Ab initio study of formohydroxamic acid isomers, their anions and protonated forms

Conformational and structural studies of N-methylacetohydroxamic acid and of its mono- and bis-chelated uranium(VI) complexes

The thermodynamics and kinetics of the cis/trans isomerism of N-methylacetohydroxamic acid (NMAH) and its conjugated base (NMA(-)) have been reinvestigated in aqueous media by (1)H NMR spectroscopy. Hindered rotation around the central C-N bond due to electronic delocalization becomes slow enough on the NMR time scale to observe both rotamers in equilibrium in D2O at room temperature. By properly assigning the methyl group resonances, evidence for the prevalence of the E over the Z form is unambiguously provided [K300=[E]/[Z]=2.86(2) and 9.63(5) for NMAH and NMA(-), respectively], closing thereby a long-lasting dispute about the most stable conformer. To that end, calculations of the chemical shifts by density functional theory (DFT), which accurately reproduced the experimental data, turned out to be a much more reliable method than the direct computation of the relative energy for each conformer. The Z ⇌ E interconversion dynamics was probed at 300 K in D2O by 2D exchange-correlated spectroscopy (EXSY), affording the associated rate constants [kZE=9.0(2) s(-1) and kEZ=3.14(5) s(-1) for NMAH, kZE=0.96(3) s(-1) and kEZ=0.10(2) s(-1) for NMA(-)] and activation barriers at 300 K [ΔG(≠)ZE=68.0 kJ mol(-1) and ΔG(≠)EZ=70.6 kJ mol(-1) for NMAH, ΔG(≠)ZE=73.6 kJ mol(-1) and ΔG(≠)EZ=79.2 kJ mol(-1) for NMA(-)]. For the first time, mono- and bis-chelated uranium(VI) complexes of NMA(-) have been isolated. Crystals of [UO2(NMA)(NO3)(H2O)2] and [UO2(NMA)2(H2O)] have been characterized by X-ray diffractometry, infrared and Raman spectroscopies.

15N-1H and 15N-13C couplings in 15N-enriched dihydroxamic acids

(15)N-enriched dihydroxamic acids (HONHCO(CH(2))(n)CONHOH, n = 0, 1, and 2) were prepared and their spectra NMR ((1)H, (13)C, (15)N) recorded in dimethyl sulfoxide (DMSO) solutions with the aim of determining (15)N coupling constants ((15)N-(1)H and (15)N-(13)C). The results supplement chemical shifts published earlier and yield additional support to the structural conclusions derived from other NMR parameters. Notably, no trace of hydroximic structures could be found in the (15)N NMR spectra of these acids. The values of (15)N-(13)C coupling constants backed by theoretical calculations support the assignments made earlier for all of the major conformers and for the minor conformer of succinohydroxamic acid. The enrichment revealed that the minor component of malonodihydroxamic acid solution previously considered to be the ZE conformer is in fact the monohydroxamic acid (HOOC-CH(2)-CO-NH-OH).

Deprotonation sites of acetohydroxamic acid isomers. A theoretical and experimental study

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).

Site selectivity in the synthesis of O-methylated hydroxamic acids with diazomethane

In this paper we report the results obtained by treating some selected hydroxamic acids with diazomethane in ethereal media. The multitask reagent diazomethane was used either as a base to induce deprotonation of the chosen hydroxamic acids or as conjugated acid which undergoes one-pot methylation processes of the generated anions. Product distributions clearly showed that a high site selectivity is expressed by the different deprotonated species in the alkylation processes. Under the adopted conditions, the prevalent site of methylation is in all the cases the oxygen of the hydroxamic acid. While in aliphatic hydroxamic acids only O-alkylation is observed, in the aromatic substrates, the NH group competes with the OH function as the nucleophilic site, although the OH reactivity still dominates.

Modeling of protonation processes in acetohydroxamic acid

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.