numerical simulation of esr spectra of nitrixides as a method of investigation. andrey kh. vorobiev...

Numerical simulation of ESR spectra of nitrixides as a method of

investigation.Andrey Kh. VorobievMoscow State University

Typical result of simulation of single ESR spectrum: qualitative accordance;small deviations from experiment.

Rus.Chem Bull.V.50, No.12, p.2384

SiON(O ) CD3

Typical result of simulation of temperaturedependence of ESR spectrum: simulation of one spectrum is satisfactory;deviations from experiment are more then experimental noise.

Practice shows that for quantitative and reliablesimulation is necessary:

0. Quality of spectra recorded (subtract base line, avoid overmodulation, saturation, etc.);

1. Simulation of a set spectra (temperature dependence, angular dependence etc).;

2. Variation of parameters using least-squire minimisation method. (It gives impartial verification of your hypothesis) .

Proposed thesisProposed thesis: QuantitativeQuantitative modelling of spectra modelling of spectra produces more reliable and informative resultsproduces more reliable and informative results

Term "Quantitative modelling" means coincidence of experimental and calculated spectra within experimental errors.

Anisotropic line width is often necessary

convolution of isotropic Gauss and Lorentz lines

convolution of axial Gauss and Lorentz lines

G=7.2GL=0.71G

GX=7.9G, GZ=4.9GLX=0.21G, LZ=1.4G

exper.calc.

Distribution on magnetic parameters prevent to quantitative simulation

TEMPON in polystyrene 110K

Example:

Admixture of species with other magnetic parametersgives marked component.

Essential trouble for simulation: distribution on mobility

TEMPON absorbed on Na-mordenite in presence of water vapor

Experimantal spectrum consist of:

rigid limit spectrum,

spectrum of slow rotation

spectrum of fast rotation

Example:

Example of set of ESR spectra:Temperature dependence.

TEMPON in porous teflon AF-2400

100-150K

150-300K

300-400K

Experimental spectra:

Attempt of simulation using model of anisotropic Brownian rotation:

Deviations are more then experimental errors!

High temperature part of dependence

Simulation of the same spectra using model of simultanious isotropic rotation and anisotropic quasilibrations of probe:

Conclusion: quantitative simulation gives additional information even in the case of low-

informative spectra

Quasilibration: avereging on frequency, limited on amplitude moves.

Moves around three axes should be taken into account in general.

Results for whole temperature dependence:movements of probes in polyvinyltrimethylsilane

N OO

We thank Prof. Kalman Hideg and Dr.Tamas Kalaifor the gift of this substance.

Probe in liquid crystal 77K

Example of simulation of set of ESR spectra:Angular dependence of ESR spectrum.

Quantitative modelling of angular dependence gives the orientation

distribution function:

X

Y

Z

X

Distribution function is presented in molecular frame

A.Kh.Vorobiev, N.A.Chumakova,J.Magn.Res. 2005, 175, No.1 pp.146

Relaxation of orientation distribution function at the annealing

X

Y

Z

X X

Y

Z

X

X

Y

Z

X X

Y

Z

X

77К 195К

226К 236К

Orientation of probe molecule relative to liquid crystal are determined by quntitative simulation of spectra.

Dr. Vladimir GurmanDr. Tatiana Klimenko Natalia Chumakova Daria Chernovatook part on presented work