The bismuth tetramer Bi₄: the ν₃ key to experimental observation

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• The spectroscopic identification of bismuth tetramer (Bi4) has been elusive.
• Understanding the structure and properties of Bi4 is crucial for advancing inorganic chemistry and materials science.

Purpose of the Study:

• To computationally investigate two constitutional Bi4 isomers (Td and C2v symmetry).
• To determine their optimized geometries, vibrational frequencies, and energetic properties.
• To predict spectroscopic data for laboratory identification.

Main Methods:

• Coupled-cluster calculations with single, double, and triple excitations (CCSD(T)) using a cc-pVQZ-PP basis set.
• Focal point analysis (FPA) with extended basis sets (cc-pV5Z-PP) and high-level correlation methods (CCSDTQ).
• Natural bond orbital (NBO) analysis to investigate electronic structure and bonding.

Main Results:

• Both Td and C2v Bi4 isomers are identified as local energetic minima.
• Predicted dissociation energy of Bi4 (Td) to 2 Bi2 is +65 kcal mol-1.
• Adiabatic energy difference between C2v and Td isomers is +39 kcal mol-1.
• Low infrared intensities for the Td isomer's fundamental frequencies (94, 123, 167 cm-1) are predicted.
• The C2v isomer exhibits unique 'long-bonding' between wing atoms, facilitated by sigma bonding in the spine.

Conclusions:

• The low predicted IR intensities for the Td isomer likely explain the experimental difficulty in observing Bi4.
• The discovery of 'long-bonding' in the C2v isomer offers new insights into chemical bonding in heavy elements.
• Computational methods provide valuable predictions for guiding experimental spectroscopic identification of elusive molecules like Bi4.