Electric properties of the water molecule in 1A1, 1B1, and 3B1 electronic states: Refined CASSCF and CASPT2 calculations

Summary The dipole moments and dipole polarizabilities of the ¹A₁, ¹B₁, and ³B₁ electronic states of the water molecule have been calculated by using the CASSCF approach followed by the evaluation of the dynamic electron correlation contribution by the second-order perturbation scheme CASPT2. All calculations have been carried out in a specifically extended ANO basis set which accounts for the Rydberg character of the two excited states. In order to estimate the correctness and accuracy of the present data a scan over a variety of different active spaces for the CASSCF wave function has been made. The present results are superior to earlier CASSCF calculations, although their qualitative features remain essentially the same. The dipole moments in ¹B₁ and ³B₁ states are predicted to be about 0.49 a.u. and 0.33 a.u., respectively, and have the opposite orientation with respect to the ground state dipole moment. The dipole polarizability tensors of the excited states are characterized by high anisotropy and are dominated by the in-plane component perpendicular to the symmetry axis. All their components are found to be about an order of magnitude larger than those of the ground state polarizability tensor. The excitation energy dependence on the choice of the active orbital space in the CASSCF reference function is also considered and the analysis of the present data concludes in the concept of what is called the mutually compatible active spaces for the two states involved in excitation. All CASPT2 results are in good agreement with the results of recent calculations carried out in the framework of the open-shell coupled cluster formalism. This agreement confirms the high efficiency of the CASSCF/CASPT2 approach to the treatment of the electron correlation effects.     

A CASSCF/CASPT2 insight into excited‐state intramolecular proton transfer of four imidazole derivatives

Excited‐state intramolecular proton transfer (ESIPT) of four imidazole derivatives, 2‐(2′‐hydroxyphenyl)imidazole (HPI), 2‐(2′‐hydroxyphenyl)benzimidazole (HPBI), 2‐(2′‐hydroxyphenyl)‐1H‐phenanthro[9,10‐d]imidazole (HPPI) and 2‐(2′‐hydroxyphenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d]imidazole (HPPPI), were studied by the sophisticated CASSCF/CASPT2 methodology. The state‐averaged SA‐CASSCF method was used to optimize their geometry structures of S₀ and S₁ electronic states, and the CASPT2 calculations were used for the calibration of all the single‐point energies, including the absorption and emission spectra. A reasonable agreement is found between the theoretical predictions and the available experimental spectral data. The forward ESIPT barriers of four target compounds gradually decrease with the increase of molecular size. On the basis of the present calculations, it is a plausible speculation that the larger the size, the faster is the ESIPT rate, and eventually, HPPPI molecule can undergo a completely barrierless ESIPT to the more stable S₁ keto form. Additionally, taking HPI as a representative example, the radiationless decays connecting the S₀ and S₁/S₀ conical intersection structures were also studied by constructing a linearly interpolated internal coordinate (LIIC) reaction path. The qualitative analysis shows that the LIIC barrier of HPI in the keto form is remarkably lower than that of its enol‐form, indicating that the former has a big advantage over the latter in the nonradiative process. © 2015 Wiley Periodicals, Inc.     

Ab initio study of the photochemistry of aminopyrimidine

Complete active space self‐consistent field (CASSCF), multi‐reference configuration interaction calculations (MR‐CISD), and equation of motion coupled‐cluster with singles and doubles (EOM‐CCSD) calculations are presented in order to elucidate the photodeactivation pathways of 6‐aminopyrimidine after vertical excitation to the S₁ ¹nπ* state. Vertical excitation energies are reported up to the S₇ state. Two S₁ excited state minima, both of ¹nπ* character, and three strongly puckered ¹ππ* minima on the crossing seam (MXS) between the S₀ and the S₁ potential energy surface were found. Nonadiabatic reaction paths are discussed by linearly interpolating between the two minima and all MXS, which explain and extend observations made in recent surface‐hopping dynamics CASSCF investigations [Barbatti and Lischka, J Phys Chem A 2007, 111, 2852]. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

The photoisomerization of 11‐cis‐retinal protonated schiff base in gas phase: Insight from spin‐flip density functional theory

This extensive theoretical study employed the spin‐flip density functional theory (SFDFT) method to investigate the photoisomerization of 11‐cis‐retinal protonated Schiff base (PSB11) and its minimal model tZt‐penta‐3,5‐dieniminium cation (PSB3). Our calculated results indicate that SFDFT can perform very well in describing the ground‐ and excited‐state geometries of PSB3 and PSB11. We located the conical intersection (CI) point and constructed the photoisomerization reaction path of PSB3 and PSB11 by using the SFDFT method. To further verify the SFDFT results, we computed the energy profiles along the constructed linearly interpolated internal coordinate (LIIC) pathways by using high‐level theoretical methods, such as the EOM‐CCSD, CR‐EOM‐CCSD(T), CASPT2, NEVPT2, and XMCQDPT2 methods. The SFDFT method predicts that the photoisomerization of PSB3 is barrierless, in accordance with previous complete‐active‐space self‐consistent‐field (CASSCF) results. However, an energy barrier is predicted along the LIIC pathways of PSB11. This finding is different from previous CASSCF results and may indicate that the photoisomerization of PSB11 in gas phase is similar to that in solution. However, the higher spin contamination of the SFDFT method in the vicinity of the CI point caused the located CI geometry to deviate from that of the real CI. In addition, the LIIC pathways are only approximations to the minimum energy path (MEP). Thus, further experimental and theoretical studies are needed to verify the existence of an energy barrier along the photoisomerization reaction path of PSB11 in gas phase. © 2013 Wiley Periodicals, Inc.     

Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections

We employed the complete active space self‐consistent field (CASSCF) and its multistate second‐order perturbation (MS‐CASPT2) methods to explore the photochemical mechanism of 2‐hydroxyazobenzene, the molecular scaffold of Sudan I and Orange II dyes. It was found that the excited‐state intramolecular proton transfer (ESIPT) along the bright diabatic ¹ππ* state is barrierless and ultrafast. Along this diabatic ¹ππ* relaxation path, the system can jump to the dark ¹nπ* state via the ¹ππ*/¹nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay ¹ππ* keto and ¹nπ* enol species to the ground state via two energetically accessible S₁/S₀ conical intersections. Finally, we encountered an interesting phenomenon in the excited‐state hydrogen‐bonding strength: it is reinforced in the ¹ππ* state, whereas it is reduced in the ¹nπ* state. The present work sets the stage for understanding the photophysics and photochemistry of Sudan I–IV, Orange II, Ponceau 2R, Ponceau 4R, and azo violet.     

Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene

The S₀ and S₁ potential energy surfaces of pentalene were studied using MMVB—a hybrid force-field/parametrized valence bond (VB) method designed to simulate CASSCF calculations for ground and covalent excited states. The results were calibrated against full CASSCF calculations. Four distinct critical points were optimized: on S₀, a C_2h minimum (with alternating single and double bonds) and a D_2h transition structure; and on S₁, a D_2h minimum and an adjacent S₁/S₀ conical intersection. A VB exchange density matrix (which is independent of the choice of the spin-coupled basis) was used to rationalize the S₀ and S₁ surface topologies. Craig defined pseudoaromatic molecules to be those with nontotally symmetric electronic ground states. For pentalene, this is true for both CASSCF and MMVB calculations: the CASSCF S₀ transition structure is an open-shell B_1x singlet, and the VB ground state is dominated by a spin-coupling which transforms as B_1g. A C_2v minimum and a D_2h transition structure were located on the CASSCF S₂ potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S₁ S₂ states of pentalene are shown to be reverse of the S₁ and S₂ states of benzene. © 1996 John Wiley & Sons, Inc.     

A CAS study on S‐loss and O‐loss dissociation mechanisms of the SO2+ ion in the C,D, and E states

In the present work, we mainly study dissociation of the C ²B₁, D²A₁, and E²B₂ states of the SO₂⁺ ion using the complete active‐space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S‐ and O‐loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S‐loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S‐loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin‐orbit couplings were calculated at the MECPs. Our calculations predict eight S‐loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O‐loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O‐loss limit. Our calculations also predict that the E²B₂ state could dissociate to the first S‐ and O‐loss limits via the A²B₂ ← E²B₂ transition. On the basis of the 13 predicted processes, we discussed the S‐ and O‐loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Ab initio quantum chemical study of the formation, decomposition and isomerization of the formaldiminoxy radical (CH2NO): comparison of the Gaussian-2 and CASPT2 techniques in the calculation of potential energy surfaces

The reaction between triplet methylene and nitric oxide, producing the formaldiminoxy (CH₂NO) radical, and the subsequent decomposition and isomerization reactions of CH₂NO have been studied using ab␣initio quantum chemical techniques that include the Gaussian-2 (G2), CASSCF and CASPT2 methods. Stationary points on the potential energy surfaces were located at MP2/6-31G(d) and CASSCF/cc-pVDZ levels of theory, while the electronic energies were determined using G2, G2(MP2), QCISD(T)/cc-pVTZ, RCCSD(T)/cc-pVTZ and CASPT2/cc-pVTZ approaches. G2 is believed to be reliable at equilibrium geometries, but the determination of certain transition state geometries and energies requires a MCSCF-based approach. The calculations suggest that CH₂NO (²A^′) forms in a barrierless reaction and could readily decompose to H+HCNO. A subsequent abstraction reaction then results in H₂+CNO. No molecular elimination channel was found. An alternative pathway is the formation of CH₂ON, which readily isomerizes to CH₂NO. Received: 8 May 1998 / Accepted: 11 August / Published online: 9 October 1998     

Insights into the Photoinduced Isomerization Mechanisms of a N,C–Chelate Organoboron Compound: A Theoretical Study

As the first discovered organoboron compound with photochromic property, B(ppy)Mes₂ (ppy=2-phenylpyridine, Mes=mesityl) displays rich photochemistry that constitutes a solid foundation for wide applications in optoelectronic fields. In this work, we investigated the B(ppy)Mes₂ to borirane isomerization mechanisms in the three lowest electronic states (S₀, S₁, and T₁) based on the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT) calculations. Our results show that the photoisomerization in the S₁ state is dominant, which is initiated by the cleavage of the B-C_ppy bond. After overcoming a barrier of 0.5 eV, the reaction pathway leads to a conical intersection between the S₁ and S₀ states (S₁/S₀)_x, from which the decay path may go back to the reactant B(ppy)Mes₂ via a closed-shell intermediate (Int1-S₀) or to the product borirane via a biradical intermediate (Int2-S₀). Although triplet states are probably involved in the photoinduced process, the possibility of the photoisomerization in T₁ state is very small owing to the weakly allowed S₁→T₁ intersystem crossing and the high energy barrier (0.77 eV). In addition, we found the photoisomerization is thermally reversible, which is consistent with the experimental observations.     

Photophysics of intramolecularly hydrogen-bonded aromatic systems: ab initio exploration of the excited-state deactivation mechanisms of salicylic acid

Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. At crucial points of the potential energy hypersurfaces, single-point energy calculations have been performed with the CASPT2 method (second-order perturbation theory based on the complete active space self-consistent field reference). Hydrogen transfer along the intramolecular hydrogen bond as well as torsion and pyramidization of the carboxy group have been identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer represents a local minimum of the S(1) energy surface, which is separated by a very low barrier from a reaction path leading to a low-lying S(1)-S(0) conical intersection via torsion and pyramidization of the carboxy group. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the carboxy group, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the two known minima of the S(0) potential energy surface (rotamer I, rotamer II). A novel structure, 7-oxa-bicyclo[4.2.0]octa-1(6),2,4-triene-8,8-diol, has been identified as a possible transient intermediate in the photophysics of salicylic acid.     

Decay Dynamics of N,N-Dimethylthioacetamide in S3(ππ*) State (cited: 1)

The decay dynamics of N,N-dimethylthioacetamide after excitation to the S₃(ππ^*) state was studied by using the resonance Raman spectroscopy and complete active space selfconsistent field method calculations. The UV-absorption and vibrational spectra were assigned. The A-band resonance Raman spectra were obtained in acetonitrile, methanol and water with the laser excitation wavelengths in resonance with the first intense absorption band to probe the Franck-Condon region structural dynamics. The CASSCF calculations were carried out to determine the excitation energies and optimized structures of the lowerlying singlet states and conical intersection point. The A-band structural dynamics and the corresponding decay mechanism were obtained by the analysis of the resonance Raman intensity pattern and the CASSCF calculated structural parameters. The major decay channel of _3,FC(ππ^*)→S₃(ππ^*)/S₁(nπ^*)→_{1(nπ^*) is proposed.}     

4-硝基咪唑A-带激发态结构动力学

通过共振拉曼光谱实验和量子化学计算的方法研究了4-硝基咪唑(4NI)A-带激发态衰变动力学. 对4NI的振动光谱、紫外电子吸收光谱、荧光光谱和共振拉曼光谱进行了指认. 在全活化空间自洽场法(CASSCF)/6-31G(d)计算水平下获得了单重激发态S₁(n_Oπ^*)和S₂(ππ^*)和势能面交叉点S₁(n_Oπ^*)/S₂(ππ^*)的优化几何结构和能量, 分析了A-带共振拉曼光谱的强度模式特征, 获得了短时结构动力学, 并结合全活化空间自洽场法(CASSCF)理论计算结果确定了4NI 在S₂(ππ^*)态衰变通道主要是S_{2, FC}→S_{2, min}(ππ^*)→S₀辐射弛豫.     

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.     

Mechanistic Photodissociation of Glycolaldehyde: Insights from Ab Initio and RRKM Calculations

Herein we report a theoretical study on mechanistic photodissociation of glycolaldehyde, HOCH₂CHO. Equilibrium structures, transition states, and intersection structures for the α‐C? C and ‐C? H bond fissions and the β‐C? O bond fission in the excited states are determined by the complete active space self‐consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations are refined by performing single‐point calculations using the multi‐state multi‐reference CASSCF second order perturbation (MS‐MR‐CASPT2) method. With a low excitation energy of 280–340 nm, the T₁ α‐C? C and β‐C? O bond fissions following intersystem crossing from the S₁ state are the predominant and comparable channels, whereas the α‐C? H bond fissions both in the S₁ and in the T₁ states are nearly prohibited due to the relevant high barriers. The rate constants for the T₁ α‐C? C and β‐C? O bond fissions are also calculated by RRKM theory. Furthermore, the S₀ reactions can occur as a consequence of intersystem crossing via T₁/S₀ intersection points resulting from the T₁ C? C and C? O bond cleavages. This photodissociation mechanism is consistent with recent experimental studies.     

A CAS SCF study of reactive interactions between Be(3 P) and H2(1ε g + )

Summary TheC _2v symmetry section of the Be(³ P)+ H₂(¹ _g ⁺ ) adiabatic energy surface is investigated by using the CAS SCF method. The small active space CAS SCF calculations in the valence approximation are followed by a perturbation treatment of the dynamic, core, and core-valence contributions in the framework of the CASPT2 method. The possibility of the nonradiative chemical deactivation of the lowest triplet state of Be by the insertion mechanism is studied. The structure of the³ B ₂ reaction intermediate BeH₂ is established. The calculations show that the symmetric dissociation of this intermediate into Be(¹ S) and 2H(² S) does not involve any barrier beyond the endothermicity of the corresponding reaction. The hydrogen abstraction mechanism via a linear configuration is shown to possess the activation barrier of about 25 mH.     

Theoretical study on low‐lying states of HAlO+ and HOAl+ cations

Some low‐lying states of HAlO⁺ and HOAl⁺ cations have been studied using the complete‐active‐space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) methods with the contracted atomic natural orbital (ANO) basis sets. The geometries of all stationary points along the potential energy surfaces were optimized at the CASSCF/ANO and CASPT2/ANO levels. The ground and the first excited states of HAlO⁺ are predicted to be X²Π and A²Σ⁺ states, respectively. It was predicted that the ground state of HOAl⁺ is X²Σ⁺ state. The A²Π state of HOAl⁺ has unique imaginary frequency. A bent local minimum M1 was found along the 1²A″ potential energy surface, and the A²Π state of HOAl⁺ should be the transition state of the isomerization reactions for M1 ? M1. The CASPT2/ANO potential energy curves of isomerization reactions were calculated as a function of HAlO bond angle. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Energetics of cytosine singlet excited-state decay paths--a difficult case for CASSCF and CASPT2

Three deactivation paths for singlet excited cytosine are calculated at the CASPT2//CASSCF (complete active space second-order perturbation//complete active space self-consistent field) level of theory, using extended active spaces that allow for a reliable characterization of the paths and their energies. The lowest energy path, with a barrier of approximately 0.1 eV, corresponds to torsion of the C5-C6 bond, and the decay takes place at a conical intersection analogous to the one found for ethylene and its derivatives. There is a further path with a low energy barrier of approximately 0.2 eV associated with the (n(N),pi*) state which could also be populated with a low energy excitation. The path associated with a conical intersection between the ground and (n(O),pi*) states is significantly higher in energy (> 1 eV). The presence of minima on the potential energy surface for the (n,pi*) states that could contribute to the biexponential decay found in the gas phase was investigated, but could not be established unequivocally.     

Efficient photochemical merocyanine-to-spiropyran ring closure mechanism through an extended conical intersection seam. A model CASSCF/CASPT2 study

A mechanism of the thermal and photochemical bleaching of merocyanine to spiropyran is proposed on the basis of CASSCF/CASPT2 calculations on the 6-(2-propenyliden)cyclohexadienone model system. Our results suggest that this photochemical transformation takes place in two steps. First, the initially pumped 1(pi-pi) S2 undergoes radiationless decay to 1(n-pi) S1 via an extended S2/S1 conical intersection seam that runs approximately parallel to the trans-to-cis isomerization coordinate, a few kilocalories per mole higher in energy. Thus, S2 --> S1 internal conversion is possible at all values of the S2 trans-to-cis reaction coordinate. Second, on the S1 potential energy surface, there is a barrierless ring closure reaction path from the S1 cis minimum that leads to a peaked S1/S0 conical intersection where the deactivation to the ground state takes place. The inertia of the moving nuclei then drives the system toward the ground-state minimum of the 2H-chromene product. Thus, the extended seam topology of the S2/S1 conical intersection and the coordinate of the branching space of the S1/S0 conical intersection are essential to explain the efficiency and high speed of this reaction.     

Initial excited‐state relaxation of locked retinal protonated schiff base chromophore. An insight from coupled cluster and multireference perturbation theory calculations

The initial S₁ excited‐state relaxation of retinal protonated Schiff base (RPSB) analog with central C11C12 double bond locked by eight‐membered ring (locked‐11.8) was investigated by means of multireference perturbation theory methods (XMCQDPT2, XMS‐CASPT2, MS‐CASPT2) as well as single‐reference coupled‐cluster CC2 method. The analysis of XMCQDPT2‐based geometries reveals rather weak coupling between in‐plane and out‐of‐plane structural evolution and minor energetical relaxation of three locked‐11.8 conformers. Therefore, a strong coupling between bonds length inversion and backbone out‐of‐plane deformation resulting in a very steep S₁ energy profile predicted by CASSCF/CASPT2 calculations is in clear contradiction with the reference XMCQDPT2 results. Even though CC2 method predicts good quality ground‐state structures, the excited‐state structures display more advanced torsional deformation leading to ca. 0.2 eV exaggerated energy relaxation and significantly red shifted (0.4–0.7 eV) emission maxima. According to our findings, the initial photoisomerization process in locked‐11.8, and possibly in other RPSB analogs, studied fully (both geometries and energies) by multireference perturbation theory may be somewhat slower than predicted by CASSCF/CASPT2 or CC2 methods. © 2018 Wiley Periodicals, Inc.