Hans V. Volden

Molecular conformation of 2,2'-bithiophene determined by gas phase electron diffraction and ab initio calculations

Abstract Gas phase electron diffraction was performed at 97–98°C on 2,2′-bithiophene and the experimental data show the existence of two conformations, anti -like and syn -like, with torsional angles of 148(3) and 36(5)° and conformational weights 56(4) and 44(4)%, respectively, which are in agreement with ab initio calculations using standard 3-21G ∗ and 6-31G ∗ basis sets.

The molecular structure of benzene derivatives, part 2: 4-chloro-benzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

The molecular structure of gaseous 4-chlorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF6-311G∗∗ level of theory. The plannar Cs symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large amplitude motion. The most important structure parameters (rg) from the joint analysis with estimated total errors (in parentheses) are: (CC)mean = 1.398(1) A, CCl = 1.734(3) A, CC(  O) = 1.482(10) A, C  O = 1.216(5) A, <CCClC = 121.0(5)°, and <CCCHOC = 120.2(8)°. A scaled molecular force field has been determined. The ground state rotational constants have been determined from microwave data.

The molecular structures of bis(pentamethylcyclopentadienyl)-calcium and -ytterbium in the gas phase; two bent metallocenes

Abstract Gas electron diffraction studies show that whereas the ligand rings in (η 5 -C 5 Me 5 ) 2 Mg, are essentially parallel, the thermal average structures of (η 5 -C 5 Me 5 ) 2 Ca and (η 5 -C 5 Me 5 ) 2 Yb are bent, the ring-centroid—metal—ring-centroid angles being 154(3)° and 158(4)°, respectively.

The molecular structure of benzene derivatives part 1. 4-Fluorobenzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

Abstract The molecular structure of gaseous 4-fluorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF 6-311 G ∗∗ and MP 2 6-31 G ∗ levels of theory. The planar Cs symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large-amplitude motion. The most important structure parameters (rg) from the joint analysis with estimates of 2σ (in parentheses) were: ( CC ) mean = 1.397(1) A , CF = 1.331(7) A , CC (= O ) = 1.488(7) A , CO = 1.195(5) A ,

The length, strength and polarity of metal-carbon bonds: dialkylzinc compounds studied by density functional theory calculations, gas electron diffraction and photoelectron spectroscopy

The molecular structures and thermodynamic functions of seven dialkyl zinc compounds, R2Zn, R = Me, Et, i-Pr, t-Bu, n-Pr, neopentyl and the silaneopentyl Me3SiCH2, of the parent hydrocarbons RH and of the radicals R have been determined by density functional theory calculations at the B3LYP/SDD level. The molecular structures of the i-Pr, t-Bu, neo-Pe and Me3SiCH2 derivatives have been determined by gas electron diffraction. Me2Zn, Et2Zn, t-Bu2Zn and neo-Pe2Zn have been studied by photoelectron spectroscopy and the ionisation energies calculated. Both experimental and calculated Zn-C bond distances were found to increase in the order Me2Zn ≈ (Me3SiCH2)2Zn < Et2Zn ≈ n-Pr2Zn ≈ neo-Pe2Zn < i-Pr2Zn < t-Bu2Zn.Calculated mean bond rupture enthalpies indicate that the strength of the Zn–C bonds decrease in the same order, viz. Me2Zn ≈ (Me3SiCH2)2Zn > Et2Zn ≈ n-Pr2Zn ≈ neo-Pe2Zn > i-Pr2Zn > t-Bu2Zn.Both bond lengths and bond strengths were found to be strongly correlated with the inductive Taft constant I, indicating that the bond strength increases and the bond length decreases with increasing electron withdrawing power of the alkyl group. Evidence from the literature indicates that bond strengths and bond lengths in homoleptic alkyl derivatives of the main group metals in Groups 12, 13 and 14 and of the transition elements in Group 4 vary in the same manner.

The molecular structure of tellurium dichloride, TeCl2, determined by gas electron diffraction

Abstract The electron diffraction pattern of the vapor from a liquid sample ( t = 210°C) of composition Te:Cl = 1.00:2.00 has been recorded. The gas jet was found to consist of TeCl 2 molecules with bond distance TeCl = 2.329(3) A and valence angle ∠ClTeCl = 97.0(6)°.

Molecular Structures of Chloro(phthalocyaninato)-aluminum(III) and -gallium(III) as Determined by Gas Electron Diffraction and Quantum Chemical Calculations : Quantum Chemical Calculations on Fluoro(phthalocyaninato)-aluminum(III) and -gallium(III), Chloro(tetrakis(1,2,5 -thiadiazole)porphyrazinato)-aluminum(III) and -gallium(III) and Comparison with their X-ray Structures

The molecular structures of chloro(phthalocyaninato)aluminum(III) (ClPcAl) and chloro(phthalocyaninato)gallium(III) (ClPcGa) have been determined by using the gas electron diffraction (GED) method and augmented by quantum chemical calculations. The molecular structures of fluoro(phthalocyaninato)aluminum(III) (FPcAl), fluoro(phthalocyaninato) gallium(III) (FPcGa), chloro(tetrakis(1,2,5-thiadiazole)porphyrazinato)aluminum(III) (ClTTDPzAl), and chloro(tetrakis(1,2,5-thiadiazole)porphyrazinato)gallium(III) (ClTTDPzGa) have been optimized at the level B3LYP with basis sets 6-31G*, 6-311++G**, and cc-pVTZ, and the structures have been compared with those obtained by X-ray diffraction. Vibrational frequencies have been calculated for all six molecules at all basis sets combinations, except B3LYP/cc-pVTZ. These calculations predict that all molecules have C 4 v symmetry with the metal atom above the plane defined by the four inner cavity N atoms and an almost planar macrocycle. The most important structure parameters (GED|B3LYP/cc-pVTZ) are h (height of the metal atom above the inner cavity) h = 50.3(32)|44.9 pm, r(Al-Cl) = 214.5(16)|217.4 pm, r(Al-N) = 197.6(9)|198.7 pm, angle(Cl-Al-N) = 104.8(9)|103.1 degrees, angle(Al-N-C) = 124.2(7)|125.9 degrees for ClPcAl, and the corresponding values for ClPcGa are h = 53.1(28)|50.2 pm, r(Ga-Cl) = 218.9(14)|222.3 pm, r(Ga-N) = 200.6(8)|202.6 pm, angle(Cl-Ga-N) = 105.4(8)|104.3 degrees, angle(Ga-N-C) = 123.8(8)|125.0 degrees. Parenthesized values are estimated error limits defined as 2.5(sigma(2)(lsq) + (0.001 x r)(2))(1/2) for distances and 2.5sigma(lsq) for angles. The title compounds are all flexible molecules with about five vibrational frequencies below 100 cm(-1).

Molecular Structure of the trans and cis Isomers of Metal-Free Phthalocyanine Studied by Gas-Phase Electron Diffraction and High-Level Quantum Chemical Calculations: NH Tautomerization and Calculated Vibrational Frequencies

The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.

Molecular Structure of Phthalocyaninatotin(II) Studied by Gas-Phase Electron Diffraction and High-Level Quantum Chemical Calculations

The molecular structure of phthalocyaninatotin(II), Sn(II)Pc, is determined by density functional theory (DFT/B3LYP) calculations using various basis sets and gas-phase electron diffraction (GED). The quantum chemical calculations show that Sn(II)Pc has C4V symmetry, and this symmetry is consistent with the structure obtained by GED at 427 degrees C. GED locates the Sn atom at h(Sn) ) 112.8(48) pm above the plane defined by the four isoindole N atoms, and a N-Sn bond length of 226.0(10) pm is obtained. Calculation at the B3LYP/ccpVTZ/cc-pVTZ-PP(Sn) level of theory gives h(Sn) ) 114.2 pm and a N-Sn bond length of 229.4 pm. The phthalocyanine (Pc) macrocycle has a slightly nonplanar structure. Generally, the GED results are in good agreement with the X-ray structures and with the computed structure; however, the comparability between these three methods has been questioned. The N-Sn bond lengths determined by GED and X-ray are significantly shorter than those from the B3LYP predictions. Similar trends have been found for C-Sn bonds for conjugated organometallic tin compounds. Computed vibrational frequencies give five low frequencies in the range of 18-54 cm-1, which indicates a flexible molecule.

The molecular structures of tris(dimethylamino)-phosphane, -arsane and -stibane, E(NMe2)3, E P, As or Sb and Me CH3, by gas electron diffraction and ab initio molecular orbital calculations

Abstract The molecular structures of E(NMe2)3, E  P, As or Sb and Me  CH3, have been determined by gas electron diffraction (GED) and ab initio molecular orbital calculations at the HF 6–31 G ∗ level. The equilibrium structures have Cs symmetry with two NMe2 ligands oriented in such a manner that the direction of the electron lone pair on each N atom is orthogonal to the direction of the lone pair on the E atom, while the third ligand is oriented in such a manner that the lone pair on the N atom is antiparallel to the lone pair on E. The coordination of the antiparallel N atom is distinctly pyramidal (sum of the valence angles = 337° by GED) while the two orthogonal N atoms are nearly planar (sum of valence angles = 353° by GED). The bond distances from E to the antiparallel N atom is two to four pm longer than the bond distances to the orthogonal N atoms, and the valence angle >NEN spanned by the orthogonal N atoms is some 10° larger than the two angles spanned by the antiparallel and one orthogonal N atom. It is suggested that the equilibrium structures are stabilized by anomeric effects, i.e. delocalization of the lone pairs of the orthogonal N atoms into antibonding σ∗(E-N) orbitals.