Influence of the bromo group on the vibrational spectra and macroscopic properties of benzophenone derivatives

The effects of a minor chemical modification such as a change in the position of a Br atom within the same phenyl ring on the optical and macroscopic properties of benzophenone derivatives are investigated by spectroscopic and calorimetry methods. More specifically, we have studied IR and Raman spectra of the two isomers of monosubstituted benzophenones: 2-bromobenzophenone (2BrBP) and 4-bromobenzophenone (4BrBP) in the wide spectral and temperature regions. It has been found that the substitution of a Br in an ortho position leads to some changes of the anharmonicity of the ν(CO) vibrations. Full geometry optimization and vibrational spectra modeling for 2BrBP and 4BrBP isolated molecules have been calculated by the density functional method (B3LYP/6-31+G(d)) using GAUSSIAN’03 software. Quantum-mechanical calculations for the isolated molecules have shown that the shape of 2BrBP molecule is strongly asymmetric in comparison with the shape of 4BrBP molecule. A change in the molecular shape translates into rather different macroscopic properties such as the crystal melting points. Namely, the melting point of 2BrBP (318 K) was found to be lower than that of 4BrBP isomorphs (358 K). Moreover, 2BrBP exhibits a large reluctance to crystallize, while 4BrBP crystallizes immediately below the melting point as a liquid is cooled.