RESEARCH
Mossbauer Spectra Give New Structural Data Linear correlation between isomer shift and the sum of ligand electronegativities aids symmetry studies of organotins Gamma-ray resonance spectroscopy (Mossbauer effect) is giving chemists new insights into the structure, symmetry, and bonding character of organometallic compounds, Dr. R. H. Herber told the Summer Symposium on Analytical Chemistry held at Cornell University, Ithaca, N.Y. The American Chemical Society's Division of Analytical Chemistry and Analytical Chemistry sponsored the 17th annual symposium. Recent studies by Dr. V. I. Goldanskii and his co-workers at the Physical Chemistry Institute of the U.S.S.R. Academy of Sciences, Moscow, have shown that there is a linear relationship between the isomer shifts observed in the Mossbauer spectra of tin tetrahalides and the sum of the electronegativities of the ligands attached to the tin atom. The more extensive studies of Dr. Herber and H. A. Stockier of Rutgers University (New Brunswick, N.J.) show that this correlation is valid for compounds of the type SnR 4 and SnAr4 as well as for tin compounds with mixed ligands. A further result of this work is the
finding that organotin compounds in which the metal-atom bonding is other than tetrahedral (sp[i hybridization) show an anomalously large ratio of quadrupole splitting to isomer shift. This result provides additional analytical evidence for pentacoordination in compounds such as ( C H 3 ) 3 S n F and (CH 3 ) 3 SnC10 4 , Dr. Herber points out. Two Parameters. The two parameters obtainable from a Mossbauer spectrum which are of most interest to chemists are the isomer shift and the quadrupole splitting. The isomer shift arises from the fact that the nucleus has a finite radius which is different in the ground state from that in the excited state. The nucleus interacts with the electron wave functions of the environment (specifically the s electrons). The extent of the interaction depends on the s electron density at the nucleus and is observed as a shift from zero velocity of the center of the resonance spectrum. It isn't possible to measure this interaction directly. However, the difference in interaction of the two en-
ergy states between the source atom and the absorber can be compared. These two atoms are involved in the recoil-free transitions. The isomer shift, which is an energy difference, can be expressed in terms of equivalent Doppler velocity. A number of correlations exist between isomer shifts and electron density at the nucleus on the basis of Mossbauer experiments with iron-57, tin-119, and gold-197. For instance, in the case of inorganic tin compounds, the range of isomer shifts is about 5 mm. per second. This corresponds to an energy of about 4.0 X 10 - 7 e.v. Since chemical bond energies usually lie between 0.1 and 5 e.v., Dr. Herber points out that the observed isomer shifts represent a very small change in the bonding environment between the source and the absorber atom. This means that the Mossbauer effect can be expected to elucidate subtle changes in bonding. Absolute values of the isomer shift are not significant; comparison of isomer shifts must be made with respect to a reference lattice.
Mossbauer Spectroscopy Uses Recoil-Free Gamma-Ray Emission Recoil-free emission and absorption of gamma radiation take place when a nuclear transition occurs without the loss of recoil energy to the environment. Since the recoil energy is proportional to the square of the transition energy and inversely proportional to the mass of the recoiling nucleus, recoil energy losses will be minimal for low energy transitions among heavy nuclei. In 1957, Dr. R. Mossbauer (Technische Hochschule, Munich) was able to demonstrate that if the excited state nucleus is bound tightly enough into a crystalline solid (so that the recoiling mass is that of the whole lattice), emission without recoil can occur. Under these conditions, the gamma ray carries off the full transition energy. If this gamma ray is absorbed by a second identical nucleus in the ground state, the transition to the excited state can be effected, provided the absorber atom again suffers no recoil energy loss. The process of recoil-free emission and absorption is a resonance phenomenon and can be detected
66
C&EN
JULY
13,
1964
experimentally by a transmission experiment. The intensity of the transmitted radiation through the absorber and onto a detector will be reduced if the radiation is resonantly absorbed by the absorber and then isotropically scattered. In a Mossbauer experiment, the transmitted radiation intensity is measured as a function of the relative source-absorber velocity (which is provided by moving one with respect to the other by a mechanical or electromagnetic drive), since the energy of the emitted gamma ray can be altered very slightly by contributing a Doppler component to the gamma ray energy. The two Mossbauer nuclides that have been most widely used in chemical studies are iron-57 and tin119. For both of these, the chemical information is contained in a spectrum which covers a range of ± 2 to 3 mm. per second. A Doppler velocity of 1 mm. per second corresponds to an energy of 8 x 1 0 s e.v. Therefore, Mossbauer spectra give information about subtle differences in the structures of molecules.
For Tetrahedrahedrally Hybridized Tin, S-Electron Density at the Tin Atom's Nucleus is a Linear Function of Ligand Electronegativity #SnF«
,* 3 SnCI, (p-CI*)3SnCI (neo)3SnCI (neo)3SnNO, (p-F*)3Snl,
(neo)4Sn T. = 298° K.
T.
78° K.
(neo) = [*-C(CH3)2-CH2-|
+0.6
+1.0 +1.4 Isomer shift (relative to Sn02) in mm./sec. - j
The isomer shift values (relative to S n 0 2 ) for organotin compounds range from about 1.0 mm. per sec. for (CH 3 ) 3 SnOH to 1.5 mm. per sec. for (CH 3 ) 2 SnCl 2 . From the relationship between electron configuration and isomer shift, the Rutgers scientists conclude that the percentage ionic character in the tin-ligand bonds doesn't change by more than 5% in this series. Moreover, the substitution of strongly electronegative groups, such as fluorine or CF 3 , either in the meta or para positions in aryltin compounds, has little or no effect on the s electron density in the tin-carbon bond. Quadrupole Splitting. The quadrupole splitting arises from the fact that a nuclear energy level of odd half-integral spin can be split into (J + 1 / 2 ) distinct levels if the electric field gradient tensor (the second derivative of the potential with respect to coordinate) is nonzero at the Mossbauer atom lattice point. In the case of both iron-57 and tin-119, the Mossbauer transition involves a (J = s/ 2 ) excited state and a (J = V 2 ) ground state. If the electric field gradient is nonvanishing, there will be a finite quadrupole splitting of the upper energy state. This interaction is observed experimentally as a splitting of the Mossbauer resonance line into a two-line pattern separated by an energy equal to the quadrupole interaction. However, if the Mossbauer atom is located in a site of cubic symmetry (tetrahedral or octahedral), the elec-
+1.8
+2.2
tric field gradient tensor vanishes. Under these conditions, only a single resonance line is observed. Organotin. The Mossbauer spectra of organotins indicate that there are no measurable quadrupole splittings in tetrahedral tin compounds such as tetraphenyltin, (neo) 4 tin (where neo refers to the neophyl group, 2-phenyl2-methylpropyl-l radical), and (C G H 1 1 ) 4 Sn. An interesting result of the work is that binuclear compounds, such as hexaphenylditin and hexa(pfluorophenyl) ditin, show no significant quadrupole splitting (less than 0.05 mm. per second). Quadrupole splitting is also absent in tris(p-fluorophenyl)tin hydride. The absence of quadrupole splitting in the binuclear tin compounds agrees with the tetrahedral configuration of the bonding orbitals of tin. But the large splittings (of the order of 2.45 mm. per second) that are observed in triaryl- and trialkyltin halides are due to the large electric field gradients at the tin atom lattice point, Dr. Herber says. A significant result of the work by Dr. Herber and Mr. Stockier is demonstrated by the ratio (identified by the symbol p) of quadrupole splitting to isomer shift. For tetrahedrally symmetrical molecules such as tetraphenyltin and stannic chloride, p values are zero because of the absence of quadrouple splitting. For compounds in which the tin atom is sp 3 -hybridized and the electric field gradient is nonvanishing, p lies between 1.0 and 1.8. But for compounds such as
(CH 3 ) 3 SnF, (CH 3 ) 3 SnOH, and n-trimethylstannylimidazole, p lies in the range 2.1 to 2.9. On this basis, the Mossbauer spectra of a number of other organotin compounds suggest nontetrahedral symmetry about the metal atom; such compounds should prove interesting for further structure determinations, Dr. Herber says. Experimental. The gamma-ray resonance spectra of the organotins are studied at liquid nitrogen temperatures since the Debye temperatures of these compounds are usually well below 300° K. This means that at room temperature the probability of observing recoil-free absorption is extremely remote; therefore, cooling of the absorbers is needed. A ceramic matrix of tin-119m dioxide, which is kept at room temperature, is used as the emitter. The equipment consists of a parabolicmotion drive coupled to a multichannel analyzer that is operated in the multiscaler mode. In this setup, all source velocities are populated simultaneously and the whole spectrum is accumulated at the same time. Solid samples are spread out in thin (20 to 50 mils) layers on an aluminum foil. The foil is thinly coated with silicone stopcock grease so that the sample can be held in a vertical position. The sample is covered with another layer of foil and the foil-sample-foil sandwich is clamped into a copper sample holder. This in turn is mechanically and thermally connected to the copper block of a standard nitrogen Dewar. Liquid samples, such as stannic chloride and trimethylethyltin, are contained in a Teflon cell which is held by screw compression in a copper sample mount. The total liquid sample thickness in this cell is about 15 mils. The sample is cooled with liquid nitrogen before the Dewar is evacuated. The relative source-absorber velocity scale is calibrated from the hyperfine spectrum of a 0.5-mil metallic iron absorber. The source for this calibration is made by diffusing cobalt57 into 2-mil palladium for one hour at 750° to 800° C. in a vacuum. The calibration constant can then be calculated from the positions of the four lines of this spectrum and the known ground-state splitting in metallic iron. The zero of motion is found from a tin119m dioxide-tin-119 dioxide Mossbauer experiment. Both components are kept at room temperature. JULY
13, 1 9 6 4
C&EN
67
