Paolo Ottaviani

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.

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.

The non-coincidence effect in binary mixtures: is a sign inversion with dilution fake or reality?

This study investigates whether benzene may be effective in promoting the sign inversion of the non-coincidence effect of the carbonyl stretching mode, v3(C=O), of acetone in the high dilution regime, as claimed in literature. We have found that benzene leaves the non-coincidence effect positive in the whole concentration range and unaltered with respect to that already observed in acetone/CCl4 mixtures at high dilution, although slightly reinforcing it. The applicability of available theories to the concentration dependence of the non-coincidence effect in benzene is discussed.

Tunneling motions of argon on chlorofluoromethane

The rotational supersonic jet Fourier transform microwave spectra of the 35Cl and 37Cl species of the molecular complex chlorofluoromethane-argon show that, in its equilibrium conformation, the argon atom is located out of the ClCF plane, interacting with the F and Cl atoms. All rotational transitions are split into several quadrupole components, each of them further split into two lines, due to the tunneling motion of the Ar atom between two equivalent positions, below and above the ClCF plane. The feasible low energy pathway between the structurally degenerate conformations is described, in a first approximation, by a circular motion around the C-Cl bond, with barriers estimated to be about 61 and 100 cm(-1).

Rotational spectrum and internal dynamics of methylpyruvate.

The rotational spectra of five isotopologues (normal and all monosubstituted (13)C species) of methylpyruvate have been measured with the pulsed jet Fourier transform microwave technique. Rotational transitions are split into quintets due to the internal rotations of the two methyl groups. The corresponding barriers to internal rotation have been determined to be V(3)(H(3)C-O) = 4.883(8) kJ mol(-1) and V(3)(H(3)C-C) = 4.657(8) kJ mol(-1), respectively. Information on the skeletal heavy atom structure has been obtained from the 15 available rotational constants.

Conformational preferences of chiral molecules: free jet rotational spectrum of 1-phenyl-1-propanol

The rotational spectra of normal and O-d species of the two most stable conformers of chiral 1-phenyl-1-propanol, obtained by free jet millimetre-wave absorption spectroscopy reveal that both conformers are stabilized by a O-H[dot dot dot]pi interaction, and have the Calpha-Cbeta-bond oriented nearly perpendicular to the plane of the benzene ring. The methyl group is trans with respect to the phenyl group for the most stable conformer (T), while it is gauche with respect to the phenyl group and entgegen with respect to the hydroxyl group for the second most stable conformer (GE). The energy difference (E(GE)-E(T)) was estimated to be 50(50) cm(-1) from relative intensity measurements.

Structure, dipole moment and large amplitude motions of 1-benzofuran

The rotational spectrum of 1-benzofuran has been investigated by three different rotational spectroscopy techniques: (i) millimeterwave absorption free jet spectroscopy, useful for a fast assignment of the spectrum; (ii) molecular beam Fourier transform microwave spectroscopy, sensitive to detect less abundant isotopic species in natural abundance; (iii) waveguide conventional microwave spectroscopy, useful for the study of intramolecular dynamics, through the rotational spectra of the vibrational satellites of low energy modes. Besides the rotational spectrum of the ground state of the normal species, the spectra of 9 singly substituted 13C and 18O isotopomers in natural abundance, and of 6 vibrational satellites, have been measured. Precise structural parameters for the molecule, as well as information on the potential energy surface of the low energy vibrations, have been obtained. The dipole moment components have been determined to be micro(a)= 0.216 (2) and micro(b)= 0.720 (3) D, respectively.