Energy cost of C−N rotation in hydroxamate group
Reference 1 reports the isomerization barrier for N‐methylacetohydroxamic acid. From the abstract:


  We have now determined the activation barrier to C–N rotation from major to minor conformation of a small secondary HA in $\ce{D2O}$ to be 67.3 kJ/mol. The HA rotational barrier scales with solvent polarity, as is observed in amides, although the HA barrier is less than that of a comparable tertiary amide in aqueous solution. 


N‐methylacetohydroxamic acid (image from PubChem):



References


Stefanie P. Sippl, Paul B. White, Charles G. Fry, Sarah E. Volk, Lingxiao Ye, Heather L. Schenck. Kinetic and thermodynamic characterization of C–N bond rotation by N‐methylacetohydroxamic acid in aqueous media. Mag Res Chem 54 (2016), 46-50. DOI: 10.1002/mrc.4318.