Hyperfine structure amines

Formation of novel free radical products at an early stage of the Maillard reaction was demonstrated by use of ESR spectrometry. Analyses of the hyperfine structures for various sugar-amino compound systems led to the conclusion that the radical products are N,N -disubstituted pyrazine cation radicals. These new pyrazine derivatives are assumed to be formed by bimolecular condensation of a two-carbon enaminol compound involving the amino reactant residue. The presence of such a two-carbon product in an early stage reaction mixture of sugar with amine was demonstrated by isolation and identification of glyoxal dialkylimine by use of TLC, GLC, NMR, MS and IR. 

The method is based on the ability of TCNQ to react with bonded nitrogen compounds (secondary, tertiary amines) to form stable ion-radical salts (IRS) under mild conditions and with high yields. Completeness of formation of stable IRS has been monitored by ESR and electronic spectroscopy. One of the most important features of this method is the formation of two-dimensional phase by TCNQ anion-radicals on the surface, if bonded donor molecules are fixed at a small distance between their anchoring points, i.e. when separation between points of fixation is in the range 1-1.5 nm. ESR spectra of such samples contain an exchange-narrowed singlet with AH = 0.5 — 1.1 Gs, and g-factor 2.0025. Such spectra are typical for crystalline samples of IRS of TCNQ [25]. If bonded donor molecules are separated by as much as 2 nm or more, a distinct phase of TCNQ IRS is not formed. ESR spectra of such samples reveal either dipole-dipole broadening or hyperfine structure. 

It is important to note that the proportional relationship between Amax, Amid, and Amin for these couplings is the same for 100% spin density, and for the present case with approximately 50% spin density. When this is so it indicates that there is no rocking motion at the radical site. This is good evidence therefore that the radical site is essentially planar. The best evidence for radical planarity comes from the analysis of the direction cosines associated with each principal values of the hyperfine coupling tensor. The direction of Amin (Table 18-2) is known to be associated with the direction of the >C-H bond, while the direction associated with the Amid indicates the direction of the n-clcctron orbital. These directions are easily calculated from the crystal structure, and are included in Table 18-2. One sees that the direction associated with Amid deviates only 2.0° from the computed perpendicular to the ring plane, while the direction of Amin, deviates only 2.8° from the computed direction of the C6-H bond. The errors listed on these values are at the 95% confidence level. This is very clear evidence that the radical shown here is planar in the solid-state. Any torsional motion of the C6-H would lead to asymmetries of the hyperfine coupling tensor, and would not produce the observed agreement between the direction cosines and the known directions obtained from the crystal structure. 

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