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 solvent's 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).

Potential energy surfaces for the intramolecular proton transfer of ground (GSIPT) and excited (ESIPT) states of 2-hydroxybenzoyl compounds were obtained. Based on the results, intramolecular proton transfer in this type of compound is strongly dependent on the distances between the oxygen atoms that bear the intramolecular hydrogen bond (IMHB). Also, the GSIPT curves for these compounds contain a single minimum that is located in the zone for the normal (enol) form. The ESIPT curves also contain a single minimum but lie in the zone for the keto form. There is no correlation between the strength of the IMHB and the proton transfer barrier through it. The energy for the excited singlet 1(n,π*) for these compounds is strongly dependent on the resonance effect of the substituent, −R, so this state is the first excited singlet only in derivatives with nearly nonresonating R. The ESIPT processes are of the proton transfer type, even though the final form possesses no zwitterionic connotations. Finally, these theoretical features are quite consistent with photophysical experimental evidence for this type of compounds.

MP2-6-31G** calculations on 1H-and 2H-indazole annular tautomers show that the 1H tautomer is more stable than the 2H one by 3.6 kcal mol-l. 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 AE(MP2-6-31G**) led to the following thermodynamic values: AIPg8( 1 H --+ 2H) = 3.9 kcal mol-l and AGG,298( 1 H + 2H) = 4.1 kcal mol-l. Electronic spectra recorded at 80 O C provided experimental evidence for the clear predominance of 1 H-indazole in the gas phase. Paper 5/03802B

The solvent basicity (SB) scale, introduced by Catalán et al. in 1996, was compared with theoretical data (V min and εmo(Vmin)), calculated at the HF-SCF 6-31G** level, and with experimental data (ΔH f of Arnett, δΔH solv of Laynez, ΔΔν̃(1)−(2) and ΔΔν̃(3)−(5) of Laurence, and ΔH g→solvent for SO2 and I2 of Benoit and Louis). The fact a SB scale is a family-independent scale results in no grouping of the solvents in parallel lines. As also shown by the results, a basicity scale constructed around a standard phenolic Brönsted acid (R-OH) such as 4-nitrophenol or 4-fluorophenol is more correct than one based on an unsubstituted amine such as 4-nitroaniline. The SB scale has a near-unity covalent-to-electrostatic ratio, so it is sensitive to both covalent and electrostatic interactions and hence approaches closely the requirements for a general basicity scale.

The first systematic comparison of structural effects on the intrinsic reactivities of carbonyl and thiocarbonyl compounds has been carried out. To this end, the gas-phase basicities (GB) of a wide variety of thiocarbonyl compounds XCSY (as well as of some carbonyl derivatives) were determined by means of Fourier transform ion cyclotron resonance spectrometry (FTICR) and SCF and MP2 ab initio calculations at different levels of accuracy were performed on 27 different neutral compounds and their protonated forms. The same set, enlarged by the inclusion of very large systems such as di-tert-butyl-and bis-( 1-adamanty1)thioketones was also investigated at the AM 1 semiempirical level in order to get a more complete view of structural effects. The agreement between the calculated and the experimental changes in thermodynamic state functions is good in all instances. Correlation analysis of the experimental data shows that (i) substituent effects on the gas-phase basicity of thiocarbonyl compounds are linearly related to those of their carbonyl homologs with a slope of 0.80 and (ii) these effects can be quantitatively analyzed in terms of polarizability, field, and resonance effects (Taft-Topsom model). Comparison of the GBs of thiocarbonyl and carbonyl compounds with solution basicities and nucleophilicities sheds light on differential structural and solvation effects. Substituent effects on both neutral and protonated species were explored by means of appropriate isodesmic reactions. These results confirm that all thiocarbonyl compounds investigated are sulfur bases in the gas phase. The features revealed by correlation analysis can be rationalized in terms of the interactions between the MOs of the substituent and the parent compound.