Energetics and Structure of Hydroxynicotinic Acids. Crystal Structures of 2-, 4-, 6-Hydroxynicotinic and 5-Chloro-6-hydroxynicotinic Acids

Revision and Extension of a Generally Applicable Group-Additivity Method for the Calculation of the Standard Heat of Combustion and Formation of Organic Molecules

The calculation of the heats of combustion ΔH°c and formation ΔH°f of organic molecules at standard conditions is presented using a commonly applicable computer algorithm based on the group-additivity method. This work is a continuation and extension of an earlier publication. The method rests on the complete breakdown of the molecules into their constituting atoms, these being further characterized by their immediate neighbor atoms. The group contributions are calculated by means of a fast Gauss-Seidel fitting calculus using the experimental data of 5030 molecules from literature. The applicability of this method has been tested by a subsequent ten-fold cross-validation procedure, which confirmed the extraordinary accuracy of the prediction of ΔH°c with a correlation coefficient R2 and a cross-validated correlation coefficient Q2 of 1, a standard deviation σ of 18.12 kJ/mol, a cross-validated standard deviation S of 19.16 kJ/mol, and a mean absolute deviation of 0.4%. The heat of formation ΔH°f has been calculated from ΔH°c using the standard enthalpies of combustion for the elements, yielding a correlation coefficient R2 for ΔH°f of 0.9979 and a corresponding standard deviation σ of 18.14 kJ/mol.

Tautomer selection through solvate formation: the case of 5-hydroxynicotinic acid

Different 5-hydroxynicotinic acid tautomers were selectively captured through solvate formation. The selectivity is lost once the memory of solvation is erased by removing the solvent from the crystal lattice.

Infrared multiple-photon dissociation spectroscopy of deprotonated 6-hydroxynicotinic acid

RATIONALE

Hydroxynicotinic acids (2-, 4-, 5- and 6-hydroxy) are widely used in the manufacture of industrial products, and hydroxypyridines are important model systems for study of the tautomerization of N-heterocyclic compounds. Here we determined the gas-phase structure of deprotonated 6-hydroxynicotinic acid (6OHNic).

METHODS

Anions were generated by electrospray ionization, and isolated and stored in a Fourier transform ion cyclotron resonance mass spectrometer. Infrared (action) spectra were collected by monitoring photodissociation yield versus photon energy. Experimental spectra were then compared with those predicted by density functional theory (DFT) and second-order Møller-Plesset (MP2) perturbation theory calculations.

RESULTS

For neutral 6OHNic, DFT and MP2 calculations strongly suggest that the 6-pyridone tautomer is favored when solvent effects are included. The lowest energy isomer of deprotonated 6OHNic, in the aqueous or gas phase, is predicted to be the 6-pyridone structure deprotonated by the carboxylic acid group.

CONCLUSIONS

The deprotonated, 6-pyridone structure is confirmed by comparison of the infrared multiple-photon photodissociation (IRMPD) spectrum in the region of 1100-1900 cm(-1) with those predicted using DFT and MP2 calculations.