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This Article Appeared in a Journal Published by Elsevier. the Attached Copy Is Furnished to the Author for Internal Non-Commerci This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/authorsrights Author's personal copy Chemical Physics Letters 574 (2013) 66–70 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett In situ spectroscopic observations of pressure-induced condensation of trimethylsilanol and behavior of dehydrated molecular water a, a,b a Ayako Shinozaki ⇑, Naoki Noguchi , Hiroyuki Kagi a Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan b Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Hiroshima 739-8527, Japan article info abstract Article history: This report is the first describing a letter of pressure-induced condensation of silanol. Raman and infrared Received 19 February 2013 absorption spectra of trimethylsilanol were observed at room temperature and at pressures up to 3.3 GPa. In final form 30 April 2013 After solidification at 0.3 GPa, the OH vibration mode shifted to lower frequencies along with increasing Available online 13 May 2013 1 pressure with a notably large pressure coefficient ( 107 cmÀ /GPa), thereby indicating that hydrogen À bonding in trimethylsilanol had strengthened prominently. Along with increased hydrogen-bond interac- tion, condensation of trimethylsilanol forming hexamethyldisiloxane and molecular water were observed. The marked shift in the OH stretching mode in the released H2O suggests considerable inter- molecular interaction with hexamethyldisiloxane. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Pressure may enhance reactions that cannot occur under ambient conditions. This report is the first describing observations of pres- Silicon compounds have been used widely in organic synthesis sure-induced changes in trimethylsilanol reported from in situ since the first synthesis of silicon compounds in the mid-19th cen- spectroscopic measurements conducted at high pressure. tury. Silicon belongs to group Iva elements in the periodic table along with carbon, but the properties and reactivity of organosil- 2. Experiments icon compounds differ notably from those of typical organic com- pounds. Among the organosilicon compounds, materials having High-pressure experiments were conducted using a diamond silanol groups (SiOH) play an important role in the preparation anvil cell (DAC) with flat-top diamond anvils (type Ia), 600 lm cu- of colloidal silica and glasses from alkoxysilanes [1], various silox- let. SUS301 and SUS304 foils were used for gaskets with thick- ane compounds [2], and so on. Silanol (SiH OH) is the simplest 3 nesses of 100 and 300 lm, respectively. A sample hole (300 lm molecule containing the silanol group. However, although many diameter) was drilled into the gasket. Liquid trimethylsilanol (pur- theoretical studies have been conducted, few reports have de- ity 99.7%; Shin-Etsu Chemical Co.) was loaded into the gasket hole scribed spectroscopic measurements of this molecule because of and then compressed without pressure medium. Pressure was its unstable characteristics which result in the rapid formation of measured using ruby fluorescence [12]. siloxanes [3,4]. Trimethylsilanol, (CH ) SiOH, is notably more sta- 3 3 In situ Raman spectroscopic measurements were performed at ble than silanol (SiH OH), although condensation of trimethylsila- 3 high pressure using a micro-Raman spectrometer consisting of an nol to hexamethyldisiloxane, (CH ) SiOSi(CH ) occurs in acidic 3 3 3 3 Ar+ laser (k = 514.5 nm), an optical microscope, a 30 cm single and alkaline solvents [5–7]. Raman and IR spectra of liquid trim- polychromator, and CCD-detectors. Raman bands of naphthalene ethylsilanol at ambient condition obtained both experimentally and neon emission lines [13] were used, respectively for calibra- and theoretically have been reported [8–10], indicating the forma- 1 tion of Raman shift in the spectral ranges from 150 to 1500 cmÀ tion of oligomer by the hydrogen bond [10]. An X-ray diffraction 1 and from 2700 to 3700 cmÀ . In addition, Raman spectra of hexam- letter of single crystals of trimethylsilanol at 80 °C showed that À ethyldisiloxane (purity 99.7%; Wako Pure Chemical Inds. Ltd.) were low-temperature phases belong to monoclinic, space group P2 /c. 1 obtained for comparison. Twelve molecules exist in the unit cell, forming the helix-like chain Infrared (IR) absorption spectra in the mid IR region were ob- by the hydrogen bond [11]. However, the phase relation and reac- tained at ambient pressure and at high pressure using a Fourier tivity of trimethylsilanol under high pressure remain unclear. transform infrared spectrometer (FT-IR) equipped with a globar source, a KBr beam splitter, and an MCT detector (Spectrum 1 Corresponding author. Address: 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, 2000; PerkinElmer Inc.) at 4 cmÀ resolution. Liquid trimethylsila- ⇑ Japan. nol at ambient pressure was placed between two CaF2 plates sep- E-mail address: [email protected] (A. Shinozaki). arated by 12 lm aluminum foil. Infrared absorption spectra at high 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.04.073 Author's personal copy A. Shinozaki et al. / Chemical Physics Letters 574 (2013) 66–70 67 pressure were measured using an IR microscope installed on the the sample hole for FT-IR measurements to avoid oversaturation FT-IR and a DAC. A few pieces of CaF2 platelets were placed into of the OH vibration mode by reducing the optical path. Near-infrared absorption spectra at high pressure were ob- tained using synchrotron radiation at BL43IR in SPring-8, Japan. An FT-IR spectrometer (Hyperion2000; Bruker Co.) was used for measurements of near infrared absorption spectra with a Ge- (a) 1 coated KBr beam splitter and an InSb detector at 4 cmÀ resolution. Hexamethyldisiloxane 1.9 GPa 3. Results and discussion ν SiOSi ν SiC3 Raman spectra of trimethylsilanol at ambient pressure resem- bled those described in previous reports. All observed peaks can be assigned to vibration modes of trimethylsilanol [8–10]. Upon compression, crystallization of trimethylsilanol was observed visu- ally at around 0.3 GPa. No remarkable change of Raman peaks was 1.6 GPa observed along with crystallization at 0.3 GPa (see Figure 1). The ν SiC3 liquid–solid transition pressure is markedly lower than that of di- Intensity methyl phenyl silanol (at 5.0 GPa) [14]. This contrast indicates that the bulky phenyl group lowers intermolecular interactions of sila- 550 600 650 700 750 800 nol considerably. At pressures higher than 0.3 GPa, irreversible Raman shift (cm-1) 1.6 GPa ν change occurred after the sample was kept in the solid phase for ν SiOSi SiC3 a certain time. Figure 1a and b show representative Raman spectra ν SiC3 After 6 days collected with increasing pressure. After the sample was kept at 1.5 GPa 1.5 GPa for 6 days, two new peaks appeared at 541 and 1 675 cmÀ , suggesting the formation of hexamethyldisiloxane (see Figure 1a). The frequencies of new peaks coincide with those of 0. 3 GPa SiO stretching mode and SiC stretching mode of solid hexame- thyldisiloxane under high pressure measured at 1.9 GPa (Figure Crystallization 1a upper). Symmetric and asymmetric CH stretching modes split 0. 1 GPa into several peaks, which is also consistent with the formation of hexamethyldisiloxane (Figure 1b). In addition, Raman spectra of 1 trimethylsilanol show that the mSiC3 mode at 630 cmÀ became Table 1 1 Vibrational frequencies (in cmÀ ) of representative Raman spectra between 0.1 and (b) Hexamethyldisiloxane 3.3 GPa. 1.9 GPa TMS HMDS 0.1 GPa 0.3 GPa 0.8 GPa 1.5 GPa 1.6 GPa 2.2 GPa 3.3 GPa dSiC3 216 233 229 233 219 221 230 dSiC3 252 259 266 271 270 272 282 qSiC3 296 297 303 306 341 346 343 qSiC3 330 354 368 377 mSi– 541 546 553 O–Si 1.6 GPa mSiC3 616 618 624 628 635 639 645 mSiC3 675 678 681 mSiC3 689 684 692 694 696 699 705 After 6 days 706 711 718 qCH3 750 749 748 749 757 759 762 (dSiOH) 767 771 772 1.5 GPa mSiO 782 784 776 785 (qCH3) q CH3 838 836 832 835 838 839 842 qCH3 875 876 874 889 896 891 893 (mSiO, 0. 3 GPa dSiOH) qCH3 928 915 916 (mSiO, Crystallization dSiOH) dCH3 1248 1229 1236 1241 1243 1242 1241 0. 1 GPa dCH3 1259 1252 1246 1257 1253 1253 1253 dCH3 1406 1406 1408 1411 1418 1423 1426 dCH3 1427 1433 1431 1434 1440 1443 1450 mCH3 2905 2915 2914 2900 2901 2914 2919 2925 2948 2944 mCH3 2959 2970 2968 2966 2979 2976 Figure 1. Representative Raman spectra of trimethylsilanol with compression. The 2974 2992 2990 top spectrum was obtained from pure hexamethyldisiloxane: (a) between 500 and 2961 2963 2977 2982 2987 3012 3010 1 1 800 cmÀ with magnified spectra of 1.6 GPa from 550 to 800 cmÀ ; (b) between 1 TMS, trimethylsilanol; HMDS, new peaks with formation of hexamethyldisiloxane. 2800 and 3200 cmÀ . Author's personal copy 68 A. Shinozaki et al. / Chemical Physics Letters 574 (2013) 66–70 weaker at 1.5 GPa with increasing pressure.
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