- Ionization of Normal Alkanes: Enthalpy, Entropy, Structural, and Isotope Effects
-
Enthalpies and entropies of ionization (ΔHi0, ΔSi0) 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 ΔSi0 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, ΔSi0 of C4-C7 n-alkanes is negligible. ΔHi0 values range from 10.35 eV (997.6 kJ mol-1) (butane) to 9.45 eV (910.9 kJ mol-1) (undecane); the incremental ΔHi0 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 ΔSi0 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 ΔHi0 as measured by the equilibrium constant K290 for n-CmD2m+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.
- Meot-Ner (Mautner), M.,Sieck, L.W.,Ausloos, P.
-
p. 5342 - 5348
(2007/10/02)
-
