Biomolecule Analogues 2-Hydroxypyridine and 2-Pyridone Base

Article pubs.acs.org/JPCA

Biomolecule Analogues 2‑Hydroxypyridine and 2‑Pyridone Base Pairing on Ice Nanoparticles Peter Rubovič,† Andriy Pysanenko,† Jozef Lengyel,† Dana Nachtigallová,‡ and Michal Fárník*,† †

J. Heyrovský Institute of Physical Chemistry, The Czech Academy of Sciences, Dolejškova 3, 182 23 Prague, Czech Republic Institute of Organic Chemistry and Biochemistry v.v.i., The Czech Academy of Sciences, Flemingovo nám. 2, 16610 Prague 6, Czech Republic

‡

S Supporting Information *

ABSTRACT: Ice nanoparticles (H2O)N, N ≈ 450 generated in a molecular beam experiment pick up individual gas phase molecules of 2-hydroxypyridine and 2-pyridone (HP) evaporated in a pickup cell at temperatures between 298 and 343 K. The mass spectra of the doped nanoparticles show evidence for generation of clusters of adsorbed molecules (HP)n up to n = 8. The clusters are ionized either by 70 eV electrons or by two photons at 315 nm (3.94 eV). The two ionization methods yield different spectra, and their comparison provides an insight into the neutral cluster composition, ionization and intracluster ion−molecule reactions, and cluster fragmentation. Quite a few molecules were reported not to coagulate on ice nanoparticles previously. The (HP)n cluster generation on ice nanoparticles represents the first evidence for coagulating of molecules and cluster formation on free ice nanoparticles. For comparison, we investigate the coagulation of HP molecules picked up on large clusters ArN, N ≈ 205, and also (HP)n clusters generated in supersonic expansions with Ar buffer gas. This comparison points to a propensity for the (HP)2 dimer generation on ice nanoparticles. This shows the feasibility of base pairing for model of biological molecules on free ice nanoparticles. This result is important for hypotheses of the biomolecule synthesis on ice grains in the space. We support our findings by theoretical calculations that show, among others, the HP dimer structures on water clusters.

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INTRODUCTION Hydrogen bonding represents the fundamental interaction in biology. It is the central structure-determining motif of nucleic acid base pairs and is essential for many biological processes, e.g., the molecular recognition.1 Therefore, the formation of intermolecular hydrogen bonds (such as the two antiparallel N−H···OC bonds in the dimers of the title molecules) is a necessary condition for the generation of biomolecules. In the search for origins of life in the universe, propositions were made that biomolecules could be synthesized on icy grains in the space.2−6 To investigate such prospects in laboratory conditions, the clustering and reactivity of molecules in cluster environments can be studied.7,8 Large water clusters composed of hundreds of water molecules (ice nanoparticles) represent a model system where a detailed molecular-level understanding of such processes can be gained. One way to approach this task is to generate a molecular beam of ice nanoparticles in vacuum which are subsequently doped by precursor molecules by a pickup technique.9−12 The experiments with the doped nanoparticles address the questions such as the following: Are the individually picked up molecules mobile on the nanoparticles, or do they stay put where they landed? Do they coagulate to clusters on the nanoparticles? How do they react upon the nanoparticle excitation with energetic photons or electrons? © XXXX American Chemical Society

We have recently investigated these questions for a series of atmospherically relevant chlorine-containing molecules on argon and ice nanoparticles.8 All the studied molecules coagulated to relatively large clusters on argon nanoparticles, but surprisingly no evidence for any clusters generated on the water−ice nanoparticles was observed. The same behavior (i.e., coagulation on argon and noncoagulation on ice nanoparticles) was observed in our photodissociation studies7,13,14 for other molecules as well. In this article we present an experimental evidence for coagulation of biologically relevant molecules on ice nanoparticles: we observed dimers of hydroxypyridine (HP) and even larger clusters composed of up to about 10 molecules generated on the nanoparticles. The evidence for the cluster generation is provided by mass spectrometry after electron ionization and photoionization. To our best knowledge, an experimental evidence for the generation of clusters by coagulation of individually picked up molecules on large water clusters has not been reported previously. Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: November 20, 2015 Revised: January 4, 2016

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DOI: 10.1021/acs.jpca.5b11359 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Both tautomers can be present in the vapor, and we cannot discriminate between them in our experiment. Therefore, we will use the abbreviation HP throughout this article, denoting both 2HP and 2PY. More discussion of HP tautomer population follows in the next section. The paper is organized as follows: a brief description of the experimental setup and theoretical methods is followed by the presentation of the experimental results, the mass spectra of HP molecules picked up on ice nanoparticles. The discussion starts with an analysis of the electron ionization and photoionization processes and of cluster fragmentation and composition. On the basis of this analysis, we further discuss the clustering processes on ice nanoparticles. The dimer generation, which prevails, is supported by the theoretical calculations. Finally the major conclusions are summarized.

It ought to be mentioned that the generation of clusters of biomolecules in water cluster was demonstrated in experiments where coexpansion of biomolecules with the water vapor was used rather than the pickup.15−17 However, in these experiments the biomolecules are incorporated inside the water cluster and the composition of the cluster and number of incorporated biomolecules are difficult to control in the supersonic expansions. The hydroxypyridine represents a molecule on which biologically relevant hydrogen bonds can be investigated. The dimer structures containing hydroxypyridine tautomers 2pyridone (2PY) and 2-hydroxypyridine (2HP) (displayed in Figure 1) represent the essential structural motifs found in

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EXPERIMENTAL AND THEORETICAL METHODS Experiment. This study was performed on CLuster Beam (CLUB) apparatus, which is a complex experiment dedicated to photochemistry,13,14,36mass spectrometry,37−39 and pickup11,12 studies of clusters in molecular beams. Detailed descriptions of the apparatus are given in the mentioned publications. However, the multiphoton ionization has been first implemented in our experiment in this study; therefore we briefly describe the mass spectrometric part of our experiment below. The ice nanoparticles were generated in a continuous supersonic expansion of heated water vapors into the vacuum through a conical nozzle (diameter d = 90 μm, opening angle α = 30°, and length l = 2 mm). The water in a resistively heated reservoir inside the vacuum chamber was kept at a temperature TR = 421 K. The nozzle was attached directly to the reservoir and heated to the constant temperature T0 = 433 K. The mean particle size under these expansion conditions could be calculated using the modified Hagena’s formula40,41 as N = 450. These nanoparticles are supposed to have internal temperatures of around 100 K.42 As a reference system we used argon nanoparticles for HP pickup. The ArN clusters were generated in a different cluster source: pure Ar was expanded through a similar nozzle of the diameter d = 60 μm at 6.0 bar stagnation pressure and T0 = 230 K, which corresponded to the mean cluster size N = 205.43 These nanoparticles have internal temperature of 37 K.44 The experimental conditions for argon and ice nanoparticles are summarized in Table 1. For comparison, we have also

Figure 1. Optimized structures of (2PY)2, (2HP)2, and 2HP·2PY. The relative energies (ΔErel) with respect to (2PY)2 are given in kcal/mol. The intermolecular hydrogen bond distances are given in Å.

larger biomolecules. Three different, strongly hydrogen-bonded structures exist: homodimers (2PY)2 and (2HP) 2 and heterodimer 2PY·2HP. As 2PY has the same H-bonding site as uracil and thymine, the (2PY)2 structure with two antiparallel N−H···OC bonds can be viewed as a model for noncanonical uracil dimer. On the other hand, 2HP shows close chemical similarity to isoguanine. This makes the mixed 2PY· 2HP dimer a relevant model compound for tautomeric DNAbase pair analogues. The HP dimer structures were studied widely in Leutwyler’s group.18−22 The 2PY dimer model system was also studied spectroscopically.23,24 Mono- and disolvated water structures with HP were also subject of numerous studies.25−30 The reasons for such interest is that the hydrogen-bonded interactions of the peptide functional group of 2PY with one and two water molecules model interactions play a central role in determining the secondary structures of proteins and nucleic acids. These weak interactions also play an important role in the intermolecular recognition processes that are crucial to most biological systems.26 The photoionization31 and excited state dynamics32 of isolated HP molecules were also investigated. 2PY and 2HP are keto−enol tautomers distinguished by H atom transfer between the N and O sites in the respective molecules. Both tautomers are very similar in energy, the free energy difference between them is 0.025 eV,25 and both are present at room temperature with equilibrium slightly shifted to the enol form. The keto−enol equilibrium was determined to be 1:2 in the gas phase.33 Sobolewski proposed a mechanism for photoinduced proton transfer between the two tautomers in an excited state called photoinduced dissociation−association.34,35 There is a large barrier for the proton transfer reaction (2.1 eV). In the solid state, the keto form is predominant; however, in solutions, both tautomers occur because of the decrease of the barrier for the proton transfer. In the presented study we evaporate a sample of pure 2HP at temperatures between 298 and 343 K to be either deposited on nanoparticles or to be coexpanded with the buffer argon gas.

Table 1. Expansion Conditions: Nozzle Diameter d, Reservoir TR and Nozzle T0 Temperatures, Stagnation Pressure for Ar Expansion P0, Average Particle Size N Generated in the Expansion, and HP pickup oven temperature TP species

d0 (μm)

T0/TR (K)

P0 (bar)

TP (K)

N

(H2O)N ArN

90 60

421/433 230

4.4 6.0

298−343 328

450 205

generated free clusters of HP molecules in seeded expansion of heated HP vapors with argon. The corresponding results are presented in Supporting Information. The molecular beam emerging from the nozzle was skimmed, and after passing a differentially pumped vacuum chamber, it entered a chamber that was equipped with home-built pickup oven containing 2HP sample (Aldrich, 97%). The pickup oven is similar to the one used in our recent studies dedicated to the B


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Figure 2. Mass spectra of HP picked up on water clusters upon 70 eV electron (a) and 315 nm (b) laser irradiation. The pickup temperature was TP = 308 K. Several cluster series are marked in the figure; for more details, refer to the corresponding text.

sodium doping of clusters.38,45 The oven was heated resistively so that HP evaporated and could be picked up by the passing nanoparticle beam. The temperature was varied between 298 and 343 K to exploit different pickup conditions, i.e., different number of HP molecules adsorbed on the particles. No published data have been found for the temperature dependence of HP vapors; thus we refer to the pickup oven temperature TP for the description of the experimental conditions in the text. It is worth noting that we evaporated a sample of pure solid 2HP in the oven. The barrier for keto−enol tautomerization in the solid state is 2.1 eV (as mentioned already in the Introduction), much higher than the energy corresponding to the highest temperature used for 2HP evaporation 343 K (0.03 eV). Attention was also paid to always work with a fresh sample of 2HP from a fridge. Therefore, the 2HP can be assumed to be the prevailing tautomer in our experiments under all evaporation temperatures. Yet some tautomerization during the solid sample heating, in the gas phase, or on the ice nanoparticle surface cannot be excluded. Therefore, we performed the theoretical calculations for interpreting our experimental results for both tautomers and we refer generally to HP (including both 2HP and 2PY) in the experiment. However, it should be noted that the experimental results are well interpreted by our calculations below, whether 2HP or 2PY or their mixed clusters are assumed. After the pickup process, the molecular beam entered a UHV chamber hosting Reflectron time of flight mass spectrometer (RTOF) mounted perpendicularly to the beam. Either electrons or photons can be used to ionize the nanoparticles in the extraction region of the RTOF. For the electron ionization (EI), we used the electron gun with frequency of 10 kHz and tunable electron energy 5−90 eV with ∼0.7 eV resolution. Here we present the spectra recorded at 70 eV, but essentially the same spectra were also recorded at 30 eV.

The photoionization (PI) was achieved by means of a UV laser radiation at the wavelength of 315 nm (3.94 eV). The cluster ionization required two photons at this wavelength, and a near-threshold ionization occurred. The isolated HP molecules were not ionized, yet the HP molecules in clusters were ionized by two-photon processes due to the lowering of their ionization energy in clusters by solvation effects. The ionization mechanism will be discussed in detail in the section Cluster Composition, Ionization, and Fragmentation. It should be mentioned that in the laser-induced-fluorescence (LIF) excitation spectra of 2HP and 2PY molecules and clusters,27,46 there were no strongly absorbing excited states identified near the wavelength of 315 nm (31 746 cm−1). Also scanning the wavelength of our laser around 315 nm did not reveal any resonance for any of the observed ion signal. Therefore, no enhancement of the ionization of HP molecules or clusters is expected due to any intermediate resonances in the excited states at this wavelength. The laser repetition rate was 10 Hz, and the pulse length was 10 ns. The laser radiation was generated by doubling a 630 nm output of a dye laser (LAS) pumped by the second harmonics (532 nm) of an Nd:YAG laser (Spectra Physics, GCR-4). The UV radiation was focused by f = 400 mm lens through a fused silica window. The average UV energy at the chamber entrance was ∼3.5 mJ/pulse. The ionized species were extracted by a 10 kV pulse and accelerated to the final kinetic energy of 8 kV. After passing the effective flight path of 0.95 m, the ions were detected on the Photonics MCP detector of 4 cm diameter in chevron configuration. The mass spectra were recorded with a typical resolution of M/ΔM ≈ 5 × 103. Calculations. We support our experimental observations with theoretical calculations. The calculations were performed for isolated HP dimers in the gas phase, for (HP)2·(H2O)n (n = 1−3) clusters and for HP dimers on ice nanoparticle surface. The ice nanoparticle surface was modeled using 47−48 water C


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The Journal of Physical Chemistry A molecules. The procedures to obtain the structure of the ice surface were described elsewhere.47 We have employed the B97D48 functional and TZVP49 basis set in the DFT calculations. The interaction energies were corrected for BSSE. The adiabatic ionization energies (IE) were calculated for HP monomers and dimers in the gas phase and also on the ice nanoparticle surface. The ice surface was modeled with 5 and 12 water molecules for the monomer and dimer calculations, respectively. The DFT calculations were performed using B3LYP50,51 functional and def2TZVPP52,53 basis set. All the calculations were performed using Gaussian 09 program package.54

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RESULTS We performed the HP pickup on water clusters with N = 450 at several pickup temperatures between 308 and 343 K; see Table 1. The results for pickup temperatures TP = 308 and 333 K are presented in this section. For comparison, we also did the pickup experiments with argon nanoparticles, and we generated the (HP)n clusters in seeded expansion with Ar as well. These results are shown in the accompanying Supporting Information. Figure 2 shows the mass spectra of HP picked up on water clusters at the pickup temperature TP = 308 K. The upper panel shows the EI spectra at 70 eV electron energy (a), and the lower one displays the PI spectra (b). The pickup temperature of 308 K corresponds to the lowest pickup pressures at which we see evidence for HP and its clusters in both EI and PI mass spectra. Several cluster series can be identified and are labeled in the spectra. The EI spectrum in Figure 2a is strongly dominated by the unprotonated HP+ monomer peak at m/z = 95 (note the break on the intensity axis to show the monomer intensity), while the peak of the protonated molecule (HP)H+ at m/z = 96 is about 8 times weaker. The protonated water cluster series (H2O)nH+ dominates the cluster mass region. Besides, there are weaker protonated series of mixed clusters HP·(H2O)nH+ up to n = 27 with a maximum for n = 6, and HP2·(H2O)nH+ series with a maximum for n = 12 (not labeled). On the other hand, the PI mass spectrum shown in Figure 2b is much richer. The dominating series here, which was not present in the EI spectrum, are the protonated (HP)nH+ clusters starting from n = 1 to n = 5. The water cluster series (H2O)nH+ is weaker reaching up to n ≤ 33, and the mixed HP−water clusters HP·(H2O)nH+ reach to n ≤ 27. There is also a weaker HPm·(H2O)nH+ series with m = 2 and even series with m = 3 (not labeled). Additionally, new species are generated upon PI, mixed pyrrole−water clusters Py·(H2O)nH+ starting at m/z = 68 for n = 0 up to n = 27. Some of the abovementioned series are shown for EI and PI in Figure 3 a and Figure 3b, respectively, where the normalized peak intensities are plotted (in logarithmic scale) as a function of the cluster size n (the intensities are normalized to the n = 1 value). The mass spectra recorded for elevated pickup temperature TP = 333 K are displayed in Figure 4. In the spectrum recorded after EI, Figure 4a, there is a dominant HP+ monomer peak followed by much weaker series of protonated (HP)nH+ clusters up to n = 7. The PI mass spectrum shows a series of protonated (HP)nH+ clusters with n ≤ 7 accompanied by a series of corresponding protonated pyrrole clusters (Py)nH+. The mass spectra for seeded expansion of HP vapors in Ar and for HP pickup on ArN nanoparticles (shown in Supporting Information) were all similar, containing series of protonated (HP)nH+ clusters only. For pickup of HP on ArN nanoparticles with the average size N¯ = 205, we can see (HP)nH+ up to n =

Figure 3. Normalized peak intensities of hydrated clusters as a function of the number of water molecules n for EI (a) and PI (b). The intensities correspond to the mass spectra displayed in Figure 2 and are normalized to n = 1. For EI, dependence of HP(H2O)nH+ (triangle) and HP2(H2O)nH+ (circle) is displayed. For PI the dependence Py(H2O)nH+ (star) is added. The distribution of the protonated water clusters (square) is included for comparison for both cases.

10 for EI and up to n = 8 for PI. For seeded expansion, we can see clusters up to n ≈ 8, for both EI and PI. All these spectra have essentially the same character with an exponential decrease of (HP)nH+ mass peak intensities with increasing n.

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DISCUSSION Cluster Composition, Ionization, and Fragmentation. The mass spectra in Figure 2 show the series of protonated water clusters. On the other hand, the mass spectra recorded under the high HP pressure conditions in Figure 4 show no evidence of water. The mass spectra do not reflect only the abundances and compositions of the neutral precursors but also the mechanism of the ionization and fragmentation processes. The spectra in Figure 4 without any water invoke an intriguing question if it is possible that the pickup of HP evaporates water from the ice nanoparticles. Can the multiple pickup corresponding to the conditions of the spectrum in Figure 4 lead to a complete water evaporation, thus changing the pure ice nanoparticle (H2O)N to a (HP)M cluster (M ≪ N) which still continues flying in the original beam direction? On the basis of our calculated binding energies in Table 2, we can estimate the number of water molecules evaporated from the ice nanoparticle upon HP pickup and coagulation. The binding energy between two water molecules was reported to be 0.137 eV.55,56 Our theoretical calculations and literature values19 yielded binding energies for HP tautomers between 0.6 and 0.9 eV. From the ratio between these energies we can assume that upon coagulation of HP molecules, at most six water molecules can be evaporated from the (H2O)N cluster. Indeed, the binding energies in the larger clusters (both water and HP) are larger than the dimer binding energies. Nevertheless we use the above ratio for a rough estimate of the maximum number of water molecules evaporated per pickup and coagulation of an HP molecule to an (HP)n cluster on the ice nanoparticle; i.e., we assume that at most six water molecules are evaporated per one HP molecule adsorbed and coagulated. D


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Figure 4. Mass spectra of HP picked up on water clusters upon 70 eV electron (a) and 315 nm laser irradiation (b). The pickup temperature was TP = 333 K. Several cluster series are marked in the figure; for more details, refer to the corresponding text.

the ionization process which leads to water evaporation rather than the pickup process. Electron Ionization. Let us first discuss the electron ionization which is a universal method and can charge any molecule within the cluster, i.e., both water and HP. The above estimate suggests that there are significantly more water molecules than HP ones in the nanoparticles. Therefore, the incoming electron is more likely to ionize a water molecule than an HP molecule. The H2O+ reacts immediately (within 40 fs and with almost 100% efficiency57,58) with a neighboring H2O to H3O+ in an exoergic process which evaporates some molecules from the nanoparticles resulting in the major (H2O)nH+ cluster fragment series in Figure 2. This is indeed a well-known pattern for the water cluster ionization.38,59 Some fraction of ions can still contain the HP molecules leading to the observed (HP)m·(H2O)nH+, m = 1, 2 fragments. Under the conditions when the ice nanoparticles are heavily loaded with HP molecules, the spectra in Figure 4, ion− molecule reactions between water and HP can be considered. Above we have argued that even at these conditions the water molecules prevail. Thus, we first assume that a water molecule is ionized initially. Both ions, H2O+ and H3O+, can react with an HP molecule via a charge transfer (CT) or proton transfer (PT) reactions, respectively. The water ionization energy is IE(H2O) = 12.6 eV for an isolated molecule and decreases to less than 11 eV due to the solvation in clusters.59 The experimental values for the ionization energies of the HP tautomers are much lower, IE(2PY) = 8.45 eV and IE(2HP) = 8.94 eV.60 Thus, the CT,

Table 2. Binding Energies between Various Species Present in Our Clusters (Our Calculated EB Values and Reference Numbers Found in Literature)a 2HP−H2O 2PY−H2O 2HP−2HP 2PY−2PY 2PY−2HP

EB (eV)

literature

0.40, 0.68 0.33, 0.49 0.83 0.86 0.75

0.33−0.4628 0.39−0.5328 0.6119 0.7619 0.5919

a

The two values for HP−H2O clusters correspond to the structures with single or double hydrogen bonds (the corresponding structures are shown in Supporting Information).

In this estimate the kinetic energy deposited into the particle by the embedded HP molecule was neglected. This energy can be estimated as the relative kinetic energy of the colliding nanoparticle and HP molecule, T =

μvr 2 , 2

where μ =

mHPMN (mHP + MN )

is the reduced mass of the system of HP molecule and (H2O)N for N = 450. The relative velocity vr was taken equal to the measured nanoparticle velocity of 1400 ms−1. This yielded T ≈ 0.02 eV, which was negligible compared to the coagulation energies. It is also worth noting that the released energy can be redistributed efficiently over all the degrees of freedom of the water nanoparticle (more than 1300). The maximum HP cluster size recorded for the higher pickup temperature (Figure 4) is n = 7. Even if we take some fragmentation upon the ionization into account, it is reasonable to assume the maximum number of HP molecules picked up on the ice nanoparticle to be around 10. For this number, 60 water molecules evaporate at most, so we are left with nanoparticles still containing about 390 water molecules and less than 10 HP molecules. These simple estimates suggest that the reason for missing water cluster peaks in the mass spectra in Figure 4 is

H 2O+ + HP → H 2O + HP+

(1)

is a highly exoergic process by about 4 eV and can readily occur. Similarly, the proton affinity (PA) of both HP tautomers30,61 is above 9.3 eV, substantially higher than that of water62 PA(H2O) = 7.2 eV, and thus the PT reaction E


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The Journal of Physical Chemistry A H3O+ + HP → H 2O + (HP)H+

also spectra of HP coexpanded with argon where many unclustered monomers are present in the molecular beam. Yet, we do not see the monomers with PI, although they are present in the corresponding EI spectra. Therefore, we can conclude that the ions in our PI mass spectra are mostly due to a twophoton ionization of solvated HP molecules, and any higher order multiphoton processes do not contribute significantly to these spectra. This exclusion of the multiphoton processes with three and more photons explains that also water molecules and pure water clusters cannot be ionized directly in the PI experiment (IE(H2O) ≥ 11 eV in clusters41,59). For these reasons PI is highly selective process in the present case, ionizing only the HP molecules in the cluster species (in (HP)n clusters and/or in the ice nanoparticles). If the higher order multiphoton processes are excluded, how can the presence of (H2O)nH+ ions in the PI spectrum in Figure 2b be explained? We have to invoke intracluster ion− molecule reactions starting with the HP ionization. A CT reaction from HP+ or PT reaction from (HP)H+ to water, i.e., reversed process to reaction 1 or reaction 2, respectively, is unlikely to happen due to their high endoergicities. Therefore, we propose a PT reaction between HP+ and water:

(2)

is exoergic by 2 eV. Therefore, if there are enough HP molecules, the charge will eventually find an HP molecule within the nanoparticle and end there. Upon this process the exoergicity of the reaction leads to the evaporation of H2O molecules. Ultimately the bare (HP)nH+ clusters can be generated as observed in the spectrum in Figure 4a. The possibility of a direct HP ionization by the incoming electron should be also considered under the high pickup pressure conditions. The ionization of an isolated molecule leads to the HP+ ion (and smaller molecular fragments not discussed here), and the ionization of HP clusters leads to the (HP)nH+ ions. The generation of the (HP)nH+ fragments in the direct EI spectra of HP clusters was demonstrated by the mass spectra of HP clusters generated in coexpansion with argon (see Figure S1 in Supporting Information). The energy released in the ionization process can again evaporate the water molecules, and thus the scenario starting with the HP ionization is also consistent with the observed mass spectra. Photoionization. Now we focus on the photoionization process. Unlike the general EI discussed above, the PI is a highly specific method in the present case. Since a single 315 nm photon energy corresponds to 3.94 eV, the energy of two photons is not sufficient to ionize an isolated HP molecule. However, the IE of molecules lowers upon solvation in clusters. For example, a lowering of IE by more than 1.5 eV was observed for water.41,59 Similarly, IE lowering by 1 eV was observed for cytosine dimer with respect to the monomer in VUV photoionization study.63 The ionization of HP in clusters by two 315 nm photons (7.88 eV) would require the IE lowering by about 1 eV, which is feasible in clusters as shown by our calculated IE summarized in Table 3. The IE of 2PY and

HP+ + H 2O → [HP − H] + H3O+

(3)

where [HP − H] denotes the hydroxypyridine molecule that lost one hydrogen atom. This reaction can be followed by subsequent HP-species evaporation yielding the pure protonated water clusters. To support this prediction, we have performed the calculations on the 2HP·(H2O)6 model. The initial neutral structure corresponds to the ground-state optimized structure of the neutral complex, see Figure 5a.

Table 3. Ionization Energies (in eV) of Isolated Molecules and in Clustersa species 2PY 2HP on ice

calcd

exptl60

8.30 8.74

8.45 8.94

2PY 2HP

7.80 7.38

(2PY)2 (2HP)2 2PY·2HP

7.64 8.07 7.48

dimers

a

Figure 5. Optimized structures of the cluster of a 2HP molecule with six water molecules in the neutral 2HP·(H2O)6 (a) and ionized 2HP· (H2O)6+ (b) forms. Starting from this neutral structure geometry the optimization of the ionized species results in the PT from 2HP, justifying the reaction 3.

Adiabatic IEs are calculated.

2HP on ice is 7.80 and 7.38 eV, respectively. Also HP clustering with another HP molecule lowers the IE: the dimer IEs for (2PY)2, (2HP)2 and 2PY·2HP are calculated at 7.64, 8.07, and 7.48 eV, respectively. Thus, two-photon processes cannot ionize HP monomers in the gas phase but the ionization of HP in any kind of clusters is feasible. This is in agreement with the experiment, where we see no evidence for the HP monomer photoionization (see below) but we indeed see the photoionization in the clusters. It is also worth noting that we do not see any evidence for the higher order multiphoton processes. Three photons (11.8 eV) would be sufficient to ionize an isolated HP molecule. Nevertheless, we do not see any HP+ ions at m/z = 95 in any PI mass spectra under our experimental conditions. This includes

Starting from this structure, the geometry optimization of the ionized species results in the PT from the ionized 2HP to the (H2O)6 cluster where H3O+ moiety is clearly visible in Figure 5b. In this context it is also worth noting, that the proton transfer in nucleobases mediated by water was observed previously.64 In the heavily doped ice nanoparticles the ionized HP molecule is always in a vicinity of another HP molecule, and thus the PT reaction 3 with water does not take place. Therefore, the mass spectra in Figure 4b exhibit only the cluster F


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The Journal of Physical Chemistry A ions of HP and its fragment Py. This is consistent with the above presented picture for EI in these nanoparticles. Hydroxypyridine Clustering on Nanoparticles. Clustering. The most important result of the present study is the evidence for the HP molecule coagulation on the ice nanoparticles. Since the molecules are picked up on the nanoparticles from the gas phase one-by-one, the presence of ion fragments containing more than one HP molecule in both EI and PI mass spectra proves the formation of (HP)n clusters. The clusters have to be generated by HP migration and coagulation on/in the ice nanoparticles prior to the ionization, since the ionization is a process that leads to the cluster fragmentation as discussed above. This is quite a surprising result, since all our previous experiments with pickup of different molecules8,13,14,65 pointed to a strong clustering on argon nanoparticles but no clustering at all on ice nanoparticles as mentioned in the Introduction. Theoretical calculations within our previous paper8 justified this lack of clustering by two reasons: first was a low mobility of the adsorbed molecules on ice nanoparticles (kinetic effect), which did not allow them to approach each other; second was a strong bond between water and adsorbed molecules which prevented the molecules from bonding with one another even if they appeared in close proximity (dynamic effect). In the present case an estimate of the dynamic effect can be made based on the binding energies of various dimer clusters summarized in Table 2. The experimentally observed binding energies between 2HP and 2PY with water are in the range of ∼0.3−0.5 eV and ∼0.4−0.5 eV, respectively,28 which is smaller than the binding energies of various HP dimers of ∼0.6−0.8 eV.19 Our calculations predict the binding energies with water in the range of ∼0.4−0.7 eV and ∼0.3−0.5 eV for 2HP and 2PY, respectively. Although their ordering is reversed compared to the experiment, they are both smaller with respect to our calculated binding energies of the dimers which are about 0.8 eV. Therefore, we do not expect the dynamic effect to hinder the HP clustering: when two HP molecules appear to be close on the ice nanoparticle, the stronger bond between them will be formed. On the other hand, our theoretical calculations suggest that multiple hydrogen bonds of water molecules to HP can be formed which can hinder the HP migration on ice. Therefore, we assume the HP cluster generation to occur at the higher pickup pressures where the ice nanoparticle coverage with HP is high and the probability of the picked up HP molecules landing close to each other and coagulating is larger. Dimers. Another important conclusion regarding the HP clustering can be drawn from the comparison of the fragment intensities for protonated monomer and larger clusters for various conditions summarized in Figure 6. We assume that the major contribution to the intensity of the protonated monomer (HP)H+ stems from the ionization of the neutral dimers (HP)2. We cannot exclude some contribution to the (HP)H+ intensity from the larger neutral clusters, yet we do not expect a large (HP)n fragmentation of the clusters upon ionization. This assumption can be supported for PI process by its threshold character which has been revealed in the section Cluster Composition, Ionization, and Fragmentation. The near-threshold photoionization does not usually lead to a large cluster fragmentation due to a low excess energy of the ionizing photons. For example, in the VUV photoionization studies of water clusters,59 the major process after the near-threshold photoionization was just the loss of the OH radical (which was eventually followed by a metastable evaporation of up to three

Figure 6. Intensities of the mass peaks corresponding to the protonated (HP)nH+ clusters as a function of n, normalized on the protonated monomer intensity for PI (blue triangles) and EI (red squares) for different conditions: (a, b) pickup of HP molecules on (H2O)N nanoparticles at the HP oven temperature TP = 308 and 333 K, respectively; (c) (HP) n clusters generated in HP/argon coexpansion; (d) pickup of HP molecules on ArN nanoparticles.

water molecules). In the present case, the (HP)n cluster ionization occurs in the large nanoparticle which acts as an efficient heat bath for the excess energy, and therefore any metastable evaporation can be suppressed. Therefore, the major channel in the near-threshold PI of the (HP)n cluster in the large nanoparticle can be assumed to be (HP)n−1H+ ion fragment. On the other hand, the 70 eV electron ionization can lead to a much larger cluster fragmentation, which has been illustrated again for water clusters even at much lower electron energies.38 However, the excess energy deposited after the electron ionization into clusters of hydrogen-bonded heteroaromatic ring molecules was shown experimentally not to exceed significantly ∼1−2 eV.66 In the present case of (HP)n cluster ionization in the larger nanoparticles the fragmentation after the EI can be largely suppressed for two reasons: First, the nanoparticle acts as an efficient heat bath as mentioned above and the excess energy can be dissipated by the water evaporation. Second, the ionization proceeds via the charge or proton transfer process as discussed in the section Cluster Composition, Ionization, and Fragmentation. The CT and PT are soft ionization processes compared to the direct EI. Besides, the similarities between EI and PI of the dependencies in Figure 6a and Figure 6b suggest that both EI and PI methods lead to a similar degree of fragmentation at least in the case of (HP)n ionization on ice nanoparticles. Therefore, we assume that the (HP)H+ ion fragment is representative of the neutral (HP)2 dimers generated on the nanoparticles. Figure 6 shows the ratio of peak intensities for (HP)nH+, n = 1−4, normalized to the n = 1 intensity for various conditions (the numerical values of the ratios can be found in Table S1 in Supporting Information). Indeed, the ratio decreases with n in all cases. However, the drop from n = 1 to n = 2 is the most dramatic for the HP clusters generated on ice nanoparticles. Figure 6a and Figure 6b illustrate this decrease for the measurements at the pickup temperatures TP of 308 and 333 K, respectively (corresponding to the spectra in Figures 2 and 4, respectively). This is compared to the case of HP coexpansion G


Article

The Journal of Physical Chemistry A with Ar in Figure 6c where the decrease is rather gradual. The intensity ratio (HP)2H+/(HP)H+ is 0.1−0.2 on ice nanoparticles, while it is 0.5−0.7 in the coexpansion (see Table S1 in Supporting Information for numerical values). This suggests that on the ice nanoparticles the dimers are generated predominantly. While the larger HP clusters are generated as well, there is a large gap between the probability of generating the HP dimer and then coagulating with further HP molecules on ice. On the other hand, the gradual decrease in intensities in seeded expansions suggests that the clusters are generated without any strong propensity for the dimers: once the HP dimer is generated in the expansion, the further HP molecules can be added gradually. This strong propensity for dimer generation on ice nanoparticles is important for the proposed synthesis of biomolecules on cosmic ice grains. The case of coagulation on ArN nanoparticles, Figure 6d, seems to be somewhere between the two above-discussed cases. The intensity ratios from EI are reminiscent of the spectra from the seeded expansions, while the PI spectra show more similarity with the spectra on ice nanoparticles. Therefore, we cannot make any conclusions concerning the propensity to dimer generation by coagulation on argon nanoparticles. To support the experimentally observed dimer generation on ice nanoparticles with theory, we have calculated the HP dimer structures on water clusters. The character of binding of the HP dimers on the ice surface has been investigated for (2PY)2 and (2HP)2 homodimers (Figure 7 a and Figure 7b, respectively) and 2PY·2HP heterodimer (Figure 7c), in which the two monomers interact via both hydrogen bonds, in the same manner as the respective gas phase minimum structures shown in Figure 1. In all cases there is an important deviation from the

planarity upon the interaction with the ice surface. In addition to the two hydrogen bonds between the two monomers, there are two hydrogen bonds which stabilize the binding of the dimers to the ice surface. This is illustrated in the right column of Figure 7 where the detailed structures and hydrogen bond lengths are shown. The calculated interaction energies show the strongest binding to the ice surface for (2PY)2. The binding of 2PY·2HP and (2HP)2 dimers is weaker by about 2 and 6 kcal/ mol, respectively. However, the dimer structures with two hydrogen bonds do not represent the energy minimum structures on the ice surface. In the clusters shown in parts a, b, and c of Figure 8 the

Figure 8. HP dimer structures optimized on ice clusters (labels as in Figure 7).

optimized dimer structures for (2PY)2, (2HP)2, and 2PY·2HP, respectively, are even more distorted from the gas phase optimized structures (Figure 1). In the case of (2PY)2 and 2PY· 2HP the dimers interact via only one intermolecular hydrogen bond. Their binding to the ice surface is, however, further strengthen by hydrogen bonding to the ice surface. As shown in the (2PY)2 structure on ice (Figure 8a), there are additional five hydrogen bonds formed with and between the water molecules which are directly bound to (2PY)2. As a result, the binding is stronger by about 4 kcal/mol compared to the relevant structure in Figure 7a. Similarly, the structure with one hydrogen bond between 2HP and 2PY and additional four hydrogen bonds formed between the 2PY·2HP dimer and the ice surface, Figure 8c, make the resulting complex by about 3 kcal/mol more stable compared to the relevant structure in Figure 7c. The binding of (2HP)2 dimer is very similar in both binding motives with very similar interaction energies; see Figure 7b and Figure 8b. Our calculations support the experimental observation according to which the HP dimers are likely to be formed on the ice nanoparticles. However, due to the multiple hydrogen bonds with water molecules, these dimer structures can be

Figure 7. Structures of HP dimers in gas-phase configurations on ice clusters: (a) (2PY)2, (b) (2HP)2, and (c) 2PY·2HP: the whole cluster used for optimization (left) and detailed view of the hydrogen bonding (right). The hydrogen bond distances are given in Å. The interaction energies (kcal/mol) with respect to the gas-phase dimers are shown. H



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quite different from the gas phase dimer structures. These structures on ice are indeed also different from the biologically relevant dimers stabilized by two hydrogen bonds.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11359. Mass spectra for seeded expansion of HP in Ar and also for HP pickup on Ar nanoparticles, and additional calculation results (PDF)

CONCLUSION

We have investigated the pickup of HP molecules on ice nanoparticles in molecular beams and the coagulation of small HP clusters on these nanoparticles. These processes were studied by mass spectrometry using electron ionization and photoionization which provided complementary information. The latter process turned out to be highly selective ionizing exclusively HP molecules solvated in clusters. The observed cluster generation and ionization processes were elucidated by theoretical calculations. The major conclusions can be summarized as follows: • Coagulation of molecules picked up on free ice nanoparticles generated in molecular beams was observed. There was no previous experimental evidence for the clustering of molecules on isolated ice nanoparticles in molecular beams; rather an evidence for nonclustering for quite a few molecules has recently been reported by our group.8 The clustering of biologically relevant molecules on ice nanoparticles is of significance for astrobiology. • A preference for HP dimers compared to larger clusters on ice nanoparticles was suggested. This dimerization was not pronounced in free HP clusters generated in coexpansion with argon. Therefore, it seems enhanced by the ice nanoparticles. This is again important for a possible generation of biologically relevant hydrogen bonds on ice nanoparticles. However, our theoretical investigations showed that the binding of the dimer in which both hydrogen bonds between the two monomers stabilize the complex is not necessarilly the most stable. In our calculations stronger binding of distorted dimers interacting via only one hydrogen bond was found. The formation of additional hydrogen bonds with water molecules stabilizes the resulting complex with respect to the binding via two hydrogen bonds between the monomers. • We demonstrated that a nonresonant multiphoton photoionization can be a highly selective tool for HP ionization in clusters. The isolated HP molecules (and also water molecules and clusters) were not ionized by PI; only the HP molecules solvated in clusters (ice nanoparticles and/or (HP)n) were ionized. • Comparison of the highly selective PI with rather universal EI yielded some information about the neutral clusters and their ionization mechanism; namely, several intracluster ion−molecule reaction schemes were proposed to substantiate the observed spectra. The present case of hydroxypyridine has been so far the only molecule experimentally proved to coagulate to clusters after the pickup on the free ice nanoparticles in molecular beams, while a number of other molecules did not coagulate and remained isolated on ice nanoparticles.8 Is the observed clustering a general behavior for molecules which can form similar binding motives as observed in HP? This question can be addressed in future studies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +420 2 6605 3206. Fax: +420 2 8658 2307. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Czech Science Foundation under Project 14-14082S. REFERENCES

(1) Jeffrey, G. A.; Saenger, W. Hydrogen Bonding in Biological Structures; Springer: Berlin, 1991. (2) Holtom, P. D.; Bennett, C. E.; Osamura, Y.; Mason, N. J.; Kaiser, R. I. A Combined Experimental and Theoretical Study on the Formation of the Amino Acid Glycine and its Isomer in Extraterrestrial Ices. Astrophys. J. 2005, 626, 940−952. (3) Herbst, E.; van Dishoeck, E. F. Complex Organic Interstellar Molecules. Annu. Rev. Astron. Astrophys. 2009, 47, 427−480. (4) Bernstein, M. P.; Dworkin, J. P.; Sandford, S. A.; Cooper, G. W.; Allamandola, L. J. Racemic Amino Acids from the Ultraviolet Photolysis of Interstellar Ice Analogues. Nature 2002, 416, 401−403. (5) Caro, G. M. M.; Meierhenrich, U. J.; Schutte, W. A.; Barbier, B.; Segovia, A. A.; Rosenbauer, H.; Thiemann, W. H.-P.; Brack, A.; Greenberg, J. M. Amino Acids from Ultraviolet Irradiation of Interstellar Ice Analogues. Nature 2002, 416, 403−406. (6) Goldman, N.; Tamblyn, I. Prebiotic Chemistry within a Simple Impacting Icy Mixture. J. Phys. Chem. A 2013, 117, 5124−5131. (7) Poterya, V.; Kočišek, J.; Lengyel, J.; Svrčková, P.; Pysanenko, A.; Hollas, D.; Slavíček, P.; Fárník, M. Clustering and Photochemistry of Freon CF2Cl2 on Argon and Ice Nanoparticles. J. Phys. Chem. A 2014, 118, 4740−4749. (8) Pysanenko, A.; Habartová, A.; Svrčková, P.; Lengyel, J.; Poterya, V.; Roeselová, M.; Fedor, J.; Fárník, M. Lack of Aggregation of Molecules on Ice Nanoparticles. J. Phys. Chem. A 2015, 119, 8991− 8999. (9) Gough, T. E.; Mengel, M.; Rowntree, P. A.; Scoles, G. Infrared Spectroscopy at the Surface of Clusters: SF6 on Ar. J. Chem. Phys. 1985, 83, 4958. (10) Fedor, J.; Poterya, V.; Pysanenko, A.; Fárník, M. Cluster Cross Sections from Pickup Measurements: Are the Established Methods Consistent. J. Chem. Phys. 2011, 135, 104305. (11) Lengyel, J.; Kočišek, J.; Poterya, V.; Pysanenko, A.; Svrčková, P.; Fárník, M.; Zaouris, D.; Fedor, J. Uptake of Atmospheric Molecules by Ice Nanoparticles: Pickup Cross Sections. J. Chem. Phys. 2012, 137, 034304. (12) Lengyel, J.; Pysanenko, A.; Poterya, V.; Slavíček, P.; Fárník, M.; Kočišek, J.; Fedor, J. Irregular Shapes of Water Clusters Generated in Supersonic Expansions. Phys. Rev. Lett. 2014, 112, 113401. (13) Poterya, V.; Lengyel, J.; Pysanenko, A.; Svrčková, P.; Fárník, M. Imaging of Hydrogen Halides Photochemistry on Argon and Ice Nanoparticles. J. Chem. Phys. 2014, 141, 074309. (14) Poterya, V.; Nachtigallová, D.; Fárník, J. L. M. Photodissociation of Aniline N−H Bond in Clusters of Different Nature. Phys. Chem. Chem. Phys. 2015, 17, 25004−25013. (15) Kim, S. K.; Lee, W.; Herschbach, D. R. Cluster Beam Chemistry: Hydration of Nucleic Acid Bases; Ionization Potentials I


Article

The Journal of Physical Chemistry A of Hydrated Adenine and Thymine. J. Phys. Chem. 1996, 100, 7933− 7937. (16) Belau, L.; Wilson, K. R.; Leone, S. R.; Ahmed, M. VacuumUltraviolet Photoionization Studies of the Microhydration of DNA Bases (Guanine, Cytosine, Adenine, and Thymine). J. Phys. Chem. A 2007, 111, 7562−7568. (17) Nosenko, Y.; Kunitski, M.; Riehn, C.; Harbach, P. H. P.; Dreuw, A.; Brutschy, B. The Structure of Adenine Monohydrates Studied by Femtosecond Multiphoton Ionization Detected IR Spectroscopy and Quantum Chemical Calculations. Phys. Chem. Chem. Phys. 2010, 12, 863−870. (18) Müller, A.; Talbot, F.; Leutwyler, S. Intermolecular Vibrations of Jet-Cooled (2-Pyridone)2: A Model for the Uracil Dimer. J. Chem. Phys. 2000, 112, 3717−3725. (19) Müller, A.; Talbot, F.; Leutwyler, S. Intermolecular Vibrations of the Jet-Cooled 2-Pyridone·2-Hydroxypyridine Mixed Dimer, a Model for Tautomeric Nucleic Acid Base Pairs. J. Chem. Phys. 2001, 115, 5192−5202. (20) Borst, D. R.; Roscioli, J. R.; Pratt, D. W.; Florio, G. M.; Zwier, T. S.; Müller, A.; Leutwyler, S. Hydrogen Bonding and Tunneling in the 2-Pyridone·2-Hydroxypyridine Dimer. Effect of Electronic Excitation. Chem. Phys. 2002, 283, 341−354. (21) Meuwly, M.; Müller, A.; Leutwyler, S. Energetics, Dynamics and Infrared Spectra of the DNA Base-Pair Analogue 2-Pyridone·2Hydroxypyridine. Phys. Chem. Chem. Phys. 2003, 5, 2663−2672. (22) Müller, A.; Losada, M.; Leutwyler, S. Ab Initio Benchmark Study of (2-Pyridone)2, a Strongly Bound Doubly Hydrogen-Bonded Dimer. J. Phys. Chem. A 2004, 108, 157−165. (23) Szyc, Ł.; Guo, J.; Yang, M.; Dreyer, J.; Tolstoy, P. M.; Nibbering, E. T. J.; Czarnik-Matusewicz, B.; Elsaesser, T.; Limbach, H.-H. The Hydrogen-Bonded 2-Pyridone Dimer Model System. 1. Combined NMR and FT-IR Spectroscopy Study. J. Phys. Chem. A 2010, 114, 7749−7760. (24) Yang, M.; Szyc, Ł.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. The Hydrogen-Bonded 2-Pyridone Dimer Model System. 2. Femtosecond Mid-Infrared Pump-Probe Study. J. Phys. Chem. A 2010, 114, 12195−12201. (25) Nimlos, M. R.; Kelley, D. F.; Bernstein, E. R. Spectroscopy, Structure, and Proton Dynamics of 2-Hydroxypyridine and Its Clusters with Water and Ammonia. J. Phys. Chem. 1989, 93, 643−651. (26) Held, A.; Pratt, D. W. Hydrogen Bonding in Water Complexes. Structures of 2-Pyridone-H2O and 2-Pyridone-(H2O)2 in Their S0 and S1 Electronic States. J. Am. Chem. Soc. 1993, 115, 9708−9717. (27) Matsuda, Y.; Ebata, T.; Mikami, N. IR-UV Double-Resonance Spectroscopic Study of 2-Hydroxypyridine and Its Hydrogen-Bonded Clusters in Supersonic Jets. J. Phys. Chem. A 2001, 105, 3475−3480. (28) Maris, A.; Ottaviani, P.; Caminati, W. Pure Rotational Spectrum of 2-Pyridone···Water and Quantum Chemical Calculations on the Tautomeric Equilibrium 2-Pyridone···Water/2-Hydroxypyridine··· Water. Chem. Phys. Lett. 2002, 360, 155−160. (29) Brause, R.; Schmitt, M.; Kleinermanns, K. Improved Determination of Structural Changes of 2-Pyridone-(H2O)1 upon Electronic Excitation. J. Phys. Chem. A 2007, 111, 3287−3293. (30) Saed, B.; Omidyan, R. Protonation Effect on the Electronic Properties of 2-Pyridone Monomer, Dimer and its Water Clusters: A Theoretical Study. J. Chem. Phys. 2014, 140, 024315. (31) Poully, J. C.; Schermann, J. P.; Nieuwjaer, N.; Lecomte, F.; Gregoire, G.; Desfrancois, C.; Garcia, G. A.; Nahon, L.; Nandi, D.; Poisson, L.; Hochlaf, M. Photoionization of 2-Pyridone and 2Hydroxypyridine. Phys. Chem. Chem. Phys. 2010, 12, 3566−3572. (32) Poisson, L.; Nandi, D.; Soep, B.; Hochlaf, M.; Boggio-Pasqua, M.; Mestdagh, J.-M. A Roaming Wavepacket in the Dynamics of Electronically Excited 2-Hydroxypyridine. Phys. Chem. Chem. Phys. 2014, 16, 581−587. (33) Beak, P.; Fry, F. S. Equilibrium Between 2-Hydroxypyridine and 2-Pyridone in the Gas Phase. J. Am. Chem. Soc. 1973, 95, 1700−1702. (34) Sobolewski, A. L. The Mechanism of Excited-State Hydrogen Transfer in 2-Hydroxypyridine. Chem. Phys. Lett. 1993, 211, 293−299.

(35) Sobolewski, A. L.; Adamowicz, L. Photophysics of 2Hydroxypyridine: An ab Initio Study. J. Phys. Chem. 1996, 100, 3933−3941. (36) Svrčková, P.; Pysanenko, A.; Lengyel, J.; Rubovič, P.; Kočišek, J.; Poterya, V.; Slavíček, P.; Fárník, M. Photodissociation Dynamics of Ethanethiol in Clusters: Complementary Information from Velocity Map Imaging, Mass Spectrometry and Calculations. Phys. Chem. Chem. Phys. 2015, 17, 25734−25741. (37) Kočišek, J.; Lengyel, J.; Fárník, M. Ionization of Large Homogeneous and Heterogeneous Clusters Generated in AcetyleneAr Expansions: Cluster Ion Polymerization. J. Chem. Phys. 2013, 138, 124306. (38) Lengyel, J.; Pysanenko, A.; Poterya, V.; Kočišek, J.; Fárník, M. Extensive Water Cluster Fragmentation After Low Energy Electron Ionization. Chem. Phys. Lett. 2014, 612, 256−261. (39) Lengyel, J.; Poterya, V.; Fárník, M. Proton Transfer and Isotope Induced Reaction in Aniline Cluster Ions. J. Mass Spectrom. 2015, 50, 643−649. (40) Bobbert, C.; Schütte, S.; Steinbach, C.; Buck, U. Fragmentation and Reliable Size Distributions of Large Ammonia and Water Clusters. Eur. Phys. J. D 2002, 19, 183−192. (41) Kočišek, J.; Lengyel, J.; Fárník, M.; Slavíček, P. Energy and Charge Transfer in Ionized Argon Coated Water Clusters. J. Chem. Phys. 2013, 139, 214308. (42) Buck, U.; Pradzinski, C. C.; Zeuch, T.; Dieterich, J. M.; Hartke, B. A Size Resolved Investigation of Large Water Clusters. Phys. Chem. Chem. Phys. 2014, 16, 6859−6871. (43) Buck, U.; Krohne, R. Cluster Size Determination from Diffractive He Atom Scattering. J. Chem. Phys. 1996, 105, 5408−5415. (44) Farges, J.; de Feraudy, M. F.; Raoult, B.; Torchet, G. Structure and Temperature of Rare Gas Clusters in a Supersonic Expansion. Surf. Sci. 1981, 106, 95. (45) Lengyel, J.; Pysanenko, A.; Rubovič, P.; Fárník, M. Sodium doping and reactivity in pure and mixed ice nanoparticles. Eur. Phys. J. D 2015, 69, 269. (46) Matsuda, Y.; Ebata, T.; Mikami, N. Population Labeling Spectroscopy for the Electronic and the Vibrational Transitions of 2Pyridone and Its Hydrogen-Bonded Clusters. J. Chem. Phys. 2000, 113, 573−580. (47) Heger, D.; Nachtigallová, D.; Surman, F.; Krausko, J.; Magyarová, B.; Brumovský, M.; Rubeš, M.; Gladich, I.; Klán, P. SelfOrganization of 1-Methylnaphthalene on the Surface of Artificial Snow Grains: A Combined Experimental-Computational Approach. J. Phys. Chem. A 2011, 115, 11412−11422. (48) Grimme, S. Semiempirical GGA-type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (49) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829. (50) Becke, A. D. Density Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (51) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (52) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−32305. (53) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D; Gaussian Inc.: Wallingford, CT, 2009. (55) Rocher-Casterline, B. E.; Ch’ng, L. C.; Mollner, A. K.; Reisler, H. Determination of the Bond Dissociation Energy (D0) of the Water J


Article

The Journal of Physical Chemistry A Dimer, (H2O)2, by Velocity Map Imaging. J. Chem. Phys. 2011, 134, 211101. (56) Samanta, A. K.; Czakó, G.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H. Experimental and Theoretical Investigations of Energy Transfer and Hydrogen-Bond Breaking in Small Water and HCl Clusters. Acc. Chem. Res. 2014, 47, 2700−2709. (57) Maršaĺ ek, O.; Elles, C. G.; Pieniazek, P. A.; Pluhařová, E.; VandeVondele, J.; Bradforth, S. E.; Jungwirth, P. Chasing Charge Localization and Chemical Reactivity Following Photoionization in Liquid Water. J. Chem. Phys. 2011, 135, 224510. (58) Svoboda, O.; Hollas, D.; Ončaḱ , M.; Slavíček, P. Reaction Selectivity in an Ionized Water Dimer: Nonadiabatic Ab Initio Dynamics Simulations. Phys. Chem. Chem. Phys. 2013, 15, 11531− 11542. (59) Belau, L.; Wilson, K. R.; Leone, S. R.; Ahmed, M. Vacuum Ultraviolet (VUV) Photoionization of Small Water Clusters. J. Phys. Chem. A 2007, 111, 10075−10083. (60) Ozeki, H.; Cockett, M. C. R.; Okuyama, K.; Takahashi, M.; Kimura, K. Structural Isomers and Tautomerism of 2-Hydroxypyridine in the Cation Ground State Studied by Zero Kinetic Energy (ZEKE) Photoelectron Spectroscopy. J. Phys. Chem. 1995, 99, 8608−8612. (61) Michelson, A. Z.; Petronico, A.; Lee, J. K. 2-Pyridone and Derivatives: Gas-Phase Acidity, Proton Affinity, Tautomer Preference, and Leaving Group Ability. J. Org. Chem. 2012, 77, 1623−1631. (62) Hunter, E. P. L.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update. J. Phys. Chem. Ref. Data 1998, 27, 413−656. (63) Kostko, O.; Bravaya, K.; Krylov, A.; Ahmed, M. Ionization of Cytosine Monomer and Dimer Studied by VUV Photoionization and Electronic Structure Calculations. Phys. Chem. Chem. Phys. 2010, 12, 2860−2872. (64) Khistyaev, K.; Golan, A.; Bravaya, K. B.; Orms, N.; Krylov, A. I.; Ahmed, M. Proton Transfer in Nucleobases is Mediated by Water. J. Phys. Chem. A 2013, 117, 6789−6797. (65) Poterya, V.; Kočišek, J.; Pysanenko, A.; Fárník, M. Caging of Cl atoms from photodissociation of CF2Cl2 in clusters. Phys. Chem. Chem. Phys. 2014, 16, 421−429. (66) Poterya, V.; Tkác,̌ O.; Fedor, J.; Fárník, M.; Slavíček, P.; Buck, U. Mass Spectrometry of Hydrogen Bonded Clusters of Heterocyclic Molecules: Electron Ionization vs. Photoionization. Int. J. Mass Spectrom. 2010, 290, 85−93.

K


Biomolecule Analogues 2-Hydroxypyridine and 2-Pyridone Base

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