Gas-phase methyl cation affinities (MCAs) for rare gases Ne, Kr, and Xe were measured with a pulsed electron-beam high-pressure mass spectrometer. The MCAs for Ne and Kr were determined to be 1.2 ± 0.3 and 19.8 ± 2.0 kcal/mol, respectively, by the observation of the clustering reaction, CH3 + + Rg = CH3 +(Rg) (Rg = Ne and Kr). The MCA of Xe was measured to be 2.0 ± 0.6 kcal/mol larger than that of N2 by the observation of the substitution reaction CH3 +(N2) + Xe = CH3 +(Xe) + N2. Based on the MCA of N2 of 44.1 kcal/mol proposed by McMahon et al., the MCA of Xe is determined to be 46.1 ± 0.6 kcal/mol. Molecular orbital calculations at six different levels consistently gave almost identical MCA values for each of the rare gases. At QCISD(T)(full)/6-311++G(2df,p)//B3LYP/6-311++G(d,p), the calculated values (all in kcal/mol) are as follows:  He, 0.6; Ne, 2.2; Ar, 15.9; Kr, 24.1; and N2, 43.2. For Xe at B3LYP/DZVP//B3LYP/DZVP, the calculated MCA is 39.0 kcal/mol. The ethyl cation affinities of Ar, Kr, and Xe were also measured. They are ∼1.7, 3.2 ± 0.3, and 6.8 ± 0.3, respectively. The stabilities of C2H5 +(Rg) and C2H5 +(N2) were discussed in terms of nonclassical (bridge) and classical (open) structures of C2H5 +.

Gas-phase ion-molecule reactions in octafluorocyclopentene (C5F8) were studied with a pulsed electron beam mass spectrometer. When a few Torr of major gas, CH4, Ar, or N2, containing approximately 10 mTorr C5F8 was ionized by 2 keV electrons, C5F8+, C5F7+, C4F6+, C4F5+, and C3F3+ were formed as major fragment ions. The interaction between those ions and C5F8 is found to be a weak electrostatic interaction. The cation...C5F8 bonding energies are around 10 kcal/mol, which were reproduced well by (U)B3LYP/6-311+G(d) calculations. The proton affinity of C5F8 (=148.6 kcal/mol by B3LYP/6-311+G(d)) was found to be smaller than that of C2H4 (=162.8 kcal/mol). In the negative mode of operation, the intense signal of C5F8- was observed during the electron pulse. This indicates that C5F8 has a positive electron affinity (1.27 eV by (U)B3LYP/6-311+G(d)). The C5F8- ion was quickly converted to a complex C10F16-. This complex did not react further with C5F8 down to 170 K. The theoretical calculation revealed that a C5F7-F-...C5F8 interaction mode in (C5F8)2- was converted to a C5F7*...C5F9- one via fluoride-ion transfer. The F- ion was found to form a strong covalent bond with C5F8, but the interaction in F-(C5F8)- - -C5F8 is a weak electrostatic interaction due to the charge dispersal in F-(C5F8). The halide ions except F- interact with C5F8 only weakly. Thermochemical stabilities for the cluster ions I-(CH3I)n (n = 1, 2) were also determined.

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