Multireference Ab Initio Calculations Research Articles

Intersystem crossing in the exit channel.

Intersystem crossing plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin-orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet-triplet intersection seam. Here we present evidence for a different intersystem crossing mechanism in the bimolecular reaction of O(3P) with alkylamines. In crossed-beam experiments, product velocity-flux maps are measured for aminoalkyl radicals produced from H abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential energy surface that could support such a complex. Multi-reference ab initio calculations suggest, instead, that intersystem crossing occurs in the exit-channel region due to the long-range dipole-dipole interaction between the nascent radical product pair coupled with the vanishing singlet-triplet splitting at long range. Intersystem crossing then leads to a deep hydroxylamine well before OH elimination.

Ab Initio Study of Covalency in the Ground versus Core-Excited States and X-ray Absorption Spectra of Actinide Complexes.

Relativistic multireference ab initio wave function calculations within the restricted active space (RAS) framework were performed to calculate metal and ligand X-ray absorption (XAS) near-edge spectroscopy (XANES) intensities for the metal M4,5 edges of [PuO2(H2O)5]2+, [AnVIO2]2+ (An = U, Np, Pu), and [AmCl6]3- and the Cl K edge of the Am complex. The extent of An(5f)-ligand bonding was determined via natural localized molecular orbital analyses of the relevant spin-orbit coupled multireference states. The calculated spectra are in good agreement with experiments and allow a detailed assignment of the observed spectral features. The XANES M4,5-edge spectra are representative of the actinide orbital covalency in the probed core-excited states, which may be different from the ground-state covalency. An assignment of ground-state An orbital covalency based on XAS spectra should therefore be made with caution.

Spin States and Other Ligand-Field States of Aqua Complexes Revisited with Multireference ab Initio Calculations Including Solvation Effects.

High-level multireference (CASPT2, NEVPT2) calculations are reported for transition metal aqua complexes with electronic configurations from (3d)1 to (3d)8. We focus on the experimentally evidenced excitation energies to their various ligand-field states, including different spin states. By employing models accounting for both explicit and implicit solvation, we find that solvation effect may contribute up to 0.5 eV to the excitation energies depending on the charge of ion and character of the electronic transition. We further demonstrate that with an adequate choice of the active space and the energetics extrapolated to the complete basis set limit, the presently computed excitation energies are in a good agreement with the experimental data. This allows us to conclusively resolve significant discrepancies reported in earlier theory works [e.g., J. Phys. Chem. C 2014 , 118 , 29196 - 29208 ]. For the benchmark set of 19 spin-forbidden and 24 spin-allowed transitions (for which experimental data are unambiguous), we find the mean absolute error of 0.15 or 0.13 eV and the maximum error of 0.56 or 0.42 eV for CASPT2 or NEVPT2 calculations, respectively. For the particularly challenging sextet-quartet gap for [Fe(H2O)6]3+, we support our interpretation by additional calculations with multireference configuration interaction (MRCI) and coupled cluster theory up to the CCSDT(Q) level. By underlining a rather subtle interplay between the solvation and correlation effects, the findings of this Article are relevant not only for modeling and interpretation of optical spectra of transition metal complexes but also in further benchmarking of theoretical methods for the challenging problem of spin-state energetics.

Effect of Heteroatoms on Field-Induced Slow Magnetic Relaxation of Mononuclear FeIII ( S = 5/2) Ions within Polyoxometalates.

In this paper, the synthesis and magnetic properties of mononuclear FeIII-containing polyoxometalates (POMs) with different types of heteroatoms, TBA7H10[(A-α-XW9O34)2Fe] (IIX, X = Ge, Si; TBA = tetra- n-butylammonium), are reported. In these POMs, mononuclear highly distorted six-coordinate octahedral [FeO6]9- units are sandwiched by two trivacant lacunary units [A-α-XW9O34]10- (X = Ge, Si). These POMs exhibit field-induced slow magnetic relaxation based on the single high-spin FeIII magnetic center ( S = 5/2). Combining experiment and ab initio calculations, we investigated the effect of heteroatoms of the lacunary units on the field-induced slow magnetic relaxation of these POMs. By changing the heteroatoms from Si (IISi) to Ge (IIGe), the coordination geometry around the FeIII ion is mildly changed. Concretely, the axial Fe-O bond length in IIGe is shortened compared with that in IISi, and consequently the distortion of the [FeO6]9- unit in IIGe from the ideal octahedral coordination geometry becomes larger than that in IISi. The effective demagnetization barrier of IIGe (11.4 K) is slightly larger than that of IISi (9.2 K). Multireference ab initio calculations predict zero-field splitting parameters in good agreement with experiment. Although the differences in the coordination geometries and magnetic properties of IIGe and IISi are quite small, ab initio calculations indicate subtle changes in the magnetic anisotropy which are in line with the observed magnetic relaxation properties.

Experimental and ab initio characterization of HC3N+ vibronic structure. I. Synchrotron-based threshold photo-electron spectroscopy.

Threshold-photoionization spectroscopy of cyanoacetylene (HC3N) and its 15N isotopologue has been investigated in the vacuum-ultraviolet range with a synchrotron-based experiment allowing to record threshold-photoelectron spectrum and photoion yield over a large energy range (from 88 500 to 177 500 cm-1, i.e., from 11 to 22 eV). Adiabatic ionization energies towards the three lowest electronic states X+2Π, A+ Σ+2, and B+ Π2 are derived from the threshold-photoelectron spectrum. A detailed description of the vibrational structure of these states is proposed leading to the determination of the vibrational frequencies for most modes. The vibrational assignments and the discussion about the electronic structure are supported by multireference ab initio calculations (CASPT2, MRCI). Unprecedented structures are resolved and tentatively assigned in the region of the B+← X transition. Exploratory calculations highlight the complexity of the electronic landscape of the cation up to approximately 10 eV above its ground state.

ChemInform Abstract: Single‐Molecule Magnet Properties of Transition‐Metal Ions Encapsulated in Lacunary Polyoxometalates: A Theoretical Study.

AbstractAll electronic structure and multireference ab initio calculations are performed to analyze the single‐molecule magnet (SMM) properties of three [M(γ‐SiW10O36)2] (M: MnIII, FeIII, CoII) complexes previously synthesized and characterized by Sato et al.

Outer sphere coordination chemistry: Unusual six-coordinate silver(I) complexes with tri-halide–iridium(III) complexes as ligands

Reaction of silver(I) with robust halido complexes of Ir(III) leads to second sphere coordination rather than halide abstraction. Two hexa-coordinated pseudo-octahedral silver(I) complexes [(CH3CN)3IrX3AgX3Ir(NCCH3)3]+ (X=Cl, Br) have been synthesized and structurally characterized as PF6− and SbF6− salts, respectively. The synthesis and crystal structure of the new, mononuclear complex, [IrBr3(NCCH3)3], are also reported along with spectroscopic characterization of [IrX3(NCCH3)3] and the trinuclear silver complexes. The electronic structures are analyzed in the framework of ligand field theory and by comparison with high-level correlated multireference ab initio calculations. The ligand field strength of the heavier halide complexes is found to be significantly less sensitive to outer-sphere complexation as compared to the pronounced outer sphere ligation effects commonly observed for fluoride complexes.

Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory.

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH3NO2 decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH3 and NO2 radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH3NO2, and for the reaction of CH3 with NO2.

Theoretical Investigation of the Reaction Mechanism of the Photoisomerization of 1,2-Dihydro-1,2-azaborine.

The photoisomerization of 1,2-dihydro-1,2-azaborine was investigated by high-level multireference ab initio and density functional theory calculations. The intermediates (IMs) and transition states (TSs) on the S(0) and S(1) states were optimized using the state-averaged complete active space self-consistent field method. The multireference configuration interaction method with the Davidson correction was used to obtain accurate energetics. Moreover, the conical intersections (CIs), which play a crucial role in photoisomerization, were also optimized. On the basis of the calculation results, the most favorable proposed reaction pathway is as follows: reactant→Franck-Condon region→TS(1) →CI→IM(0) →TS(0P) →product. The product was not directly formed through the CI, and the IM(0) existed on the S(0) state. These results show that the isomerization of 1,2-dihydro-1,2-azaborine involves both photoreactions and thermal reactions. The calculated results clarify recent experimental observations.

Assessment of density functional theory based ΔSCF (self-consistent field) and linear response methods for longest wavelength excited states of extended π-conjugated molecular systems.

Computational investigation of the longest wavelength excitations in a series of cyanines and linear n-acenes is undertaken with the use of standard spin-conserving linear response time-dependent density functional theory (TD-DFT) as well as its spin-flip variant and a ΔSCF method based on the ensemble DFT. The spin-conserving linear response TD-DFT fails to accurately reproduce the lowest excitation energy in these π-conjugated systems by strongly overestimating the excitation energies of cyanines and underestimating the excitation energies of n-acenes. The spin-flip TD-DFT is capable of correcting the underestimation of excitation energies of n-acenes by bringing in the non-dynamic electron correlation into the ground state; however, it does not fully correct for the overestimation of the excitation energies of cyanines, for which the non-dynamic correlation does not seem to play a role. The ensemble DFT method employed in this work is capable of correcting for the effect of missing non-dynamic correlation in the ground state of n-acenes and for the deficient description of differential correlation effects between the ground and excited states of cyanines and yields the excitation energies of both types of extended π-conjugated systems with the accuracy matching high-level ab initio multireference calculations.

Trinuclear Early/Late‐Transition‐Metal Thiolate Complexes

The thiolato-bridged cobalt(II)/vanadium(II) complexes [Li(thf)4]2[Co2V(SPh)8] and [Li(15-crown-5)(thf)]2[Co2V(SPh)8] as well as a related nickel(II)/vanadium(II) complex [Li(15-crown-5)(thf)]2[Ni2V(SPh)8] have been prepared by treatment of CoCl2 (NiCl2) and VCl3 {[VCl3(thf)3]} with LiSPh in THF without and in the presence of 15-crown-5, respectively. The crystal structures of the trinuclear compounds were determined by X-ray analysis of single crystals and investigated by density functional theory. Both give evidence for one very short Ni–V distance (259.41 pm) in [Li(15-crown-5)(thf)]2[Ni2V(SPh)8]. The dark-coloured compounds display absorption bands over a wide energy range with an absorption onset lying in the near-infrared region at approximately 2000 nm. Magnetic measurements show strong antiferromagnetic coupling of the metal ions through acute phenylthiolate bridges (M–S–M < 76.1°). Ab initio multireference calculations together with mapping approaches reveal that the magnetic properties of the anionic metal complexes can be described by different effective spin Hamiltonians that indicate different types of intramolecular interactions between the metal ions. In [Co2V(SPh)8]2–, we found at low temperatures an effective S = 3/2 behaviour with small zero-field splitting (ZFS) caused by a strong, symmetric antiferromagnetic exchange interaction. A weak direct Ni–V bond in [Ni2V(SPh)8]2– generates an effective two-centre system that consists of an effective spin of S = 1/2 from the weakly bound Ni–V pair and a pseudospin S = 1 located on the other nickel site, antiferromagnetically coupled to an S = 1/2 ground state.

Strong electron correlation in UO2−: A photoelectron spectroscopy and relativistic quantum chemistry study

The electronic structures of actinide systems are extremely complicated and pose considerable challenges both experimentally and theoretically because of significant electron correlation and relativistic effects. Here we report an investigation of the electronic structure and chemical bonding of uranium dioxides, UO2(-) and UO2, using photoelectron spectroscopy and relativistic quantum chemistry. The electron affinity of UO2 is measured to be 1.159(20) eV. Intense detachment bands are observed from the UO2(-) low-lying (7sσg)(2)(5fϕu)(1) orbitals and the more deeply bound O2p-based molecular orbitals which are separated by a large energy gap from the U-based orbitals. Surprisingly, numerous weak photodetachment transitions are observed in the gap region due to extensive two-electron transitions, suggesting strong electron correlations among the (7sσg)(2)(5fϕu)(1) electrons in UO2(-) and the (7sσg)(1)(5fϕu)(1) electrons in UO2. These observations are interpreted using multi-reference ab initio calculations with inclusion of spin-orbit coupling. The strong electron correlations and spin-orbit couplings generate orders-of-magnitude more detachment transitions from UO2(-) than expected on the basis of the Koopmans' theorem. The current experimental data on UO2(-) provide a long-sought opportunity to arbitrating various relativistic quantum chemistry methods aimed at handling systems with strong electron correlations.

Multireference ab initio study on the low-lying excited states of isoselenocyanic acid, HNCSe

The electronic spectra of HNCSe were studied by performing multireference ab initio calculations at CASPT2 level of theory taking into account spin-orbit (SO) coupling. Our highly accurate calculation indicated that theoretically determined geometric parameters and harmonic vibrational frequencies for the ground state X 1 A ′ are consistent with observed experimental data. We present the oscillator strengths, electronic and structural changes accompanying the excitation process. The bonding characteristics in the low-lying excited states were analyzed on the basis of the valence molecular orbitals (MOs). And the excited states of HNCSe exist in the complex of Se and HNC radical pair. According to our calculations, the ground-state of HNCSe + is linear, and the value of the ionization potential of HNCSe has been found. The existence of bound excited anion states has been found for the first time in HNCSe -.

Communication: Ultraviolet photodissociation dynamics of the simplest Criegee intermediate CH2OO

The velocity and angular distributions of O (1)D photofragments arising from UV excitation of the CH2OO intermediate on the B (1)A' ← X (1)A' transition are characterized using velocity map ion imaging. The anisotropic angular distribution yields the orientation of the transition dipole moment, which reflects the π∗ ← π character of the electronic transition associated with the COO group. The total kinetic energy release distributions obtained at several photolysis wavelengths provide detail on the internal energy distribution of the formaldehyde cofragments and the dissociation energy of CH2OO X (1)A' to O (1)D + H2CO X (1)A1. A common termination of the total kinetic energy distributions, after accounting for the different excitation energies, gives an upper limit for the CH2OO X (1)A' dissociation energy of D0 ≤ 54 kcal mol(-1), which is compared with theoretical predictions including high level multi-reference ab initio calculations.

State-Dependent Electron Delocalization Dynamics at the Solute-Solvent Interface: Soft-X-Ray Absorption Spectroscopy andAb InitioCalculations

Non-radiative decay channels in the L-edge fluorescence spectra from transition metal-aqueous solutions give rise to spectral dips in X-ray transmission spectra. Their origin is unraveled here using partial and inverse partial fluorescence yields on the micro-jet combined with multi-reference ab initio electronic structure calculations. Comparing Fe2+, Fe3+, and Co2+ systems we demonstrate unequivocally that spectral dips are due to a state-dependent electron delocalization within the manifold of d-orbitals.

Multireference Ab Initio Study of the Ground and Low-Lying Excited States of Cr(CO)2 and Cr(CO)3

We investigate the ground and low-lying excited states of unsaturated chromium carbonyls, Cr(CO)2 and Cr(CO)3, using multiconfigurational ab initio perturbation theory. Unlike other chromium carbonyls, there are discrepancies between the experiment and theory on the identity of the ground states of Cr(CO)2 and Cr(CO)3. From multireference ab initio calculations considering the full valence orbitals of Cr(CO)2 and Cr(CO)3, the differences in the molecular structures of their various electronic states are explained by the electronic structure analysis. On the basis of the result from CASPT2 and MS-CASPT2 calculations, we propose that the ground states of Cr(CO)2 and Cr(CO)3 are the (5)Πg and (1)A1 states, respectively, addressing the ambiguity regarding their ground states. In addition, the multiconfigurational ab initio perturbation theory calculations reveal that (1) the energy gaps between the ground and first low-lying excited states of Cr(CO)2 and Cr(CO)3 are quite small and (2) the first low-lying excited states of Cr(CO)2 and Cr(CO)3 have the same spin multiplicities as the ground states of CrCO and Cr(CO)2, respectively, which are the products of ligand dissociation. As a result, the apparent spin-forbidden dissociation of Cr(CO)2 and Cr(CO)3 into CrCO and Cr(CO)2, respectively, are likely to be facilitated by thermal excitation of the ground states of Cr(CO)2 and Cr(CO)3 into their first low-lying excited states, which then actually undergoes the spin-allowed dissociation to the ground states of CrCO and Cr(CO)2 with the same spin multiplicities.

A non-adiabatic wavepacket dynamical study of the low energy charge transfer process in the S3+ + H collision

The collisional system S3++H→S2++H+ has been studied using a time-dependent wavepacket methodology in two-dimensions. Using available potential energy surfaces and coupling matrix elements obtained from multireference ab initio calculations, five non-adiabatically coupled electronic states of 1Π symmetry have been included in the dynamical simulations. The collision has been studied in the low energy regime of 1–10eV. The wavepacket treatment accounts for the description of the collision including all possible impact parameters simultaneously in all spatial directions of the considered plane. The molecular movie of the scattered products arising from charge transfer yields the angular distribution and the total charge transfer probability vs. initial kinetic energy of the projectile. A detailed analysis of the forward, sideways, and backward scattering mechanisms showing different patterns is presented.

Communication: Theoretical exploration of Au++H2, D2, and HD reactive collisions

A quasi-classical study of the endoergic Au(+)((1)S) + H(2)(X(1)Σ(g)(+)) → AuH(+) ((2)Σ(+)) + H((2)S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au(+)+H+H, is the dominant reaction channel for energies above the H(2) dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion.

Correction to “Binding of CO, NO, and O2 to Heme by Density Functional and Multireference ab Initio Calculations”

Correction to “Binding of CO, NO, and O₂ to Heme by Density Functional and Multireference ab Initio Calculations”

What is not required to make a single molecule magnet

The widely accepted assumption that the development of more efficient single molecule magnets must involve ever higher total spin values has so far driven synthetic efforts towards molecular clusters of increasing nuclearity. In the present paper it is suggested that it might be worthwhile to reconsider this approach. There is evidence from theory and experiment to suggest that the race for multinuclear complexes with higher total spin might not necessarily be fruitful as a strategy for maximizing the magnetic relaxation barrier. Instead, we propose that more effort should be directed in understanding the parameters involved in maximizing the anisotropy of small, perhaps even mononuclear, molecules. Using multi-reference ab initio calculations we demonstrate the theory that can be applied and the principles of the computational approach for representative mononuclear complexes. Such small units may subsequently be employed as building blocks for the controlled assembly of larger and maximally anisotropic single molecule magnets.