No abstract

Structural, energetic, and spectroscopic data derived in this work aim at the setup of an “experimentally validated” database for amino acids and polypeptides conformers. First, the “cheap” composite scheme (ChS, CCSD­(T)/(CBS+CV)MP2) is tested for evaluation of conformational energies of all eight stable conformers of glycine, by comparing to the more accurate CCSD­(T)/CBS+CV computations (Phys. Chem. Chem. Phys. 2013, 15, 10094–10111 and J Mol. Model. 2020, 26, 129). The recently proposed jun-ChS (J. Chem. Theory and Comput. 2020, 16, 988–1006), employing the jun-cc-pVnZ basis set family for CCSD­(T) computations and CBS extrapolation, yields conformational energies accurate to 0.2 kJ·mol–1, at reduced computational cost with respect to aug-ChS employing aug-cc-pVnZ basis sets. The jun-ChS composite scheme is further applied to derive conformational energies for three dipeptide analogues Ac-Gly-NH2, Ac-Ala-NH2, and Gly-Gly. Finally, dipeptide conformational energies and semiexperimental equilibrium rotational constants along with the CCSD­(T)/(CBS+CV)MP2 structural parameters (J. Phys. Chem. Lett. 2014, 5, 534–540) stand as the reference for benchmarking of selected density functional methodologies. The double-hybrid functionals B2-PLYP-D3­(BJ) and DSD-PBEP86, perform best for structural and energetic characterization of all dipeptide analogues. From hybrid functionals CAM-B3LYP-D3­(BJ) and ωB97X-D3­(BJ) represent promising methods applicable for larger peptide-based systems for which computations with double-hybrid functionals are not feasible.

The conformations and molecular structures of diisopropyl disulfide have been studied by high-resolution microwave spectroscopy and quantum chemical calculations. Three conformers, G′GG′, G′GT, and GGG′, have been observed in the jet expansion. The global minimum, G′GG′, adopts a configuration with the G′ orientation of H–C–S–S and S–S–C–H and the G orientation of C–S–S–C showing the C 2 symmetry. The rotational spectra of monosubstituted 13C and 34S isotopologues have also been recorded for G′GG′, leading to an accurate structural determination of this conformer. Two additional 34S isotopologues have also been measured for G′GT. The relative energies of three observed conformers calculated at the MP2/6-311++(d,p) level of theory are within 2 kJ mol–1, while the relative intensity measurements suggested their population ratio to be N G′GG′ /N G′GT /N GGG′ ≈ 5:3:2.