Determination of the proton affinity of carbon dioxide by photoionization mass spectrometry

The adiabatic ionization energies and COOH⁺ appearance energies for a series of carboxylic acids have been measured by dissociative photoionization mass Spectrometry. Using the stationary electron convention, a heat of formation of 143.5 ± 0.7 kcal mol^?1 (1 kcal = 4.184 kJ) at 298 K is derived for the carboxyl cation in the gas phase, which corresponds to an absolute proton affinity for carbon dioxide of 128.1 ± 0.7 kcal mol^?1.     

Energetics of the loss of H2 from [C3H7]+

The internal energies of [C₃H₇]⁺ ions contributing to the metastable peak [C₃H₇]⁺ → [C₃H₅]⁺ + H₂ are higher (by perhaps > 100 kJ mol^?1) than those of the ion contributing to the threshold current in appearance energy measurements on [C₃H₅]⁺. The measured appearance energy may lead to an underestimation of the activation energy, i.e. negative ‘kinetic shift’, due to quantum, mechanical tunnelling. The distribution of energy released in the decomposition can be explained on the basis that much of the reverse activation energy and a statistical proportion of the excess energy is released as translation.     

Formation of the 3-Pentanone ion from ionized propyl propanoate through Ion-Neutral complexes

Formation of the 3-pentanone ion (3) from ionized propyl propanoate through the complex [C₂H₅CO^{+ ?}OC₃H₇] is proposed based on data obtained by photoionization. The threshold and energy dependence for forming 3 relative to those for related processes support this proposal. The threshold for forming 3 coincides with that predicted for forming [CH₃CH₂CO^{+ ?}CH₂CH₃], suggesting that that complex is also an intermediate in the pathway to 3. 3-Pentanone ion formation is important much further above threshold than is alkane elimination through [RCO^{+ ?}R] complexes. This adds to evidence that reactions between the partners in ion-dipole complexes take place over a wider energy range than do such reactions in complexes containing nonpolar neutral partners.     

Heat of formation for the SH radical by photoionization mass spectrometry

Photoionization mass spectrometry has been used to measure the appearance energies for [C₂H₅]⁺ from ethanethiol, [C₃H₇]⁺ from 2-propanethiol and [C₃H₅]⁺ from 2-methylthiirane. From the known thermochemistry of these cations and their precursor molecules, a 298 K heat of formation of 138.6±0.4 kJ mol^?1 for the SH radical has been derived.