Quang Shen

3-Chloro-1-propene (allylchloride): gas-phase molecular structture and conformation as determined by electron diffraction

Abstract A gas-phase study of 3-chloro-1-propene by electron diffraction showed that the most abundant conformer was gauche with a torsional angle τ=120.8(5.4)°, relative to τ = 0 for syn from when all heavy atoms are in the same plane and the chlorine atom is eclipsing the double bond. At 20 and 90°C the amount of the gauche conformer was found to be 82(9) and 78(15)%, respectively. The least-squares refinements of the molecular structure were assisted by a few constraints taken from ab intio calculations. At 90°C the following gauche structure was obtained in terms of ra and Lα parameters: r(C=C) = 1.341(5) A, r(=CC) = 1.506(6) A, r(CCl) = 1.803(5) A, (r(CH)) = 1.081(17) A, LC=CC = 122.6(1.3)°, LCCCl = 110.0(1.0)°, LClCH = 106.6(2.4)°. Error limits are given as 2σ (σ includes uncertainty due to correlation and systematic errors in the data). The syn form was found to have 2 large root-mean-square torsional amplitude, 20–25°. In connection with the structure analysis a relatively simple valence force field was developed.

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).

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 structure and pseudorotation of cyanocyclopentane as determined by gas phase electron diffraction

Abstract The structure of cyanocyclopentane has been determined by gas phase electron diffraction. The molecule was found to have a low barrier to pseudorotation with two minima corresponding to C S envelope conformations with the CN in the quatorial and axial positions. The best least squares value obtained for the barrier to pseudorotation was 240(330) cal mol −1 , and the energy difference between the equatorial and axial conformers was found to be 180(330) cal mol −1 with the equatorial form being preferred. The puckering amplitude for the five-membered ring was found to be 0.42(28) A, and an average CC(ring) distance of 1.544(1) A was obtained. Other parameters obtained from least squares analysis of the experimental data include: r g (CCN) = 1.477(7) A, r g (CN) = 1.160(2) A, r g (CH) = 1.102(6) A, avg = 107.2(3.2)°. The results obtained are in excellent agreement with related cyclopentyl compounds, and with the microwave spectroscopic results obtained for cyanocyclopentane.

The molecular structures of 1,1-dimethylsilacyclobutane and 1,1,3,3-tetramethyl-1,3-disilacyclobutane as determined by gas-phase electron diffraction

Abstract The molecular structures of 1,1-dimethylsilacyclobutane (DMSICB) and 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSICB) have been studied by gas-phase electron diffraction at room temperature. Both molecules exist in a puckered conformation. The major geometrical parameters obtained are as follows: for DMSICB, r(CH) = 1.117(4) A, r(CC) = 1.565(4) A, r(SiC) av = 1.880(2) A, Delt(SiC) = [r(SiC) — r(SiCm)] = 0.004(15) A, ∠CSiC = 76.6(0.8)°, ∠CmSiCm = 113(2)°, ∠HCSi = 113.5(1.6)°, ∠HCH = 111(4)°, and Flap [angle between the planes formed by atoms C1,Si,C3 and C1,C2,C3, where planar = 0°] = 34.2(3.0)°; for TMDSICB, r(CH) = 1.080(5) A, r(SiC)av = 1.884(2) A, Delt(SiC) = 0.017(3) A, ∠CSiC = 90.1(6)°, ∠CmSiCm = 117.4(2.6)°, ∠HCSi =112.5(1.7)°, and Flap = 22.1(2.2)°.

Molecular structure and conformation of gaseous vinyldimethylchlorosilane as determined by electron diffraction

Abstract The molecular structure of vinyldimethylchlorosilane has been determined by gas phase electron diffraction at room temperature. The least squares values of the bond lengths ( r g ) and bond angles (∠ α ) are : r(CH) = 1.086(6) A, r(CC) = 1.347(5) A, r(SiC=) = 1.838(6) A, r(SiC) = 1.876(3) A, r(SiCl) = 2.078(2) A, ∠CCSi = 127.8° (1.2) and ∠=CSiCl = 107° (1). Models with pure syn form and a mixture of syn and gauche gave equally good agreement with the diffraction data.

The molecular structure and conformational composition of epichlorohydrin as determined by gas phase electron diffraction

Abstract The molecular structure of gaseous epichlorohydrin has been investigated using electron diffraction data obtained at 67°C. The conformational composition at this temperature is such that the molecules exist predominantly in a gauche -2 conformer (where the CCl bond is 160° away from the CO) bond). Refinements showed that 33% (σ = 4) of the molecule exist in the gauche -1 form. The important distances ( r g ) and angle (∠α) with the associated uncertainties are r (CH) = 1.095(5) A, r (CO) = 1.442(3) A, r (CC) = 1.475(8) A, r (CC M ) = 1.523(7) A, r (CCl) = 1.788(2) A, ∠CCO = 114° (1), ∠CCC M = 119°(1), ∠ClCC = 108.9° (7), and Tau(ClCCO) = −150°(10) ( gauche -2) and Tau(ClCCO) = 78° (10) ( gauche -1).

The molecular structure of methyl chlorothioformate by gas-phase electron diffraction and microwave spectroscopy data

Abstract Methyl chlorothioformate, CH 3 SC(O)Cl, has been investigated by using data from gas-phase electron diffraction (ED) and microwave spectroscopy (MW). The majority of the molecules (94(6)%) at 298 K have a planar syn form with a CSCO torsion angle of 0° (SC bond eclipsing the CO bond), but small amounts (6(6)%) of a planar anti form may also be present. The values obtained for bond distances ( r g ) valence angles (∠ α ) from the combined ED/MW investigation where results from ab initio molecular orbital calculations ( HF 6-31 G ∗ ) are used as constraints are: r( CH ) = 1.104(14) A , r( CO ) = 1.191(3) A , r( OCS ) = 1.751(8) A , r( CCl ) = 1.787(12) A , r( SCH 3 ) = 1.812(8) A , ∠SCO = 127.9(15)°, ∠SCCl = 109.2(28)°, ∠CSC = 99.0(3)°, ∠SCH 6 = 106.3° (ab initio value), ∠SCH 7 = 110.2° (ab initio value), and ∠H 7 CH 8 = 110.0° (ab initio value).

The molecular structure and pseudorotational motion of 1,1-difluorosilacyclopetane as determined by gas-phase electron diffraction

Abstract The structure of 1,1-difluorosilacyclopentane has been studied by gas-phase electron diffraction. The molecule is found to have a barrier of pseudorotion of 2.25(90) kcal mol −1 . The potential function has minimum at the twist form ( C 2 ) symmetry and maxima at the envelope forms. The major bond distances (itr) g ) and valence angles obtained from the least-squares refinements with error estimates are as follow: r (CH) = 1.128(7) A, r (CC) av = 1.553(15) A, r (SiF) = 1.582(6) A, r (SiC) = 1.853(3) A, ∠(CSiF) = 113.4′(3), ∠CCC = 106°(1), and Tau(C1C2C3C4) = 56.0°(32).

The molecular structure and pseudorotational motion of cyclopentylsilane as determined by electron diffraction

Abstract The molecular structure of cyclopentylsilane has been studied by gas-phase electron diffraction at a nozzle temperature of −10°C. The molecules is found to be undergoing pseudorotational motion. The most stable conformer is in an envelope form ( C s symmetry) with the silyl group positioned at the equatorial position. The axial form is found to be 750 cal mol −1 higher in energy. The major geometrical parameter values ( r g and ∠ a ) obtained from the least aquares analyses are: r (CC) = 1.548(2) A, q = 0.409(20) A. r (SiH) = 1.490(14) A, r (SiC) = 1.874(4) A, ∠SiCC = 113.1°(6) and ∠HCC = 110.2°(8).