Discussion
The characteristic feature of an electron spin resonance (ESR) spectrum of frozen aqueous solution of CuSO4·5H2O at 77 K is
Answer: A
The characteristic feature is \(g_{\parallel} \gt g_{\perp}\).
In frozen solution, Cu(II) (d9) is surrounded by water molecules in a tetragonally elongated octahedron, the Jahn-Teller distortion. The single hole lies in the \(d_{x^{2}-y^{2}}\) orbital, and for that ground state spin-orbit coupling gives an axial spectrum with \(g_{\parallel} \gt g_{\perp} \gt 2.0023\) (typically about 2.4 and 2.1).
\(g_{\parallel} \lt g_{\perp}\) is the signature of a \(d_{z^{2}}\) ground state, as in a tetragonally compressed geometry. Equal g values would mean an isotropic, undistorted environment, which the Jahn-Teller effect rules out for d9. Three different g values describe a rhombic site, not the characteristic axial spectrum of hydrated copper(II).
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