Surface-enhanced Raman, normal Raman and infrared spectra of chlorobenzoic acids

IR and Raman spectra of p-, m-, and o-chlorobenzoic acids (CBAs) have been recorded in solid and in different solutions. Vibrational assignments have been proposed based essentially on group frequency, band shape, intensity, and some internal consistency of CBAs. Examinations of the carboxylic acid and the carboxylate vibrations show that chlorobenzoic acids (CBA's) exist as hydrogen-bonded dimers in solid and monomeric acids in organic solvents such as diethyl ether, chloroform, and ethanol, while existing as carboxylate ions in sodium hydroxide solutions. Surface-enhanced Raman (SER) spectra of CBAs adsorbed on silver-coated capillary tube and silver-coated glass plates have also been collected. Experimental results indicate that the detected adsorption species exist as carboxylates on silver surfaces. Explanation of the dependency of the SER intensity of p-chlorobenzoic acid on acidity has been suggested on the basis of the chemical mechanism. From the experimental Raman data, the degree of enhancement is estimated to be 1.0 x 10(5) for the p-chlorobenzoic acid adsorbed on the substrates. The adsorption behaviours of these acids on silver surfaces are discussed.