Photoelectron Spectroscopy of the Trimethylene- methane Negative Ion. The Singlet−Triplet Splitting of Trimethylenemethane

The photoelectron spectrum of the trimethylenemethane (TMM) negative ion is described. The electron affinity of TMM is found from the spectrum to be 0.431 6 0.006 eV, and the energy difference between the X ̃ 3A92 and b̃ 1A1 states of TMM is determined to be 16.1 6 0.2 kcal/mol. The energy difference between the lowest energy triplet and singlet states is estimated to be 13–16 kcal/mol. The enthalpy of formation of TMM is measured to be 70 6 3 kcal/mol, and the C–H bond enthalpy in 2-methylallyl radical is 90 6 2 kcal/mol. Previously unobserved vibrational frequencies of 425, 915, and 1310 cm21 are found for the triplet state of TMM, whereas a frequency of 325 cm21 is found for the singlet state. In addition, an overtone peak is observed for the triplet state at 1455 cm21, and both states contain peaks that are assigned to bands arising from excited vibrational levels of the ion. (J Am Soc Mass Spectrom 1999, 10, 800–809)