Mattar's Research Interests

 

Our research program involves the study of the formation mechanisms, structure-bonding relationships, properties and chemical reactivity of selected paramagnetic reaction intermediates. They are sulfur-nitrogen heterocycles and organometallic transition metal half-sandwich complexes.

These intermediates are fundamental building blocks that react to form important new magnetic, conducting and spin-electronic materials. We would like to understand how their structure and bonding influences their magnetic, electronic, chemical and physical properties. 

This is done by generating the intermediates and determining their experimental parameters, such as hyperfine (A) and g tensors by electron paramagnetic resonance (EPR), vibrational frequencies by Raman and infrared (IR) spectroscopy and their electronic excitation energies by ultraviolet-visible (UV-VIS) spectroscopy. These parameters are also calculated by hybrid density functional, CAS-SCF or configuration interaction (CI) methods. The agreement between the computed and experimental parameters results in a comprehensive and clear bonding picture from which concrete conclusions are made about the role of the intermediate in the mechanism of the overall formation reaction.

We have successfully used this technique in the past. Organic semiquinone free radical intermediates that are involved in single electron transfer reactions of biological importance are also being studied by the same methods.

When the reactive intermediates are short-lived, they are trapped in rare-gas matrices at low temperatures (8-12 K). By matrix-isolating them, their experimental parameters are determined, via EPR, IR and UV-VIS spectroscopy, without the necessity of expensive ultra-fast techniques.

Electron magnetic resonance, such as EPR, ELDOR and ENDOR, spectroscopy are our most important tools and wecontinue to develop new methods of improving our home-built pulsed and conventional multi-frequency (4.5 - 35.0 GHz) spectrometers. These include new pulsed electron-spin-echo techniques, phase-modulation detection and dielectric resonator probes that increase the spectrometers’ sensitivities by a factor of 30. Thisl enhances their spectral resolution and the detection of smaller amounts of intermediates.

 

My research Interests, during the past five years, has been focused on:

Role of Magnetic Inequivalency in Solid State EPR and ENDOR Spectroscopy.

Atoms in a molecule may be spatially, magnetically or totally equivalent. These equivalencies are important when computing, understanding and interpreting a molecule’s electronic and magnetic properties. A spectroscopist’s first fundamental objective is to identify the different types of equivalent atoms in a molecule.  For example, in methane the first thing one notes is that its 4 hydrogens are spatially equivalent.  Degenerate energy levels in a molecule are a direct result of its equivalent atoms.   In turn, these degeneracies determine the number of lines and unique intensity patterns of the molecule’s solid state spectra (NMR, EPR, ENDOR etc.).  We were the first to establish five general and fundamental relationships among spatial equivalency, magnetic equivalency, total equivalency and the molecule’s point group symmetry [S. M. Mattar, 1998, Chem. Phys. Lett., 287, 608].  It was proven that, in general, spatially equivalent atoms in a molecule are totally and magnetically equivalent only if they are related to one another by a center of inversion.  Therefore, while a molecule can have a large number of spatially equivalent atoms, magnetically equivalent ones can only exist in pairs. We have shown that magnetic inequivalency manifests itself in the EPR spectra of single crystals and in matrix-isolated organometallic compounds [S.M. Mattar, R.S. Sammynaiken, 1997, J. Chem. Phys.,, 106, 1094.], dithiazol-2-yl radicals [43], nitroxide spin labels [44], and organic radicals [60].  Inequivalency effects are also evident when computing the spin Hamiltonian tensor components. These may be the gyromagnetic (g), and nuclear hyperfine (A) tensors [(Chem. Phys. Lett., 1999, 300, 545), 44-46, 60].  As a result of magnetic inequivalency, the interpretation of a paramagnetic molecule’s EPR and ENDOR spectra requires that all nine components of each of its Spin Hamiltonian tensors must be determined experimentally and appropriately computed.

Multi-Frequency EPR spectrometers and High Sensitivity Resonators

Second order effects in EPR spectra show up as variations in the nuclear hyperfine splittings, intensities and line widths. These perturbations complicate the appearance of a solid state spectrum and render it more difficult to simulate.  Fortunately, second order effects can be minimized by operating at higher frequencies.  However, sometimes the EPR spectral resolution of transition metal complexes and organometallic compounds deteriorates as the frequency is increased [Ann. Rev. Biophys. Bioeng, 1982, 11, 391].  In these cases it is advantageous to acquire additional spectra at lower frequencies.  Thus, EPR spectra taken at low, medium and high frequencies complement each other and their simulation leads to more accurate spin Hamiltonian tensors.  In this regard, we have already built three spectrometers that operate at 4.5, 9.0 and 15.0 GHz (another at 35 GHz is almost complete). We have used the multi-frequency approach to get accurate experimental g and A tensor components of bicyclic 1,3,2-dithiazolyl radicals [S. M. Mattar, A. D. Stephens. 2000, Chemical Physics Letters, 327, 409-419, S. M. Mattar, 2005, Chemical Physics Letters, 405, 382-388].   These were then compared with the corresponding accurately computed tensors leading to excellent correlation between theory and experiment.  Concurrently, we have designed and constructed a novel dielectric resonator for EPR spectroscopy by inserting a pair of stacked ceramic rings in an intact TE102 rectangular cavity [S. M. Mattar, and A. Emwas, 2003, Chemical Physics Letters, 368, 724-731 ].  The new probe, termed DR/TE102, is tunable over the range of 8.0-10.0 GHz. It greatly enhances the spectrometer sensitivity and has superior signal-to-noise ratios that are at least 24-30 times larger than the original cavity in this extended frequency range. The resonator’s performance was tested using DPPH, TEMPONE, MnO and Cu(II)-biuret complexes.  Thus EPR spectra of very small amounts of paramagnetic organic, inorganic samples and spin labeled biomolecules may be obtained without resorting conventional cavities or loop gap resonators. We have also recently designed dielectric resonator probes, which display excellent resolution and sensitivity, for our home-built spectrometers that operate around 4.5 GHz and 15.0 GHz.

Correlation of the Experimental and Computed Spin Hamiltonian Tensors of Cyclic Dithiazolyl Free Radicals

Mono-, bi- and tri-cyclic free radicals containing S and N are precursors for conducting polymers, spin-electronic (spintronic) devices and molecular magnets. A basic building block for these compounds and devices is the 1,3,2-dithiazolyl (DTA) moiety.  The bulk magnetic properties of DTA radicals, which determine if they are effective molecular magnets and spintronic devices, depend on their solid state structure and aggregation mechanisms.  In order to understand these processes, one must first determine how their electronic and magnetic properties are influenced by their bonding, electronic structure and spin density distribution.  Thus, their experimental and computed A and g tensors should be correlated.  Apart from one early study, we have lead the way by systematically computing the A and g tensor components of mono-, bi-, and tri-cyclic DTA radicals by hybrid-density functional (HDF) UB1LYP methods. They are all in excellent agreement with those determined experimentally by EPR spectroscopy [(Chem. Phys. Lett., 1999, 300, 545), 43, 45, 55, 57].  The computed and experimental g and A tensors differ by < 1.0 part-per-thousand and  < 1 Gauss of their experimental values respectively. Thus, for the first time, g and A tensors of large bicyclic and tricyclic hetero radicals that contain S-N bonds are accurately calculated without resorting to post Hartree-Fock techniques.  This gave us confidence to compute the magnetic properties of larger polycyclic S-N inorganic radicals [53, 56].

Effect of Vibrations, Rotations and Solvent on the g and Hyperfine Tensors

The accurate computation of a radical’s g and A tensors must take into account its vibrations and the rotation of any of its functional groups.  In addition, the surrounding solvent molecules may also play a crucial role.  Consequently we have studied these effects on a set of benchmark radicals.  They are the widely used TEMPONE [44] and DTBN [46] nitroxide spin labels, the standard g tensor marker DPPH [58], 1,4-benzo-semiquinone [50] and 2-methyl-benzosemiquinone (MeBzSQ) [48]. The effects of solvents and bending modes are calculated using HDF techniques in conjunction with the PCM and COSMO methods. They reproduce the experimental trends very well [44, 46, 48, 50, 58]. To accurately estimate the MeBzSQ methyl protons’ hyperfine coupling constants, rotational averaging of the methyl group had to be considered [48]. In a matrix, where the radical’s atoms are immobilized, they are not magnetically equivalent. This must be taken into account when simulating the EPR line shapes of randomly oriented DTBN molecules isolated in an argon matrix [46]. Analysis of these line shapes points to the necessity of using pulsed EPR spectroscopy to avoid adiabatic rapid passage (scanning the magnetic field too fast) [46].  This is the main reason for building the pulsed spectrometers.

The Oxidation-Reduction Properties of Semiquinone Free Radicals

Semiquinone radicals play crucial roles in important biochemical process such as photosynthesis and respiration.  They are readily generated by the oxidation of hydroquinones or the reduction of quinones.  To understand their unique but complex REDOX reaction mechanisms, we have identified at least two intermediate species when tetrahydroquinone is oxidized to anthraquinone [(Chem. Phys. Lett., 1999, 306, 249), 51,54].  Another one of our goals is to use quinones and hydroquinones as precursors to generate stable free radicals that are, in turn, used to react with other ligands to form new species that can be matrix-isolated. Accordingly, we have reacted quinones with SNS+ to generate novel S-N containing heterocyclic radicals [53].  In the pursuit of stable semiquinones, we have demonstrated by EPR that although the bianthrone radical anion is in intimate contact with O2, it is totally resistant to chemical oxidation.  To the best of our knowledge this is the first reported semiquinone radical anion that is stable in the presence of O2 [J. Phys. Chem. A, 1997, 101,8227].