Table 2.

Signal‐to‐noise (SN_rel) ratios (relative to ¹H=100 %) and gain and loss factors for indirect (by CPMG‐HSQMBC) versus direct ⁷⁷Se NMR detection.

77Se detection method	77Se direct	1H[77Se] CPMG‐HSQMBC
signal halfwidth	3.0 Hz	1.5 Hz
SNrel=SN/√number‐of‐scans	0.077 %	1.763 %
f gain=SNrel(1H[77Se])/SNrel(77Se)[a]	–	23
f theor=(γ H/γ Se)⋅(γ H/γ Se)1.5[b]	–	62.5
f 100 %=100 %/7.63 %[c]	13.1	13.1
SN100 %=SNrel⋅f 100 % [d]	1.0 %	23.1 %
f INEPT=√SN100 %(1H[77Se])[e]	–	48.1 %
f coil,Se =SN100 %⋅f theor [f]	63.5 %	–

Spectra were recorded with the same FID resolution (0.5 Hz), receiver gain, long total interscan delay (6.3 s>3⋅T1), and were processed identically after exponential line broadening with the indicated natural signal halfwidth. For the CPMG‐HSQMBC experiment, the SN of H1 was multiplied by 2 to compensate a 50 % loss from gradient coherence selection. [a] f _gain is the experimentally achieved gain with the CPMG‐HSQMBC indirect versus direct ⁷⁷Se detection scheme. [b] f _theor is the theoretical gain for indirect ⁷⁷Se detection with an ¹H↔⁷⁷Se out‐and‐back transfer scheme, relative to direct ⁷⁷Se detection. [c] f _100 % is the gain from 100 % enrichment of ⁷⁷Se. [d] SN₁₀₀=SN_rel compensated for natural ⁷⁷Se isotope abundance. [e] f _INEPT is the transfer efficiency of each CPMG‐INEPT step in the CPMG‐HSQMBC experiment (with Δ _opt=70 ms, B_CPMG=1371 Hz, δ _CPMG=157.3 μs). [f] f _coil,Se is the outer‐coil ⁷⁷Se detection sensitivity relative to inner‐coil ¹H detection (BRUKER 600 MHz TBI probehead).