Influence of Prototropic Reactions on the Absorption and Fluorescence Spectra of Methyl p-dimethylaminobenzoate and Its Two Ortho Derivatives

The influence of prototropic reactions on the spectral characteristics of methyl p-dimethylaminobenzoate (I) and its o-methoxy (II) and o-hydroxy (III) derivatives has been studied using steady-state spectroscopic technique and quantum-chemical calculations. This study concerns the solvent-induced shift of the absorption, locally excited (LE) and intramolecular charge transfer (ICT) fluorescence bands in the neat tetrahydrofuran (THF) and its hydrochloric acid solutions at different HCl concentrations. On the basis of the experimental results and quantum-chemical calculations, it was shown that in a hydrochloric acid solution the studied molecules exist as a mixture of neutral, mono-, and dicationic forms. Additionally, the results of spectroscopic measurements were used to calculate, according to the Benesi-Hildebrand method, the equilibrium constants of protopropic reactions in the ground, S0, and excited, S1, states. Our findings predestine molecules I and II to be used as acid fluorescence probes in a region of 0–2.5 M of [H+] concentrations

THE LINESHAPE OF INFRARED-MICROWAVE TWO-PHOTON TRANSITIONS: THE ν2\nu_{2} BAND OF 15NH3∗^{15}NH_{3}\ast

Author Institution: Department of Chemistry, Michigan State University East LansingA computer-coupled infrared-microwave two-photon spectrometer has been used to record the lineshapes of transitions in $^{15}NH_{3}$ as a function of sample pressure and microwave power. The frequency of the $CO_{2}$ laser is stabilized by monitoring the saturation dip in the fluorescence from a $CO_{2}$ sample in the laser cavity. A laboratory computer steps the microwave frequency and records the spectrum. The computer also records the frequency variation of the microwave power, which can be used to normalize the data to constant power. The lineshapes, as analyzed by means of the expression for a Voigt profile, appear to show significant changes in the center frequency and the Lorentz width as the microwave power is varied. The nature of these variations will be described and compared to predictions from the theory of two-photon absorption in a three-level system. $^{*}$This work was supported by the National Science Foundatio