Multiconfigurational Perturbation Theory Research Articles

Spin-orbit ab initio investigation of the photolysis of o-, m-, and p-bromotoluene

The photodissociations of o-, m-, and p-bromotoluene were investigated by ab initio and spin-orbit ab initio calculations. The possible photodissociation mechanisms at 266 and 193 nm were clarified by multistate second order multiconfigurational perturbation theory (MS-CASPT2) calculated potential energy curves, vertical excitation energies, and oscillator strengths of low-lying states. The dissociation products with spin-orbit-coupled states of Br(*)((2)P(12)) and Br((2)P(32)) were identified by the MS-CASPT2 method in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) calculations. The effects of methyl rotation and substituent on the photodissociation mechanism were discussed.

Recent applications of a QM/MM scheme at the CASPT2//CASSCF/AMBER (or CHARMM) level of theory in photochemistry and photobiology

The excited-state properties of chemically different chromophores embedded in diverse protein environments or in solution can be nowadays correctly evaluated by means of a hybrid quantum mechanics/molecular mechanics (QM/MM) computational strategy based on multiconfigurational perturbation theory and complete-active-space-self-consistent-field geometry optimization. In particular, in this article we show how a QM/MM strategy has been recently developed in our laboratory and has been successfully applied to the investigation of the fluorescence of the green fluorescent protein (GFP) and how the same strategy (embedding the chromophores in methanol solution) has been combined with retrosynthetic analysis to design a prototype light-driven Z/E molecular switch featuring a single reactive double bond and the same electronic structure and photoisomerization mechanism of the chromophore of the visual pigment Rhodopsin.

Relative energy of the high-(T2g5) and low-(A1g1) spin states of the ferrous complexes [Fe(L)(NHS4)]: CASPT2 versus density functional theory

High-level ab initio calculations using multiconfigurational perturbation theory [complete active space with second-order perturbation theory (CASPT2)] were performed on the transition energy between the lowest high-spin (corresponding to (5T2g) in Oh) and low-spin (corresponding to 1A1g in Oh) states in the series of six-coordinated Fe(II) molecules [Fe(L)(NHS4)], where NHS4 is 2,2'-bis(2-mercaptophenylthio)diethylamine dianion and L=NH3, N2H4, PMe3, CO, and NO+. The results are compared to (previous and presently obtained) results from density functional theory (DFT) calculations with four functionals, which were already shown previously by Casida and co-workers [Fouqueau et al., J. Chem. Phys. 120, 9473 (2004); Ganzenmuller et al., ibid. 122, 234321 (2005); Fouqueau et al., ibid. 122, 044110 (2005); Lawson Daku et al., ChemPhysChem 6, 1393 (2005)] to perform well for the spin-pairing problem in these and other Fe(II) complexes, i.e., OLYP, PBE0, B3LYP, and B3LYP*. Very extended basis sets were used both for the DFT and CASPT2 calculations and were shown to be necessary to obtain quantitative results with both types of method. This work presents a sequel to a previous DFT/CASPT2 study of the same property in the complexes [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+ [Pierloot et al., J. Chem. Phys. 125, 124303 (2006)]. The latter work was extended with new results obtained with larger basis sets and including the OLYP functional. For all considered complexes, the CASPT2 method predicts the correct ground state spin multiplicity. Since experimental data for the actual quintet-singlet (free) energy differences are not available, the performance of the different DFT functionals was judged based on the comparison between the DFT and CASPT2 results. From this, it was concluded that the generalized gradient OLYP functional performs remarkably well for the present series of ferrous compounds, whereas the success of the three hybrid functionals varies from case to case.

Spin-orbit ab initio calculation of photodissociation of methyl iodide

The effective potential energy curves correlated to 2I03/2 and 2I01/2* atom limits are calculated using second order spin-obit multiconfigurational quasidegenerate perturbation theory (SO-MCQDPT). The absorption spectra of CH3I molecule are calculated, photodissociation processes of CH3I molecule are analyzed and the quantum yields of 2I01/2* atom are estimated. The present calculations can be used to interpret the experimental results.

Approach to the Atmospheric Chemistry of Methyl Nitrate and Methylperoxy Nitrite. Chemical Mechanisms of Their Formation and Decomposition Reactions in the Gas Phase

Potential energy surfaces, minimum energy reaction paths, minima, transition states, reaction barriers, and conical intersections for the most important atmospheric reactions of methyl nitrate (CH(3)ONO(2)) and methylperoxy nitrite (C(3)HOONO) on the electronic ground state have been studied (i) with the second-order multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary points and (ii) with the density functional theory (DFT). The proposed mechanism explains the conversion of unreactive alkyl peroxy radicals into alkoxy radicals: CH(3)O(2) + NO <=> CH(3)OONO <=> CH(3)O + NO(2) left arrow over right arrow CH(3)ONO(2). Additionally, several discrepancies found in the comparison of the results obtained from the two employed approaches are analyzed. CASPT2 predicts that all dissociation reactions into radicals occur without an extra exit energy barrier. In contrast, DFT finds transition states for the dissociations of cis- and trans-methylperoxy nitrite into CH(3)O + NO(2). Furthermore, multiconfigurational methods [CASPT2 and complete active space SCF (CAS-SCF)] predict the isomerization of CH3ONO2 to CH3OONO to occur in a two-step mechanism: (i) CH(3)ONO(2) --> CH(3)O + NO(2); and (ii) CH(3)O + NO(2) --> CH(3)OONO. The reason for this has to do with the coupling of the ground electronic state with the first excited state. Therefore, it is demonstrated that DFT methods based on single determinantal wave functions give an incorrect picture of the aforementioned reaction mechanisms.

Identifying the low-lying electronic states of anthracene-9,10-endoperoxide

The electronic excited states of anthracene-9,10-endoperoxide giving rise to singlet oxygen generation ( 1O 2) or homolytic O–O cleavage have been unambiguously characterized using multi-configurational second order perturbation theory MS-CASPT2/CASSCF, with inclusion of solvent interactions, femtosecond polarization resolved UV/IR pump–probe, and steady-state fluorescence experiments. The S 1 state, responsible for homolytic O–O cleavage, is observed at 34 400 cm −1. Determination of the transition dipole moment direction of the 37 000 cm −1 band, confirms the assignment of the S 4 state, from which 1O 2 is generated, and allows assigning the other electronic states. Furthermore, it is demonstrated that APO can emit after electronic excitation.

Computational studies on the dimers and the thermal dimerization of norbornadiene

In this study, 14 norbornadiene (NBD) dimers and the thermal dimerization mechanism were studied using the hybrid density functional theory (B3LYP) and the second-order multiconfigurational perturbation theory (CASPT2). In the process of dimerization, the biradical stationary points were located using the unrestricted, broken-spin, symmetry approach. The pathways were divided into eight parts to aid the analysis of their mechanisms. Our results indicated that the process for the formation of the cage-like heptacyclo[6.6.0.0.260.3130.411059.01014] tetradecane (HCTD, D14) is highly exothermic (92.15 kcal/mol), indicating that D14 is the most stable NBD dimer. However, the formation of D14 is very difficult to achieve kinetically because of a higher barrier in the thermal dimerization. On the contrary, the formation of exo-cis-exo (D5) is kinetically favorable, but thermodynamically unfavorable at higher temperature. Therefore, the combination of both thermodynamic and kinetic factors indicated that the formation of exo-exo (D9), which resembles the product of the pseudo-Diels-Alder addition, is most likely in the NBD dimerization.

Theoretical investigation of the energies and geometries of photoexcited uranyl(VI) ion: A comparison between wave-function theory and density functional theory

In order to assess the accuracy of wave-function and density functional theory (DFT) based methods for excited states of the uranyl(VI) UO2(2+) molecule excitation energies and geometries of states originating from excitation from the sigma(u), sigma(g), pi(u), and pi(g) orbitals to the nonbonding 5f(delta) and 5f(phi) have been calculated with different methods. The investigation included linear-response CCSD (LR-CCSD), multiconfigurational perturbation theory (CASSCFCASPT2), size-extensivity corrected multireference configuration interaction (MRCI) and AQCC, and the DFT based methods time-dependent density functional theory (TD-DFT) with different functionals and the hybrid DFTMRCI method. Excellent agreement between all nonperturbative wave-function based methods was obtained. CASPT2 does not give energies in agreement with the nonperturbative wave-function based methods, and neither does TD-DFT, in particular, for the higher excitations. The CAM-B3LYP functional, which has a corrected asymptotic behavior, improves the accuracy especially in the higher region of the electronic spectrum. The hybrid DFTMRCI method performs better than TD-DFT, again compared to the nonperturbative wave-function based results. However, TD-DFT, with common functionals such as B3LYP, yields acceptable geometries and relaxation energies for all excited states compared to LR-CCSD. The structure of excited states corresponding to excitation out of the highest occupied sigma(u) orbital are symmetric while that arising from excitations out of the pi(u) orbitals have asymmetric structures. The distant oxygen atom acquires a radical character and likely becomes a strong proton acceptor. These electronic states may play an important role in photoinduced proton exchange with a water molecule of the aqueous environment.

On the applicability of multireference second‐order perturbation theory to study weak magnetic coupling in molecular complexes

The performance of multiconfigurational second-order perturbation techniques is established for the calculation of small magnetic couplings in heterobinuclear complexes. Whereas CASPT2 gives satisfactory results for relatively strong magnetic couplings, the method shows important deviations from the expected Heisenberg spectrum for couplings smaller than 15-20 cm(-1). The standard choice of the zeroth-order CASPT2 Hamiltonian is compared to alternative definitions published in the literature and the stability of the results is tested against increasing level shifts. Furthermore, we compare CASPT2 with an alternative implementation of multiconfigurational perturbation theory, namely NEVPT2 and with variational calculations based on the difference dedicated CI technique.

Ab initio characterization of C5

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X1Sigmag+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the nu7(piu) anharmonic fundamental has been calculated to be at 102 cm(-1), which validates the assignment to C5 of the pattern of bands centered at 102 cm(-1) observed with the ISO telescope.

An Opsin Shift in Rhodopsin: Retinal S0−S1 Excitation in Protein, in Solution, and in the Gas Phase

We considered a series of model systems for treating the photoabsorption of the 11-cis retinal chromophore in the protonated Schiff-base form in vacuum, solutions, and the protein environment. A high computational level, including the quantum mechanical-molecular mechanical (QM/MM) approach for solution and protein was utilized in simulations. The S0-S1 excitation energies in quantum subsystems were evaluated by means of an augmented version of the multiconfigurational quasidegenerate perturbation theory (aug-MCQDPT2) with the ground-state geometry parameters optimized in the density functional theory PBE0/cc-pVDZ approximation. The computed positions of absorption bands lambdamax, 599(g), 448(s), and 515(p) nm for the gas phase, solution, and protein, respectively, are in excellent agreement with the corresponding experimental data, 610(g), 445(s), and 500(p) nm. Such consistency provides a support for the formulated qualitative conclusions on the role of the chromophore geometry, environmental electrostatic field, and the counterion in different media. An essentially nonplanar geometry conformation of the chromophore group in the region of the C14-C15 bond was obtained for the protein, in particular, owing to the presence of the neighboring charged amino acid residue Glu181. Nonplanarity of the C14-C15 bond region along with the influence of the negatively charged counterions Glu181 and Glu113 are found to be important to reproduce the spectroscopic features of retinal chromophore inside the Rh cavity. Furthermore, the protein field is responsible for the largest bond-order decrease at the C11-C12 double bond upon excitation, which may be the reason for the 11-cis photoisomerization specificity.

Theoretical investigation on o-, m-, and p-chlorotoluene photodissociations at 193 and 266nm

Photodissociations of the o-, m-, and p-chlorotoluene at 193 and 266 nm were investigated by ab initio calculations with and without spin-orbit interaction. The experimentally observed photodissociation channels were clearly assigned by multistate second order multiconfigurational perturbation theory (MS-CASPT2) calculated potential energy curves. The dissociation products with spin-orbit-coupled states of Cl*(2P1/2) and Cl(2P3/2) were identified by MS-CASPT2 in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) calculations. The effects of methyl rotation and substituent on the photodissociation mechanism were discussed in detail.

A CASPT2//CASSCF study of the quartet excited state $$\tilde{a}^{4}A^{\prime\prime}$$ of the HCNN radical

Complete active space self-consistent field and second-order multiconfigurational perturbation theory methods have been performed to investigate the quartet excited state \({\tilde{a}}^{4}{A^{\prime\prime}}\) potential energy surface of HCNN radical. Two located minima with respective cis and trans structures could easily dissociate to CH \(({\tilde{a}}^{4}\Sigma^{-})\) and \(N_{2} ({\tilde{X}}^{1}\Sigma_{\rm g}^{+})\) products with similar barrier of about 16.0 kcal/mol. In addition, four minimum energy crossing points on a surface of intersection between \({\tilde{a}}^{4}A^{\prime\prime}\) and X (\(X={\tilde{X}}^{2}A^{\prime\prime}\) and \({\tilde{A}}^{2}A^{\prime}\)) states are located near to the minima. However, the intersystem crossing \({\tilde{a}}^{4}A^{\prime\prime} \rightarrow X\) is weak due to the vanishingly small spin–orbit interactions. It further indicates that the direct dissociation on the \({\tilde{a}}^{4}{A^{\prime\prime}}\) state is more favored. This information combined with the comparison with isoelectronic HCCO provides an indirect support to the recent experimental proposal of photodissociation mechanism of HCNN.

Dependence ofN-Nitrosodimethylamine Photodecomposition on the Irradiation Wavelength: Excitation to the S2State as a Doorway to the Dimethylamine Radical Ground-State Chemistry

The photochemistry of N-nitrosodimethylamine after excitation to the S(1) and S(2) states has been studied with the complete active space self-consistent field method (CASSCF) and the second-order multiconfigurational perturbation theory (CASPT2). The calculated vertical transitions agree with the experiment: the S(0) --> S(1) transition occurs at 3.29 eV (f = 0.003 au), the S(0) --> S(2) transition at 5.30 eV (f = 0.17 au) and the first excited triplet state is computed at 2.48 eV. Solvent effects have been reproduced by means of PCM calculations. It is shown that excitation to S(1) and S(2) yields the same photoproducts: (CH(3))(2)N (1(2)B(1)) and NO (X(2)Pi). However, while on S1 the process is adiabatic, the process on S(2) implies an ultrafast decay through a planar S(2)/S(1) conical intersection. Our calculations are consistent with the reversibility of the N-N dissociation after irradiation at 363.5 nm and the observed dimethylamine radical reactions when irradiated at 248 nm, namely, H extrusion, a one-step process (41.3 kcal/mol), and methyl radical extrusion, a two-step process (44.0 kcal/mol and 31.5 kcal/mol). Finally, two more aspects are considered: (i) the topology of the T(1) surface where two minima have been found to correlate with the phosphorescence emission band and (ii) the influence of tautomerizations which is shown to be neglectable.

Tracking the excited-state time evolution of the visual pigment with multiconfigurational quantum chemistry.

The primary event that initiates vision is the photoinduced isomerization of retinal in the visual pigment rhodopsin (Rh). Here, we use a scaled quantum mechanics/molecular mechanics potential that reproduces the isomerization path determined with multiconfigurational perturbation theory to follow the excited-state evolution of bovine Rh. The analysis of a 140-fs trajectory provides a description of the electronic and geometrical changes that prepare the system for decay to the ground state. The data uncover a complex change of the retinal backbone that, at approximately 60-fs delay, initiates a space saving "asynchronous bicycle-pedal or crankshaft" motion, leading to a conical intersection on a 110-fs time scale. It is shown that the twisted structure achieved at decay features a momentum that provides a natural route toward the photoRh structure recently resolved by using femtosecond-stimulated Raman spectroscopy.

Spin‐Orbit Ab Initio Investigation of the Ultraviolet Photolysis of Diiodomethane

The UV photodissociation (<5 eV) of diiodomethane (CH(2)I(2)) is investigated by spin-orbit ab initio calculations. The experimentally observed photodissociation channels in the gas and condensed phases are clearly assigned by multi-state second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space-state interaction potential energy curves. The calculated results indicate that the fast dissociations of the first two singlet states of CH(2)I(2) and CH(2)I--I lead to geminate-radical products, CH(2)I (.)+I((2)P(3/2)) or CH(2)I (.)+ I*((2)P(1/2)). The recombination process from CH(2)I--I to CH(2)I(2) is explained by an isomerization process and a secondary photodissociation reaction of CH(2)I--I. Finally, the study reveals that spin-orbits effects are significant in the quantitative analysis of the electronic spectrum of the CH(2)I--I species.

Effects of water re-location and cavity trimming on the CASPT2//CASSCF/AMBER excitation energy of Rhodopsin

We have used quantum-mechanics/molecular-mechanics computations based on ab initio multiconfigurational perturbation theory to determine and rationalize the effect of the re-location of one crystallographic water molecule on the vertical excitation energy of the visual pigment rhodopsin. It is found that the re-location of one water molecule to the opposite side of the 11-cis retinal chromophore leads to a large 0.7–0.8 A contraction in the chromophore—counterion salt-bridge distance. In spite of this structural effect, the change in excitation energy is found to be limited (< 1.5 kcal mol−1). Through an analysis of different rhodopsin models in terms of “components” (isolated chromophore, isolated chromophore—counterion ion-pair and models deprived of the counterion charges) we show that the limited change of the excitation energy can be related to a displacement of the retinal chromophore to a different spot of the protein cavity.

Spin–orbit splitting in the ground X2 Π electronic state of the and molecular ions

Potential energy curves for electronic states 2Π1/2 and 2Π3/2 of the and molecular ions have been calculated in the framework of the spin–orbit multi-configuration quasi-degenerate perturbation theory (SO-MCQDPT) under model core potential (MCP) methods. In the present theoretical study, the calculated potential energy curves were fitted to analytical potential energy functions (APEF) using the Murrell–Sorbie potential function, from the which accurate spectroscopic parameters were derived. The spin–orbit coupling constants for and were determined to be 463.990 and 545.734 cm−1, respectively, at the MCQDPT/MCP level, which are in good agreement with the experimental values. In addition, the theoretical spin–orbit coupling constants for S (3P2,1,0) were also found to be consistent with the experimental results.

Spin–orbit ab initio investigation of the photodissociation of CH 2Cl 2

The photodissociation of dichloromethane (CH 2Cl 2) has been investigated by spin–orbit ab initio calculations. The potential energy curves were calculated by multi-state second order multiconfigurational perturbation theory in conjunction with spin–orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO). The calculated results clearly assigned the experimentally observed photodissociation channels from the first and second singlet excited states leading to CH 2Cl + Cl( 2P 3/2) and CH 2Cl + Cl ∗( 2P 1/2). The reason of higher Cl ∗/Cl ratios in vibrationally mediated photodissociation than in vibrationless one was discussed.

The nature of the [20.0]Σ+1 electronic state of RhB: A multiconfigurational study

Multiconfigurational second order perturbation theory, with extended atomic basis sets and inclusion of scalar relativistic effects, was employed to investigate the low-lying (1)Sigma(+) electronic states of RhB. The [20.0] (1)Sigma(+) state is represented by a single configuration, mid R:[ellipsis (horizontal)]10sigma(2)11sigma(1)5pi(4)2delta(4)12sigma(1), derived from a single excitation (11sigma-->12sigma) from the ground state, which defines its electronic nature. A new excited state, coined as [9.0] (1)Delta (R(0)=1.786A, DeltaG(12)=792 cm(-1)), located 9221 cm(-1) above the X(1)Sigma(+) state, and described by the |...10sigma(2)11sigma(2)5pi(4)2delta(3)12sigma(1)> electronic configuration, was also identified.