Unlocking n-alk-1-ynes conformers: Quantum "trigger finger" versus "stiff joint" conformations

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• Conventional models simplify n-alk-1-yne (CnA) molecular conformation to an all-planar structure.
• Understanding molecular conformation is crucial for interpreting spectroscopic data and designing molecular electronic devices.

Purpose of the Study:

• To conduct a comprehensive quantum chemical analysis of molecular conformation in n-alk-1-ynes.
• To investigate the existence and energetics of different rotamers at the acetylenic terminus.
• To elucidate the kinetic and thermodynamic factors governing CnA conformation and their implications for molecular electronics.

Main Methods:

• Quantum chemical calculations were employed to analyze the molecular conformation of n-alk-1-ynes.
• The study focused on identifying and characterizing rotamers at the acetylenic terminus.
• Rotational energy barriers and kinetic profiles were computationally investigated.

Main Results:

• Two near-isoenergetic rotamers were identified: a planar (Cs) and a skewed (C1) conformation.
• A high rotational energy barrier (≈150 meV) was observed, arising from steric and electronic factors near the sp center.
• A unique kinetic profile, termed the Quantum "Trigger Finger" (α rotation), leads to a ≈50%:50% Cs/C1 ensemble, contrasting with the alkyl chain's "Stiff Joint" (δ rotation).

Conclusions:

• The conformational degeneracy of n-alk-1-ynes necessitates ensemble averaging for spectroscopic data interpretation.
• The slow interconversion between rotamers allows for kinetic trapping and conformer enrichment.
• This research redefines the understanding of the alkyne anchor, offering a framework for accurate spectroscopic analysis and conformational control in molecular electronics.