Infrared spectra of small anionic water clusters from density functional theory and wavefunction theory calculations

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Spectroscopy

Background:

• Anionic water clusters are important in atmospheric and solution chemistry.
• Accurate calculation of infrared (IR) spectra is crucial for understanding molecular properties.
• The choice of theoretical method and basis set significantly impacts spectral accuracy.

Purpose of the Study:

• To investigate the convergence of calculated IR spectra for small anionic water clusters (W/D(-)(N=2-6)).
• To compare the performance of density functional theory (B3LYP) and wavefunction theory (MP2) methods.
• To assess the impact of diffuse functions on the accuracy of IR intensity calculations.

Main Methods:

• Systematic theoretical studies using B3LYP and MP2 levels of theory.
• Analysis of infrared (IR) spectra convergence with increasing diffuse functions.
• Examination of electronic structure and dynamic IR dipole moment behavior.

Main Results:

• MP2 method shows convergence for relative IR intensities with fewer diffuse functions for larger clusters (N=4-6).
• B3LYP method fails to show convergence for relative IR intensities across all cluster sizes.
• Increasing diffuse functions improve B3LYP electronic structure convergence, but it remains inadequate for IR intensity calculations.

Conclusions:

• MP2 is a more reliable method than B3LYP for calculating IR intensities of dipole-bound electron systems.
• The convergence behavior is linked to the electronic structure convergence at the MP2 level.
• Careful selection of theoretical methods is essential for accurate spectroscopic predictions.