Infrared Spectroscopy of the Astrochemically Relevant Protonated

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Infrared Spectroscopy of the Astrochemically Relevant Protonated Formaldehyde Dimer J. Philipp Wagner, David C. McDonald, and Michael A. Duncan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10573 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry

J. Phys. Chem. A

Infrared Spectroscopy of the Astrochemically Relevant Protonated Formaldehyde Dimer

J. Philipp Wagner, David C. McDonald II, and Michael A. Duncan* Department of Chemistry, University of Georgia, 140 Cedar Street, Athens, GA, 30602, U.S.A. Email: [email protected]

ABSTRACT The protonated formaldehyde dimer (H2CO)2H+ was generated in an electrical discharge and supersonic expansion of argon saturated with formalin solution vapor. Its infrared spectrum was measured in the region from 900–4000 cm–1 employing infrared laser photodissociation and messenger atom tagging. Comparison of the experiment to quantum chemical computations at the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level reveals that the experimentally observed structure is the head-to-tail dimer and not the more stable proton-bound dimer. This is consistent with the usually observed C–O bond formation upon formaldehyde oligomerization under acidic conditions in solution and resembles the structure of the neutral (H2CO)2 dimer in the gas phase. There is no evidence for the formation of other isomers, most notably protonated glycolaldehyde, that could result from covalent bond formation. These findings may be relevant to a proposed carbohydrate formation mechanism in the interstellar medium starting from protonated formaldehyde dimer.

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1. Introduction Formaldehyde (H2CO) is a well known, abundant polyatomic organic in the interstellar medium (ISM).1 Although it is relatively unreactive in the gas phase, formaldehyde’s interaction with its interstellar protonated analogue (H2COH+)2 provides access to a rich chemistry due to the general importance of barrierless ion-molecule reactions in space.3-8 It has been proposed that a reaction beginning with the protonated dimer (Ia) could potentially lead to protonated glycolaldehyde (IIa) in the ISM, which formally corresponds to a two-carbon aldose (Scheme 1).9,10 Otherwise, in mass spectrometry and computational chemistry the structure of the ion is usually attributed to the proton-bound dimer (Ia).11-14 With new techniques in ion infrared spectroscopy, it has become possible to determine the structures of more complex organic ions in the gas phase.15-20 Therefore, to investigate this structural issue, we set out to prepare the ion of the stoichiometry C2H5O2+ in the gas phase starting from formaldehyde vapor and probe its structure with infrared laser photodissociation spectroscopy. In solution, formaldehyde undergoes two kinds of oligomerization reactions. While C–O bond formation is preferred under acidic conditions, yielding linear polyoxymethylenes or cyclic oxanes, basic conditions favor reaction over the carbon atoms producing glycolaldehyde and monosaccharides.21 The latter reaction was described by Butlerow as early as 1861 and is generally referred to as the formose reaction.22 This reaction is now generally regarded to be important for the formation of prebiotic molecules.23,24 Breslow and others have rationalized its mechanism as a sequence of aldol reactions.25,26 However, the first step of formaldehyde dimerization cannot be explained this way and the participation of "activated formaldehyde" has been invoked.26 Most interestingly, glycolaldehyde was detected towards Sagittarius B2(N) by millimeter-wave spectroscopy27,28 causing speculation about a gas phase version of the formose 2 ACS Paragon Plus Environment

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reaction in the interstellar medium. Given the abundance of protonating agents like H3+ in space,29 a Nazarov-type cyclization of proton-bound formaldehyde dimer with subsequent [1,2]H-shift (Scheme 1) has been proposed to be a source of protonated glycolaldehyde.9,10 Although this idea is tempting, the proposed reaction seems unlikely based on simple polarity arguments. A flow drift tube experiment in which H3O+ was reacted with formaldehyde gas yielded an ion with the m/z ratio of protonated glycolaldehyde, but no structural evidence was provided.10 On the other hand, it was demonstrated that in its unimolecular decomposition protonated glycolaldehyde exclusively loses water and not formaldehyde.30 In a similar vein, protonated glyceraldehyde and dihydroxyacetone (C3 aldose and ketose, respectively) lost formaldehyde in unimolecular decomposition, but the back reactions were computed to have non-negligible barriers.31 A more likely pathway to interstellar glycolaldehyde might be grainsurface hydrogenation of carbon monoxide in combination with C−C bond formation by radical recombination,32 similar to the formation of interstellar formaldehyde itself.33,34 The starting point for the proposed reaction sequence (Scheme 1) is the proton-bound dimer of formaldehyde, in which a proton links the two oxygens in an O−H+−O structural motif. Such proton-bound dimers have been studied for many years with gas-phase infrared spectroscopy in order to develop a microscopic understanding of proton accommodation and proton transport.35-49 Infrared bands associated with the shared proton (νsp) can occur in a broad spectral range and their frequencies have been found to correlate with the difference in proton affinity of the two involved bases. In symmetrical systems, νsp is found near or below 1000 cm–1 and is often split into multiplets due to coupling with other vibrational modes.40 In this regard, the aggregation of aldehydes upon protonation was investigated with mass spectrometry. Whereas cluster ions with up to 11 monomer units have been observed for acetaldehyde,50 in the



formaldehyde case only the dimer was found.11 Quantum chemical studies of the (H2CO)2H+ complex assigned the lowest energy structure to be the proton-bound dimer displaying a double well minimum at low levels of theory.12,13 When correlation is included into the computations, the proton transfer barrier collapses to a single minimum with equal proton sharing.10 Spectroscopically, only the protonated formaldehyde monomer has been studied so far.51-53 Our group investigated the chemically related protonated acetone dimer with infrared photodissociation spectroscopy, and assigned its structure to a proton-bound dimer.44 The proton stretch in that symmetric system was fond at 900 cm-1, with a series of intense combination bands extending to higher frequencies just above that.44 We therefore aim to further elucidate the gas phase chemistry of this important ion with a combined infrared and computational study.

2. Methods Experimental Ions of the composition C2H5O2+ were generated by a pulsed electrical discharge in a supersonic expansion of 5% hydrogen in argon saturated with formalin solution vapor at ambient temperature, as used in a recent study of the formaldehyde radical cation.54 The argon tagged complex of this ion was mass selected in a reflectron time-of-flight spectrometer and its infrared spectrum was recorded with laser photodissociation spectroscopy.20 Infrared transitions were explored in the range of 900−4000 cm−1 with a Nd:YAG-pumped infrared OPO/OPA laser system (LaserVision). For the low energy region (900−2050 cm−1), a AgGaSe2 crystal was used in a second stage of difference frequency generation. The action spectra were collected as fragment ion yield from argon loss as a function of the energy of the laser's photon energy.


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Computational Conceivable non-covalently bound isomers of protonated formaldehyde dimer were explored and optimized at the MP2/cc-pVTZ level of theory utilizing the Gaussian09 program package.55,56 Additionally, geometries of other molecular ions of the same stoichiometry were localized at the same level.57 We chose this level of theory because a recent theoretical study on the neutral formaldehyde dimer revealed that MP2 binding energies differ by no more than about 0.2 kcal mol−1 from those obtained with explicitly correlated CCSD(T)-F12 theory.58 MP2 also seems a reasonable choice since it has been found that it performs better for hydrogen-bonded systems than the more modern SCS-MP2,59 and DFT often has difficulties when it comes to processes related to proton accommodation and transfer.60 Although DFT methods with dispersion-corrected functionals are often employed for improved energetics of non-covalent systems, we have found that such methods are no better for the vibrational patterns of small ions.47 However, to explore this issue further, we also conducted DFT computations at the B3LYP-D3(BJ)/cc-pVTZ level on these various structures, as shown in the Supporting Information. There was no significant difference in the vibrational spectra predicted by these different methods. For each of the methods employed, harmonic vibrational frequencies were computed to ascertain the nature of the stationary points on the potential energy surfaces (PES). For comparison to experimental infrared spectra, an empirical scaling factor of 0.961 was applied to the MP2 frequencies, derived from a comparison of computed frequencies for neutral formaldehyde to its known vibrations.61 Zero point vibrational energies (ZPVE) are included in all the energies reported here (∆H0). For further refinement of energies, CCSD(T)/cc-pVQZ single points were computed on top of the optimized structures.62,63



3. Results and Discussion The experimentally recorded photodissociation spectrum of the (H2CO)2H+Ar ion is presented in the upper black trace in Figure 1. The spectrum is dominated by a strong band at 3401 cm−1 in the O−H stretching region accompanied by two weaker features at 3344 and 3560 cm−1. A band at 3145 cm−1 indicates a C−H stretch at an unsaturated carbon center. Three bands at 1618, 1683 and 1710 cm−1 suggest the presence of one or more carbonyl groups. In the fingerprint region, various transitions of medium (1489, 1248 and 983 cm−1) to weak (1399 and 1162 cm−1) intensity are found. There are no strong infrared transitions near or below 1000 cm−1 like that expected for a proton-bound dimer although such a band might still be found outside the experimental range. On the contrary, the dominant absorption at 3401 cm−1 indicates that there is a free or weakly coordinated hydroxy group. To further understand this spectrum, we explored possible isomers of protonated formaldehyde dimer in which the monomeric subunits are still intact. The optimized structures are displayed as complexes Ia-d in Figure 2. Recognizing the possibility of reactions in the plasma, molecular ions isobaric to protonated formaldehyde dimer were optimized as well and are shown as structures IIa-e (Figure 2). For the non-covalent ions it turns out that the C2h symmetric proton-bound dimer Ia is the most stable isomer in agreement with a previous theoretical study.13 We compute a D0 binding energy of formaldehyde to protonated formaldehyde of 31.6 kcal mol−1. In contrast to earlier lower level computations, this ion does not display a symmetric double well potential, but exhibits a single minimum with equal O−H distances (C2h).13,14 The cis-rotamer Ib (C2v), which could lead to the proposed Nazarov cyclization, turns out to be a transition state 2.9 kcal mol−1 higher in energy. The head-to-tail dimer Ic featuring a C−O connectivity is only 3.9 kcal mol−1 less stable than Ia. In this structure,


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the nucleophilic, non-protonated aldehyde oxygen attacks the carbonyl group carbon of the protonated monomer at an O–C–O angle of 110.8° close to the "Bürgi-Dunitz approach trajectory" known for carbonyl reactions.64 Also, the structure exhibits a non-classical CH−O hydrogen bond providing added stability to this configuration. The C−O bond length of the protonated aldehyde unit is substantially elongated to 1.33 Å, indicating that the double bond character of the attacked carbonyl group is greatly reduced. This is also expected to lower the C−O vibrational frequency significantly compared to a normal carbonyl (e.g., 1628 cm-1 in the protonated formaldehyde monomer53). Dimer Ic would not lead to a sugar-like structure upon reaction, but rather to an oxane (IIb) due to the preformed connectivity. Finally, a less important CH−O bound isomer Id was calculated at an energy of 17.7 kcal mol−1. The two most important covalently bound isomers result from C−C and C−O bond formation, respectively: protonated glycolaldehyde IIa and protonated oxane IIb (formally 1,3dioxethane). While formation of protonated glycolaldehyde is favorable by 7.1 kcal mol−1 relative to Ia, the reaction to IIb is highly endothermic (+29.2 kcal mol−1), probably due to a high strain in the four-membered ring.65 Other chemically meaningful, isobaric ions are less likely to form and would require substantial molecular rearrangement. The most stable of these is protonated acetic acid IIc that is 35.7 kcal mol−1 lower in energy than Ia. The second most stable ion is protonated methyl formate (IId, −17.3 kcal mol−1). Both acetic acid and methyl formate are known molecules in the ISM.66,67 The least energetically favorable ion is the peroxide IIe (+82.7 kcal mol−1) which would formally derive from a [2 + 2] cycloaddition. Figure 1 shows a comparison of our experimental spectrum to the scaled harmonic infrared spectra predicted for these various isomeric structures. This comparison suggests that the ion detected in our experiment is not the expected proton-bound formaldehyde dimer Ia.



Dimer Ia lacks an appropriate high-energy mode to explain the strongest band at 3401 cm−1 (green trace, Figure 1). The computed spectral pattern of protonated glycolaldehyde also does not match the observed spectrum (purple trace, Figure 1). Moreover, ion IIa displays an intramolecular ionic hydrogen bond which often appears as a broadened band in the spectrum;68,69 we do not observe such a feature. The other covalent isomers IIb-e can also be excluded on the basis of similar spectral comparisons. Figure S14 in the Supporting Information shows the computed infrared spectra of all the covalent structures IIa-e compared to the experiment. As the formalin solution used is stabilized with methanol, protonated methanolcarbon monoxide complexes could conceivably form and contribute to the experimental IR spectrum. However, the spectral comparison in Figure S15 in the Supporting Information also makes it possible to exclude this heterogeneous complex. The only ion whose predicted infrared spectrum is in convincing agreement with the observed vibrational transitions is the head-to-tail dimer Ic, whose argon tagged isomer we also present in Figure 1 (red and blue trace, respectively). Band assignments for the argon tagged ion based on scaled harmonic frequencies are given in Table 1. The strongest infrared transition for isomer Ic at 3401 cm−1 is assigned to the O−H stretch of the protonated molecule. The experimentally observed frequency shows a large deviation from the scaled harmonic MP2 computation. This suggests that this mode either displays a very large anharmonicity or that the electronic structure treatment of the non-covalent interaction is still insufficient. A more pronounced nucleophilic interaction would increase the single bond character of the attacked protonated formaldehyde converting it into an alcohol. This would be accompanied by an expected spectral blue-shift of the O−H stretch to the region typically observed for these functional groups (e.g. in methanol at 3681 cm−1).61 If the interaction is less


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evolved, the absorption should be closer to that of a protonated carbonyl group. In the case of the argon tagged protonated formaldehyde monomer, this absorption was seen at 3182 cm−1.53 These considerations suggest that the intermolecular C−O bond length would elongate when higher levels of theory are applied or that there is a bond-stretch isomer with a greater intermolecular distance. Unfortunately, all attempts to localize such a structure failed. More sophisticated electronic structure treatments and consideration of anharmonic effects could improve the agreement between experiment and theory in future studies. The band at 3560 cm−1 probably corresponds to a combination of the O−H with the Ar−H stretching. An analogous band 159 cm−1 to the blue of the O−H stretch was observed for the argon tagged protonated formaldehyde monomer.53 This is in excellent agreement with the interval of 158 cm−1 that we observe here. The relatively intense band at 3344 cm−1 is not explained easily. One might consider the possibility of a Fermi resonance resulting from mixing with overtones of the vibrations around 1700 cm−1, but they are probably not intense enough to cause such an effect. Other isomers in which the argon binds to different sites of the molecule would be blue-shifted from the main O−H stretching feature. Also, the presence of further rotamers with respect to the intermolecular C−O bond of Ic that might cause this absorption can be considered unlikely based on a relaxed potential energy scan (Figure S18, Supporting Information). The band at 3145 cm−1 can be assigned to the asymmetric C−H stretch of the non-protonated formaldehyde. The carbonyl stretch of the attacking formaldehyde unit is assigned to the broad band at 1618 cm−1. Unfortunately, the two accompanying bands at 1683 and 1710 cm−1 are not easy to understand. The carbonyl stretch of the protonated formaldehyde is expected to be at a much lower frequency because of its reduced double bonding, as noted earlier. The modes below 1500 cm−1 are difficult to describe as their character is often a mixture of H−C−H scissoring, wagging and



rocking, H−O−C deformation and C−O bond stretching (of the protonated formaldehyde unit). All these modes are similar in energy which can give rise to complex coupling and calls for a better theoretical treatment in the future. Even though there are unassigned bands, the main features of the experimental infrared spectrum are understandable. We therefore conclude that the C2H5O2+ in our experiment is unlikely to be the proton-bound dimer of formaldehyde or the protonated glycolaldehyde ion, but instead corresponds to the previously unanticipated head-to-tail dimer Ic. This is surprising, as this complex is computed to be less stable than its competing structural isomers. Therefore, one might speculate that this structure forms for kinetic reasons in the ionized and rapidly-cooled supersonic expansion. Because of its large excess in the expansion, it is conceivable that argon attaches to the protonated formaldehyde monomer first, blocking the O−H group for further interactions. Because of the low temperature, the second formaldehyde would then attach to the carbonyl group in an antiparallel fashion. If this is the case, tagging with a more weakly bound atom such as neon may make it possible for the second formaldehyde molecule to displace the rare gas and find its more stable binding site. We tried to make such a neon-tagged species, but were unfortunately not able to produce enough of this ion for study. Another possibility is that the neutral formaldehyde dimer (H2CO)2 forms first in the expansion and then is subsequently protonated. The structure seen here is essentially the same as that of the neutral dimer according to a microwave study,70 which is in agreement with computations at high levels of theory.71 The efficiency of formation for the neutral dimer should vary with the formaldehyde concentration and beam conditions, also perhaps providing some control over the ion cluster growth. Again, our attempts to produce other spectra by varying these conditions were unsuccessful. The most stable proton-bound structure would likely be produced by a reaction of the protonated monomer


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with a neutral monomer. A bias against this chemistry is conceivable if the monomer concentration in the beam is low compared to larger formaldehyde aggregates. Polymers of formaldehyde are of course well known, but these species would have extremely low vapor pressures compared to monomers under our room-temperature conditions. Additionally, we are confident that formaldehyde monomers are present as they are intense in the mass spectrum produced (Figure S1) and we have used these same conditions previously to measure spectra for the protonated formaldehyde monomer and the formaldehyde cation.53,54 Even allowing for these potential kinetic effects, it is remarkable that there is no signal indicating formation of the stable proton-bound dimer structure. In other ion systems that we have studied previously, we have found several examples of co-existing isomers separated by activation barriers that precluded the equilibration to the most stable structures.20 However, this is the first system in which we find no evidence for the most stable isomer. It should be noted, that the strong shared proton stretch vibration characteristic of the proton-bound dimer structure is notoriously difficult to handle computationally.35-48 It is conceivable that this vibration occurs at a frequency significantly lower than that predicted by theory, and this would then be lower than the range covered by the experiment. If the predicted spectrum is accurate in other respects, there would not be any strong vibrations for this isomer in the higher frequency region. It could therefore be possible that the proton-bound dimer structure is present, but just not detected. An argument against this possibility is that most proton-bound dimer spectra studied in the past,35-48 especially including that for the corresponding acetone species,44 have extensive vibrational progressions extending to higher frequencies from the proton stretch fundamental. These additional strong bands correspond to the strong couplings of the proton stretch with other low frequency modes. Such progressions would fall in the range of the present spectrum, but we also



do not detect such bands. The simplest interpretation of the overall experiment is therefore that we have produced primarily isomer Ic. It is instructive to consider the implication of the present work for astrochemistry. The formation of interstellar protonated glycolaldehyde as in Scheme 1 requires the existence of the proton-bound dimer form of (CH2O)2H+, which was not observed in this experiment. However, the present experiment includes the complicating factors of argon tagging and the presence of high concentrations of neutral formaldehyde, either one of which could conceivably account for the inefficient formation of the proton-bound structure. Neither of these factors would apply in the conditions of interstellar space. Therefore, it is not clear from the present results what the likelihood is for the formation of proton-bound dimer structures in space. This would presumeably involve the reaction of a protonated monomer with a neutral monomer. Such reactions could be studied with standard ion-molecule methods in different kinds of mass spectrometers or flowing afterglow instruments. However, the reaction would need subsequent spectroscopy to verify the structure. Our present instrumentation does not allow such an experiment, but it may be possible in other labs in the future.

4. Conclusion and Outlook The protonated formaldehyde dimer has been studied with infrared photodissociation spectroscopy and computational chemistry. Comparison of the spectrum for the argon tagged ion with scaled harmonic frequency computations at the MP2/cc-pVTZ level reveals that the structure most likely corresponds to a non-covalently bound head-to-tail dimer Ic and not to the expected proton-bound dimer Ia or its proposed reaction product protonated glycolaldehyde IIa. An anharmonic vibrational as well as a higher level electronic structure treatment of ions Ia and


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Ic would be desirable to better understand the experimental spectrum, although such calculations would be challenging. It would also be helpful to explore different ion-molecule reaction conditions to investigate the formation mechanism for the unexpected structure Ic and the possible formation of the more stable Ia structure.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. It includes the complete citation for reference 55, the mass spectrum, computed geometric structures and electronic energies for different isomers, comparison of the experimental spectrum to those of other isomers, and additional comparisons to spectra predicted by DFT.

Author Information Corresponding Authors *Email: [email protected]

Acknowledgments We gratefully acknowledge support for this work by the National Science Foundation (MAD grant CHE-1464708). J.P.W. acknowledges the Alexander von Humboldt Foundation for a Feodor Lynen Postdoctoral Fellowship.



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Table 1. Comparison of observed and scaled (0.961) harmonic infrared transitions in cm−1 at the MP2/cc-pVTZ level together with the approximate description of the mode. experimental ω (I)a

approximate description

3560

-

O−H stretch + Ar−H combination

3401

3597 (391)

O−H stretch

3145

3143 (17)

asym. C−H stretch, non-protonated formaldehyde

1710

-

1683

-

1618

1639 (59)

C=O stretch, non-protonated formaldehyde

1489

1439 (52)

H−C−H scissoring, non-protonated formaldehyde

1452

1366 (23)

H−O−C bending/H−C−H rocking

1399

1328 (31)

H−C−H wagging, protonated formaldehyde

1248

1225 (97)

H−C−H rocking, non-protonated formaldehyde

1162

1192 (45)

H−O−C deformation/C−O stretch, protonated formaldehyde

983

1026 (25)

H−O−C bending/H−C−H rocking

a

Intensities in km·mol−1



Figure Captions

Scheme 1. Proposed gas phase dimerization reaction of proton-bound formaldehyde dimer with subsequent rearrangement to protonated glycolaldehyde drawn in analogy to Figure 5 in reference 9.

Figure 1. Comparison of the experimental infrared photodissociation spectrum of the C2H5O2+Ar ion and the predicted scaled harmonic spectra of the most relevant isomers employing MP2 theory together with a cc-pVTZ basis set.

Figure 2. Structures and energies (∆H0) of ions of the stoichiometry C2H5O2+ at CCSD(T)/ccpVQZ//MP2/cc-pVTZ level including ZPVE.


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Scheme 1.



Figure 1.


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Figure 2.



TOC Graphic:


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Infrared Spectroscopy of the Astrochemically Relevant Protonated

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