Infrared detection of the 1,3-dimethyl cyclopentadienyl cation, an isomer of protonated toluene

Toluenium and Other Gaseous Methylbenzenium Ions: Complex Interplay of Protonated Arenes and Cyclo‐olefins

“…From the experimentally determined gas-phase basicity of 6-methylfulvene, 4, the heat of formation of ions [4+H] + was found to exceed that of ions p-[1+H] + by 43 kJ/ mol (Mormann, Salpin, & Kuck, 1999). In line with this, independent computational work on these C 7 H 9 + ions gave a difference of 39 kJ/mol (Mormann, Salpin, & Kuck, 2006;Mosley, Young, & Duncan, 2015), and the stabilities of the parent fulvenium and benzenium isomers, C 6 H 7 + , were calculated to differ by 40 kJ/mol (Bouchoux, Yáñez, & Mó, 1999). It should be noted that the above-mentioned nonconjugated tautomer of ion [4+H] + , the 4-vinylcyclopent-1-en-3-yl cation, x, was calculated to be much less stable than [4+H] + and the formally anti-aromatic ethylcyclopentadienyl isomer, C 2 H 5 -cyclo-C 5 H 4 + (not shown in Scheme 13) would be even more so (Schröder et al, 2006).…”

“…A C 7 H 9 + isomer that had been overlooked in all the efforts undertaken over several decades was suggested some years ago by Duncan et al (Mosley, Young, & Duncan, 2015), who detected the 1,3-dimethylcyclopentadienyl cation aa in the mixture with other C 7 H 9 + isomers by infrared spectroscopy (see below and Fig. 10).…”

“…Putative but never observed 1,1-elimination of H 2 from the toluenium ions, p-[1+H] + , m-[1+H] + and o-[1+H] + , that would produce the corresponding tolylium cations (e.g., c) is not included. -References to individual data for C 7 H 9 + ions (for C 6 H 7 + and C 8 H 11 + ions in italics): [a] Kuck (1990a), Maksić (1998), Ishikawa (2004), Kuck (2005b), Dopfer (2006), Mormann (2006), Schröder (2006), Chiavarino (2008), Douberly (2008), Mosley (2015), , Wang (2018) Kuck, Schneider, Grützmacher (1985), Schröder (2006); [l] Kuck, Schneider, Grützmacher (1985), Ignatev (2004, Dopfer (2006), Schröder (2006, Sekiguchi (2009), Wang (2018); [m] Williams (1974b), Kuck, Schneider, Grützmacher (1985); Kuck (1990a), Maksić (1998), Ishikawa (2004), Kuck (2005b), Mormann (2006, Schröder (2006), Wang (2018).…”

“…Since then and following Dewar's AM1 semiempirical work published in 1986 (Dewar & Dieter, 1986), numerous further computational studies have accompanied experimental investigations. The following stability order with some margins has been established over the years: p- (Maksić & Eckert-Maksić, 1998;Ishikawa et al, 2004;Dopfer et al, 2006;Mormann, Salpin, & Kuck, 2006;Schröder et al, 2006;Chiavarino et al, 2008;Sekiguchi et al, 2009;Mosley, Young, & Duncan, 2015;Wang et al, 2018) (Fig. 10, [a 1 ]−[a 4 ]).…”

“…As mentioned above, cation [3'+H] + is undoubtedly the least stable among the gaseous C 7 H 9 + isomers studied by experiment. Calculations place its heat of formation at 97 kJ/mol (Mormann, Salpin, & Kuck, 2006), 103 kJ/mol (Mosley, Young, & Duncan, 2015) and even 173 kJ/mol (Schröder et al, 2006) above that of the para-toluenium ion, p-[1+H] + . As shown in Scheme 13 (path d), the tricyclic ion [3'+H] + formed by protonation of norbornadiene, 3, can undergo ring opening to give the 4-vinylcyclopent-1-en-3-yl cation, x.…”