Microwave spectra ofs-cis-Glyoxal (CHO−CHO) molecules in excited vibrational states

The microwave spectra of s-cis-glyoxal molecules in the excited states of torsional (v_t=1,2) and bending (v_bend=1) vibrations have been studied. For the v_t=1 state, the rotational constants have been refined and the quartic constants of centrifugal distorition have been determined. For the excited v_t=2 and v_bend=1 states, the rotational constants have found. For the ground vibrational state, the rotational constants and the quartic constants of centrifugal distortion have been refined. The vibrational frequencies have been determined from the relative intensities of rotational transitions. Ufa Scientific Center of Russian Academy of Sciences, Physics Department. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp.418–423, May–June, 1995. Translated by I. Izvekova     

Microwave spectrum,centrifugal perturbation,dipole moment,and conformation of 4-methyl-1,3-dioxane

The microwave spectrum of 4-methyl-1,3-dioxane is studied in a frequency range from 16 GHz to 40 GHz. The rotational transitions of a-, b-, and c-types with J ≤ 57 are identified. The rotational constants (MHz) A = 4802.335(2), B = 2376.163(1), C = 1738.852(1) and the quartic constants of the centrifugal distortion are found for the ground vibrational state of the molecule. The components of the dipole moment (D) μ _a = 0.73(1), μ_b = 1.32(1), μ_c = 1.36(1) and the total dipole moment μ = 2.03(1) are determined. The experimental data obtained correspond to the chair conformation of the molecule with the equatorial orientation of the methyl group.     

Structure and spectra of 1,3-dioxanes. microwave spectrum, structural parameters, and ab initio calculations of 5-methyl-1,3-dioxane

In the microwave spectrum of a 5-methyl-1,3-dioxane sample, the rotational transitions of a and c types with 4 ≤ J ≤ 11 for five isotopomers of a molecule with ¹³C and ¹⁸O isotopes in different sites are identified in a frequency range of 18 GHz to 42 GHz. The spectroscopic constants of isotopomers are found. The substituted r _s and effective r _o structural parameters of 5-methyl-1,3-dioxane are determined. By the B3PW91/aug-cc-pVDZ density functional method the equilibrium structure of the molecule is calculated. The results of quantum chemical calculations are compared to the experimental data.     

Microwave spectrum,centrifugal perturbation,dipole moment,and conformation of 5-methyl-1,3-dioxane

The microwave spectrum of 5-methyl-1,3-dioxane was studied in the frequency range 12–35 GHz. The a and c type rotation transitions with J≤30 were identified. The rotational constants A = 4658.5244(33) MHz, B = 2383.3930(12) MHz, and C = 1724.28907(88) MHz and the quartic constants of the centrifugal distortion of the molecule in the ground vibrational state were determined. The components of the dipole moment were found, μ _a = 1.76 ± 0.01 D and μ _c = 1.10 ± 0.01 D; the net dipole moment of the molecule is μ = 2.08 ± 0.01 D. 5-Methyl-1,3-dioxane was calculated by the B3PW91/aug-cc-pVDZ density functional theory method. The calculated data are compared with the experimental data. The most stable conformation is the chair conformation with an equatorial orientation of the methyl group.     

Conformational analysis of <Emphasis Type="Italic">cis-</Emphasis> and <Emphasis Type="Italic">trans</Emphasis>-isomers of 2,5-dimethyl-1,3-dioxane

The conformational isomerization of cis- and trans-isomers of 2,5-dimethyl-1,3-dioxane was investigated by means of ab initio RHF//6-31G(d) and PBE//ccpVDZ quantum-chemical methods. It has been shown that in comparison with cis-form the potential energy surface of trans-isomer has a less number of stationary points and includes the main minimum belonging to the diequatorial chair form. In the case of cisisomer the main minimum corresponds to the C 2e5a chair form. The values of potentional barriers of conformational isomerization were also determined. On the basis of experimental (NMR ¹H) and theoretical vicinal coupling constants we determine the value ΔG ⁰ of chair-chair inversion for the trans-isomer molecule.     

Structure and spectra of 1,3-dioxanes. II. Microwave spectrum, structural parameters, and ab initio calculations of 2-methyl-1,3-dioxane

The microwave spectra of five isotopomers with the ¹³C and ¹⁸O natural abundance isotopes of the 2-methyl-1,3-dioxane molecule (22–50 GHz) were studied. Rotational transitions of a and c types with 4 ≤ J ≤ 12 were identified. The rotational constants and the substitution r _s and effective r _o structural parameters of the molecule were determined. Ab initio calculations on 2-methyl-1,3-dioxane were performed with molecular structure optimization. The results of quantum-chemical calculations at different levels are compared with experimental data.     

Structure and spectra of 1,3,2-dioxaphospholenes. 1. Microwave spectra of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene and its isotopomers in the ground and excited vibrational states

The microwave spectrum of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene (1) was studied in the frequency range from 7 to 53 GHz. Rotational transitions of the parent molecule in the ground and eleven excited vibrational states and those of its mono-substituted ³⁷Cl, ¹³C_Me, and ¹³C_Cycle isotopomers in the ground vibrational state were identified. Rotational constants and partial r _s-structure were obtained. The quartic centrifugal distortion constants, dipole moment components _a = 3.8D and _c = 0.24D (the total dipole moment is 3.81D), and the ³⁵Cl quadrupole coupling constants were determined for the parent molecule. The fine structure of the microwave transitions in the parent molecule was analyzed under the assumption of noninteracting methyl groups. The height of the barrier to internal rotation (V ₃₀ = V ₀₃ = 665 cm^–1) and the frequency of torsional vibrations ( = 167 cm^–1) were found. The frequencies of two lowest vibrational modes corresponding to deformation vibrations of the five-membered ring were estimated (100 cm^–1) from the relative intensities of rotational transitions for different vibrational states.