Molecular Structure and Conformational Composition of 1-[(Methylthio)methyl]-2-nitrobenzene (MTMNB): A Theoretical and Experimental Study

The conformational composition of gaseous MTMNB and the molecular structures of the rotational forms have been studied by electron diffraction at 130^∘C aided by results from ab initio and density functional theory calculations. The conformational potential energy surface has been investigated by using the B3LYP/6-31G(d,p) method. As a result, six minimum-energy conformers have been identified. Geometries of all conformers were optimized using MP2/6-31G(d,p), B3LYP/6-31G(d,p), and B3LYP/cc-pVTZ methods. These calculations resulted in accurate geometries, relative energies, and harmonic vibrational frequencies for all conformers. The B3LYP/cc-pVTZ energies were then used to calculate the Boltzmann distribution of conformers. The best fit of the electron diffraction data to calculated values was obtained for the six conformer model, in agreement with the theoretical predictions. Average parameter values (r_a in angstroms, angle α in degrees, and estimated total errors given in parentheses) weighted for the mixture of six conformers are r(C–C) = 1.507(5), r(C–C)_{ring, av} = 1.397(3), r(C–S)_av = 1.814(4), r(C–N) = 1.495(4), r(N–O)_av = 1.223(3), ∠(C–C–C)_ring = 116.0–122.5, ∠ C6–C4–C7 = 118.2(4), ∠ C–C–S = 113.6(6), ∠ C–S–C = 98.5(12), ∠ N–C–C4 = 121.9(3), ∠(O–N–C)_av = 116.8(3), ∠ O–N–O = 127.0(4). Torsional angles could not be refined. Theoretical B3LYP/cc-pVTZ torsional angles for the rotation about C–N bond, φ_C−N, were found to be 30.5–36.5^∘ for different conformers. As to internal rotation about C–C and C–S bonds, values of φ_C−C = 68–118^∘ and φ_C−S = 66–71^∘ were obtained for the three most stable conformers with gauche orientation with respect to these bonds. Some conclusions of this work were presented in a short communication in Russ. J. Phys. Chem. 2005, 79, 1701.     

Thermal, spectral and AFM studies of calcium silicate hydrate-polymer nanocomposite material

A non-ionic polymer (poly(vinyl alcohol) (PVA)) has been incorporated into the inorganic layers of calcium silicate hydrate (C–S–H) during precipitation of quasicrystalline C–S–H from aqueous solution. C–S–H and a C–S–H-polymer nanocomposite (C–S–HPN) material were synthesized and characterized by X-ray fluorescence (XRF), energy dispersive spectroscopy (EDS), ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) and ¹³C cross-polarization nuclear magnetic resonance (¹³C CP NMR) spectroscopy, atomic force microscopy (AFM), thermal conductivity, thermogravimetric analysis (TG) and differential thermal analysis (DTA). Thermal conductivity of PVA, C–S–H and C–S–HPN material was studied in the temperature range 25–50°C. C–S–HPN materials exhibited the highest thermal conductivity at 25 and 50°C. The thermal conductivity increases from 25 to 50°C are 7.03, 17.46 and 14.85% for PVA, C–S–H and C–S–HPN material, respectively. Three significant decomposition temperature ranges were observed on the TG curve of C–S–HPN material.     

Conformational analysis of carbonic anhydrase inhibitors using ab initio molecular orbital methods. 1. Rotational isomerism in methane sulfonamide anion, CH3-SO2-NH−

Ab initio calculations have been performed on methane sulfonamide anion. Geometries have been optimized using Hartree-Fock basis sets up to 6-31+G*, and single-point calculations employing those Hartree-Fock geometries have been performed at levels up to MP2/6-311++G**. In addition, geometry optimizations for the 0°, 90°, 150°, and 180° conformers have been carried out at the MP2/6-31G*, MP2/6-31+G*, and MP2/6-311++G** levels. Vibrational frequencies have been calculated using the HF/4-31G*, MP2/6-31G*, and MP2/6-31+G* geometries. All calculations at or above the 4-31G* level agree that H—N—S—C ˜90° is the global minimum. The H—N—S—C = 180° conformer is clearly higher in energy although the relative energy of this conformer varies from 0.36 to 1.03 kcal/mol for the post-HF calculations depending on basis set. The H—N—S—C = 180° conformer appears to be a very shallow local minimum. However, the potential energy surface is quite flat in this region, and the highest-level calculations, including MP2 optimizations and vibrational frequency analysis, are ambiguous on this point. The conformer with an H—N—S—C torsion of 0° is a transition state with a relative energy ˜8 kcal/mol. Received: 3 December 1996 / Accepted: 2 January 1997     

Electron Transfer Dissociation (ETD) of Peptides Containing Intrachain Disulfide Bonds

The fragmentation chemistry of peptides containing intrachain disulfide bonds was investigated under electron transfer dissociation (ETD) conditions. Fragments within the cyclic region of the peptide backbone due to intrachain disulfide bond formation were observed, including: c (odd electron), z (even electron), c-33 Da, z + 33 Da, c + 32 Da, and z–32 Da types of ions. The presence of these ions indicated cleavages both at the disulfide bond and the N–Cα backbone from a single electron transfer event. Mechanistic studies supported a mechanism whereby the N–Cα bond was cleaved first, and radical-driven reactions caused cleavage at either an S–S bond or an S–C bond within cysteinyl residues. Direct ETD at the disulfide linkage was also observed, correlating with signature loss of 33 Da (SH) from the charge-reduced peptide ions. Initial ETD cleavage at the disulfide bond was found to be promoted amongst peptides ions of lower charge states, while backbone fragmentation was more abundant for higher charge states. The capability of inducing both backbone and disulfide bond cleavages from ETD could be particularly useful for sequencing peptides containing intact intrachain disulfide bonds. ETD of the 13 peptides studied herein all showed substantial sequence coverage, accounting for 75%–100% of possible backbone fragmentation.     

Azabicyclo[3.3.1]nonanone: a case when weak interactions are preferred over strong hydrogen bonds

Derivatives of azabicyclo[3.3.1]nonanone tend to prefer for weak interactions in the crystal over strong N–H···O hydrogen bonds. The main stabilizing forces in the investigated azatricyclo[7.3.1.0^2,7]trideca-trienone derivatives are C–H···O, N–H···π and C–H···π interactions, leading to interesting structural patterns. The azabicyclo[3.3.1]nonanone ring adopts chair-envelope conformation having exo-C2,C4-aromatic substituents. Amino NH is in trigonal pyramidal configuration. The interesting stereochemistry of azabicyclo[3.3.1]nonanone, driving exceptional preference for weaker interactions over strong hydrogen bonds serves a useful example toward engineering and design strategy, and structure prediction methodologies.     

Effect of sacrificial electron donors on hydrogen generation over visible light–irradiated nonmetal-doped TiO2 photocatalysts

Hydrogen generation over carbon-, nitrogen- and sulfur-doped TiO₂ semiconductor photocatalysts (represented as C–TiO₂, N–TiO₂ and S–TiO₂, respectively) under visible light irradiation has been achieved using various sacrificial electron donors, namely triethanolamine, diethanolamine, monoethanolamine, triethylamine, MeOH, EtOH, EDTA, l-ascorbic acid and phenol. The highest initial rate of H₂ production was found to be in the range 1,000–2,200 μmol/g/h at ambient conditions when triethanolamine was used as sacrificial electron donor. The efficacy of hydrogen production over these photocatalysts depends strongly on the nature of the sacrificial electron donor and decreases in the following order: C–TiO₂ > S–TiO₂ > N–TiO₂. The results of the present studies suggest that the rate of H₂ production is not simply governed by the reduction potential of the sacrificial electron donor but also by the kinetic barrier of the electron transfer process.     

Ab initio study of the ground state and conformational stability of peroxyacetyl nitrate in internal rotation about the peroxide bond

The molecular and electronic structure of the ground state of peroxyacetyl nitrate (PAN) was calculated by the unrestricted Hartree-Fock-Roothaan method with the use of the standard 3–21G and 6–31G basis set. The potential curve of the internal rotation about the peroxide bond of PAN was calculated with the 6–31G basis set. The curve contains two maxima. The ground state of PAN is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in planes that are perpendicular to each other (the dihedral angle ϱ(COON) is 89.9°). The calculated barriers to rotation are 19.6 and 66.8 kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 600–604, April, 1998.     

Aromaticity in heterocycles: new HOMA index parametrization

Abstract  

A new parametrization for the Harmonic Oscillator Model of Aromaticity (HOMA) index to determine aromaticity of heterocycles is introduced. The new HOMA for Heterocycle Electron Delocalization (HOMHED) is based on the experimental data from electron diffraction X-ray for the reference molecules used to estimate the simple, double, and optimal bond lengths. Bond length of “pure” single and double bonds of non-conjugated systems or systems without π-electrons and/or n-electron delocalization were considered. The HOMHED index was determined for a series of five and six heterocycles with C–C, C–N, C–O, C–S, N–N, N–O, and N–S bonds. The π-electron delocalization of these heterocycles was determined by Krygowski-reformulated HOMA and HOMHED and it was proved that HOMHED worked in line with HOMA for all heterocycles, except those containing oxygen, which were found to be weak aromatic from Krygowski rHOMA calculations.     

Self-assembly of a [2]Pseudorotaxane Composed of Cucurbit[6]uril into Linear Pseudopolyrotaxanes by N–H···O, C–H···O and π···π Interactions

The [2]pseudorotaxane of cucurbit[6]uril (Q6) with 1,6-bis(imidazol-1-yl)hexane dihydrobromide was synthesized and its crystal structure was described. The structure of [2]pseudorotaxane was mainly stablized by host–guest C–H···O interactions. Self-assembly of the [2]pseudorotaxane produces infinite one-dimensional chains with intermolecular N–H···O, C–H···O, and π···π interactions; thus, a linear non-covalent pseudopolyrotaxane is formed.     

Effect of a thin spreading solvent film on the efficiency of the hexadecan-1-ol monolayer deposited for water evaporation retardation

The influence of a thin spreading solvent film (ethanol, diethyl ether, and three fractions of petroleum ether boiling at 30–60 °C, 60–90 °C, and 90–120 °C) on the properties of hexadecan-1-ol (C₁₆H₃₃OH) monolayers at the air—water interface was studied. The specific evaporation resistance and the surface pressure were determined to describe the spreading behavior of the C₁₆H₃₃OH monolayers. The physical properties of the solvents and the images obtained in an atomic force microscope were examined. The time of establishing the equilibrium spreading surface pressure of monolayers can be reduced using a more volatile solvent with a lower boiling point and a lower relative density. The influence of the monolayer nature on water evaporation corresponds to the order of changing the solvent spreading rate: petroleum ether (30–60 °C) > diethyl ether > ethanol > petroleum ether (60–90 °C) > petroleum ether (90–120 °C). The monolayers formed upon petroleum ether (30–60 °C) spreading form a film with a less deficient and relatively planar surface. When ethanol is used as a spreading solvent, water evaporation is accelerated rather than retarded, while petroleum ether (30–60 °C) is more appropriate for this purpose.     

Synthesis, structural characterization and thermal properties of three copper(II) complexes based on aryl hydrazide ligands

The thiosemicarbazide and hydrazide Cu(II) complexes, [Cu₃L₂¹(py)₄Cl₂] (1), [Cu(HL²)py] (2) and [Cu(HL³)py] (3), (H₂L¹ = 1-picolinoylthiosemicarbazide, H₃L² = N′-(2-hydroxybenzylidene)-3-hydroxy-2-naphthohydrazide, H₃L³ = 2-hydroxy-N′-((2-hydroxy-naphthalen-1-yl)methylene)benzohydrazide) have been prepared and characterized through physicochemical and spectroscopic methods as well as X-ray crystallography. Complex 1 has a centrosymmetric structure with –N–N– bridged Cu₃ skeleton. Neighboring molecules are linked into a 3D supermolecular framework by π–π stacking interactions, N–H···Cl and C–H···Cl hydrogen bonds. Complexes 2 and 3 have similar planar structures but different dimers formed by concomitant Cu···N and Cu···O interactions, respectively. Solvent accessible voids with a volume of 391 ?³ are included in the structure of complex 2, indicating that this complex is a potential host candidate. Thermogravimetric analysis shows that the three complexes are stable up to 100 °C.     

Electron-density studies on hydrogen bonding in chromone derivatives. Part II: comparative study

The experimental electron density of a chromone derivative was determined from a multipole refinement of 100 K X-ray synchrotron data and complemented by theoretical calculations with experimental and optimized geometry. Atomic and topological properties were obtained using the Quantum Theory of Atoms in Molecules approach. The examination of topological parameters unambiguously showed π-delocalization within the H-bonded ring. The application of source function analysis confirmed the intramolecular N–H···O hydrogen bond to be a resonance-assisted hydrogen bond. The topological study confirmed the covalent nature of N–H···O interaction and the electrostatic nature of weak C–H···O interactions.     

Internal rotation in squaramide and related compounds. A theoretical ab initio study

The structural and energetic changes associated with C–N bond rotation in a squaric acid derivative as well as in formamide, 3-aminoacrolein and vinylamine have been studied theoretically using ab initio molecular orbital methods. Geometry optimizations at the MP2(full)/6-31+G* level confirmed an increase in the C–N bond length and a smaller decrease in the C=O length on going from the equilibrium geometry to the twisted transition state. Other geometrical changes are also discussed. Energies calculated at the QCISD(T)/6-311+G** level, including zero-point-energy correction, show barrier heights decreasing in the order formamide, squaric acid derivative, 3-aminoacrolein and vinylamine. The origin of the barriers were examined using the atoms-in-molecules approach of Bader and the natural bond orbital population analysis. The calculations agree with Pauling's resonance model, and the main contributing factor of the barrier is assigned to the loss of conjugation on rotating the C–N bond. Finally, molecular interaction potential calculations were used to study the changes in the nucleophilicity of N and O (carbonyl) atoms upon C–N rotation, and to obtain a picture of the abilities of the molecules to act in nonbonded interactions, in particular hydrogen bonds. The molecular interaction potential results confirm the suitability of squaramide units for acting as binding units in host–guest chemistry. Received: 13 March 2002 / Accepted: 23 June 2002 / Published online: 21 August 2002     

Sulfenylation reactions activated by thionyl and sulfuryl chlorides

Reactions ofN-(4-nitrophenylthio)morpholine,N,N′-thiobismorpholine, andN,N′-dithiobismorpholine with thionyl chloride or sulfuryl chloride at −40°C afford electrophilic chlorosulfenylating reagents, which add to the C=C bond of norbornene in high yields. Semiempirical quantum-chemical calculations and comparison of the relative reactivity of the sulfenylating complexes formed upon activation of arenesulfenamides by sulfur and phosphorus oxohalides were performed. The mechanism of the reactions is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2516–2518, December, 1998.     

FTIR study of the sol–gel synthesis of cementitious gels: C–S–H and N–A–S–H

The study explored the compatibility between the main product of Portland cement hydration and the main product of the alkali activation of fly ash: C–S–H and N–A–S–H gels, respectively. Both gels were synthesized with laboratory reagents at different pH values. Blends of the two were synthesized as well, using the sol–gel procedure. All the gels were characterized with Fourier transform IR spectroscopy (FTIR). The gels synthesized with this procedure were shown to precipitate together with a silica-rich gel. In addition, the pH level was found to play a determinant role in both C–S–H and N–A–S–H gel synthesis. The C–S–H gel is the major phase formed at pH > 11 and N–A–S–H gel for pH > 12. The results relating to the joint synthesis of the two (C–S–H and N–A–S–H) gels were not conclusive. Technique used for the characterization failed to differentiate between them in the blended material.

Thermo-kinetics study of orange peel in air

Thermal degradation of orange peel was studied in dynamic air atmosphere by means of simultaneous TG-DSC and TG-FTIR analysis. According to the obtained thermal profiles, the orange peel degradation occurred in at least three steps associated with its three main components (hemicellulose, cellulose and lignin). The volatiles compounds evolved out at 150–400 °C and the gas products were mainly CO₂, CO, and CH₄. A mixture of acids, aldehydes or ketones C=O, alkanes C–C, ethers C–O–C and H₂O was also detected. The E _α on α dependence reveled the existence of different and simultaneous processes suggesting that the combustion reaction is controlled by oxygen accessibility, motivated by the high evolution low-molecular-mass gases and volatile organic compounds. These results could explain the non-autocatalytic character of the reactions during the decomposition process.     

Dipole moments and molecular conformations ofO-vinyl- andO-ethylacetoximes in solutions

Dipole moments of the molecules ofO-vinyl- andO-ethylacetoximes in octane and THF were measured at different temperatures. In octane, the equilibrium mixture of stableO-vinylacetoxime conformers consists of forms with different polarity. Polarity of these conformers, which are due to rotation about the C−O and N−O bonds, is mainly determined by the relative position of the lone electron pairs of nitrogen and oxygen atoms. The fraction of the polar conformer increases as temperature increases, as well as on going from octane to THF. Saturation of the vinyl group in theO-vinylacetoxime molecule nearly halves the molecular dipole moment in the nonpolar medium, which radically differentiates this compound from vinyl ethers. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1685–1690, September, 1999.     

Leaving group effect in the sulfonyl transfer reactions of aryl benzenesulfonates with Grignard reagents

The C–S coupling reactions of aryl benzenesulfonates with phenylmagnesium bromide in THF:toluene (7:10) at 90 °C have been studied. A Hammett-type kinetic study of the leaving group effect of aryloxy groups provides a conclusive support for a two step addition-elimination mechanism in which C–S bond formation is rate determining step.     

Molecular structure and conformation of triphenylsilane from gas-phase electron diffraction and theoretical calculations, and structural variations in H4?nSiPhn molecules (n?=?1–4)

The molecular structure of triphenylsilane has been investigated by gas-phase electron diffraction and theoretical calculations. The electron diffraction intensities from a previous study (Rozsondai B, Hargittai I, J Organomet Chem 334:269, 1987) have been reanalyzed using geometrical constraints and initial values of vibrational amplitudes from calculations. The free molecule has a chiral, propeller-like equilibrium conformation of C ₃ symmetry, with a twist angle of the phenyl groups τ = 39° ± 3°; the two enantiomeric conformers easily interconvert via three possible pathways. The low-frequency vibrational modes indicate that the three phenyl groups undergo large-amplitude torsional and out-of-plane bending vibrations about their respective Si–C bonds. Least-squares refinement of a model accounting for the bending vibrations gives the following bond distances and angles with estimated total errors: r _g(Si–C) = 1.874 ± 0.004 ?, 〈r _g(C–C)〉 = 1.402 ± 0.003 ?, 〈r _g(C–H)〉 = 1.102 ± 0.003 ?, and ∠_aC–Si–H = 108.6° ± 0.4°. Electron diffraction studies and MO calculations show that the lengths of the Si–C bonds in H_4−n SiPh _n molecules (n = 1–4) increase gradually with n, due to π → σ*(Si–C) delocalization. They also show that the mean lengths of the ring C–C bonds are about 0.003 ? larger than in unsubstituted benzene, due to a one hundredth angstrom lengthening of the C_ipso–C_ortho bonds caused by silicon substitution. A small increase of r(Si–H) and decrease of the ipso angle with increasing number of phenyl groups is also revealed by the calculations.     

Detailed study of the γ-radiolysis of nitrilotriacetic acid in a simulated, mixed nuclear waste

Gamma radiolysis of a simulant of an actual mixed waste, composed of nitrilotriacetic acid (NTA) and an inorganic matrix, at γ-doses ranging from 0–7.5·10⁶±10% R in a⁶⁰Co-source, resulted in total NTA degradation, but 10.8% of the simulant's original organic content remained. Radiolysis yielded 4 chelator fragments and 2 carboxylic acids, all formed at different rates. The chelator fragment N-(Methylamine)iminodiacetic acid (MAIDA) dominated at low γ-doses, while N-[N′-amino(2-iminoethyl)]iminodiacetic acid (AIEIDA) dominated at high γ-doses. Chemical degradation control studies revealed that the harsh chemistry of the inorganic matrix caused only 20.7% NTA degradation and formation of only 2 chelator fragments, while the heat generated by the⁶⁰Co-source (95°C) had little additional impact.