A.O. Diallo

Rotational isomerism in 3-bromo, 2-methylpropene studied by i.r. spectroscopy

Abstract From analyses of i.r. spectra of the title compound, CH 2 C(CH 3 )CH 2 Br, it was shown that the molecular system behaves at room temperature as a mixture of three rotational isomers in very different proportions, with a clear predominance of the apparently less polar form in all phases. The energy difference between two of these rotamers in the liquid phase was estimated to be 0.35 kcal/mol under experimental conditions where the concentration of the third form was negligibly small. A normal coordinate analysis was carried out, assuming the molecules in their most symmetrical configuration (point group C S ), and the calculated frequencies were used as a guide in assigning the observed fundamental bands.

Vibrational spectra and structure of cyclopropane-d1 and dicyclopropyl ketone

Abstract Infrared spectra have been obtained for cyclopropane- d 1 , C 3 H 5 D(CP- d 1 ), in the vapor phase and for dicyclopropyl ketone (C 3 H 5 ) 2 CO (DCPK) in the liquid state. Raman spectra of the latter compound in the liquid and solid phases have also been recorded. By comparing the experimental with the theoretical spectra of CP- d 1 it has been possible to assign most of the fundamental frequencies of the molecule. Additionably, empirical spectra—structure correlations from the literature for cyclopropyl derivatives containing the COX moiety (X=halogen) have been used to investigate the conformational preference of DCPK. The experimental results confirm that the molecular configuration with the carbonyl group cis to the planes of the two rings is the predominant species in the liquid state, though evidence for a second conformer, which we tentatively identify as the cis—trans conformer, is presented.

Infrared spectra and conformation of cyclopropylamine, CPA

Abstract Infrared spectra have been obtained for CPA, C3H5NH2, in the liquid state at room temperature and in the vapor state at a series of temperatures. The spectra confirm the presence of only one rotational isomer of this molecule, the cis form (symmetry Cs). Approximate values for the NH stretching force constant and the HNH angle have been calculated from the observed NH2 stretching frequencies of the compound. The results have been compared with the analogous parameters of related molecules of known structure in order to investigate the nature of the bonding of the nitrogen to the cyclopropyl ring. The present i.r. spectral data are consistent with the absence of conjugation between the amino group and the ring established by ab initio calculations on the CPA molecule.

Rotational isomerism in 3-methyl, 3-butenenitrile(methallyl cyanide)

Abstract Infrared spectra were obtained for the title compound, CH2C (CH3) CH2 CN, in the liquid and solid states. The rotational isomerism of the molecule was investigated on the basis of solvents and temperature dependence of the absorption spectra. The results revealed the coexistence of three conformational isomers in the liquid state. The energy difference between two of them, the more and the less polar forms as inferred from the solvents variation study, was estimated to be 1.35 kcal/mol (mean value). A normal coordinate analysis assuming valence force field constants and structural parameters was carried out and the calculated frequencies were used as a guide in approximately assigning the observed fundamentals to their normal modes.

Infrared spectra and rotational isomerization of 1-bromo-3-methyl-2-butene

Abstract Infrared spectra obtained for the title compound, (CH 3 ) 2 CCH. CH 2 Br, have revealed the co-existence of two rotational isomeric forms in the fluid phases, the geometrical structures of which cannot be ascertained by use of vibrational spectroscopy alone. The energy ditterence between the two isomers was found to be 1.1 kcal mol −1 (mean value) in the liquid state. Some of the i.r. fundamental bands were assigned using the characteristic absorption frequencies of localized group.

Spectres de vibration des groupements hydroxyles dans les alcools primaires mononitres et le methy-2-propanol-1

Abstract The absorption spectra of Me2CHCH2OH, MeCH(NO2)CH2OH, Et·CH(NO2)CH2OH, Me2C(NO2)CH2OH, Me·C·NO2(CH2OH)2 Et·CNO2(CH2OH)2 and CNO2(CH2OH)3 in CCl4 solution, have been studied in the 3 μ region. In all the compounds studied two distinct v (OH) bands corresponding to the monomeric molecules were observed. The magnitude of the frequency shift and the reversal of the relative intensities of the doublet components with an increase in the number of hydroxyl groups indicate the existence of weak intramolecular interactions between the nitro and hydroxyl groups.

Infrared absorption spectra of 2,2-dibromobutane

Abstract The possible existence of geometrically isomeric forms of the title compound, CH3CBr2CH2CH3, has been investigated by i.r. spectroscopy. It was found that the general features of the spectra can be interpreted reasonably in terms of only one conformation of the molecules in the liquid state. Further confirmation of this view, based on a brief comparison of the liquid phase spectra of CH3CBr2CH2CH3 and CH3CHBrCHBrCH3 is given. Vibrational assignments have been made for most of the fundamental bands assuming Cs molecular symmetry.

Infrared spectra and conformational structure of 2-methoxypropene

Abstract The rotational isomerism of the title compound, CH2C(CH3)OCH3, has been studied by i.r. spectroscopy. The spectra reveal the presence of two conformational isomers in the fluid phases. The energy difference between them is estimated to be 0.85 kcal/mole in the liquid state. From the shapes of the band envelopes in the vapour phase spectra evidence is obtained that the more stable isomer has a planar configuration of the heavy atoms. A vibrational assignment of the observed fundamental bands has been attempted on the basis of characteristic group frequencies.

Vibrational spectra and molecular structure of 2,2,3,3-tetrabromobutane

Abstract The i.r. and Raman spectra of MeBr 2 CCBr 2 Me, in the crystalline state, are reported. The i.r. spectra of the same substance in dilute solutions in nonpolar solvents have also been obtained. Taking the CBr stretching bands as a guide to qualitative understanding of the experimental results it is shown that point group C 2 h is a reasonable working approximation to the molecular symmetry. On this basis it has been possible to obtain nearly complete vibrational assignment for the molecule.

Infrared spectra and conformations of 2,3-dimethyl- 2,3-dinitrobutane and 2,2,3,3-Tetranitrobutane

Abstract The following molecules: MeC(NO 2 ) 2 Me, NO 2 Me 2 CCMe 2 NO 2 and (NO 2 ) 2 MeCCMe(NO 2 ) 2 have been investigated by infrared spectroscopy with the aim of determining whether the title compounds can be characterized by definite conformations. On the basis of a comparison of the CN valence frequencies within the series the conclusion is drawn that samples of both compounds, studied under various conditions, do not consist of single molecular species having a center of symmetry. Arguments are cited in favour of a gauche conformation for 2,3-dimethyl-2,3-dinitrobutane and of an equilibrium mixture of trans-gauche isomers for 2,2,3,3-tetranitrobutane, in the solid phases.