Towards eco-friendly biofuel design: Atmospheric transformation and divergent ecological risk assessment of dimethylbutanol isomers

Branched saturated higher alcohols such as 3,3-dimethyl-1-butanol (3,3DM1ButOH) and 3,3-dimethyl-2-butanol (3,3DM2ButOH) are promising oxygenated biofuel additives, yet their atmospheric transformation pathways and associated ecological risks remain poorly constrained. Here, we integrate density functional theory with multi-structural canonical variational transition state theory including small-curvature tunneling to elucidate the ·OH-initiated oxidation mechanisms and kinetics of both isomers over atmospherically relevant conditions. Four distinct H-abstraction channels are identified for each alcohol, with reactivity governed by hydroxyl position: the β -secondary H in 3,3DM1ButOH and the α-carbinol H in 3,3DM2ButOH dominate abstraction. At 298 K, computed total rate coefficients—5.29 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ (3,3DM1ButOH) and 1.07 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (3,3DM2ButOH)—agree well with experimental data and reveal that 3,3DM2ButOH reacts nearly twice as fast due to its activated α-hydrogen. Subsequent peroxy radical (RO₂·) chemistry is mapped under varying NO_x regimes: RO₂· + NO dominates in polluted environments, yielding multifunctional carbonyls (e.g., aldehydes, enols, ketones) and organic peroxynitrite (ROONO), while RO₂· + HO₂· and RO₂· + RO₂· pathways under low-NO_x conditions produce hydroperoxides (ROOH) and dimeric products. Critically, environmental risk assessment using EPA-based toxicity models reveals divergent hazard trajectories: H-shift-driven autoxidation channels amplify developmental and aquatic toxicity, whereas radical termination via RO₂· + HO₂·/RO₂· yields readily biodegradable, low-toxicity products. The RO₂· + NO route exhibits mixed risk outcomes. Overall, low-NO_x conditions mitigate integrated ecological risk despite comparable degradation rates. These findings bridge atmospheric chemical mechanisms with ecological risk assessment, providing a scientific foundation for the development of eco-friendlier biofuel additives with reduced environmental hazards.