L. V. Vilkov

The molecular structure of gaseous monobromobenzene

Abstract The molecular structure of gaseous monobromobenzene has been studied by the electron diffraction method. The molecular geometry was determined by a conjoint analysis based on electron diffraction intensities and microwave rotational constants, assuming C 2v molecular symmetry. The angular distortion of the benzene ring mainly affects the internal angle at the ipso carbon atom: this angle is determined to be ∠ α (C 2 C 1 C a ) = 121.5(4)° which, as expected for an electronegative substituent, is significantly larger than 120°. The other geometrical parameters are: r a (C 1 Br) = 1.898(1) A, r a (C 1 C 2 ) = 1.394(3) A, r n (C 2 C 1 ) = 1.396(5) A, r a (C 2 C 1 ) = 1.394(7) A r a (C 2 H 2 ) = 1.097(3) A, r a (C 2 H 2 ) = 1.086(3) A, r a (C 4 H 9 ) = 1.085(3) A, ∠ α C 3 = 119.0(7)°, ∠ α C 1 C 2 H 7 = 121.7(1.1)° and ∠ α C 4 C 3 H 3 = 120.5(1.1)°. The r o α (CH) bond lengths are assumed to be equal and are refined in one group. Parenthesized values are one standard deviation from the least-squares refinement.

Electron diffraction study on the molecular structure of benzyl chloride and benzyl bromide in the vapour phase

The molecular geometries and conformations of benzyl chloride and bromide, C6H5CH2Cl and C6H5CH2Br have been investigated by electron diffraction. The following geometrical parameters (in terms of ra values) were determined, with uncertainties in parentheses, referring to the last significant number C6H5CH2Cl C16H5CH2Br The C-X bonds lengthen in systems  C-CH2X and C-CH2X as compared with those in C-CH2X. The experimental data could be approximated equally well with two conformational models. In one the average structures have the angles of rotation around the C-C bond ϕ = 67.5(45)° and 74.2(13)° for the chlorine and bromine derivatives, respectively (ϕ = 0 when the C-X bond is in the plane of the benzene ring), with relatively large torsional amplitudes. The other model has potentials of internal rotation of with V2 = 1.5 and 2 kcal mol−1 for C6H5CH2Cl and C6H5CH2Br, respectively, with a minimum at ϕ = 90° and relatively smaller vibrational amplitudes.

The influence of torsional vibrations on the molecular configuration determined by gas electron diffraction

Abstract As a consequence of intramolecular vibrations distorted apparent structures may result from an electron diffraction analysis of molecules possessing symmetrical equilibrium configuration. The amount of torsional distortion gives information concerning the barrier height to internal rotation. An approach is suggested to estimate barrier heights on the basis of average torsional angles as determined from electron diffraction, and expressions of the rotation-dependent distances as obtained from a Taylor expansion by neglecting higher order terms.

An electron diffraction study of 3-methyldiaziridine and 1,2-dimethyldiaziridine

Abstract The structures of the title compounds, diaziridines, (the first to be studied in the gas phase) have been determined by electron diffraction. The following principal structural parameters were obtained with the estimated standard deviations parenthesized: 3-methyldiaziridine, N-C = 1.489(9) A, N-N = 1.444(13) A, C-C = 1.505(16) A, C-H = 1.107(5) A, α =∠ (C-C, NCN) = 61.3° (0.9); 1,2-dimethyldiaziridine, (parameters of the cycle CN 2 were assumed from the previous molecule), N-C (methyl) = 1.445(3) A, C-H = 1.108(9) A, ∠ C-N-Me = 112.0° (0.5), the two methyl groups are in the trans position. Vibrational amplitudes were also determined for all important distances.

The molecular structure and barrier to internal rotation of p-bromonitrobenzene, determined by gas-phase electron diffraction

Abstract The molecular structure of gaseous p -bromonitrobenzene has been studied by the electron diffraction method. The torsion of the nitro group has been treated as a large-amplitude motion, and the barrier to internal rotation was found to be 4.2(8) kcal mol −1 . The angular distortion of the benzene ring mainly affects the internal angles ∠ α CC Br C = 122.6(2)° and ∠ α CC NO2 C = 121.6(2)° which, as expected for electronegative substituents, are significantly larger than 120°. The other geometrical parameters are: r a (NO) = 1.239(2) A, r a (CN) = 1.454(4) A, r a (CBr) = 1.896(2) A, r a (C 1 C 2 ) = 1.393(4) A, r a (C 2 C 3 ) = 1.402(4) A, r a (C 3 C 4 ) = 1.395(3) A, r a (CH) = 1.095(7) A, ∠ α CNO = 117.5(2)°. Values in parentheses are one standard deviation from the least-squares refinement using a diagonal weight matrix.

Molecular structures of acetylene derivatives of tin: Part II. Gas phase electron diffraction study ofbis(trimethylstannyl)acetylene, Me3SnCCSnMe3

Abstract A gas phase electron diffraction study of bis(trimethylstannyl)acetylene is reported. The ra structure refines to the following parameters (bond lengths in nm, valence angles in degrees): The numbers in parentheses are three times the standard deviation values. The difference between the bond lengths in the bis(trimethyl-stannyl) acetylene molecule is less significant than with trimethylstannylacetylene. The observed departure from linearity in the Sn-CC-Sn fragment is seemingly due to the shrinkage effect.

Molecular structures of acetylene derivatives of tin: Part III. Gas phase electron diffraction study of tetrakis(trifluoropropynyl)-tin, Sn(CC-CF3)4

Abstract A gas phase electron diffraction study of tetrakis(trifluoropropynyl)tin is reported. The model, based on T d symmetry for the carbon—tin skeleton and C 3v symmetry for the CF 3 groups, refines to the following parameters (bond lengths, r a , in nm; valence angles in degrees): Sn—C0.2070(7), CC 0.1215(6), C—C 0.1460(7), C—F 0.1343(2), CCF 111.3(0.2). The uncertainties (given in parentheses) represent three times the standard deviation values. The results obtained point to practically free rotation of the CF 3 groups. The presence of electronegative CF 3 causes shortening of the Sn-C bonds in Sn(CC—CF 3 ) 4 from Me 3 SnCCH and Me 3 SnCCSnMe 3 . The triple CC bond length is larger than in hexafluoro-2-butyne and nearly the same as in dimethylacetylene.

Electron diffraction study of meta-dinitrobenzene and meta-chloronitrobenzene in gas phase

Abstract The following geometric parameters were determined on the basis of the sM(s) curves: 1. 1) for m-C6H4(NO2)2 rg(C … C)av = 1.382(2), rg(C-N) = 1.461(6), rg(N=O) = 1.225(2)A, / ONO = 125.3(0.7)o, / CCNO2C= 121.8(1.0),o,φ(CN) = 23o(3)o, / CNO2CCNO2 = 118.5(1.5)o 2. 2) for m-ClC6H4NO2 rg(C … C)av = 1.388(3), rg(CN) = 1.442(10), rg(NO) = 1.243(3) rg(CCl) = 1.746(6)A, / ONO = 122.6(1.0), / CCNO2C = 123.0(1.5)o, / CCClC = 121.5(1.2)o, / CClCCNO2 = 118.3(1.5)o, φ(CN) = 13(6)o. Amplitudes of vibrations were calculated from estimated force fields. Comparison with structures of other nitro aromatic derivatives showed that they agree in general but the CCl bond in m-ClC6H4NO2 is longer than in p-ClC6H4NO2.

A comparison of amplitudes and shrinkage corrections for C6Cl3(NO2)3 calculated using conventional and new procedures

Abstract A harmonic force field for C6Cl3(NO2)3 has been estimated, and the calculated and experimental vibrational frequencies are compared. The vibrational amplitudes and shrinkage corrections have been calculated using the conventional technique based on normal coordinate analysis (1) and the new technique of taking into consideration non-linear terms in the transformation between the Cartesian and internal coordinates (2). The shrinkage corrections for bonded distances calculated using technique (2) are substantially (an order of magnitude) smaller than those calculated with the conventional procedure (1). The opposite effect is observed for non-bonded distances depending on internal rotations, which is more consistent with the physical sense of the influence of vibrational motions on the molecular structure.

Gas phase electron diffraction study of the molecular structure of 6,6-dinitro-2,2-diphenic acid

Abstract The 6,6′-dinitro-2,2′-diphenic acid molecule has been studied by gas phase electron diffraction. The structure analysis (based on C i symmetry for the molecule as a whole and D 6h , C 2v and C s symmetries for the phenyl rings, the nitro and carboxyl groups, respectively) gives the following parameters (bond lengths, r a , in A; angles in degrees): ; CC ⪕ = 1.505 (14); CO = 1.212(8); CO = 1.409(21); CCO = 126.6(1.6); CCO = 111.8(1.3); the angle between the planes of the two phenyl rings, 71.2(0.7); the torsional angle of nitro groups, 30.4(1.4); the torsional angle of carboxyl groups, 25.1(1.5). The uncertainties given in parentheses represent three times the standard deviation values. The results are compared with the structural data on biphenyl derivatives, monomers and dimers of organic acids. The observed distances between the oxygen atoms of nitro groups and the oxygen atoms of hydroxyl groups are 2.693(26) A which may offer strong support for intramolecular hydrogen bonding.