AB-INITIO CALCULATION OF THE CONFORMATIONS AND VIBRATIONAL-SPECTRA OF2-PHENYLBUTANE

Molecules with internal degrees of rotation are of particular interest to understand the behavior of synthetic or natural polymers. With the extremely rapid increase in floating point performance of modern comp uters we are able to calculate a subspace of the Born-Oppenheimer hype rsphere for quite large molecules with ab initio methods. We choose 2- Phenylbutane as an elementary model for Polystyrene (PS), because it p ossesses two internal degrees of freedom, that are highly relevant for PS. These are the rotation of the benzene ring and the chain like mot ion of the ethylgroup. We minimized the energy of one conformation wit h re spect to all internal coordinates and used different basis functi on sets. From that the standard gaussian type basis 3-21G elaborates a s an optimum function set regarding computing time and accuracy. With the help of multidimensional search algorithms, that are available in the GAMESS [1] and GAUSSIAN92 [2] program packages, we were able to ca lculate 4 different conformations of the 2-Phenylbutane that are energ etically relevant (figures 1a-d). In a more extensive search we show t hat it is possible to calculate the saddle point structures between th e minima without restricting the minimization to any coordinate. The c alculated intrinsic reaction coordinate (IRC) between the saddle point s and the minima provide a detailed view in the Born-Oppenheimer hyper sphere of 2-Phenylbutane (figure 2), A three dimensional representatio n of this energy landscape in dependence of the two internal rotationa l degrees of freedom is calculated that gives a further inside into th e flexibility of the studied molecule (figure 3). Our investigation pr oofs that the assumption of rigid rotors in molecules with more than 1 internal degree of rotation is - especially for PS - a crude approxim ation. We verify the level of energy relaxation in the calculated conf ormations by performing a normal coordinate analysis at the minimum an d saddle point structures. The calculated polarized and depolarized Ra man spectra are in agreement with our experimental data (figure 4). We characterized the measured bandstructure of the strong symmetric ring breathing vibration as a superposition of the vibrations of different conformers. The coupling of the two CCC-bending modes of the chain le ads to a normal vibration whose frequency is highly dependent on the c onformation of the chain (figure 5), which can be used to monitor the internal rotation of 2-Phenylbutane with Raman spectroscopy [3]. The a nalysis of the normal vibrations of 2-Phenylbutane with ab initio calc ulations give a quantitative view of the complex vibrational Raman spe ctra With the help of these investigation we are able to show in an Ra man experiment the freezing of an internal mode of 2-Phenylbutane duri ng the glass transition [4].