An ESR and ENDOR study of tunneling rotation of a hindered CH3 group in C2H5 radicals trapped in xenon matrices at 4.2 K: Environment effect on internal rotation

Toriyama, Kazumi; Iwasaki, Machio; Nunome, Keichi; Muto, Hachizo

doi:10.1063/1.442267

Cited by 25 publications

(30 citation statements)

References 22 publications

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“…[33][34][35][36] On the basis of this model, EPR and ENDOR spectra of organic radicals containing methyl rotors have been extensively described. [37][38][39][40][41][42][43] Those studies used the hyperfine splittings and the thermal evolution of the magnetic resonance signals to characterize the shape and depth of the rotational well that causes the hindrance of the methyl motion.…”

Section: Analysis Of the Results: Methyl Rotation Modelmentioning

confidence: 99%

Methyl rotors in flavoproteins

Martínez

Alonso

García-Rubio

et al. 2014

Phys. Chem. Chem. Phys.

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In this contribution we present the study of the thermal dependence of the ENDOR spectra of flavodoxin at low temperatures which reveals the dynamics of the methyl groups bound to the flavin moiety in flavoproteins. The methyl groups behave as quantum rotors locked by a deep rotational well and undergoing a tunneling process. At room temperature, methyl rotors are locked and the hopping motion is slow. This picture of the dynamics of the methyl groups of the flavin ring is quite different from the one usually accepted and has relevant consequences on the understanding of the mechanisms of flavoproteins.

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Section: Analysis Of the Results: Methyl Rotation Modelmentioning

confidence: 99%

Methyl rotors in flavoproteins

Martínez

Alonso

García-Rubio

et al. 2014

Phys. Chem. Chem. Phys.

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“…6 The μ þ is produced 100% spin polarized at a nuclear accelerator (TRIUMF in the present study) and this polarization can be effectively transfered to a free radical by Mu addition reactions, and then sensitively monitored by the μSR (muon spin relaxation or resonance) technique. [6][7][8][9][10][11][12][13][14] The most extensive studies of Hfcc in alkyl radicals to date has been for the ethyl radical and its isotopomers, both by EPR 1,5,12,15 and μSR 12,14 (see also citations in ref 11). Of relevance here are the torsional barriers to internal rotation.…”

Section: Introductionmentioning

confidence: 99%

Isotope Effects and the Temperature Dependences of the Hyperfine Coupling Constants of Muoniated sec-Butyl Radicals in Condensed Phases

et al. 2011

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Reported here is the first μSR study of the muon (A(μ)) and proton (A(p)) β-hyperfine coupling constants (Hfcc) of muoniated sec-butyl radicals, formed by muonium (Mu) addition to 1-butene and to cis- and trans-2-butene. The data are compared with in vacuo spin-unrestricted MP2 and hybrid DFT/B3YLP calculations reported in the previous paper (I), which played an important part in the interpretation of the data. The T-dependences of both the (reduced) muon, A(μ)′(T), and proton, A(p)(T), Hfcc are surprisingly well explained by a simple model, in which the calculated Hfcc from paper I at energy minima of 0 and near ±120° are thermally averaged, assuming an energy dependence given by a basic 2-fold torsional potential. Fitted torsional barriers to A(μ)′(T) from this model are similar (~3 kJ/mol) for all muoniated butyl radicals, suggesting that these are dominated by ZPE effects arising from the C−Mu bond, but for A(p)(T) exhibit wide variations depending on environment. For the cis- and trans-2-butyl radicals formed from 2-butene, A(μ)′(T) exhibits clear discontinuities at bulk butene melting points, evidence for molecular interactions enhancing these muon Hfcc in the environment of the solid state, similar to that found in earlier reports for muoniated tert-butyl. In contrast, for Mu−sec-butyl formed from 1-butene, there is no such discontinuity. The muon hfcc for the trans-2-butyl radical are seemingly very well predicted by B3LYP calculations in the solid phase, but for sec-butyl from 1-butene, showing the absence of further interactions, much better agreement is found with the MP2 calculations across the whole temperature range. Examples of large proton Hfcc near 0 K are also reported, due to eclipsed C−H bonds, in like manner to C−Mu, which then also exhibit clear discontinuities in A(p)(T) at bulk melting points. The data suggest that the good agreement found between theory and experiment from the B3LYP calculations for eclipsed bonds in the solid phase may be fortuitous. For the staggered protons of the sec-butyl radicals formed, no discontinuities are seen at all in A(p)(T), also demonstrating no further effects of molecular interactions on these particular proton Hfcc.

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“…The measured value of the isotropic hfi, 141, is about 60 MHz and provides the most direct way of determining spin densities in organic free radical^.^ McConnell's study of the anisotropic hfi in malonic acidlo demonstrated that the sign of a is negative.…”

Section: Introductionmentioning

confidence: 99%

ENDOR studies at 4.2 K of the radicals in malonic acid single crystals

McCalley¹,

Kwiram²

1993

J. Phys. Chem.

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Proton hyperfine tensors based on ENDOR measurements at 4.2 K are reported for five distinct radical types in X-irradiated crystals of partially deuterated malonic acid. A total of 30 tensors are reported reflecting slightly different orientations of the damage products in the crystal: four tensors for RCHR; four for (RCH&; ten for ( R C H Z )~; four for the u radical, (RCH2C-O)u; six relatively isotropic proton tensors; and two d i k e tensors. R = -COOH throughout. The classic RCHR radical is found in two conformations at 4.2 K, separated by a f 1 2 O rotation of the >C-H direction with respect to the C-C-C plane of the room temperature neutron diffraction structure. It is hypothesized that the RCHR radical observed at room temperature is undergoing rapid librational motion between these potential energy minima (separated by 24O) thereby explaining the observed asymmetry of the dipolar hyperfine tensor. The proton hyperfine tensors for the RCH2 radicals whose plane is nearly parallel to the malonic acid C-C-C plane were measured to a precision of f50 kHz, allowing an accurate determination of the anisotropy difference between the two protons. From this difference, the spin density delocalization onto the carboxyl oxygens has been deduced and compared with the results of ab initio calculations. The set of radicals with relatively isotropic proton tensors are tentatively assigned to stabilized forms of a malonic acid reduction product, in which an extra electron is trapped on one of the carboxyl groups.

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An ESR and ENDOR study of tunneling rotation of a hindered CH3 group in C2H5 radicals trapped in xenon matrices at 4.2 K: Environment effect on internal rotation

Cited by 25 publications

References 22 publications

Methyl rotors in flavoproteins

Methyl rotors in flavoproteins

Isotope Effects and the Temperature Dependences of the Hyperfine Coupling Constants of Muoniated sec-Butyl Radicals in Condensed Phases

ENDOR studies at 4.2 K of the radicals in malonic acid single crystals

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