Abstract:The IR absorption spectrum of salicylhydroxamic acid (sha) isolated in an argon matrix at 12-21 K has been recorded. Calculations of all the possible entgegen and zusammen keto conformers of sha have been performed at the B3LYP/6-311++G(d,p) level of theory. The computed energies, optimized structures, and relative abundances show that sha exists in the matrix as the z,z,z,z-conformer [relative to the C-O(-H), C-C(-N), C-N and N-O bonds, respectively] and is stabilized by two intramolecular H-bonds. Photofragm… Show more
“…The A- Z -amide structure appears to be most stable in the gas phase; its most stable conformer, the B- Z -amide, lies 5.4 kcal/mol above the corresponding A-form. The A- Z -amide conformer becomes stabilized through formation of a strong intramolecular O P H···O C H-bond; actually, the most stable A- E -amide conformer lies 4.5 kcal/mol above the A- Z- amide form, in good agreement with the value reported by Kaczor et al 1 Most Stable Neutral and Anionic Structures of A-SHA Amide…”
Section: Resultssupporting
confidence: 88%
“…The upfield shift observed when the temperature was raised can be put down to a reduction of the hydrogen bond strength present in the substrate structure. The most pronounced variation of the (N)H E proton signal suggests formation of E − E aggregates such as that shown in Figure a, similar to the dimer structures put forward for amides; it should be noted that dimer formation has been suggested for other hydroxamic acids. ,,,, The somewhat more modest variation of the other signals in Figure a, will be discussed below. The structure depicted in Figure a requires that, even if the monomers are in the Z conformation (as suggested by the theoretical calculations), those should adopt the E conformation for the dimerization process to occur.…”
The acid-base behavior and self-aggregation of salicylhydroxamic (SHA) and p-hydroxybenzohydroxamic acids (PHBHA)have been investigated by UV and 1HNMR spectroscopy, respectively. The acid-base parameters, measured in H2O at 25 degrees C and I=0.1 M, were pK1=7.56, pK2=9.85 for SHA and pK1=8.4, pK2=9.4 for PHBHA. The 1H NMR signals for salicylhydroxamic and p-hydroxybenzohydroxamic acids measured in acetone indicate that both acids self-aggregate according to a mechanism where two monomers produce planar E-E dimers stabilized by horizontal H-bonds. Further dimer aggregation yields sandwich-like tetramer structures stabilized by vertical H-bonds and pi-pi interactions. The p-hydroxybenzohydroxamic tetramers, less stable than those of salicylhydroxamic, contain two water molecules in their structures. The gas-phase structures of salicylhydroxamic acid and its anions were investigated by ab initio calculations using the density functional theory at the B3LYP/AUG-cc-pVDZ level. The SHA most stable gas-phase conformer is the A-Z amide, a structure with all three phenolate (OP), carboxylate (OC), and hydroxamate (OH) oxygen atoms in the cis position. The B-Z amide, with the OP oxygen trans to OC, lies 5.4 kcal above the A-Z amide. The most stable monoanion is the N-deprotonated A-Z amide.
“…The A- Z -amide structure appears to be most stable in the gas phase; its most stable conformer, the B- Z -amide, lies 5.4 kcal/mol above the corresponding A-form. The A- Z -amide conformer becomes stabilized through formation of a strong intramolecular O P H···O C H-bond; actually, the most stable A- E -amide conformer lies 4.5 kcal/mol above the A- Z- amide form, in good agreement with the value reported by Kaczor et al 1 Most Stable Neutral and Anionic Structures of A-SHA Amide…”
Section: Resultssupporting
confidence: 88%
“…The upfield shift observed when the temperature was raised can be put down to a reduction of the hydrogen bond strength present in the substrate structure. The most pronounced variation of the (N)H E proton signal suggests formation of E − E aggregates such as that shown in Figure a, similar to the dimer structures put forward for amides; it should be noted that dimer formation has been suggested for other hydroxamic acids. ,,,, The somewhat more modest variation of the other signals in Figure a, will be discussed below. The structure depicted in Figure a requires that, even if the monomers are in the Z conformation (as suggested by the theoretical calculations), those should adopt the E conformation for the dimerization process to occur.…”
The acid-base behavior and self-aggregation of salicylhydroxamic (SHA) and p-hydroxybenzohydroxamic acids (PHBHA)have been investigated by UV and 1HNMR spectroscopy, respectively. The acid-base parameters, measured in H2O at 25 degrees C and I=0.1 M, were pK1=7.56, pK2=9.85 for SHA and pK1=8.4, pK2=9.4 for PHBHA. The 1H NMR signals for salicylhydroxamic and p-hydroxybenzohydroxamic acids measured in acetone indicate that both acids self-aggregate according to a mechanism where two monomers produce planar E-E dimers stabilized by horizontal H-bonds. Further dimer aggregation yields sandwich-like tetramer structures stabilized by vertical H-bonds and pi-pi interactions. The p-hydroxybenzohydroxamic tetramers, less stable than those of salicylhydroxamic, contain two water molecules in their structures. The gas-phase structures of salicylhydroxamic acid and its anions were investigated by ab initio calculations using the density functional theory at the B3LYP/AUG-cc-pVDZ level. The SHA most stable gas-phase conformer is the A-Z amide, a structure with all three phenolate (OP), carboxylate (OC), and hydroxamate (OH) oxygen atoms in the cis position. The B-Z amide, with the OP oxygen trans to OC, lies 5.4 kcal above the A-Z amide. The most stable monoanion is the N-deprotonated A-Z amide.
“…Infrared spectra of aqueous solutions of BHA and SHA have been recorded at pH = 1.66 and I = 1 M. Moreover, the spectra of the ligand and Iron(III) mixtures in the stoichiometric ratios C M /C L = 1 and 2 have been recorded also at pH = 1.66 ([H + ] = 0.022 M), where the extent of complex formation is highest. Concerning the Iron(III)/BHA system, Figure 12A shows that the band corresponding to the NÀH vibrational frequency (3300 cm À1 ) 36,37 does not display any remarkable change on going from BHA alone to mixtures of BHA and Fe(ClO 4 ) 3 . In particular, the spectra of solutions in which both the metal and the ligand are mixed in the ratios C M / C L = 1 and C M /C L = 2 are very similar.…”
Section: ' Resultsmentioning
confidence: 99%
“…Concerning the Iron(III)/SHA system, comparison of the spectrum of SHA alone with those of the C M /C L = 1 and C M /C L = 2 mixtures (Figure 12B) shows that, like in the case of the Iron(III)/BHA control system, Iron(III) binds to SHA through the O,O sites. Moreover, comparison of the spectral behavior of the C M /C L = 1 and C M /C L = 2 mixtures reveals that the bands corresponding to the phenol O P ÀH (3463 cm À1 ) and NÀH stretching (3300 cm À1 ), 36,37 which for C M /C L = 1 mixtures are well evident (although overlapped) between 3000 and 3400 cm À1 , become reduced to a large extent for the C M /C L = 2 mixture. This outcome indicates that the second Iron(III) ion in the binuclear complex is bound to SHA through the O P ,N site.…”
The equilibria and the kinetics of the binding of Iron(III) to salicylhydroxamic (SHA) and benzohydroxamic (BHA) acids have been investigated in aqueous solution (I = 1 M (HClO(4)/NaClO(4)), T = 298 K) using spectrophotometric and stopped-flow methods. Whereas Iron(III) forms a 1:1 complex (ML) with BHA, it forms both ML and M(2)L complexes with SHA. The presence of M(2)L in aqueous medium is corroborated by FTIR measurements. The reactive form of Iron(III) is the hydrolyzed species FeOH(2+), which binds to the O,O site in ML and to the O,O and O(P),N (P = phenolate) sites in M(2)L, inducing full deprotonation of the latter. The reaction pathway is discussed in terms of a multistep mechanistic scheme in which the metal-ligand interaction is coupled to hydrolysis and self-aggregation steps of Iron(III). The observation and characterization of M(2)L as a stable species is important because it contains the -Fe-O-N-Fe- sequence, which constitutes the repetitive motif of the SHA-based metallacrown ring and provides the rationale for 12-MC-4 metallacrowns. In the framework of this study, the kinetics of the Iron(III) dimerization and trimerization have also been investigated using the stopped-flow method to perform dilution jumps. The reaction scheme put forward involves two parallel steps (FeOH(2+) + FeOH(2+) and Fe(3+) + FeOH(2+)) that lead to formation of the Fe(2)(OH)(2)(4+) dimer and a slower step (FeOH(2+) + Fe(2)(OH)(2)(4+)) to form the trimer species. The kinetics of the last step have been investigated here for the first time, and the results deduced indicate that, of the two possible trimer structures reported in the literature, Fe(3)(OH)(3)(6+) and Fe(3)(OH)(4)(5+), the latter prevails by far.
“…The IR absorption spectrum of salicylhydroxamic acid in Ar matrices at 12-21 K has also been recorded. 386 This exists as a Z,Z,Z,Z-conformer relative to the C-O(-H), C-C(-N), C-N and N-O bonds and it is stabilised by two intramolecular H bonds. Photofragmentation occurs in matrices with o-hydroxyphenylisocyanate being the major product.…”
the Progress of Chemistry, Section C. The report is divided into sections: weak adducts and conformational isomers; reactions of atoms; high-temperature molecules; photochemistry; biological molecules; astrochemistry. The last two sections reflect the fact that matrix isolation now extends into areas outside the traditional mainstream disciplines of chemistry. There are a number of areas where substantial progress has been made during the period of this report. First, the routine use of laser-ablation to generate atoms and to initiate atomic reactions in matrices has increased the scope of matrix isolation to synthesise many new molecular species in inorganic chemistry. Laser-ablation allows ground state atoms to be more easily studied than is the case when atoms are produced by thermal methods. Moreover, this procedure allows atoms to be generated from materials where thermal production is difficult. Secondly, the manufacture of more efficient closed-cycle helium refrigerators allows hydrogen matrices to be deposited readily. A significant body of work therefore has focussed upon metal hydrides, dihydrogen complexes and other hydrogen species in matrices. Underpinning all of this experimental work are theoretical calculations, without which the correct identification of most matrix isolation molecules would remain dangerously ambiguous. Infrared spectroscopy remains-as it has for the past 50 years-the workhorse of matrix isolation studies. It does seem that this situation will not change-it is impossible to conceive of another spectroscopic method that combines the necessary sensitivity to study molecules at very low dilution with the structural information required to make a reasonably certain identification.
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