Kolbjørn Hagen

Molecular structure and conformation of gaseous bromoacetyl chloride and bromoacetyl bromide as determined by electron diffraction

Abstract Bromoacetyl chloride and bromoacetyl bromide are studied by gas phase electron diffraction at nozzle-tip temperatures of 70°C and 77°C, respectively. Both compounds exist as mixtures of anti and gauche conformers. The mole fraction anti, with uncertainties estimated at 2σ, was found to be 0.474(0.080) for bromoacetyl chloride and 0.615(0.069) for bromoacetyl bromide. The results for the distance (ra)and angle (∠α) parameters, with parenthesized uncertainties of 2σ including estimated uncertainty in the electron wave length and correlation effects are as follows: (1) bromoacetyl chloride, r(C-H) = 1.086(0.062) A, r(CO) = 1.188(0.009) A, r(C-C) = 1.519(0.018) A, r(C-Cl) = 1.789(0.011) A, r(C-Br) = 1.935(0.012) A, ∠C-CO = 127.6(1.3)°, ∠C-C-Cl = 111.3(1.1)°, ∠C-C-Br = 111.0(1.5)°, ∠H-C-H = 109.5°(assumed), \ /o (gauche torsion angle relative to 0° for the anti form) = 110.0°(assumed); (2) bromoacetyl bromide, r(C-H) =1.110(0.088) A, r(C=O) = 1.175(0.013) A, r(C-C) = 1.513(0.020) A, r(CO-Br) = 1.987(0.020) A, r(CH2-Br) = 1.915(0.020) A, ∠C-CO = 129.4(1.7)°, ∠CH2-CO-Br = 110.7(1.5)°, ∠CO-CH2-Br = 111.7(1.8)°, ∠H-C-H = 109.5°(assumed), ∠o (gauche torsion angle relative to 0° for the anti form) = 105.0°(assumed). The structural results are discussed in connection with the structures of related molecules.

Organometallic precursors to the formation of GaN by MOCVD: structural characterisation of Me3Ga · NH3 by gas-phase electron diffraction

Abstract The molecular structure of Me3Ga · NH3 [I] has been studied by gas-phase electron diffraction at 25°C. The experimental data are fitted by a model in which the C3GaN core of the molecule has C3v symmetry. The molecule was defined in terms of four bond distances, three valence angles and two torsion angles. Of the bond distances three were refined (rg(GaN) = 2.161(22)°, rg(GaC) = 1.979(3) A, rg(CH) = 1.109(7) A). It was necessary to hold the fourth bond distance at an assumed value [rg(NH) = 1.045 A]. Two of the valence angles were refined (NGaC = 101.8(62)°, GaCH = 111.3(16)°) with the third (GaNH) being held at 109.0°. The torsion angle HN / GaC was held at 60.0° while the remaining torsion angle HC / GaN was refined to 37.5(224)°. The dependent angle CGaC was 115.9(42)°, so the C3Ga fragment is not far from planar, which is in accord with the lone pair from the nitrogen atom being donated into the pz orbital on the gallium atom. This suggestion is supported by the gas-phase and low temperature infra-red spectroscopic data that are reported. Evidence is also presented suggesting the GaN bond is weak and thus it is not surprising that when NH3 and Me3Ga are used to grow GaN it is necessary to use NH3 / Me3Ga ratios greater than one.

1,1,2,2-tetrabromodisilane: gas-phase molecular structure and conformational composition as determined by electron diffraction

Abstract The molecular structure of 1,1,2,2-tetrabromodisilane has been investigated using gas-phase electron diffraction data obtained at 110°C. At this temperature the molecules exist as a mixture of about equal parts ( X = 0.5 ±0.2) of the two conformers with the HSiSiH torsion angle equal to 180° ( anti ) or 60° ( gauche ). Assuming that the two conformers differ in their geometries only in the torsion angle φ, some of the important distance ( r a ) and angle (∠ α ) parameters are: r (SiSi) = 2.349(19) A, r (SiBr) = 2.205(5) A, r (SiH) = 1.485 A (assumed), ∠BrSiBr = 110.1(1.6)°, ∠SiSiBr = 107.1(1.2)° ∠SiSiH = 108.6° (assumed). The error limits are 2σ. The observed conformational composition ( X anti = 0.5(0.2)) corresponds to an energy difference between the conformers of Δ E = E ( gauche ) — E ( anti ) = 0.5 ± 0.6 kcal mol −1 , assuming Δ S = R ln2.

On the planarity of styrene and its derivatives: the molecular structures of styrene and (Z)-β-bromostyrene as determined by ab initio calculations and gas-phase electron diffraction

Abstract The molecular structures of styrene and (Z)-β-bromostyrene have been studied in the gas phase at nozzle temperatures of 303 and 338 K respectively. For both molecules the electron diffraction data were consistent with the results from ab initio calculations which described the vinyl torsional motion, near the planar configurations, in terms of a double minimum potential function with barriers of 243 cal mol−1 (styrene) and 430 cal mol−1 (bromostyrene) at the planar form, and with the minimum energy forms 27° (styrene) and 39° (bromostyrene) away. The perpendicular barriers were calculated to 2.73 kcal mol−1 (styrene) and 1.10 kcal mol−1 (bromostyrene). The important distances (ra) and angles (∠α) obtained from least squares refinements of the electron diffraction data are as follows: styrene, r(CH)Av = 1.102(7) A, r( CC ) = 1.355(16) A , r( CC ) Ph = 1.399(2) A , r( CC ) = 1.475(23) A , ∠CCC = 126.9(24)°; and bromostyrene, r(CH)Av = 1.082(13) A, r(CC) = 1.331(20) A, r( CC ) Ph = 1.400(2) A , r(CC) = 1.465(20) A, r( CBr ) = 1.893(8) A , ∠CCC = 132.8(23)°, ∠BrCC = 125.7(15)°, ∠C2C1C7 = 123.9(33).

1,2-DIIODODISILANE AND 1,1,2,2-TETRAIODODISILANE : GAS-PHASE MOLECULAR STRUCTURE AND CONFORMATIONAL COMPOSITION AS DETERMINED BY ELECTRON DIFFRACTION

Abstract The molecular structures of 1,2-diiododisilane (DIDS) at 55°C and 1,1,2,2,-tetraiododisilane (TIDS) at 155°C have been studied using gas-phase electron diffraction data. These molecules exist as mixtures of anti and gauche conformers. The observed conformational compositions were (percentage of gauche ): 76(16) % for DIDS, and 60 (29%) for TIDS. Assuming Δ S = R ln 2, the conformational energy differences δ E ( gauche—anti ) are: −0.3 (DIDS) and +0.2 (TIDS) with error limits ±0.6 kcal mol−1. Bond lengths ( r g ) and valence angles (〈 α ) with estimated 2 σ uncertainties are for 1,2-diiododisilane: r (SiSi)=2.380(34)A, r(SiI)=2.429(13)A, ∠ISiSi=107.5(1.2)°, and φ( gauche ) =58(31)°. For 1,1,2,2-tetraiododisilane we found r (SiSi) =2.389(37)A, r (SiI) =2.440(9)A, ∠SiSiI= 107.2(1.0)°, ∠ISiI = 111.4(0.6)°, and φ( gauche )=61(27)°.

Molecular structure and conformation of gaseous chlorocarbonylsulfenyl chloride, ClSCOCl, as determined by electron diffraction

Abstract Gaseous chlorocarbonylsulfenyl chloride, ClSCOCl, has been investigated at 35°C by electron diffraction. The major conformer has the chlorine atoms anti to each other. A small amount (6.5 ± 9.9%) of a second form may also be present. For the anti form the bond distances ( r a ) and valence angles (∠α) are as follows: r (CO) = 1.183(5) A, r (CCl) = 1.749(8) A, r (CS) = 1.791(9) A, r (SCl) = 2.010(4) A, ∠SCO = 126.9(2.0)°, ∠SCCl = 106.0(2)° and ∠CSCl = 100.6(4)°.

The molecular structure of selenonyl fluoride, SeO2F2, and sulfuryl fluoride, SO2F2, as determined by gas-phase electron diffraction

Abstract The molecular structure of selenonyl fluoride (SeO2F2) and sulfuryl fluoride (SO2F2) has been studied by gas-phase electron diffraction. The geometries of both molecules are consistent with predictions of VSEPR (valence-shell electron-pair repulsion) theory. The results for the more important distance (ra), bond angle, and r.m.s. amplitude (l) parameters with estimated uncertainties estimated at 2σ are for SeO2F2 r(Se = 0) = 1.575 A (0.002), r(Se-F) = 1.685 A (0.002), ∠OSeO = 126.2° (0.5), ∠FSeF = 94.1° (0.5), l(Se = 0) = 0.0440 A (0.0046), l(Se-F) = 0.0472 A (0.0042), and for SO2F2 r(S = 0) = 1.397 A (0.002), r(S-F) = 1.530 A (0.002), ∠OSO = 122.6° (1.2), ∠FSF = 96.7° (1.1), l(S = 0) = 0.0331 A (0.0015), l(S-F) = 0.0393 A (0.0018).

The structure and conformation of dichloroacetyl chloride as determined by gas-phase electron diffraction

Abstract The structure and conformation of dichloroacetyl chloride have been determined by gas-phase electron diffraction at nozzle temperatures of 20 and 119°C. The molecules exist as a mixture of two conformers with the hydrogen and oxygen atoms syn and gauche to each other. The composition (mole fraction of syn form) of the vapor was found to be 0.72 ± 0.06 and 0.73 ± 0.12 at 20 and 119°C, respectively, corresponding to almost equal energy for the two forms. The results for the distance ( r g ), angle ∠α and r.m.s. amplitude ( l ) parameters obtained at the two temperatures are entirely consistent. At 20°C the more important parameters, with estimated uncertainties of 3σ are: r (C-H) = 1.062(0.049)A, r (C0) = 1.189(0.003)A, r (C-C) = 1.535(0.008)A, r (CO-Cl) = 1.752 (0.009)A, r (CHCl-Cl) = 1.771(0.004)A, ∠C-CO = 123.3(1.3)°, ∠C-CO-Cl = 113.9 (5.9)°, ∠C-CHCl—Cl = 109.5(1.5)°, ∠C1-C-Cl = 111.7(0.5)°, ∠Cl-C-H = 108.0(1.5), φ 1 (HCCO torsion angle in the syn conformer) = 0.0° (assumed), φ 2 (HCCO torsion angle in the gauche conformer) = 138.2(5.1)°.

Molecular structure and conformation of chloroacetaldehyde as determined by gas-phase electron diffraction

Abstract The molecular structure and conformation of chloroacetaldehyde have been studied by gas-phase electron diffraction at a nozzle temperature of 42°C. The molecules exist as a mixture of two conformers with the chlorine and the oxygen atoms anti and syn to each other. The anti form dominates (94(7)%) at 42°C. The torsional potential for the anti form was found to have a flat and wide minimum with a small hump at the anti position. The results for the distance ( r a ) and angle ( α ) parameters, with estimated uncertainties of 2σ, are: r (CO) = 1.206(3) A, r (CC) = 1.521(5) A, r (CCl) = 1.782(4) A, 〈 r (CH)〉 = 1.093(12) A, ∠CCO = 123.3(6)°, ∠CCCl = 110.4(3)°, ∠CCOH = 112.4(38)°, ∠CCHClH = 110.3(15)° and ∠HCH = 109.5° (assumed).

Molecular structure and conformational equilibrium of gaseous thiophene-2-aldehyde as studied by electron diffraction and microwave, infrared, Raman and matrix isolation spectroscopy

Thiophene-2-aldehyde has been investigated by microwave, IR and Raman spectroscopy and by electron diffraction of the vapour. The compound was also isolated in argon and nitrogen matrices and studied by IR spectroscopy. Two conformers were identified, a more stable planar isomer with the S atom syn to the 0 atom and a less stable planar (or near planar) anti form. Assuming that the geometry of the two forms differs only in the S-C-C=0 torsion angle and, assuming the thiophene ring to have C,, symmetry, the electron diffraction study gives the following result for some of the distances (ra) and angles (&): r(C-H) = 1.114(20) A, r(C=O) = 1.224(7) a, r(C-S) = 1.717(4) a, r(C=C) = 1.375(7) A, r(C-C) (in th e ring) = 1.431(15) A, r(C-COH) = 1.466(16) a, LC=C-COH = 126.4(1.3)“, f.C=C-S = 111.8(4)‘, LC=C-H = 129.2(4.0)“, IC--C=O = 123.7(g)“, and Lo (S-C-C=0 torsion angle in the anti conformer) = 158.5 (23.1): At 94°C the observed amount of the conformer with 0 and S syn was 80.5(7.9)%, and the syn conformer had a r.m.s. torsional amplitude of vibration of 7 = 17.2(13.6)“. Assuming ASo = 0, the obtained amount of syn corresponds to AH” = 4.3 5 1.6 kJ mol-’ . The matrix isolation study included the use of a heatable nozzle, which made it possible to trap different conformational equilibria in the matrices. By comparing absorbances obtained for different temperatures, the enthalpy difference between the conformers was estimated as 4.1 f 0.4 kJ mol-‘, in very good agreement with the electron diffraction result. The matrix data support the extensive IR and Raman study of the other phases (vapour, liquid, solution and solid). In the crystalline solid the preferred conformation was found to be syn. The microwave investigation shows the planarity of the syn isomer, whereas the anti form was not detected. The spectral data have hence been interpreted in terms of C, symmetry, and a normal coordinate analysis has been carried out for both conformers. The final structural refinement was based upon electron diffraction intensities in combination with the microwave rotational constants. The vibrational amplitude parameters applied were derived from the force field calculations.