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.
JOURNAL DE PHYSIQUE
Colloque C6, supplCment au no 12, Tome 35, DCcembre 1974, page C6-255
HYPERFINE INTERACTION PARAMETERS FOR l 2lSb AND lZ7IIN (CH3),SbX3-, (X = C1, Br, I) (n = 0,1,2,3) J. P. DEVORT, J. P. SANCHEZ, J. M. FRIEDT and G. 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-, (n = 0, 1, 2, 3 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 1271. 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(n = 0, 1, 2, 3 and X = C1, Br, I) are discussed In terms of the geometrical 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 parameters 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 resonance in lZ1Sb (spin states 512-712) in the set of compounds (CH3),SbX3-, where n = 0, 1, 2, 3 and X = C1, Br or I. In addition, we have measured the lZ71resonance 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 measurement 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 12'" Te sources, respectively, and the absorber thicknesses were about 8 mg Sb/cm2 and 8 mg 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 at 4.2 K of (a) : (CH3)2SbCIand (6) : 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 lZ1Sbin (CH3),SbX3-,. The isomer shift (IS) vaIues refer to Ca121mSn03at 4.2 K. The parameters remeasuredfor Sb(CH,), at 4.2 K are e2 q, Q = 15.25 1 0.20 mm/s and y = 0 IS = - 8.53 0.05 mm/s,
+
literature [3] 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 prepared. The experimental results of lZ1Sb and "'1 are summarized in Tables I and 11.
tion the principal component of the electric-fieldgradient (efg) tensor is proportional to [5] : ( - NO + NX)
3 cos a '
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 a 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. Parameters of lZ7Iat 4.2 K in SbI, and (CH,)SbI, Assuming that the angle a is approximately a constant within the series, the decrease of the absolute value of e2 q, Q on changing the CI- anion by Brand 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 3. Interpretation and discussion. - 3 . 1 QUADRU- its negative sign in all the compounds whereas its POLE INTERACTIONS IN SbX3 (X = C1, Br) AND absolute value decreases from the chloride to the iodide. Sb(CH,),. -The large value for the asymmetry The intermolecular interactions vanish in Sb(CH,), ; parameter (y) deduced from the lZ7IMossbauer spectra as a result, No is considerabiy increased and this in SbI, ~evealsthe existence of strong intermolecular accounts for the higher value of I e2 q, Q I interactions between the Sb ion and the I ion of a neigh- (= 15.25 mm/s) observed in this compound. bouring molecule. From the lZ7Ihyperfine interaction 3.2 QUADRUPOLE INTERACTIONS IN (CH,)SbX, AND parameters one deduces that the ionicity of the intramolecular Sb-I bond is 53 % whereas the inter- (CH,),SbX. - Strong intermolecular interactions also molecular character is 14 % (see appendix). Similar occur in these organo-halides of Sb(II1) as revealed by intermolecular interactions are known to occur in the the large value of q deduced from the spectra of lZ7Iin other two halides from previous NQR investiga- (CH3)Sb12 (Table 11). Interpreting the results of the tions [2]. They however cannot exist in Sb(CH,), as lZ7Ispectra, one deduces that the intramolecular Sb-I deduced from the bonding characters of the CH, bond ionicity is 52 %, whereas the intermolecular ligand. The analysis of the 12'Sb spectra in all the SbX, bonding character amounts to 16 % (see appendix). The 12'Sb data reveal that the sign of the quadrucompounds reveal a small y (Table I). One may hence assume an axial molecular symmetry with a C, axis pole interaction in the two types of organo-halides is along the hybrid orbital carrying an electron doublet opposite whereas the magnitudes are nearly equal. The (i. e. the z axis). Using Townes and Dailey approxima- 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 bonding 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 r, and the angle a between MA, MB directions and the xy plane. The values of eq, and y obtained by diagonalizing the efg tensor are represented in figure 3 as functions of the angle a 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 figure is given for MA,B and the lower one for MAB,. It appears that for certain values of a and of r,/r (for
)x
FIG.
2. - Geometrical parameters used for the point charge model in the compounds MA2B.
example for a = 12.50 and ro/r = 1) the sign of eq, changes for the MA,B and MAB, molecules, although the absolute value is approximately constant. The
FIG. 3. - Variation of the efg principal component (eq,) and of the asymmetry parameter (q) as functions of the angle a and for various values of rolr in MA2B. a and b are the charges on ligands A and B.
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 quadrupole interaction parameters and the changes in 0 observed in (CH,)SbX, and (CH,),SbX. A similar variation has been reported in some nitrogen compounds from 14N NQR experiments [5, 61. The knowledge of the detailed crystal structure would be required in order to perform more quantitative evaluation of the experimentally observed trends using either the precise hybrid orbitals or molecular orbital theory.
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;-[7]. One may thus conclude that the different trends are related to the difference in geometries of these compounds and that in (CH3),SbX3-,the intermolecular 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 compound 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.
3.3 ISOMER SHIFTS. - For a given halogen within a series (CH,),SbX, -,the lZ1sb isomer shift increases with the number n of methyl groups bound to the antimony 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 C1by Br- or by I- for a constant value of q 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 experimentaliy 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
Appendix. - Using Townes and Dailey approximation to interpret the quadrupole interaction parameters at the 1271site 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]
[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, E. G. and FERNELIUS, W. C., J. Znorg. 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.
[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.
IS = 3.07 h, - 0.5 h,
+ 0.16
one defines the ionicity of the iodine bond as
and the proportion of intermolecular interaction as :
