X-ray diffraction and ENDOR investigation of hydrogen bonding in malonic acid-urea 1: 1 adduct: II. Endor

The ENDOR spectra of ?-irradiated single crystals of the urea-malonic acid 1 : 1 adduct were recorded at room temperature. The hyperfine coupling tensor of the ? proton of the HOOC-CH-COOH free radical formed by irradiation were measured as well as the hyperfine coupling tensors of a number of hydrogen bonded protons. The tensor elements are compared with values calculated according to the McConnell and Strathdee procedure taking into account the spin distribution in a ?- orbital extending over the whole molecule. The position of the proton belonging to a very short hydrogen bond is discussed on the basis of its dipolar and isotropic interaction.