Formohydroxamic Acid Research Articles

The interaction of formohydroxamic acid with carbon monoxide: FTIR matrix isolation and quantum chemistry studies

Abstract Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) and carbon monoxide have been recorded. The experimental results indicate formation of three isomeric complexes. In two complexes the NH or OH groups of formohydroxamic acid are attached to the carbon atom of the CO molecule and in the third complex the NH group interacts with the oxygen atom of CO. The formohydroxamic acid complex with two CO molecules is also trapped in the matrix. One of the two CO molecules interacts with the NH group and the second one with the OH group of HCONHOH, in both cases the site of interaction is the carbon atom of CO. Theoretical studies of the structure and spectral characteristics of the complexes were carried out on the DFT(B3LYP)/6-311++G(2d,2p) level. The calculated vibrational frequencies for the complexes present in the matrices are in good agreement with the experimental data. The calculations show also an additional potential energy minimum corresponding to the complex in which the OH group of formohydroxamic acid is attached to the oxygen atom of carbon monoxide.

Cis–trans isomerism of the keto tautomer of formohydroxamic acid

The trans (1E) isomer of the formohydroxamic acid is identified for the first time and the population ratio of the trans (1E) and cis (1Z) isomers is estimated to be 0.035±0.009 in the gas phase at room temperature on the basis of the matrix infrared isolation spectroscopy.

Matrix Infrared Spectra and ab Initio Calculations of the Formohydroxamic Acid Complexes with HF and HCl

Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) or its isotopic analogues (HCONDOD, HCO15NHOH) and hydrogen halides (HF, HCl) have been recorded. The experimental results indicate formation of a very strong binary complex in which the hydrogen halide molecule acts as a proton donor toward the carbonyl group of formohydroxamic acid. The H−X stretches and several perturbed HCONHOH, HCONDOD, and HCO15NHOH vibrations were identified for the two HX complexes; for the HF complex two librational HF modes were also observed. Theoretical studies of the structure and spectral characteristics of the HCONHOH···HF and HCONHOH···HCl complexes were carried out on the electron correlation level with the 6-311++G(2d,2p) basis set. The calculated vibrational frequencies for the complexes present in matrixes are in good agreement with the experimental data. The calculations demonstrate also the stability of the other three binary complexes. In the cyclic structure the HX molecule acts as a proton donor for the CO group and a proton acceptor for the NOH group, and in the two less stable structures the HX is attached to the oxygen or nitrogen atom of the NOH group.

Photodecomposition of formohydroxamic acid. Matrix isolation FTIR and DFT studies

Primary dissociation pathways of formohydroxamic acid have been investigated in argon and xenon matrices. The irradiation of the HCONHOH/Ar(Xe) matrices with full output of an Xe arc lamp leads to the formation of hydrogen bonded HNCO–H2O and NH2OH–CO complexes. Two structures were identified for the isocyanic acid–water complex, in the first one HNCO is hydrogen bonded to an oxygen atom of the water molecule and in the second, water serves as a proton donor toward the nitrogen atom of HNCO. For the hydroxylamine–carbon monoxide complex the spectra prove the existence of the structure with the OH group attached to the carbon atom and also suggest the existence of the structure with the NH2 group interacting with the carbon atom. The identification of the products is confirmed by 15N isotopic substitution and by DFT calculations (at the B3LYP level of the theory with 6-311++G(2d,2p) basis set) of the structure and vibrational frequencies of the identified complexes.

The Reduction of Pu(IV) by Formohydroxamic Acid in Nitric Acid

Hydroxamic acids can act as both complexing agents and reducing agents for actinide ions. In HNO3, Pu(IV) ions form strong dark red complexes with formo- and aceto-hydroxamic acids, which on standing convert to blue Pu(III) nitrate ions. In acid solution, these hydroxamic acids also hydrolyse to the parent carboxylic acid and hydroxylamine. The rate of hydrolysis varies depending on the substituents within the hydroxamate and whether the hydroxamate is free or bound to a metal ion centre. We have qualitatively shown that the observed rate of reduction of the Pu(IV)-FHA complex is most probably controlled by the hydrolysis reactions of the hydroxamic acid.

Estudo semi-empírico de ácidos hidroxâmicos: ácido formoidroxâmico e derivados do aleloquímico dimboa

Open chain hydroxamic acid (Hx) can exist as Z and E diastereomers of two tautomers, hydroxamic acid and hydroximic acid. The conformational stability of the formohydroxamic acid isomers evaluated by PM3 compared better to ab initio results from the literature than AM1 results. Structural data of the cyclic Hx 2,4-dihydroxy-7-metoxy-2H-1,4-benzoxazin-3(4 H)-one (DIMBOA) obtained by both semiempirical methods compared well to ab initio results. pKa data from the literature for derivatives of the aldolic isomer of DIMBOA were compared to the stability of the anions resulting from the loss of protons of their phenol and hydroxamic acid groups, determined as the difference in heat of formation between anionic and neutral forms, calculated by AM1 and PM3 methods. Good correlations between theoretical and experimental data were obtained for both semiempirical methods.

Hydroxamic chelates of boric acid, a density functional study

A density functional study of the neutral and ionic chelates of boric acid with formohydroxamic acid is performed. Optimum structures were determined for the chelates, and the IR spectrum for the bis(hydroxamate) boron chelate was compared to the experimental spectrum obtaining an excellent agreement. Two bands that were unassigned experimentally were identified with the aid of the theoretical calculations. The enthalpies of formation of formohydroxamic acid (−50.2±2 kcal/mol) as well as those of the bis(hydroxamate), hydroxamate–hydroximate and bis(hydroximate) boron chelates (3.8±2, −145±2 and −155±2 kcal/mol, respectively) were determined.

Ab Initio Study of Proton Transfer between Protonated Formohydroxamic Acid and Water Molecules

Proton transfer between protonated formohydroxamic acid (FAH)+ and water molecules (H2O−FAH−H2O)+ is studied theoretically. In a proton-relay mechanism, the carbonyl oxygen in formohydroxamic acid (HCONHOH, FA) accepts a proton from the hydronium ion (H3O+) and releases a proton either from the amine group (N−H) or the N-hydroxyl (N−OH) of formohydroxamic acid to another H2O molecule. The movement of the two protons is not simultaneous but through a stepwise process. The catalytic effect of the H2O molecule in reducing the proton-transfer barrier compared to the catalytic effect of pure formohydroxamic acid through intramolecular proton transfer is substantial. The transfer barriers of the two protons in a relay process are asymmetric. The proton on either the amine group (N−H) or the N-hydroxyl (N−OH) of FAH+ will be transferred to the second H2O molecule, and the proton transfer is determined by the lengths RN···O and RO···O, which are related to the distance between FAH+ and the second H2O molecule. At the same RN···O and RO···O, the proton on amino group (N−H) has a transfer energy barrier lower than that of the proton on the N-hydroxyl site by about 4.1 kcal/mol. If we elongate the distance between the two water molecules and let protonated FA shuttle freely as a proton carrier between the two water molecules, then the energy barrier for the movement of protonated FA increases slowly with the increase of the distance between two H2O's separated by no more than 10 Å. However, the proton-transfer barrier between protonated FA and water is independent of the separated distance of the two water molecules.

Structure, reactivity and vibrational spectra of formohydroxamic and silaformohydroxamic acids: a comparative ab initio study

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the structure, gas-phase acidities and vibrational spectra of formohydroxamic acid HC(O)NHOH, and its sila derivative HSi(O)NHOH. The geometries of various tautomers and rotamers of formohydroxamic and silaformohydroxamic acids, their anions and protonated forms were optimized at the MP2(FC)/6-31G(†) level of theory. The calculations showed that the molecules of neutral acids should exist in several forms very close in energy. The molecules of neutral formohydroxamic and silaformohydroxamic acids exist in more stable keto form. The formohydroxamic tautomer is by about 20kJmol−1 more stable than the hydroximic structure. The result of the effect of the carbon-by-silicon substitution is considerable destabilization of the hydroximic tautomer. In the structure of the neutral form and anion of the parent hydroxamic acid the intramolecular hydrogen bond stabilizes the structure and makes the most stable conformations more planar. In both acids the HON− anion is more stable than the O-anion, hence formohydroxamic and silaformohydroxamic acids in the gas phase are N-acids. Formohydroxamic acid is a weak acid with calculated acidity of about 1427kJmol−1. The acidity increases in the order: HSiONHOH<HCONHOH. The proton affinity of formohydroxamic acid was computed to be −821.0kJmol−1. Silaformohydroxamic acid is a stronger base with computed proton affinity ΔH298=−883.0kJmol−1. The fundamental vibrational frequencies have been computed using the MP2(Full)/6-31G(d) method. Sila substitution produces an appreciable shift of the corresponding harmonic vibrational frequencies of formohydroxamic acid.

A preliminary study of the reduction of Np(VI) by formohydroxamic acid using stopped-flow near-infrared spectrophotometry

The kinetics of the reduction of NpO2 2+ to NpO2 + in nitric acid aqueous solution by formohydroxamic acid (FHA) were studied to allow more accurate flow sheet modelling of neptunium separation from uranium and plutonium in an Advanced PUREX process. The rate of the reaction was monitored using stopped-flow spectrophotometry with near-infrared detection. The conditional reduction rate in 2 M nitric acid at 22°C could be described by the following equation: -d [NpO2 2+]/dt = k [NpO2 2+][FHA](M/s) with k = 1.17×103 M-1 s-1 at [H+] = 2.0 M. Comparison with other data available in the literature, indicates that formohydroxamic acid is an unusually strong reducing agent for NpO2 2+.

Thermodynamic stability of carbonyl and silacarbonyl anions R–M-22O [M=C, Si, R=H, F, Cl, OH, NH2, CH3, N(H)OH]. A comparative ab initio study

Ab initio molecular orbital methods at the CBS–Q level of theory have been used to study the structure and gas-phase stability of carbonyl and silacarbonyl anions R–M−O [M=C, Si, R=H, F, Cl, OH, NH2, CH3, N(H)OH]. The geometries of carbonyl, silacarbonyl acids and their anions were optimized at the MP2(FC)/6-31G() level of theory. The calculations showed that the thermodynamic stability of the carbonyl and silacarbonyl anions depends on both the type of substituent R and the possibility for competitive existence of O- and N-anions resulting from the monoionization of the multiple ionization sites. Thus formic acid, formamide, acetaldehyde and formohydroxamic acid are O- or N-acids. The result of the effect of the carbon-by-silicon substitution is considerable stabilization of the silaformyl anion. The relative stability between Si- and O-anions of silanoic acid is substantially lowered. Silaformamide, methylsilanone and silaformohydroxamic acid behave as Si-acids. The investigated acids are weak acids with calculated acidity of about 1400–1650 kJ mol−1. For C-acids the acidity increases in the order: H2CO⩽CH3CHO<NH2CHO<HCOOH<HC(O)NHOH<FCHO<ClCHO. Silaacids were found more acidic than their carbon counterparts. Their Si-acidity order is: CH3SiHO⩽NH2SiHO⩽H2SiO<HSiOOH<HSi(O)NHOH<FSiHO<ClSiHO.

Actinide complexation in the purex process

In the PUREX process Pu(IV) and U(VI) nitrate complexes are extracted into 30% TBP/OK (30% tributylphosphate in odourless kerosene) as UO2(NO3)2.2TBP and Pu(NO3)4.2TBP. The actinides can then be selectively stripped back into dilute nitric acid to yield separate uranium and plutonium products. BNFL are currently developing strategies to improve this process culminating in the development of single cycle flowsheets for an Advanced Reprocessing plant. Two areas highlighted for further investigation are dibutylphosphate (HDBP) complexation of actinides and new methods of Np(IV) and Pu(IV) stripping from the U(VI) product stream. Complexation of tetravalent actinides (An(IV)) with hydrophilic organic ligands such as formohydroxamic acid (FHA) can prevent extraction into 30% TBP/OK. This process can be used to selectively strip Np(IV) and Pu(IV) in the presence of U(VI). An(IV) complexation has been investigated by near infra-red spectroscopy and selective stripping demonstrated by batch solvent extraction experiments and the determination of distribution coefficients.

Neptunium(IV) and uranium(VI) complexation by hydroxamic acids

Hydroxamic acids complex more readily with Ac(IV) than Ac(VI). Formohydroxamic acid (FHA) is a hydrophilic organic ligand which can readily complex with Np(IV), as indicated by near infrared spectroscopy. Distribution experiments have also shown that FHA can strip Np(IV) from 30% TBP/OK into aqueous nitric acid. In contrast U(VI) does not complex as strongly with FHA and the reaction is greatly inhibited by nitric acid, as observed by UV-Vis spectroscopy. After stripping Np(IV) it has also been proven, by 13C NMR spectroscopy, that FHA can be decomposed to gaseous products in concentrated nitric acid leaving no organic waste products in solution. This experimental evidence proves that FHA can be used to selectively strip Np(IV) from a 30% TBP/OK solution of Np(IV) and U(VI) into aqueous nitric acid and is thus a viable new reagent for inclusion in an advanced PUREX process.

The formation of hydrophilic Np(IV) complexes and their potential application in nuclear fuel reprocessing

A series of organic ligands have been screened for their effectiveness as complexants for Np(IV) in a neptunium rejection stage of an advanced PUREX process. Four of these species, formohydroxamic acid, acetohydroxamic acid, glycolic acid and pyruvic acid, readily form hydrophilic complexes with Np(IV) and can strip the actinide from 30% TBP/OK (30% tributylphosphate in odourless kerosene) into nitric acid. Near infra-red spectroscopy has been used to monitor Np(IV) complexation in nitric acid. Distribution experiments have been undertaken between nitric acid and 30% TBP/OK to examine the effect of ligand and nitric acid concentration on Np(IV) stripping. Finally, it has been shown that the extractability of U(VI) is unaffected by the presence of these ligands and all can be used to selectively strip Np(IV) from a U(VI) product stream in an advanced PUREX process.

Ab Initio Study of Intramolecular Proton Transfer in Formohydroxamic Acid

Interconversion of five isomeric tautomers of formohydroxamic acid via intramolecular proton transfer has been examined by ab initio theoretical calculation. The transfer potential surfaces, the global isomeric structures, and the transition geometries of intramolecular proton transfer were determined by the MP2/6-31+G** level of calculation. The energy was further analyzed by a single point calculation, MP2/6-31++G**//MP2/6-31+G**, and the use of G2 theory. Not counting the unstable charge separating species, the order of stability of these tautomers calculated at the HF level was 1E > 1Z > 2Z > 2E, and it shifted to 1Z > 1E > 2Z > 2E at the MP2 level, where 1Z and 1E are keto forms, while 2Z and 2E are iminol forms. Further investigation using G2 theory redirects the order to be 1Z > 2Z > 1E > 2E. The strength of the intramolecular hydrogen bond and the effect of dipole moment are the two major factors to dominate the acidity of formohydroxamic acid. Judging from the transition barrier of intramolecular...

Conformational and tautomeric equilibria of formohydroxamic acid in the gas phase and in aqueous solution

The extent of tautomerism and conformational isomerism in formohydroxamic acids was explored using ab initio calculations. Several new conformers were identified, as well as a new tautomeric form that may be viewed as the N-oxide of imidic acid. In most cases, these new isomers are less stable than those previously discussed in the literature. Hydration effects were explored using AM1-SM2 and PM3-SM3 calculations. The new N-oxide tautomers were found to be preferentially stabilized compared to the gas phase, but not to the extent of making them competitive with the global minimum.

Gas-Phase Structure and Acidity of Formohydroxamic Acid and Formamide: A Comparative ab Initio Study

Gas-Phase Structure and Acidity of Formohydroxamic Acid and Formamide: A Comparative ab Initio Study

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

The geometries of various tautomers and rotamers of formohydroxamic acid, its anions and protonated forms were optimized at several ab initio levels, using the 4–31G, 4–31 +G and 6–31G ∗ (MP2/6-31 +G ∗∗ for selected species) basis sets. The calculations predict that the molecule of neutral acid should exist in several forms very close in energy: one of them is the hydroximic tautomer, which has not yet been found experimentally. The two most stable rotamers of the hydroxamic tautomer are non-planar with the OH hydrogen atom out of the plane of the heavy atoms. The original non-planar conformation of hydroxylamine is thus partly retained and not overcome even by the intramolecular hydrogen bond. In the structure of the anion the hydrogen bond is stronger and more important; it makes the most stable conformations planar. Of the possible tautomers of the anion the N-anion is more stable than the O-anion, hence hydroxamic acids are predicted to behave as N-acids in the gas phase. The protonated forms are again non-planar; the most stable form is protonated on the carbonyl oxygen atom. The calculated internal coordinates, dipole moments, and ionization energies are in reasonable agreement with experimental findings, obtained mostly on more complex hydroxamic acids. Calculated energy barriers for rotation about the C-N bond in formohydroxamic acid lie in the range of 90–135 kJ mol −1 .

Studies on the nitroso-glyoxylate reaction. Relative hydroxamic acid production by glyoxylate, pyruvate, and formaldehyde in reactions with 4-nitrosobiphenyl.

The pH rate profiles for the reactions of 4-nitrosobiphenyl with three carbonyl substrates in aqueous buffers were determined by use of chromatographic and spectrophotometric methods. Glyoxylate and formaldehyde caused the conversion of 4-nitrosobiphenyl to N-(4-biphenyl)-formohydroxamic acid, while pyruvate resulted in the production of N-(4-biphenyl)acetohydroxamic acid. The dramatic effect of pH on the kinetics of these reactions provided considerable information concerning the nature of these reactions. The reactions with pyruvate and formaldehyde displayed similar pH rate profiles and were significant only at acidic pH. Glyoxylate displayed a pH rate profile that differed markedly from those of pyruvate and formaldehyde as the pH was increased beyond 2.0. The ability of glyoxylate to convert 4-nitrosobiphenyl to the hydroxamic acid increased rapidly in the pH range 2.0-4.0, above which the pH dependency was constant. This biphasic appearance of the pH rate profile was unique to glyoxylate, since the reactions of pyruvate and formaldehyde became extremely slow as solution neutrality was approached. A second substrate, 4-chloronitrosobenzene, displayed similar pH rate profiles in its reactions with these carbonyl substrates. For 4-nitrosobiphenyl, hydroxamic acid formation by glyoxylate was 10(4) times faster than that by pyruvate at neutral pH, but only about 3-fold faster at pH 1.0. The appearance of the pH rate profile for glyoxylate suggested that this alpha-oxo acid reacts with nitrosoarenes at neutrality via a pathway that is insignificant for pyruvate or formaldehyde. Thus, the nitroso-glyoxylate reaction is unique to this alpha-oxo acid under physiological pH conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular orbital study of the structures of hydroxamic acids

Semiempirical (AM1 and PM3) and ab initio Hartree-Fock and Moeller-Plesset (MP2) calculations using the 3-21G, 3-21G(d), 6-31G(d,p), 6-311G(d,p) basis sets are reported for the E and Z conformations of both the keto and iminol tautomers of formohydroxamic acid. Semiempirical and Hartree-Fock calculations are also reported for several substituted hydroxamic acids. The HF calculations tend to favor the non-hydrogen-bonding E forms, while the MP2 calculations predict both of the Z forms to be close in energy