Evaluation of Thiol Raman Activities and pK_a Values Using Internally Referenced Raman-Based pH Titration

Thiols, including organothiol and thiol-containing biomolecules, are among the most important classes of chemicals that are used broadly in organic synthesis, biological chemistry, and nanosciences. Thiol pK_a values are key indicators of thiol reactivity and functionality. Reported herein is an internally referenced Raman-based pH titration method that enables reliable quantification of thiol pK_a values for both mono- and dithiols in water. The degree of thiol ionization is monitored directly using the peak intensity of the S–H stretching feature in the 2600 cm^–1 region relative to an internal reference peak as a function of the titration solution’s pH. The thiol pK_a values and Raman activity relative to its internal reference were then determined by curve fitting the experimental data with equations derived on the basis of the Henderson–Hasselbalch equation. Using this Raman titration method, we determined for the first time the first and second thiol pK_a values for 1,2-benzenedithiol in water. This Raman-based method is convenient to implement, and its underlying theory is easy to follow. It should therefore have broad application for thiol pK_a determinations and verification.