In this Chapter, we highlight the power of vibrational spectroscopy as central technique to investigate the structure and reactivity of two relevant families of nitrogen-containing heterocyclic molecules: hydantoins, and mercaptoimidazoles. Infrared spectroscopy is used in connection with the matrix isolation technique to investigate the structures of the isolated molecules and their photochemistry, while both infrared and Raman spectroscopies, supplemented by thermodynamics, microscopy and diffraction techniques, are used to investigate neat condensed phases of the compounds and transitions between these phases. The experimental studies are supported by extensive computational studies, which include several approaches for detailed analysis of the electron density.

This investigation has been performed within the Project PTDC/QEQ-QFI/3284/2014 – POCI-01-0145-FEDER-016617, funded by the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT) and FEDER/COMPETE 2020-EU. The Coimbra Chemistry Centre (CQC) is supported by FCT, through the project UI0313/QUI/2013, also cofunded by FEDER/COMPETE 2020-EU. E.M.B. and B.A.N. thank FCT for the grant CCMAR/BI/0013/2017, within Project PTDC/MAR-BIO/4132/2014, the PhD. grant SFRH/BD/129852/2017, respectively. R.F. and G.O.I. acknowledge the financial support through the project MATIS - Materiais e Tecnologias Industriais Sustentáveis (FCT and CENTRO-01-0145-FEDER-000014.