Solvatochromic shifts of dihydroxybenzenes. Complexes of solute solvent association

MARIA C. ALMANDOZ; MATIAS I. SANCHO; SONIA E. BLANCO

Cordoba (Argentina)

Conferencia; 9 Conferencia Latinoamericana de Fisicoquímica Orgánica; 2007

Asociación Argentina de Investigación Fisicoquímica - Sociedad Argentina de Investigación en química Orgánica. Universidad Nacional de Rio Cuarto

The dihydroxybenzenes (DHB) are compounds of great importance, they are part of the molecular structures that characterize numerous benzophenones and flavonoids which have a large pharmaceutical, biochemical, medical and industrial significance. To detect the existence of some molecular solute-solvent interactions and to quantify their relative strengths, the UV spectral changes of resorcinol (R) and catechol (C) in protic and aprotic solvents were studied. UV spectra were registered in acetonitrile (ACN), ethanol (EtOH), methanol (MeOH) and water with a Shimadzu UV 160A spectrophotometer. The solvatochromic shifts were measured at 25°C with respect to cyclohexane (Cy), the solvent used as reference. The permittivities (e) of these solvents at the indicated temperature are, 36.34, 24.55, 32.63, 78.39 and 2.023, respectively. UV spectrum of R in Cy shows a main band at 274.3nm and a shoulder at 280 nm. The recorded shifts were: 0.7 nm (ACN), 2 nm (EtOH), 1.9 nm (MeOH) and -0.5 nm (water) for the main band and 0.4 nm (ACN), 1.4 nm (EtOH), 0.9 nm (MeOH) and 1.6 nm (water) for the shoulder. The main band of C in Cy is observed at 276 nm. The registered solvatochromic shifts were: -0.1 nm (ACN), 0.9 nm (EtOH), 1.6 nm (MeOH) and -1.2 m (water).The following reaction model was proposed to explain the spectroscopic shifts of R and C, DHB + S DHB-S complex (1). In this equation, DHB represents R and C, S are the solvents used and DHB-S is the complex of solute-solvent association of stoichiometric composition 1:1. The thermodynamic stability of all the species described in Eq. (1), and the pertinent equilibrium constants (K<==>C), were theoretically calculated by means of the B3LYP/6-31G(d) (Becke hybrid three-parameter nonlocal exchange functional combined with the functional Lee-Yang-Parr dynamic correlation) method that makes use of the IPCM model (Isodensity Polarizable Continuum Model). The following relationships between the magnitudes Ln KC (theoretical), lmax (wavelength of higher absorption) and ε were obtained: (a) For resorcinol: Ln KC = 3.8006 lmax  1034.6 (Band I, r=0.9677) (2); Ln KC = 3.4954 lmax  969.95 (Sh, r=0.9953) (3); Ln KC = ?0.1877 e +19.909 (r=0.9793) (4); (b) For catechol: Ln KC = 2.6123 lmax  711.33 (5).It is concluded that the solvent polarity is the main property that determines the spectral changes of R while in C the intermolecular H-bonds play an important role in the determination of the solvatochromic shifts observed. Likewise, the equilibrium constants calculated are suitable to quantify the solute-solvent molecular interactions.