Ab initio studies of structural features not easily amenable to experiment: Part 20. Structural aspects of ethyl groups

The molecular structures of a number of stable conformations of ethanol, ethylamine, methylethyl ether, methylethylamine and of the ethyl anion have been determined by ab initio geometry optimizations using Pulay's Force method on the 4–21G level. The calculated geometries characterize the extent to which structural groups in a molecule are sensitive to asymmetries in their environment. Characteristic structural trends are consistently found for the CH bond distances and CCH angles in the C₂H₅ groups of trans-ethanol, trans-methylethyl ether and in the ethyl anion. They differ from those previously found for C₂H₅ groups in hydrocarbons. There is qualitative disagreement between the trends calculated for CH bond distances in trans-ethanol and trans-methylethyl ether and those found in the microwave substitution structures of these compounds. Since the substitution parameters are unresolved because of relatively large experimental or model uncertainties, it is presently impossible to decide whether this discrepancy is the result of computational or experimental deficiency. The methyl groups in methylethyl ether and methylethylamine exhibit the characteristic structural distortions which are usually found for CH₃ groups adjacent to electron lone pairs. The CC bond distances in C₂H₅ in the systems studied here are sensitive to the conformational arrangement of ethyl relative to the rest of a system in a way which can be rationalized by orbital interactions involving antibonding orbitals on sp³-hybridized carbon atoms. The calculated conformational stabilities agree qualitatively with experimental trends, except in the case of ethanol where the trans — gauche energy difference is small (about 0.1 kcal mol^?1) and within the uncertainties of the calculations. Our conformational energies for CH₃CH₂NH₂ are in disagreement with a previous ab initio investigation based on a comparison of unoptimized standard geometries. In general, the agreement between calculated structural parameters and corresponding reliable experimental values is very good in all comparable cases.     

The crystal and molecular structure of hexadecaphenyloctasiloxyspiro(9,9)titanate(IV)

The crystal and molecular structures of the title compound have been determined by single crystal X-ray diffraction methods. In the spiro molecule, the metal atom has a geometry very close to tetrahedral, with OTiO angles of 107.9–111.0(2)° and very short TiO bonds of length 1.777–1.791(5)Å. The two TiO₅Si₄ rings have different, ill-defined conformations; the SiO bond lengths and SiOSi angles are similar to those in (SiO)_n rings.     

Ab initio studies of structural features not easily amenable to experiment : Part 40. Structural and conformational investigations of 2-butanone and 2-pentanone

The ab initio gradient refined 4-21G geometries of two conformations of 2-butanone and of six conformations of 2-pentanone are reported. The C---C---C=O torsional energies of both systems were determined with geometry optimization at each point and are compared with those previously calculated for some homologous aldehydes and carboxylic acids. In agreement with the structural trends known for C---H bonds in methyl groups adjacent to C=O, it is found that a C---C bond eclipsing an adjacent C=O bond is more stable and shorter than in a skew position (C---C---C= O = 120°). The sum total of the 4-21G results available for various systems may support the following general rule: in X--- C---C=O systems the C---X bond is relatively short when syn-coplanar with C==O (X---C---C= O = 0°), and relatively long when skew with C=O (X---C---C=O 120°).     

Substituent effect on geometric and electronic structure of benzenesulfonic acid: gas-phase electron diffraction and quantum chemical studies of 4-CH3C6H4SO3H and 3-NO2C6H4SO3H molecules

A combined gas-phase electron diffraction/mass-spectrometric and quantum chemical (B3LYP/cc-pVTZ, MP2/cc-pVTZ) study of the molecular structures of para-methylbenzenesulfonic acid (4-MBSA) and meta-nitrobenzenesulfonic acid (3-NBSA) was carried out. On the basis of mass spectrometric analysis, it was found that the substituted benzenesulfonic acids are thermostable at least up to 431(3) K. The fragmentations of 4-MBSA and 3-NBSA molecules under electron impact were analyzed. Quantum chemical calculations show that the 4-MBSA molecule exists as an enantiomeric pair, which is formed as a result of rotation of OH group about the S–O(H) bond. The 3-NBSA molecule has two conformers with different orientations of the O–H bond with respect to the nitro group and two corresponding enantiomers. The equilibrium configurations of 4-MBSA and both conformers of 3-NBSA have similar structures of the SO₃H group, with the O–H bond eclipsing one of the S=O bonds. Selected experimental bond distances for 4-MBSA/3-NBSA are (Å) r _h1(C–C)_av = 1.403(3)/1.395(4); r _h1(C–S) = 1.765(5)/1.784(5); r _h1(S=O)_av = 1.433(4)/1.438(4); and r _h1(S–O) = 1.618(4)/1.620(4). The potential functions for the internal rotation of SO₃H, OH, and CH₃ or NO₂ groups were calculated, and the transition states between enantiomers (conformers) were determined. The influence of substituent's nature on molecular geometry as well as on the energies of frontier orbitals and red-ox properties of the compounds is discussed. The inductive and mesomeric substituent effects were estimated from the donor–acceptor interaction energies of the natural bond orbitals of substituent and benzene frame. The correlation between group electronegativities and cooperative energetic characteristics of inductive and mesomeric effects of substituents is shown.     

Spannungsenergie und konformation sterisch gehinderter olefine berechnungen von überbrückten derivaten des tetra-tert-butylethylens

Strain energy relaxation by in plane bending is not eflective in tetra-tert-butylethyethylene 1 because of the repulsions between geminal tert-butyl groups. When rings are closed between the geminal tert-butyl groups, the repulsions across the double bond are relieved. The strain energies and conformations of such molecules have been evaluated by empirical force field calculations (molecular mechanics) A close correlation is found between ring size and strain energy: strain increases stepwise with ringsize. In the most stable conformations, the methyl groups adopt a “staggered” arrangement across the double bond. In compounds containing a six-membered ring this can be achieved only in (strongly preferred) twist-boat conformations. The double bond in such molecules is nearly planar when the rings are smaller than 6-membered, but exhibits a torsion of up to 16.5° when 6-membered rings are present.     

Molecular and electronic structure of disiloxane,an ab initio MO study

The bond lengths and angels obtained by means of a 4–31G basis agree with electron diffraction data. The calculated SiOSi bending potential, showing a minimum for the linear arrangement, is discussed with regard to available experimental information. Calculated dipole moments and ionization potentials are also in reasonable agreement with experimental data. Comparison is made with STO-3G and INDO results which both overestimate the stability of cyclic structures.     

Steric and electronic effects on the conformations of n-butane derivatives with trichlorosilyl, silyl and trichloromethyl groups

The molecular structure and conformation of 1,1,1,4,4,4-hexachloro-1,4-disilabutane in the gas-phase have been determined by electron diffraction and computational methods. The lowest-energy conformation has the trichlorosilyl groups anti to one another. The gauche conformation also has a shallow potential minimum, but lies about 19 kJ mol-1 above the anti form. Calculations on related butane derivatives, in which terminal methyl groups have been replaced by CCl3, SiH3 and SiCl3 groups, reveal that the conformational preferences are primarily caused by steric interactions between the terminal groups, and that it is the presence of chlorine atoms that destabilises gauche conformations. The electronegativity of the chlorine atoms has only small effects, mainly limited to the SiCl bond lengths.     

Current research in gas-phase electron diffraction at the University of Antwerp

The gas-phase molecular structures of norbornane and methyl vinylether have been investigated by joint analysis of electron diffraction, infrared, Raman and microwave spectroscopic data. Constraints were taken from the completely relaxed ab-initio (4–21G) geometry. A range of models was investigated which fit to all the available data. For methyl vinylether the quadratic force field was determined by numerical differentiation of the energy gradient and used to calculate vibrational quantities. Also, features of our new electron diffraction unit are illustrated. A new scheme of densitometric data collection is used, based on a modified ELSCAN 2500 densitometer, and a Z8-microprocessor.     

Electron diffraction study of the trichloromethyltrichlorogermane molecular structure and estimation for torsional barrier

The molecular geometry (in terms of r_a and r_g internuclear distances) and mean amplitudes of vibration of CCl₃GeCl₃ have been determined by electron diffraction. The bond lengths are similar to those found in analogous molecules. Although bond angles of unambiguous physical definition have not been determined it is established that the carbon and germanium bond configurations deviate little from the regular tetrahedral arrangement. The molecule performs large amplitude motion around the carbon-germanium bond. The torsional barrier was estimated to be 1.1 kcal mole^?1 using J. Karle's method [8].     

Molecular structure and conformation of diisopropylamine studied by gas electron diffraction combined with vibrational spectroscopy and molecular mechanics

The molecular structure and conformation of diisopropylamine have been determined by gas electron diffraction with the aid of vibrational spectroscopy and molecular mechanics calculations.Only one conformer with the skeletal geometry of C₂ symmetry has been detected. The dihedral angle, CNCH, has been determined to be 52(4)°. The difference between the NCC angles at the gauche and trans positions with respect to the opposite NC bond is 2.4°. The CNC bond angle, 120.1(10)°, and the CN bond length, 1.470(4) Å, are 8.3° and 0.014 Å larger than the corresponding values of dimethylamine respectively     

The proton transfer process observed in the structure analysis and DFT calculations of (E)-2-ethoxy-6-[(2-methoxyphenylimino)methyl]phenol

The crystal and molecular structures of an o-hydroxy Schiff base derivative, (E)-2-ethoxy-6-[(2-methoxyphenylimino)methyl]phenol, have been determined by single crystal X-ray diffraction analyses at 296 and 100 K. The results from temperature-dependent structural analysis regarding the tautomeric equilibrium of the compound were interpreted with the aid of quantum chemical calculations. To clarify the tautomerization process and its effects on the molecular geometry, the gas-phase geometry optimizations of two possible tautomers of the title molecule, its OH and NH form, were achieved using DFT calculations with B3LYP method by means of 6-31 + G(d,p) basis set. In order to describe the potential barrier belonging to the phenolic proton transfer, nonadiabatic Potential Energy Surface (PES) scan was performed based on the optimized geometry of the OH tautomeric form by varying the redundant internal coordinate, O–H bond distance. The Harmonic Oscillator Model of Aromaticity (HOMA) indices were calculated in every step of the scan process so as to express the deformation in the aromaticities of principal molecular moieties of the compound. The results show that there is a dynamic equilibrium between the aromaticity level of phenol and chelate ring and furthermore π-electron coupling affecting overall molecule of the title compound. Charge transfer from phenol ring to pseudo-aromatic chelate ring increases with increasing temperature, whereas π-electron transfer from chelate ring to anisole ring is decreased as temperature increases. The most strength intramolecular H-bonds are observed for conformers close to transition state.     

Electric dipole moment studies of carboxylic ester conformations: alkyl acetates,benzoates, 2,4,6-trimethyl-benzoates and 2,3,5,6-tetramethylbenzoates

Electric dipole moments μ in benzene at 30 °C have been determined (Table 3) on methyl, ethyl, isopropyl and t-butyl esters of the title compounds to determine steric effects on conformation in solution. Experimental moments were compared with those calculated for various possible conformations by a 3-dimensional vectorial addition method using bond moments and bond angles. The experimental moments for the alkyl acetates were best interpreted in terms of an out-of-plane deviation of the alkyl group from an s-trans conformation caused by steric interference between the alkyl group and the carbonyl oxygen and increasing in the series from methyl to t-butyl. The dihedral angles 0 (deviations) were calculated using a vector addition method. An increase in the moments of the benzoate series over the acetates was interpreted in terms of conjugative interaction between phenyl and carbonyl groups. Angles of twist φ for the benzoates and trimethylbenzoates were calculated using the Braude-Sondheimer equation. A decrease in the moments of the methyl, ethyl, and isopropyl trimethyl-benzoates as compared with the benzoates was interpreted in terms of steric interference between ortho methyls and both oxygens. The decrease in the angles of twist from methyl to t-butyl for the trimethylbenzoates was tentatively explained by greater steric interaction of the alkyl group with both carbonyl oxygen and ortho methyls, which forces adoption of a more coplanar arrangement between the ring and the carbonyl group than for the other alkyl derivatives, this interaction increasing with the size of the alkyl group. Dipole moments for 2,3,5,6-tetramethylbenzoates were nearly the same as for corresponding trimethyl-benzoates, thus showing no conclusive evidence for operation of a “buttressing” effect.     

Molecular structure study of 1,2,3-trimethyldiaziridine by means of gas electron diffraction method

The molecular structure of 1,2,3-trimethyldiaziridine has been determined from the gas-phase electron diffraction data supplemented spectral and quantum chemical calculations. The configuration of studied compound incorporates trans-position of methyl groups attached to nitrogen atoms of diaziridine cycle. The following principal structural parameters were determined (r_h1 bond lengths in Å, bond angles in degrees with 3σ in parentheses): r(N–C), 1.489(9); r(N–N), 1.480(15); r(C–C), 1.503(15); ∠NCN, 61.5(9); ∠(H₃C)CN, 124.0(15). The obtained structural parameters of 1,2,3-trimethyldiaziridine were compared with those for structural analogues. The gaseous standard enthalpy of formation of 1,2,3-trimethyldiaziridine was estimated to be 176.2?±?5.0 kJ/mol.

     

Effect of methyl groups substitution on the strength of intramolecular hydrogen bonding of naphthazarin: DFT and NBO studies

A density functional theory (DFT) study-based method B3LYP/6-311++G** was carried out to investigate the methyl groups substitution effect on the structure and the strength of intramolecular hydrogen bonding in naphthazarin (NZ) (5,8-dihydroxy-1,4-naphthoquinone). The full geometry optimization of molecular structures, the difference between the energies of hydrogen-bonded and non-hydrogen-bonded rotamers, and the proton chemical shift of the hydroxyl groups in NZ and its methyl substituents obtained at the B3LYP/6-311++G** level. The vibrational frequencies of all samples and their deuterated analogues were calculated at the same theoretical level. The ¹H chemical shifts for NZ and its methyl substituents were computed at the B3LYP/6-311++G** level using the gauge-including atomic orbital method. Furthermore, in order to investigate the changes in bond order, electron density, electron delocalization, and steric effects caused by methyl substituents, natural bond orbital analysis were carried out at the B3LYP/6-311++G** level. After comparing these effective parameters in methyl substituents with those of their parent, NZ, we concluded that, in general, intramolecular hydrogen bonding strength increases by substituting methyl groups in the different positions of NZ.     

Ab initio studies of structural features not easily amenable to experiment. 18. Conformational analysis and molecular structure of glycine methyl ester

The structures of four conformations of the methyl ester of glycine were determined by standard single-determinant molecular orbital (MO ) calculations using Pulay's force method and the 4-21G basis set. The most stable conformation of this compound has a symmetry plane which contains all the heavy atoms; it is stabilized by hydrogen bonds between the NH₂ group and the carbonyl oxygen; it corresponds to the most stable, stretched form of free glycine. The structural parameters in the different conformations can vary significantly (bond distance by more than 0.02 Å and bond angles by up to 15°). The structural changes which are caused in glycine by esterification are discussed and some of them are interpreted in terms of hyperconjugative π-electron delocalization.     

Internal Rotation and Equilibrium Structure of the Bromonitromethane Molecule According to Gas Electron Diffraction Data and Quantum Chemical Calculations

The structure and internal rotation of the bromonitromethane molecule are studied using electron diffraction analysis and quantum chemical calculations. The electron diffraction data are analyzed within the models of a general intramolecular anharmonic force field and quantum chemical pseudoconformers to account for the adiabatic separation of a large amplitude motion associated with the internal rotation of the NO₂ group. The following experimental bond lengths and valence angles are obtained for the equilibrium orthogonal configuration of the molecule with C_s symmetry: r_e(N=O) = 1.217(5) Å, r_e(C–N) = 1.48(2) Å, r_e(C–Br) = 1.919(5) Å, ∠_еBr–C–N = 109.6(9)°, ∠_еO=N=O = 125.9(9)°. The equilibrium geometry parameters are in good agreement with CCSD(T)/cc-pVTZ calculations. Thermally averaged parameters are calculated using the equilibrium geometry and quadratic and cubic quantum chemical force constants. The barrier to internal rotation cannot be determined reliably based on the electron diffraction data used in this work. There is a 82% probability that the equilibrium configuration with orthogonal C–Br and N=O bonds is most preferable, and internal rotation barrier does not exceed 280 cm^-1, which agrees with CCSD(T)/cc-pVTZ calculations.     

Do halogen and methyl substituents have electronic effects on the structures of simple disilanes? An experimental and theoretical study of the molecular structures of the series X3SiSiMe3 (X = H, F, Cl and Br)

The gas-phase molecular structures of a series of halogen-substituted disilanes [X₃SiSiMe₃ (X = H, F, Cl and Br)], 1,1,1-trimethyldisilane (H₃SiSiMe₃), 1,1,1-trifluoro-2,2,2-trimethyldisilane (F₃SiSiMe₃), 1,1,1-trichloro-2,2,2-trimethyldisilane (Cl₃SiSiMe₃) and 1,1,1-tribromo-2,2,2-trimethyldisilane (Br₃SiSiMe₃), have been determined in the gas phase by electron diffraction. Ab initio calculations at the HF and MP2 level were used to support the experimental investigation using the SARACEN method. All of the investigated structures were determined to adopt a staggered structure with C _3v symmetry. The effect of substitution on the Si–Si bond and the Si–Si–X bond angle was investigated and these results were compared to results obtained from a recent study of halogen-substituted disilanes [X₃SiSiXMe₂ (X = F, Cl, Br and I)] to consider the effect of the methyl groups on the substituted disilanes.     

Konformative Beweglichkeit Flexibler Ringsysteme–XII Modellrechnungen zur Ringinversion bei Methylsubstituierten Cyclohexanen

For the interpretation of experimental data on the activation energy and free activation enthalpy for the inversion of cyclohexane and its di-, tetra- and hexa-methyl derivatives, model calculations were made to determine the ‘relative’ energies of the ground, intermediate and transition states of the molecules. For this purpose Hendrickson's model was extended so that with internal molecular variables (bond lengths, valence and torsional angles) the topography and the ‘relative’ energy of every possible unsymmetrical conformation could be included. To obtain optimal agreement between the calculated values and the experimental results a total of 17 different combinations of potential functions for deformation of valence angles, torsional angles and H? H interactions were used. By application of the extended calculating procedure it was found that for cyclohexane the half-chair conformation is not, as until now assumed, the only transition conformation in chair inversion, but that there are numerous other unsymmetrical transition conformations with similar energies. The calculations for methyl cyclohexanes showed that for molecules with synaxial arrangement of methyl groups the relative energy of the chair form is considerably increased. The chair form is however still the most stable, even in the case of 1,1,3,3,5,5-hexamethylcyclohexane. The most favourable twist conformations are about 2.6 to 6.5 kcal/mole energy richer. Calculation of activation energies showed that, with synaxial arrangement of two or more methyl groups, the relative energy of the transition conformation is less markedly increased than is that of the ground state, with the result that the activation energy is reduced in comparison with that for cyclohexane.     

Polystyrene models. III. modeling backbone/side-group interactions by an ab initio study of 2,4-diphenylpentane

The Structure and the energy of the stable conformations of the two possible stereochemical configurations of 2,4-diphenylpentane are obtained using the ab initio molecular orbital theory. The objective was to mimic the possible structures and determine the corresponding energies of the dyads of syndiotactic and isotactic polystyrene and, consequently, to study the interactions between the phenyl groups. The results of complete geometry optimization showed significant changes in geometrical parameters compared with those expected from the ideal hydrocarbon structure. The steric strain is most pronounced in some of the gauche conformations where large (approximately 40°) distortions of the backbone torsional angles and/or simultaneous phenyl group rotations in the range of 30°–40° away from its global minimum position may occur. In addition to the discussion of the geometrical parameters, the corresponding dipole moments are also calculated and differences related to the various structures and discussed. © 1993 John Wiley & Sons. Inc.     

Inter- and Intramolecular Aryl–Aryl Interactions in Partially Fluorinated Ethylenedioxy-bridged Bisarenes**

Several ethylenedioxy-bridged bisarenes with a variety of type and number of aryl groups were synthesized to study non-covalent dispersion-driven inter- and intramolecular aryl–aryl interactions in the solid state and gas phase. Intramolecular interactions are preferably found in the gas phase. DFT calculations with and without dispersion correction show larger interacting aromatic groups increase the stabilization energy of folded conformers and decrease the intermolecular centroid–centroid distance. Single-molecule structures generally adopt folded conformations with short intramolecular aryl–aryl contacts. Gas electron diffraction experiments were performed exemplarily for 1-(pentafluorophenoxy)-2-(phenoxy)ethane. A new procedure for structure refinement was developed to deal with the conformational complexity of such molecules. The results are an experimental confirmation of the existence of folded conformations of this molecule with short intramolecular aryl–aryl distances in the gas phase. Solid-state structures are dominated by stretched structures without intramolecular aryl–aryl interactions but interactions with neighboring molecules.