A molecular orbital treatment of the electronic spectra of some tryptamines

The electronic absorption spectra of tryptamine, 5-methoxytryptamine, 6-fluorotryptamine, N-acetyl-5-hydroxytryptamine, gramine, and melatonin were investigated. The observed transitions were π–π*, and the values of band maxima and intensity reflected an extent of interaction between the indole ring and the alkylamine side chain. Molecular orbital calculations at the level of INDO /S –CI were performed on all the studied molecules. State functions and transition energies were calculated. The correspondence between the experimental and theoretical results was satisfactory. © 1992 John Wiley & Sons, Inc.     

The electronic absorption spectra of pyridine azides, solvent-solute interaction

The electronic absorption spectra of: 2-, 3-, and 4-azidopyridines have been investigated in a wide variety of polar and non-polar solvents. According to Onsager model, the studied spectra indicate that the orientation polarization of solvent dipoles affects the electronic spectrum much stronger than the induction polarization of solvent dipoles. The effect of solvent dipole moment predominates that of solvent refractive index in determining the values of band maxima of an electronic spectrum. The spectra of azidopyridines differ basically from these of pyridine or mono-substituted pyridine. Results at hand indicate that the azide group perturbs the pyridine ring in the case of 3-azidopyridine much more than it does in the case of 2-azidopyridine. This result agrees with the predictions of the resonance theory. Although the equilibrium <==> azide tetrazole is well known, yet the observed spectra prove that such an equilibrium does not exist at the studied conditions. The spectra of the studied azidopyridines are characterized by the existence of overlapping transitions. Gaussian analysis is used to obtain nice, resolved spectra. All the observed bands correspond to pi-->pi* transitions, n-->pi* may be overlapped with the stronger pi-->pi* ones.     

Theoretical investigation of the decomposition of acyl azides: Molecular orbital treatment

The thermal decomposition of formyl, acetyl, and benzoyl azides to the corresponding isocyanate and nitrogen has been treated theoretically using ab initio molecular orbital calculations at the Møller–Plesset type 2 (MP2)(full)/6‐31G* level. The reaction is stimulated by elongation of N N bond and is followed until the formation of the isocyanate and expulsion of nitrogen. The decomposition of formyl azide proved to be a concerted one‐step reaction without the formation of a nitrene intermediate. In contrast, the conversion of both acetyl and benzoyl azides to the corresponding isocyanate and nitrogen is a two‐step reaction, and a nitrene intermediate is formed. One transition state is located and identified during the course of the conversion of formyl azide, but two transition states are located and identified during the course of the conversion of acetyl and benzoyl azides. The thermodynamic functions, ΔE_r and ΔH_r, of the studied reactions are calculated. The results predict that the ease of conversion of the acyl azide to the isocyanate and nitrogen goes in the order: formyl azide > acetyl azide > benzoyl azide. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Electronic absorption spectra of some nicotinamides and nicotinic acids. Molecular orbital treatment

The electronic absorption spectra of the position isomers nicotinamide and isonicotinamide, nicotinic acid, and isonicotinic acid were investigated, together with the spectra of thionicotinamide, N-methyl nicotinamide and nicotinic acid N oxide. Apparent differences in the spectra of the position isomers were interpreted in terms of the torsion angle between the planes of the molecule, the height of the barrier to internal rotation, and the results of molecular orbital (MO) calculations. The largest perturbation effect was observed in the case of thionicotinamide whereas the smallest effect was observed in the case of nicotinic acid N oxide. MO calculations have indicated the existence of overlapping transitions. The observed transitions proved to be π-π* transitions, none of the n-π* was observed. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64 : 689–701, 1997     

Mass spectra of some sulphonyl azides

A series of sulphonyl azides decomposed upon electron-impact with loss of azide radical. This fragmentation was in marked contrast to the photochemical behaviour of the compounds. An interesting ortho-extrusion process occurred in the mass spectrum of 2,4,6-triisopropylbenzenesulphonyl azide.     

The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.     

Vibrational and electronic absorption spectra of some trisubstituted benzenes

Despite our studies on the Π* ← Π systems in some trisubstituted benzenes, the available data are limited, especially concerning two heavy and one light or one heavy and two light substituents. We therefore report and interpret the vapour phase electronic absorption, infrared and Raman spectra of 1-bromo-2,4-difluoro-, 1-bromo-3,4-difluoro-, 1-bromo-3,5-difluoro-, 1,3-dichloro-2-fluoro- and 1,3-dichloro-4-fluoro-benzenes.     

Infrared and electronic absorption spectra of some trisubstituted benzenes

The electronic absorption spectra of 2,3-, 2,4-, 2,5-, 2,6- and 3,4-difluorobenzonitriles, 3,4-difluoroaniline and 3,4-difluoroanisole in the ultraviolet region in vapour phase have been recorded on medium quartz and Hilger large quartz spectrographs and on a Hitachi model 150-20 UV-VIS ratio recording spectrophotometer. All the molecules investigated have exhibited two π* ← π band systems corresponding to ¹B_2u ← ¹A_1g (λ2600 Å) and ¹B_1u ← ¹A_1g (λ2100 Å) systems of benzene. The infrared absorption spectra of all the molecules studied have also been recorded and analysed. These infrared data have been taken to help analyse the u.v. spectra of the molecules studied.     

Studies on the electronic absorption spectra of some monoazo derivatives

The electronic absorption spectra of a series of azo dye compounds containing -Cl, -SO3H and -OH groups were recorded in twenty one solvents with different polarities. The solvents were selected to cover a wide range of parameters (refractive index, dielectric constant and hydrogen bonding ability). Solvent-induced effect on the maximum absorption band frequencies of azo dye compounds are described in terms of solute-solvent interactions. Multiple linear regression equation for νmax was performed using polarizability-polarity parameters and Kamlet-Taft solvatochromic parameters. The results help to assign the solvent-solute interactions and the solvatochromic potential of the investigated compounds. It is concluded that the electronic of the substituent and hydrogen bonding acceptor ability of the solvents are the major factors to the interaction in solutions.     

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.

     

Preparation of polyfunctional acyl azides

A general synthesis of acyl azides from the corresponding N-acyl benzotriazoles is described. The procedure affords acyl azides in good yields and avoids the use of acid activators and NO+ equivalents typically employed to synthesize these compounds from acid chlorides and hydrazides, respectively.     

The electronic absorption spectra of nine substituted phenols

A new technique—photoacoustic spectroscopy (PAS)—is used for the first time to record the u.v.-vis spectra of nine substituted phenols. The π-π* electronic transitions analogous to benzene first primary and secondary transitions could be detected from the PAS spectra in comparison with the u.v. solution and vapour phase spectra recorded by conventional methods. Detection of singlet → triplet absorptions from the PAS spectra, with significant intensity, is considered to be an important feature which transitions are in general either not observed or observed with weak intensity, by conventional methods. The analyses show that a few excited state combinations observed in the u.v.-vis PAS spectra compare well with such combinations in the ground state observed in the near i.r. PAS spectra of the molecules.     

Syntheses and electronic absorption spectra of some new penta- and di-methine cyanine dyes

The reaction of a ratio of 1 mole of 5-chloro-3-methyl-1-phenyl pyrazolo-4[2(4)]-dimethine cyanine ( 1a,b ) or (5-chloro-4-formyl-3-methyl-1-phenylpyrazole) 2 with 2 moles of an appropriate 2(4)-heterocyclic quaternary salt afforded the novel pentamethine cyanine dyes ( 3a–3e ). Other reactions of 2 with heterocyclic nitrogen bases, followed by condensation of the newly formed quaternary nitrogen salts ( 4a–4c ) with methoheterocyclic iodide salts provided the corresponding unsymmetrical dimethine cyanine dyes ( 5a–5g ). The new pentamethine and dimethine cyanine dyes were identified by elemental analyses and IR and ¹H-NMR spectral data. The visible absorption spectra of some selected dyes were investigated in pure and mixed solvents as well as in universal buffer solutions. The measure of the variation of absorbance with pH was utilized for the determination of the pK_a values for compounds 3b, 3d, 3e, 5e , and 5f , and these data are discussed. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8 : 259–266, 1997.     

Electronic structure of the peptide linkage. II. A molecular orbital treatment of the electronic spectra of benzohydroxamic acids

The electronic absorption spectra of some benzohydroxamic acids have been analyzed and interpreted using the results of molecular-orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. Comparison between the spectra of benzohydroxamic acids and that of simple amides led to conclusions regarding the structural differences between the two classes of compounds.     

Application of molecular orbital theory to the interpretation of x-ray absorption spectra of platinum complexes

X-ray L_III-absorption edges of platinum in nine octahedral complexes have been recorded using a bent crystal spectrograph. The edge features of the discontinuities have been interpreted with the help of qualitative molecular orbital diagrams. A correlation between the energy separation of the first two absorption maxima and the spectrochemical series of the ligands has been arrived at.     

Solvatochromic behavior of the electronic absorption spectra of some azo derivatives of amino pyridines

The electronic absorption spectra of a series of the entitled eight compounds containing groups with variable electronic characters were recorded. The solvents were selected to cover a wide range of parameters (refractive index, dielectric constant and hydrogen bonding capacity). The electronic transitions are assigned and the solvent induced spectral shifts have been analyzed in relation to the different solute-solvent interaction mechanisms using computational chemistry. The regression analysis is applied for correlation parameters. The phenomenon of tautomerism is explained. The electronic character of the substituent as well as the chemistry of the solvent are the major factors for the solvatochromic behavior.     

Ultraviolet absorption spectra of some alkylchloramines

The ultraviolet spectra of six alkylchloramines are determined between 220 and 320 nm. The wavelengths of maximum absorption are 244 nm (? = 458 l mol^?1 cm^?1) for monochloramine, 252 nm (? = 372 l mol^?1 cm^?1) for methylchloramine, 251 nm (? = 378 l mol^?1 cm^?1) for ethylchloramine, 250 nm (? = 368 l mol^?1 cm^?1) for isopropylchloramine, 264 nm (? = 354 l mol^?1 cm^?1) for dimethylchloramine and 262 nm (? = 315 l mol^?1 cm^?1) for diethylchloramine.     

Switchable electronic coupling in model oligoporphyrin molecular wires examined through the measurement and assignment of electronic absorption spectra

The use of a quinone functionality in the linkage unit of laterally bridged oligoporphyrins as a switch for controlling electronic coupling between the termini is examined. The quinone-bridged bisporphyrin P(2)TA-O(2) was synthesized by condensation of 2 equiv of the dione 2,3-dioxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin with 2,3,5,6-tetraamino-1,4-benzoquinone. The electronic absorption spectra of P(2)TA-O(2) and its conjugated benzenoid analogue P(2)TA are measured and assigned, in conjunction with the spectra of the fragment monomers and porphyrin-bridge compounds. Band homologies and CASPT2 calculations are used to make the assignments. Chemically, the dimer in one case is bridged by a through-conjugated, pi-delocalized 1,4,5,8-tetraazaanthracene molecule. This is shown to display significant inter-porphyrin coupling, with an observed difference in the exciton couplings of the B(x) and B(y) bands being ca. 0.18 eV. However, the other dimer is bridged using a derivative in which the central ring is converted to a cross-conjugated, pi-localized quinonoid form; this molecule displays no observable inter-porphyrin coupling. This scenario provides a paradigm for the use of molecular electronic devices in sensing, control, and high-capacity relatively low-speed data storage applications.