Infrared and Raman spectroscopy of 9,9′-spirobifluorene, bis(2,2′-biphenylene)silane, and bis(2,2′-biphenylene)germane. Vibrational assignment by depolarization measurement and HF and density functional theory studies

The infrared and Raman spectra of 9,9′-spirobifluorene (SBFL), bis(2,2′-biphenylene)silane (BBPS), and bis(2,2′-biphenylene)germane (BBPG) are measured, and the vibrational frequencies and modes for these molecules are assigned by ab initio Hartree-Fock and Becke 3-Lee-Yang-Parr (B3LYP) density functional theory (DFT) calculations using the 4-31G and 3-21G basis sets for SBFL and for BBPS and BBPG, respectively. Assignment of some of the vibrational modes of SBFL is also confirmed by solution Raman spectroscopy with the depolarization method. Comparison of the calculated and experimental vibrational spectra reveals that the DFT calculations are quite accurate in predicting the vibrational frequencies, intensities, and modes. It is found that the central Si and Ge atoms dominantly enhance some of the IR intensities of the vibrations involving only the M-C4 (M = Si, Ge) antisymmetric stretching motion. The enhancement of some of the infrared intensities is accounted for by the increase of the electric dipole moment involving the M-C⁴ vibrations due to the presence of the small electronegative Si and Ge atoms in the central position.