Imaging Reaction Dynamics of F–(H2O) and Cl–(H2O) with CH3I

The dynamics of microhydrated nucleophilic substitution reactions have been studied using crossed beam velocity map imaging experiments and quasiclassical trajectory simulations at different collision energies between 0.3 and 2.6 eV. For F–(H2O) reacting with CH3I, a small fraction of hydrated product ions I–(H2O) is observed at low collision energies. This product, as well as the dominant I–, is formed predominantly through indirect reaction mechanisms. In contrast, a much smaller indirect fraction is determined for the unsolvated reaction. At the largest studied collision energies, the solvated reaction is found to also occur via a direct rebound mechanism. The measured product angular distributions exhibit an overall good agreement with the simulated angular distributions. Besides nucleophilic substitution, also ligand exchange reactions forming F–(CH3I) and, at high collision energies, proton transfer reactions are detected. The differential scattering images reveal that the Cl–(H2O) + CH3I reaction also proceeds predominantly via indirect reaction mechanisms.


Low collision energy
During F -(H 2 O) preparation, full mass separation could not be achieved and all integer masses from 35 to 40 amu have been observed, that is FH n Oand H n F -2 for n = 0, 1, 2.
Reactant compositions for three measurements a (148 000 counts for ion images), b (strong FHO -, 5600 counts) and c (strong HF -2 , 5600 counts) relative to 100 parts F -(H 2 O) are listed in Table S2.1. Ion beam characterizations were performed every few hours and averaged.
Acceleration behind the trap results in partial mass separation and tabulated values reflect the observed composition at the moment of product ion imaging during reactive scattering.
The ratios are therefore proportional to true fractions with a bias towards heavier masses as the light and fast species earlier in the beam contribute to the observed reactivity.  Observed product mass spectra are presented in Fig. S2.1 a and quantified in Table S2.2.
The broad tail of apparently higher masses to 142.1 amu is attributed to Iexcept for the HCIflux at 140 amu. It mainly arises from decomposition of long living FCH 3 Iintermediates as discussed in the main text. In principal, the contribution of specific reactants to the mass spectrum can be removed by combining observations with different reactant composition as with observed normalized branchings O x i of the i'th product and derived product branchings B i (ideally originating from a single reactant). If one or two specific product branchings B i vanish in the absence of one or two reactants in question, the condition B i = 0 determines the coefficients (r 1 , r 2 , 0) or (r 1 , r 2 , r 3 ) to remove their contribution by combining two or three S2.2 data sets. Coefficients r i and resulting product branching ratios in Table S2.2 correspond to the derived mass spectra in Fig. S2.1 b. In the following, the relevance of the different reactants to the observed data is discussed. We exclude FOand H 2 F -2 that are mostly absent in the reactant composition associated with the main data set a.
A strong presence of FHOin data set b gives rise to HCIand FIOHformation at the cost of Iand FCH 3 I -. The obvious condition to remove the F -(H 2 O) contribution from the FHOdata is the disappearance of the I -(H 2 O) product in its absence. However, as the mass resolution does not allow to unambiguously separate I -(HO) and I -(H 2 O), the subtraction leads to erroneously negative estimates of the FCH 3 Iproduct branching. We take this as evidence that FCH 3 Iformation from FHOis negligible to derive the "b − a" data in Table S2 involves covalent bond cleavage such that a high reaction barrier is expected. The resulting "a − b" data may be affected by F -2 and HF -2 as will be further discussed. Recovery of the observed data set a from the derived product branching ratios "b − a" and "a − b" with the coefficients in Table S2.2 implies a 6(3) % contribution of FHOto the observed products.   The ratio of these contributions to the reactant composition of data set a in Table S2.1 (with an uncorrected bias towards heavier masses) is proportional to the rate constants.

Higher collision energies
The reactions F -+ CH 3 I and F -(H 2 O) + CH 3 I have been measured at four different collision energies ranging from 0.3 to 2.6 eV to allow for a direct comparison of experimental data under similar conditions. Product mass spectra are presented in Fig. S2    the IOproduct in the same order of magnitude while the CH 2 Iproduct branching remains unchanged. At the same time, the HF -2 fraction was suppressed by more than a factor of three and proton transfer to HF -2 is energetically at the edge. We therefore attribute proton transfer to the F -(H 2 O) + CH 3 I reaction.