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The rotational spectrum of the weak complex between argon and pyrrole has been observed by use of a Fourier transform microwave spectrometer with a pulsed supersonic nozzle molecular beam source. The complex is a nearly symmetric prolate top with the Ar atom located above the plane of pyrrole, 3.553 (10) A from the pyrrole center of mass. The line between the Ar atom and the pyrrole center of mass makes an angle of 5 . 5 (20)' with the normal to the pyrrole plane and is displaced toward the N atom. The dipole moment of the complex, 1.707 (3) D, is smaller than that of free pyrrole, 1.767 (2) D, and the negative end of the complex's dipole moment is rotated from the pyrrole plane toward the Ar atom. The I4N quadrupole coupling constants have been determined. Based on the centrifugal distortion constant 0, and a pseudcdiatomic model, the binding energy of the complex is estimated to be 300 cm-I. The dipole moment and I4N quadrupole coupling constants of pyrrole were redetermined.The laser-induced fluorescence (LIF) spectra of trans-stilbene in a supersonic free jet have been investigated to discuss the vibrational level structure in the SI state. Since the rotational temperature is reduced to as low as 1.5 K, the congested vibronic bands can be resolved to uncover the level structure in the SI state. The rotational band contours have been measured to determine structural parameters of the SI state. It is concluded that in the SI state the C=C bond length, r c e , is larger by 0.05 A and the C=C-C angle, t9c,c--c, is smaller by 10' than those in the So state. Most of the transition lines to the SI vibrational levels above 800 cm-' exhibit splittings caused by Fermi coupling. The number of the observed split bands is larger in a transition to a higher Si vibrational level. Above 1200 cm-' from the band origin, the line broadening is found for respective lines, and this is attributed to a congestion caused by Fermi coupling with dense background levels. The observed change in the level structure caused by 13C or D isotope substitution is ascribed to differences between the level coupling schemes in the normal and isotopic species.

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The rotational spectra of benzyl alcohol and of its OD isotopologue have been assigned and measured in a supersonic expansion, either with pulsed-jet Fourier transform microwave or free jet absorption millimeter wave spectroscopy. The spectrum is consistent with a gauche conformation of the oxygen atom, characterized by a theta (OC(7)-C(1)C(2)) dihedral angle of approximately 55 degrees. Such a configuration is 4-fold degenerate, corresponding to minima with theta approximately +/-60 degrees, +/-120 degrees. The four equivalent minima are separated by two kinds of barrier, corresponding to theta = +/-90 degrees, and 0 or 180 degrees. Only the theta = +/-90 degrees barriers are low enough to generate a tunneling splitting, which has been measured in a spectrum strongly perturbed by tunneling interactions. The observed splittings diminish considerably upon deuterium substitution. The tunneling splittings are consistent with a barrier about 280 cm(-1) and high level ab initio calculations predicting a 320 cm(-1) barrier.

As part of our continuing spectroscopic program of measuring the rotational spectra of large gas-phase molecules that may exist in observable quantities in the interstellar medium, we have studied the millimeter-and submillimeterwave spectrum of the alcohol n-propanol (n-CH 3 CH 2 CH 2 OH ), a straight-chain molecule with one more carbon atom than ethanol, which has already been detected in hot molecular cores. Although the molecule is thought to exist in five different conformers, we have only studied what is most likely to be the Gauche-trans (Gt) conformer, and analyzed over 2800 lines through 375 GHz in frequency. These lines have been added to both previously and newly measured rotational lines at lower frequencies. The global data set has been fitted to experimental accuracy, allowing us to determine accurate spectral parameters so as to predict useful frequencies for many more transitions of the conformer.

Saturated hydrocarbons have structures with completely staggered bonds and dihedral angles of 180 degrees . Substituting hydrogen by fluorine results in a slight shift from 180 degrees , giving rise to a helical structure. X-ray diffraction studies on fibers and computational studies on perfluoroalkanes estimate a dihedral angle of about 17 degrees from the trans position. The rotational spectra of perfluoropentane and its three (13)C isotopomers have been observed and assigned using a pulsed-jet Fourier transform microwave spectrometer. The rotational constants for the parent species are A 990.6394(3) MHz, B 314.0002(1) MHz, and C 304.3703(1) MHz, respectively. The determination of an exact dihedral angle has been challenging, as the helical twist has proven to be quite sensitive to the structural inputs and constraints. A series of r(0) structures incorporating various model constraints and a Kraitchman analysis gives a range of 13-19 degrees for the torsional angle. An objective approach, which only assumes overall C(2) symmetry, is to scale the principal coordinates from ab initio models by the square root of the ratio of the observed second moments to the computed second moments. The scaled structures of computed models at various levels of theory reproduce the parent second moments exactly and the (13)C second moments very well, giving a dihedral angle of 17 +/- 1 degrees from trans. The microwave spectrum of perfluoropropane has also been observed and assigned. The rotational constants are A 1678.5982(9) MHz, B 900.1968(10) MHz, and C 955.3216(11) MHz, respectively. Unlike longer perfluoroalkanes, perfluoropropane has a nonhelical, C(2v) structure. Computations are in excellent agreement with experimental results.

In ethane the facts of a staggered equilibrium structure and a 2.9 kcal/mol torsional barrier are established. 1 However, the origin of the barrier to internal rotation in ethane remains controversial. This debate centers around the relative importance of hyperconjugation vs steric/exchange interactions in determining the staggered structure and torsional barrier. A distinction between the competing models is that steric interactions primarily depend on the distance between interacting atoms, whereas hyperconjugation depends also on the symmetry (orientation) of the interacting orbitals. The orbital orientation distinction suggests another perspective on the barrier. The staggered gauche and trans conformers of butane are a corollary of the staggered ethane structure. 3-Hexyne (CH 3 CH 2 CtCCH 2 CH 3 ) is butane elongated by a cylindrically symmetrical CtC spacer, which separates the ethyl groups. If hyperconjugation were the dominant interaction in this butane analogue, the equilibrium structure would be staggered. But if steric interactions were dominant, the structure would be eclipsed. The equilibrium conformation of 3-hexyne is experimentally determined and its structure is syneclipsed, C 2V , not staggered.