Conjugation within the oximino group: A semiquantitative estimation

1988 ◽  
Vol 53 (5) ◽  
pp. 1018-1032 ◽  
Author(s):  
Otto Exner ◽  
Václav Jehlička
Keyword(s):  
Dipole Moment ◽  
Bond Order ◽  
Simple Formula ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Bond Lengths ◽  
Group A ◽  
Structural Database ◽  
A Charge ◽  
Bond Character

The n-π conjugation within the oximino group was estimated on the basis of dipole moments and of bond lengths. The dipole moments were measured on oximes I and O-methyloximes II in benzene solution, the C=N and N-O bond lengths were retrieved from the Cambridge Structural Database for all available derivatives. The results differ somewhat in the quantitative sense but can be always interpreted in terms of the classical mesomeric formulae IV ↔ V. This formalism postulates – in agreement with experiments – a lowered C=N bond order, some double bond character of the N-O bond, planarity of the C=NOR group, and an excess dipole moment (μm) oriented in the sense of a charge transfer from O toward C. Quite different results were obtained for O-acetyloximes III. While their bond lengths could agree with a weakened conjugation, the excess dipole moment is oriented from C towards O and cannot be expressed by any simple formula. Hence the mesomeric formulae may represent an acceptable description of the actual charge distribution in many cases but not in all.

The Electrostatic Moments of a Charge Distribution

1997 ◽  
Author(s):  
Philip Coppens
Keyword(s):  
Dipole Moment ◽  
Charge Distribution ◽  
Dipole Moments ◽  
Practical Importance ◽  
Diffraction Method ◽  
Quadrupole Moments ◽  
Charge Densities ◽  
Moment Vector ◽  
Experimental Values ◽  
A Charge

The moments of a charge distribution provide a concise summary of the nature of that distribution. They are suitable for quantitative comparison of experimental charge densities with theoretical results. As many of the moments can be obtained by spectroscopic and dielectric methods, the comparison between techniques can serve as a calibration of experimental and theoretical charge densities. Conversely, since the full charge density is not accessible by the other experimental methods, the comparison provides an interpretation of the results of the complementary physical techniques. The electrostatic moments are of practical importance, as they occur in the expressions for intermolecular interactions and the lattice energies of crystals. The first electrostatic moment from X-rays was obtained by Stewart (1970), who calculated the dipole moment of uracil from the least-squares valence-shell populations of each of the constituent atoms of the molecule. Stewart’s value of 4.0 ± 1.3 D had a large experimental uncertainty, but is nevertheless close to the later result of 4.16 ± 0.4 D (Kulakowska et al. 1974), obtained from capacitance measurements of a solution in dioxane. The diffraction method has the advantage that it gives not only the magnitude but also the direction of the dipole moment. Gas-phase microwave measurements are also capable of providing all three components of the dipole moment, but only the magnitude is obtained from dielectric solution measurements. We will use an example as illustration. The dipole moment vector for formamide has been determined both by diffraction and microwave spectroscopy. As the diffraction experiment measures a continuous charge distribution, the moments derived are defined in terms of the method used for space partitioning, and are not necessarily equal. Nevertheless, the results from different techniques agree quite well. A comprehensive review on molecular electric moments from X-ray diffraction data has been published by Spackman (1992). Spackman points out that despite a large number of determinations of molecular dipole moments and a few determinations of molecular quadrupole moments, it is not yet widely accepted that diffraction methods lead to valid experimental values of the electrostatic moments.

Bond order-bond length relationships in conjugated hydrocarbons - the microwave spectrum and structure of 3,4-dimethylenecyclobutene

1983 ◽  
Vol 36 (4) ◽  
pp. 639 ◽  
Author(s):  
RD Brown ◽  
PD Godfry ◽  
BT Hart ◽  
AL Ottrey ◽  
M Onda ◽  
...  
Keyword(s):  
Dipole Moment ◽  
Ab Initio ◽  
Bond Length ◽  
Bond Order ◽  
Microwave Spectrum ◽  
Basis Set ◽  
Molecular Orbital Calculations ◽  
Bond Orders ◽  
Bond Lengths ◽  
Typical Error

The microwave spectrum of the benzene isomer 3,4-dimethylenecyclobutene including spectra of all possible single 13C-substituted and sufficient singly and doubly D-substituted species to give a complete r5 geometry, have been measured and analysed. An estimate of the re geometry has also been derived. The additional precise CC bond lengths obtained for an unsubstituted, conjugated hydrocarbon enable us to examine bond order-bond length relationships more thoroughly than has previously been possible. The CC bond lengths exhibit a noticeably better correlation with SCFMO bond orders than with simple H�ckel bond orders. Further confirmatory measurements of the dipole moment of dimethylenecyclobutene have been made. Ab initio molecular orbital calculations using a 6-31G basis set give an optimized geometry with CC bond lengths within 2 pm of the r5 values. The computed dipole moment agrees almost exactly with experiment but a corresponding calculation on fulvene is discrepant with experiment by 0.16 D, which is probably a more typical error.

A Study of d-Orbital Effects in Esters of Trifluorothiolacetic Acid. A Comparison of Evidence from Dipole Moment and Kinetic Data

1974 ◽  
Vol 52 (17) ◽  
pp. 3113-3118 ◽  
Author(s):  
E. Bock ◽  
A. Queen ◽  
S. Brownlee ◽  
T. A. Nour ◽  
M. N. Paddon-Row
Keyword(s):  
Dipole Moment ◽  
Trifluoroacetic Acid ◽  
Kinetic Data ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Aqueous Acetone ◽  
Electron Release ◽  
Hydrolysis Of ◽  
D Orbitals

The dipole moments of a series of esters of trifluoroacetic acid and trifluorothiolacetic acid have been determined in benzene solution at 25 °C. The results are consistent with conjugative and hyperconjugative electron release by the hydrocarbon groups to the d-orbitals of sulfur. This conclusion is supported by data for the hydrolysis of these compounds in water or aqueous acetone.

Dipole moments of some ethylene derivatives

1967 ◽  
Vol 45 (10) ◽  
pp. 1097-1100 ◽  
Author(s):  
E. Bock ◽  
E. F. Dojack
Keyword(s):  
Dipole Moment ◽  
Carbon Tetrachloride ◽  
Solvent Effect ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Vinyl Ketone ◽  
Methyl Vinyl Ketone ◽  
Methyl Vinyl ◽  
Vinyl Bromide ◽  
Reported Data

The dipole moments of the ethylene derivatives acrylonitrile, chloroacrylonitrile, methyl vinyl ketone, vinyl bromide, 1,1-dichloroethylene, and tribromoethylene were determined in benzene solution at 20 °C. The measured dipole moments were corrected for the solvent effect by the method of Looyenga. In addition the applicability of the Looyenga equation for the solvent effect was tested on earlier reported data for 20 different molecules, whose dipole moments were derived from dilute benzene and carbon tetrachloride solutions, respectively. The corrected dipole moment values were compared with earlier corrected values calculated according to the method of Le Fevre and Le Fevre and with reported gas values. It was found that for most compounds both methods of correction yielded dipole moment values which differed by 0–10% from the gas values.

The conformation of aryl carbonates. A dipole moment study

1981 ◽  
Vol 46 (4) ◽  
pp. 856-860 ◽  
Author(s):  
Otto Exner ◽  
Václav Jehlička
Keyword(s):  
Dipole Moment ◽  
Benzene Solution ◽  
Dipole Moments

The dipole moments of diaryl carbonates I-V in benzene solution are consistent with the Z,Z-conformation (A) in which the conformation of simple esters is repeated in the two moieties. The E,Z-form (B) was not detected; it may be either present as a quite small fraction or only with specific, overcrowded structures.

Conformation around several equivalent bonds. Polymethoxy derivatives of benzene

1983 ◽  
Vol 48 (4) ◽  
pp. 1030-1041 ◽  
Author(s):  
Otto Exner ◽  
Václav Jehlička
Keyword(s):  
Dipole Moment ◽  
Crystalline Phase ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Graphical Comparison ◽  
Methoxy Groups ◽  
Derivatives Of

Dipole moments of benzene methoxy derivatives I-IX were measured in benzene solution and discussed in terms of the conformation around the Car-O bonds, exploiting the method of graphical comparison and systematic introduction of substituents. In the case of 1,2-dimethoxybenzene several explanations are in accord with the dipole moment data. However, considering all available evidence, most probable seems to be the presence of two conformers, one of which is non-planar with two non-equivalent methoxy groups (IC). The conformation of 1,2,3-trimethoxybenzene is similar to that found for many derivatives in the crystalline phase (VC), only the outer methoxy groups are probably slightly deviating from the coplanar position.

A physical Study on the Aromaticity of 4H-Pyran-4-one, 9H-Xanthen-9-one, and related sulphur Compounds

1986 ◽  
Vol 41 (10) ◽  
pp. 1250-1257 ◽  
Author(s):  
Henri Lumbroso ◽  
Jacqueline Curé ◽  
Michel Evers
Keyword(s):  
Dipole Moment ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Moment Analysis ◽  
Sulphur Compounds ◽  
Physical Study

Dipole moment analysis of 4H-pyran-4-one and 9H-xanthen-9-one, and their sulphur analogues, and of 9H-selenoxanthen-9-one, enables a discussion on their aromaticities. In the present work, the dipole moments of tetrahydro-4H-pyran-4-one, tetrahydrothio-4H-pyran-4-one, tetrahydroseleno- 4H-pyran-4-one, 9(10H)-anthracen-9-one, 9H-xanthen-9-one, 9H-thioxanthen-9-one and 9H-selenoxanthen-9-one were measured in benzene solution at 30.0 °C.

Dipole moments and conformation of peroxy esters

1981 ◽  
Vol 46 (2) ◽  
pp. 490-497 ◽  
Author(s):  
Božo Plesničar ◽  
Otto Exner
Keyword(s):  
Dipole Moment ◽  
Dihedral Angle ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Ester Group ◽  
Model Compounds ◽  
Tert Butyl ◽  
Butyl Esters

Dipole moments of sixteen substituted peroxybenzoic esters III, IV were measured in benzene solution and interpreted in terms of the conformation around the O-O bond. The dipole moment of the CO3R group is different for R = methyl and R = tert-butyl, viz. 9.0 and 10.4 . 10-30 C m, respectively. Its solution into components was carried out making reference to the dipole moments of substituted benzoic esters as model compounds. As a result the dihedral angle τ = C-O-O-C was estimated to 160-180° in the case of tert-butyl peroxy esters IV and to 110-150° in the case of methyl peroxy esters III. The mesomeric dipole moment expressing the conjugation within the ester group was estimated to 0.8 or 1.5 . 10-30 C m for methyl or tert-butyl esters, respectively.

On the molecular conformation of the bis-1,3-dioxolyl molecule in decahydronaphthalene and benzene solutions

1977 ◽  
Vol 55 (9) ◽  
pp. 1440-1443 ◽  
Author(s):  
Ernst Bock ◽  
Garnette Sutherland ◽  
David McKinnon ◽  
Edward Tomchuk
Keyword(s):  
Dipole Moment ◽  
Temperature Dependence ◽  
Electric Dipole ◽  
Molecular Conformation ◽  
Benzene Solution ◽  
Dipole Moments ◽  
Conformational Behavior ◽  
Electric Dipole Moments ◽  
Solvation Theory

The electric dipole moments of the bis-1,3-dioxolyl molecule were determined in dilute decahydronaphthalene solution at 25 and 100 °C and in dilute benzene solution at 25 °C. From the temperature dependence of the dipole moment it is concluded that the trans rotamer of the molecule is more stable in both solvents than the gauche rotamer by ∼4.5 kJ mol−1. The conformational behavior of the molecule is discussed in terms of a recently proposed solvation theory.

Dipole moment and electron distribution of the amide group

1980 ◽  
Vol 45 (9) ◽  
pp. 2410-2416 ◽  
Author(s):  
Otto Exner ◽  
Zlata Papoušková
Keyword(s):  
Dipole Moment ◽  
Electron Distribution ◽  
Dipole Moments ◽  
Amide Group ◽  
Group A

Dipole moments of five substituted N,N-dimethylbenzamides Ia-Ie were measured in benzene and dioxan solutions. The group moment of 12.7 . 10-30 C m (at an angle of 80° to the C(1)-C bond) was resolved into components and a mesomeric moment of 4.7 . 10-30 C m (in the direction from N to C) was derived, accounting for the conjugation within the dimethylamide group. A smaller mesomeric moment in the same direction was found for the unsubstituted amide group. The results are but roughly consistent with an interpretation in terms of the mesomeric formulae A ##e B.

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