Carbyne Instability is Not a Chemistry Problem: The Phonon Connectivity Deficit and The Path to Ambient-Stable sp-Carbon Chains

For five decades, the failure to synthesize ambient-stable carbyne, the linear-chain allotrope of sp-hybridized carbon, has been attributed to the chemical reactivity of its terminal bonds toward oxygen, moisture, and crosslinking. This work argues that the problem has been incorrectly framed: carbyne instability is fundamentally a kinetics problem of vibrational energy redistribution, not a chemistry problem. A strictly one-dimensional chain lacks the phonon channels needed to disperse vibrational energy fast enough to prevent bond dissociation, even in a chemically inert environment. This reframing yields a precise stability criterion: the rate of vibrational energy redistribution from the reactive stretching modes must exceed the intrinsic bond-dissociation rate. In an isolated chain this rate falls several orders of magnitude short, so engineering the anharmonic coupling between the chain and its environment, rather than relying on chemical end-capping alone, is the rational path to stability. The astronomical spectroscopic record of interstellar and circumstellar carbon chains offers an independently validated archive of environments satisfying this criterion over billions of years of exposure, providing a template for rational design. Recent independent studies spanning several unrelated experimental and theoretical methodologies converge on the same conclusion: physical coupling of a carbyne chain to an external reservoir, not chemical passivation of its ends, governs stability. Building on this reframing, an astro-chemically informed, machine-learning-compatible inverse-design strategy is proposed to translate a target stability requirement directly into structural and environmental design parameters, replacing decades of chemical trial-and-error with a systematic, falsifiable engineering approach toward ambient-stable sp-carbon chains.