Fig. 2.

(a) UV-vis spectra of 1 (0.4 mM) (black line) and 2 (0.4 mM) (red line) in CH₃CN/CH₃OH (1:1) at 0 °C. (b) ESI-MS of 2-¹⁸O. Inset shows isotope distribution patterns for 2-¹⁸O (blue line) and 2-¹⁶O (red line). (c) Resonance Raman spectra of 2 (4 mM) obtained upon excitation at 407 nm in CH₃CN/CH₃OH (1:1) at −20 °C. Red/blue lines: samples prepared with H₂¹⁶O₂/H₂¹⁸O₂; green line: difference spectrum, (¹⁶O–¹⁸O). The peak marked with “s” is ascribed to the solvents. (d) X-band EPR spectrum of 2 recorded in CH₃CN/CH₃OH (1:1) at 4.3 K. Spectroscopic settings: frequency = 9.10 GHz, microwave power = 1 mW, modulation frequency = 100 kHz.