Temporary anion states of pyrimidine and halopyrimidines

The empty-level electronic structures of pyrimidine and its 2-chloro, 2-bromo, and 5-bromo derivatives have been studied with electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS) in the 0-5 eV energy range. The spectral features were assigned to the corresponding anion states with the support of theoretical calculations at the ab initio and density functional theory levels. The empty orbital energies obtained by simple Koopmans' theorem calculations, scaled with empirical equations, quantitatively reproduced the energies of vertical electron attachment to π* and σ* empty orbitals measured in the ET spectra and predicted vertical electron affinities close to zero for the three halo derivatives. The total anion currents of the halo derivatives, measured at the walls of the collision chamber as a function of the impact electron energy, presented intense maxima below 0.5 eV. The mass-selected spectra showed that, in this energy, range the total anion current is essentially due to halide fragment anions. The DEA cross sections of the bromo derivatives were found to be about six times larger than that of the chloro derivative. The absolute cross sections at incident electron energies close to zero were evaluated to be 10(-16)-10(-15) cm(2).     

Electron attachment to dye-sensitized solar cell components: cyanoacetic acid

The energies of electron attachment associated with temporary occupation of the lower-lying virtual orbitals of cyanoacetic acid (CAA), proposed as a possible component of dye-sensitized solar cells, and its derivative methyl cyanoacetate (MCA) are measured in the gas phase with electron transmission spectroscopy (ETS). The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies (VAEs). The vertical and adiabatic electron affinities are also evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. Dissociative electron attachment spectroscopy (DEAS) is used to measure the total anion current as a function of the incident electron energy in the 0-4 eV energy range, and the negative fragments generated through the dissociative decay channels of the molecular anion are detected with a mass filter. In both compounds only two intense fragment anion currents are observed, that due to loss of a hydrogen atom from the molecular anion ([M - H](-)) and that due to formation of CN(-). In CAA the former signal displays a very sharp feature at 0.68 eV, assigned to a vibrational Feshbach resonance arising from coupling between a dipole bound anion state and a temporary σ* anion state.     

Dissociative electron attachment to di-tert-butylperoxide, artemisinin, and beta-artemether

The gas-phase dissociative electron attachment spectra of di-tert-butylperoxide (DBP) and the antimalarial polycyclic peroxides artemisinin and beta-artemether are presented for the first time. The total anion currents measured at the walls of the collision chamber and the mass selected anion currents are reported in the 0-6 eV energy range. Electron attachment to DBP produces an intense current, peaking at 1.3 eV, due to the C4H9O- negative fragment, in line with the strongly O-O antibonding character of the singly occupied orbital of the parent molecular anion and the small (if any) thermodynamic energy threshold predicted by B3LYP calculations for the formation of this anion fragment. A five times less intense signal, with m/e = 57 and a maximum at 0.7 eV, is also observed. The calculations exclude that this signal can be associated with the C4H9- negative fragment, whereas they support its assignment to the C3H5O- species, generated by simultaneous dissociation and loss of a methane molecule from the parent molecular anion. In DBP, artemisinin, and beta-artemether, currents corresponding to the parent molecular anion are not detected, indicating that its survival time is shorter than the time required (about 10-6 s) to pass through the mass filter. In the latter two compounds, where simple O-O bond breaking does not generate separate fragments, the anion currents are much weaker than in DBP and the maximum total anion current peaks at zero energy.     

Temporary anion states and dissociative electron attachment in diphenyl disulfide

The temporary anion states of gas-phase diphenyl disulfide are characterized by means of electron transmission (ET) and dissociative electron attachment (DEA) spectroscopies. The measured energies of vertical electron attachment are compared to the virtual orbital energies of the neutral state molecule supplied by MP2 and B3LYP calculations with the 6-31G basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the attachment energies measured in the ET spectrum. The first anion state of diphenyl disulfide is stable, thus escaping detection in ETS. The vertical and adiabatic electron affinities, evaluated with B3LYP/6-31+G calculations as the energy difference between the neutral and anion states, are predicted to be 0.37 and 1.38 eV, respectively. The anion current displayed in the DEA spectrum has a sharp and intense peak at zero energy, essentially due to the C6H5S- negative fragment. In agreement, according to the calculations, the localization properties of the first anion state are strongly S-S antibonding, and the energetic requirement for its dissociation along the S-S bond is fulfilled even at zero energy.     

Temporary anion states and dissociative electron attachment to isothiocyanates

The temporary anion states of isothiocyanates CH3CH2=C=S (and CH3CH2N=C=O for comparison), C6H5CH2N=C=S, and C6H5N=C=S are characterized experimentally in the gas phase for the first time by means of electron transmission spectroscopy (ETS). The measured vertical electron attachment energies (VAEs) are compared with the virtual orbital energies of the neutral-state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental VAEs. The first VAE is also closely reproduced as the total energy difference between the anion and neutral states calculated at the B3LYP/6-31+G* level. Due to mixing between the ring and N=C=S pi-systems, C6H5N=C=S possesses the best electron-acceptor properties, and its lowest-lying anion state is largely localized at the benzene ring. The anion states with mainly pi*C=S and pi*N=C character lie at higher energy than the corresponding anion states of noncumulated pi-systems. However, the electron-acceptor properties of isothiocyanates are found to be notably larger than those of the corresponding oxygen analogues (isocyanates). The dissociative electron attachment (DEA) spectra show peaks close to zero energy and at 0.6 eV, essentially due to NCS- negative fragments. In spite of the energy proximity of the first anion state in phenyl isothiocyanate to the DEA peak, the zero-energy anion current in the benzyl derivative is about 1 order of magnitude larger.     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X??(2)Σ(+)) reveals transitions to the ground electronic state (X??(1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 A?. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X??(2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.     

Low-energy electron capture by 6-Aza-2-thiothymine: investigations by electron attachment and electron transmission spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.     

Electronic state spectroscopy of c-C5F8 explored by photoabsorption, electron impact, photoelectron spectroscopies and ab initio calculations

The electronic transitions and resonance-enhanced vibrational excitations of octafluorocyclopentene (c-C5F8) have been investigated using high-resolution photoabsorption spectroscopy in the energy range 6-11 eV. In addition, the high-resolution electron energy loss spectrum (HREELS) was recorded under the electric dipolar excitation conditions (100 eV incident energy, approximately 0 degrees scattering angle) over the 5-14 eV energy loss range. A He(I) photoelectron spectrum (PES) has also been recorded between 11 and 20 eV, allowing us to derive a more precise value of (11.288 +/- 0.002) eV for the ground neutral state adiabatic ionization energy. All spectra presented in this paper represent the first and highest resolution data yet reported for octafluorocyclopentene. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.     

Resonance electron attachment and long-lived negative ions of phthalimide and pyromellitic diimide

Resonance attachment of low energy (0-15 eV) electrons to imide-containing molecules, phthalimide (PTI) and pyromellitic diimide (PMDI), was investigated in the gas-phase by means of Electron Transmission Spectroscopy (ETS) and Dissociative Electron Attachment Spectroscopy (DEAS). Among a variety of low intensity negatively charged fragments formed by DEA, in both compounds the dominant species was found to be a long-lived (μs) parent molecular anion formed at zero energy. In addition, in PMDI long-lived molecular anions were also observed at 0.85 and 2.0 eV. The experimentally evaluated detachment times from the molecular anions as a function of incident electron energy are modeled with a simple computational approach based on the RRKM theory. The occurrence of radiationless transitions to the ground anion state, followed by internal vibrational relaxation, is believed to be a plausible mechanism to explain the exceptionally long lifetime of the PMDI molecular anions formed above zero energy.     

Electron-acceptor properties of phospha-alkynes as probed by electron transmission and dissociative electron attachment spectroscopies

Electron transmission spectroscopy is applied for the first time to a phospha-alkyne derivative, (CH₃)₃CCP. Electron attachment to the π^* LUMO in the gas-phase occurs at 0.63 eV. Formation of the temporary π^* anion state is also observed in measurements of the total anion current, as a function of the incident electron energy. Comparison with the ethyne analogue shows that replacement of a CH group with a P atom causes a very strong increase (2.3 eV) of the vertical electron affinity. Simple Koopmans’ theorem calculations reproduce the experimental data.     

Limonene: electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, electron scattering, He(I) photoelectron spectroscopy and ab initio calculations

Electronic state spectroscopy of limonene has been investigated using vacuum ultraviolet photoabsorption spectroscopy in the energy range 5.0-10.8 eV. The availability of a high resolution photon beam (~0.075 nm) enabled detailed analysis of the vibrational progressions and allowed us to propose, for the first time, new assignments for several Rydberg series. Excited states located in the 7.5-8.4 eV region have been studied for the first time. A He(I) photoelectron spectrum has also been recorded from 8.2 to 9.5 eV and compared to previous low resolution works. A new value of 8.521 ± 0.002 eV for the ground ionic state adiabatic ionisation energy is proposed. Absolute photoabsorption cross sections were derived in the 10-26 eV range from electron scattering data. All spectra presented in this paper represent the highest resolution data yet reported for limonene. These experiments are complemented by new ab initio calculations performed for the three most abundant conformational isomers of limonene, which we then used in the assignment of the spectral bands.     

The triplet state of cytosine and its derivatives: electron impact and quantum chemical study

The excitation of the lowest electronic states and vibrational excitation of cytosine (C) have been studied using electron energy loss spectroscopy (EELS, 0-100 eV) with angular analysis. The singlet states have been found to be in good agreement with UV-VIS absorption results on sublimed films, slightly blueshifted by about 0.1 eV. The EEL spectra recorded at residual energy below 2 eV show clear shoulders at energy losses of 3.50 and 4.25 eV (+/-0.1 eV). They are assigned to the lowest triplet electronic states of cytosine. Energies and molecular structures of the lowest-lying triplet state of C and its methylated and halogenated 5-X-C, 6-X-C, and 5-X, 6-X-C substituted derivatives (X=CH3, F, Cl, and Br) have been studied using quantum chemical calculations with both molecular orbital and density functional methods, in conjunction with the 6-311++G(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets. The triplet-singlet energy gap obtained using coupled-cluster theory [CCSD(T)] and density functional theory (DFT) methods agrees well with those derived from EELS study. The first C's vertical triplet state is located at 3.6 eV, in good agreement with experiment. The weak band observed at 4.25 eV is tentatively assigned to the second C's vertical triplet excitation. For the substituted cytosines considered, the vertical triplet state is consistently centered at 3.0-3.2 eV above the corresponding singlet ground state but about 1.0 eV below the first excited singlet state. Geometrical relaxation involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.6-1.0 eV in favor of the equilibrium triplet. The lowest-lying adiabatic triplet states are located at 2.3-3.0 eV. Halogen substitution at both C(5) and C(6) positions tends to reduce the triplet-singlet separations whereas methylation tends to enlarge it. The vibrational modes of triplet cytosine and the ionization energies of substituted derivatives were also evaluated.     

Effect of intramolecular hydrogen bonding on the empty level structure of gas-phase salicylaldehyde:electron affinities and anion yield upon electron impact

The gas-phase empty level structures of γ-butyrolactone, α-hydroxy-γ-butyrolactone, benzaldehyde, 3-hydroxybenzaldehyde and 2-hydroxybenzaldehyde (salicylaldehyde) are studied by means of electron transmission spectroscopy (ETS) and ab initio 6-31G^** calculations. The most stable conformer of salicylaldehyde is characterised by intramolecular hydrogen bonding which causes relevant geometrical variations and, in particular, a lengthening of the C=O double bond. The empty π^* molecular orbitals of salicylaldehyde with large contribution from the carbonyl group are significantly stabilised with respect to their counterparts in benzaldehyde and in the 3-hydroxy derivative. Dissociative electron attachment spectroscopy (DAS) did not reveal formation of negative fragments at low energy, that is, the presence of dissociative decay channels for the π^* resonances observed in the ET spectra. The total anion current measured at the walls of the collision chamber (close to the electron beam), as a function of the incident electron energy, displays signals peaking at zero energy and at about 0.8 eV in benzaldehyde and in the two hydroxy derivatives. However, the only signal detected using a quadrupole mass filter (which requires a lifetime ≥10⁻⁶ s) is a zero energy peak in salicylaldehyde, due to the molecular anion.     

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.     

Photoelectron spectroscopy of anions at 118.2 nm: observation of high electron binding energies in superhalogens MCl4- (M=Sc, Y, La)

High energy photon is needed for photoelectron spectroscopy (PES) of anions with high electron binding energies, such as superhalogens and O-rich metal oxide clusters. The highest energy photon used for anion PES in the laboratory has been 157 nm (7.866 eV) from F2 eximer lasers. Here, we report an anion PES experiment using coherent vacuum ultraviolet radiation at 118.2 nm (10.488 eV) by tripling the third harmonic output (355 nm) of a Nd:YAG laser in a XeAr cell. Our study focuses on a set of superhalogen species, MCl(4) (-) (M=Sc, Y, La), which were expected to possess very high electron binding energies. While the 157 nm photon can only access the ground state detachment features for these species, more transitions to the excited states at binding energies higher than 8 eV are observed at 118.2 nm. The adiabatic detachment energies are shown to be, 6.84, 7.02, and 7.03 eV for ScCl(4) (-), YCl(4) (-), and LaCl(4) (-) eV, respectively, whereas their corresponding vertical detachment energies are measured to be 7.14, 7.31, and 7.38 eV.     

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.     

Electron Momentum Spectroscopy of Ethanethiol Complete Valence Shell

The binding energy spectra and electron momentum distributions for the complete valence orbitals of ethanethiol were measured for the first time by binary (e, 2e) electron momentum spectroscopy employing non-coplanar symmetric kinematics at an impact energy of 1200 eV plus binding energy. The experimental results are generally consistent with the theoretical calculations using density functional theory and Hartree-Fock methods with various basis sets. A possible satellite line at 17.8 eV in binding energy spectrum was observed and studied by electron momentum spectroscopy.     

Anion photoelectron spectroscopy of C(5)H(-)

Anion photoelectron spectroscopy is performed on the C(5)H(-) species. Analogous to C(3)H(-) and C(3)D(-), photodetachment transitions are observed from multiple, energetically close-lying isomers of the anion. A linear and a cyclic structure are found to have electron binding energies of 2.421+/-0.019 eV and 2.857+/-0.028 eV, respectively. A cyclic excited state is also found to be 1.136 eV above the linear (2)Pi C(5)H ground state. Based on our assignments of the observed transitions and previous calculations on the energetics of neutral C(5)H isomers, the cyclic (1)A(1) anion state is found to lie 0.163 eV below the (3)A linear anion.     

Quantum chemical modeling of photoadsorption properties of the nitrogen-vacancy point defect in diamond

Quantum chemical calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV(0) and NV(-)) have been performed employing various theoretical methods and basis sets and using finite model NC(n)H(m) clusters. Unpaired electrons in the ground doublet state of NV(0) and triplet state of NV(-) are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV(-) are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with the first absorption band calculated at approximately 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV(0), we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (approximately 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (approximately 3.2 eV and higher).