The fluorescence spectra of the title compound have been measured in ethanol and in aqueous solutions of different p H . In ethanol the neutral molecule's fluorescence (at 360 nm) is observed, whereas in aqueous solutions emission occurs from the anion (at 428 nm) and from a " tautomer ic " species (at 460 nm), but not from the neutral molecule. An aqueous solution at p H 3, which contains abou...

A thorough analysis of the single and double proton transfer and the internal rotations of neutral indigo and its dianionic leucoindigo form has been performed for the ground and first singlet excited electronic states using, respectively, DFT and TDDFT state-of-the-art methods. Our theoretical analysis discloses that the diketo isomer is the most stable one in the ground state of indigo but no...

The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DF...

bonds [either N–H···O (keto-amine tautomer) or N···H–O (phenol-imine tautomer)] can exist. It is claimed that the hydrogen-bond type depends neither on the stereochemistry of the molecule nor on the sort of substituent bonded to the N imino atom, but on the kind of aldehyde used.1 In the solid state, it is also claimed that only an intramolecular N–H···O type hydrogen bond is present in naphtha...

In his book The Double Helix, James Watson describes an early proposal for DNA structure in which equal bases pair with each other. He writes: ‘‘My scheme was torn to shreds the following noon. Against me was the awkward chemical fact that I had chosen the wrong tautomeric forms for guanine and thymine.’’ This fact was pointed out to him by the American crystallographer Jerry Donahue who ‘‘ . ....

The title compound, C7H14N2Si, crystallizes in a tetra-gonal space group and exists as an N-H⋯N hydrogen-bonded tetra-mer, formed around the crystallographic fourfold rotoinversion axis. The mol-ecular identity is clearly the 5-tri-methyl-silyl-3-methyl-1H-pyrazole tautomer and the structure is isomorphous with that of 5-tert-butyl-3-methyl-1H-pyrazole [Foces-Foces & Trofimenko (2001 ▶). Acta C...

Schiff-base macrocycles that combine keto-enamine and enol-imine tautomers within the same ring were synthesized and characterized. These new macrocycles form host-guest complexes with organic cations and unexpectedly form a lyotropic liquid crystal in organic solvents such as chloroform and toluene.

For molecules with mobile hydrogen atoms, the result of quantitative structure-activity relationships (QSARs) as well as quantum chemical geometry optimisation depends on the position of the respective hydrogen atoms. Thus to obtain reliable results, tautomerism needs to be taken into account. With our in-house software ChemProp, we set up four different kinds of tautomerism and generated the t...

A method is presented for assigning protonation states and hydrogen atom geometry to macromolecular structures. Tautomer and ionization state (including metal ligation) are taken into account during a large scale free energy optimization. This discrete optimization is performed with a Unary Quadratic Optimization procedure. The thermodynamic theory and the results of several computational valid...