Physicochemical Studies on Isoxazoles

Compared with other potentially aromatic five-membered heterocycles, few theoretical studies on isoxazoles have been reported.262 Del Re, using 

Surprisingly, the enthalpy of combustion of isoxazole was determined only very recently.270 For isoxazole, AH° (298.15 K) = —(394.70 + 0.12) kcalth mol-1, from which the enthalpy of formation in the gas phase was derived as AHf (g) = 18.78 0.13 kcalth mol-1. The enthalpies of combustion of 3-amino-5-methylisoxazole and 5-amino-3,4-dimethylisoxazole have also been determined.271 Thermodynamic parameters for isoxazole have been derived from vibrational spectra using the harmonic oscillator-rigid rotor approximation.272,273 Analysis of the rotational spectra of isotopic forms of isoxazole, studied by double resonance modulated microwave spectroscopy, has given the molecular dimensions shown in Fig. 1.274,275 

Berthier, L. Praud and J. Serre, The Jerusalem Symposia on Quantum Chemistry and Biochemistry, Vol. 1, p. 40. Isr. Acad. Sci. Humanities, Jerusalem, 1970. 

The dimensions thus established are generally similar to those determined by X-ray crystallography for several substituted isoxazoles (see, for example, Blanpain et a/.276,277). No significant distortion appears to occur on coordination with copper (I).137 

Several studies of both the H and, 3C-NMR spectra of isoxazoles have been reported. For isoxazole itself the parameters are recorded in Table Xj 278-283 anaiySjs Gf the H-NMR spectrum of isoxazole in a liquid crystal solution also gave the dipole-dipole coupling constants D35 = -64 + 2 Hz D45 = -300 2 Hz D34 = -487 2 Hz.281 

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