No abstract

Examination of circular dichroic and phosphorus nuclear magnetic resonance spectra showed that poly(dA-dT)-poly(dA-dT) exhibited an ethanol-induced transition to the A form in an Na+ containing medium like natural DNAs. A mere replacement of the Na+ by Cs+ counterions meant that the polynucleotide was with a little cooperativity transformed into a novel conformation displaying a deep negative band in the long wavelength part of the CD spectrum. The presence of very low concentration of Cs2+ shifted the midpoint of the transition to a lower content of ethanol.

It was found that three crystalline forms of ethylene glycol dibenzoate can be prepared. Infrared and Raman spectra of these three forms, as well as of the glassy and liquid states, were measured. From 3JHH coupling constants obtained by analysis of the 13C satellite band of the -CH2- group in 1H NMR spectra, and from the 3JCH coupling constants of the -CO.O.CH2- fragment obtained by analysis of the carbonyl band in 13C NMR spectra it was found that in the liquid state the -CH2-CH2- group exists predominantly in the gauche conformational structure, and the bonds C-O-C-C assume predominantly a trans orientation. The results of the analysis of NMR and vibrational spectra were used for the structural interpretation of conformationally sensitive bands in vibrational spectra of ethylene glycol dibenzoate.

Infrared and Raman spectra of ethyl benzoate and diethyl terephthalate were measured in their crystalline forms, in the glassy state and in the melt. With liquid samples, the NMR coupling constants 3 J CH of the group -CO.O.CH2-were also measured. By analysis of these measurements it was found that in the liquid state of ethyl benioate the following conformers are present: forms with a fully planar structure; forms with the methyl group twisted out of the aromatic and ester group plane by rotation about the O-C2HS bond; and forms with non-planar mutual , orientation of the ester group and the aromatic ring, formed by rotation about the Car-CO or CO-O bonds. The first two forms are aJso present in the crystalline modifications of ethyl benzoate. Diethyl terephthalate in the liquid state is present in forms analogous to those of ethyl benzoate; in addition, it contains isomers differing by the mutual orientation of the two ester groups on the aromatic ring.As part of our studies of the conformational structure of poly(ethylene terephthalate) model • compounds, in our previous paper! we have investigated the relation between conformational structure and vibrational spectra of methyl benzoate and dimethyl terephthalate; we have discussed the problem of conformer generation in these molecules by rotation about the bonds Car-C; CO-O, and of the mutual cis-trans isomerism of the ester groups of dimethyl terephthalate. In ethyl benzoate and diethyl terephthalate, additional rotations are possible about the O-C2HS bond. Complete infrared and Raman spectra of ethyl benzoate have so far been measured in the liquid state 2 -6; only in the range of lattice vibrations (10-200 cm -!), Raman spectra have also been measured in the crystalline state 7 . Infrared and Raman spectra of diethyl terephthalate have only been measured in the crystalline state S ,6,S. These spectra have been interpreted partly on the basis of analogy with the spectra of similar molecules 2 -4 ,6,s, partly by comparison of the experimental band frequencies with the normal vibrations calculated from secular equations s . The influence of conformational structure upon vibrational spectra has not been analyzed so far. By X-ray analysis it was found 9 that diethyl terephthalate in the crystalline state contains molecules which are essentially planar, with a trans mutual orientation of the two ester groups on the aromatic ring. Measuremen~s of dipole moments! 0 have indicated that diethyl terephthalate in benzene solution contains approximately equal populations of molecules with cis and trans mutual orientation of ester groups; for ethyl benzoate a structure with the ester group twisted ou' t of the plane of the aromatic ring!! has been proposed.

Infrared and Raman spectra of the crystalline forms and of the liquid state of acetophenone, methylbenzoate and dimethyl terephthalate were measured. From these measurements it was found that acetophenone in the liquid state is present in a single conformational form, and that methyl benzoate in the liquid state contains, in addition to the major conformer with a planar structure and trans form of the ester group, also a minor conformer of different geometrical structure. Dimethyl terephthalate in the melt contains approximately equal populations of the conformers with the mutual trans and cis orientation of the two ester groups. For all the observed structures, the conformationally sensitive bands were assigned.