Intramolecular hole-transfer in protonated anthracene.

Excitation spectra of protonated and deuteronated anthracene are obtained by triple-resonance dissociation spectroscopy. Very cold cations, protonated/deuteronated exclusively at the 9-position, are generated from two-colour two-photon threshold ionisation of 9-dihydroanthracenyl radicals (C14H11). The excitation spectra reveal rich structure, not resolved in previous studies, that is assigned based on anharmonic and Herzberg-Teller coupling calculations. This work reveals that the excitation of protonated anthracene induces a symmetry-breaking intramolecular charge-transfer process along a Marcus-Hush coordinate, where the positively charged hole hops from the central bridging sp2 carbon, onto one of the aromatic rings. Signatures of this charge-transfer event are observed in the excitation spectrum, through active Herzberg-Teller progressions.

Quantum-Induced Symmetry Breaking in the Deuterated Dihydroanthracenyl Radical.

The hydrogen-atom adduct with anthracene, 9-dihydroanthracenyl radical (C14H11), and its deuterated analogue have been identified by laser spectroscopy coupled to time-of-flight mass spectrometry, supported by time-dependent density functional theory calculations. The electronic spectrum of 9-dihydroanthracenyl radical exhibits an origin band at 19115 cm-1 and its ionization energy was determined to be 6.346(1) eV. The spectra reveal a low-frequency vibrational progression corresponding to a mode described by a butterfly inversion. In the deuterated analogue, a zero-point-energy imbalance along this coordinate is found to lead to a doubling of the observed spectral lines in the progression. This is attributed to quantum-induced symmetry breaking as previously observed in isotopologues of CH5+.

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.

Hydrogen-atom attack on phenol and toluene is ortho-directed.

The reaction of H + phenol and H/D + toluene has been studied in a supersonic expansion after electric discharge. The (1 + 1') resonance-enhanced multiphoton ionization (REMPI) spectra of the reaction products, at m/z = parent + 1, or parent + 2 amu, were measured by scanning the first (resonance) laser. The resulting spectra are highly structured. Ionization energies were measured by scanning the second (ionization) laser, while the first laser was tuned to a specific transition. Theoretical calculations, benchmarked to the well-studied H + benzene → cyclohexadienyl radical reaction, were performed. The spectrum arising from the reaction of H + phenol is attributed solely to the ortho-hydroxy-cyclohexadienyl radical, which was found in two conformers (syn and anti). Similarly, the reaction of H/D + toluene formed solely the ortho isomer. The preference for the ortho isomer at 100-200 K in the molecular beam is attributed to kinetic, not thermodynamic effects, caused by an entrance channel barrier that is ∼5 kJ mol(-1) lower for ortho than for other isomers. Based on these results, we predict that the reaction of H + phenol and H + toluene should still favour the ortho isomer under elevated temperature conditions in the early stages of combustion (200-400 °C).

Resonance-Enhanced 2-Photon Ionization Scheme for C2 through a Newly Identified Band System: 4(3)Πg-a(3)Πu.

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.

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.

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.

Electronic spectroscopy of diatomic VC.

Resonant two-photon ionization spectroscopy has been applied to diatomic VC, providing the first optical spectrum of this molecule. The ground state is determined to be a (2)Δ3/2 state that arises from the 1σ(2)1π(4)2σ(2)1δ(1) configuration. The r0" ground-state bond length is 1.6167(3) Å. The manifold of excited vibronic states in the visible portion of the spectrum is quite dense, but two possible vibrational progressions have been identified. It is noted that VC joins CrC, NbC, and MoC as species in which the metal ns-based 3σ orbital is unoccupied, resulting in large dipole moments in the ground states of these molecules. In the corresponding 5d metal carbides, however, the 3σ orbital is occupied, leading to different ground electronic states of the 5d congeners, TaC and WC.

ZrFe, a sextuply-bonded diatomic transition metal?

Diatomic ZrFe has been spectroscopically investigated for the first time, with the optical spectrum recorded in the range from 13890 cm(-1) to 18870 cm(-1). In the region from 13890 to 17500 cm(-1), a single exceptionally weak vibrational progression is found. Band origins, excited state vibrational frequencies and anharmonicities, excited state lifetimes, and the ground state vibrational interval, ΔG"(1/2), are reported for the five most abundant isotopomers. For the most abundant species, (90)Zr(56)Fe (47.2%), these values are: T(0) = 13931.9(1.2) cm(-1), ω'(e) = 325.05(54) cm(-1), ω'(e)x'(e) = 1.589(40) cm(-1), and ΔG"(1/2) = 452.2 cm(-1). Rotationally resolved studies have revealed ground and excited state rotational constants and Ω values, bond lengths and rotation-vibration constants, giving B(0)" = 0.138786(30) cm(-1) and r(0)" = 1.87685(20) Å for (90)Zr(56)Fe. The ground state and all observed excited states have Ω = 0. On the basis of the short bond length, the ground state of ZrFe is assigned as a nominally sextuply bonded (1)Σ(+) (Ω = 0(+)) state deriving from the 1σ(2)1π(4)2σ(2)1δ(4) electronic configuration. Above 18000 cm(-1), the spectrum becomes much more intense and congested, indicating the onset of electronically allowed transitions in this region.

Electronic spectroscopy and electronic structure of diatomic TiFe.

Diatomic TiFe, a 12 valence electron molecule that is isoelectronic with Cr(2), has been spectroscopically investigated for the first time. In addition, the first computational study that includes the ground and excited electronic states is reported. Like Cr(2), TiFe has a (1)Σ(+) ground state that is dominated by the 1σ(2) 2σ(2) 1π(4) 1δ(4) configuration. Rotationally resolved spectroscopy has established a ground state bond length of 1.7024(3) Å, quite similar to that found for Cr(2) (r(0) = 1.6858 Å). Evidently, TiFe exhibits a high degree of multiple bonding. The vibronic spectrum is highly congested and intense to the blue of 20,000 cm(-1), while two extremely weak band systems, the [15.9](3)Π(1) ← X (1)Σ(+) and [16.2](3)Π(0+) ← X (1)Σ(+) systems, are found in the 16,000-18,500 cm(-1) region. The bond lengths, obtained by inversion of the B(e)' values, and vibrational frequencies of the two upper states are nearly identical: 1.886 Å and 344 cm(-1) for [15.9](3)Π(1) and 1.884 Å and 349 cm(-1) for [16.2](3)Π(0+). The measured spin-orbit splitting of the (3)Π state is consistent with its assignment to the 1σ(2) 2σ(2) 1π(4) 1δ(3) 2π(1) configuration, as is also found in the ab initio calculations.

Resonant two-photon ionization spectroscopy of jet-cooled tantalum carbide, TaC.

The optical spectrum of diatomic TaC has been investigated for the first time, with transitions recorded in the range from 17,850 to 20,000 cm(-1). Six bands were rotationally resolved and analyzed to obtain ground and excited state parameters, including band origins, upper and lower state rotational constants and bond lengths, Fermi contact parameter b(F) for the ground state, and lambda doubling parameters for the excited states. The ground state of TaC was found to be X (2)Sigma(+), originating from the 1sigma(2)2sigma(2)1pi(4)3sigma(1) electronic configuration, in which only the valence orbitals arising from the Ta(5d+6s) and C(2s+2p) orbitals are listed. All of the rotationally resolved and analyzed bands were found to originate from the ground state, giving B(0)"=0.489 683(83) cm(-1), r(0)"=1.749 01(15) A, and b(F)"=0.131 20(36) cm(-1) (1sigma error limits) for (181)Ta (12)C. Comparison of the Fermi contact parameter to the atomic value shows that the 3sigma orbital is approximately 75% Ta 6s in character. The other group 5 transition metal carbides, VC and NbC, have long been known to have 1sigma(2)2sigma(2)1pi(4)1delta(1), (2)Delta ground states, with low-lying 1sigma(2)2sigma(2)1pi(4)3sigma(1), (2)Sigma(+) excited states. The emergence of a different ground state in TaC, as compared to VC and NbC, is due to the relativistic stabilization of the 6s orbital in Ta. This lowers the energy of the 6s-like 3sigma orbital in TaC, causing the 1sigma(2)2sigma(2)1pi(4)3sigma(1), (2)Sigma(+) state to fall below the 1sigma(2)2sigma(2)1pi(4)1delta(1), (2)Delta state.

Resonant two-photon ionization spectroscopy of jet-cooled OsC.

The optical spectrum of diatomic OsC has been investigated for the first time, with transitions recorded in the range from 17 390 to 22 990 cm(-1). Six bands were rotationally resolved and analyzed to obtain ground and excited state rotational constants and bond lengths. Spectra for six OsC isotopomers, 192 Os 12C (40.3% natural abundance), 190 Os 12C(26.0%), 189 Os 12C(16.0%), 188 Os 12C(13.1%), 187 Os 12C(1.9%), and 186 Os 12C(1.6%), were recorded and rotationally analyzed. The ground state was found to be X 3 Delta 3, deriving from the 4 delta 3 16 sigma 1 electronic configuration. Four bands were found to originate from the X 3 Delta 3 ground state, giving B 0"=0.533 492(33) cm(-1) and r 0 "=1.672 67(5) A for the 192 Os 12C isotopomer (1 sigma error limits); two of these, the 0-0[19.1]2<--X 3 Delta 3 and 1-0[19.1]2<--X 3 Delta 3 bands, form a vibrational progression with Delta G' 1/2=953.019 cm(-1). The remaining two bands were identified as originating from an Omega"=0 level that remains populated in the supersonic expansion. This level is assigned as the low-lying A 3 Sigma 0+ (-) state, which derives from the 4 delta 2 16 sigma 2 electronic configuration. The OsC molecule differs from the isovalent RuC molecule in having an X 3 Delta 3 ground state, rather than the X 2 delta 4, 1 Sigma+ ground state found in RuC. This difference in electronic structure is due to the relativistic stabilization of the 6s orbital in Os, an effect which favors occupation of the 6s-like 16 sigma orbital. The relativistic stabilization of the 16 sigma orbital also lowers the energy of the 4 delta 2 16 sigma 2, 3 Sigma(-) term, allowing this term to remain populated in the supersonically cooled molecular beam.