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Low temperature Raman study of dimethylacetylene
Journal of Molecular Structure 482–483 (1999) 653–659
Low temperature Raman study of dimethylacetylene V. Mohacˇek Grosˇev*, K. Furic´ Rud⁄er Bosˇkovic´ Institute, Bijenicka 54, P.O. Box 1016, 10 000 Zagreb, Croatia Received 24 August 1998; received in revised form 18 December 1998; accepted 21 December 1998
Abstract On the basis of low temperature Raman spectra from 10 to 170 K of polycrystalline dimethylacetylene (DMA), two stable and two metastable crystalline phases were detected. One can be identified with a previously reported metastable phase [N. Prasad, R. Kopelman, Chem. Phys. Lett. 20 (1973) 513–516] and can be observed from 10 to 75 K, when it transforms into another metastable phase. This one, in turn, transforms at 115 K into the stable C2/m phase. To enable better assignment of the observed internal modes, especially those involving methyl group, matrix isolation of DMA at 10 K was performed. In the recorded Raman spectra at matrix ratios of 1:100, 1:50 and 1:25, the strongest bands observed are of a1s symmetry. Besides them, we were able to detect bands at 218 and 1040 cm 21 of e1d symmetry, which are only infrared active when no intermolecular interactions are present. The Raman spectrum of DMA as a thin solid film was also recorded. The bands’ positions are found not to deviate significantly in comparison with the spectra of the matrix isolated sample, and thanks to the better signal to noise ratio, several additional bands were detected. Among those is the n 8 at 1147 cm 21 of a4s symmetry. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Dimethylacetylene; Raman spectroscopy; Phase transition; Matrix isolation
1. Introduction Dimethylacetylene (DMA), or 2-butyne, is known for its very low barrier to internal rotation in the gas phase: 62 J/mol [1]. Solid state studies on DMA include heat capacity measurements [2], X-ray diffraction [3,4], NMR studies [5,6], vibrational spectroscopy [7,8], neutron [9,10,11] and electron diffraction experiments [12]. In this work we focus our attention on vibrational spectroscopy experiments. In the first part we discuss stable and unstable crystal phases of DMA, and in the second part we discuss the Raman spectra both of the
* Corresponding author. Tel./fax: 1 385-1-4680-112. E-mail address: [email protected] (V. Mohacˇek Grosˇev)
matrix isolated sample and of DMA as a thin solid film. The early heat capacity measurements [2] discovered an anomaly at 154 K. Subsequent X-ray diffraction experiments showed that both crystal structures above and below 154 K are tetragonal. The higher temperature crystal structure was found to be P42/ mnm with two molecules per unit cell, whereas the lower temperature phase was discovered to be P41212 with four molecules per unit cell. We shall denote the higher temperature crystal phase as phase I, and the lower temperature crystal phase as phase II. Recently a neutron powder diffraction study on DMA was published, which states that the lower crystal phase is not tetragonal, but of monoclinic symmetry, space group C2/m, with two molecules per unit cell [11]. The authors have not yet completely resolved the
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00023-X
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V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
symmetry of phase I, but suggest it is of rhombohedral symmetry with three molecules per unit cell. This phase, as they say, shows a lot of diffuse scattering which indicates a highly disordered system. Also, there is an evidence of a non-tetragonal crystal structure of DMA coming from an electron diffraction study on a jet-cooled sample [12]. We focus our attention on vibrational spectroscopy experiments. In their low temperature Raman study, Prasad and Kopelman [7] provided low frequency spectra of three crystal phases— phase I, phase II and a new unknown metastable phase. Their discovery was disputed by Butler and Newbury [8] who claimed that the metastable phase was an artefact produced by the undercooling of phase I in phase II. Our experimental evidence confirms the results of Prasad and Kopelman and moreover indicates new phase transitions connected to the metastable phase. Another interesting problem is the degeneracy of methyl bending and stretching modes observed for the free DMA molecule. The assignment of the observed Raman and infrared bands for a molecule in the vapour or gas phase was the subject of several studies [1,13–15,16]. From the analysis of the complex rotation-torsional structure of all the degenerate methyl fundamentals, the authors conclude that the degeneracy is fourfold, that is the modes are of G(s) symmetry if the extended molecular symmetry 1 is used. By preparing the matrix with a group G36 low matrix ratio (MR) one could obtain a system of weakly interacting DMA molecules. From the number and the position of the observed bands one could estimate the strength of the intermolecular interaction— if the interaction is weak, one could obtain the answer to the degeneracy of a given mode much quicker than when compared to the rather cumbersome interpretation of the observed vapour–gas spectra. As the matrix isolation technique is suitable for providing data on bands of very weak intensity in the gas phase [17–20] we recorded Raman spectra of DMA at 10 K in argon matrices with MRs of 1:100, 1:50, and 1:25. We also recorded the spectrum of DMA as a thin solid film. As all molecular conformations are possibly present in the amorphous film, one can in principle observe all vibrational bands. In our spectrum of the thin solid film the majority of bands are of a1s and e2d symmetry (fundamentals),
and of A1s symmetry (overtones). We were able to observe some a4s and e1d fundamentals.
2. Experimental DMA (99% pure) was purchased from Aldrich and used in Raman measurements without further purification. Low temperature Raman spectra of polycrystalline DMA were recorded using a CTI Cryogenics (Helix, Cryodine) Model 21 cryostat with a closed cycle helium refrigerator. The capillary containing approximately 20 ml of the sample was sealed under vacuum after the sample was frozen by immersing the tube in liquid nitrogen. The capillary was placed in a specially constructed mount on the cold finger [21]. Matrix isolated samples were prepared by the “slow spray on” technique using the same cryostat as described earlier, but using a different attachment to the cryostat head [19]. For matrix isolation, a gaseous mixture of DMA and argon was prepared by allowing a fixed amount of DMA (20 ml) to evaporate at room temperature. Argon from its own bottle (3 dm 3) and under a known pressure was allowed to mix in the sample tube together with the DMA vapour. In this way we obtained a 1:100 DMA to argon mixture. The sample tube was then attached to the needle valve on the cryostat. The cryostat was cooled to 10 K and the valve was opened to one turn (15 turns correspond to the maximum flow). The good quality thin films that formed on the gold plated surface on the cryostat head were obtained with deposition times from 10–15 min. Matrices with the MR of 1:50 and 1:25 were formed in a similar way, only increasing the amount of DMA two and four times, respectively. The ratio of integrated intensities of the 374 cm 21 band in different matrices is in good agreement with this. For the preparation of the thin solid film, a small amount of pure DMA was stored in a bottle. Several degasing procedures were applied to eliminate the air from the sample tube, which was then attached to the needle valve and the whole procedure continued as in the matrix isolation experiment. Raman spectra were recorded with a triple monochromator model DILOR Z24. For excitation a COHERENT Innova 100 laser operating at 514.5 nm was used, the laser power being 100 and 200 mW. Spectra were recorded sequentially, the
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
655
Fig. 1. Low frequency Raman spectra of stable crystalline phases of dimethylacetylene obtained by slow cooling (20 K/h). 160 K:phase I, 120 K and 10 K:phase II. Asterisk denotes a laser plasma line.
smallest step was 0.2 cm 21 in the phonon spectra, and 1 cm 21 in the spectra of the matrix isolated sample. The slitwidths used were 2, 3 and 5 cm 21.
3. Results and discussion 3.1. Polycrystalline stable and metastable phases Low frequency Raman spectra of polycrystalline phases are shown in Figs. 1 and 2. In Fig. 1, the Raman spectra of the sample that was slowly cooled from 160 to 10 K (cooling rate < 20 K/h) are shown. The spectrum recorded at 160 K is in accordance with the spectrum of phase I of Butler and Newbury [8], and the spectra at 120 and 10 K correspond to phase II from the same reference. The phase transition between these two phases is reported to lie at 154 K [2]. Subsequent X-ray diffraction studies of phase I at 223 K [3] and of phase II at 123 K [4] stated that there
Fig. 2. Low frequency Raman spectra of DMA rapidly cooled (cooling rate ( < 250 K/h) to 10 K and then slowly heated (spectra from bottom up). Asterisk denotes a laser plasma line, asterisk in brackets indicates a coincidence of a laser plasma line with a phonon (checked with the 488.8 nm excitation line).
is no change of volume at the transition point and claimed that the space group of phase I was P42/ mnm with two molecules per unit cell, and that of phase II was P41212 with four molecules per unit cell. On the contrary, neutron powder diffraction study by Ibberson and Prager [11] stated that phase II has the C2/m space group with two molecules per unit cell, whereas the symmetry of the higher temperature phase was not fully determined—they stated it is rhombohedral with three molecules per unit cell. For phase I they found strong evidence of a highly disordered structure. Using the group theory, we can determine the number of optical lattice modes for each space group proposed. The results are shown in Table 1. Comparing the number of observed bands with the
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
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Table 1 Optical phonons and their Raman and infrared activity for the various space groups proposed for the stable crystalline phases of DMA Phase II Space group Reference Mole per unit cell Optical phonons Raman active Infrared active
Phase I
C2/m [11] Z2 2Ag % 4Bg % Au % 2Bu 2Ag % 4Bg Au % 2Bu
P4121 [4] Z4 7A2 % 7E 7E 7A2 % 7E
predicted ones, we find one phonon band in the Raman spectrum of phase I at 160 K, and three bands in the low frequency spectrum of phase II at 10 K. For both phases the number of observed phonons is less than that predicted by the group theory. This could be explained with the near degeneracy of Rx and Ry librations, as well as Tx and Ty translations, which probably overlap in phase I and coincide in the limit of our resolution at 10 K (phase II). The librations Rz (around the acetylene bond) are Raman inactive if the space group P42/mnm for phase I or P41212 for phase II are supposed, but are allowed as Ag modes in the C2/m space group. In our opinion, the polarizability change involved in Rx or Ry librations is much greater than the change of the a zz component—therefore the strong bands at 104 and 138 cm 21 of phase II at 10 K are attributed to Rx or Ry librations. The small band at 50 cm 21 could be the Rz libration, however further information on the intermolecular potential in solid DMA is needed.
R [11] Z3 — — —
P42/mnm [3] Z2 A2g % 2Eg % A2u % Eu 2Eg A2u % Eu
In Fig. 2, the Raman spectra of DMA obtained after the sample was rapidly cooled from room temperature to 10 K (cooling rate < 250 K/h) and then slowly heated, are shown. Comparing the bands thus observed at 10 K (see Table 2) with the bands of phase II at 10 K (showed in Fig. 1) and with those of the metastable phase detected by Prasad and Kopelman [7], we conclude that our fastest cooling was not fast enough to eliminate the bands of phase II from those of the new phase. Indeed we have a mixture of phase II and a new phase, which we denote II 00 . Bands’ positions in the phase II 00 are in full agreement with the data from Ref. [7]. On heating, discontinuities are observed at <75 K and at <115 K. At 75 K, the phase II 00 goes into a new phase denoted II 0 , and at 115 K phase II 0 transforms into phase II. Temperature dependence of observed Raman phonons is given in Figs. 2 and 3, where all phases observed in the rapidly cooled sample are indicated. To obtain pure
Table 2 Observed phonon Raman frequencies of DMA at different temperatures (cm 21). s-strong, m-medium, w-weak Rapidly cooled, cooling rate < 250 K/h 10 K II
phases 1
II 00
90 K II
phases 1
II 0
Slowly cooled, cooling rate < 20 K/h 10 K phase 170 K phase II I
156 w 146 mw
138 ms
146 s
126 ms 118 m 114 m 108 mw
104 ms
108 s 80 s
76 w 70 s
70 s 53 w 14 mw
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
Fig. 3. Temperature dependence of the observed positions of the phonons for rapidly cooled sample to 10 K and then heated. II 00 — lower metastable phase, II 0 —higher metastable phase, II: lower stable phase, I: higher stable phase.
phases II 00 and II 0 , one should use higher cooling rate for the preparation of the sample. 3.2. Matrix isolated sample The Raman spectra of matrix isolated DMA with
657
MR of 1:100, 1:50 and 1:25, together with the spectrum of the thin solid film are shown in Fig. 4. The position and assignments of all the observed bands are presented in Table 3 and are in accordance with the data from the literature [22,23]. Matrix isolated samples have nitrogen and oxygen from the air, but otherwise there is a very satisfactory low background and no other impurities. As the concentration of DMA is increased, one finds the shift of observed frequencies towards lower wavenumbers. For example, the CH3 symmetric stretching band appears at 2937 cm 21 for the MR of 1:100, at 2931 cm 21 for the MR of 1:50, 2928 cm 21 for the MR 1:25 and at 2921 cm 21 in the thin solid film. There is also an interesting increase in the intensity of the CH3 asymmetric stretching band at ,2970 cm 21 compared to the intensity of the CH3 symmetric stretching band, as the concentration of DMA is increased. As a result of larger intermolecular distances in the matrix and therefore weaker intermolecular interaction, the positions of the bands observed using matrix isolation are much closer to those of the free molecule [1,16] than to those found in the crystal phases [8]. Both in the matrix samples and in the thin solid film we were able to observe Raman forbidden bands of e1d (218 cm 21, 1040 cm 21) and a4s (1147 cm 21) symmetry [22]. The weak band at 555 cm 21 was assigned as 2 n 5, torsion overtone (the
Fig. 4. Raman spectra of DMA at 10 K as a thin solid film (the spectrum at the top), and matrix isolated (lower spectra, MR indicated above each spectrum).
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Table 3 Observed Raman frequencies of DMA for different MRs and in thin solid film (cm 21). (s-strong, m-medium, w-weak, v-very, as-asymmetric, br-broad) Matrix isolated, matrix ratios 1:100 1:50
2978 m 2937 vs
Thin solid film 10 K
Mode
Symmetry
3057 wm 2960 s 2921 s,as.
n 2 1 n 15 n 13 n1 2 n 10 2 n 14 2 n3 n3 1 n8 n 14 1 n 15 n 3 1 n 15 n (N2) 2 n8 n2 n 2 – n 12 n (O2) n 14 n 15 1 n 16 2 n4 n3 n8 3 n 16 n 4 1 n 16 n 11 n 15 n 2–n 3 2 n 16 n 14 – n 4 n4 2 n5 n 16 n 12
E2d e2d a1s A1s A1s A1s A4s A1s E2d
1:25
2875 mw 2742 w
2973 ms,br 2931 vs 2902 sh 2873 m 2742 w
2966 s,br 2928 vs 2896 sh 2868 m 2742 m
2334 m 2329 s 2256 ms
2327 s 2321 m,as 2247 ms,br
2326 ms 2316 ms 2240 s
1558 m 1448 m
1380 m
1552 mw 1448 m 1405 w 1391 mw 1378 m
1551 mw 1447 m 1404 w 1390 mw 1378 ms
< 1040 vw
1029 vw
1038 vw 1029 vw
775 w
775 w
776 w
698 w
698 mw
697 mw
374 s
378 s 219 vw
377 vs 218 vw
closest combination—that of n 12 and n 16 would lie at t 590 cm 21 and would be of A4s symmetry). In Fig. 5 we show the Raman spectrum of the thin solid film in the region from 950 to 1600 cm 21, where the methyl bending and methyl rocking fundamentals appear. The band at 1448 cm 21 is very broad—no splitting of the fourfold degenerate mode for the asymmetric bending is observed. The weak band at 1408 cm 21 is assigned to a combination of n 15 (1028 cm 21 in the matrix, symmetry e2d) and n 16 (378 cm 21 in the matrix, symmetry e2d). The strong band at 1391 cm 21 of medium intensity is assigned as 2 n 4 (n 4 appears at 697 cm 21 in the matrix, symmetry a1s), and the band at 1379 cm 21 is the n 3, CH3 symmetric bending fundamental. The only clear indication of the breaking of the fourfold degenerate
2863 m 2737 w 2521 vw 2473 vw 2408 vw 2312 ms 2236 s 2200 vw 1448 m,br 1408 w 1391 m 1379 m 1147 vw 1131 vw 1069 vw 1040 sh 1028 m 871 vw 778 w 758 vw 697 m 555 vw 378 vs 224 w,as.,br
A1s a1s E1d e2d A1s A1s a1s a4s E2d E2d e1d e2d A1s A1s E2d a1s A1s e2d e1d
methyl mode into two modes is the appearance of two bands at 1028 and 1040 cm 21 (Fig. 5).
4. Conclusion Using low temperature Raman spectroscopy, temperature dependence of DMA phonon spectra were studied. Besides two previously known stable phases, phases I and II, two metastable phases were observed. At 75 K they transform one into another, and at 115 K the higher metastable phase transforms into the phase II (C2/m). The results of matrix isolation experiments with DMA suggest that there is no significant shift of the position of the observed Raman bands when the MR is
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
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Fig. 5. Raman spectra of DMA as a thin solid film at 10 K (1600–950 cm 21).
increased from 1:100 to 1:25 compared to the bands of the thin solid film. In the film, the splitting of the fourfold degenerate methyl rocking band into two bands takes place, and several infrared active bands were observed. Acknowledgements This work was supported by the Ministry of Science and Technology of the Republic of Croatia, grant no. 00980303. References [1] P.R. Bunker, J.W.C. Johns, A.R.W. McKellar, C. di Lauro, J. Mol. Spectrosc. 162 (1993) 142–151. [2] D.M. Yost, D.W. Osborne, C.S. Garner, J. Am. Chem. Soc. 63 (1941) 3492–3496. [3] E. Pignataro, B. Post, Acta Cryst. 8 (1955) 672–674. [4] M.G. Miksic, E. Segerman, B. Post, Acta Cryst. 12 (1959) 390–393. [5] S. Albert, J.A. Ripmeester, J. Chem. Phys. 57 (1972) 5336– 5339. [6] F.L. Givens, W.D. McCormick, J. Chem. Phys. 66 (1977) 5829–5831.
[7] N. Prasad, R. Kopelman, Chem. Phys. Lett. 20 (1973) 513– 516. [8] I.S. Butler, M.L. Newbury, Spectrochim. Acta 36A (1980) 453–458. [9] B. Alefeld, A. Kollmar, Phys. Lett. 57A (1976) 289–290. [10] K.J. Tilli, B. Alefeld, Mol. Phys. 36 (1978) 287–298. [11] R.M. Ibberson, M. Prager, Acta Cryst. B51 (1995) 71–76. [12] J. Huang, W. Lu, L.S. Bartell, J. Phys. Chem. 99 (1995) 11147–11151. [13] C. di Lauro, P.R. Bunker, J.W.C. Johns, A.R.W. McKellar, J. Mol. Spectrosc. 184 (1997) 177–195. [14] J. Pliva´, A.S. Pine, S. Civisˇ, J. Mol. Spectrosc. 180 (1996) 15– 25. [15] P.R. Bunker, C. di Lauro, Chem. Phys. 190 (1995) 159–169. [16] P. Messler, H.W. Schroetter, K. Sarka, J. Raman Spectrosc. 25 (1994) 647–653. [17] F.D. Verdarame, J.A. Lannon, L.E. Harris, W.G. Thomas, E.A. Lucia, J. Chem. Phys. 56 (1972) 2638–2648. [18] A. Serrallach, R. Meyer, Hs.H. Gu¨nthard, J. Mol. Spectrosc. 52 (1974) 94–129. [19] K. Furic´, V. Mohacˇek, M. Mamic´, Spectrochim. Acta 49A (1993) 2081–2087. [20] D. Hallamasek, E. Babka, E. Kno¨zinger, J. Mol. Struct. 408/ 409 (1997) 125–132. [21] V. Mohacˇek, K. Furic´, M. Dakkouri, M. Grosser, J. Phys. Chem. 96 (1992) 11042–11047. [22] P.R. Bunker, J. Chem. Phys. 47 (1967) 718–739. [23] P. Messler, Diplomarbeit, Ludwig–Maximilians University, Munich, 1991.
Low temperature Raman study of dimethylacetylene V. Mohacˇek Grosˇev*, K. Furic´ Rud⁄er Bosˇkovic´ Institute, Bijenicka 54, P.O. Box 1016, 10 000 Zagreb, Croatia Received 24 August 1998; received in revised form 18 December 1998; accepted 21 December 1998
Abstract On the basis of low temperature Raman spectra from 10 to 170 K of polycrystalline dimethylacetylene (DMA), two stable and two metastable crystalline phases were detected. One can be identified with a previously reported metastable phase [N. Prasad, R. Kopelman, Chem. Phys. Lett. 20 (1973) 513–516] and can be observed from 10 to 75 K, when it transforms into another metastable phase. This one, in turn, transforms at 115 K into the stable C2/m phase. To enable better assignment of the observed internal modes, especially those involving methyl group, matrix isolation of DMA at 10 K was performed. In the recorded Raman spectra at matrix ratios of 1:100, 1:50 and 1:25, the strongest bands observed are of a1s symmetry. Besides them, we were able to detect bands at 218 and 1040 cm 21 of e1d symmetry, which are only infrared active when no intermolecular interactions are present. The Raman spectrum of DMA as a thin solid film was also recorded. The bands’ positions are found not to deviate significantly in comparison with the spectra of the matrix isolated sample, and thanks to the better signal to noise ratio, several additional bands were detected. Among those is the n 8 at 1147 cm 21 of a4s symmetry. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Dimethylacetylene; Raman spectroscopy; Phase transition; Matrix isolation
1. Introduction Dimethylacetylene (DMA), or 2-butyne, is known for its very low barrier to internal rotation in the gas phase: 62 J/mol [1]. Solid state studies on DMA include heat capacity measurements [2], X-ray diffraction [3,4], NMR studies [5,6], vibrational spectroscopy [7,8], neutron [9,10,11] and electron diffraction experiments [12]. In this work we focus our attention on vibrational spectroscopy experiments. In the first part we discuss stable and unstable crystal phases of DMA, and in the second part we discuss the Raman spectra both of the
* Corresponding author. Tel./fax: 1 385-1-4680-112. E-mail address: [email protected] (V. Mohacˇek Grosˇev)
matrix isolated sample and of DMA as a thin solid film. The early heat capacity measurements [2] discovered an anomaly at 154 K. Subsequent X-ray diffraction experiments showed that both crystal structures above and below 154 K are tetragonal. The higher temperature crystal structure was found to be P42/ mnm with two molecules per unit cell, whereas the lower temperature phase was discovered to be P41212 with four molecules per unit cell. We shall denote the higher temperature crystal phase as phase I, and the lower temperature crystal phase as phase II. Recently a neutron powder diffraction study on DMA was published, which states that the lower crystal phase is not tetragonal, but of monoclinic symmetry, space group C2/m, with two molecules per unit cell [11]. The authors have not yet completely resolved the
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00023-X
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V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
symmetry of phase I, but suggest it is of rhombohedral symmetry with three molecules per unit cell. This phase, as they say, shows a lot of diffuse scattering which indicates a highly disordered system. Also, there is an evidence of a non-tetragonal crystal structure of DMA coming from an electron diffraction study on a jet-cooled sample [12]. We focus our attention on vibrational spectroscopy experiments. In their low temperature Raman study, Prasad and Kopelman [7] provided low frequency spectra of three crystal phases— phase I, phase II and a new unknown metastable phase. Their discovery was disputed by Butler and Newbury [8] who claimed that the metastable phase was an artefact produced by the undercooling of phase I in phase II. Our experimental evidence confirms the results of Prasad and Kopelman and moreover indicates new phase transitions connected to the metastable phase. Another interesting problem is the degeneracy of methyl bending and stretching modes observed for the free DMA molecule. The assignment of the observed Raman and infrared bands for a molecule in the vapour or gas phase was the subject of several studies [1,13–15,16]. From the analysis of the complex rotation-torsional structure of all the degenerate methyl fundamentals, the authors conclude that the degeneracy is fourfold, that is the modes are of G(s) symmetry if the extended molecular symmetry 1 is used. By preparing the matrix with a group G36 low matrix ratio (MR) one could obtain a system of weakly interacting DMA molecules. From the number and the position of the observed bands one could estimate the strength of the intermolecular interaction— if the interaction is weak, one could obtain the answer to the degeneracy of a given mode much quicker than when compared to the rather cumbersome interpretation of the observed vapour–gas spectra. As the matrix isolation technique is suitable for providing data on bands of very weak intensity in the gas phase [17–20] we recorded Raman spectra of DMA at 10 K in argon matrices with MRs of 1:100, 1:50, and 1:25. We also recorded the spectrum of DMA as a thin solid film. As all molecular conformations are possibly present in the amorphous film, one can in principle observe all vibrational bands. In our spectrum of the thin solid film the majority of bands are of a1s and e2d symmetry (fundamentals),
and of A1s symmetry (overtones). We were able to observe some a4s and e1d fundamentals.
2. Experimental DMA (99% pure) was purchased from Aldrich and used in Raman measurements without further purification. Low temperature Raman spectra of polycrystalline DMA were recorded using a CTI Cryogenics (Helix, Cryodine) Model 21 cryostat with a closed cycle helium refrigerator. The capillary containing approximately 20 ml of the sample was sealed under vacuum after the sample was frozen by immersing the tube in liquid nitrogen. The capillary was placed in a specially constructed mount on the cold finger [21]. Matrix isolated samples were prepared by the “slow spray on” technique using the same cryostat as described earlier, but using a different attachment to the cryostat head [19]. For matrix isolation, a gaseous mixture of DMA and argon was prepared by allowing a fixed amount of DMA (20 ml) to evaporate at room temperature. Argon from its own bottle (3 dm 3) and under a known pressure was allowed to mix in the sample tube together with the DMA vapour. In this way we obtained a 1:100 DMA to argon mixture. The sample tube was then attached to the needle valve on the cryostat. The cryostat was cooled to 10 K and the valve was opened to one turn (15 turns correspond to the maximum flow). The good quality thin films that formed on the gold plated surface on the cryostat head were obtained with deposition times from 10–15 min. Matrices with the MR of 1:50 and 1:25 were formed in a similar way, only increasing the amount of DMA two and four times, respectively. The ratio of integrated intensities of the 374 cm 21 band in different matrices is in good agreement with this. For the preparation of the thin solid film, a small amount of pure DMA was stored in a bottle. Several degasing procedures were applied to eliminate the air from the sample tube, which was then attached to the needle valve and the whole procedure continued as in the matrix isolation experiment. Raman spectra were recorded with a triple monochromator model DILOR Z24. For excitation a COHERENT Innova 100 laser operating at 514.5 nm was used, the laser power being 100 and 200 mW. Spectra were recorded sequentially, the
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
655
Fig. 1. Low frequency Raman spectra of stable crystalline phases of dimethylacetylene obtained by slow cooling (20 K/h). 160 K:phase I, 120 K and 10 K:phase II. Asterisk denotes a laser plasma line.
smallest step was 0.2 cm 21 in the phonon spectra, and 1 cm 21 in the spectra of the matrix isolated sample. The slitwidths used were 2, 3 and 5 cm 21.
3. Results and discussion 3.1. Polycrystalline stable and metastable phases Low frequency Raman spectra of polycrystalline phases are shown in Figs. 1 and 2. In Fig. 1, the Raman spectra of the sample that was slowly cooled from 160 to 10 K (cooling rate < 20 K/h) are shown. The spectrum recorded at 160 K is in accordance with the spectrum of phase I of Butler and Newbury [8], and the spectra at 120 and 10 K correspond to phase II from the same reference. The phase transition between these two phases is reported to lie at 154 K [2]. Subsequent X-ray diffraction studies of phase I at 223 K [3] and of phase II at 123 K [4] stated that there
Fig. 2. Low frequency Raman spectra of DMA rapidly cooled (cooling rate ( < 250 K/h) to 10 K and then slowly heated (spectra from bottom up). Asterisk denotes a laser plasma line, asterisk in brackets indicates a coincidence of a laser plasma line with a phonon (checked with the 488.8 nm excitation line).
is no change of volume at the transition point and claimed that the space group of phase I was P42/ mnm with two molecules per unit cell, and that of phase II was P41212 with four molecules per unit cell. On the contrary, neutron powder diffraction study by Ibberson and Prager [11] stated that phase II has the C2/m space group with two molecules per unit cell, whereas the symmetry of the higher temperature phase was not fully determined—they stated it is rhombohedral with three molecules per unit cell. For phase I they found strong evidence of a highly disordered structure. Using the group theory, we can determine the number of optical lattice modes for each space group proposed. The results are shown in Table 1. Comparing the number of observed bands with the
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
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Table 1 Optical phonons and their Raman and infrared activity for the various space groups proposed for the stable crystalline phases of DMA Phase II Space group Reference Mole per unit cell Optical phonons Raman active Infrared active
Phase I
C2/m [11] Z2 2Ag % 4Bg % Au % 2Bu 2Ag % 4Bg Au % 2Bu
P4121 [4] Z4 7A2 % 7E 7E 7A2 % 7E
predicted ones, we find one phonon band in the Raman spectrum of phase I at 160 K, and three bands in the low frequency spectrum of phase II at 10 K. For both phases the number of observed phonons is less than that predicted by the group theory. This could be explained with the near degeneracy of Rx and Ry librations, as well as Tx and Ty translations, which probably overlap in phase I and coincide in the limit of our resolution at 10 K (phase II). The librations Rz (around the acetylene bond) are Raman inactive if the space group P42/mnm for phase I or P41212 for phase II are supposed, but are allowed as Ag modes in the C2/m space group. In our opinion, the polarizability change involved in Rx or Ry librations is much greater than the change of the a zz component—therefore the strong bands at 104 and 138 cm 21 of phase II at 10 K are attributed to Rx or Ry librations. The small band at 50 cm 21 could be the Rz libration, however further information on the intermolecular potential in solid DMA is needed.
R [11] Z3 — — —
P42/mnm [3] Z2 A2g % 2Eg % A2u % Eu 2Eg A2u % Eu
In Fig. 2, the Raman spectra of DMA obtained after the sample was rapidly cooled from room temperature to 10 K (cooling rate < 250 K/h) and then slowly heated, are shown. Comparing the bands thus observed at 10 K (see Table 2) with the bands of phase II at 10 K (showed in Fig. 1) and with those of the metastable phase detected by Prasad and Kopelman [7], we conclude that our fastest cooling was not fast enough to eliminate the bands of phase II from those of the new phase. Indeed we have a mixture of phase II and a new phase, which we denote II 00 . Bands’ positions in the phase II 00 are in full agreement with the data from Ref. [7]. On heating, discontinuities are observed at <75 K and at <115 K. At 75 K, the phase II 00 goes into a new phase denoted II 0 , and at 115 K phase II 0 transforms into phase II. Temperature dependence of observed Raman phonons is given in Figs. 2 and 3, where all phases observed in the rapidly cooled sample are indicated. To obtain pure
Table 2 Observed phonon Raman frequencies of DMA at different temperatures (cm 21). s-strong, m-medium, w-weak Rapidly cooled, cooling rate < 250 K/h 10 K II
phases 1
II 00
90 K II
phases 1
II 0
Slowly cooled, cooling rate < 20 K/h 10 K phase 170 K phase II I
156 w 146 mw
138 ms
146 s
126 ms 118 m 114 m 108 mw
104 ms
108 s 80 s
76 w 70 s
70 s 53 w 14 mw
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
Fig. 3. Temperature dependence of the observed positions of the phonons for rapidly cooled sample to 10 K and then heated. II 00 — lower metastable phase, II 0 —higher metastable phase, II: lower stable phase, I: higher stable phase.
phases II 00 and II 0 , one should use higher cooling rate for the preparation of the sample. 3.2. Matrix isolated sample The Raman spectra of matrix isolated DMA with
657
MR of 1:100, 1:50 and 1:25, together with the spectrum of the thin solid film are shown in Fig. 4. The position and assignments of all the observed bands are presented in Table 3 and are in accordance with the data from the literature [22,23]. Matrix isolated samples have nitrogen and oxygen from the air, but otherwise there is a very satisfactory low background and no other impurities. As the concentration of DMA is increased, one finds the shift of observed frequencies towards lower wavenumbers. For example, the CH3 symmetric stretching band appears at 2937 cm 21 for the MR of 1:100, at 2931 cm 21 for the MR of 1:50, 2928 cm 21 for the MR 1:25 and at 2921 cm 21 in the thin solid film. There is also an interesting increase in the intensity of the CH3 asymmetric stretching band at ,2970 cm 21 compared to the intensity of the CH3 symmetric stretching band, as the concentration of DMA is increased. As a result of larger intermolecular distances in the matrix and therefore weaker intermolecular interaction, the positions of the bands observed using matrix isolation are much closer to those of the free molecule [1,16] than to those found in the crystal phases [8]. Both in the matrix samples and in the thin solid film we were able to observe Raman forbidden bands of e1d (218 cm 21, 1040 cm 21) and a4s (1147 cm 21) symmetry [22]. The weak band at 555 cm 21 was assigned as 2 n 5, torsion overtone (the
Fig. 4. Raman spectra of DMA at 10 K as a thin solid film (the spectrum at the top), and matrix isolated (lower spectra, MR indicated above each spectrum).
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Table 3 Observed Raman frequencies of DMA for different MRs and in thin solid film (cm 21). (s-strong, m-medium, w-weak, v-very, as-asymmetric, br-broad) Matrix isolated, matrix ratios 1:100 1:50
2978 m 2937 vs
Thin solid film 10 K
Mode
Symmetry
3057 wm 2960 s 2921 s,as.
n 2 1 n 15 n 13 n1 2 n 10 2 n 14 2 n3 n3 1 n8 n 14 1 n 15 n 3 1 n 15 n (N2) 2 n8 n2 n 2 – n 12 n (O2) n 14 n 15 1 n 16 2 n4 n3 n8 3 n 16 n 4 1 n 16 n 11 n 15 n 2–n 3 2 n 16 n 14 – n 4 n4 2 n5 n 16 n 12
E2d e2d a1s A1s A1s A1s A4s A1s E2d
1:25
2875 mw 2742 w
2973 ms,br 2931 vs 2902 sh 2873 m 2742 w
2966 s,br 2928 vs 2896 sh 2868 m 2742 m
2334 m 2329 s 2256 ms
2327 s 2321 m,as 2247 ms,br
2326 ms 2316 ms 2240 s
1558 m 1448 m
1380 m
1552 mw 1448 m 1405 w 1391 mw 1378 m
1551 mw 1447 m 1404 w 1390 mw 1378 ms
< 1040 vw
1029 vw
1038 vw 1029 vw
775 w
775 w
776 w
698 w
698 mw
697 mw
374 s
378 s 219 vw
377 vs 218 vw
closest combination—that of n 12 and n 16 would lie at t 590 cm 21 and would be of A4s symmetry). In Fig. 5 we show the Raman spectrum of the thin solid film in the region from 950 to 1600 cm 21, where the methyl bending and methyl rocking fundamentals appear. The band at 1448 cm 21 is very broad—no splitting of the fourfold degenerate mode for the asymmetric bending is observed. The weak band at 1408 cm 21 is assigned to a combination of n 15 (1028 cm 21 in the matrix, symmetry e2d) and n 16 (378 cm 21 in the matrix, symmetry e2d). The strong band at 1391 cm 21 of medium intensity is assigned as 2 n 4 (n 4 appears at 697 cm 21 in the matrix, symmetry a1s), and the band at 1379 cm 21 is the n 3, CH3 symmetric bending fundamental. The only clear indication of the breaking of the fourfold degenerate
2863 m 2737 w 2521 vw 2473 vw 2408 vw 2312 ms 2236 s 2200 vw 1448 m,br 1408 w 1391 m 1379 m 1147 vw 1131 vw 1069 vw 1040 sh 1028 m 871 vw 778 w 758 vw 697 m 555 vw 378 vs 224 w,as.,br
A1s a1s E1d e2d A1s A1s a1s a4s E2d E2d e1d e2d A1s A1s E2d a1s A1s e2d e1d
methyl mode into two modes is the appearance of two bands at 1028 and 1040 cm 21 (Fig. 5).
4. Conclusion Using low temperature Raman spectroscopy, temperature dependence of DMA phonon spectra were studied. Besides two previously known stable phases, phases I and II, two metastable phases were observed. At 75 K they transform one into another, and at 115 K the higher metastable phase transforms into the phase II (C2/m). The results of matrix isolation experiments with DMA suggest that there is no significant shift of the position of the observed Raman bands when the MR is
V. Mohacˇek Grosˇev, K. Furic´ / Journal of Molecular Structure 482–483 (1999) 653–659
659
Fig. 5. Raman spectra of DMA as a thin solid film at 10 K (1600–950 cm 21).
increased from 1:100 to 1:25 compared to the bands of the thin solid film. In the film, the splitting of the fourfold degenerate methyl rocking band into two bands takes place, and several infrared active bands were observed. Acknowledgements This work was supported by the Ministry of Science and Technology of the Republic of Croatia, grant no. 00980303. References [1] P.R. Bunker, J.W.C. Johns, A.R.W. McKellar, C. di Lauro, J. Mol. Spectrosc. 162 (1993) 142–151. [2] D.M. Yost, D.W. Osborne, C.S. Garner, J. Am. Chem. Soc. 63 (1941) 3492–3496. [3] E. Pignataro, B. Post, Acta Cryst. 8 (1955) 672–674. [4] M.G. Miksic, E. Segerman, B. Post, Acta Cryst. 12 (1959) 390–393. [5] S. Albert, J.A. Ripmeester, J. Chem. Phys. 57 (1972) 5336– 5339. [6] F.L. Givens, W.D. McCormick, J. Chem. Phys. 66 (1977) 5829–5831.
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