Cyanoacetylene adsorption on amorphous and crystalline water ice films: Instigation through matrix isolation and quantum study

The structure and energy properties of the 1:1 complexes formed between cya noacetylene and H2O (D2O) are investigated using FT-IR matrix isolation spe ctroscopy and ab initio calculations at the MP2/6-31G(d,p) level. Cyanoacet ylene adsorption and desorption on amorphous ice film are monitored by FT-I R using the temperature-programmed desorption method. In an argon matrix, t wo types of 1:1 complexes are observed. The first one corresponds to the NH structure, which involves a hydrogen bond with the terminal nitrogen of cy anoacetylene. The second corresponds to the HO form, which involves a hydro gen bond from the cyanoacetylene to the, oxygen of water. This last complex is the more stable (DeltaE = -8.1 kJ/mol.). As obtained in argon matrixes, two kinds of adsorption site are observed between HC3N and ice. The first one, stable between 25 and 45 K is characterized by a nu (OH) shift similar to the one observed in matrix for the NH complex. The second, stable at hi gher temperatures (between 45 and 110 K), corresponds to an interaction wit h the dangling oxygen site of ice and is similar to the HO complex observed in matrix. From theoretical, calculations (DFT method, combined with a pla ne wave basis set and ultrasoft pseudopotentials), it is shown that, for th is adsorption site, the HC3N moiety is flattened on the ice surface and sta bilized by a long-distance interaction (similar to3 Angstrom) between one d angling OH and the pi system of the C equivalent toC triple bond. The HC3N desorption occurs between 110 and 140 K, and the associated desorption ener gy is 39 kJ/mol. This value is in good agreement with the first principle c alculation, based on density functional theory and ultrasoft pseudopotentia ls (34 kJ/mol). These calculations confirm the electrostatic nature of the interaction forces. A small amount of cyanoacetylene is incorporated into t he bulk and desorbs at the onset of the ice crystallization near 145 K. In these two kinds of experiments, HC3N acts as both an electrophilic and a nu cleophilic molecule.