Research ArticleChemistry

Stretching vibration is a spectator in nucleophilic substitution

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Science Advances  06 Jul 2018:
Vol. 4, no. 7, eaas9544
DOI: 10.1126/sciadv.aas9544


  • Fig. 1 Stationary points along the reaction coordinate of the reaction F + CH3I for the SN2 and the proton transfer reaction channel [taken from (33)].

    Both reaction channels proceed via the same pre-reaction complex, in which the fluoride anion is hydrogen-bonded to an H atom. The transition state of the SN2 is submerged, whereas a barrier exists for the proton transfer. Intrinsic reaction coordinate computations find three interconnected proton transfer pathways. The [FH--CH2I] transition state of the most direct pathway, which is marked in green, is used for the SVP analysis. The relative translational collision energies are marked with the light orange lines, and the total translational and vibrational energy of the CH-stretch excited reactants is marked with the bright orange line. The inset shows a sketch of the scattering experiment with CH3I excited in the symmetric (a1) CH stretching mode at 2971 cm−1 or 0.37 eV.

  • Fig. 2 Time-of-flight traces of the reaction products of the F + CH3I reaction for two different relative collision energies.

    The conversion to atomic mass units (amu) is plotted on the upper horizontal axis. At the lower collision energy, we performed every other ion-molecule crossing with the infrared excitation laser present. The difference signal is plotted in red.

  • Fig. 3 Illustration of the overlap of the overlap of the molecular beam, ion beam, and laser beam and laser beams in the center of the velocity map imaging spectrometer.

    From the overlap integral, we determined the excited fraction of the neutral molecules in the reaction volume.

  • Fig. 4 Differential scattering data for the reaction F + CH3I → CH2I + HF for ground state and vibrationally excited CH3I molecules compared to results from QCT simulations.

    (A) Center-of-mass angle and velocity differential cross sections for CH2I ions at 0.71 eV (top) and 1.17 eV (bottom) collision energy. We obtained the distributions at 0.71 eV collision energy by subtracting the contribution of ground-state reactions from the signal for reactions obtained with CH3I (v1 = 1) vibrationally excited molecules. The red circle indicates the kinematic cutoff. Relative orientations of the velocity vectors are indicated in the Newton diagram. (B) Measured (black line) and simulated (red line) angular distributions are shown for vibrationally excited and ground-state reactants. Both velocity integrated angular distributions show an isotropic component and a small amount of forward and backward scattering. (C) Measured internal energy distributions (black line) for the product species are in good overall agreement with the QCT simulations (red line). The internal energy is expected to be locked up in CH2I, as previous simulations already showed that HF acquired almost no internal excitation (35). We found a similar degree of internal excitation for CH2I for experiments with and without IR excitation.


  • Table 1 Product ion counts with and without vibrational excitation for the SN2 and proton transfer reaction channels at a collision energy of 0.71 eV.
    Counts (IRon)Counts (IRoff)(IRon-IRoff)Rel. differenceRelative rate increase*
    SN2389,726388,5521174 ± 882(0.30 ± 0.23)%0.21 ± 0.16
    Proton transfer1,191911280 ± 46(31 ± 5)%22 ± 4

    *The rate increase is based on an excited fraction of (1.4 ±0.5)% (see the Materials and Methods section for details).

    • Table 2 SVP values for normal modes of CH3I projected onto the reaction coordinates at transition states for the proton transfer and the nucleophilic substitution channel.
      Proton transferNucleophilic substitution
      CI stretch0.220.78
      CH3 rocking0.090.00
      CH3 umbrella0.760.24
      CH3 deformation0.300.00
      CH symmetric stretch0.330.03
      CH asymmetric stretch0.100.00

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