Agreement Is Not Assignment: Dynamic Observables and the Limits of Structural Validation

Area of Science:

• Chemical Physics
• Spectroscopy
• Computational Chemistry

Background:

• Interpreting spectroscopic data requires agreement between experimental results and theoretical calculations.
• Agreement alone does not specify the molecular structure probed, which can be an average over vibrations, tunneling, or excited states, not just a single equilibrium geometry.

Purpose of the Study:

• To introduce a diagnostic framework for identifying the specific structural object measured by rotational spectroscopy.
• To differentiate between equilibrium structures and dynamic nuclear motion effects in spectroscopic analysis.

Main Methods:

• Combining high-accuracy ab initio equilibrium structures with perturbative anharmonic corrections.
• Employing one-dimensional variational treatments for low-barrier coordinates.
• Analyzing isotope-resolved experimental-theory residual patterns.

Main Results:

• Improved agreement highlights discrepancies arising from vibrational treatments or structural references, not just electronic structure errors.
• Distinct structural assignment regimes emerge from residual patterns, with specific "residual-collapse factors" indicating the nature of the molecular system.
• Dynamic averaging is necessary for systems like glycine conformer II and malonaldehyde to achieve isotope balance.

Conclusions:

• Agreement in spectroscopy becomes chemically meaningful only after identifying the nuclear-motion regime.
• The framework clarifies structural assignments for various systems, from compact covalent frameworks to noncovalent complexes and radicals.
• This approach is crucial for accurate structural interpretation in high-resolution spectroscopy.