ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Solvent effects on low-lying absorptions and vibrational spectra of thieno[3,4-b]pyrazines: the role of unconventional C–H···N bonds

Marcos V. A. Damasceno, Vinícius Manzoni, Lucas Modesto-Costa, Geanso M. Moura, Jordan Del Nero, Alberto Torres, and Rodrigo Gester

Universidade Estadual de Mato Grosso, Tangará da Serra, Brazil

E-mail: gester@unifesspa.edu.br

Abstract: Difunctionalized thieno[3,4-b]pyrazines are a class of fused-ring systems used as precursors for copolymers and since these molecules were first synthesized, there is a long discussion about their photophysics and vibrational properties. For 2,3-dimethylthieno[3,4-b]pyrazine, different degrees of solvation were combined to determine how the environment tunes these molecular properties, which can allow future applications. The present results indicate that unconventional C–H···N hydrogen bonds have great influence in both UV–Vis and IR spectra. The low-energy region is composed by two close n–π* and π–π* absorptions, with the lowest excitation being associated with an n–π* line. This absorption is covered by an intense π–π* explaining the reason why n–π* absorptions were never observed for these polymeric compounds. Another effect of the weak C–H···N bond is to tune the highest vibrational frequencies corresponding to C–H stretching modes of the solute’s methyl groups providing strong and uncommon blueshifts of ~ 15 cm⁻¹ and better stabilizing the solute in the liquid environment.

Keywords: UV–Vis spectra ; Vibrational frequencies ; Unconventional hydrogen bond ; Solvent effects 

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-019-00703-2

Chemical Papers 73 (6) 1519–1527 (2019)