title: Theoretical model of the interaction of glycine with hydrogenated amorphous carbon (HAC).
abstract: A theoretical model of hydrogenated amorphous carbon (HAC) is developed and applied to study the interaction of glycine with HAC surfaces at astronomical temperatures. Two models with different H content are tried for the HAC surface. The theory is applied at the Density Functional Theory (DFT) level, including a semiempirical dispersion correlation potential, d-DFT or Grimme DFT-D2. The level of theory is tested on glycine adsorption on a Si(001) surface. Crystalline glycine is also studied in its two stable phases, α and β, and the metastable γ phase. For the adsorption on Si or HAC surfaces, molecular glycine is introduced in the neutral and zwitterionic forms, and the most stable configurations are searched. All theoretical predictions are checked against experimental observations. HAC films are prepared by plasma enhanced vapor deposition at room temperature. Glycine is deposited at 20 K into a high vacuum, cold temperature chamber, to simulate astronomical conditions. Adsorption takes place through the acidic group COO(-) and when several glycine molecules are present, they form H-bond chains among them. Comparison between experiments and predictions suggests that a possible way to improve the theoretical model would require the introduction of aliphatic chains or a polycyclic aromatic core. The lack of previous models to study the interaction of amino-acids with HAC surfaces provides a motivation for this work.
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain. vicente.timon@csic.es.
authors.forename: Vicente
authors.initials: V
authors.lastname: Timón
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain. vicente.timon@csic.es.
authors.forename: Óscar
authors.initials: Ó
authors.lastname: Gálvez
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain. vicente.timon@csic.es.
authors.forename: Belén
authors.initials: B
authors.lastname: Maté
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain. vicente.timon@csic.es.
authors.forename: Isabel
authors.initials: I
authors.lastname: Tanarro
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain. vicente.timon@csic.es.
authors.forename: Víctor J
authors.initials: VJ
authors.lastname: Herrero
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain. vicente.timon@csic.es.
authors.forename: Rafael
authors.initials: R
authors.lastname: Escribano
mesh_terms: D000327:Adsorption; D002244:Carbon / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry*; D006860:Hydrogen Bonding; D006865:Hydrogenation; D008962:Models, Theoretical; D012825:Silicon / Q000737:chemistry; D013499:Surface Properties; D013696:Temperature
chemical_list: D002244:Carbon; D005998:Glycine; D012825:Silicon