Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra

The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the pi --> pi* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n --> pi* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvent's dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding.     

Synthesis and photochromic properties of indolinospirochromenes with electron-donor substituents in the indoline part of the molecule

1,3,3-Trimethylspiro(indoline-2,2'-[2H]chromenes) with CH₃ and CH₃O groups in the 4–7 positions and NO₂ and CH₃O groups in the 6 and 8 positions were obtained by the reaction of 4,6-, 4,7-, 5,6-, and 6,7-dimethoxy- and 4-methyl-7-methoxy-2-methyleneindolines and 1,3,3,4,5,6,7-heptamethyl-2-methyleneindoline with 3- and 5-nitrosalicylaldehydes and their derivatives. Most of the compounds have photochromic properties. The introduction of electron-donor groups into the indoline fragment of the spirochromene molecules changes the rate of the dark reaction within the limits of one order of magnitude and has a small effect on the position of the long-wave absorption band of the photomerocyanine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1220–1224, September, 1982.     

Effect of substituents on the fluorescence and absorption spectra of coumarins

The absorption and fluorescence emission of substituted coumarins have been studied in different solvents. It is observed that the substituents alter both the absorption and fluorescence emission maxima. These shifts are discussed in terms of the change in mobility of π-electrons caused by the nature and position of the substituent group in the parent compound.     

Effect of dimerization on vibrational spectra of eumelanin precursors

We have synthesized a compound ideally suited to the study of structure-function relationships in eumelanin synthesis. N-methyl-5-hydroxy-6-methoxy-indole (MHMI) has key functional groups strategically placed on the indole framework to hinder binding in the 2, 5, 6 and 7 positions. Thus, the dimer bound exclusively in the 4-4' positions was isolated and characterized. In order to study the difference in vibrational structure between the MHMI monomer and dimer, Raman spectra were acquired of both compounds, as well as indole, indole-2-carboxylic acid and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Peaks were assigned to particular vibrational modes using B3LYP density functional theory calculations, and experimental and theoretical spectra displayed good agreement. Addition of functional groups to either benzene or pyrrole rings in the indole framework impacted vibrational spectra attributed to vibrations in either ring, and in some cases, peaks appearing unchanged between two compounds corresponded to different contributing vibrations. Dimerization resulted in an expected increase in the number of vibrational modes, but not a significant increase in the number of apparent peaks, as several modes frequently contributed to an individual observed peak. Comparison of spectral features of the monomer and dimer provides insight into eumelanin photochemistry, but final conclusions depend on the planarity of oligomeric structure in vivo.     

The vibrational spectra and conformational properties of liquid dimethyl oxalate

The infrared spectra of dimethyl oxalate and dimethyl oxalate-d₆ have been recorded in the liquid phase. These spectra are compared with both the infrared and Raman spectra recorded in solid phase. It is shown that dimethyl oxalate exists as a conformational equilibrium mixture in the liquid phase. The most stable conformer is that present exclusively in the crystal, the planar, cis-ester C_2h structure. The second conformer is probably one formed by rotation about the carbonyl carbon-oxygen bonds of the molecule.     

Electronic absorption and vibrational spectra and nonlinear optical properties of L-valinium succinate

FT-Raman and FTIR techniques have been applied to investigate the potential nonlinear optical material l-valinium succinate. A detailed interpretation of the vibrational spectra was carried out with the aid of potential energy distribution analysis. Density functional theory is applied to explore the nonlinear optical properties of the molecule. Good consistency is found between the calculated results and the experimental data for IR and Raman spectra. The natural bond orbital analysis confirms the occurrence of strong intramolecular N-H?O hydrogen bonding.     

The vibrational spectra and crystallographic properties of CsPF6

Indications are that CsPF₆(I) at ambient conditions is cubic with a possible space group of Fm3m-O⁵_h. A slight distortion of the unit cell cannot, however, be ruled out. Assuming Fm3m symmetry the Raman spectra of CsPF₆(I) are consistent with a disordered model in which the PF^?₆ ions are tilted away from the crystallographic axes. The phase transition which occurs below 90 K in CsPF₆ is reflected in the vibrational spectra and further significant changes occur below 60 K particularly in the Raman bands. It is not yet clear whether these changes represent the establishment of long-range order or whether a further phase of CsPF₆ exists below 60 K. A possible structure for CsPF₆ at very low temperatures is discussed.     

Effect of solvents and substituents on the distribution coefficients of alkyl-substituted β-diketones and their copper chelates

The distribution ratios of alkyl-substituted β-diketone derivatives and their copper(II) chelates between water and a number of common water-immiscible solvents were investigated, in order to establish the effect of substituents and solvents on the extraction. A plot of the logarithms of the distribution coefficients of the β-diketones and of their copper chelates as a function of the number of carbon atoms in the molecule, gave a linear relationship. The relationship between the distribution coefficients of the copper chelate (P_CuA2) and those of β-diketone (P_HA) was shown to be log P_CuA2=2 log P_HA +const.     

Effect of substituents on electron energy redistribution in uracil derivatives and their ionization in polar solvents

It is assumed that the chemical reactivity may be described in terms of the appropriate energy density calculated for each atom. Indices of energy density for deprotonization of uracil, 5-nitro- and 5-aminouracil are presented. The results of a quantum chemical calculation of the deprotonization energy are in qualitative agreement with experimental pK and ΔH values.     

Effect of substituents on the permeation properties of polyamide membranes

Aromatic polyamides, designed for evaluation as gas separation membranes, were processed into dense films, whose properties were measured with special emphasis on their mechanical and thermal properties. The polymers had been synthesized from monomers bearing side substituents, such as methyl, iso-propyl or tert-butyl, and various hinge-like connecting linkages of p-phenylene moieties, which yielded amorphous aromatic polyamides, with improved solubility, high glass transition temperatures (over 250 °C) and excellent mechanical properties (tensile strength about 100 MPa, and moduli about 2.0 GPa). The permeability of the polymer films were investigated using helium, oxygen, nitrogen, carbon dioxide and methane. Gas permeability typically increased with increasing free volume, and, in general, free volume could be related to the chemical structure. The analysis of the transport parameters (permeability, diffusivity and solubility coefficients) as a function of the chemical structure, confirmed the predominant role of the side substituents and of the linking groups connecting phenylene units on the permeation properties. Besides, a molecular modelling study carried out via computational chemistry, made it clear that an acceptable theoretical explanation can be given of how the nature of hinge groups between aromatic rings can affect torsional mobility and gas diffusion of aromatic polyamides.

The experimental aromatic polyamides of this report, as a whole, showed a favourable combination of permeability–selectivity, better than that of conventional polyamides and that of most engineering thermoplastics, confirming the hypothesis that the incorporation of side bulky substituents is a convenient approach to hinder the inherently efficient chain packing of polyamides.     

Prediction of vibrational spectra from microwave spectra

Explicit and simple relations are derived for the Kivelson-Wilson parameters which can directly generate the vibrational spectrum of XY₂ bent type molecules. These relations are also shown to generate the above parameters for the different isotopes of a parent molecule. These predictions are verified in the case of some molecules of the above symmetry for which experimental microwave studies have been made.     

Effect of fluorine substituents on the properties of nylon copolymers

Nylon 610/65F and nylon 610/65 copolymers were prepared by melt polycondensation from the diethylester of sebacic acid (10) and hexafluoroglutaric acid (5F) or glutaric acid (5) with hexamethylene diamine (6). Reduced specific viscosities of nylon 610/65F were lower than those of nylon 610/65. The crystallinity measured by WAXS and the melting point measured by DTA were depressed by the copolymerization. The melting point depression of nylon 610/65F was much larger than that of nylon 610/65. The thermal decomposition temperature measured by TG was also depressed significantly for nylon 610/65F. The residue at 600°C was increased by the fluorine substitution. The copolymers with higher fluorine content became flame retardant. The solubility was appreciably affected by the fluorine substitution. The contact angle for nylon 610/65F varied in a complicated manner with increasing 65F content and tended to be higher than the corresponding nylon 610/65.     

Effect of solvents on the electronic absorption spectra of some substituted diarylformazans

Summary The electronic absorption spectra of some substituted diarylformazans in organic solvents of varying polarities are studied. The absorption bands are assigned to the corresponding electronic transitions in the molecules. The solvent effects on the intramolecular charge transfer bands are discussed using various solvent parameters. The charge transfer nature of the bands was also confirmed by the solvent effects on the absorption spectra of some formazans. Molecular orbital calculations using MNDO-PM3 were performed and indicate atrans configuration as the favoured geometry.

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Lösungsmitteleinflüsse auf die Absorptionsspektren einiger substituierter Diarylformazane

Zusammenfassung Die Absorptionsspektren einiger substituierter Diarylformazane werden in organischen Lösungsmitteln unterschiedlicher Polarität untersucht. Die Absorptionsbanden werden den entsprechenden Elekronenübergängen in den Molekülen zugeordnet. Die Lösungsmitteleffekte auf die intramolekularencharge-transfer-Banden werden unter Verwendung verschiedener Lösungsmittelparameter diskutiert. Diecharge-transfer-Natur der Banden wurde durch den Einfluß der Lösungsmittel auf die Absorptionsspektren einiger Formazane bestätigt. MO-Rechnungen (MNDO-PM3) wurden durchgeführt und bestätigen einetrans-Konfiguration als bevorzugte Geometrie.