Linear and Helical Carbonic Acid Clusters.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Carbonic acid crystallization is understood to initiate via linear or ribbon-like oligomerization.
• The precise structural motifs governing early-stage crystallization remain an active area of research.

Purpose of the Study:

• To investigate a newly identified helical spiral motif as a competing structure in carbonic acid crystallization.
• To compare the energetic and spectroscopic properties of the helical spiral motif against the established ribbon motif.

Main Methods:

• Utilized combined density functional theory (DFT) and coupled-cluster theory (CC theory) for high-level electronic structure calculations.
• Calculated relative energies, sequential binding energies, and electronic spectra for both ribbon and helical spiral motifs.
• Analyzed spectral features to identify potential experimental discriminators.

Main Results:

• The helical spiral motif exhibits a notable energy divergence from the ribbon structure, approximately 0.2 eV per dimer addition (∼4 kcal/mol).
• Distinct, high-intensity absorption features were identified: 9.16 eV (135 nm) for the ribbon and 7.11 eV (175 nm) for the spiral.
• These spectral differences provide a basis for experimental differentiation.

Conclusions:

• A helical spiral structure represents a viable, competing pathway in carbonic acid crystallization.
• Electronic spectroscopy offers a powerful tool to experimentally distinguish between ribbon and helical spiral motifs.
• This work clarifies early-stage structural preferences in solid carbonic acid formation.