The molecular structure of N-methylsuccinimide studied by gas-phase electron diffraction (GED) and quantum-chemical methods

The molecular structure of N-methylsuccinimide was studied by the GED method at a nozzle temperature of 69–73°C. Anharmonic vibrational corrections to thermal-average r _a bond lengths, Δ(r _a − r _e), were calculated using the quadratic and cubic force constants from B3LYP/6-31G(df, p) calculations. The molecular skeleton was found to be planar within measurement errors. Some structural effects were likely caused by the conjugation of the N atom with two C=O bonds. The equilibrium geometric parameters derived from the experimental data and those from MP2/cc-pVTZ(seg-opt) calculations were in close agreement.     

Conformation dependence of pKa’s of the chromophores from the purple asFP595 and yellow zFP538 fluorescent proteins

Two members of the green fluorescent protein family, the purple asFP595 and yellow zFP538 proteins, are perspective fluorescent markers for use in multicolor imaging and resonance energy-transfer applications. We report the results of quantum based calculations of the solution pK_a values for selected protonation sites of the denatured asFP595 and zFP538 chromophores in the trans- and cis-conformations in order to add in the interpretation of photo-physical properties of these proteins. The pK_a values were determined from the theromodynamic cycle based on B3LYP/6-311++G(2df, 2p) calculations of the gas phase free energies of the molecules and the B3LYP/6-311++G(d, p) calculations of solvation energies. The results show that the pK_a’s of the protonation sites of the chromophore from asFP595 noticeably depend on the isomer conformation (cis- or trans-), while those of zFP538 are much less sensitive to isomerization.     

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The r _g (C–F) bond is shortened to 1.307(13) Å in comparison with r _g (C–F) = 1.356(4) Å in C₆H₅F.     

Conformational Stability of Propenoyl Bromide from Temperature-Dependent Infrared Spectra of Krypton Solutions,Ab Initio Calculations,and r 0 Structural Parameters of the Propenoyl Halides (F,Cl, Br)

Variable temperature (–100 to –150°C) studies of the infrared spectra (3500–400 cm^–1) of propenoyl bromide, CH₂=CHCBrO, dissolved in liquid krypton, have been carried out. Utilizing six different conformer pairs, an enthalpy difference of 204 ± 20 cm^–1 (2.44 ± 0.24 kJ/mol) was obtained, with the anti conformer (carbonyl bond trans to C=C bond) the more stable form. At ambient temperature, there is approximately 28 ± 2% of the syn conformer present. The anti conformer also remains in the infrared and Raman spectra of the polycrystalline solid. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies, are reported for both conformers from MP2/6-31G(d) ab initio calculations. The potential function governing the conformational interchange has been obtained from the MP2/6-31G(d) ab initio calculations. The conformational stabilities were calculated from a variety of basis sets and at the highest level of calculations, MP2/6-311 + (2df,2pd), the anti conformer is predicted to be more stable by 178 cm^–1, which is in excellent agreement with the experimental results. The r ₀ adjusted structural parameters have been obtained for propenoyl fluoride and chloride from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. Several of the parameters for the chloride are significantly different than those proposed from an electron diffraction investigation. The results of these spectroscopic, structural, and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

Spectra and Structures of Silicon-Containing Compounds. XXIV.* Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,Barriers to Internal Rotation,and Ab Initio Calculations of Ethylsilane

The infrared (3200 to 400 cm^–1) and Raman (3200 to 20 cm^–1) spectra of gaseous and solid ethylsilane, CH₃CH₂SiH₃, have been recorded. Additionally, the Raman spectrum of the liquid has been obtained with quantitative depolarization values. The SiH₃ torsional mode has been observed as sum and difference bands with the silicon-hydrogen stretching vibration. Utilizing the torsional fundamental frequency of 132 cm^–1 the threefold periodic barrier of 590 cm^–1 (7.06 kJ/mol) has been obtained. Utilizing the frequencies of the silicon-hydrogen stretches, Si-H bond distances of 1.485 and 1.484 Å have been obtained for the bonds gauche and trans to the methyl group, respectively. Using previously reported rotational constants from seven different isotopomers, the r ₀ parameters have been calculated and are compared to the corresponding r _s parameters. A complete vibrational assignment is proposed that is consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities as well as the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31G(d), 6-311 + G(d,p), and 6-311+G(2d,2p) basis sets at levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. The results are discussed and the theoretical values are compared to the experimental values when appropriate.     

Spectra and Structure of Silicon-Containing Compounds. XXV. Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,and Ab Initio Calculations of Ethyl Chlorosilane-Si-d2

The infrared (3200 to 400 cm^–1) spectra of gaseous and solid and Raman (3200 to 20 cm^–1) spectra of liquid and solid ethyl chlorosilane-Si-d₂, CH₃CH₂SiD₂Cl, have been recorded. Both the gauche and trans conformers have been identified in the fluid phases, but only the gauche conformer remains in the solid phase. Variable temperature (–105 to –150°C) studies of the infrared spectra of CH₃CH₂SiH₂Cl dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 78±11 cm^–1 (0.93±0.13 kJ/mol), with the gauche conformer the more stable form. Utilizing the frequencies of the silicon-hydrogen stretches, from the chlorosilane-Si-d isotopomer, Si—H bond distances of 1.481 and 1.480 Å have been obtained for the gauche conformer and 1.481 Å for the trans conformer. Complete vibrational assignments are proposed for both isotopomers which are consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities and the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31(d), 6-311++G(d,p), and 6-311+G(2d,2p) basis sets with full electron correlation by the Moller–Plesset (MP) perturbation method to second order. Continuing the previously reported rotational constants from five different isotopomers and the ab initio predicted structural parameters, adjusted r ₀ parameters have been calculated, which are compared to the corresponding r _s parameters. The results are discussed and the theoretical values are compared to the experimental values when appropriate.Taken in part from the dissertation of Y. E. Nashed, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree     

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.     

Investigation of the structure and energy of β-diketonates. XIV. Composition of overheated vapors and the molecular structure of monomeric yttrium tris-hexafluoroacetylacetonate Y(C5O2HF6)3

Simultaneous electron diffraction and mass spectrometry along with a quantum chemical (DFT/B3LYP) calculation are applied to study the molecular structure of yttrium tris-hexafluoroacetylacetonate Y(hfa)₃. The superheating of the vapor in a double two-temperature effusion cell shows that up to a temperature of ∼200°C ions containing from one to three metal atoms are formed, and the most intensive ion has the stoichiometry of (Y₂L₅)⁺ at a temperature below ∼120°C. The monomer starts to noticeably decompose at temperatures above 330°C.The electron diffraction patterns of monomers are obtained at T _exp = 208(5)°C. According to the results of theoretical and experimental investigations, Y(hfa)₃ molecule has D ₃-symmetry. The rotation angle of triangular O-O-O faces with respect to their position in the regular prism is equal to 14.4(1)°C. The values of internuclear distances and valence angles (r _h1-geometry) are: r(Y-O) = 2.259(6) Å, r(C-O) = 1.263(6) Å, r(C-C_r) = 1.413(4) Å, r(C-C_F) = 1.531(4) Å, r(C-F) = 1.344(3) Å, O-Y-O = 75.2(2)°, O-C-C_F = 113.8(2)°, C-C_F-F = 112.4(2)°. The results of quantum chemical calculations are well consistent with the experimental data. Original Russian Text Copyright ? 2007 by G. V. Girichev, V. V. Rybkin, N. V. Tverdova, S. A. Shlykov, N. P. Kuz’mina, and I. G. Zaitseva __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 48, No. 5, pp. 871–879, September–October, 2007.     

Conformational properties of <Emphasis Type="Italic">ortho</Emphasis>-nitrobenzenesulfonamide in gas and crystalline phases. Intra- and intermolecular hydrogen bond

A combined gas-phase electron diffraction and quantum chemical (B3LYP/6-311+G**, B3LYP/cc-pvtz, MP2/cc-pvtz) study of molecular structure of 2-nitrobenzenesulfonamide (2-NBSA) was carried out. Quantum chemical calculations showed that 2-NBSA has four conformers, two of which are stabilized by intramolecular hydrogen bond. The latter (with the S–N bond in a close to orthogonal position around the phenyl ring and differing from each other by staggered or eclipsed positions of the N–H and S=O bonds in the SO₂NH₂ group) presented in a saturated vapor over 2-NBSA at T = 433 (3) K in commensurable amounts. Experimental internuclear distances (Ǻ) for the staggered conformer are (?): r _h1(C–H)_av. = 1.071(9), r _h1(C–C)_av. = 1.390(4), r _h1(C–S) = 1.789(8), r _h1(S=O)_av. = 1.427(6), r _h1(S–N) = 1.644(6), r _h1(N–O)_av. = 1.221(4), r _h1(C′–N) = 1.487(8), r _h1(N–H)_av. = 1.014. Calculations at B3LYP/cc-pvtz level were performed to determine the structure and the energies of the transition states between conformers. It was shown that the conformer structures of free molecule differ from those of a molecule stabilized by intermolecular hydrogen bonds in a crystal. Influence of a substituent X (X = –CH₃, –NO₂) on conformational features of the ortho-substituted benzenesulfonamide was established.     

Theoretical study of reaction mechanism for Se + O<Subscript>3</Subscript>

The reaction mechanism of Se + O₃ on the singlet potential energy has been investigated at CCSD(T)/6-311++G(2df,2pd) level of theory based on the geometric parameters optimized at the B3LYP/6-311++G(3df,3pd) level of theory. The calculated results show that the reactants are firstly associated into the adduct Se–O₃ with any intrinsic barrier. Subsequently, through a variety of transformations of isomer Se–O₃, two kinds of products P₁(SeO₃(D_3h)) and P₂(SeO + ³O₂) are obtained. The breakage and formation of the chemical bonds in the reaction have been studied by the topological analysis of electronic density. The topological analysis results show that the ring transitional structure region does not only occur in cis-OSeOO → SeO₃(C_s) process but also occur in SeO₃(C_s) → SeO₃(D_3h).     

Conformational Studies of Cyclopropylmethyl Ketone from Temperature-Dependent FT–IR Spectra of Xenon Solutions

Variable temperature (–60 to –100°C) studies of the infrared spectra (3500–400 cm^–1) of cyclopropylmethyl ketone, c-C₃H₅C(CH₃)O, dissolved in liquid xenon have been recorded. Utilizing several doublets due to the cis and near-trans conformers, the enthalpy difference has been determined to be 269 ± 26 cm^–1 (3.22 ± 0.31 kJ/mol) with the cis conformer (oxygen atom cis to the three-membered ring) the more stable rotamer. From these data it is estimated that 79 ± 3% of the cis form is present at ambient temperature. Ab initio calculations have been carried out with different basis sets up to 6-311+G(2df,2pd) at the restricted Hartree–Fock and/or with full electron correlation by the perturbation method to second order (MP2) from which structural parameters and conformation stabilities have been determined. These calculations support the experimental conformational conclusions that the cis form is the more stable conformer. A complete vibrational assignment is given for the cis conformer, which is supported from a normal coordinate calculation utilizing ab initio force constants. Several of the fundamentals of the near-trans conformer have been identified and assigned. Adjusted r ₀ structural parameters have been obtained from combined ab initio predicted values and previously reported rotational constants from the microwave investigation. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

Ab initio study of reactions of hydroxyl radicals with chloro‐ and fluoro‐substituted methanes

Ab initio calculations at the unrestricted Hartree–Fock (UHF) level have been performed to investigate the hydrogen abstraction reactions of ^? OH radicals with methane and nine halogen‐substituted methanes (F, Cl). Geometry optimization and vibrational frequency calculations have been performed on all reactants, adducts, products, and transition states at the UHF/6‐31G* level. Single‐point energy calculations at the MP2/6‐31++G* level using the UHF/6‐31G* optimized geometries have also been carried out on all species. Pre‐ and postreaction adducts have been detected on the UHF/6‐31G* potential energy surfaces of the studied reactions. Energy barriers, ΔE^?, reaction energies, ΔE_r, reaction enthalpies, ΔH_r, and activation energies, E_a, have been determined for all reactions and corrected for zero‐point energy effects. Both E_a and ΔH_r come into reasonable agreement with the experiment when correlation energy is taken into account and when more polarized and diffuse basis sets are used. The E_a values, estimated at the PMP2/6‐31++G* level, are found to be in good agreement with the experimental ones and correctly reproduce the experimentally observed trends in fluorine and chlorine substitution effects. A linear correlation between E_a and ΔH_r is obtained, suggesting the presence of an Evans–Polanyi type of relationship. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 426–440, 2001     

Comparison of the molecular structures of five benzene derivatives as determined independently by gas-phase electron diffraction in two different laboratories: a perspective

The merits and limitations of gas-phase electron diffraction as a technique for determining accurately the structures of small organic molecules are demonstrated by comparing the molecular structures of five benzene derivatives studied in two different laboratories. The existence of many possible representations of the structure of a molecule (r _a, r _g, r _α, r _α⁰, r _e, …), implying differences in the physical meaning of geometrical parameters, is discussed in some detail as it may cause difficulties in a comparison. Also illustrated are different ways of estimating and expressing uncertainties in the geometrical parameters from a gas-phase electron diffraction study.     

Conformational Stability from Variable-Temperature Infrared Spectra of Krypton Solutions, Ab Initio Calculations, and r o Structural Parameters of Chlorocyclopentane

The infrared spectra (3500–400 cm^–1) of krypton solutions of chlorocyclopentane, c-C₅H₉Cl, at variable temperatures (–101 to –150°C) have been recorded and the fundamental vibrations of the axial conformer and several of those for the equatorial form have been assigned. Utilizing two pairs of fundamentals for the two conformers in the krypton solution, an enthalpy difference of 145±15 cm^–1 (1.73±0.18 kJ-mol^–1) has been obtained with the axial conformer the more stable form. It is estimated that there is 67±2% of the axial conformer present at ambient temperature. Convincing spectroscopic evidence shows that a significant percentage of the chlorocyclopentane molecules are undergoing pseudorotation at ambient temperature. The conformational stabilities, harmonic force constants, fundamental frequencies, infrared intensities, and Raman activities have been obtained from MP2/6-31G(d) calculations with full electron correlation and these quantities have been compared to the experimental values when appropriate. The optimized geometries and conformational stabilities have also been obtained from ab initio MP2 calculations as well as by density functional theory (DFT) by the B3LYP method with several different basis sets. The adjusted r ₀ structural parameters have been obtained for both conformers by combining the ab initio data with the previously reported microwave rotational constants. These new values of the structural parameters for both conformers are compared to those previously reported from electron diffraction and microwave studies. These results are compared to the corresponding quantities of some similar molecules.     

DFT studies of ESR parameters for N?O centered radicals,N‐alkoxyaminyl and aminoxyl radicals

Theoretical calculations of ESR parameters for aminoxyl radicals have been widely studied using the density functional theory (DFT) calculations. However, the isomer N‐alkoxyaminyl radicals have been limitedly studied. With the use of experimental data for 46 N‐alkoxyaminyl and 38 aminoxyl radicals, the isotropic ¹⁴N hyperfine coupling constants (a_N) and g‐factors have been theoretically estimated by several DFT calculations. The best calculation scheme of a_N for N‐alkoxyaminyl radicals was PCM/B3LYP/6‐31 + + G(d,p) (R² = 0.9519, MAE = 0.034 mT), and that for aminoxyl radicals was PCM/BHandHLYP/6‐31 + + G(3df,3pd) (R² = 0.9336, MAE = 0.057 mT). For aminoxyl radicals, the solvation models in calculations enhanced the accuracy of reproducibility. In contrast, for N‐alkoxyaminyl radicals the calculations with solvation models provided no improvement. The differences in the best functionals between two types of radicals were thought to come from the contribution ratios of neutral and dipolar canonical structures in resonance forms. The a_N for N‐alkoxyaminyl radicals that were stabilized by small contribution of dipolar canonical structures could be precisely reproduced by B3LYP with only 20% HF exact exchange. In contrast, the a_N for aminoxyl radicals stabilized by large contribution of dipolar canonical structures was well reproduced by BHandHLYP with 50% HF exchange. The best calculation scheme of g‐factors was IEFPCM/B3LYP/6‐31 + G(d,p) (R² = 0.9767, MAE = 0.0001) for not only aminoxyl but also N‐alkoxyaminyl radicals. Copyright © 2011 John Wiley & Sons, Ltd.     

The influence of steric hindrance on conformation properties and molecular structure of 2,4,6-trinitrobenzenesulfonic acid: gas electron diffraction and quantum chemical calculations

A combined gas-phase electron diffraction and quantum chemical (B3LYP/cc-pVTZ, MP2/cc-pVDZ) study of molecular structure of 2,4,6-trinitrobenzenesulfonic acid (2,4,6-tri-NBSA) was carried out. Quantum chemical calculations showed that 2,4,6-tri-NBSA possesses six conformers, which form three pairs of enantiomers with the relative energy of 0, 4.4/3.9, and 2.5/2.5 kcal/mol. It was experimentally established that at T = 444(5) K a saturated vapor over 2,4,6-tri-NBSA is, predominantly (up to 93 mol.%), represented by a low-energy enantiomers II and II′ characterized by intramolecular hydrogen bond between an H atom of the hydroxyl group and one of the O atoms of the NO₂ group. Experimental internuclear distances for the low-energy enantiomers are (?): r _h1(C–C)_av. = 1.387(4), r _h1(C–S) = 1.811(6), r _h1(S=O)_av. = 1.424(4), r _h1(S–O) = 1.579(4), r _h1(N–O)_av. = 1.214(3), r _h1(C–N)_av. = 1.491(5). Geometry of the conformer II points on existance of strong steric interactions between SO₂OH group and two ortho-nitro groups. Analysis of the orbital interactions between the substituents and benzene ring was carried out. Geometric parameters and energies of transition states between conformers were calculated (B3LYP).     

Microcalorimetric study of DNA–Cu(II)TOEPyP(4) porphyrin complex

The influence of new water soluble cationic metalloporphyrin Cu(II)TOEPyP(4) (meso-tetra-(4-N-oxyethylpyridyl)), analogue of Cu(II)TMPyP(4), on thermodynamic stability of DNA at various molar ratios of r = porphyrin/DNA b.p. (0 < r < 0.12) has been studied. It has been shown that Cu(II)TOEPyP(4) is a strong stabilizing agent for calf thymus DNA increasing its melting temperature from 75.5 to 99.5 °C, in the range 0 < r < 0.06. The melting enthalpy (∆H _m) does not change in the range 0.002 < r < 0.06 and it equals to 11.6 ± 0.8 cal/g. At r > 0.07, ∆H _m and T _m decrease, and at r = 0.12 they equal to 6.4 ± 0.6 cal/g and 92.5 °C, accordingly. We suggest that such centers of binding are the well documented 5′CG3′ sites and G-quadruplex at r < 0.01, and negatively charged phosphate groups at r > 0.01. On the basis of ∆H _m invariability with simultaneous increase of T _m in the range 0.002 < r < 0.06, it is shown that the DNA-Cu(II)TOEPyP(4) complex melting is not of an enthalpic nature but of an entropic one. The two-phase helix–coil transition of DNA at r < 0.01 is considered as a result of porphyrin redistribution in the melting process.     

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.     

Energy Storage by Poly(<Emphasis Type="Italic">o</Emphasis>-anisidine) Matrix During Oxidative Hydrolysis

The chromaticity of poly(o-anisidine) (POAN) doped with different acids (HA), HA-doped POAN, has been studied by the spectrophotometric technique and the results were substantiated by molecular mechanics (MM+) calculations. The observed absorbance decrease (λ around 720 nm, dark green coloration) with increasing concentration of the inorganic oxidizing agent (KMnO₄) can be attributed to the oxidative hydrolysis mechanism. The oxidative hydrolysis constant (K _h) is highly dependent on the strength of the acid used. The HClO₄-doped POAN matrix has the ability to store about 128.878 kJ⋅g⁻¹ chromogenic energy (CE) at the wavelength 720 nm in a condensed lightweight form. MM+ calculations suggest that the potential energy (PE) in kJ⋅mol⁻¹ of the optimum molecular geometric (OMG) structure of the HClO₄-doped POAN matrix is at least two (2.052) times more stable than the OMG of the base form (POAN-EB) of the POAN matrix. Kinetic parameters of the oxidative hydrolysis reaction of the HA-doped POAN matrix were deduced from absorbance variations with time. The results of computer-oriented kinetic analysis indicate that the rate-controlling step for HA-doped POAN oxidative hydrolysis is governed by the Ginstling-Bronshein equation that represents three-dimensional diffusion (D4). Activation parameters for the oxidative hydrolysis of the HClO₄-doped POAN matrix were computed and discussed.     

Trimethylsilyl trichloroacetate vibrational,structural and electronic properties and their comparison with related acetates

The molecular structure of trimethylsilyl trichloroacetate, CCl₃C(O)OSi(CH₃)₃, was determined by ab initio (MP2) and DFT calculations using 6-31G(d), 6-311G(d,p), 6-311++G(d,p) and 6-311++G(3df,3pd) basis sets. The infrared and Raman spectra for the liquid phase were also recorded and the bands observed assigned to the vibrational normal modes. The study was completed using natural bond orbital (NBO) analysis and atoms in molecules (AIM) calculations. The comparison between the calculated molecular geometrical parameters, conformation and vibrational properties and those measured for CX₃C(O)OR [X = F, Cl and R = CH₃, Si(CH₃)₃] was of particular interest in order to check the behavior of the CO and CO with respect to the different substitutions. The experimental vibrational data, along with calculated theoretical force constants, were used to define a scaled quantum mechanical force field for the target system that enabled us to estimate the measured wavenumbers with a final root-mean-square deviation of 8.92 cm⁻¹.