Olha Krechkivska

Resonance-Enhanced 2-Photon Ionization Scheme for C2 through a Newly Identified Band System: 43Πg–a3Πu

We report the observation of a new band system of C2, namely, the 4(3)Πg-a(3)Πu system. The bands, observed by resonant 2-photon ionization spectroscopy and time-of-flight mass spectrometry, were identified through a synergy of high-level ab initio computation and double-resonance spectroscopy. Two bands are firmly identified, 1-3 and 0-2, allowing the 4(3)Πg origin to be placed at 51496.44 cm(-1). The 4(3)Πg state is characterized as having a single bond, with a vibrational frequency of about 1268 cm(-1), and an equilibrium bond length of 1.57 Å. The state is predicted to exhibit a barrier to dissociation, with a rotational constant that unusually increases with vibrational excitation up to a maximum before decreasing at higher vibrational excitation. The new band system allows us to probe the a(3)Πu state of C2 through a straightforward 1 + 1 REMPI scheme.

The ionization energy of C2

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionization energy of C2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4(3)Πg ← a(3)Πu band system of C2, the ionization energy of the lowest rovibronic level of the a(3)Πu state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionization energy of the forbidden origin of the a(3)Πu state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionization energy of X(1)Σg(+) state C2 is 11.866(5) eV.

Ionization energies of three resonance-stabilized radicals: cyclohexadienyl (dn, n = 0, 1, 6, 7), 1-phenylpropargyl, and methylcyclohexadienyl.

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.

H and D Attachment to Naphthalene: Spectra and Thermochemistry of Cold Gas-Phase 1-C10H9 and 1-C10H8D Radicals and Cations

Excitation spectra of the 1H-naphthalene (1-C10H9) and 1D-naphthalene (1-C10H8D) radicals, and their cations, are obtained by laser spectroscopy and mass spectrometry of a skimmed free-jet expansion following an electrical discharge. The spectra are assigned on the basis of density functional theory calculations. Isotopic shifts in origin transitions, vibrational frequencies and ionization energies were found to be well reproduced by (time-dependent) density functional theory. Absolute bond dissociation energies, ionization energies and proton affinities were calculated using high-level quantum chemical methods.

First observation of the 3Πg3 state of C2: Born-Oppenheimer breakdown

The 33Πg state of the dicarbon molecule, C2, 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 C2 spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 43Πg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 33Πg state of C2 is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d3Πg state.

The eΠg3 state of C2: A pathway to dissociation

The lowest 13 vibrational levels, v = 0-12, of the eΠg3 state of the C2 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 C2, 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 C2 between 6.1803 and 6.2553 eV, in agreement with the Active Thermochemical Tables.

Jet-Cooled Spectroscopy of Ortho-Hydroxycyclohexadienyl Radicals

The electronic spectra of the ortho-hydroxycyclohexadienyl radical have been observed following the supersonic expansion of the electric discharge products of phenol and water. Hydrogen atoms, split from water, add to the phenol ring at the ortho position, generating syn and anti rotamers with respect to the hydroxyl group. The D1 ← D0 transitions were recorded by resonance-enhanced multiphoton ionization spectroscopy. The spectrum of each isomer was isolated through hole-burning spectroscopy. The assignment and symmetry of the excited state are evaluated through ab initio calculations and are employed to assign each spectrum. Both rotamers are calculated to have a puckered ring in the excited state, leading to C1 symmetry. The spectrum of the anti isomer is assigned well using this symmetry; however, the syn isomer is assigned better in the C s symmetry of the ground state. We use Duschinsky matrices as a tool for the spectroscopist to determine which point group to use when ab initio calculations are ambiguous.