HYPERFINE INTERACTION PARAMETERS FOR 121Sb AND 127I IN (CH3)nSbX3-n (X = Cl, Br, I) (n = 0, 1, 2, 3)

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HYPERFINE INTERACTION PARAMETERS FOR 121Sb AND 127I IN (CH3)nSbX3-n (X = Cl, Br, I) (n

= 0, 1, 2, 3)

J. Devort, J. Sanchez, J. Friedt, G. Shenoy

To cite this version:

J. Devort, J. Sanchez, J. Friedt, G. Shenoy. HYPERFINE INTERACTION PARAMETERS FOR

121Sb AND 127I IN (CH3)nSbX3-n (X = Cl, Br, I) (n = 0, 1, 2, 3). Journal de Physique Colloques,

1974, 35 (C6), pp.C6-255-C6-258. �10.1051/jphyscol:1974634�. �jpa-00215791�

JOURNAL DE PHYSIQUE

Colloque C6, supplCment au

Tome 35, DCcembre 1974, page C6-255

HYPERFINE INTERACTION PARAMETERS FOR

lSb AND lZ7I IN (CH3),SbX3-, (X

C1, Br, I) (n

0,1,2,3)

J. P. DEVORT, J. P. SANCHEZ, J. M. FRIEDT and

K. SHENOY Laboratoire de Chimie NuclCaire

Centrz de Recherches NuclCaires, B. P. 20, 67037 Strasbourg Cedex, France

RksumB.

- Les variations de deplacement isomerique et d'interaction quadrupolaire au site de I2lSb dans la serie de composCs (CH3),SbX3-,

et X

C1, Br, I) sont discutks en fonction de la geometrie moleculaire, du changement d'ionicite des liaisons Sb-ligand et des interactions intermolBculaires

ces interactions intermoleculaires ont ete mises en evidence dans les composes SbI3 et CNsSbI2 par spectroscopie Mossbauer sur

L'Bvolution des parametres d'interaction hyperfine au site de l2lSb est interpretee dans I'approximation de Townes et Dailey ainsi qu'a i'aide d'un mod8le de charges ponctuelles.

Abstract. -

The variations of the isomer shift and quadrupole interaction at the 121Sb nucleus in the series (CH3)%SbX3-n

and

C1, Br,

are discussed In terms of the geo- metrical molecular parameters, the changes in ionicity of the Sb to ligand bonds and the varying degree of intermolecular interactions. The occurence of the latter interactions is clearly inferred from the I271 measurements in SbI3 and (CH3)SbIz. The evolution of the hyperfine interaction para- meters at the 121Sb nucleus is discussed in the Townes and Dailey approximation and using a point charge model.

1. Introduction. - The series of mixed antimony (111) organo-halides provides an interesting example to study the systematic dependence of the hyperfine interaction parameters on the chemical environment and the molecular geometry. We report here the Mossbauer investigations using the 37.15 keV reso- nance in lZ1Sb (spin states 512-712) in the set of compounds (CH3),SbX3-, where

0, 1, 2, 3 and X

C1, Br or I. In addition, we have measured the

lZ71 resonance spectra in SbI, and (CH3)SbI,.

In these compounds, a single molecule can be assumed to have a trigonal pyramidal form. Thus, the antimony atom forms sp3 hybrid orbitals with the neighbours, the fourth orbital being occupied by a lone pair. Comparing with the known structures of the corresponding arsenic compounds [I], one may well assume that the 3 angles, viz., X-%-CH,, CH,:S~-CH, and X-$-X, are nearly equal within each molecule.

Slight deviations in these angles would be due to the presence of intermolecular interactions in the solids, which have been evidenced through the NQR measure- ment on halogens in some of these compounds 121.

2. Experimental.

The spectra have been measured using a conventional electromechanical drive on a multichannel analyser in the time mode, keeping both sources and absorbers at 4.2 K. The 12'Sb and '''I spectra were measured against CalZ1"SnO3 and

Zn

Te sources, respectively, and the absorber

thicknesses were about 8 mg Sb/cm2 and 8

I/cm2.

The halides, (CH,),SbX and (CH,)SbX, with X

C1 and Br, have heen obtained either by reaction of CH,X on metallic Sb or by thermal decomposition of the corresponding Sb(V) organo-halide. In each of these preparations both the halides were present which could be identified from their characteristic quadrupole patterns (see Fig. 1). In fact, it is not clear from the

FIG. 1. -

Mossbauer spectra

(CH3)2SbCI

CH3SbC12 (containing some impurities of (CHs)*SbCl).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974634

C6-256 J. P. DEVORT, J. P. SANCHEZ, J. M. FRlEDT AND G. K. SHNOEY

Hyperfine interaction parameters at 4.2 K of lZ1Sb in (CH3),SbX3-,. The isomer shift (IS) vaIues refer to Ca121mSn03 at 4.2 K. The parameters remeasured for Sb(CH,), at 4.2 K are

IS

- 8.53 + 0.05 mm/s, e2 q, Q

15.25 1 0.20 mm/s and

literature

if the pure compounds can be isolated. The measured spectra were least-squares fitted using a transmission integral [4] including two quadrupole patterns of lZ1Sb. The analysis of the spectra measured on samples containing different amount of these halides yielded identical values for the hyperfine parameters.

In the case of iodide, we were able to obtain pure (CH3)Sb12

however, (CH,),SbI could not be pre- pared.

The experimental results of lZ1Sb and "'1 are summarized in Tables I and 11.

Parameters of l Z 7 I at 4.2 K

SbI, and (CH,)SbI,

3. Interpretation and discussion. - 3 . 1

QUADRU-

POLE INTERACTIONS IN

SbX3 (X

C1, Br)

Sb(CH,),. -The large value for the asymmetry parameter (y) deduced from the l Z 7 I Mossbauer spectra in SbI, ~eveals the existence of strong intermolecular interactions between the Sb ion and the I ion of a neigh- bouring molecule. From the lZ7I hyperfine interaction parameters one deduces that the ionicity of the intra- molecular Sb-I bond is 53 % whereas the inter- molecular character is 14 % (see appendix). Similar intermolecular interactions are known to occur in the other two halides from previous NQR investiga- tions [2]. They however cannot exist in Sb(CH,), as deduced from the bonding characters of the CH, ligand. The analysis of the 12'Sb spectra in all the SbX, compounds reveal a small y (Table I). One may hence assume an axial molecular symmetry with a C, axis along the hybrid orbital carrying an electron doublet (i. e. the z axis). Using Townes and Dailey approxima-

tion the principal component of the electric-field- gradient (efg) tensor is proportional to [5]

3 cos

-

+ NX)

Here, No and Nx represent the electron populations of the hybrid orbital carrying the lone electron pair and along the Sb-X direction, respectively and

is the X-Sb-X angle.

The negative sign of the principal component of the efg tensor reveals that No > Nx, i. e. that there is an excess of electron population in the hybrid orbital along the z axis compared to that in the three orbitals along the Sb-X directions.

Assuming that the angle a is approximately a cons- tant within the series, the decrease of the absolute value of e2 q,

on changing the CI- anion by Br- and 1- can be attributed to an increasing strength of intermolecular interactions in that order (which decreases No) associated with a decrease of the Sb-X ionicity (which increases N,). Thus

No + Nx) keeps its negative sign in all the compounds whereas its absolute value decreases from the chloride to the iodide.

The intermolecular interactions vanish in Sb(CH,),

as a result, No is considerabiy increased and this accounts for the higher value of I e2

I

(=

15.25 mm/s) observed in this compound.

3.2

(CH,)SbX,

(CH,),SbX.

Strong intermolecular interactions also occur in these organo-halides of Sb(II1) as revealed by the large value of

deduced from the spectra of lZ7I in (CH3)Sb12 (Table 11). Interpreting the results of the lZ7I spectra, one deduces that the intramolecular Sb-I bond ionicity is 52 %, whereas the intermolecular bonding character amounts to 16 % (see appendix).

The 12'Sb data reveal that the sign of the quadru-

pole interaction in the two types of organo-halides is

opposite whereas the magnitudes are nearly equal. The

asymmetry parameter 11 practically vanishes in

HYPERFINE INTERACTION PARAMETERS FOR lZISb AND 1271

(CH,)SbX, and is close to unity in (CH,),SbX

(Table I).

Due to the low molecular symmetry, the principal efg axis becomes arbitrarily oriented relative to the bond- ing directions and hence for this case the Townes and Dailey approximation becomes impracticable. To account for the experimental trends in the quadrupole interaction, we have thus attempted to use a point charge model.

Assuming the molecular model shown in figure 2,

the efg components were calculated as a function of the following parameters

a charge of - 2 on the electron pair at a distance ro from the Sb(II1) ion, a charge, a, on the ligand A at a distance r, a charge, b, on the ligand B at the same distance

and the angle a between MA, MB directions and the

plane. The values of eq, and

obtained by diagonalizing the efg tensor are

represented in figure 3 as functions of the angle

and

for various values of the ratio ro/r. Changes of the

strength of intermolecular interactions would be

accounted for by different values of ro/r. The upper example for a

12.50 and ro/r

1) the sign of eq, figure is given for MA,B and the lower one for MAB,. changes for the MA,B and MAB, molecules, although It appears that for certain values of a and of r,/r (for the absolute value is approximately constant. The

FIG. 3.

-

C6-258 J. P. DEVORT, J. P. SANCHEZ, J. M. FRIEDT AND G. K. SHENOY

change in the sign of eq, furthermore implies a sharp

variation in the asymmetry parameter q. The point charge model thus qualitatively accounts for the qua- drupole interaction parameters and the changes in

observed in (CH,)SbX, and (CH,),SbX. A similar variation has been reported in some nitrogen com- pounds from 14N NQR experiments [5, 61.

The knowledge of the detailed crystal structure would be required in order to perform more quantita- tive evaluation of the experimentally observed trends using either the precise hybrid orbitals or molecular orbital theory.

3.3

ISOMER

For a given halogen within a series (CH,),SbX, -, the lZ1sb isomer shift increases with the number n of methyl groups bound to the anti- mony ion. Thus, the s electron density at the Sb nucleus is higher in the (CH,),SbX compounds as compared to (CH,)SbX,. This trend, along with the fact that the Sb-CH, bond is more covalent than the Sb-X bond, reveals that the shielding effect of the p electrons on the s density at the nucleus represents the dominant parameter in these compounds.

The decrease in the isomer shift when replacing C1- by Br- or by I - for a constant value of

reveals an increase in the s electron density at the lZ1Sb nucleus in that order, possibly due to a decrease in the p shell population. Chlorine being more electronegative than iodine one expects a larger withdrawal of electrons from the Sb ion in the former halide. The experimen- taliy observed trend in the isomer shift would then indicate a larger s character of the Sb-X bonds so that the direct effect of the s shell depopulation would be predominant compared to the decreasing shielding

[I] SKINNER, H. A. and SWTTON, L. E., Trans. Faraday Soc. 40 (1943) 164.

[2] OGAWA, S., J. Phys. Soc. Japan 13 (1958) 613.

[3] MAIER, L.,

ROCHOW,

Nucl. Chem. 16 (1961) 213.

[4] SHENOY, G. K., FRIEDT, J. M., N d . Znstr. Meth. 116 (1974) 573.

[5] LUCKEN, E. A. C., Nuclear Quadrupole Coupling Constants (Academic Press London and New York) 1969 p. 219.

effect of the p electrons. This explanation would however contradict to one suggested earlier to account for the changes in the isomer shift when increasing the number of halide ions, i. e. when changing the number n in (CH,),SbX,-, for constant X. It furthermore appears that the dependences of the isomer shift on the ligand electronegativity are in reverse order in the present compounds as compared to the series SbX;

and s~,x;-

One may thus conclude that the diffe- rent trends are related to the difference in geometries of these compounds and that in (CH3),SbX3 -, the inter- molecular interactions are important. Due to the increased intermolecular bonding in the iodide as compared to the chloride, the p electron depopulation of the Sb ion might be increased in the former com- pound which would indeed result in a decreased isomer shift. One cannot however decide on the basis of the present results about the relative importance of the two mechanisms.

Appendix. - Using Townes and Dailey approxima- tion to interpret the quadrupole interaction parameters at the 1271 site as a function of the electron populations in the p orbitals and knowing the relation between the isomer shift and the number of electron holes in the s and p shells [8]

IS

h, - 0.5 h, + 0.16 one defines the ionicity of the iodine bond as

and the proportion of intermolecular interaction as

[6] VAGA, S., J. Chenz. Phys. 60 (1974) 3884 and references given there in.

[7] DONALDSON, J. D., TRICKER, M. J. and DALE, B. W., J.

Chem. Soc. (Dalton) 893 (1972).

DONALDSON, J. D., KJEKSHUS, A., NICHOLSON, D. G. and TRICKER, M. J., Acta Chem. Scand. 26 (1972) 3215.

[8] BUKSHPAN, S., GOLDSTEIN, C. and SONNINO, T., J. Chen7.

Phvs. 49 (1968) 5477.