Snefrid Gundersen

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.

Structure and barrier of internal rotation of biphenyl derivatives in the gaseous state: Part 3. Structure of 4-fluoro-, 4,4′ -difluoro-, 4-chloro - and 4,4′-dichlorobiphenyl

Abstract Gas-phase electron diffraction structures of the four title compounds have been determined. The structure parameters were found to be: 4-Fluorobiphenyl: r(CF) = 1.365(4), r(CC)ave = 1.396(1), r(C1C1′) = 1.497(3), r(CH)ave = 1.097(2), ∠C6C1C2 = 118.8(5), ∠C1C2C3 = 121.8(3), ∠C6′C1′C2′ = 118.4(6), ∠C1′C2′C3′ = 120.8(2) 4-Chlorobiphenyl: r(CCl) = 1.735(2), r(CC)ave = 1.397(1), r(C1C1′) = 1.480(4), r(CH)ave = 1.099(2), ∠C6C1C2 = 118.6(6), ∠C1C2C3 = 121.3(5), ∠C6′C1′C2′ = 117.4(6), ∠C1′C2′C3′ = 120.7(3) 4,4′-Difluorobiphenyl: r(CF′) = 1.349(2), r(C1C2) = 1.413(3), r(C2C3) = 1.393(3) r(C3C4′) = 1.387(2),r(C1C1′) = 1.483(4), r(CH)ave = 1.085(3), ∠C6C1C2 = 117.8(3) ∠C1C2C3 = 121.3(2) 4,4′-Dichlorobiphenyl: r(CC1) = 1.736(1), r(C1C2) = 1.405(4), r(C2C3) = 1.385(3), r(C3C4) = 1.401(3), r(C1C1′) = 1.494(5), r(CH)ave = 1.093(3), ∠C6C1C2 = 118.1(3), ∠C1C2C3 = 121.4(3) Distances, ra, are given in Angtroms and angles, ∠α, in degerees refering to the dynamic model. Both static and dynamic models have been applied in the investigation of the large amplitude motion about the inter-ring CC bond. All title compounds are non planar. The dynamic model (applied potential function V(o)) = 1 2 V3(1 - cos 2o) + 1 2 V4 (1 - cos 4o) gave dihedral angles 44.8(0.8), 44.1(1.1), 45.0(1.0) and 45.2(1.5)°, and the Fourier coefficients V1 and V4 equal to 0.3(1.3) and -11.0(3.1) 0.9(1.2) and -7.2(2.8), 0.0(1.3) and -9.5(2.7), -0.3(1.8) and -8.3(3.1) kJ mol−1 respectively for 4-fluoro-, 4-chloro-4,4′-difluoro- and 4,4′-dichlorobiphenyl. The numbers in parentheses are one standard deviation as given by least-squares refinements using a diagonal weight matrix.

Structure and barrier to internal rotation of biphenyl derivatives in the gaseous state: Part 2. Structure of 3,3′-dibromo-, 3,5,4′ -tribronio- and 3,5,3′5′ -tetrabromobiphenyl

Abstract Gas-phase electron diffraction structures of the title compounds have been determined. The structure parameters were found to be: 3,3′-Dibromobiphenyl: r(Br) = 1.892(2), r(CC) ave = 1.398(1), r(ClCl′) = 1.504(5), r(CH)ave = 1.093(5), ∠C6C1C2 = 121.4(4), ∠C2C3Br = 119.4(4). 3,5,4′-Tribromobiphenyl: r(C3Br3) = 1.885(2), r(C4Br4′) = 1.892(2), r(C1C2) = 1.396(1), r(C2C3) = 1.399(1), r(C3C4) = 1.396(1), r(C1′C2′) = 1.394(1), r(C2′C3′) = 1.398(1), r(C3′C4′) = 1.394(1), r(C1C1′)_ = 1.511(9), r(C2H2) = 1.065(6), r(C4H4) = 1.070(6), r(C3′H3′) = 1.066(6), ∠C2C1C6 = 120.5(8), ∠C1C2C3 = 118.9(6), ∠C2′C1′C6′ = 120.4(1.0), ∠C1′C2′C3′ = 120.0(2). 3,5,3′5′ -Tetrabromobiphenyl: r(CBr) = 1.889(1), r(C1C2) = 1.395(5), r(C2C3) = 1.389(7), r(C3C4) = 1.406(9), r(C1C1′) = 1.513(9), r(CH) = 1.062(6), ∠C2C1C6= 120.2(5), ∠C1C2C3 = 119.5(2). Distances ra, are given in Angstroms and angle, ∠α, in degrees referring to the dynamic model. Both static and dynamic models have been applied in investigating the large amplitude motion about the inter-ring CC bond. All title compounds are non-planar. The dynamic model (using potential function V(o) = built1 2 V2 (1 −cos 2o) + built1 2 V4(1 − cos 4o)) gave dihedral angles of 43.8(1,3)°, 42.4(2.6)° and 43.7(0.8)°, and Fourier coefficients V2 and V4 equal to 0.8(0.9) and −5.0(1.8), −1.6(1.2) and 4.5(3.7), 1.3(0.8) and −7.0(1.5) kJ mol−1, respectively for 3,3′ -dibromo-,3,5,4,′-tribromo- and 3.5.3′,5′,-tetrabromo-biphenyl. The numbers in parentheses are one standard deviation as given by least-squares refinements using a diagonal weight matrix.

Apparent wavelengths of the Oslo electron diffraction apparatus according to diffraction patterns from gaseous benzene

Abstract The electron wavelength of the short camera distance electron diffraction diagrams of benzene was calibrated against the CC distance of the molecule. This wavelength, when applied to the long camera distance data, repeatedly gives a CC distance about 0.3% longer than the applied calibration distance. The effect is demonstrated by the analysis of benzene data from eight plates recorded at each of the two applied camera distances. Our presently applied photometer procedures and numerical data reduction, which includes digital Fourier filtering of the photometer data, are described. A new variant of the autocorrelation power spectrum is illustrated for the benzene data.

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The rg(C–F) bond is shortened to 1.307(13) Å in comparison with rg(C–F) = 1.356(4) Å in C6H5F.

Gas electron diffraction data: A representation of improved resolution in the frequency domain, a background correction for multiplicative and additive errors, and the effect of increased exposure of the photographic plates

Abstract An improved version of the previously proposed modified autocorrelation power spectrum is described. The enhanced resolution of this spectrum in the frequency domain is demonstrated for lead tetrachloride and benzene data. A background which may simultaneously correct gas electron diffraction data for both multiplicative and additive long periodic (low frequency) errors is suggested. Application of this background revealed systematic non-constant multiplicative errors in our data. The multiplicative corrections previously done to the calculated intensities of lead tetrachloride by modifications of the Pb scattering factor could now be substituted by a multiplicative background correction to the experimental intensities. For weakly exposed benzene data, the applied ‘blackness correction’ seemed to introduce a linear multiplicative correction of negative slope. The need for this correction disappeared for benzene data when the exposure of the photographic plates was increased.

Inductive effects on bridging Ga-Cl distances: The molecular structure of the dichloro(methyl)gallium dimer, [Me2Cl2Ga2(μ-Cl)2], determined by gas electron diffraction

Abstract The gas-phase electron diffraction data for [Me 2 Cl 2 Ga 2 (μ-Cl) 2 ] are consistent with a trans model of C 2 h symmetry and bond distances ( r a ) GaC = 194.9(7) pm, GaCl t = 212.9(3) pm and GaCl b = 233.9(3) pm.

Molecular structure and conformations of bicyclopentyl, C5H9C5H9, as studied by electron diffraction, molecular mechanics and ab initio methods

Abstract The molecular structure of bicyclopentyl was established by gas phase electron diffraction, and it is compared with that calculated previously by molecular mechanics (MM3) and currently by ab initio methods (6–31G∗). There is good agreement between theory and experiment for the structural parameters, and the conformational ratio, measured with a low precision, is in fair agreement with that calculated by both methods. Experiment and theory agree that the major conformer is equatorial, equatorial, anti (ee, anti). Comparisons are made with conformations of similar bicyclic (bicyclopropyl, bicyclobutyl and bicyclohexyl) and open-chain (sym-tetramethylethane) compounds. The main structural parameters measured for the ee, anti conformer are as follows: CCave, 1.541(4) A; CH, 1.121(12) A; bond angles: C2C1C5, 103.7(3)°; C1C2C3, 107.9(5)°; C2C1C6, 113.3(3)°; CCH, 112.7(10)°; HCH 106.3(8) A. Additionally, the CC bond length between the rings is assessed to be 1.545(10) A. Similar bond lengths in related bicyclic molecules are found to correlate with the ring CCC bond angles.

A reinvestigation of the molecular structure of dimethyl-N-nitramine by gas electron diffraction, ab initio calculations of the molecular geometry and the force field and vibrational spectra

Abstract The molecular structure of dimethyl- N -nitramine was reinvestigated with gas-phase electron diffraction (ED) and ab initio calculations. Ab initio calculations using different basis sets and HF, MP2 and DFT all predict a molecule with C s symmetry and a pyramidal amine N bond configuration. The vibrational spectra were interpreted from the scaling of the harmonic force field, and vibrational amplitudes required for the ED analysis were calculated from this scaled force field. The following values ( r g bond lengths in A and ∠ α angles in degrees with errors equal to three standard deviations) were found for the main parameters: r (N–O)=1.232(3), r (N–N)=1.387(3), r (N–C)=1.466(3), r (C–H) ave =1.114(9), ∠CCN=116.1(6), ∠CNC=122.4(27), ∠ONO=127.6(12), ∠NCH ave =109.9(18). The sum of the bond angles around the amine N atom is 354.6(28)°. The geometrical parameters obtained from the ED analysis are in agreement with the ab initio calculations except that a more pyramidal amine N bond configuration is predicted by ab initio.