X-ray spectra and electronic structure of solid ammonia

Intermolecular interactions in solid ammonia are investigated in a combined X-ray spectral and quantum chemical study. Theoretical NK_α spectra are constructed on the basis of MNDO calculations of the ammonia molecule and (NH₃)₇ and (NH₃)₁₃ clusters modeling solid ammonia; the spectra are in satisfactory agreement with the experimental X-ray spectra. Fragment analysis of the clusters with respect to the central ammonia molecule is carried out. It is shown that intermolecular electronic interactions in solid ammonia are most effective in MOs of e symmetry (σ-binding of nitrogen and hydrogen atoms). The fragment 2a₁ orbital contributes to the MO structure of the clusters to the least extent. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 721–726, July–August, 1996. Translated by L. Smolina     

The Existence of Hexafluoroarsenic(V) Acid

The homogeneous mixture of anhydrous hydrogen fluoride aHF and antimony pentafluoride AsF₅ is known as a superacidic system. The high acidity is derived from the formation of [H₂F]⁺ [AsF₆]^?. No experimental evidence exists for the existence of the free acid molecule HAsF₆. The reaction of trimethylsilyl N,N‐dimethylcarbamate in the binary system aHF/AsF₅ led to decomposition of trimethylsilyl N,N‐dimethylcarbamate at ?50 °C to dimethylammonium hexafluoridoarsenate and cocrystallization of HAsF₆. The single‐crystal X‐ray structure displays an HAsF₆ molecule involved in an asymmetric hydrogen bridge to the hexafluoridoarsenate anion. As a result of the incalculable situation in the crystal lattice, the molecular structure of HAsF₆ is calculated by quantum chemical structure optimization of the extreme cases of [FHF‐AsF₅]^? (strong hydrogen bond) and HAsF₆ (no hydrogen bond) at the PBE1PBE/6‐311G(3df,3pd) level of theory.     

Analysis of relaxation on polypropyleneglycols above vitrification temperature

The dielectric spectra of polypropyleneglycols H-(C₃H₆O) _{N p} -OH (PPGs), where N _p = 1, 2, 3, 7, 12, 17, 20, 34, 69, were analyzed in terms of the Dissado Hill (DH) cluster model above the vitrification temperatures. In PPGs, the structural clusters are associates formed by intra- and intermolecular hydrogen bonds. The activation processes of cleavage and formation of intermolecular hydrogen bonds in clusters, when the total number of intermolecular hydrogen bonds changes, are characterized by the parameter n _DH. The fluctuation processes of simultaneous exchange of molecules between adjacent clusters correspond to redistributions of intermolecular hydrogen bonds between clusters, when only the position but not the total number of intermolecular hydrogen bonds changes, and are characterized by the parameter m _DH. The relaxation time τ_DH at 303 K and 423 K and the parameters n _DH and m _DH of the dielectric spectra were calculated. The activation energies of relaxation in the range 210–323 K were determined. The mean statistic squares of the dipole moments of clusters 〈μ_c²〉 and di-PG, PPG-425 (N _p = 7), and PPG-2025 (N _p = 34) molecules 〈μ_m²〉 at 303 K and 423 K were calculated. The number of the units of the oxypropylene chains involved in relaxation was determined. The dependence of the parameters of the DH model, relaxation energies, 〈μ_c²〉 and 〈μ_m²〉 on N _p were studied.     

Modeling of the mechanism of one-electron transfer from the perylene molecule to the oxygen molecule <Superscript>3</Superscript>O<Subscript>2</Subscript> in the HF medium

The thermodynamic parameters of the formation of the perylene radical cation in anhydrous hydrogen fluoride containing dissolved dioxygen were calculated by the ab initio method MP2. The protonated product of HF autoprotolysis was modeled as the H(FH)₃⁺ cluster. The ³O₂ molecule was found to bind to the linear H(FH)₃⁺ cluster via a hydrogen bond. As the charge and multiplicity of the system change upon the capture of an electron, the oxygen-hydrogen fluoride cluster complex undergoes rearrangement to yield the hydroperoxyl radical OOH incorporated in a cycle formed by HF molecules. The free energy of electron transfer from the perylene molecule to the ³O₂ molecule in the HF medium is about −38 kcal/mol.     

Effect of accelerated protons on the molecular-topological structure and thermal stability of poly(vinylidene fluoride)

The bombardment of poly(vinylidene fluoride) (PVDF) with accelerated protons in vacuum results in the release of gaseous products and the destruction of cluster junctions in its pseudo-network structure, which pass to the crystalline ones of three modifications with different temperatures and rates of melting, the molecular mass of crystallized chains, and their weight fractions. The composition of gaseous products indicates that, upon proton bombardment, the detachment of fluorine or hydrogen atoms yielding H₂ and HF occurs as the main process, and the scission of the carbon chain hardly takes place in this case.     

Quantum monte carlo study of the energetics of small hydrogenated and fluoride lithium clusters

An investigation of the energetics of small lithium clusters doped either with a hydrogen or with a fluorine atom as a function of the number of lithium atoms using fixed‐node diffusion quantum Monte Carlo (DMC) simulation is reported. It is found that the binding energy (BE) for the doped clusters increases in absolute values leading to a more stable system than for the pure ones in excellent agreement with available experimental measurements. The BE increases for pure, remains almost constant for hydrogenated, and decreases rapidly toward the bulk lithium for the fluoride as a function of the number of lithium atoms in the clusters. The BE, dissociation energy as well as the second difference in energy display a pronounced odd–even oscillation with the number of lithium atoms. The electron correlation inverts the odd–even oscillation pattern for the doped in comparison with the pure clusters and has an impact of 29%–83% to the BE being higher in the pure cluster followed by the hydrogenated and then by the fluoride. The dissociation energy and the second difference in energy indicate that the doped cluster Li₃H is the most stable whereas among the pure ones the more stable are Li₂, Li₄, and Li₆. The electron correlation energy is crucial for the stabilization of Li₃H. © 2016 Wiley Periodicals, Inc.     

The nature of transitions in electronic absorption spectra of radical cations of dipyrroles with phenylene bridging groups

The electronic absorption spectra of radical cations of dipyrroles with a phenylene bridge were studied by laser flash photolysis and quantum chemical methods. Intense absorption bands of the radical cations in the visible region (λ_max ≈ 500 nm, ε_max > 2 · 10⁴ L mol⁻¹ cm⁻¹) are caused by excitation of electrons from single occupied MOs to the LUMO. In the near IR region, calculations predict additional, relatively intense (f≈ 0.27–0.29) electronic transitions associated with excitation of electrons from low-lying MOs to the single occupied MO.     

A density functional theory study on the Ag<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>H (<Emphasis Type="Italic">n</Emphasis> = 1–10) clusters

Density functional GGA-PW91 method with DNP basis set is applied to optimize the geometries of Ag _n H (n = 1–10) clusters. For the lowest energy geometries of Ag _n H (n = 1–10) clusters, the hydrogen atom prefers to occupy the two-fold coordination bridge site except the occupation of single-fold coordination site in AgH cluster. After adsorption of hydrogen atom, most Ag _n structures are slightly perturbed and only the Ag₆ structure in Ag₆H cluster is distorted obviously. The Ag–Ag bond is strengthened and the strength of Ag–H bond exhibits a clear odd–even oscillation like the strength of Au–H bond in Au _n H clusters, indicating that the hydrogen atom is more favorable to be adsorbed by odd-numbered pure silver clusters. The adsorption strength of small silver cluster toward H atom is obviously weaker than that of small gold cluster toward H atom due to the strong scalar relativistic effect in small gold cluster. The pronounced odd–even alternation of the magnetic moments is observed in Ag _n H systems, indicating that the Ag _n H clusters possess tunable magnetic properties by adsorbing hydrogen atom onto odd-numbered or even-numbered small silver cluster.     

Stretching vibrations and structure of (HF) n (n=4–8) clusters

IR spectra of 24 structural isomers of (HF) _n (n=4–8) clusters were calculated in the framework of semiempirical theory of polyatomic molecule vibrations. Based on the results obtained and available experimental data it is proposed that (HF) _n associates comprising 3–5-membered cycles with attached monomeric HF units are present in molecular beams and gas phase.Ab initio calculations performed by the SCF method show the existence of local minima corresponding to such structures on the potential energy surface of (HF) _n clusters (n=4–6). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 435–443, March, 1997.     

Ab initio investigation of water clusters (H2O)n (n = 2–34)

Various properties of typical structures of water clusters in the n = 2–34 size regime with the change of cluster size have been systematically explored. Full optimizations are carried out for the structures presented in this article at the Hartree–Fock (HF) level using the 6‐31G(d) basis set by taking into account the positions of all atoms within the cluster. The influence of the HF level on the results has been reflected by the comparison between the binding energies of (H₂O)_n (n = 2–6, 8, 11, 13, 20) calculated at the HF level and those obtained from high‐level ab initio calculations at the second‐order Møller–Plesset (MP2) perturbation theory and the coupled cluster method including singles and doubles with perturbative triples (CCSD(T)) levels. HF is inaccurate when compared with MP2 and CCSD(T), but it is more practical and allows us to study larger systems. The computed properties characterizing water clusters (H₂O)_n (n = 2–34) include optimal structures, structural parameters, binding energies, hydrogen bonds, charge distributions, dipole moments, and so on. When the cluster size increases, trends of the above various properties have been presented to provide important reference for understanding and describing the nature of the hydrogen bond. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Crystal Structure and Spectroscopic Investigations of POF3

Phosphoryl fluoride was characterized by Raman spectroscopy and X‐ray diffraction analysis. The X‐ray structure was obtained by in‐situ crystallization. Phosphoryl fluoride crystallizes in the trigonal space group P$\bar{3}$ m1 with two formula units in the unit cell. In the crystal structure zigzag chains are observed which are formed by intermolecular P–O contacts. The Raman spectra of neat and matrix isolated POF₃ display an extra line, which indicates intermolecular interaction in the solid state. Therefore quantum chemically calculation of a POF₃ oligomer was performed. The theoretical calculation indicates that the extra Raman line is caused by side splitting of the P–O valence vibration.     

Crystal Structure and Vibrational Spectra of ClSO2NHC(O)F

The reaction of chlorosulfonyl isocyanate (ClSO₂NCO) with anhydrous hydrogen fluoride (aHF) leads to the formation of ClSO₂NHC(O)F. The title compound with a melting point of –38 °C is characterized by vibrational spectroscopy and a single crystal structure analysis. It crystallizes in the tetragonal space group I4₁/a with 16 formula units per unit cell. a = 11.1115(2) Å, c = 16.5654(6) Å. The experimental data are supported by quantum‐chemical calculations on the PBE1PBE/6‐311G(3pd,3df) level of theory.     

Making and breaking of small water clusters: A combined quantum chemical and molecular dynamics approach

We present a combined quantum chemical and molecular dynamics study of cyclic and noncyclic water n-mers ([(H₂O]_n, n = 2–6) at four different temperatures and showcase that the dynamics of small water clusters can reproduce the known properties of bulk water reasonably well. We investigate the making and breaking of the water clusters by computing the hydrogen bond strengths, average lifetimes, and relative stabilities, which are important to understand the complex solution dynamics. We compare the behavior of water clusters in the gas phase and in the solution phase as well as the variation in the properties as a function of cluster size and highlight the notably more interesting cluster dynamics of the water trimer when compared to the other water clusters. © 2019 Wiley Periodicals, Inc.     

Analysis of nuclear quantum effects on hydrogen bonding

The impact of nuclear quantum effects on hydrogen bonding is investigated for a series of hydrogen fluoride (HF)n clusters and a partially solvated fluoride anion, F-(H2O). The nuclear quantum effects are included using the path integral formalism in conjunction with the Car-Parrinello molecular dynamics (PICPMD) method and using the second-order vibrational perturbation theory (VPT2) approach. For the HF clusters, a directional change in the impact of nuclear quantum effects on the hydrogen-bonding strength is observed as the clusters evolve toward the condensed phase. Specifically, the inclusion of nuclear quantum effects increases the F-F distances for the (HF)n=2-4 clusters and decreases the F-F distances for the (HF)n>4 clusters. This directional change occurs because the enhanced electrostatic interactions between the HF monomers become more dominant than the zero point energy effects of librational modes as the size of the HF clusters increases. For the F-(H2O) system, the inclusion of nuclear quantum effects decreases the F-O distance and strengthens the hydrogen bonding interaction between the fluoride anion and the water molecule because of enhanced electrostatic interactions. The vibrationally averaged 19F shielding constant for F-(H2O) is significantly lower than the value for the equilibrium geometry, indicating that the electronic density on the fluorine decreases as a result of the quantum delocalization of the shared hydrogen. Deuteration of this system leads to an increase in the vibrationally averaged F-O distance and nuclear magnetic shielding constant because of the smaller degree of quantum delocalization for deuterium.     

Production and characterization of thermally-annealed large clusters by high-pressure laser-vaporization technique

We have developed a high-pressure laser-vaporization cluster source, which enables us to produce large, annealed clusters. Large carbon clusters up to C ₂₀₀₀ ⁺ , annealed sodium fluoride clusters of Na⁺(NaF)_n, and ammonium iodide clusters of NH ₄ ⁺ (NH₄I)_n have been produced by this technique. Annealed ammonium iodide clusters show a transition from hydrogen bonded complexes to ionic crystals as the cluster size increases. The characterization of the present technique and the production processes of the large, annealed clusters are discussed.     

Hydrogen fluoride vibrational energy distributions for the reactions of fluorine atoms with cyclanes

The hydrogen fluoride infrared chemiluminescence produced by the reactions of fluorine atoms with cyclopropane, cyclopentane, and cyclohexane have been studied. The emission data were used to determine the vibrational energy distributions for the abstraction of hydrogen from the secondary carbon–hydrogen bonds of these small cyclic hydrocarbons. The fraction of reaction exothermicity going into vibrational excitation of hydrogen fluoride was as follows: c-C₃H₆, 45%; c-C₅H₁₀, 53%; c-C₆H₁₂, 49%. The slightly lower fraction for the cyclopropane system may indicate that its radical reorganization energy is not completely available for excitation of product HF.     

All-electron relativistic calculations on hydrogen atom adsorption onto small copper clusters

An all-electron scalar relativistic calculation on Cu _n H (n = 1–13) clusters has been performed by using density functional theory with the generalized gradient approximation at the PW91 level. Our results reveal that the hydrogen atom prefers to occupy the two fold coordination site for Cu _n H (n = 2, 4–6, 8, 10–13) clusters, the single fold coordination site for Cu _n H (n = 1, 3, 7) and the three fold coordination site for Cu₉H cluster. For all Cu _n H clusters, only the Cu₁₁ structure in Cu₁₁H is distorted obviously. After adsorption, the Cu–Cu bond is strengthened and the Cu–H bond of odd-numbered Cu _n H clusters is relatively stronger than that of adjacent even-numbered Cu _n H clusters. The Cu–Cu bond-length and Cu–H bond-length for all Cu _n H clusters of our work are significantly shorter than those of previous work. This discrepancy can be explained in terms of the scalar relativistic effect. The most favorable adsorption between small copper clusters and hydrogen atom takes place in the case that hydrogen atom is adsorbed onto an odd-numbered pure Cu _n cluster and becomes Cu _n H cluster with even number of valence electrons. The odd–even alteration of magnetic moments is observed in Cu _n H clusters and may provide the material with tunable code capacity of “0” and “1” by adsorbing a hydrogen atom onto odd- or even-numbered copper clusters.     

Theoretical study of silicon–sulfur clusters (SiS2)n− (n=1–6)

The possible geometrical structures and relative stability of silicon–sulfur clusters (SiS₂) (n=1–6) are explored by means of density functional theory (DFT) quantum chemical calculations. We also compare DFT with second‐order Møller–Plesset (MP2) and Hartree–Fock (HF) methods. The effects of polarization functions, diffuse functions, and electron correlation are included in MP2 and B3LYP quantum chemical calculations, and B3LYP is effective in larger cluster structure optimization, so we can conclude that the DFT approach is useful in establishing trends. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂)_n⁻ are analyzed by B3LYP. As a result, the regularity of the (SiS₂)_n⁻ cluster growing is obtained, and the calculation may predict the formation mechanism of the (SiS₂)_n⁻ cluster. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 280–290, 2001     

Non-alkane behavior of cyclopropane and its derivatives: characterization of unconventional hydrogen bond interactions

Eight cyclopropane derivatives (Δ − R) have been modeled, with R = −H, −CH₃, −NH₂, −C ≡ CH, −C ≡ CCH₃, −OH, −F and −C ≡ N. All geometries have been fully optimized at the MP2/ AUG-cc-pVTZ level of calculations. Natural bond orbital analyses reveal extra p character (sp^λ, λ > 3) in the C-C bonds of the cyclopropyl rings. The banana-like σ _CC bonds in the rings are described in detail. Alkene-like complexes between Δ − R molecules and hydrogen fluoride are identified. These weakly bonded complexes are formed through unconventional hydrogen bond interactions between the hydrogen atom in the HF molecule and the carbon–carbon bonds in the cyclopropane ring. A topological analysis of the electronic charge density and its Laplacian has been used to characterize the interactions. The possible relevance of such complexes in the modeling of substrate–receptor interactions in some anti-AIDS drugs is discussed. Contribution to the Serafin Fraga Memorial Issue.     

Theoretical study of silicon–sulfur clusters (SiS2)n (n = 1–6)

 The possible geometrical structures and relative stability of (SiS₂) _n (n=1–6) silicon–sulfur clusters are explored by means of density functional theory quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂) _n are analyzed by the same method. As a result, the regularity of the (SiS₂) _n cluster growth is obtained, and the calculation may used for predicting the formation mechanism of the (SiS₂) _n cluster. Received: 17 November 1999 / Accepted: 3 November 2000 / Published online: 3 May 2001