They propose pentafluorobenzylpyridinium, with an E0 value of 2.95 eV calculated at the CCSD(T)/6-311++G(d,p)//M06-2X-D3/6-311++G(d,p) level of theory. Its dissociation mechanism is shown in Figure . The C7HF4 + fragment observed at m/z 161 is minor compared to the F5-Bz + fragment and should be summed up with the latter. It does not introduce errors in the survival yield values. There were also attempts to determine E0 values experimentally . The most accurate first-principles experimental determination of E0 values and activation entropy were obtained using guided ion beam tandem mass spectrometer (GIBMS, results in Table ) . The four measured E0 values agree with the calculations of DeBord, and with E0 calculations at the MP2(full)/6-311+G(2d,2p) level with correlation of all electrons and with counterpoise correction for the basis set superposition error . The GIBMS study confirms that the hypothesis of the loose transition state is valid. In its supporting information, the same paper also provides frequency sets for the precursor ion and for the transition state of p-OCH3, p-CH3, p-H and p-NO2 benzylpyridiniums that match the activation entropies but are derived from a variational study and not by a rough scaling of the precursor ion frequencies. These frequency lists are precious for RRKM calculations, which are necessary to calculate kinetic shifts, use the characteristic temperature method, or for any type of master equation modelling.