Gas Phase Spectra Research Articles

Al2O4](-), a Benchmark Gas-Phase Class II Mixed-Valence Radical Anion for the Evaluation of Quantum-Chemical Methods.

The radical anion [Al2O4](-) has been identified as a rare example of a small gas-phase mixed-valence system with partially localized, weakly coupled class II character in the Robin/Day classification. It exhibits a low-lying C2v minimum with one terminal oxyl radical ligand and a high-lying D2h minimum at about 70 kJ/mol relative energy with predominantly bridge-localized-hole character. Two identical C2v minima and the D2h minimum are connected by two C2v-symmetrical transition states, which are only ca. 6-10 kJ/mol above the D2h local minimum. The small size of the system and the absence of environmental effects has for the first time enabled the computation of accurate ab initio benchmark energies, at the CCSDT(Q)/CBS level using W3-F12 theory, for a class-II mixed-valence system. These energies have been used to evaluate wave function-based methods [CCSD(T), CCSD, SCS-MP2, MP2, UHF] and density functionals ranging from semilocal (e.g., BLYP, PBE, M06L, M11L, N12) via global hybrids (B3LYP, PBE0, BLYP35, BMK, M06, M062X, M06HF, PW6B95) and range-separated hybrids (CAM-B3LYP, ωB97, ωB97X-D, LC-BLYP, LC-ωPBE, M11, N12SX), the B2PLYP double hybrid, and some local hybrid functionals. Global hybrids with about 35-43% exact-exchange (EXX) admixture (e.g., BLYP35, BMK), several range hybrids (CAM-B3LYP, ωB97X-D, ω-B97), and a local hybrid provide good to excellent agreement with benchmark energetics. In contrast, too low EXX admixture leads to an incorrect delocalized class III picture, while too large EXX overlocalizes and gives too large energy differences. These results provide support for previous method choices for mixed-valence systems in solution and for the treatment of oxyl defect sites in alumosilicates and SiO2. Vibrational gas-phase spectra at various computational levels have been compared directly to experiment and to CCSD(T)/aug-cc-pV(T+d)Z data.

GAS PHASE ABSORPTION SPECTROSCOPY OF AND IN A CRYOGENIC ION TRAP: COMPARISON WITH ASTRONOMICAL MEASUREMENTS*

ABSTRACT Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation is responsible for four diffuse interstellar bands (DIBs). These absorptions correspond to the strongest bands of the lowest electronic transition. The gas phase spectrum below is reported here for the full wavelength range encompassed by the electronic transition. The absorption spectrum of , with its origin band at , has been obtained under similar laboratory conditions. Observations made toward the reddened star were used in a specific search for the absorption of these fullerene cations in diffuse clouds. In the case of , one further band in the astronomical spectrum at is identified, increasing the total number of assigned DIBs to five. Numerous other absorptions in the laboratory spectrum are found to lie below the astronomical detection limit. Special emphasis is placed on the laboratory determination of absolute absorption cross-sections. For this directly yields a column density, , of in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the electronic transition in the range 7000–8000 Å is spread over many features of similar strength. Absorption cross-section measurements indicate that even for a similar column density, the individual absorption bands of will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of to the equivalent width.

Light Absorption of Contacted Molecules: Insights and Impediments from Atomistic Simulations

The absorption spectrum of a prototypical organic molecule (hydroquinone) in contact with semi-infinite metallic nanowires is investigated by means of atomistic parameter-free simulations. We employ an open boundary Liouville-van Neumann approach in conjunction with approximate time-dependent density functional theory to reveal changes in absorbance with respect to the gas phase spectrum. It is found that molecule–metal hybridization leads to state specific shifts in the band maxima and significant broadening. In addition, the line shape acquires a nonsymmetrical Fano character for strong molecule–metal coupling. As the strong plasmonic response of the metal leads overshadows molecular excitations, we show that construction of a differential cross section for the junction with and without embedded molecule effectively uncovers hidden spectral features.

Elucidation of the CO-Release Kinetics of CORM-A1 by Means of Vibrational Spectroscopy.

CO-releasing molecules (CORMs) are developed for investigations of the interaction between the signaling molecule carbon monoxide (CO) and cells or tissue. Prior to their application these molecules must be fully characterized with respect to their CO-release mechanism. One widely used CORM for biological application is sodium boranocarbonate (CORM-A1), which shows pH-dependent CO liberation. The complete reaction mechanism of CORM-A1 is not fully understood yet. Therefore, in this contribution time-resolved gas-phase IR spectroscopy is used to monitor the headspace above decaying CORM-A1 solutions at four different pH values (5.8 to 7.4). Borane carbonyl is found as an intermediate in the gas phase, which is formed during CORM degradation and further decays to CO. Concentration profiles of a pseudoconsecutive first-order reaction are successfully fitted to specific band areas of the measured gas-phase spectra, and the rate constants are obtained. The production of borane carbonyl is strongly pH dependent (half-lives between 5 and 106 min), whereas the decay of borane carbonyl in the gas phase is nearly constant with a half-life of about 33 min. The ratio of liberated CO molecules per CORM-A1 is determined to be 0.91±0.09, and boric acid is identified as further end product.

An experimental NEXAFS and computational TDDFT and ΔDFT study of the gas-phase core excitation spectra of nitroxide free radical TEMPO and its analogues

Core-hole spectroscopy adds to the fundamental understanding of the electronic structure of stable nitroxide free radicals thus paving way for a sensible design of new analogues with desired functionalities. We study the gas-phase C 1s, N 1s and O 1s excitation spectra of three nitroxide free radicals - TEMPO and two of its amide-substituted analogues - using the experimental NEXAFS technique and the theoretical TDDFT and ΔDFT methods in the unrestricted setting. The short-range corrected SRC1-BLYP and SRC2-BLYP exchange-correlation functionals are used with TDDFT, and the standard B3LYP functional with ΔDFT. The TDDFT-based detailed spectral assignment includes the valence, mixed valence-Rydberg and Rydberg portions of the spectra from the onset of absorptions to the vicinity of the core-ionization thresholds. The relative overlaps between the experimental and TDDFT-modelled spectra are reasonably good, in the range of 0.7-0.8, 0.6-0.8, and 0.7-0.8 for the C 1s, N 1s, and O 1s spectra, respectively. The extent of spin contamination within the unrestricted framework and its effect on the accuracy of the calculated excitation energies and dipole intensities are discussed in detail. It is concluded that, despite the sizeable spin contamination, the presently used methods are capable of predicting the core-excitation spectra of comparatively large free radical species fairly reliably over a wide spectral range.

The anharmonic quartic force field infrared spectra of three polycyclic aromatic hydrocarbons: Naphthalene, anthracene, and tetracene.

Current efforts to characterize and study interstellar polycyclic aromatic hydrocarbons (PAHs) rely heavily on theoretically predicted infrared (IR) spectra. Generally, such studies use the scaled harmonic frequencies for band positions and double harmonic approximation for intensities of species, and then compare these calculated spectra with experimental spectra obtained under matrix isolation conditions. High-resolution gas-phase experimental spectroscopic studies have recently revealed that the double harmonic approximation is not sufficient for reliable spectra prediction. In this paper, we present the anharmonic theoretical spectra of three PAHs: naphthalene, anthracene, and tetracene, computed with a locally modified version of the SPECTRO program using Cartesian derivatives transformed from Gaussian 09 normal coordinate force constants. Proper treatments of Fermi resonances lead to an impressive improvement on the agreement between the observed and theoretical spectra, especially in the C-H stretching region. All major IR absorption features in the full-scale matrix-isolated spectra, the high-temperature gas-phase spectra, and the most recent high-resolution gas-phase spectra obtained under supersonically cooled molecular beam conditions in the CH-stretching region are assigned.

Vibrational assignments and conformer stability determination of cyclobutyldichlorosilane by variable temperature Raman spectra in krypton solution

The infrared spectrum (3100–300cm−1) in the gas phase and Raman spectrum (3100–150cm−1) of the liquid of cyclobutyldichlorosilane (c-C4H7SiHCl2) have been recorded. Variable temperature (−143°C to −123°C) studies of the Raman spectra (3100–300cm−1) dissolved in liquid krypton have been carried out. From these data, the experimental stabilities of the conformer have been determined to be Eq-g>Ax-t>Eq-t>Ax-g. The enthalpy difference between Eq-g and Ax-t has been determined from band pairs to be 85±10cm−1 (1.01±0.11kJmol−1) with the Eq-g conformer the more stable form. The enthalpy difference between Eq-g and Eq-t is 116±43cm−1 (1.39±0.52kJmol−1) and the enthalpy difference between Eq-g and Ax-g has been determined to be 138±42cm−1 (1.66±0.51kJmol−1) with the Eq-g conformer as the more stable form. The percentage of the equatorial gauche conformer is estimated to be 47% at ambient temperature. To support the spectroscopic study, ab initio calculations by the Møller–Plesset perturbation method to second order MP2(full) and density functional theory calculations by the B3LYP method have been carried out. The infrared intensities, Raman activities, vibrational frequencies and band contours have been predicted from MP2(full)/6-31G(d) calculations and these theoretical values are compared to the experimental ones when available. The conformational stabilities have been predicted from theoretical calculations with basis sets up to 6-311+G(2d,2p) from both MP2(full) and density functional theory calculations by the B3LYP method. The results are discussed and compared to the corresponding properties of some related molecules.

Communication: Evidence for dipole-bound excited states in gas-phase I(-) ⋅ MI (M = Na, K, Cs) anionic salt microclusters.

We report the first UV laser photodissociation spectra of gas-phase I(-) ⋅ MI (M = Na, K, Cs) alkali halide anionic microclusters. The photodepletion spectra of these clusters display strong absorption bands just below the calculated vertical detachment energies, indicative of the presence of dipole-bound excited states. Photoexcitation at the peak of the transition to the dipole-bound excited state results in production of a primary [MI](-) photofragment along with a less intense I(-) ion. The photofragmentation mechanism of the excited state cluster is discussed in the context of an initial dipole-bound excited state that subsequently relaxes via a vibrational Feschbach resonance. The experiments described have been performed in an electrospray source laser-interfaced quadrupole ion-trap instrument and demonstrated for the first time that dipole-bound excited states can be identified in the relatively high-collision environment of a quadrupole ion-trap, in particular for systems with large dipole moments associated with the presence of charge separation. This indicates considerable potential for future experiments that identify dipole-bound excited states as a "low-resolution" structural probe of biomolecules and molecular charge separation using the instrumentation employed in this work.

Photoelectron spectroscopy on the charge reorganization energy and small polaron binding energy of molecular film

Understanding of electron-phonon coupling as well as intermolecular interaction is required to discuss the mobility of charge carrier in functional molecular solids. This article summarizes recent progress in direct measurements of valence hole-vibration coupling in ultrathin films of organic semiconductors by using ultraviolet photoelectron spectroscopy (UPS). The experimental study of hole-vibration coupling of the highest occupied molecular orbital (HOMO) state in ordered monolayer film by UPS is essential to comprehend hole-hopping transport and small-polaron related transport in organic semiconductors. Only careful measurements can attain the high-resolution spectra and provide key parameters in hole-transport dynamics, namely the charge reorganization energy and small polaron binding energy. Analyses methods of the UPS HOMO fine feature and resulting charge reorganization energy and small polaron binding energy are described for pentacene and perfluoropentacene films. Difference between thin-film and gas-phase results is discussed by using newly measured high-quality gas-phase spectra of pentacene. Methodology for achieving high-resolution UPS measurements for molecular films is also described.

Toward laser-induced vibrational emission spectroscopy of C+60

SynopsisC+60 has been proposed to be responsible for two of the diffuse interstellar bands (DIBs), the absorption features observed in the visible-to-near-infrared spectra of the interstellar medium. However, a confirmation requires laboratory gas-phase spectra, which are so far not available. We plan to develop a novel spectroscopy technique that will allow us to obtain the first gas-phase spectra of C+60, and that will be applicable to other complex organic molecules such as polycyclic aromatic hydrocarbons. The current status of the experimental setup, the ideas behind the measurement scheme and the preparatory work toward its implementation will be presented.

The electronic structure of VO in its ground and electronically excited states: A combined matrix isolation and quantum chemical (MRCI) study.

The electronic ground and excited states of the vanadium monoxide (VO) molecule were studied in detail. Electronic absorption spectra for the molecule isolated in Ne matrices complement the previous gas-phase spectra. A thorough quantum chemical (multi-reference configuration interaction) study essentially confirms the assignment and characterization of the electronic excitations observed for VO in the gas-phase and in Ne matrices and allows the clarification of open issues. It provides a complete overview over the electronically excited states up to about 3 eV of this archetypical compound.

Laboratory confirmation of C60(+) as the carrier of two diffuse interstellar bands.

The diffuse interstellar bands are absorption lines seen towards reddened stars. None of the molecules responsible for these bands have been conclusively identified. Two bands at 9,632 ångströms and 9,577 ångströms were reported in 1994, and were suggested to arise from C60(+) molecules (ref. 3), on the basis of the proximity of these wavelengths to the absorption bands of C60(+) measured in a neon matrix. Confirmation of this assignment requires the gas-phase spectrum of C60(+). Here we report laboratory spectroscopy of C60(+) in the gas phase, cooled to 5.8 kelvin. The absorption spectrum has maxima at 9,632.7 ± 0.1 ångströms and 9,577.5 ± 0.1 ångströms, and the full widths at half-maximum of these bands are 2.2 ± 0.2 ångströms and 2.5 ± 0.2 ångströms, respectively. We conclude that we have positively identified the diffuse interstellar bands at 9,632 ångströms and 9,577 ångströms as arising from C60(+) in the interstellar medium.

Electronic structure of nitrogen-containing intracomplex nickel(II) compounds based on ultraviolet photoelectron spectra and density functional theory

The electronic structure of nickel(II) bis-acetylacetoiminate and ethylene-bis-acetylacetoiminate is studied at the density functional theory level. Based on the quantum chemical calculations in the Koopmans’ theorem approximation, the bands of gas phase spectra of the studied complexes are assigned. The effect of amine substitution in nickel acetylacetonate on the character of the ligand bonding with the complexing agent is established.

Communication: Does a single CH3CN molecule attached to Ru(bipy)3(2+) affect its absorption spectrum?

Tris(bipyridine)ruthenium(II) (Ru(bipy)3 (2+)) is a prototypical transition metal coordination complex whose photophysical properties have attracted considerable attention. A much debated issue is whether the metal-to-ligand charge transfer (MLCT) transition that accounts for the complex's beautiful red color is fully delocalized across all three bipyridine ligands or located on just one ligand. Here, we show based on gas-phase action spectroscopy that attachment of a single acetonitrile molecule does not change the absorption spectrum from that of the bare ions, which is indicative of a delocalized state. However, the gas-phase spectra of the bare and one solvent molecule complexes are significantly blueshifted relative to that obtained in bulk acetonitrile, which suggests that in solution the polarizability of many solvent molecules working together can localize the MLCT state. Our data clearly show that more than one solvent molecule is needed to break the symmetry of the MLCT excited state and reproduce its solution-phase characteristics.

Self-association of organic solutes in solution: a NEXAFS study of aqueous imidazole.

N K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of imidazole in concentrated aqueous solutions have been acquired. The NEXAFS spectra of the solution species differ significantly from those of imidazole monomers in the gas phase and in the solid state of imidazole, demonstrating the strong sensitivity of NEXAFS to the local chemical and structural environment. In a concentration range from 0.5 to 8.2 mol L(-1) the NEXAFS spectrum of aqueous imidazole does not change strongly, confirming previous suggestions that imidazole self-associates are already present at concentrations more dilute than the range investigated here. We show that various types of electronic structure calculations (Gaussian, StoBe, CASTEP) provide a consistent and complete interpretation of all features in the gas phase and solid state spectra based on ground state electronic structure. This suggests that such computational modelling of experimental NEXAFS will permit an incisive analysis of the molecular interactions of organic solutes in solutions. It is confirmed that microhydrated clusters with a single imidazole molecule are poor models of imidazole in aqueous solution. Our analysis indicates that models including both a hydrogen-bonded network of hydrate molecules, and imidazole-imidazole interactions, are necessary to explain the electronic structure evident in the NEXAFS spectra.

Positively Charged Phosphorus as a Hydrogen Bond Acceptor.

Phosphorus (P) is an element that is essential to the life of all organisms, and the atmospheric detection of phosphine suggests the existence of a volatile biogeochemical P cycle. Here, we investigate the ability of P to participate in the formation of OH···P hydrogen bonds. Three bimolecular alcohol-trimethylphosphine complexes have been detected. Initially, the complexes were detected using matrix isolation spectroscopy, which favors complex formation. Subsequently, the fundamental OH-stretching vibration was observed in room-temperature gas-phase spectra. On the basis of our measured OH-stretching frequency red shifts and quantum chemical calculations, we find that P is an acceptor atom similar in strength to O and S and that all three P, O, and S atoms are weaker acceptors than N. The quantum chemical calculations show that both H and P in the OH···P hydrogen bond have partial positive charges, as expected from their electronegativities. However, the electrostatic potentials show a negative potential area on the electron density surface around P that facilitates formation of hydrogen bonds.

Molecular structure, spectroscopic properties, NLO, HOMO–LUMO and NBO analyses of 6-hydroxy-3(2H)-pyridazinone

The molecular structure and relative stabilities of the six possible isomers of 6-hydroxy-3(2H)-pyridazinone (DHP) in the gas phase and in solutions of different polarities are predicted using the B3LYP/6-311++G(d,p) method. The oxo-hydroxo isomer is the most stable form in the gas phase and in solution. These results agree with our reported X-ray structure. The effect of solvents on the spectroscopic properties of the most stable isomer has been studied using the polarized continuum method (PCM) at the same level of theory. The vibrational spectra of the compound studied are calculated and compared with the experimentally measured FTIR spectra. The electronic spectra in gas phase and in solution were calculated using the TD-DFT method. The most intense absorption band is predicted at 312.4nm and belongs mainly to a π→π* transition. In polar solvents, this spectral band undergoes a hypsochromic shift. Two stable dimer forms were calculated at same level of theory. Dimer A is more stable than dimer B, by 6.66kcalmol−1. The former is stabilized by stronger OH⋯O H-bonds compared to the weaker NH⋯O interactions in the latter. The effect of these H-bonding interactions on the molecular structure and vibrational spectra of these compounds are predicted. NBO analyses were carried out to investigate the stabilization energy of various inter- and intramolecular charge transfer interactions within the systems studied.

Spectroscopy and structure of the simplest actinide bonds.

Understanding the influence of electrons in partially filled f- and d-orbitals on bonding and reactivity is a key issue for actinide chemistry. This question can be investigated by using a combination of well-defined experimental measurements and theoretical calculations. Gas phase spectroscopic data are particularly valuable for the evaluation of theoretical models. Consequently, the primary objectives of our research have been to obtain gas phase spectra for small actinide molecules. To complement the experimental effort, we are investigating the potential for using relativistic ab initio calculations and semiempirical models to predict and interpret the electronic energy level patterns for f-element compounds. Multiple resonance spectroscopy and jet cooling techniques have been used to unravel the complex electronic spectra of Th and U compounds. Recent results for fluorides, sulfides, and nitrides are discussed.

Critical interpretation of CH- and OH- stretching regions for infrared spectra of methanol clusters (CH₃OH)n (n = 2-5) using self-consistent-charge density functional tight-binding molecular dynamics simulations.

Vibrational infrared (IR) spectra of gas-phase O-H⋅⋅⋅O methanol clusters up to pentamer are simulated using self-consistent-charge density functional tight-binding method using two distinct methodologies: standard normal mode analysis and Fourier transform of the dipole time-correlation function. The twofold simulations aim at the direct critical assignment of the C-H stretching region of the recently recorded experimental spectra [H.-L. Han, C. Camacho, H. A. Witek, and Y.-P. Lee, J. Chem. Phys. 134, 144309 (2011)]. Both approaches confirm the previous assignment (ibid.) of the C-H stretching bands based on the B3LYP/ANO1 harmonic frequencies, showing that ν3, ν9, and ν2 C-H stretching modes of the proton-accepting (PA) and proton-donating (PD) methanol monomers experience only small splittings upon the cluster formation. This finding is in sharp discord with the assignment based on anharmonic B3LYP/VPT2/ANO1 vibrational frequencies (ibid.), suggesting that some procedural faults, likely related to the breakdown of the perturbational vibrational treatment, led the anharmonic calculations astray. The IR spectra based on the Fourier transform of the dipole time-correlation function include new, previously unaccounted for physical factors such as non-zero temperature of the system and large amplitude motions of the clusters. The elevation of temperature results in a considerable non-homogeneous broadening of the observed IR signals, while the presence of large-amplitude motions (methyl group rotations and PA-PD flipping), somewhat surprisingly, does not introduce any new features in the spectrum.

Dynamics of endo- vs. exo-complexation and electronic absorption of calix[4]arene-Ar2

The dynamics of the calix[4]arene cone conformer (cax[4]) and calix[4]arene-Ar2 complexes (cax[4]-Ar2) at three temperatures (T=20, 100, and 200K) was investigated through Born–Oppenheimer molecular dynamics (BOMD). Thermal effects on the endo- vs. exo-complexation dynamics, the vibrational dynamics of the homodromic OH bond array, and the electronic absorption spectra are investigated. In comparison with results for the electronic spectra of gas-phase optimized structures, thermal effects on the electronic absorption spectra lead to a red-shift and broadening of the low energy excitation maxima.