Probing the Chemical Stability of Aniline under High Pressure - The

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Probing the Chemical Stability of Aniline Under High-Pressure Marcelo Medre Nobrega, Marcia L.A. Temperini, and Roberto Bini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12924 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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Probing the Chemical Stability of Aniline Under High-Pressure

Marcelo M. Nobrega*a,b, Marcia L. A. Temperinia and Roberto Binib,c,d a

Departamento de Química Fundamental, Instituto de Química da Universidade de São

Paulo (USP), C.P. 26077- CEP 05513-970 -São Paulo, SP, Brazil b

LENS, European Laboratory for Nonlinear Spectroscopy, Via Nello Carrara 1, 50019

Sesto Fiorentino (FI), Italy c

Dipartimento di Chimica“Ugo Schiff”dell’Università degli Studi di Firenze, Via della

Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy d

ICCOM-CNR, Institute of Chemistry of OrganoMetallic Compounds, National

Research Council of Italy,Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Firenze, Italy Tel.: + 55 11 3091 3890; fax: + 55 11 3091 3890. E-mail: [email protected]

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Abstract Aniline is an important molecule from fundamental point of view because it is the prototype of aromatic amines, but it is also important for industry due to its highly exploited conducting polymer in the materials engineering field. High pressure and temperature studies on the chemical stability of aniline are mandatory to figure out how to trigger aniline’s reactivity in reduced space for a possible practical exploitation of high pressure in the synthesis of novel materials, such as highly ordered polyaniline. Synchrotron X-ray diffraction (XRD) experiments allowed the aniline’s equation of state to be expanded up to 16.3 GPa. UV-VIS absorption and Fourier Transformed IR (FTIR) experiments at different pressure and temperature showed an anomalous chemical stability of aniline with respect to other aromatic systems, likely due to the hydrogen bonds arrangement. Reactivity has been laser induced showing, for short irradiation, the formation of a limited amount of saturated chain-like structures that collapse, once further irradiated, into a 3D amorphous extended network.


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Introduction Solid-state high-pressure studies of aromatic and heteroaromatic systems have attracted great attention in the last years/decades since the reactivity of such compounds typically forms saturated, smaller volume and/or confined polymers and extended amorphous networks.1–3 Generally, the reduction of the volume per molecule caused by high pressure conditions emphasizes the topochemical constraints, whereas the structure of the final product reflects the symmetry of the molecular crystal from which it is formed or at least the relative molecular arrangement. 4 For instance, this is the case of the polymerization of crystalline ethylene to form, once compressed in monoclinic phase I, crystalline orthorhombic polyethylene.5 Within aromatic systems, aniline is one of the most important monosubstituted benzenes, quite important for industry, since it is the precursor of one of the most studied and important conducting polymers: polyaniline (PANI) 6–12 and it is a common structural motif in biological systems.13 Aniline is a colorless liquid at ambient conditions (0.1 MPa and 298 K) crystallizing at low pressure into the orthorhombic phase-II, space group Pna21 containing 2 molecules per cell, and no additional phase transition have been observed up to 7.3 GPa.14 Aniline presents a particular substituent group, NH2, in the benzene ring, which stabilizes the crystal phase by forming NH···N H-bonds as illustrated in Figure 1. Additionally to the NH···N H-bonds, the strongest interactions in aniline phase-II are those formed by NH···π, which are as important as H-bonds, followed by the CH···π contacts.14 Calculations have indicated that the intermolecular interaction energy for pairs of aniline molecules connected by H-bonds is in the range of – 9 to -16 kJ mol-1.

14

It is known that heteroaromatics systems, in particular, those that present

such strong interactions are more stable than their counterpart that does not contain a heteroatom.



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Figure 1. Crystal structure of aniline phase-II at 0.84 GPa projected along the b-axis evidencing the H-bonds between NH···N contacts along the a-axis. Adapted from ref. 14.

So far, very little is known about the reactivity of N heteroaromatic systems which are interesting candidates for the synthesis of N bearing amorphous hydrogenated carbons and carbon nitrides. Remarkable examples of different kind of reactions involving simple molecules are the formation of amorphous hydrogenated carbon from benzene and s-triazine,

4,15,16

the reaction between red phosphorus and water, 17 and the

formation of polymeric nitrogen.

18

In addition, the two simplest N heteroaromatic

systems studied so far, pyridine and triazine, in spite of the close resemblance show a quite different behaviour essentially related to the H-bonding arrangement. In pyridine the strong H-bonds prevent the reactivity which is limited to the crystal defects,

2

whereas in triazine a complex competition between the lattice dynamics and the change with pressure of the electronic properties determines a cooperative extension of the reaction to the entire crystal.1 Laser-irradiation is frequently used to trigger the reactivity in high pressure experiments because the production of reactive excited molecules could allow overcoming the large energy barriers characteristic of stable systems, as well as, the barriers easily formed at high pressure which tends to hinder the reactions. In this


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framework, the reactivity of a system can be indeed induced by electronic excitation using visible or UV radiation. High-pressure and high-temperature spectroscopic and diffratometric studies directed to understand the structural and electronic relationship and how this is modified by pressure are mandatory to figure out how to trigger aniline’s reactivity in reduced space. The results of these studies would open a practical exploitation of high pressure in the synthesis of novel materials, such as highly oriented polyaniline.19 In this paper we have performed an extensive study of crystalline aniline phaseII by X-ray diffraction, UV-VIS absorption spectroscopy and IR spectroscopy. The main results can be summarized in: i) aniline’s equation of state was extended to 16.3 GPa; ii) an anomalous chemical stability was observed in a quite broad pressure and temperature range probably due to the peculiar hydrogen bond arrangement; and iii) reactivity was laser induced at two different pressures and the results have pointed out to a limited extension of the reaction possibly occurring through the crystal defects and giving rise to a 2D chain-like structure resembling an alkane polymeric structure for short irradiation cycles, whereas the 2D product structure collapsed into a 3D amorphous extended network for longer irradiation times.

Experimental Section Aniline (C6H5NH2, Merck) was distilled under reduced pressure prior to use and was loaded into a MDAC (membrane diamond anvil cell) equipped with IIa type diamonds and stainless steel gasket. The sample dimension was 150 µm in diameter and about 50 µm thick. In some measurements to reduce the strong sample absorption the aniline thickness was reduced by pressing KBr into the sample chamber producing a pellet whose surface was successively scratched. Afterwards, liquid aniline (sample



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thickness ranging from 10 to 20 µm) and a ruby chip were added above the KBr pellet. High-temperature experiments were performed by using resistively heated cells and rhenium gasket. The temperature was measured with a precision of ±0.1 K by a K- type thermocouple placed close to the diamonds. FT-IR absorption measurements were recorded with an instrumental resolution of 1 cm-1 using a Bruker-IFS 120 HR spectrometer modified for high-pressure measurements.20 The ruby fluorescence was excited using few milliwatts of a 532 nm laser line from a Nd:YAG laser source. UVVIS absorption measurements as a function of pressure were performed in a home-made UV-VIS spectrometer and the attenuated intensity was compared to the intensity measured in the MDAC loaded with KBr. X-ray diffraction powder patterns as a function of pressure were measured in the XDS beamline at the Brazilian Synchrotron Light Laboratory (LNLS) with a monochromatic beam ( λ = 0.6199 Å) and a MARCCD Marmosaic 225 image detector. The 2D diffraction patterns were analyzed and integrated using the FIT2D software. 21

Results and Discussion X-ray diffraction. Aniline presents two crystalline phases. Phase-I, monoclinic (P21/c), is formed on cooling the liquid at ambient pressure down to 267 K, whereas, phase-II is obtained by increasing pressure at room temperature. As reported recently, the strongest intermolecular interactions in the orthorhombic phase-II, space group Pna21 containing 2 molecules in each asymmetric unit, are those developing between molecules connected by NH···π and NH···N, hydrogen bonds, followed by the CH···π contacts. 14 Crystalline phase-II was investigated so far up to 7.3 GPa.14 Since the instability threshold of aromatic molecules is generally above 10 GPa, 22 we have measured angle dispersive x-ray diffraction patterns at ambient temperature (298 K) as a function of


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pressure up to 16.3 GPa using synchrotron radiation to gain insight about the stability of aniline, Figure 2. As the pressure increases, a remarkable broadening and weakening of the diffraction peaks occurs but no evidence of phase transitions or reactivity up to the maximum pressure was gained by this study. The peak intensities decrease with increasing pressure indicating a cristallinity decrease of aniline phase-II, also considering that no compression medium was employed, it is not completely lost at the highest pressure reached. Upon decompression the starting crystal quality is not recovered (see Figure 2 lower traces).

Figure 2. Pressure evolution of the x-ray diffraction pattern of aniline at ambient temperature (298 K) upon compression upper (black) traces and decompression lower (gray) traces.

The evolution of the cell parameters as a function of pressure was obtained by the unit cell refinement of the powder diffraction data.

23

These data are presented in

Figure 3. Lattice parameters decreased with pressure presenting a, b and c-axes compression by 10.8, 7.8 and 8.8 % respectively, representing a volume decrease of about 25%. Funnel et al.

14

have observed a compression in volume by ~26 % at 7.3



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GPa, our results indicate that increasing pressure up to the highest value achieved 16.3 GPa caused an additional 9 % compression in volume, reaching a final compression of ~35 % with respect to the initial volume (V0). A compression up to 7.3 GPa corresponds to a shortening by 10 and 13% in the NH···π and NH···N contacts and brought one of the NH···N and the CH···π contacts to a destabilizing region with energies of the order of + 1.1 kJ mol-1 and + 6.6 kJ mol-1, respectively.

14

Reaching a pressure of about 16

GPa one can expect that these contacts were further reduced by about 3%. An interesting result is the behaviour of the c parameter which remains constant above ∼10 GPa. The incompressibility along the c direction is possibly related to the fact that both NH···N contacts are entering into a repulsive region. The c direction is the one where the H-bonds are directed forming a sort of a chain connecting the molecules (see Figure 1) evidencing how the presence of those strong directional interactions causes a substantial non-isotropic compression which could reflect in the selection of the directions along which instability is driven. c direction is related to the translational movement of the molecules favouring the stacking of the rings, which would be expected to affect the π-π* electronic transition as will be discussed below. Lattice parameters were used to determine the volume data, which were fitted with a 3rd order Vinet equation of state (EOS),

24

Figure 3, the values of V0, B0 and C0

used in the fit are also presented. We have expanded the previous EOS reported up to 7.3 GPa

14

and these data are also presented in Figure 3 for comparison purposes. The

EOS and its parameters, isothermal bulk modulus (B0), its derivative against pressure at P = O and the V0 derived from fitting the volume versus pressure, 5.44 ± 0.4 GPa, 9.88 ± 0.2 and 1056 ± 6.7 Å3, respectively, nicely agrees with the previous ones.

14

Additionally, the B0 value (5.44 GPa) indicates that aniline is a soft solid and is similar


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to other aromatic molecules such as, benzene (5.5 GPa) 25 and triazine (6.3 GPa).1

Figure 3. Top panels: Ambient temperature pressure evolution of the lattice parameters, c, a and b for aniline phase II. Bottom panel: Experimental unit cell volume as a function of pressure of phase II aniline at 298 K and literature data up to 7.3 GPa. The data fitted by a 3rd order Vinet equation (full red line), and the values of V0, B0 and C0 used in the fit are also presented. Literature data from ref. 14.

UV-VIS absorption. The highest energy occupied molecular orbitals (HOMO) of aniline has a π character, while the lowest energy unoccupied molecular orbital (LUMO) is π*.12 The four lower frequency bands in the electronic spectrum have been assigned, in order of increasing energy, to the transitions to the following states 1 1A" (4.40 eV, 280 nm), 3 1A' (5.39 eV, 230 nm), 4 1A" (6.40 eV, 193 nm), 6 1A' (6.88 eV, 180 nm) all of them having ππ* character.12 The intensity of these bands is increasing



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with the transition energy with an oscillator strength increasing 5 times for the second transition and 25 times for the third and the fourth, with respect to the lowest energy transition. The UV-VIS electronic absorption spectrum of aniline measured at ambient condition using water as solvent with the same set-up employed for the measurements in the DAC is presented in Figure 4 - blue trace. According to Honda et al.

12

the

electronic transition which we could follow as a function of pressure was the ππ*, 1 1A" that was observed at 280 nm (4.40 eV). Aniline electronic absorption spectrum was measured on a few microns thick sample, in order to avoid signal saturation, as a function of pressure up to 16.7 GPa, Figure 4. As the pressure is increased a red shift and a broadening of the electronic transition to the 1 1A" state were observed, perfectly reversible during decompression. The red shift of the electronic transition to the 1 1A" could be related to the considerable compression of the c axis that, as already stated, favors the stacking of the rings. With increasing pressure a quite consistent increase of the background is observed. This is likely the low frequency wing of the stronger higher frequency transitions which red shifts on compression. In Figure 5 the pressure dependence of the wavelength maximum of the absorption band relative to the transition to the 1 1A" state is reported. The pressure shift of this band is approximately linear, with a slope of ca. 1.7 nm / GPa, and it is perfectly reversible during the decompression step. This behavior suggests that no chemical modifications take place in this pressure range.


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Figure 4. UV-VIS electronic absorption spectra of aniline at ambient condition in water (lower panel: blue trace) and at several pressures indicated in the figure. Lower panel: spectra recorded during the compression. Upper panel: spectra recorded during the decompression.



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Figure 5. Pressure evolution of the wavelength maximum of the absorption band assigned to the π-π* transition of aniline. Full symbols: compression. Open symbols: decompression

Infrared spectra. A better insight about the chemical stability of aniline at high pressure can be gained by the pressure evolution of the IR absorption spectrum. The spectra recorded during an isothermal compression-decompression cycle up to 30.2 GPa at 298 K are reported in Figure 6. As discussed previously, aniline crystallizes into phase-II by compressing the fluid at ambient temperature. The assignment of aniline infrared bands has been extensively discussed in several papers (see refs

26–28

and

references therein). Upon increasing pressure, a broadening, a blue shift and a decrease in the intensities of the bands are clearly observed. Up to the highest pressure achieved, no notable change is observed in the infrared spectrum of aniline that could indicate any reactivity under this condition. Upon decompression, there are no indications of new chemical species and the spectral features of aniline are recovered although a considerable weakening of the entire spectrum is observed likely ascribable to the partial amorphization already evidenced by XRD experiments.


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Particularly interesting is the high frequency region. The frequency of the mode related to the asymmetric stretching of the NH2 group (3415 cm-1 at 1.6 GPa) does not appreciably change during the entire compression-decompression cycle. On the contrary, the frequency of the mode related to the symmetric stretching of the NH2 group (3343 cm-1 at 1.6 GPa) is highly sensitive to the applied pressure, blue shifting 115 cm-1 from 1.6 to 30.2 GPa, crossing the band of the NH2 asymmetric stretching around 13 GPa. This behaviour is likely due to the fact that one hydrogen atom of the NH2 group participates in the NH···N bond (Figure 1), which could make the two modes differently sensitive to the compression effect. Lastly, a remarkable red shift of the band at 3220 cm-1, marked with a red trace in Figure 6, is observed. There is a lack of consensus in the literature about the assignment of this band, being assigned as the harmonic of the band at 1609 cm-1,28 βNH2, or as the NH stretching contaning the H atom involved in the NH···N hydrogen bond.27 Because of the considerable red shift observed one could guess that the assignment to the hydrogen bond is consistent. Otherwise, if one considers the assignment as the harmonic of the βNH2 the red shift should be observed in its counterpart at lower frequency (at 1609 cm-1) which is not the case.



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Figure 6. Infrared absorbance spectra of aniline acquired during the compression (black traces) and decompression (gray traces) at 298 K. Red trace highlights the redshift of the NH stretching related to the hydrogen bonded NH···N group. The high pressure chemical stability of aniline was also tested by combining high-pressure and high-temperature. An isobaric heating up to 530 K was performed at 12.5 GPa but also in this case no indications of a chemical reaction was revealed. However, this experiment provides insight about the assignment of the band at 3220 cm1

. The frequency shift of this band is presented in Figure 7b. For comparison purposes

the frequency shift of the band during the room temperature isothermal compression is also presented in Figure 7a. A blue shift of the band during the isobaric heating is observed showing that increasing temperature the thermal motion heavily affects the strength of the hydrogen bonds, consequently lowering the ordering of the system. The


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frequency of the band at 1609 cm-1 remained practically unaltered in the temperature range 300 – 525K thus confirming that the band observed at 3220 cm-1 is due to the NH···N stretching of the hydrogen bonding and that the pressure increase to 12 GPa favours the interaction between the NH···N involved in the hydrogen bonding, giving rise to a sort of chains of the molecules directed along the c-axis 14. Above 12 GPa the NH···N stretching band starts to become broader, spreading out in the 3200 - 3000 cm-1 range and its intensity decreases until the completely disappearance around 20 GPa. These results indicate a loss in the long range order that has been maintained by the hydrogen bonding, probably due to a non-hydrostatic stress that the crystals are experiencing under such high pressures. For the sake of clarity, in Figure 7c the pressure evolution of the NH···N contacts in aniline phase-II is reported. The NH···N contact at a given pressure was calculated by adjusting the atomic positions with respect to the changes in the lattice parameters at that pressure, which were determined by the unit cell refinement of the powder diffraction data. A substantial decrease of the H-bonding is observed up to 11 GPa, reaching values in the order of 1.97 Å at this pressure. Above 11 GPa, the NH···N distance further slightly decrease up to 16.3 GPa. The discussions of the behaviour of the c parameter that remained roughly constant above ∼10 GPa and the IR feature to which the NH···N band is assigned suggests that 1.97 Å is a kind of a limit contact for the NH···N and further compression in the system leads to a disordering in the crystal.



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Figure 7. (a) Frequency shift o the band at 3220 cm-1during an isothermal compression at 298 K (red filled circles) and (b) during an isobaric heating at 12.5 GPa (blue filled squares).(C) Pressure evolution of the NH···N contacts (black filled squares).

X-ray diffraction and spectroscopic data suggest that aniline is chemically stable well beyond 20 GPa. XRD indicated a progressive deterioration of the crystal quality not reversible upon decompression. The UV-VIS data provided fundamental information about the stability of aniline up to the highest pressure reached. In fact there are no hints in the electronic spectrum of a possible destabilization of the ring structure. This result is particular interesting because a dramatic change of the electron density can be the reason of the instability of the molecule at such conditions. As reported in the s-triazine case, the electronic mechanism played a fundamental role in driving the reactivity of these molecules. In that case, it was verified that the energies of the nπ* and ππ* excited states got closer with compression, accounting for the destabilization of the aromatic ring and driving the reaction.

29

Considering the FTIR up to the highest

pressure achieved, no notable change is observed in the infrared spectrum of aniline that could indicate any reactivity under this condition, in accordance with XRD and UV-VIS results. Unfortunately, due to the high fluorescence of the aniline with increasing pressure and of the recovered samples it was not possible to perform Raman experiments which would be useful for a full characterization of aniline’s behaviour.

PhotoInduced reactivity. It is known that the reactivity of aromatics and heteroaromatics can be easily triggered by using electronic photo-excitation. This technique is not only effective in reducing the reaction threshold but it is also proved to



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be effective in the selection of the reactive pathway, because of the specific structural and electronic changes.30,31 Combining high-pressure and electronic excitation could help us to understand aniline’s reactivity towards the synthesis of low-dimensional ordered aniline-derived materials with improved properties, and this method was never tested in aniline. In order to overcome aniline’s stability, a freshly loaded sample was compressed to 0.7 GPa and irradiated with 50 mW of the 350 nm line of an Ar ion laser. The absorption at 350 nm likely takes place through a two-photon process, since the lowest electronic transition, to the 1 1A" state, is located around 280 nm at ambient conditions (see discussion presented in the UV-VIS absorption section). For short irradiation times (0.5 h) it is already possible to notice the appearance of a set of bands in the 2800 3000 cm-1 region, Figure 8A. These absorptions are characteristic of the C-H stretching bands involving saturated sp3 hybridized carbon atoms. The most remarkable feature is the intensity of the bands related to CH2 groups. The presence of these groups cannot be related to an extended network, as in the benzene case,4,32 but more likely to a 2D chainlike alkane polymeric structure in which part of the H atoms have migrated to terminal groups, suggested by the presence of the CH3 stretching band.1,16 Due to the saturation of the sample signal in the sp2 hybridized carbon region (3000 – 3200 cm-1) it is not possible to infer about the presence of C-H groups from Csp2 in the product. The intensity of the product bands increases with irradiation time in the first 3-4 hours, however, even after 6 h irradiation the reaction extension was very limited being reacted only about 15 % of the sample. This result could indicate that the reaction proceeds through the defects present in the sample and at this pressure and irradiation conditions the reaction does not proceed over the entire sample.


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Another freshly loaded sample was compressed to 5.0 GPa and irradiated up to 13.0 h, difference spectra for several irradiation times are presented in Figure 8B. As in the case of the lower pressure experiment the reaction was identified by the set of bands in the 2800 - 3000 cm-1 region characteristic of C-H stretching of unsaturated C atoms. Also in this case, the reaction proceeded through the sample only during irradiation. The product formed is characterized, up to 7 h of irradiation, by a spectrum very similar to that obtained at 0.7 GPa although with slightly broader bands likely related to the effect of pressure. Additional 6 h of irradiation cause a relevant growth of the full C-H structure but with a more pronounced intensification of the R3C-H stretching mode. This occurrence suggests that for longer irradiation, and likely also for the higher pressure conditions, we have the collapse of the product structure into a 3D amorphous extended network. At 5.0 GPa, after 13h irradiation approximately 45% of the sample has been reacted, a considerable amount that clearly indicates a massive extension of the reaction. An optical image of the freshly loaded sample and the recovered sample after 13 h irradiation shows a darkening of the sample likely due to the formation of graphitic regions. The appearance of the product seeds is likely driven, in close analogy with the benzene case,31 by the formation of excimers. Aniline excimers have been reported to form following the excitation of dimers whose most stable conformation has a head-tohead orientation, with the phenyl rings stacked roughly parallel but with slightly displaced geometry and a distance between the ring centers of about 4 Å.33 This configuration closely recalls the arrangement of the molecules in the crystal along the c axis where the ring centers distance is 4.6 and 4.0 Å at 0.7 and 5.0 GPa, respectively, distances therefore fully compatible with the excimer formation. In addition, the barrier for energy transfer from the excited state of the dimer to the excimer state is estimated



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to be very low in jet cooled aniline.33 The only available data concerning the effect of pressure on the decay channels in the excited states are again the ones measured in benzene where, from the change of the relative intensities of monomer and excimer emission as a function of pressure, a consistent reduction with increasing pressure of the energy barrier between the excited monomer and the excimer was deduced.31 From these analogies with the benzene case and supported by the structural data, we can therefore confidently speculate that the reaction should be triggered by excimers whose formation is driven by the two-photon excitation of aniline molecules and by the favorable geometrical configuration realized along the c axis with increasing pressure. The results of the laser induced reactivity suggest the possibility to control the chemical behavior by tuning the pressure and irradiation time pointing to the attainment of a polymeric like structure or an amorphous extended carbon network containing NH2 groups, depending on the conditions. The idea of controlling the chemical structure by tuning pressure and irradiation conditions is exciting since it would be a promising tool for a possible practical exploitation of high pressure in the synthesis of novel materials with improved and optimized properties.


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Figure 8. Difference spectra obtained by subtracting from each spectrum the one obtained before the beginning of the irradiation. (A) Induced reactivity at 0.7 GPa and (B) induced reactivity at 5.0 GPa. Inset: Optical image of the sample before irradiation (time = 0h) and after 13.0 h irradiation.

Conclusions In summary, x-ray diffraction and spectroscopic data evidence the chemical stability of aniline upon compression up to about 30 GPa. As mentioned, XRD indicated a progressive deterioration of the crystal quality of aniline phase-II not reversible upon decompression. UV-VIS data indicated a high stability of aniline’s electronic structure up to the highest pressure reached, differing from other aromatics in which the energies of the nπ* and ππ* excited states got closer with compression driving its reactivity. No



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notable changes were observed in the infrared spectrum up to the highest pressure reached. As reported for the pyridine case, the strong H-bonds may prevent the reactivity of aniline in a broad pressure and temperature range. This is particular interesting because it emphasizes how the presence of these strong directional interactions causes a substantial non-isotropic compression which reflects in the directions along which instability is driven. UV laser irradiation at moderate pressures gives rise to a 2D chain-like structure resembling a saturated polymeric structure as results by a set of bands in the 2800 - 3000 cm-1 region characteristic of C-H stretching of unsaturated C atoms. The limited extension of this reaction, likely triggered by the formation of excimers, could indicate an active role of crystal defects. Longer irradiation of the sample and higher pressures cause the collapse of the 2D structure into a 3D amorphous extended network. Once again, understand the structural and electronic relationship and how this is modified by pressure are mandatory to figure out how to trigger aniline’s reactivity in reduced space for a possible practical exploitation of high pressure in the synthesis of novel materials, such as highly oriented 2D polyaniline.

Acknowledgements The authors acknowledge the Brazilian agencies CNPq and FAPESP (Grant Nos. 2012/13119-3, 2014/15107-8, 2015/09763-2) for fellowships and financial support. We also acknowledge the Brazilian Synchrotron Light Laboratory (LNLS) for granting access to XDS beamline facility, and the Deep Carbon Observatory initiative (Extreme Physics and Chemistry of Carbon: Forms, Transformations and Movements in Planetary Interiors) funded by the Alfred P. Sloan Foundation.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. No competing financial interests have been declared.

References (1)

Citroni, M.; Fanetti, S.; Bazzicalupi, C.; Dziubek, K.; Pagliai, M.; Nobrega, M. M.; Mezouar, M.; Bini, R. Structural and Electronic Competing Mechanisms in the Formation of Amorphous Carbon Nitride by Compressing S -Triazine. J. Phys. Chem. C 2015, 119 (51), 28560–28569.

(2)

Fanetti, S.; Citroni, M.; Bini, R. Structure and Reactivity of Pyridine Crystal under Pressure. J. Chem. Phys. 2011, 134 (20).

(3)

Fitzgibbons, T. C.; Guthrie, M.; Xu, E.; Crespi, V. H.; Davidowski, S. K.; Cody, G. D.; Alem, N.; Badding, J. V. Benzene-Derived Carbon Nanothreads. Nat. Mater. 2014, 14 (1), 43–47.

(4)

Ciabini, L.; Santoro, M.; Gorelli, F. A.; Bini, R.; Schettino, V.; Raugei, S. Triggering Dynamics of the High-Pressure Benzene Amorphization. Nat. Mater. 2007, 6 (1), 39–43.

(5)

Chelazzi, D.; Ceppatelli, M.; Santoro, M.; Bini, R.; Schettino, V. PressureInduced Polymerization in Solid Ethylene. J. Phys. Chem. B 2005, 109 (46), 21658–21663.

(6)

Macdiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Polyaniline: A New Concept in Conducting Polymers. Synth. Met. 1987, 18 (1–3), 285–290.

(7)

Chiang, J.-C.; MacDiarmid, A. G. “Polyaniline”: Protonic Acid Doping of the Emeraldine Form to the Metallic Regime. Synth. Met. 1986, 13 (1–3), 193–205.

(8)

Nobrega, M. M.; Ceppatelli, M.; Temperini, M. L. A.; Bini, R. Pressure-Induced Reactivity in the Emeraldine Salt and Base Forms of Polyaniline Probed by FTIR and Raman. J. Phys. Chem. C 2014, 118 (47), 27559–27566.

(9)

Nobrega, M. M.; Martins, V. L.; Torresi, R. M.; Temperini, M. L. a. One-Step Synthesis, Characterization, and Properties of Emeraldine Salt Nanofibers Containing Gold Nanoparticles. J. Phys. Chem. C 2014, 118 (8), 4267–4274.

(10)

Nobrega, M. M.; Souza, K. S.; Andrade, G. F. S.; Camargo, P. H. C.; Temperini, M. L. A. Emeraldine Salt Form of Polyaniline as a Probe Molecule for Surface Enhanced Raman Scattering Substrates Excited at 1064 Nm. J. Phys. Chem. C 2013, 117 (35), 18199–18205.

(11)

Nobrega, M. M.; Izumi, C. M. S.; Temperini, M. L. a. Probing Molecular Ordering in the HCl-Doped Polyaniline with Bulk and Nanofiber Morphology by



Page 24 of 26

24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Their Thermal Behavior. Polym. Degrad. Stab. 2015, 113 (0), 66–71. (12)

Honda, Y.; Hada, M.; Ehara, M.; Nakatsuji, H. Excited and Ionized States of Aniline: Symmetry Adapted Cluster Configuration Interaction Theoretical Study. J. Chem. Phys. 2002, 117 (5), 2045–2052.

(13)

Spesyvtsev, R.; Kirkby, O. M.; Fielding, H. H. Ultrafast Dynamics of Aniline Following 269–238 Nm Excitation and the Role of the S2(π3s/πσ*) State. Faraday Discuss. 2012, 157, 165.

(14)

Funnell, N. P.; Dawson, A.; Marshall, W. G.; Parsons, S. Destabilisation of Hydrogen Bonding and the Phase Stability of Aniline at High Pressure. CrystEngComm 2013, 15 (6), 1047–1060.

(15)

Ciabini, L.; Santoro, M.; Bini, R.; Schettino, V. High Pressure Photoinduced Ring Opening of Benzene. Phys. Rev. Lett. 2002, 88 (8).

(16)

Citroni, M.; Fanetti, S.; Bini, R. Pressure and Laser-Induced Reactivity in Crystalline S-Triazine. J. Phys. Chem. C 2014, 118 (19), 10284–10290.

(17)

Ceppatelli, M.; Bini, R.; Caporali, M.; Peruzzini, M. High-Pressure Chemistry of Red Phosphorus and Water under near-UV Irradiation. Angew. Chemie - Int. Ed. 2013, 52 (8), 2313–2317.

(18)

Eremets, M. I.; Gavriliuk, A. G.; Trojan, I. A.; Dzivenko, D. A.; Boehler, R. Single-Bonded Cubic Form of Nitrogen. Nat. Mater. 2004, 3 (8), 558–563.

(19)

Mahmood, J.; Lee, E. K.; Jung, M.; Shin, D.; Choi, H.-J.; Seo, J.-M.; Jung, S.M.; Kim, D.; Li, F.; Lah, M. S.; et al. Two-Dimensional Polyaniline (C 3 N) from Carbonized Organic Single Crystals in Solid State. Proc. Natl. Acad. Sci. 2016, 113 (27), 7414–7419.

(20)

Bini, R.; Ballerini, R.; Pratesi, G.; Jodl, H. J. Experimental Setup for Fourier Transform Infrared Spectroscopy Studies in Condensed Matter at High Pressure and Low Temperatures. Rev. Sci. Instrum. 1997, 68 (8), 3154.

(21)

Hammersley, a. P.; Svensson, S. O.; Hanfland, M.; Fitch, a. N.; Hausermann, D. Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan. High Press. Res. 1996, 14 (4–6), 235–248.

(22)

Bini, R.; Schettino, V. Materials Under Extreme Conditions. In Materials Under Extreme Conditions; 2014; pp 79–158.

(23)

Holland, T. J. B.; Redfern, S. a T.; Street, D. Unit Cell Refinement from Powder Diffraction Data : The Use of Regression Diagnostics. Mineral. Mag. 1997, 61 (February), 65–77.

(24)

Vinet, P.; Ferrante, J.; Smith, J. R.; Rose, J. H. A Universal Equation of State for Solids. J. Phys. C Solid State Phys. 1986, 29 (6), 2963–2969.

(25)

Ciabini, L.; Gorelli, F. A.; Santoro, M.; Bini, R.; Schettino, V.; Mezouar, M. High-Pressure and High-Temperature Equation of State and Phase Diagram of Solid Benzene. Phys. Rev. B 2005, 72 (9), 94108.


Page 25 of 26


25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(26)

Wojciechowski, P. M.; Zierkiewicz, W.; Michalska, D.; Hobza, P. Electronic Structures, Vibrational Spectra, and Revised Assignment of Aniline and Its Radical Cation: Theoretical Study. J. Chem. Phys. 2003, 118 (24), 10900.

(27)

Badawi, H. M.; Förner, W.; Ali, S. A. A Comparative Study of the Infrared and Raman Spectra of Aniline and O-, M-, P-Phenylenediamine Isomers. Spectrochim. Acta - Part A Mol. Biomol. Spectrosc. 2013, 112, 388–396.

(28)

Tripathi, G. N. R. The Vibrational Factor Group Splittings in Crystalline Aniline. J. Chem. Phys. 1980, 73 (11), 5521–5530.

(29)

Fanetti, S.; Citroni, M.; Bini, R. Tuning the Aromaticity of S-Triazine in the Crystal Phase by Pressure. J. Phys. Chem. C 2014, 118 (25), 13764–13768.

(30)

Citroni, M. Laser-Induced Selectivity for Dimerization Versus Polymerization of Butadiene Under Pressure. Science (80-. ). 2002, 295 (5562), 2058–2060.

(31)

Citroni, M.; Bini, R.; Foggi, P.; Schettino, V. Role of Excited Electronic States in the High-Pressure Amorphization of Benzene. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (22), 7658–7663.

(32)

Ciabini, L.; Santoro, M.; Bini, R.; Schettino, V. High Pressure Reactivity of Solid Benzene Probed by Infrared Spectroscopy. J. Chem. Phys. 2002, 116 (7), 2928– 2935.

(33)

Yeh, J.; Shen, T.; Nocera, D. G.; Leroi, G. E.; Lansing, E. Resonance TwoPhoton Ionization Spectroscopy of the Aniline Dimer. J. Phys. Chem. 1996, 100, 4385–4389.



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Probing the Chemical Stability of Aniline under High Pressure - The

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