Assimo Maris

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.

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.

Proton Transfer in Homodimers of Carboxylic Acids: The Rotational Spectrum of the Dimer of Acrylic Acid

The dimer of acrylic acid can exist in two forms, depending on the entgegen or zusammen orientations of the two allyl groups. The latter one (zusammen) has a permanent value of the μ(b) dipole moment component, which allowed measuring its pulsed jet Fourier transform microwave (MW) spectrum. From the tunneling splitting, originating in the concerted proton transfer of the two carboxylic hydrogen atoms and measured for four isotopologues of such a bimolecule, we could determine the barrier and dynamics of the proton transfer.

Free jet rotational spectrum and Ar inversion in the dimethyl ether–argon complex

Abstract The results of free jet millimeter-wave absorption and molecular beam Fourier transform microwave investigations of dimethyl ether–Ar are reported. The Ar atom lies in the σ v symmetry plane of dimethyl ether at a r 0 -distance of 3.58 A and the line connecting the argon atom to the center of mass ( cm ) of dimethyl ether (Ar- cm ) forms a r 0 -angle of 77° with the O- cm line. All rotational transitions are split into two component lines, due to the tunnelling of Ar from above to below the C–O–C plane. The corresponding inversion splitting has been used to determine the barrier to inversion. Some additional information on the pathways of the Ar inversion and the potential energy function have been obtained from ab initio calculations.

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.

Pure rotational spectrum of 2-pyridone⋯water and quantum chemical calculations on the tautomeric equilibrium 2-pyridone⋯water/2-hydroxypyridine⋯water

Abstract The free-jet millimeter-wave spectrum of the complex 2-pyridone⋯water has been measured. The water is strongly linked to the partner group through a double hydrogen bond. We could not observe the spectrum of 2-hydroxypyridine⋯water (2-hydroxypyridine is more abundant than 2-pyridone in the tautomeric mixture). This failure is explained by density functional and ab initio calculations on the relative stability of the two 1:1 complexes with water. Estimates of the dissociation energy have been obtained both from the centrifugal distortion effects and from quantum chemical calculations.

Features of the C-H...N weak hydrogen bond and internal dynamics in pyridine-CHF3.

The structural and energetic features of the C H···N interaction and the internal dynamics of the pyridine–trifluoromethane molecular complex in its normal, N and C species are here reported. They have been obtained from analysis of the pure rotational spectra of the complex generated in a supersonic expansion. Weak hydrogen bonds (WHB) are a major topic in hydrogen-bond research. The energies of these interactions lie within a few kJ mol 1 and approach those of van der Waals forces, but have the same directional properties of “classical” hydrogen bonds. This behavior was recently shown for the C H···O, C H···F C, C H···S, and C H···p linkages by investigating the rotational spectra of several hydrogenbonded molecular complexes generated in supersonic jets. With this technique, precise information on the energetics and on the structural and dynamical aspects of such interactions is obtained in an environment free from the intermolecular interactions that take place in condensed media. Investigation of the rotational spectrum of benzene–trifluoromethane has shown that this complex is a symmetric top, with the two moieties held together through a C H···p interaction, and thus provided information on this kind of WHB. When replacing benzene with pyridine, two sites of high electronic density become available in the ring, so the two adducts shown in Figure 1 could be plausibly formed for the pyridine–trifluoromethane (Py–CHF3) complex.

Molecular Recognition of Chiral Conformers: A Rotational Study of the Dimers of Glycidol

Two homochiral dimers of glycidol, deriving from two different conformers, have been characterized by rotational spectroscopy in a supersonic expansion.

Rotational spectrum, dynamics, and bond energy of the floppy dimethylether⋯neon van der Waals complex

The equilibrium conformation, dynamics, and dissociation energy of the weakly bonded dimethylether⋯Ne complex have been deduced from its pulsed jet millimeter wave spectrum. Each rotational transition is split into two component lines. The corresponding vibrational splitting has been determined to be ΔE=807.2(9) MHz. The inversion barrier between the two equivalent minima with Ne above or below the COC plane has been calculated from this datum to be 16 cm−1. The dissociation energy of the complex is estimated, from the centrifugal distortion constant DJ, to be ca. 1.0 kJ/mol.

Intramolecular hydrogen bonds and conformational properties of biogenic amines: A free-jet microwave study of tyramine

The conformational properties of tyramine, an important biologically active molecule, were studied by free jet microwave absorption spectroscopy aided by quantum chemical calculations. Structural assignments were made on the basis of ab initio predicted spectral properties calculated at the MP2/6-31G* level of theory in the minima found in the conformational hypersurface and four conformational species were detected in the jet expansion. The most stable conformers are the folded ones in which the amino hydrogen atoms interact with the aromatic π cloud showing that the conformational properties of tyramine are mainly determined by non-bonding interactions.