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Isolated and solvated chiroptical behavior in conformationally flexible butanamines
Paul M Lemler1,2, Clayton L Craft1,3, Corina H Pollok1,4
1Department of Chemistry, Yale University, New Haven, Connecticut, USA.
Chirality
|August 7, 2023
Summary
This study investigated the optical activity of flexible aliphatic amines, revealing that solvent polarity significantly impacts their chiroptical response. Understanding these effects is crucial for predicting molecular behavior in different environments.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Computational Chemistry
Background:
- Flexible aliphatic amines exhibit complex conformational landscapes.
- Understanding their optical activity is key to predicting molecular behavior.
- Solvent effects can significantly perturb chiroptical properties.
Purpose of the Study:
- To probe the nonresonant optical activity of (2R)-3-methyl-2-butanamine (R-MBA) and (2R)-(3,3)-dimethyl-2-butanamine (R-DMBA).
- To examine the roles of conformational isomerism and solvent perturbations on optical rotatory dispersion (ORD).
- To compare experimental results with theoretical predictions from quantum chemical calculations.
Main Methods:
- Measured optical rotatory dispersion (ORD) in six different solvents.
- Employed density-functional theory (DFT) and coupled-cluster (CC) methods for theoretical calculations.
- Utilized the polarizable continuum model (PCM) for implicit solvation effects.
- Calculated ensemble-averaged ORD profiles using the independent-conformer ansatz.
Main Results:
- Observed uniformly negative rotatory powers for R-MBA and R-DMBA across the 320-590 nm region.
- Chiroptical response magnitude increased with decreasing solvent dielectric constant.
- Vapor-phase specific optical rotations were best reproduced by high-polarity, low-polarizability solvents like acetonitrile and methanol.
- Theoretical models showed reasonable agreement for isolated species but less satisfactory results for solution-phase, especially in polar solvents.
Conclusions:
- Solvent-mediated changes in transition state energies and structures impact the validity of the independent-conformer ansatz in solution.
- The study highlights the challenges in accurately modeling the chiroptical properties of flexible molecules in condensed phases.
- Further refinement of theoretical models is needed to account for complex solvent-solute interactions.
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