Hydrogen bonding and proton localization in complexes of carboxybetaines with phenols and carboxylic acids

Complexes of betaine (BET) with 2,6-dichloro-4-nitrophenol (DCNP), pentachlorophenol (PCP) and trifluoroacetic, trichloroacetic, dichloroacetic, chloroacetic and maleic acids and of pyridine betaine (PBET) with DCNP in solution and in the solid state were studied by UV and FTIR spectroscopies and X-ray analysis. The crystal of BET·DCNP is triclinic, space group P1̄, a = 7.1770(10) Å, b = 10.001(2) Å, c = 11.241(2) Å, α = 108.81(3)°, β = 100.06(3)°, γ = 106.82(3)°, Z = 2; the final R value is 0.033 for 1871 observed reflections. Protonated betaine and 2,6-dichloro-4-nitrophenolate are linked by an O(2)Htctdot;O(1) hydrogen bond with an Otctdot;O distance of 2.424(3) Å and the O(2)Htctdot;O(1) angle is 159(3)°. The broad absorption in the solid state FTIR spectra of the investigated complexes varies with ΔpKa, and is typical of complexes with strong hydrogen bonds. The UV spectra of phenol complexes in acetonitrile show a typical absorption for Btctdot;HA and B+Htctdot;A− species. In less polar dichloromethane, only molecular complexes are present. An exception is PBET·DCNP, where B+Htctdot;A− species appear in both solvents. The agreement between the UV and IR data is good.