Jason A. Sonk

Bioinspired Five-Coordinate Iron(III) Complexes for Stabilization of Phenoxyl Radicals†

Considerable effort has been directed towards the integration of biomimetic principles into molecular materials that have customized and controllable properties. The notion of stimulus-triggered molecular switching between two or more ground states of comparable energy is particularly relevant because such switching leads to detectable electronic and structural changes. Coordination complexes that merge transition-metal ions with ligands that stabilize organic radicals are among the most promising candidates for redox-responsive switching processes. Among the electroactive ligands that have been well characterized, those that contain phenolate moieties are significant because of their synthetic versatility and redox accessibility. This importance has been highlighted by studies on metal–phenoxyl complexes that have several geometries. Iron(III) complexes that contain phenolates tend to favor an octahedral geometry and are electrochemically reversible, but usually do not withstand multiple redox cycles. Thus, an understanding of the alternative geometries of such complexes becomes a necessary strategy for the future development of redox switches. We are investigating bioinspired designs that incorporate the basic geometries that are present in redox-versatile enzymes, such as tyrosine hydroxylase and intradiol dioxygenase, in which five-coordinate iron(III) centers support radical-based mechanisms for generating l-3,4-dihydroxyphenylalanine (l-DOPA) and cleaving catechol-type rings, respectively. We have reported the behavior of high-spin iron(III) complexes that are confined to low-symmetry, pentadentate N2O3 environments. [6] In these complexes, the assignment of oxidation states becomes challenging because of the contributions of ligandand metal-centered orbitals to the same redox process, and the presence of five unpaired electrons. Nonetheless, we have shown that high oxidation states are unavailable to the metal ion, and that the ligand supports up to three consecutive oxidations, which leads to antiferromagnetic interactions. Relative to octahedral fields, these fivecoordinate environments are expected to yield low-degeneracy molecular orbitals (MOs) that are sensitive to subtle but noticeable structural changes in the ligands. These changes should lead to orbital rearrangements that modify the sequence by which phenolate oxidations occur. Herein, we investigate the behavior of the five-coordinate species [FeL] (1) and [FeL] (2, Scheme 1), in which a low-symmetry ligand field is purposefully enforced around the 3d metal ion by the N2O3 ligands. Ligands [L ] and [L] both contain N2O3 environments with three phenolate moieties, denoted A, A’, and B; phenolates A and A’ share the same amine group and are chemically equivalent, whereas phenolate B is attached to either an azomethine group in L or to a methylamine group in L. Both species have four accessible ground states: [FeL]/[FeL] , [FeLC], [FeLCC], and [FeLCCC]. The aim of this study is to determine the sequence in which each of the phenolate rings is oxidized in the presence of the azomethine and the methylamine groups, and to test the feasibility of consecutive, multielectronic oxidations by ion-pairing effects with the supporting electrolyte. This study is intended to contribute to the fundamental understanding of the redox and electronic behavior of high-spin 3d 5 ions in five-coordinate ligand fields, and provide significant insight into bioinspired redox cycling. Complexes 1 and 2 were synthesized as previously described and crystals that were suitable for analysis by [*] M. M. Allard, J. A. Sonk, Dr. M. J. Heeg, Prof. H. B. Schlegel, Prof. C. N. Verani Department of Chemistry, Wayne State University 5101 Cass Ave. Detroit, MI 48202 (USA) E-mail: cnverani@chem.wayne.edu Homepage: http://chem.wayne.edu/veranigroup/

Strong field ionization rates simulated with time-dependent configuration interaction and an absorbing potential

Ionization rates of molecules have been modeled with time-dependent configuration interaction simulations using atom centered basis sets and a complex absorbing potential. The simulations agree with accurate grid-based calculations for the ionization of hydrogen atom as a function of field strength and for charge resonance enhanced ionization of H2(+) as the bond is elongated. Unlike grid-based methods, the present approach can be applied to simulate electron dynamics and ionization in multi-electron polyatomic molecules. Calculations on HCl(+) and HCO(+) demonstrate that these systems also show charge resonance enhanced ionization as the bonds are stretched.

TD-CI simulation of the electronic optical response of molecules in intense fields II: comparison of DFT functionals and EOM-CCSD.

Time-dependent configuration interaction (TD-CI) simulations can be used to simulate molecules in intense laser fields. TD-CI calculations use the excitation energies and transition dipoles calculated in the absence of a field. The EOM-CCSD method provides a good estimate of the field-free excited states but is rather expensive. Linear-response time-dependent density functional theory (TD-DFT) is an inexpensive alternative for computing the field-free excitation energies and transition dipoles needed for TD-CI simulations. Linear-response TD-DFT calculations were carried out with standard functionals (B3LYP, BH&HLYP, HSE2PBE (HSE03), BLYP, PBE, PW91, and TPSS) and long-range corrected functionals (LC-ωPBE, ωB97XD, CAM-B3LYP, LC-BLYP, LC-PBE, LC-PW91, and LC-TPSS). These calculations used the 6-31G(d,p) basis set augmented with three sets of diffuse sp functions on each heavy atom. Butadiene was employed as a test case, and 500 excited states were calculated with each functional. Standard functionals yield average excitation energies that are significantly lower than the EOM-CC, while long-range corrected functionals tend to produce average excitation energies slightly higher. Long-range corrected functionals also yield transition dipoles that are somewhat larger than EOM-CC on average. The TD-CI simulations were carried out with a three-cycle Gaussian pulse (ω = 0.06 au, 760 nm) with intensities up to 1.26 × 10(14) W cm(-2) directed along the vector connecting the end carbons. The nonlinear response as indicated by the residual populations of the excited states after the pulse is far too large with standard functionals, primarily because the excitation energies are too low. The LC-ωPBE, LC-PBE, LC-PW91, and LC-TPSS long-range corrected functionals produce responses comparable to EOM-CC.

TD-CI Simulation of the Electronic Optical Response of Molecules in Intense Fields: Comparison of RPA, CIS, CIS(D), and EOM-CCSD

A number of different levels of theory have been tested in TD-CI simulations of the response of butadiene interacting with very short, intense laser pulses. Excitation energies and transition dipoles were calculated with linear-response time-dependent Hartree-Fock (also known as the random phase approximation, RPA), configuration interaction in the space of single excitations (CIS), perturbative corrections to CIS involving double excitations [CIS(D)], and the equation-of-motion coupled-cluster (EOM-CC) method using the 6-31G(d,p) basis set augmented with n = 0-3 sets of diffuse sp functions on all carbons and only on the end carbons [6-31 n+ G(d,p) and 6-31(n+)G(d,p), respectively]. Diffuse functions are particularly important for transitions between the pseudocontinuum states above the ionization threshold. Simulations were carried out with a three-cycle Gaussian pulse (ω = 0.06 au, 760 nm) with intensities up to 1.26 × 10(14) W cm(-2) directed along the vector connecting the end carbons. Depending on the basis set, up to 500 excited states were needed for the simulations. Under the conditions selected, the response was too weak with the 6-31G(d,p) basis set, and the difference between levels of theory was more pronounced. When two or three set of diffuse functions were included on all of the carbons, the RPA, CIS, and EOM-CC results were comparable, but the CIS(D) response was too large compared to the more accurate EOM-CC calculations. Even though the frequency of the pulse is not resonant with any of the ground-to-excited transitions, excitations to valence and pseudocontinuum states occur readily above a threshold in the intensity.

TD-CI Simulation of the Strong-Field Ionization of Polyenes

Ionization of ethylene, butadiene, hexatriene, and octatetraene by short, intense laser pulses was simulated using the time-dependent single-excitation configuration-interaction (TD-CIS) method and Klamroths heuristic model for ionization (J. Chem. Phys.2009, 131, 114304). The calculations used the 6-31G(d,p) basis set augmented with up to three sets of diffuse sp functions on each heavy atom as well as the 6-311++G(2df,2pd) basis set. The simulations employed a seven-cycle cosine pulse (ω = 0.06 au, 760 nm) with intensities up to 3.5 × 10(14) W cm(-2) (E(max) = 0.10 au) directed along the vector connecting the end carbons of the linear polyenes. TD-CIS simulations for ionization were carried out as a function of the escape distance parameter, the field strength, the number of states, and the basis set size. With a distance parameter of 1 bohr, calculations with Klamroths heuristic model reproduce the expected trend that the ionization rate increases as the molecular length increases. While the ionization rates are too high at low intensities, the ratios of ionization rates for ethylene, butadiene, hexatriene, and octatetraene are in good agreement with the ratios obtained from the ADK model. As compared to earlier work on the optical response of polyenes to intense laser pulses, ionization using Klamroths model is less sensitive to the number of diffuse functions in the basis set, and only a fraction of the total possible CIS states are needed to model the strong field ionizations.

Synthesis, Characterization, and Theoretical Studies of Metal Complexes Derived from the Chiral Tripyridyldiamine Ligand Bn-CDPy3

Synthesis and characterization of metal complexes of the chiral tripyridyldiamine ligand Bn-CDPy3 (1), derived from trans-1,2-diaminocyclohexane, are described, along with theoretical studies that support the experimental data. These studies confirm that a single coordination geometry, out of five possible, is favored for octahedral complexes of the type [M(Bn-CDPy3)Cl], where M equals Co(III), Fe(II), and Zn(II). A combination of X-ray crystallographic and NMR spectroscopic methods was used to define the structures of the complexes [Co(Bn-CDPy3)Cl]Cl(2) (5), [Fe(Bn-CDPy3)Cl]X (X = FeCl(4), Cl, ClO(4), 6-8), and [Zn(Bn-CDPy3)Cl](2)ZnCl(4) (9) in the solid state and in solution. Experimental and theoretical data indicate that the most stable coordination geometry for all complexes possesses the Cl group trans to a basic amine donor and three pyridyl donors adopting the mer geometry, with two pyridyl N-donors adopting a coplanar geometry with respect to the M-Cl bond and the third pyridyl donor perpendicular to that axis. Calculations indicate that the ability to favor a single geometry is born from the chiral ligand, which prefers to be in a single conformation in metal complexes due to steric interactions and electronic factors. Calculated structures of the complexes were used to locate key interactions among the various diastereomeric complexes that are proposed to create an energetic preference for the coordination geometry observed in the metal complexes of 1.

Predicted Chemical Activation Rate Constants for HO2 + CH2NH: The Dominant Role of a Hydrogen-Bonded Pre-reactive Complex

The reaction of methanimine (CH2NH) with the hydroperoxy (HO2) radical has been investigated by using a combination of ab initio and density functional theory (CCSD(T)/CBSB7//B3LYP+Dispersion/CBSB7) and master equation calculations based on transition state theory (TST). Variational TST was used to compute both canonical (CVTST) and microcanonical (μVTST) rate constants for barrierless reactions. The title reaction starts with the reversible formation of a cyclic prereactive complex (PRC) that is bound by ∼11 kcal/mol and contains hydrogen bonds to both nitrogen and oxygen. The reaction path for the entrance channel was investigated by a series of constrained optimizations, which showed that the reaction is barrierless (i.e., no intrinsic energy barrier along the path). However, the variations in the potential energy, vibrational frequencies, and rotational constants reveal that the two hydrogen bonds are formed sequentially, producing two reaction flux bottlenecks (i.e., two transition states) along the reaction path, which were modeled using W. H. Millers unified TST approach. The rate constant computed for the formation of the PRC is pressure-dependent and increases at lower temperatures. Under atmospheric conditions, the PRC dissociates rapidly and its lifetime is too short for it to undergo significant bimolecular reaction with other species. A small fraction isomerizes via a cyclic transition state and subsequent reactions lead to products normally expected from hydrogen abstraction reactions. The kinetics of the HO2 + CH2NH reaction system differs substantially from the analogous isoelectronic reaction systems involving C2H4 and CH2O, which have been the subjects of previous experimental and theoretical studies.

Numerical Bound State Electron Dynamics of Carbon Dioxide in the Strong-Field Regime

Time-dependent Hartree-Fock simulations for a linear triatomic molecule (CO(2)) interacting with a short IR (1.63 eV) three-cycle pulse reveal that the carrier-envelope shape and phase are the essential field parameters determining the bound state electron dynamics during and after the laser-molecule interaction. Analysis of the induced dipole oscillation reveals that the envelope shape (Gaussian or trapezoidal) controls the excited state population distribution. Varying the carrier envelope phase for each of the two pulse envelope shapes considerably changes the excited state populations. Increasing the electric field amplitude alters the relative populations of the excited states, generally exciting higher states. A windowed Fourier transform analysis of the dipole evolution during the laser pulse reveals the dynamics of state excitation and in particular state coupling as the laser intensity increases.

HO + OClO Reaction System: Featuring a Barrierless Entrance Channel with Two Transition States.

Chlorine-containing compounds play a significant role in the troposphere and are key players in the stratosphere. The free radical compound OClO reacts with HO free radicals, but the existing experimental kinetics data are limited and uncertain. In the present theoretical investigation, the reaction mechanism, rate constants, and product branching ratios for the HO + OClO reaction system were computed over wide temperature and pressure ranges and compared with the existing experimental data. Stationary points on the singlet potential energy surface (PES) were calculated at high levels of theory, and the kinetics parameters were computed using several methods, including variational transition state theory (VTST) and RRKM/master equation techniques. The computed PES is in reasonable agreement with previous calculations, and the computed rate constants and branching ratio are in good agreement with the recent experiments. The results are used as the basis for recommendations for atmospheric chemistry modeling. The PES along the reaction path forming the peroxy bond has a steplike structure and only a very weakly bound prereactive complex, and yet it still supports two transition states along the reaction path. This feature may also be present in other reactions in which electrostatic forces align the approaching reactants in an unfavorable orientation at long distances, thus requiring a dramatic geometry change before reaction can take place.