CASPT2 Level Of Theory Research Articles

Photochemistry of Protonated Nitrosamine: Chemical Inertia of NH2NOH+ Versus Reactivity of NH3NO+

The photochemical behavior of the protonated simplest nitrosamine [NH2NO-H](+) has been addressed by means of the CASPT2//CASSCF methodology in conjunction with the ANO-L basis sets. The relative stability of the different tautomers, namely, (1) NH2NOH(+), (2) NH3NO(+), and (3) NH2NHO(+), has been considered, and the corresponding tautomerization transition states have been characterized. With respect to the most chemically relevant species, it has been found that NH2NOH(+) corresponds to a bound structure, while NH3NO(+) corresponds to an adduct between NH3 and NO(+) at both CASSCF and CASPT2 levels of theory. Vertical transition calculations and linear interpolations on the homolytic dissociation of NH3NO(+) in combination with previous results on neutral nitrosamine [J. Chem. Phys. 2006, 125, 164311] and neutral N,N-dimethylnitrosamine [J. Org. Chem. 2007, 72, 4741] indicate that, in acidic diluted solutions, the protonation of nitrosamine takes place on the excited surface. The N-N dissociation channels have been studied both in ground and first excited singlet state. An S1/S0 conical intersection is found to be responsible for the photostability of NH2NOH(+). On the contrary, NH3NO(+) is photochemically unstable as its first excited state is purely dissociative. The latter species is characterized by a twofold reactivity: the formation of nitrosyl cation (NO(+)) in the ground state and the photorelease of physiologically relevant nitric oxide radical (NO) in its first excited state.

A Theoretical Study of the Kinetics of the Benzylperoxy Radical Isomerization

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

Potential energy functions for an intramolecular proton transfer reaction in the ground and excited state

We describe the development of empirical potential functions for the study of the excited state intramo- lecular proton transfer reaction in 1-(trifuloroacetylamino)- naphtaquinone (TFNQ). The potential is a combination of the standard CHARMM27 force field for the backbone structure of TFNQ and an empirical valence bond formalism for the proton transfer reaction. The latter is parameterized to repro- duce the potential energies both in the ground and the excited state, determined at the CASPT2 level of theory. Parameters describing intermolecular interactions are fitted to reproduce molecular dipole moments computed at the CASSCF level of theory and to reproduce ab initio hydrogen bonding ener- gies and geometries for TFNQ-water bimolecular complexes. The utility of this potential energy function was examined by computing the potentials of mean force for the proton trans- fer reactions in the gas phase and in water, in both electronic states. The ground state PMF exhibits little solvent effects, whereas computed potential of mean force shows a solvent stabilization of 2.5 kcal mol −1 in the product state region, suggesting proton transfer is more pronounced in polar sol- vents, consistent with experimental findings.

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.

MAGNETIC EXCHANGE IN POLYNUCLEAR TRANSITION METAL SYSTEM: AB INITIO CASPT2 AND DENSITY FUNCTIONAL THEORY STUDY ON TRIANGULAR COPPER(II) COMPLEXES

The magnetic exchange interactions for five representative triangular Copper(II) complexes: antiferromagnetic Cu 3( TiPB )6 (1), [ Cu 3(μ3- OH )( aaat )3( H 2 O )3]2+ (2), [ PPN ]2 [ Cu 3(μ3- O )(μ- pz )3 Cl 3] (3), [ PPN ][ Cu 3(μ3- OH )(μ- pz )3 Cl 3] (4) and ferromagnetic [ Cu 3(2- CH 3 C 6 H 4 CO 2)4{( C 2 H 5)2 NC 2 H 4 O }2 H 2 O ] (5) are investigated by using density functional theory combined with broken-symmetry approach (DFT-BS) and ab initio CASPT2 method. Our calculated results show that DFT-BS has remarkable dependence on the particular chosen XC functionals and is system-dependent, while the calculations at CASPT2 level of theory are able to give the accurate magnetic coupling constants. Qualitatively, the two theoretical methods reproduce consistently the linear correlation between the magnetic coupling constants and the departure of the (μ3- O ) oxygen atom from the { Cu3 } plane in the complexes (3) and (4). Spin population analyses reveal that the DFT-BS method overestimates the spin electronic delocalization from the Cu(II) center to the bridging ligands.

A butterfly like motion as a clue to the photophysics of thioxanthone

A theoretical, quantum-chemical study of the thioxanthone (TX) molecule has been performed at the MP2 and CASPT2 levels of theory. Geometries, absorption energies, and transition and state properties have been investigated. Two conformers have been found very close in energy with planar (P) and non-planar (NP) structures, the latter characterized by a dihedral angle θ = 173.3° representing the bend of the side benzene rings and an out-of-plane angle ϕ = 4° of the C O bond. Large changes in the relative positions and properties of the two low-lying electronic absorption bands, ππ * and nπ *, are computed when changing the conformation. As a consequence of the analysis of the TX absorption spectrum it is suggested that TX should be viewed as dynamically changing its conformation between the planar and the non-planar conformations. We coined such behavior as a “butterfly like motion”. This dynamic view of the TX structure enables a plausible explanation of the photophysical properties observed for the system.

On the Low‐Lying Excited States of sym‐Triazine‐Based Herbicides

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.

Charge-transfer transitions in protein circular dichroism spectra

Two peptide dimers ( N-acetyl-glycine- N′methylamide and a hydrogen-bonded dimer of N-methylacetamide) were considered as models for charge-transfer transitions in proteins. Electronic structure calculations at the CASSCF and CASPT2 levels of theory were performed to determine the transition densities and excitation energies for these systems in two separate configurations. These transition properties were incorporated within the matrix method which couples the individual monomer transitions in a protein. The effects of including the charge-transfer transitions on circular dichroism spectra were considered for ideal α-helix and β-sheet structures.

Magnetic coupling interaction under different spin multiplets in neutral manganese dimer: CASPT2 theoretical investigation

In order to theoretically study on the magnetic coupling interaction in the Mn–Mn dimer, quantum chemical calculations on the equilibrium Mn–Mn distance R e, total energy E T, binding energies B e and exchange interaction energies E( S) under different spin multiplets were performed at CASPT2 level of theory. It is found that the calculated R e, E T, and B e values are correlated with the spin multiplet, and with increasing the total spin S, R e increases, but E T and B e exhibit a decrease trend. The exchange interaction energies E( S) deviate significantly from the `Lande interval rule'. This unusual magnetic phenomenon is primarily attributed to the biquadratic j( S a · S b ) 2 term contribution in spin Hamiltonian for the Mn–Mn dimer.

Coordination of the neptunyl ion with carbonate ions and water: a theoretical study.

The results of a study on the ground-state of monocarbonate, bicarbonate, and tricarbonate complexes of neptunyl using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to neptunium in the formal oxidation state (V) have been fully optimized at the CASPT2 level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. Some water molecules have been explicitly included in the calculation. This study is consistent with the hypothesis that the monocarbonate complex has a pentacoordinated structure with three water molecules in the first coordination shell and that the bicarbonate complex has a hexacoordinated structure, with two water molecules in the first coordination shell. The typical bond distances are in good agreement with experimental results. The tricarbonate complex was studied with explicit counterions, which resulted in somewhat longer Np-carbonate bond distances than experiment indicates.

Structure and bonding in Hartwig stretched eta(3)-hydridoborate sigma-complex of niobium(III).

Ab initio calculations at the SCF, MP2, CASSCF, and CASPT2 levels of theory with basis sets using atomic pseudopotentials have been carried out for the stretched eta(3)-hydridoborate sigma-complex of niobium, [Cl2Nb(H2B(OH)2)], in order to investigate the nature and energetics of the interaction between the transition metal and the eta(3)-hydridoborate ligand. The geometry of the complex [Cl2Nb(H2B(OH)2] and its fragments [Cl2Nb](+) and [H2B(OH)2](-) were optimized at SCF and CASSCF levels. These results are consistent with [Cl2Nb(eta(3)-H2B(OH)2)] being a Nb(III) complex in which both hydrogen and boron of the [eta(3)-H2B(OH)2](-) ligand have a bonding interaction with the niobium preserving stretching B-H bond character. The calculated values of DEF (energy required to restore the fragment from the equilibrium structure to the structure it takes in the complex) for [Cl2Nb](+) are 5.35 kcal/mol at SCF, 3.27 kcal/mol at MP2, 4.80 kcal/mol at CASSCF, and 2.82 kcal/mol at CASPT2 and for [H2B(OH)2](-) 21.13 kcal/mol at SCF, 23.85 kcal/mol at MP2, 20.69 kcal/mol at CASSCF, and 23.48 kcal/mol at CASPT2. Values of INT (stabilization energy resulting from the coordination of distorted ligand to the metal fragment) for the complex [Cl2Nb(H2B(OH)2)] are -239.35 kcal/mol at SCF, -260.00 kcal/mol at MP2, -230.76 kcal/mol at CASSCF, and -252.60 kcal/mol at CASPT2. For the complex [(eta(5)-C5H5)2Nb(H2B(OH)2)], calculations at the SCF and MP2 levels were carried out. Values of INT for [(eta(5)-C5H5)2Nb(H2B(OH)2)] are -169.93 kcal/mol at SCF and -210.62 kcal/mol at MP2. The results indicate that the bonding of the [eta(3)-H2B(OH)2](-) ligand with niobium is substantially stable. The electronic structures of [Cl2Nb(H2B(OH)2)], [(eta(5)-C5H5)2Nb(H2B(OH)2)], and its fragments are analyzed in detail as measured by Mulliken charge distributions and orbital populations.

Stabilities, Excitation Energies, and Dissociation Reactions of CF2Cl2 and CF2Br2: Quantum Chemical Computations of Heats of Formation of Fluorinated Methanes, Methyls, and Carbenes

The heats of formation of a range of chlorofluoro-, bromofluoro-, and iodofluoromethanes, methyls, and carbenes were computed using the Gaussian-3 (G3) and Gaussian-2 (G2) methods. From a number of computed isodesmic reaction enthalpies, the heats of formation (at 298 K) of the key methyl species in the decomposition reactions of difluoromethanes, namely CF2Cl, CF2Br, and CF2I, were determined as −64.7 ± 2.0, −52.3 ± 2.0, and −39.3 ± 2.0 kcal mol-1, respectively, while our recommended heat of formation for CI4 is 77.0 ± 3.0 kcal mol-1. Using the available thermochemistry the energetics of the dissociation reactions of CF2X2 (X = Cl, Br, I) corresponding to the product channels CF2X + X and CF2 + X2 (as well as CF2 + 2X) were thus computed and compared. The energetics favor the molecular channel but only by ∼10−12 kcal mol-1 in the case of CF2Cl2 and CF2Br2. However, no transition states were found in these systems for the molecular elimination of Cl2 and Br2. By contrast, H2 elimination in CF2H2 was found to be favored by ∼ 40 kcal mol-1 (at the G2 level) with the corresponding transition state predicted to be 17.3 kcal mol-1 below the radical products CHF2 + H. The vertical excitation energies of CF2Cl2 and CF2Br2 (to the lowest singlet and triplet B1, A2, B2, and A1 excited states) were computed using a range of methods including CASPT2 and EOM-CCSD in conjunction with the cc-pVTZ basis set. The results obtained for the lowest energy, viz., Ã(1B1) ← X̃(1A1), transitions are consistent with the available experimental data. The excited state potential energy surfaces, as calculated at the CASPT2 level of theory, are found to be repulsive with respect to a single C−Cl or C−Br bond stretch, but have shallow local minima in C2v symmetry, i.e., when both C−Cl or C−Br bonds are stretched to ∼2.3 Å, that suggest the existence of weakly bound metastable states.

CASSCF, CASPT2, and MRCI investigations of formyloxyl radical (HCOO.)

The structures of the electronic states of HCOO. were optimized at CASSCF and CASPT2 levels of theory with generally contracted basis sets of atomic natural orbital type: [8s4p]/[3s2p] for hydrogen and [14s9p4d]/[4s3p2d] for carbon and oxygen atoms. Vibrational frequencies were calculated to verify the characteristics for some of the optimized stationary points. MRCI calculations were carried out for the σ radicals at the CASPT2 optimized geometries to provide a test on the higher-order correlation effects. At the CASPT2 level, the lowest electronic state of HCOO. is the 2A1 state. The electronic energy of the 2B2 state is 6 kJ/mol above. The broken symmetry 2A′ state is predicted to lie 25k J/mol above the 2A1 state and to be a transition structure for rearrangement to HOCO.. Single-point MRCI calculations yield the reverse order for the C2v symmetric states, 2A1 state being predicted to be 9.5 kJ/mol above the 2B2 state. The 2A′ state is nearly degenerate with the 2A1 state. Relative energies predicted at the (unoptimized) MRCI level are almost identical to those found by single-reference CI calculations.

A theoretical study on the structure of acetonitrile (CH3CN) and its anion CH3CN−

The electronic and geometrical structures of the CH3CN molecule and its anion CH3CN− are calculated at the UHF, UMP2, UMP4, CASSCF and CASPT2 levels of theory. The anion CH3CN− is confirmed to possess a dipole-bound state. The electron affinity (EA) of CH3CN is computed to be ≈ 6 meV. The potential energy surfaces (PES) of both the neutral and anionic species are computed as functions of the distance between carbon atoms of the CH3 and CN constituents. It is found that the neutral molecule possesses the avoiding crossing between two lowest singlet states having different dissociation limits, namely CH3(doublet) + CN(doublet) and CH3+(singlet) + CN−(singlet). The anionic PES is always lower than the neutral one and possesses an appreciable barrier of ≈ 0.8 eV at the distance range of 2.0–2.5 Å between the CH3 and CN groups. This barrier separates the dipole-bounded and “normal” bound states of the CH3CN− anion. After passing the barrier there is a local minimum at the anion PES with respect to which the adiabatic EA of the neutral CH3CN molecule is negative of ≈ −1.6 eV.