Effects of Substitution of OH Group by F Atom for Conformational Preferences of Fluorine-Substituted Analogues of (R,R)-Tartaric Acid, Its Dimethyl Diester, Diamide, and N,N,N‘,N‘-Tetramethyl Diamide. Ab Initio Conformational Analysis

High-level ab initio methods up to MP2/6-311++G**//RHF/6-31G* have been used to characterize the conformations of isolated molecules of (2S,3S)-2,3-dideoxy-2,3-difluorotartaric acid (FTA) and its dimethyl diester (FME), diamide (FAM), and N,N,N‘,N‘-tetramethyldiamide (FTMA). A wide range of possible structures (84 for FTA and 63 for FME, FAM, FTMA) has been surveyed at the RHF/3-21G level. At the highest level of theory, 23 conformers were located for FTA, 15 for FME, 9 for FAM, and 11 for FTMA. Electronic correlation has been included with the relatively large basis set 6-311G, augmented with polarization and diffuse functions, to calculate MP2/6-311++G**//RHF/6-31G* single-point energies for all the conformers. Frequency analysis and thermochemical calculations have been performed at the RHF/6-31G* level and the results have been utilized to assess gas-phase populations of conformers at 298 K for the studied molecules. Moreover, SM5.4 solvation model was used to assess Gibbs free energies of conformers both in water and in chloroform. The obtained results are compared to those from previous studies of (R,R)-tartaric acid and its derivatives and analyzed in terms of effects of substitution of the hydroxyl group by the fluorine atom. It seems that substitution of the OH group by an F atom leads to greater conformational diversity of the molecules studied, mainly because the F atom cannot act as a hydrogen bond donor. From our results, it appears that if hydroxyl groups of (R,R)-tartaric acid are involved in intermolecular interactions, like in crystals or polar solvents, then the conformational preferences of these compounds are similar to the conformational preferences of isolated molecules of their dideoxydifluoro analogues.