Heat of Formation of the Allyl Ion by TPEPICO Spectroscopy

The 0 K onset of C(3)H(6) --C(3)H(5)(+) + H(*) is measured by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. From the onset (11.898 +/- 0.025 eV) the heat of formation of the allyl ion (CH(2)CHCH(2)(+)) is determined to be DeltaH degrees (f,0K) = 967.2; DeltaH degrees (f,298K) = 955.4 +/- 2.5 kJ mol(-1). The value is significantly more positive than prior determinations, and resolves a discrepancy between measurements of the allyl radical and allyl ion heats of formation and recent highly precise measurements of the allyl radical adiabatic ionization energy. The new allyl ion heat of formation leads to a new proton affinity for propadiene (allene) of 765.0 +/- 2.6 kJ mol(-1). An attempt is made to determine the CH(3)CCH(2)(+) heat of formation by measuring the 0 K onset of 2-ClC(3)H(5) --C(3)H(5)(+) + Cl(*). However, C(3)H(5)(+) appears at too low an energy to be the higher energy CH(3)CCH(2)(+) structure. Rather, 2-ClC(3)H(5)(+) undergoes a concerted hydrogen transfer and Cl-loss via an intramolecular S(N)2 like mechanism to produce the allyl ion. The 0 K onset of 3-ClC(3)H(5) --C(3)H(5)(+) + Cl(*) (11.108 +/- 0.010 eV) is measured to determine the 3-ClC(3)H(5) heat of formation (DeltaH degrees (f,0K) = 14.9; DeltaH degrees (f,298K) = 1.1 +/- 2.7 kJ mol(-1)). 3-ClC(3)H(5)(+) is suggested to readily isomerize to trans 1-ClC(3)H(5)(+) prior to dissociation.