The Microwave Spectrum of Oxazole

Abstract

The dipole moments and the quadrupole coupling constants of the normal and the three mono-deuterated species of oxazole have been measured. The dipole moment of 1.50 ± 0.03 D is found to deviate by 10.8° from the external bisector of the CNC angle towards the C(2) carbon atom. The principal values of the quadrupole coupling tensor are determined as Xzz = -4.04^ 0.02 MHz and %Xx -1-66 =L 0.02 MHz along the axes in the molecular plane, so that the gradient perpendicular to this plane is %yy = 2.38 MHz. The direction of the largest gradient deviates by 5.7° from the external bisector of the CNC angle towards the carbon atom C(4).An attempt is made to correlate these results and the geometry of oxazole with the electron distribution in this molecule.

A Reanalysis of 14N Nuclear Quadrupole Coupling and Methyl Internal Rotation in the Rotational Spectra of Monomethyl Oxazoles and Isoxazoles

“…For oxazole [11] and isoxazole [13] complete 14 Nquadrupole coupling tensors or, equivalently, the principal tensor elements x xx , X yy and thus the angle < (z, a) between the principal quadrupole tensor axis z and the principal inertial axis a, were determined. As the off-diagonal coupling elements x a b the monomethyl derivatives could not be fixed spectroscopically, I could not diagonalize the 14 N-quadrupole tensors in the respective molecular principal inertia axes system to make possible a direct comparison of principal tensor elements, but had to resort to tensor transformation methods.…”

“…Bond angles (in degrees °) and lengths (in A) have been obtained by fitting 13 structural parameters, i.e. six angle and seven length parameters, to nine sets of rotational constants of the most abundant normal isotopic species and eight isotopically single-substituted derivatives (oxazole: [11], isoxazole: [14]). Fixed bond angles in square brackets.…”

“…seven distances and six angles, to nine sets of rotational constants of the oxazole [11] and isoxazole [14] normal isotopic species and eight monosubstituted isotopomers, respectively, whose microwave spectra had originally been assigned for the purpose of r sstructure determination. It is therefore easily understood that the available data did not suffice for a complete isoxazole restructure fit, so that I had to fix the H-atom bond angle in 5-position at its restructure value because otherwise it would have resulted with great uncertainty (110(12)°).…”

“…(CH 3 , ring) correspondingly, as pointed out below. With the projection angles £ (z, a) resulting from the optimized structures of the methyl derivatives, I transformed the principal oxazole [11] and isoxazole [13] 14 N-quadrupole tensor elements y zz and y xx into the principal inertia axes system of the respective methyl compound according to The experimental diagonal coupling elements and Xll P are also given in Table 4 with their triple standard deviations for a numerical comparison with the projected values. The angle <£ (CH 3 , ring) in block " < (a, i)" of Table 4 was arrived at by parametrically varying a £(CH 3 ,ring) taken over as £ (H, ring) from the respective parent molecule as starting-point, until the angle between methyl-bond axis and inertial axis a coincided with the absolute value of < (a, i).…”

“…We used the computer programs HT1NQ, QUAD, and DH14KS. The results of these analyses are contained in Table 3 together with some comparative data for the parent molecules oxazole [11] and isoxazole ( [12,13]) as far as they were accessible from literature. The precision and convergence of the fitting calculations prove a first order perturbation treatment to be fully sufficient in our cases.…”