Molecular Structure of PrBr3 and HoBr3 According to Simultaneous Electron Diffraction and Mass Spectrometry Experiment

The saturated vapors of praseodymium and holmium tribromides were investigated for the first time by electron diffraction with mass spectral monitoring at 1100(10) and 991(10) K. It is established that the molecules have a pyramidal effective configuration with bond angles Br–Pr–Br = 114.7(10)° and Br–Ho–Br = 115.3(11)°. Given the low deformation vibration frequencies of lanthanide tribromide molecules, the insignificant pyramidality of the r_g configuration may correspond to the planar equilibrium geometry of D_3h symmetry for the molecules. The internuclear distances r_g(Pr–Br) = 2.696(6) and r_g(Ho–Br) = 2.594(5) point to the lanthanide compression effect. The vibration frequencies of PrBr₃ and HoBr₃ molecules were estimated from electron diffraction data.     

Structural analysis and probing the conformational space of dansylamide by means of gas-phase electron diffraction and quantum chemistry

Dansylamide is perhaps the most ubiquitous fluorophore due to its donor-acceptor bifunctionality and its ability to form intra- and intermolecular hydrogen bonding. Among the diversity of its applications is the development of new generation of biosensors for the in vivo monitoring of traces of metals. The structure and conformational stability of dansylamide in the gas phase were investigated for the first time by a combined gas-phase electron diffraction-mass spectrometry (GED/MS), complemented by quantum chemical calculations. GED data indicate that different skewed conformers exist at T?=?464 K, which are characterized by the deviation of two S–N bonds from the perpendicular orientation relative to the naphthalene plane. Maybe the most indicative structural parameters for electronic interactions between the donor-acceptor substituents and the aromatic naphthalene and the subsequent stabilization of the favorable skewed eclipsed-syn conformer are the dihedral angles C₉–C₁–S–N and C₁₀–C₅–N–C with the experimentally determined values of 66.8° (32) and 68.1° (72), respectively. The role of –SO₂NH₂ by forming intramolecular hydrogen bonds was scrutinized by employing the natural bond orbital approach (NBO), quantum theory atoms in molecules (QTAIM), and molecular electrostatic potential (MESP). The non-planarity of the naphthalene skeleton due to the electronic interactions with the substituents and its consequence for the fluorescence activity of dansylamide have been discussed.     

Structure and energy studies of Β-diketonates. 5. structure of the Y(DPM)2 radical from gas-phase electron diffraction data

The overheated vapor of yttrium tris-dipivaloyl methanate was studied by combined electron diffractometry and mass spectrometry. It was established that at 760 K the vapor of Y(DPM)₃ contains the Y(DPM)₂ molecular form. The r_a, r_g, and r_a parameters of Y(DPM)₂ were determined, and it was found that the chelate system of Y(DPM)₂ has a planar structure. Possible reasons for differences in the intemuclear distances of the Y(DPM)₂ and Y(DPM)₃ molecular forms are discussed. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 3, pp. 480–488, May–June, 1997.     

IR Spectra of N,N′-Ethylene-Bis(salicylaldiminates) and N,N′-ethylene-Bis(acetylacetoniminates) of Ni(II), Cu(II), and Zn(II)

Vibrational spectra of N,N′-ethylene-Bis(salicylaldiminaates) and N,N′-ethylene-Bis(acetylacetoniminates) of nickel (II), copper (II), and zinc (II) are studied experimentally (IR spectroscopy, 400–4000 cm⁻¹) and theoretically (B3LYP), band assignment is given, and the distribution of potential energy of normal vibrations in internal coordinates is studied. Differences between vibrational spectra of the complexes are discussed. Thermodynamic functions of gas-phase complexes corresponding to temperatures of 298 and 800 K are calculated.     

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).     

Molecular structure and bonding in octamethylporphyrin tin(II), SnN4C28H28

Gas-phase electron diffraction was applied for the molecular structure determination of octamethylporphyrin tin(II), SnN(4)C(28)H(28), at the temperature of 706(10) K. The molecule was found to possess C(4v) symmetry with the Sn atom 1.025(30) ? above the plane of the N atoms and the following main internuclear distances (r(h1), ?): Sn-N = 2.301(9), C(α)-N = 1.360(8), C(α)-C(β) = 1.453(4), C(α)-C(m) = 1.395(4), C(β)-C(CH3) = 1.498(4). Quantum chemical calculations, DFT (B3LYP, BP86, PBE, PBE0) with cc-pVDZ, cc-pVTZ and cc-pVQZ basis sets reproduce the experimental bond distances with accuracy within 0.03 ?. According to NBO(B3LYP/cc-pVTZ) analysis, the direct donation gives a prevailing contribution to Sn-N bonding, decreasing the net charge on Sn from formal +2 to +1.28. The substitution effects at the pyrrole rings are discussed. The ability of different theoretical methods to predict the structure of this compound is analyzed.     

Molecular structure and conformations of benzenesulfonamide: gas electron diffraction and quantum chemical calculations

The molecular structure and conformational properties of benzenesulfonamide, C6H5SO2NH2, were studied by gas electron diffraction (GED) and quantum chemical methods (MP2 and B3LYP with different basis sets). The calculations predict the presence of two stable conformers with the NH2 group eclipsing or staggering the SO2 group. The eclipsed form is predicted to be favored by about 0.5 kcal/mol. According to GED, the saturated vapor over solid benzenesulfonamide at a temperature of 150(5) degrees C consists of the eclipsed conformer. The GED intensities, however, possess a very low sensitivity toward the vapor composition, and contributions of the anti conformer of up to 75% (at the 0.05 level of significance) or up to 55% (at the 0.25 level of significance) cannot be excluded. The molecule possesses C(sS) symmetry with the S-N bond perpendicular to the ring plane.     

Structure and vibration frequencies of gaseous europium dibromide

Molecular structure of EuBr₂ was studied by electron diffraction and mass spectrometry at 1373(20) K. The molecule has a nonlinear equilibrium configuration and is characterized by the following effective parameters: r_g(Eu-Br) = 2.767(6) ?, r_g(Br-Br) = 5.11(5) ?, l_g(Eu-Br) = 0.109(2) ?, l_g(Br-Br) = 0.388(5) ?, Z_g(Br-Eu-Br) = 135.0(3.5)°. The vibration frequencies v₁ = 225(10) cm^-1 and v₂ = 40(4) cm^-1 were found using electron diffraction data. Translated from Zhumal Strukturnoi Khimii, Vol. 41, No. 4, pp. 718-724, July-August, 2000