Interaction of Acetonitrile with the Surfaces of Amorphous and Crystalline Ice

The adsorption of acetonitrile (CH 3 CN) on ultrathin films of ice under ultrahigh vacuum was investigated with temperature-programmed desorption mass spectrometry (TPD) and Fourier transform infrared reflection absorption spectroscopy (FTIRAS). Two types of film were studied, amorphous and crystalline. On the amorphous films, two states of adsorbed acetonitrile were observed by TPD and FTIRAS. One of the states is attributed to acetonitrile that is hydrogen bonded to a free OH group at the ice surface; the other state is assigned to acetonitrile that is purely physisorbed. Evidence for the hydrogen-bonded state is two-fold. First, there is a large kinetic isotope effect for desorption from H 2 O- and D 2 O-ice: the desorption temperatures from ice- h 2 and ice- d 2 are ∼161 and ∼176 K, respectively. Second, the C⋮N stretching frequency (2265 cm - 1 ) is 16 cm - 1 greater than that of physisorbed acetonitrile, and it is roughly equal to that of acetonitrile which is hydrogen bonded to an OH group at the air−liquid water interface. On the crystalline films, there is no evidence for a hydrogen-bonded state in the TPD spectra. The FTIRAS spectra do show that some hydrogen-bonded acetonitrile is present but at a maximum coverage that is roughly one-sixth of that on the amorphous surface. The difference between the amorphous and crystalline surfaces cannot be attributed to a difference in surface areas. Rather, this work provides additional evidence that the surface chemical properties of amorphous ice are different from those of crystalline ice.