Georges Roussy

Spectres de rotation de la molécule de chlorobenzène: II. Variétés isotopiques multisubstituées et structure géométrique précise

Abstract In order to complete the r s structure of chlorobenzene given in a preceding paper, a variety of isotopic species of this molecule were synthesized and their microwave spectra studied. This made twenty isotopic species available, enabling the determination of the geometrical parameters by a least squares method. Fitting only differences of moments of inertia either for monosubstituted species or to multiply substituted species gave the same result. They hardly differ from the r o values and agree with the r s values obtained by the Kraitchman equations. The resulting error limits were reduced, however. The following structural parameters were obtained, C 1 C 2 = 1.399 A, C 2 C 3 = 1.386 A, C 3 C 4 = 1.3976 A, C 1 Cl = 1.7248 A, C 2 H 2 = 1.080 A, C 3 H 3 = 1.081 A, C 4 H 4 = 1.081 A, C 6 C 1 C 2 = 120° 16, C 1 C 2 C 3 = 119°78, C 2 C 3 C 4 = 120°24, C 3 C 4 C 5 = 119°80, C 1 C 2 H 2 = 119°45, C 2 C 3 H 3 = 119°76. The structure of the ring differs significantly from C 6 symmetry. The deformation can be regarded as a compression of the position C 1 while the angle of C 2 H 2 bond is also changed.

Etude de la structure geometrique de la molecule de para-chloroaniline par spectroscopie microonde

Abstract The microwave spectra of the normal, mono and di N -deuterated species of para 35 Cl and 37 Cl aniline have been observed. The rotators associated with the two first states of the amino group inversion and with the three first states of the chlorine atom out of the plane vibration have been assigned. A geometrical structure has been calculated from the moments of inertia of all isotopic species in the ground state, the out of plane angle of the NH 2 group being 42.5°. The electronic interaction between the chlorine atom and the amino group has been pointed out.

Microwave spectroscopy and nuclear magnetic resonance study of the molecular geometry of 2-fluorophenylisocyanate

Abstract Analysis of the microwave spectra of the vibrational ground state of 2-fluorophenyl-isocyanate in the gaseous state shows the existence of a single planar cis conformer. Rotational spectra of the excited torsional states out of the plane and spectra of in-plane vibrations have also been studied. MINDO III type theoretical calculations corroborate the experimental results. Similarly, the configuration of 2-fluorophenylisocyanate in the liquid phase has been studied by NMR using the chemical shift reagent, Eu(fod)3. Induced paramagnetic shifts have been measured on the 13C spectra, for different quantities of the reagent. The results clearly show a higher paramagnetic shift for the carbon bonded to the fluorine atom than for the other nuclei of the aromatic cycle (with the exception of the quaternary carbon), indicating the probable presence of the cis isomer.

The microwave spectrum of 4-pyridine carbaldehyde (isonicotinealdehyde)

Abstract The microwave spectrum of 4-pyridine carbaldehyde has been investigated in the region 8 to 40 GHz. Rotational transitions have been observed and assigned for the ground state and two excited states of the torsion mode. Analysis yields precise rotational constants ( A = 5519.04 ± 0.08, B = 1559.17 ± 0.03, C = 1216.11 ± 0.02 MHz) which prove the molecule to be planar. Centrifugal distortion constants have also been obtained. Analysis of the observed 14 N quadrupole fine structure yields the following quadrupole coupling constants (in MHz): χ aa = −4.67 ± 0.09; χ bb = 1.19 ± 0.26; χ cc = 3.48 ± 0.26. The electric field gradient about the nitrogen nucleus is thus similar to that of pyridine.

Spectres de rotation des especes hexa- et octo-deuteriees de la molecule de cis-but-2-ene

Resume Les spectres de rotation de cis -2-3-H 2 - d 6 -but-2-ene et cis-d 8 -but-2-ene ont ete observes dans le domaine de frequence 18–40 GHz. La contribution de la rotation interne des toupies est negligeable. La validite de la barriere haute est confirmee en observant que tous les transitions de rotation sont des triplets symetriques. Les constantes de rotation rigides ont ete determinees precisement et la geometrie de la molecule cis -but-2-ene est discutee.

Microwave spectra of cis and trans 2-furylisocyanate conformers

Abstract The microwave spectrum of 2-furylisocyanate has been obtained in the frequency region from 8 to 40 GHz. This spectrum is attributed to the ground state of the cis and trans configurations. The rotational constants for both ground vibrational states have been determined. Two sets of vibrational satellites are observed and assigned to the modes of C-N torsion and CNC bending. The microwave results show and MINDO/3 calculations confirm that the barrier between the cis and trans conformers is high and that the cis conformer is more stable than the trans by 3 kcal mol −1 .

Conformational analysis of gaseous 1-oxa-spiro [2.5] octane by microwave spectroscopy and molecular mechanics calculations

Abstract Microwave experiments have been coupled with molecular mechanics calculations to obtain the gas-phase structure of 1-oxa-spiro [2.5] octane. The molecule is found to exist as a mixture of two chair conformational isomers. The effects of spiro substitution upon the conformational behavior of the cyclohexane ring are discussed and related to the corresponding computed data.

6,6-dimethyl-1-oxa-spiro[2.5]octane. Conformational equilibrium in the gas phase as studied by microwave spectroscopy and molecular mechanics calculations

Abstract The gas-phase molecular structure of 6,6-dimethyl-1-oxa-spiro[2.5]octane has been investigated by joint analysis of the microwave spectrum and molecular mechanics calculations. Both experimental and theoretical data concur showing the presence of two stable conformational isomers having a chair cyclohexane ring. A set of structural parameters which reproduce the ground state rotational constants has been derived. The influence of 1,4 disubstitution is discussed in terms of deformations of the cyclohexane ring.

High J rotational transitions free of centrifugal distortion

Abstract It has been observed that the centrifugal distortion correction of the frequency of b Q XXX transitions cancels out at certain high J values. These J values are identified with the roots of the derivative of the transition frequency function with respect to the asymmetry parameter. It is proved that these J values are also the roots of the derivative of the transition frequency function with respect to the quantum number, J . Thus they may be readily determined by the rigid rotator approximation and can also be predicted and assigned without analysing the centrifugal distortion of the rotator.

The gas-phase molecular structure of 1-thia-spiro [2.5] octane: A microwave and molecular mechanics study

Abstract This paper describes the results on the gas-phase molecular structure of 1-thia-spiro [2.5] octane probed by a combination of microwave experiments and molecular mechanics calculations. Of the two theoretically predicted conformational isomers only one has been detected experimentally showing a pseudoequatorial orientation for the sulfur atom. A set of geometrical parameters which gives a good fit to the rotational constants has been derived. In agreement with molecular mechanics predictions, we conclude that the cyclohexane ring has a slightly flattened chair conformation.