the temperature dependence of the raman and infrared spectra of 1,4,7-trithiacyclononane

ELSEVIER

Journal of

Journal of Molecular Structure 378 (1996) 165-175

MOLECULAR STRUCTURE

The temperature dependence of the Raman and infrared spectra of 1,4,7-trithiacyclononane

Y.S. Park”, H.F. Shurvellbt* “Department of Oncology. Queen’s University, Kingston, Ont., K7L 3N6, Canada

bDepartment of Chemistry, Queen’s University, Kingston, Ont.. K7L 3N6. Canada

Received 4 August 1995; accepted 9 October 1995

Abstract

The temperature dependence of the Raman and infrared spectra of solid 1,4,7-trithiacyclononane (TTCN) has been studied from 10 K to the melting point (355 K). Raman and infrared spectra of the liquid have been recorded from the melting point to 390 K. Differences between the spectra of the compound in the liquid and solid states indicate that there is a change from the conformation of symmetry D1 in the liquid phase to one of C, symmetry in the solid. As the temperature is lowered from room temperature to 10 K, splitting of many bands in the Raman and infrared spectra is observed. This indicates that a further lowering of symmetry occurs at low temperatures. Anomalies in the solid-state frequency versus temperature plots have been observed for several internal modes. It is suggested that a structural phase change occurs in the crystalline solid near 225 K. Possible structures of the low temperature phase are discussed.

Keywords: Trithiacyclononane; Raman; Infrared; Low temperature; Structure

1. Introduction

Raman and infrared (IR) spectra of the cyclic thioether 1,4,7- trithiacyclononane (TTCN) in the liquid, solution and solid states were reported recently [l]. The spectra were consistent with a molecular conformation of symmetry 03 in the pure liquid at 390 K and in solution in Ccl4 at room temperature. However, a conformation of lower symmetry C, was indicated for the TTCN molecule in the solid state at room temperature. The molecular structure of TTCN obtained from a semi-empirical calculation [l] is shown in Fig. 1.

* Corresponding author.

TTCN is the sulphur analogue of the cyclic ether 1,4,7-trioxacyclononane (TOCN). Borgen et al. [2] listed the possible conformers of TOCN. It is rea- sonable to expect that TTCN has the same conformers as TOCN. In the notation of Dale [3], there are two [333] conformations of symmetry D3 and C,, respectively. There are also two structures denoted [12222] of C, symmetry and three unsymmetrical [234] conformations.

The crystal structure of the solid compound at room temperature was determined from an X-ray diffraction study to be rhombohedral with space group R3c (C&), with six molecules per unit cell [4]. This study found that the TTCN molecule assumes a conformation with C, symmetry in the solid at room temperature.

0022-2860/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)09 173-4

166 Y.S. Park, H.F. ShurveN/Journal of Molecular Structure 378 (1996) 165-175

2. Experimental

Fig. 1. The molecular structure of TTCN.

We report here Raman and IR spectra of TTCN in the solid state at various temperatures between 10 K and the melting point (355 K). Spectra of the liquid between the melting point and 390 K were included in Ref. [I]. A detailed study of the temperature dependence of Raman and IR bands of the solid compound is reported here. This study was made to look for evidence of structural phase changes associated with conformational changes from the conformer of C3 symmetry to those of lower symmetry.

1,4,7-Trithiacyclononane (TTCN), 98%, mp 78-81°C was purchased from Aldrich Chemical Co., and was used without further purification. To record the Raman spectra of the solid, pellets of the powdered polycrystalline sample were made, or powder was loosely packed in a capillary tube. The samples were then mounted on a copper block in an Air Products and Chemicals Inc., Model HV- 202 Displex closed-cycle helium refrigeration system. Variable temperatures between 10 K and 300 K were obtained using this system. To obtain Raman spectra of the solid above 300 K, a home- made heated cell was used.

Raman spectra were recorded using a Jarrell- Ash double monochromator equipped with photon counting electronics. The spectra were excited by the 514.5 nm line of a Spectra-Physics Model 171 argon ion laser operating at powers of lOO- 400 mW at the sample.

Solid-state IR spectra were recorded at room temperature from thin crystalline films. The films were obtained by melting the solid between CsI plates and allowing the liquid to crystallize. Low temperature IR spectra were obtained from 13-mm discs containing TTCN dispersed in CsI. The closed cycle helium refrigerator described above

10 K

WAVENUMBER (CM -’ )

Fig. 2. The Raman (lower) and IR (upper) spectra of TTCN at 10 K from 3000 to 2800 cm-’ and from 1500 to 950 cm-’

Y.S. Park, H.F. ShurvelljJournal of Molecular Structure 378 (1996) 165-175 157

was used to obtain temperatures down to 10 K. A Raman spectrum for a free TTCN molecule with Perkin-Elmer model 9836 IR spectrometer was 0s symmetry is 29 (10 Ai + 19 E), whereas 38 used to record the spectra. peaks are predicted for the structure with C, syiri-

metry and 57 for the C, conformer. In Table 1 it’ is seen that in the Raman spectrum of liquid TTCN

3. Results and discussion at 390 K, 21 distinct bands are observed between 150 and 3000 cm-‘. In the solid state at 330 K, 29

Raman and IR spectra of TTCN at temperatures bands are assigned to internal modes, while at 10 K above and below the melting point were included the number of bands increases to 46. The three new in Ref. [l]. Raman and IR spectra of TTCN at bands in the solid-state spectrum at 330 K (122,67 10 K are compared in Figs. 2 and 3. Observed wavenumbers (cm-‘) in the Raman and IR spectra of

and 54 cm-‘) are believed to be due to lattice modes, since they are not observed in the spectrum

the liquid at 390 K and the solid at 295 K and 10 K of the liquid. The other additional bands observed are listed in Table 1. The qualitative assignments in the solid-state spectrum at 330 K are attributed given in Table 1 are based on the regions of the to the lower site symmetry (Cs) of the TTCN mole- spectrum in which the bands occur [5]. Assign- cule in the crystal structure. The further increase in ments to symmetry species of the D3 structure the number of observed bands at 10 K is attributed (liquid phase) are based on calculated frequencies to a further lowering of symmetry in a new low and infrared intensities from Ref. [l]. In addition, temperature crystalline phase. those bands that split into doublets at low tempera- The IR spectrum of liquid TTCN at 390 K is tures (see later) are assigned to E-modes of the D3 quite different from that of the solid at room tem- structure. perature [ 11. Approximately the same total number

Assuming that TTCN has D3 symmetry in the liquid phase, the 38 normal vibrations comprise 10

of bands is observed, but the frequencies and intensities differ considerably. This is clear evidence that

Ai + 9 A2 + 19 E modes. The 10 A, modes are a conformational change occurs when the com- Raman-active, the 9 A2 modes are IR-active and pound changes from the liquid to the solid phase. the 19 E modes are both Raman- and IR-active. In many cases the frequencies of the observed Thus the total expected number of peaks in the bands in the Raman and IR spectra of the solid

10 K

800 600 400 200 -1

WAVENUMBER (CM )

Fig. 3. The Raman (lower) and IR (upper) spectra of TTCN at 10 K from 950 to 600 cm-’ and from 500 to 150 cm-‘.

168 Y.S. Park, H.F. Shurvell/Jotcrnal of Molecular Structure 378 (1996) 165-175

Table 1 Observed wavenumbers (cm-‘)’ and assignmentsb of the infrared and Raman spectra of liquid and solid TTCN

Infrared Raman Assignment

Liquid Solid Solid Liquid Solid Solid (390 K) (295 K) (10 K) (390 K) (330 K) (10 K)

2972wsh 2920ssh 2930msh

2909~s

2801m

1412~s

1275~s 1205w

- 1013vw

997vw 935m 923m

907m

860m 838s 818ms

693sh 675m 641vw 627~

435w 409w 392~ 306vw

2968~

2926~~

2898~s

2806m

1455vs 1420~ 1413sh 1408s 1295m - 1282~~

1191w 1184w

1136m - 1128m -

987w

924s

878s

838s 824vw

705vw

671m 638vw 620~ 455vw

416~

2968~ 2958wsh

2924~s 291 lwsh

2894s

2804m

1460s 1423~ 1413m 1408m 1300m

1285m

1278ms

1192~ 1185~

1138~ 1132m 1127m

988~

923s

aaovw

840s 827vw

672s

622m 457vw

415s

2936sh

2902s

14lOm 1294~

1205~~

1188~~ 1151vw

lOlOvw

912~ 88Ovw

- 84Ovw

684~ 672sh 642~ 598~

429vw

301sh

2964m 2858sh

2923s 2916s -

2895~s 2835~~ 2825~~ 2803~

1456~

1412sh 1408ms 13oow

128Ow

1191sh 1188m

-

1015w

-

918~

879w

838vw 822m

-

703w

670s 639vwsh 619~~s 454w

416~

308vw

2964s 2958m 2932~ 2920~~s 2918sh 2901s 2891~~s 2842~~ 2830~~ 2804~ 1469~ 1464m 1426~~ 1423vwsh 1408s 1304w 1297wsh 1285wsh 128Ow

1205vw 1192msh l189m

1138vwsh ll34w 1127wsh 1018~ 1014w

925~ 918~

88lm

841vw 826m 767~ 760~ 705w, br

673s 641~ 622~~s 457m

415w

316m

CH str. (E) CH str. (E) CH str. CH str. (E) CH str. CH str. (E) CH str. CH str. (E) CH str. (A,) CH str. (A,) CH2 def. (E) CH2 def. CH2 def. (At) CH2 def. (A,) CH2 def. (E) CH2 rock (E) CHs rock CH2 rock (E) CH2 rock CH2 rock (A,) CH2 rock (A,) CH2 rock (E) CH2 rock C-C str. (E) C-C str. C-C str. (A,) C-C str. CH2 twist (E) CH2 twist CH2 twist (A?) CH2 twist (A,) CH2 twist (E) CH1 twist CH2 wag (El CHI wag CH2 wag 6%) CH2 wag (AI 1 CH2 wag (E) C-S str. (E) C-S str. C-S str. (A2) C-S str. C-S str. (E) C-S str. (At) C-S str. (E) Ring def. Ring def. (E) Ring def. (A,) Ring def. (As) Ring def. (A,)

Y.S. Park. H.F. Shurvell/Journal of Molecular Structure 378 (1996) 165-175 169

Table 1 Continued

Infrared

Liquid (390 K)

Solid (295 K)

Solid (10 K)

Raman

Liquid (390 K)

Solid (330 K)

Solid (10 K)

Assignment

-

246s.h

-

-

- - 285w 290m Ring def. 278m - 284~ Ring def. (At)

251vw - 253vw 25lw Ring def. (A,) - 235vw - - Ring def. (E) - 205vw - Ring def. (A,)

184m 191m Ring def. 159m 168m 174m Ring def. (E)

- 121w 128w External mode - - 67s -70c External mode

54w 58vw External mode

’ Relative experimental intensities are denoted by: v = very, s = strong, m = medium, w = weak and sh = shoulder. b Assignments and symmetry species are given for the liquid phase conformation of D, symmetry. ’ Obscured by a grating ghost.

coincide within experimental error. In the liquid phase spectra, there are several bands in the Raman spectrum which have no IR counterparts, and several IR absorptions with no corresponding Raman lines. This is consistent with a D, conformation for the free TTCN molecule in the liquid phase.

Information on crystal structure can often be obtained from splitting patterns of IR and Raman bands observed for internal modes. The total number of observed external modes and the observed splittings can be compared with the predictions from a Factor Group analysis [6]. A correlation diagram relating the symmetry species of the free molecule (assumed to belong to point group 0s) to those of the site group C3 and factor group (unit cell group), CsV, is shown in Table 2. Table 2 A correlation diagram for the high temperature form of solid TTCN

Free molecule Site Unit cell D3 c3 C3”

10 AI W A UR RI 9 A2 (W 7 r;;ER)

19 E (IR, R) - E (IR, R) -E (IR, R) Totals: 28 IR, 29 R 38 (IR, R) 38 (IR, R)

Predictions: No splittings of E-modes. Additional bands in both IR and Raman.

3.1. Temperature dependence of spectra

The temperature dependence of several bands in the Raman and IR spectra of TTCN is shown in Figs. 4-7. The main spectroscopic effect of lowering the temperature of a crystalline material is a sharpening of the bands in the spectrum. This is clearly seen in the low temperature spectra. Closer

2950 2900 2850 2800 WAVENUMBER (CM-’ )

Fig. 4. The temperature dependence of some bands in the Raman spectrum of TTCN.

170 Y.S. Park, H.F. Slrurveell/Joumal of Molecular Structure 378 (1996) 165-175

examination of the spectra reveals moderate shifts to higher frequencies of most bands as the temperature is lowered. These shifts can be attributed to a contraction of the unit cell, which results in a small increase in force constants and consequently the frequencies are slightly higher at lower temperatures. Splittings of many bands are observed in both Raman and IR spectra at 10 K. The detailed temperature dependence of the spectra has been studied.

3.2. Raman spectra

Shifts and splittings of some bands in the Raman spectra of TTCN as the temperature is varied between 295 K and 10 K are shown in Figs. 4 and 5. The temperature dependence of lines in the CH stretching region is seen in Fig. 4. Two new bands appear in the spectrum at 10 K. Below 230 K, a weak shoulder appears near 2930 cm-’ on the high frequency side of the strong line at 2922 cm-‘. This becomes a weak line at 2933 cm-’ in the 10 K spectrum. The shoulder at 2915 cm -’ on the low frequency side of the 2922 cm-’ band shifts to 2920 cm-’ and is barely detectable under the main band at 10 K. The very strong band at 2895 cm-’ develops a distinct shoulder at temperatures below 230 K. At 10 K, the main band shifts to 2891 cm-’ , while the shoulder becomes a strong sharp peak at 2901 cm-‘.

In the room temperature Raman spectrum of TTCN (Fig. 2 of Ref. [1]) sharp lines are observed at 1456,1300, 1280, 1015,918 and 703 cm-‘. These lines are all observed as doublets in the low temperature spectra of Fig. 2. The 1456 cm-’ becomes a doublet at 1469/1464 cm-‘. The weak pair of lines at 1300 and 1280 cm-’ each have distinct shoulders in the 10 K spectrum. A line of medium intensity at 1015 cm-’ in the room temperature spectrum becomes a sharp doublet at 1018/ 1014 cm-’ at 10 K. The weak line at 918 cm-’ is resolved into a doublet with the main peak at 918 cm-’ and a weaker peak at 925 cm-‘. A weak line at 703 cm-’ becomes a broader unre- solved doublet at 10 K. A new doublet at 767/ 760 cm-’ is observed in the low temperature spectrum. This feature is absent in the room temperature spectrum.

300 250 200 150

WAVENUMBER (CM -1

)

Fig. 5. The temperature dependence of some bands in the Raman spectrum of TTCN.

In Fig. 5 a noticeable change is observed in the Raman spectrum of TTCN in the region 350- 150 cm-’ as the temperature is lowered. All bands in this region sharpen and shift to higher wavenumbers. A weak peak at 283 cm-’ sharpens and shifts to 291 cm-’ with an increase in intensity as the temperature is lowered. A shoulder which first begins to appear at 230 K, becomes resolved into a sharp line at 285 cm-’ in the 10 K spectrum.

The splitting into doublets in the low temperature Raman spectra can be explained by a lifting of the degeneracy of the E-modes in a new structure of lower symmetry.

3.3. Infrared spectra

Many shifts and splittings are observed in the IR spectra of TTCN at low temperatures. Some of these are shown in Figs. 6 and 7. In Fig. 6 it is seen that the strong band at 1454 cm-’ in the room temperature spectrum shifts to 1459 cm-‘, while the shoulder at 1458 cm-’ shifts by an equal amount to 1463 cm-’ at 10 K. A weak absorption at 1426 cm-’ shifts to 1430 cm-‘. It is also seen in Fig. 6 that at 205 K a new band appears at 1300 cm-‘. This band sharpens and shifts

Y.S. Park. H.F. Shurvell/Journal of Molecular Structure 378 (1996) 165-175 171

298 K

1480 1460 1440 1300 1280 1260

WAVENUMBER (CM-’ ) WAVENUMBER (CM-’ )

Fig. 6. The temperature dependence of some bands in the IR spectrum of TTCN.

slightly to a higher wavenumber on cooling the sample to 10 K. The doublet observed at 1281/ 1275 cm-’ shifts to 12&l/1278 cm-‘. While the separation between the components of this doublet does not change, the relative intensities of the components are reversed on cooling from 298 to 10 K. Clearly, a strong new band, which was not present

at room temperature, has appeared in the 10 K spectrum. A new weak absorption also appears at 1260 cm-’ in the low temperature spectrum.

The most obvious feature of Fig. 7 is the appear- ance of a new IR band at 1132 cm-‘. This band grows between the components of the room temperature doublet 1136/l 128 cm-‘. The 1136 cm-’

1160 1140 1120 1100 840 830 820

WAVENUMBER (CM-’ ) WAVENUMBER (CM-’ )

Fig. 7. The temperature dependence of some bands in the IR spectrum of ITCN.

172 Y.S. Park, H.F. Shurvell/Journal o/Molecular Structure 378 (1996) 165-I 75

component of this doublet shifts slightly to higher frequency, while the 1128 cm-’ component moves down by an equal amount as the temperature is lowered to 10 K. The new band at 1132 cm-’ becomes the strongest component of the resulting triplet in the low temperature spectrum. Bands at 838 and 823 cm-’ shift up to 840 and 827 cm-’ as the temperature is lowered from 298 to 10 K. A new weak band appears near 820 cm-’ in the low temperature spectrum.

3.4. Crystal phase changes

When frequencies of both internal and external modes are plotted against temperature, smooth curves indicate the absence of any crystal phase change. However, discontinuities in such curves indicate a transformation to a new crystal phase at the temperature of the discontinuity. Splitting of bands may originate at this temperature.

In Fig. 8, the frequencies of some Raman bands in the CH stretching region are plotted as a func- tion of temperature. There are discontinuities near

200 K in the frequency versus temperature plots and a splitting of one band is observed.

Fig. 9 shows the temperature versus frequency plots of IR bands near 1460 cm-‘. Again anomalies are observed near 200 K. A similar plot for the 285 cm-’ Raman band is shown in Fig. 10. A splitting of this band is observed near 200 K.

3.5. Phase transitions in TTCN

The changes observed in both Raman and IR spectra as the sample is heated through 350 K can be attributed to a first-order solid to liquid phase change. The observed changes in the spectra are reversed when the sample is cooled through 350 K. This phase change is accompanied by a conformational change from a structure of symmetry D3 to a conformation of symmetry C3 [l].

The anomalies shown in both Raman and IR spectra near 200 K and the subsequent splitting of many bands can be attributed to a structural phase transition in which the site symmetry is lowered by a further conformational change in the

2940

^ 2920 5

5 I 3 $ 3 2900

0 0 0 0 0

0 0 0 0 q o0 0 0 q 0

v v v v

0 0 0 o

V

0 00 0

0 50 100 150 200 250 300 350 400

TEMPERATURE (K)

Fig. 8. Wavenumber versus temperature plots for some bands in the CH stretching region of the Raman spectrum of TTCN.

Y.S. Park, H.F. Slwvell/Journal of Molecular Structure 378 (1996) 165-175 173

1470

5 m 1460

5 5 z 3

1450

O 0 0

00 000 0

0

0 0

0 00 0 00 00 00 00 0

0

0 0 0 0 00

0 50 100 150 200 250 300 350

TEMPERATURE (K)

Fig. 9. Wavenumber versus temperature plots for bands near 1460 cm-’ in the IR spectrum of TTCN.

TTCN molecule to a conformation of symmetry C, or to an unsymmetrical structure.

3.6. The low temperature phase

The crystal structure of the new low temperature phase could remain rhombohedral with a CJV unit cell if the site symmetry is lowered to Ci and the new C’s” unit cell contains four molecules. In this case the TTCN molecule would have to assume one of the three unsymmetrical [234] conformations. For this structure, a doubling of the number of external modes would be predicted. However, this is not observed. On the other hand, in order for the TTCN molecule to have one of the two structures of C2 symmetry, the crystal structure would have to change. Any of the orthorhombic structures with a unit cell symmetry C,, could have a site of symmetry C2 and two molecules per unit cell. In both cases dicussed above, the degeneracy of the E-modes of the free TTCN molecule would be lifted. This would give rise to splitting of all bands assigned to E-modes in both infrared and Raman spectra.

A diagram showing the correlations between the

symmetry species of the free molecule group (D3), the site group (C,), and a possible factor group (C,,) is shown in Table 3. It is seen that splitting is predicted for the degenerate modes at the site. Further (unit cell) splitting is predicted, but this is expected to be much less important than the site splitting and is not expected to be observable in the spectra reported here.

Table 3 A correlation diagram for a possible low temperature form of solid TTCN

Free molecule D3

Site c2

Unit cell C2”

/ A, W, R)

10 AI (RI A UR RI --A2 (RI

I9 E (IR, R) B W R) --I (IR, R)

9 A2 (IN \ B2 OR, R)

Totals 28 IR, 29 R 57 (IR, R) 76IR, 114R

Predictions: Splittings of E-modes at the site. Additional (unit cell) splittings.

174 Y.S. Park, H.F. Shurvell/Journal o/ Molecular Structure 378 (1996) 165-175

280

0 0 0 0 0

0 0 0 0

0 0

O 0 O 0 0 0

I I I I I I I I

0 50 100 150 200 250 300 350 400

TEMPERATURE (K)

Fig. 10. A wavenumber versus temperature plot for the 285 cm-’ Raman band of TTCN.

3.7. External (lattice) modes

Two molecules per spectroscopic unit cell give rise to six external modes, of which three are

acoustic modes. The three optical modes are Raman-active and are seen at 122, 67 and 54 cm-’ in the room temperature spectrum of Fig. 11. These bands shift to 130, approx. 70 and

I 300 250 200 150 100 50

-1 WAVENUMBER (CM )

Fig. 11. Raman spectra of the external mode region of TTCN at 298 K and 10 K: E, external mode; l , grating ghost.

Y.S. Park, H.F. Shurvell/Journal of Molecular Structure 378 (1996) 165-175 175

130 cm-’ at 10 K. They are also much weaker at 10 K than at 298 K. The weakening of these modes could be attributed to mode softening associated with the phase transition, which appears to occur near 200 K.

4. Conclusions

The temperature dependence of the external and internal modes of 1,4,7-trithiacyclononane (TTCN) has been studied. As the temperature is lowered, dramatic changes in the Raman and IR spectra are observed at the melting point. This is associated with a conformational change from a structure of symmetry D3 in the liquid phase to a conformation of symmetry C, in the solid.

On cooling the solid to 10 K, all Raman and IR bands associated with internal modes become shar- per and generally shift to higher frequencies. Many Raman and IR bands split into doublets at low temperatures. Three low-frequency Raman bands which are not present in the spectrum of liquid TTCN are assigned to external modes. These bands appear to weaken as the temperature is lowered. This behaviour could be associated with mode softening. The frequency vs. temperature curves for the internal modes are generally non- linear and there are some anomalies in these curves near 200 K. These observations indicate that a structural phase change occurs near 200 K. Detailed studies of the pressure and temperature

dependence of Raman and IR spectra in this region might further clarify the nature of the phase transition.

The observed splitting of many bands assigned to E-modes in the low temperature spectra can be explained by a transition to a new crystal phase of lower symmetry. One possibility is an orthorhom- bit structure with factor group CzV, site symmetry C, and two molecules per unit cell. Single-crystal Raman experiments and low-temperature X-ray diffraction studies are needed to determine the structure of the phase transition.

Acknowledgements

This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada.

References

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Sot. Commun., (1974) 243. [3] J. Dale, Acta Chem. Stand., 27 (1973) 1115. [4] R.S. Glass, G.S. Wilson and W.N. Setzer, J. Am. Chem.

Sot., 102 (1980) 5068. [S] G. Socrates, Infrared Characteristic Group Frequencies,

2nd. edn., Wiley, Chichester, 1994. [6] W.G. Fateley, F.R. Dollish, N.T. McDevitt and F.F. Bent-

ley, Infrared and Raman Selection Rules for Molecular and Lattice Vibrations, Wiley-Interscience, New York, 1972.

the temperature dependence of the raman and infrared spectra of 1,4,7-trithiacyclononane

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