Fig. 2. Synthesis and spectroscopic study of model MARS probes.

a Synthetic methods to build asymmetric 9-cyanonpyronins. Reaction conditions: i. H₃PO₄, 90 °C, overnight. ii. (1) BCl₃, DCM, overnight; (2) H₃PO₄/PPA, 90 °C, 4 h; (3) FeCl₃/HCl(aq), DCM, 2 h. iii. (1) BCl₃, DCM, overnight; (2) n-BuLi, THF, −78 °C, overnight. b Structures of the model compounds used for spectroscopic study. The first set was used to verify the effect of ring expansion and the second set was used for isotope and core atom effect. c Fitted and normalized epr-SRS spectra of compound 1–5 at triple bond region. Curves were Voight fitted to obtain accurate peak positions. d Normalized epr-SRS spectra of compound 6a–d, 7a–d, and 8a–d at triple bond region. e Additivity and commutativity of three individual effects induced by isotope doping, core atom substitution, and ring expansions. The three axis represents three dimensions of structural modifications, which are mutually orthogonal.