Ionization of Normal Alkanes: Enthalpy, Entropy, Structural, and Isotope Effects

Article abstract of DOI:10.1021/ja00408a011

Enthalpies and entropies of ionization (ΔH_i⁰, ΔS_i⁰) of C4 to C11 normal alkanes were determined from charge-transfer equilibrium measurement between 300 and 420 K by using photoionization high-pressure mass spectrometry.Large negative ΔS_i⁰ values are observed in C7 and larger n-alkanes, from -4.7 cal mol^-1 K^-1 (-19.6 J mol^-1 K^-1) in heptane to -13.9 cal mol^-1 K^-1 (-58.1 J mol^-1 K^-1) in undecane; in contrast, ΔS_i⁰ of C4-C7 n-alkanes is negligible. ΔH_i⁰ values range from 10.35 eV (997.6 kJ mol^-1) (butane) to 9.45 eV (910.9 kJ mol^-1) (undecane); the incremental ΔH_i⁰ values also suggest the occurence of an effect that stabilizes C7 and higher but not the lower molecular ions.Analogy with disubstituted alkanes suggests that the negative ΔS_i⁰ values and excess stabilization in C7 and higher alkane ions are due to constrained cyclic conformations which result from noncovalent intramolecular bonding between the terminal -C2H5 groups in the large, flexible molecular ions.These effects are more pronounced in n-alkanes than in 2-methylalkanes.Isotope effects on ΔH_i⁰ as measured by the equilibrium constant K₂₉₀ for n-CmD2m+2⁺ + n-CmH2m+2 ->/<- n-CmH2m+2⁺ + n-CmD2m+2 are significant for ethane (k₂₉₁ = 4.5) but decrease with increasing m: in propane K₂₉₀ = 3.2 and in hexane and octane K₂₉₁ = 1.0.However, the isotope effects in cyclic alkanes are much larger than in corresponding normal alkanes: in cyclohexane, K₃₂₁ = 3.3 compared with that in n-hexane, were K₃₂₀ = 1.0.