1554
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
P. H. Kasai and D. McLeod
Electron Spin Resonance Study of Intermetallic Molecules AgM (M = Group 2 Metal) Paul H. Kasai" and D. McLeod, Jr. Union Carbide Corporation, Tarrytown Technical Center, Tarrytown, New York 1059 I (Received February 9, 1978) Publication costs assisted by Union Carbide Corporation
A series of intermetallic diatomic molecules AgM (M = Mg, Ca, Sr, Ba, Zn, Cd, and Hg) were generated in argon matrices and were examined by ESR spectroscopy. For all the AgM examined the hyperfine coupling tensors to the Ag nucleus were found to be isotropic. It is shown that the unpaired electron resides in an orbital given essentially by an antibonding combination of the valence s orbitals of the Ag and M atoms. The coupling tensors determined for the magnetic Cd and Hg nuclei are slightly anisotropic,indicating small admixture (-10%) of the valence pr orbital of the atom M. The analysis of the g tensors also indicated a similar amount of admixture of the pt orbital in each AgM examined.
Introduction The nature of metal-metal bonds found between metal atoms in complex coordination compounds1 as well as those existing between ligand free metal has been the subject of many recent investigations. The homonuclear diatomic molecules of various metals have been examined by mass spectroscopy of the vapor phase3 and by optical spectroscopy using the matrix isolation t e ~ h n i q u e . ~ Electron spin resonance (ESR) spectra of heteronuclear, intermetallic diatomic molecules, if attainable, would be particularly elucidative of the interaction that could occur between a pair of metal atoms. When metal atoms, M, are condensed in inert gas matrices a t liquid helium temperature, the diatomic species, M2, are found in a quantity much larger than that expected from the vapor phase compo~ition.~ The diffusion of metal atoms within the quasi-liquid surface layer existing during matrix deposition is thought to be responsible for the extra dimerization process. It follows then that a sufficient amount of heteronuclear diatomic molecules may be generated and stabilized in rare gas matrices by co-condensation of two different metal atoms. The present paper describes the successful generation by this technique and observation by ESR of a series of intermetallic diatomic molecules, Ag-M, formed between the Ag atoms and the group 2A (Mg, Ca, Sr, and Ba) and 2B (Zn, Cd, and Hg) atomsa5 The electronic configuration of Ag is 4d1° 5s1, and those of group 2A and 2B are ns2and ( n- l)d1° ns2,respectively. The configuration of the heteronuclear diatomic molecule Ag-M, if formed, is thus expected to be:u uS*lwhere u8 and us* are the bonding and antibonding u orbitals arising essentially from the valence s orbitals of Ag and the group 2 atom, M. The observed ESR spectra were found to be in full accord with this prediction. Experimental Section The cryostat-spectrometer assembly that would permit trapping of high temperature vapor phase species in a rare-gas matrix at -4 K and observation of the resulting matrix by ESR has been described previously.6 In the present series of experiments the Ag atoms were vaporized from a resistively heated tantalum cell and were trapped in argon matrices together with the atoms of other metal independently vaporized from the second tantalum cell. The tantalum cells contained the metals in granular form and were heated to a temperature at which the vapor pressure of the respective metal would be 0.1-0.5 Torr. In the cases of AgSr, AgBa, and AgHg, a sufficient amount of the diatomic species was produced only when 0022-3654f 7812082-1554$01.0010
the matrix was irradiated simultaneously with a highpressure xenon lamp (Oriel, 1kW)during deposition. The simultaneous irradiation must facilitate the diffusion of heavier and/or more polarizable atoms through the surface region. Only a small increase (-10%) in the ESR signal of AgM was noted when the matrix was irradiated with the same light after the deposition. The frequency of the ESR spectrometer locked to the sample cavity was 9.410 GHz and all the spectra were obtained while the matrices were maintained at -4 K. Results The molecular symmetry of AgM dictates that their ESR spectra be compatible with an axially symmetric spin Hamiltonian of the form: = gilPHzS2 + g l P ( H x S x + H Y S Y ) + AIII,SZ + A i ( I x S x + I y s y ) + B i l L ' &
%pin
WL'SX +
W
Y
1
+ (1)
where Ail and A, represent the hyperfine coupling tensor to the Ag nucleus, and the last two terms involving BIIand B , are to be added when the atom M possesses a magnetic nucleus. The ESR spectrum of Ag atoms (4d1° 599 isolated in an argon matrix had been studied earlier.7 It consists of two sets of sharp, isotropic doublets with the respective spacings of 650 and 750 G attributed to the couplings to lo7Ag(natural abundance = 51%, I = 1/2, p = -0.1130/3~) and lo9Ag (natural abundance = 4970, I = 1/2, p = -0.1299PN). AgM (A4= Mg, Ca, Sr, and Ba). Figures 1-4 show the ESR spectra observed from argon matrices containing the Ag atoms and the atoms of group 2A elements, Mg, Ca, Sr, and Ba, respectively. In each case p e notes, in addition to the pair of doublets due to the Ag atoms described above, two additional sets of doublets with considerably smaller separations. The latter signals are attributed to the heteronuclear diatomic molecules, AgM (M = Mg, Ca, Sr, and Ba). The intensities of the AgM doublets relative to those of the Ag atoms were in the range of 2-5 % . The spectrum of AgMg (Figure 1) appears essentially isotropic, while those of AgSr and AgBa (Figures 3 and 4) show the anisotropy expected from an ensemble of randomly oriented molecules having an axially symmetric g tensor. The sharper peaks on the negative side of the latter spectra are thus assigned as the perpendicular components. The anisotropy of the g tensors of these molecules, however, is not enough to separate the parallel components 0 1978 American Chemical Society
ESR
Study of Intermetallic Molecules
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
1555
TABLE I: Spin Hamiltonian Parameters of AgM M Mg Ca
Sr Ba
Zn
Cd
Hg
IP of M, eV 7.64 6.11 5.69 5.21 9.39 8.99 10.43
&la
gla
Ail(= A l h b GHz
1.9823 1.9538 1.9905 1.9711
0.837 0.435 0.372 0.218 1.324 1.327
2.0001 1.9982 2.0025 2.0014 1.9958
1.9136
1.562
BLC
Bll
GHz
GHz
2.18 (50.20) 2.84 (10.20)
1.99
( f 0.01) 2.53 (f 0.01)
Ag atom 7.57 1.9998 1.809 Accuracy: 10.0002. For M = Mg and Ca the observed spectra were analyzed as being isotropic. For M = Sr and Ba, the parallel components were not resolved. Coupling to lo7Ag. Accuracy: 10.003 GHz. Coupling to '"Cd or 199Hg. a
Figure 1. ESR spectrum of AgMg generated in an argon matrix. The sharp, off-scale doublets are due to the Ag atoms.
Figure 3. ESR
spectrum of AgSr generated in an argon matrix.
AgCa
AgBa
1
Figure 4. ESR Figure 2. ESR
spectrum of AgCa generated in an argon matrix.
from the dominant perpendicular signals. Figures 1-4 also revealed a gradual decrease of the doublet separations and a high-field shift of the resonance positions with the increasing atomic number of M in AgM. The g values (g,) and the hyperfine coupling constants ( A , ) to lo7Agdetermined from the observed spectra are given in Table I. AgZn. Figure 5a shows the ESR spectrum of an argon matrix containing the Ag and Zn atoms. In addition to the Ag atom doublets, one notes the presence of two additional sets of doublets attributable to the heteronu-
1
I
spectrum of AgBa generated in an argon matrix,
clear diatomic species AgZn. As indicated in Figure 5b, the spectral features expected from an axially symmetric spin Hamiltonian are completely resolved. The spacings of the AgZn doublets are only slightly less than those of the Ag atoms. A large hyperfine interaction, such as that encountered here, prevents an accurate evaluation of the spin Hamiltonian parameters from the usual second-order solutions of the matrix. When the magnetic field is parallel to the symmetry axis, the secular determinent derived from Hamiltonian (1)is tridiagonal and, thus, can be expanded exactly using a continued fraction technique.* From the eigenvalues expressed in the continued fraction form, the following "continued
1556
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
P. H. Kasai and D. McLeod
Figure 6. ESR spectrum of AgCd generated in an argon matrix.
Y
L1
-I8
'
u I p l n
**
8
( ' O S b )
-----+
,
';b
u A+"
%ET Figure 5. ESR spectra of AgZn generated in an argon matrix: (a) shown in scale to indicate the relative intensities of the AgZn and Ag atom signals; (b) observed with a higher gain in order to indicate the assignment.
expressions" were obtained for the parallel components of AgM with a nonmagnetic M nucleus. Figure 7. ESR spectrum of AgCd observed with a higher gain and a wider scan range. The brackets indicate the satellites due to "'Cd and Ii3Cd nuclei. Vl12
where Hli O = h v / g I i P 1711 = A l / 2 g i i P
If lAll- All
