Search

A study was conducted to explore the interaction of molecular hydrogen with the small molecules HF, [H.sub.2]O, N[H.sub.3], and LiOH. The interaction energy for the simultaneous interaction of [H.sub.2] with two LiOH molecules has not exceeded the value obtained for the interaction with a single LiOH molecule.

Binuclear square planar Ni(II) complexes, formed by two tridentate ligands with two imine-nitrogens coordinating two nickel atoms, have presented new types of eidge-tile-like chiral compounds that are stable in the appropriate bent conformation. The new compounds with ridge-tile-like chiral topology are shown to be reasonably stable and hence their chemistry, physical properties and applications are analyzed.

The synthesis of [Si.sub.5][Mes.sub.6] (Mes = 2,4,6-trimethylphenyl) consisting of an archetypal [1.1.1] cluster core featuring two ligand-free, ' inverted tetrahedral' bridgehead silicon atoms, is described. Reactivity studies have provided evidence for both closed-shell and radical-type reactivity and have thus confirmed the unusual nature of the stretched silicon-silicon bond in the ' nonclassical' molecule.

The correlated coupled-cluster theory with singles and doubles (CCSD) approach is employed for the calculation of the reaction barrier height for hydrogen abstraction by the hydroperoxyl radical from methane. The reactions of the [C.sub.n][H.sub.2n+2], where n = 2-4 alkanes, are also studied.

The article describes the benchmark, near basis set limit fc-CCSD(T) equilibrium harmonic frequencies for a set of 12 closed-shell and five open-shell molecules, which is computed numerically by using the CCSD(T)-R12 method. The corrections due to core-valance correlation are used for assessing the accuracy of the benchmark data, along with corrections from scalar relativistic effects.

The effect of basis set on second order Moller-Plesset perturbation (MP2) of several hydrogen-bonded complexes is studied. The possibilities of utilizing basis set extrapolations for geometries and dissociation energies are discussed along with the results of R12 methods and density functional and local correlation methods.

An analytical, full-dimensional, and global representation of the potential energy surface of ammonia in the lowest adiabatic electronic state is developed, and parameters are determined by adjustment to ab initio data and thermochemical data for several low-lying dissociation channels. The representation is distinguished by giving a qualitatively correct description of the potential energy for very large amplitude displacements of the nuclei from equilibrium.

The larger aromatic systems should be favored over single benzene rings in order to increase the interaction of dihydrogen with aromatic systems. The investigations of different ways to store hydrogen are described.

The article presents a comprehensive review on the explicitly correlated electrons molecules developed in the early quantum mechanics. It discusses the historical background and basics of F12 and R12 theories of nonrelativistic electronic wave functions. It mentions the Hy-CI and Hylleraas-type wave functions which proved to be helpful in the calculations of the accurate structure of electrons.

The synthesis of C∧C∧P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of Pt II and Pd II via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of Pt II as well as Pd II emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S 0 → T 1 ) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S 0 → T 1 excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S 0 → S n → T 1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.

We present a study of the structural evolution of tantalum cluster anions Ta n - , 6 ≤ n ≤ 13 using a combination of trapped ion electron diffraction (TIED) experiments with a variety of electronic structure methods. A genetic algorithm has been employed to establish a set of likely structures for each cluster, their geometries and energetics have been studied by density functional theory (DFT), random phase approximation, and two-component (2C) DFT methods, which include spin-orbit coupling. We find octahedral structures for Ta 6 - and Ta 8 - as well as structures based on the pentagonal bipyramid (Ta 7 - and Ta 9 - ). Ta 10 - -Ta 12 - are defective icosahedral structures and Ta 13 - is a distorted icosahedron. For most clusters, we find a good agreement between the theoretically predicted ground-state structures, especially those determined by the 2C method and the TIED results.

Orange, transparent crystals of [BMIm] 2 [Mn(CO) 3 (GeI 3 ) 3 ] (BMIm: 1-butyl-3-methylimidazolium) are obtained by reacting GeI 4 , [Mn(CO) 6 ][AlCl 4 ], and Ph 3 GeH in the ionic liquid [BMIm][NTf 2 ]. [Mn(CO) 6 ][AlCl 4 ] and triphenylgermane turn out to be essential as reactive carbonyl precursors and for the reduction of GeI 4 . According to X-ray structure analysis based on single crystals, the title compound exhibits a novel {MnGe 3 } cluster unit with Mn-Ge single bonds and surprisingly short distances (236-241 pm). Although sensitive to oxygen/moisture, the carbonyl compound is stable up to a temperature of 150 °C. Mass spectrometry (MS) shows [Mn(CO) 3 (GeI 3 ) 2 ] - , [Mn(CO) 3 (GeI 3 )I] - , and [GeI 3 ] - as decomposition fragments in the gas phase. In addition to crystal structure analysis and MS, the title compound is characterized by energy-dispersive X-ray spectroscopy (EDXS), thermogravimetry (TG), optical spectroscopy (UV-visible), infrared spectroscopy (FT-IR), and density functional theory (DFT) calculations.

The performance of the Bethe-Salpeter equation (BSE) approach for the first-principles computation of singlet and triplet excitation energies of small organic, closed-shell molecules has been assessed with respect to the quasiparticle energies used on input, obtained at various levels of GW theory. In the corresponding GW computations, quasiparticle energies have been computed for all orbital levels by means of using full spectral functions. The assessment reveals that, for valence excited states, quasiparticle energies obtained at the levels of eigenvalue-only self-consistent (ev GW) or quasiparticle self-consistent theory (qs GW) are required to obtain results of comparable accuracy as in time-dependent density-functional theory (TDDFT) using a hybrid functional such as PBE0. In contrast to TDDFT, however, the BSE approach performs well not only for valence excited states but also for excited states with Rydberg or charge-transfer character. To demonstrate the applicability of the BSE approach, computation times are reported for a set of aromatic hydrocarbons. Furthermore, examples of computations of ordinary photoabsorption and electronic circular dichroism spectra are presented for (C 60 ) 2 and C 84 , respectively.

The recently introduced pump-probe fragmentation action spectroscopy reveals a unique observation of excited state vibrational coherence (430-460 fs) in the isolated metal complex [Ag 2 (Cl)(dcpm) 2 )] + (dcpm = bis(dicyclohexylphosphino)methane) containing the [Ag 2 Cl] + scaffold. After photoexcitation by an 1 XMCT transition (260 nm) in an ion trap, an unexpected correlation between specific fragment ions (loss of HCl/Cl - vs loss of dcpm) and the phase of the wave packet is probed (1150 nm). Based on ab initio calculations, we assign the primary electronically excited state and ascribe the observed coherences (72-78 cm -1 ) to contain predominantly Ag-Ag stretch character. We propose specific probe absorption and vibronic coupling at the classical turning points to switch remarkably early on between the different fragmentation pathways. The overall excited state dynamics are fitted to a multiexponential decay with time constants: 0.2-0.4/3-4/19-26/104-161 ps. These findings open new perspectives for further dynamics investigations and possible applications in photocatalysis.

Theoretical description of phosphorescence lifetimes in the condensed phase requires a method that takes into account both spin-orbit coupling and solvent-solute interactions. To obtain such a method, we have coupled our recently developed two-component coupled-cluster method with singles and approximated doubles to a polarizable environment. With this new method, we investigate how different solvents effect the electronic phosphorescence energies and lifetimes of 4H-pyran-4-thione.

NH···π hydrogen bonds occur frequently between the amino acid side groups in proteins and peptides. Data-mining studies of protein crystals find that ∼80% of the T-shaped histidine···aromatic contacts are CH···π, and only ∼20% are NH···π interactions. We investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene (Im·Bz) complex as a model for the NH···π interaction between histidine and phenylalanine. Ground- and excited-state dispersion-corrected density functional calculations and correlated methods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energy structure with an NH···π hydrogen bond to the Bz ring; the NH bond is tilted 12° away from the Bz C6 axis. IR depletion spectra support the T-shaped geometry: The NH stretch vibrational fundamental is red shifted by -73 cm(-1) relative to that of bare imidazole at 3518 cm(-1), indicating a moderately strong NH···π interaction. While the S0(A1g) → S1(B2u) origin of benzene at 38 086 cm(–1) is forbidden in the gas phase, Im·Bz exhibits a moderately intense S0 → S1 origin, which appears via the D(6h) → Cs symmetry lowering of Bz by its interaction with imidazole. The NH···π ground-state hydrogen bond is strong, De=22.7 kJ/mol (1899 cm–1). The combination of gas-phase UV and IR spectra confirms the theoretical predictions that the optimum Im·Bz geometry is T shaped and NH···π hydrogen bonded. We find no experimental evidence for a CH···π hydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···π geometry of the Im·Bz complex is very different from the majority of the histidine·aromatic contact geometries found in protein database analyses, implying that the CH···π contacts observed in these searches do not arise from favorable binding interactions but merely from protein side-chain folding and crystal-packing constraints. The UV and IR spectra of the imidazole·(benzene)2 cluster are observed via fragmentation into the Im·Bz+ mass channel. The spectra of Im·Bz and Im·Bz2 are cleanly separable by IR hole burning. The UV spectrum of Im·Bz2 exhibits two 000 bands corresponding to the S0 → S1 excitations of the two inequivalent benzenes, which are symmetrically shifted by -86/+88 cm(-1) relative to the 000 band of benzene

[BMIm]2[{PbMn(CO)5)}6I8] (BMIm: 1-butyl-3-methylimidazolium) is obtained by ionic liquid mediated reaction of PbI2 and Mn2(CO)10. Central is a cubelike (Pb6I8) unit containing a nonfilled Pb6 octahedron. Each Pb of this (Pb6I8) unit is terminated on its outside by Mn(CO)5, exhibiting Pb-Mn metal-to-metal bonding (280 pm). Structurally, the (Pb6I8) unit is similar to the well-known octahedral (M6Xn) cluster-type family (M = Zr, Nb, Ta, Mo, W; X = Cl, Br, I). In contrast to most similar cluster compounds, such as W6Br12 ([W6Br8]Br2/1Br4/2, according to Niggli notation) or the carbonyl cluster [Sn6{Cr(CO5)6}](2-), however, the nonfilled central Pb6 octahedron in [{PbMn(CO)5)}6I8](2-) does not exhibit any metal-to-metal bonding. Structure and bonding of the title compound are validated by single-crystal structure analysis, energy-dispersive X-ray analysis (EDX), infrared spectroscopy (FT-IR), and density functional theory (DFT) calculations. Based on the isolobal principle, electronegativity considerations, bond lengths, and DFT calculations including Mulliken population analysis and natural population analysis (NPA), in sum, the charge distribution of Pb is best reflected by an oxidation state of +1.

The strength of interactions between CO2 and 23 building blocks of metal-organic frameworks are reported in this paper. This theoretical study is based on an incremental, explicitly correlated coupled-cluster scheme with interference effects. This scheme allows the accurate calculation of molecular complexes such as zinc acetate (32 non-hydrogen atoms) at the CCSD(T) level, close to the basis set limit. Higher CO2 affinity for complexes with nitrogen-containing heterocycles is predicted from the calculated interaction energies. The good agreement between the interaction energies obtained from the CCSD(T) scheme and DFT-D3 is discussed.

Dynamical effects on the mechanochemistry of linear alkane chains, mimicking polyethylene, are studied by means of molecular dynamics simulations. Butane and octane are studied using density-functional theory (DFT), whereas higher homologues are studied using a simple one-dimensional model in which the molecules are represented by a linear chain of Morse potentials (LCM). The application of a fixed external force to a thermodynamically pre-equilibrated molecule leads to a preference for cleavage of the terminal C-C bonds, whereas a sudden application of the force favors bond breaking in the central part of the chain. In all cases, transition-state theory predicts higher bond-breaking rates than found from the more realistic molecular dynamics simulations. The event of bond dissociation is related to dynamic states involving symmetric vibrational modes. Such modes do in general have lower frequencies of vibration than antisymmetric modes, which explains the deviation between the statistical theory and the dynamics simulations. The good qualitative agreement between the DFT and LCM models makes the latter a useful tool to investigate the mechanochemistry of long polymer chains.

The photophysics of chromophores is strongly influenced by their environment. Solvation, charge state, and adduct formation significantly affect ground and excited state energetics and dynamics. The present study reports on fluorescence emission of rhodamine B cations (RhBH + ) and derivatives in the gas phase. Substitution of the acidic proton of RhBH + by alkali metal cations, M + , ranging from lithium to cesium leads to significant and systematic blue shifts of the emission. The gas-phase structures and singlet transition energies of RhBH + and RhBM + , M = Li, Na, K, Rb, and Cs, were investigated using Hartree-Fock theory, density functional methods, second-order Møller-Plesset perturbation theory, and the second-order approximate coupled-cluster model CC2. Comparison of experimental and theoretical results highlights the need for improved quantum chemical methods, while the hypsochromic shift observed upon substitution appears best explained by the Stark effect due to the inhomogeneous electric field generated by the alkali ions.

Gas-phase measurements using mass-spectrometric techniques allow determination of the luminescence properties of selected molecular systems with knowledge of their exact composition. Furthermore, isolated luminophores are unaffected by matrix effects like solvent interactions or crystal packing. As a result, the system complexity is reduced relative to the condensed phase and a direct comparison with theory is facilitated. Herein, we report the intrinsic luminescence properties of nonanuclear europium(III) and gadolinium(III) 9-hydroxyphenalen-1-one (HPLN)-hydroxo complexes. Luminescence spectra of [Eu9(PLN)16(OH)10](+) ions reveal an europium-centered emission dominated by a 4-fold split Eu(III) hypersensitive transition. The corresponding Gd(III) complex, [Gd9(PLN)16(OH)10](+), shows a broad emission from a ligand based triplet state with an onset of about 1000 wavenumbers in excess of the europium emission. As supported by photoluminescence lifetime measurements for both complexes, we deduce an efficient europium sensitization via PLN-based triplet states. The luminescence spectra of the complexes are discussed in terms of a square antiprismatic europium/gadolinium core structure as suggested by density functional computations.

We present gas-phase dispersed photoluminescence spectra of europium(III) 9-hydroxyphenalen-1-one (HPLN) complexes forming adducts with alkali metal ions ([Eu(PLN)3M](+) with M = Li, Na, K, Rb, and Cs) confined in a quadrupole ion trap for study. The mass selected alkali metal cation adducts display a split hypersensitive (5)D0 → (7)F2 Eu(3+) emission band. One of the two emission components shows a linear dependence on the radius of the alkali metal cation whereas the other component displays a quadratic dependence thereon. In addition, the relative intensities of both components invert in the same order. The experimental results are interpreted with the support of density functional calculations and Judd-Ofelt theory, yielding also structural information on the isolated [Eu(PLN)3M](+) chromophores.