Fluorescence and absorption spectra of 10 -4 M thymine in aqueous solutions at different pH values are reported at room temperature. The shape of the fluorescence spectrum is found to change when the excitation wavelength is varied. The absorption spectra for the aqueous pH 4-8 thymine solutions in the range 240-300 nm are believed to include the diketo tautomer T1 and the enol-keto tautomer T2. Steady-state fluorescence spectra and ab initio calculations support the existence of these neutral species in their ground state. The lifetimes of the excited states of the T1 and T2 tautomers are 0.72 and 3.87 ns, respectively. Aqueous thymine solutions with pH values below pH 3 contain a protonated enol-keto form of T2 in addition to the major diketotautomer T1 in its neutral form. The lifetime of the protonated enol-keto form is 4.2 ns. On the other hand, thymine solutions with pH values above pH 8 contain mixtures of the neutral forms of T1 and T2 as well as their monoanioic forms. However, aqueous thymine solutions at pH values above pH 11 contain a 1:1 equilibrium mixture of the monoanionic forms 1-HTand 3-HT -. The lifetimes for these monoanionic forms are expected to be shorter than 0.05 ns.

Level III (case control study).

An ESR study of interactions between the nonmesomorphic nitroxide spin probe solutes PD-tempone (PDT) and tempo-palmitate (TP) and homologues of the p-n-alkyl-p′-cyanobiphenyl series of liquid-crystals has been carried out. The characteristics of the ESR spectra obtained show that the constraint for rotation about the x axis in normal liquids is reinforced for TP and relaxed for PDT in liquid crystals to the extent of making PDT switch to y axis rotation in liquid crystals. Evidence for such reinforcement/relaxation is provided by large and small hyperfine splitting, a, values for TP and PDT, respectively, in liquid-crystal solvents.These a values were rationalized on the basis of the nature of solute and solvent structures and interactions. This study also demonstrates that the g factor of the probe is sensitive to the structural and conformational characteristics of the liquid-crystal solvent molecules with g increasing as the degree of out of planarity of the benzene rings in their core increases and exhibiting an odd-even effect that is attributable to the polarizabilities of their alkyl tails. Finally, our results cast doubt on the generally held view that the greater the structural similarity between the probe molecule and the liquid-crystal solvent molecules the better is the probe at reflecting the liquid-crystalline properties of the solvent.

An extensive study of interactions between nitroxide spin probe solutes and the liquid crystalline solvent 4-cyano-4'-n-pentylbiphenyl(5CB) has been carried out. The visual results show that, at the spin probe concentrations used in the ESR experiment, the spin probe does not affect the sharpness of the nematic-to-isotropic transition temperature. As expected the ESR results show that both the hyperfine splitting a and the g-value depend on the ordering matrix parameter. Moreover, hyperfine splitting values were used to arrive a t values for the order parameter of PD-Tempone and solvent potential parameters at different temperatures. The line-width analysis of the ESR spectra of PD-Tempone in the nematic and isotropic phases of 5CB was carried out to examine the different factors affecting the relaxation mechanism of PD-Tempone. Our analysis shows that anisotropy in both the nematic and isotropic phases, orientational fluctuation on either side of the nematic-to-isotropic transition, and the ordering potential in the nematic and crystal phases are the main factors influencing the relaxation mechanism of PD-Tempone.

No abstract