Ground State Of Cation Research Articles

Vinylidene-acetylene cation isomerization investigated by large scale ab initio calculations

The planar isomerization routes of the vinylidene/acetylene cation in the lowest electronic states are accurately examined for the first time, by using large scale MRCI and CCSDT calculations in a complementary way. They are compared with the similar calculations performed for the neutral ground state isomerization. An accurate value of the adiabatic ionization potential of vinylidene (11.26 eV) is predicted. The vinylidene cation lowest state, 1 2A1, follows an almost flat pathway with a shallow secondary minimum on the 1 2A' potential energy surface, before suddenly dropping to the stable acetylene cation ground state, X 2Piu. It is therefore confirmed to be completely unstable with respect to isomerization. The first excited state of the vinylidene cation, 1 2B1, which also correlates with the 2Piu ground state of acetylene cation along a 2A' isomerization route, has been studied at the same level of calculation. This 1 2B1 state is lying only 0.15 eV above the 1 2A1 state, and exhibits a potential energy barrier of 0.55 eV which explains the earlier assignment of this symmetry to the ground state of vinylidene cation. In addition to large scale calculations, a comprehensive description of the important steps of isomerization drawn from a very simple model involving monoconfigurational states is presented. In particular, the behavior of one unique orbital, namely, the 5a1 outer molecular orbital, is shown to completely govern the molecular geometry and energy evolution along the isomerization route of the ground state cation C2H2+.

Bending energy level structure and quasilinearity of the X̃+B13 ground electronic state of NH2+

The bending level structure of the quasilinear X+ 3B1 ground electronic state of the amidogen cation NH2+ was studied by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy using a near-infrared vacuum-ultraviolet two-photon ionization sequence via selected rovibronic levels of the A 2A1 state of NH2. The careful selection of the intermediate levels permitted to optimize the transition intensities to the lowest vibrational levels of the cation in the photoionization step and to overcome the low sensitivity of previously employed single-photon ionization schemes. For the first time, all bending levels of the cationic ground state with quantum numbers upsilon2,lin + < or =4, N+ < or =4, and /K+/ < or =2 could be observed, enabling a detailed characterization of the large-amplitude bending vibration. The rotational structure corresponds to that of an effectively linear molecule in all observed vibrational levels. The bending vibrational structure which shows marked deviations from a harmonic behavior was analyzed in terms of a semirigid bender model. The bending potential function was obtained from a fit to the experimental data. The height of the barrier at the linear geometry and the bond angle at the potential minimum were determined to be 231.8(22) cm(-1) and 152.54(4) degrees , respectively, and all bending levels are located above the maximum of the barrier.

High-Resolution Mass-Analyzed Threshold Ionization Study of Deuterated Derivatives of Bis(η6-benzene)chromium

Two-color multiphoton mass-analyzed threshold ionization (MATI) spectra of jet-cooled (η6-C6D6)2Cr and (η6-C6D5H)(η6-C6D6)Cr have been obtained for the first time with excitation of the sandwich molecules via vibronic components of the 3dz2 → R4px,y Rydberg transition. The ionization energies determined with extremely high accuracy (±0.0004 eV) on the basis of the MATI experiment are 5.4558 and 5.4562 eV, respectively. The simultaneous measurement of the two MATI spectra in one experiment reveals clearly the increase in the ionization energy caused by substitution of one deuterium atom in (η6-C6D6)2Cr with hydrogen. The MATI experiment demonstrates a nonlinear dependence of the ionization potential on the number of deuterium atoms in the bis(η6-benzene)chromium molecule. Deuteration appears to decrease both the ionization energy and the R4px,y Rydberg term value. The decrease in the term value is a result of the Jahn−Teller instability of the R4px,y Rydberg state. The ground-state cation vibrational frequencies of the totally symmetric valence metal−ligand stretch ν21 and CH umbrella vibration ν11as determined from the MATI spectra are, respectively, 249 ± 3 and 594 ± 3 cm-1 for (η6-C6D6)2Cr+ and 251 ± 3 and 595 ± 3 cm-1 for (η6-C6D5H)(η6-C6D6)Cr+. These values correspond to the free gas-phase cations.

Vacuum Ultraviolet Photoionization of C3

Photoionization efficiency (PIE) curves for C(3) molecules produced by laser ablation are measured from 11.0 to 13.5 eV with tunable vacuum ultraviolet undulator radiation. A step in the PIE curve versus photon energy, obtained with N(2) as the carrier gas, supports the conclusion of very effective cooling of C(3) to its linear (1)Sigma(g)(+) ground state. The second step observed in the PIE curve versus photon energy could be the first experimental evidence of the C(3)(+)((2)Sigma(g)(+)) excited state. The experimental results, complemented by ab initio calculations, suggest a state-to-state vertical ionization energy of 11.70 +/- 0.05 eV between the C(3)(X(1)Sigma(g)(+)) and the C(3)(+)(X(2)Sigma(u)(+)) states. An ionization energy of 11.61 +/- 0.07 eV between the neutral and ionic ground states of C(3) is deduced using the data together with our calculations. Accurate ab initio calculations are performed for both linear and bent geometries on the lowest doublet electronic states of C(3)(+) using Configuration Interaction (CI) approaches and large basis sets. These calculations confirm that C(3)(+) is bent in its electronic ground state, which is separated by a small potential barrier from the (2)Sigma(u)(+) minimum. The gradual increase at the onset of the PIE curve suggests a geometry change between the ground neutral and cationic states. The energies between several doublet states of the ion are theoretically determined to be 0.81, 1.49, and 1.98 eV between the (2)Sigma(u)(+) and the (2)Sigma(g)(+),( 2)Pi(u), (2)Pi(g) excited states of C(3)(+), respectively.

Ionization Spectroscopy of a DNA Base: Vacuum-Ultraviolet Mass-Analyzed Threshold Ionization Spectroscopy of Jet-Cooled Thymine

The high-resolution ionization spectroscopy of DNA bases is reported for the first time. Vacuum-UV mass-analyzed threshold ionization (VUV-MATI) spectrum of jet-cooled thymine provides not only the most precise ionization potential but also its vibrational structure in the ground cationic state.

The Maximin Principle of π-Radical Packings

A two-term model is proposed for hydrocarbon and N-containing pi-radicals which are in close contact with one another. The first term is attractive (due to partially occupied frontier pi-orbitals), and the second, repulsive (due to hard-core repulsion between close-lying atoms). This model is applied to dimers where intermolecular contacts are closer than <0.95 x the sum of the atomic van der Waals radii. The maximin principle is proposed. The maximin principle states that the lowest energy conformation maximizes overlap of the frontier orbitals while simultaneously minimizing intermolecular contacts. A Hückel Hamiltonian, the mu(2)-Hamiltonian, which contains the above attractive and repulsive terms, is applied. The interaction surfaces of two pi-hydrocarbon radical cations were calculated for the three systems known crystallographically to contain cations in close contact: naphthalene, fluoranthene, and pyrene. The global minima of these surfaces correspond to the experimentally determined structures. The mu(2)-Hamiltonian energy surfaces of the naphthalene cation dimer are qualitatively similar to those calculated at the RHF/6-311G(d,p) and MP2/6-311G(d,p) levels. The maximin principle is applied to N-containing pi-radicals. Except in the case of tetracyanoethene, the maximin principle correctly predicts the most common dimer crystal packing. (MgPc)(NO(3)).0.5THF and (MgPc)(ReO(4)).1.5THF (Pc = phthalocyanine) were prepared: both new crystal structures follow the maximin principle. The maximin principle is used to suggest the dimer cation ground state of oligoacenes, cations important as organic hole-based semiconductors.

Comprehensive Photochemistry and Photophysics of Land‐ and Marine‐based β‐carbolines Employing Time‐resolved Emission and Flash Transient Spectroscopy

The photophysical and photochemical behavior of Norharmane (Norh), Harmane (Hara) and Harmine (Hari) and their cations have been examined as a function of the nature of the solvent. Time-resolved emission in nonprotic polar solvents showed fluorescence for all and also phosphorescence for Hari. All emissions were assigned as those of the neutral molecules. Norh and Hari showed fluorescence of both the neutral and the cation in methanol as well as phosphorescence of the neutral while Hari also had fluorescence of the zwitter ion. In ethanol, Norh and Hari displayed fluorescence and phosphorescence of the neutral. The ground-state cations of Norh and Hari exhibited fluorescences of the cation and Hari also had a phosphorescence (cation). The flash transient spectra in nonprotic solvents of all three carbolines had long-lived triplet transients only of the neutral. Triplet and singlet oxygen yields were quite high, 0.31-0.40. Direct excitation of any of the cations gave only the cation triplet. The triplet yields of the cations appear to be low (0.01-0.10 range). Theoretical calculations were done relative to location of triplet states. Some new information will be reported on other naturally occurring differently substituted marine-based beta-carbolines. The impact of all of the foregoing observations on the photosensitizing potential of all compounds is discussed.

Energetics and chemical bonding of the 1,3,5-tridehydrobenzene triradical and its protonated form

Quantum chemical calculations were applied to investigate the electronic structure of the parent 1,3,5-tridehydrobenzene triradical (C 6H 3, TDB) and its anion ( C 6 H 3 - ) , cation ( C 6 H 3 + ) and protonated form ( C 6 H 4 + ) . Our results obtained using the state-averaged complete active space self-consistent-field (CASSCF) followed by second-order multi-state multi-configuration perturbation theory, MS-CASPT2, and MRMP2 in conjunction with the large ANO-L and 6-311++G(3df,2p) basis set, confirm and reveal the followings: (i) TDB has a doublet 2A 1 ground state with a 4B 2– 2A 1 energy gap of 29 kcal/mol, (ii) the ground state of the C 6 H 3 - anion in the triplet 3B 2 being 4 kcal/mol below the 1A 1 state. (iii) the electron affinity (EA), ionization energy (IE) and proton affinity (PA) are computed to be: EA = 1.6 eV, IE = 7.2 eV, PA = 227 kcal/mol using UB3LYP/6-311++G(3df,2p) + ZPE; standard heat of formation Δ H f(298 K, 1 atm) (TDB) = 179 ± 2 kcal/mol was calculated with CBS-QB3 method. An atoms-in-molecules (AIM) analysis of the structure reveals that the topology of the electron density is similar in all compounds: hydrogens connect to a six-membered ring, except for the case of the 2A 2 state of C 6 H 4 + (MBZ +) which is bicyclic with fused five- and three-membered rings. Properties of the chemical bonds were characterized with Electron Localization Function (ELF) analysis, as well as Wiberg indices, Laplacian and spin density maps. We found that the radicals form separate monosynaptic basins on the ELF space, however its pair character remains high. In the 2A 1 state of TDB, the radical center is mainly localized on the C1 atom, while in the 2B 2 state it is equally distributed between the C3 and C5 atoms and, due to the symmetry, in the 4B 2 state the C1, C2 and C3 atoms have the same radical character. There is no C3–C5 bond in the 2A 1 state of TDB, but the interaction between these atoms is strong. The ground state of cation C 6 H 3 + (DHP), 1A 1, is not a diradical and has a doubly aromatic character. Aromaticity of the different compounds was studied within the ELF framework and the standard deviation of the bond lengths and bond orders. The Jahn–Teller distorted 2A 1 and 2B 2(C 2v) states of TDB were found to exhibit an aromaticity comparable to that of benzene. Overall protonation of the TDB reinforces the stability of the low-spin doublet states, the classical Hund’s rule is not obeyed. In a view, these species could better be regarded as radicals than triradicals.

Structure of Gas Phase Radical Cation of 1,3,6,8-Tetraazatricyclo[4.4.1.13,8] Dodecane Determined from Zero Kinetic Energy Photoelectron Spectroscopy

We report gas-phase vibrational spectroscopy of the ground-state cation of 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane (TTD) using two-color two-photon zero kinetic energy photoelectron spectroscopy. From the distribution of active vibrational modes and comparisons between the experiment and theoretical simulation, we offer proof that the cationic state and the first electronically excited state have the same D(2d) symmetry.

High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopic study of the two lowest electronic states of the ozone cation O3+

The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of jet-cooled O3 has been recorded in the range 101,000-104,000 cm(-1). The origins of the X 1A1-->X+ 2A1 and X 1A1-->A+ 2B2 transitions could be determined from the rotational structure of the bands, the photoionization selection rules, the photoionization efficiency curve, and comparison with ab initio calculations. The first adiabatic ionization energy of O3 was measured to be 101,020.5(5) cm(-1) [12.524 95(6) eV] and the energy difference between the X+ 2A1 (0,0,0) and A+ 2B2 (0,0,0) states was determined to be DeltaT0=1089.7(4) cm(-1). Whereas the X-->X+ band consists of an intense and regular progression in the bending (nu2) mode observed up to v2+=4, only the origin of the X-->A+ band was observed. The analysis of the rotational structure in each band led to the derivation of the r0 structure of O3+ in the X+ [C2v,r0=1.25(2) A,alpha0=131.5(9) degrees ] and A+[C2v,r0=1.37(5) A,alpha0=111.3(38) degrees ] states. The appearance of the spectrum, which is regular up to 102,300 cm(-1), changes abruptly at approximately 102,500 cm(-1), a position above which the spectral density increases markedly and the rotational structure of the bands collapses. On the basis of ab initio calculations, this behavior is attributed to the onset of large-amplitude motions spreading through several local minima all the way to large internuclear distances. The ab initio calculations are consistent with earlier results in predicting a seam of conical intersections between the X+ and A+ states approximately 2600 cm(-1) above the cationic ground state and demonstrate the existence of potential minima at large internuclear distances that are connected to the main minima of the X+ and A+ states through low-lying barriers.

Electronic excitation and ionisation of 1-azabicyclo[2.2.2]octane (ABCO): evidence for a twisted structure from experiment and theory

The first excited neutral and ground cationic states of 1-azabicyclo[2.2.2]octane (ABCO) have been studied using a combination of (1 + 1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. In common with the related tertiary alkyl amines, analysis of both REMPI and ZEKE spectra indicate that the overriding impulse upon electronic excitation and ionisation is for a flattening of the nitrogen end of the molecule, albeit hindered by the cage. In addition, the activity of the torsion mode (19) in both REMPI and ZEKE spectra suggests geometry changes along the torsion coordinate between the three electronic states sampled in these experiments. Ab initio molecular orbital calculations conducted at the MP2/cc-pVDZ level of theory yield minimum energy geometries with CNCC dihedral angles of 6.011° and 7.86° for the S 0 and D 0 states, respectively, which indicates that the torsion coordinate is described by a double minimum potential. However, the degree of perturbation to the vibrational eigenvalues suggests that the zero point level lies a few wavenumbers above a small barrier and that ABCO is pseudo-twisted with a vibrationally averaged C 3v symmetry.

Observation of rotamers of m-aminobenzoic acid: Zero kinetic energy photoelectron and hole-burning resonantly enhanced multiphoton ionization spectroscopy

We report studies of supersonically cooled m-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. Two conformers have been identified and characterized using the hole-burning method in the REMPI experiment. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S(1)) of the neutral species and those of the ground state cation (D(0)) have been assigned, and the adiabatic ionization potentials have been determined for both conformers. The REMPI spectra are dominated by in-plane motions of the substituents and ring deformation modes. A propensity of Deltav=0, where Deltav is the change in vibrational quantum number from the S(1) to the D(0) state, is observed in the ZEKE spectra. The origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. Comparisons of these results with those of p-aminobenzoic acid will be analyzed.

The effects of changes in the electronic environment of the 2-aminopyridine–argon van der Waals complex

The first excited neutral and ground cationic states of the 2-aminopyridine–argon van der Waals complex have been studied using a combination of (1 + 1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. In addition to an updated ionisation energy of 65 265 ± 3 cm −1, an analysis of the intermolecular vibrational structure observed in both REMPI and ZEKE spectra supports an assignment to a π-bound complex. The pyramidal conformation of the amino group in the electronic ground state allows for the possibility of anti and syn π-bound conformers: this is discussed in the context of the related molecule, aniline–Ar.

Two-color two-photon REMPI and ZEKE spectroscopy of supersonically cooled o-aminobenzoic acid

We report studies of supersonically cooled o-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy. Vibrational modes of the first electronically excited state (S 1) of the neutral species and those of the ground state cation (D 0) have been assigned, and the ionization threshold has been determined. The results are analyzed in comparison with the other two isomers of aminobenzoic acid, mainly in terms of electron back donation of the two substituents in both the S 1 and the D 0 states. The change in the intramolecular hydrogen bond upon electronic excitation and ionization will also be discussed.

High-lying Rydberg states of vinyl bromide studied by two-photon resonant ionization spectroscopy

High-lying Rydberg states of vinyl bromide at 70 000–79 500 cm −1 have been investigated using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Seven Rydberg series, including one s, three p, two d, and one f, have been identified from fitting the term values of observed electronic states to Rydberg formula rendering the ionization energy of 79 194 ± 17 cm −1 (9.819 ± 0.002 eV). All of the seven series converge to the same ionization-energy limit, corresponding to the ground state of vinyl bromide cation. The adiabatic ionization energy of vinyl bromide determined from this study is in excellent agreement with the values reported recently by other methods.

Two-color two-photon REMPI and ZEKE photoelectron spectroscopy of jet-cooled 2-chloropyrimidine

We report studies of supersonically cooled 2-chloropyrimidine (2-ClP) using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S 1) of the neutral species and those of the ground state cation (D 0) have been assigned, and the adiabatic ionization potential has been determined to be 77 106 ± 4 cm −1 (9.5600 ± 0.0005 eV). These results will be discussed in comparison with pyrimidine and other related compounds. Although the rate of intersystem crossing is affected by halosubstitution, 2-ClP and pyrimidine have qualitatively similar photophysical behaviors.

Theoretical Investigation of Jahn−Teller Dynamics in the 2E‘ Electronic Ground State of the Cyclopropane Radical Cation

The static and dynamic aspects of (E x e)-Jahn-Teller (JT) interactions in the electronic ground state (X 2 E') of the cyclopropane radical cation are investigated with the aid of an ab initio based quantum dynamical approach. The valence photoelectron spectrum of cyclopropane pertinent to an ionization to the X 2 E' electronic manifold of its radical cation is calculated and compared with the most recent experimental recording of Holland and co-workers using He I and synchrotron radiation as excitation sources [J. Electron Spectrosc. Relat. Phenom. 2002, 57, 125]. A model diabatic Hamiltonian up to a quadratic vibronic coupling scheme and ab initio calculated coupling parameters are employed in the quantum dynamical simulations. Despite some minor details, the theoretical results are in good accord with the observed bimodal shape of the photoelectron band. The observed splitting of the maxima of ∼0.78 eV in the bimodal profile compares well with our theoretical value of ∼0.76 eV. A strong first-order JT activity of the degenerate vibrational modes is discovered, which results in the distinct twin structure of the photoelectron band, indicating transitions to both the sheets of the so-called Mexican hat potential energy surface of the X 2 E' electronic ground state of the radical cation. Two Condon active (A' 1 ) and three JT active (E') vibrational modes are found to contribute mostly to the nuclear dynamics in this electronic manifold. The low-energy progression in the photoelectron band is found to be mainly caused by the degenerate CH 2 wagging and ring deformation modes. While the linear vibronic coupling scheme overestimates the observed spacing in the low-energy progressions, it leads to a very good agreement with the overall shape of the observed band. The effect of quadratic coupling terms of the Hamiltonian on this low-energy progression is also discussed.

Theoretical study of the hydrogen atom transfer in the heterodimer indole-ammonia and comparison with experimental results.

Ab initio calculations on the heterodimer C8H6NH...NH3 are carried out for its ground, the excited pisigma*, and the ground cationic electronic states, enabling the description of hydrogen or proton transfer, respectively. Two-dimensional quantum-dynamical computations on the pisigma* potential surface help one to understand the mechanism and the time scale of the hydrogen transfer. Subsequent decay processes are discussed depending on the vibrational excitation of the ammonium constituent. Finally, the theoretical results obtained are used for the interpretation of the time-dependent signals observed in femtosecond pump-probe experiments.

Ionization from a double bond: rovibronic photoionization dynamics of ethylene, large amplitude torsional motion and vibronic coupling in the ground state of C2H4+.

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X-->X+ transition in ethylene and ethylene-d4 have been recorded at a resolution of 0.09 cm(-1). The spectra provide new information on the large amplitude torsional motion in the cationic ground state. An effective one-dimensional torsional potential was determined from the experimental data. Both C2H4+ and C2D4+ exhibit a twisted geometry, and the lowest two levels of the torsional potential form a tunneling pair with a tunneling splitting of 83.7(5) cm(-1) in C2H4+ and of 37.1(5) cm(-1) in C2D4+. A model was developed to quantitatively analyze the rotational structure of the photoelectron spectra by generalizing the model of Buckingham, Orr, and Sichel [Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] to treat asymmetric top molecules. The quantitative analysis of the rotational intensity distributions of allowed as well as forbidden vibrational bands enabled the identification of strong vibronic mixing between the X+ and A+ states mediated by the torsional mode nu(4) and a weaker mixing between the X+ and B+ states mediated by the symmetric CH2 out-of-plane bending mode nu7. The vibrational intensities could be accounted for quantitatively using a Herzberg-Teller-type model for vibronic intensity borrowing. The adiabatic ionization energies of C2H4 and C2D4 were determined to be 84 790.42(23) cm(-1) and 84 913.3(14) cm(-1), respectively.

Quantifying ground-state complexation between Ag+ and polycyclic aromatic hydrocarbons in dilute aqueous solution via fluorescence quenching

Interactions of Ag+ with naphthalene and pyrene in aqueous solution were investigated using ultraviolet (UV) absorption and steady-state and time-resolved fluorescence spectroscopies. Small red-shifts in the two primary absorption bands of naphthalene and pyrene were observed in the presence of high concentrations of Ag+, indicating that ground-state cation–aromatic π electron interactions occurred. Ag+ complexation constants (K1 and K2) for naphthalene were determined directly from steady-state and time-resolved fluorescence data, whereas the formation of a pyrene–Ag+ exciplex required an additional correction to remove its interference on apparent pyrene complexation constants. The correction utilized ratios of the exciplex and monomer preexponential factors obtained from pyrene fluorescence decay curves measured at several emission wavelengths that were impacted to different degrees by the exciplex emission. The novel approach developed here to quantify ground-state complexes between Ag+ and polycyclic aromatic hydrocarbons (PAHs) offers new opportunities to investigate weak metal–organic complexes such as those resulting from cation–π interactions.