Evolution of the energy landscape for <i>n</i> -alkanes: <i>n</i> -C14H30 to <i>n</i> -C20H42

Abstract

Unbranched n-alkanes provide useful models for investigating how conformational complexity evolves with chain length. In this work, we examine the energy landscapes of n-C14H30 to n-C20H42 with a Δ-corrected transferable many-body permutationally invariant polynomial potential, explored with the Cambridge Energy Landscape software. Basin-hopping global optimization was used to identify low energy minima, and discrete path sampling was employed to construct kinetic transition networks connecting the conformational families. We find that the lowest energy family changes systematically with chain length: extended all-trans structures are most favorable for n-C14H30, hairpin structures are favored for n-C15H32 to n-C17H36, and a more compact folded conformation becomes the global minimum on the MB-PIP surface from n-C18H38 onward. Additional DLPNO-CCSD(T1)-F12 calculations suggest that the hairpin conformer is actually more stable for these longer chains, but the compact conformations are still relevant. Analysis of the pathways connecting representative minima reveals that interconversion proceeds through multistep paths that become increasingly complex with chain length. Disconnectivity graphs further reveal competing low energy funnels, indicating that increasing chain length reorganizes, not only relative stability, but also the underlying landscape. For n-C18H38, we further analyzed first-passage times and heat capacity signatures to characterize the kinetic and thermodynamic features of the multifunnel organization. Relaxation to the global minimum occurs over a broad range of timescales, and a low-temperature heat capacity feature arises from configurational redistribution among competing low energy minima and funnels. Overall, this study provides a detailed picture of how folding behavior, landscape organization, and the associated kinetics and thermodynamics evolve with chain length in n-alkanes.