The spectrum of isopropyl alcohol in the millimeter region was recorded using a hybrid radio-spectrometer. A total of 1278 rotational and vibration-rotational transitions of its gauche-conformer were identified. Rotational constants and quartic centrifugal constants were refined. Sextic centrifugal constants and all six internal rotation constants were determined. The tunnel splitting parameter was refined. The reduced Hamiltonian of internal rotation is used for the first time in full format and describes sufficiently well the vibration-rotational spectrum of the gauche-conformer of isopropyl alcohol, which is complicated by internal rotation.Key words: radio-spectrometer, Hamiltonian, internal rotation, spectrum in the millimeter region.Introduction. Isopropanol, (CH 3 ) 2 CHOH, consists of two methyls and one hydroxyl that undergo internal rotation around the C-C and C-O single bonds, respectively. The rotational degree of freedom around the C-O bond gives rise to trans-and gauche-conformers. The gauche-conformer does not have a plane of symmetry and exists as two rotamers, the structures of which are mirror reflections of each other. They interconvert via tunneling of the hydroxyl H atom through the cis-barrier (the plane of symmetry of the methyls). A change of orientation of the OH group shifts considerably the internal axes and alters the direction of the dipole moment components relative to the principal axes of the molecule. This causes the rotational spectrum to become very complicated. It becomes impossible to describe it within the framework of the accepted model of a nonrigid asymmetric top. Internal rotation of these groups produces all possible rotation-vibrational interactions that perturb the rotational spectrum of isopropanol. Torsional vibrations relative to the cis-barrier of the hydroxyl H atom lift the double degeneracy of the torsion-vibrational levels, which results in the formation of two energetically nonequivalent torsion-vibrational symmetric (+) and antisymmetric (-) states.
We have studied the microwave rotational spectrum of the Tt conformer of the propyl alcohol molecule n-CH 3 CH 2 CH 2 OH in the frequency range 37.0-78.0 GHz. Theoretical treatment of the spectrum was carried out using Watson′s A-reduced rotational Hamiltonian. We identifi ed 78 rotational transitions with values of the rotational quantum number up to J = 37 inclusive. We have refi ned the rotational and centrifugal constants of the molecule.Keywords: microwave rotational spectrum, centrifugal distortion, Watson's Hamiltonian.Study of the microwave spectrum of the propanol molecule n-CH 3 CH 2 CH 2 OH (n-propanol) began in the 1970s [1][2][3][4][5][6]. In this paper, with the objective of refi ning the rotational and centrifugal constants of the Tt conformer of the n-propanol molecule, we have continued our study of its rotational spectrum in the frequency range 37.0-78.0 GHz for higher values of the rotational quantum numbers J (J ≤ 37). The refi ned rotational and centrifugal constants of the studied molecule should make it signifi cantly easier to further study the rotational spectra of this molecule and to search for certain spectral lines in the atmosphere and interstellar space.The n-propanol molecule is an asymmetric top type of molecule, close to a prolate symmetric top with χ = -0.978. It contains one methyl group, two methylene groups, and one hydroxyl group with a axis located near the symmetry axis and c axis perpendicular to the symmetry plane, in which the dipole moments of the molecule μ b (1.54 D) and μ a (0.21 D) lie. Conformational diversity, strong rotational-vibrational coupling, and considerable centrifugal distortion [7] appreciably complicate the spectrum of propyl alcohol. Therefore study of the rotational spectrum of the n-propanol molecule is still timely.In this paper, we carried out theoretical treatment of the spectrum using Watson′s A-reduced rotational Hamiltonian [8]. The calculations show that for a more detailed description of the rotational spectrum of a molecule with high J, we need to include higher order centrifugal constants in the calculation. The expression for Watson′s A-reduced Hamiltonian includes all the quartic, sextic, and octic plus fi ve dectic centrifugal distortion constants.The least-squares method was used to successively fi t the calculated frequencies of the newly identifi ed transitions to their experimental values, starting from low J. All the calculations were performed in a second axial representation. We identifi ed 78 rotational transitions with values of the rotational quantum number J ≤ 37. As a result, we were able to appreciably refi ne the rotational and centrifugal constants of the molecule. Table 1 gives the calculated and experimental frequencies of the identifi ed transitions in the n-propanol molecule; Table 2 gives the rotational and centrifugal constants, and also indicates the inaccuracies in the calculation of these parameters.The rotational and centrifugal constants found for the n-propanol molecule can be used to calculate to high accura...
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