Effect of Pressure on 4-Toluenesulfonyl Azide Studied by Raman

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Effect of Pressure on 4-Toluenesulfonyl Azide Studied by Raman Scattering and Synchrotron X-ray Diffraction Junru Jiang, Xuefeng Li, Peifen Zhu, Dongmei Li, Xue Han, Qiliang Cui, and Hongyang Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11016 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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The Journal of Physical Chemistry

Effect of Pressure on 4-Toluenesulfonyl Azide Studied by Raman Scattering and Synchrotron X-ray Diffraction Junru Jiang,a Xuefeng Li,a Peifen Zhu,b Dongmei Li,a Xue Han,a Qiliang Cuia and Hongyang Zhu*a

a

State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin,

130012, China. b

Department of Physics and Engineering Physics, The University of Tulsa, Tulsa,

Oklahoma 74104, United States

*(H.Z.) Telephone: +8643185168881. Fax: +8643185168881. E-mail: [email protected].

ACS Paragon Plus Environment


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Abstract The effect of high pressure on the phase transition behaviors of 4-toluenesulfonyl azide (C7H7N3O2S, 4-TsN3) have been investigated by Raman scattering and angle-dispersive X-ray diffraction (ADXRD) measurements in diamond anvil cells up to ~ 15.6 GPa at room temperature. The liquid 4-TsN3 (phase I) begins to transform into solid state (phase II) at 0.7 GPa, and turns to phase III at about 2.7 GPa, then going to phase IV at about 6.3 GPa. The phase IV of 4-TsN3 finally starts to turn into an amorphous state above 10.6 GPa. The first phase transition (phase I - II) of 4-TsN3 is triggered by the rearrangement of C-H…π interaction, and the second phase transition (phase II - III) is attributed to the conformational change, then the rotation of sulfonyl leads to the third phase transition (phase III - IV). The variation of sulfonyl has an influence on the behavior of azide group which will bend and further decompose upon compression. In the process of amorphization, the lattice structure of 4-TsN3 abnormally expanded, which may be caused by the change of C-H…π interactions. We anticipate that the high pressure study of 4-TsN3 provides information towards further understanding and optimizing synthesis conditions of the polymeric nitrogen using azides as starting materials, especially using organic azides.


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1. Introduction Organic azides are attractive materials for physics and chemistry because they own the inherent properties of both energetic groups and active groups.1-3 They have important industrial applications not only as reagents for the preparation of superconducting materials but also as high-energy sources in propellants and explosives.4-6 Particularly, under the actions of pressure and temperature, organic azides are of potential use as precursors in forming highly energetic polymeric nitrogen. Since the firstly successful formation of polymeric nitrogen in molecular nitrogen above 140 GPa, it is urgent to search for new generation of polymeric nitrogen at relative low pressure.7 Previous studies on inorganic azides have shown the pressure-induced polymerization above 120 GPa and the theoretical calculations further confirmed the mechanism responsible for the polymerization, which was caused by the hybridization of electron orbital.8-12 However, the high polymerization pressure restricts the widespread practical applications. Accordingly, we focus on the high-pressure behaviors of organic azide which is considered more likely to the hybridization of the electron orbit under high pressure than that in azide ion of inorganic azide. The azide group of organic azides is bent and asymmetric which is comprised of two resonance structures.13, 14 Compound with resonance structures can reflect all the properties of a molecule which is a superposition of two structures described by conventional bonding patterns.15 Resonance structures have an impact on the high-pressure behavior of molecule. For example, benzene molecules are close to each other under pressure, which causes polymerization into dense solid-state amorphous carbon mateirals.16 We believed that the unique electronic structures of azide group effectively affect the structural stability and have a significant impact on



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the phase transitions and the behavior of organic azides. In our recent studies, we found that the amorphization pressure of trimethyltin azide (about 27 GPa) and benzyl azide (about 25 GPa) is much lower than that of inorganic azide (sodium azide to 120 GPa).14, 17 The azide ions of lithium azide have been calculated to transform into amorphous-like nitrogen state of zigzag chains or “N6” ring in the process of formation of polymeric nitrogen network.18, 19 In this respect, the bent azide group might readily transform into amorphous and enhance the propensity of nitrogen polymerization.20 Up to now, few high-pressure polymerization studies of organic azide have been reported, and most of the researches are concentrated on inorganic azide. Given the lack of research knowledge about the high-pressure behavior of azide group and the potential applications, development and improvement of organic azide materials is highly desirable in the later study. In this paper, we report a comprehensive high-pressure investigation of the behavior of 4-toluenesulfonyl azide (C7H7N3O2S, 4-TsN3) at room temperature using Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) techniques up to 15.6 and 15.3 GPa, respectively. Our work makes clear the variation of azide group and the pressure-induced phase changes of 4-TsN3, which will advance our fundamental understanding of the structural properties of organic azides. 2. Experimental and Computational Details. The liquid 4-TsN3 sample (purity > 98 %) was purchased from Adamas-beta without further purification before utilization. The high-pressure experiments were performed in a set of symmetric diamond anvil cells (DACs) with 400 µm in diameter. T301 stainless steel sheets were served as gaskets with sample chambers of 120 µm in diameter and 60µm in thickness. A ruby ball, along with sample, was loaded into the


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sample chamber for in situ pressure calibration according to the fluorescence shift of the ruby R1 line. No pressure-transmitting medium was used in the experiments. The high-pressure Raman spectra of 4-TsN3 were recorded in a backscattering geometry using Acton SpectraPro500i spectrograph equipped with the liquid nitrogen cooled CCD detector. The 532 nm wavelength laser generated by the frequency-doubled diode-pumped Nd: vanadate laser was utilized to excite the sample. The high-pressure synchrotron ADXRD experiments were conducted at the 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF) with a beam wavelength of 0.6199 Å. The diffraction patterns were collected using a MAR345 image plate detector. The FIT2D software was used for the analysis of two-dimensional XRD images, yielding one-dimensional intensity versus diffraction angle 2θ patterns. All the indexing and refinements were performed using the Reflex module in Materials Studio software. The calculation of the Raman spectrum at ambient pressure was performed by using the density functional theory (DFT) method with the local density approximation of Perdew-Wang (LDA/PWC) provided by DMol3 package. The optimized geometry of 4-TsN3 at the ground state (in vacuo) was carried out without symmetry constraints. The frequencies of Raman modes were calculated, and ensured that the optimized geometry was a true minimum. 3. Results and Discussion The experimental Raman spectra of liquid 4-TsN3 were recorded from 50 to 3200 cm-1 at ambient conditions as shown in Figure 1a. As the vibrational modes of the same groups have similar characteristic frequencies in different compounds, the vibrations of methyl, phenyl, sulfonyl, and azide groups were analyzed on the basis of the literature data of toluene, benzenesulfonyl, sulfonyl azide, and organic azide.21-26



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For further assignment, the vibrational property of 4-TsN3 was calculated using DFT method, and the calculated Raman spectra of 4-TsN3 are presented in Figure 1b. The observed and calculated Raman bands corresponding to the assigned vibrational modes are summarized in Table 1. Table 1. The Vibrational Mode Assignments of 4-TsN3, the Raman Bands from Experiment and Calculation are Listed Separately.

Experimental (cm-1)

Calculated (cm-1)

78

79/88

159

137

ν ring-SO2-N3

184

160

ν ring-SO2-N3

213

202

ν SO2-ring

291

286/303

γ Ring

322

333

γ Ring+ ω CH3

352

354

ρ CH3

371

386

ν S-N

383

393

ν C-CH3

450

440

τ SO2

Assignments

γ Ring

461 506

527

γ Ring

544

534

δ SO2

600

582

δ N3

637

622

ν C-C

667

661

ν CH3−Ring


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753

746

γ N3

805

802

γ C-H

819

820

γ C-H

850

Combination 1017/1026

τ CH3

1091

1090/1103

Ring breathing

1171

1160

δ C-H

1195

1182

νs SO2

1218

1228

δ C-H

1302

1276

νs N3

1310

1311

νs N3

1367

ω CH3

1383

1385/1402

νas SO2

1454

1437/1446

δ CH3

1497

1495

δ C-H

1598

1594

ν C-C

2131

2077

νas N3

2879

2882

νs CH3

2933

2955

νas CH3

2983

2983

νas C-H νs C-H

3039 3058

3021

νs C-H

3069

3046

νs C-H



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γ, out-of-plane bending; ω, wagging; ρ, rocking; ν, stretching; τ, twisting; δ, bending; νs, symmetric stretching; νas, asymmetric stretching.

Figure 1. (a) The experimental and (b) calculated Raman spectra of 4-TsN3 at ambient pressure. The assignments of all the vibrational modes are listed above each band. The shadow area labeled with “Diamond” is the spectral region that will be blocked by diamond after the usage of DAC to generate high pressure. The orange spectrum marked with “×5” in part b means the magnification of 5 times for the blue one in the dash rectangle. The in situ high-pressure Raman scattering of 4-TsN3 have been performed from ambient conditions up to 15.6 GPa. The selected Raman scattering patterns of 4-TsN3 at various pressure are displayed in Figure 2a-2c. In Figure 2a, the Raman shifts of 4-TsN3 in the frequency region of 50-150 cm-1 at ambient pressure are representative broad peak for liquid. With increasing pressure to 0.7 GPa, five new Raman peaks at 71/86/114/124/141 cm-1 were designated into the external modes as marked with diamonds (♦). As the appearance of external modes signify that the liquid 4-TsN3 starts to crystallize, these significant changes of external modes of


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4-TsN3 indicate the first phase transformation from liquid to solid state at 0.7 GPa. The Raman spectra above 150 cm-1 represent the internal modes which efficiently reflect the variation of groups in molecule. As shown in Figure 2c at 0.7 GPa, the peak marked by pound (#) at 2983 cm-1 split into two bands, and a new peak labeled with diamond (♦) appeared at 3108 cm-1 assigned to symmetric stretching mode of ring C-H (νs C-H), suggesting 4-TsN3 begins the transformation from phase I to phase II at 0.7 GPa. With the pressure increased to 1.3 GPa, several new peaks were observed at 1464, 1585, 2912, and 3086/3120 cm-1 which were identified as CH3 bending mode (δCH3), ring C-C stretching mode (νC-C), CH3 symmetric stretching mode (νsCH3), and ring C-H symmetric stretching modes (νsC-H), respectively. Concurrently, three peaks of CH3 bending mode (δCH3), ring C-H bending mode (δC-H), and CH3 symmetric stretching mode (νsCH3) disappeared as marked with down arrows (↓). These changes indicate the completion of first phase transition from phase I to phase II, which is accompanied with solidification.



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Figure 2. Selected high-pressure Raman spectra of 4-TsN3 in the spectral regions of (a) 50-540 cm-1, (b) 540-1280 cm-1, and (c) 1430-3180 cm-1, respectively. D0 means the Raman spectrum of 4-TsN3 released to 0 GPa. The solid diamonds (♦) and the down arrow (↓) represent the appearance and disappearance of Raman peaks, respectively. The pound (#) denotes the splitting of peak. With further increasing pressure to 2.7 GPa, the new Raman peaks of external modes showed up at 74/81/135 cm-1 (Figure 2a) accompanied by the disappearance of original external mode at 163 cm-1 as labeled with down arrow (↓). Other four new peaks of internal modes emerged at 216/240, 379 and 707 cm-1 (Figure 2a and 2b), which were assigned to SO2-ring stretching (νSO2-ring), SO2-N3 stretching (νS-N), and CH3-ring stretching (νCH3-ring) modes, respectively. It is suggested that 4-TsN3 undergoes the second phase transition from phase II to phase III at 2.7 GPa. On further compression to 6.3 GPa, two new Raman peaks of external modes were observed at 92/169 cm-1 (Figure 2a) along with the change of relative intensity for other external modes, and new Raman internal modes appeared at 612 and 1195/1208 cm-1 (Figure 2b) which were identified as N3 bending mode (δN3) and SO2 symmetric stretching modes (νsSO2), respectively. It signified the transformation from phase III to phase IV of 4-TsN3 at 6.3 GPa. Upon increasing pressure to 10.6 GPa, most of the Raman bands of 4-CBSA are shown in blue shift. However, in Figure 2c, the symmetric and asymmetric stretching modes of CH3 and ring C-H (νsCH3, νasCH3, νsC-H, νasC-H) became extremely weak and difficult to recognize, meaning the tendency of the amorphization in 4-TsN3 at 10.6 GPa. The vibration modes of azide group (δN3, γN3, νasN3) almost disappeared at 12.5 GPa, which might because the decomposition of azide group. As the pressure


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increased up to 15.6 GPa, the highest pressure of this work, all the Raman bands almost vanished, suggesting 4-TsN3 turned into amorphous state completely. The discontinuities of the spectral features of phase transitions can be directly perceived from the pressure-dependent vibrational frequencies displayed in Figure 3a, 3b and 3c. The vertical dash line at 0.7 GPa account for the pressure of the first phase transition from phase I to phase II, which are detected from the new emerging external modes. The dash lines at 2.7 and 6.3 GPa are responsible for the suggested pressures of the second phase transition (phase II to phase III) and the third phase transition (phase III to phase IV), respectively. Above 10.6 GPa, most of the Raman modes disappeared implying the beginning of amorphization in 4-TsN3.

Figure 3. Raman shifts of 4-TsN3 as a function of pressure in the range of (a) 50-440 cm-1, (b) 440-1280 cm-1 and (c) 1430-3180 cm-1, respectively. The vertical dash lines



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suggest the pressures of phase transitions. The red vertical dash lines show the amorphization pressure of 4-TsN3. Pressure plays a vital role in reducing the distance between molecules of 4-TsN3. In the liquid phase of 4-TsN3, molecules are free to arrange and C-H…π interaction is the mainly intermolecular interaction. The C-H…π interaction is a weak hydrogen bond occurring between C-H bond of CH3 group and π electron system of benzene ring and have an important effect on determining the conformation.27 The C-H…π interaction strengthens under pressure and makes the molecular arrangement reach the limit of stability that changes the 4-TsN3 molecule and the solid state crystalline generates. This can be verified by the significant changes of the Raman external modes. In the pressure range of 0.7-1.3 GPa, the variations of Raman internal modes are primarily associated with the vibrations of CH3 and benzene ring, which clarify the reasonableness of the first phase transition. The change of ring C-H stretching mode (νsC-H) and C-C stretching mode (νC-C) suggest the distortion of benzene ring.28, 29 The variations of CH3 bending mode (δCH3) and stretching modes (νsCH3) imply the modification of CH3 group. All these transformations are attributed to the rearrangement of C-H…π interaction, the reason for the first phase transition from phase I to phase II, which lead to a different molecular conformation. The possible molecular conformations of phase I and phase II for 4-TsN3 are exhibited in Figure 4. There are several possible distinct conformations in 4-TsN3 that are sensitive to pressure. At 2.7 GPa, the new appeared stretching modes among methyl, phenyl, sulfonyl and azido are related to the conformational change of 4-TsN3, indicating the second phase transition from phase II to phase III. These stretching modes suggest the elongation or shortening of bond distance among each group in phase III. The analogous phenomena of conformational change have been observed in the studies of


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chain organics.30,

31

For n-heptane, pressure perturbs the distribution of molecular

conformations which induces the extension of chain length and the expansion or contraction of bond angle.31, 32 Similarly, it is perfect presented that the second phase transition of 4-TsN3 is just indicated by the change of Raman mode of νSO2-ring, νS-N, and νCH3-ring at 2.7 GPa. The probable molecular conformation of phase III is shown in Figure 4.

Figure 4. Probable molecular conformation of 4-TsN3 in phase I, phase II, phase III, and phase IV. The blue, white, red, green, and yellow spheres denote C, H, O, N, and S atoms, respectively. For 4-TsN3, the new SO2 stretching mode (νsSO2) at 6.3 GPa indicates the rotation of sulfonyl, which is responsible for the third phase transformation from phase III to phase IV.14 It has been reported that the azide group was affected by the adjacent group under pressure, which would bent and cyclize.33, 34 Thus, the change of sulfonyl has an influence on the behavior of azide group that can be directly explained by the evolution of the N3 vibration modes. At ambient pressure, the azide group of 4-TsN3 is bent and asymmetric. In the third phase transition, the new bending mode of azide group (δN3) can be interpreted as the bending of azide group.35 In other word, the bent azide group further bends accompanied with the decreasing angle of N1-N2-N3. The possible molecular conformation of phase IV is displayed in Figure 4. In order to confirm the pressure-induced phase transitions of 4-TsN3, the high-pressure synchrotron ADXRD measurements were carried out up to 15.3 GPa.



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The liquid to solid phase transition can be directly verified by the two-dimensional XRD images and the microscopic image of 4-TsN3 as demonstrated in Figure 5. In Figure 5a, 4-TsN3 was in the liquid phase and the diffraction pattern was a wide diffraction ring without any diffraction peak at ambient pressure. With increasing pressure to 0.7 GPa, the transparent sample darkened and the crystal showed up as can be seen from Figure 5b. These phenomena suggest the occurrence of the first phase transition (liquid to solid phase) of 4-TsN3 at 0.7 GPa. To obtain more information on high pressure phases, the Pawley refinements of the diffraction patterns were performed, and the refinement results are exhibited in Figure 7. The pattern at 0.7 GPa of phase II describes with monoclinic structure with P2 space group, as shown in Figure 7a. Upon compression, the diffraction peaks marked by pound (#) gradually approach their adjacent peaks and completely merge with them at 2.5 GPa with compression, suggesting the second phase transition from phase II to phase III.36 For phase III, the number of the reflection reduces compared with phase II, and it may transform to higher-symmetry orthorhombic structure as illustrated in Figure 7b. With the pressure increased to 6.2 GPa, two new diffraction peaks are noticed as labeled with arrows which means the third phase transition (phase III to phase IV) occurs at this pressure. In light of phase IV has more diffration peaks than phase III, we propose that the third high pressure phase has a monoclinic structure with P2 space group with the refinement result as displayed in Figure 7c. Above 10.3 GPa, four diffraction peaks signed as asterisks (∗) start to shift toward lower angle until 14.2 GPa, which could be attributed to the abnormal expansion of 4-TsN3.37-39 Along with the transition, the intensity of diffraction peaks decrease and several peaks tend to disappear gradually, indicating the amorphous trend in 4-TsN3. Up to the highest pressure of 15.3 GPa, 4-TsN3 completely transforms into amorphous state. When


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pressure decompression to 0 GPa, the amorphous state is retained deduced from the solid in the sample chamber of 4-TsN3 as exhibited in Figure 5c. The released spectra of Raman and ADXRD also suggest that the amorphization of 4-TsN3 is irreversible.

Figure 5. Two-dimensional XRD images and microscopic image of 4-TsN3 at 0, 0.7 and D0 GPa, respectively. The variations of d-spacing of the diffraction peaks under high pressure are plotted in Figure 6b to intuitively reflect the relevant phase transitions. At 0.7 GPa, the appearance of diffraction peaks suggested the first phase transition from phase I to phase II. The second phase transformation from phase II into phase III could be observed through the changing slope of the pressure dependence d-spacing at 2.5 GPa. The slope of phase III is smaller than that of phase II, indicating 4-TsN3 becomes more difficult to be compressed in phase III. The d-spacing of diffraction peaks of phase III 4-TsN3 linearly decreased with increasing pressure until the new peaks appeared at 6.2 GPa. It indicates that the third phase transformation from phase III into phase IV occurs at 6.2 GPa. With further compression above 10.3 GPa, the



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d-spacing of the main diffraction peaks gradually increased implying the abnormal expansion of the structure.

Figure 6. (a) Representative ADXRD patterns of 4-TsN3 at selected pressures. The bottom pattern is collected after releasing pressure as marked by D0. The arrows mean the new diffraction peaks. The pounds illustrate the peaks approached their adjacent peaks. The asterisks represent the peaks shifted to lower angles. The inset in part a is magnified of the dash rectangular. (b) D-spacing as a function of pressure at room temperature. The vertical dash lines represent the pressure of phase transition. The red vertical dash line exhibits the amorphization pressure of 4-TsN3.


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Figure 7. Pawley refinement results of the XRD patterns of high pressure phases. (a) Monoclinic structure (P2) at 0.7 GPa with cell parameters of a = 20.8991 Å, b = 4.9506 Å, c = 8.3503 Å, and β = 89.80492°. (b) Orthorhombic structure (P222) at 2.5 GPa with cell parameters of a = 19.3259 Å, b = 7.8397 Å, and c = 4.6230 Å. (c) Monoclinic structure (P2) at 6.2 GPa with cell parameters of a = 15.0146 Å, b = 6.9093 Å, c = 7.6326 Å, and β = 106.9832°. The intermolecular interactions dominate the physical and chemical properties of organic compounds. For example in organic hydrogen-bonded compounds, pressure



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disturbs the cooperation of hydrogen bonds giving rise to the abnormal extension in a certain direction.37, 40, 41 The C-H…π interactions (a feature similar to hydrogen bonds) also contribute to the structure changes of 4-TsN3 under pressure. Above 10.3 GPa, the change of C-H…π interactions distorted the unit cell along with the amorphous trend in 4-TsN3, which was confirmed by the changes of the spectra profile of Raman external modes and the broad bands of XRD. This transition pressure is consistent with that of the abnormal expansion of the structure. Hence, we propose that the abnormal expansion of 4-TsN3 may be caused by the change of C-H…π interactions. Previous study of benzyl azide has shown that the rotation of methylene group causes the rotation of azide group.14 While for 4-TsN3, the rotation of sulfonyl leads to the bending of azide group. At 12.5 GPa, the spectral feature of azide group is absent manifesting the decomposition of azide group, which is an important intermediate in the process of nitrogen polymerization. The decomposition pressure of azide group in 4-TsN3 is lower than that in benzyl azide. The different behavior of azide group between benzyl azide and 4-TsN3 may be due to the different substitute groups which are connected to azide group. It is considered that the bending process of azide group is beneficial to the hybridization of electron orbit because of the approach of terminal nitrogen atoms of adjacent azide groups.20 Compared to the linear and symmetric azide ions, the bent and asymmetric azide groups are easier to bend and decompose under high pressure that may be associated with the polymerization at a relatively low pressure. Thus, the unique property of 4-TsN3 is conductive to investigate the formation of polymeric nitrogen in organic azides. 4. Conclusion In summary, we have studied the pressure-induced phase transitions of 4-TsN3 using in situ Raman scattering and ADXRD techniques under high pressure up to 15.6


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and 15.3 GPa, respectively. All the Raman vibrational modes of 4-TsN3 at ambient pressure are assigned combined our experiment and theoretical calculation. With increasing pressure, 4-TsN3 transforms from phase I to phase II (liquid phase to solid phase) at 0.7 GPa, and turns to phase III at 2.7 GPa, then converts to phase IV at 6.3 GPa. Upon compression to 10.6 GPa, phase IV is going to amorphous state. The vibrational analyses reveal that the first phase transition arises from the rearrangement of C-H…π interactions, the second phase transition is a conformational change, and the rotation of sulfonyl group is responsible for the third phase transition. The azide group bends during the third phase transition and subsequent decomposition with compression. During the amorphization of 4-TsN3, the abnormal expansion of structure is induced by the change of C-H…π interactions. Upon pressure release to ambient conditions, the amorphous phase of 4-TsN3 is retained, which signifies the decomposition of azide group is irreversible. The high-pressure study of 4-TsN3 might provide a significant step forward in the formation of polymeric nitrogen in organic azide. Acknowledgements This work is supported by the National Natural Science Foundation of China (11304111). Synchrotron ADXRD measurements were performed at 4W2 HP-Station, Beijing Synchrotron Radiation Facility (BSRF). The authors declare no competing financial interest. References 1. Xie, S.; Zhang, Y.; Ramström, O.; Yan, M. Base-Catalyzed Synthesis of Aryl Amides from Aryl Azides and Aldehydes. Chem. Sci. 2016, 7, 713-718.



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Figure 1. (a) The experimental and (b) calculated Raman spectra of 4-TsN3 at ambient pressure. The assignments of all the vibrational modes are listed above each band. The shadow area labeled with “Diamond” is the spectral region that will be blocked by diamond after the usage of DAC to generate high pressure. The orange spectrum marked with “×5” in part b means the magnification of 5 times for the blue one in the dash rectangle. 287x152mm (300 x 300 DPI)



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Figure 2. Selected high-pressure Raman spectra of 4-TsN3 in the spectral regions of (a) 50-540 cm-1, (b) 540-1280 cm-1, and (c) 1430-3180 cm-1, respectively. D0 means the Raman spectrum of 4-TsN3 released to 0 GPa. The solid diamonds (•) and the down arrow (↓) represent the appearance and disappearance of Raman peaks, respectively. The pound (#) denotes the splitting of peak. 342x201mm (300 x 300 DPI)


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Figure 3. Raman shifts of 4-TsN3 as a function of pressure in the range of (a) 50-440 cm-1, (b) 440-1280 cm-1 and (c) 1430-3180 cm-1, respectively. The vertical dash lines suggest the pressures of phase transitions. The red vertical dash lines show the amorphization pressure of 4-TsN3. 254x177mm (300 x 300 DPI)



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Figure 4. Probable molecular conformation of 4-TsN3 in phase I, phase II, phase III, and phase IV. The blue, white, red, green, and yellow spheres denote C, H, O, N, and S atoms, respectively. 726x129mm (72 x 72 DPI)


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Figure 5. Two-dimensional XRD images and microscopic image of 4-TsN3 at 0, 0.7 and D0 GPa, respectively. 170x155mm (72 x 72 DPI)



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Figure 6. (a) Representative ADXRD patterns of 4-TsN3 at selected pressures. The bottom pattern is collected after releasing pressure as marked by D0. The arrows mean the new diffraction peaks. The pounds illustrate the peaks approached their adjacent peaks. The asterisks represent the peaks shifted to lower angles. The inset in (a) is magnified of the dash rectangular. (b) D-spacing as a function of pressure at room temperature. The vertical dash lines represent the pressure of phase transition. The red vertical dash line exhibits the amorphization pressure of 4-TsN3. 203x201mm (300 x 300 DPI)


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Figure 7. Pawley refinement results of the XRD patterns of high pressure phases. (a) Monoclinic structure (P2) at 0.7 GPa with cell parameters of a = 20.8991 Å, b = 4.9506 Å, c = 8.3503 Å, and β = 89.80492°. (b) Orthorhombic structure (P222) at 2.5 GPa with cell parameters of a = 19.3259 Å, b = 7.8397 Å, and c = 4.6230 Å. (c) Monoclinic structure (P2) at 6.2 GPa with cell parameters of a = 15.0146 Å, b = 6.9093 Å, c = 7.6326 Å, and β = 106.9832°. 152x203mm (300 x 300 DPI)


Effect of Pressure on 4-Toluenesulfonyl Azide Studied by Raman

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