JOURNAL OF RAMAN SPECTROSCOPY, VOL. zyxwvutsr 21, 169-175 (1990) Application of Raman Spectroscopy in Recognition of the Two Crystalline Forms of Terephthalic Acid L. Colombo* Ruder BoHkovik Institute, Zagreb, Yugoslavia V. VolovSek Technical Faculty, University of Zagreb, Zagreb, Yugoslavia K. Furik Ruder BoSkovii: Institute, Zagreb, Yugoslavia J. R. Durig University South Carolina, Columbia, South Carolina, USA Solid terephthalic acid exists in two polymorphic forms with both structures being triclinic. Raman spectroscopic studies have been made in order to recognize the presence of form I and/or form I1 in the sample. INTRODUCTION zyxwvutsrq The polymorphism of solid terephthalic acid (TPHA) results in the existence of two triclinic structures (forms I and 11), which are simultaneously present in the sample at all temperatures.’ The relative concentrations of the two forms depend on the conditions and speed of crystallization,’ and the two forms do not have the same efficiency in the procedure of polymerization. Therefore, it is of interest to obtain data which could be the basis of the spectroscopic recognition of the two forms. An excellent basis for this study was provided by Gerasimov et zyxwvutsrq al.’ who studied the low-frequency Raman spectra of the two forms. They studied samples obtained under different conditions of crystallization with some of their samples containing almost only one crystalline form. In this way they were able to determine the frequencies of the lattice modes of each particular form in the temperature range 293-423 K. For practical use it is often difficult to obtain a well resolved low- frequency spectrum, in particular when working with powdered samples. Hence, in our opinion, it is useful to find molecular transitions which are sensitive to the molecular packing which could be used to identify the crystalline form. Comparison of our low-frequency spectrum with those given by Gerasimov et al.’ indicated that our sample, from Columbia Organic Chemicals, at room temperature contains mostly TPHA in form 11. As the transition from form I1 to form I can be effected by compressing the sample,2 we tried to obtain the same effect by cooling. Some of these spectra are presented in Fig. 1, where the appearance of new bands with decreasing temperature is evident. In Fig. 2 the tem- perature dependence of observed frequencies in the low- temperature region is shown, together with the high-temperature measurements of Gerasimov et zyxwvu aZ.’ zyxwvu * Auther to whom zyxwvutsrq correspondence should be addressed. 0377-0484/90/03014947 sOS.00 Q 1990 by John Wiky & Sons, Ltd The slopes of all the curves agree fairly well for the two temperatures ranges investigated, which indicates that the new bands appearing in our spectra at lower tem- peratures correspond to TPHA in form I. Subsequently we made systematic investigations of the internal spectrum at low temperatures in the range 300-20 K. The molecular and crystal structures .of I I 150 50 S/cm-’ Figure 1. Low-frequency spectrum of the TPHA sample used in the present study. Spectra were recorded in the temperature range 3OC-20 K. Received 20 July 1989 Accepted 31 October 1989