Probing the molecular and electronic structure of capsaicin: A spectroscopic and quantum mechanical study

A. Alberti, V. Galasso, B. Kovač, A. Modelli, Fabio Pichierri

Research output: Contribution to journalArticlepeer-review

12 Citations (Scopus)

Abstract

The conformational preferences of capsaicin were investigated by using the hybrid meta density functional theory (DFT) method MPWBlK. Its flexible, pendant side chain allows for a multitude of conformations only slightly different in energy. The distinctive vibrational features of the most stable conformers were characterized. To elucidate the most favorable reaction sites of capsaicin for radical scavenging, various homolytic bond-dissociation energies were also calculated. Of the possible radical intermediates, the allyl and benzyl radicals are energetically preferred. The filled and empty electronic structures of capsaicin were investigated by exploiting the photoelectron and electron-transmission spectra also of reference molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron-attachment energies of capsaicin was proposed. The frontier n molecular orbitais are concentrated over the vanillyl moiety, with a modest influence of the amidic - aliphatic chain. The (negative) first vertical electron affinity is predicted to be similar to that of benzene. The absorption spectrum of capsaicin and its change by conversion into a phenolic deprotonated anion (modest bathochromic displacement) or a phenoxyl neutral radical (from colorless to red) were interpreted with time-dependent DFT calculations. ESR measurements following chemical or electrochemical reduction of capsaicin did not lead to detection of the corresponding radical anion. The spectra show fragmentation of the original molecule and formation of a variety of radical species which are believed to have a semiquinonic structure.

Original languageEnglish
Pages (from-to)5700-5711
Number of pages12
JournalJournal of Physical Chemistry A
Volume112
Issue number25
DOIs
Publication statusPublished - 2008 Jun 26

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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