Generation of covalent and electrostatic complexes in association reactions of tert-butyl cation with small organics

Pulsed ionization High-Pressure Mass Spectrometry (PHPMS) has been used to investigate the thermochemistry of the association reactions of t-C₄H₉⁺ with various neutral molecules in the gas phase. The behavior of the logarithm of the association equilibrium constant as a function of inverse reaction temperature (a van't Hoff plot) has been examined over a broad temperature range to yield accurate thermochemical data for the enthalpy and entropy of association. All of the systems investigated show either a pronounced break or a subtle curvature in the van 't Hoff plot. This behavior is consistent with two coexisting isomeric forms of the association adduct, viz, (i) a low-temperature, covalently bound isomer characterized by larger -ΔH° and -ΔS° values and (ii) a high-temperature, electrostatically bound isomer of lower -ΔH° and -ΔS° values. Experimental thermochemical data for the association reactions are reported. From the low-temperature data, proton affinities (PA) of t-C₄H₉OCH₃ (205.1 kcal/mol) and t-C₄H₉OC₂H₅ (214.0 kcal/mol) were determined. Ab initio calculations carried out in conjunction with the experimental study of some of these association reactions support the experimental results. The results obtained for t-C₄H₉⁺ onto (CH₃)₂O revealed the existence of two broad energy plateaus when attempting to optimize different electrostatically bound adduct structures. Structures corresponding to these energy plateaus are proposed to be possible transition states characterized by unconventional ionic hydrogen bonds. Theoretical thermochemical data for the association reactions investigated are reported.