Dipole moments and spectroscopic properties of methyl-4-nitropyridine N-oxides

Summary Molecular dipole moments and dipole moments of interaction of seven methyl derivatives of 4-nitropyridine N-oxides were determined in benzene solution. Polar and spectroscopic (¹³C NMR and UV/Vis) manifestations of intramolecular interaction indicate that the methyl groups modify the electronic interaction between the NO and NO₂ groups mainly through steric strain.

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Die polaren und spektroskopischen Eigenschaften von Methyl-4-nitropyridin-N-oxiden

Zusammenfassung Dipolmomente und Wechselwirkungsdipolmomente von sieben 4-Nitropyridin-N-oxiden wurden in benzolischen Lösungen gemessen. Die polaren und spektroskopischen Daten (¹³C NMR und UV/Vis) deuten darauf hin, daß die Methylgruppen die elektronische Wechselwirkung zwischen N-oxid und Nitrogruppe hauptsächlich durch sterische Hinderung beeinflussen.

     

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.     

A dipole moment study of the conformational properties of diaryl diselenides and related compounds

The electric dipole moments of the diaryl diselenides (RC₆H₄)₂Se₂ (R  H, 4-F, 4-Br, 4-CH₃, 3-F) were measured in benzene solution at 25 and 45°C. The conformations of these compounds were deduced by matching experimental moments with values calculated for a variety of possible conformations. In the dissolved state the diselenides exist at 25°C in fixed “skew” conformations characterized by dihedral angles of 75–106° between the CSeSe planes, corresponding to the conformational energy minima. At 45°C oscillations about the SeSe bonds are excited in the diphenyl and bis(4-methylphenyl) diselenides, whereas the 4-bromophenyl derivative exhibits free rotation. The fluoro compounds have temperature-independent dipole moments, suggesting “rigid conformations” with dihedral angles of 106° (4-F) and 74.4° (3-F). An analysis of the dipole moments at 25 and 45°C obtained for the compounds (RC₆H₄)₂X₂ (R  H, 3-F, 4-F, 4-Br, 4-CH₃; X  S, Se, Te) showed that the conformational properties of these derivatives change on passing from X  S to X  Te. The observed variations are explicable in terms of a decreasing repulsion between the lone electron pairs of the chalcogen atoms on going from the disulfides to the ditellurides and a concomitant reduction of the energy barrier to rotations about the XX bonds.     

Dipole moments of phenyl,benzo, and benzo[b]thiopheno derivatives of thiophene; 1, 4-dithiin,and thianthrene and their sulfones

The dipole moments of a series of phenyl, benzo-, and benzo[b]thiopheno-derivatives of thiophene, 1,4-dithiin, and thianthrene have been determined. An increase in the dipole moments of the phenyl derivatives of thiophene with an increase in the number of phenyl groups is connected with the existence of conjugation between the thiophene and benzene rings, a quantitative evaluation of the nature of which is given. It is suggested that the structure of the 1, 4-dithiin ring is close to coplanar.     

The polar nature of 2,1,3-benzoxadiazole, -benzothiadiazole, -benzoselenadiazole and derivatives as determined by their electric dipole moments

Permanent electric dipole moments have been determined in benzene solutions at 25° for the oxygen, sulfur, and selenium series of 2,1,3-benzodiazoles. The derivatives studied contained 4, 5, and 6 substituted methyl, chloro, nitro, and amino groups. The dipole moment data of the derivatives were analyzed relative to that of the parent species which allowed deductions to be made about the mesomeric structures operating in the compounds. The mesomeric charge transfer increases regularly from the oxygen to the selenium structures, being nearly undetected in 2,1,3-benzoxadiazole derivatives and very pronounced in the 2,1,3-benzoselenadiazoles.     

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.     

给、吸电子基团对吡嗪衍生物电子结构影响的DFT研究

采用B3LYP方法在6—31G^#基组水平上优化了对位取代吡嗪衍生物的几何构型，利用TD—DFT方法计算了它们的前线分子轨道能级和电子光谱．结果表明，带有给、吸电子基团对吡嗪衍生物与苯、吡啶相比，也具有很好的共轭性；随着分子共轭链的增长，分子的偶极矩增大，前线分子轨道能级差减小，最大吸收波长发生红移．对于具有相同共轭链的同分异构体，推电子基团与具有给电子性质的共轭链相连，则分子的电荷转移明显，导致偶极矩增大，前线分子轨道能级间的电子跃迁更容易；吸电子基团与具有给电子性质的共轭链相连，情况正好相反，这些结果对分子设计有重要意义。     

Polar aspects of intramolecular interactions in 2‐amino‐5‐nitro‐4‐methylpyridines and 2‐amino‐3‐nitro‐4‐methylpyridines

The dipole moments of twelve 2‐N‐substituted amino‐5‐nitro‐4‐methylpyridines ( I‐XII ) and three 2‐N‐substituted amino‐3‐nitro‐4‐methylpyridines ( XIII‐XV ) were determined in benzene. The polar aspects of intramolecular charge‐transfer and intramolecular hydrogen bonding were discussed. The interaction dipole moments, μ_int, were calculated for 2‐N‐alkyl(or aryl)amino‐5‐nitro‐4‐methylpyridines. Increased alkylation of amino nitrogen brought about an intensified push‐pull interaction between the amino and nitro groups. The solvent effects on the dipole moments of 2‐N‐methylamino‐5‐nitro‐4‐methyl‐( I ), 2‐N,N‐dimethylamino‐5‐nitro‐4‐methyl‐ ( II ) and 2‐N‐methylamino‐3‐nitro‐4‐methylpyridines ( XIII ) were different. Specific hydrogen bond solute‐solvent interactions increased the charge‐transfer effect in I , but it did not disrupt the intramolecular hydrogen bond in XIII.     

苯甲酰膦酸二甲酯及其衍生物的密度泛函研究

采用密度泛函方法,在6-311G(d,p)基组下,对苯甲酰膦酸二甲酯及其衍生物进行研究.优化了化合物的几何构型,研究了在几何结构、电子结构、偶极距、前线轨道等方面性质.结果表明,这些化合物都倾向于CO和PO处于trans-空间构型,苯环和羰基形成大的共轭体系且共平面.中性化合物和离子化合物由于结构上的差异性,导致了在性质上有较大的区别.离子化合物相对来说,电荷更集中,偶极距更大.这些结果为以苯甲酰膦酸二甲酯及其衍生物的合成及相关研究提供了理论参考.     

All-valence-electron scf-mo calculations on the electronic structure and reactivities of some halogenated benzene and azabenzene derivatives

All-valence-electron CNDO/2 SCF-MO calculations have been performed on some fluoro, chloro and methoxy derivatives of benzene, pyridine and the diazines. The computed charge distributions and the dipole moments are discussed and compared with available experimental data. The relative basicities of the derivatives of pyridine and the diazines have been investigated and an attempt made to correlate the results with known pK_a values. Localisation energies for the nucleophilic substitution of halogen by methoxide ion have also been calculated and the predicted relative reactivities compared with experimental observations, where these were available.     

Hexasubstituted Benzenes with Ultrastrong Dipole Moments

Hexasubstituted benzenes have been synthesized with the highest known dipole moments, as determined by dielectric spectroscopy and DFT methods. Based on the preparation of 4,5‐diamino‐3,6‐dibromophthalonitrile, combined with a novel method to synthesize dihydrobenzimidazoles, these benzene derivatives have dipole moments in excess of 10 debye. Such dipole moments are desirable in ferroelectrics, nonlinear optics, and in organic photovoltaics. Structure determination was achieved through single‐crystal X‐ray crystallography, and the optical properties were determined by UV/Vis absorption and fluorescence spectroscopy.     

Novel macrocyclic uracil derivatives: Structure in solid and solution

Isomeric pyrimidinophanes containing uracil moieties and nitrogen atoms in bridges have been synthesized and characterized by a variety of methods both in solid and in solution. Unambiguous assignment of mutual arrangement of C(4)_pyrO groups at different pyrimidine rings in isomers is made by X-ray diffraction. In solutions the arrangement of C(4)_pyrO groups of the isomeric pyrimidinophanes results in different dipole moments. Based on dipole moments simulations mutual orientation of uracil units in isomers is analysed. UV-data are discussed in terms of hypo- or hyperchromic effect.     

Organo group VB chemistry: Part VII. On the electric dipole moments of some substituted triarylphosphine derivatives R3PY,with Y = free electron pair,O and S

The dipole moments of a series of quadruply-coordinated phosphorus derivatives of the general formula R₃PY where R = C₆H₅, 4-XC₆H₄, 3-XC₆H₄ (X = F, Cl) and Y = O, S, are reported. This consistent set of data is used to discuss the suitable values for the whole of group moments μ(C₆H₅-P), bond moments μ(X-C₆H₅), coordination bond moments μ(P → Y), valence angle β and conformation angles Φ.     

Triplet state dipole moments of aminobenzonitriles

The triplet state dipole moments mu(T) of a series of 4-amino- and 3-aminobenzonitriles in cyclohexane, benzene, and 1,4-dioxane are recalculated from previously published [J. Phys. Chem. 1992, 96, 10809] time-resolved microwave conductivity data, on the basis of newly measured intersystem crossing yields. For 4-(dimethylamino)benzonitrile (DMABN), the following values are now determined for mu(T): 8.3 D (cyclohexane), 8.9 D (benzene), and 9.7 D (1,4-dioxane), as compared with the previously reported dipole moment of 12 D for the first and the last solvent. With the other aminobenzonitriles, similar mu(T) data are obtained, between 6.9 D for 4-aminobenzonitrile (ABN) in n-hexane and 10.0 D for 4-(di-n-decylamino)benzonitrile (DDABN) in 1,4-dioxane. The increase of mu(T) observed for all aminobenzonitriles when going from cyclohexane via benzene to 1,4-dioxane may indicate that their triplet dipole moments become larger with increasing solvent polarity. The present mu(T) of DMABN, between 8.3 and 9.7 D, although larger than the ground state dipole moment mu(0) of 6.6 D, is somewhat smaller than that of the locally excited (LE) state (9.9 D) but considerably smaller than the dipole moment of the intramolecular charge transfer (ICT) state (17 D). By comparing these mu(X) data with the frequency (CN) of the cyano vibration in each state, it appears that at least for DMABN in the triplet state (CN) is not a reliable indication of the extent of charge transfer as compared with the other states S0, LE, and ICT.     

Dipole moments of nitro derivatives of indazole

The dipole moments of indazole and a number of its nitro derivatives in dioxane solution were measured. The directions of the vectors of the dipole moments of indazole in the tautomeric 1-H and 2-H forms were determined to solve the problems associated with the tautomerism and the position of the substituents in the molecules. The experimental values of the dipole moments, of the investigated compounds were compared with the values calculated via a vector additive scheme. As a result it was shown that the mononitro and dinitro derivatives of indazole exist primarily in the 1-H tautomeric form, whereas the trinitro and tetranitroindazoles exist in the 2-H tautomeric form. The ratios of the isomers were calculated for compounds that have dipole moments intermediate between the 1-H and 2-H forms. A deviation between the experimental and calculated dipole moments of compounds containing two nitro groups in the ortho position was established; this may be explained by disruption of the coplanarity of the molecules or by deviation from additivity because of the introduction of strong electron-acceptor substituents.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1669–1671, December, 1977.     

Solvent effect on tautomeric equilibria and dipole moments of some alpha substituted benzoylacetanilides

Based on dipole moment measurements, a study was made of the keto-enol equilibria of five alpha substituted benzoylacetanilide derivatives in two non-polar and one polar solvents at 30°C. The enol mole fractions of the compounds investigated were determined by analysis of their ultraviolet spectra. The same type of study was extended to salicylanilide for its resemblance to the enol tautomer of these compounds. A deduction of the most probable configurations of the keto and enol forms was tentatively made through a comparison between the dipole moments of some suggested configurations for these forms of benzoylacetanilide and those measured in benzene for the alpha methyl benzoylacetanilide and salicylanilide, respectively. These inferred configurations were used to calculate the dipole moments of both tautomers of the other compounds, and consequently, the dipole moment values of their tautomeric mixtures in dioxane and chloroform. A comparison between the dipole moment values of their tautomeric mixtures in dioxane and chloroform. A comparison between the dipole moment values calculated in this way and those measured indicated a solute-solvent interaction with dioxane, which chloroform behaved as a non-interacting solvent.     

Molecular Orbital Calculations on 2, 4, 6-Trinitrotoluene

Three ab initio calculations (HF/6-3IG, HF/6-3IG*, and HF/6-3IG**) on 2,4,6-trinitrotoluene were made, The results compare well with xray data, except dihedral angles of NO₂ relative to the plane of the benzene ring. The deviations are attributed to packing forces and steric effects in the crystal. The most stable structure was a torsional angle 10° of the methyl top with the benzene ring, unlike toluene. The rotational barriers of the methyl top and the 4-nitro group are small. Hydrogen bonding, dipole moments and total atomic charges arc calculated.     

Dipole moment,isomerization, and intramolecular hydrogen bond in some benzene derivatives (computations)

The dipole moment vectors of some benzene monoderivatives are calculated to test the method used in an attempt to explain the dipole moments of o-, m- and p-substituted phenols (X-C₆H₄-OH; X = -OH, -F, -NO₂, -CHO, -COOH, -CH₃). The vectors seem to include contributions from a mixture of planar conformers whose populations obey a Boltzmann-type distribution. The calculated intramolecular interaction moment is proposed as a simple criterion indicating the strength of the intramolecular hydrogen bond in o-phenols.     

Differentiating action of solvents on the strength of CH-acids

The reasons for the differentiating action of solvents on the relative kinetic acidity of CH-acids (hydrocarbons and their derivatives), determined by measuring the kinetics of the isotopic exchange of hydrogen, have been examined. The effect is associated with deviations from Brönsted's relationship, which depend on the characteristic features of the structure and differences in the solvation of the acids, their anions, or the transition states of the reactions is which the acids are taking part. It has been shown that the differentiating effect is observed for a large number of substances with different structures (aliphatic-aromatic hydrocarbons, benzene derivatives, and five-membered heterocyclic compounds and their derivatives) when the relative kinetic acidities of the substances in solvents belonging to different groups are compared. One group includes t-butyl alcohol, liquid ammonia, and cyclohexylamine, and the second includes dimethyl sulfoxide and methyldi(methoxymethyl)phospine oxide. The solvents differ in their proton-donating properties and in the values of their dipole moments and dielectric constants. Different values are obtained for the deuterium—tritium kinetic isotopic effects for reactions carried out in solvents belonging to different groups.     

Molecular conformation of pyridinic aromatic esters : I. Electronic absorption spectra and dipole moments of the phenylic and pyridin1c esters of the benzoic- and pyridine-carboxylic acids

Nine pyridine aromatic ester derivatives have been prepared, six of which were previously unreported. The electronic absorption spectra have been determined in methanol and cyclohexane solution and band assignment is briefly discussed. The dipole moments were measured in benzene at 25 °C and are compared with values calculated using a vectorial model. Comparison suggests the most probable conformation of derivatives which have the nitrogen either in ortho or meta position in one or both rings of the molecule.