INTRAMOLECULAR HYDROGEN-BONDS AND FERMI RESONANCE IN N-SUBSTITUTED N'-(2-FLUOROBENZOYL)THIOUREA DERIVATIVES

The presence of intramolecular NH...O=C bonds was proved in N-substituted N'-(2-fluorobenzoyl)-thiourea derivatives by analysis of their infrared spectra. The intramolecular vibrational effects bring about a shift of the nu(NH) vibrational band to lower frequencies to the extent that the band gets into a vicinity to the delta(NH) overtone and Fermi resonance occurs between them. In addition, the nu(NH) vibration is also affected by Fermi resonance with the nu(CO) + delta(NH) combination. Double absorption bands were observed in the nu(CO) region for some of the derivatives. Based on perturbation theory applied to three-level interactions and using the Langseth and Lord equations, the band frequencies corrected for Fermi resonance were calculated and the hydrogen bond strength was examined in the compounds studied.