Roland Erickson

EPR and ENDOR studies of NOx and Cu2+ in zeolites: bonding and diffusion

The diffusion and bonding of NOx (x=1, 2) and Cu2+ species in zeolites are reviewed, based mainly on our own research. The molecular motion of adsorbed NO2 has been examined with EPR in several zeolite samples and analyzed using the slow-motional EPR theory. In X- and Y-type zeolites the broadening of the spectra with temperature could be analyzed by simulations using a rotational diffusion model in agreement with earlier results in Vycor glass and Cu-metal. For the diffusion of NO2 in Na-mordenite and Na-ZSM-5 the broadening of the spectra with increased temperature could be better simulated with the Heisenberg type of spin exchange model. The exchange was attributed to the interaction between NO2 molecules diffusing along the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increasing Si/Al ratio of the zeolite. The effect was attributed to the hindrance against diffusion by Na+, the amount of which increases with a decreased Si/Al ratio. The rates increased with increasing number of H2O molecules adsorbed on the zeolite because of weaker interaction between NO2 and the surface. A more detailed slow-motional analysis indicated that at each temperature a distribution of diffusion rates occurs. NO adsorbed on Na-A zeolite was found to be present as monomer at low pressure ( 100 mbar). The triplet was absent in calcium ion-exchanged A-type zeolite indicating that the NO–NO species depends on Na+ for its stability. The detailed structure of a Cu2+ complex with water molecules in Cu-ZSM-5 zeolite was characterised with ENDOR at 4 K. The complex has an axial distorted octahedral structure with two water molecules at a longer distance in axial position and four molecules in the equatorial positions. The Cu2+ complexes with ammonia and deuterated ammonia have been investigated with electron nuclear double resonance (ENDOR). Simulations of the ENDOR spectra with 1H, 2H and 14N hyperfine interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.

Analysis of powder EPR and ENDOR spectra of the biphenyl radical cation on H-ZSM-5 zeolite, silica gel and in CFCl3 matrix

Abstract The biphenyl radical cation absorbed on H-ZSM-5 zeolite and silica gel and isolated in frozen CFCl 3 has been studied by EPR and ENDOR spectroscopy. The recorded spectra can be explained by anisotropy of proton hyperfine interactions in para and ortho position and of the g -tensor. The experimental results indicate that the cation has a nearly planar structure, with a twist angle of less than 10°. The ENDOR spectra are analysed by computer simulations. A simple theory to calculate ENDOR transition moments to first order in disordered systems is presented. The simulations show that the observed ENDOR line positions correspond to principal hyperfine tensor values of the para and ortho ring protons.

Simulation of ENDOR spectra of radicals with anisotropic hyperfine and nuclear quadrupolar interactions in disordered solids

Abstract A general theory to calculate ENDOR spectra of S = 1 2 radicals in single crystal or disordered solids is presented. The theory is suitable for paramagnetic systems with n interacting nuclei which can be described by the spin-Hamiltonian H = β B g S + Σ i=1 n I i A i S + I i Q i I i − g i β N BI i . There are restrictions on the relative magnitude of the hyperfine, quadrupolar and nuclear Zeeman interactions, nor on the relative orientation of principal axes of the tensors. ENDOR transition frequencies and intensities are calculated with perturbation theory under the assumption that the electron Zeeman interaction is dominating. A simple formula for first order transition moments, including the effect of the quadrupolar coupling, is presented. The impact on the transition moment of the orientational distribution of the radio-frequency field in single crystal and disordered systems is taken into account. The method is tested against experimental proton- and 14N-ENDOR data of neutral and ionic radicals in disordered solids.

Ionic radicals on silica surfaces — an EPR, ENDOR and ESE study of benzene radical cations adsorbed on HY and silica gel

Abstract Electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and electron spin echo, (ESE) spectroscopy have been used to characterise radical cations of benzene (benzene-d6 and benzene-d1), generated by ionising radiation, and stabilised on silica gel and HY molecular sieve surfaces. The electronic structure and dynamic features of the benzene cation were found to be different from those in the case of stabilisation in a Freon matrix (CFCl3). The monomeric cation was found to undergo pseudorotation at 3.5 K with hyperfine structure (h.f.s.) constants due to six averaged hydrogen nuclei. Simulations of the time-domain deuterium electron spin echo envelope modulation (ESEM) (C6D+6 cation) gave the following h.f.s. constants: the perpendicular component of the axially symmetric tensor T⊥ = −0.8 MHz, and the isotropic component a = 2.17 MHz, consistent with ENDOR results at 105 K. It was concluded that the monodeuterated benzene cation ‘slowed down’ the rotation at 3.5 K, although not enough to allow an analysis in the rigid limit. ENDOR spectra of the protonated and deuterated monomeric and dimeric benzene radical cations on silica gel and HY molecular sieve surfaces are presented. On HY molecular sieve the ENDOR results revealed two types of dimer, one stabilised at low temperature (below 110 K) characterised by h.f.s. constants of 4.8 and 8.9 MHz, and the other stabilised at higher temperatures with an h.f.s. constant of 6.5 MHz. Only the latter type of dimer could be detected on silica gel. In addition to hyperfine splittings from ring protons, a smaller hyperfine splitting attributed to hydroxyl protons situated on the surface was determined from the ENDOR results.

An EPR, ENDOR and ESEEM study of the benzene radical cation in CFCl3 matrix: isotopic substitution effects on structure and dynamics

A combination of EPR lineshapes, ENDOR and ESEEM was performed to investigate possible Jahn–Teller effects in mono- and perdeuterated benzene radical cation in a polycrystalline CFCl3 matrix. Replacement of one proton by a deuteron was utilized to exclude significantly fast quantum pseudorotation and overall rotary motion below 77 K. Clear evidence was obtained for static Jahn–Teller distortion at temperatures up to 30 K, with major spin densities on two para positions (1 and 4) in agreement with a b2g(χs+) localized orbital. From ENDOR measurements of C6H5D+, the isotropic and dipolar coupling constants for protons occupying both high and low spin density positions were accurately measured. Additional ESEEM experiments and simulations confirmed the results, indicating further that deuteron-isotope substitution does not disturb appreciably the fundamental dynamics of the benzene radical benzene. The hyperfine tensors of C6D6+ in the Jahn–Teller distorted configuration obtained by ESEEM were analogous to those of fully protonated compounds, but the components were scaled by the magnetic moment ratio of deuterons to protons.

The structure of a copper(II) complex with water in Cu-ZSM-5 at 4 K by ENDOR

Abstract Electron spin resonance and electron nuclear double resonance were used to characterize the detailed structure of a copper complex with water molecules in Cu-ZSM-5 zeolites at 4 K. This Letter highlights the structure of the complex. It has an axial distorted octahedral symmetry. The distances between the copper ion and protons within the water molecules are 2.54 and 2.69 A. Accordingly, the water molecules can be divided into two groups. Two water molecules at the longer distance are in the axial position and four other water molecules at the shorter distance are in the equatorial position with respect to the Cu ion.

ESR line shapes and methyl rotation in the acetic acid anion radical

Abstract The CH 3 COOD − and CD 3 COOD − radical anions have been studied by ESR in irradiated crystal acetic acid in the temperature region 77–170 K. The line-shape variations which are due to methyl rotation are simulated with a program based on the Liouville direct method. The activation energy to the motion is evaluated and is found not to be appreciably affected by susbtitution of the methyl protons by deuterons. The activation energies are determined to be 2.2 and 2.5 kcal/mol respectively. Indications of hydrogen bond effects on the activation barrier are also found.

The [2H8]THF radical cation in CF3CCl3 and CFCl3. An EPR and ENDOR study

The structure and dynamics of the radical cation of deuteriated tetrahydrofuran ([2H8]THF) stabilized in the freon matrices CFCl3 and CF3CCl3 have been investigated by means of EPR and ENDOR spectroscopy. The EPR and ENDOR results of the rigid structure give two pairs of strongly interacting β-deuterium hf splittings [aD= 1.36 (2 D) and aD= 0.60 mT (2 D)] consistent with the protonated analogue [Kubodera et al., J. Phys. Chem., 1981, 85, 2583 (ref. 1)]. An additional hf splitting (0.9–1.1 mT) assigned to the matrix fluorine was resolved above ca. 110 K in CFCl3. The temperature dependent EPR lineshapes (77–145 K) involving the exchange between four coupled (I= 1) nuclei have been analysed. A two-site model explained qualitatively the alterating line-widths at intermediate temperatures and resulted in an activation energy of ca. 1.7 kcal mol–1 A non-perturbative approach adopting a four-site model to account for puckering motion was tested. The dynamics were also manifested as a reversible temperature effect for the ENDOR. The relative abundance of resonances due to an averaged (hf) structure become predominant as the temperature is increased.

ESR Lineshape for Exchanging Anisotropic Spin Systems

The ESR lineshape of one electron spin coupled by anisotropic hyperfine interaction to a number of nuclei is calculated when at least two different configurations of the molecule interchange according to a given reaction scheme. The spectrum calculation includes also the anisotropic electron- zeeman and the nuclear quadrupole interaction. The lineshape, which simulates a nonsaturated CW experiment, is calculated numerically by a direct method and includes the nonsecular terms of the electron spin. The density matrix theory, combined with Liouville formalism, is used allowing both for an accurate treatment of spin dynamics as well as detailed description of the reorganization of the system during exchange [1]. The exchange is treated phenomenologically by a sudden-jump, markov model in which the steady-state populations are calculated prior to the lineshape and are used for the lineshape calculation. For a given overall molecular orientation each configuration (site) is characterized by a set of three strength constants and three Euler angles per interaction [2]. This basic system is used to simulate a single crystal case or is appropriately averaged to simulate a powder sample. The above theory is compared with two experiments.