Molecular structure of 1,5-diazabicyclo[3.1.0]hexane as determined by gas electron diffraction and quantum-chemical calculations

The equilibrium molecular structure and conformation of 1,5-diazabicyclo[3.1.0]hexane (DABH) has been studied by the gas-phase electron-diffraction method at 20 degrees C and quantum-chemical calculations. Three possible conformations of DABH were considered: boat, chair, and twist. According to the experimental and theoretical results, DABH exists exclusively as a boat conformation of C s symmetry at the temperature of the experiment. The MP2 calculations predict the stable chair and twist conformations to be 3.8 and 49.5 kcal mol(-1) above the boat form, respectively. The most important semi-experimental geometrical parameters of DABH (r(e), A and angle)e), deg) are (N1-N5) = 1.506(13), (N1-C6) = 1.442(2), (N1-C2) = 1.469(4), (C2-C3) = 1.524(7), (C6-N1-C2) = 114.8(8), (N5-N1-C2) = 107.7(4), (N1-C2-C3) = 106.5(9), and (C2-C3-C4) = 104.0(10). The natural bond orbital (NBO) analysis has shown that the most important stabilization factor in the boat conformation is the n(N) --> sigma*(C-C) anomeric effect. The geometry calculations and NBO analysis have been performed also for the bicyclohexane molecule.     

Molecular structure of (cyanomethoxy)(dimethylamino)methane as studied by gas-phase electron diffraction and quantum-chemical calculations

The molecular structure of (cyanomethoxy)(dimethylamino)methane in the gas phase is investigated by means of gas-phase electron diffraction and quantum-chemical studies for the first time. It is shown that in the gaseous state, (cyanomethoxy)(dimethylamino)methane is a mixture of gauche-anti (65%) and anti-gauche (35%) conformers. The slight influence of the anomeric effect on the compound’s structural parameters was noted.     

Molecular structure of the macrocyclic copper(II) chelate with 6,7,13,14-tetramethyl-3,10-dithio-1,2,4,5,8,9,11,12-octaazatetradecatetraene-1,5,7,11 according to quantum-chemical DFT calculations

The key parameters of the molecular structure of macrotetracyclic (NNNN)-coordinated chelate of Cu(II) with 6,7,13,14-tetramethyl-3,10-dithio-1,2,4,5,8,9,11,12-octaazatetradecatetraene-1,5,7,11 have been calculated by means of the DFT-based B3LYP6-31G(d) method in GAUSSIAN-09 software. The complex may be formed via self-assembly of Cu(II), thiocarbohydrazide, and diacetyl in the gelatin-immobilized matrix implants. The bond lengths, bond angles, and torsion angles are presented; it has been shown that the metal chelate and the chelating ligand are almost planar.     

Electronic structure of adducts of Nd(III) carboxylate complexes determined by DFT and XPS methods

X-ray photoelectron spectroscopy and quantum chemistry methods are used to study Nd(tol)₃ and Nd(сor)₃ carboxylate complexes and their adducts with 1,10-phenanthroline (Nd(tol)₃Phen and Nd(сor)₃Phen₂). The electronic structure and specific features of the nature of chemical bonds are studied, as well as the effect of 1,10-phenanthroline on the electronic structure of the adduct. We propose the band assignment of the valence band of the XPS spectra of all compounds.     

Molecular orbital calculation of the electronic structure of poly(N-vinylcarbazole)

Energy levels and molecular orbital wave functions of π-electrons in a carbazole ring are calculated by the ω-method. The result agrees well with observed optical absorption spectra of carbazole and poly(N-vinylcarbazole) (PVCA). Molecular orbital wave functions are obtained for a PVCA chain in a 3/1 helix. The overlap integral between adjacent carbazole rings is evaluated for each molecular orbital of the ring. The result affords evidence for hole conduction in PVCA and explains the photoconductivity spectrum under high de field.     

Molecular structure and conformation of gaseous vinyldimethylchlorosilane as determined by electron diffraction

The molecular structure of vinyldimethylchlorosilane has been determined by gas phase electron diffraction at room temperature. The least squares values of the bond lengths (r_g) and bond angles (∠_α) are : r(CH) = 1.086(6) Å, r(CC) = 1.347(5) Å, r(SiC=) = 1.838(6) Å, r(SiC) = 1.876(3) Å, r(SiCl) = 2.078(2) Å, ∠CCSi = 127.8° (1.2) and ∠=CSiCl = 107° (1). Models with pure syn form and a mixture of syn and gauche gave equally good agreement with the diffraction data.     

Molecular structure of bis(acetylacetonato)nickel(II) in the gas phase as determined from electron diffraction data

The molecular structure of bis(acetylacetonato)nickel(II) has been determined by a sector-microphotometer gaseous electron-diffraction method. The experimental data were found to be consistent with a monomeric square-planar structure. The structural parameters of the chelate were determined as follows: ∠ ONiO = 93.6 ± 1.1°, r_g(Ni-O) = 1.876±0.005A Å, r_g(C-0) = 1.273± 0.007 Å, r_g(C-C_ring) = 1.401 ± 0.010 Å, r_g(C-C_methyl) = 1.504 ± 0.013 Å. The mean amplitudes of vibration and the shrinkage effects were calculated from normal-vibration treatment using the Urey-Bradley force field.     

2-Bornyl cations stabilized by metal clusters: synthetic and X-ray structural and quantum-chemical studies

Data on syntheses and structures of 2-bornyl cations stabilized by metal clusters are presented.This work was reported at the conference Modern Problems of Organometallic Chemistry (May 8–13, 1993, Moscow) and is a part of the work prepared for publication in collaboration with H. El Hafa, M. Nikol'ski, G. Jaouen, J. Waissermann, L. Lee, and M. J. Mc Glinchey.Ecole Nationale Superieure de Chimie, 11 rue Pierre et Marie Curie, 76231 Paris, France.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 589–593, April, 1994.     

DFT quantum-chemical study of the hydrolysis products of Fe(II) and Fe(III) aqua-complexes

The quantum-chemical DFT calculations of thermodynamic characteristics of the reactions of formation of binuclear dihydroxobridging [Fe(H₂O)₄(μ-OH)₂Fe(H₂O)₄] ⁿ⁺ and oxobridging [Fe(H₂O)₅(μ-O)Fe·(H₂O)₅] ⁿ⁺ (n = 2, 4) cations, the hydrolysis products of cations [Fe(H₂O)₆] ^m+ (m = 2, 3). It is shown that effects of solvation lead to higher energetic stability of the dihydroxobridging binuclear compounds in aqueous solutions.     

Molecular structure and force field of boron tribromide as determined from combined analysis of gas electron diffraction and spectroscopic data and supported by quantum-chemical density-functional calculations

The thermal-average parameters of BBr₃ at 21(1) °C were obtained from a conventional analysis of gas electron diffraction (GED) data (r_g(B---Br) = 190.0(4) pm). The equilibrium structure and the force constants were refined from a joint analysis of the GED intensities and vibrational frequencies using different approximations. The simplest approximation (quadratic potential function in rectilinear coordinates) is suitable for the refinements of the equilibrium bond length (r^h_e(B---Br) = 189.6(4) pm) and the force constants of BBr₃. The molecule is planar within the error limits. Quantum-chemical density-functional calculations supported planarity of the molecule.     

Molecular structure of bis(acetylacetonato)beryllium in the gas phase as determined from electron diffraction data

The molecular structure of bis(acetylacetonato)beryllium has been determined by gas electron diffraction. The experimental data were found to be consistent with the D_2d model in which the oxygen atoms are arranged tetrahedrally around the central beryllium atom (∠OBeO = 106.0 ± 1.0°). The structural parameters are as follows: r_g(Be-O) = 1.615 ± 0.006 Å, r_g (C-O) = 1.270 ± 0.004 Å, r_g (C-C_ring) = 1.397 ± 0.004 Å, r_g (C-C_meth) = 1.499 ± 0.005 Å. The mean amplitudes of vibration were calculated from the normal-vibration treatment using the modified Urey—Bradley force field     

Molecular structure of N-chlorosuccinimide studied by gas-phase electron diffraction and quantum-chemical methods

The molecular structure of N-chlorosuccinimide has been studied by GED method at the nozzle temperature of 116 °C. Vibrational corrections to the r _a parameters, Δ(r _a − r _e), have been calculated using the scaled quadratic and cubic force constants from B3LYP/6-31G(df,p) calculations. The force field scaling has been carried out using the IR and Raman spectra of the solid N-chlorosuccinimide. The molecular skeleton and the bond conformation around nitrogen were found to be planar within large experimental errors. The equilibrium geometrical parameters derived from the experimental data assuming C _2v molecular symmetry and those from MP2(fc)/6-311G(3df,2pd) calculations are in a good agreement.     

Theoretical DFT study of the structure and chemical activity of small indium(III) oxide clusters

The B3LYP/Lanl2dz and B3LYP/SDD levels of DFT have been used to describe the structural properties of small stoichiometric indium(III) oxide clusters. It was shown that the most stable structures for the monomer and dimer are linear and cubic, respectively, in origin. The most stable trimer is due to the formation of three eight-membered and two six-membered rings with alternation of In and O atoms. Among neutral and monocation tetrameric structures, formation of an ??arrowhead?? isomer is energetically less favorable than the global minimum structure that has eight six-membered and six four-membered rings. In the pentamer and octamer, a few centers of higher coordination number and a variety of In?CO bond lengths are observed. The other centers cannot be fitted to the characteristic bixbyite structure, however, so the larger octamer cannot be a good model for mimicking the properties of the In₂O₃ crystal structure. An H-terminated cluster model consisting of In₁₃O₂₇H₁₅ is proposed that well describes basic features of indium oxide and tin-doped indium oxide (ITO) structures.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.     

Molecular structure of 2,2,4,4,6,6-hexamethyl-1,3,5-trimethylenecyclohexane as determined by gas-phase electron diffraction

The molecular structure of 2,2,4,4,6,6-hexamethyl-1,3,5-trimethylenecyclohexane has been determined in the gas phase at a nozzle tip temperature of 340 K. The electron diffraction data were found to be consistent with a model where the cyclohexane ring adopts a distorted twist-boat conformation. The averaged geometrical parameters (r(g) and 90 degree angle (alpha)) obtained from least squares analysis are r(C=C) = 1.346(4) A, r(C-C)(ring) = 1.537(1) A, r(C-C)(Me) = 1.543(1) A, 90 degree angle C(6)C(1)C(2) = 117.5(11) degrees, 90 degree angle C(1)C(2)C(3) = 113.1(12) degrees, and 90 degree angle MeCMe = 108.2(13) degrees. The experimental results are consistent with the results from HF/6-311G(d) and MP2/6-311G(d) calculations where the distorted twist-boat form is found to be lower in energy than the chair form by 9.85 and 10.7 kcal/mol, respectively.     

Molecular structure and conformation of gaseous chloroacetyl chloride as determined by electron diffraction

Chloroacetyl chloride is studied by gas-phase electron diffraction at nozzle-tip tempera- tures of 18, 110 and 215°C. The molecules exist as a mixture of anti and gauche confor- mers with the anti form the more stable. The composition (mole fraction) of the vapor with uncertainties estimated at 2σ is found to be 0.770 (0.070), 0.673 (0.086) and 0.572 (0.086) at 18, 110 and 215°C, respectively. These values correspond to an energy difference with estimated standard deviation ΔE^o = E^o_g -E^o_a = 1.3 ± 0.4 kcal mol^?1 and an entropy difference ΔS^o = S^o_g -S^o_a = 0.7 ± 1.1 cal mol^?1 K^?1. Certain of the diffraction results permit the evaluation of an approximate torsional potential function of the form 2V = V₁(1 - cos φ) + V₂(1 - cos 2φ) + V₃(1 - cos 3φ); the results are V₁ = 1.19 ± 0.33, V₂ = 0.56 ± 0.20 and V₃ = 0.94 ± 0.12, all in kcal mol^?1. The results for the distance (r_a), angle (_∠α) and r.m.s. amplitude parameters obtained at the three temperatures are entirely consistent. At 18°C the more important parameters are, with estimated uncertainties of 2σ, r(C-H) = 1.062(0.030) Å, r(CO) = 1.182(0.004) Å, r(C-C) = 1.521(0.009) Å. r(CO-Cl) = 1.772(0.016) Å, r(CH₂-Cl) = 1.782(0.018) Å, ∠C-C-0 = 126.9(0.9)°, ∠CH₂-CO-C1 = 110.0(0.7)°,∠CO-CH₂-C1 = 112.9(1–7)°, ∠H-C-H = 109.5° (assumed), ∠φ (gauche torsion angle relative to 0° for the anti form) = 116.4(7.7)°, δ (r.m.s. amplitude of torsional vibration in the anti conformer) == 17.5(4.2)°.     

1-Chloro-1-(trichlorovinyl)cyclopropane : Molecular structure and conformation in the gaseous phase as determined by electron diffraction

A gas phase electron diffraction investigation of 1-chloro-1-(trichlorovinyl)cyclopropane at 65° showed the existence of one conformer having a perpendicular arrangement of the cyclopropyl- and the trichlorovinylgroup, with torsional angle τ = 91(3)° relative to τ = 180° for the antiperiplanar conformer (CCl-CClanti arrangement). Both the experimental data and a molecular mechanics calculation indicate the lower barrier toward the antiperiplanar form,less than 40 kJ · mol^-1. The second barrier was by molecular mechanics calculated to 60 kJ · mol^-1.     

DFT quantum-chemical calculations of nitrogen oxide chemisorption and reactivity on the Cu(100) surface

A DFT quantum-chemical study of NO adsorption and reactivity on the Cu₂₀ and Cu₁₆ metal clusters showed that only the molecular form of NO is stabilized on the copper surface. The heat of monomolecular adsorption was calculated to be ΔH _m = ?49.9 kJ/mol, while dissociative adsorption of NO is energetically unfavorable, ΔH _d = + 15.7 kJ/mol, and dissociation demands a very high activation energy, E _a = + 125.4 kJ/mol. Because of the absence of NO dissociation on the copper surface, the formation mechanism of the reduction products, N₂ and N₂O, is debatable since the surface reaction ultimately leads to N-O bond cleavage. As the reaction occurs with a very low activation energy, E _a = 7.3 kJ/mol, interpretation of the NO direct reduction mechanism is both an important and intriguing problem because the binding energy in the NO molecule is high (630 kJ/mol) and the experimental studies revealed only physically adsorbed forms on the copper surface. It was found that the formation mechanism of the N₂ and N₂O reduction products involves formation (on the copper surface) of the (O_adN-NO_ad) dimer intermediate that is chemisorbed via the oxygen atoms and characterized by a stable N-N bond (r _N-N ～1.3 Å). The N-N binding between the adsorbed NO molecules occurs through electron-accepting interaction between the oxygen atoms in NO and the metal atoms on the “defective” copper surface. The electronic structure of the (O_adN-NO_ad) surface dimer is characterized by excess electron density (ON-NO)^δ? and high reactivity in N-O_ad bond dissociation. The calculated activation energy of the destruction of the chemisorbed intermediate (O_adN-NO_ad) is very low (E _a = 5–10 kJ/mol), which shows that it is kinetically unstable against the instantaneous release of the N₂ and N₂O reduction products into the gas phase and cannot be identified by modern experimental methods of metal surface studies. At the same time, on the MgO surface and in the individual (Ph₃P)₂Pt(O₂N₂) complex, a stable (O_adN-NO_ad) dimer was revealed experimentally.