Water solvent effects using continuum and discrete models: The nitromethane molecule, CH3NO2.

Area of Science:

• Computational chemistry
• Theoretical spectroscopy
• Quantum mechanics

Background:

• Nitromethane (CH3NO2) exhibits two dark n → π* and one intense π → π* valence transitions.
• Understanding these electronic transitions is crucial for predicting molecular behavior.

Purpose of the Study:

• To theoretically investigate the gas-phase and solvated electronic transitions of nitromethane conformers.
• To evaluate various computational methods for accurately describing solvation effects on electronic spectra.

Main Methods:

• Employed time-dependent density functional theory (TDDFT), configuration interaction (CIS(D)), symmetry-adapted-cluster CI (SAC-CI), complete active space second order perturbation theory (CASPT2), and algebraic-diagrammatic construction (ADC(2)).
• Utilized polarizable continuum model (PCM), conductor-like screening (COSMO), and sequential quantum mechanics/molecular mechanics (S-QM/MM) for solvation modeling.
• Investigated the impact of explicit water molecules in S-QM/MM, establishing hydrogen bond formation.

Main Results:

• Gas-phase CASPT2, SAC-CI, and ADC(2) methods showed excellent agreement with experimental and theoretical data.
• Continuum models (PCM, COSMO) provided good approximations, with PCM/CASPT2/SAC-CI/B3LYP and COSMO/ADC(2) showing particular promise.
• The S-QM/MM approach with 24 explicit water molecules, using ADC(2), yielded highly accurate results, surpassing TDDFT.

Conclusions:

• The ADC(2) method combined with the S-QM/MM approach offers the best balance of accuracy and computational cost for polar solvent calculations.
• Explicitly modeling the solvation shell is critical for precise electronic transition predictions in polar environments.
• This study provides a robust computational framework for studying solvated molecules.