Schematic representation of the vibrationally induced reactions expected for bare (top) and Ar solvated (bottom) CH3NO2 −·CH3I complex ions (see text).
Photodissociation action spectra of CH3NO2 −·CH3I (upper trace) and CH3NO2 −·CD3I (lower trace) obtained by monitoring the formation of I− photoproducts (see text). The dashed lines mark CH stretching vibrational modes attributed to the CH3I component of the complex.
Calculated conformations of [CH3NO2, CH3, I]− (see text).
Comparison of the experimental spectrum of CH3NO2 −·CH3I·Ar (top) and simulated spectra for conformations I (center) and II (bottom). Note that the peak at ∼2950 cm−1 for conformation I contains two close-lying calculated bands (see Table I ), which gives a good account for the splitting of the experimentally observed band in this region.
Change in photofragmentation channels in CH3NO2 −·CH3I·Ar n from charge transfer (red) to Ar evaporation (blue) with increasing Ar solvation (see text). The number of Ar atoms, n, is shown in each panel.
Schematic representation of the potential energy surface along the reaction coordinate of dissociative electron transfer from nitromethane anion to methyl iodide (energies are to scale). (a) Adiabatic electron affinity of nitromethane (172 meV, from Ref. 17 ). (b) Binding energy of the CH3NO2 −·CH3I entrance channel complex (490 meV, calculated for conformation I, this work). (c) Estimated lower limit of the barrier height towards electron transfer (230 meV, experimental from Ar solvation data, this work). (d) Relative energy of the exit channel complex; the solid line shows I−·CH3NO2·CH3 · (conformation III, calculated at 500 meV, this work), the dotted line represents the I−·CH3NO2CH3 · SN2 product (conformation IV, calculated at ∼1150 meV, this work). (e) Exit channel asymptote to form I− and neutral products for the two possible exit channel complexes; the full line shows the I− + CH3NO2 + CH3 · asymptote, based on the heat of reaction for dissociative electron attachment to CH3I (620 meV, from Ref. 21 ), the dotted line represents I− + CH3NO2CH3 · asymptote (calculated at ∼1550 meV, this work).
Calculated potential energy curve (top) and charge on the iodine atom (bottom) as a function of the C–I bond distance. The full squares in the top trace are calculated with only the C–I bond distance fixed, letting all other coordinates relax. The open diamonds represent calculations where the out-of-plane angle of the methyl group of the CH3I molecule (θ oop ) was fixed at 10.0°, corresponding to the geometry at a C–I bond distance of ∼2.575 Å in the unrestricted calculation. The open squares are data points calculated with the same angle fixed at 5.5°, corresponding to the geometry at 2.625 Å C–I bond distance in the unrestricted calculation. We note that θ oop changes very rapidly as the transition state is approached. The resulting approximated diabatic potential energy curves are shown as dotted lines in the inset of the top panel, the structures shown as insets from left to right represent the entrance channel, transition state, and exit channel geometries, respectively.
Peak positions (in cm−1) for the dominant vibrational features in CH3NO2 −·CH3I; experimental values for the CH3NO3 −·CD3I complex are given in parentheses where observed; calculated (scaled harmonic approximation) values are given for the all-H complex only, with calculated intensities (in km/mol). Vibrational assignments are based on structure I in Figure 3 .
Article metrics loading...
Full text loading...