CASPT2 Level Research Articles

Microscopic Origin of Different Hydration Patterns of para-Nitrophenol and Its Anion: A Study Combining Multiconfigurational Calculations and the Free-Energy Gradient Method.

A theoretical study of the solvatochromic shifts of para-nitrophenol ( pNP) and para-nitrophenolate anion ( pNP-) in aqueous solution is presented using a QM/MM methodology with molecular dynamics simulation. The optimized structures in aqueous solution are obtained using both the polarizable continuum and the free-energy gradient methods. For pNP, the calculated redshifts at the CASPT2 (12,10) level are, respectively, 0.71 and 0.94 eV, in good agreement with the experimental ones (0.80-0.83 eV), whereas for pNP-, they are small. The difference between the solvatochromic shifts of pNP and pNP- is calculated as 0.71 eV in good agreement with the experimental one (0.79-0.81 eV). Finally, these shifts are understood in terms of the solvent effect on the solute structure, accurately calculated by the present theoretical treatment.

A quantitative tool to establish magic number clusters, ε3, applied in small silicon clusters, Si2-11.

The present work focuses on establishing a function to rank the stability of small silicon clusters to characterize their magic numbers. This function is composed by a thermodynamic descriptor, the atomization Gibbs free energy, and indirect kinetic descriptors, the highest occupied molecular orbital energy and the lowest excitation energy of each system. The silicon clusters geometries were optimized using density functional theory within a hybrid meta-GGA approximation (M06), while the electronic energy was corrected by single-point calculation using CASPT2 level of theory to obtain the molecular properties. Both methodologies were combined with polarized diffused triple zeta, 6-311++G(3df,3pd), basis set for all atoms. Some molecular properties and their combinations were considered to create the aforementioned function to represent the clusters chemical stability and their magic numbers. The chosen stability ranking function, called ε3, presents results in agreement with the previous mass spectrometry experimental data identifying 4, 6, 7 and 10 as magic numbers for small silicon clusters. We believe this stability ranking function can be useful to study other intramolecular atomic and molecular clusters. Graphical abstract Stability ranking function,ε31, applied on Sin (n = 2 - 11) clusters showing Fukui's functions for the Sin (n = 2 - 11) obtained by the electronic density difference through CASPT2//M06/6-311++G(3df,3pd) with an isosurface value equal to 0.003.

Analysis of acetic acid gas phase reactivity: Rate constant estimation and kinetic simulations

The gas phase reactivity of acetic acid was investigated combining first principle calculations with kinetic simulations. Rate constants for the unimolecular decomposition of acetic acid were determined integrating the 1D master equation over a Potential Energy Surface (PES) investigated at the M06-2X/aug-cc-pVTZ level. Energies were computed at the CCSD(T)/aug-cc-pVTZ level using a basis set size correction factor determined at the DF-MP2/aug-cc-pVQZ level. Three decomposition channels were considered: CO2 + CH4, CH2CO + H2O, and CH3 + COOH. Rate constants were computed in the 700–2100 K and 0.1–100 atm temperature and pressure ranges. The simulations show that the reaction is in fall off above 1200 K at pressures smaller than 10 atm. Successively, the PESs for acetic acid H-abstraction by H, OH, OOH, O2, and CH3 were investigated at the same level of theory. Rate constants were computed accounting explicitly for the formation of entrance and exit van der Waals wells and their collisional stabilization. Energy barriers were determined at the CASPT2 level for H-abstraction by OH of the acidic H, since it has a strong multireference character. The calculated rate constant is in good agreement with experiments and supports the experimental finding that at low temperatures it is pressure dependent. The calculated rate constants were used to update the POLIMI kinetic model and to simulate the pyrolysis and combustion of acetic acid. It was found that acetic acid decomposition and the formation of its direct decomposition products can be reasonably predicted. The formation of secondary products, such as H2 and C2 hydrocarbons, is underpredicted. This suggests that reaction routes not incorporated in the model may be active. Some hypotheses are formulated on which these may be.

Ultrafast photodissociation dynamics of 1,4-diiodobenzene.

The photodissociation dynamics of 1,4-diiodobenzene is investigated using ultrafast time-resolved photoelectron spectroscopy. Following excitation by laser pulses at 271 nm, the excited-state dynamics is probed by resonance-enhanced multiphoton ionization with 405 nm probe pulses. A progression of Rydberg states, which come into resonance sequentially, provide a fingerprint of the dissociation dynamics of the molecule. The initial excitation decays with a lifetime of 33 ± 4 fs, in good agreement with a previous study. The spectrum is interpreted by reference to ab initio calculations at the CASPT2(18,14) level, including spin-orbit coupling. We propose that both the 5B1 and 6B1 states are excited initially, and based on the calculations, we identify diabatic spin-orbit coupled states corresponding to the main dissociation pathways.

Gas-Phase Ion Chemistry of Metalloporphyrin Anions with Molecular Oxygen: Probing the Influence of the Oxidation and Spin State of the Central Transition Metal by Experiment and Theory.

We performed a comprehensive gas-phase experimental and quantum-chemical study of the binding properties of molecular oxygen to iron and manganese porphyrin anions. Temperature-dependent ion-molecule reaction kinetics as probed in a Fourier-transform ion-cyclotron resonance mass spectrometer reveal that molecular oxygen is bound by, respectively, 40.8 ± 1.4 and 67.4 ± 2.2 kJ mol-1 to the FeII or MnII centers of isolated tetra(4-sulfonatophenyl)metalloporphyrin tetraanions. In contrast, FeIII and MnIII trianion homologues were found to be much less reactive-indicating an upper bound to their dioxygen binding energies of 34 kJ mol-1. We modeled the corresponding O2 adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O2 binding on the FeII or MnII centers and suggested that O2 binds as a superoxide anion.

Redox-Dependent Metal-Metal Bonding in Trinuclear Metal Chains: Probing the Transition from Covalent Bonding to Exchange Coupling.

The synthesis and physical properties of two new cationic tri-metallic chains, [(PEt3 )3 RuCl3 M'Cl3 Ru(PEt3 )3 ]1+ , M'=Rh and Ir are reported. These are isostructural with a previously reported 17-electron all-ruthenium analogue, but replacing a d5 RuIII ion in the central position with d6 RhIII /IrIII has a significant impact on the nature of the metal-metal interactions. All three materials have been characterized electrochemically at the 18-, 17- and 16-electron levels. X-ray crystallography and spectroelectrochemistry, complemented by electronic structure analysis at the DFT and CASPT2 levels, indicate that whilst the presence of a RuIII ion in the center of the chain allows multi-center covalent bonding to develop, a closed-shell RhIII /IrIII ion pushes the system towards the exchange-coupled limit, where the outer Ru centers are only weakly interacting. This family of three isostructural compounds reveals how changes in metal composition can have subtle effects on physical properties of systems that lie close to the localized/delocalized borderline.

Imidoylnitrenes R'C(═NR)-N, Nitrile Imines, 1H-Diazirines, and Carbodiimides: Interconversions and Rearrangements, Structures, and Energies at DFT and CASPT2 Levels of Theory.

The structures, energies, and rearrangements of imidoylnitrenes H-C(═NH)-N, H2N-C(═NH)-N, Ph-C(═NH)-N, H-C(═NPh)-N, and MeO-C(═NCN)-N (10a-e) are investigated at DFT and CASPT2 levels of theory. Imidoylnitrenes are potentially formed by pyrolysis or photolysis of azides, tetrazoles (6, 6'), or sydnones. Unlike most acylnitrenes, the imidoylnitrenes 10 have triplet ground states. The first excited states are the open-shell singlets (OSSs), lying between ca. 4 and 20 kcal mol-1 above the triplets at the CASPT2 level. The second excited states are the closed-shell singlets (CSSs), lying >50 kcal mol-1 higher in energy. The OSS imidoylnitrenes can ring-close to 1H-diazirines 9 with very low activation energies (2-12 kcal mol-1), and the 1H-diazirines can then rearrange to nitrile imines 8 with activation energies of 37-48 kcal mol-1. Conversely, nitrile imines generated directly by pyrolysis or photolysis of 2,5-substituted tetrazoles 6 can rearrange to 1H-diazirines 9 and imidoylnitrenes 10 with activation energies of 37-60 kcal mol-1. Finally, the imidoylnitrenes 10 can rearrange to carbodiimides 11 with modest activation barriers of 12-20 kcal mol-1. Calculated vibrational data, UV-vis spectra, and spin densities in the triplet states are also reported, and zero-field splitting parameters |D/hc| in the range 0.9-1 cm-1 and nonzero |E/hc| values are predicted.

A CASSCF/CASPT2 investigation on electron detachments from ScSi n- (n=4-6) clusters.

In this work, the low-lying states of several isomers of ScSi n-/0 (n=4-6) were investigated with the B3LYP functional and CASPT2 method. The ground states of the anionic clusters were predicted to be the singlet states of the trigonal bipyramid ScSi4- (A-ScSi4-), the face-capped trigonal bipyramid ScSi5- (A-ScSi5-), and the pentagonal bipyramid ScSi6- (A-ScSi6-) isomer. Based on the anionic ground states, all the relevant electron detachment processes were identified. The corresponding adiabatic and vertical detachment energies (ADEs and VDEs) of the anionic clusters were computed at the CASPT2 level. The calculated results were used to interpret all the important features in the photoelectron spectra of ScSi n- (n=4-6) clusters. Franck-Condon factor simulations were also performed based on the B3LYP geometries, vibrational frequencies, and normal modes to explain the shapes of the first bands in the spectra.

Iminocyclohexadienylidenes: Carbenes or Diradicals? The Hetero-Wolff Rearrangement of Benzotriazoles to Cyanocyclopentadienes and 1H-Benzo[b]azirines.

The thermal rearrangements of benzotriazole 1 to fulvenimine 4 and 1H-benzazirine 7 are investigated at DFT and CASPT2 levels of theory. Ring opening of benzotriazole 1 to 2-diazo-cyclohexadienimine 2 followed by N2 elimination affords Z- and E-2-iminocyclohexadienylidenes 3, which have triplet ground states (3A″). The open-shell singlet (OSS) (1A″) and closed-shell singlet (CSS) (1A') of 3 lie ∼15 and 40 kcal/mol higher in free energy, respectively. The OSS 3 (1A″) is best described as a 1,3-diradical, whereas the CSS (1A') has the character of a carbene. A hetero-Wolff rearrangement of OSS 3 yields fulvenimine 4, which is a precursor of cyanocyclopentadiene 5, with a calculated activation barrier of 38 kcal/mol at the CASPT2(8,8) level, whereby there is a surface crossing from the OSS to the CSS near the transition state. The barrier for cyclization to 1H-benzo[b]azirine 7 is only ∼13 kcal/mol. Therefore, reaction paths involving the singlet iminocyclohexadienylidene diradicals 3 will necessarily cause equilibration with 1H-benzazirine 7 prior to ring contraction to iminofulvene 4 and cyanocyclopentadiene 5, in agreement with experimental observations based on 13C labeling. The thermolysis of 1-acetylbenzotriazole 7 leads to the analogous N-acetyl-diazocyclohexadienimines 8, N-acetyliminocyclohexadienylidene diradicals 9, and N-acetylfulvenimine 10. The E-N-acetyliminocyclohexadienylidene E9 ring closes to the N-acetyl-1H-benzazirine 11 prior to ring contraction to N-acetylfulvenimine 10, and the Z-N-acetyl-2-diazocyclohexadienimine Z8 ring closes to 2-methylbenzoxazole 12. 1H-benzazirines are predicted to be spectroscopically observable species.

Theoretical Method for an Accurate Elucidation of Energy Transfer Pathways in Europium(III) Complexes with Dipyridophenazine (dppz) Ligand: One More Step in the Study of the Molecular Antenna Effect.

A theoretical protocol to study the sensitization and emission mechanism in lanthanide compounds on the basis of multireference CASSCF/PT2 calculations is proposed and applied to [Eu(NO3)3(dppz-CN)] and [Eu(NO3)3(dppz-NO2)] compounds synthesized and characterized herein. The method consists of a fragmentation scheme where both the ligand and the lanthanide fragments were calculated separately but at the same level of theory, using ab initio wave-function-based methods which are adequate for the treatment of quasi-degenerate states. This is based on the fact that the absorption is ligand-localized and the emission is europium-centered. This characteristic allowed us to describe the most probable energy transfer pathways that take place in the complexes, which involved an ISC between the S1 to T1 ligand states, energy transfer to 5D2 in the lanthanide fragment, and further 5D0 → 7FJ emission. For both compounds, the triplet and 5D2 states were determined at the CASPT2 level to be around ∼26000 and ∼22400 cm-1, respectively. This difference is in the optimal range for the energy transfer process. Finally, the emissive state 5D0 was found at ∼18000 cm-1 and the emission bands in the range 550-700 nm, in quite good agreement with the experimental results.

Conformational analysis of N-methylacetamide molecule in the ground and excited electronic states

Equilibrium geometry parameters, harmonic vibrational frequencies and barriers to conformational transitions and conformer energy differences for N-methylacetamide (CH3CONHCH3) molecule in S0, S1 and T1 electronic states were calculated in MP2 (S0) and CASPT2 (S1, T1) levels of theory. It is known that there are two planar (or effectively planar) conformers of N-methylacetamide in the ground electronic state. It is proved that the electronic excitation to aforementioned electronic states resulted in both CCON and HNCC fragments becoming pyramidal and rotated around «central» CN bond. One-, two- and three-dimensional potential energy surface (PES) sections by different large amplitude motions (LAM) coordinates were obtained by MP2 (S0) and CASPT2 (S1, T1) methods with cc-pVTZ basis set. On 2D PES sections by «central» torsion (CN) and one of the CCON or HNCC fragments’ inversion coordinates there are six different minima. Using PES sections as the potentials in model Hamiltonians, different anharmonic vibrational problems were solved and the frequencies of large amplitude vibrations were estimated variationally.

Observation of H displacement and H2 elimination channels in the reaction of O(3P) with 1-butene from crossed beams and theoretical studies

We report preliminary combined experimental/theoretical results on O(3P)+1-butene reaction dynamics with focus on atomic hydrogen displacement and molecular hydrogen elimination channels. Dynamics and relative yield of the ethylvinoxy+H and ethylketene+H2 product channels are characterized in crossed beam experiments. Stationary points and energetics of triplet/singlet C4H8O potential energy surfaces (PESs) are calculated at CCSD(T)/CBS and CASPT2 level. O(3P) attack occurs on both unsaturated C-atoms with preference for the less substituted one leading, among other products, to C2H5CHCHO+H via an exit barrier on the triplet PES, and to C2H5CHCO+H2 via a very high exit barrier on the singlet PES following intersystem crossing.

Electronic Transition of Ferrocenium: Neon Matrix and CASPT2 Studies

Electronic absorptions of ferrocenium starting at 632.5 nm were measured in a 6 K neon matrix following mass-selective deposition of the ions. The spectrum shows clear vibrational structure and provides the best-yet resolved view of the electronic states of this cation. The absorption system is identified as the 1 2E1′ ← X 2E2′ transition (D5h symmmetry) on the basis of vertical excitation energies and oscillator strengths calculated at the CASPT2 level. Vibrational bands in the spectrum are assigned with the aid of the ground-state frequencies calculated with the DFT method.

New Insight into the Photoisomerization Process of the Salicylidene Methylamine under Vacuum.

The deactivation process of salicylidene methylamine in the gas phase has been explored using static calculations (CASSCF, CASPT2, and CC2) and on-the-fly surface hopping dynamics simulations (CASSCF). Five minimum energy conical intersections (MECIs) were located upon the geometry optimization calculations. One corresponds to the excited state intramolecular proton transfer (ESIPT) process, and the remaining four arise from CN bond rotational motion. Our calculation results found that the molecule prefers to decay to the ground state through the four rotational motion related MECIs rather than the ESIPT related one. This mechanistic scenario is verified by the energy profiles connecting the Franck-Condon point and the MECIs at CASSCF, CASPT2, and CC2 levels. Our proposed new decay mechanism can explain the previous experimental findings of femtosecond pump-probe photoionization spectroscopy and can provide additional guidance to the rational design of photochemically switchable molecules.

Nitrene–Nitrene Rearrangement under Thermal, Photochemical, and Electron‐Impact Conditions: The 2‐Azidopyridines/Tetrazolo[1,5‐a]pyridines

15N‐Labeling demonstrates that the two nitrogen atoms in the 2‐pyridylnitrene radical cation 2·+ become equivalent prior to fragmentation in the mass spectrometer. Furthermore, the mass spectra of 6‐ and 7‐tetrazolo[1,4‐a]pyridine are identical, as are those of 5‐ and 8‐tetrazolo[1,5‐a]pyridine, thereby again demonstrating interconversion of the nitrogen atoms in 2‐pyridylnitrenes. These rearrangements parallel the reactions established under thermal (flash vacuum pyrolysis) and photochemical condition. Calculations of the energies of ground and transition states at the CASPT2(7,8) level support the notion that 2‐pyridylnitrenes undergo very easy and exothermic ring expansion to 1,3‐diazacycloheptatetraene 3, both in the neutrals and the radical cations. In addition, the ring opening to 4‐cyanobutadienylnitrene 4 can take place in both the neutrals and the radical cations with modest activation barriers.

Quantum Chemical Study of the Low-Lying Electronic States of VSi3(-/0) Clusters and Interpretation of the Anion Photoelectron Spectrum.

The geometrical and electronic structures of VSi3(-/0) clusters have been investigated with the DFT, CCSD(T), and CASSCF/CASPT2 methods. The results showed that the suitable functional to identify the ground states of VSi3(-/0) clusters is not the B3LYP but the BP86. At the BP86, CCSD(T), and CASPT2 levels, the ground state of the anionic cluster was the (1)A' ((1)A1) of tetrahedral η(3)-(Si3)V(-) isomer, while that of the neutral cluster was the 1(2)A' and 1(2)A″ (1(2)E) of the same isomer. The 1(2)A' and 1(2)A″ of the tetrahedral η(3)-(Si3)V isomer were the results of the Jahn-Teller distortions of the 1(2)E in C3v symmetry. All three bands in the photoelectron spectrum of the VSi3(-) cluster were interpreted by one-electron detachments from the (1)A' anionic ground state on the basis of the BP86, CCSD(T), and CASPT2 methods. The calculated adiabatic and vertical detachment energies were in agreement with the experimental values. The broad shape of the first band was explained by Franck-Condon factor simulations for the (1)A' → 1(2)A' and (1)A' → 1(2)A″ transitions within the tetrahedral η(3)-(Si3)V(-/0) isomers.

Avoided Crossing in Lithium Chloride Using CASPT2 and MRCI Level of Theory

Avoided Crossing in Lithium Chloride Using CASPT2 and MRCI Level of Theory

Reaction Dynamics of O((3)P) + Propyne: II. Primary Products, Branching Ratios, and Role of Intersystem Crossing from Ab Initio Coupled Triplet/Singlet Potential Energy Surfaces and Statistical Calculations.

The mechanism of the O((3)P) + CH3CCH reaction was investigated using a combined experimental/theoretical approach. Experimentally the reaction dynamics was studied using crossed molecular beams (CMB) with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy. Theoretically master equation (ME) simulations were performed on a potential energy surface (PES) determined using high-level ab initio electronic structure calculations. In this paper (II) the theoretical results are described and compared with experiments, while in paper (I) are reported and discussed the results of the experimental study. The PES was investigated by determining structures and vibrational frequencies of wells and transition states at the CASPT2/aug-cc-pVTZ level using a minimal active space. Energies were then determined at the CASPT2 level increasing systematically the active space and at the CCSD(T) level extrapolating to the complete basis set limit. Two separate portions of the triplet PES were investigated, as O((3)P) can add either on the terminal or the central carbon of the unsaturated propyne bond. Minimum energy crossing points (MECPs) between the triplet and singlet PESs were searched at the CASPT2 level. The calculated spin-orbit coupling constants between the T1 and S0 electronic surfaces were ∼25 cm(-1) for both PESs. The portions of the singlet PES that can be accessed from the MECPs were investigated at the same level of theory. The system reactivity was predicted integrating stochastically the one-dimensional ME using Rice-Ramsperger-Kassel-Marcus theory to determine rate constants on the full T1/S0 PESs, accounting explicitly for intersystem crossing (ISC) using the Landau-Zener model. The computational results are compared both with the branching ratios (BRs) determined experimentally in the companion paper (I) and with those estimated in a recent kinetic study at 298 K. The ME results allow to interpret the main system reactivity: CH3CCO + H and CH3 + HCCO are the major channels active on the triplet PES and are formed from the wells accessed after O addition to the terminal and central C, respectively; (1)CH3CH + CO and C2H3 + HCO are the major channels active on the singlet PES and are formed from the methylketene and acrolein wells after ISC. However, also a large number of minor channels (∼15) are active, so that the system reactivity is quite complicated. The comparison between computational and experimental BRs is quite good for the kinetic study, while some discrepancy with the CMB estimations suggests that dynamic non-ergodic effects may influence the system reactivity. Channel specific rate constants are calculated in the 300-2250 K and 1-30 bar temperature and pressure ranges. It is found that as the temperature increases the H abstraction reaction, whose contribution is negligible in the experimental conditions, increases in relevance, and the extent of ISC decreases from ∼80% at 300 K to less than 2% at 2250 K.

Ab initio insights on photophysics of 9-methylhypoxanthine

ABSTRACTIn this work, the low-lying electronic singlet states of 9-methylhypoxanthine (9MHPX) were explored by the complete active space self-consistent-field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations, and the conical intersections between the optically bright excited S1 state and ground S0 state were optimised by the two-root state-averaged SA-2-CASSCF approach. These studies indicate that four slightly different kinds of S1/S0 conical intersections are identified computationally for 9MHPX, corresponding to four main internal conversion pathways, respectively, all of which are found to show the comparable timescales according to dynamics simulations. At the CASPT2 level, four bright ππ* transitions of 9MHPX are calculated to locate at 4.47, 5.35, 5.97 and 6.30 eV, respectively, responsible for the available experimental absorption peaks of 9MHPX in the vapour phase (4.41, 5.19, 6.05 and 6.42 eV). Though one relatively weak ππ* transition computed at 5.69 eV is not observed in the vapour phase, it is in accordance with the circular dichroism measurement of another hypoxanthine derivative deoxyinosine 5'-phosphate near 5.51 eV.

Nile blue and Nile red optical properties predicted by TD-DFT and CASPT2 methods: static and dynamic solvent effects

The absorption and emission spectra of the fluorescent dyes Nile blue (NB) and Nile red (NR), widely used in biology and histology, were simulated with different methods, considering the effect of water as solvent. The aforementioned dyes are extremely significant because they also act as DNA sensitizers and hence can be used in photodynamic therapy. Especially, time dependent-density functional theory (TD-DFT) including different functionals, and ab initio single-state- and multi-state-complete active space perturbation theory (SS- and MS-CASPT2) including the effect of the basis set, were considered. The solvent environment was taken into account statically and dynamically: static optical properties were calculated with the polarizable continuum model as vertical transitions from the ground state equilibrium geometry, while dynamic properties were obtained by performing ground state molecular dynamics of NB and NR in explicit water, followed by hybrid quantum mechanics/molecular mechanics calculations of a statistical number of geometries along the trajectory. The results show that a dynamic treatment is required in order to reproduce the experimental absorption spectra, since the static approach gives rise to a hypsochromic shift of ca. 0.3 eV for NB and 0.2 eV for NR, at the TD-DFT and CASPT2 level of theory. This can be explained in terms of out-of-plane vibrational normal modes, which are properly taken into account only in the dynamic approach. Moreover, an exhaustive description of the charge transfer character in the excited state is given, at both TD-DFT and CASPT2 levels of theory.