Vibrational spectroscopy and ab initio MO study of the molecular structure and vibrational spectra of α- and γ-pyrones

Abstract

The molecular structures and vibrational spectra of the monomeric forms of a-and g-pyrones were investigated by ab initio MO calculations, undertaken at the HF/6-31G p level of theory, and vibrational spectroscopy, including matrix-isolation FTIR spectroscopy. A complete assignment of the vibrational spectra of the studied compounds isolated in an argon matrix, at 8 K, or in the condensed phases, at room temperature, is presented and the vibrational data correlated with some important structural parameters. It is shown that the intermolecular interactions in the room temperature condensed phases do not affect the structure and vibrational properties of the studied molecules strongly, though the vibrational results clearly reveal minor changes induced in the carbonyl groups that indirectly affect the electron distribution in the whole pyrone rings, leading to an increase in their pelectron delocalization. For the isolated monomers, both structural and vibrational results point to a relatively weak p-electron delocalization in both a-and g-pyrone moieties. q

FTIR Spectroscopic and Theoretical Study of the Photochemistry of Matrix‐isolated Coumarin

“…From the analysis of the two datasets (calculated vs experimental) it can be concluded that the calculated geometry for the monomer in vacuum is in excellent agreement with the X‐ray data, indicating that, in the condensed phase, the intermolecular interactions introduce only relatively unimportant effects on the electron distribution in the coumarin molecules. It is worth noticing that this result follows the trend previously observed for both α ‐ and γ ‐pyrone (45), where intermolecular interactions in the condensed phases were also found to affect the structure and vibrational properties of the molecules only very slightly.…”

“…On the whole, the structural results clearly demonstrate that π ‐delocalization in the pyrone sub‐unit of coumarin is very limited, being even less important than in α ‐pyrone (45). An interesting way to evaluate the level of π ‐electron delocalization is the aromaticity index HOMA (Harmonic Oscillator Measure of Aromaticity), defined by Kruszewski and Krygowski (46–48): where n is the number of bonds considered, and α is an empirical constant (for C−C and C−N bonds α= 257.7 and 93.52, respectively) fixed to have HOMA = 0 for a model nonaromatic system, and HOMA = 1 for a system with all bonds equal to an optimal value R opt , which is assumed to be achieved for a fully aromatic system.…”

“…Table 1 also shows the equilibrium geometry calculated for α ‐pyrone (45) at the DFT(B3LYP)/6‐311++G(d,p) level of theory. Comparison of the structural parameters obtained for the two molecules at the same level of theory (in particular, bond lengths) allowed us to shed light on the influence of the phenyl ring of coumarin on the electronic properties of its pyrone ring.…”

DFT calculations and electronic absorption spectra of some, α- and γ-pyrone derivatives

“…The structural and vibrational spectroscopy data for the ground state demonstrated that no extensive π-system electron delocalization occurs in the pyrone moiety. Comparison of the spectroscopic results obtained for the monomeric species with those obtained for the room temperature condensed phase reveal that intermolecular interactions in these phases do not affect the structure and vibrational properties of pyrone derivatives [16].…”

The Influence of Monogaloidosubstitution on the Structure and Dynamics of Sixmember Cyclic Compounds

“…4, является предсказательной. Располагаем только экспериментальными данными [14] для базового соединения -Г-П.…”