M. Traetteberg

The molecular structure of N-benzylidene-aniline

Abstract The molecular structure of N-benzylidene-aniline has been studied experimentally by the gas electron diffraction method, and also by molecular mechanics calculations. Both approaches gave the same results for the most stable conformer of the free molecule. The phenyl ring bonded to the carbon end of the CN bond was found to be coplanar with this bond, while the other phenyl ring was extensively (ca. 52°) rotated about the NΦ bond.

Structure and conformations of isoprene by vibrational spectroscopy and gas electron diffraction

Abstract The structure of the isoprene (2-methyl-buta-1,3-diene) molecule has been studied by vibrational spectroscopy and gas electron diffraction techniques. A normal coordinate analysis was used to calculate the vibrational frequencies for the anti ( s-trans ), gauche and syn ( s-cis ) conformers of the molecule. Assignments of some frequencies of the anti form were revised. The experimental bands at 1255, 635, 555, 419 and 311 cm −1 are assigned as fundamentals of the gauche conformer with a high degree of confidence. Among the structural parameters determined by electron diffraction are the following bond distances (in A): CC 1.340, CC 1.463, CC methyl 1.512, CH 1.076 and C methyl H 1.110. The abundance of the anti form was determined to be 95.3%, while the rest (4.7%) was interpreted as the gauche form with a dihedral angle of 73.5°.

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).

Potential function of internal rotation for isoprene from ab initio calculations and experimental data

Abstract Ab initio computations of the potential energy curve of internal rotation around the central single CC bond of isoprene have been performed at the Hartree—Fock level with a 3·21G basis set. The similarity of the slope of the curve obtained and the potential energy curve calculated for a more complete basis set ( 7s3p/4s2p ) [Kavana-S2ebo, J. Mol. Struct. 106 (1984) 259] is discussed. The values of the Pitzer function F (φ), its Fourier expansion coefficients, and coefficients of the potential energy expansion were calculated from data given in the above reference. The correction of the potential energy expansion coefficients was carried out from frequencies of torsional “hot” bands of isoprene and torsional overtone of its second rotational isometric form. It was shown that the isoprene second isomer is realized as a gauche -form. The potential energy expansion coefficients were obtained as follows: V 1 = 399.9, V 2 = 1330.22, V 3 = 781.8 and V 4 = −175.8 cm −1 .

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.

Bonding properties of bicyclopropylidene: gas phase electron diffraction study and X-ray crystal structure analysis

Abstract The CC double bond length in bicyclopropylidene (1) was found to be 131.4(1) and 130.4(8) pm by electron diffraction at 20° and X-ray diffraction at −40°C respectively. This value is in remarkable agreement with that obtained by molecular mechanics calculations; it is significantly shorter than the double bond in normal olefins.

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.