Biagio Velino

Microwave spectrum and ab initio calculations of ethylbenzene: potential energy surface of the ethyl group torsion

The weak rotational spectrum of the ground and three vibrational states belonging to the ethyl group torsional progression of ethylbenzene has been measured using conventional microwave spectroscopy. The small dipole moment (μa = 0·49(4), μc = 0·2(1) D) has been determined from a second order Stark effect, and is compared with a variety of ab initio calculations at the RHF/6-31G, MP2/6-31G, RHF/6-31G*, MP2/6-31G* and RHF/6-31G** levels. The potential energy surface for the rotation of the ethyl group with respect to the ring has also been estimated on the basis of vibrational satellite intensities, shifts of rotational constants upon torsional excitation, and of ab initio calculations. A conformation in which the ethyl group is perpendicular to the nominal ring plane is found to be the most stable.

Reinvestigation of the microwave spectrum of cyanocyclobutane: assignment of the axial conformer

Abstract The microwave spectrum of the axial conformer of cyanocyclobutane has been assigned on the basis of its ab initio structure. From dipole moment and relative intensity measurements it has been possible to determine the relative energy with respect to the previously assigned equatorial conformer: E (axial) − E (equatorial) = 258 ± 50 cm −1 .

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.

Conformation of dimethoxymethane: roles of anomeric effects and weak hydrogen bonds. A free jet microwave study

The rotational spectrum of dimethoxymethane has been measured by free jet millimetre-wave absorption spectroscopy. It corresponds to a distorted, but C2 symmetric, configuration with the two methyl groups gauche with respect to the far oxygen atom. Each rotational transition is split into several, allowed and forbidden, component lines due to the two methyl groups internal rotations. The corresponding V3 barrier has been determined to be 1.58 kcal mol−1. The “distorted” shape is attributable to the anomeric effect, but also to intramolecular weak C–H⋯O hydrogen bonds.

Conformational equilibrium and internal hydrogen bonding in 2-methylallyl alcohol: Detection of a second conformer by microwave spectroscopy on the basis of ab initio structure calculations

Abstract On the basis of 4-21G ab initio geometry refinements the microwave spectrum of a second conformer of 2-methylallyl alcohol, CH2C(CH3)CH2OH, has been assigned. In a previous investigation a conformer with the oxygen atom skew with respect to the CC bond was detected [W. Caminati and A. C. Fantoni, Chem. Phys.105, 59–67 (1986)]. In the present study the conformer with the oxygen atom syn to the double bond and with the hydroxyl hydrogen tunneling through the heavy atom plane has been measured.

The rotation-inversion spectrum of 15NH3

Abstract The rotation-inversion spectrum of 15 NH 3 has been recorded between 38 and 280 cm −1 with a resolution of about 0.03 cm −1 . By combining the present results with the inversion frequencies obtained by microwave spectroscopy, the following main rotational and centrifugal distortion constants were derived (in cm −1 ): State B D J D JK v inv = 0 9.92235 0.0008493 −0.001575 v inv = 1 9.91742 0.0008336 −0.001532

Microwave spectrum and ab initio calculations of indazole

Abstract The microwave spectrum of indazole and its ND isotopomer has been investigated at 70°C. The tautomer with the NH group closer to the phenyl ring is found to be the more stable both by the analysis of the rotational spectrum and ab initio calculations. The second tautomer is calculated at HF/4–21G level of theory to be 2800 cm −1 higher in energy. Information on the dipole moment and the low energy vibrational excited states has also been obtained.

Microwave spectrum of benzimidazole

Abstract The microwave spectra of benzimidazole and of its N-D monodeuterated isotopic species, obtained at 120°C, show the full planarity of the molecule. Information on the lower energy vibrational excited states, and on the dipole moment have also been obtained.

Rotational spectrum and molecular properties of pyridine xenon

The rotational spectra of six isotopologues of pyridine-xenon, two isotopes of the nitrogen atom ((14)N and (15)N) in pyridine with three isotopes of the rare gas atom ((129)Xe, (131)Xe, and (132)Xe) have been measured by pulsed jet Fourier transform microwave spectroscopy. The complex has a structure with the xenon atom located in the plane of symmetry perpendicular to the aromatic ring plane. Its distance from the center of mass of pyridine is 3.81 A, and it is tilted--with respect to the c principal axis of pyridine--by 7 degrees toward the N atom. The (14)N and (131)Xe nuclear quadrupole coupling constants have been determined for the isotopologues containing these nuclei. Information on the dynamics of the Xe van der Waals motions was obtained from the centrifugal distortion and from the changes in the planar moments of inertia in going from pyridine to pyridine...Xe.

A study of the large amplitude motions of indoline through microwave spectroscopy and ab initio calculations : evidence of ring puckering and butterfly motion coupling

We report the gas phase microwave observation and subsequent analysis of the ring puckering level splitting of indoline and its N-D isotopomer. The spectra of the four lowest vibrational states are modelled through a two-dimensional vibrational Hamiltonian to furnish a coherent picture. According to this new picture, five-membered rings with a single double bond and five-membered rings in which the double bond is fused with a benzenic ring share a similar low ring puckering barrier. Discrepancies with previous work are ascribed to the use of a one-dimensional vibrational Hamiltonian. Ab initio calculations concur with the present analysis.