Proton Transfer Reactions between Methanol and Formic Acid

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Proton Transfer Reactions Between Methanol and Formic Acid Deposited on Free Ar Nanoparticles N

Andriy Pysanenko, Francisco Gamez, Karolina Farnikova, Eva Pluharova, and Michal Fárník J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b05372 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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The Journal of Physical Chemistry

Proton Transfer Reactions Between Methanol and Formic Acid Deposited on Free ArN Nanoparticles Andriy Pysanenko, Francisco Gámez, Karolína Fárníková, Eva Pluhaˇrová,∗ and Michal Fárník∗ J. Heyrovský Institute of Physical Chemistry, The Czech Academy of Sciences, Dolejškova 3, 182 23 Prague, Czech Republic E-mail: [email protected]; [email protected]

Phone: +0420 2 6605 3206. Fax: +420 2 8658 2307

∗

To whom correspondence should be addressed

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The Journal of Physical Chemistry 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

Abstract We have sequentially picked up two astrochemicaly relevant Brønsted acids (methanol and formic acid) on the surface of argon nanoparticles acting as a cold support. Photoionization and electron ionization yield (HCOOH)x H+ , (CH3 OH)x H+ as well as mixed protonated clusters. Experiments with perdeuterated methanol CD3 OD demonstrate notable proton transfer to formic acid acting as a proton acceptor in addition to the proton transfer from formic acid which is, perhaps, a more intuitive one. We, therefore, for the first time observed reactions between two different complex molecules adsorbed individually on argon nanoparticles. The experimental results are compared with state-of-the-art quantum chemistry calculations showing that both CH3 OH·+ and HCOOH·+ radical-cations resulting from ionization can act as efficient proton donors and neutral CH3 OH and HCOOH as proton acceptors. According to the theoretical calculations, the C-H bond cleavage in the radical-cation should be more favorable than the O-H bond cleavage. Both channels are observed and distinguished in the experiments with CD3 OH and CH3 OD. Our detailed mechanism of formation of the CH3 O· and CH2 OH· radicals contributes to understanding of astrochemistry in the protostellar medium.

Introduction About 10 % of the matter in galaxies is constituted by interstellar medium which represents seemingly hostile environment for chemistry due to generally low temperatures and low molecular densities. 1,2 The average density of molecules in a Giant Molecular Cloud is about ∼103 cm−3 and it can increase up to ∼106 cm−3 in the densest part. Nevertheless, such densities correspond to very low collision rates of ∼10−3 −10−6 s−1 , i.e. about a collision per hours or even 100 days. Due to the cold environment (10-100 K), colliding molecules usually do not carry enough energy to overcome barriers for reactions between neutral molecules. Therefore, the gas-phase chemistry in interstellar medium proceeds mostly via ion-molecule

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reactions which are governed by attractive potentials with only centrifugal barriers. 3 Presence of a variety of complex compounds could not be solely explained by such homogeneous reaction mechanisms. Instead, the chemical synthesis of larger molecules from simple precursors is facilitated by surfaces of dust and ice grains where these species reside for a very long time due to the low temperatures. 1,4 Eventually, some reactions between neighboring molecules on the surface can be triggered by an incoming radiation. As model systems for particles in the interstellar medium we use clusters in molecular beams in vacuum which are often frozen solids (e.g., ArN 5–7 or (H2 O)N ice nanoparticles 8,9 ). Different molecules can be deposited on these particles via pickup processes as the nanoparticles fly in a molecular beam in vacuum. 10,11 Reactions between adsorbed molecules can be triggered by electrons or UV-photons and the ionized products can be detected and analyzed via mass spectrometry and/or various spectroscopic techniques. 11–13 Note that, even though large helium nanodroplets (HeN with N ≥ 103 ) are also a popular choice for studying ionmolecule reactions upon ionization inside the nanodroplets 14,15 or reactions between neutral species, 16,17 they are less suitable proxies for interstellar particles because of their superfluid character and their tendency to attract embedded species towards their center. Coagulation of molecules of one kind and reactions between them upon cluster ionization were investigated on large argon and water 18–21 or ammonia 22 nanoparticles. However, to the best of our knowledge, no reactions between two different molecules adsorbed individually in subsequent pickup processes on such clusters have been reported. We aim at the first investigation of reactions between two different complex species adsorbed on ArN nanoparticles. We selected methanol and formic acid because of their abundances in the interstellar space. 23 Different reactions involving these species as well as their protonated forms can occur 23,24 and they are still being investigated. Recently, formation of their aduct or methyl formate were characterized and suggested to be one of the potential routes to this interstellar ester. 24 As prototypical representatives of different classes of organic Brønsted acids, methanol and formic acid serve as proxies for studying proton

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transfer (PT) which is a frequent reaction in astrochemistry. 25,26 Here we are interested in PT initiated by ionization by electrons and/or photons. It can not be a priori decided in which direction will the proton transfer occur. Moreover, it can originate from the cleavage of the O-H bond or the C-H bond as anticipated from previous studies of single-component clusters. 22,27,28 In order to reveal which of the above processes take place and a potential interplay between them, we collect mass spectra of clusters containing deuterated species and interpret the results with the help of ab initio calculations. The paper is organized as follows. We first describe the experimental methods and theoretical calculations. Next, we report experimental results for clusters constituted from molecules of one kind. Then we continue with mixed clusters and interpret the observations by ab initio calculations. We discuss possible proton transfer pathways and the interplay between the C–H and the O–H bond cleavage in the radical-cations. Finally, we offer concluding remarks.

Experimental and theoretical methods Experiment We investigated pick-up and subsequent ionization of methanol (CH3 OH, CD3 OD, CH3 OD, CD3 OH) and formic acid (HCOOH) on argon clusters. The experiments were carried out on a versatile cluster beam (CLUB) apparatus that allows for different cluster experiments. 11 In the present experiments, ArN nanoparticles are generated by supersonic continuous expansion of argon at a stagnation pressure of P0 through a conical nozzle kept at a controlled temperature T0 = 213 K. Two different nozzles were exploited in the present experiments with a full opening angle of α=30◦ , length of 2 mm, and the nozzle throat diameters d0 = 55 µm and 100 µm. For large ArN clusters, the mean cluster size can be determined from the expansion conditions using Hagena’s scaling laws 29–31 modified by Buck. 32 The formula can be also found in our recent review. 11 Varying the nozzles and stagnation pressure P0 , the mean 4


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¯ = 90 and 440 was covered without any qualitative difference cluster size range between N between results of our experiments. Therefore, we present here only the results obtained for ¯ ≈ 270 that correspond to d0 = 55 µm, P0 = 6 bar, and ArN clusters of the mean size N T0 = 213 K. The large ArN clusters concerned here represent cold matrices with temperatures around 40 K, and are thus definitely frozen. 5–7 Therefore, we assume that the picked up molecules are adsorbed on the solid argon ice surface, however, we cannot exclude a possibility of the molecules submerging in the nanoparticle surface. Nevertheless, considering this possibility would not change our results and their discussion below. The cluster beam passed through a skimmer into a pickup chamber filled with the first pickup gas. This was either vapor of methanol (Lachner 99.99%) or formic acid (SigmaAldrich, ≥95%). The pressure in the pickup chamber was controlled with an ionization pressure gauge as reported previously. 33,34 For the experiments with deuterated methanol CD3 OD (Sigma-Aldrich, ≥99.8%), CD3 OH (99.8%) and CH3 OD (99.5%), the inlet line was flushed with heavy water and filled with the deuterated methanol for a long period (days) in order to suppress the H/D exchanges on the metal surfaces. This procedure was found previously to diminish the isotope exchange. 22 Then the clusters passed through another differentially pumped vacuum chamber which could be filled with the second pickup gas – the vapor of formic acid in this case. After the second pickup chamber, the clusters passed another differentially pumped chamber (velocity map imaging chamber – not used in the present experiment) and entered the reflectron time-of-flight (TOF) mass spectrometer chamber. Here, the clusters were ionized either by electrons (EI) or photons (PI). Our TOF mass spectrometer was described previously. 35,36 In the EI mode the clusters were ionized with 70 eV electrons from an electron gun with a frequency of 5 kHz. The 3 µs long ionization pulse was followed by 0.5 µs delay before the ions were extracted by 2 kV pulse into the time-of-flight region, and subsequently they were accelerated to 8 keV. After about 95 cm flight path, the ions were detected with a multichannel plate, and the mass spectra were recorded. In the PI mode, the clusters were

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ionized by 193 nm (6.4 eV) photons from a ArF excimer laser (Excistar XS500, Coherent) at 100 Hz frequency. The laser radiation with energy 2 mJ in a pulse of 5 ns duration was focused into the ionization region of the TOF with an f = 40 cm fused silica lens.

Theoretical calculations In order to provide molecular pictures for the mass spectra we computationally investigated the neutral and protonated clusters consisting of methanol, formic acid or both compounds and radicals and radical-cations of methanol and formic acid. Specifically, we studied HCOOH·+ and CH3 OH·+ radical cations, HCOO· , COOH· , CH3 O· , and CH2 OH· radicals, neutral closed-shell clusters (CH3 OH)X ·(HCOOH)Y , X + Y ≤ 3, protonated closed-shell clusters (CH3 OH)x ·(HCOOH)y H+ , x + y ≤ 3, and CH3 O− and HCOO− anions. We call the structures with x + y = 2 dimers and x + y = 3 trimers. The structures were optimized at the M06-2X/aug-cc-pVDZ level. 37 The initial geometries for optimization were obtained from molecular dynamics simulations described below. Frequency analysis was performed to confirm the local minimum. All reported energies include the zero-point energy correction. Relative energies of several isomers of selected clusters were recalculated using the CCSD(T)/aug-cc-pVTZ method. Table S1 in the Supporting Information (SI) shows generally a good agreement between the two methods. The ab initio calculations were performed using the Gaussian 09 program package. 38 The configurational space of the clusters was explored by the Born-Oppenheimer molecular dynamics in the CP2K program package. 39,40 The electronic structure of the system was treated by the revPBE 41 functional with the D3 dispersion correction. 42 The core electrons were described by the Goedecker-Tetter-Hutter norm-conserving pseudopotentials 43 and Kohn-Sham orbitals were expanded in a TZV2P MOLOPT and basis set. 44 A cutoff of 400 Ry was used for the auxiliary plane wave basis set. The temperature was maintained at 300 K by the CSVR thermostat. 45 For each kind of the dimer and trimer we collected at least one trajectory of the length of 20 ps with a time step of 0.5 fs. Snapshots were spaced by 1 ps 6


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and each structure was then optimized by the M06-2X method. The resulting energy span of the local minima was up to 10 - 20 kJ/mol depending on the kind of cluster. Later, we report only the lowest lying energy isomers, and we include also some less favorable structures that we found in the literature.

Results and discussion Our main goal is to investigate the ionization-initiated proton transfer between formic acid and methanol, in particular, in which direction it occurs and whether both C-H and O-H bonds can donate protons. As a reference, we start with the pickup and ionization of pure compounds separately and then we proceed to the mixed system. By means of theoretical calculations we characterize the structures of observed clusters and systematically compare reaction energies of potential proton transfer pathways. Finally, we follow selected routes experimentally using deuterated compounds.

Individual pickup of formic acid or methanol on ArN Methanol: First, we analyze the pickup and ionization of methanol on ArN . Figure 1 shows the mass spectra of ArN with adsorbed methanol after the electron ionization in the top panel, and photoionization in the bottom one. Both spectra are dominated by the protonated methanol progressions (CH3 OH)x H+ . The spectra demonstrate that methanol molecules adsorbed individually on ArN are mobile and coagulate to clusters with more than 10 constituents. This behavior on ArN was observed previously for many different molecules. 18–21 The ionization of the hydrogen bonded clusters then yields the protonated fragments and Ar atoms evaporate completely from the ion fragments. Chemical structures of neutral and protonated methanol monomer, dimer and trimer are depicted in Figure S1 in the SI. The molecules in the neutral structures tend to maximize the number of hydrogen bonds. However, protonated clusters possess the chain-like structures which is in accord with

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a previous study. 46 In addition to that, protonation shrinks the oxygen-oxygen distance. 1x106 (CH OH) H 3 x

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Figure 1: Mass spectra of methanol picked up on Ar clusters after EI (top) and PI (bottom). Protonated (CH3 OH)n H+ cluster series is labeled. An arrow points to a peak at m/z = 47 assigned to protonated dimethylether (CH3 )2 OH+ (see the text). The character of the spectra suggests that EI leads to somewhat larger cluster fragmentation than PI (this is similar for formic acid clusters below). It is worth noting, that this does not simply reflect a higher energy of the ionizing electrons (70 eV) compared to the photon energy (2×6.4 eV), since opposite behavior (i.e. larger fragmentation in PI compared to EI) was observed for clusters deposited on ice nanoparticles in some cases previously. 20,47 In the case of EI, which is a general ionization method relatively insensitive to the species, the more abundant nanoparticle constituents were ionized and the deposited cluster was ionized via a charge transfer. On the other hand, in PI the deposited molecules represented the chromophores and were ionized directly by the photons. Apparently, in the present case, the charge transfer from Ar+ generated by EI leads to a larger fragmentation of the deposited cluster than the PI. Besides the protonated series (CH3 OH)x H+ , there is a peak at m/z = 47 assigned to 8


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protonated dimethylether (CH3 )2 OH+ . This molecule can be generated by intracluster dehydration reaction already in protonated methanol dimer (CH3 OH)2 H+ → (CH3 )2 OH+ + H2 O, 48 or by ion-molecule reaction (CH3 OH)H+ + CH3 OH→ (CH3 )2 OH+ + H2 O. 49 Formic acid: Second, we present the mass spectra of formic acid adsorbed on ArN using EI and PI in the top and bottom panel of Figure 2, respectively. Once again, the spectra are dominated by the protonated series (HCOOH)x H+ and the PI spectra are less fragmented than the EI ones. The protonated formic acid can exist as four isomers (Figure 3), the three lower lying ones differ by the value of the H-C-O-H dihedral angle. A scan along this coordinate gives a barrier of about 60 kJ/mol for the transformation of the less favorable conformer. There are different opinions about the optimal structure of the protonated dimer. 50,51 Our calculations suggest that the chain-like structure is more stable than the structure with one strong and one weak hydrogen bond. Similarly to methanol, protonated formic acid trimer also prefers the chain-like structure over the structure with one additional hydrogen bond and a motif resembling the stable neutral carboxylic acid dimer. 3x105 (HCOOH) H

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Figure 2: Mass spectra of formic acid picked up on Ar clusters after EI (top) and PI (bottom). Protonated (HCOOH)n H+ cluster series is labeled. 9



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Another weaker series is present (especially in PI spectrum) shifted by m/z = 18 to higher values, assigned to the hydrated clusters (HCOOH)x ·(H2 O)H+ . These ions are due to the water contamination in the formic acid evaporated into the pickup chamber. However, the small amount of H2 O in our formic acid sample (≤2.5%) does not influence our results concerning the reactions between methanol and formic acid on ArN . Due to the general species-insensitive nature of ionization by electrons mentioned above, the EI spectra reflect the degree of water contamination better than the PI ones, where the hydrated (HCOOH)x ·(H2 O)H+ peaks are enhanced perhaps by a direct ionization of water in the two-photon processes. 52 a

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Figure 3: Structures of selected isomers of protonated formic acid clusters with relative energies. Monomers are displayed in the first row, dimers in the second row and trimers in the third row.

Reactions between methanol and formic acid on ArN Next, we want to initiate reactions between methanol and formic acid successively adsorbed on ArN in two separated pickup chambers. At first we use CH3 OH and HCOOH to simply demonstrate that dissimilar molecules are able to cluster on the ArN . From missing or newly

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appearing peaks in both PI and EI spectra we might also speculate about the preferred direction of the proton transfer. Figure 4 shows the mass spectra after EI (top) and PI (bottom) of ArN with both molecules. The spectra contain the same series of protonated fragment methanol (CH3 OH)x H+ (circles) and formic acid (HCOOH)x H+ (triangles) as in Figs. 1 and 2, respectively. 1.5x10

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Figure 4: The EI (top) and PI (bottom) mass spectra from sequential methanol and formic acid picked up on ArN . Series of protonated methanol (CH3 OH)x H+ (circles) and formic acid (HCOOH)x H+ (triangles) are labeled, analogical to the ones in Figs. 1 and 2. It is interesting to note that the pure (HCOOH)x H+ fragments are actually missing in the PI spectrum (within our signal-to-noise ratio), even though the (HCOOH)x neutral clusters are clearly present on ArN as proved by the EI spectrum. Photoionization of HCOOH shall take place at least to some extent, since the (HCOOH)x H+ ions are observed upon PI of (HCOOH)X ·ArN in Fig. 2. Therefore, we may ask, why are the (HCOOH)x H+ ions suppressed in the PI spectra? Preferential photoionization of methanol would yield CH3 OH·+ ions in the first place. The ratio of one-photon absorption cross sections at 193 nm is in fact larger for an iso11



lated methanol molecule compared to formic acid, σ(CH3 OH):σ(HCOOH) ≈ 4. 53,54 The final preference for the two-photon ionization of methanol could be even significantly higher than that inferred from the one-photon absorption cross sections. Higher probability of photoionization of methanol followed by exclusive proton transfer to another methanol molecule could, in principle, explain the lack of the (HCOOH)x H+ ions. Such justification is, however, not valid, because CH3 OH·+ is an efficient proton donor for both CH3 OH and HCOOH as will be shown later, and thus even an exclusive photoionization of methanol would yield also some (HCOOH)x H+ ions via the proton transfer. An alternative explanation is the preferential proton transfer from ionized formic acid to CH3 OH in comparison with HCOOH, i.e., HCOOH·+ + CH3 OH→ HCOO· + (CH3 OH)H+ . This is in accord with the fact that such reaction is more exothermic than the proton transfer to HCOOH (see Tab. 1 for the reaction energies between the most stable isomers). On the other hand, it is not obvious at this point why it should be preferred only upon PI compared to EI, since the ion-molecule reaction is essentially independent of the way the HCOOH·+ ion was generated. Unless different isomers of HCOOH·+ ions with different reactivities were formed upon PI and EI. Therefore we hypothesize that different isomers of HCOOH+ 2 (Fig. 3) accompanied by different reaction energies are generated in PI and EI processes. Proton transfer from HCOOH·+ in the arrangement resembling the lowest-lying (HCOOH)2 structure leads to the HCOOH+ 2 structure (Fig. 3 c). Such reaction is by 18 kJ/mol less exothermic than the optimal variant and by 25 kJ/mol less exothermic than the proton transfer to methanol. Addition of more energy during EI enhances populations of other intermolecular HCOOH arrangements which makes the formation of the most stable HCOOH+ 2 isomer (Fig. 3 a) feasible and, thus, brings the reaction energy much closer to the value for the proton transfer to methanol. See Fig. S4 in the SI for more details. This tentative explanation of the observed experimental fact that the (HCOOH)x H+ ion series is strongly suppressed in the photoionization spectra is, however, not the focus of our present study. We focus on the mixed species below.

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In addition to the single-component clusters, the mass spectra in Figure 4 contain mixed series constituted from both methanol and formic acid molecules that are not explicitly labeled in sake of clarity. Their relative intensities are displayed in Figure 5 as a function of the cluster size x. Open circles correspond to protonated methanol-formic acid cluster accompanied by x-methanol molecules (CH3 OH·HCOOH)(CH3 OH)x H+ , and closed circles correspond to the same mixed dimer accompanied by x-formic acid molecules (CH3 OH·HCOOH)(HCOOH)x H+ . Analogously, square symbols correspond to the cluster with two methanol and two formic acid molecules (CH3 OH·HCOOH)2 accompanied by either x-methanol molecules (open squares) or x-formic acid molecules (closed squares). Top and bottom panels of Figure 5 correspond to EI and PI, respectively. In the legend, M and FA stand for methanol and formic acid, respectively. Clearly, the methanol and formic acid molecules coagulate to relatively large mixed clusters on ArN nanoparticles and their ionization yields mixed protonated species. It is interesting to note that protonated fragments with one formic acid and many methanol molecules (CH3 OH·HCOOH)(CH3 OH)x H+ are more abundant than the fragments with one methanol and many formic acid molecules (CH3 OH·HCOOH)(HCOOH)x H+ . In the case of photoionization spectra, the later series extends only up to x = 3 formic acid molecules. This can point to a comparably lower loading of the ArN nanoparticles with formic acid molecules. Experimentally, we can determine the pickup gas pressures and the beam path lengths in the pickup chambers, however, there are many uncertainties involved in pickup cross sections and sticking probabilities of the molecules on nanoparticles and the beam scattering. Therefore we cannot reliably estimate the relative abundances of the adsorbed molecules on ArN . We have performed these experiments while changing pickup pressures of both molecules (within certain limits given by the cluster beam attenuation) and changing the pickup order, however, the typical results corresponded to the ones presented in Figures 4 and 5. The pickup pressures were adjusted so that the cluster beam attenuation corresponded to about one third with addition of each gas in the pickup cell, nevertheless, this did not guarantee

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the same amount of methanol and formic acid molecules picked up on the nanoparticles. However, the lower abundance of the clusters containing formic acid in the photoionization spectra bottom panel of Figure 5 is again consistent with our previous suggestion that the methanol molecules are ionized more efficiently by the photoionization process than the formic acid ones. (M FA)(M) H x

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Figure 5: Integrated peak intensities for series (CH3 OH·HCOOH)y (CH3 OH)x H+ and (CH3 OH·HCOOH)y (HCOOH)x H+ for y = 1, 2 as a function of x from methanol and formic acid picked up on ArN clusters. Top panel EI, bottom PI. M and FA stand for methanol and formic acid, respectively, in the legend.

The (CH3 OH·HCOOH)H+ protonated heterodimer has a nearly linear structure with the proton preferentially located at the methanol moiety (Figure 6). The two lowest-energy structures of (CH3 OH·HCOOH)(HCOOH)H+ are depicted in the middle panel of Figure 6. They again adopt the chain-like structure with the proton located on the central molecule. Interestingly, formic acid and methanol have nearly the same tendency to be the protonated species in the center of the cluster. The structures of (CH3 OH·HCOOH)(CH3 OH)H+ at the

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bottom of Figure 6 follow the same trend, i.e., almost linear arrangement with a very small preference for formic acid to be the central protonated molecule.

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Figure 6: Structures of protonated mixed clusters. Heterodimer is depicted in the first row, isomers of CH3 OH·HCOOH)(HCOOH)H+ and CH3 OH·HCOOH)(CH3 OH)H+ with relative energies in the second and third row. We have shown experimentally that subsequent pickup and ionizaton of formic acid and methanol leads to the formation of protonated clusters of pure compounds as well as their mixtures. The theoretical calculations suggest that independently on the composition, protonated clusters tend to form chain-like structure, whereas neutral structures prefer to maximize number of hydrogen bonds. However, whether the proton transfer reaction between different molecules takes place or not, can not be answered from the results presented so far. The (CH3 OH)x H+ and (HCOOH)x H+ series can be explained by proton transfer reactions between the molecules of the same kind. Even the mixed series (CH3 OH·HCOOH)y (CH3 OH)x H+ and (CH3 OH·HCOOH)y (HCOOH)x H+ could still be explained by the proton transfer reactions between the molecules of the same kind generating the protonated clusters of one kind, and the molecules of the other kind just sticking to 15



them. To get deeper insight into the occurring reactions, we perform additional experiments with isotopicaly labeled compounds.

Experiments with deuterated methanols The main purpose of the present experiment was to initiate reactions between methanol and formic acid on argon nanoparticles. In order to describe the possible pathways of a proton transfer reaction between methanol and formic acid, we perform the experiments with perdeuterated methanol CD3 OD. To further elucidate the exact origin of the transferred proton (the O-H or C-H bond), we also used CH3 OD and CD3 OH. The results are shown in Fig. 7. In the case of perdeuterated methanol CD3 OD, the spectra contained (HCOOH)x D+ ions in addition to the (HCOOH)x H+ series which is illustrated in the top panel of Fig. 7, where the region of protonated formic acid monomer, dimer and trimer are shown in detail. This is an unequivocal proof that we observe for the first time a reaction between different molecules adsorbed in the argon clusters. When the pressure of methanol was increased in the pickup chamber (black full line spectrum), the corresponding deuterated peaks (HCOOH)x D+ increase significantly as illustrated for , x = 1, 2 and 3 in Fig. 7 a), top panel. The deuterated formic acid peaks clearly originated from the deuteron transfer from methanol to formic acid. It is worth noting, that (CD3 OD)x H+ ions originating from the proton transfer from formic acid to methanol were observed in the spectra as well. As this is the less interesting reaction pathway (more intuitive acid→base proton transfer), it is documented by the corresponding experimental data only in the SI Figure S3. To assess the accessibility of possible proton transfer pathways, we calculated corresponding reaction energies (Table 1). We start with the intuitive cleavage of the O-H bond in the radical-cation and subsequent proton transfer to the neutral molecule. Such process is indeed exothermic for the reaction (3): CH3 OH·+ + HCOOH→ CH3 O· + (HCOOH)H+ (-35 kJ/mol) which is in line with the formation of (HCOOH)D+ . Both CH3 OH·+ and HCOOH·+

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CD OD 3

1x10

mbar

+

mbar

-4

3

2

(FA) H

(FA) H

+

1.0

+

+

3

1.5

-4

4x10

(FA) D

2

(FA) D

+

(FA)D

a)

(FA)H

Relative Intensity

+

2.0

0.5

0.0

Relative Intensity

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


CH OD

b)

3

1.0 CD OH 3

0.5

0.0 47

48

93

94

139

140

m/z

Figure 7: Background corrected EI spectra in the regions of protonated/deuterated formic acid monomer, dimer and trimer. The mass peaks in individual panels are normalized on (HCOOH)x H+ ions, x = 1, 2 and 3. Top panels (a) show the pickup of perdeuterated methanol CD3 OD. Black full line corresponds to the spectrum with increased methanol pressure in the pickup cell. Bottom panels (b) correspond to the pickup of CD3 OH (red dashed) and CH3 OD (green full line). radical-cations are efficient proton donors and neutral molecules proton acceptors as is indicated by the reaction energies (1 - 4); the most exothermic combination is the last one. Proton transfer between the closed-shell cation and the neutral molecule proceeds in the + direction from HCOOH+ 2 to CH3 OH2 (5), but it is much less exothermic than in case of

the radical-cation. Pronounced endothermicity of the reactions (6 - 9) shows that proton transfer between the neutral molecules is impossible. Isomers of the formic acid radical are depicted in the top part of Figure 8 (a,b,c). The structures resulting from the H+ abstraction from the C-H bond of the radical-cation (a, b) are more stable than the one resulting from the O-H bond cleavage (c) in agreement with previous studies. 55 The same trend is observed for the CH2 OH· vs. CH3 O· radicals. Proton transfer from the C-H group of the radical-cation is thermodynamicaly more favorable than from the O-H group as is illustrated by the reaction energies (10 - 13) that are analogous to (1 - 4). However, it is less likely for the reactants to adopt arrangement suitable for the C-H

17



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bond cleavage than for the O-H bond cleavage because of the less favorable intermolecular interactions, thus the dominant route does not have to be solely determined by the reaction thermodynamics. This is exemplified for the proton transfer from CH3 OH·+ to HCOOH in Fig. S5 in the SI. Table 1: Reaction energies for proton transfer between monomers of formic acid and methanol and their radical-cations. Only the lowest energy structures are considered. The zero-point vibrational energy correction is included. No. 1 2 3 4 5 6 7 8 9 10 11 12 13

Reaction CH3 OH·+ HCOOH·+ CH3 OH·+ HCOOH·+ HCOOH2 + CH3 OH HCOOH HCOOH CH3 OH CH3 OH·+ HCOOH·+ CH3 OH·+ HCOOH·+

+ + + + + + + + + + + + +

CH3 OH HCOOH HCOOH CH3 OH CH3 OH CH3 OH HCOOH CH3 OH HCOOH CH3 OH HCOOH HCOOH CH3 OH

a

→ → → → → → → → → → → → →

·

CH3 O + · HCOO + CH3 O· + · HCOO + HCOOH + CH3 O− + − HCOO + HCOO− + CH3 O− + · CH2 OH + COOH· + · CH2 OH + COOH· +

b

0 kJ/mol

d

+

CH3 OH2 HCOOH2 + HCOOH2 + CH3 OH2 + CH3 OH2 + CH3 OH2 + HCOOH2 + CH3 OH2 + HCOOH2 + CH3 OH2 + HCOOH2 + HCOOH2 + CH3 OH2 +

Energy (kJ/mol) -43 -46 -35 -53 -7 846 695 688 853 -77 -112 -70 -119

c

6 kJ/mol

66 kJ/mol

e

0 kJ/mol

35 kJ/mol

Figure 8: Structures of the isomers of formic acid (top) and methanol (bottom) radicals with their relative energies. In order to follow the interplay between the possible proton transfer from the methyl or the hydroxyl group experimentally, we performed measurements also with the CD3 OH and CH3 OD molecules. The results are shown in the bottom panels of Fig. 7 (b). The 18


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deuterated peaks (HCOOH)x D+ , x = 1 − 3, occur in both cases and we have also checked that their intensity increases with increasing pressure of the deuterated methanols. Clearly, the deuteron transfer from both groups CD3 and OD occurs, with the hydroxyl group being perhaps slightly more favorable. However, one has to bear in mind that it is notoriously difficult to assure exactly the same conditions in the different pickup experiments. Nevertheless, the deuteron/proton transfer from both methyl and hydroxyl groups of methanol is an unambiguous experimental observation supported by the quantum chemical calculations. This is an interesting observation, since our recent experiments with ionization of methanol adsorbed on ammonia clusters 22 did not show any evidence for the proton transfer from CH3 group of methanol. In the methanol-ammonia case, this channel was also predicted by the theoretical calculations as the most favorable energetically, 22 yet our experiment with deuterated methanols did not reveal any evidence for proton transfer from the methyl group. Finally, it is also worth noting that the dimethyether peak mentioned in the methanol pickup spectra above was observed in the spectra after the pickup of both molecules, methanol and formic acid. In the case of the perdeuterated methanol, the peak is observed at m/z = 54, (CD3 OCD3 )D+ . Some contribution at m/z = 53 can be also seen, that would correspond to the protonated dimethylether (CD3 OCD3 )H+ . The proton H+ could originate from formic acid pointing to another reaction between methanol and formic acid. However, we cannot entirely exclude some H-D exchange on surfaces in the methanol reservoir and pickup cell. Thus the H+ could also originate from some small CD3 OH contamination of the methanol in the pickup cell which, however, cannot influence the proton transfer between methanol and formic acid discussed above.

Conclusions In a cluster beam experiment, we pickup different molecules, methanol and formic acid, on ¯ ≈ 270, clusters and investigate reactions between them triggered by either large ArN , N

19



electron ionization or photoionization. As far as we know, this is the first time that the reaction between different molecules adsorbed on ArN is reported. Both molecules picked up individually by the nanoparticles coagulate to clusters on ArN and their ionization results in protonated series (CH3 OH)x H+ and (HCOOH)x H+ in the mass spectra. Electron ionization leads to more fragmentation that photoionization in all cases. Pickup of both, methanol and formic acid, yields the protonated series of individual as well as mixed clusters, pointing to the generation of the mixed clusters on ArN . Theoretical calculations suggest chain structures of the small protonated clusters, while the neutral clusters prefer cyclic structures with maximum hydrogen bonds. Experiments with perdeuterated metanol CD3 OD demonstrate deuteron D+ transfer from methanol radical cation to formic acid resulting in (HCOOH)x D+ series as well as perhaps a more conventional proton transfer from formic acid to methanol yielding (CD3 OD)x H+ . Both channels were confirmed theoretically as exoergic ones. In addition, experiments with partially deuterated metanols CH3 OD and CD3 OH were implemented to reveal the proton transfer reactions from either hydroxyl or methyl group. The quantum chemical calculations predicted the proton transfer from C–H bond to be more exoergic. Experimentally, both channels are observed. The above proton transfer reactions yielding methoxy radical CH3 O· and hydroxymethyl radical CH2 OH· , which are released from the nanoparticles, are interesting and topical in the astrochemical context. Both radicals were suggested to participate in the formation of complex organic molecules (COMs) in methanol-containing ices in hot protostellar envelopes 56 as well as in cold dark clouds. 57–59 Recently, CH3 O· was observed toward a cold core of the B1-b low-mass protostar together with other COMs including formic acid. 58 Formation of these species on dust grains with methanol content, their ejection aided by cosmic rays and subsequent secondary photon-induced processes were debated. Our detailed mechanism of possible pathways leading to both CH3 O· and CH2 OH· radicals on nanoparticles contributes to understanding of astrochemistry in the protostellar medium.

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Acknowledgement The authors thank for support of the Czech Science Foundation (GAČR) grant No.: 1704068S. Access to computing and storage facilities owned by parties and projects contributing to the National Grid Infrastructure MetaCentrum provided under the program "Projects of Large Research, Development, and Innovations Infrastructures" (CESNET LM2015042), is greatly appreciated.

Supporting Information Available SI contains further experimental results for the system containing CD3 OD and HCOOH, benchmark calculations, computational results for methanol clusters, formic acid dimer, heterodimers, heterotrimers, and Cartesian coordinates of optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry PT HCOOH×+ + CH3OH ® HCOO× + CH3OH2+ ArN

hn

HCOOH + CH3OH×+ ® HCOOH2+ + CH3O× e-

HCOOH + CH3+OH× ® HCOOH2+ + CH2OH×

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Proton Transfer Reactions between Methanol and Formic Acid

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