Idowu Iweibo

Electronic absorption spectra and structures of aniline and its 4-chloro, pentafluoro and pentachloro derivatives

Abstract The relative sensitivities of oscillator strengths and transition energies in an electronic band system to solvent and substitutent perturbations have allowed the assignment of the electronic bands in pentachloroaniline, pentafluoroaniline, 4-chloroaniline and aniline. While the 1 L b , 1 L a and 1 B b transitions are distinct in pentachloroaniline, the 1 L a and 1 B b transitions appear to be mixed in pentafluoroaniline, 4-chloroaniline or aniline. In all these compounds, except aniline, the 1 L b transition shows vibrational bands which are most distinct in nonpolar solvents. Analysis of the vibrational contours and Franck—Condon envelope showed changes in equilibrium nuclear configuration as the origin of the vibrational bands and that in pentachloroaniline and pentafluoroaniline different vibrational modes are responsible for intensity borrowing characteristic of the 1 L b transition. Determination of excited state dipole moment relative to ground states by solvent shift of the transition energy of an electronic band reveals that the excited states of all transitions have more charged character, the charge transfer character of the 1 L a transition being dominant.

Protein fluorescence and electronic energy transfer in the determination of molecular dimensions and rotational relaxation times of native and coenzyme-bound horse liver alcohol dehydrogenase

The intrinsic fluorescence lifetimes of horse liver alcohol dehydrogenase (EC 1.1.1.1) and pig heart isocitrate dehydrogenase (EC 1.1.1.42) have been determined to be 5.36 ns and 4.84 ns, respectively. When reduced coenzyme is bound, the fluorescence lifetime of alcohol dehydrogenase is reduced to 4.98 ns while that of isocitrate dehydrogenase remains unchanged. Oxidized coenzymes have no effect on fluorescence lifetimes of alcohol and isocitrate dehydrogenases. This virtual constancy of protein fluorescence lifetimes has allowed the conclusion to be reached that in protein-ligand complexes with equilibrium constants in the range of 10(4)-10(6) M(-1), the static mode of quenching is substantial. The observation of resonance energy transfer in alcohol dehydrogenase-NADH complex facilitates the determination of the distance between tryptophan and the reduced nicotinamide ring involved in the transfer as 30.6 A, compared to the effective molecular radius of 36.2 A for alcohol dehydrogenase. The increased rotational relaxation times of coenzyme-bound alcohol dehydrogenase relative to the unliganded form (sigmah = 72 ns) indicate in this protein structural fluctuations occurring in the time range of nanoseconds.

Excited state electric dipole polarizabilities and moments by solvent spectral frequency shifts: Aniline, phenol and naphthalene

Abstract The solvent spectral frequency shift theory of A be in its rigorous or unqualified form has been used to determine the electric dipole polarizabilities and moments of some of the excited electronic states of aniline, phenol and naphthalene. The results of the present analysis show the internal consistency of A be s solvatochromic method and are largely in reasonable agreement with those determined by means of electro-optical measurements and/or molecular orbital calculations.

Vibronic absorption spectra of acenaphthene in solution and its excited state electric dipole moments and polarizabilities

Abstract The vibronic spectra of acenaphthene in solution have been studied in detail in the region 27778–50000 cm−1. A vibronic analysis of the two longest-wavelength absorption bands was made to reveal the vibrational modes that contribute to the enhancement of the intensities of these bands. The oscillator strengths of the various electronic transitions and the electric dipole moments and polarizabilities of several excited states were determined, the latter two by the solvent spectral frequency shift method.

The theory of electronic intensity in solution or condensed media

Based on microscopic and macroscopic models for the description of the electronic intensity of an ensemble of absorbing molecules in solution or condensed media, rigorous and general expressions have been derived for the integral Einstein B coefficient and the oscillator strength by means of the classical and the quantum mechanical methods. The two approaches are shown to yield in all essential points the same results. The present expressions are formulated to accommodate various field models and molecular shapes and to permit the distinction and/or selection of a model that best describes a spectroscopic or molecular property or the physical condition of the molecular ensemble. The merit of the present description relative to some previous derivations is examined.

Solvent Perturbation of the Electronic Intensity of Solvated Absorbing Molecules

In this paper, we present a general expression originating from quantum-mechanical perturbation treatment of electronic intensities and Hamiltonian operators for the system (HA and HB) of an absorber and a perturber respectively. The expression is related to the Longuet-Higgins’ definition of solute-solvent interaction and fitted into linear regression mode for the determination of transition polarizabilities of 9H-xanthene, 9H-xathone and 9H-xanthione. The result conforms to those earlier obtained when all possible interaction modes are considered.

Vibrational spectra and torsional barrier of 2,4,6-triiodophenol

Abstract The i.r. and Raman spectra of 2,4,6-triiodophenol (s-TIP) have been recorded and assignments proposed for the frequencies of the normal modes. Some absorption bands useful for characterizing symmetrically-trisubstituted halophenols have also been identified. In CCl 4 solution, no evidence could be found in favour of self-association of s-TIP molecules. The barrier height hindering the rotation of the OH group about the phenyl-O bond was estimated to be 2068±20 cm −1 .