Conformational energies, rotational barrier heights and molecular structures in C(CH2X)4 molecules (X=F, Cl, Br)

The conformational energies, rotational barrier heights and molecular structures in C(CH₂X)₄ molecules (X=F, Cl, Br) based on molecular-mechanics calculations have been obtained. The results from these calculations are compared with the experimental gas-phase results.     

Determination of compositions and isomer structures in mixtures of cis- and trans-1,2-dichloroethene (ClHC=CHCl) by gas-phase electron diffraction

Using gas-phase electron diffraction it has been demonstrated that a composition of known isomer mixtures can be determined with error limits of about 5%, all relevant structural parameters being refined simultaneously by the least-squares method. If, however, corresponding bond distances and valence angles have erroneously been assigned equal values in the two isomers, a large increase in the least-squares error limits from 5% to 12% is noticed. Apparently innocent assumptions about some of the parameters can lead to incorrect isomer composition and to too small error limits as estimated by the least-squares formulae.

From the reinvestigation of pure cis-1,2-dichloroethene the following bond distances (r_a) and valence angles () were determined: r(C---H) = 1.090(29) Å, r(C=C) = 1.345(6) Å, r(C---Cl) = 1.716(4) Å, C=C---Cl = 123.8(2)°, C=C---H = 119.4(26)°. Error limits are 2σ.     

Structure and conformation of 1-Bromopropane: A gas-phase electron-diffraction investigation using microwave-spectroscopy data and results from ab initio calculations as constraints

1-Bromopropane has been studied by gas-phase electron diffraction (ED) at 24C. Earlier published values for rotational constants from microwave spectroscopy (MW), together with results from ab initio molecular-orbital calculations, have been included in the ED analysis. Two conformers with C-C-C-Br torsion angles of 180 (anti) or 66.0(17) (gauche) have been observed. The results obtained for the bond distances (r _g) and valence angles () from this combined ED/MW analysis, with the ab initio results used as constraints are r (C-H)=1.114(9) å,r(C₁-C₂)=1.521(5) å,r(C₂-C₃)=1.535(5) å,r (C₁-Br)=1.962(6) å, <(C-C-C)anti,=110.0(11), <(C-C-C)gauche=113.3(11), < (C-C-Br)anti=111.1(6), < (C-C-Br)gauche=112.1(6), <C₂-C₁-H=112.1 (ab initio value), <C₂-C₃-H=111.4 (ab initio value), <H-C₂-H=107.0 (ab initio value). Error limits are given as 2, where (standard deviation) includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The observed amountof gauche conformer was 64(14)%. Using the entropy difference between conformers obtained in the ab initio calculations, this composition corresponds to an energy difference of E=E _anti —E _gauche=0.03(36) kcal/mol. The results are compared with those earlier obtained for other 1-halopropanes.     

Conformational structures, energies, rotational barrier heights and torsional force constants in halogenated methyl-silanes obtained by molecular-mechanics calculations

Conformational structures, energies, barrier heights and torsional force constants in halogenated (F, Cl, Br) methyl-silanes have been obtained by molecular-mechanics calculations. A comparison of the ethan-like molecules with central bonds C---C, Si---C and Si---Si has been made. It is predicted that, for halogenated methyl-silanes, the difference in conformational energy between anti and gauche conformers is small.     

The morse curve as a non-bonded potential function

The morse curve has considerable advantages over the exp-6 and 12-6 potentials as a non-bonded potential function. The three potential functions are compared and the equivalent Morse curves of several commonly used steric potentials are given.     

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, 18C, 18C, and 23C, 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 (r _g) and angles () from the combined ED/ab initio study for the GA conformer of 1-chlorobutane, with estimated 2 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), <C₂C₃C₄=112.0(6), <CCCl=112.3(5). 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), <C₂C₃C₄=112.8(11),<CCBr=112.1(14). 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), <C₂C₃C₄=115.3(15), <CCI=110.2(14). 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.