The ionization energies for three resonance-stabilized radicals are determined: cyclohexadienyl, 1-phenylpropargyl, and methylcyclohexadienyl. The recommended ionization energies are, respectively, 6.820(1), 6.585(1), and 7.232(1) eV. That of cyclohexadienyl is found to be just 0.02 eV above a high level ab initio calculation [Bargholz, A.; Oswald, R.; Botschwina, P. J. Chem. Phys. 2013, 138, 014307], and that of 1-phenylpropargyl is found within the stated error of a recent experimental determination [Holzmeier, F.; Lang, M.; Hemberger, P.; Fischer, I. ChemPhysChem 2014, DOI: 10.1002/cphc.201402446]. The ionization energy of the methylcyclohexadienyl radical is consistent with the ortho isomer. Ionization energies of a range of isotopologues of cyclohexadienyl radical are given, along with their D1 ← D0 origin band positions, which indicate a blue shift of 18 cm(-1) per deuterium atom substituted. The ionization energy of cyclohexadienyl, along with the calculated bond dissociation energy of Bargholz et al., affords a new estimate of the 0 K proton affinity of benzene: 739.7 ± 2.0 kJ/mol. The ionization energies are discussed in terms of the interplay between radical and cation stabilization energies.

No abstract

The 3Π state of the dicarbon molecule, C, has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 4Π state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 3Π state of C is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound dΠ state.

The lowest 13 vibrational levels, v = 0-12, of the eΠg3 state of the C molecule have been measured by laser-induced fluorescence of new bands of the Fox-Herzberg system. The newly observed levels, v = 5-12, which span the eΠg3 electronic state up to and beyond the first dissociation threshold of C, were analyzed to afford highly accurate molecular constants, including band origins, and rotational and spin-orbit constants. The spin-orbit coupling constants of the previously published lowest five levels are revised in sign and magnitude, requiring an overhaul of previously published molecular constants. The analysis is supported by high level ab initio calculations. Lifetimes of all observed levels were recorded and found to be in excellent agreement with ab initio predicted values up to v = 11. v = 12 was found to exhibit a much reduced lifetime and fluorescence quantum yield, which is attributed to the onset of predissociation. This brackets the dissociation energy of ground state XΣg+1 C between 6.1803 and 6.2553 eV, in agreement with the Active Thermochemical Tables.

The excitation spectrum of very cold, isomerically pure protonated naphthalene is obtained by resonant dissociation spectroscopy. The cations are prepared by a pair of laser pulses which ionize 1-hydronaphthyl radicals at threshold, thereby creating only cations of a particular isomer in the vibrational ground state. Due to the small bandwidth of the first excitation laser, only the lowest rotational states are selected. The cold cation spectrum reveals a rich structure, some of which had previously been attributed to hot bands. However, the nature of the prepared cations is such that no hot bands appear, and the structure is assigned to a′ modes, and a′ combinations of a″ modes in the C s point group. The 12 excited-state frequencies extracted agree well with those calculated previously at the RI-CC2 level. [