Ab initio molecular dynamics study for C–Br dissociation within BrHC–C≡C adsorbed on an Ag(111) surface: a short-time Fourier transform approach

The reaction dynamics for C–Br dissociation within BrHC–C≡CH adsorbed on an Ag(111) surface has been investigated by combining density functional theory-based molecular dynamics simulations with short-time Fourier transform (STFT) analysis of the dipole moment autocorrelation function. Two possible reaction pathways for C–Br scission within BrHC–C≡CH have been proposed on the basis of different initial structural models. Firstly, the initial perpendicular orientation of adsorbed BrHC–C≡CH with a stronger C–Br bond will undergo dynamic rotation leading to the final parallel orientation of BrHC–C≡CH to cause the C–Br scission, namely, an indirect dissociation pathway. Secondly, the initial parallel orientation of adsorbed BrHC–C≡C with a weaker C–Br bond will directly cause the C–Br scission within BrHC–C≡CH, namely, a direct dissociation pathway. To further investigate the evolution of different vibrational modes of BrHC–C≡CH along these two reaction pathways, the STFT analysis is performed to illustrate that the infrared (IR) active peaks of BrHC–C≡CH such as vCH [2956 cm(s) and 3020 cm(as)], v≡CH (3320 cm) and vC≡C (2150 cm) gradually vanish as the rupture of C–Br bond occurs and then the resulting IR active peaks such as C=C=C (1812 cm), ω-CH (780 cm) and δ-CH (894 cm) appear due to the formation of HC=C=CH which are in a good agreement with experimental reflection adsorption infrared spectrum (RAIRS) at temperatures of 110 and 200 K, respectively. Finally, the total energy profiles indicate that the reaction barriers for the scission of C–Br within BrHC–C≡CH along both direct and indirect dissociation pathways are very close due to a similar rupture of C–Br bond leading to a similar transition state.