ESR spectra assigned to Ag3 molecules have been produced by codepositing atomic silver with excess nitrogen at temperatures close to 4.2 K. The spectra are characterized by an axially symmetric spin Hamiltonian having g∥ =1.9933(3) and g⊥ =1.9558(3) and, for 107Ag3, A∥(1) =310.8(6) G with A⊥(1) =310.1(6) G and A∥(2) =76.0(2) G with A⊥(2) =72.6(2) G for the apical(1) and basal(2) nuclei, respectively. The axial symmetry of the spectra is believed to imply that the trimer is rotating about one axis. There is no evidence for a pseudorotating trimer spectrum. The isotropic spin populations are ρ5s(1) =0.51 and ρ5s(2) =0.12 implying an acute angled geometry with ground state symmetry 2A1. This is in sharp contrast to the obtuse angled isomer (2B2 ground state) found for Ag3 in a C6D6 matrix. For Ag3(2A1) there is little p character on the apical atom but a 10%–15% p hybridization on each of the two basal nuclei.
REFERENCES
1.
2.
3.
4.
5.
The ESR spectra of in adamantane are characteristic of a pseudorotating isomer. The magnetic parameters are surprisingly similar to those of Ref. 4. See:
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7.
Both and have negative nuclear moments. For simplicity, however, we consistently denote hf constants and related parameters as being positive if these correspond to positive spin populations.
8.
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11.
12.
For a comprehensive review of ESR powder line shapes see
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This program was written by Dr. P. H. Kasai and modified to run on either an IBM CS9000 or a VAX 11/780 computer.
14.
15.
W. Weltner, Jr., Magnetic Atoms and Molecules (Van Nostrand, New York, 1983).
16.
All hf constants are expressed in Gauss using the conversion factor ergs ( Bohr magneton). Some authors use See Ref. 15.
17.
For the maximum inequivalency is estimated to be 3–4 G, a relatively small value compared to the ESR linewidth of 5–6 G. In addition, this effect is probably largest for those orientations where there is little ESR intensity. For a discussion of the conditions under which two nuclei are completely equivalent see Ref. 11.
18.
P. Kusch and V. W. Hughes, Handbuch der Physik, edited by S. Flugge (Springer, Berlin, 1959).
19.
Handbook of Physics and Chemistry, edited by R. C. Weast (Chemical Rubber, Cleveland, 1975).
20.
The parallel features are less well resolved and in many instances overlapped by the much stronger perpendicular transitions. However, the (six) observed parallel features do occur at their predicted field positions.
21.
H. Kopfermann, Nuclear Moments (Academic, New York, 1958).
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A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Oxford University, London, 1970).
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For the structural parameters of Ref. 37, the rotational constants for are and where A, B, and Ccorrespond (in Fig. 2) to axes y, z, and x, respectively. For with and (see Table D‐4 of Ref. 15) so that a rotation about the a axis seems most likely (Ref. 23). In a matrix, the axis of rotation need not correspond exactly to a principal axis of inertia. See:
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Although both and have experimental uncertainties comparable to their difference, the two errors are correlated. Measurements made on several independently calibrated spectra all gave
26.
One further contribution to the dipolar tensor is the interaction of an isotropic spin population on one nucleus with the magnetic moments of adjacent nuclei. For the maximum interaction is estimated to be 0.03 G.
27.
Spin populations derived in this manner do not take into account matrix interactions or the effects of orbital contraction or expansion on molecule formation. As shown by the atom data of Table HI, the former can contribute errors of several percent. For some radicals, orbital overlap effects introduce appreciable core orbital contributions to the hf constants. See L. B. Knight, Jr., A. Ligon, R. W. Woodward, D. Feller, and E. R. Davidson, J. Am. Chem. Soc. (in press).
28.
The anisotropic parameter is proportional to the mean inverse cube radius of a silver 5p orbital (Ref. 15). From the fine structure (fs) splitting of the first excited state of atomic silver one finds Alternative values may be obtained by an appropriate scaling of for ground state Ga. Using both fs and hf splittings, the values obtained for silver are and 3.30 a.u., respectively. Accordingly, the average of these values (one standard deviation in parentheses) was adopted: giving G. A similar analysis has been made for the other Group IB elements and will be discussed in a separate article. See: D. M. Lindsay and P. H. Kasai, J. Magn. Reson. (submitted).
29.
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B. M. Gimarc, Molecular Structure and Bonding (Academic, New York, 1979).
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34.
A deviation from axial symmetry comparable to the ESR linewidth (5–6 G) would be difficult to detect. This corresponds to an orthorhombic g tensor with two elements differing by no more than or about 7% of
35.
However is mainly s in character and the contribution to from the orbital of the p manifold may not be negligible. By contrast, both and belong to the p manifold and it is probably the former that determines
36.
37.
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W. Andreoni and J. L. Martins, Surf. Sci. (in press). The relative stabilities of the and states and the calculated spin populations were provided by Dr. J. L. Martins.
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For example the inversion splitting of the first excited vibrational state of is over an order of magnitude smaller in than in either a rare gas matrix or in the gas phase. See
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S. M. Mattar and G. A. Ozin, J. Am. Chem. Soc. (in press). The calculated spin populations were provided by Dr. S. M. Mattar.
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