Solvent effects on the thioamide rotational barrier: an experimental and theoretical study

Area of Science:

• Computational Chemistry
• Physical Organic Chemistry
• Spectroscopy

Background:

• Understanding C-N bond rotation is crucial for comprehending molecular structure and reactivity.
• Amide resonance influences rotational barriers, but solvent effects are less understood.
• Thioamides, sulfur analogs of amides, present unique electronic properties affecting rotational dynamics.

Purpose of the Study:

• To investigate the influence of solvent polarity on C-N rotational barriers in N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA).
• To compare experimental NMR data with theoretical ab initio calculations for gas and solution phases.
• To elucidate the relationship between molecular dipole moments, solvent properties, and rotational barrier heights.

Main Methods:

• Selective inversion recovery Nuclear Magnetic Resonance (NMR) spectroscopy to measure rotational barriers.
• Ab initio calculations at the G2(MP2) theoretical level, including corrections for vibrational motions.
• Reaction field theory to model solvation effects and solvent polarity.

Main Results:

• Gas-phase rotational barriers calculated theoretically closely matched experimental values.
• Reaction field theory accurately predicted solution-phase barriers for aprotic, nonaromatic solvents.
• Solvent effects on thioamide rotational barriers were more pronounced than on amides, linked to larger ground-state dipole moments and polarity-dependent dipole moment changes.

Conclusions:

• Solvent polarity plays a significant role in modulating C-N rotational barriers in thioamides.
• The increased barrier in thioamides compared to amides is attributed to their higher ground-state dipole moments and greater dipole moment changes with solvent polarity.
• The study provides insights into the origin of C-N rotational barriers and their connection to amide resonance concepts.