No abstract

The structural and spectroscopic effects of hydrogen bonding on isolated 2,2,2-trifluoroethanol (TFE) and its molecular complexes are theoretically investigated at the MP2/aug-cc-pVDZ level. As a result, previous interpretations of the relative stability of the trans and gauche conformers of the isolated molecule are revised. We show that the prevalence of the gauche form is due to a decrease of repulsion forces, rather than to the formation of an intramolecular hydrogen bond. We find that the instability of the trans geometry is caused by repulsion forces between the oxygen electronic pair and the fluorine atom clouds, which are significantly stronger in trans-TFE. Molecular agents capable of weakening the repulsion produce stabilization. These results lead to a reinterpretation of the stabilizing factors of halogenated compounds. To analyze complexation, two small molecules (water and ammonia) have been chosen. Water can form four different molecular aggregates with TFE. The most stable corresponds to a species where H 2 O acts as hydrogen donor and TFE presents the cis-gauche conformation, forming two intramolecular hydrogen bonds. For NH 3 , the cis-gauche conformation loses stability, because of steric hindrance. In this case, TFE varies the relative stability of its conformers, with trans-TFE becoming the preferred structure. Hydrogen bond formation between NH 3 and trans-TFE produces vibrational shifts of -354, -17, and +518 cm -1 for the OH stretching, the OH bending, and the OH torsion, in agreement with the experimental findings. We found complexation to produce an important variation of the position of the infrared bands corresponding to the hydroxyl group.

This work presents a theoretical study of acetohydroxamic acid and its protonation processes using ab initio methodology at the MP2(FC)/cc-pdVZ level. We find the amide form more stable than the imidic tautomer by less than 1.0 kcal mol(-)(1). For comparison with the experimental data, a three-dimensional conformational study is performed on the most stable tautomer (amide). From this study, the different barriers to rotation and inversion are determined and the intramolecular hydrogen bond between the OH group and the carbonyl oxygen is characterized. The electrostatic potential distribution shows three possible sites for electrophilic attack, but it is shown that only two of them, the carbonyl oxygen and the nitrogen atoms, are actual protonation sites. The protonation energy (proton affinity) is obtained from the results of the neutral and charged species. Proton affinities for the species charged on the carbonyl oxygen and the nitrogen atoms are estimated to be 203.4 and 194.5 kcal mol(-)(1), respectively. The development of a statistical model permits the quantification of DeltaG (gas-phase basicity) for the two protonation processes. In this way, the carbonyl oxygen protonated form is found to be more stable than that of the nitrogen atoms by 8.3 kcal mol(-)(1) at 1 atm and 298.15 K, due to the enthalpic contribution. As temperature increases, the proportion of the nitrogen protonated form increases slightly.

Theoretical (ab initio calculations) and experimental (NMR, spectrophotometric, and potentiometric measurements) investigations of the isomers of acetohydroxamic acid (AHA) and their deprotonation processes have been performed. Calculations with the Gaussian 98 package, refined at the MP2(FC)/AUG-cc-pVDZ level considering the molecule isolated, indicate that the Z(cis) amide is the most stable form of the neutral molecule. This species and the less stable (Z)-imide form undergo deprotonation, giving rise to two stable anions. Upon deprotonation, the E(trans) forms give three stable anions. The ab initio calculations were performed in solution as well, regarding water as a continuous dielectric; on the basis of the relative energies of the most stable anion and neutral forms, calculated with MP2/PCM/AUG-cc-pVDZ, N-deprotonation of the amide (Z or E) structure appeared to be the most likely process in solution. NMR measurements provided evidence for the existence of (Z)- and (E)-isomers of both the neutral and anion forms in solution. Comparisons of the dynamic NMR and NOESY (one-dimensional) results obtained for the neutral species and their anions were consistent with N-deprotonation, which occurred preferentially to O-deprotonation. The (microscopic) acid dissociation constants of the two isomers determined at 25 degrees C from the pH dependence of the relevant chemical shifts, pK(E) = 9.01 and pK(Z) = 9.35, were consistent with the spectrophotometric and potentiometric evaluations (pK(HA) = 9.31).