Study of low-energy electron-induced dissociation of 1-Propanol

The fragmentation of 1-propanol resulting from dissociative electron attachment has been explored across an energy range of 3.5 to 16 eV. Four distinct ion species are identified: $\text{H}^{-}$, $\text{O}^{-}$, $\text{OH}^{-}$, and $\text{C}{3}\text{H}{7}\text{O}^{-}$. The $\text{OH}^{-}$ ion exhibited a prominent peak near 8.7 eV, along with a small hump near 5.6 eV. Complementary channels led to the formation of the $\text{H}^{-}$ and $\text{C}{3}\text{H}{7}\text{O}^{-}$ ions. Both these two ions exhibit a sharp peak near 6 eV and broad overlapping resonances between 7 to 12 eV. The observed ion yields of distinct dissociation fragments in this study, when compared with those from previously studied alcohols, suggest site-specific fragmentation of alcohols during dissociative electron attachment. To gain a deeper understanding of the dissociation pathways, Density Functional Theory~(DFT) calculations were conducted, revealing the threshold energies for each channel. These threshold energies aligned well with the experimental uncertainties.

💡 Research Summary

This paper presents a comprehensive investigation of dissociative electron attachment (DEA) to 1‑propanol (CH₃CH₂CH₂OH) over the electron‑energy range of 3.5–16 eV. Using a segmented time‑of‑flight (TOF) mass spectrometer equipped with a pulsed, magnetically collimated electron beam (10 kHz repetition, 200 ns pulse width, ~0.8 eV energy resolution), the authors recorded the yields of four anionic fragments: H⁻ (1 amu), O⁻ (16 amu), OH⁻ (17 amu), and C₃H₇O⁻ (59 amu). The experimental setup was refined compared with earlier work, notably by extending the flight tube and adding additional einzel lenses, which improved ion‑collection efficiency, especially for the low‑mass H⁻ ion.

The ion‑yield curves reveal distinct resonance features. OH⁻ shows a dominant broad maximum at 8.7 eV together with a smaller hump at 5.6 eV. H⁻ and C₃H₇O⁻ each display a sharp peak near 6 eV and a broad, overlapping structure spanning roughly 7–12 eV; fitting the H⁻ yield with three Gaussian components yields peaks at 6.5, 8.7, and 10.9 eV. The rise in all yields above ~13.5 eV is attributed to ion‑pair dissociation. The detection of O⁻, absent in earlier DEA studies on propanol, underscores the increased sensitivity of the present apparatus.

To interpret the observed channels, the authors performed quantum‑chemical calculations at the B3LYP/aug‑cc‑pVTZ level using Gaussian 16. They optimized neutral and anionic fragments, computed zero‑point‑energy‑corrected electronic energies, and derived thermodynamic thresholds for each dissociation pathway. For H⁻ formation, two two‑body channels were considered: (1) cleavage of the O‑H bond yielding H⁻ + CH₃CH₂CH₂O (threshold 3.33 eV) and (2) cleavage of a C‑H bond yielding H⁻ + CH₂CH₂CH₂OH (threshold 3.30 eV). A three‑body channel (H⁻ + CH₂CH₂CH₂O + H) has a higher threshold of 5.31 eV. For OH⁻ formation, the primary route is C‑O bond rupture, giving OH⁻ + CH₃CH₂CH₂ (threshold ≈1.9 eV), with an alternative pathway involving loss of a hydrogen atom (threshold ≈2.45 eV). Calculated thresholds agree with experimental peak positions within 0.1–0.3 eV, confirming that the observed resonances correspond to Feshbach‑type temporary negative ion (TNI) states that decay by bond cleavage.

The authors compare these findings with DEA studies on smaller alcohols (methanol, ethanol). The 6 eV H⁻ resonance is essentially invariant across the series, indicating that O‑H bond cleavage is governed by a similar Feshbach resonance regardless of carbon chain length. In contrast, the broader resonance near 9–10 eV grows in intensity with longer chains, reflecting the increasing contribution of C‑H and C‑C σ* orbitals to the TNI manifold. The detection of O⁻ in 1‑propanol, absent in earlier work, suggests that experimental sensitivity plays a crucial role in observing low‑cross‑section channels.

From an applied perspective, 1‑propanol is a promising bio‑fuel additive: it possesses a higher octane rating (108 AKI) and greater energy density (24 MJ L⁻¹) than ethanol. Understanding its DEA behavior is relevant for plasma‑assisted ignition, fuel‑injector ionization, and high‑temperature combustion environments where low‑energy electrons are abundant. The facile O‑H bond cleavage at ~6 eV implies that even modest electron fluxes can generate H⁻ and OH⁻ radicals, potentially influencing flame chemistry, radical pool dynamics, and pollutant formation.

The paper acknowledges experimental limitations: the 0.8 eV electron‑energy spread prevents full resolution of closely spaced resonances, and the ion‑collection efficiency for H⁻ remains sub‑optimal despite improvements. Theoretical limitations are also noted; while DFT captures thermodynamic thresholds well, a more rigorous treatment of electron correlation and non‑adiabatic dynamics (e.g., multi‑reference methods or time‑dependent wave‑packet simulations) would be required to fully map the potential energy surfaces of the transient negative ions.

In summary, this work delivers the first detailed DEA cross‑section data for 1‑propanol, identifies four anionic fragments with their respective resonance structures, validates the assignments with high‑level DFT calculations, and situates the results within the broader context of alcohol chemistry and fuel‑related plasma processes. The data provide a valuable benchmark for future theoretical modeling and for the design of combustion systems where electron‑induced chemistry plays a role.