Madhulata Shukla

Electronic spectra of adenine and 2-aminopurine: an ab initio study of energy level diagrams of different tautomers in gas phase and aqueous solution

Ground and lowest two singlet excited state geometries of four tautomeric forms (N9H, N7H, N3H and N1H) of each of adenine and 2-aminopurine (2AP) were optimized using an ab initio approach employing a mixed basis set (6-311 + G* on the nitrogen atom of the amino group and 4-31G basis set on the other atoms). Excited states were generated employing configuration interaction involving single electron excitations (CIS). Subsequently, the different species were solvated in water employing the self-consistent reaction field (SCRF) approach along with the corresponding gas phase optimized geometries. Thus the observed absorption and fluorescence spectra of adenine and 2AP have been explained successfully. It is concluded that both the N9H and N7H forms of 2AP would contribute to absorption and fluorescence spectra. Further, the fluorescence of 2AP would be absorbed by its cation in which both the N9 and N7 atoms are protonated, the fluorescence of which can have an anti-Stokes component. Among the different tautomers of adenine, the N9H form would be present dominantly in the ground state in aqueous solutions but the N7H form would be produced by energy transfer and subsequent fluorescence. The N3H form of adenine appears to be responsible for the observed absorption near 300 nm by its solutions intermittently exposed to ultraviolet radiation. The rings of the different species related to 2AP and adenine remain almost planar in the pi-pi* and n-pi* singlet excited states as in the ground state. The pyramidal character of the amino group is usually less in the pi-pi* excited states than that in the corresponding ground or n-pi* excited states. Molecular electrostatic potential (MEP) maps of the molecules provide useful clues regarding phototautomerism.

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.

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.

A Comparative Study of Piperidinium and Imidazolium Based Ionic Liquids: Thermal, Spectroscopic and Theoretical Studies

Ionic liquids (ILs) comprise an extremely broad class of molten salts that are attractive for many practical applications because of their useful combinations of properties [1-3]. The ability to mix and match the cationic and anionic constituents of ILs and functionalize their side chains. These allow amazing tenability of IL properties, including conductivity, viscosi‐ ty, solubility of diverse solutes and miscibility/ immiscibility with a wide range of solvents. [4] Over the past several years, room temperature ILs (RTILs) has generated considerable excitement, as they consist entirely of ions, yet in liquid state and possess minimal vapour pressure. Consequently, ILs can be recycled, thus making synthetic processes less expensive and potentially more efficient and environmentally friendly. Considerable progress has been made using ILs as solvents in the areas of monophasic and biphasic catalysis (homoge‐ neus and heterogeneous).[5-6] The ILs investigated herein provides real practical advantag‐ es over earlier molten salt (high temperature) systems because of their relative insensitivity to air and water. [6-7] A great deal of progress has been made during last five years towards identifying the factors that cause these salts to have low melting points and other useful properties.[8] ILs are subject of intense current interest within the physical chemistry com‐ munity as well. There have been quite a lot of photophysical studies in ionic liquids. [8] The most important properties of ionic liquids are: thermal stability, low vapour pressure, elec‐ tric conductivity, liquid crystal structures, high electro-elasticity, high heat capacity and in‐ flammability properties enable the use of ionic liquids in a wide range of applications, as shown in Figure 1. It is also a suitable solvent for synthesis, [5, 8, 9-12] catalysis [6, 8, 13] and purification. [14-18] It is also used in electrochemical devices and processes, such as re‐ chargeable lithium batteries and electrochemical capacitors, etc.[19] Rechargeable Lithium

Interactions and Transitions in Imidazolium Cation Based Ionic Liquids

Ionic liquids (ILs) raise considerable research interest not only as promising new solvents for the replacement of conventional solvents in synthesis, but also as new liquid materials.[1-2] It appears from the recent breakthroughs that the novel properties of various ILs are of more interest than mere application as ‘green solvent’ in traditional organic chemistry. ILs are now rediscovered to be whole new materials with many wonderful properties, much of it are yet to be discovered. The defining characteristic of ILs is of their constitutions molecular ions as their building blocks as opposed to molecules in the traditional solvents. In other words, ILs or molten salts in general are defined as liquids composed of ions only, either at room temperature or at elevated temperatures (below 100C). ILs are rather unique in the sense that in addition to ionic and covalent interactions, there are relatively weaker interactions such as H-bondings, and π-stacking, which are not commonly found in conventional solvents.[3-4] The nature of the forces in different ILs may however differ from one another and mainly control their physical properties. As the properties of any material depends on the structure of molecules in different phases, it is very important to understand the structural features of ILs in depth. Researchers have paid considerable attention towards pyridinium, imidazolium and pyrrolidinium based ILs in addition to ammonium and phosphonium ILs. Pyridinium based ILs are heterocyclic aromatic compounds proven to have great potential in organic synthesis and biocatalyst. Compared with the imidazolium based ILs, few studies have examined the biodegradability of pyridinium based ones.[5] Due to biodegradable property of these pyridinium and pyrrolidinium ILs, it has been extensively studied.[6-12] On the other hand, pyrrolidinium ILs are mainly involved in dye sensitized solar cell and batteries.[ 11] Since structure plays the vital role for any application, recently detailed x-ray scattering studies have been reported on the pyrrolidinium cations based ILs while varying the length of the alkyl chain attached with the ring.[12] Interestingly, diffraction pattern shows signature of intermediate range ordering similar to that of in imidazolium based ILs. Among all ionic liquids, imidazolium cation based ILs are the most extensively studied ILs, and therefore our discussion will mainly be confined with imidazolium cation based ILs. As of true ‘designer solvent’, it has been observed that a small variation in imidazolium cation (such as increase or decrease in alkyl chain length) alters their physical properties

An unusual way to synthesize 1-nitronaphthalene from 1-amino-5-nitronaphthalene

Reductive removal of amine substituent from aromatic organic molecules constitutes an important process in organic synthesis (March, 1992). Reactions involving such reductive processes are particularly useful in aromatic chemistry because of the strong directing effects associated with amine substituent. Many reagents have been suggested to replace the primary aromatic amino group by hydrogen, though no method is found to be universally applicable. The yields vary considerably with both the method and the amine used. In most of the cases deaminations are carried out using the diazotized amines. While usually the diazonium salts are not isolated, there are reports of stabilized dry diazonium salts, in one case, e.g., diazonium fluoroborate (Mitsuhashi et al., 2000). Generally, diazotization of the primary aromatic amine followed by decomposition in presence of an appropriate reducing agent leads to the desired product. Graham and Roe have used ethanol in presence of zinc powder for different types of diazonium fluoborates (Roe and Graham, 1952) for the deamination. Sodium borohydride and hypophosphorous acid has also been preferred in recent times for the same purpose (Mitsuhashi et al., 2000). In this communication we are reporting for the first time, the deamination from one of the smallest nitropolycyclic aromatic hydrocarbons (NPAHs), 5-