ABSTRACT
The rotational spectra of four (GT, TT, TG, and GG) of the five possible conformers of 1-fluorobutane have been assigned by combining free jet and conventional microwave spectroscopy. The geometry optimization was performed at the MP2 (full) level of theory with the 6-31G (d) and 6-311G (d, p) basis sets and by using the B3LYP (3df, 3pd) density functional method. The relative stability of the five rotamers is calculated at the QCISD (T)/6-311G (d, p) level of theory. In spite of the fact that ab initio calculations indicated the unobserved GG' conformer to be more stable than at least one of the observed conformers it was not possible to detect its rotational spectrum. GT and TG are the most and the least stable species, respectively. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. The overall conformational equilibrium is governed by the two-dimensional potential energy surface of the skeletal torsions MeC-CC and FC-CC, which have been evaluated by a flexible model analysis, based on the experimental values of the relative conformational and vibrational energy spacings, and on the shifts of second moments of inertia upon conformational change and vibrational excitation. The relative energy of the fifth stable conformer (GG') was determined to be 333 cm(-1) from flexible model calculations, and to be 271 cm(-1) from the most accurate ab initio calculations.
ABSTRACT
The rotational spectrum of CD2I2 was measured and analyzed by combining results from two different millimeter wave spectrometers. Low-J transitions were measured with a free jet spectrometer at conditions of completely resolved hyperfine structure over the frequency range 59-69 GHz. High-J rotational transitions were measured at room temperature at frequencies 167-338 GHz and for J" up to 190". Spectroscopic constants in the sextic rotational Hamiltonian of CD2I2 have been determined, and the analysis of the observed hyperfine structure yielded all components in the inertial and in the principal nuclear quadrupole coupling tensors for the iodine nuclei. The rotational constants for CD2I2 and those for CH2I2 from Z. Kisiel, L. Pszczólkowski, W. Caminati, and P. G. Favero (1996. J. Chem. Phys. 105, 1778-1785) have been used to evaluate a full r0 structure and have been combined with the results of ab initio calculations to also evaluate the average r* structure: r(CI) = 2.1364 Å, r(CH) = 1.078 Å, <(ICI) = 113.83 degrees, and <(HCH) = 113.3 degrees. The current results complete the first analysis of a rotational spectrum of a molecule containing two iodine nuclei. Copyright 1998 Academic Press.
ABSTRACT
The rotational spectrum of p-anisaldehyde in a supersonic expansion has been investigated in the frequency range 60-78 GHz. Transitions up to J = 57 measured for the anti and syn conformers have been used to determine complete sets of fourth-order centrifugal distortion constants. Methyl group internal rotation splittings have also been observed for some of the lines and have yielded the respective barriers for both conformers. A vibrational satellite observed for each of the two conformers has been assigned to the respective first excited methoxy torsional state. The jet conditions, in particular the stagnation pressure of the carrier gas, can be adjusted to control the degree of cooling achieved in the expansion. This possibility has been exploited in the present work to enhance the intensities of the observed vibrational satellites. Applying a two-dimensional flexible model to the methoxyl and aldehydic groups torsions, the potential energy and structural deformation parameters transferred from anisole and benzaldehyde have been found to be suitable to describe these motions in p-anisaldehyde. Mixing of anti and syn conformations has been found to be insignificant within the first 82 calculated torsional eigenstates and therefore less likely to explain the intermediate bands observed in the low-resolution microwave spectrum than the internal vibrational relaxation suggested by R. K. Bohn, M. S. Farag, C. M. Ott, J. Radhakrishnan, S. A. Sorenson, and N. S. True (1992, J. Mol. Struct. 268, 107-121). A qualitative discussion of relaxation among the torsional states and its effect on the rotational spectrum is given. Copyright 1997 Academic Press. Copyright 1997Academic Press
