Thermodynamics and structure of selected species

Spectroscopic studies in the gas phase and very high level ab initio calculations indicate that this species is an oblate symmetric top belonging to the molecular point group (Fig. 3). The experimentally estimated C-H distance is 1.087 A, the computed value (1.089 A) being in excellent agreement. 

The standard enthalpy of formation given in Table 1 is based on the experimental value of the adiabatic ionization potential of the methyl radical and has been adjusted to a series of fundamental enthalpies of formation. Very recently,the onset of the formation of CH from methane was determined quite accurately by means of the PFI-PEPICO technique. This datum, combined with other experimental results, yields the accurate 0 K dissociation energies (Dq) for CH4 and CH, Do(H-CH3) = 103.479 0.023 and Do(H-CH ) = 39.321 0.092 kcal mol  

Reliable experimental values for PA(CH4) and GB(CH4) are available, particularly from HPMS and flowing afterglow studies.When combined with the (rather accurate) data for the neutral species, they provide the thermodynamic data given in Table 1. The results of G2 ab initio calculations (Ref. 83 and this study) are also reported. 

Within the Bom-Oppenheimer approximation, computational studies at extremely high levels show that the global minimum, 2a(Q), has a Q symmetry and that it can be viewed as an H2 molecule tightly bound to a methyl cation (three-center two-electron bonds). Two other structures, 2b(Q) and 2c(Cv), respectively, of symmetries Q and C2V, are transition states. 2b(Q) involves the rotation of the H2 moiety and allows the exchange of the two hydrogen atoms. 

2-Metbylpropan-2-ylium (tert-butyl), tert-C4Hj (6) 169.9 0.9 73.4 2.0  

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THERMODYNAMIC STABILITIES OF CARBENIUM IONS 5 Thermodynamics and structure of selected species... 

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