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 +.

Ion/molecule reactions in octafluorocyclobutane (c-C4F8) were studied using a high-pressure mass spectrometer. The thermochemical stabilities of the cluster ions of halide ions (X−) with c-C4F8 were measured. While the F− ion forms a covalent bond with c-C4F8, the interaction between other halide ions with c-C4F8 is mainly electrostatic. Theoretical calculation revealed that the halide ions interact not with the lowest unoccupied molecular orbital but with the next lowest unoccupied molecular orbital of the c-C4F8 molecule in the most stable cluster ions X−(c-C4F8). The electron affinity of c-C4F8 was measured to be 24.2±2.3 kcal/mol (1.05±0.10 eV) by observing the equilibria for reaction of SF6−+c-C4F8=c-C4F8−+SF6. While the sound equilibrium for that reaction was established in the temperature region from ∼350 K down to the lowest temperature measured (∼150 K), that was not established in the higher temperature region above 350 K. This was attributed to the existence of an isomer for c-C4F8− whose electron detachment energy is smaller than 1.05±0.10 eV. By the measurement of thermochemical stabilities of [(O2)m(c-C4F8)n]− (m+n=1–3), the lower electron affinity of c-C4F8 was determined to be 12.0±1.2 kcal/mol (0.52±0.05 eV). The lower limit of the proton affinity of c-C4F8 was estimated to be 130 kcal/mol.

6 ) n , respectively, become faster at lower temperature. This is due to the existence of an entropy barrier for the formation of cluster ions. The C 3 F 5 + ion was found to form cluster ions readily with C 3 F 6 solvent molecules. Thermochemical stabilities for C 3 F 5 + (C 3 F 6 ) n with n ) 1 and 2 could be determined. The proton affinity (PA) of C 3 F 6 was found to be smaller but close to that of C 2 H 4 (162.6 ( 1.5 kcal/mol). The G2MP2-calculated PA is 157.26 kcal/mol. Lone-pair orbitals of the CF 3 substituent are electronic-charge donor sites to C 2 F 4 + and C 3 F 5 + . The polymerization reactions of C 3 F 6 initiated by F -, C 3 F 5 -, and C 3 F 6were observed. Those reactions became faster with a decrease of temperature. The high reactivity of C 3 F 6 in the negative-mode ion/molecule reactions is ascribed to the perfluoro effect. The halide ions Cl -, Br -, and Iwere found to form cluster ions with C 3 F 6 . Thermochemical stabilities for X -(C 3 F 6 ) n (X -) Cl -, Br -, and I -) have been determined. A slight charge transfer in the complex Cl -fC 3 F 6 results in the fairly strong bond energy (12.6 kcal/mol) for the cluster.