Sonia Melandri

Observation of the rotational spectra of van der Waals complexes by free jet absorption millimeter wave spectroscopy: pyridine-argon

Abstract A free jet millimeter wave spectrometer has been modified in order to observe the rotational spectra of van der Waals molecular complexes. Here the free jet millimeter wave spectrum of the pyridine-argon molecular complex, as investigated in the 60–78 GHz frequency range, is reported.

The C-F⋅⋅⋅H-C “Anti-Hydrogen Bond” in the Gas Phase: Microwave Structure of the Difluoromethane Dimer

: A shortening of the C-H bond lengths and a blue shift of the C-H stretching frequencies for the C-F small middle dot small middle dot small middle dotH-C groups indicates that anti-hydrogen bonds are present the difluoromethane dimer. The most stable conformer has three such interactions (shown schematically).

A microwave free jet absorption spectrometer and its first applications

Abstract A free jet absorption microwave spectrometer working in the frequency range 50–80 GHz has recently been built in our laboratory. Different experiments can be performed with the system: the microwave radiation can be focussed on the supersonic expansion either with a lens corrected horn system or with a semiconfocal Fabry-Perot interferometer. The microwave radiation can be either parallel or perpendicular to the jet, and Stark modulation or double resonance experiments (MWMW or RFMW, respectively) are possible. As first applications, studies on complicated conformational equilibria, tautomeric equilibria and molecules with puzzling nuclear hyperfine structure (for example, a molecule containing two iodine atoms) have been performed.

Intermolecular Hydrogen Bonding between Water and Pyrazine

Completely planar is the hydrogen-bonded complex of pyrazine and water (see sketch), which was obtained by supersonic expansion and investigated by rotational spectroscopy. The water molecule lies in the plane of the aromatic ring, and the lone pair of electrons on the nitrogen atom functions as the acceptor in the N⋅⋅⋅H-O hydrogen bond, not-as in the corresponding benzene complex-the π electrons.

Free jet absorption millimeter wave spectrum of the pyrimidine—argon molecular complex

Abstract The free jet millimeter wave spectrum of the pyrimidine-argon molecular complex has been investigated in the 60–78 GHz frequency range. Estimates are given of the force constants for the Van der Waals vibrations, as well as of Coriolis and mass dispersion effects related to the argon vibrations parallel to the ring plane.

Die C-F⋅⋅⋅H-C-„Anti-Wasserstoffbrückenbindung” in der Gasphase: mikrowellenspektroskopisch ermittelte Struktur des Difluormethan-Dimers

Eine Verkurzung der C-H-Bindungen und eine Blauverschiebung der Frequenzen der C-H-Streckschwingungen in den C-F⋅⋅⋅H-C-Gruppen deuten darauf hin, das im Difluormethan-Dimer Anti-Wasserstoffbruckenbindungen vorliegen. Das stabilste Konformer des Dimers enthalt drei derartige Brucken (siehe Bild).

PSEUDOROTATION PATHWAY AND EQUILIBRIUM STRUCTURE FROM THE ROTATIONAL SPECTRUM OF JET-COOLED TETRAHYDROFURAN

The rotational spectrum of jet-cooled tetrahydrofuran has been investigated in the 8–18 and 60–78 GHz frequency ranges. Only the four lowest vibrational levels remain populated in the jet conditions. Two pseudorotation vibrational spacings have been measured directly to be ΔE01=21 307.71(3) MHz and ΔE23=61 205.69(3) MHz. They have been used, together with the lowest 10 measured far infrared transition frequencies and with the effective moments of inertia of the ground state and their shifts upon excitation observed in this work, to determine the potential energy function for pseudorotation and the associated structural relaxations. The potential energy barriers are 45 cm−1 at the envelope conformation of the oxygen and 16 cm−1 at the twisted conformation of the Cβ–Cβ′ bond relative to the CαOCα′ plane. The four symmetrically equivalent equilibrium structures are close to the envelope conformations of the CH2 groups next to the oxygen atom. While the C–O bond lengths and the local CH2 angles appeared to re...

Free jet absorption millimetre-wave spectrum and model calculations of phenol–water

Abstract The free jet millimeter-wave spectra of three isotopomers of the 1:1 complex between phenol and water have been measured. The water molecule has been reliably located in the complex from these experimental data. The two-dimensional potential energy surface of the internal rotation and inversion of water in the complex has been evaluated by B3LYP/6-31G** and MP2/6-31G** quantum chemical calculations, and adapted with a flexible model to interpret the spectroscopically observed tunneling effects of the two motions.

Hydrogen bonding, structure, and dynamics of benzonitrile–water

Rotational transitions with high quantum numbers J and K of the 1:1 complex of benzonitrile with H2O and D2O have been investigated in the frequency range 60–78 GHz with the free jet absorption microwave technique to get detailed information on the unusual hydrogen bond and on the dynamics of the large amplitude motions of the water moiety. With respect to previous microwave studies [V. Storm, D. Consalvo, and H. Dreizler, Z. Naturforsch. A 52, 293 (1997); R. M. Helm, H.-P. Vogel, H. J. Neusser, V. Storm, D. Consalvo, and H. Dreizler, 52, 655 (1997); V. Storm, H. Dreizler, and D. Consalvo, Chem. Phys. 239, 109 (1998)] the position of the water oxygen has been confirmed and the planar configuration of the complex has been determined. The distance of the oxygen atom to the ortho hydrogen is 2.48 A, the angle to the ortho C–H bond is 144° and the angle between the free hydrogen atom of water with the same C–H bond is 164°. A coupled analysis of the 0+ and 0− states observed for the normal species was perform...

Microwave spectroscopy of hydroquinone: The rotational spectrum of the cis conformer

The microwave spectra of hydroquinone and of its O–D mono‐ and di‐deuterated species have been investigated in the frequency range 8–40 GHz. Only the cis rotamer, which possesses a permanent dipole moment, has been observed. For this conformer the rotational and quartic centrifugal distortion constants have been obtained and the dipole moment has been measured. No doubling due to internal rotation between the two equivalent minima of the hydroxyl groups has been resolved.