Pirkko Bakken

Molecular mechanics calculations of conformational structures, energies, rotational barrier heights and torsional force constants in halogenated disilanes, hexachloroethane and trichloromethyl-trichlorosilane

Results from molecular-mechanics calculations of conformational structures, energies, barrier heights and torsional force constants in halogenated disilanes, Cl3CCCl3 and Cl3SiCCl3 are given, together with force constants and reference values for bond lengths and bond angles in such molecules.

Cross-conjugated polyenes: Part III. The molecular structures of gaseous bis(4,4-dimethyl-2,5-cyclohexadiene-1-ylidene) and 4,4-dimethyl-l-methylene-2,5-cyclohexadiene

Abstract The molecular structures of bis(4,4-dimethyl-2,5-cyclohexadiene-1-ylidene)(1)and 4,4-dimethyl-1-methylene-2,5-cyclohexadiene(2)have been studied by the gas electron-diffraction method. Both molecules were found to have non-planar carbocyclic rings that may be described by C 6 -C 1 -C 2 &.z.dbnd;C 3 dihedral angles of 9.7 and 7.9° respectively for the former and latter compounds. The C double bond at the bridge in the pentaene molecule (1)was found to be significantly longer than those in the ring [1.382(8) vs. 1.350(3) A].

The molecular structure, conformation and vibrational amplitudes of 1,5-hexadiyne (bipropargyl) in the vapour phase

Abstract The molecular structure and conformation of 1,5-hexadiyne have been studied by the gas electron diffraction method. The molecules were found to exist in an anti/gauche conformational mixture, with the anti conformer very predominant (75 ± 10% anti conformer). The observed conformational composition was correlated with the magnitude of the gauche dihedral angle and the mean vibrational amplitudes of the C1C5 and C1C6 distances of the anti conformer.

Structure and conformation of gaseous butyronitrile: C–H⋯π interaction?

Abstract The molecular structure and conformations of butyronitrile have been studied experimentally by the gas electron diffraction method. A conformational mixture of 75.1% gauche and 24.9% anti , with standard deviation equal to 6.0%, was observed. Results from ab initio MP2/6-31G ∗ optimization calculations are in excellent agreement with those observed.

Unexpected conformational behavior of gaseous 1-pentyne ☆

Abstract The molecular structure and conformations of 1-pentyne have been studied experimentally by the gas electron diffraction method. A conformational mixture of 68.6% gauche and 31.4% anti , with standard deviation equal to 4.5%, was observed. Results from ab initio MP2/6-31G ∗ optimization calculations are in excellent agreement with those observed.

Interaction between silyl groups and acetylenes

Abstract The interaction between silyl groups and acetylenes is studied using gas phase electron diffraction, ab initio quantum chemical calculations, and qualitative chemical reasoning. For brevity, we limit our attention to trimethylsilylacetylene and 1,2-bis(trimethylsily)acetylene and related silylated and methylated acetylenes.

The molecular structure of benzyl fluoride — a combined theoretical and experimental study

Abstract The molecular geometry and conformation of benzyl fluoride have been studied by ab initio and molecular mechanics calculations, by NMR and by gas electron diffraction. The CPh dihedral angle, θ, was shown by the latter method to be 52.3°. The observed vibrational amplitudes indicate mean oscillations of ca. 7° about the equilibrium CPh dihedral angle. The NMR study gave coupling constants for benzyl fluoride consistent with either a single conformation with the CPh dihedral angle equal to 54° or a preferred non-planar conformation with a small rotational barrier. The molecular mechanics calculations gave a minimum energy conformation at θ ≅ 60° and a barrier to rotation of ca. 0.4 kcal mol−1. The ab initio calculations were also carried out for several values of CPh dihedral angle, θ. The energy profile as a function of θ was found to be very flat with the planar conformer being most stable with an energy barrier of 0.24 kcal mol−1.

Non-bonded potentials for the atom—atom interactions Cl⋯OC and H⋯OC as derived from gas-phase data using molecular-mechanics calculations

Abstract Experimental gas-phase data on conformational energies, structures and barrier heights for the molecules HOCCH 2 Cl, ClOCCH 2 Cl, ClOCCHCl 2 , HOCCH 3 , ClOCCH 3 , BrOCCH 3 , ClH 2 COCH 3 and (CH 3 ) 2 CO have been used to established parameters for the potentials Cl⋯O(carbonyl) and H⋯O(carbonyl) within the Morse formulation.

Conformational analysis of the halomethyl cyclopropanes and the halosilyl cyclopropanes by molecular mechanics calculations

Abstract Using non-bonding atom⋯atom interaction potentials derived from gas-phase data on haloalkanes, the torsional potentials in the molecules XH 2 CC 3 H 5 and XH 2 SiC 3 H 5 have been calculated. The molecules have stable gauche and syn conformations. Our calculations show that for FH 2 CC 3 H 5 syn is the conformer of lowest energy, while for the remaining XH 2 CC 3 H 5 molecules (X=Cl,Br,I) gauche is the low-energy form. In contrast to this, syn is the low-energy form of all XH 2 SiC 3 H 5 molecules X=(F,Cl,Br,I). The molecular mechanics results agree with the available observations on XH 2 CC 3 H 5 compounds (X=Cl,Br,I). Conformational energies, rotational barrier heights and torsional force constants have been calculated.

Torsional potentials and conformational structures in the halogen substituted molecules HOCCH2X, HOCCHX2, XOCCH2X, XOCCHX2, XH2CCOCH3, X2HCCOCH3, (XH2C)2CO, (X2HC)2CO, and (X3C)2CO (X = Cl, Br) as determined by molecular-mechanics calculations

Abstract Parameter values of the non-blonded atom⋯atom interaction Br⋯OC have been established from gas-phase data. Conformational energies, structures, torsional barrier heights and torsional force constants of the title compounds have been estimated. For BrOCCH 2 Br, (Cl 3 C) 2 CO and (ClH 2 C) 2 CO the calculated results are compared with gas-phase observations. It is suggested that hexachloroacetone ought to be reinvestigated by electron diffraction.