Polarity of Hydrogen Bonds on Carbonyl Groups

From the dipole moments of the complexes and of the separate compounds the di-pole increments Δμ were computed which result from the complexation of a series of phenols of increasing acidity with several compounds with a carbonyl group: a ketone, an ester, a carbonate, an ureine and two amides. The computations were made for several possible values of the angle between the C = 0 axis and the direction of the hydrogen bond going from 100° to 140°. For a given proton acceptor Δμ increases when the pKa of the proton donor decreases. The derivatives -d Δμ/d pKa increase with the basicity of the donor. The dipole increment Δμ also increases with the complexation constant Kab and with the frequency shift ΔνOH. Both derivatives d Δμ/d log Kab, and d Δμ/d ΔνOH increase when the basicity of the proton acceptor becomes higher. From the Badger-Bauer equation relations between Δμ and the complexation enthalpy ΔHab are obtained. The evolution of Δμ with increasing energy of the hydrogen bond in the whole range of ΔHab cannot be represented by a linear equation but the curves must show an upwards curvature. From ΔνOH the displacement of the proton is deduced and the corresponding contribution ΔμH in the dipole increment is computed. For a given energy of the bond ΔμH is clearly less important for the O-H … O = C bonds than for the O-H … N bonds but the dipole increment due to electron displacements is higher. This last increment is mainly due to the rearrangement of the electron clouds in the proton acceptor.