Methods to distinguish tautomeric cases from static ones

Tautomerism may occur both in the gas phase, namely the liquid and condensed phases, and in the solid state, namely in the ground state as well as in the excited state. This chapter provides an overview and tools by discussing different techniques for achieving disclosure of tautomerism in new types of compounds. The techniques considered are UV, NMR, isotope effects on chemical shifts, infra red (IR) spectroscopy, fluorescence, density functional theory (DFT) calculations, and X-ray as well as analytical techniques such as principal component analysis and fishing algorithms. Theoretical calculations are treated in a number of cases such as energy differences of the two forms, calculations of chemical shifts of tautomeric species, and calculation of IR vibrational frequencies. Single molecules of fluorescent molecules can be studied by means of confocal microscopy in combination with azimuthally or radially polarized laser light.