RAPID COMMUNICATIONS PHYSICAL REVIEW A 86, 020701(R) (2012) Shape resonance spectra of lignin subunits Eliane M. de Oliveira, 1 Sergio d’A. Sanchez, 2 M´ arcio H. F. Bettega, 2 Alexandra P. P. Natalense, 1 Marco A. P. Lima, 1,3 and M´ arcio T. do N. Varella 4 1 Laborat´ orio Nacional de Ciˆ encia e Tecnologia do Bioetanol (CTBE/CNPEM), CP 6170, 13083-970, Campinas, S˜ ao Paulo, Brazil 2 Departamento de F´ ısica, Universidade Federal do Paran´ a, CP 19044, 81531-990, Curitiba, Paran´ a, Brazil 3 Instituto de F´ ısica “Gleb Wataghin”, Universidade Estadual de Campinas, 13083-859, Campinas, S˜ ao Paulo, Brazil 4 Instituto de F´ ısica, Universidade de S ˜ ao Paulo, CP 66318, 05315-970, S˜ ao Paulo, S˜ ao Paulo, Brazil (Received 17 July 2012; published 20 August 2012) We report integral cross sections for elastic electron scattering by the lignin subunits phenol, guaiacol, and p-coumaryl alcohol. Our calculations employed the Schwinger multichannel method with pseudopotentials and indicate three to four π ∗ shape resonances for each of these systems, suggesting that low-energy electrons could efficiently transfer energy into the lignin matrix. We also discuss dissociation mechanisms based on the calculated cross sections, available experimental data, virtual orbital analysis, and the knowledge on electron interactions with biomolecules. Our results point out a physical-chemical basis for electron-driven biomass delignification. The latter would be an essential step for efficient biofuel production from lignocellulosic materials. DOI: 10.1103/PhysRevA.86.020701 PACS number(s): 34.80.Bm Replacing fossil fuels for biofuels from renewable sources is a viable way to reduce greenhouse gas emissions. A success- ful example is the large-scale use of sugarcane ethanol to power light vehicles, especially after the development of flex-fuel en- gines that can run on any mixture of gasoline and ethanol [1]. A major goal to optimize biofuel production, either ethanol [2] or butanol [3], would be the development of high-yield methods to obtain fermentable sugars from lignocellulosic biomass, e.g., leaves, straw, and bagasse. Even though the cellulose basic unit is a fermentable sugar (β -D-glucose), lignocellulose is a composite material resistant to chemical or enzymatic hydrolysis. A dense hydrogen bonding network stabilizes the cellulose crystals that pack into fibers (∼45% of lignocel- lulose content), which are tightly embedded within hemicellu- lose (∼30% content) and lignin (∼25% content) [4]. Two key aspects regarding the biofuel industry would thus be biomass pretreatment technologies, namely bio- or physical-chemical processes that can improve the efficiency of subsequent hy- drolysis, and the biorefinery concept, i.e., using the feedstock to produce high-value chemicals [5–7] along with biofuels. Several pretreatment strategies have been proposed to expose cellulose chains, such as steam explosion, alkaline hydrolysis, and organosolv processes, among others [8]. Alter- native technologies could be based on low-cost atmospheric- pressure plasmas [9–11] as the reactive species generated in discharge environments can increase the surface energy of cellulose and lignin films [10], and even allow for the real-time control of biomass delignification [11]. These promising results also draw attention to electron interactions with lignocellulose components. Free electrons can reach the substrate in atmospheric-pressure dielectric barrier discharge apparatuses [12], and low-pressure plasmas have long been applied in industrial processes, e.g., polymer surface modi- fication [13]. Low-energy electrons (20 eV) are known to induce dissociative processes that damage biomolecules either in gas or condensed phase [14,15]. The underlying mechanism is the formation of transient negative ions (resonances), since energy can efficiently be transferred into nuclear degrees of freedom upon electron attachment, leading to significant vibration excitation and dissociation. Much of the current knowledge on electron-driven DNA damage was gained from studies on subunits, such as bases and sugars, as the attachment occurs in specific sites of the chain [14,16–18]. At low energies, the dominant mechanism for dissociative electron attachment (DEA) would involve shape resonances, i.e., anion states formed by adding an elec- tron to the molecule ground state (into virtual orbitals) [19,20], specifically, the formation of long-lived π ∗ anions (attachment to π ∗ virtual orbitals) diabatically coupled to dissociative σ ∗ anions arising from antibonding virtual orbitals localized on polar bonds. From these facts, lignin would be expected to play an essential role in electron interactions with ligno- cellulose. While cellulose and hemicellulose are saturated polysaccharides, lignin is an aromatic copolymer that can give rise to long-lived π ∗ resonances. The lignin monomers (monolignols), namely, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, are derived from phenol and differ in the degree of methoxylation, such that polar σ OH and σ CO bonds are also abundant. This Rapid Communication surveys the low-energy shape resonance spectra of the lignin components phenol, guaiacol, and p-coumaryl alcohol (shown in Fig. 1), as obtained from elastic electron scattering cross sections. Based on well-known results for biomolecules, these subunits would be expected to provide essential information on electron-induced damage. Results for other lignin monomers, to be published elsewhere, also indicate that relevant aspects of the collision dynamics can be learned from the systems addressed here. The reported results provide insight into fundamental electron-transfer processes that might be of help for biomass delignification, a key pretreatment step that can yield value-added chemicals [6], and they will hopefully motivate other groups to further investigate electron interactions with lignocellulose. Integral cross sections (ICSs) were obtained with the par- allel version [21] of the Schwinger multichannel method with pseudopotentials (SMCPP) [22]. This variational approach to the T -matrix was discussed in detail elsewhere [21,22] and relies on a discrete trial set to expand the scattering state. The present calculations were performed in two approxima- tions, namely, static-exchange (SE) and static-exchange plus 020701-1 1050-2947/2012/86(2)/020701(4) ©2012 American Physical Society