Experimental and theoretical studies of the standard oxidation potentials of triethylamine and triethanolamine

Tertiary aliphatic amines epitomized by triethylamine and triethanolamine (TEOA) are sacrificial donors used in photocatalytic CO 2 reductions in acetonitrile. Knowing their standard oxidation potential ( E 0 ox ) in acetonitrile helps design efficient catalytic systems. However, reported values either do not exist or do not agree with each other. We determined the values to be 1.05 and 1.21 V (vs. aqueous NHE) by the inflection-point method (Vullev et al. J. Electrochem. Soc., 2019, H3175), which agrees very well with the calculated results (1.09 and 1.20 V, respectively) using the M06SMD-s method, we presented in this work. To calculate E 0 ox values, we proposed three methods at M06-2X/6-31G * level without using thermodynamic cycles. They differ in the way of calculating free energies in solvent ( G s ). All of them are accurate; however, the M06SMD-s method is the most efficient. It obtains the related G s based on single-point calculations of the gas-phase geometries using the SMD solvent model. The calculated E 0 ox values for seven benzene derivatives and five N-containing compounds are very accurate with an average absolute error of 30 mV. In addition, our calculations reveal that TEOA molecules can associate with each other through hydrogen bonds. The standard equilibrium constant ( K 0 ) for forming the dimer reaches 1.63 × 10 6 . Furthermore, we proved that the E 0 (Fc + /Fc) (Fc = ferrocene) to be 0.629 V versus NHE, which can be obtained accurately using a wide range of conditions. The Ag + /Ag (Ag|0.010 M AgNO 3 , 0.090 M [NBu 4 ]PF 6 ) reference electrode potential was determined to be 0.544 V (NHE).