High Overtones of C-H Stretching Vibrations in Isoxazole, Thiazole, and Related Methyl and Dimethyl Derivatives Giuseppe Sbrana* ,† and Maurizio Muniz-Miranda ‡ Centro di Studio sui Composti Eterociclici, and Dipartimento di Chimica, UniVersita′di Firenze, Via G. Capponi 9, Firenze, I-50121 Italy ReceiVed: April 17, 1998; In Final Form: June 26, 1998 The high overtone spectra of liquid isoxazole and thiazole, two five-membered heterocyclic molecules with different aromaticity characters, have been investigated in the C-H stretching region by visible absorption spectroscopy. The local-mode model describes satisfactorily the C-H stretching vibrations at the fifth and sixth quanta of excitation, where the localized character is increased. Each overtone band generally displays a number of peaks corresponding to the different types of C-H oscillators. Comparison between the overtone spectra of the parent molecules and those of the methyl derivatives provided sufficient information for a conclusive assignment of the progressions. Deconvolution of the fifth overtone bands gave further evidence regarding the shape and the width of the distinct peaks. Anharmonicity values of the overtone vibrations, C-H bond lengths, and force constants were obtained from the local-mode analysis and discussed in comparison with the results obtained from the normal-mode approximation. A correlation between overtone frequencies and chemical shifts in proton magnetic resonance, generally observed in aromatic compounds, has been found only for thiazole, whose aromaticity is responsible for a significant electron current in the ring. Introduction The local-mode model has been satisfactorily applied to highly excited vibrational states because it offers a more appropriate description of the X-H oscillators, in terms of uncoupled anharmonic local modes, than the usual harmonic normal-mode approximation. 1-5 High overtones of the X-H vibrations (X ) C, N, O) were found to have properties consistent with the chemical environment of the oscillators; 6-9 hence the study of the vibrational overtones can provide useful information on the photodissociation mechanism, radiation-less transitions of molecules, and chemical reactivity of the bonds involved in the vibrations. 10,11 This type of experimental study offers also the possibility for the investigation of the potential energy surface whose anharmonicity may be properly corrected by determining exactly the mechanical frequencies of the molecules. Two conditions are necessary for the success of this model, a weak coupling among the oscillators and a large separation, in terms of vibrational energy, from the remaining modes of the molecule. The main coupling between the local modes derives from the kinetic energy, but it is weak if the mass ratio between X and H is very small, as in case of C-H, O-H, or N-H stretching vibrations. On the other hand, coupling due to potential energy is significant only for delocalized bonds. At low energies, as in the case of low values of the vibrational quantum number, coupling between oscillators occurs, even if weakly, and the ordinary normal-mode description, in terms of harmonic oscillators, can be considered a valid approximation. At high energies, as in case of high overtones, the local-mode model becomes the most appropriate because of the quenching of interbond coupling due to the increasing of the bond anharmonicity with the quantum number. In this paper, we report a study of the C-H stretching overtone progressions in isoxazole and thiazole, five-membered rings containing two heteroatoms. Both molecules possess three C-H bonds which do not undergo the same chemical environ- ments; hence three distinct overtone bands are expected in the absorption spectra. On the contrary, in furan and thiophene, two C-H stretching overtone bands are predicted in agreement with the presence in the ring of two sets of equivalent C-H oscillators. 12,13 Isoxazole and thiazole present significant dif- ferences in the spectroscopic and chemical properties. The different electron distribution in the ring is responsible for the larger aromaticity of thiazole with respect to isoxazole, whose heteroatoms strongly impair the π-electron delocalization. Since the frequency of a high-energy overtone band is specific to an isolated chemical bond, bond lengths and force constants of the C-H oscillators were accurately evaluated by the analysis of the overtone progressions and compared with the results obtained from the normal-mode approximation. Experimental Section All compounds were obtained from Aldrich and Sigma (2,4- dimethylthiazole), dried on molecular sieves, and purified by repeated distillations. The spectra of the liquids at room temperature were recorded in the 3300-300 nm region on a Cary 5 spectrophotometer equipped with near-IR and visible light sources. In the overtones region of ∆V CH ) 2-4, cells with 1- to 10-mm path lengths were used, while 4- or 5-cm path length cells (Helma) were employed for ∆V CH ) 5 or 6. Higher signal intensities of the extremely weak fifth overtones were obtained by properly assembling two quartz cells (4- and 5-cm path length) in order to have an optical path of 9 cm. The structures of the broad bands were analyzed through a computational deconvolution procedure in terms of Gaussian bands. Computed areas of the deconvoluted peaks differ from † Centro di Studio sui Composti Eterociclici. ‡ Dipartimento di Chimica. 7603 J. Phys. Chem. A 1998, 102, 7603-7608 S1089-5647(98)01926-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/09/1998