Dipole moments of thiophene analogs of chalcones and their vinylogs

The dipole moments of 25, unsaturated ketones of the thiophene series are measured in benzene at 25°, and their most probable coplanar conformations established. For the systems studied it is shown that, in the static state, the 2-thienyl group exhibits an electron-donating effect, greater than those of 2-furyl and phenyl. Introduction of electron-donating and electron-accepting substituents into the molecule of 1-(thienyl-2)-3-phenylpropenones as well as shift of the carbonyl group away from the aryl one towards the heterocyclic ring, leads in general to an increase in dipole moment. A linear relationship is found for the changes in dipole moment of isomeric thiophene ketones due to electron-donating substituents. Dipole moments of propen-3-ones correlate satisfactorily with Hammett parameters for substituents, and intermolecular distances.     

Dipole moments of some substituted benzaldehydes. Conformational preference of substituents ortho to the aldehyde group

The dipole moments of a number of substituted benzaldehydes are measured in benzene solution. The angle which the dipole axis of the CHO group makes with the axis of rotation of the group is determined. The observed moments of the ortho-substituted benzaldehydes are compared with the moments calculated for free rotation as well as fors-trans ands-cis orientations of the -CHO group.o-Fluorobenzaldehyde exists mostly in thes-trans conformation.o-Chloro-,o-bromo-ando-nitro-benzaldehydcs also exist in thes-trans conformation; their observed dipole moments are even lower than the values calculated fors-trans forms, indicating mutual induction of the ortho substituents. Though 2,5-dichlorobenzaldehyde is expected to have the same dipole moment as benzaldehyde, the observed moment is significantly lower due to mutual induction of the ortho substituents. 2,5-Dimethylbcnzaldehyde has, however, almost the same moment as benzaldehyde. The dipole moment ofo-methoxybcnzaldchyde is considerably higher than the values calculated for boths-cis ands-trans conformations. An explanation is given for this.o-Hydroxybenzaldehyde exists exclusively in thes-cis form due to internal H-bonding.     

Conformations of highly hindered aryl ethers: XV. Mesomeric moments in m-dinitrobenzenes

The dipole moments of fourteen m-dinitrobenzenes with different substituents (OMe, CN, CH₃, CHO, Cl, CF₃, COOMe, F, Br, NH₂) in various positions were measured in order to study mesomeric interactions. Inclusion of conformational considerations permitted estimates to be made for the mesomeric moments of these groups at given positions. The values obtained could be applied with consistent results to a total of forty m-dinitrobenzenes, including others previously reported. The mesomeric moment increments deduced, compared with substituted benzenes possessing no nitro groups, were found to be only slightly larger than in non-nitro substituted benzenes (compared with substituted alkanes), which would indicate a reluctance of the extended π system of m-dinitrobenzene to interact with other substituents present, in any way other than by direct extension of the π network.One apparent case of intermolecular electron donor-acceptor complex formation was detected. The measured dipole moment of o-iodoanisole indicates a planar anti conformation for the methoxyl group.     

An examination of hyperconjugative and electrostatic effects in the hydride reductions of 2-substituted-4-tert-butylcyclohexanones

To better understand electronic effects on the diastereoselectivity of nucleophilic additions to the carbonyl group, a series of 2-X-4-tert-butylcyclohexanones (X = H, CH(3), OCH(3), F, Cl, Br) were reacted with LiAlH(4). Reduction of ketones with equatorial substituents yields increasing amounts of axial alcohol in the series for X [H < CH(3) < Br < Cl < F < OCH(3)]. These data cannot be explained by steric or chelation effects or by the theories of Felkin-Anh or Cieplak. Instead, an electrostatic argument is introduced: due to repulsion between the nucleophile and the X group, axial approach becomes energetically less favorable with an increase in the component of the dipole moment anti to the hydride approach trajectory. The ab initio calculated diastereoselectivities were close to the experimental values but did not reproduce the relative selectivity ordering among substituents. For reduction of ketones with axial substituents, increasing amounts of axial alcohol are seen in the series for X [Cl < Br < CH(3) < OCH(3) < H < F]. After some minor adjustments are made, this ordering is consistent with both the electrostatic model and Felkin-Anh theory. Cieplak theory cannot account for these data regardless of adjustments. Ab initio calculated diastereoselectivities were reasonably accurate for the nonpolar substituents but were poor for the polar substituents.     

Dipole moments and structures of 1.5-diphenyl-3-aryl-2-pyrazolines and their formyl derivatives

The dipole moments of 1,5-diphenyl-3-aryl and 1-(p-formylphenyl)-3-aryl-5-phenyl-2-pyrazolines were measured in benzene solutions at 25°C and were estimated theoretically by a combination of quantum-chemical calculations by the self-consistent-field MO method within the Pariser-Parr-Pople approximation and the vector-additive scheme. It is shown that polarization of the N₁-N₂, N₁-C_arand C = N bonds makes a significant contribution to the resulting moment of 1,3,5-triphenyl-2-pyrazoline. The introduction of an electron-acceptor aldehyde group in the para position of the N-phenyl ring changes the polarization of these bonds substantially and leads to the appearance of considerable interaction. When electron-donor substituents are introduced into the 3-aryl grouping, one observed a similar but weaker effect that changes regularly as the electron donor properties of the substituents increase.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 106–110, January, 1977.The authors thank A. A. Sukhorukov for his assistance in the quantum-chemical calculations.     

Infrared spectra of thiophene analogs of chalcones and their vinylogs

The IR spectra of 45 thiophene analogs of chalcones and vinylogs, and (2-thienyl)phenylketone are determined in the 1700-650 cm^–1 region, and frequency assignments made. It is confirmed that the carbonyl group and aliphatic double bond are so situated with respect to one another that there is approximation to the S-cis form. It is shown that in the systems studied, 2-thienyl has an electron-donating effect, exceeding that of 2-furyl. It is also found that for 1-(2-thienyl)-3-arylpropen-1-ones the carbonyl group frequency shift due to substituents tends to correlate with the Hammett parameters.     

Structure and basicity of ω-substituted ethynyl 2-thienyl ketones

By a comparison of the calculated and experimental dipole moments of ethynyl 2-thienyl ketones, it was shown that the S,O-cis conformation is the most probable one. The basicities of these compounds were estimated by means of IR spectroscopy from the shift of the band of the phenolic OH stretching vibration. The correlation relationships between the dipole moments of the ketones, their basicities, and the association constants of propiolic acids that have identical substituents attached to the triple bond were established. It is noted that the experimental characteristics found do not correlate with the different sorts ofσ substituent constants; this provides evidence for a difference in the mechanism of the interaction of the atomic groups in a number of acetylenic and aromatic molecules.     

Improved mesophase stability of benzoxazole derivatives via dipole moment modification

By modifying the molecular dipole moments with lateral monofluorine substituent, improved mesophase stabilities were obtained for novel benzoxazole derivatives, 2-(4?-alkoxy-3-fluorobiphenyl-4-yl)-benzoxazole liquid crystals (coded as nPPF(3)Bx). The series of nPPF(3)Bx with lateral monofluorine substituent ortho to benzoxazole group have larger calculated dipole moments by about 2 D than the corresponding fluorine-substituted analogs (compounds I) with lateral monofluorine ortho to alkoxy group; it is interesting to note that they show lower melting and clearing points but better mesophase stability with wider mesophase ranges for the molecules with both polar (NO₂, Cl) and nonpolar (CH₃, H) terminal groups. Meanwhile, compounds nPPF(3)Bx show greater red-shifted photoluminescence emissions than compounds I, which suggest that π–π interaction between molecules is reinforced by the enhanced dipole–dipole interaction caused by increased dipole moments. These results suggest that modification of the molecular dipole moment is an effective method to improve the mesophase stability of the classical mesogenic compounds.     

Experimental studies of light-induced charge transfer and charge redistribution in (X(2)-bipyridine)Re(I)(CO)(3)Cl complexes

Stark emission spectroscopy, transient DC photoconductivity (TDCP), and ground-state dipole moment measurements have been used to evaluate charge transfer (CT) within various (X(2)-bipyridine)Re(I)(CO)(3)Cl complexes following (3)MLCT excited-state formation. The Stark technique reports on vector differences between ground-state (mu(g)) and excited-state (mu(e)) dipole moments, while TDCP, when combined with independently obtained mu(g) information, reports on scalar differences. For systems featuring collinear, same-signed ground- and excited-state dipole moments, the scalar and vector differences are equivalent. However, for the low symmetry systems studied here, they are distinctly different. The vector difference yields the effective adiabatic one-electron-transfer distance (R(12)), while the combined vector and scalar data yield information about dipole rotation upon ground-state/excited-state interconversion. For the systems examined, charge transfer distances are substantially smaller than geometric electron-donor/electron-acceptor site separation distances. The measured distances are significantly affected by changes in acceptor ligand substituent composition. Electron-donating substituents decrease CT distances, while electron-withdrawing substituents increase CT distances, as do aromatic substituents that are capable of expanding the bipyridyl ligand (acceptor ligand) pi system. The Stark measurements additionally indicate that the CT vector and the transition dipole moment are significantly orthogonal, a consequence of strong polarization of the Re-Cl bond (orthogonal to the metal/acceptor-ligand plane) in the ground electronic state and relaxation of the polarization in the upper state. The ground-state Re-Cl bond polarization is sufficiently large that the overall ground-state scalar dipole moment exceeds the overall excited-state scalar dipole moment, despite transfer of an electron from the metal center to the diimine ligand. This finding provides an explanation for the otherwise puzzling negative solvatochromism exhibited in this family of compounds. Combining TDCP and Stark results, we find that the dipole moment can be rotated in some instances by more than 90 degrees upon (3)MLCT excited-state formation. The degree of rotation or reorientation can be modulated by changing the identity of the acceptor ligand substituents. Reorientational effects are smallest when the compounds feature aromatic substituents capable of spatially extending the pi system of the acceptor ligand.     

Dipole moments. Structure and conformation of asymmetrically 4,5-substituted 1-(α-aroyloxyarylideneamino)-1,2,3-triazoles

The dipole moments of several asymmetrically 4,5-substituted 1-(α-aroyloxyarilideneamino)-1,2,3-triazoles, produced by oxidation of bis-aroylhydrazones of α-dicarbonyl compounds, were measured in benzene solution. The structure and the probable conformation of each of these compounds was determined by comparing their experimental and theoretical dipole moments. It has been found that when one of the substituents R₁ or R₂ is an aryl group, it is located at the 4-position of the triazole ring. Considering the conformation of the ester-imino function, it has been found to be the same as that of symmetrically substituted derivatives.     

Dipole moments of indole analogs of chalcones

The most probable planar conformations of 22 indole analogs of chalcones were established on the basis of dipole moment measurements. The introduction of substituents into the phenyl ring of 1-(3-indolyl)-3-phenylpropenones causes a change in the dipole moments, which, in the 3-propenone series, correlates with the Hammett constants.Translated from Khimiya Geterotsiklicheskih Soedinenii, No. 8, pp. 1083–1086, August, 1972.     

Conformational analysis of aromatic derivatives of 2,3-dihydro-1H-1,5-benzodiazepine

The PMR spectra and dipole moments of some 2,4-disubstituted 2,3-dihydro-1H-1,5-benzodiazepines have been examined. It is shown that seven-membered rings with an aromatic or heterocyclic substituent in the 2-position have the boat configuration, the substitutent preferentially being equatorially oriented. It has been found that the changes in the chemical shifts and spin coupling constants for the protons of the CH-CH₂ fragment in the dihydrobenzodiazepine ring are mainly due to differences in the electronegativity of the substituents in the 2- and 4-positions. Some dipole moment vectors have been calculated for the dihydrobenzodiazepine ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1662–1666, December, 1984.     

Discotic hexa-peri-hexabenzocoronenes with strong dipole: synthesis, self-assembly and dynamic studies

Strong dipole moments have been built into two hexa-peri-hexabenzocoronene (HBC) derivatives (1 and 2) originating from the push-pull structure of the molecules with one electron-donating and one electron-withdrawing substituent. The influence of dipole moment on the self-assembly of HBCs in solution and in bulk has been investigated.     

Electron-donor substituents on the dppz-based ligands to control luminescence from dark to bright emissive state in Ir(III) complexes

The structures and electronic properties of a series of cyclometalated Ir(III) complexes with substituted dppz ancillary ligand (dppz: dipyrido[3,2-a:2′,3′-c]phenazine) including tert-butyl, pyrrolidine ring, and alkoxys substituents as donor group have been theoretically study. With the aim of highlight the attractive qualities of each system for their use as luminescent material, the effects of the electron-donor substituents onto dppz was evaluated on the structural, charge transport, absorption, and phosphorescent properties. The effect of the substituent was studied on the modulation of the bright and dark triplet states. Main results show that ortho-methoxy and tert-butyl substituents act as lower electron-donating groups, then, efficient electron donation ability was displayed with alkoxy (ethoxy and propoxy) substituents. The best performance was found in a complex with pyrrolidine ring according since their phosphorescence is favored, highlighting their larger electric transition dipole moment value, proposing this system as potential candidate to LEC technology.     

Dipole moments of some isoxazole derivatives

The dipole moments of isoxazole and 13 of its halo, amino, and nitro derivatives were measured. A method is proposed for vector calculation for estimating the direction of the dipole moment of the isoxazole ring. It is demonstrated that phenyl substituents in the 3 and 5 positions are electron donors with respect to the isoxazole ring. It was found that the dipole moments of 4-substituted 3,5-dimethylisoxazoles correlate satisfactorily with the Hammett constants.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 460–463, April, 1972.     

5,10-A2B2-type meso-substituted porphyrins--a unique class of porphyrins with a realigned dipole moment

Current applications in porphyrin chemistry require the use of unsymmetrically substituted porphyrins. Many current industrial interests in optics and biomedicine require systems with either push-pull (electron-donating and -withdrawing groups) or amphiphilic systems (hydrophobic and hydrophilic groups). In this context we present the class of 5,10-A(2)B(2)-type porphyrins for which two different substituents are positioned in diagonally opposite meso positions. Thus, the intramolecular dipole moment in these tetrapyrroles is positioned along a β-β vector passing through two pyrrole rings. This is opposite to the situation of the frequently used 5,15-A(2)BC porphyrins for which the dipole moment is oriented along a meso-meso axis. We have elaborated syntheses of the 5,10-A(2)B(2) porphyrins by using transition-metal-catalyzed transformations of 5,10-A(2) porphyrins or direct substitutions reactions thereof; this gives the target molecules in 22-77% overall yields. The compounds exhibit interesting structural, spectroscopic, and optical features and can serve as building blocks for new porphyrin arrays and applications.     

Preparation and properties of lateral monofluoro-substituted benzoxazole-based mesogenic compounds

A series of 2-(3?-fluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-benzoxazole liquid crystals (coded as nPF(3)PBx) were prepared, where a lateral fluorine substituent, as well as methyl, chlorine and nitro terminal groups, was introduced into the molecules to investigate the effects of different polar substituents on the liquid crystal properties. The mesomorphic and photophysical properties were investigated. The results show that compounds nPF(3)PBx have enantiotropic mesophases; meanwhile, they exhibit UV–vis absorption bands with maxima at 323–326 nm and photoluminescence emission peaks at 389–395 nm, respectively. It is noted that nPF(3)PBx with terminal polar groups or electron-withdrawing groups (NO₂, Cl) display higher clearing temperatures and wider mesophase range than those of the corresponding homologues with terminal non-polar groups or electron-donating groups (CH₃, H). Meanwhile, compared with two lateral fluorine-substituted analogues containing 3,5-difluorophenyl unit, lateral monofluoro-substituted nPF(3)PBx display enhanced mesophase range both in heating and cooling except for terminal methyl-substituted compounds, as well as show obvious red-shifted UV–vis absorption bands and photoluminescence emission, which are attributed to the enhanced dipole–dipole interaction caused by increased dipole moment.     

Molecular Dipole Moment and Interaction Dipole Moment in 2-N-Substituted Amino-3(or 5)-methyl-4-nitropyridines and Their N-Oxides

The dipole moments of 2-alkylaminoand 2-alkylnitramino-3(or 5)-methyl-4-nitropyridines and their N-oxides have been measured, as well as calculated, using vectorial summation of the group moments and by ab initio method. The estimated interaction dipole moments, _int, have been discussed in terms of the electronic and steric effects and intramolecular hydrogen bonding. Introduction of the NHCH₃ group to position 2 in 4-nitropyridine strengthens the conjugation between the nitro group and ring nitrogen. The lack of such strengthening in 4-nitropyridine-N-oxides is explained as being due to formation of intramolecular hydrogen bond between the N-oxide group and the hydrogen of alkylamino group. Introduction of the nitramino group does not lead to marked modification of the charge distribution in a molecule. This fact may be explained by much weaker electron-donating ability of this group in comparison with the alkylamino group.     

AM1 semiempirical searching for suitable thiophene derivatives that will enable thiophenes to act as a diene in the diels-alder reactions

The AM1 semiempirical method was used for theoretical searching of activation of thiophene as a diene for the Diels-Alder reaction. The reactivity of thiophene, electron-withdrawing and electron-donating substituted thiophenes, as well as the S-methylthiophenium ion were studied as the diene for Diels-Alder reactions by evaluating their frontier orbital energies and by calculating reaction barriers with activated and deactivated dienophiles. It was demonstrated that slight activation of the thiophene ring can be obtained with both electron-donating and electron-withdrawing groups attached to the thiophene ring. It was predicted that the actual transformation of thiophenes into the corresponding S-methylthiophenium anions is the best means of activating the thiophenes. The calculated activation energies for normal (non-activated) dienophiles are moderate so mild reaction conditions are predicted. If dienophiles are activated with electron-donating substituents, AM1 calculations predict a two step cycloaddition reaction with a very small activation barrier.     

Investigation of the effect of <Emphasis Type="Italic">π</Emphasis>–<Emphasis Type="Italic">π</Emphasis> stacking interaction on the properties of –CONH<Subscript>2</Subscript> functional group of benzamide

The effect of π–π stacking between substituted benzene and benzamide on the properties of –CONH₂ functional group, as an important unit in the drugs activities, was studied at the M06-2X/6-311++G(d,p) level of theory. All substituents enhanced the π–π interaction energies, where a reasonably good correlation was found between the interaction energies and Hammett constants of substituents. A linear correlation is observed between the sum of electron charge density at the cage critical point ∑ρ _ccp obtained from the atoms in molecules (AIM) analysis and the interaction energies, where both values grow up with electron-withdrawing substituents (EWSs). The electrostatic potential around the O and N atoms, the natural charges, and the dipole moment of C=O bond have been calculated to predict the ability of functional group on the electrophilic and nucleophilic attacks. The charge transfer increases the electrostatic potential around the benzamide functional group in the presence of electron-donating substituents (EDSs). EWSs increase the acidity of the N atom of the –NH₂ group; a linear relationship is observed between the acidity calculated with the molecular electrostatic potential around the N atom and the natural valence orbital energies.