HYPERFINE MATRIX SHIFT AND EPR - LINESHAPE ANISOTROPY OF

METHYL RADICALS TRAPPED IN

SOLID Ne, Ar, Kr AND p-H2 MATRICES.

YURIJ A. DMITRIEV1 and NIKOLAS-PLOUTARCH BENETIS2

1A. F. Ioffe Physical-Technical Institute, St. Petersburg, Russia

2Department of Pollution Control, Technological Educational Institution, TEI, West Macedonia, Kozani 501 00, Greece.

The aim of the study is:

to correlate the EPR experimental spectral anisotropy data and hyperfine interaction (hfi) matrix shifts to the matrix-radical interaction

Why methyl radicals, CH3?

1.exceptionally small momentum of inertia2.kinetic energies of the lowest rotational states comparable to kT3.quantum statistics instead of Boltzmann statistics could be expected

at liquid He temperatures

#2

To obtain a sample of a frozen gas with trapped methyl radicals, we used the deposition technique. The samples were obtained by gas condensation on the thin-walled bottom, 2, of a quartz finger, 3, inserted into the EPR cavity, 1. The finger may be filled with liquid He at 1.5 – 4.2 K or cooled by He vapor to obtain temperatures in the range of 4.2 – 40 K.

#3Experimental technique

EPR spectrum anisotropy

 As an example of the anisotropy, the EPR spectrum of CH3 radicals in solid Ar is presented.

The anisotropy reveals itself through different amplitudes and widths of different hyperfine components.

#4

#5

1 2 3 40.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0A

mp

litu

de,

arb

. un

.

Component number

Ar Kr Ne H

2

To compare the variation of the anisotropy of the EPR spectrum of CH3 radicalstrapped at T ≤ 4.2 K in different solid gas matrices the amplitudes of the hyperfine components versus the component number for Ne, Ar, Kr and H2 were all plotted together.

The component number increases going from low (MF = 3/2) to high (MF = -3/2) magnetic field. The first component is used as a reference by setting all the amplitudes equal, and presenting all the rest relative the first.

#6The figure shows the EPR spectrum ofCH3 radicals in p-H2 matrix which revealed no anisotropy.

EPR spectrum of CH3 radicals trapped in p-H2 matrix and recorded at 4.2 K. The insert demonstrates the shape of MF = ½ component recorded at smaller scan of the magnetic field and the spectrometer time constant increased three times in comparison with the panoramic spectrum.

To have an idea about the effect of thesample temperature on the spectrumanisotropy of CH3 radicals in solid gases,a study, in Ar matrix, at temperatures above 4.2 K has been carried out.

5 10 15 20 25 300,0

0,4

0,8

1,2

1,6

2,0

AV

I/AI

T, K

Temperature dependence of the amplitude ratio of the high-field component (fourth line) to the amplitude of the low-field one (first line) measured in several runs. Circles present data at temperatures above 4.2 K, while the triangle summarizes measurements made at liquid helium temperatures.

In this stage of our consideration, we need a quantitative measure of the extent of the anisotropy. We based our search for that measure on the dependence of the inverse square root of the hyperfine component amplitude, A0, on the nuclear spin projection, MF:

#7

It is seen from the above expression that b and c describe the anisotropy, while constant a constitutes the isotropic part of the expression. We found the ratio, Arel:

22/10 FF McMbaA

a

cb

AA

AAArel

2/1max0

2/1min0

2/1max0

2/1min0

applied as the quantitative measure of the anisotropy. Here, A0max and A0min are set equal to the amplitudes of the largest and smallest experimental hf-components, respectively. The constants b and c are believed to be proportional to the squared interaction between the radical and the matrix particle.

The next figure tests possible correlation between anisotropy of the EPR spectrum for CH3 radicals and the attraction (van der Waals) interaction of the radicals with matrix particles. The noble- gas matrices are grouped away the least interacting H2 matrix.

#8

The figure suggests that the vdW attraction between the radical and the matrix particles hindering radical rotation could not be the major cause responsible for increasing the spectral anisotropy.

0,05 0,10 0,15 0,20 0,25

0,0

0,1

0,2

0,3

0,4

NeAr

Kr

H2

Are

l, arb

. un

.

EV

2, 10-6 2

Next we tested the effect of repulsion (Pauli) interaction between the CH3 radical and matrix particles on the EPR spectrum anisotropy.

The good correlation between Arel and Ep2 suggests that the repulsion is

the major source of the anisotropy and, hence, the major source of the hindering of the radical rotation.

#9

0,00 0,01 0,02 0,03 0,04 0,05 0,06

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

NeAr

Kr

H2

Are

l, ar

b. u

n.

Ep

2, 10-6 2

1. T. Kiljunen, E. Popov, H. Kunttu, J. Eloranta “Rotation of methyl radicals in solid krypton matrix.” J. Chem. Phys. 130 (2009) 164504.

2. Mc Kenzie, J.-C. Brodovitch, K. Ghandi, B.M. Mc Collum, P. W. Percival “Hyperfine Coupling in Methyl Radical Isotopomers” J. Phys. Chem. A 111: 42 (2007) 10625 – 10624

#10Hyperfine constant matrix shift

The shift was previously discussed in the literature [2]. Based on these results, it may be supposed that, approximately, the hf constant depends linearly on the van-der-Waals interaction of the methyl radical with matrix particles. With our experimental data, we investigated the dependence of the hf constant on the attraction and repulsion radical – matrix interactions. In the next figure, the averaged hf- component spacing, δH, versus –EV, is plotted.

0 1 2 3 4 5 622,8

23,0

23,2

23,4

23,6

23,8

Ne

Ar

Kr

H2

free

H,

G

-EV,

0

3. F. J. Adrian “Matrix Effect on the Electron Spin Resonance Spectra of Trapped Hydrogen Atoms” J. Chem. Phys. 32 (1960) 972 – 981

While the overall tendency in the previous figure is nearly linear, the deviation from linearity suggests that probably another mechanism of the matrix shift should also be taken into consideration.

In the present work it was assumed that, generally, the mechanism of the shift would be in line with Adrian’s approach introduced previously [3] and applied to matrix-isolated H atoms. He showed that the van-der-Waals, vdW, interaction decreases the H-atom hf- coupling constant while the Pauli exclusion forces increase the constant.

Let us suppose that this is the case for the methyl radical also. Then, it would be more appropriate to plot δH against EV + βEp, where β is a parameter accounting for the repulsion.

#11

Several charts were plotted with different β and for each chart the standard deviation (SD) from the linear fit for δH was measured

#12

-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0

0,02

0,04

0,06

0,08

0,10

0,12S

D

βmin = 1.63 was obtained when the Standard Deviation reaches its minimum

Based on βmin, the spacing between CH3 hf-components affected by both attraction and repulsion was plotted

#13

0,0 0,5 1,0 1,5 2,0 2,5

22,9

23,0

23,1

23,2

23,3

23,4

23,5

23,6

23,7

Ne

Ar

Kr

H2

free

H,

G

-(EV+1.63E

p),

Results

1. It is found that the barrier exists to the rotation of CH3 radicals even in matrices of spherical particles and with high symmetry cages (except p-H2).

2. Repulsion forces between the radical and matrix particles make major contribution to the hindering of the radical rotation.

3. Attraction between the radical and the matrix particles is the major source of the matrix shift of the hf constant, but the repulsion should also be taken into account.

Sincere gratitude to

The Organizing Committee for the opportunity to present the talk

Professor Benetis for the fruitful collaboration

The Russian Foundation for Basic Research for a partial financial support of the study