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Stability and partial oligomerization of naphthalene under high pressure at room temperature
Accepted Manuscript Frontiers article Stability and partial oligomerization of naphthalene under high pressure at room temperature Ayako Shinozaki, Koichi Mimura, Tamihito Nishida, Toru Inoue, Satoshi Nakano, Hiroyuki Kagi PII: DOI: Reference:
S0009-2614(16)30715-1 http://dx.doi.org/10.1016/j.cplett.2016.09.042 CPLETT 34183
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
11 July 2016 9 September 2016 13 September 2016
Please cite this article as: A. Shinozaki, K. Mimura, T. Nishida, T. Inoue, S. Nakano, H. Kagi, Stability and partial oligomerization of naphthalene under high pressure at room temperature, Chemical Physics Letters (2016), doi: http://dx.doi.org/10.1016/j.cplett.2016.09.042
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Stability and partial oligomerization of naphthalene under high pressure at room temperature Ayako Shinozakia, *, Koichi Mimuraa, Tamihito Nishidaa, Toru Inoueb, Satoshi Nakanoc, and Hiroyuki Kagid a
Department of Earth and Planetary Sciences, Graduate School of Environmental Studies,
Nagoya University, Nagoya 464-8601, Japan b
Geodynamics Research Center, Ehime University, Bunkyo-cho, Matsuyama, Ehime
790-8577, Japan c
National Institute for Materials Science, Namiki, Tsukuba, Ibaraki 305-0044, Japan
d
Geochemical Research Center, Graduate School of Science, The University of Tokyo,
Hongo, Tokyo 113-0033, Japan
*Corresponding author: Ayako Shinozaki. Tel.: +81-52-789-2524, fax: +81-52-789-3033 E-mail: [email protected]
1
Abstract The stability and pressure-induced chemical reactions of naphthalene were investigated at room temperature at pressures up to 23 GPa. In-situ X-ray diffraction (XRD) measurements indicated that naphthalene retained its crystal structure up to ~20 GPa, whereas a solid amorphous phase was observed in the recovered samples. Based on microanalysis of the recovered samples using Gas Chromatograph Mass Spectrometer (GC/MS), naphthalene dimer and trimer isomers were observed at pressures exceeding 15 GPa. The dimers were classified as products of simple dimerization,
naphthylation,
and condensation,
similar
to the case of the
pressure-induced dimerization of benzene, indicating a similar dimerization mechanism for naphthalene. Keywords: High pressure, polycyclic aromatic hydrocarbons, XRD, GC/MS
2
1. Introduction Pressure is an important factor promoting chemical reactions of organic compounds. Pressure-induced oligomerization and polymerization for various substances have been reported [1-6]. The behavior of aromatic hydrocarbons under high pressure has also been investigated because of their distinctive two-dimensional molecular structure and delocalized π-electrons. In case of benzene, which is the simplest aromatic hydrocarbon, several pressure-induced phase changes were reported [7-12], together with the irreversible formation of an amorphous phase [9,10,12-14]. In addition, a pressure-induced dimerization of benzene was theoretically and experimentally predicted [15-19]. The formation of naphthalene, biphenyl, and benzene dimers at room temperature was observed by GC/MS analysis for the quenched samples exposed to pressures exceeding 13 GPa, whereas most of the benzene remained [20]. Interestingly, a one-dimensional sp3-carbon nanothread was detected in the recovered products after compression at ~20 GPa and room temperature [21]. Oligomerization, polymerization, and amorphization are thought to be promoted by the decreasing distance between the neighboring benzene molecules at increased pressure, and the above-mentioned reactions can take place upon reaching some threshold of the
3
neighboring distance [14,20]. Naphthalene (C10H8) features a fused pair of benzene rings. The 1- and 2-positions of naphthalene exhibit different reactivity, unlike in fully symmetric benzene. Naphthalene crystals have a monoclinic structure (P21/a) at pressures from ambient to ~13 GPa [22-25]. Theoretical calculations suggest the absence of phase transitions up to 20 GPa [26]. Moreover, irreversible chemical reactions of naphthalene were reported by the disappearance of its IR peaks at 45.2 GPa [27]. Dimerization of naphthalene was observed from the recovered sample after compression up to 16 GPa and room temperature [28], although the molecular structure of the dimer was not evaluated as well as the pressure dependence of its formation. At high pressure and temperature, chemical reactions of naphthalene were also reported. Based on in-situ XRD measurements, diffraction peaks of naphthalene were observed up to 873 K at ~7 GPa, while graphite and diamond were formed above that temperature [28-30]. In the samples recovered from ~7 GPa and 773–873 K, several naphthalene oligomers were detected by Matrix Assisted Laser Desorption Ionization analysis [28,31]. Samples recovered after shock experiments of naphthalene at pressures up to 33.7 GPa and a temperature of 1660 K showed presence of methylation, phenylation, and naphthylation
4
products based on GC/MS analysis, as well as a dichloromethane-insoluble dark residue [32]. However, distinguishing the effects of pressure from those of temperature in these studies is difficult, since both of these parameters were varied. In this study, pressure-induced chemical reactions of naphthalene were investigated. We performed in-situ XRD measurements to investigate the stability of naphthalene crystals at high pressure and room temperature using a diamond anvil cell (DAC). Subsequently, high-pressure experiments were conducted using a multi-anvil apparatus, and the reaction products in the recovered samples were qualitatively identified by GC/MS to characterize the pressure-induced oligomerization of naphthalene. 2. Experimental For DAC experiments, a pair of diamond anvils with 450 μm culets was used. Stainless steel was used for the gasket after pre-compression to approximately 100 μm thickness. A sample hole with a ~200 μm diameter was drilled in the gasket. Powdered naphthalene (C10H8, Wako, purity 98.0%) was loaded in the sample chamber, and Ne gas was loaded using a gas loading apparatus [33]. The pressure was measured using the ruby fluorescence method [34]. The XRD patterns were obtained at BL-18C of the
5
Photon Factory of the High Energy Accelerator Research Organization (KEK) at room temperature. A typical X-ray wavelength was ca. 0.6128 Å, calibrated with the CeO2 standard. The X-ray beam was collimated to 100 µm in diameter. To obtain a sample suitable for GC/MS analysis, we used a multi-anvil apparatus: a Kawai-type 3000 ton press (OEANGE-3000 at GRC, Ehime University). Tungsten carbide second stage anvils with 5 mm truncated edge length were used. Semi-sintered MgO doped with CoO and a gold capsule (5 mm outer diameter, 4.6 mm inner diameter) were used as the pressure medium and sample capsule, respectively. To prevent organic contaminants, the pressure medium and the sample capsule were cleaned with acetone and heated in an oven at 450 °C for 3 h prior to encapsulating naphthalene. Pressure was calibrated in advance using the phase transitions of standard materials (ZnS, 15.6 GPa; GaAs, 18.7 GPa; GaP, 23.0 GPa). A naphthalene sample (C10H8, Sigma-Aldrich, purity ≥ 99.7%) of about 50 mg weight was used as a starting material for high-pressure experiments. Four experiments were performed (at 10, 15, 20, and 23 GPa), with a preservation time of 1 h at each maximal pressure. The samples were decompressed to ambient pressure and dissolved in distilled dichloromethane (CH2Cl2) with the sample capsule, in order to prevent volatilization of the products
6
formed. The reaction products inside the gold capsule were extracted in distilled dichloromethane. The solution was initially filtered to remove fragments of the pressure medium and the sample capsule. The concentrated reaction products were analyzed by a GC/MS spectrometer (JMS-K9; JEOL Co.) equipped with a capillary column (0.25 mm inner diameter × 30 m, 0.25 µm film thickness, HP-5; Agilent Technology Co.). The GC column temperature was programmed similarly to our previous study [20]. Helium carrier gas (1 mL/min) was used. Ionization voltage of the mass spectrometer was set to 70 eV, and an m/z scan range from 40 to 500 was used. Methyl laurate (C13H26O2), methyl stearate (C19H38O2), and methyl triacontanate (C31H62O2) were used as internal standards for the analysis. 3. Results and discussion First, in-situ XRD measurements using DAC were performed to investigate the stability of naphthalene crystals under high pressure. Figure 1a shows representative XRD patterns with increasing pressure. Diffractions of naphthalene were observed at least up to 19.8 GPa, indicating that naphthalene retained its crystal phase at that pressure, whereas the peaks became weaker and broader with increasing pressure,
7
compared to the diffraction from neon, a pressure medium. It is possible that the peak broadening and the decrease in diffraction intensity are attributed to a pressure-induced chemical reaction, such as partial amorphization, or a change of hydrostaticity of the sample. The diffraction patterns could be indexed to the monoclinic structure [24,25] up to 19.8 GPa, indicating that the crystal structure was not considerably changed. Naphthalene diffraction peaks were also observed in absence of the pressure medium during compression to 21.3 GPa and decompression. When the sample was decompressed to ambient pressure and the DAC was opened, some solid residue was observed after the naphthalene was sublimated and thus removed from the sample capsule. No diffraction peaks were observed for the solid product (see Figure 1b), suggesting pressure-induced irreversible reaction leading to the formation of an amorphous phase. To identify the product of the pressure-induced reaction from naphthalene, high-pressure experiments were performed using a multi-anvil apparatus, and the recovered samples were analyzed using GC/MS. In all of the recovered samples, naphthalene dominated over the reaction products. Figure 2 shows the representative mass chromatograms (MCs) of the reaction products with m/z = 87, 228, 254, and 256.
8
In the sample recovered from 10 GPa, peaks 2 and 7 were observed, which were also detected in the starting material. In addition, weak peaks 8 and 9 were observed concomitantly with the internal standard peak (methyl stearate). These peaks were not detected in the starting materials, and therefore probably corresponded to the products formed at high pressure, even though their intensities are lower than those of peaks 2 and 7. In the sample compressed to 15 GPa, five new peaks (peaks 1 and 3–6) were observed in addition to the intense peaks 2, 7, 8, and 9. For the sample compressed to 20 GPa, weak peaks were detected in addition to intense peaks 1–9, indicating that the number of reaction products increased with pressure. In contrast, increasing the pressure to 23 GPa did not further change the types of products formed. In a sample compressed to 20 GPa for 11h, the same products were detected as that obtained from a sample compressed to 20 GPa for 1 h. The result indicates that a long preservation time is not necessary for the irreversible chemical reaction and has a negligible effect on the molar yield. Mass spectra (MS) of peaks 2 and 9 in Fig. 2, which have [M]+ = 254, can be clearly identified as 1,1'-binaphthyl and 2,2'-binaphthyl, respectively, based on comparison of their retention time and mass spectra with those of the internal standard
9
(Fig. S1a and b). Peak 7 has [M]+ = 228, attributable to benz[a]anthracene (Figure S1c). The chemical structures of the other products (peaks 1, 3–6, and 8) were suggested based on comparison of their mass spectra with the database (NIST 02) installed in the GC/MS software, because of the lack of a suitable standard. Peak 8, with [M]+ = 254 was assigned to 1,2'-binaphthyl, an isomer of 1,1'- and 2,2'-binaphthyl (Figure 3a). Peaks 1 and 3–6 exhibited similar mass spectra with [M]+ = 256 (Figure 3b) and were assigned to the isomeric naphthalene dimers (C20H16), such as dihydro-binaphthyl, whereas their spectra clearly differed from those of other aromatic hydrocarbons with the
same
molecular
weight,
such
as
dimethylbenz[a]anthracene
and
ethylbenz[a]anthracene. Most of the other trace products detected from 20 and 23 GPa samples were likely to have [M]+ = 228 or 256, although the exact molecular structures were not determined because of low peak intensities. The present results indicate the occurrence of naphthalene dimerization even at room temperature above 15 GPa, whereas trace amounts of binaphthyl isomers were detected at 10 GPa. The number of formed dimers increased with increasing pressure up to 20 GPa. The dimers formed can be divided into three types. The first one is a simple dimerization product from a pair of naphthalene molecules (m/z = 256, C20H16), with its
10
molar weight being exactly twice that of naphthalene. The second type represents isomers of binaphthyl (m/z = 254, C20H14), formed by naphthylation with release of hydrogen atoms and preserved aromatic character. The areas of 1,2’-binaphthyl peaks in the total ion chromatograms (TICs) are about twice as large as those of 1,1’- and 2,2’-binaphthyl peaks in 20 and 23 GPa samples, indicating that 1,2’-binaphthyl is easier to form at
high pressure, compared to
1,1’- and 2,2’-binaphthyl.
Benz[a]anthracene, formed by condensation with release of two methylene groups, represents the third dimer type. The TIC peak area of benz[a]anthracene is remarkably larger than that of other condensation products, such as chrysene, tetracene, and benz[c]phenanthrene. Similar dimerization mechanisms were also found for the pressure-induced dimerization of benzene at room temperature above 13 GPa, where benzene dimers, biphenyl and naphthalene were formed [20]. The oligomerization of naphthalene probably occurs via a mechanism similar to oligomerization of benzene, where the intermolecular distance shortens with increasing pressure and at some point exceeds the reaction threshold [14,20]. In addition to the various dimers of naphthalene, some peaks assignable to trimers were detected above 15 GPa. Figure 4 shows a representative MS of the trimer,
11
with [M]+ = 384 (molecular ion peak) and a distinctive base peak at m/z = 228. This observation implies that the trimer contains a C18H12 substructure, such as benz[a]anthracene. One of the possible molecular structures of this product is depicted in the upper right part of Fig. 4, featuring a benz[a]anthracene substructure bonded to a naphthalene substructure sandwiching two methylene groups. In addition, several products with [M]+ = 382 and 384 were detected, considered to be isomers and derivatives of the trimer formed above 15 GPa. Formation of highly polymerized products, such as carbon nanothreads, can be expected as insoluble fractions in organic solvates, similarly to the case of benzene compression [20,21]. However, the formation of dichloromethane-insoluble products was not observed in the present study even at 23 GPa. Since the dichloromethane solution of the reaction products was turbid due to the fine particles from the pressure medium (MgO doped with CoO), it was difficult to determine the presence of other dichloromethane-insoluble materials. Thus, it remained unclear whether heavier oligomers, such as carbon nanothreads, were formed in the present study. Both XRD and GC/MS analyses showed that naphthalene was still present even at around 20 GPa, indicating that the oligomerization occurred only partially,
12
similarly to the cases of pressure-induced oligomerization of benzene and L-alanine [20,35]. This result suggests that the intermolecular distance between neighboring naphthalene molecules might not exceed the reaction threshold in average, but is likely to shorten locally, e.g., around lattice defects and deformed grain boundaries. In addition, the distance between neighboring naphthalene molecules fractionates by molecular vibration. Under such conditions, the intermolecular distance partially exceeds the reaction threshold, and oligomerization occurs. Chemical reactions of naphthalene were also reported for shock compression, where both temperature and pressure were dynamically elevated [32]. Dark matter comprised of amorphous carbon was predominantly formed, with subsidiary formation of methylation, phenylation, and naphthylation products. Although naphthylation products were detected for both the shock experiment and the present static experiment at room temperature, the methylation and phenylation products were not detected for the present experiment, since the formation of methyl and phenyl functionalities requires decomposition of the naphthalene molecule. During the shock experiments, pyrolysis reaction occurred, leading to the decomposition of naphthalene in addition to its pressure-induced oligomerization [32]. In contrast, the decomposition of naphthalene
13
was limited in the static compression experiment at room temperature. Additionally, the relative yields of binaphthyl isomers for the present static compression at room temperature, the shock compression [32], and the thermal decomposition at ambient pressure [36,37] differed from each other. As shown in the present study, 1,2’-binaphthyl is the most abundant product at high pressure and room temperature, compared to minor amounts of 1,1’- and 2,2’-binaphthyl. For the shock compression experiments at pressures up to 26.7 GPa, the relative yield of 2,2’-binaphthyl compared to other binaphthyl isomers is considerably higher than that observed for static compression in the present study [32]. Based on the thermal reactions of naphthalene, oligomerization is most likely to promote at the α-position [37], suggesting the 1,1’-binaphthyl is a major reaction product. These variations of the ratio of naphthylation isomers could be induced by differences in the reaction mechanism and the aggregation state of naphthalene. In thermal polymerization, a free radical intermediate is believed to induce the reaction [36]. In contrast, a cycloaddition reaction is believed to occur in the pressure-induced reaction [18,38]. The distance between the neighboring naphthalene molecules and their geometry are considered to be different between high-pressure and high-temperature conditions, since naphthalene is solid
14
under the high-pressure conditions at room temperature, while it melts during the thermal reaction at ambient pressure. 4. Conclusion In this study, the stability and chemical reactions of naphthalene were investigated at high pressure and room temperature. In-situ XRD measurements indicated that naphthalene crystals remained up to around 20 GPa, while a solid amorphous phase was observed in the recovered samples. The GC/MS analysis of the samples recovered from multi-anvil experiments indicated that various dimer and trimer isomers were formed above 15 GPa, The products formed by two naphthalene molecules were classified as simple dimerization, naphthylation, and condensation products, in analogy to the pressure-induced dimerization of benzene at room temperature. These results suggest that the pressure-induced oligomerization of both naphthalene and benzene occurs in similar pressure, regardless of difference in their molecular symmetry, and similar oligomerization mechanism was thought to control, where the intermolecular distance shortens with increasing pressure and exceeds the reaction threshold. The difference of relative yields of binaphthyl isomers formed during the present static compression at room temperature, the shock compression, and heating at ambient pressure suggested
15
that different reaction mechanisms and naphthalene states were involved. Acknowledgments We thank Dr. Shinmei and Mr. Kakizawa for supporting the high-pressure experiments using multi-anvil apparatus. This work was supported by the Joint Usage/Research Center PRIUS, Ehime University. This study was financially supported by JSPS KAKENHI (Grant No. 25287147, 15J03619). The synchrotron radiation facility was used for XRD measurements at BL18C of KEK, PF (Proposal No. 2013G686, 2014G695). A. Shinozaki was supported by a JSPS Research Fellowship for Young Scientists.
16
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Figure captions Fig. 1 (a) Representative XRD patterns as a function of pressure. A diffraction line of Ne was observed above 4.6 GPa. (b) Representative XRD patterns of the sample at 21.3 GPa and the sample recovered after cell opening. Fig. 2 Representative mass chromatograms of m/z = 87, 228, 254, and 256 for the samples compressed to 10, 15, 20, and 23 GPa with that from of the starting material. I.S.: internal standard (methyl stearate). Fig. 3 (a) Mass spectrum of 1,2’-binaphthyl (peak 8 in Fig. 2), (b) representative mass spectrum of the naphthalene dimer (peak 1 in Fig. 2). Fig. 4 Representative mass spectrum of the naphthalene trimer obtained from the recovered sample (20 GPa) with a possible molecular structure.
Fig. S1 Mass spectra (compared to the corresponding standards) of (a) 1,1’-binaphthyl (peak 2 in Fig. 2) and (b) 2,2’-binaphthyl (peak 9 in Fig. 2), (c) benz[a]anthracene (peak 7 in Fig. 2).
24
Figure
Ne 19.8 GPa 18.4 GPa 16.7 GPa 15.2 GPa 13.9 GPa 12.5 GPa 9.7 GPa 8.8 GPa 7.2 GPa Ne
5.7 GPa 4.6 GPa
31-1 31-3
11-3 12-2
1.5 GPa 020 210 021 120
200,21-1 21-2
20-2 11-2
20-1
11-1 110
2.9 GPa
Fig. 1a
21.3 GPa
The recovered sample
Fig. 1b
Figure
100%
7
I.S.
50%
46
9
23 GPa
2 1
0%
8
48
50
4
3
5 6
52 54 Retention time (min)
100%
56
58
8
7
60
20 GPa 9
I.S.
50%
2 1
0%
46
48
50
4 3
5 6
52 54 Retention time (min)
56
58
60
I.S.
100%
7
15 GPa 8
50%
9
2 1 0%
46
48
50
3
45 6
52 54 Retention time (min)
56
58
60
I.S.
100%
10 GPa
50%
7 2
0%
46
48
50
9
8
52 54 Retention time (min)
56
58
60
I.S.
100%
starting material
50%
7 2 0%
46
48
50
52 54 Retention time (min)
56
58
60
Fig. 2
Figure
254
100 %
Peak 8
50 %
126 113
0%
238 226
100
100
200
300 m/z
400
500
Fig.3a
Figure
256
100 %
Peak 1
50 % 128 241 228 141 120 113 77 101
0%
100
165 215
200
300
400
500
m/z Fig.3b
Figure
228
100 %
128
Mw=384
228
50 %
0%
128 115 100
384
202 200
m/z
300
400
500 Fig.4
Main reaction products 15-23 GPa Room temperature
Naphthalene dimers Benz[a]anthrene
Naphthalene
MW=256
MW=228
MW=128
Binaphthyls MW=254
Highlights Naphthalene retained its crystal phase at around 20 GPa with a amorphous phase. Naphthalene partially oligomerized above 15 GPa at room temperature. The main reaction products could be divided into three types. 1,2’-binaphthyl is the most abundant product in three binaphthyl isomers.
The oligomerization would be promoted with decreasing intermolecular distance.
25
S0009-2614(16)30715-1 http://dx.doi.org/10.1016/j.cplett.2016.09.042 CPLETT 34183
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
11 July 2016 9 September 2016 13 September 2016
Please cite this article as: A. Shinozaki, K. Mimura, T. Nishida, T. Inoue, S. Nakano, H. Kagi, Stability and partial oligomerization of naphthalene under high pressure at room temperature, Chemical Physics Letters (2016), doi: http://dx.doi.org/10.1016/j.cplett.2016.09.042
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Stability and partial oligomerization of naphthalene under high pressure at room temperature Ayako Shinozakia, *, Koichi Mimuraa, Tamihito Nishidaa, Toru Inoueb, Satoshi Nakanoc, and Hiroyuki Kagid a
Department of Earth and Planetary Sciences, Graduate School of Environmental Studies,
Nagoya University, Nagoya 464-8601, Japan b
Geodynamics Research Center, Ehime University, Bunkyo-cho, Matsuyama, Ehime
790-8577, Japan c
National Institute for Materials Science, Namiki, Tsukuba, Ibaraki 305-0044, Japan
d
Geochemical Research Center, Graduate School of Science, The University of Tokyo,
Hongo, Tokyo 113-0033, Japan
*Corresponding author: Ayako Shinozaki. Tel.: +81-52-789-2524, fax: +81-52-789-3033 E-mail: [email protected]
1
Abstract The stability and pressure-induced chemical reactions of naphthalene were investigated at room temperature at pressures up to 23 GPa. In-situ X-ray diffraction (XRD) measurements indicated that naphthalene retained its crystal structure up to ~20 GPa, whereas a solid amorphous phase was observed in the recovered samples. Based on microanalysis of the recovered samples using Gas Chromatograph Mass Spectrometer (GC/MS), naphthalene dimer and trimer isomers were observed at pressures exceeding 15 GPa. The dimers were classified as products of simple dimerization,
naphthylation,
and condensation,
similar
to the case of the
pressure-induced dimerization of benzene, indicating a similar dimerization mechanism for naphthalene. Keywords: High pressure, polycyclic aromatic hydrocarbons, XRD, GC/MS
2
1. Introduction Pressure is an important factor promoting chemical reactions of organic compounds. Pressure-induced oligomerization and polymerization for various substances have been reported [1-6]. The behavior of aromatic hydrocarbons under high pressure has also been investigated because of their distinctive two-dimensional molecular structure and delocalized π-electrons. In case of benzene, which is the simplest aromatic hydrocarbon, several pressure-induced phase changes were reported [7-12], together with the irreversible formation of an amorphous phase [9,10,12-14]. In addition, a pressure-induced dimerization of benzene was theoretically and experimentally predicted [15-19]. The formation of naphthalene, biphenyl, and benzene dimers at room temperature was observed by GC/MS analysis for the quenched samples exposed to pressures exceeding 13 GPa, whereas most of the benzene remained [20]. Interestingly, a one-dimensional sp3-carbon nanothread was detected in the recovered products after compression at ~20 GPa and room temperature [21]. Oligomerization, polymerization, and amorphization are thought to be promoted by the decreasing distance between the neighboring benzene molecules at increased pressure, and the above-mentioned reactions can take place upon reaching some threshold of the
3
neighboring distance [14,20]. Naphthalene (C10H8) features a fused pair of benzene rings. The 1- and 2-positions of naphthalene exhibit different reactivity, unlike in fully symmetric benzene. Naphthalene crystals have a monoclinic structure (P21/a) at pressures from ambient to ~13 GPa [22-25]. Theoretical calculations suggest the absence of phase transitions up to 20 GPa [26]. Moreover, irreversible chemical reactions of naphthalene were reported by the disappearance of its IR peaks at 45.2 GPa [27]. Dimerization of naphthalene was observed from the recovered sample after compression up to 16 GPa and room temperature [28], although the molecular structure of the dimer was not evaluated as well as the pressure dependence of its formation. At high pressure and temperature, chemical reactions of naphthalene were also reported. Based on in-situ XRD measurements, diffraction peaks of naphthalene were observed up to 873 K at ~7 GPa, while graphite and diamond were formed above that temperature [28-30]. In the samples recovered from ~7 GPa and 773–873 K, several naphthalene oligomers were detected by Matrix Assisted Laser Desorption Ionization analysis [28,31]. Samples recovered after shock experiments of naphthalene at pressures up to 33.7 GPa and a temperature of 1660 K showed presence of methylation, phenylation, and naphthylation
4
products based on GC/MS analysis, as well as a dichloromethane-insoluble dark residue [32]. However, distinguishing the effects of pressure from those of temperature in these studies is difficult, since both of these parameters were varied. In this study, pressure-induced chemical reactions of naphthalene were investigated. We performed in-situ XRD measurements to investigate the stability of naphthalene crystals at high pressure and room temperature using a diamond anvil cell (DAC). Subsequently, high-pressure experiments were conducted using a multi-anvil apparatus, and the reaction products in the recovered samples were qualitatively identified by GC/MS to characterize the pressure-induced oligomerization of naphthalene. 2. Experimental For DAC experiments, a pair of diamond anvils with 450 μm culets was used. Stainless steel was used for the gasket after pre-compression to approximately 100 μm thickness. A sample hole with a ~200 μm diameter was drilled in the gasket. Powdered naphthalene (C10H8, Wako, purity 98.0%) was loaded in the sample chamber, and Ne gas was loaded using a gas loading apparatus [33]. The pressure was measured using the ruby fluorescence method [34]. The XRD patterns were obtained at BL-18C of the
5
Photon Factory of the High Energy Accelerator Research Organization (KEK) at room temperature. A typical X-ray wavelength was ca. 0.6128 Å, calibrated with the CeO2 standard. The X-ray beam was collimated to 100 µm in diameter. To obtain a sample suitable for GC/MS analysis, we used a multi-anvil apparatus: a Kawai-type 3000 ton press (OEANGE-3000 at GRC, Ehime University). Tungsten carbide second stage anvils with 5 mm truncated edge length were used. Semi-sintered MgO doped with CoO and a gold capsule (5 mm outer diameter, 4.6 mm inner diameter) were used as the pressure medium and sample capsule, respectively. To prevent organic contaminants, the pressure medium and the sample capsule were cleaned with acetone and heated in an oven at 450 °C for 3 h prior to encapsulating naphthalene. Pressure was calibrated in advance using the phase transitions of standard materials (ZnS, 15.6 GPa; GaAs, 18.7 GPa; GaP, 23.0 GPa). A naphthalene sample (C10H8, Sigma-Aldrich, purity ≥ 99.7%) of about 50 mg weight was used as a starting material for high-pressure experiments. Four experiments were performed (at 10, 15, 20, and 23 GPa), with a preservation time of 1 h at each maximal pressure. The samples were decompressed to ambient pressure and dissolved in distilled dichloromethane (CH2Cl2) with the sample capsule, in order to prevent volatilization of the products
6
formed. The reaction products inside the gold capsule were extracted in distilled dichloromethane. The solution was initially filtered to remove fragments of the pressure medium and the sample capsule. The concentrated reaction products were analyzed by a GC/MS spectrometer (JMS-K9; JEOL Co.) equipped with a capillary column (0.25 mm inner diameter × 30 m, 0.25 µm film thickness, HP-5; Agilent Technology Co.). The GC column temperature was programmed similarly to our previous study [20]. Helium carrier gas (1 mL/min) was used. Ionization voltage of the mass spectrometer was set to 70 eV, and an m/z scan range from 40 to 500 was used. Methyl laurate (C13H26O2), methyl stearate (C19H38O2), and methyl triacontanate (C31H62O2) were used as internal standards for the analysis. 3. Results and discussion First, in-situ XRD measurements using DAC were performed to investigate the stability of naphthalene crystals under high pressure. Figure 1a shows representative XRD patterns with increasing pressure. Diffractions of naphthalene were observed at least up to 19.8 GPa, indicating that naphthalene retained its crystal phase at that pressure, whereas the peaks became weaker and broader with increasing pressure,
7
compared to the diffraction from neon, a pressure medium. It is possible that the peak broadening and the decrease in diffraction intensity are attributed to a pressure-induced chemical reaction, such as partial amorphization, or a change of hydrostaticity of the sample. The diffraction patterns could be indexed to the monoclinic structure [24,25] up to 19.8 GPa, indicating that the crystal structure was not considerably changed. Naphthalene diffraction peaks were also observed in absence of the pressure medium during compression to 21.3 GPa and decompression. When the sample was decompressed to ambient pressure and the DAC was opened, some solid residue was observed after the naphthalene was sublimated and thus removed from the sample capsule. No diffraction peaks were observed for the solid product (see Figure 1b), suggesting pressure-induced irreversible reaction leading to the formation of an amorphous phase. To identify the product of the pressure-induced reaction from naphthalene, high-pressure experiments were performed using a multi-anvil apparatus, and the recovered samples were analyzed using GC/MS. In all of the recovered samples, naphthalene dominated over the reaction products. Figure 2 shows the representative mass chromatograms (MCs) of the reaction products with m/z = 87, 228, 254, and 256.
8
In the sample recovered from 10 GPa, peaks 2 and 7 were observed, which were also detected in the starting material. In addition, weak peaks 8 and 9 were observed concomitantly with the internal standard peak (methyl stearate). These peaks were not detected in the starting materials, and therefore probably corresponded to the products formed at high pressure, even though their intensities are lower than those of peaks 2 and 7. In the sample compressed to 15 GPa, five new peaks (peaks 1 and 3–6) were observed in addition to the intense peaks 2, 7, 8, and 9. For the sample compressed to 20 GPa, weak peaks were detected in addition to intense peaks 1–9, indicating that the number of reaction products increased with pressure. In contrast, increasing the pressure to 23 GPa did not further change the types of products formed. In a sample compressed to 20 GPa for 11h, the same products were detected as that obtained from a sample compressed to 20 GPa for 1 h. The result indicates that a long preservation time is not necessary for the irreversible chemical reaction and has a negligible effect on the molar yield. Mass spectra (MS) of peaks 2 and 9 in Fig. 2, which have [M]+ = 254, can be clearly identified as 1,1'-binaphthyl and 2,2'-binaphthyl, respectively, based on comparison of their retention time and mass spectra with those of the internal standard
9
(Fig. S1a and b). Peak 7 has [M]+ = 228, attributable to benz[a]anthracene (Figure S1c). The chemical structures of the other products (peaks 1, 3–6, and 8) were suggested based on comparison of their mass spectra with the database (NIST 02) installed in the GC/MS software, because of the lack of a suitable standard. Peak 8, with [M]+ = 254 was assigned to 1,2'-binaphthyl, an isomer of 1,1'- and 2,2'-binaphthyl (Figure 3a). Peaks 1 and 3–6 exhibited similar mass spectra with [M]+ = 256 (Figure 3b) and were assigned to the isomeric naphthalene dimers (C20H16), such as dihydro-binaphthyl, whereas their spectra clearly differed from those of other aromatic hydrocarbons with the
same
molecular
weight,
such
as
dimethylbenz[a]anthracene
and
ethylbenz[a]anthracene. Most of the other trace products detected from 20 and 23 GPa samples were likely to have [M]+ = 228 or 256, although the exact molecular structures were not determined because of low peak intensities. The present results indicate the occurrence of naphthalene dimerization even at room temperature above 15 GPa, whereas trace amounts of binaphthyl isomers were detected at 10 GPa. The number of formed dimers increased with increasing pressure up to 20 GPa. The dimers formed can be divided into three types. The first one is a simple dimerization product from a pair of naphthalene molecules (m/z = 256, C20H16), with its
10
molar weight being exactly twice that of naphthalene. The second type represents isomers of binaphthyl (m/z = 254, C20H14), formed by naphthylation with release of hydrogen atoms and preserved aromatic character. The areas of 1,2’-binaphthyl peaks in the total ion chromatograms (TICs) are about twice as large as those of 1,1’- and 2,2’-binaphthyl peaks in 20 and 23 GPa samples, indicating that 1,2’-binaphthyl is easier to form at
high pressure, compared to
1,1’- and 2,2’-binaphthyl.
Benz[a]anthracene, formed by condensation with release of two methylene groups, represents the third dimer type. The TIC peak area of benz[a]anthracene is remarkably larger than that of other condensation products, such as chrysene, tetracene, and benz[c]phenanthrene. Similar dimerization mechanisms were also found for the pressure-induced dimerization of benzene at room temperature above 13 GPa, where benzene dimers, biphenyl and naphthalene were formed [20]. The oligomerization of naphthalene probably occurs via a mechanism similar to oligomerization of benzene, where the intermolecular distance shortens with increasing pressure and at some point exceeds the reaction threshold [14,20]. In addition to the various dimers of naphthalene, some peaks assignable to trimers were detected above 15 GPa. Figure 4 shows a representative MS of the trimer,
11
with [M]+ = 384 (molecular ion peak) and a distinctive base peak at m/z = 228. This observation implies that the trimer contains a C18H12 substructure, such as benz[a]anthracene. One of the possible molecular structures of this product is depicted in the upper right part of Fig. 4, featuring a benz[a]anthracene substructure bonded to a naphthalene substructure sandwiching two methylene groups. In addition, several products with [M]+ = 382 and 384 were detected, considered to be isomers and derivatives of the trimer formed above 15 GPa. Formation of highly polymerized products, such as carbon nanothreads, can be expected as insoluble fractions in organic solvates, similarly to the case of benzene compression [20,21]. However, the formation of dichloromethane-insoluble products was not observed in the present study even at 23 GPa. Since the dichloromethane solution of the reaction products was turbid due to the fine particles from the pressure medium (MgO doped with CoO), it was difficult to determine the presence of other dichloromethane-insoluble materials. Thus, it remained unclear whether heavier oligomers, such as carbon nanothreads, were formed in the present study. Both XRD and GC/MS analyses showed that naphthalene was still present even at around 20 GPa, indicating that the oligomerization occurred only partially,
12
similarly to the cases of pressure-induced oligomerization of benzene and L-alanine [20,35]. This result suggests that the intermolecular distance between neighboring naphthalene molecules might not exceed the reaction threshold in average, but is likely to shorten locally, e.g., around lattice defects and deformed grain boundaries. In addition, the distance between neighboring naphthalene molecules fractionates by molecular vibration. Under such conditions, the intermolecular distance partially exceeds the reaction threshold, and oligomerization occurs. Chemical reactions of naphthalene were also reported for shock compression, where both temperature and pressure were dynamically elevated [32]. Dark matter comprised of amorphous carbon was predominantly formed, with subsidiary formation of methylation, phenylation, and naphthylation products. Although naphthylation products were detected for both the shock experiment and the present static experiment at room temperature, the methylation and phenylation products were not detected for the present experiment, since the formation of methyl and phenyl functionalities requires decomposition of the naphthalene molecule. During the shock experiments, pyrolysis reaction occurred, leading to the decomposition of naphthalene in addition to its pressure-induced oligomerization [32]. In contrast, the decomposition of naphthalene
13
was limited in the static compression experiment at room temperature. Additionally, the relative yields of binaphthyl isomers for the present static compression at room temperature, the shock compression [32], and the thermal decomposition at ambient pressure [36,37] differed from each other. As shown in the present study, 1,2’-binaphthyl is the most abundant product at high pressure and room temperature, compared to minor amounts of 1,1’- and 2,2’-binaphthyl. For the shock compression experiments at pressures up to 26.7 GPa, the relative yield of 2,2’-binaphthyl compared to other binaphthyl isomers is considerably higher than that observed for static compression in the present study [32]. Based on the thermal reactions of naphthalene, oligomerization is most likely to promote at the α-position [37], suggesting the 1,1’-binaphthyl is a major reaction product. These variations of the ratio of naphthylation isomers could be induced by differences in the reaction mechanism and the aggregation state of naphthalene. In thermal polymerization, a free radical intermediate is believed to induce the reaction [36]. In contrast, a cycloaddition reaction is believed to occur in the pressure-induced reaction [18,38]. The distance between the neighboring naphthalene molecules and their geometry are considered to be different between high-pressure and high-temperature conditions, since naphthalene is solid
14
under the high-pressure conditions at room temperature, while it melts during the thermal reaction at ambient pressure. 4. Conclusion In this study, the stability and chemical reactions of naphthalene were investigated at high pressure and room temperature. In-situ XRD measurements indicated that naphthalene crystals remained up to around 20 GPa, while a solid amorphous phase was observed in the recovered samples. The GC/MS analysis of the samples recovered from multi-anvil experiments indicated that various dimer and trimer isomers were formed above 15 GPa, The products formed by two naphthalene molecules were classified as simple dimerization, naphthylation, and condensation products, in analogy to the pressure-induced dimerization of benzene at room temperature. These results suggest that the pressure-induced oligomerization of both naphthalene and benzene occurs in similar pressure, regardless of difference in their molecular symmetry, and similar oligomerization mechanism was thought to control, where the intermolecular distance shortens with increasing pressure and exceeds the reaction threshold. The difference of relative yields of binaphthyl isomers formed during the present static compression at room temperature, the shock compression, and heating at ambient pressure suggested
15
that different reaction mechanisms and naphthalene states were involved. Acknowledgments We thank Dr. Shinmei and Mr. Kakizawa for supporting the high-pressure experiments using multi-anvil apparatus. This work was supported by the Joint Usage/Research Center PRIUS, Ehime University. This study was financially supported by JSPS KAKENHI (Grant No. 25287147, 15J03619). The synchrotron radiation facility was used for XRD measurements at BL18C of KEK, PF (Proposal No. 2013G686, 2014G695). A. Shinozaki was supported by a JSPS Research Fellowship for Young Scientists.
16
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23
Figure captions Fig. 1 (a) Representative XRD patterns as a function of pressure. A diffraction line of Ne was observed above 4.6 GPa. (b) Representative XRD patterns of the sample at 21.3 GPa and the sample recovered after cell opening. Fig. 2 Representative mass chromatograms of m/z = 87, 228, 254, and 256 for the samples compressed to 10, 15, 20, and 23 GPa with that from of the starting material. I.S.: internal standard (methyl stearate). Fig. 3 (a) Mass spectrum of 1,2’-binaphthyl (peak 8 in Fig. 2), (b) representative mass spectrum of the naphthalene dimer (peak 1 in Fig. 2). Fig. 4 Representative mass spectrum of the naphthalene trimer obtained from the recovered sample (20 GPa) with a possible molecular structure.
Fig. S1 Mass spectra (compared to the corresponding standards) of (a) 1,1’-binaphthyl (peak 2 in Fig. 2) and (b) 2,2’-binaphthyl (peak 9 in Fig. 2), (c) benz[a]anthracene (peak 7 in Fig. 2).
24
Figure
Ne 19.8 GPa 18.4 GPa 16.7 GPa 15.2 GPa 13.9 GPa 12.5 GPa 9.7 GPa 8.8 GPa 7.2 GPa Ne
5.7 GPa 4.6 GPa
31-1 31-3
11-3 12-2
1.5 GPa 020 210 021 120
200,21-1 21-2
20-2 11-2
20-1
11-1 110
2.9 GPa
Fig. 1a
21.3 GPa
The recovered sample
Fig. 1b
Figure
100%
7
I.S.
50%
46
9
23 GPa
2 1
0%
8
48
50
4
3
5 6
52 54 Retention time (min)
100%
56
58
8
7
60
20 GPa 9
I.S.
50%
2 1
0%
46
48
50
4 3
5 6
52 54 Retention time (min)
56
58
60
I.S.
100%
7
15 GPa 8
50%
9
2 1 0%
46
48
50
3
45 6
52 54 Retention time (min)
56
58
60
I.S.
100%
10 GPa
50%
7 2
0%
46
48
50
9
8
52 54 Retention time (min)
56
58
60
I.S.
100%
starting material
50%
7 2 0%
46
48
50
52 54 Retention time (min)
56
58
60
Fig. 2
Figure
254
100 %
Peak 8
50 %
126 113
0%
238 226
100
100
200
300 m/z
400
500
Fig.3a
Figure
256
100 %
Peak 1
50 % 128 241 228 141 120 113 77 101
0%
100
165 215
200
300
400
500
m/z Fig.3b
Figure
228
100 %
128
Mw=384
228
50 %
0%
128 115 100
384
202 200
m/z
300
400
500 Fig.4
Main reaction products 15-23 GPa Room temperature
Naphthalene dimers Benz[a]anthrene
Naphthalene
MW=256
MW=228
MW=128
Binaphthyls MW=254
Highlights Naphthalene retained its crystal phase at around 20 GPa with a amorphous phase. Naphthalene partially oligomerized above 15 GPa at room temperature. The main reaction products could be divided into three types. 1,2’-binaphthyl is the most abundant product in three binaphthyl isomers.
The oligomerization would be promoted with decreasing intermolecular distance.
25