Transient Resonance Raman and ab Initio MO Calculation Studies of the Structures and Vibrational Assignments of the T₁ State and the Anion Radical of Coumarin and Its Isotopically Substituted Analogues

Abstract

Transient resonance Raman and absorption spectra of the anion radical CM•- and lowest excited triplet state 3CM* of coumarin were measured. Vibrational assignments of the ground state S0, CM•-, and 3CM* were performed based on both the frequency shifts on isotopic (18O, 13C, and deuterium) substitutions and normal-coordinate calculations using the force constants obtained by ab initio molecular orbital computations. The vibrational assignments and ab initio MO calculations provided much information on the structures of S0, CM•-, and 3CM*. It is concluded that the C(3)C(4) bond of the pyrone ring is lengthened markedly in CM•- and drastically in 3CM*. This suggests that the C(3)C(4) bond is one of the reactive sites of coumarin and furocoumarins in their photochemical reactions, which is in good accord with the formation of cyclobutane-type adducts with pyrimidine bases, particularly with thymine, through the C(3)C(4) bond of furocoumarins and the C(5)C(6) bond of pyrimidine bases. The CO bond was found to be moderately lengthened both in CM•- and 3CM*. The moderate lengthening of the CO bond of 3CM* is consistent with the π−π* character of 3CM*.

The DFT and MP2 based computational scrutiny on blue-shifted H–F stretching vibrational frequencies in hydrogen-fluoride complexes with nitriles: Insights into the decisive role of intermolecular hydrogen bonding (IMHB) in ground and electronic excited states

“…Also, it is important to point out that the bond strength is controlled predominately by the HOMO-LUMO gap. Besides, the energy values of LUMO, HOMO and HOMO-LUMO energy gap help to characterize the chemical reactivity and kinetic stability of the molecule (Uesugi et al, 1997). It is well known that a molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is also termed as soft molecule (Powell et al, 2004;Rauk, 2001;Streitwieser, 1961).…”

Studies of the Chemical Reactivity of a Series of Rhodanine Derivatives by Approaches to Quantum Chemistry

“…The difference in energy between the HOMO and the LUMO, called the energy gap, is an important stability factor for the structures [49]. The HOMO-LUMO energy gap helps characterize the chemical reactivity and kinetic stability of the molecule [50]. A molecule with a high energy gap (ΔE) is less polarizable and is generally associated with low chemical reactivity and high kinetic stability [51].…”

Molecular Structure, Vibrational Spectra and Chemical Shift Properties of C12H9ClO4 and C12H9BrO4 Crystals by Density Functional Theory and ab initio Hartree-Fock Calculations

“…They have demonstrated a great variety of biological properties as anti-inflamatories [3], anti-bacterials [4] and anti-helmintics [5]. Coumarin and its derivatives are known to exhibit photosensitizing properties [6]. They have been proposed in HIV [7] and cancer [8] treatment, as well as being inhibitors of monoaminooxidase [9,10].…”

“…In order to understand 0932-0784 / 09 / 1100-0745 $ 06.00 c 2009 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com the photosensitizing mechanism of furocoumarins, it is important to obtain information on the structures and dynamics of the excited states and photolytically generated transient species of coumarin. Due to its structural similarities with psoralen, coumarin serves as a useful model for elucidating spectroscopic characteristics of the transient species of psoralen and its derivatives [6]. Non-covalent interactions are involved in most of the molecular recognition processes [18].…”