Vibrational properties and first hyperpolarizability of cyclopentadiene homologues C4H4XH2 (X=C, Si, Ge, Sn): ab initio HF and DFT investigation

IR and Raman spectra and static electronic and vibrational first hyperpolarisabilities, β, of the cyclopentadiene homologues C₄H₄XH₂ (X=C, Si, Ge, Sn) have been calculated by conventional ab initio and DFT-B3LYP methods using Sadlej POL and correlation consistent Dunning basis sets to investigate on the role of the heteroatom on the response property. The pure vibrational contribution to β has been evaluated within the double harmonic oscillator approximation. A clear heavy atom effect was found in the vibrational frequencies and intensities localised on the XH₂ and C–X–C fragments, which decrease and increase, respectively, as the size of the heteroatom increases. Both β^e and β^v show an increase along the series, the vibrational contribution to β being modest in cyclopentadiene, but substantial in metalloles, especially for silole, where it is about two times greater than the electronic counterpart. The β^v/β^e ratio is strongly dependent on the basis set, decreasing as the quality of the basis set increases. Both β^v and total β^e+β^v values of the cyclopentadiene homologues are higher than the corresponding ones in the furan series, except for stannole and tellurophene which have close total β^e+β^v values.