CASSCF Calculations Research Articles

Asymmetry within the Fe(NO)2 moiety of dithiolate dinitrosyl iron complexes

Dinitrosyl iron complexes have earned attention due to their applications as nitric oxide donors as well as their interesting electronic structures. Typical Fe–NO electromerism issues are complicated in this case by the presence of a second nitrosyl ligand. Each of the formal oxidation states of iron in such complexes – from Fe(III) to Fe(0) – have been shown to be subject to electromerism. Here, broken-symmetry density functional theory data are shown, that reveal asymmetry within iron dithiolate dinitrosyl models in several formal oxidation states. CASSCF calculations on selected models confirm the multiconfigurational character of the Fe(NO)2 moiety, with the largest contributor at only 44% of the total weight of the wave function. This asymmetry is most noticeable in the excited states; it is further enhanced by solvation and reveals itself even in the ground state in dynamics calculations as well as in reaction pathways connecting the dinitrosyl state to a putative hyponitrite adduct.

Redox and linkage isomerism with ligands relevant to oxidative and nitrosative stress in cobalamin

Presented here is an extensive DFT study on the interaction of both Co(II) and Co(III), base-on and base-off cobalamins with such ligands as dioxygen, superoxo, hydroperoxo, NO, NO+, HNO and NO2−. Dioxygen is predicted to bind to Co(II) but unlikely to form a stable complex with Co(III)cobalamins, in line with the fact that such an adduct has not been observed experimentally. Formally hexacoordinated Co(II) models bound to anions (hydroperoxo, superoxo) are predicted to be possible, but partial charges show that such adducts would instead contain a Co(III) state, with the extra electron either on the corrin or the ligand, and not a ‘true’ Co(II). Binding of a nitro group to Co(III)cobalamins is thermodynamically favored over nitrito binding, in line with the complexes thus far characterized by X-ray crystallography, which only give the Co(III)–N bound isomer. NO is predicted here to bind to both Co(II) and Co(III), with the first preferred. Although it is known that NO will not react with Co(III)–H2O cobalamins (the actual species present in neutral or acidic media), which is in accordance with experimental data, HNO appears more favored to bind to cobalamins (both Co(II) and Co(III)) compared to water and hence these complexes are a viable target for the future. Furthermore, CASSCF calculations were performed in order to obtain more detailed views on problematic electronic structures; these are discussed in comparison (and, mostly in agreement) to the DFT results.

Qualitative Estimation of the Single-Electron Transfer Step Energetics Mediated by Samarium(II) Complexes: A “SOMO–LUMO Gap” Approach

Lanthanide II organometallic complexes usually initiate reactions via a single-electron transfer (SET) from the metal to a bonded substrate. Extensive mechanistic studies were carried out for lanthanide III complexes in which no change of oxidation state is involved. Some case-dependent strategies were reported by our group in order to account for a SET event in organometallic computed studies. In the present study, we show that analysis of DFT orbital spectra allows differentiating between exothermic and endothermic electron transfer. This methodology appears to be general; it allows differentiating between lanthanide centers and substituent effects on metallocenes. For that purpose, we considered mainly various samarocene adducts as well as a SmI2 complex explicitly solvated by THF. Comparison between DFT methods and ab initio (CAS-SCF and HF) computational level revealed that the SOMO-LUMO gap computed at the DFT B3PW91 level, in combination with small-core RECPs and standard basis sets, offers a qualitative estimation of the energetics of the SET that is in line with both CAS-SCF calculations and experimental results when available. This orbital-based approach, based on DFT calculation, affords a fast and efficient methodology for pioneer exploration of the reactivity of lanthanide(II) mediated by SET.

Dicobalt-μ-oxo Polyoxometalate Compound, [(α2-P2W17O61Co)2O]14–: A Potent Species for Water Oxidation, C–H Bond Activation, and Oxygen Transfer

High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbon-hydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt(IV)-μ-oxo polyoxometalate compound [(α2-P2W17O61Co)2O](14-), [(POMCo)2O], has now been isolated and characterized by the oxidation of a monomeric [α2-P2W17O61Co(II)(H2O)](8-), [POMCo(II)H2O], with ozone in water. The crystal structure shows a nearly linear Co-O-Co moiety with a Co-O bond length of ∼1.77 Å. In aqueous solution [(POMCo)2O] was identified by (31)P NMR, Raman, and UV-vis spectroscopy. Reactivity studies showed that [(POMCo)2O]2O] is an active compound for the oxidation of H2O to O2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon-hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)2O]2O] is best defined as a compound having two cobalt(III) atoms with two oxidized oxygen atoms.

Mechanisms of Laser-Induced Reactions of Stacked Benzene Molecules: A Semiclassical Dynamics Simulation and CASSCF Calculation

The response to ultrashort laser pulses of two stacked benzene molecules has been studied by semiclassical dynamics simulation; two typical pathways were found following excitation of one of the benzene molecules by a 25 fs (FWHM), 4.7 eV photon. With a fluence of 40.49 J/m2, the stacked molecules form a cyclobutane benzene dimer; the formation of the two covalent bonds linking two benzenes occurs asynchronously after the excimer decays to electronic ground state. With a fluence of 43.26 J/m2, only one bond is formed, which breaks about 50 fs after formation, followed by separation into the two molecules. The deformation of benzene ring is found to play an important role in the bond cleavage.

Spectroscopic determination of crystal field splittings in lanthanide double deckers

We have investigated the crystal field splitting in the archetypal lanthanide-based single-ion magnets and related complexes (NBu4)+[LnPc2]−·2dmf (Ln = Dy, Ho, Er; dmf = N,N-dimethylformamide) by means of far infrared and inelastic neutron scattering spectroscopies. In each case, we have found several features corresponding to direct crystal field transitions within the ground multiplet. The observation of three independent peaks in the holmium derivative enabled us to derive crystal field splitting parameters. In addition, we have carried out CASSCF calculations. We show that exploiting the interplay of CASSCF calculation (for the composition of the states) and advanced spectroscopic measurements (for accurate determination of the energies) is a very powerful approach to gain insight into the electronic structure of lanthanide-based single-molecule magnets.

New insight into the potential energy landscape and relaxation pathways of photoexcited aniline from CASSCF and XMCQDPT2 electronic structure calculations

There have been a number of recent experimental investigations of the nonadiabatic relaxation dynamics of aniline following excitation to the first three singlet excited states, 1(1)ππ*, 1(1)π3s/πσ* and 2(1)ππ*. Motivated by differences between the interpretations of experimental observations, we have employed CASSCF and XMCQDPT2 calculations to explore the potential energy landscape and relaxation pathways of photoexcited aniline. We find a new prefulvene-like MECI connecting the 1(1)ππ* state with the GS in which the carbon-atom carrying the amino group is distorted out-of-plane. This suggests that excitation above the 1(1)π3s/πσ* vertical excitation energy could be followed by electronic relaxation from the 1(1)ππ* state to the ground-electronic state through this MECI. We find a MECI connecting the 1(1)π3s/πσ* and 1(1)ππ* states close to the local minimum on 1(1)π3s/πσ* which suggests that photoexcitation to the 1(1)π3s/πσ* state could be followed by relaxation to the 1(1)ππ* state and to the dissociative component of the 1(1)π3s/πσ* state. We also find evidence for a new pathway from the 2(1)ππ* state to the ground electronic state that is likely to pass through a three-state conical intersection involving the 2(1)ππ*, 1(1)π3s/πσ* and 1(1)ππ* states.

Calculation of Heats of Formation for Zn Complexes: Comparison of Density Functional Theory, Second Order Perturbation Theory, Coupled-Cluster and Complete Active Space Methods.

Heats of formation were predicted for nine ZnX complexes (X= Zn, H, O, F2, S, Cl, Cl2, CH3, (CH3)2) using fourteen density functionals, MP2 calculations and the CCSD and CCSD(T) coupled-cluster methods. Calculations utilized the correlation consistent cc-pVTZ and aug-cc-pVTZ basis sets. Heats of formation were most accurately predicted by the TPSSTPSS and TPSSKCIS density functionals, and the BLYP, B3LYP, MP2, CCSD and CCSD(T) levels were among the poorest performing methods based on accuracy. A wide range of Zn2 equilibrium bond distances were predicted, indicating that many of the studied levels of theory may be unable to adequately describe this transition metal dimer. To further benchmark the accuracy of the density functional methods, high-level CASSCF and CASPT2 calculations were performed to estimate bond dissociation energies, equilibrium bond lengths and heats of formation for the diatomic Zn complexes and the latter two quantities were compared with the results of DFT, MP2 and coupled-cluster calculations as well as experimental values.

Infrared, Raman, and Ultraviolet Absorption Spectra and Theoretical Calculations and Structure of 2,6-Difluoropyridine in Its Ground and Excited Electronic States

The infrared and Raman spectra of 2,6-difluoropyridine (26DFPy) along with ab initio and DFT computations have been used to assign the vibrations of the molecule in its S0 electronic ground state and to calculate its structure. The ultraviolet absorption spectrum showed the electronic transition to the S1(π,π*) state to be at 37,820.2 cm(-1). With the aid of ab initio computations the vibrational frequencies for this excited state were also determined. TD-B3LYP and CASSCF computations for the excited states were carried out to calculate the structures for the S1(π,π*) and S2(n,π*) excited states. The CASSCF results predict that the S1(π,π*) state is planar and that the S2(n,π*) state has a barrier to planarity of 256 cm(-1). The TD-B3LYP computations predict a barrier of 124 cm(-1) for the S1(π,π*) state, but the experimental results support the planar structure. Hypothetical models for the ring-puckering potential energy function were calculated for both electronic excited states to show the predicted quantum states. The changes in the vibrational frequencies in the two excited states reflect the weaker π bonding within the pyridine ring.

Influence of the Torsion Angle in 3,3′-Dimethyl-2,2′-bipyridine on the Intermediate Valence of Yb in (C5Me5)2Yb(3,3′-Me2-bipy)

The synthesis and X-ray crystal structures of Cp*2Yb(3,3′-Me2bipy) and [Cp*2Yb(3,3′-Me2bipy)][Cp*2YbCl1.6I0.4]*CH2Cl2 are described. In both complexes, the NCCN torsion angles are approximately 40°. The temperature-independent value of nf of 0.17 shows that the valence of ytterbium in the neutral adduct is multiconfigurational, in reasonable agreement with a CASSCF calculation that yields a nf value of 0.27; that is, the two configurations in the wave function are f13(π*1)1 and f14(π*1)0 in a ratio of 0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV below the triplet state (nf accounts for f-hole occupancy; that is, nf = 1 when the configuration is f13 and nf = 0 when the configuration is f14). A correlation is outlined between the value of nf and the individual ytterbocene and bipyridine fragments such that, as the reduction potentials of the ytterbocene cation and the free x,x′-R2-bipy ligands approach each other, the value of nf and therefore the f13:f14 ratio reaches a maximum; conversely, the ratio is minimized as the disparity increases.

Theoretical study of the extremely small torsional barriers of thiophenol in the ground and the first excited electronic states

The extremely small (around 0.80kcal/mol) barrier for the –SH rotation of thiophenol is studied by using MP2, CASSCF(12,11), and three DFT (B3LYP, HCTH, and ωB97X-D) methods with various basis sets. The 6-311++G(3df,3pd) turns out to be the proper minimal basis-set for reliable calculations. CCSD(T) and CASPT2//CASSCF calculations confirm that the planar conformation of the –SH is the minimum-energy structure of not only the ground S0 but also the first excited S1 electronic states. The calculated torsional barrier of the S1, 1.89kcal/mol, is also quite small.

Computational Evidence for Heavy-Atom Tunneling in the Bergman Cyclization of a 10-Membered-Ring Enediyne

DFT and CASSCF calculations for the cyclization of (3Z)-cyclodec-3-en-1,5-diyne were carried out to investigate heavy-atom tunneling. At 37 °C, tunneling was computed to enhance the rate by 38-40% over the transition-state theory rate. Intramolecular (12)C/(13)C kinetic isotope effects were predicted to be substantial, with a steep temperature dependence. These results are discussed in relation to recent experimental findings that show heavy-atom tunneling at moderate temperatures. The calculations point to the possibility of a simple computational test for the likelihood of heavy-atom tunneling using standard quantum-chemical information.

Water-Assisted Self-Photoredox of 3-(Hydroxymethyl)benzophenone: An Unusual Photochemistry Reaction in Aqueous Solution

An unusual photochemistry of water-assisted self-photoredox of 3-(hydroxymethyl) benzophenone 1 has been investigated by CASPT2//CASSCF computations. The water-assisted self-photoredox is found to proceed via three sequential reactions: an excited-state intermolecular proton transfer (ESIPT), a photoinduced deprotonation, and a self-redox reaction. Upon photoexcitation at 243 nm, the system of 1 is taken to the Franck-Condon region of a short-distance charge transfer (SCT) state of S(SCT)((1)ππ*) and then undergoes ESIPT with a small barrier of ∼3.4 kcal/mol producing the intermediate 2. Subsequently, the singlet-triplet crossing (STC) of STC ((1)ππ*/(3)ππ*) relays 2 by intersystem crossing to the T(SCT)((3)ππ*) state followed by a deprotonation reaction overcoming a moderate barrier of ∼8.0 kcal/mol and finally produces the triplet biradical intermediate 3. Another moderate barrier (∼5.8 kcal/mol) in the T(SCT)((3)ππ*) state has to be overcome so as to relax to a second singlet-triplet crossing STC(T/S0) that allows an efficient spin-forbidden decay to the ground state. The self-redox reaction aided by water molecules occurs with tiny barriers in the S0 state via two steps, protonation of the benzhydrol carbon to produce intermediate 4 and then deprotonation from the benzylic oxygen to yield the final product 3-formylbenzhydrol 5.

Low-Lying Electronic States of Irn Clusters with n = 2–8 Predicted at the DFT, CASSCF, and CCSD(T) Levels

Low-lying structures of the small iridum clusters Ir(n) (n = 2-8) were optimized using DFT methods. Ir2 and Ir3 were also optimized using the CASSCF method. MRCI-SD (for Ir2) energies and CCSD(T) (for Ir2 and Ir3) energies of the leading configurations from the CASSCF calculations were done to predict the low-lying states. The normalized atomization energies (<AE>) for Ir(n) (n = 2-8) were calculated at the CCSD(T) level up to the complete basis set (CBS) limit in some cases using the B3LYP optimized geometries. The ground state for Ir2 is predicted to be (5)Δg, and the ground state of Ir3 is linear (2)Δg, with the D3h(4)A"1 state ~10 kcal/mol higher in energy at the CASSCF level without core-valence corrections and ~15 kcal/mol higher at the CCSD(T)/CBS level with spin-orbit and core-valence corrections. Inclusion of the spin orbit corrections in the normalized bond dissociation energies <AE> for Ir(n) is critical and will decrease the <AE> by ~15 kcal/mol for n ≥ 4. The <AE> for Ir(n) increases as n increases in general, and the <AE> is far from convergence to the bulk value at n = 8. The average coordination number (CN) and average bond length for the low energy Ir(n)clusters are far from being converged to the bulk values by n = 8.

A multi-sheeted three-dimensional potential-energy surface for the H-atom photodissociation of phenol

Extending the previous work of Lan et al. [J. Chem. Phys., 122, 224315 (2005)], a multi-state potential model for the H atom photodissociation is presented. All three "disappearing coordinates" of the departing H atom have been considered. Ab initio CASSCF computations have been carried out for the linear COH geometry of C(2v) symmetry, and for several COH angles with the OH group in the ring plane and also perpendicular to the ring plane. By keeping the C6H5O fragment frozen in a C(2v)-constrained geometry throughout, we have been able to apply symmetry-based simplifications in the constructions of adiabatic model. This model is able to capture the overall trends of twelve adiabats at both torsional limits for a wide range of COH bend angles.

Decisive interactions that determine ferro/antiferromagnetic coupling in {3d–4f} pairs: a case study on dinuclear {V(iv)–Gd(iii)} complexes

The emerging class of mixed transition metal and lanthanide {3d-4f} complexes have gained more interest in recent years in the field of molecular magnetism. The key to success in this class of compounds lies in the nature of their observed magnetic coupling, which is mostly ferromagnetic. However several exceptions have emerged in recent years which makes understanding the origin of magnetic coupling crucial. DFT and CASSCF calculations have been performed on a structurally similar pair of {V(iv)-Gd(iii)} complexes to underpin the dilemma of ferro/antiferromagnetic exchange interaction. We have chosen two structurally similar complexes, [L(1)V(O)Gd(H(2)O)(NO(3))(3)] (1); which displays a ferromagnetic interaction (J = +1.5 cm(-1)) between the {V(iv)-Gd(iii)} pair, while complex [L(2)V(O){(CH(3))(2)CO}Gd(NO(3))(3)], (2) (see text for descriptions of L(1) and L(2)) exhibits an antiferromagnetic exchange (J = -2.6 cm(-1)). The DFT calculations yield J values of +2.0 cm(-1) and -0.7 cm(-1) for complexes 1 and 2 respectively and these values are in good agreement with the experimental values. CASSCF calculations have also been performed to understand the nature of the interaction in these complexes. The MO and NBO analysis demonstrate the importance of Gd(iii) vacant 5d orbitals which contribute to the ferromagnetic part of the J values in this class of complexes. The extensive magneto-structural correlations developed suggests that a combination of two parameters, the V-O-Gd angle and the V-O-Gd-O dihedral angle, control the sign as well the magnitude of the J values. We have extended our studies to a tetranuclear [L(3)V(O)Gd(hfac)(2)(CH(3)OH)(2)](2) complex to validate the proposed mechanism and the developed correlation. Our calculations also reveal that weak interactions are playing an important role in predicting the ground state for large polynuclear complexes.

Electronic and Magnetic Properties of Kremer's tris-Hydroxo Bridged Chromium Dimer: A Challenge for DFT.

We present computations of the zero field splitting constants in a tris-hydroxo bridged chromium dimer (Kremer's dimer). A comparison is given of broken symmetry density functional theory (DFT) and multiconfigurational ab initio methods for evaluating ZFS constants. Kremer's dimer is known to be antiferromagnetically coupled, with the spin ladder order of E(S = 0) < E(S = 1) < E(S = 2) < E(S = 3). The B3LYP functional gives the order E(S = 0) < E(S = 3) < E(S = 1) < E(S = 2), and similar results are obtained for other density functionals (PBE, M06, M06-L, and TPSS). In contrast, we find that simple CASSCF calculations yield a correct spin ladder. DFT poorly reproduces the experimental D splitting values, while the CASSCF technique coupled with quasi-degenerate perturbation theory qualitatively reproduces D for all the spin states. State-optimized orbitals result in more accurate spin state energies and D values compared to state-averaged orbitals. Inclusion of spin-spin coupling is found to be essential for both the magnitude and sign of D. The rhombic splitting parameter is found to be near zero, which is comparable to experimental results for which the analysis assumed C3h symmetry.

Heteroaryl Oxenium Ions Have Diverse and Unusual Low-Energy Electronic States

The electronic state orderings and energies of heteroaryl oxenium ions were computed using high-level CASPT2//CASSCF computations. We find that these ions have a number of diverse, low-energy configurations. Depending on the nature of the heteroaryl substituent, the lowest-energy configuration may be open-shell singlet, closed-shell singlet, or triplet, with further diversity found among the subtypes of these configurations. The 2- and 3-pyridinyl oxenium ions show small perturbations from the phenyl oxenium ion in electronic state orderings and energies, having closed-shell singlet ground states with significant gaps to an n,π* triplet state. In contrast, the 4-pyridinyl oxenium ion is computed to have a low-energy nitrenium ion-like triplet state. The pyrimidinyl oxenium ion is computed to have a near degeneracy between an open-shell singlet and triplet state, and the pyrizidinyl oxenium ion is computed to have a near-triple degeneracy between a closed-shell singlet state, an open-shell singlet state, and a triplet state. Therefore, the ground state of these latter heteroaryl oxenium ions cannot be predicted with certainty; in principle, reactivity from any of these states may be possible. These systems are of fundamental interest for probing the spin- and configuration-dependent reactivity of unusual electronic states for this important class of reactive intermediate.

Spectroscopy of the Simplest Criegee Intermediate CH2OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra

CH(2)OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas-phase studies on CH(2)OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH(2)OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH(2)OO. In order to encourage more direct studies on CH(2)OO and other Criegee intermediates, the electronic and photoelectron spectra of CH(2)OO have been simulated using high level electronic structure calculations and Franck-Condon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH(2)OO were calculated with TDDFT, EOM-CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low-lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH(2) OO corresponds to the B(1)A' ← X(1)A' absorption. It is a broad band in the region 250-450 nm showing extensive structure in vibrational modes involving O-O stretching and C-O-O bending. Evidence is presented to show that the electronic absorption spectrum of CH(2)OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH(2)OO was prepared in this earlier work from the reaction of CH(2)I with O(2) and that the assignment of the observed spectrum solely to CH(2)IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH(2)OO consists of two overlapping vibrationally structured bands corresponding to one-electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.

Re‐examining the Mechanisms of Competing Pericyclic Reactions of 1,3,7‐Octatriene

DFT (both B3LYP and M06-2X), CASSCF, and CASPT2 calculations were used to investigate competing [3, 3] and [3, 5] sigmatropic shifts and intramolecular [4+2] cycloaddition of 1,3,7-octatriene. In accord with previous results on 1,5-hexadiene, CASSCF calculations found both stepwise and concerted pathways for the [3, 3] rearrangement. For the competing [3, 5] sigmatropic rearrangement, CASSCF and CASPT2 calculations revealed three stepwise pathways with similar barriers. UB3LYP and UM06-2X calculations predicted a different potential energy landscape: no stepwise [3, 3] pathway, only two competing [3, 5] sigmatropic shifts, and an intramolecular Diels-Alder cycloaddition/homolytic ring-opening pathway. Significant lowering of barriers for all rearrangements was predicted for some 1,3,7-octatrienes with substituents at the 4- and 7-positions.