The proton NMR spectrum of nirmatrelvir (1) (obtained prior to treatment with MTBE) in dimethylsulfoxide solvent showed mainly one rotamer, with only ca. 5% of the minor rotamer present. DFT calculations on conformers of nirmatrelvir were carried out looking to support observations that nirmatrelvir and related compounds featuring the bicyclic proline residue in 1 exist as two rotamers about the tertiary amide bond that are unusually slow to equilibrate at room temperature as seen in the NMR spectrum (see SI-2). Calculations at the B3LYPD3BJ/6-31+G (d,p) level in the gas phase and in acetonitrile using the SMD solvation model showed a predominance of the rotamer with the tertiary amide carbonyl group participating in a cyclic, 7-membered ring-containing a hydrogen bond to the NH of the pyrrolidinone side-chain amide (Figure ). The structure of the minor rotamer (calculated in acetonitrile) is shown in Figure with two marginal longdistance, non-linear hydrogen bonds. The rotational free energy barrier for nirmatrelvir in acetonitrile was calculated to be 25.25 kcal mol -1 at 298 K, significantly higher than the experimental barrier of 17.77 kcal mol -1 for acetamide in acetonitrile solvent.