José Luis G. De Paz

On the Molecular Structure and UV/vis Spectroscopic Properties of the Solvatochromic and Thermochromic Pyridinium-N-Phenolate Betaine Dye B30

Quantum chemical calculations as well as vis absorption and fluorescence measurements of the pyridinium-N-phenolate betaine dye B30, dissolved in 1-chlorobutane at temperatures between 343 and 77 K, shed more light on the solvatochromism, thermosolvatochromism, and photophysical behavior of this probe dye, formerly used to establish an empirical scale of solvent polarity, called E(T)(30) or E(T)(N) scale. A new calculated gas-phase E(T)(30) value is reported. Complementary to recent work of Kharlanov and Rettig (J. Phys. Chem. A 2009, 113, 10693-10703), it is shown that fluorescence of B30 in 1-chlorobutane solution is observable already at temperatures just below the solvents melting point and not only at 77 K. Analogous to increasing solvent polarity, decreasing solvent temperature leads to a large hypsochromic shift of the vis absorption band of B30, dissolved in 1-chlorobutane (Deltalambda = -245 nm from 797 nm at 343 K to 552 nm at 77 K). This thermosolvatochromism can be easily seen: the solution color changes from greenish yellow (343 K) to magenta-violet (77 K).

An AB initio comparative study of the electronic properties of sulfonamides and amides

Abstract A comparative ab initio study of amides and sulfonamides has been carried out using minimal, extended and polarization basis sets. Rotational barriers and tautomerism are discussed comparatively. The two main conclusions of this study are the absence in sulfonamides of a conjugation of the type present in planar amides and the relative insensitivity of the total energy to nitrogen hybridization in sulfonamides in contrast with amides.

Importance of aromaticity on the relative stabilities of indazole annular tautomers: an ab initio study

MP2-6-31G** calculations on 1H- and 2H-indazole annular tautomers show that the 1Htautomer is more stable than the 2H one by 3.6 kcal mol–1. In the case of 1H-indazole, the excellence of MP2-6-31G** results is confirmed by the fact that its microwave rotational constants are reproduced with great accuracy. The addition of the thermal energy correction and entropy effects to ΔE(MP2-6-31G**) led to the following thermodynamic values: ΔH298(1H→ 2H)= 3.9 kcal mol–1 and ΔG0298(1H→ 2H)= 4.1 kcal mol–1. Electronic spectra recorded at 80 °C provided experimental evidence for the clear predominance of 1H-indazole in the gas phase.

Molecular structure of a unique UV stabilizer: Tinuvin P

The gas-phase and crystal structures of Tinuvin P were established by ab initio MP2-6-31G** level calculations and X-ray diffraction, respectively; both structures were found to include a strong intramolecular hydrogen bond. The IR transitions for this compound were determined theoretically and experimentally, as were the corresponding Raman activities. Intercyclic torsion about the N(1)–C(1) bond exhibits a very low vibrational frequency (13 cm -1 ). Based on the photophysical evidence obtained, the crystalline state of Tinuvin P may not be a suitable photophysical model for the compound in solution.

Theoretical study of the structure and tautomerism of N 1 -unsubstituted pyrazoles in the solid state

Ab initio theoretical calculations have been performed on the pyrazole cyclic dimer, trimer and tetramers as well as on linear oligomers, assuming that there is no relaxation of the geometry during the proton transfer. The ground state and a wide variety of transition states, corresponding to different pathways for intermolecular proton transfer, have been explored and the results compared with experimental data from crystallography and solid state NMR spectroscopy. For the simplest case of the dimer, the reaction path corresponding to a double proton transfer has been explored as well as the effect of relaxing the geometry.

Influence of changes in dipole moments of a system with intramolecular coupling on the four-wave mixing signal

In this work we study the changes in the Four-Wave Mixing (FWM) signal with the modification of the permanent and transition dipolar moments of states in a coupled basis (under the inclusion of intramolecular coupling phenomenon), due to the changes in different parameters, such as coupling parameters v and Vo, strength constant Δ, and transition dipolar moment between the uncoupled states m12. Changes in the intensity and position of the resonances in the FWM signal spectra are observed with the variation in the parameters mentioned above.

Modification in the Relaxation Times and Strength Constant of potential curves in Four-Wave Mixing

In this work we have studied the changes in the Four-Wave Mixing (FWM) signal with the changes in the ratio between the longitudinal and transversal relation times T1/T2. Moreover, we have studied too the modifications in the FWM signal with the changes in the strength constant of the crossed harmonic potential curves that describe the two- levels system employed to this study. We have included the permanent dipolar moments of the states in the uncoupled basis. The rotating wave approximation is neglected to observe the process out of the resonance region. We observed changes in the shape, intensity and position in the resonances of the FWM signal spectra, with the changes in the ratio T1T2 and the strength constant (delta) .

Intramolecular coupling effects on the four-wave mixing signal

In this work we have studied the effect of the intramolecular coupling on the spectral properties of an ensemble of two-level systems carrying homogeneously distributed resonance frequencies, considering permanent dipole moments non zero for a Malachite Green solution.