14N Quadrupole Double Resonance in Some Substituted Hydroxamic Acids

Article abstract of DOI:10.1021/ja00183a020

14N quadrupole coupling constants and asymmetry parameters have been measured in a number of hydroxamic acids by double-resonance field-cycling techniques based on either irradiation in zero magnetic field or cross relaxation.The compounds all display high asymmetry parameters.Those in which this quantity is greater than 0.9 show remarkable line shapes for the two lower 14N frequencies (ν_y, ν_z) in their irradiation spectra.They are explained in terms of a thermal-mixing mechanism, which generates polarization of the 1H dipolar levels when these nearly degenerate frequencies are strongly irradiated in zero field, and then subsequently modified by level crossing when the sample is returned to high field to measure the remaining 1H signal.Ab initio SCF-MO calculations of the 14N quadrupole tensor in a group of molecules at the orientation found in crystals of acetohydroxamic acid hemihydrate and oxalodihydroxamic acid are in reasonable agreement with experiment and predict that in all the hydroxamic acids studied the maximum principal component is negative and closely parallel to the direction of the 2p_? orbital.