Unlocking n-alk-1-ynes Conformers: Quantum "Trigger Finger" versus "Stiff Joint" Conformations

Abstract

Molecular conformation in n-alk-1-ynes (CnA) is conventionally simplified to an all-planar structure. We report a comprehensive quantum chemical analysis revealing two near-isoenergetic rotamers at the acetylenic terminus: planar (C$_s$) and skewed (C$_1$). The high, symmetric rotational energy barrier ($\approx 150$\,meV) arises from unique steric relief near the $\mathrm{sp}$ center coupled with electronic stabilization of C$_1$. This creates a unique kinetic profile: a Quantum ``Trigger Finger'' ($α$ rotation) that enforces an $\approx 50\%:\,50\%$ $\mathrm{C}_s/\mathrm{C}_1$ ensemble, sharply contrasting with the thermodynamically biased ``Stiff Joint'' ($δ$ rotation) of the alkyl chain. This structural degeneracy necessitates ensemble averaging for spectroscopic data interpretation, while the slow interconversion permits kinetic trapping and intentional conformer enrichment during synthesis and molecular junction fabrication. Our work redefines the alkyne anchor, providing a blueprint for accurate interpretation of spectroscopic data and achieving conformational control in molecular electronics.

Figures (3)

• Figure 1: Optimized geometries for 1-decyne (C8A) and fluorinated decyne ($\mathrm{F2-C8A}$). The C atoms defining the $\alpha$- and $\delta$-dihedral angles are highlighted in green. (a, b) Planar (C$_s$) and skewed (C$_1$) $\alpha$-conformers of C8A. (c) Eclipsed transition state ($\alpha \approx 120^\circ$). (d, e) Planar (C$_s$) and skewed (C$_1$) $\alpha$-conformers of $\mathrm{F2-C8A}$. (f) Nonplanar conformer with internal $\mathrm{gauche}$ motif ($\delta \approx 62^\circ$). IUPAC numbering is shown in panel (a).
• Figure 2: Conformational energy profile for the terminal $\alpha$ dihedral angle ($\angle[\ce{C2,C3,C4,C5}]$). Profiles shown are in vacuo for various chain lengths $n$ (a) M06-2X and (b) B3LYP, and (c) M06-2X/IEFPCM for C8A in representative solvents. The consistent, nearly symmetric and $\approx 150$ meV barrier separates the near-isoenergetic planar (C$_s$) and skewed (C$_1$) minima. Solvent effects are negligible.
• Figure 3: Conformational energy profile for the internal $\delta$ dihedral angle ($\angle[\ce{C_{k},C_{k+1},C_{k+2},C_{k+3}}]$) along the alkyl backbone. Profiles shown are in vacuo for various chain lengths $n$ (a) M06-2X and (b) B3LYP, and (c) M06-2X/IEFPCM for C8A in solvents. The profile shows a thermodynamic preference for the $\mathrm{anti}$ (planar) state ($\approx 20$ meV) and an asymmetric barrier ($\approx 110$ meV vs. $\approx 130$ meV), significantly lower than the symmetric $\alpha$-barrier (\ref{['fig:outer-dihedral']}).