Tautomerism and microsolvation in 2-hydroxypyridine/2-pyridone.

The Fourier transform microwave spectra of the hydrated forms of the tautomeric pair 2-pyridinone/2-hydroxypyridine (2PO/2HP) have been investigated in a supersonic expansion. Three hydrated species, 2PO-H2O, 2HP-H2O, and 2PO-(H2O)2, have been observed in the rotational spectrum. Each molecular complex was confidently identified by the features of the ¹4N quadrupole hyperfine structure of the rotational transitions. The presence of water affects the tautomeric equilibrium -N═C(OH)- ↔ -NH-C(═O)-, which is shifted to the enol form for the bare molecules 2PO/2HP but to the keto tautomer for the hydrated forms.

Conformational impact of aliphatic side chains in local anaesthetics: benzocaine, butamben and isobutamben.

We studied the impact of aliphatic side chains on the stability and conformational landscape of the local anaesthetics benzocaine, butamben and isobutamben, combining high-resolution rotational spectroscopy in the microwave and millimetre regions and molecular modelling. The study reveals the connections between alkyl chain flexibility and molecular conformations.

Conformational equilibrium and potential energy surface of 1-fluorobutane by microwave spectroscopy and Ab initio calculations

The rotational spectra of four (GT, TT, TG, and GG) of the five possible conformers of 1-fluorobutane have been assigned by combining free jet and conventional microwave spectroscopy. The geometry optimization was performed at the MP2 (full) level of theory with the 6-31G (d) and 6-311G (d, p) basis sets and by using the B3LYP (3df, 3pd) density functional method. The relative stability of the five rotamers is calculated at the QCISD (T)/6-311G (d, p) level of theory. In spite of the fact that ab initio calculations indicated the unobserved GG' conformer to be more stable than at least one of the observed conformers it was not possible to detect its rotational spectrum. GT and TG are the most and the least stable species, respectively. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. The overall conformational equilibrium is governed by the two-dimensional potential energy surface of the skeletal torsions MeC-CC and FC-CC, which have been evaluated by a flexible model analysis, based on the experimental values of the relative conformational and vibrational energy spacings, and on the shifts of second moments of inertia upon conformational change and vibrational excitation. The relative energy of the fifth stable conformer (GG') was determined to be 333 cm(-1) from flexible model calculations, and to be 271 cm(-1) from the most accurate ab initio calculations.

The C-F small middle dot small middle dot small middle dotH-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).

Internal dynamics features in the free jet rotational spectrum of the acetaldehyde-Kr molecular complex.

The structure and the dynamics of internal motions in the complex formed between acetaldehyde and Kr are studied by free jet absorption microwave spectroscopy performed in the range 60-78 GHz. The fourfold structure of each rotational line is evidence of the vibration-rotation coupling between the overall rotation of the complex, a tunneling motion of the Kr atom between two equivalent positions and the internal rotation of the methyl group in the acetaldehyde moiety. The four sets of transitions could be fitted with a coupled Hamiltonian which allows for the Coriolis interaction obtaining the energy separation between the vibrational energy levels related to the tunneling motion, while the observed splittings due to the methyl group internal rotation were analyzed independently with an appropriate model. The potential energy barriers for the tunneling motion and the internal rotation of the methyl group have been calculated and the interaction of the rare gas atom with the acetaldehyde moiety is reflected in the change of the V(3) barrier to internal rotation in going from the molecule to the weakly bound complex.

Large Amplitude Motions in 2,3-Cyclopentenopyridine

Large amplitude motions in 2,3-cyclopentenopyridine have been investigated by microwave spectroscopy and ab initio calculations. The rotational spectra of the ground and of several low energy vibrational excited states have been assigned. The potential energy functions of the ring puckering and 1,3-ring-twisting motions have been obtained by applying a flexible model to the experimental data. Copyright 1997 Academic Press. Copyright 1997Academic Press

Millimeter-Wave Free Jet Absorption Spectrum of SD Methylthioglycolate: Description of the SH Torsion Double Minimum Potential

The free jet absorption millimeter-wave spectrum of methylthioglycolate-SD has been investigated in the 60-78 GHz frequency range. The double-minimum potential associated with the SD group torsion generates a vibrational spacing Delta01 = 8285(5) MHz between the O+ and O- states of the A sublevel. This splitting, compared to the corresponding value of 18 953 MHz for the normal species, suggests a large involvement of the sulfur atom in the motion. A model is given to describe this motion. Copyright 1997 Academic Press. Copyright 1997Academic Press

Rotational Spectrum of CD2I2

The rotational spectrum of CD2I2 was measured and analyzed by combining results from two different millimeter wave spectrometers. Low-J transitions were measured with a free jet spectrometer at conditions of completely resolved hyperfine structure over the frequency range 59-69 GHz. High-J rotational transitions were measured at room temperature at frequencies 167-338 GHz and for J" up to 190". Spectroscopic constants in the sextic rotational Hamiltonian of CD2I2 have been determined, and the analysis of the observed hyperfine structure yielded all components in the inertial and in the principal nuclear quadrupole coupling tensors for the iodine nuclei. The rotational constants for CD2I2 and those for CH2I2 from Z. Kisiel, L. Pszczólkowski, W. Caminati, and P. G. Favero (1996. J. Chem. Phys. 105, 1778-1785) have been used to evaluate a full r0 structure and have been combined with the results of ab initio calculations to also evaluate the average r* structure: r(CI) = 2.1364 Å, r(CH) = 1.078 Å, <(ICI) = 113.83 degrees, and <(HCH) = 113.3 degrees. The current results complete the first analysis of a rotational spectrum of a molecule containing two iodine nuclei. Copyright 1998 Academic Press.

Free Jet Absorption Millimeter Wave Spectrum of Pyrrolidine: Assignment of a Second, Equatorial, the Most Stable Conformer.

The equatorial conformer of pyrrolidine has been discovered while investigating the rotational free jet spectrum of the pyrrolidine-water adduct. It is more stable than the axial species, previously assigned with conventional microwave spectroscopy (W. Caminati, H. Oberhammer, G. Pfafferott, R. R. Filgueira, and C. H. Gomez, 1984. J. Mol. Spectrosc., 106, 217-266). The assignment of the equatorial conformer was missed in that microwave investigation because of the accidentally zero or almost zero value of its µa dipole moment component; its low J µc-type transitions, very weak in the room temperature spectrum, are the strongest lines in the jet. Copyright 1998 Academic Press.