The molecular structure and conformation of acetamides in the vapour phase: Part III. Thioacetamide (ethanethioamide)

Thioacetamide has been studied by electron diffraction in the gas phase, utilizing a new nozzle construction and using a broad electron beam. The molecule has C_s symmetry, and one C-H bond eclipses the CS bond. The most important structural parameters are: r_g(C-N) = 135.6(3) pm, r_g(C—C) = 151.2(4) pm, r_g(CS) = 164.7(3) pm,∠_αCCS = 122.9(3)° and ∠_αCCN = 114.8(4)°. Parenthesized values are one standard deviation where correlation among data and uncertainty in the electron wavelength have been included. The methyl barrier, V₃, is found from the electron diffraction data to be 4.56 kJ mol^?1. This corresponds to a torsional frequency of 131 cm^?1.     

The geometry of the propynol cation,HC3O+

The geometry of a potential interstellar molecule, the propynol cation, HC₃O⁺, is determined by split-valence plus polarization (6-31G*) SCF method with the inclusion of electron correlation effects by third order Møller—Plesset perturbation theory (MP3). The 6-31G* MP3 geometry is: r_e(CO) = 1.125, r_e(CC) = 1.351, r_e(CC) = 1.214, r_e(CH) = 1.078 A, and thus a B_e of 4.421 GHz.     

A theoretical study of the bound electronic states of the C−2 negative ion

Configuration interaction calculation are employed to study the X ²Σ⁺_g, A ²Π_u, B ²Σ⁺_u, ⁴Σ⁺_u and ⁴Δ_u states of the C^?₂ ion. The results are in good quantitative agreement with experimental findings for the Herzberg—Lagerquist (²Σ⁺_u-²Σ⁺_g) bands and predict a T_e value for the ²Π_u state of only 0.40 eV; corresponding transition moment results are obtained as a function of CC distance. The Cl electron affinity is 3.43 eV, in good agreement with the most recent experimental estimate for this quantity.     

The molecular structure of thiocarbonylbromide

An electron diffraction investigation of SCBr₂ has yielded the following r_α⁰ parameters: r_SC= 1.597(5) Å, r_CBr = 1.894(9) Å and ?BrCBr = 111.6°. These values are compared with other carbonyl and thiocarbonyl halides.     

Rotational analysis of the A—X bands of S+2

The S⁺₂ (A²Π_u-X²Π_g) emission system from sulphur monochloride in a helium flowing afterglow has been analysed in the 5000–6000 Å region. Molecular constants for the S⁺₂ (A²Π_u, X²Π_g) states have been determined and are compared with previous estimates. Equilibrium bond lengths of S⁺₂ are found to be: X²Π_g,r_e = 1.8226 ± 0.0010 Å and A²Π_u, r_e = 2.0441 ± 0.0013 Å.     

The isomers of 3-methyl pentene-2; configurational assignment and molecular structure by electron diffraction

The two isomers of 3-methyl pentene-2 have been investigated in the vapour phase by electron diffraction. The higher boiling isomer (70.4 °C) has the E-configuration and the isomer boiling at 67.7 °C has the Z-configuration. Bond distances in the E-isomer are: r(CC) = 1.349, r(Csp²-Csp³) = 1.511, r(C-C) = 1.551, r(C-H) = 1.116 Å; in the Z-isomer: r(CC) = 1.344, r(Csp²-Csp³) = 1.508, r(C-C) = 1.553, r(C-H) = 1.114 Å. In both compounds the Csp³-Csp³ bond is at approximately right angles to the plane containing the double bond. The possibility of non-planar arrangements around the double bond is discussed.     

The effect of complex stability on the rates of transport,uptake, and release of ions by macrocyclic carriers in liquid membrane systems

Several macrocyclic carriers were used in the measurements of the rates of transport, uptake, and release of potassium picrate in a chloroform membrane system. For the carrier cryptand 2.2.2, the rates were determined for each of the other alkali metal picrates (Na⁺, Rb⁺ and Cs⁺). With an increase in the stability constant of the metal—carrier complex, the rate of ion uptake, J_u, increased and the rate of ion release, J_r, decreased. Among the carriers studied, 18-crown-6 (18C6), whose J_u and J_r values were not small, gave a maximum rate of transport of potassium picrate through the liquid membrane. The rate of transport decreased with stability constant values higher or lower than that for the K⁺—18C6 complex. For cryptand 2.2.2 complexes with high stability, the J_r value was very small compared with the J_u value and decreased in the reverse order to that for the uptake, Na⁺ > Rb⁺ > K⁺, in which the rate of transport decreased. These results suggest that the rate of transport is controlled by the rate of uptake in the region of low stability constant and by the rate of release in that of high stability constant.     

Density functional theory of the chemical potential of atoms and its relation to electrostatic potentials and bonding distances

Using an approximate density functional pseudopotential method we have confirmed several relations proposed earlier by P. Politzer, R.G. Parr and D.R. Murphy (J. Chem. Phys.79, 3859 (1983)), who combined theoretical and experimental information. The main result is that the radial distancer _μ where the chemical potential μ is equal to the electrostatic potential in the free atom, is of especial significance for the bonding properties of the atom.r _μ is linearly related to the nonpolar covalent radius and to the Wigner-Seitz radius of the solid phase. On the other hand μ is linearly related to the electrostatic potentialV _Q (r _μ ) due to the net charge inside a sphere of radiusr _μ in the free atom. Finally we have discovered that a linear relation exists in the free atom between (?μ)^1/2 and ρ(r _μ )^1/3.     

The molecular structure of seleno- and thiocarbonyl difluoride

The geometric structures of seleno- and thiocarbonyl difluoride, Se=CF₂ and S=CF₂, studied in the gas phase by electron diffraction, are presented. For S=CF₂ the electron diffraction data are combined with microwave rotational constants. The following geometric parameters were derived: for SeCF₂ (r_α⁰ values) Se-C = 1.743(3) Å, C-F = 1.314(2) Å and ? FCF = 107.5(0.4)° ; for SCF₂ (r_av values) S-C = 1.589(2) Å, C-F = 1.316(2) Å and FCF = 107.1(0.2)° . Uncertainties are 3σ values of the least squares analyses.     

Electron diffraction study of the trichloromethyltrichlorogermane molecular structure and estimation for torsional barrier

The molecular geometry (in terms of r_a and r_g internuclear distances) and mean amplitudes of vibration of CCl₃GeCl₃ have been determined by electron diffraction. The bond lengths are similar to those found in analogous molecules. Although bond angles of unambiguous physical definition have not been determined it is established that the carbon and germanium bond configurations deviate little from the regular tetrahedral arrangement. The molecule performs large amplitude motion around the carbon-germanium bond. The torsional barrier was estimated to be 1.1 kcal mole^?1 using J. Karle's method [8].     

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.     

Theoretical study of the electronic transition moment for the d 3IIga 3IIu band system of the C2 molecule

Large scale ab initio SCF and CI calculations are employed to study the potential curves for the d ³II_g, a ³II_u and X¹Σ⁺_g states of the C₂ radical. The electronic transition moment R_e′e″ for the Swan bands (d ³II_ga ³II_u) is calculated in various AO and MO basis sets as a function of the internuclear CC distance. The form of the Σ|R_e′e″|² curve is in very good agreement with that obtained recently from measurements of Danylewych and Nicholls and Tatarczyk et al.; the calculated value for Σ|R_e′e″² at 2.44 bohr is found to be 5.2 au² compared to the most recent experimental values of |R_e(r_oo)|² = 3.57 au² of Tatarczyk et al.     

Interstate coupling and dynamics of excited singlet states of isolated diphenylbutadiene

In this paper, we report on absolute fluorescence quantum yields from photoselected vibrational states of jet-cooled 1,4-diphenylbutadiene for excess vibrational energies, E_v = 0−7500 cm⁻¹, above the apparent electronic origin of the S₁(2A_g) state. The pure radiative lifetimes, τ_r, of the strongly scrambled S₂(1B_u)—S₁(2A_g) molecular eigenstates (E_v = 1050−1800 cm⁻¹) show a marked dilution effect, (τ_r/τ_r(S₂) ≈ 40), being practically identical with the τ_r values from the S₁(2A_g) manifold (E_v = 0–900 cm⁻¹), which is affected by near-resonant vibronic coupling to S₂(1B_u) and exhibiting the dynamic manifestations of the intermediate level structure. Isomerization rates in the isolated molecule, which do not exhibit vibrational mode selectivity, were recorded over the energy range 0–6600 cm⁻¹ above the threshold.     

The structure of trimethoxymethane in the gas phase,an electron diffraction,ab initio and molecular mechanics study

The structure of trimethoxymethane in the gas phase was studied by electron diffraction, ab initio molecular orbital calculations and molecular mechanics. The molecule was found to exist almost exclusively as an asymmetric all-staggered TGG conformer. The electron diffraction structural parameters (r_g distances, r_α angles) as obtained from geometrically consistent r_α-refinements are: r(C-O) central 1.382(6) Å, r(C-O) terminal 1.418(6) Å, r(C-H) 1.112(1) Å, ∠(O-C-O) in the gauche—gauche chain 115.0(1.0)°, in the gauche-anti chains 109.2(0.6)° ∠(C-O-C) 114.3(0.6)°, ∠(O-C-H)_Me 109.9(0.3)°, methyl torsion 68(6)°. The structure is adequately reproduced by molecular mechanics calculations applying Allinger's force field. The structures of methoxymethanes can be explained in terms of the anomeric effect. This is confirmed by ab initio calculations.     

Ultraviolet emission of molecular chlorine

The UV emission of Cl₂ from a new valence-shell state having 0⁺_u symmetry (T_c ≈ 59774 cm^?1, r_c ≈ 3.0 Å) was observed by focusing ≈ 500 nm laser radiation to gaseous chlorine. Excitation was achieved by virtual two-photon absorption from the B ³Π_0⁺u state formed by single-photon absorption stepwisely. The emission spectra showed transitions to the ground state as well as to the repulsive grade estate dissociating to Cl²P) + Cl(²P) products.     

Multi-reference configuration interaction calculations on the systems Xe 2 + and Xe 3 +

Potential curves for the ground (²Σ _u ⁺ ) and the three lowest excited states of the Xe ₂ ⁺ dimer ion (²Π _g ,²Π _u ,²Σ _g ⁺ ) have been calculated using pseudopotentials in MRD-CI (multi-reference single anddouble excitationconfigurationinteraction) calculations. Spin-orbit interaction — leading to the six states 1.(1/2) _u , 1.(3/2) _g , 1.(3/2) _u , 1.(1/2) _g , 2.(1/2) _u , 2.(1/2) _g — has been taken into account using a semiempirical technique [1]. Subsequently, starting with a relaxed Xe ₂ ⁺ ion in its ground state, the potential energy surface for the system Xe-Xe ₂ ⁺ was studied. We found that the collinear approach of the Xe atom leads to the most stable geometry. This is a linear symmetric molecule with bond lengths of 6.38 bohr. In the bestT-shaped structure, the Xe atom is 7.83 bohr away from the midpoint of the Xe ₂ ⁺ (r=6.1 bohr) dimer. The calculated binding energy of 0.25 eV for the equilibrium structure of the Xe ₃ ⁺ molecule (i.e. the linear symmetric geometry), is in very good agreement with experimental results of 0.27 ± 0.02 eV [2].     

Molecular structure of n-butane: calculation of vibrational shrinkages and an electron diffraction re-investigation

A normal coordinate analysis was carried out based on the force field of Schachtschneider and Snyder in order to calculate all amplitudes of vibration and shrinkage corrections for n-butane. The results are tabulated to aid diffraction analyses of related substances. A vapor-phase electron diffraction reinvestigation of n-butane led to experimental measurements of the principal amplitudes of vibration and to the following molecular parameters (± 3σ ): r_g(C-C) = 1.531(2)Å, r_g(C-H)= 1.117(5)Å, ∠CCC (trans. gauche average) = 113.8(4)°, ∠CCH (ave) = 111.0(5)° , gauche CCCC dihedral angle 65(6)°, % trans conformer = 54 ± 9%, and ΔG° (gauche— trans) = 497 ± 220 cal mol^?1.     

Electron diffraction and vibrational spectroscopic investigation of the molecular structure of (chloromethyl)trichlorosilane

An electron diffraction analysis of the molecular structure of the title compound has been carried out, and related vibrational spectroscopic measurements and calculations have been made. The main bond lengths (r_g and bond angles r_α) are as follows: SiCl, 202.8(2); SiC, 185.1(10); CCl, 179.4(11); CH, 111.2(18) pm; SiCCl, 111.7(4);l ClSiC, 109.95(21)°. The conformation of the molecule is staggered. The barrier to internal rotation is estimated to be around 10 kJ mol^?1.     

Molecular structure and conformation of gaseous 2-chloro-3-fluoro-1-propene

2-Chloro-3-fluoro-1-propene has been studied by electron diffraction, and the molecule was found to exist in equilibrium between a syn and a gauche conformation, with the syn conformation as the most stable. The most important structure parameters with standard deviation are: r_g(CC) = 1.338(6) Å,r_g(C—C) = 1.505(5) Å, r_g(C—F) = 1.378(4) Å, r_g(C-Cl) = 1.743(3) Å, ∠CC—Cl = 123.0(7)°, ∠CC—C = 125.6(6)° and ∠C—C—F = 111.2(8)°.A force field was determined by a least-squares refinement to vibrational frequencies. Mean square amplitudes of vibration and perpendicular amplitude correction coefficients have been calculated. The mean square amplitudes of vibration from the electron diffraction data are in very good agreement with the values calculated from the spectroscopic data.     

Small-angle maxima and cation-cation distances. Differences between iron nitrate and perchlorate solutions

X-ray diffraction data of aqueous solutions of iron(III) nitrate show a small-angle peak at an s value of about 0.5–0.9 A^?1. A constant relationship between peak position and concentration leads to reject the hypothesis of a quasi-lattice structure for the solutions investigated. On the contrary, no peak is present in the same s range of diffraction data of iron (III) perchlorate solutions.