Rotational isomerism in open-chain polychlorinated hydrocarbons: Problems in predicting conformations from carbon-chlorine stretching frequencies

Normal coordinate analysis calculations have been made to determine the carbon-chlorine stretching (v(C-Cl)) frequencies of model rotational isomers of 1,2-dichloroethane, 1,2-dichloropropane, 1,3-dichloropropane, 1,5-dichloropentane and 1,2,3-trichloropropane. The calculations were based on prior calculations on models of a number of primary and secondary monochlorohydrocarbons. The models were derived from the real isomers by ignoring the hydrogen atoms and considering only the heavy atom skeletal structure. The possible effect on v(C-Cl) frequencies of coupling between C-C1 stretching and adjacent CH₂ rocking modes in some isomers was taken into account where necessary. The results, and in some cases the frequency of the highest ∠CCC deformation mode, are used to establish the conformations of rotational isomers present in these molecules in the vapour, liquid and crystalline states. Conclusions are drawn concerning the applicability of known v(C-Cl) frequency structure correlations for monochlorohydrocarbons to polysubstituted chlorohydrocarbons. v(C-Cl) frequency correlations and assignments have also been made for several chloropropanes containing one CCl₃ group.     

Rotational isomerism in 1,3-dimercaptopropane

It is well known that the molecules of 1,2-dihaloethanes [1], 1-halopropanes [2] and 1,2-dihalopropanes [3] exist in more than one rotational isomeric form trans (T) and gauche (G) in the liquid state and only in the trans (T) form in the crystalline state at low temperature. In the case of 1,3-dihalopropanes [4], however, the molecules exist in dynamic equilibrium of three types of rotameric forms, TT, TG and GG in the liquid phase, while GG is the only stable form in the crystalline phase. The number of rotational conformers present in 1,2-ethanedithiol [5], 1-propanethiol [6] and 1,2-propanedithiol [7] and their stabilities in the liquid and crystalline solid phases are observed to be similar to those of the corresponding haloalkanes [1–3]. Since such studies with 1,3-dimercaptopropane have not yet been made, its infrared spectrum in the liquid state and its Raman spectra in the liquid and solid states have been investigated. The present communication reports the results and their analyses with reference to the possibility of rotational isomerism in the molecule.     

Rotational isomerism in 1-pentyne

Infrared and Raman spectra have been obtained and normal coordinate calculations have been made for 1-pentyne. It has been shown that this compound exists in two molecular conformations. A force field has been obtained that can be transferred to other 1-alkynes.     

Rotational isomerism in 1,4-disilabutane

Raman spectra of 1,4-disilabutane (DSB) are recorded in both liquid and solid states. IR spectra in the gas and solid phases are also recorded. Two rotational isomers, trans and gauche, are present in the gas and pure liquid phases. An assignment of the main features of the IR and Raman spectra is made, with the aid of normal coordinate calculations for the two isomers. The temperature dependence of the Raman spectrum of DSB is studied in the range 77–300 K. The enthalpy difference between the two isomers is 5.01 ± 0.31 kJ mol^?1. This value is expected to be higher for gaseous DSB.     

Effect of fluorine substitution on stretching vibrations of the azoxy group in azoxybenzenes

The IR and Raman spectra of azoxybenzene (I) and a number of its fluoroderivatives have been analysed, including the ¹⁵N and ¹⁸O labelled samples and complexes of some azoxybenzenes with SbCl₅. On the basis of the calculated frequencies and forms of normal vibrations of compound (I) in the valence force field, the vibration spectrum has been interpreted. The presence and number of fluorine atoms in the phenyl fragments of azoxybenzenes have practically no effect on the stretching frequencies of the azoxy group but considerably decrease the band intensity of vibrations of the azoxy and phenyl groups in the Raman spectra. Introduction of fluorine into the aromatic rings of azoxybenzenes considerably complicates complex formation with SbCl₅ at oxygen of the azoxy group.     

On the red shift of OH stretching region vibrations in ice and water

The nature of the red shift of frequencies of the fundamental modes, v₁ and v₃, in the OH stretching region of the vibrational spectrum of ice (and, possibly, in water) under H-bonding formation is explained in the framework of the continuum approach for the one-dimensional infinite chain of water molecules represented as dipolar and polarizable OH oscillators vibrating in the definite force field. The explicit expressions in the form of the generalized cnoidal nonlinear waves describing these fundamental modes and obtained as the solutions of the nonlinear Klein–Gordon equation of motion, and their red shifts that are consistent with experimental observations, are presented.     

Rotational isomerism in fluoro- and chloro-benzaldehydes

The results of the study of rotational isomerism in the nπ^* electronic spectra of o- and m-substituted fluoro- and chlorobenzaldehydes are reported. It has been found that trans-O is the more stable rotamer in the ground electronic states as well as in the lowest lying singlet and triplet electronic states in all four molecules.     

Rotational isomerism in molecules of genetic interest

In this paper the role of Raman Spectroscopy and Circular Dichroism in the determination of the secondary structure of nucleic acids as a function of base composition and interaction with platinum anticancer drugs is discussed. It is pointed out that these various differences in secondary structure occur because of different rotational isomers which are present in these biologically active large molecules.     

C-I stretching vibrations in iodobenzenes

Comparison of the calculated and experimental IR and Raman spectra of a series of iodobenzenes showed that the C-I stretching vibrations for these compounds correspond to a very highly polarized Raman band at 150–270 cm^–1 (=0.1). The position of this band depends on the mass of the para substituent and relative position of the fluorine and iodine atoms in the molecule. The UV absorption spectral data indicate an interaction of the iodine atom and para substituents through the -system. Opposite substituent effects on the change in intensity of the B-band in the UV spectra of iodobenzenes and tetrafluoroiodobenzenes were noted.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2637–2639, November, 1991.     

Rotational isomerism of some 1-substituted silatranes

Conclusions It was shown by the DM method that the conformation of the side chain is realized in the 1--halo-propylsilatranes, the gauche form is realized predominantly in the 1-thioethoxymethylsilatranes, while a trans orientation of the C-S bond with respect to the silatrane fragment is realized in the 1--thioethoxye-thylsilatranes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7. pp. 1671–1673, July, 1977.     

Rotational isomerism and conformational energies in 2-haloethanols

The Raman spectra of 2-fluoroethanol have been obtained as a function of temperature in the liquid and in the solid phases. The results of these experiments have provided substantial evidence for the existence of trans conformers in the liquid phase. The above study, coupled with analysis of the temperature dependence of the C-X stretching vibrations of the other haloethanols in the pure liquid, has permitted, for the first time, a determination of ΔH for all four members of the series. An interpretation is offered to explain the observed trends in the conformational energy differences.