Document Detail


Acido-base behavior of hydroxamic acids: experimental and ab initio studies on hydroxyureas.
MedLine Citation:
PMID:  18950143     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
The values of Ka, DeltaSa, and DeltaHa for deprotonation of hydroxyurea (HU) and N-methylhydroxyurea (NMHU), as targeted compounds, and for betainohydroxamic acid, were potentiometrically determined. Although NMHU has two and HU even three deprotonation sites, the measurements confirm that they behave as weak acids with a single pK a approximately 10. Comparison with analogous thermodynamic parameters previously determined for series of monohydroxamic acids reveals deviations from a DeltaSa, vs DeltaHa plot for HU and NMHU, raising the question of the dissociation site of hydroxureas in water. In addition to the deprotonation of the hydroxyl oxygen, ab initio calculations performed at the MP2/6-311++G(d,p) level of theory for these two compounds indicate a notable participation of the nitrogen deprotonation site in HU. The calculations for the isolated, monohydrate, trihydrate, and decahydrate molecular and anionic forms of hydroxyureas support the importance of hydrogen bonding in the gas and aqueous phases. The hydroxylamino nitrogen in HU is the most acidic site in water, contributing approximately 94% to the overall deprotonation process at 25 degrees C. On the contrary, the hydroxylamino oxygen is by far the most favored deprotonation site in NMHU, contributing almost 100% in aqueous medium. The predicted participations of two deprotonation sites in HU, calculated at the MP2/6-311++G(d,p) level of theory, combined with the calculated relative reaction enthalpy and entropy for the deprotonation, satisfactorily explain the observed deviation from linearity of DeltaHa vs DeltaSa, plot. There is no such a simple explanation for acid-base behavior of NMHU.
Authors:
Ivana Vinković Vrcek; Ivan Kos; Tin Weitner; Mladen Birus
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't     Date:  2008-10-25
Journal Detail:
Title:  The journal of physical chemistry. A     Volume:  112     ISSN:  1520-5215     ISO Abbreviation:  J Phys Chem A     Publication Date:  2008 Nov 
Date Detail:
Created Date:  2008-11-14     Completed Date:  2009-02-06     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9890903     Medline TA:  J Phys Chem A     Country:  United States    
Other Details:
Languages:  eng     Pagination:  11756-68     Citation Subset:  IM    
Affiliation:
Faculty of Pharmacy and Biochemistry, University of Zagreb, Zagreb, Croatia.
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MeSH Terms
Descriptor/Qualifier:
Hydroxamic Acids / chemistry*
Hydroxyurea / chemistry*
Linear Models
Potentiometry
Protons
Quantum Theory
Software
Thermodynamics
Chemical
Reg. No./Substance:
0/Hydroxamic Acids; 0/Protons; 127-07-1/Hydroxyurea

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