HYDROGEN-BONDING AND THE ULTRAFAST TIME RESPONSE IN CARBOXYLIC-ACIDS

We present a preliminary study of Rayleigh-wing results performed on t wo carboxylic acids, namely formic acid (HCOOH) and acetic acid (CH3CO OH) at T = 18 degrees C. The obtained data are compared with those ded uced from two derivatives (methyl formate, HCOOCH3 and methyl acetate, CH3COOCH3) in which the OH active sites for the interactions have bee n substituted with the non-interacting CH3 methyl group. This study al lowed us to enter in detail the dynamic contribution to the wing in th e picosecond time domain. Recently the Reverse Monte Carlo method and Xray and neutron diffraction have determined that short branching chai ns are formed for the stronger acid. In the case of CH3COOH, due to it s steric hindrance, the closed dimeric species are favoured at lower t emperature and they tend to open when the temperature increases, as in dicated by the Kirkwood correlation factor. The optical response for t he two acids has shown the presence of a zero-frequency Lorentzian ban d plus a definite librational shoulder, and an exponential collision i nduced contribution. This behaviour is peculiar for H-bonded associate d liquids and verifies the Keys and Kivelson's model very well. In the case of the non-associated isomorphous liquids a reorientational Lore ntzian band and a Gaussian contribution connected to inertial effects, also zero-frequency centred, fit the spectral wing, together with a v ibrational band centred at omega approximate to 300 cm(-1). Furthermor e, a preliminary study of the OH stretching region vs. temperature for the acetic acid, allowed us to obtain information about the aggregati ve processes existing in this system.