Jorge H. Rodriguez

First-principle computation of zero-field splittings: Application to a high valent Fe(IV)-oxo model of nonheme iron proteins

We report the computational implementation of a combined spin-density-functional theory and perturbation theory (SDFT-PT) methodology for the accurate calculation of zero-field splittings (ZFS) in complexes of the most diverse nature including metal centers in proteins. We have applied the SDFT-PT methodology to study the cation of the recently synthesized complex [Fe(IV)(O)-(TMC)(NCCH(3))](OTf)(2), [J. Rohde et al., Science 299, 1037 (2003)] which is an important structural and functional analog of high-valent intermediates in catalytic cycles of nonheme iron enzymes. The calculated value (D(Theory)=28.67 cm(-1)) is in excellent agreement with the unusually large ZFS reported by experiment (D(Exp)=29+/-3 cm(-1)). The principal component D(zz) of the ZFS tensor is oriented along the Fe(IV)=oxo bond indicating that the oxo ligand dominates the electronic structure of the complex.

A direct method for locating minimum-energy crossing points (MECPs) in spin-forbidden transitions and nonadiabatic reactions

An efficient computational method for locating minimum-energy crossing points (MECPs) between potential-energy surfaces in spin-crossover transitions and nonadiabatic spin-forbidden (bio)chemical reactions is introduced. The method has been tested on the phenyl cation and the computed MECP associated with its radiationless singlet-triplet spin crossover is in good agreement with available data. However, the convergence behavior of the present method is significantly more efficient than some alternative methods which allows us to study nonadiabatic processes in larger systems such as spin crossover in metal-containing compounds. The convergence rate of the method obeys a fast logarithmic law which has been verified on the phenyl cation. As an application of this new methodology, the MECPs of the ferrous complex [Fe(ptz)(6)](BF(4))(2), which exhibits light-induced excited spin state trapping, have been computed to identify their geometric and energetic parameters during spin crossover. Our calculations, in conjunction with spin-unrestricted density-functional calculations, show that the transition from the singlet ground state to a triplet intermediate and to the quintet metastable state of [Fe(ptz)(6)](BF(4))(2) is accompanied by unusually large bond-length elongations of the axial ligands ( approximately 0.26 and 0.23 A, respectively). Our results are consistent with crystallographic data available for the metastable quintet but also predict new structural and energetic information about the triplet intermediate and at the MECPs which is currently not available from experiment.

Accurate Calculation of Zero-Field Splittings of (Bio)inorganic Complexes: Application to an {FeNO}7 (S = 3/2) Compound

Iron nitrosyl complexes with {FeNO}7 (S = 3/2) configuration have a complex electronic structure and display remarkable but not fully understood spectroscopic properties. In particular, {FeNO}7 (S = 3/2) complexes have very large zero-field splittings (ZFSs), which arise from strong spin-orbit coupling, a relativistic effect. The accurate prediction and microscopic interpretation of ZFSs in transition metal complexes can aid in the interpretation of a vast amount of spectroscopic (e.g., Mössbauer and electron paramagnetic resonance) and other experimental (e.g., magnetic susceptibility) data. We report the accurate calculation of the sign and magnitude of ZFSs for a set of representative diatomic molecules based on a combined spin density functional theory and perturbation theory (SDFT-PT) methodology. In addition, we apply the SDFT-PT methodology to accurately calculate the magnitude and sign of the ZFS parameters of an {FeNO}7 (S = 3/2) complex and to interpret its spectrocopic data. We find that the principal component Dzz of the ZFS tensor is very closely oriented along the Fe-N(O) bond, indicating that nitric oxide dominates the very intricate electronic structure of the {FeNO}7 (S = 3/2) compound. We find a direct correlation between electronic delocalization along the Fe-N(O) bond, which is due to pi-bonding, and the large ZFS.

Ground- and excited-state electronic structure of an iron-containing molecular spin photoswitch

The electronic structure of the cation of [Fe(ptz)(6)](BF(4))(2), a prototype of a class of complexes that display light-induced excited-state spin trapping (LIESST), has been investigated by time-independent and time-dependent density-functional theories. The density of states of the singlet ground state reveals that the highest occupied orbitals are metal centered and give rise to a low spin configuration Fe(2+)(3d(xy) ( upward arrow downward arrow)3d(xz) ( upward arrow downward arrow)3d(yz) ( upward arrow downward arrow)) in agreement with experiment. Upon excitation with light in the 2.3-3.3 eV range, metal-centered spin-allowed but parity-forbidden ligand field (LF) antibonding states are populated which, in conjunction with electron-phonon coupling, explain the experimental absorption intensities. The computed excitation energies are in excellent agreement with experiment. Contrary to simpler models we show that the LF absorption bands, which are important for LIESST, do not originate in transitions from the ground to a single excited state but from transitions to manifolds of nearly degenerate excited singlets. Consistent with crystallography, population of the LF states promotes a drastic dilation of the ligand cage surrounding the iron.

A quantum Monte Carlo and density functional theory study of the electronic structure of peroxynitrite anion

Single point calculations of the ground state electronic structure of peroxynitrite anion have been performed at the optimized cis geometry using the restricted Hartree–Fock (RHF), Moller Plesset second order perturbation theory (MP2), generalized gradient approximation density functional theory (GGA DFT) in the B3LYP form and two quantum Monte Carlo (QMC) methods, variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC). These calculations reveal differences in atomization energies estimated by B3LYP (287.03 kcal/mol), MP2 (290.84 kcal/mol), and DMC, 307.4(1.9) kcal/mol, as compared to experiment, 313(1) kcal/mol. The error associated with MP2 and B3LYP methods is attributed largely to differential recovery of correlation energies for neutral nitrogen and oxygen atoms relative to the oxygen and peroxynitrite anions. In addition, basis set studies were carried out to determine potential sources of error in MP2 and B3LYP valence energy values. Our studies indicate that MP2 and B3LYP valence energies a...

Negative magnetoresistance and spin filtering of spin-coupled di-iron-oxo clusters

Spin-dependent transport has been computationally studied for an open-shell singlet di-iron-oxo cluster. Currents and magnetoresistances have been investigated as a function of spin state within the nonequilibrium Greens function approach. The applied bias can be used to tune the sign of the observed magnetoresistance. A colossal magnetoresistance has been determined for hydrogen anchoring. Applied biases lower than 0.3 V, in conjunction with sulfur anchoring, induce a negative magnetoresistance due to lowering of the anchor-scatterer tunneling barrier. The di-iron-oxo cluster displays nearly perfect spin filtering for parallel alignment of the iron magnetic moments due to the energetic proximity, relative to the Fermi level, of its highest occupied molecular orbitals.

Hydrogen-bonded intermediates and transition states during spontaneous and acid-catalyzed hydrolysis of the carcinogen (+)-anti-BPDE.

Understanding mechanisms of (+)-anti-BPDE detoxification is crucial for combating its mutagenic and potent carcinogenic action. However, energetic-structural correlations of reaction intermediates and transition states during detoxification via hydrolysis are poorly understood. To gain mechanistic insight we have computationally characterized intermediate and transition species associated with spontaneous and general-acid catalyzed hydrolysis of (+)-anti-BPDE. We studied the role of cacodylic acid as a proton donor in the rate limiting step. The computed activation energy (ΔG‡) is in agreement with the experimental value for hydrolysis in a sodium cacodylate buffer. Both types of, spontaneous and acid catalyzed, BPDE hydrolysis can proceed through low-entropy hydrogen bonded intermediates prior to formation of transition states whose energies determine reaction activation barriers and rates.