Interaction between closed- and open-shell molecules. VI. 1H and 13C contact shifts and molecular orbital studies on the hydrogen bond of nitroxide radical

The ¹H and ¹³C Fermi contact shifts induced by the hydrogen bond with di‐tert‐butyl nitroxide(DTBN) radical have been observed for various proton donor molecules $(\mathrm{X–H})$ . The upfield contact shifts of the X–H proton and downfield ¹³C contact shifts of the donor molecules were interpreted in terms of the spin polarization mechanism of electron spin transfer from DTBN to the X–H molecule. The formation constants, enthalpies, limiting ¹H and ¹³C contact shifts, and spin densities on the H and C atoms were also determined for the 1$\mathrm{X–H}\mathrm{·····}\mathrm{DTBN}$ hydrogen bond interaction from ¹H and ¹³C contact shift measurements at various temperatures. The theoretical studies on this closed and open‐shell bimolecular system were also performed by unrestricted Hartree‐Fock SCF MO(INDO method) calculations. The hydrogen bond energies and spin densities on the X–H molecules were well reproduced by the MO calculations.