Neutral Ground State Research Articles

Photodetachment-photoelectron spectroscopy of disulfanide: the ground and first excited electronic state of HS2 and DS2

Photodetachment-photoelectron spectra of HS(2)(-) and DS(2)(-) are presented. The obtained electron affinities (EA (HS(2)) = 1.916 +/- 0.015 eV and EA (DS(2)) = 1.918 +/- 0.015 eV) are in good agreement with earlier results (S. Moran and G. B. Ellison, J. Phys. Chem., 1988, 92, 1794). Photodetachment into the neutral ground state of the radicals HS(2) and DS(2) leads to excitation of the S-S stretching mode, whereas photodetachment into the first excited state causes a S-S-H bending and a weak S-H stretching motion. Additionally, weak peaks of S(2) were observed in the spectra of HS(2) and DS(2), leading to the conclusion that photodetachment with 2.77 eV (448 nm) causes dissociation of HS(2)(-) and DS(2)(-) into S(2)(-) and H/D. Taking 2.77 eV as an upper limit for D(0)(S(2)(-)-H) a lower limit for Delta(f)H(0)(HS(2 (g))) of 100.8 kJ mol(-1) is obtained which is an improvement on the literature value of 107.145 +/- 10.46 kJ mol(-1).

Chirality-dependent hydrogen bond direction in jet-cooled (S)-1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM): IR-ion dip vibrational spectroscopy of the neutral and the ion

The structural modifications of (S)-1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM) upon ionisation have been investigated in jet-cooled conditions, by means of laser-induced fluorescence, REMPI, and IR-UV ion-dip spectroscopy of the neutral ground state and the ion. These results combined with ab initio calculations, support the presence in the jet of two low-energy conformers of THIQM. In the most stable Conformer I, the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position. In this case, the nitrogen atom is in (S) configuration. Conformer II shows the opposite NHO hydrogen bond from the hydrogen atom in the equatorial position of nitrogen to the OH group. In this case, the nitrogen atom is in (R) configuration. This chirality dependence of the hydrogen bond direction is lost upon ionisation. While ionisation of Conformer II reinforces the NHO hydrogen bond, ionisation of Conformer I induces its isomerisation to the same ion as Conformer II, i.e. a change in hydrogen bond direction.

Neutral Möbius Aromatics: Derivatives of the Pyrrole Congener Aza[11]annulene as Promising Synthetic Targets

AbstractAmino‐ and methyl‐disubstituted derivatives of an aromatic 12π electron pyrrole homologue Möbius 1H‐aza[11]annulene are predicted computationally at the B3LYP/6‐31G* level of theory to be distinctly aromatic and a neutral ground state on its potential surface. The parent annulene has a nonaromatic conformation, which is 6.0 kcal mol–1 lower in energy at the CCSD(T)/D95*//B3LYP/6‐31G* level of theory than a structurally very similar Möbius conformation. After derivatization, however, geometry optimizations of the two lowest‐energy – Möbius and non‐Möbius – heteroannulene conformations both converged two a single Möbius minimum, whereas two (Möbius and non‐Möbius) distinct difluoro‐ substituted species could be located 1.8 kcal mol–1 apart in energy at the B3LYP level. The Möbius minimum is 7.5 kcal mol–1 lower in energy at the coupled cluster level than a “Hückel topology” form derived from a known bridged 1H‐[4,9]methano[11]annulene. Several possible reactive modes are investigated in order to assess the relative stability of the Möbius parent annulene. In addition, we report formal novel examples for charged and neutral Möbius triplet aromatic annulenic and heteroannulenic systems, some of which also exhibit distinct aromatic properties. Further fundamental insights into the phenomenon of “Möbius aromaticity” are suggested. The aromaticity of Möbius annulenes is for the first time linked to wavefunction boundary conditions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Excitonic spin-state preservation mediated by optical-phonon resonant excitation in a single quantum dot

High degree of excitonic spin-state preservation during energy relaxation is demonstrated in a single quantum dot. Optical-phonon resonances and corresponding suppression of spin relaxation are clearly identified as dip structures in photoluminescence excitation spectra probed by the positive trion emission. Consequently, under the longitudinal optical-phonon resonant excitation, a distinguishably high degree of circular polarization up to $\ensuremath{\sim}0.85$ is achieved. Extended spin-relaxation time in the positive trion ground state compared to that in neutral exciton ground state is also revealed.

Diversity and evolution of coral fluorescent proteins.

GFP-like fluorescent proteins (FPs) are the key color determinants in reef-building corals (class Anthozoa, order Scleractinia) and are of considerable interest as potential genetically encoded fluorescent labels. Here we report 40 additional members of the GFP family from corals. There are three major paralogous lineages of coral FPs. One of them is retained in all sampled coral families and is responsible for the non-fluorescent purple-blue color, while each of the other two evolved a full complement of typical coral fluorescent colors (cyan, green, and red) and underwent sorting between coral groups. Among the newly cloned proteins are a “chromo-red” color type from Echinopora forskaliana (family Faviidae) and pink chromoprotein from Stylophora pistillata (Pocilloporidae), both evolving independently from the rest of coral chromoproteins. There are several cyan FPs that possess a novel kind of excitation spectrum indicating a neutral chromophore ground state, for which the residue E167 is responsible (numeration according to GFP from A. victoria). The chromoprotein from Acropora millepora is an unusual blue instead of purple, which is due to two mutations: S64C and S183T. We applied a novel probabilistic sampling approach to recreate the common ancestor of all coral FPs as well as the more derived common ancestor of three main fluorescent colors of the Faviina suborder. Both proteins were green such as found elsewhere outside class Anthozoa. Interestingly, a substantial fraction of the all-coral ancestral protein had a chromohore apparently locked in a non-fluorescent neutral state, which may reflect the transitional stage that enabled rapid color diversification early in the history of coral FPs. Our results highlight the extent of convergent or parallel evolution of the color diversity in corals, provide the foundation for experimental studies of evolutionary processes that led to color diversification, and enable a comparative analysis of structural determinants of different colors.

Effect of Noncovalent Interactions on the n-Butylbenzene···Ar Cluster Studied by Mass Analyzed Threshold Ionization Spectroscopy and ab initio Computations

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB...Ar clusters, and red shifts for anti-BB...Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I...Ar and BB-V...Ar were determined as 70052 and 69845 +/- 5 cm (-1), respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I...Ar and BB-V...Ar were obtained as 650 and 558 cm (-1), respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB...Ar in the excited neutral S 1 and the D 0 cationic ground states.

Vibrational and photoionization spectroscopy of biomolecules: Aliphatic amino acid structures

The aliphatic amino acids glycine, valine, leucine, and isoleucine are thermally placed into the gas phase and expanded into a vacuum system for access by time of flight mass spectroscopy and infrared (IR) spectroscopy in the energy range of 2500-4000 cm(-1) (CH, NH, OH, and stretching vibrations). The isolated neutral amino acids are ionized by a single photon of 10.5 eV energy (118 nm), which exceeds by less than 2 eV their reported ionization thresholds. As has been reported for many hydrogen bonded acid-base systems (e.g., water, ammonia, alcohol, acid clusters, and acid molecules), the amino acids undergo a structural rearrangement in the ion state (e.g., in simplest form, a proton transfer) that imparts sufficient excess vibrational energy to the ion to completely fragment it. No parent ions are observed. If the neutral ground state amino acids are exposed to IR radiation prior to ionization, an IR spectrum of the individual isomers for each amino acid can be determined by observation of the ion intensity of the different fragment mass channels. Both the IR spectrum and fragmentation patterns for individual isomers can be qualitatively identified and related to a particular isomer in each instance. Thus, each fragment ion detected presents an IR spectrum of its particular parent amino acid isomer. In some instances, the absorption of IR radiation by the neutral amino acid parent isomer increases a particular fragmentation mass channel intensity, while other fragmentation mass channel intensities decrease. This phenomenon can be rationalized by considering that with added energy in the molecule, the fragmentation channel populations can be modulated by the added vibrational energy in the rearranged ions. This observation also suggests that the IR absorption does not induce isomerization in the ground electronic state of these amino acids. These data are consistent with theoretical predictions for isolated amino acid secondary structures and can be related to previous IR spectra of amino acid conformers.

The structure and energetics of (GaAs)n, (GaAs)n−, and (GaAs)n+ (n=2–15)

Electronic and geometrical structures of neutral, negatively, and positively charged (GaAs)n clusters are computed using density functional theory with generalized gradient approximation. All-electron computations are performed on (GaAs)2-(GaAs)9 while effective core potentials (ECPs) are used for (GaAs)9-(GaAs)15. Calibration calculations on GaAs and (GaAs)9 species support the use of the ECP for the larger clusters. The ground-state geometries of (GaAs)n(-) and/or (GaAs)n+ are different from the corresponding neutral ground-state geometry, except for n=7, 9, 12, 14, and 15, where the neutral and ions have similar structures. Beginning with n=6, all atoms are three coordinate, except for (GaAs)10+ and (GaAs)13+. For the larger species, there is a competition between fullerenes built from hexagons and rhombi and geometrical configurations where Ga-Ga and As-As bonds are formed, which results in the formation of pentagons. As expected, the static polarizability varies in the order of anion>neutral>cation, but the values are rather similar for all three charge states. The thermodynamic stability for the loss of GaAs is reported.

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.

Supramolecular Approach to the Synthesis of [60]Fullerene−Metal Dithiocarbamate Complexes, {(MII(R2dtc)2)x·L}·C60 (M = Zn, Cd, Hg, Fe, and Mn; x = 1 and 2). The Study of Magnetic Properties and Photoconductivity

Coordination assemblies formed by metal(II) dithiocarbamates MII(R2dtc)2 (M = Zn, Cd, Hg, Mn, and Fe; R = Me, Et, iPr, nPr, and nBu) and nitrogen-containing ligands (L = DABCO, DMP, and HMTA) are shown to be sterically well consistent with the spherical shape of fullerenes and are effectively cocrystallized with them to form complexes {(MII(R2dtc)2)x·L}·C60 (x = 1 and 2) with layered or three-dimensional (3D) packing of fullerene molecules (1−11). For the first time the molecular structures of the MII(R2dtc)2 assemblies in the complexes with C60 are presented (including those with M = Mn, Fe, and Hg). According to optical and electron paramagnetic resonance (EPR) spectroscopy 1−11 have a neutral ground state. Magnetic susceptibilities of {[MII(Et2dtc)2]2·DABCO}·C60·(DABCO)2 (M = Mn and Fe) follow the Curie–Weiss law in the 50–300 K range with the Weiss constants Θ = 0.35 and 1.70 K, respectively. Magnetic moments of the complexes equal to 8.23 and 6.88 μB at 300 K correspond to the high-spin state of the ...

Characterization of cyclic and linear C3H− and C3H via anion photoelectron spectroscopy

Anion photoelectron spectroscopy of C3H- and C3D- is performed using both field-free time-of-flight and slow electron velocity-map imaging. We observe and assign transitions originating from linear/bent (l-C3H) and cyclic (c-C3H) anionic isomers to the corresponding neutral ground states and low-lying excited states. Transitions within the cyclic and linear manifolds are distinguished by their photoelectron angular distributions and their intensity dependence on the neutral precursor. Using calculated values for the energetics of the neutral isomers [Ochsenfeld et al., J. Chem. Phys. 106, 4141 (1997)], which predict c-C3H to lie 74 meV lower than l-C3H, the experimental results establish c-C3H- as the anionic ground state and place it 229 meV below l-C3H-. Electron affinities of 1.999+/-0.003 and 1.997+/-0.005 eV are determined for C3H and C3D from the X 2B2<--X 1A1 photodetachment transition of c-C3H. Term energies for several low-lying states of c-C3H and l-C3H are also determined. Franck-Condon simulations are used to make vibrational assignments for the bands involving c-C3H. Simulations of the l-C3H bands were more complicated owing to large amplitude bending motion and, in the case of the neutral A 2Pi state, strong Renner-Teller coupling.

Calculations ofNd−binding energies and photodetachment partial cross sections

The electron affinity for neodymium is determined from relativistic configuration interaction calculations to be 169 meV, and six additional bound excited states are also predicted. These $6p$ attachments to the $4{f}^{4}6{s}^{2}$ neutral ground-state configuration are carefully analyzed with respect to approximate $LS$ of the total configuration and $LSJ$ of the $4{f}^{4}$ subgroup. This analysis is used to drastically reduce the basis size of the calculations and preselect channels for potentially large photodetachment partial cross sections. Estimations of the effects of the mixing of resonance states in the partial cross-section calculations suggest that for incident photon energies $\ensuremath{\sim}2.0\text{ }\text{eV}$ or larger, $6s$ detachments to excited $4{f}^{4}6s6p$ thresholds will represent the dominant channels of the total cross section.

The low-lying electronic states of ArXe+ and their potential energy functions

Photoionization and pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of ArXe have been recorded between 96 400 and 108 200 cm(-1) following resonance-enhanced two-photon excitation via selected vibrational levels of the C 1 and D 0+ Rydberg states. The PFI-ZEKE photoelectron spectra consist of three vibrational progressions corresponding to the X 1/2<--X 0+, A1 3/2<--X 0+, and A2 1/2<--X 0+ transitions. From these progressions, adiabatic ionization energies, equilibrium internuclear distances, and vibrational constants have been derived for the lowest three electronic states of ArXe+. The photoionization spectra reveal long progressions of autoionizing Rydberg states converging to the lowest vibrational levels of the A1 3/2 state. A potential model has been developed that enables a global description of the low-lying electronic states of the heteronuclear rare gas dimer ions. The model explicitly treats the effects of the spin-orbit, charge-exchange, and long-range interactions. This model was used to obtain potential energy functions for all six low-lying electronic states of ArXe+ from the experimental positions of the vibrational levels of the X 1/2, A1 3/2, and A2 1/2 states relative to the ground neutral state and existing spectroscopic data on the B 1/2, C1 3/2, and C2 1/2 states.

Redox and debromination reactions of brominated hypericin

AbstractPhototoxic and radical‐generating debromination reactions of monobrominated hypericin with bromine at one of four possible positions were investigated using density functional theory. The study was performed on two closely lying conformational isomers, differing in the relative orientations of the two anthracene units of the hypericin core. Calculated adiabatic electron affinities show that the molecules have the ability to, in aqueous solution, extract an electron from the surrounding. The electron might then be passed on to molecular oxygen, forming reactive superoxide radical anions. If electron extraction from the molecule does not occur in this step, the molecule might dissociate, generating a negatively charged bromine as a leaving group and a hypericin radical capable of forming direct binding to biological molecules. This reaction was found possible for those species substituted by Br at two of the four positions, with barriers of ∼13 kcal/mol in aqueous solution. Debromination was not found energetically possible for neither the neutral ground state compounds nor the bay‐deprotonated species. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

ON THE EVALUATION OF THE RIGOROUS ELECTROSTATIC POTENTIAL/ATOMIC ENERGY RELATIONSHIP

A series of atomic energy formulas that relate atomic energies to the electrostatic potentials V0 at nuclei are obtained by a series of polynomial and series fits of V0 versus nuclear charge (Z). Density functional and Hartree–Fock V0 are used for a series of fits that involve an isoelectronic series of anions, cations, and neutral ground state atoms to approximate atomic energies. Comparisons to the exact energies were performed in order to demonstrate the efficacy of the rigorous expressions.

Perfluorocyclobutane electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, electron impact,HeIphotoelectron spectroscopy, andab initiocalculations

The electronic state spectroscopy of perfluorocyclobutane $(c\text{\ensuremath{-}}{\mathrm{C}}_{4}{\mathrm{F}}_{8})$ has been investigated using high resolution vacuum ultraviolet (vuv) photoabsorption spectroscopy in the energy range $6.0--11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The electron energy loss spectrum (EELS) was also recorded in the nonelectric dipolar interaction mode ($100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ incident energy, 10\ifmmode^\circ\else\textdegree\fi{} scattering angle) over the $8--14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ energy-loss range and the excited states in the $11--14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ spectral region have been observed. An $\mathrm{He}\phantom{\rule{0.2em}{0ex}}I$ photoelectron spectrum recorded between 11.0 and $19.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is compared with earlier lower resolution results. This has allowed us to derive a more precise value of $12.291\ifmmode\pm\else\textpm\fi{}0.002\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the ground neutral state vertical ionization energy. All spectra presented in this paper represent the highest resolution data yet reported for perfluorocyclobutane, to the best of our knowledge. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.

IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters

IR spectra of phenol-Arn (PhOH-Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus pi bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, nuOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH-Ar2 as well as cationic PhOH+-Ar have a pi-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+-Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This pi-->H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The pi-bound nuOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound nuOH band. The analysis of the picosecond IR spectra demonstrates that (i) the pi-->H site switching is an elementary reaction with a time constant of approximately 7 ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100 cm(-1)), (iii) both the position and the width of the H-bound nuOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from pi bonding to H bonding in the PhOH+-Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.

Trapping of Neutral Rubidium with a Macroscopic Three-Phase Electric Trap

We trap neutral ground-state rubidium atoms in a macroscopic trap based on purely electric fields. For this, three electrostatic field configurations are alternated in a periodic manner. The rubidium is precooled in a magneto-optical trap, transferred into a magnetic trap, and then translated into the electric trap. The electric trap consists of six rod-shaped electrodes in cubic arrangement, giving ample optical access. Up to 10;{5} atoms have been trapped with an initial temperature of around 20 microkelvin in the three-phase electric trap. The observations are in good agreement with detailed numerical simulations.

Diradicals, antiaromaticity, and the pseudo-Jahn-Teller effect: Electronic and rovibronic structures of the cyclopentadienyl cation

The electronic and rovibronic structures of the cyclopentadienyl cation (C(5)H(5) (+)) and its fully deuterated isotopomer (C(5)D(5) (+)) have been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy and ab initio calculations. The vibronic structure in the two lowest electronic states of the cation has been determined using single-photon ionization from the X (2)E(1) (") ground neutral state and 1+1(') resonant two-photon ionization via several vibrational levels of the A (2)A(2) (") excited state. The cyclopentadienyl cation possesses a triplet ground electronic state (X(+) (3)A(2) (')) of D(5h) equilibrium geometry and a first excited singlet state (a(+) (1)E(2) (')) distorted by a pseudo-Jahn-Teller effect. A complete analysis of the Emultiply sign in circlee Jahn-Teller effect and of the (A+E)multiply sign in circlee pseudo-Jahn-Teller effect in the a(+) (1)E(2) (') state has been performed. This state is subject to a very weak linear Jahn-Teller effect and to an unusually strong pseudo-Jahn-Teller effect. Vibronic calculations have enabled us to partially assign the vibronic structure and determine the adiabatic singlet-triplet interval (1534+/-6 cm(-1)). The experimental spectra, a group-theoretical analysis of the vibronic coupling mechanisms, and ab initio calculations were used to establish the topology of the singlet potential energy surfaces and to characterize the pseudorotational motion of the cation on the lowest singlet potential energy surface. The analysis of the rovibronic photoionization dynamics in rotationally resolved spectra and the study of the variation of the intensity distribution with the intermediate vibrational level show that a Herzberg-Teller mechanism is responsible for the observation of the forbidden a(+) (1)E(2) (')<--A (2)A(2) (") photoionizing transition.

Hydration Process of Na2- in Small Water Clusters: Photoelectron Spectroscopy and Theoretical Study of Na2-(H2O)n

Photoelectron spectroscopy (PES) of Na2- (H2O)n (n < or = 6) was investigated to examine the solvation of sodium aggregates in small water clusters. The PES bands for the transitions from the anion to the neutral ground and first excited states derived from Na2 (1(1)Sigmag+) and Na2 (1(3)Sigmau+) shifted gradually to the blue, and those to the higher-excited states correlated to the 3(2)S + 3(2)P asymptote dropped down rapidly to the red and almost degenerated on the 1(3)Sigmau+-type band at n = 4. Quantum chemical calculations for n up to 3 showed that the spectra can be ascribed to structures where one of the Na atoms is selectively hydrated. From the electron distributions, it is found that the Na- -Na+(H2O)n- -type electronic state grows with increasing cluster size, which can be regarded as a sign of the solvation of Na2- with ionization of the hydrated Na.