P.C. Mishra

Electronic structures and spectra of two antioxidants: uric acid and ascorbic acid

Abstract Electronic absorption and fluorescence spectra of aqueous solutions of two well known antioxidants, uric acid and ascorbic acid (vitamin C), have been studied at different pH. The observed spectra have been interpreted in terms of neutral and anionic forms of the molecules with the help of molecular orbital calculations. The N 3 site of uric acid has been shown to be the most acidic. Fluorescence of uric acid seems to originate from an anion of the molecule in a wide pH range. Around pH 3, both the neutral and anionic forms of ascorbic acid appear to be present in aqueous solutions. In aqueous media, ascorbic acid appears to get converted easily to its dehydro form and this conversion does not seem to be reversible. An anion of dehydroascorbic acid seems to be formed on heating dehydroascorbic acid in aqueous solutions.

Interaction of guanine with oxygen and further evidence for its asymmetric double-well potential surfaces

Abstract Prolonged irradiation of guanine in aqueous solution at its absorption peak wavelengths has been shown to cause its complexation with dissolved oxygen, which eventually results in the formation of a stable product, possibly an oxide of the molecule. No dye was used to effect the reaction. It has been shown that the 1 ∑ + g state of oxygen may be involved in the process and a large number of bands belonging to its 1 ∑ + g ue5f8 3 ∑ − g system have been observed in absorption. The complexation of guanine with oxygen and the following reaction are quite slow and occur over a period of several days. The double-well nature of the ground state potential surface of guanine seems to play an important role in this process and the molecule oscillates back and forth between the two minima. No information is available as to the role of the 1 Δ g excited state of oxygen in the events observed here. Additional evidence in favour of the double-well nature of the ground and excited state potential surfaces of guanine is presented.

CNDO-CI study of electronic structure and molecular geometry in ground and excited states

The modified CNDO method of Del Bene and Jaffe (CNDO/s) and the CNDO/2 and INDO methods due to Pople have been used to study the electronic structure and geometry of mono- and p-difluorobenzenes in the ground and the first excited singlet states. Bond orders, hybrid populations and transition energies together with the corresponding oscillator strengths have been given. Ground and excited state rotational constants and changes in bond lengths in the excited state as compared to the ground state have also been presented and discussed in the light of experimental observations.

Electronic spectra and structures of some biologically important xanthines

Abstract Electronic absorption and fluorescence spectra of aqueous solutions of xanthine, caffeine, theophylline and theobromine have been studied at different pH. The observed spectra have been interpreted in terms of neutral and ionic forms of the molecules with the help of molecular orbital calculations. At neutral and acidic pH, the spectra can be assigned to the corresponding most stable neutral forms, with the exception that the fluorescence of xanthine at acidic pH appears to originate from the lowest singlet excited state of a cation of the molecule. At alkaline pH, xanthine and theophylline exist mainly as their monoanions. In xanthine and theophylline at alkaline pH, fluorescence originates from the lowest singlet excited state of the corresponding anion. However, in caffeine and theobromine, even at alkaline pH, fluorescence belongs to the neutral species. On the whole, the properties of xanthine are quite different from those of the methyl xanthines.

Vibrational structures and intensity distributions in the electronic absorption spectra of nucleic acid bases: evidence for non-planarity of guanine

Abstract Electronic absorption spectra of uracil, thymine, cytosine, adenine and guanine in the 260 nm region have been studied in dilute aqueous solutions. Vibronic features of the spectra have been resolved and compared with those of benzene. It has been shown that the spectra of the bases arise from a single electronic transition in each case. This is particularly important for guanine since in this case a strong dip in intesity is observed around 260 nm and the presence of two systems has been suggested by other authors. The dip in intensity has been explained in this work using the Franck—Condon principle in terms of non-planarity, geomatrical flexibility of the molecule at the CC double bond where the two rings join and asymmetric double-well nature of the ground and excited state potential surfaces. This model has been confirmed by demonstrating that the dip in intensity does not appear when the sample is in thin film form wherein flexibility of the molecule would be lost. An analysis of substitution effects on the spectra of purine, 2-aminopurine, hypoxanthine and guanine also supports the model.

Complexation of guanine with dissolved oxygen in solution and study of excited-state lifetime

Abstract It has been shown that guanine forms a complex with the dissolved oxygen in a solution even without ultraviolet (UV) irradiation, although, if used, UV irradiation can accelerate the complexation due to the formation of singlet excited oxygen. Triplet-singlet transitions in guanine become allowed due to its complexation with oxygen. Electronic excitations of the complex cause jumping of the guanine molecule from the deeper well of the ground state potential surface to the shallower well of the same surface. The guanine—oxygen complex shows two new fluorescence peaks besides the 350-nm fluorescence from the first singlet excited state of guanine, at around 400 and 450 nm, which have been explained as originating from the second triplet and first triplet excited states of the molecule, respectively. On repeated irradiation of the guanine solution, the 450-nm fluorescence becomes prominent, while the other two are gradually suppressed. The 450-nm fluorescence has two components, one with a small lifetime (ca. 0.23 ns) and the other with a much longer lifetime (12.6 ns).

Electronic spectra of adenine: interaction with dissolved oxygen in solution, oscillation and intensification of n-π* transition

Abstract The electronic absorption and fluorescence spectra of adenine were studied in aqueous solution at two different pH values, and in the presence and absence of oxygen dissolved in the solution as a function of time. Because of interaction with the dissolved oxygen in the solution, adenine shows a slow oscillation similar to that observed previously for guanine; eventually, the spectrum is significantly modified and a strong absorption band appears near 305 nm which has been interpreted as a n -π* transition in adenine. It has been concluded that, like guanine, adenine has asymmetric double-well potential surfaces in its ground and excited states, but the double-well nature of the potential surfaces is not as prominent as it is in guanine. It has been shown that the rare (protonated) tautomer of adenine, the presence of which at low pH has been suggested previously, fluoresces near 370 nm, while the normal species, under interaction with oxygen, fluoresces near 345 nm from the singlet n -π* excited state.

Excited state dipole moments and geometries of the bases of nucleic acids

Abstract CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μhyb(σ), μch and μπ has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different. Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.

Ultraviolet spectra and structure of the isomeric dihydroxy- and diamino-substituted benzenes

The semi-empirical all-valence-electron SCF-LCAO-MO-CI method in its Del Bene and Jaffe formalism has been applied to the study of the electronic absorption spectra and structure of isomeric dihydroxy- and diamino-substituted benzenes. The calculated π-π* singlet excitation energies and corresponding oscillator strengths have been compared with earlier π-electron results and discussed in the light of experimental findings. Ionization potentials, electron affinities, ground-state atomic orbital populations and net electronic charges of atoms, have also been presented.

Ultraviolet spectra and structure of the isomeric fluoro and chloro toluenes

Abstract The semi-empirical scf-mo-ci method, in its Pariser-Parr-Pople formalism, has been applied to the electronic structure of isomeric chloro and fluoro toluenes. Mullikens hyperconjugative hypothesis for the methyl group was employed. The calculated π-π ★ singlet excitation energies, oscillator strengths and charge densities are discussed in the light of the available experimental data.