Dye Probes of Nanoclusters in Liquids

Calculations have been carried out to optimize the structure of Van der Waals complexes of methanol with N-methyl-2-nitroaniline, a dye capable of shifts in visible and ultraviolet spectra that depend on (1) solvent dielectric, (2) solvent shell structure, and (3) hydrogen bonding to a slight extent. Hartree–Fock–Roothaan calculations with various basis sets and single-excitation configuration interaction (SCI) are compared to Density–Functional–Theory Time-Dependent Hartree–Fock (DFT-TD) results for three low-energy ultraviolet electronic transitions. Energy-minimized structures are reported for a trimeric complex of two methanol-one water as found using a 6-311G** basis indicating two possible hydrogen-bonding schemes. The effect of a dielectric medium on the ultraviolet spectrum is compared to gas-phase clusters. Electronic transitions are also given for the dye-probe complexed with four or five methanol molecules finding good agreement with observed shifts in the ultraviolet spectrum as found with the TDHF-DFT formalism for the lowest energy transition near 425nm.     

Origin of the attraction in aliphatic C-H/pi interactions: infrared spectroscopic and theoretical characterization of gas-phase clusters of aromatics with methane

An attractive intermolecular interaction between an aliphatic C-H bond and a pi-electron system (C-H/pi interaction) was characterized on the basis of infrared spectroscopy and high level ab initio calculations. Infrared spectroscopy was applied to several isolated methane clusters with benzene, toluene, p-xylene, mesitylene, and naphthalene in the gas phase, and the spectral changes of the C-H stretch bands in the methane moiety upon the cluster formation were observed. In the theoretical approach, interaction energies of the clusters were evaluated by high-level ab initio calculations. The forbidden symmetric C-H stretch transition weakly appeared in the IR spectra of the clusters, and it confirmed the small deformation of the methane moiety from the T(d)() symmetry, which was predicted by the ab initio calculations. On the other hand, the degenerated asymmetric C-H stretch band showed complicated splitting, which is qualitatively interpreted by a hindered rotor model. Low-frequency shifts upon the cluster formation were seen in the symmetric C-H stretch frequency, though the magnitude of the shifts was extremely small and no clear correlation with the interaction energy was found. On the other hand, the size of the calculated interaction energy well correlates with the polarizability of aromatics. The S(1)-S(0) electronic transition of the aromatic moiety was also observed, and it showed low-frequency shifts upon cluster formation. These results support the dominance of the dispersion interaction over the electrostatic and charge-transfer terms in the aliphatic C-H/pi interaction.     

Quinoidal/Aromatic Transformations in π‐Conjugated Oligomers: Vibrational Raman studies on the Limits of Rupture for π‐Bonds

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene‐vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π‐conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two‐well BLA–energy curve of their ground electronic state as a function of the bond‐length‐alternation pattern (BLA). The connection among BLA values, π‐conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one‐dimensional linear π‐electron delocalized systems such as quinoidal (polyene) and aromatic characters.     

Study of Wave Functions and Radiation Transitions in a Rydberg Polar Cluster Using the Continuum Model

A continuum model describing highly excited (Rydberg) electronic states in clusters composed of polar molecules was proposed. On the basis of this model, the wave functions of Rydberg electronic states of clusters were calculated for a wide range of characteristic cluster parameters. These states are not hydrogen-like and can be described using the quantum defect theory. This fact indicates that radiative transitions can be forbidden within certain ranges of cluster parameters. The quantum defects in clusters and the lifetimes of different states were calculated. The possibility of formation of metastable Rydberg states and anomalous spectral characteristics of Rydberg clusters was shown.     

Rate of radiationless electronic transitions for molecules in the condensed phase

Rate constants of radiationless electronic transitions in a diatomic molecule in a crystal at non-zero temperature are calculated. The electronic terms of the molecule are simulated by the Morse potential. The crystal vibrations are assumed to be harmonic. The calculations are done under the assumption that perturbation theory is applicable to the operator inducing the electronic transitions. The vibrational interaction of the molecule with the medium is not supposed to be small. The results explain certain experimental data on the radiationless electronic transitions in aromatic hydrocarbons.     

Helium clusters seeded with CO molecules: new results for HeN-13C18O and the approach to the nanodroplet limit

Infrared spectra of helium clusters seeded with doubly substituted carbon monoxide molecules, 13C18O, have been studied in order to complement recent helium nanocluster results and to determine whether additional isotopic data would help to separate vibrational and rotational contributions to the observed transitions. The experiments were made by direct infrared absorption in pulsed supersonic jet expansions using a tunable diode laser probe in the region of the fundamental band (approximately 2045 cm-1 for 13C18O). Even with data on the R0 transitions from four CO isotopomers, it was found that a clear and consistent separation of vibration and rotation could not be achieved for HeN-CO clusters in the size range N approximately 10-20. Isotope shifts observed for clusters with 13C18O (relative to 12C16O) were found to be close to the sums of the shifts previously determined for 13C16O and 12C18O. The new measurements generally supported previous assignments of cluster size, but some modifications for the range N=14-16 are suggested here. New measurements for HeN-12C16O under conditions favoring larger clusters (high backing pressure and low jet temperature) showed that individual transitions could be resolved even at N approximately 50. For larger clusters, a partly resolved "lump" of transitions was observed to approach the nanodroplet limit.     

Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra

The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the pi --> pi* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n --> pi* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvent's dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding.     

Linear and circular dichroism detection of magnetic torque on matrix isolated small manganese clusters

Matrix isolated atoms and small manganese clusters have been isolated in krypton matrices at concentrations lower than 10^?3. Absorption and polarized light measurements have been conducted simultaneously on these samples. Defined electronic transitions show distinct circular or linear dichroïsm signals only after the application of an external magnetic field. Clusters belonging to these bands have an axial structure with a plane of symmetry perpendicular to their magnetic moment. On the basis of previous ESR results we are concluding that three of the defined electronic transitions are due to Mn₅ cluster.     

Evaluation of franck-condon factors for nonradiative singlet-triplet transitions in aromatic molecules

Dependence of the probability of nonradiative T S transitions in aromatic molecules on electronic transition energy, temperature, and parameters relating the electronic excitation to molecular vibrations is investigated on the basis of general formulas for the probabilities of the transitions. An evaluation is made of the real parameters by means of the vibrational structure of the phosphorescence spectrum and the probability of T S transitions in benzene, anthracene, naphthalene, and benzophenone.The author is indebted to E. E. Nikitin and to Prof. N. D. Sokolov for their discussion of the work.     

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3) clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Ru(n)Au(m)](0/+) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations.     

Structure of the phenylacetylene-water complex as revealed by infrared-ultraviolet double resonance spectroscopy

The structure of the phenylacetylene-water complex has been elucidated based on spectral shifts in electronic and vibrational transitions. Phenylacetylene forms a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds, which is different from both the benzene-water and acetylene-water complexes, even though phenylacetylene combines the features of both benzene and acetylene. Formation of such a complex can be rationalized on the basis of cooperativity between the two sets of hydrogen bonds.     

A theoretical study of electronic spectra of radical cations of some dihydroxynaphthalenes

Electronic transition energies of radical cations of 1.2-, 1.3-, 1.6-, and 1.7?dihydroxynaphthalenes are calculated using an open-shell SCF method with configuration interaction. The results are critically analyzed and a correlation diagram is given that shows the energy-shift and intensity variation in the electronic transitions when moving from one system to another, thus revealing the characteristic behavior of the transitions depending on the positions of the hydroxyl substituents. An interesting relation connecting the electronic spectroscopy with the UV photoelectron spectroscopy is suggested on the basis of which the first ionization potentials (IPS ) of the substituted aromatic systems can be inferred from the calculated energy of the A-type (HOMO → LUMO ) transitions for their radical cations. Furthermore, the predictability of the IP s is found to be considerably increased with the incorporation of “molecular size” in the regression.     

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.     

The boron connection: a parallel description of aromatic, bonding, and structural characteristics of hydrogenated silicon-carbon clusters and isovalent carboranes

The aromatic, bonding, and structural characteristics of the Si 4C 2H 2-C 2B 4H 6, Si 2C 4H 4-C 4B 2H 6, and other Si n C 2H 2-C 2B n H n+2 ( n = 1, 2, 3, 5) isovalent pairs are studied using density functional theory (DFT) and coupled cluster methods to fully illustrate the homology of the two species. This homology, which is based on the replacement of the carborane B-H units by isovalent Si atoms, is extended to all three characteristics (structural, electronic, and aromatic) and includes all three lowest-energy structures of the isovalent pairs. This type of "boron connection", which has been tested for silicon clusters recently, seems to be a valid and extremely useful concept. For the aromatic properties of the Si n C 2H 2-C 2B n H n+2 species, expressed through the nucleus independent chemical shifts (NICS), a strange odd-even effect with respect to the number of Si atoms is observed which seems rather difficult to explain. To help possible future identification and characterization of the Si n C 2H 2 clusters, their infrared, Raman, and optical excitation spectra are calculated within the framework of DFT, using the 6-311+G(2d, p) basis set. It is expected that the present results would facilitate the exploitation of the well-known carborane and metallacarborane chemical properties and applications for the design and development of novel silicon-carbon-based composite materials.     

Density functional study of structural, electronic, and optical properties of small bimetallic ruthenium-copper clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Cu(n)Ru(m)](+/0/-) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 nonlocal hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Cu(n Ru(m)](+/0/-) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and assignments of all principal electronic transitions were made and interpreted in terms of contribution from specific molecular orbital excitations.     

Isotope shift studies in the second spectrum of neodymium in UV region

Isotope shift (IS) Δσ (142–150) have been measured in 92 spectral lines of Nd⁺ in the region 3290–3955 Å, employing recording Fabry-Perot spectrometer and highly enriched isotopic samples excited in a liquid-nitrogen cooled hollow cathode. Earlier IS measurement in UV region of Nd II spectrum is available for only a few lines, New classification has been made for several of the spectral lines studied presently. We have also revised the earlier suggested classification for some of the Nd II lines on the basis of observed isotope shifts. The transitions presently studied mostly involve high lying odd parity energy levels above 30,000 cm^?1 having neither electronic configuration assignment nor term isotope shifts (ΔT) evaluated. The present study would thus enable to evaluate ΔT values of some of these high lying odd levels, which should be helpful in suggesting possible electronic configurations.     

Density functional analysis of geometries and electronic structures of gold-phosphine clusters. The case of Au4(PR3)42+ and A(u4μ2-I)2(PR3)4

Geometries, ligand binding energies, electronic structure, and excitation spectra are determined for Au(4)(PR(3))(4)(2+) and Au(4)(μ(2)-I)(2)(PR(3))(4) clusters (R = PH(3), PMe(3), and PPh(3)). Density functionals including SVWN5, Xα, OPBE, LC-ωPBE, TPSS, PBE0, CAM-B3LYP, and SAOP are employed with basis sets ranging from LANL2DZ to SDD to TZVP. Metal--metal and metal--ligand bond distances are calculated and compared with experiment. The effect of changing the phosphine ligands is assessed for geometries and excitation spectra. Standard DFT and hybrid ONIOM calculations are employed for geometry optimizations with PPh(3) groups. The electronic structure of the gold--phosphine clusters examined in this work is analyzed in terms of cluster ("superatom") orbitals and d-band orbitals. Transitions out of the d band are significant in the excitation spectra. The use of different basis sets and DFT functionals leads to noticeable variations in the relative intensities of strong transitions, although the overall spectral profile remains qualitatively unchanged. The replacement of PMe(3) with PPh(3) changes the nature of the electronic transitions in the cluster due to low-lying π*-orbitals. To reproduce the experimental geometries of clusters with PPh(3) ligands, computationally less expensive PH(3) or PMe(3) ligands are sufficient for geometry optimizations. However, to predict cluster excitation spectra, the full PPh(3) ligand must be considered.     

Electronic Transitions in Transition Metal Compounds at High Pressure

Very high pressure is becoming increasingly important for investigating electronic structure. The relative shift in energy of electronic orbitals which is commonly observed at high pressure can frequently lead to a new ground state for the system. These electronic transitions may result in changes in electrical, optical, or magnetic properties as well as changes in chemical reactivity. Electronic transitions in metals and insulator-metal transitions have been widely studied by physicists. Recently, it has been found that electronic transitions in aromatic hydrocarbons and their electron donor-acceptor complexes can induce chemical reactivity and lead to the formation of new classes of hydrocarbons. Electronic transitions in transition metal complexes may lead to changes in spin state; both increase and decrease in multiplicity with increasing pressure have been observed. In addition, it has been shown that Fe(III ) and Cu(II ) reduce at high pressure in a variety of compounds. The behavior of these transition metal ions is described in some detail in relation to the general area of high pressure and electronic structure.