ENDOR Spectroscopy.

Q-band CW EPR spectra were collected at 2 K on a spectrometer with a helium immersion Dewar as previously reported.³⁵ Q-band pulsed Davies, Mims,³⁶ and ReMims³⁷ ENDOR spectra were collected at 2 K on a spectrometer with a helium immersion Dewar as previously reported.³⁸ The intensity of the ENDOR response for the Mims sequence, [π/2-τ-π/2-T(rf)-π/2-τ-detect],³⁶ follows the response function, I(A) ~ 1 − cos(2πAτ),^36,27 and as a result, the signals at Aτ = n, n = 0, 1, …, are suppressed (“blind spots”).³⁶ The Doan/ReMims sequence, [π/2-τ₁-π/2-T(rf)-π/2-τ₂-π-(τ₁+τ₂)-detect], allows use of a short preparation interval τ₁ and study of a wider range of hyperfine values without “blind spot” distortions.³⁷ The simulations of 2D field-frequency plots of ENDOR spectra collected across the EPR envelopes of E₀ and hi-CO, presented below, were carried out with lab-written programs as described.^39–41 Details of the analysis required to derive ¹³C-carbide hyperfine tensors are presented in the Supporting Information (SI).