Where Vibrations Agree and Excitations Argue: DFT and TD-DFT Characterization of Crototropone

Introduction

Crototropone is a tropone derivative isolated from Croton zehntneri roots collected in Pernambuco, northeastern Brazil. The plant, known as canela-de-cunhã, is used medicinally in teas and infusions. Experimental IR and UV–visible data are available, but we found no quantum-chemical study of the compound.

Methods

Using ORCA 6.1, the lowest-energy conformer from a GOAT/GFN2-xTB search was pre-optimized with r²SCAN-3c and refined at PBE0-D4/def2-TZVP in the gas phase and methanol (SMD), the solvent used for the experimental UV–visible spectrum. Frequency calculations confirmed minima and provided IR and Raman spectra. Singlet excitations were computed in methanol by TD-DFT using B3LYP, PBE0, CAM-B3LYP, M06-2X, and ωB2PLYP with a perturbative doubles correction, followed by natural transition orbital (NTO) analysis.

Results

Calculated IR and Raman spectra placed the conjugated stretching region at 1648 cm⁻¹ in the gas phase and 1623–1644 cm⁻¹ in methanol, close to the reported 1620 cm⁻¹ band. TD-DFT spectra agreed qualitatively with reported bands at 211, 227, and 287 nm. Several intense transitions, with f values up to 0.794, formed a broadened envelope around 227 nm. S1 predictions ranged from 320.2 nm with B3LYP to 291.5 nm with ωB2PLYP; CAM-B3LYP and M06-2X were closer to 287 nm than PBE0 and B3LYP. NTOs showed that S1 remained dominated by a single pair, whereas S2 retained a two-pair, more method-dependent character. No method reproduced the weak band reported at 397 nm.

Conclusions

The continuum-solvated neutral monomer qualitatively captured the vibrational and 200–320 nm absorption features, but not the weak 397 nm band.