Temperature dependence of vibrational modes of CH3CC(ads) and I(ads) coadsorbed on Ag(111): ab initio molecular dynamics approach.

Ab initio molecular dynamics simulations accompanied by a Fourier transform of the dipole moment (aligned perpendicular to the surface) autocorrelation function are implemented to investigate the temperature-dependent infrared (IR) active vibrational modes of CH3C(β)C(α)(ads) and I(ads) when coadsorbed on an Ag(111) surface at 200 and 400 K, respectively. The analytic scheme of the Fourier transform of a structural coordinate autocorrelation function is used to identify two distinguishable IR active peaks of C(β)C(α) stretching, which are characterized by two types of dynamic motion of adsorbed CH3C(β)C(α)(ads) at 200 K, namely, the motion of the tilted CC(β)C(α) axis and the motion of the stand-up CC(β)C(α) axis. These two recognisable IR active peaks of C(β)C(α) stretching are gradually merged into one peak as a result of the dominant motion of the stand-up CC(β)C(α) axis as the temperature increases from 200 to 400 K. The calculated intensities of the IR active peaks of the asymmetrical deformation mode of CH3 and the asymmetrical stretching mode of CH3, with their dynamic dipole moments nearly perpendicular to the CC(β)C(α) axis, become relatively weak; however, the symmetrical deformation mode of CH3 and the symmetrical stretching mode of CH3, with their dynamic dipole moments randomly directed away from the CC(β)C(α) axis, will not have direct correspondence between the intensities of their IR active peaks and the angle between the Ag(111) surface and the CC(β)C(α) axis as the temperature increases from 200 to 400 K. Finally, the increased flipping from the motion of the tilted CC(β)C(α) axis to the motion of the stand-up CC(β)C(α) axis followed by its diffusion, resulting from the increasing temperature from 200 to 400 K or even higher, seems to be the initial event that initiates the alkyne self-coupling reaction that leads to the final production of H3CCCCCCH3.