Rotation spectra of isotopic forms of isoxazole (ONCHCHCH) have been studied in their natural abundance by double resonance modulated microwave spectroscopy. Earlier data have been refined and extended to include the three deuterium forms, also in natural abundance. From the eight isotopic species the complete structure of isoxazole has been determined. Bond distances (in Å) are O(1) –N(2) =1.399±0.001, N(2) –C(3) =1.309±0.002, C(3) –C(4) =1.425±0.002, C(4) –C(5) =1.356±0.001, C(5) –O(1) =1.344±0.001, C(3) –H(3) =1.077±0.001, C(4) –H(4) =1.074±0.001, and C(5) –H(5) =1.075±0.002. The ring angles, beginning at the oxgyen atom, are (uncertainties <0.2°) 108.8°, 105.3°, 112.3°, 103.0°, and 110.6°; the angles between the CH- and adjacent ring bonds are C(4)C(3)H(3) =129.1±0.3°, C(3)C(4)H(4) =128.5±0.4° and C(4)C(5)H(5) =133.4±0.5°. During the course of the structure study, satellite spectra due to each of the eleven bending vibrations of isoxazole were assigned and correlated with ir work through relative intensity measurements. The reassignment of the lowest in-plane bending mode as ν13=844 cm−1 is suggested. On the basis of the results on isoxazole and other molecules the likely impact of the DRM technique on structure determinations by microwave spectroscopy is considered.
From its rotation spectrum propionic acid (CH3CH2COOH) is shown to exist predominantly in a cis−conformation in which the methyl group eclipses the carbonyl bond and with the hydroxyl hydrogen between the two oxygen atoms. The dipole moment of this rotamer is μ = 1.55±0.03 D (μa = 0.19 D, μb = 1.54 D). From the variation of its components under isotopic substitution the dipole moment deviates by (5.3±1.5) ° from the C = O bond toward the hydroxyl group. The barrier hindering internal rotation of the methyl group has been determined for four isotopic forms with the result V3 = 2340±30 cal/mole for CH3CH2COOH and CH3CH2COOD and V3 = 2370±30 cal/mole for CH3CD2COOH and CH3CD2COOD. Satellite spectra from 24 different vibrational levels have been analyzed by double resonance techniques. The five lowest vibrational modes of cis−propionic acid have been assigned: C−C torsion at 64±3 cm −1, CH3 internal rotation at 190.4 cm−1, CCC deformation at 270±15 cm−1, with tentative vibrational assignments of δ (CCO) at 530±30 cm−1, τ (OH) at 547±20 cm−1.
Double resonance modulation (DRM) microwave spectroscopy has been used to determine the complete substitution structure of oxazole. Isotopic species spectra associated withWith uncertainties below 0.002 Å and 0.1° for distances and angles, respectively, the structure parameters of oxazole are as follows:0(1)-C(2) = 1.357C(2)-N(3) = 1.291N(3)-C(4) = 1.395C(4)-C(5) = 1.352C(5)-O(1) = 1.369C(2)-H(2) = 1.075C(4)-H(4) = 1.075C(5)-H(5) = 1.073The geometry of oxazole is compared with that of isoxazole and with the structures of the closely related compounds furan and 1,3,4-oxadiazole.
Microwave double resonance modulation has been used for the first time to observe all the isotopic species spectra that are needed for a structure determination by the microwave method. From spectra of the normal and thirteen isotopically labelled species of cis−propionic acid the substitution structure of this molecule has been determined completely. Bond lengths (in Å) are as follows; Cketo−Cmethylene = 1.509±0.002, Cmethylene−Cmethyl = 1.523±0.003, C=O = 1.210±0.001, C−O = 1.352±0.002, O−H = 0.970±0.001, (C−H)methylene = 1.098±0.002, (C−H)methyl = 1.088±0.002. The nonbonded distance between the oxygen atoms is 2.245±0.001 Å. The bond angles are (in degrees): & CCC = 112.7±0.1, & (HCH)methylene = 106.4±0.2, & (HCH)methyl = 108.6±0.2, & (CCH)methyl =110.0±0.3, & C=O = 125.8±0.2, & CC−O = 111.8±0.1 and & COH = 105.8±0.2. The plane HCH of the methylene hydrogens is tilted by 4.9±0.1° from the external bisector of the CCC−angle towards the hydroxyl oxygen atom. Spectra corresponding to the first excited state of the large amplitude, torsional vibration around the central carbon−carbon bond have been analyzed for all 14 isotopic forms, enabling the extrapolation to torsional equilibrium moments. Structure calculations based on these moments show that the torsional vibration has no significant influence on the ground state structure. For the normal and four deuterated species the effects of the three lowest vibrations could be eliminated from the ground state constants. The corresponding structure calculations suggest that the out−of−plane carbon−hydrogen bonds derived from ground state constants contain a contribution of 0.005 and 0.002 Å, respectively, for the methylene and methyl hydrogens due to the combined zero−point effects of these three vibrations. The heavy skeleton structure of propionic acid appears little affected, however.
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.
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