Electron Diffraction Study of the Molecular Structure of 6,6′-bis>(1,5-Diazabicyclo[3.1.0]hexane)

The molecular structure of 6,6-bis(1,5-diazabicyclo[3.1.0]hexane) was investigated by gas-phase electron diffraction using quantum-chemical calculations. Two conformations were found: boat for bicyclic fragments and anti relative to the exocyclic bond.     

Molecular structure of 1,10-dicarba-closo-decaborane(10), 1,10-C2B8H10, by gas-phase electron diffraction

The molecular structure of the title compound has been determined by gas-phase electron diffraction, assuming that the C₂B₈ unit has the form of a bicapped square antiprism (D_4d symmetry). The amplitudes of vibration and the shrinkage corrections were calculated from the force field transferred mainly from 1,6-C₂B₄H₆. The molecular parameters (r_g, φ_α) and uncertainties (3σ) are: B-C = 1.602(2), B2-B3 (basal) = 1.850(5), B2-B6 (equatorial) = 1.829(4), B-H = 1.164(14), C-H = 1.14(2) and ∠C-B-H = 117.5(1.8)°. Comparisons are made with structural data for other carboranes studied in the gas phase.     

Automatic gain control in mass spectrometry using a jet disrupter electrode in an electrodynamic ion funnel

We report on the use of a jet disrupter electrode in an electrodynamic ion funnel as an electronic valve to regulate the intensity of the ion beam transmitted through the interface of a mass spectrometer in order to perform automatic gain control (AGC). The ion flux is determined by either directly detecting the ion current on the conductance limiting orifice of the ion funnel or using a short mass spectrometry acquisition. Based upon the ion flux intensity, the voltage of the jet disrupter is adjusted to alter the transmission efficiency of the ion funnel to provide a desired ion population to the mass analyzer. Ion beam regulation by an ion funnel is shown to provide control to within a few percent of a targeted ion intensity or abundance. The utility of ion funnel AGC was evaluated using a protein tryptic digest analyzed with liquid chromatography Fourier transform ion cyclotron resonance (LC-FTICR) mass spectrometry. The ion population in the ICR cell was accurately controlled to selected levels, which improved data quality and provided better mass measurement accuracy.     

Electron diffraction determination of the vapour phase molecular structure of azetidine, (CH2)3NH

The electron diffraction study of azetidine yielded the following main geometrical parameters (r_a structure): dihedral angle (the angle between the C-C-C and C-N-C planes) φ = 33.1 ± 2.4°, r(C-N) = 1.482 ± 0.006Å, r(C-C) = 1.553 ± 0.009Å, r(C-H) = 1.107 ± 0.003Å, ∠C-N-C = 92.2 ± 0.4°, ∠C-C-C = 86.9 ± 0.4° and ∠C-C-N = 85.8 ± 0.4°.     

Molecular Structures of Acetylene Derivatives of Tin. VIII Trimethylstannylacetylene: Use of Results of Quantum-Chemical Calculations and Spectroscopic Measurements in Analysis of Gas-Phase Electron Diffraction Data

Structural analysis of electron diffraction data on trimethylstannylacetylene, (CH₃)₃SnCCH (1), obtained in the previous investigation (the nozzle temperature being 22°C), has been performed with consideration of nonlinear kinematic effects at the first-order level of perturbation theory (h1). The geometry and force field of 1 have been calculated by the RHF and MP2 (frozen core) methods. The effective core potential in SBK form and the optimized 31G* valence basis set have been applied in the case of Sn atom. The 6-311G** basis set have been used for carbon and hydrogen atoms. Vibrational spectra of the light and two deuterated isotopomers of 1 have been interpreted using the C _3v equilibrium molecular symmetry. For this purpose, the procedure of scaling the quantum-chemical force field by fitting the calculated frequencies to the experimental ones has been employed. The root-mean-square (RMS) vibrational amplitudes and shrinkage corrections used in the electron diffraction analysis have been calculated from the scaled quantum-chemical force field. It has been shown that flexibility of the linear fragment in 1 decreases considerably compared to that of the symmetrically substituted acetylene fragment in the (CH₃)₃SnCCSn(CH₃)₃ molecule (2). Using these data, we refined the geometrical parameters of 1 in terms of a static C _3v symmetry molecular model. The following r _h1 values have been obtained (the bond distances are given in Å and the valence angles in degrees): Sn—C_Me 2.147(7), Sn—C2.096(17), CC 1.237(11), C_Me—H (av.) 1.091(4), C_Me—Sn-C107.1(7), Sn—C_Me—H (av.) 113.4(6). The values in parentheses are experimental total errors including least-squares standard deviation values and scale uncertainties. The structural parameters of linear fragments in both ethynyl derivatives of Sn 1 and 2 are found to be virtually equal.     

The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane,Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations

Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V ₀[(/ _e )² – 1]² with V ₀ = 1.0 ± 0.5 kcal/mol and _e = 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r _a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)_av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.