Abstract
Experimental IR spectroscopic data for the N–H stretching mode frequencies for several types of tri-substituted ureas containing benzyl and/or phenyl substituents as well as theoretical results from B3LYP/6–31G(d,p) computations on selected compounds provide sufficient evidence to determine the conformational state of these molecules. Two types of N–H bands may be found the spectra: (a) A type band due to a classical trans conformation (t rans I) of the CO NH structure; (b) B type band arising from an alternative trans form ( trans II), in which the N–H band is involved in a hydrogen bond like interaction with the aromatic ring at the neighbouring nitrogen atom (benzyl or phenyl substituents). The N–H band of trans I CO NH structure is observed at frequencies higher than 3460 cm −1, the actual position depending on weather the non-substituted N–H group is linked to aryl or alkyl substituents. The N–H band of the trans II rotameric structure is observed at 3430–3420 cm −1.
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