Kirsten Aarset

Molecular structure and conformational composition of 1-Chlorobutane, 1-Bromobutane, and 1-Iodobutane as determined by gas-phase electron diffraction and ab initio calculations

Gas-phase electron diffraction (ED), together with ab initio molecular orbital calculations, have been used to determine the structure and conformational composition of 1-chlorobutane, 1-bromobutane, and 1-iodobutane. These molecules may in principle exist as mixtures of five different conformers, but only three or four of these were observed in gas phase at temperatures of the ED experiments, 18‡C, 18‡C, and 23‡C, respectively. The observed conformational compositions (1-chlorobutane, 1-bromobutane, and 1-iodobutane) were AA (13 ± 12%, 21 ± 14%, 19 ± 17%), GA (60±13%, 33±32%, 17±31%), AG (12±16%, 8±12%, <1%), and GG (12 ±16%, 38± 34%, 64±31%). A and G denotesanti andgauche positions for the X-C1-C2-C3 (X=Cl, Br, I), and the C1-C2-C3-C4 torsion angles. The results for the most important distances (rg) and angles (∠α) from the combined ED/ab initio study for the GA conformer of 1-chlorobutane, with estimated 2σ uncertainties, arer(C1-C2)=1.519(3)å,r (C2-C3)=1.530(3) å,r (C3-C4)=1.543(3) å,r (C1-Cl)=1.800(4) å, <C1C2C3=114.3(6)ℴ, <C2C3C4=112.0(6)ℴ, <CCCl=112.3(5)ℴ. The results for the GA conformer of 1-bromobutane arer (C1-C2)=1.513(4) å,r (C2-C3)=1.526(4) å,r (C3-C4)=1.540(4) å,r(C1-Br)=1.959(8) å, <C1C2C3=115.3(11)ℴ, <C2C3C4=112.8(11)ℴ,<CCBr=112.1(14)ℴ. The results for 1-chlorobutane and 1-bromobutane are compared with those from earlier electron diffraction investigations. The results for the GA conformer of 1-iodobutane arer (C1-C2)=1.506(5) å,r (C2-C3)=1.518(5) å,r (C3-C4)=1.535(5) å,r (C1-I)=2.133(11) å, <C1C2C3=116.8(15)ℴ, <C2C3C4=115.3(15)ℴ, <CCI=110.2(14)ℴ. Differences in length between the different C-H bonds in each molecule, between the different C-C bonds, between the different CCH angles, and between the different CCC angles were kept constant at the values obtained from the ab initio calculations.

Hydrogen bonding in the gas-phase: the molecular structures of 2-hydroxybenzamide (C7H7NO2) and 2-methoxybenzamide (C8H9NO2), obtained by gas-phase electron diffraction and theoretical calculations.

The structures of 2-hydroxybenzamide (C7H7NO2) and 2-methoxybenzamide (C8H9NO2) have been determined in the gas-phase by electron diffraction using results from quantum chemical calculations to inform restraints used on the structural parameters. Theoretical methods (HF and MP2/6-311+G(d,p)) predict four stable conformers for both 2-hydroxybenzamide and 2-methoxybenzamide. For both compounds, evidence for intramolecular hydrogen bonding is presented. In 2-hydroxybenzamide, the observed hydrogen bonded fragment is between the hydroxyl and carbonyl groups, while in 2-methoxybenzamide, the hydrogen bonded fragment is between one of the hydrogen atoms of the amide group and the methoxy oxygen atom.

Molecular structures and conformational compositions of 2-chlorobutane and 2-bromobutane; an investigation using gas-phase electron-diffraction data and ab initio molecular orbital calculations ☆

Abstract The structure and conformational composition of 2-chlorobutane and 2-bromobutane have been studied by gas-phase electron diffraction (GED) at 25°C, together with ab initio molecular orbital calculations (HF/6-311+G(d,p)). These molecules may exist as three distinguishable conformers (G+, A, and G−). The symbols refer to anti (A) with a torsion angle Φ2(X8–C2–C3–C4) of about 180° and gauche (G+ and G−) with torsion angles Φ2(X8–C2–C3–C4) of about +60° and 300°(−60°), respectively. It was not possible; from our GED-data alone, to accurately determine the conformational composition because the distance distributions for two of the conformers (G+ and G−) are very similar. The conformational composition for 2-chlorobutane obtained from the ab initio calculations (G+ 62%, A 25% G− 13%) was found to fit the experimental data quite well. For 2-bromobutane the ab initio calculated conformational composition (G+ 58%, A 28% G− 14%) did not, however, fit the experimental data. Here a much better fit was obtained by using only 10% of the A conformer and using the relative energy for the two gauche conformers, as obtained in the ab initio molecular orbital calculations, to calculate the relative amounts of the two gauche forms (G+ 73%, A 10% G− 17%). The results for the principal distances (rg) and angles ∠α for the G+ conformer of 2-chlorobutane, with estimated 2σ uncertainties, obtained from the combined GED/ab initio study are: r( C 1 – C 2 )=1.524(3) A , r( C 2 – C 3 )=1.528(3) A , r( C 3 – C 4 )=1.539(3) A , r( C – Cl )=1.812(3) A , r( C – H ) ave =1.098(4) A , ∠C1C2C3=111.5(16)°, ∠C2C3C4=113.3(5)°, ∠C1C2C1=110.4(9)°. The results for the G+ conformer of 2-bromobutane are: r( C 1 – C 2 )=1.526(4) A , r( C 2 – C 3 )=1.530(4) A , r( C 3 – C 4 )=1.540(4) A , r( C – Br )=1.982(5) A , r( C – H ) ave =1.111(8) A , ∠C1C2C3=112.5(16)°, ∠C2C3C4=114.6(15)°, ∠C1C2Br=110.1(16)°. Only average values for r(C–C), r(C–H), ∠CCC, and ∠CCH could be determined in the least-squares refinements, the differences between these parameters in the same conformer, and between the different conformers, were kept constant at the values obtained in the ab initio molecular orbital calculations.

An evaluation of the use of a commercial scanner to obtain experimental data produced by gas-phase electron diffraction and recorded on photographic plates

Abstract A commercial scanner (Agfa Arcus II) has been used to retrieve electron-diffraction data from photographic plates. The data thus obtained from five different molecules has been analysed and the results compared with the original published data. Excellent agreement was observed between bond distances and amplitudes obtained from refinements on data collected from this scanner, a similar scanner and a micro-densitometer. It is planned to use the Agfa Arcus II scanner for future measurement of electron-diffraction intensity data from photographic plates.

Molecular structure and conformational composition of 1,1-dichlorobutane: a gas-phase electron diffraction and ab initio investigation

Abstract Gas-phase electron diffraction data obtained at 23°C, together with results from ab initio molecular orbital calculations ( HF 6-31 G(d) ). were used to determine the structure and conformational composition of 1,1-dichlorobutane. Of the five distinguishable conformers (AA, G + A, AG +, G + G + and G + G −), the G + A conformer was found to be the low-energy form, and the investigation also indicated that certain amounts of the AA and G + G − conformers might be present. The symbols describing the conformers refer to torsion about the C 1 C 2 and C 2 C 3 bonds, anti (A) with H 5 C 1 C 2 C 3 and C 1 C 2 C 3 C 4 torsion angles of 180° and gauche (G + or G −) with torsion angles of + 60° or 300° (−60°) respectively. The results for the principal distances ( r g ) and angles (∠ α ) from the combined electron diffraction/ab initio study for the G + A conformer, with estimated 2σ uncertainties, were as follows: r( C 1  C 2 ) = 1.521(4) A , r( C 2  C 3 ) = 1.539(4) A , r( C 3  C 4 ) = 1.546(4) A , r( C  Cl 6 ) = 1.782(3) A , r( CCl 7 ) = 1.782(3) A , 〈r( CH )〉 = 1.106(6) A , ∠C 1 C 2 C 3 = 114.4(13)°, ∠C 2 C 3 C 4 = 112.5(13)°, ∠CCCl 6 = 110.4(7)°, ∠CCCl 7 = 111.9(7)°, 〈∠CCH〉 = 108.9(47)°. Only average values for r (CC), r (CCl), r (CH), ∠CCC, ∠CCX and ∠CCH were determined in the least-square refinements; the differences between the values for these parameters in the same conformer and between the different conformers were kept constant at the values obtained from the ab initio molecular orbital calculations.

The molecular structure of hexamethyldigermane determined by gas-phase electron diffraction with theoretical calculations for (CH3)3M-M(CH3)3 where M = C, Si, and Ge.

Gas-phase electron diffraction (GED) data together with results from ab initio molecular orbital calculations (HF and MP2/6-311+G(d,p)) have been used to determine the structure of hexamethyldigermane ((CH(3))(3)Ge-Ge(CH(3))(3)). The equilibrium symmetry is D(3d), but the molecule has a very low-frequency, large-amplitude, torsional mode (phiCGeGeC) that lowers the thermal average symmetry. The effect of this large-amplitude mode on the interatomic distances was described by a dynamic model which consisted of a set of pseudoconformers spaced at even intervals. The amount of each pseudoconformer was obtained from the ab initio calculations (HF/6-311+G(d,p)). The results for the principal distances (r(a)) and angles (angle(h1)) obtained from the combined GED/ab initio (with estimated 1sigma uncertainties) are r(Ge-Ge) = 2.417(2) A, r(Ge-C) = 1.956(1) A, r(C-H) = 1.097(5) A, angleGeGeC = 110.5(2) degrees, and angleGeCH = 108.8(6) degrees. Theoretical calculations were performed for the related molecules ((CH(3))(3)Si-Si(CH(3))(3) and (CH(3))(3)C-C(CH(3))(3)).

Torsion potentials and conformational structures in 1,4-halobutane (F,Cl,Br,I) as determined by molecular mechanics calculations

Abstract Ten halogenated butanes XCH 2 CH 2 CH 2 CH 2 X (X  F,Cl,Br,I) and XCH 2 CH 2 CH 2 CH 2 Y (X  F,Cl,Br, I and Y  F,Cl,Br,I; X ≠ Y) have been studied by molecular mechanics calculations. For all XCH 2 CH 2 CH 2 CH 2 X (X  F,Cl,Br,I) molecules the torsional potentials obtained show ten minima and all XCH 2 CH 2 CH 2 CH 2 Y (X  F,Cl,Br,I and Y = F,Cl,Br,I: X ≠ Y) compounds show 14 minima. Values for energy differences are given for all conformers. The conformational energy differences are in the range 0.1–4.0 kcal mol −1 . Barrier heights between conformers, and structural parameters are given for the most important conformers.

Torsional potentials and conformational structures in chloro-substituted propanals as determined by molecular mechanics calculations

Abstract The conformational structures, energies, rotational barrier heights and torsional force constants of chloro-substituted propanals were obtained using molecular mechanics calculations based on non-bonding interatomic potentials derived from gas-phase data. Most of these molecules exist as complex conformational mixtures at room temperature.