The ionization dynamics of an aminopyridine dimer (AP)(2) has been investigated by means of the direct ab initio molecular dynamics (MD) method. It was found that the reaction process was composed of three steps after the vertical ionization of (AP)(2): dimer approach, proton transfer and energy relaxation. The timescales of these processes were 50-100, 10-20, and 200 fs, respectively. The timescale of the dimer approach was dependent on the initial separation between AP(+) and AP. After the ionization, AP approached gradually the ionized AP(+). The proton of AP(+) was transferred to AP at the nearest intermolecular distance, while the potential energy was quickly dropped according to the proton transfer. The energy relaxation of the dimer cation was significantly faster than that of the monomer cation. The mechanism of ionization dynamics of (AP)(2) was discussed on the basis of the theoretical results.
The ion-molecule reaction, CH(3)CN(+) + CH(3)CN → CH(3)CNH(+) + CH(2)CN, has been investigated using the threshold electron-secondary ion coincidence (TESICO) technique. Relative reaction cross sections for two microscopic reaction mechanisms, i.e., proton transfer (PT) from the acetonitrile ion CH(3)CN(+) to neutral acetonitrile CH(3)CN and hydrogen atom abstraction (HA) by CH(3)CN(+) from CH(3)CN, have been determined for two low-lying electronic states, (2)E and (2)A(1) of the CH(3)CN(+) primary ion. The cross section for PT of the (2)A(1) state was smaller than that of the (2)E state, whereas that of HA are almost the same in the two states. Ab initio calculations showed that the dissociation of the C-H(+) bond of CH(3)CN(+) is easier in the (2)E state than that in the (2)A(1) state. The direct ab initio molecular dynamics (MD) calculations showed that two mechanisms, direct proton transfer and complex formation, contribute the reaction dynamics.
Abstract. Ab-initio MO calculations have been carried out for hydrogen-dissociation reactions HCNH, i.e. HCNH → H + HCN (I) and HCNH → HNC+H (II), in order to elucidate the branching ratio of HCN/HNC on the ground state potential energy surface. The calculations showed that the transition state for reaction I is lower in energy than that of reaction II. The "bare" barrier heights for channel I and II were calculated to be 33.5-34.8 kcal/mol and 38.5-40.7 kcal/mol, respectively. The energy difference between transition states I and II was calculated to be 3.7-6.9 kcal/mol, meaning that reaction I preferentially occurs in the threshold energy region. Rice-Ramsperger-Kassel-Marcus (RRKM) theory including the tunneling effect indicated that reaction I is more favorable than reaction II at lower energy region, if tunneling effects are included in the rate calculations. On the other hand, the higher energy region above ca. 50 kcal/mol, channel II became dominant. The branching ratio (HCN/HNC) was calculated to be 0.3 at E = 4.4 eV, which corresponds to the electron affinity of HCNH + . The mechanism of the reaction is discussed on the basis of theoretical results.
The fluorescence emission spectra of fluorobenzene, p-difluorobenzene and hexafluorobenzene excited in collisions with electrons were measured for the study of the dynamics of the exciting aromatic system. These spectra showed a band assignable to the S1→S0 transition of the parent molecule. The bands of fluorobenzene and p-difluorobenzene were intense and showed a pronounced vibrational structure and the intense vibrational transitions were optically allowed. Whereas, that of hexafluorobenzene was broad and weak and did not show any clear vibrational structure.
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