Neutral MF occurs as syn and anti conformers (also denoted Z and E rotamers) with Cs symmetry, with the syn rotamer being more stable (by ∼20 kJ mol−1) and well separated from the anti rotamer by a significant energy barrier of ∼35 kJ mol−1 for anti 





 syn isomerization (Wilmshurst 1957; Curl 1959; Blom & Günthard 1981; Müller et al. 1983; Bagno & Scorrano 1996; Neill et al. 2011, 2012; Zeegers-Huyskens & Kryachko 2011; Ferro-Costas & Mosquera 2013). The zero-kinetic-energy photoelectron spectrum of MF provides an accurate estimate for its adiabatic ionization energy and vibrational frequencies of the radical cation ground state (Waterstradt et al. 1994). The electronic excitation spectrum of MF is very similar to that of acetic acid featuring three valence transitions (Nunes et al. 2010). In contrast to MF and its radical cation (MF+), only very limited information is available for H+MF. Theoretical studies indicate the preference for protonation at the carbonyl oxygen (CO) over the methyl ester (OMe) by around 80 kJ mol−1, and the CO-protonated tautomer has four rotamers within 20 kJ mol−1 (Zeegers-Huyskens & Kryachko 2011; Ferro-Costas & Mosquera 2013). Numerous mass spectrometric studies have investigated the production of H+MF via various ion–molecule reactions and its fragmentation by metastable decay and collisions (Harrison & Tsang 1976; Benoit & Harrison 1977; Hopkinson et al. 1979; van Baar et al. 1986; Horn et al. 2004; Lawson et al. 2012), and the measured proton affinity (PA) is reported as PA = 782.5 kJ mol−1 (Hunter & Lias 1998). However, the site of protonation and the rotational conformation of H+MF could not be established from these mass spectrometric measurements. The dominant fragment channel of H+MF is decarbonylation (loss of CO), whereby the C and O atoms come from the CO group. Significantly, there are no spectroscopic data available for H+MF and its clusters. To this end, our combined spectroscopic and DFT approach provides the first reliable experimental evidence for the preferred protonation site in the isolated molecule and the isomer assignment along with the intermolecular interaction of this prototypical protonated aliphatic ester with nonpolar ligands. There are a few experimental data for H+MF in the condensed phase. However, it is well known that solvation and counter ions can have a drastic effect on both the position and energetics of protonation in the various possible tautomers and rotamers. Previous 1H-NMR spectra of H+MF revealed three OH+ resonances in a super acid solution, demonstrating protonation at the CO group with three rotamers (although their configurations could not be established) (Fraenkel 1961; Olah et al. 1967). IR, Raman, and 1H/13C-NMR spectra of single crystal salts of H+MF with 





 and 





 show only a single isomer (the most stable (ts) conformer), which forms a strong ionic hydrogen bond (H band) between the OH+ group and the anion (Minkwitz et al. 2000). The characteristic OH stretch frequency is thus largely redshifted and thus not identified in both the IR and Raman spectra of the salts. Also, the IR intensities of the CH stretches are too weak to be observed.
Citation: Lawson et al. 2012
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