Formamide complexes with selected azabenzenes: A discussion on their thermodynamic properties from Raman analyses

Spectral changes in the v1v1 region of pyridine (py) and pyrimidine (pd) at 991 cm−1 have been observed by means of a temperature dependent Raman study of the equimolar mixtures of formamide (FA)/py and FA/pd. The thermodynamic functions of the respective H-bonded complexes are determined and discussed as a function of the physical and chemical properties of the corresponding azabenzenes. The equilibrium constant values for the 1:1 FA:py complex are much greater than those obtained for the 2:1 FA:pd complex. This result clearly shows that py is a stronger base than pd, although their dipole moment (μ  ) values are almost identical. This last parameter seems to play an important role with respect to the band shifts (Δv) as well as to the formation enthalpies (ΔH°) of these complexes. The values of −43.6 J mol−1 K−1 for 1:1 FA:py and −75 J mol−1 K−1 for 2:1 FA:pd indicate that the whole hydrogen bonding process is entropically unfavorable. The high molecular association extent in these systems is likely the explanation to the significantly smaller band shifts in magnitude as compared to ternary systems.