostomers [(C₆H₉]Mo(CO)₃] ( Mn , 1 and Mn , 2 ), the comprehensive density functional theory (DFT) theoretical investigations were performed. Results indicated that the Mo β-agostomer 1 is only favorable by 0.5 kcal mol⁻¹ than Mo γ-agostomer 2 , and the Gibbs barrier for their interconversion was 9.1 kcal mol⁻¹. A slightly higher Gibbs barrier of 12.7 kcal mol⁻¹ for the isomerization between the Mn α/ϵ-agostomers was also obtained. The relatively strong agostic interactions in Mo β-agostomer 1 and Mn α-agostomer 1 were further verified by the AIM (Atoms-In-Molecules) analyses and the NAdOs (natural adaptive orbitals) analyses. The findings on the agostic interaction presented in this study are believed to benefit the understandings of the agostic interaction involved catalytic processes and to promote the development of new organometallic complexes.     

London Dispersion Effects in a Distannene/Tristannane Equilibrium: Energies of their Interconversion and the Suppression of the Monomeric Stannylene Intermediate

Reaction of {LiC₆H₂−2,4,6-Cyp₃⋅Et₂O}₂ (Cyp=cyclopentyl) ( 1 ) of the new dispersion energy donor (DED) ligand, 2,4,6-triscyclopentylphenyl with SnCl₂ afforded a mixture of the distannene {Sn(C₆H₂−2,4,6-Cyp₃)₂}₂ ( 2 ), and the cyclotristannane {Sn(C₆H₂−2,4,6-Cyp₃)₂}₃ ( 3 ). 2 is favored in solution at higher temperature (345 K or above) whereas 3 is preferred near 298 K. Van't Hoff analysis revealed the 3 to 2 conversion has a ΔH=33.36 kcal mol⁻¹ and ΔS=0.102 kcal mol⁻¹ K⁻¹, which gives a ΔG^{300 K}=+2.86 kcal mol⁻¹, showing that the conversion of 3 to 2 is an endergonic process. Computational studies show that DED stabilization in 3 is −28.5 kcal mol⁻¹ per {Sn(C₆H₂−2,4,6-Cyp₃)₂ unit, which exceeds the DED energy in 2 of −16.3 kcal mol⁻¹ per unit. The data clearly show that dispersion interactions are the main arbiter of the 3 to 2 equilibrium. Both 2 and 3 possess large dispersion stabilization energies which suppress monomer dissociation (supported by EDA results).     

Mutual isomerization of cyclopentyl and 1-Penten-5-y1 radicals

Unimolecular reactions of mutual isomerization of cyclopentyl and 1-penten-5-yl radicals have been investigated by chemical activation. The radicals were generated by adding energized hydrogen atoms (E_H about 23 kcal mol⁻¹) to the double bond of either cyclopentane or 1,4-pentadiene. Based on the extensive steady-state RRKM calculations employing the experimental data from this work as well as from the literature, the threshold energies for the cyclopentyl ring opening and closure are 32 ± 0.3 and 16.2 ± 0.3 kcal mol⁻¹, respectively. The entropy of activation for the ring opening is close to zero.     

MS-CASPT2 calculation of excess electron transfer in stacked DNA nucleobases

Calculations using the complete active space self-consistent field (CASSCF) and complete active space second-order perturbation (CASPT2) methods, and the multistate formulation of CASPT2 (MS-CASPT2), are performed for the ground and excited states of radical anions consisting of two pi-stacked nucleobases. The electronic couplings for excess electron transfer (EET) in the pi-stacks are estimated by using the generalized Mulliken-Hush approach. We compare results obtained within the different methods with data derived using Koopmans' theorem approximation at the Hartree-Fock level. The results suggest that although the one-electron scheme cannot be applied to calculate electron affinities of nucleobases, it provides reasonable estimates for EET energies. The electronic couplings calculated with KTA lie between the CASPT2 and the MS-CASPT2 based values in almost all cases.     

Photolytic Decarbonylation of Oxalyl Diisothiocyanate in Solid Argon Matrices to syn-anti Carbonyl Diisothiocyanate and Its Isomerization

Oxalyl diisothiocyanate, ((CO)NCS)₂, has been studied in solid argon matrices at 4.2 K with the aid of infrared (IR) spectroscopy. The spectra show mainly signals attributed to the most stable anti-anti conformer, which is corroborated by comparison to computed anharmonic fundamental IR transitions. Upon irradiation with 254 nm UV light, oxalyl diisothiocyanate eliminates carbon monoxide under formation of carbonyl diisothiocyanate, CO(NCS)₂. This reaction is only slightly exothermic by 0.4 kcal mol⁻¹ at the DLPNO-CCSD(T)/def2-QZVPP//B3LYP-D3/def2-TZVPP level of theory. Remarkably, photolysis produces mostly the less stable syn-anti conformer of carbonyl diisothiocyanate. Subsequent annealing at 30 K for two minutes results in a structural relaxation to the 0.7 kcal mol⁻¹ more stable syn-syn conformer confirming a low torsional barrier height between the isomers.     

Understanding the Deactivation Pathways of Iridium(III) Pyridine-Carboxiamide Catalysts for Formic Acid Dehydrogenation

The degradation pathways of highly active [Cp*Ir(κ²-N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO₃), 2 undergoes partial protonation of the amide moiety, inducing rapid κ²-N,N to κ²-N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ²-N,N key intermediate of FA dehydrogenation ( I_NH ), bearing a N-protonated pica, can easily transform into the κ²-N,O analogue ( I_NH2 ; ΔG^≠≈11 kcal mol⁻¹, ΔG ≈−5 kcal mol⁻¹). Intramolecular hydrogen liberation from I_NH2 is predicted to be rather prohibitive (ΔG^≠≈26 kcal mol⁻¹, ΔG≈23 kcal mol⁻¹), indicating that FA dehydrogenation should involve mostly κ²-N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c . DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.     

Thermal isomerization of acetylnitrene: a quantum-chemical study

The electronic structure and pathways of thermal isomerization of formylnitrene and acetylnitrene were studied by the B3LYP/6-311G(d,p) density functional method and ab initio G2(MP2,SVP) computational procedure using the geometries obtained from B3LYP calculations. According to G2 calculations, both nitrenes have singlet ground states while the energies of the corresponding triplet states are 2.8 and 5.7 kcal mol^–1 higher. For acetylnitrene, the activation barrier to the nitrene isocyanate isomerization was estimated at 28.9 kcal mol^–1 (G2). Calculations revealed no pathway for single-step isomerization of nitrene into cyanate in both systems. The formation of methyl cyanate from isocyanate is thermodynamically unfavorable (E = 26.5 kcal mol^–1) and requires a high activation barrier (89.4 kcal mol^–1) should be overcome. Based on the results obtained, the pathways of transformation of nitrene formed in thermal decomposition of acetyl azide (Curtius rearrangement) were analyzed.     

The role of Ag(I) ions in the electronic spectroscopy of adenine–cytosine mispairs: A MS-CASPT2 theoretical study

Adenine–cytosine (AC) mispairs have been theoretically studied with MS-CASPT2//CASSCF methods in the presence and absence of Ag ions. The electronically excited states of the most stable AC mispair in the reverse-Wobble (RW) conformation have been compared with those of different Ag(I)–AC complexes, including (i) metalated RW conformations, and (ii) the most stable structures in gas phase which contain the Ag ion bridging A and C. The spectra of these complexes are characterized by charge-transfer (CT) and strong locally excited (LE) states. The metal-to-metal, metal-to-ligand, and Rydberg transitions are very weak in comparison to the nucleobase transitions. Attending to the LE and CT states, and except for the shifts induced by the presence of the Ag, the electronic spectrum of metalated AC mispairs resembles the one of the RW, showing two intense LE bands around 4.5 and 5.5 eV, corresponding to transitions within the adenine and cytosine π-system, respectively. Additionally TD-DFT results obtained with the B3LYP functional are compared with MS-CASPT2//CASSCF calculations. The results clearly evidence the weakness of TD-DFT to describe long range exchange interactions leading to strongly underestimated CT states.