- Email: [email protected]
Separation of chlorocyclohexane's axial and equatorial conformer infrared spectrum by isothermal relaxation in the glass→liquid transition region
MOLSTR 10704
Journal of Molecular Structure 479 (1999) 237–243
Separation of chlorocyclohexane’s axial and equatorial conformer infrared spectrum by isothermal relaxation in the glass ! liquid transition region S. Ru¨disser, G. Fleissner, A. Pichler, A. Hallbrucker, E. Mayer* Institut fu¨r Allgemeine, Anorganische und Theoretische Chemie, Universita¨t Innsbruck, A-6020 Innsbruck, Austria Received 26 May 1998; accepted 13 July 1998
Abstract A new low-temperature method enables separation of the infrared spectrum of a mixture of conformers into the distinct spectra of its constituents, and its merits are demonstrated here for chlorocyclohexane (Cl–CH). Nonequilibrium population of Cl–CH’s axial (a) and equatorial (e) conformers was generated by rapid quenching of their vapours into the glassy state. Interconversion of conformers was accomplished by isothermal relaxation in the glass ! liquid transition, and was followed by Fourier-transform infrared spectroscopy. These spectra were separated into the distinct spectra of the a and e-conformer. Assignment of infrared bands of the separated spectra to the normal vibrations of the conformers is consistent with those reported in the literature with two exceptions. 䉷 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Vibrational spectroscopy is one of the most effective physical methods employed for conformational studies (for reviews see Refs. [1–3]). Numerous compounds exist as mixtures of conformers in the vapour, liquid, solution and glassy state, whereas the crystal usually contains one conformer only. Therefore, the spectrum of one conformer can often be easily obtained from the crystal. Spectra of the other conformers can be obtained indirectly from disappearance of their bands on crystallization, from changes in conformer population with temperature, pressure and solvent, and from relaxation of nonequilibrium conformer populations [2–5]. * Corresponding author. Tel.: ⫹43-512-5075110; Fax: ⫹43-5125072934. E-mail address: [email protected] (E. Meyer)
We have recently shown that spectral features of distinct conformers can be separated by a new method where nonequilibrium conformer population is generated by rapid quenching of a sample into the glassy state [6,7]. On subsequent heating up to the glass ! liquid transition region [8,9], interconversion of conformers on relaxation towards equilibrium is followed isothermally by Fourier-transform infrared (FT–IR) spectroscopy. In a next step, spectral features of the conformers can be separated by Hirschfeld’s ‘ratio method’ [10–13]. Our first application was separation of B-DNA’s IR spectrum into those of its conformational substates [6,7]. Here we use chlorocyclohexane (Cl–CH) as a model compound and report the distinct spectra of its conformers. Liquid and gaseous Cl–CH consist of a mixture of axial (a) and equatorial (e) conformers whereas the anisotropic crystal consists only of the econformer [1–3]. Therefore, infrared (IR) and Raman
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00874-6
238
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
Two publications have to be mentioned in connection with our study: Fishman et al. investigated the isothermal relaxation kinetics of Cl–CH’s econformer to the a-conformer; they had generated a nonequilibrium conformer population by dissolving a crystal containing only the e-conformer at low temperatures in liquid propane [4]. Lin and Koenig separated by the ‘ratio method’ the spectra of the trans and gauche isomers in the disordered and crystalline phase of poly(ethylene terephthalate) [13].
2. Experimental
Fig. 1. (top) Structural relaxation seen in IR spectra of a quenched Cl–CH film on isothermal annealing at 140 K: (a) recorded after 10 min; and (b) after 30 min. (bottom) Difference spectrum (b-a) 3fold enlarged.
spectra of the e-conformer are well known [14–24]. All attempts to crystallize the a-conformer failed and its spectrum is deduced indirectly as mentioned earlier [20]. In liquid Cl–CH the a and e-conformer interconvert rapidly at ambient temperature but are frozen-in in the glassy state below ⬃130 K [3–5]. At 140 K interconversion of Cl–CH’s conformers occurs on time scales of up to 1 h and can therefore be studied by convential FT–IR spectroscopy. The spectral changes on isothermal annealing are reported in the form of IR difference spectra, where positive peaks indicate formation of the e-conformer and negative peaks disappearance of the a-conformer. The advantage of isothermal relaxation is that changes of band shape and peak position with temperature are avoided. Difference spectra recorded at several temperatures were useful in the assignment of conformer bands especially in spectral regions of overlapping bands but changes of band shape and peak positions with temperature can lead to artifacts [24,25].
Cl–CH (from Merck–Schuchardt, Nr. 820287, ⬎98%) was used as received. Glassy Cl–CH was prepared by admitting Cl–CH vapours from a reservoir held at 298 K through a fine metering valve into a high vacuum system, where the vapours condensed on a CsI window precooled to 78 K. Deposition time was 2.5 min, and intensity of the Cl–CH band centered at 729 cm ⫺1 was 0.70 absorbance units. The temperature of the copper holder and CsI window was regulated with a thermocontroller (AP Paar, Model TTKHC) and remained constant within ^0.1⬚. The spectra were recorded in situ, with base pressure ⬍10 ⫺6 mbar. Evaporation of Cl–CH is negligible for temperatures up to 140 K. IR spectra were recorded in transmission on BioRad’s FTS-45 model at 2 cm ⫺1 resolution (UDR1, DTGS detector; zero-filling factor 2; low-pass filter at 1.12 kHz; triangular apodization), by coadding 100 scans. Data processing was performed with the GRAMS 32 software. The spectra are drawn on the same scale, with the enlargement factors given in the Figures. Vertical bars indicate the ordinate scale in absorbance units. The scaling factors necessary for separation of the recorded spectra into the spectra of the a and econformer were determined from changes in band areas of the bands centered at 558 and 512 cm ⫺1 (see Fig. 1). The band area of the a-conformer band at 558 cm ⫺1 decreased on isothermal relaxation from 1 to 0.915 (band area obtained by integration, with break points for two-point baseline set at 589.2 and 539.1 cm ⫺1), whereas the band area of the econformer band at 512 cm ⫺1 increased from 1 to
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
239
Fig. 2. The separated spectra of the a and e-conformer showing their n22 (left) and n23 (right) normal vibrations. The first-derivative-like feature in the a-conformer spectrum at ⬃ 728 cm ⫺1 is the most intense artifact.
1.026 (break points for two-point baseline set at 539.1 and 500.5 cm ⫺1). These scaling factors were then used for separation in the same manner as described in Ref. [10,11].
3. Results and discussion Nonequilibrium distribution of a and e-conformers was generated by quenching of the vapours of Cl–CH on a CsI window held at 78 K as described before [2,26–30]. But here we studied isothermal interconversion of conformers at low temperatures. Fig. 1 shows the spectral IR region of Cl–CH from 490 to 590 cm ⫺1 recorded at 140 K. The two bands centered at 558 and 512 cm ⫺1 from the a and e-conformer (n23, a 0 symmetry, [20]) have often been used to study conformer populations. These bands are not welllocalized vibrations but seem to involve the C–Cl stretching vibration [20]. Spectrum a (solid line) was recorded after 10 min, spectrum b (broken) after 30 min. Their difference (b-a, enlarged 3-fold) shows decreasing intensity of the band centered at 558 cm ⫺1 and increasing intensity of the band at 512 cm ⫺1,
which indicates conversion of the a into the econformer on relaxation toward equilibrium. Conversion of the a into the e-conformer on relaxation at 140 K is consistent with Bugay et al.’s report of thermodynamic parameters of the conformers in rapid equilibrium [21]. For the axial , equatorial conformer equilibrium, the equilibrium constant (Keq [equatorial]/[axial]) is 2.93 at 298 K and 5.4 at 140 K (calculated from thermodynamic quantities listed in Table 1 of Ref. [21]).Therefore, relaxation of the frozen-in nonequilibrium conformer population toward equilibrium must occur via conversion of axial into equatorial Cl–CH. The ratio of band areas of the difference curve allows one to calculate the ratio of extinction coefficients of the a and e-conformer bands at 558 and 512 cm ⫺1 (Ja/Je). The Ja/Je value of 3.3 thus calculated is consistent with the ratio of 3.2 ^ 0.1 reported by Fishman et al. from isothermal kinetics of Cl–CH dissolved in propane at 143.5 K (see Fig. 3 in Ref. [4]). In a next step the spectra of Fig. 1 were separated into the the spectral components of the a and econformer of Cl–CH. Hirschfeld’s ‘ratio method’
240
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
Fig. 3. (a and b) Spectra of the separated a and e-conformer of Cl–CH, and (d) the Cl–CH spectrum recorded at 140 K after 10 min which is identical with the sum of the separated a and e-conformer spectra; (c) for comparison the difference spectrum shown in Fig. 1. Spectrum (a) is enlarged 2-fold and the difference spectrum (c) 10-fold.
allows resolution of ‘‘the spectrum of a mixture of an unknown small number of unknown constituents in unknown proportions into the spectra of these constituents without isolating them’’, provided that each unknown constituent absorbs dominantly in a particular spectral region [10,11]. Spectra of an original mixture and of mixture(s) following some fractionation procedure of the components are necessary. The method was applied to unknown mixtures of two components only and the scaling factors necessary for quantitative analysis were obtained by ratioing two spectra. This method is simple and rapid, but it does not correct properly for the spectral background. Diem and Krimm [12] improved the accuracy of the Hirschfeld-method by about one order of magnitude. Their method is based on the scaling factors required to null the components, one at a time, in successive absorbance subtraction of the two spectra. We use their method and determine the scaling coefficients from the changes in integrated band areas of the two bands centered at 558 and 512 cm ⫺1 on relaxation and conformer interconversion. Fig. 2 (right) shows the result of this separation for
the two bands of the a and e-conformer at 558 and 512 cm ⫺1. Separation is not perfect and some residual feature resembling a first derivative curve is present in the e-conformer spectrum at ⬃ 558 cm ⫺1. This type of artifact occurs also in other spectral regions of the separated spectra. The most intense artifact occurs at ⬃728 cm ⫺1 in the separated spectrum of the aconformer [see Fig. 2 (left)]. The bands centered at 684 and 728 cm ⫺1 (n22, a 0 symmetry, see Ref. [20]) have often been used in the vibrational analysis of Cl– CH’s a and e-conformer (see e.g. Refs. [22,23]). The artifact occurs in the a-conformer spectrum at the position where the e-conformer has its intense band. Similar artifacts were observed before in the spectra of separated components [11]. They were attributed to the following causes: (i) Allara [31] has shown that band shapes can be changed even in the absence of intermolecular interactions as a result of the effects of differences in the real part of the refractive indices of the pure components [12,31]. ‘‘The magnitude of this anomaly will depend on the optical properties of the two pure compnents. With given components, the magnitude of the anomaly will
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243 Table 1 Comparison of chlorocyclohexane’s a and e-conformer frequencies from their distinct separated spectra a with observed b and calculated b frequencies [cm ⫺1]. Axial obs. a
obs. b
calc. b
Equatorial obs. a obs. b
calc. b
a0 n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14 n15 n16 n17 n18 n19 n20 n21 n22 n23 n24
2940 (2915) (2903) 2893 2862 2862 (2839) 1459 1439 1428 1340 1329 1265 (1259) ⬃1214 1098 1030 1015 869 859 808 684 558 472
2946 2921 2908 2889 2860 2860 2845 1461 1439 1429 1343 1326 1268 1260 1216 1099 1030 1015 869 859 807 685 559 473
2956 2922 2916 2914 2857 2853 2851 1456 1437 1422 1345 1343 1283 1251 1226 1105 1026 985 870 853 806 680 552 482
(2954) 2935 (2914) (2904) (2875) 2856 2856 1464 (1453) ? 1353 1333 1269 1258 1217 1133 1032 994 889 851 818 728 512 435
2955 2946 2908 2903 2865 2860 2860 1466 1449 1429 1353 1333 1268 1259 1216 1133 1026 994 889 852 819 732 512 436
2956 2922 2916 2914 2857 2853 2851 1456 1440 1425 1351 1340 1265 1251 1221 1121 1020 992 892 839 819 733 508 428
a 00 n28 n29 n30 n31 n32 n33 n34 n35 n36 n37 n38 n39 n40 n41 n42 n43 n44 n45 n46
2932 2893 2862 (2839) 1447 1439 1358 (1347) 1340 1329 1272 1146 1118 (1082) 1030 923 864 (779) 456
2943 2903 2865 2845 1452 1436 1358 1353 1340 1321 1273 1146 1119 1075 1030 921 862 775 454
2917 2913 2855 2851 1440 1434 1368 1350 1342 1320 1261 1145 1144 1083 1034 928 856 792 461
2935 (2925) (2875) 2856 (1453) 1447 1366 1353 1341 1301 1258 1187 1088 1075 1050 920 871 790 (443)
2943 2930 2865 2860 1449 1439 1366 1353 1340 1302 1260 1185 1089 1075 1052 917 877 789 440
2918 2913 2855 2852 1448 1434 1383 1351 1345 1299 1245 1182 1100 1068 1051 916 886 787 457
a
This work, peak frequencies from Figs. 2 and 3, peak positions in brackets are read from second derivative curves. b From Table 4 in Ref. [18]. Normal vibrations below 400 cm ⫺1 are not included in Table 1.
241
increase as the difference in the compositions of the two solutions increases and as the absolute intensities of the bands increases’’ [12]. (ii) Spectral artifacts can arise from intermolecular interactions because band shapes can be altered on changing the composition of the mixture [13]. (iii) A first derivative feature can occur in a difference spectrum when a band slightly shifts its peak position [25]. This situation could arise when a weak band of one component is superposed by an intense band of the second component, and its band shape is altered by changes in intermolecular interactions. Fig. 3 shows the separated spectra of Cl–CH’s a and e-comformer from 2990 to 2820 cm ⫺1 and from 1500 to 780 cm ⫺1 (curves a and b). The spectrum of the conformer mixture of Cl–CH recorded at 140 K after 10 min is shown as curve d. The difference of the spectra recorded after 30 and 10 min is labelled as curve c (identical with the difference curve in Fig. 1, bottom). Spectrum d is identical with the sum of spectra a and b. The difference spectrum has positive and negative peaks at those positions where the e and a-conformers have bands. The spectral artifacts occur mainly in the separated spectrum of the a-conformer. Since the a to e-conformer population ratio is 0.30, the a-conformer is dissolved in the e-conformer and spectral artefacts by intermolecular interaction with the econformer are more likely than vice versa. Spectra recorded at 140 K after shorter times than those used for Figs. 1–3 (i.e. after 10 and 20 min) gave the same separated a and e-conformer spectra (not shown). Very similar separated spectra of the a and e-conformer were also obtained from glassy Cl–CH made by so-called hyperquenching of micrometersized droplets [32–35]. The a and e-conformer of Cl–CH both belong to point group Cs, and 27 of the 48 normal vibrations are of a 0 and 21 of a 00 symmetry. Woldbæk’s [20] report of the IR and Raman spectral data of Cl–CH in the liquid and crystalline state is probably the most extensive one. He also performed a normal coordinate analysis for both conformers, and tentatively assigned all normal vibrations of both conformers (see Table 4 in Ref. [20]. In Table 1 we list our assignment of the normal vibrations of the a and e-conformer obtained from curves a and b in Fig. 3 (peak positions marked), and we compare it with Woldbæk’s [20] observed and calculated frequencies. Peak positions
242
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
obtained from our separated conformer spectra vary at most by a few wavenumbers from those reported before, and the IR band intensities are similar to those collected in Table 1 of Ref. [20]. Therefore, our separation into distinct spectra of Cl–CH’s a and e-conformer spectra seems to be reasonable, despite the spectral artifacts pointed out and discussed earlier. Our results differ only in two cases from previously reported work (marked by arrows): first, a band of medium intensity at ⬃1429 cm ⫺1 was attributed to the e-conformer (assigned as n10, see Table 1 in Ref. [20]) but it is absent in our spectrum obtained by separation. Curves a and b of Fig. 3 show that in this spectral region only the a-conformer has a band at 1428 cm ⫺1 of medium intensity. In the IR and Raman spectrum of crystalline Cl–CH recorded at 90 K, a weak band was observed at 1429 cm ⫺1 and a medium intensity band at 1431 cm ⫺1 (see Table 1 in Ref. [20]). Assignment of this band to the e-conformer was probably made because the anisotropic crystal contains only the e-conformer. We suggest that the ⬃ 1430 cm ⫺1 band in the crystal is caused by crystal field splitting and thus, observation of this band in both the liquid and crystalline state is accidental. Second, the separated a-conformer spectrum contains a band at 2962 cm ⫺1 of medium intensity (see Fig. 3, curve a, marked) which is not assigned to the aconformer in Table 1 of Ref. [20]. In Cl–CH dissolved in CCl4, a strong IR band is centered at 2955 cm ⫺1 which was assigned to n1 of the econformer. Overlap of bands is most pronounced in this C–H stretching band region and therefore, a band of the minor component can easily be missed. A weak band is also observable at 2961 cm ⫺1 in the fourthderivative of the Cl–CH spectrum recorded at 140 K. We suggest that the a-conformer band at 2962 cm ⫺1 should be considered as normal vibration in the assignment. Our method is obviously not limited to IR spectroscopy, and Raman spectroscopic studies with polarization measurements will be an important extension in the future. The basic requirement for the applicability of this method is the absence of crystallization. This can be achieved either by using a sufficiently low temperature, or by dissolving the compound in a solvent which prevents crystallization. The latter approach was used by Fishman et al. with the aim to
determine the glass ! liquid transition temperature of a solvent by using as a probe freezing-in of conformer interconversion in a solute [3–5]. Thermograms provide a quick test to see whether the glass ! liquid transition is separated sufficiently from the onset temperature of crystallization. Nonequilibrium population of conformers can be obtained either by rapid cooling of vapours, or by so-called hyperquenching of a liquid or a solution into the glassy state [32–35].
Acknowledgements We are grateful for financial support by the ‘Forschungsfo¨rderungsfonds’ of Austria (project P12319-PHY).
References [1] W.J. Orville-Thomas (Ed.), Internal Rotation in Molecules, 1974, Wiley, London, NY. [2] P. Klaeboe, Vibrational Spectrosc. 9 (1995) 3. [3] A.I. Fishman, A.A. Stolov, Spectrochim. Acta 49A (1993) 1435. [4] A.I. Fishman, A.A. Stolov, A.B. Remizov, Spectrochim. Acta 41A (1985) 505. [5] A.I. Fishman, S.Y. Guseva, A.B. Remizov, A.A. Stolov, O.E. Zgadzai, Spectrochim. Acta 42A (1986) 1247. [6] S. Ru¨disser, A. Hallbrucker, E. Mayer, J. Am. Chem. Soc. 119 (1997) 12251. [7] S. Ru¨disser, PhD Thesis, Universita¨t Innsbruck, 1997. [8] S. Ru¨disser, A. Hallbrucker, E. Mayer, J. Phys. Chem. 100 (1996) 458. [9] S. Ru¨disser, A. Hallbrucker, E. Mayer, G.P. Johari, J. Phys. Chem. 101 (1997) 266. [10] T. Hirschfeld, Anal. Chem. 48 (1976) 721. [11] J.L. Koenig, Spectroscopy of Polymers, ACS Professional Reference Book, American Chemical Society, Washington, DC, 1992, pp. 67–68. [12] H.E. Diem, S. Krimm, Appl. Spectrosc. 35 (1981) 421. [13] S.-B. Lin, J.L. Koenig, J. Polym. Science, Polym. Phys. Ed. 20 (1982) 2277. [14] P. Klæboe, J.J. Lothe, K. Lunde, Acta Chem. Scandinavia 10 (1956) 1465. [15] C.G. Opaskar, S. Krimm, Spectrochim. Acta A 23 (1967) 2261. [16] A.B. Remizov, L.M. Sverdlov, Izv. Vyssh. Uchebn. Zaved. Fiz. 11 (1968) 150. [17] P. Klæboe, Acta Chem. Scandinavia 23 (1969) 2641. [18] J. Sˇtokr, B. Schneider, J. Jakesˇ, J. Mol. Structure 15 (1973) 87. [19] S.D. Christian, J. Grundnes, P. Klaboe, J. Am. Chem. Soc. 97 (1975) 3864. [20] T. Woldbæk, Acta Chem. Scandinavia A 36 (1982) 641.
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243 [21] R. Borsdorf, S. Ferner, R. Salzer, Chem. Monthly 113 (1982) 827. [22] D.J. Gardiner, C.J. Littleton, N.A. Walker, J. Raman Spectrosc. 18 (1987) 9. [23] D.E. Bugay, C.H. Bushweller, Ch.T. Danehey Jr., St Hoogasian, J.A. Blersch, W.R. Leenstra, J. Phys. Chem. 93 (1989) 3908. [24] E. Nodland, F.O. Libnau, O.M. Kvalheim, H.-J. Luinge, P. Klæboe, Vibrat. Spectrosc. 10 (1996) 105. [25] D.B. Parry, M.G. Samant, O.R. Melroy, Appl. Spectrosc. 45 (1991) 999. [26] M. Squillacote, R.S. Sheridan, O.L. Chapman, F.A.L. Anet, J. Am. Chem. Soc. 97 (1975) 3244. [27] P. Felder, Hs.H. Gu¨nthard, J. Mol. Struct. 60 (1980) 297.
243
[28] A.J. Barnes, J. Mol. Struct. 113 (1984) 161. [29] V. Vidya, K. Sankaran, K.S. Viswanathan, Chem. Phys. Lett. 258 (1996) 113. [30] A.J. Barnes, in: A.J. Barnes, W.J. Orville-Thomas, A.Muller, R. Gaufres, (Eds.), Matrix Isolation Spectroscopy, Reidel, Dordrecht, 1981, chap. 23. [31] D.L. Allara, Appl. Spectrosc. 33 (1979) 358. [32] E. Mayer, J. Appl. Phys. 58 (1985) 663. [33] G.P. Johari, A. Hallbrucker, E. Mayer, Nature 330 (1987) 552. [34] G.P. Johari, A. Hallbrucker, E. Mayer, J. Phys. Chem. 93 (1989) 2648. [35] E. Mayer, in: M.-C. Bellissent-Funel, J.C. Dore (Eds.), Hydrogen Bonded Networks, Kluwer Academic Publ., Netherlands, 1994, pp. 355–372.
Journal of Molecular Structure 479 (1999) 237–243
Separation of chlorocyclohexane’s axial and equatorial conformer infrared spectrum by isothermal relaxation in the glass ! liquid transition region S. Ru¨disser, G. Fleissner, A. Pichler, A. Hallbrucker, E. Mayer* Institut fu¨r Allgemeine, Anorganische und Theoretische Chemie, Universita¨t Innsbruck, A-6020 Innsbruck, Austria Received 26 May 1998; accepted 13 July 1998
Abstract A new low-temperature method enables separation of the infrared spectrum of a mixture of conformers into the distinct spectra of its constituents, and its merits are demonstrated here for chlorocyclohexane (Cl–CH). Nonequilibrium population of Cl–CH’s axial (a) and equatorial (e) conformers was generated by rapid quenching of their vapours into the glassy state. Interconversion of conformers was accomplished by isothermal relaxation in the glass ! liquid transition, and was followed by Fourier-transform infrared spectroscopy. These spectra were separated into the distinct spectra of the a and e-conformer. Assignment of infrared bands of the separated spectra to the normal vibrations of the conformers is consistent with those reported in the literature with two exceptions. 䉷 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Vibrational spectroscopy is one of the most effective physical methods employed for conformational studies (for reviews see Refs. [1–3]). Numerous compounds exist as mixtures of conformers in the vapour, liquid, solution and glassy state, whereas the crystal usually contains one conformer only. Therefore, the spectrum of one conformer can often be easily obtained from the crystal. Spectra of the other conformers can be obtained indirectly from disappearance of their bands on crystallization, from changes in conformer population with temperature, pressure and solvent, and from relaxation of nonequilibrium conformer populations [2–5]. * Corresponding author. Tel.: ⫹43-512-5075110; Fax: ⫹43-5125072934. E-mail address: [email protected] (E. Meyer)
We have recently shown that spectral features of distinct conformers can be separated by a new method where nonequilibrium conformer population is generated by rapid quenching of a sample into the glassy state [6,7]. On subsequent heating up to the glass ! liquid transition region [8,9], interconversion of conformers on relaxation towards equilibrium is followed isothermally by Fourier-transform infrared (FT–IR) spectroscopy. In a next step, spectral features of the conformers can be separated by Hirschfeld’s ‘ratio method’ [10–13]. Our first application was separation of B-DNA’s IR spectrum into those of its conformational substates [6,7]. Here we use chlorocyclohexane (Cl–CH) as a model compound and report the distinct spectra of its conformers. Liquid and gaseous Cl–CH consist of a mixture of axial (a) and equatorial (e) conformers whereas the anisotropic crystal consists only of the econformer [1–3]. Therefore, infrared (IR) and Raman
0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00874-6
238
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
Two publications have to be mentioned in connection with our study: Fishman et al. investigated the isothermal relaxation kinetics of Cl–CH’s econformer to the a-conformer; they had generated a nonequilibrium conformer population by dissolving a crystal containing only the e-conformer at low temperatures in liquid propane [4]. Lin and Koenig separated by the ‘ratio method’ the spectra of the trans and gauche isomers in the disordered and crystalline phase of poly(ethylene terephthalate) [13].
2. Experimental
Fig. 1. (top) Structural relaxation seen in IR spectra of a quenched Cl–CH film on isothermal annealing at 140 K: (a) recorded after 10 min; and (b) after 30 min. (bottom) Difference spectrum (b-a) 3fold enlarged.
spectra of the e-conformer are well known [14–24]. All attempts to crystallize the a-conformer failed and its spectrum is deduced indirectly as mentioned earlier [20]. In liquid Cl–CH the a and e-conformer interconvert rapidly at ambient temperature but are frozen-in in the glassy state below ⬃130 K [3–5]. At 140 K interconversion of Cl–CH’s conformers occurs on time scales of up to 1 h and can therefore be studied by convential FT–IR spectroscopy. The spectral changes on isothermal annealing are reported in the form of IR difference spectra, where positive peaks indicate formation of the e-conformer and negative peaks disappearance of the a-conformer. The advantage of isothermal relaxation is that changes of band shape and peak position with temperature are avoided. Difference spectra recorded at several temperatures were useful in the assignment of conformer bands especially in spectral regions of overlapping bands but changes of band shape and peak positions with temperature can lead to artifacts [24,25].
Cl–CH (from Merck–Schuchardt, Nr. 820287, ⬎98%) was used as received. Glassy Cl–CH was prepared by admitting Cl–CH vapours from a reservoir held at 298 K through a fine metering valve into a high vacuum system, where the vapours condensed on a CsI window precooled to 78 K. Deposition time was 2.5 min, and intensity of the Cl–CH band centered at 729 cm ⫺1 was 0.70 absorbance units. The temperature of the copper holder and CsI window was regulated with a thermocontroller (AP Paar, Model TTKHC) and remained constant within ^0.1⬚. The spectra were recorded in situ, with base pressure ⬍10 ⫺6 mbar. Evaporation of Cl–CH is negligible for temperatures up to 140 K. IR spectra were recorded in transmission on BioRad’s FTS-45 model at 2 cm ⫺1 resolution (UDR1, DTGS detector; zero-filling factor 2; low-pass filter at 1.12 kHz; triangular apodization), by coadding 100 scans. Data processing was performed with the GRAMS 32 software. The spectra are drawn on the same scale, with the enlargement factors given in the Figures. Vertical bars indicate the ordinate scale in absorbance units. The scaling factors necessary for separation of the recorded spectra into the spectra of the a and econformer were determined from changes in band areas of the bands centered at 558 and 512 cm ⫺1 (see Fig. 1). The band area of the a-conformer band at 558 cm ⫺1 decreased on isothermal relaxation from 1 to 0.915 (band area obtained by integration, with break points for two-point baseline set at 589.2 and 539.1 cm ⫺1), whereas the band area of the econformer band at 512 cm ⫺1 increased from 1 to
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
239
Fig. 2. The separated spectra of the a and e-conformer showing their n22 (left) and n23 (right) normal vibrations. The first-derivative-like feature in the a-conformer spectrum at ⬃ 728 cm ⫺1 is the most intense artifact.
1.026 (break points for two-point baseline set at 539.1 and 500.5 cm ⫺1). These scaling factors were then used for separation in the same manner as described in Ref. [10,11].
3. Results and discussion Nonequilibrium distribution of a and e-conformers was generated by quenching of the vapours of Cl–CH on a CsI window held at 78 K as described before [2,26–30]. But here we studied isothermal interconversion of conformers at low temperatures. Fig. 1 shows the spectral IR region of Cl–CH from 490 to 590 cm ⫺1 recorded at 140 K. The two bands centered at 558 and 512 cm ⫺1 from the a and e-conformer (n23, a 0 symmetry, [20]) have often been used to study conformer populations. These bands are not welllocalized vibrations but seem to involve the C–Cl stretching vibration [20]. Spectrum a (solid line) was recorded after 10 min, spectrum b (broken) after 30 min. Their difference (b-a, enlarged 3-fold) shows decreasing intensity of the band centered at 558 cm ⫺1 and increasing intensity of the band at 512 cm ⫺1,
which indicates conversion of the a into the econformer on relaxation toward equilibrium. Conversion of the a into the e-conformer on relaxation at 140 K is consistent with Bugay et al.’s report of thermodynamic parameters of the conformers in rapid equilibrium [21]. For the axial , equatorial conformer equilibrium, the equilibrium constant (Keq [equatorial]/[axial]) is 2.93 at 298 K and 5.4 at 140 K (calculated from thermodynamic quantities listed in Table 1 of Ref. [21]).Therefore, relaxation of the frozen-in nonequilibrium conformer population toward equilibrium must occur via conversion of axial into equatorial Cl–CH. The ratio of band areas of the difference curve allows one to calculate the ratio of extinction coefficients of the a and e-conformer bands at 558 and 512 cm ⫺1 (Ja/Je). The Ja/Je value of 3.3 thus calculated is consistent with the ratio of 3.2 ^ 0.1 reported by Fishman et al. from isothermal kinetics of Cl–CH dissolved in propane at 143.5 K (see Fig. 3 in Ref. [4]). In a next step the spectra of Fig. 1 were separated into the the spectral components of the a and econformer of Cl–CH. Hirschfeld’s ‘ratio method’
240
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
Fig. 3. (a and b) Spectra of the separated a and e-conformer of Cl–CH, and (d) the Cl–CH spectrum recorded at 140 K after 10 min which is identical with the sum of the separated a and e-conformer spectra; (c) for comparison the difference spectrum shown in Fig. 1. Spectrum (a) is enlarged 2-fold and the difference spectrum (c) 10-fold.
allows resolution of ‘‘the spectrum of a mixture of an unknown small number of unknown constituents in unknown proportions into the spectra of these constituents without isolating them’’, provided that each unknown constituent absorbs dominantly in a particular spectral region [10,11]. Spectra of an original mixture and of mixture(s) following some fractionation procedure of the components are necessary. The method was applied to unknown mixtures of two components only and the scaling factors necessary for quantitative analysis were obtained by ratioing two spectra. This method is simple and rapid, but it does not correct properly for the spectral background. Diem and Krimm [12] improved the accuracy of the Hirschfeld-method by about one order of magnitude. Their method is based on the scaling factors required to null the components, one at a time, in successive absorbance subtraction of the two spectra. We use their method and determine the scaling coefficients from the changes in integrated band areas of the two bands centered at 558 and 512 cm ⫺1 on relaxation and conformer interconversion. Fig. 2 (right) shows the result of this separation for
the two bands of the a and e-conformer at 558 and 512 cm ⫺1. Separation is not perfect and some residual feature resembling a first derivative curve is present in the e-conformer spectrum at ⬃ 558 cm ⫺1. This type of artifact occurs also in other spectral regions of the separated spectra. The most intense artifact occurs at ⬃728 cm ⫺1 in the separated spectrum of the aconformer [see Fig. 2 (left)]. The bands centered at 684 and 728 cm ⫺1 (n22, a 0 symmetry, see Ref. [20]) have often been used in the vibrational analysis of Cl– CH’s a and e-conformer (see e.g. Refs. [22,23]). The artifact occurs in the a-conformer spectrum at the position where the e-conformer has its intense band. Similar artifacts were observed before in the spectra of separated components [11]. They were attributed to the following causes: (i) Allara [31] has shown that band shapes can be changed even in the absence of intermolecular interactions as a result of the effects of differences in the real part of the refractive indices of the pure components [12,31]. ‘‘The magnitude of this anomaly will depend on the optical properties of the two pure compnents. With given components, the magnitude of the anomaly will
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243 Table 1 Comparison of chlorocyclohexane’s a and e-conformer frequencies from their distinct separated spectra a with observed b and calculated b frequencies [cm ⫺1]. Axial obs. a
obs. b
calc. b
Equatorial obs. a obs. b
calc. b
a0 n1 n2 n3 n4 n5 n6 n7 n8 n9 n10 n11 n12 n13 n14 n15 n16 n17 n18 n19 n20 n21 n22 n23 n24
2940 (2915) (2903) 2893 2862 2862 (2839) 1459 1439 1428 1340 1329 1265 (1259) ⬃1214 1098 1030 1015 869 859 808 684 558 472
2946 2921 2908 2889 2860 2860 2845 1461 1439 1429 1343 1326 1268 1260 1216 1099 1030 1015 869 859 807 685 559 473
2956 2922 2916 2914 2857 2853 2851 1456 1437 1422 1345 1343 1283 1251 1226 1105 1026 985 870 853 806 680 552 482
(2954) 2935 (2914) (2904) (2875) 2856 2856 1464 (1453) ? 1353 1333 1269 1258 1217 1133 1032 994 889 851 818 728 512 435
2955 2946 2908 2903 2865 2860 2860 1466 1449 1429 1353 1333 1268 1259 1216 1133 1026 994 889 852 819 732 512 436
2956 2922 2916 2914 2857 2853 2851 1456 1440 1425 1351 1340 1265 1251 1221 1121 1020 992 892 839 819 733 508 428
a 00 n28 n29 n30 n31 n32 n33 n34 n35 n36 n37 n38 n39 n40 n41 n42 n43 n44 n45 n46
2932 2893 2862 (2839) 1447 1439 1358 (1347) 1340 1329 1272 1146 1118 (1082) 1030 923 864 (779) 456
2943 2903 2865 2845 1452 1436 1358 1353 1340 1321 1273 1146 1119 1075 1030 921 862 775 454
2917 2913 2855 2851 1440 1434 1368 1350 1342 1320 1261 1145 1144 1083 1034 928 856 792 461
2935 (2925) (2875) 2856 (1453) 1447 1366 1353 1341 1301 1258 1187 1088 1075 1050 920 871 790 (443)
2943 2930 2865 2860 1449 1439 1366 1353 1340 1302 1260 1185 1089 1075 1052 917 877 789 440
2918 2913 2855 2852 1448 1434 1383 1351 1345 1299 1245 1182 1100 1068 1051 916 886 787 457
a
This work, peak frequencies from Figs. 2 and 3, peak positions in brackets are read from second derivative curves. b From Table 4 in Ref. [18]. Normal vibrations below 400 cm ⫺1 are not included in Table 1.
241
increase as the difference in the compositions of the two solutions increases and as the absolute intensities of the bands increases’’ [12]. (ii) Spectral artifacts can arise from intermolecular interactions because band shapes can be altered on changing the composition of the mixture [13]. (iii) A first derivative feature can occur in a difference spectrum when a band slightly shifts its peak position [25]. This situation could arise when a weak band of one component is superposed by an intense band of the second component, and its band shape is altered by changes in intermolecular interactions. Fig. 3 shows the separated spectra of Cl–CH’s a and e-comformer from 2990 to 2820 cm ⫺1 and from 1500 to 780 cm ⫺1 (curves a and b). The spectrum of the conformer mixture of Cl–CH recorded at 140 K after 10 min is shown as curve d. The difference of the spectra recorded after 30 and 10 min is labelled as curve c (identical with the difference curve in Fig. 1, bottom). Spectrum d is identical with the sum of spectra a and b. The difference spectrum has positive and negative peaks at those positions where the e and a-conformers have bands. The spectral artifacts occur mainly in the separated spectrum of the a-conformer. Since the a to e-conformer population ratio is 0.30, the a-conformer is dissolved in the e-conformer and spectral artefacts by intermolecular interaction with the econformer are more likely than vice versa. Spectra recorded at 140 K after shorter times than those used for Figs. 1–3 (i.e. after 10 and 20 min) gave the same separated a and e-conformer spectra (not shown). Very similar separated spectra of the a and e-conformer were also obtained from glassy Cl–CH made by so-called hyperquenching of micrometersized droplets [32–35]. The a and e-conformer of Cl–CH both belong to point group Cs, and 27 of the 48 normal vibrations are of a 0 and 21 of a 00 symmetry. Woldbæk’s [20] report of the IR and Raman spectral data of Cl–CH in the liquid and crystalline state is probably the most extensive one. He also performed a normal coordinate analysis for both conformers, and tentatively assigned all normal vibrations of both conformers (see Table 4 in Ref. [20]. In Table 1 we list our assignment of the normal vibrations of the a and e-conformer obtained from curves a and b in Fig. 3 (peak positions marked), and we compare it with Woldbæk’s [20] observed and calculated frequencies. Peak positions
242
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243
obtained from our separated conformer spectra vary at most by a few wavenumbers from those reported before, and the IR band intensities are similar to those collected in Table 1 of Ref. [20]. Therefore, our separation into distinct spectra of Cl–CH’s a and e-conformer spectra seems to be reasonable, despite the spectral artifacts pointed out and discussed earlier. Our results differ only in two cases from previously reported work (marked by arrows): first, a band of medium intensity at ⬃1429 cm ⫺1 was attributed to the e-conformer (assigned as n10, see Table 1 in Ref. [20]) but it is absent in our spectrum obtained by separation. Curves a and b of Fig. 3 show that in this spectral region only the a-conformer has a band at 1428 cm ⫺1 of medium intensity. In the IR and Raman spectrum of crystalline Cl–CH recorded at 90 K, a weak band was observed at 1429 cm ⫺1 and a medium intensity band at 1431 cm ⫺1 (see Table 1 in Ref. [20]). Assignment of this band to the e-conformer was probably made because the anisotropic crystal contains only the e-conformer. We suggest that the ⬃ 1430 cm ⫺1 band in the crystal is caused by crystal field splitting and thus, observation of this band in both the liquid and crystalline state is accidental. Second, the separated a-conformer spectrum contains a band at 2962 cm ⫺1 of medium intensity (see Fig. 3, curve a, marked) which is not assigned to the aconformer in Table 1 of Ref. [20]. In Cl–CH dissolved in CCl4, a strong IR band is centered at 2955 cm ⫺1 which was assigned to n1 of the econformer. Overlap of bands is most pronounced in this C–H stretching band region and therefore, a band of the minor component can easily be missed. A weak band is also observable at 2961 cm ⫺1 in the fourthderivative of the Cl–CH spectrum recorded at 140 K. We suggest that the a-conformer band at 2962 cm ⫺1 should be considered as normal vibration in the assignment. Our method is obviously not limited to IR spectroscopy, and Raman spectroscopic studies with polarization measurements will be an important extension in the future. The basic requirement for the applicability of this method is the absence of crystallization. This can be achieved either by using a sufficiently low temperature, or by dissolving the compound in a solvent which prevents crystallization. The latter approach was used by Fishman et al. with the aim to
determine the glass ! liquid transition temperature of a solvent by using as a probe freezing-in of conformer interconversion in a solute [3–5]. Thermograms provide a quick test to see whether the glass ! liquid transition is separated sufficiently from the onset temperature of crystallization. Nonequilibrium population of conformers can be obtained either by rapid cooling of vapours, or by so-called hyperquenching of a liquid or a solution into the glassy state [32–35].
Acknowledgements We are grateful for financial support by the ‘Forschungsfo¨rderungsfonds’ of Austria (project P12319-PHY).
References [1] W.J. Orville-Thomas (Ed.), Internal Rotation in Molecules, 1974, Wiley, London, NY. [2] P. Klaeboe, Vibrational Spectrosc. 9 (1995) 3. [3] A.I. Fishman, A.A. Stolov, Spectrochim. Acta 49A (1993) 1435. [4] A.I. Fishman, A.A. Stolov, A.B. Remizov, Spectrochim. Acta 41A (1985) 505. [5] A.I. Fishman, S.Y. Guseva, A.B. Remizov, A.A. Stolov, O.E. Zgadzai, Spectrochim. Acta 42A (1986) 1247. [6] S. Ru¨disser, A. Hallbrucker, E. Mayer, J. Am. Chem. Soc. 119 (1997) 12251. [7] S. Ru¨disser, PhD Thesis, Universita¨t Innsbruck, 1997. [8] S. Ru¨disser, A. Hallbrucker, E. Mayer, J. Phys. Chem. 100 (1996) 458. [9] S. Ru¨disser, A. Hallbrucker, E. Mayer, G.P. Johari, J. Phys. Chem. 101 (1997) 266. [10] T. Hirschfeld, Anal. Chem. 48 (1976) 721. [11] J.L. Koenig, Spectroscopy of Polymers, ACS Professional Reference Book, American Chemical Society, Washington, DC, 1992, pp. 67–68. [12] H.E. Diem, S. Krimm, Appl. Spectrosc. 35 (1981) 421. [13] S.-B. Lin, J.L. Koenig, J. Polym. Science, Polym. Phys. Ed. 20 (1982) 2277. [14] P. Klæboe, J.J. Lothe, K. Lunde, Acta Chem. Scandinavia 10 (1956) 1465. [15] C.G. Opaskar, S. Krimm, Spectrochim. Acta A 23 (1967) 2261. [16] A.B. Remizov, L.M. Sverdlov, Izv. Vyssh. Uchebn. Zaved. Fiz. 11 (1968) 150. [17] P. Klæboe, Acta Chem. Scandinavia 23 (1969) 2641. [18] J. Sˇtokr, B. Schneider, J. Jakesˇ, J. Mol. Structure 15 (1973) 87. [19] S.D. Christian, J. Grundnes, P. Klaboe, J. Am. Chem. Soc. 97 (1975) 3864. [20] T. Woldbæk, Acta Chem. Scandinavia A 36 (1982) 641.
S. Ru¨disser et al. / Journal of Molecular Structure 479 (1999) 237–243 [21] R. Borsdorf, S. Ferner, R. Salzer, Chem. Monthly 113 (1982) 827. [22] D.J. Gardiner, C.J. Littleton, N.A. Walker, J. Raman Spectrosc. 18 (1987) 9. [23] D.E. Bugay, C.H. Bushweller, Ch.T. Danehey Jr., St Hoogasian, J.A. Blersch, W.R. Leenstra, J. Phys. Chem. 93 (1989) 3908. [24] E. Nodland, F.O. Libnau, O.M. Kvalheim, H.-J. Luinge, P. Klæboe, Vibrat. Spectrosc. 10 (1996) 105. [25] D.B. Parry, M.G. Samant, O.R. Melroy, Appl. Spectrosc. 45 (1991) 999. [26] M. Squillacote, R.S. Sheridan, O.L. Chapman, F.A.L. Anet, J. Am. Chem. Soc. 97 (1975) 3244. [27] P. Felder, Hs.H. Gu¨nthard, J. Mol. Struct. 60 (1980) 297.
243
[28] A.J. Barnes, J. Mol. Struct. 113 (1984) 161. [29] V. Vidya, K. Sankaran, K.S. Viswanathan, Chem. Phys. Lett. 258 (1996) 113. [30] A.J. Barnes, in: A.J. Barnes, W.J. Orville-Thomas, A.Muller, R. Gaufres, (Eds.), Matrix Isolation Spectroscopy, Reidel, Dordrecht, 1981, chap. 23. [31] D.L. Allara, Appl. Spectrosc. 33 (1979) 358. [32] E. Mayer, J. Appl. Phys. 58 (1985) 663. [33] G.P. Johari, A. Hallbrucker, E. Mayer, Nature 330 (1987) 552. [34] G.P. Johari, A. Hallbrucker, E. Mayer, J. Phys. Chem. 93 (1989) 2648. [35] E. Mayer, in: M.-C. Bellissent-Funel, J.C. Dore (Eds.), Hydrogen Bonded Networks, Kluwer Academic Publ., Netherlands, 1994, pp. 355–372.