title: Conformational and structural studies of N-methylacetohydroxamic acid and of its mono- and bis-chelated uranium(VI) complexes.
abstract: 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.
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB), UMR 6302, CNRS, Université de Bourgogne Franche-Comté, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Brandès
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB), UMR 6302, CNRS, Université de Bourgogne Franche-Comté, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France.
authors.forename: Alejandra
authors.initials: A
authors.lastname: Sornosa-Ten
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB), UMR 6302, CNRS, Université de Bourgogne Franche-Comté, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France.
authors.forename: Yoann
authors.initials: Y
authors.lastname: Rousselin
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB), UMR 6302, CNRS, Université de Bourgogne Franche-Comté, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France.
authors.forename: Mickael
authors.initials: M
authors.lastname: Lagrelette
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB), UMR 6302, CNRS, Université de Bourgogne Franche-Comté, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Stern
authors.affiliation: Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), UMR 8516, CNRS, Université de Lille - Sciences et Technologies, 59655 Villeneuve d'Ascq Cedex, France.
authors.forename: Aurélien
authors.initials: A
authors.lastname: Moncomble
authors.affiliation: Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), UMR 8516, CNRS, Université de Lille - Sciences et Technologies, 59655 Villeneuve d'Ascq Cedex, France.
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Cornard
authors.affiliation: Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB), UMR 6302, CNRS, Université de Bourgogne Franche-Comté, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France. Electronic address: michel.meyer@u-bourgogne.fr.
authors.forename: Michel
authors.initials: M
authors.lastname: Meyer
mesh_terms: D002614:Chelating Agents / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006877:Hydroxamic Acids / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D013059:Spectrum Analysis, Raman; D014501:Uranium / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D002614:Chelating Agents; D056831:Coordination Complexes; D006877:Hydroxamic Acids; D014867:Water; D014501:Uranium; C006358:acetohydroxamic acid
keywords: EXSY spectroscopy; Hydroxamic acid; Raman spectroscopy; Uranium(VI) complexes; X-ray crystal structures