Structure and molecular spectra of ethyl benzoate and diethyl terephthalate

Abstract

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.

Vapour pressure of diethyl phthalate

“…Potential functions were estimated by the molecular mechanics method MM2 [35] and adjusted for consistency with the calculated vibrational frequencies and experimental values of the gas entropy. Qualitative accordance of the calculated frequencies with the IR and Raman spectra [36,37] was observed, as well as with the vibrational frequencies of diethyl terephthalate [38,39].…”

A new Raman spectroscopy‐based method for monitoring the crystallinity ratio of polyethylene terephthalate

“…The band at 998 cm À1 characterizes the "trans" conformations of the C-C bond of the ethylene segment of PET. [10,39] According to Donnay, these conformations exist mainly in the crystalline phase (94%). [16] We can notice an increase of this criterion then a decrease allowing to follow globally the crystallinity ratio.…”

“…The crystallization of PET can take place only if all conformations are "trans." [10][11][12] Several authors have established criteria for the study of crystallinity, orientation of amorphous and crystalline phases and damage in the case of a range of polymers based only on Raman spectroscopy. [13][14][15] These criteria were validated by comparing them to results obtained by other techniques such as X-ray diffraction or infrared spectroscopy.…”

“…These conformations have been linked to the different phases of semi‐crystalline polymer. [ 10 ] The crystallization of PET is highly dependent on the conformational states of the macromolecule. There are three bonds involved, first the C–O bond between the terephthalic group and the ester group.…”

“…And finally, the third is the ester groups of the terephthalic acid unit where both “ trans ” and “ cis ” conformations are possible. The crystallization of PET can take place only if all conformations are “ trans .” [ 10–12 ]…”