High field ENDOR as a characterization tool for functional sites in microporous materials

Abstract

The determination of the details of the spatial and electronic structure of functional sites (centers) in any system, be it in materials chemistry or in biology, is the first step towards understanding their function. When such sites happen to be paramagnetic in any point of their activity cycle, the tool box offered by a variety of high resolution electron paramagnetic resonance (EPR) spectroscopic techniques becomes very attractive for their characterization. This tool box has been considerably expanded by the developments in high field (HF) EPR in general, and HF electron nuclear double resonance (ENDOR), in particular. These have led to numerous new applications in the fields of biology, physics, chemistry and materials sciences. This overview focuses specifically on recent applications of pulsed HF ENDOR spectroscopy to microporous materials, such as zeotype materials, presenting the new opportunities it offers. First, a brief description of the theoretical basis required for the analysis of the HF ENDOR spectrum is given, followed by a description of the pulsed techniques used to record spectra and assign the signals, along with a brief presentation of the required instrumentation. Next, specific applications are given, including transition metal ions and complexes exchanged into zeolite cages, transition metal substitution into frameworks of zeolites, aluminophosphate molecular sieves, and silicious mesoporous materials, the interaction of NO with Lewis sites in zeolite cages and trapped S. We end with a discussion of the advantages and the shortcomings of the method and conclude with a future outlook.

HYSCORE and DEER with an upgraded 95GHz pulse EPR spectrometer

“…In the last decade numerous applications of high field CW EPR and ENDOR have been reported and the virtues of measurements at a high field have been well demonstrated and established [1][2][3][4][5][6]. Recently, it has also been demonstrated that distance measurements between paramagnetic centers by pulsed DEER techniques can also be performed at high fields [9][10][11][12].…”

“…In the past, due to widespread use of proton related ESEEM measurements it was often assumed that ESEEM spectroscopy is best performed at low frequencies (2)(3)(4)(5)(6)(7)(8)(9) to decrease Zeeman frequencies of nuclei and therefore facilitate the simultaneous excitation of forbidden and allowed EPR transitions necessary for the generation of the ESEEM effect. This led to the design and construction of low frequency pulse EPR spectrometers such as S-and C-band spectrometers [18][19][20][21][22].…”

Rational Design of Engineered Multifunctional Heterogeneous Catalysts. The Role of Advanced EPR Techniques

“…A review on the advantages of high field ENDOR in the characterization of functional sites in microporous materials can be found in Ref. [9], while some case studies are reported in ref [10][11][12][13]. Even though the application of these techniques is limited to cryogenic temperatures and, at present, not applicable under operando conditions, socalled hyperfine spectroscopy (ESEEM, ENDOR, HYSCORE) allows monitoring the distribution of the unpaired electrons wave function, which is one of the most useful of the molecular ''fingerprints'' obtained from an EPR spectrum, over first and second coordinative shells.…”

Determining the Secondary Structure of Membrane Proteins and Peptides Via Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy

“…It was first observed over a half century ago and has been particularly useful in characterizing organic radicals, metal complexes, and biomolecules with paramagnetic centers (Brückner, 2010; Goldfarb, 2006). However, with the development of site-directed spin-labeling (SDSL) techniques to target biological systems, there has been a significant increase in the application of EPR spectroscopy to study protein structure and dynamics (Alexander, Bortolus, Al-Mestarihi, Mchaourab, & Meiler, 2008; Altenbach, Flitsch, Khorana, & Hubbell, 1989; Fanucci & Cafiso, 2006; Hirst, Alexander, McHaourab, & Meiler, 2011; Hubbell, Gross, Langen, & Lietzow, 1998; Hubbell, López, Altenbach, & Yang, 2013; Sahu, McCarrick, & Lorigan, 2013; Sahu, McCarrick, Troxel, et al, 2013).…”