M.V. Popik

The molecular structure of benzene derivatives part 1. 4-Fluorobenzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

Abstract The molecular structure of gaseous 4-fluorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF 6-311 G ∗∗ and MP 2 6-31 G ∗ levels of theory. The planar Cs symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large-amplitude motion. The most important structure parameters (rg) from the joint analysis with estimates of 2σ (in parentheses) were: ( CC ) mean = 1.397(1) A , CF = 1.331(7) A , CC (= O ) = 1.488(7) A , CO = 1.195(5) A ,

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.

Extension of a regularizing algorithm for the determination of equilibrium geometry and force field of free molecules from joint use of electron diffraction, molecular spectroscopy and ab initio data on systems with large-amplitude oscillatory motion

Abstract The previously developed integrated algorithm for the joint treatment of gas-phase electron diffraction and vibrational spectroscopic data is extended to include systems with large-amplitude oscillatory motion. In addition, the treatment is augmented by the inclusion of microwave rotational constants. As in the previous work, the analysis of data from experimental sources is guided by quantum mechanical molecular geometry and force field optimization results. The computed force field matrix can be corrected empirically with the aid of suitable scale factors. Centrifugal distortion corrections to interatomic distances are included. The standard deviations of the parameters determined and the correlation coefficients can now be estimated. The principal design of the developed computer program is outlined, and some methodological problems associated with diffraction analysis of molecules with large-amplitude motion are discussed. To provide an example of a problem susceptible to attack by the present method an account is made of the re-analysis of diffraction data for 4-fluorobenzaldehyde collected earlier on the Balzers apparatus in Oslo.

An electron diffraction study of the molecular structure of gaseous bicyclo[3.3.1]nonane

Abstract A gas-phase electron diffraction study of the title compound, carried out at 65°C, has found no statistically significant evidence for the presence of any conformer other than the double chair. The geometrical parameters were found to be: r g (CC) av = 1.538(1) A, r g (CH) av = 1.116(3) A, ∠C1C9C5 = 111.5(1.1)°, ∠C2C3C4 = 111.2(1.4)°, ∠HCH = 107.0(2.6)°, ∠θ = 121.5(0.8)°, ∠ϕ = 42.6(2.5)°. In an attempt to refine electron diffraction data for models with three non-equivalent CC distances, two sets of molecular parameters were obtained. Amplitudes of vibration were calculated from an approximate force field and the uncertainties in the fixed amplitudes were analyzed. The resulting structural parameters, chosen on the basis of molecular mechanical calculations, were: ( r g structure for bond lengths, r α structure for angles) C1C2 = 1.559(10) A, C2C3 = 1.541(10) A, C1C9 = 1.480(9) A, CH = 1.114(3) A, ∠C1C9C5 = 110.1(2.3)°, ∠C2C3C4 = 113.2(2.7)°, ∠HCH = 111.4(3.5)°, ∠θ = 124.1(0.9), ∠ϕ = 40.0(2.0)°. Comparison with structures of other bicyclo[ n.m. 1]alkanes has been made.

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.

A nonparametric determination of the internal rotation potential and the molecular geometry of o-chloroanisole using gas electron diffraction and data from spectroscopy and ab initio calculations

Abstract Structural parameters of o -chloroanisole have been determined using a dynamic model from electron diffraction together with vibrational spectroscopic data and ab initio calculations. A new approach to calculate the nonparametric torsional potential of the methoxy group based on Tikhonovs method of regularization was applied. The nonparametric torsional potential gives a two-minimum function with respect to the rotation energy around the CO bond. The most stable conformation corresponds to a planar form with the torsional angle ϕ = 0° and another to an orthogonal form with ϕ = 90°. The energy difference between these two conformers is found to be 0.9–1.0 kcal mol −1 and the barrier height at ϕ = 65° is 1.4–1.6 kcal mol −1 . These values are in good agreement with those calculated by the ab initio method. The main geometrical parameters ( r a in A, ∠ α in degrees and errors given as three times the standard deviation from least squares are as follows: r (CC) mean = 1.398(4); r (OC Ph ) = 1.358(36); r (OC Me ) = 1.426(21); r (OCl) = 1.733(4); r (CH) Ph = 1.086(6); r (CH) Me = 1.095(6); ∠CC O C Cl = 118.7(2.2); ∠C O CC = 119.9(2.5); ∠C O C Cl C = 121.5(1.1); ∠COC = 117.6(2.6); ∠C O CCl = 119.1(2.1); ∠CCO = 124.7(1.2). The structural results are compared with those obtained for similar compounds. Stereochemical peculiarities in ortho derivatives of anisole which result in the existence of an orthogonal conformer are discussed.

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.

A new approach to the nonparametric determination of internal rotation potential from electron diffraction data as applied to reinvestigation of the molecular structure of m-bromonitrobenzene

Abstract A nonparametric numerical algorithm is suggested for calculation of the torsional potential from gas electron diffraction data using Tikhonovs method of regularization. A new approach to determine the admissible interval of the regularization parameter using Hamiltons statistical test has been considered. Applied to this reinvestigation of m -bromonitrobenzene it was confirmed that this algorithm only allows the determination of the form of the torsional potential function near its minimum. The barrier to internal rotation was estimated to be 4.6-5.4 kcal mol −1 as interpolated by Fourier series with proper parameterization. The most important structural parameters ( r a in Angstroms, ∠ α in degrees) are: r (CC) av = 1.399(3), r (CN) = 1.459(16), r (NO) = 1.244(3), r (CBr) = 1.884(6), r (CH) = 1.099(20), ∠CC N C = 123.9(1.4), ∠C N CC Br = 116.8(1.5), ∠C N CC = 116.6(1.9), ∠CNO = 118.8(0.8). The structure parameters are compared with those obtained previously for o -, m -, and p -bromonitrobenzenes.

Molecular structure of gaseous trimethylgermylformate from electron diffraction

Abstract The molecular structure of trimethylgermylformate (TMGF) has been determined by electron diffraction in the gas phase. The principal parameters, bond lengths ( r a , A), bond and torsional angles (deg) with the least-squares standard deviations in parentheses, are as follows: GeC, 1.919(21); GeO, 1.820(28); CO, 1.213(76); CO, 1.326(42); CH(methyl), 1.109(80); C6H8, 1.12(fixed); GeOC, 117.6(23); OGeC, 104.9(7); GeCH, 109.8(80); OCO, 125.4(59); CGeC, 113.6(7); OCH, 127(fixed); GeOCO, 81.2(63); CGeOC, 35.5(38); HCGeO, 73.2(52). It is found that the GeOCO fragment of TMGF is not planar, and the HCO plane is close to the perpendicular position with respect to the GeOC plane. This result stands out from the series of previously obtained data for similar molecules and suggests that the conformation of TMGF is controlled mainly by steric effects. The conformation of TMGF from electron diffraction is in agreement with molecular mechanics calculations.

The conformational equilibrium in bicyclo(3.3.1)nonane at 65–400°C by electron diffraction and molecular mechanics

Abstract The amount of boat-chair conformation (5–25 %) in equilibrium with the chair-chair has been measured and calculated.