Probing Axial Water Bound to Copper in Tutton Salt Using Single Crystal ¹⁷O-ESEEM Spectroscopy

Area of Science:

• Solid-state chemistry
• Magnetic resonance spectroscopy
• Quantum chemistry

Background:

• Electron spin-echo envelope modulation (ESEEM) is a technique used to study interactions between unpaired electrons and nearby nuclei.
• Cu(II)-doped Tutton salts are model systems for studying metal-ligand interactions in coordination compounds.
• Understanding the coordination environment of metal ions is crucial in various fields, including biochemistry and materials science.

Purpose of the Study:

• To detect and analyze electron spin-echo envelope modulation (ESEEM) signals from axial water bound to Cu²⁺.
• To experimentally determine the hyperfine and quadrupole coupling tensors of the ¹⁷O nucleus.
• To investigate the structural implications of Jahn-Teller distortion in Cu(II)-doped Tutton salts.

Main Methods:

• Electron spin-echo envelope modulation (ESEEM) spectroscopy was performed on Cu(II)-doped ¹⁷O-water-enriched potassium zinc sulfate hexahydrate (Tutton salt) crystals.
• Magnetic field orientation dependences of low-frequency modulations were measured.
• Quantum chemical calculations were employed for comparative analysis.

Main Results:

• ESEEM signals attributed to axial water interacting with Cu²⁺ were successfully detected and analyzed.
• The hyperfine tensor exhibited near-axial symmetry, with the largest value oriented perpendicular to the metal equatorial plane.
• Experimental determination of the ¹⁷O quadrupole coupling tensor and its principal axis frame was achieved, showing close correlation with water geometry.
• The results support the model of Jahn-Teller distortion in Cu²⁺-doped Tutton salts.

Conclusions:

• This study provides one of the few experimental confirmations of the principal axis frame of the ¹⁷O quadrupole tensor in a metal-ligand system.
• The findings highlight the weak interaction between axial water and the Cu²⁺ d_x2-y2 orbital.
• The results have implications for understanding water dynamics in the active sites of copper enzymes and proteins.