Reexamining Al₂H₂: Evidence That the Unidentified Butterfly Ground-State Isomer Was Previously Observed

Area of Science:

• Computational Chemistry
• Spectroscopy
• Materials Science

Background:

• Interest in aluminum dihydride (Al2H2) declined after the early 2000s, creating a gap between theoretical predictions and experimental observations.
• Previous experimental studies observed spectral features attributed to Al2H2, but lacked definitive identification of its structure.

Purpose of the Study:

• To reconcile theoretical calculations with experimental infrared (IR) data for aluminum dihydride (Al2H2).
• To provide definitive evidence for the butterfly global minimum structure of Al2H2 and identify key vibrational frequencies.
• To investigate the potential existence and spectroscopic signatures of anionic Al2H2 structures.

Main Methods:

• High-level computational chemistry methods, including Coupled Cluster Singles Doubles with Perturbation Theory (CCSD(T)) and Coupled Cluster Singles Doubles Triples with perturbative quadruples (CCSDT(Q)).
• Extensive basis sets up to augmented quadruple zeta were employed for accurate geometry and energy calculations.
• Vibrational Perturbation Theory 2 (VPT2) was used to compute fundamental frequencies for neutral and anionic Al2H2 isomers.

Main Results:

• Confirmed the butterfly global minimum structure of Al2H2 and its experimental IR assignments, reconciling theory and experiment.
• Identified a critical b2 symmetry Al-H stretching frequency at 1096 cm⁻¹, matching an unassigned experimental feature at 1090 cm⁻¹.
• Calculations indicate the linear Al2H2 structure is not an equilibrium geometry, challenging previous assignments.
• Characterized two anionic Al2H2 structures (butterfly ground state and monobridged local minimum) for the first time.

Conclusions:

• The study successfully bridges the gap between theory and experiment for Al2H2, confirming its butterfly structure.
• The identified vibrational frequencies provide a clear signature for experimental detection and verification.
• Anionic Al2H2 structures present promising targets for future mass-selected gas-phase spectroscopy.