The Photochemistry of Crystalline Nitromethane under Static Pressure

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The Photochemistry of Crystalline Nitromethane Under Static Pressure Samuele Fanetti, Margherita Citroni, Naomi Falsini, and Roberto Bini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11287 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

The Photochemistry of Crystalline Nitromethane Under Static Pressure Samuele Fanetti,†,‡ Margherita Citroni,∗,†,‡ Naomi Falsini,† and Roberto Bini†,‡ †European Laboratory for Nonlinear Spectroscopy (LENS), via Nello Carrara 1, 50019 Sesto Fiorentino, Italia ‡Dipartimento di Chimica, Universit`a di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino, Italia E-mail: [email protected]

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Abstract The high-pressure chemical reactivity of nitromethane, under irradiation with visible and near-UV laser light, was investigated by in-situ FTIR spectroscopy in a diamond anvil cell. The reactivity was probed at different pressures (0.2, 1.2, 5.0, 15.3, and 28.0 GPa) and with different excitation wavelengths (514, 458, and 350 nm), all absorbed through a two-photon process. Insight into the reaction mechanism was gained by measuring the near-UV absorption spectrum of nitromethane as a function of pressure up to 32 GPa, the threshold pressure above which it reacts spontaneously in the absence of electronic excitation. We were thus able to determine the pressureevolution of the two lowest-energy transitions (σ → π ∗ and a singlet-triplet transition). The information obtained from the absorption spectra together with the reactivity data allowed to locate the red absorption edge of the higher-energy π → π ∗ transition and its pressure shift. The excitation of the σ → π ∗ transition is not able to induce any photochemical reaction in the crystal, whereas excitation of the π → π ∗ transition is very effective at all the pressures probed. Comparing the products of the reactions induced at the different pressures, we observe as a general trend: the higher the reaction pressure, the lower is the relative amount of small oxidized compounds such as CO2 , N2 O, and ammonium carbonate, and the larger is the amount of N-methylformamide. The latter, a prototype prebiotic molecule containing the simplest peptidic bond, was reported as the main product in the purely pressure-induced reaction in the absence of light (P≥32 GPa).

Introduction Nitromethane is the simplest nitro organic compound, and is considered an insensitive explosive. Compression is a typical trigger to overcome the activation barrier and induce detonation. The pressure-induced decomposition of this simple molecular system can thus be used as a model to understand the reactivity of more complex organic explosives containing the nitro-group. Nitromethane decomposition has in fact been largely studied experimentally 2


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under static 1–3 and dynamic compression, 4–7 and has been the subject of several computational works aimed at understanding the mechanisms of shock-induced reactivity. 8–15 The reaction onset in the solid phase is suggested to occur through an associative step involving the cooperation of neighboring molecules. Experimentally, this hypothesis is supported by the reaction kinetics becoming faster with increasing the pressure, 2,3 and by the role of small amounts of amines in sensitizing the crystal towards decomposition. 16–19 In fact, the presence of basic molecules implies the formation of aci-anions of nitromethane, which are the predicted moieties triggering the reactivity in neat compressed nitromethane. 3,12 When a chemical reaction is activated by the pressure in a molecular crystal, the first steps are expected to take place along the crystal coordinates where the intermolecular interactions are strongest. 20 However, hydrogen-bonded crystals make an exception to this very general rule. It has been shown in fact in several cases that the crystals where H bonds develop under pressure become chemically more stable 21–25 with respect to compression than other crystals with similar functional groups and intermolecular distances but where the H bonds are absent. Nitromethane belongs to the H-bonded class of solids, in fact structural studies showed that, upon compression, the interactions that build up in the crystal are driven by the strengthening of hydrogen bonds. 26 Between 1 and ∼10 GPa we assist to a progressive ordering of the molecules into an eclipsed conformation of the methyl groups and the formation of an extended H-bonded network, ending with the formation of ionic structures and with an irreversible chemical transformation taking place above 30 GPa. The reaction takes place through the formation of a disordered polymeric network, which decomposes on decompression giving N-methylformamide as the main product, with small amounts of H2 O and CO2 . 3 The formation of N-methylformamide, the simplest molecule containing a peptidic bond, is remarkable in the search for possible synthetic pathways for biomolecules from abiotic materials in the early Earth during astronomical impacts, where irradiation with UV and visible light may also have played a role. Elucidating the physical and chemical changes induced by pressure in crystalline nitromethane is then important to explain the

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role of H-bonding and the onset of chemistry in a model crystal, and to understand aspects of the detonation processes. Solid state reactivity can be efficiently induced photochemically, by exciting electronic transitions. The combination of photochemical effects and high pressure can give rise to selective new reactions, which have been investigated for several molecular systems, in the liquid or in the solid state. In particular, two-photon absorption was shown to be effective in activating the reactivity under pressure, 27,28 with the advantage of using visible light and, due to the small absorption cross section, of avoiding sample damage. Two-photon absorption creates a very small population of excited molecules, that is however able to initiate a reaction in a condensed phase. It has thus been possible to control and monitor a variety of chemical reactions including selective polymerizations 29,30 or the formation of molecular hydrogen. 31 Photochemical activation is also able to induce reactions at milder pressures than required in the purely pressure-induced chemistry, making it possible to envisage and design reaction paths compatible with industrial production. Moreover, photon-assistance allows to explore the reactivity at different pressures, i.e. at different densities, which in some cases, as in s-triazine, 32 brought to the formation of different products. To understand photochemistry in compressed solids, in addition to the knowledge of the crystal structure we need a precise characterisation of the pressure-evolution of the excited electronic states. The photochemistry of nitromethane has been extensively studied, mainly in the gas phase. Excitation of the lowest-energy singlet electronic transitions leads to photodissociation into CH3 and electronically excited or ground-state NO2 radicals and the subsequent formation of NO and O. 33–48 The formation of methylnitrite (CH3 ONO) and its following decomposition to CH3 O and NO, and then formaldehyde and HNO, has also been reported, 47–49 and a similar photochemistry has also been observed in the liquid phase. 50–52 In the solid state, the photolysis of nitromethane at low temperatures trapped in solid Ar matrix 53,54 or as a bulk neat crystal 55–58 yields several molecular products deriving from the recombination of the initially formed radicals, favored by the cage effect provided by the

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solid environment. In particular, relatively complex molecules were formed in the photolysis of nitromethane bulk crystals: nitrosoalkanes and organic nitrites up to C3 H7 NO and C3 H7 ONO, and species deriving from the association of two nitromethane molecules. 57 The effects of photochemical activation in crystalline nitromethane under high pressure were briefly investigated by static 3 and dynamic 7 compression experiments. In both cases, laser irradiation favored the reactivity. In particular under static conditions, irradiation at 458 nm lowered the pressure stability limit from ∼30 to 15-18 GPa. 3 Theoretical works have also addressed the involvement of excited electronic states in shock-induced reactivity. 9–11 In this work we investigated the photochemistry of nitromethane under irradiation with different visible and near-UV lines at different pressures up to 28 GPa, and we characterized for the first time the near-UV absorption spectrum of the nitromethane crystal under static pressure, up to the spontaneous decomposition threshold (32 GPa). The separation of the pressure and photochemical effects is mandatory to get insight into the reaction mechanism, and to improve our understanding of natural and laboratory synthetic processes. These measurements elucidate the role of the lowest excited electronic states and the role of density in tuning the formation of different reaction products, in particular N-methylformamide, ammonium carbonate, and small gaseous compounds.

Methods Nitromethane was purchased from Aldrich (≥99.0 %) and used without further purification. The liquid sample was loaded into a membrane diamond anvil cell (DAC) equipped with ultra-low fluorescence synthetic IIa type diamonds. Rhenium gasket were used for all the samples. For the studies of reactivity the gaskets were indented to a thickness of 50 - 80 µm. For the measurement of the UV-vis absorption spectrum we either used a gasket thickness of 30 µm or we deposited liquid nitromethane on a KBr pellet previously scratched. In the latter case the thickness of the nitromethane film, around 15 µm, was estimated by the IR

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absorbance in the overtones region (4000 - 5000 cm−1 ). The initial sample diameter was 150 µm. The pressure was measured by the ruby fluorescence method, using as excitation source a laser diode at 532 nm (Thorlabs CPS532) attenuated to around 2 mW. The near UV - visible spectra were measured with a spectrometer expressely designed for UV-vis absorption measurements in the DAC. The source is a Xenon lamp (Hamamatsu L10725), attenuated by UV fused silica neutral density filters with varying optical densities, and filtered by a shortpass filter with cutoff wavelength at 450 nm to reject light out of the desired spectral range. The light is focused onto the sample and collected by a couple of Al coated 90◦ off-axis parabolic mirrors (with reflected focal lens of 50.8 mm), and then focused by an Al coated 90◦ off-axis parabolic mirrors into a 1/8m monochromator (Newport 77250), with F/number = F/3.7. The light is dispersed by a ruled grating (Newport 77304) with 600 lines/mm blazed at 200 nm, and collected on a CCD (Hamamatsu S9971-1006UV). The resulting spectral resolution is 3 nm. The infrared spectra were measured in the mid-IR region 600-5000 cm−1 with a spectral resolution of 1 cm−1 with a Bruker IFS-120HR FTIR spectrometer modified to allow in situ measurements in the DAC. 59 The FTIR spectra were measured just after loading the cell to check the initial sample purity, during compression and decompression, and before and after each irradiation cycle to observe the presence, identity, and amount of reaction products. Different Ar+ laser lines (514.5, 457.9, and 350 nm) were used to induce the photochemical reaction. The beam was focused with a UVFS lens with focal length of 200 mm, the sample was placed at a distance of ∼190 mm allowing the full and nearly homogeneous irradiation of the whole sample. The values of laser power indicated in the text for the irradiation of the sample, refer to the power measured just after the focussing lens.

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Results Electronic absorption spectra The near UV spectrum of nitromethane has been characterized in the gas phase and consists of a band centered at 270 nm ( = 10 M−1 cm−1 ) and a much stronger band centered at 198 nm ( =5000 M−1 cm−1 ). 41 The higher energy transition is assigned to a π → π ∗ involving the NO2 moiety, while the lower-energy one has been assigned to a σ(CN) → π ∗ (N O) 42 or to a n→ π ∗ (N O) 60 involving a nonbonding electron located on an O atom. In fact, both transitions are located in the same spectral region. 40 We measured near UV- visible spectra on several samples of different thickness. In Fig. 1 we show some of the spectra recorded in two different compression experiments, on a pure sample with initial thickness of 23 µm (up to 15 GPa), and on nitromethane deposited on a KBr pellet with ∼10 µm thickness (up to 32 GPa). As shown in Figure 2, the experimental spectra were fit with two bands, i.e. the minimum number of Voigt profiles able to accurately reproduce them, and their evolution was monitored with increasing the pressure. In the following, the transition observed at 270 nm is referred to as a σ → π ∗ , although this assignment is not definitive. 43 Three spinforbidden transitions to a 3 ππ ∗ , a 3 σπ ∗ , and a 3 nπ ∗ contribute to the second, weaker band, observed at ∼325 nm. 40 Upon solidification (∼1 GPa) we observe a marked blue-shift of the spectrum. With further compression, as can be seen in Fig. 2c, the maximum of the σ → π ∗ transition red-shifts by an amount barely exceeding the measurement uncertainties from the solidification up to ∼8 GPa . The red shift is more pronounced above 8 GPa, up to the maximum pressure probed, above which the sample reacts. Overall, from solidification to the pressure-induced reaction threshold, the red shift amounts to 12 nm, i.e. 0.4 nm/GPa (with no shift between 0 and 8 GPa and a 0.5 nm/GPa shift in the 8 - 32 GPa pressure range). The second peak maximum (singlet-triplet transitions) undergoes a continuous blue shift going from the liquid phase to the crystal and up to ∼5 GPa, and a red shift at higher pressures. During the compression

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3 3 2 .0 2 6 .4 1 5 .8 1 1 .0 7 .2 G 1 .0 G 1 .0 G 0 .1 G

2

A

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G P a G P a G P a G P a P a P a P a ( liq ) P a ( liq )

1

0 2 5 0

3 0 0

3 5 0 w a v e le n g th ( n m )

4 0 0

4 5 0

Figure 1: Selected near UV - visible spectra of nitromethane measured during compression. The spectra at 26.4 and 32.0 GPa have been measured on a thinner sample - a nitromethane layer deposited on a KBr pellet- and have been multiplied by a factor 2 to account for the different thickness of the two samples. To obtain this factor, we superimposed two spectra measured at the same pressure (15.7 GPa) on the two samples. we also observe an intensification of the spectrum, which becomes saturated at λ ≤ 270 nm above ∼15 GPa, likely because of a red-shift or broadening of the higher-energy π → π ∗ absorption band. This band is outside our spectral range but is much stronger than the σ → π ∗ and even a relatively small shift can affect our spectra. For this reason the peak wavelengths reported in Fig. 2c might be slightly overestimated above 12-15 GPa. All the spectral changes observed are perfectly reversible with decreasing pressure for the sample compressed up to 15 GPa. The sample compressed at 32 GPa partially reacted at the highest pressure, as observed from the FTIR spectrum and in agreement with previous reports, 3 and its electronic spectrum was not measured on decompression.

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3 4 0

2 .0 1 5 .8 G P a

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3 2 0

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2 6 0 a

0 .0

2 5 0

3 0 0 3 5 0 w a v e le n g th ( n m )

c

0

4 0 0

5

1 0

1 5

2 0

2 5

3 0

3 5

P (G P a )

Figure 2: Left panels: Examples of fit of the spectrum performed using Voigt profiles, for the liquid (a) and for the solid (b). The wavelength values of the peak maxima obtained from the fit are reported as a function of the pressure in panel (c). Full square symbols (green) are from the UV spectrum of liquid nitromethane in a quartz cuvette 0.1 mm thick at room conditions, measured with a Perkin Elmer spectrophotometer. Empty circles are from the ∼20 µm thick sample compressed to 15 GPa. Empty triangles are from the ∼10 µm thick layer on KBr, compressed to 32 GPa. The error bars represent the standard deviation from the average for the peak maximum value, obtained by fitting procedures using different baselines and different cut-off wavelength on the blue side of the spectrum.

Photochemical reactions The photochemical reactivity was probed at different pressures with the 514.5 nm and the 457.9 nm lines, and the UV multiline centered at 350 nm of an Ar+ laser (see Methods section). The purpose of this investigation is twofold. First, by inducing the reactivity at different pressures, we can investigate the possible relationship between the density of the starting crystal and the reaction evolution and products. Second, by characterizing the chemical stability under irradiation with different wavelengths, we have an independent way to get insight into the location of the absorption edge.

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1 5 .3 G P a , 5 1 4 n m , 5 3 0 m W

2 4 h 1 8 h 3 h 0 h

A

2

1

a 0

b

2 8 .3 G P a 5 1 4 n m 2 6 0 m W 2

A

3 h 0 h 0

c

5 .0 G P a 3 5 0 n m 2

1 h , 5 0 m W 1 h , 2 5 m W 0 h

A

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0 5 0 0

1 0 0 0

1 5 0 0

2 5 0 0

3 0 0 0

fre q u e n c y (c m

3 5 0 0 -1

4 0 0 0

4 5 0 0

5 0 0 0

)

Figure 3: FTIR spectra measured during some of the photoinduced reactions. The labels report the hours of irradiation corresponding to each spectrum. a) Slow reaction induced at 15.3 GPa by irradiation with the 514.5 nm line (530 mW). b) Fast reaction induced at 28.3 GPa by irradiation with the 514.5 nm line (260 mW). c) Reaction induced at 5.0 GPa by irradiation at 350 nm, using different laser powers (25 and 50 mW) showing the strongly nonlinear dependence of the process on the photon flux.

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1 0 0

2 8 .3 G P a

5 .0 G P a

1 5 .3 G P a

1 .2 G P a

3 5 0 n m 5 0 m W

5 1 4 n m 2 6 0 m W

8 0

r e a c te d s a m p le ( % )

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6 0 3 5 0 n m 1 0 0 m W

4 5 8 n m 2 6 0 m W

4 0

5 1 4 n m 5 3 0 m W

2 0

b a

0 0

1

2

3

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1 0

2 0

c

3 0

0 .0

0 .5

1 .0

0

d 2

4

6

8

1 0

ir r a d ia tio n tim e ( h )

Figure 4: In each panel is reported the percentage amount of reacted nitromethane as a function of the irradiation time, as obtained by the fit of the combination bands of nitromethane in the region 4000-5000 cm−1 . (a) At 28.3 GPa, under irradiation at 514 nm, 260 mW. (b) At 15.3 GPa, under irradiation at 514 nm, 530 mW (green symbols) and at 458 nm, 260 mW (blue symbols). (c) At 5.0 GPa, under irradiation at 350 nm, 50 mW. (d) At 1.2 GPa, under irradiation at 350 nm, 100 mW. Each set of data was fit to a linear dependence (black lines). In (d) the fit was performed only up to 3h, because at longer irradiation times a saturation effect was observed (see text).

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Irradiation with the 514.5 nm line at 5.3 GPa (4h with 250 mW) and at 10.0 GPa (3h with 260 mW and 3 h with 520 mW) did not induce any reactivity. When the irradiation was performed at 15.3 GPa, for 3h with 260 mW, a very slow transformation was observed leading to the changes of the IR spectrum already observed and discussed in reference 3 : the growth of two very broad (∼ 600 cm−1 ) absorptions centered at 1300 and at 3200 cm−1 respectively, indicating the formation of a disordered compound. The irradiation was then continued for a total of 24h with 530 mW, regularly checking the FTIR spectrum as shown in Fig. 3a. The reactivity was also probed, with the same excitation wavelength, on a new sample at 28.3 GPa where it was found to be extremely fast: after 3h of irradiation with 260 mW the FTIR spectrum appeared completely saturated (Fig. 3b) . The photoinduced reactivity was probed at 15.5 GPa under irradiation with the 457.9 nm line (260 mW). The kinetics was intermediate between those shown above and the spectrum confirms that the product is the same as previously observed under similar conditions. 3 Table 1: Nitromethane consumption rate, k (h−1 mW−2 ) reaction conditions k (h−1 mW−2 )

28.3 GPa 15.3 GPa 15.5 GPa 12 GPa (ref 3 ) 5.0 GPa 1.2 GPa 514 nm 514 nm 458 nm 514 nm 350 nm 350 nm 3.44

0.011

0.251

0

284

24.2

The irradiation with the UV multiline at 350 nm was very effective in inducing a reaction at any pressure probed: in the liquid phase at 0.2 GPa and in the crystal at 1.2 and 5.0 GPa. The sample irradiated at P=5.0 GPa for 2 hours with 200 mW shows a FTIR spectrum completely saturated by the products absorptions. A second sample was used to explore the dependence of the amount of reacted sample as a function of the incident laser power. This is a crucial point to understand if the visible or near-UV light is absorbed by a one photon or a two-photon absorption process, and thus to identify which excited state is involved. As can be seen in Fig. 3c, the dependence is strongly nonlinear. After 1h of irradiation with 25 mW 12


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the amount of reacted nitromethane was less than 10 %, while after 1h of irradiation with 50 mW the amount of reacted sample was ∼ 70 %. Thus with double energy we have more than four times the amount of product. Even if the dependence of the nitromethane consumption rate on the laser power is halfway between quadratic and cubic, we can confidently conclude that two-photon absorption of the 350 nm laser line is responsible for photo-activation. In fact, while the two-photon absorption of light certainly follows a quadratic dependence on the laser power, the reactivity may show an autocatalytic behavior. The amount of reacted sample was estimated by fitting the combination bands of nitromethane in the spectral region 4000 - 5000 cm−1 , where there is no saturation (Figure 3). The fit was performed using the minimum number of Voigt profiles able to reproduce the overall spectrum in this region. For each experiment the total area was reported as a function of irradiation time as shown in Figure 4. During the time intervals between irradiation cycles, i.e. in the absence of laser light, the reaction did never proceed and this time is therefore not considered in the analysis. In each experiment, the percentage of reacted nitromethane grows linearly with irradiation time, except in the reaction performed at 1.2 GPa, with 100 mW at 350 nm (Fig. 4d) where we observe a saturation in the amount of reacted sample, due to the impossibility to distinguish the combination bands from the spectral background when the reacted fraction exceeds 80%. Each data set was then fit to a linear dependence (except where the saturation was observed). In Table 1 we report the slopes of the linear regressions, normalized by the square of the incident power to account for the quadratic dependence of the amount of absorbed light. From these values it is evident how the excitation wavelength has a dominant role, rather than pressure, in determining the consumption rate constant.

Identification of the products In the following we report the outcome of the photo-assisted reactions performed at the different pressures (1.2, 5.0, 15.0, and 28.0 GPa in the crystal phase, and at 0.2 GPa in the 13



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G P a P a P a P a

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b a

0 5 0 0

1 0 0 0

1 5 0 0

2 5 0 0

3 0 0 0

3 5 0 0

4 0 0 0 2 3 0 0

2 4 0 0

2 5 0 0

fre q u e n c y (c m -1 )

Figure 5: FTIR spectra measured during the decompression of the sample reacted at 15.3 GPa under irradiation with the 457.9 nm line. In the inset: zoom of the region of CO2 asymmetric stretching mode.

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2

a

A

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b

0

c

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0


3 0 0 0 -1

3 5 0 0

4 0 0 0

)

Figure 6: Spectra recovered at room conditions (open cell) from the reactions at 1.2 GPa (a) and 5 GPa (b), compared with the spectrum of ammonium carbonate from the NIST chemistry webbook (c).

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a

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b c d

1 e

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f

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0

5 0 0

1 0 0 0

1 5 0 0

2 5 0 0


3 0 0 0 -1

3 5 0 0

4 0 0 0

)

Figure 7: Upper panel: FTIR spectra of the products of the reaction at 5 GPa (a) and of the reaction at 15 GPa (b) compared with the spectra of N-methyl formamide (c) measured in a DAC as a thin layer on KBr 3 and of nitromethane (d). Middle panel: FTIR spectra of the volatile fraction obtained as difference between closed cell and opened cell spectra, from the reactions at 1.2 GPa (e) and 5 GPa (f), compared to the spectrum of N-methyl formamide (c) measured in a DAC as a thin layer on KBr. 3 Lower panel: FTIR spectrum of the product obtained from the reaction in the liquid phase at 0.2 GPa (g). The bars indicate the frequencies of the identified or proposed reaction products. Dark gray: CO2 . Cyan: N2 O. Red: Methylformate (HCOOCH3 ). Green: HNO. Light gray: HNCO. Yellow: formaldehyde (H2 CO). Orange: methyl nitrite (CH3 ONO). Violet: H2 O.

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liquid phase). In all the experiments, upon decompression we observe a gradual decrease of the overall MIR absorbance, as shown in Fig. 5. At the cell opening, the entire sample, or a part of it, is lost thus revealing the presence of solid and volatile products. All the spectra have been measured in the DAC, including those after cell opening, thus the region 1900-2200 cm−1 is not accessible due to the diamond second order phonon absorption.

Solid products A solid product was recovered, after opening the cell, from the crystal-phase reaction performed at the lowest pressures (1.2 GPa). As shown in Fig. 6, this spectrum (trace a) matches almost perfectly the spectrum of ammonium carbonate (trace c), with some H2 O likely contributing to the absorptions at 1640 and above 3200 cm−1 . The same product was also obtained, but in a smaller quantity, from the reactions performed at 5.0 GPa (trace b in Fig. 6). No trace of this product was found in the reactions performed at higher pressures. However, in all the spectra measured after the photochemical reaction (including the one performed in the liquid phase), a broad spectral background is observable, which includes two broad absorptions centered at 1470 and 3120 cm−1 (FWHM 500 and 900 cm−1 respectively) with some more structured peaks superimposed (upper panel of Fig. 7). These broad features could be due to the superposition of the absorptions of many volatile compounds, but a small solid component cannot be exculded even if it is lost at the cell opening.

Volatile products The volatile fraction of the product is best investigated by subtracting the spectrum of the solid part, measured after cell opening, to the spectrum measured just before opening the cell. Two examples (products of the reactions at 1.2 and 5.0 GPa) are shown in Figure 7, middle panel. For the reaction performed in the liquid phase, all the products were lost at cell opening, and we analysed the FTIR spectrum measured on the closed cell (bottom panel of Figure 7). The products identified are listed in the following.

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• CO2 forms in all experiment during decompression below ∼ 2 GPa (Figure 5). Its absorptions at 665 and 2350 cm−1 become saturated in most experiments. CO2 is also responsible for two sharp bands at 3600 and 3705 cm−1 , due to the combination modes ν3 +ν1 , ν3 +2ν2 , split by Fermi resonance. • N2 O is easily recognizable by its strongest bands at 2220 and 1285 cm−1 which also appear clearly at the end of decompression. The former is in a region partly masked by the diamond phonon, whereas the latter is a sharp band that is in a spectral region mostly free from other sample’s absorptions (in Fig. 5 it appears as a sharp shoulder of the residual ν8 band of nitromethane). Sharp absorption bands of N2 O are also detected at 584 and 3488 cm−1 . • Methyl formate, HCOOCH3 is only observed in the product of the reaction performed in the liquid phase, together with the bands of CO2 and N2 O. This compound is recognizable from the intense C=O stretching band at 1730 cm−1 , the two sharp absorptions at 1164 and 1212 cm−1 and several weaker absorptions (Fig. 7). This molecule is completely absent in the products obtained in the crystal phase, as they all lack any band above 1700 cm−1 . • Additional absorptions can be observed, giving rise to a broad background and to sharp structured absorptions superimposed. The products expected by photolysis of nitromethane in Ar matrix 54 include HNO, HNCO, H2 CO (formaldehyde), and CH3 ONO (methyl nitrite). The presence of all these products is consistent, as for frequency values and relative intensities, with the spectrum reported in Fig. 7, trace e. • Residual nitromethane can also contribute the spectrum with the absorptions at ∼1400 cm−1 (ν8 , ν11 + ν14 , ν7 ) and 1550 cm−1 (ν11 + ν13 , ν4 , ν11 + ν12 ). • N-methylformamide, the main product recognized in the purely pressure-induced reac-

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tion, 3 is present in our products (middle and upper panel of Fig. 7). It is interesting to notice that the relative amount of this compound increases with the reaction pressure (see traces a and b in Fig. 7), while the amount of CO2 and N2 O decreases. Observed with an optical microscope, all the products are transparent. In the lowest pressure experiments (liquid, and solid at 1 GPa) before opening the cell we could observe small dots in the sample likely due to gas bubbles. At cell opening everything was lost in the majority of cases. Where non volatile products were recovered (mostly ammonium oxalate) these were only few drops forming transparent films on the diamond surfaces.

Discussion The near-UV spectrum of crystalline nitromethane has been investigated in this work for the first time up to the spontaneous decomposition pressure. The experimental determination of the pressure behavior of the lowest excited electronic states is per se a relevant result for this model molecular system. Nitromethane has in fact been the subject of many theoretical studies addressing the lowest excited electronic states under shock 9–14 or static pressure, 61,62 and the results can be compared with our experimental findings. The computations locate the band gap in nitromethane crystal at 3.28 eV (378 nm) 10 or 3.61 eV (344 nm) 61 at room pressure. Upon compression to 20 GPa, the predicted red-shift amounts to 0.4 eV (2.6 nm/GPa) 10 or 0.23 eV (1.1 nm/GPa), 11,61 much larger values than those observed in this work. Two experimental works have previously reported the near-UV spectrum of solid nitromethane under shock. 6,16 The spectral range was limited to 300 - 500 nm, and the sample thickness was tens of microns. Due to spectral saturation, increasing with compression, the authors had only access to red edge of the σ → π ∗ transition, thus overestimating (1.8 nm/GPa) its red-shift. In this work the absorption could be studied avoiding saturation (Fig. 1), The band assigned to the σ → π ∗ (peaked at 270 nm in the liquid and at 262 nm in the crystal) red-shifts by 0.4 nm/GPa in the range 0-32 GPa. The higher-energy π → π ∗ 19



transition, 36 peaked at 193 nm in the liquid, contributes to the progressive intensification in the blue region of our spectra with increasing the pressure, particularly above 12-15 GPa. This intensification could in fact be due to either a marked red-shift of the π → π ∗ transition, or just to its intensification and broadening. The former hypothesis is supported by the reaction rate data as a function of pressure and wavelength, as discussed later. The π → π ∗ and the σ → π ∗ transitions thus show a quite different pressure-shift, the former moving faster to lower energies and the latter being almost insensitive to the increasing density. In terms of molecular orbitals, this behaviour can be described as a progressive destabilization of the π orbitals, delocalised but mostly residing on the NO2 moiety, while the σ orbitals are less affected by compression of the crystal. This is consistent with the H-bonded structure of the nitromethane crystal that stronlgy involves the nitro group. In fact, by closer examination of our spectra we can observe that the σ → π ∗ transition is almost insensitive to the compression from solidification up to ∼8 GPa (Fig. 2c). In this pressure range the crystal structure of nitromethane undergoes a continuous change consisting of a progressive ordering of the methyl groups into an eclipsed conformation. 26 During this structural transformation (1 - 10 GPa), the intermolecular distances change rapidly with compression: the nearest neighbor O · · · H distances decrease from 2.8 to 2.3 ˚ A, and this should remarkably affect the orbitals located on the nitro group. On the other hand, these interactions are not strong enough to affect the σ orbitals. This observation also supports the assignment adopted for the observed transition, insensitive to compression in this pressure range, to a σ → π ∗ rather than to a n → π ∗ . The σ → π ∗ orbitals start being affected by compression above 8 GPa (Fig. 2c), when the blocking of the methyl groups in the eclipsed conformation is completely achieved. Remarkably, a sudden change in the vibrational signatures of the C-N stretching mode was also observed at the same pressure. 26 A different response to compression of molecular orbitals residing on different parts of the molecule was previously observed in the triazine 63 and pyridine 24 crystals. In triazine the H-bond is absent, and the n orbitals are insensitive to pressure up to the reaction threshold (8 GPa) while the higher-energy π → π ∗ shows a

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faster red-shift with pressure. As opposite, in pyridine crystals the increasing involvement in H bonding strongly stabilizes the n orbitals giving rise to an energy inversion of the n → π ∗ and π → π ∗ transition at the origin of the fluorescence emission at high pressure. The exact spectral shift of the π → π ∗ transition is not accessible in our study, but we can make qualitative estimations based on indirect data, i.e. the crystal photochemistry induced at the different pressures. According to the strongly non linear dependence of the consumption rate of nitromethane on the laser power, two-photon absorption is likely the process driving the photoinduced reactivity. The excited transitions are thus 257, 229, and 175 nm when exciting with the 514, 458, 350 nm lines respectively. The 350 nm line is therefore resonant with the π → π ∗ transition (centered at 198 nm in the liquid phase) at all pressures including room pressure, and is in fact able to induce an almost complete photochemical transformation at any pressure probed (P≥0.2 GPa). Similarly, if excitation of the σ → π ∗ transition would contribute to the reactivity, then the 514 nm line should trigger a reaction even at low pressure. Instead, with this excitation line we observe an appreciable reaction rate only above ∼15 GPa (see Table 1), when the edge of the π → π ∗ transition enters the spectral region around 260 nm, as seen by the marked increase in the absorbance in the near-UV spectra. The absence of reactivity through excitation of the σ → π ∗ is in agreement with gas 33,34,37,38,44,47 and liquid phase 52 experiments, which report a reduced quantum yield of photodissociation (27%) under excitation at 266 nm. Our impossibility to induce a reaction by exciting the σ → π ∗ transition can thus be related to the limited quantum yield of the photodissociation, but also a low two-photon absorption cross section could be responsible. There are no available data in the literature, to our knowledge, on the two-photon absorption cross sections of the two transitions to further prove this hypothesis. This study shows that the pressure has a minor effect on the nitromethane reactivity with respect to the excitation characteristics (wavelength and power), as far as the reaction rate is concerned. As can be seen in Table 1, for any given wavelength the nitromethane consump-

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tion rate per square laser power increases with the pressure, but the effect of wavelength is much more pronounced. For instance, the reaction performed at 28 GPa (close to the spontaneous reaction threshold) induced by the 514 nm line has a rate per square laser power 100 times slower than that of the reaction induced at 5 GPa by the 350 nm line. By photochemically inducing the chemical transformation at different pressures we are able to investigate the effect of density on the type and relative amounts of the products. When the reaction is performed in the solid state, the final products are very similar for all the pressures probed. However, as can be appreciated from Fig.7, the higher the pressure at which the reaction is induced, the larger is the amount of N-methylformamide obtained and the lower is the amount of the volatile products CO2 and N2 O. This behavior is consistent with what observed for the purely-pressure induced reaction, 3 performed at 32 or 35 GPa, where the main product was N-methylformamide, along with water and a small amount of CO2 . Ammonium carbonate is clearly detected in the product of the reaction at 1.2 GPa and in a smaller amount in the product of the reaction at 5.0 GPa. It is not possible to assess the relative amount of this salt formed in higher pressure conditions, because of its possible loss at cell opening, and its bands are too broad to be clearly detected in the spectrum measured before opening the cell. Interestingly, in the reaction performed in the liquid phase, and thus characterized by lower structural constrains and by longer free mean path for the radicals produced in the photodissociation events, we have observed products which are not detected in the reactions induced in the crystal phase, such as, for example, methyl formate. However, the quite large number of molecules likely involved in the reaction process makes any speculation about the reaction path extremely difficult.

Conclusions A systematic study of the high-pressure photoinduced reactivity of nitromethane, as a function of pressure and irradiation wavelength, allowed to gain insight into the electronic states

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involved in the process and into the role of density in selecting the products. Photodissociation from the ππ ∗ state is the triggering event for the reaction, whose evolution is however ruled by the density, in view of the different products obtained at the different pressure conditions explored. At low pressure, a larger degree of decomposition is observed, with the massive formation of CO2 , N2 O, and ammonium carbonate. On the contrary, at higher pressure a more extended product is formed, which decomposes during the decompression into more complex molecules. The higher is the pressure probed, the more is the reaction similar to the purely-pressure induced reaction, 3 where the main product is N-methylformamide. The formation of this prototype prebiotic molecule from a completely abiotic material thus needs a high static pressure, which appears to be a key requisite not replaceable with a different trigger such as photolysis. The measurement of the UV absorption spectrum as a function of pressure, besides the identification of the excited states involved in the reaction, allows to relate the electronic to the structural properties. In fact, the σ → π ∗ transition is unperturbed during the formation of a network of H-bonded molecules, characterized by the achievement of an ordered crystalline structure with the methyl groups blocked in the eclipsed conformation. 26 The achievement of this configuration, at ∼8-10 GPa, reflects in dramatic changes of the vibrational features related to the CN stretching mode 26 and coincides, as shown here, with a change in the pressure response of the σ → π ∗ transition energy. This also confirmed the adopted assignment of this transition, as the involved σ molecular orbital is mainly localised on the C-N bond and starts being affected by pressure only when the rigid H bond network is formed.

Acknowledgement The work was supported by the Deep Carbon Observatory initiative (Extreme Physics and Chemistry of Carbon: Forms, Transformations, and Movements in Planetary Interiors, from the Alfred P. Sloan Foundation), by MIUR (grant FIRB - Futuro in Ricerca 2010

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RBFR109ZHQ), and by Fondazione Cassa di Risparmio di Firenze under the project “Chimica Ultraveloce ad Altissima Pressione”.

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S2 S1 S0

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S2 S1 S0


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The Photochemistry of Crystalline Nitromethane under Static Pressure

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