High-Pressure Photoinduced Reactivity of CH3OH and CD3OH | The

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High-Pressure Photoinduced Reactivity of CH3OH and CD3OH Samuele Fanetti,† Matteo Ceppatelli,† Margherita Citroni,†,‡ and Roberto Bini*,†,‡ † ‡

LENS, European Laboratory for Nonlinear Spectroscopy, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy Dipartimento di Chimica dell’Universit
a degli Studi di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Firenze, Italy ABSTRACT: The room-temperature reactivity of liquid methanol induced by twophoton absorption of near UV photons (350 nm) was studied as a function of pressure. Different chemical reactions were triggered by the radical species produced through the population of the lowest electronic excited singlet state because of its dissociative character. Experiments were performed at room temperature between 0.1 and 1.8 GPa on CH3OH and between 0.2 and 1.5 GPa on CD3OH. Different irradiation cycles were performed at constant pressure conditions, and FTIR and Raman spectra were measured to monitor the reaction evolution. Methoxymethanol and methylformate were the main products and the only ones detected in all the experiments. Ethylene glycol formed only at low pressure (0.2
0.3 GPa), whereas small amounts of methane, water, and unsaturated (CdC) species were also detected independently of the reaction pressure. Only dissociation along the O
H and C
O coordinates was relevant in the investigated pressure range. Ethylene glycol, methoxymethanol, and methylformate derive from the dissociation channel involving the O
H bond cleavage, whereas methane and unsaturated species come from the dissociation along the C
O bond. The comparison of the results obtained for the two isotopomers at the different investigated pressures allowed the identification of three different reactive paths that, starting from the methoxy radical, lead to the formation of the main products. The important effect of pressure on the reaction evolution could suggest a modification of the potential energy surface of the lowest electronic excited state along the O
H coordinate on increasing pressure.

1. INTRODUCTION The combination of high-pressure and selective electronic excitation has been recently employed to trigger several chemical reactions in molecular fluids, crystals, and mixtures.1
3 The electronic and structural modifications characterizing the optically excited molecules enable these species to trigger a chemical reaction with the neighboring molecules by exploiting the high density conditions realized through the application of pressure. Examples concerning the synthesis of polymers,4,5 amorphous extended materials,6
9 and molecular products3,10,11 have been reported. Besides fundamental aspects, several other factors make this approach interesting for applicative purposes. Although many of the previously mentioned reactions also take place by applying only pressure, the threshold pressure for the chemical transformation can be considerably lowered through the combined use of pressure and resonant radiation with specific electronic transitions. It is remarkable that in all the cases the excitation is realized by two-photon (TP) absorption processes. The advantages are 2-fold: on one hand visible or near UV radiation can be used to reach the excited states of small simple molecules, often lying in the vacuum UV spectral region, and on the other hand the small cross section of TP transition allows a reduced concentration of excited species and then a certain control of the reactivity. Among these kinds of high-pressure reactions, a noteworthy class regards mixtures in which one of the molecular components, activated by the optical excitation, triggers a chemical reaction in a host material otherwise stable at the P
T conditions r 2011 American Chemical Society

of the experiment. Experiments performed both in water fluid mixtures and in clathrate hydrates of model molecules, like N2 and CO,12 and of small hydrocarbons, like acetylene, ethane, and propene,10 have demonstrated the efficiency of water molecules in acting both as reactant and as photoactivated high-pressure initiator. The dissociative character of its lowest electronic excited singlet state, reached by TP absorption of near UV photons, allows the production of hydroxyl radicals and hydrogen atoms that, exploiting the high density conditions attainable at high pressure, trigger a chemical reaction with the sorrounding molecules. A comparable energy and a similar dissociative character of the excited states characterizes also the simplest alcohols, methanol and ethanol.13,14 Experimental and theoretical evidence indicates that the lowest electronic excited state of methanol is dissociative along the O
H coordinate and weakly bonding along the C
O one.15 Photodissociation studies of jet-cooled ethanol indicate the split of the O
H bond as the primary dissociation channel.14 These features make these simple alcohols excellent candidates for observing photoinduced reactions at high pressure. The presence of alkyl radicals can indeed originate an interesting chemistry also in the pure systems. No such experimental or Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: June 14, 2011 Revised: September 7, 2011 Published: October 18, 2011 2108

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The Journal of Physical Chemistry C theoretical studies are available for methanol, whereas the photoinduced reactivity of pure liquid ethanol under high-pressure conditions has been recently reported.11 In the ethanol case, twophoton absorption of near UV radiation triggers a complex reactivity ascribable to the two previously described dissociation channels. The homolytic split of the O
H bond is confirmed as the main dissociation channel in all the investigated pressure range. The reaction exhibits a remarkable dependence on pressure likely due to the molecularity characterizing the different reactive paths and to the free mean path of the radicals, both processes being ruled by density. At pressures of a few MPa, molecular hydrogen is the main product, whereas other products become dominant with increasing pressure. Among these, ethane, 2-butanol, 2,3-butanediol, and 1,1-diethoxyethane derive from the ethoxide and ethyl radicals produced in the photodissociation of ethanol, whereas CH4, H2O, and CO2 have been identified as deriving by disproportion of ethanol and of the reaction products. The synthesis of a considerable amount of molecular hydrogen is particularly relevant also in view of the low pressure required, of the absence of any chemicals other than the reactant, and of the renewable character of the starting material with net zero carbon emission for H2 generation.16
18 Nevertheless, some important questions remained unanswered after the ethanol study. The first issue concerns the possible role of the photodissociation along the C
H coordinate, which is an important dissociation channel in the ground state,19 but negligibly contributes in the excited state of the isolated molecule.13 An increased importance of this process with rising pressure could not be ruled out a priori. A second issue is related to the formation of 2-butanol and 2,3-butanediol among the products observed in the photoinduced reaction of fluid ethanol. Their formation was explained on the basis of an intermediate step in which the radical is stabilized by the charge delocalization on the ethyl group,11 a process that should be less important in the methanol case. These are the main reasons that motivated an extensive study of the high-pressure photoinduced reactivity of methanol and one of its isotopomers (CD3OH) in the liquid phase.

2. EXPERIMENTAL SECTION Liquid methanol by Merck (99.9%) and CD3OH from SigmaAldrich (99.8% D) were loaded into a membrane diamond anvil cell (MDAC) by filling the gasket after cooling the cell in a nitrogen atmosphere. The cell was equipped with IIa type diamonds and a stainless steel gasket. The gaskets were indented to the desired thickness (40
70 μm) and then drilled to produce a 150 μm diameter hole. A small ruby chip was placed on a diamond for pressure calibration by the ruby fluorescence method. The reaction was photoinduced by performing irradiation cycles of different duration using the multiline emission of a cw Ar ion laser centered at 350 nm with a power of 400 mW effectively reaching the sample. Care was taken to homogeneously irradiate the whole sample. FTIR absorption measurements were performed with a Bruker-IFS 120 HR spectrometer modified for high-pressure measurements.20,21 The instrumental resolution was 1 cm
1. Raman spectra were measured in a backscattering geometry by using 10
40 mW of the 647.1 nm line of a Kr+ laser. The different regions of the sample could be probed with a spatial resolution better than 10 μm. The scattered light was dispersed by a single-stage monochromator (900 grooves/ mm) and analyzed by a CCD detector with a resulting instrumental resolution of 0.7 cm
1.

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Figure 1. Selected regions of the FTIR difference absorption spectra (see text) measured as a function of the irradiation time at 0.3 GPa on a sample of CH3OH having initial thickness of 70 μm.

3. RESULTS At ambient temperature, liquid methanol is chemically stable, solidifying at 3.5 GPa, even though the liquid can be easily supercompressed.22,23 Different freshly loaded samples of CH3OH and CD3OH were used in all the experiments. After the loading, the samples were compressed to the desired pressure, and both IR and Raman spectra were measured before starting the irradiation. We used the 350 nm laser line to irradiate the sample because the onset of the ambient pressure absorption spectrum of liquid methanol is around 7 eV (∼178 nm). This energy threshold is very similar to that of ethanol24 in which the reaction was triggered by using the same laser wavelength and power exceeding 100 mW.11 In all the irradiation cycles, we used an effective incident power of 400 mW on the sample, higher than in the ethanol case, because for methanol the reaction was extremely slow for lower laser power. Monitoring of the reaction by IR spectroscopy showed that the reaction proceeded only under irradiation, thus ruling out any chemical reaction in the absence of irradiation. 3.1. Methanol CH3OH. The photoinduced reactivity of CH3OH was studied at four different pressures: 0.1, 0.3, 1.2, and 1.8 GPa. A few hours of irradiation at 0.1 GPa produced the appearance of barely detectable new absorption bands around 900 and 1200 cm
1. Longer irradiation cycles were therefore performed on a new sample at slightly higher pressure (0.3 GPa). Since a considerable amount of unreacted methanol was present in the sample, the absorption spectrum was dominated by the methanol bands, making the detection of the weak product bands difficult. We therefore used difference spectra to identify the product bands. These spectra were obtained by subtracting from the spectrum recorded after each irradiation cycle the reference spectrum measured at the same pressure before irradiating the sample. These difference spectra are reported as a function of the irradiation time in Figure 1. The formation of several product bands in the same spectral regions as in the 0.1 GPa experiment was observed. The irradiation time was found to be extremely important, as the spectral signatures of the products became clearly visible only after 7 h of irradiation. Fluorescence heavily interfered with the measurement of Raman spectra. The fluorescence was likely originated by a thin product layer formed on the diamond surface in contact with the sample on the side where the laser was focused. 2109

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Figure 2. Selected regions of the FTIR difference absorption spectra (see text) measured as a function of irradiation time at 1.2 GPa on a sample of CH3OH having initial thickness of 37 μm.

To reduce the saturation of the IR absorption bands of methanol in the 1000
1100 cm
1 spectral region, the experiment at 1.2 GPa was performed on a sample of reduced thickness, 37 μm instead of 70 μm employed in both the lower-pressure experiments. The difference spectra measured in this experiment are reported as a function of the irradiation time in Figure 2. Despite the reduced sample thickness, the intensity of the product bands was higher, approximately by a factor of 2, than in the 0.3 GPa experiment, thus indicating an increased reactivity. In addition, some bands (866, 887, and 1091 cm
1) observed both at 0.1 and 0.3 GPa were absent at 1.2 GPa. By comparing Figures 1 and 2, it can be appreciated how the sample thickness reduction made it possible to get a nice quality of the difference spectra even between 1050 and 1100 cm
1. Also in this case, the Raman spectra presented a very strong fluorescence background allowing the detection of only one peak at 917 cm
1 ascribable to the products. Four irradiation cycles for a total of 23 h were performed at 1.8 GPa on a freshly loaded sample. The difference spectra are similar to those measured at 1.2 GPa except for a small blue shift of the peak maxima and a considerable weakening of all the absorption bands of the products that, assuming the absorption cross sections are unaffected by the small pressure change, indicate a reduction of the reactivity in going from 1.2 to 1.8 GPa . 3.1.1. Spectral Analysis and Product Identification. Voigt profiles have been used to fit the difference absorption spectra. This analysis provided the peak frequency and the integrated area of each product band. Among the possible compounds attainable from the photodissociation of the methanol molecule, we selected those with IR bands matching the frequencies and the relative intensities of the bands observed in our spectra. A first product was easily identified through the three bands at 866, 887, and 1091 cm
1, present in the two lower-pressure experiments and missing at higher pressure. These absorptions in fact nicely match the most intense low-frequency IR bands of ethylene glycol. Another product, methylformate, was identified by the IR absorption bands peaked at 911, 1163, 1212, and 1727 cm
1 (1.2 GPa values). This species also accounts for the 917 cm
1 Raman peak, the most intense Raman band of methylformate. Most of the remaining peaks coincide with those detected in a study about the dissociation of methanol induced by carbon dioxide laser radiation and assigned to methoxymethanol.25 This

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Figure 3. Comparison between the FTIR difference absorption spectrum measured on a methanol sample at 1.2 GPa after 37 h of irradiation (red trace) and that measured at the same pressure on a 5:5:1 (CH3OH/ H2O/CH2O) molar mixture to which the spectrum relative to a 1:1 (CH3OH/H2O) molar mixture was subtracted (blue trace). The spectrum of the products obtained in the photoinduced reaction of methanol has been multiplied by an arbitrary factor to fit the intensities of the reference mixture spectrum.

compound is the stable form of formaldehyde, the primary product of the ground state photoreaction, in methanol at ambient conditions.26 We purchased a liquid mixture of methanol, water, and formaldehyde, 10%, 53.5%, and 36.5% in weight, respectively (Riedel de H€aen), and we enriched it with methanol and water to obtain a 5:5:1 (CH3OH:H2O:CH2O) molar mixture, which is the concentration reported to be optimal for the methoxymethanol formation.26 The IR spectrum of the compressed mixture after subtraction of the 1:1 CH3OH/H2O spectrum is reported for comparison with that measured at the same pressure (1.2 GPa) in our photoinduced experiment (Figure 3). The excellent agreement allows the identification of methoxymethanol among the products. The frequencies of the bands detected in the different experiments and assigned to the mentioned products are reported in Table 1. Few other weak bands, not ascribable to these substances, were observed in the IR spectra. A band around 1600 cm
1, very weak in the experiments performed up to 1.2 GPa but quite strong in that at 1.8 GPa, can be due to the stretching of CdC bonds. These species are those presumably responsible for the sample fluorescence that limits the Raman data collection. The broad band around 1660 cm
1 is likely due to the bending mode of water. The weakness of this band is remarkable considering that in the ethanol case the amount of water produced in the reaction was significantly higher.11 Finally, the weak band in the CdO stretching region (1713 cm
1) is likely due to traces of small aldehydes. 3.2. Deuterated Methanol CD3OH. In the attempt to understand the high-pressure photoinduced reactivity mechanisms of methanol, we also investigated one of its deuterated isotopomers, CD3OH. Three experiments were performed at ambient temperature and 0.2, 1.0, and 1.5 GPa. At 0.2 GPa, the sample was irradiated for a total of 47 h in six different cycles. The difference spectra relative to this experiment are reported in Figure 4. All the product bands could be assigned by considering the deuterated compounds corresponding to the products identified in the fully hydrogenated sample: ethylene glycol, methoxymethanol, and methylformate. In addition, the identification of CD3H is worth 2110

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Table 1. Peak Frequencies (cm
1) of the Product Bands Obtained by the Analysis of the IR Spectra Measured in the Different Experiments and by Ambient Pressure Literature Data CH3OH 1 bar ethylene glycol

methoxy methanol

0.3 GPa

864a

866

a

882

1.2 GPa

1.8 GPa

1 bar

0.2 GPa

1.0 GPa

1.5 GPa

897a

908

887

946a

947

1084a

1091

1063a 1197a

1063 1196

925b

922

833c

831

834

836

1065

1109c

1096

1099

1097

1142c

1140

1140

1141

834e

822

824

826

1091e 1199e

1090 1200

1090 1195

1091 1198

b

1067

methyl formate

CD3OH

HO(CD2)2OH

925

927

1123b

1127

1125

1200b

1198

1199

1201

1303b

1305

1305

1305

910d

907

911

912

1158d 1208d

1156 1209

1163 1212

1212

1728d

1728

1727

1726

CD3OCD2OH

CD3OCDO

CD3H

1698e

1693

1693

1693

1740e

1737

1740

1739

1003 f

1002

1036 f

1034

1035

1031

a

Liquid data from ref 27. b Our data, see text. c Data obtained in an Ar matrix at 10 K.28 d Liquid data from ref 29. e Solution data from ref 30. f Gas data from ref 31.

Figure 4. Evolution of the FTIR difference absorption spectra measured at 0.2 GPa in CD3OH as a function of the irradiation time. The negative peak appearing at 887 cm
1 is relative to CD3OH and reflects the reactant consumption. The absorbance scale of the spectra reported in the two central panels is obtained by multiplying the scale on the left by the factors reported in each panel.

mentioning. In the fully hydrogenated methanol, the detection of methane through the strongest IR band at 1306 cm
1 (ν4 bending mode) was prevented by the presence of methoxymethanol which possesses a quite strong band peaked at 1303 cm
1 (ambient pressure value). The complete assignment is reported in Table 1. The relevant difference with the reaction induced in fully hydrogenated methanol is represented by the presence of a broad band centered at 2470 cm
1. This band is due to the O
D stretching mode, and it could be due to CD3OD or to deuterated water, even though water was barely detectable in the nondeuterated methanol. Some weak bands were also observed in the Raman spectrum: at 737 cm
1 a peak ascribable to ethylene glycol, one at 830 cm
1 due to methoxymethanol,

and one at 3612 cm
1 unambiguously assigned to the stretching of the HD molecule. This is the only experiment, considering both methanol isotopomers, where we detected molecular hydrogen. The same irradiation cycles, for a total of 47 h, were also performed at 1.0 and 1.5 GPa. The products observed in these cases were the same as those identified in the 0.2 GPa experiment, with the only exception of ethylene glycol that, as also observed in the fully hydrogenated sample, did not form at higher pressure. In addition, in analogy to the fully hydrogenated methanol, a dependence on pressure of the reactivity was observed. In fact, the amount of reacted methanol, and consequently of the products, remarkably increased in going from 0.2 to 1.0 GPa but decreased again on further compression. 2111

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Figure 5. Evolution with the irradiation time of the integrated area of some selected bands of the products at the three different pressures where the reaction in CH3OH was investigated. The bands of methoxymethanol and methylformate correspond to C
O
C and CdO stretching modes, respectively. The dotted lines represent the best linear fit of the absorption data, and the corresponding slopes are reported in Table 2.

Figure 6. Evolution with the irradiation time of the integrated area of some selected bands of the products and of the percentage of CD3OH consumed at the three different pressures where the reaction was investigated. The amount of deuterated methanol consumed is obtained by the ratio between the integrated area of the 887 cm
1 (CD3 rocking) measured after each irradiation cycle (At) and the reference value obtained before starting the irradiation (A0) as (A0
At)/A0. The bands of methoxymethanol and methylformate correspond to the same stretching modes reported in Figure 5. The dotted lines represent the best linear fit of the absorption data, and the corresponding slopes are reported in Table 2.

4. DATA ANALYSIS The evolution of the integrated area with the irradiation time was studied for several bands of the products. We reported some of these data in Figures 5 and 6 for CH3OH and CD3OH, respectively. The bands selected for methoxymethanol and methylformate were chosen because they correspond to pure stretching modes, CdO in methylformate and C
O
C in methoxymethanol,28 so that the effects of the isotopic substitution on the transition intensity can be reasonably neglected. In the case of deuterated methanol, we were also able to follow the intensity decrease of the methanol band at 887 cm
1 and therefore to calculate the percentage of reacted methanol with

irradiation time from the ratio between the integrated area after and before sample irradiation. This was not possible for fully hydrogenated methanol due to saturation of most of the bands or to their overlap with strong products bands. The formation rates of methoxymethanol and methylformate indicate an increase of the reactivity with pressure, reaching a maximum around 1.0 GPa in deuterated methanol and 1.2 GPa in fully hydrogenated methanol, and abruptly decreasing on further compression. The pressure values corresponding to the maximum reactivity are obviously only indicative because the reaction was studied only at three different pressures. On the contrary, the amount of deuterated methane (Figure 6) forming in the reaction is almost 2112

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Table 2. Slope Values Obtained by the Linear Fit of the Evolution with the Irradiation Time of the Integrated Area of the Bands Corresponding to the C
O
C Stretching of Methoxymethanol and to the CdO Stretching of Methylformate Reported in Figures 5 and 6 C
O
C str. methoxymethanol CH3OH (922 cm
1)

CdO str. methylformate

CD3OH (831 cm
1)

CH3OH (1728 cm
1)

CD3OH (1693 cm
1)

P (GPa)

dA/dt

P (GPa)

dA/dt

P (GPa)

dA/dt

P (GPa)

dA/dt

0.3

0.032

0.2

0.061

0.3

0.019

0.2

0.010

1.2

0.124

1.0

0.068

1.2

0.221

1.0

0.019

1.8

0.035

1.5

0.019

1.8

0.010

1.5

0.002

insensitive to pressure. Unsaturated species deserve a separate comment. Here the integrated area of the 1600 cm
1 absorption band (the data for hydrogenated methanol are reported in Figure 5) grows during the first irradiation hours to remain constant, or even decrease, for longer irradiation time. This behavior is likely ascribable to the photoinduced reaction of these species on further irradiating the sample. However, the most important observation regards the decrease of the amount of both methoxymethanol and methylformate going from hydrogenated to deuterated methanol. This is particularly relevant for methylformate, in which the depletion with deuteration was markedly pronounced. In the attempt to quantify these differences, we fitted the evolution with the irradiation time of the integrated absorbance of methoxymethanol and methylformate by a linear law. The slope values resulting from the fit are reported in Table 2.

5. DISCUSSION As already reported for ethanol,11 also fluid methanol reacts once irradiated at high pressure by near UV light (350 nm). Although some differences exist between the two photoinduced reactions, the trigger mechanism appears the same. The near UV light is absorbed through a two-photon process; in fact, the onset of the ambient pressure absorption spectrum of methanol, in the gas,32 liquid,24 and low-temperature solid32 phases, is around 7 eV (∼178 nm), with the first absorption band centered at about 8.2 eV (∼151 nm). This band has been assigned to the electronic transition from the 2p orbital localized on the oxygen atom to the 3s(OH) Rydberg state.24 Since the absorption cross sections in CH3OH and CD3OH perfectly match in this energy range,15 no differences are expected in the excitation mechanisms of the two isotopomers. This excited state is reported to be dissociative along the O
H coordinate and weakly bonding along the C
OH one.15 Dissociation along the O
H coordinate has been demonstrated to be the major channel following irradiation at 193 nm,13 but also the competing split of the C
O bond has been reported to contribute to the methanol photodissociation in condensed phases, at 175 nm in solid Ne and Ar matrices33 and at 157 nm in pure methanol at 90 K.34 As the excitation wavelength decreases, H elimination from the methyl group is reported to increase, likely because dissociation initiates in the higher energy and longer-lived 3p Rydberg state.35 Direct C
H bond cleavage has been demonstrated to occur in the ground state, in competition with the lower enthalpy C
O bond breaking, in infrared multiphoton dissociation of CH3OH and CD3OH.19 The radicals produced by the two-photon-induced dissociation can either trigger the reaction with neighboring molecules or

further rearrange to give unstable intermediates. As already anticipated, although ethanol and methanol exhibit strong similarities concerning the electronic properties, the dissociation mechanism, and the reaction evolution, some important differences should be remarked. Methanol is less reactive than ethanol. The incident power employed in methanol experiments was greater by a factor 1.5, corresponding to more than a factor 2 in the expected amount of products according to the quadratic dependence on the incident power expected for a dissociation process driven by TP absorption. Nevertheless, the products were barely detectable after a few hours of irradiation, requiring almost twice the amount of time to give absorptions comparable to those observed in ethanol.11 Another important difference between the two reactions is represented by the hydrogen formation, which was abundant at low pressure in the ethanol case, whereas it was never detected in methanol with the only exception of the experiment performed in CD3OH at 0.2 GPa in which the vibron band of HD was observed. Furthermore, long irradiation cycles caused in ethanol the decomposition of the products which instead was never observed in methanol. This is likely the reason why only very small amounts of water and CO2, the latter barely visible only in the 1.2 GPa experiment after 37 h of irradiation, were observed in the methanol case. Finally, an important difference between the two systems is represented by the product fluorescence that limits, in the methanol case, the Raman analysis. The fluorescence was likely due to the formation of unsaturated species, revealed by the CdC stretching mode observed at 1598 cm
1, which were not present in the ethanol case. These are also the only species decomposing for long irradiation times (see Figure 5). The three dissociation channels occurring in gaseous methanol are listed below13 _ þ H _ CH3 OH f CH3 O

ðΔH° ¼ 103:2 kcal=molÞ

ð1Þ

_ 3 þ OH _ CH3 OH f CH

ðΔH° ¼ 93:0 kcal=molÞ

ð2Þ

_ 2 OH þ H _ CH3 OH f CH

ðΔH° ¼ 98:2 kcal=molÞ

ð3Þ

Channel (1) has been found to be by far the main dissociation path (86%), whereas the competing source of H atoms, channel (3), is negligible especially for low incident intensity.13 Since we photodissociated the methanol molecules by TP processes, this was certainly our case. Although these mechanisms and their relative weight were determined for gaseous methanol, the very similar absorption spectra of gaseous and liquid methanol as well as the successful interpretation of the high-pressure reactivity of fluid ethanol on the gas data basis11 make us confident on their employment in the discussion of the present results. The contribution of 2113

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The Journal of Physical Chemistry C channel (3) can be confidently ruled out. In fact, among the detected products, ethylene glycol is the only one that could derive from the hydroxymethyl radical; however, in this case we should observe in deuterated methanol also the formation of D2, whereas only HD was detected, indirectly remarking the importance of channel (1). Methane and species containing CdC bonds were likely obtained through channel (2). Also in this case, the very limited amount of these products confirms the possibility to rely on the gas data also for the condensed phase. As a matter of fact, methane (CD3H) was clearly detected only in the deuterated sample through its strongest IR absorption band at 1034 cm
1 (0.2 GPa). Methane instead was not detected in CH3OH likely because the corresponding IR band at 1301 cm
1 overlapped with a band of the main reaction product methoxymethanol. It is interesting to note that the amount of CD3H was almost insensitive to pressure (Figure 6), in contrast to all the other products. Dissociation along channel (1) can account for all the main products observed: ethylene glycol, methoxymethanol, and methylformate. Ethylene glycol can form according to a two-step mechanism where, after the photodissociation, the rearrangement of the methoxy radical gives the hydroxymethyl radical. The barrier to this rearrangement has been calculated to be 36 kcal/mol.36 A similar process has been invoked also in the ethanol case to explain the formation of 2-butanol and 2,3butanediol and their progressive inhibition with rising pressure. The situation is analogous in methanol, where ethylene glycol was clearly detectable only at the lowest investigated pressures. This pressure dependence can be related both to an increase of the rearrangement barrier and to a shortening of the radical lifetime due to density that prevents the rearrangement. Methoxymethanol can form from the direct reaction of the methoxy radical with another methanol molecule. Nevertheless, both methoxymethanol and methylformate can also be produced from the methoxy radical according to a two-step mechanism in which the first one consists of hydrogen elimination to give formaldehyde which further reacts giving the two species. This process is reasonable because the barrier for the formaldehyde formation is about 20 kcal/mol,13,37,38 and our excitation energy exceeds the absorption onset of the lowest electronic excited state by approximately the same amount. The comparison of the reactivity data obtained on CH3OH and CD3OH can help to understand this point. Looking at the formation rate of methoxymethanol and methylformate at the different pressures (Table 2), a general reduction on going from hydrogenated to deuterated methanol can be observed. Nevertheless, this reduction is rather different for the two reaction products, not exceeding a factor 2 for methoxymethanol, whereas it is much larger and significantly increasing with pressure for methylformate (1 order of magnitude at 1.2 GPa). The reduction of the formation rate in the deuterated isotopomer indicates a kinetic isotopic effect (KIE) and therefore the involvement of a C
H bond split in the ratelimiting step of the reaction. This result could support the formation of methylformate through the rearrangement of the methoxy radical to formaldehyde, identifying this one as the ratelimiting step of the reaction because of the higher energetic barrier to be overcome in the deuterated species. The more limited effect on the methoxymethanol formation must be therefore ascribed to the fact that methoxymethanol mainly forms through the direct reaction of the methoxy radical with another methanol molecule, which is not affected by the isotopic substitution, whereas the reactive path having formaldehyde as an intermediate is a minor, but not negligible, contribution.

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However, these considerations are not able to explain why the formation of methoxymethanol and methylformate is strongly affected by pressure, presenting a maximum yield around 1 GPa and decreasing on further compression. It is interesting to note that this behavior is characteristic of the products ascribed to the dissociation channel (1), whereas the formation of methane and of unsaturated species, occurring through the C
O bond splitting channel, seems not to be affected by pressure. It is unlikely that geminal recombination could act differently on the proposed dissociation channels. Even structural aspects alone, like the continuous enforcement of the hydrogen bonds with rising pressure,39,40 cannot account for the quite selective pressure effects on the two dissociation channels nor for the presence of a maximum in the products yield. A modification with pressure of the electronic excited state could instead determine such effects by changing the dissociative or binding character along a specific coordinate. As the products deriving from channel (1) are concerned, the noticeable greater reactivity observed on rising pressure to about 1 GPa could be related to an increased densitydriven effectiveness of the methoxy radicals in inducing a chemical reaction. On further increasing pressure, the sudden and remarkable drop in the observed reactivity could be instead related to changes in the dissociation efficiency, due to an increased binding character of the excited state along the O
H coordinate.

6. CONCLUSION The reactivity of liquid methanol induced by two-photon absorption of near UV radiation was studied as a function of pressure between 0.1 and 1.8 GPa. Exploiting the dissociative character of the lowest electronic excited state to generate radicals and the high density conditions attainable at high pressure, a complex reactivity strongly dependent on pressure was observed. The parallel study of CH3OH and of one of its isotopomers CD3OH allowed the identification of the C
O and O
H bonds splitting as the only relevant dissociation channels. As already observed in the high-pressure photoinduced reaction of ethanol, dissociation along the O
H coordinate is by far the main channel. Among the identified products, ethylene glycol, obtained only at the lowest investigated pressure, methoxymethanol, and methyformate derive from the main dissociation channel, whereas methane and unsaturated hydrocarbons form from the splitting of the C
O bond. Ethylene glycol was not observed at high pressure because its formation requires a rearrangement of the methoxy radical to the hydroxymethyl radical, and as also observed in ethanol, such a mechanism is likely inhibited by pressure. The comparison between the results obtained on the hydrogenated and deuterated methanol suggests that methoxymethanol is mainly produced by the direct reaction of the methoxy radical with a neighbor methanol molecule. A secondary, less important, channel leading to the methoxymethanol formation involves the rearrangement to formaldehyde that also leads to the methylformate formation. By increasing pressure, all the reactions involving the methoxy radical were remarkably enhanced, likely due to a density effect which increases the probability of reactive events. On the other hand, the maximum yield of these products was reached around 1.0 GPa, indicating that density is not the only factor ruling the reactivity. The remarkable reactivity depletion observed at higher pressure for the products deriving from the dissociation along the O
H coordinate, but not for those coming from the C
O dissociation, could indicate that the dissociative character of the excited energy 2114

dx.doi.org/10.1021/jp205563d |J. Phys. Chem. C 2012, 116, 2108–2115

The Journal of Physical Chemistry C surface along the O
H coordinate is reduced at high pressure. These results remark the important role of pressure in modifying the electronic properties of simple model molecules. Further studies are mandatory to better elucidate this important topic.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +39-055-4572489 (4573079). Fax: +39-055-4572451. E-mail: roberto.bini@unifi.it.

’ ACKNOWLEDGMENT Supported by the European Union under Contract RII3CT2003-506350, given by the Italian Ministero dell’Universit
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