The bismuth tetramer Bi4: the ν3 key to experimental observation

Abstract

The spectroscopic identification of Bi₄ has been very elusive. Two constitutional Bi₄ isomers of T_d and C_2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the T_d isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi₄ (T_d) into two Bi₂ and the adiabatic energy difference between the C_2v and T_d isomers of Bi₄. These quantities are predicted to be +65 kcal mol⁻¹ and +39 kcal mol⁻¹, respectively. Two electron vertical excitation energies between the T_d and C_2v electronic configurations are computed to be 156 kcal mol⁻¹ for the T_d isomer and 9 kcal mol⁻¹ for the C_2v isomer. The most probable approach to laboratory spectroscopic identification of Bi₄ is via an infrared spectrum. The predicted fundamentals (cm⁻¹) with harmonic IR intensities in parentheses (km mol⁻¹) are 94(0), 123(0.23), and 167(0) for the T_d isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi₄ has yet to be observed. Through natural bond orbital analysis, the C_2v isomer of Bi₄ was discovered to exhibit “long-bonding” between the furthest apart ‘wing’ atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the ‘spine’ atoms of the C_2v isomer.