Experimental and theoretical spectroscopic characterization, NLO response, and reactivity of the pharmacological agent spilanthol and analogues

Abstract

Spilanthol is the most abundant pharmacologic agent present in Acmella oleracea, a popular Amazonian plant usually called as Jambu. We performed an experimental and theoretical investigation on several electronic properties connected to the molecular structure of spilanthol (S1) and the other two analogs (S2 and S3). Our FT-IR analysis is not conclusive but suggests the existence of a resonance structure that oscillates between the NH = C - O to the NH - C = O conformations involving the amide nitrogen and carbon atom of the carbonyl group. We confirmed this proposal for all alkylamides by using appropriate DFT/6-311++G(d,p) level of theory and confirmed that the NH = C - O enters with greater width. Our analysis also shows that the UV–vis spectra are strongly affected by the number of C ≡ C bonds, and suffer an uncommon blue shift from S1 to the S3 structure. Our nonlinear optical (NLO) investigation based on DFT methods shows that the chromophores could have some use in optoelectronic due to they present second-hyperpolarizability of ∼72×10−36 esu, in competition with urea, PNA, and some chalcones. The analysis of the global reactive descriptors shows that spilanthol and its two analogs are classified as strong electrophilic chromophores with ω > 1.5 eV. However, the systematic inclusion of the C ≡ C bonds from S1 to S3 turns the system less nucleophile. While spilanthol is a strong nucleophile with N > 3.0 eV, the remaining S2 and S3 chromophores can be classified as moderate nucleophilic systems with 2.0 ≤ N ≤ 3.0 eV. These results suggest that S2 and S3 molecules have great potential as pharmacological agents.