Evidence of ultra-fast dissociation in ammonia observed by resonant Auger electron spectroscopy

We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the N1s→4a₁ core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z+1 approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH₂^* and H fragments.     

Molecular structure of acetyl cyanide as studied by gas electron diffraction

The structure of acetyl cyanide has been determined by making joint use of the electron diffraction intensities measured in the present study and the rotational constants reported by Krisher and Wilson. The thermal average bond distances are: r_g(C-H) = 1.116±0.011 Å, r_g(CN) = 1.167±0.010 Å, r_g(C=O) = 1.208±0.009 Å, r_g(=C-C) = 1.477±0.008 Å and r_g(C-C_methyl) = 1.518±0.009 Å. The bond angles in the zero-point average structure (r_av) are: (C_methyl-C=O) = 124.6±0.7°, (C-C-C) = 114.2±0.9°, (C-CN) = 179.2±2.2° and (H-C-H) = 109.2±0.7°. The uncertainties represent the estimated limits of experimental error. The C-C single bond placed between the double and triple bonds is longer than those in vinylacetylene, acrylonitrile and propynal. Other structural parameters are also compared with those in related molecules. The infrared and Raman spectra of this molecule have been measured, and Urey-Bradley force constants have been determined.     

The structure of cyclopentene oxide determined by gas phase electron diffraction

The r_g structure of cyclopentene oxide has been determined by the simultaneous least squares analysis of electron diffraction and microwave spectroscopic data. The investigation has reaffirmed previous studies indicating that the molecule prefers a boat conformation. The methylene and epoxide flap angles obtained are 152.3±2.1° and 104.7±1.0° respectively. Other structural parameters determined are: r_g (C-H avg.) = 1.120±0.004 Å; r_g (C-C avg.) = 1.538±0.002 Å; r_g (C-O) = 1.443±0.003 Å, and r_g (C-C) = 1.482±0.004 Å for the carbon-carbon bond in the three membered epoxide ring. These results compare favorably with the reported structures of ethylene oxide and cyclohexene oxide. A tentative rationalization of the unusual boat conformation is also offered.     

Molecular structure of carbonyl bromide as studied by gas electron diffraction

The molecular structure of COBr₂ has been determined as follows by an analysis of electron diffraction intensity: r_g(CO) = 1.178 ± 0.009 Å, r_g(C-Br) = 1.923 ± 0.005 Å and θ°_α(Br-C-Br) = 112.3 ± 0.4°. The uncertainties represent estimated limits of error. The observed systematic trends in the bond lengths and bond angles in carbonyl and thiocarbonyl halides are discussed.     

An investigation of the molecular structure of cycloheptanone by gas phase electron diffraction

The electron diffraction data of cycloheptanone, collected at 371 K, can be explained using a model of partial pseudorotation, with the symmetrical twist—chair as the mean structure. Ther_g, r_α-structure is characterized by r(C-C) = 1.536 Å, r(C=O) = 1.219 Å, r(C-H) = 1.124 Å, xxxCC(sp²)C = 117.3°, xxx(CCC = 115.5° and xxx(HCH = 103.2°. Approximate values for the constants of the pseudorotation potential are included.     

Molecular structures of propene and 3,3,3-trifluoropropene determined by gas electron diffraction

The structures of propene and 3,3,3-trifluoropropene have been studied by electron diffraction intensities measured in the present study and rotational constants reported in the literature. The following average structures have been determined: For propene, r_g(CC) = 1.342 ± 0.002 Å, r_g(C-C) = 1.506 ± 0.003 Å, r_g(C-H)_vinyl = 1.104 ± 0.010 Å, r_g(C-H)_methyl = 1.117 ± 0.008 Å, ∠(C-CC) = 124.3 ± 0.4°, ∠(CC-H) = 121.3 ± 1.4°, and ∠(C-C-H) = 110.7 ± 0.9°; for trifluoropropene, r_g(CC) = 1.318 ± 0.008 Å, r_g(C-C) = 1.495 ± 0.006 Å, r_g(C-H)= 1.100 ± 0.018 Å, r_g(C-F) = 1.347 ± 0.003 Å, ∠(C-CC) = 125.8 + 1.1°, ∠(C-C-F) = 112.0 ± 0.2°, where the valence angles refer to the r_av structure, and the uncertainties represent estimated limits of experimental error. A simple set of quadratic force constants for each molecule has been estimated. Regular trends have been observed in the CC and C-C bond distances and the C-CC angles in these and related molecules. Significant differences between the CC, C-C and C-F distances and the C-C-F angle in trifluoropropene and in hexafluoroisobutene reported by Hilderbrandt et al. have been indicated.     

Indicator of efficiency of experiment in gas electron diffraction

The ineffectiveness of the traditional probe electron-diffraction experiment on free molecules (atoms), which prevents the development of the diffraction structural method in gas phase electron diffraction, has been shown. The application of a molecular beam enabled the determination of a quantitative performance indicator of the scattering process using the given experimental equipment. The quantitative value of the effective cross section on residual gas molecules σ_ext(S _max, Å^?1) has been suggested as a technical characteristic for the electron diffraction equipment complex. A nomographic chart for determining the number of molecules of any substance has been drawn up in order to obtain a given density of scattered charge on the detector within the maximum sector radium. The author refers to the GED community with the need for agreement on a unified estimation of the efficiency of electron scattering experiment on the free molecules.     

Molecular potential functions from joint analysis of gas electron diffraction and spectroscopic data

A joint analysis of electron diffraction and spectroscopic data is carried out for BF₃, PBr₃, AsBr₃, SbCl₃, SeO₂ and ClO₂ in terms of the harmonic force fields. The scheme of analysis was extended to include the following spectroscopic observables: vibrational frequencies, rotational, Coriolis coupling and centrifugal distortion constants and, whenever available, those for the isotopic species. For ClO₂ a simplified anharmonic analysis was also performed, the anharmonic spectroscopic observables being involved in this case.

Compelling evidence has been presented that the conventional harmonic approximation for the force field in terms of rectilinear internal coordinates yields the simplest satisfactory representation of diffraction and spectroscopic observations. However, a consistently better fit to experimental data was found when natural curvilinear internal coordinates were used. It is shown here that for the systems considered the joint analysis of data from various sources ensures that reliable and accurate values for equilibrium distances and force field parameters are obtained. The optimized values of spectroscopic constants, structural and force field parameters obtained are presented and compared with those available from the literature.     

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.     

A gas electron diffraction study of the molecular structure of cis-stilbene

The cis-Stilbene molecule is found to possess C₂ symmetry and may be described as having a propeller-like conformation with the phenyl groups rotated ca. 43° about the C-φ bonds. The deviation from planarity is found to be more extensive than predicted by most theoretical calculations. The steric strain in the molecule is also revealed by large valence angles at the central carbon-carbon double bond (∠CC-C: 129.5°).     

The radicals formed by photoinduced electron transfer from hypocrellins to electron acceptors

The photoinduced electron transfer from excited singlet and triplet states of hypocrellins to three electron acceptors, namely, methyl viologen chloride (MV), tetrachloro-p-benzoquinone (TCQ) and 2,3-dichloro-5,6-dicyan-p-benzoquinone (DDQ), has been investigated by fluorescence and time-resolved transient absorption spectra. In acetonitrile solution, DDQ and TCQ quenched the fluorescence and T-T absorption of hypocrellins (HA and HB) efficiently, while neither fluorescence nor T-T absorption of them could be quenched by MV. The quenching resulted from the electron transfer between excited hypocrellins and the electron acceptors was controlled by diffusion. The rate constants of electron transfer from excited singlet and triplet of HA to DDQ are 9.20×10¹⁰ dm³ mol^?1 s^?1 and 1.28×10⁹ dm³ mol^?1 s^?1, respectively. The transient absorption spectra of the formed radical cations of hypocrellins are reported.     

A gas electron diffraction study of the molecular structure of trans-stilbene

The molecular structure of trans-stilbene has been studied by the gas electron diffraction method. Unlike the approximately planar structure with C_i symmetry found for the solid state, the molecule in the gas phase was found to be non-planar and to possess C₂ symmetry. The phenyl groups were found to be rotated ca. 30° about the C-φ bonds. The non-planarity of the molecule is, however, not so large as to seriously influence the resonance energy.