## 3 Results
### Unimolecular Reactions of Proton-Bound Solvent-MF Clusters  
The most common primary reaction products discussed above are the proton-bound complexes between MF and a solvent molecule. We were able to independently generate MF(W)H\({}^{+}\), MF(M)H\({}^{+}\), MF(E)H\({}^{+}\), MF(M)\({}_{2}\)H\({}^{+}\), MF(E)\({}_{2}\)H\({}^{+}\), and MF(E)\({}_{3}\)H\({}^{+}\) in the electrospray source and obtain CID breakdown curves for each. They are shown in Figures 3 and 4, along with initial assignments. A common dissociation product is protonated methyl formate as expected from the ion-molecule reaction results in Figure 1. We have previously investigated the unimolecular chemistry of MFH\({}^{+}\), which leads to the formation of \(m/z\) 33 (loss of CO to form protonated methanol) [25]. In the case of MF(M)H\({}^{+}\), a competing loss of CH\({}_{4}\) is observed, which could form either a proton-bound complex between M and CO\({}_{2}\), M(CO\({}_{2}\))H\({}^{+}\), or MF and OH, MF(OH)\({}^{+}\). For MF(E)H\({}^{+}\), the similarity between the proton affinities (PAs) of MF (782.5 kJ mol\({}^{-1}\)) and E (776.4 kJ mol\({}^{-1}\)) [26] leads to competing cleavage of both sides of the hydrogen bond, Figure 3c. The MF(M)\({}_{2}\)H\({}^{+}\) cluster ion dissociates by sequential loss of both M molecules. A minor channel involves the loss of MF from the initially formed MF(M)H\({}^{+}\) product ion, Figure 4a. As was observed with the MF(E)H\({}^{+}\) dimer ion, the MF(E)\({}_{2}\)H\({}^{+}\) trimer ion exhibits competitive loss of E and MF, with the tetrameric MF(E)\({}_{3}\)H\({}^{+}\) ion undergoing sequential E loss. Evident from the breakdown curves is that these cluster ions undergo facile decomposition. In each case (except MF(E)\({}_{2}\)H\({}^{+}\)) there is significant dissociation between 0 and 0.5 eV E\({}_{\text{COM}}\). Thus, even under the high-pressure limit of 1 atm, these ions can decompose. The primary unimolecular reaction of these complexes is the simple dissociation of a hydrogen bond. Relaxed potential energy scans of this dissociation for two examples,MF(W)H\({}^{+}\) and MF(E)\({}_{3}\)H\({}^{+}\), were carried out to explore the potential for reverse activation barriers due to the possible structural rearrangement of the complexes, Figure 5. Evident from both scans is that these encounter complexes can easily fragment without a reverse barrier, which means that the relative product energies in Figure 2 direct the relative abundance of the competing reactions. Unique among the clusters is MF(M)H\({}^{+}\), which undergoes a loss of CH\({}_{4}\). This reaction was explored computationally, and the reaction pathway is shown in Figure 6 in comparison to the simple loss of methanol (and subsequent dissociation of MFH\({}^{+}\)). The MF methyl group first pulls away from the complex leading to the TS at 1.98 eV. It then interacts with the carbon H atom on the central HOC(H)O moiety leading to a molecule of methane. Due to the low binding affinity of the methane to the resulting ionic complex, it is very mobile and can wander the skeleton. The methane complex shown in Figure 6 at \(-\)0.49 eV only has a binding energy of 0.2 eV. The competing loss of methanol via a barrierless H-bond cleavage results in MFH\({}^{+}\), which is due to the higher proton affinity of methyl formate (782.5 kJ mol\({}^{-1}\)) compared to methanol (754.3 kJ mol\({}^{-1}\)) [25], having an energy requirement of 1.25 eV. The MFH\({}^{+}\) then dissociates by loss of CO [25]. The barrier heights to CH\({}_{4}\) loss vs. M loss are consistent with the breakdown diagram that shows M loss to be the lower energy-threshold pathway. Figure 5: Relaxed potential energy scan of the dissociation of **(a)** MFWH\({}^{+}\) and **(b)** MF(E)\({}_{3}\)H\({}^{+}\) at the B3 LYP/6-311+G(d,p) level of theory.