## 3 Results  
### Solvent Cluster Ion/Methyl Formate Reactions  
Representative mass spectra resulting from the ion-molecule reactions R1-R5 of proton-bound solvent clusters and neutral methyl formate (MF) are shown in Figure 1. Water = W, methanol = M, ethanol = E, and methyl formate = MF.  
The mass spectra recorded from the reaction between methyl formate and water dimer ion (R1) showed several peaks: _m/z_ 79, which represent the proton-bound water-methyl formate complex; _m/z_ 61, which would nominally be protonated methyl formate, and _m/z_ 19 ion due to WH\({}^{+}\). The overall reaction scheme is summarized in Figure 2a at the B3 LYP/6-311+G(d,p) level of theory. Formation of the encounter complex is exothermic by 1.31 eV, and the resulting loss of W exothermic by 0.53 eV (relative to the reactants), which means the encounter complex is not observed in the experiment. Based on the relative energies, WH\({}^{+}\) is most likely to arise from the dissociation of the initial (W)\({}_{2}\)H\({}^{+}\) complex, as it would not be competitive to generate it from MF(W)H\({}^{+}\).  
Figure 1: Representative mass spectra of neutral methyl formate reacting with (**a**) water dimer ion (_m/z_ 37, R1), (**b**) water trimer ion (_m/z_ 55, R2), (**c**) methanol dimer ion (_m/z_ 65, R3), (**d**) ethanol dimer ion (_m/z_ 93, R4), and (**e**) ethanol trimer ion (_m/z_ 139, R5). The red asterisk highlights the reacting cluster ion.  
Similar peaks were observed in the ion-molecule reactions with the proton-bound water trimer ions (\(m\)/\(\mathrm{\SIUnitSymbolZ}\) 55) and from all of the solvent cluster ion reactions, Figure 1. In each case the initial encounter complex was not observed because the low binding-energy (Figure 2) results in a high enough rate constant for dissociation that there is not enough time or pressure to collisionally stabilize the complex (pressure in the collision cell, monitored at the inlet to the cell, was only 10\({}^{-4}\) mbar). Evident from the computational results summarized in Figure 2 is that the highest binding-energy encounter complex is MF(E)\({}_{2}\)H\({}^{+}\), 0.89 eV, but this is still not enough to allow it to be observed. RRKM calculations were performed for the solvent-loss reaction from each encounter complex, and the resulting k(E) vs. E curves are compared in Figure S1. For example, the calculated dissociation energy for the MF(W)\({}_{2}\)H\({}^{+}\) complex was 0.78 eV. The rate constant for the dissociation is greater than \(1\times 10^{6}\) s\({}^{-1}\), only 0.2 eV above this threshold. In a thermal system, the encounter complexes have an internal energy distribution that extends to between 0.60 and 1.00 eV (Figure S2), which means that they would be stable species at atmospheric pressure.  
Also observed in the reactive mass spectra, aside from those shown in Figure 2, are secondary reactions of protonated MF and dissociation products of the solvent cluster ions themselves (Table S1).  
Figure 2: Relative energy (B3 LYP/6-311+G(d,p)) of the major product ions resulting from the reaction between neutral methyl formate with (**a**) the water cluster dimer and (**b**) the water cluster trimer, (**c**) the methanol dimer, (**d**) the ethanol dimer and (**e**) the ethanol trimer.