Some observations of various 1,8 naphthoquino-dimethane biradicals by electron spin resonance spectroscopy

Photolysis of 8-methyl-1-naphthyldiazomethane at 4 K gives the triplet ESR spectrum of 1,8-naphthoquinodimethane but not that of 8-methyl-1-naphthylcarbene. Photolysis of either 8-methyl-1-azido-naphthalene or 8-amino-1-azidonaththalene does produce the triplet biradical spectra of 8-imino-1-naphthoquinomethane and 1,8-diiminonaphthoquinodimethane, in addition to the triplet nitrene spectra of 8-methyl-1-nitreno-naphthalene and 8-amino-1-nitreno-naphthalene. Curie Law analyses demonstrate that these biradicals are all ground state triplets. Hückel and INDO calculations pertaining to the biradicals are reported. Large charge transfer interactions were observed in the ESR spectra of 8 substituted-1-naphthyl nitrenes.     

Modeling triplet flavin-indole electron transfer and interradical dipolar interaction: a perturbative approach

A benchmark biochemical reaction is here theoretically investigated by means of a perturbative approach in order to model each reaction step. The reaction is the flavin-indole electron transfer, involving also a spin-state relaxation of the ionic complex. The whole reaction path is modeled and the kinetics of the process is studied. The dipolar interaction between the two radicals is explicitly considered during the dynamic evolution of the system in order to investigate the proper conditions for the triplet-to-singlet transition to occur.     

Pulsed electron-electron double resonance on multinuclear metal clusters: assignment of spin projection factors based on the dipolar interaction

The interaction between two paramagnetic metal centers, a [3Fe-4S](+) cluster and a [NiFe] center, is investigated in the hydrogenase from Desulfovibrio vulgaris Miyazaki F by pulsed ELDOR (electron-electron double resonance). The distance between the metal centers is known from X-ray crystallography. The experimental dipolar spin-spin interaction deviates from the value expected for two point-dipoles located at the centers of the metal clusters. An extended spin-coupling model accounting for the spin coupling in the [3Fe-4S](+) cluster yields the observed interaction under the assumption of a particular magnetic coupling scheme for the three Fe ions. These results demonstrate that pulsed ELDOR can be used to gain insight into the inner structure of a multinuclear metal cluster.     

Electron-enhanced vibrational spectroscopy: a theoretical approach.

Enhancement of the Raman scattering and IR absorption activities due to the electron-attachment was investigated for water systems by DFT calculations. DFT calculation of a 6-ring water cluster system that included the diffusive nature of electrons well reproduced the Raman enhancement effects and Raman shifts of the OH stretching modes observed in experiments. Based on the same model and calculations, enhancement of the IR absorption activity was also studied and was found to also be improved. Furthermore, the same calculation revealed that the enhancement can be also expected not only in the OH stretching but also in the lower wavenumber region. The enhancement factors for the various vibrational modes of the OH groups range from 10(2) - 10(5) thanks to the electron addition. Based on the coincidence between the theoretical model and the experimental results for the Raman signals and theoretical prediction for IR absorption, new enhancement techniques based on an electron-attachment in both Raman scattering and IR absorption, denoted as "electron-enhanced vibrational spectroscopy (EEVS)", is proposed, where molecular polarizability itself is modulated by the strong electrostatic field induced by neighboring electrons.     

A new approach to determining the geometry of weakly coupled radical pairs from their electron spin polarization patterns

Analytical expressions for the spin polarized EPR lineshapes of weakly coupled radical pairs (RPs) are derived as functions of the angles between the anisotropic g-tensors of the radicals and the vector describing the dipolar coupling. It is shown that with a singlet precursor the EPR signal of the RP can be written as a linear function of the dipolar coupling. Under these conditions, the calculated powder spectrum can be expressed as a linear combination of four powder spectra, which are independent of the geometry of the RP. To reproduce the experimental spectra the optimal set of coefficients can be found by least-squares fitting. The advantage of this approach is that the four powder spectra must only be calculated once. This treatment shows very clearly the restrictions placed on the information obtainable from such spectra. Most importantly, a unique set of angles can only be obtained if the absolute amplitude of the spectrum is known. In general, the calculated spectrum is related to the experimental spectrum by an unknown, arbitrary scaling factor. In this case, sets of angles consistent with the data are obtained. Possible strategies for obtaining unique geometric information are discussed and demonstrated with the experimental data for the state P+*(865)Q-*(A) in Zn-substituted bacterial reaction centres.     

Nuclear magnetic resonance proton dipolar order relaxation in thermotropic liquid crystals: a quantum theoretical approach

By means of the Jeener-Broekaert nuclear magnetic resonance pulse sequence, the proton spin system of a liquid crystal can be prepared in quasiequilibrium states of high dipolar order, which relax to thermal equilibrium with the molecular environment with a characteristic time (T1D). Previous studies of the Larmor frequency and temperature dependence of T1D in thermotropic liquid crystals, that included field cycling and conventional high-field experiments, showed that the slow hydrodynamic modes dominate the behavior of T1D, even at high Larmor frequencies. This noticeable predominance of the cooperative fluctuations (known as order fluctuations of the director, OFD) could not be explained by standard models based on the spin-lattice relaxation theory in the limit of high temperature (weak order). This fact points out the necessity of investigating the role of the quantum terms neglected in the usual high temperature theory of dipolar order relaxation. In this work, we present a generalization of the proton dipolar order relaxation theory for highly correlated systems, which considers all the spins belonging to correlated domains as an open quantum system interacting with quantum bath. As starting point, we deduce a formulation of the Markovian master equation of relaxation for the statistical spin operator, valid for all temperatures, which is suitable for introducing a dipolar spin temperature in the quantum regime, without further assumptions about the form of the spin-lattice Hamiltonian. In order to reflect the slow dynamics occurring in correlated systems, we lift the usual short-correlation-time assumption by including the average over the motion of the dipolar Hamiltonian together with the Zeeman Hamiltonian into the time evolution operator. In this way, we calculate the time dependence of the spin operators in the interaction picture in a closed form, valid for high magnetic fields, bringing into play the spin-spin interactions within the microscopic time scale. Then, by adopting the spin-temperature density operator to represent the collective state of the spin system, and removing the traditional hypothesis of high temperature, we deduce an expression for the first order quantum contribution to T1D (-1), in terms of spectral densities, with coefficients in form of spin traces. The properties that distinguish our result from the high-temperature T1D (-1) are as follows. (a) It is exclusively associated to cooperative fluctuations. (b) Because of its quantum character, it relies on both considering the lattice degrees of freedom quantum mechanically and including the spin-spin interactions in the microscopic time scale. With regard to the average dipolar Hamiltonian, only the nonsecular part plays a relevant role. (c) Associated with the structure of the spin operator involved in the quantum contribution, a term arises which is proportional to the number of spins in the correlated molecular domains, showing that the quantum contribution may be of macroscopic size in highly correlated systems. When applied to nematic liquid crystals, the new term exhibits the typical nu(-1/2) Larmor frequency dependence through the spectral density of the OFD, in consistence with the experimental results.     

On the theoretical determination of the electron affinity of ozone

Summary Multiconfigurational electron correlation methods have been analyzed in order to theoretically compute the electron affinity (EA) of ozone. The near-degeneracy correlation effects, which are so important in O₃ and O ₃ ^– , have been described using complete active space (CAS) SCF wave functions. Remaining dynamic correlation effects are computed using second-order perturbation theory (the CASPT2 method). The best calculated adiabatic value (including zero-point energy corrections), 2.19 eV, is about 0.09 eV larger than the experimental value. Comparative studies using size-consistent coupled pair functional approaches (CPF and ACPF) have also been performed. The harmonic frequencies in O ₃ ^– have been determined to be: ₁=992, ₂=572, and ₃=879 cm^–1, which gives a zero-point energy of 0.151 eV.     

Pulsed field ionization electron spectroscopy and molecular structure of aluminum uracil

Al-uracil (Al-C4H4N2O2) was synthesized in a laser-vaporization supersonic molecular beam source and studied with pulsed field ionization-zero electron kinetic energy (ZEKE) photoelectron spectroscopy and density functional theory (DFT). The DFT calculations predicted several low-energy Al-uracil isomers with Al binding to the diketo, keto-enol, and dienol tautomers of uracil. The ZEKE spectroscopic measurements of Al-uracil determined the ionization energy of 43 064(5) cm-1 [or 5.340(6) eV] and a vibrational mode of 51 cm-1 for the neutral complex and several vibrational modes of 51, 303, 614, and 739 cm-1 for the ionized species. Combination of the ZEEK spectrum with the DFT and Franck-Condon factor calculations determined the preferred isomeric structure and electronic states of the Al-uracil complex. This isomer is formed by Al binding to the O4 atom of the diketo tautomer of uracil and has a planar Cs symmetry. The ground electronic states of the neutral and ionized species are 2A' ' and 1A', respectively. The 2A' ' neutral state has a slightly shorter Al-O4 distance than the 1A' ion state. However, the 1A' ion state has stronger metal-ligand binding compared to the 2A' ' state. The increased Al-O4 distance from the 2A' ' state to the 1A' state is attributed to the loss of the pi binding interaction between Al and O4 in the singlet ion state, whereas the increased metal-ligand binding strength is due to the additional charge-dipole interaction in the ion that surpasses the loss of the pi orbital interaction.     

Mechanisms of exchange modulation in trimethylenemethane-type biradicals: the roles of conformation and spin density

The molecular structures and magnetic properties of six dinitroxide biradicals are described. Five of the dinitroxides are trimethylenemethane-type (TMM-type) biradicals; that is, the intramolecular exchange parameter, J, is modulated by a carbon-carbon double bond. However, the efficacy of the carbon-carbon double bond as an exchange coupler is determined by the molecular conformation. Our results show that the exchange parameters correlate with phenyl-ring torsion angles (phi) via a simple Karplus-Conroy-type relation: J = 44 cos(2) phi - 17. Comparison of these results to those obtained for our isostructural series of bis(semiquinone) biradicals shows that both the magnitude of J and the resistance of ferromagnetic J to bond torsions is proportional to the spin density adjacent to the exchange coupler.     

Nanostructure-driven analyte-interface electron transduction: a general approach to sensor and microreactor design

A concept describing the nanostructure-directed dynamics of acid/base interaction and the balance between physisorption and chemisorption on an extrinsic semiconductor interface is evaluated and compared for n- and p-type semiconductors. The inverse hard/soft acid/base (IHSAB) concept, as it complements the HSAB concept, defines the nature of a dominant physisorption behavior and enables the creation of a matrix of controllable interactions. The technology results in the coupling of Lewis acid/base chemistry with the extrinsic semiconductor majority carriers. Nanoporous silicon layers facilitate the application of nanostructured metal/metal oxides, which provide sensitivity and selectivity for the modified interface. Applied fractional depositions can produce a dominant reversible physisorptive (sensors) or chemisorptive (microreactors) interaction at the semiconductor interface as the nanostructures act as antennas to focus the interaction. The dynamic natures of n- and p-type silicon are evaluated and compared, by focusing on the controlled manipulation of these semiconductors as they are modified with nanostructures and interact with the gas-phase analytes. The observed semiconductor responses correlate well with the temperature dependence of the extrinsic semiconductor, the population of the donor or acceptor levels, and the inherent mobilities of electrons. The response of the modified n-type semiconductors is found to exceed that of comparable p-type systems. The IHSAB concept can be extended to assess the properties of several additional semiconductor interfaces including nanowires. The results obtained not only pertain to sensor and microreactor array design, but also suggest the importance of the dynamic changes created, as the majority charge-carrier concentrations are manipulated and the Fermi energies are modified through chemical interaction.     

Unmasking melon by a complementary approach employing electron diffraction, solid-state NMR spectroscopy, and theoretical calculations-structural characterization of a carbon nitride polymer

Poly(aminoimino)heptazine, otherwise known as Liebig's melon, whose composition and structure has been subject to multitudinous speculations, was synthesized from melamine at 630 degrees C under the pressure of ammonia. Electron diffraction, solid-state NMR spectroscopy, and theoretical calculations revealed that the nanocrystalline material exhibits domains well-ordered in two dimensions, thereby allowing the structure solution in projection by electron diffraction. Melon ([C(6)N(7)(NH(2))(NH)](n), plane group p2 gg, a=16.7, b=12.4 A, gamma=90 degrees, Z=4), is composed of layers made up from infinite 1D chains of NH-bridged melem (C(6)N(7)(NH(2))(3)) monomers. The strands adopt a zigzag-type geometry and are tightly linked by hydrogen bonds to give a 2D planar array. The inter-layer distance was determined to be 3.2 A from X-ray powder diffraction. The presence of heptazine building blocks, as well as NH and NH(2) groups was confirmed by (13)C and (15)N solid-state NMR spectroscopy using (15)N-labeled melon. The degree of condensation of the heptazine core was further substantiated by a (15)N direct excitation measurement. Magnetization exchange observed between all (15)N nuclei using a fp-RFDR experiment, together with the CP-MAS data and elemental analysis, suggests that the sample is mainly homogeneous in terms of its basic composition and molecular building blocks. Semiempirical, force field, and DFT/plane wave calculations under periodic boundary conditions corroborate the structure model obtained by electron diffraction. The overall planarity of the layers is confirmed and a good agreement is obtained between the experimental and calculated NMR chemical shift parameters. The polymeric character and thermal stability of melon might render this polymer a pre-stage of g-C(3)N(4) and portend its use as a promising inert material for a variety of applications in materials and surface science.     

On the carrier properties of perfluoropolyether-betaine mixed vesicles: the contribution of electron spin resonance spectroscopy

The carrier properties of mixed unilamellar vesicles of fluorocarbon-hydrocarbon surfactants built up with the ammonium salt of a perfluoropolyether and n-dodecylbetaine were investigated by electron spin resonance spectroscopy (ESR). The ESR-active lipophilic nitroxide 5-doxylstearic acid (5-DSA) was used as a model of a lipophilic drug to be carried and delivered into cell membranes. Healthy and malignant colorectal tissues were used as the target cells. Cell suspensions of living tissues were studied in physiological conditions. For the maintenance of the surgically removed tissues, McCoy’s 5A culture medium was used. 5-DSA probe was rapidly delivered from perfluoropolyether (PFPE)/betaine mixed vesicles to the membranes of both healthy and malignant colorectal cells. The analysis of the computed 5-DSA ESR line shapes ensured that no spectral differences occurred when 5-DSA was either introduced directly into the cells or through the intervention of vesicles. This well agreed for an unmodified physical status of the membranes where the probe was mainly localized.     

The electron transfer rate of large TPA based compounds: a joint theoretical and electrochemical approach

A series of triphenylamine (TPA) based compounds is investigated by means of density functional theory and cyclic voltammetry. Using the Nicholson's formalism, the measured deltaE(p) are correlated with B3LYP/6-31G* calculated reorganisation energies (lambda), elucidating the trend followed by the electron transfer rate of these compounds. Besides the direct dependency upon the dimension of the cationic fragment contributing to the hole stabilisation, the lambdas are tuned by the symmetry local to the TPA units, as evidenced by the structural relaxation of the cations. MDTAB shows the interesting combination of low ionisation potential (IP) and low lambda. This can make this compound interesting for practical applications in organic light emitting diode (OLEDs) devices, due to the direct correlation of the IP and lambda with the hole transfer efficiency to the anode, along with the hole mobility.     

Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap ΔEs-T= Es - ET is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shownthat when the radical centers R(R=H2C- ,H2N - or HN'- ) are twisted sufficiently far out ofconjugation with the benzene ring, ΔEs-T tends to zero or is negative, i.e. ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho-or para-phenylene coupling unit.     

Infrared spectroscopy of small sodium-doped water clusters: interaction with the solvated electron

The measured vibrational OH-stretch spectra of size-selected Na(H2O)n clusters for n=8, 10, 16, and 20 are compared with first-principle calculations, which account for the interaction of the sodium cation, the electron, and the water molecules with the hydrogen-bonded network. The calculated harmonic frequencies are corrected by comparing similar results obtained for pure water clusters with experiment. The experimental spectra are dominated by intensity peaks between 3350 and 3550 cm(-1), which result from the interaction of the H atoms with the delocalized electron cloud. The calculations, which are all based upon the average spectra of the four lowest-energy isomers, indicate that most of the peaks at the lower end of this range (3217 cm(-1) for n=8) originate from the interaction of one H atom with the electron distribution in a configuration with a single hydrogen-bonding acceptor. Those at the upper end (3563 cm(-1) for n=8) come from similar interactions with two acceptors. The doublets, which arise from the interaction of both H atoms with the electron, appear in the red-shifted part of the spectrum. They are with 3369/3443 cm(-1) quite pronounced for n=8 but slowly vanish for the larger clusters where they mix with the other spectral interactions of the hydrogen-bonded network, namely, the fingerprints of the free, the double, and the single donor OH positions known from pure water cluster spectroscopy. For all investigated sizes, the electron is sitting at the surface of the clusters.     

A new theoretical approach for the determination of molecular orientation persistence length of adsorbed nanofilms by FTIR reflectance spectroscopy

FTIR-Reflectance experiments have proved to be a powerful tool for the determination of molecular orientation in thin films adsorbed onto highly reflecting metals. We propose an original method for determination of the persistence length of molecular orientation in the film. This approach is based on the fact that molecular orientation persists only over a given distance from the geometrical interface. This distance is called the “persistence length of molecular orientation”. We then suppose that the nanofilm adsorbed is stratified and consists of an oriented layer (in the near-interface region) plus an isotropic one. Correlation between infrared reflection absorption band intensities and simulated band intensities allows experimentators to determine accurate molecular orientation and persistence length of orientation of a considered functional group. This is accomplished by using various IR reflection angles and p-polarization state of the incident IR wave. Film thickness and complex refractive index spectra, n(v) − i · k(v), are only needed to deduce calculated specular reflectance intensities.     

Effect of thick fixed-charge layers on electrostatic interaction — a theoretical approach

The electrostatic interaction pressure of charged surface layers is considered qualitatively and quantitatively. In the case of mutual penetration of the surface layers in addition to Maxwell stress and osmotic resp. hydrostatic pressure an isotropic stress on the fixed charges carrying molecules of the surface layers has to be taken into account. The derivation of the pressure-distance equations is given starting from both thermodynamic/electrostatic and hydrostatic/electrostatic principles. A possible biological significance of the additional stress is discussed emphasizing its role in modifying the structure of surface layer molecules.List of symbols e ₀ elementary charge - k Boltzmann constant - n _i concentration of theith ionic species in the bulk solution - P hydrostatic pressure - P hydrostatic pressure in the bulk volume (× ) - P ^h integration constant, independent on ×:P ^h =P(h) - T absolute temperature - Z _i electrovalence of theith ionic species - thickness of the surface layer - , ₀ relative and absolute permittivities - II(×) osmotic pressure at position × - II osmotic pressure in the bulk solution (× ) - osmotic pressure in the symmetry plane of interacting identical surface layers (electric field strength equals zero) - integration constant, independent on ×: - _e ^h electrostatic component of the disjoining pressure _e ^h = _e (h) - (×) mobile charge density profile (cations and anions of the electrolyte) - (×) fixed charge density profile - ^t(x) total charge density profile ( ^t = +) - ¹(x) fixed charge density profile of one of the two surface layers ( ¹(×) 0 for 0×) - (×) electric potential profile