Quantum Tunneling Observed Without Its Characteristic Large Kinetic Isotope Effects

Quantum tunneling observed without its characteristic large kinetic isotope effects

Tetsuya Hama1, Hirokazu Ueta2, Akira Kouchi, and Naoki Watanabe

Institute of Low Temperature Science, Hokkaido University, Sapporo 060–0819, Japan

Edited by Eric Herbst, University of Virginia, Charlottesville, VA, and accepted by the Editorial Board May 4, 2015 (received for review January 20, 2015)

Classical transition-state theory is fundamental to describing chem- KIE can be much smaller than that intrinsically associated with ical kinetics; however, quantum tunneling is also important in tunneling in cases when the isotopically insensitive process de- explaining the unexpectedly large reaction efficiencies observed in termines the reaction rate. In other words, an observation of a many chemical systems. Tunneling is often indicated by anomalously small KIE under a given condition does not necessarily exclude large kinetic isotope effects (KIEs), because a particle’s ability to tun- tunneling, and tunneling may occur unrecognized in chemical nel decreases significantly with its increasing mass. Here we exper- reactions on condensed phases. – imentally demonstrate that cold hydrogen (H) and deuterium (D) Using in situ IR reflection absorption spectroscopy (Methods; – atoms can add to solid benzene by tunneling; however, the observed See also Supporting Information and Figs. S1 S3), we previously H/D KIE was very small (1–1.5) despite the large intrinsic H/D KIE of showed that H atoms can add to an amorphous solid benzene tunneling (≳100). This strong reduction is due to the chemical kinet- (C6H6) surface by tunneling to form cyclohexane (C6H12)at20K ics being controlled not by tunneling but by the surface diffusion of (8). The present study investigates the KIEs associated with tunneling in the following hydrogenation/deuteration reactions of the H/D atoms, a process not greatly affected by the isotope type. – Because tunneling need not be accompanied by a large KIE in surface amorphous solid C6H6 over a wide temperature range (10 50 K): and interfacial chemical systems, it might be overlooked in other + ð Þ → ð Þ = · −1 [R1] systems such as aerosols or enzymes. Our results suggest that sur- C6H6 H D C6H7 C6H6D Ea 18.2 kJ mol , face tunneling reactions on interstellar dust may contribute to the

deuteration of interstellar aromatic and aliphatic hydrocarbons, which C6H7 + HðDÞ → C6H8ðC6H6D2Þ, [R2] CHEMISTRY could represent a major source of the deuterium enrichment ob- − served in carbonaceous meteorites and interplanetary dust parti- C H + HðDÞ → C H ðC H D Þ E = 6.3 kJ · mol 1, [R3] cles. These findings could improve our understanding of interstellar 6 8 6 9 6 6 3 a physicochemical processes, including those during the formation of + ð Þ → ð Þ [R4] the solar system. C6H9 H D C6H10 C6H6D4 ,

quantum tunneling kinetic isotope effect heterogeneous reactions −1 | | | C6H10 + HðDÞ → C6H11ðC6H6D5Þ Ea = 10.5 kJ · mol , [R5] reaction dynamics | astrochemistry

C6H11 + HðDÞ → C6H12ðC6H6D6Þ, [R6] unneling arises from the wave nature of matter, allowing Tparticles to penetrate barriers that are impossible to over- where Ea is the activation barrier for H-atom addition in the gas come classically. Because the de Broglie wavelength is inversely phase (9, 10). The radical recombination reactions R2, R4, and proportional to particle momentum, tunneling becomes notice- R6 are barrierless on the surface. We exposed amorphous C6H6 able in small masses and at low temperatures. The de Broglie wavelengths for hydrogen (H, 9.8–1.8 Å) and deuterium (D, 6.9– Significance 1.3 Å) at 10–300 K exceed the scale of the typical widths of ∼ activation barriers in chemical reactions ( 1 Å), which invalid- Quantum tunneling, an important phenomenon in many sur- ates a purely classical description of their motion in chemistry + − face and interfacial chemical processes, is strongly dependent (1, 2). Hydrogen, including its ionic forms (H and H ), is present on the isotope of the tunneling atom. However, surface tun- in water and most organic compounds, and kinetic measurements neling during the hydrogenation/deuteration of solid benzene have established that tunneling occurs in reactions involving hy- at 15–25 K is accompanied by an almost semiclassical kinetic drogen in gas (1), liquid (1, 3, 4), and solid phases (4). Tunneling isotope effect (KIE) of 1–1.5, which is much lower than that has also been recognized as a significant factor in reactions on intrinsic to tunneling (≳100), because isotopically insensitive surfaces and at interfaces; for example, proton transfer reactions surface diffusion of the adsorbed atoms controls the chemical at the air/water interface (5) and enzyme catalysis (3, 6). More- kinetics. Our results suggest that tunneling has been unrec- over, tunneling reactions (e.g., CO + H or D) on interstellar dust ognized in studies of the chemistry of condensed phases, and are crucial in explaining the abundances of organic molecules such small-KIE tunneling may account for the unexplained fast reac- as methanol and their deuterated isotopologues observed in cold tions of hydrogen and deuterium observed in surface/interface and dense interstellar regions (≤100 K), such as molecular clouds, chemical systems such as aerosols, enzymes, and interstellar where thermally activated reactions rarely occur (2, 7). dust grains. A particle’s ability to tunnel through a barrier decreases drastically with its increasing mass. This means different isotopes of a Author contributions: T.H., A.K., and N.W. designed research; T.H. performed research; given element show very different tunneling behaviors, and larger T.H., H.U., A.K., and N.W. analyzed data; and T.H., H.U., A.K., and N.W. wrote the paper. kinetic isotope effects (KIEs) than those expected from semi- The authors declare no conflict of interest. classical theory have been regarded as a reliable indicator of tun- This article is a PNAS Direct Submission. E.H. is a guest editor invited by the Editorial neling (1). However, chemical reactions involving tunneling on Board. condensed phases are usually accompanied by other surface pro- 1To whom correspondence should be addressed. Email: [email protected]. cesses such as adsorption, diffusion, and desorption. These are 2Present address: National Institute for Materials Science, Tsukuba 305–0047, Japan. generally thermal processes, and thus much less sensitive to the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. reactant isotope than tunneling is (2). Therefore, the observed 1073/pnas.1501328112/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1501328112 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 samples at 10–50 K to cold H or D atoms at 120 K. In situ IR spectroscopy revealed that cyclohexane or deuterated cyclohexane are efficiently formed by reactions R1–R6. Given the activation barriers and low temperatures, these reactions proceeded via tunneling (8, 11); however, we observed only small KIEs. The ratio of the hydrogenation and deuteration rates (H/D) was 1–1.5 at 15–25 K, whereas deuteration by tunneling typically occurs at a rate more than two orders of magnitude smaller than that of the comparable hydrogenation (11). This indicates that the isotopically insensitive surface processes of the atoms physisorbed on solid C6H6 masked the tunneling KIE, despite tunneling’s providing a classically anomalous reaction efficiency. The present study is also the first report to our knowledge of the nonenergetic deuteration of aromatic hydrocarbons at low temperature. We discuss the importance of our findings for astrochemistry and geochemistry in relation to the origin of deuterium enrichment observed in extraterrestrial materials such as interstellar aromatic/aliphatic hydrocarbons, carbonaceous meteorites, and interplanetary dust particles (12–17), the chemistry of which influences our understanding of interstellar physicochemical processes, including the formation of the solar system (18–20). Results

Fig. 1A shows the IR spectra of amorphous C6H6 and C6H12 at 20 K. The column density (the amount of a substance per unit area integrated along a path perpendicular to the surface) was − estimated to be 6 × 1015 cm 2. For reference, the column density 14 −2 of monolayer coverage of crystalline C6H6 is (6–8) × 10 cm (Supporting Information). Fig. 1 B and C show the difference spectra after H or D atom exposure for up to 180 min, respectively. In the difference spectrum B, the C H absorption bands at 3,000–3,100, − 6 6 1,480, and 1,036 cm 1 decreased, and new peaks for the C H − 6 12 products appeared at 2,800–3,000 and 1,453 cm 1. This indicates the formation of C6H12 by reactions R1–R6 with the consumption − of C6H6. After 180-min exposure, both the C6H6 consumption Fig. 1. Typical infrared reflection–absorption spectra at 3,200–800 cm 1. ∼ × and the C6H12 formation saturated at column densities of 9 (A, upper spectrum) Amorphous solid benzene (C6H6) at 20 K before atomic 14 −2 10 cm at 20 K (Figs. S4 and S5), namely 15% of the C6H6 exposure. (A, lower spectrum) Amorphous solid cyclohexane (C6H12) de- molecules reacted with the H atoms. This value is higher than the posited at 20 K. (B and C) Difference spectra for amorphous solid C6H6 at column density of monolayer coverage of crystalline C6H6,suggest- 20 K after 180-min exposure to H or D atoms, respectively. ing a large amount of reactive C6H6 on the surface. We speculate that amorphous C H had 3D island structures and a larger surface 6 6 Δ Δ area than crystalline C H (8,21).ConsumptionofCH was also atom exposure for 180 min ( C6H6_H or C6H6_D)(Fig.3B). At 6 6 6 6 – observed upon exposure to D atoms (Fig. 1C). Although the as- 30 50 K, PH was two to five times larger than PD.Valuesof Δ signment of the products is difficult from the IR spectra, tempera- C6H6_H were also two to four times larger than those of Δ – Δ ture-programmed desorption mass spectrometry showed that the C6H6_D at 30 50 K. PH and C6H6_H were greatest at 20 K. Δ Δ main products were partially deuterated cyclohexane (C6H6D6) Remarkably, both PH/PD and C6H6_H/ C6H6_D lay between with further-deuterated cyclohexane (C H D ,CH D ...C H D , 1.0–1.5 at 15–25 K despite the intrinsic H/D KIE associated with 6 5 7 6 4 8 6 1 11 J C6D12)(Fig. S6). This shows that D atom addition to C6H6 oc- tunneling ( 100) (11). These small differences between addition curredtoformC6H6D6 by tunneling. Subsequent H–Dsub- by H or D tunneling clearly show that isotopically insensitive stitution reactions of C6H6D6 yielded C6H5D7...C6D12 also surface processes strongly contributed to the outcome of these by tunneling (Figs. S7 and S8). Throughout the H atom expo- surface reactions. Note that heating the sample to 50 K slightly sure, C6H12 was the dominant product (Fig. 1 and Figs. S4 and S5). altered the surface structure but did not greatly affect PH or PD This indicates that reaction R1 is the rate-limiting step, because it (Fig. S4). – Δ has the highest barrier of reactions R1–R6. Here, we concentrate At the lowest studied temperatures (10 12 K), PH and C6H6_H Δ Δ on the KIE of H and D atoms on C6H6 consumption. decreased, whereas both PH/PD and C6H6_H/ C6H6_D increased Fig. 2 plots the variation in the column density of amorphous (Figs. 2 and 3). At 10–12 K, H2 or D2 ejected as undissociated C6H6 (ΔC6H6) during exposure to H or D atoms at temperatures molecules from the atomic source can efficiently physisorb onto of 10–50 K. Absolute rate constants of the surface reactions the amorphous C6H6 surface (2). Therefore, we tested the effect of could not be determined owing to the surface heterogeneity and adsorbed molecules on the hydrogenation/deuteration reactions −3 to the difficulty of measuring the surface number density of via additional H2 or D2 codeposition at (1–2) × 10 Pa (Fig. 4). atoms. Hence, we used the reaction probabilities of C6H6 per At 15 K, the variations of C6H6 were similar both with and without incident H or D atom (PH or PD) during the initial 3-min ex- the additional H2 or D2 codeposition. However, C6H6 con- posure to evaluate the KIE (Fig. 3A), where the subscripts H and sumption decreased at 10 K when additional H2 was deposited on D represent H and D atoms, respectively. PH or PD were cal- the amorphous C6H6 surface. In the D/D2 codeposition exper- culated as the ratio of C6H6 consumption to the fluence of either iment, the decrease in C6H6 consumption was observed at 12 K. H or D atoms. For reference, the fluences of H and D atoms These results show that the atomic addition reactions are inhibited after 3-min exposure were estimated to be 8.1 × 1016 and 6.5 × by competing long-term adsorption of molecules on the surface at − 1016 cm 2, respectively, assuming a flux of H atoms of 4.5 × 1014 10–12 K, and reaction R1 at 10–12 K is controlled by the ad- − − − − cm 2·s 1 and of D atoms of 3.6 × 1014 cm 2·s 1 (Figs. S2 and S3). sorption of atoms that overcame the inhibiting effect of the ad- We also plotted the amounts of C6H6 consumed after H or D sorbed molecules.

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1501328112 Hama et al. Downloaded by guest on October 1, 2021 Fig. 2. Variations in the column density of C6H6 consumption (ΔC6H6) at surface temperatures of 10–50 K. ○, H atom exposure; △, D atom exposure. CHEMISTRY

Discussion of H atoms, respectively. [H] should be determined by the balance Before discussing the KIE of the hydrogenation/deuteration of between the atomic flux, the atom’s adsorption probability, and the amorphous C6H6, we briefly describe the reaction mechanism. loss of the atom by desorption and recombination following sur- There are two major mechanisms for surface reactions: the Eley– face diffusion. Adsorption and desorption of H and D atoms inter- Rideal (ER) mechanism, which is a direct reaction between a acting with the surface through van der Waals forces only have particle from the gas phase and an adsorbate on the surface, and semiclassical KIEs owing to the lower zero-point energy of a D the Langmuir–Hinshelwood (LH) mechanism, which is the re- atom than an H atom. Smoluchowski (22–24) theoretically showed action of two species after adsorption and diffusion (i.e., ther- that the tunneling diffusion of physisorbed H atoms is strongly malization) on the surface (2). The ER mechanism should provide suppressed on an amorphous surface because of the nonperiodic a yield that is largely independent of the surface temperature, adsorption potential sites with different energy depths (25). We whereas the yield of a LH reaction should reflect the adsorption previously observed a small KIE during the surface diffusion of time of atoms on the surface. Fig. 3B shows that the values of H and D atoms on amorphous solid water (25). Vidali, Pirronello, ΔC6H6_H and ΔC6H6_D strongly depend on surface temperature. – and coworkers also reported the thermally activated diffusion of H They increased at low temperatures (15 25 K), until competitive and D atoms on silicates and amorphous carbon (26–28). Even on adsorption of H2 or D2 occurred at 10–12 K (Fig. 4). This suggests that reaction R1 occurred mainly via the LH pathway. A large a crystalline metal surface, surface defects and steps strongly sup- barrier exists for reaction R1, and we previously showed that press the tunneling diffusion of H atoms (29). Hence, the surface − diffusion of H and D atoms on the amorphous C6H6 solid can be C6H6 C6H6 intermolecular interactions only act as inhibitors J through steric hindrance (8). Hence, reaction R1 mainly proceeds limited by thermal hopping with a small KIE (kdiff_H kdiff_D). In on the surface, not in the bulk. The LH mechanism at low tem- addition, there should be a small difference between [H] and [D]. peratures (10–50 K) also indicates that the atomic H and D ad- The factor κ (0 K κ K 1) represents the probability of an atom ditions require tunneling (11). Our previous study also found that reacting with a C6H6 molecule by tunneling instead of undergoing the reactivity of C6H6 with H atoms is inhomogeneous across its a competing process, such as escaping from the reaction site by amorphous surface (8). Both reactive and nonreactive C6H6 mole- diffusive hopping or desorption (30): cules are distributed on the surface; C6H6 becomes less reactive as its number of neighboring C H molecules increases, and dan- ktunnel 6 6 κ = , [2] gling C6H6 molecules that lack near neighbors are the most ktunnel + kdiff + kdes reactive (8). In addition, the surface becomes covered with cyclohexane products following the atomic addition reactions where k and k represent the intrinsic tunneling and de- (Fig. 1). Therefore, the tunneling addition reaction requires atoms tunnel des sorption rates of an atom, respectively. The KIE on the C6H6 to encounter and stay with reactive C6H6 before thermal de- consumption (r =r ) is written as sorption; that is, atomic diffusion is essential. H D Neglecting the surface diffusion of C6H6, a rate equation for rH −κH kdiff H ½H½C6H6 the consumption of C6H6 by reaction with H atoms (rH) via the KIE = = rD −κD kdiff D ½D½C6H6 LH mechanism can be expressed as d½C H = ktunnel H [3] r = 6 6 = −κ k ½H½C H , [1] + + H H diff H 6 6 ktunnel H kdiff H kdes H d t ktunnel D + kdiff D + kdes D kdiff H ½H where [C6H6], [H], and kdiff_H represent the surface number den- × . ktunnel D kdiff D ½D sity of reactive C6H6 molecules and H atoms and the diffusion rate

Hama et al. PNAS Early Edition | 3of6 Downloaded by guest on October 1, 2021 the range 10–50 K. The inverse of the Arrhenius equation predicts a change of many orders of magnitude in the adsorption time of an atom physisorbed on amorphous C6H6: The variation is probably −10 around 1/kdes = (10–10 )sat10–50 K (Fig. 3C and Fig. S9). We first consider the case of kdes and kdiff being much larger than ktunnel (ktunnel kdiff and kdes). This means that the average adsorption time of an atom on a C6H6 site (τads ∝ 1/kdes and 1/kdiff)is short compared with the average time required for tunneling through the barrier of reaction R1 (τtunnel = 1/ktunnel, thus τads τtunnel) (Fig. 5). This condition is valid when the surface temperature is sufficiently high. In this case, the values of κH and κD are both small (1/3), and the ratio ðκH= κDÞ can be approximated as κ + + H = ktunnel H ktunnel D kdiff D kdes D κD ktunnel H + kdiff H + kdes H ktunnel D k k + k ≈ tunnel H diff D des D . ktunnel D kdiff H + kdes H [4]

Eq. 4 can be rewritten as a product of the KIEs owing to tunneling and other surface processes: k k + k k ½H KIE ≈ tunnel H diff D des D diff H . [5] ktunnel D kdiff H + kdes H kdiff D ½D

A D atom requires a longer time for tunneling than an H atom does (τtunnel_H τtunnel_D and thus ktunnel_H ktunnel_D). Hence, Eqs. 4 and 5 indicate that κH κD and that large KIEs appear in the observed rates owing to ðktunnel H=ktunnel DÞ (Fig. 5). In fact, Fig. 3 shows large KIEs at 30–50 K (PH/PD = 2–5). However, these values were much smaller than the calculated tunneling

Fig. 3. Surface temperature dependence of hydrogenation and deutera-

tion of amorphous C6H6 solid samples at 10–50 K. (A) Reaction probabilities per incident H atom (PH, ○) and D atom (PD, △) during initial 3-min exposure. (B)C6H6 consumption after 180-min exposure to H atoms (ΔC6H6_H, ●) or D atoms (ΔC6H6_D, ▲). The H/D ratios of the reaction probabilities (PH/PD) Δ Δ □ and of the C6H6 consumption ( C6H6_H/ C6H6_D) are also plotted ( and ▀ , respectively). (C) Estimated desorption rate (kdes) and tunneling rate (ktunnel) of H atoms as a function of surface temperature. kdes is calculated with the Arrhenius equation, kdes = ν exp(Edes ∕ kB T), where ν, kB, T, and Edes represent the frequency factor, Boltzmann constant, surface temperature, and de-

sorption energy, respectively. ktunnel for the C6H6 + H → C6H7 reaction was calculated with the Eckart model. d is the barrier width parameter. See Supporting Information and Fig. S9 for details.

This rate equation is certainly an oversimplification for surface reactions on amorphous surfaces. Nevertheless, it provides a convenient qualitative explanation of the present results. Adsorption probabilities (ap) of atoms and molecules are generally high on molecular solid surfaces at low temperatures (2, 31). Molecular dynamics calculations showed that ap = 0.8 and 0.4 for H atoms with an incident energy of 100 K on an amorphous solid water surface at 10 and 70 K, respectively (32). Efficient adsorption can be also expected on the amorphous C H surface, considering the stronger van der Waals interaction 6 6 − − of H with C H (2.5 kJ·mol 1) than with H O (0.6 kJ·mol 1) and 6 6 2 − the greater physisorption energy of H on graphite (3.9 kJ·mol 1) − as a solid benzene analog than on water ice (3.3 kJ·mol 1)(33–37). These values of ap are much greater than the obtained reaction −3 probabilities PH and PD of <7 × 10 at 15–50 K (Fig. 3A), Fig. 4. Variations in the column density of C6H6 consumption (ΔC6H6) with suggesting that adsorption is not the dominant rate-limiting respect to exposure time to (A) H atoms and (B) D atoms. Open symbols process. The kinetics appear to be predominantly controlled by indicate results without additional (A)H2 or (B)D2 codeposition at the sur- the subsequent surface diffusion and tunneling reactions. Tun- face temperature of 10 K (○), 11 K (□), 12 K (△), and 15 K (▽). These plots neling through the barrier is almost temperature-independent are identical to those in Fig. 2. Filled symbols indicate results with additional −3 (Fig. 3C and Fig. S9) (1, 11). In contrast, thermal diffusion and (A)H2 or (B)D2 codeposition at (1–2) × 10 Pa by background deposition at desorption are strongly correlated with surface temperature in surface temperatures of 10 K (●), 11 K ( ▀ ), 12 K (▲), and 15 K (▼).

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1501328112 Hama et al. Downloaded by guest on October 1, 2021 findings can also explain the unexpected KIEs observed in other chemical reactions on surfaces. For example, the experimentally deduced KIE value for the addition of H/D atoms by tunneling to solid CO at 15 K (12.5) is 20 times smaller than that theoretically calculated at low temperature in the gas phase (250) (2, 38, 39). This discrepancy should be partly attributable to the isotopically insensitive surface processes of the H and D atoms. Fig. 4 shows that reaction R1 at 10–12 K is controlled by the adsorption of atoms that overcame the inhibiting effect of the adsorbed molecules. The KIEs observed at 10–12 K (Figs. 2 and 4) should be attributable to a semiclassical KIE caused by D2’s having a larger adsorption energy than H2 on the amorphous C6H6, which resulted in [D] being smaller than [H]. The adsorption and ac- cumulation of molecules over long periods also support large values of τads for atoms at low temperatures. In summary, the addition of H and D to amorphous solid C6H6 formed cyclohexane or deuterated cyclohexane by reactions R1– R6. The reactions proceeded via tunneling at low temperatures of 10–50 K. Reaction R1 showed KIEs that depended on the surface temperature; their values (1–1.5 at 15–25 K) were considerably J Fig. 5. Representation of H and D atom additions to amorphous solid C6H6. smaller than the intrinsic KIE associated with tunneling ( 100 at Processes influencing the KIEs of the addition are summarized in the lower 10–50 K). Our results show that the overall KIE at a given tem- τ τ ∝ panel. ads is the average adsorption time of an atom on a C6H6 ( ads 1/kdes perature is influenced by the effects of the reactants’ adsorption, τ and 1/kdiff). tunnel is the average time required for tunneling through the diffusion, and tunneling. We found that the intrinsic KIE asso- τ = barrier of reaction R1 ( tunnel 1/ktunnel). ktunnel, kdiff, and kdes represent the ciated with tunneling can be almost completely masked by surface rates of the tunneling reaction, diffusion, and desorption of atoms, re- processes that are insensitive to the isotope of the reactant atoms. spectively. κ (0 K κ K 1) is the probability of an atom reacting by tunneling The detection of hydrogen tunneling often relies on the obser- rather than undergoing a competing process (e.g., escaping from the re- vation of large KIEs. However, some reactions on condensed CHEMISTRY action site by diffusive hopping or desorption) when it encounters a C6H6 phases can be studied only around room temperature or within a molecule: κ = k /(k + k + k ). The subscripts H and D represent tunnel tunnel des diff limited range of temperatures, and they are often controlled by H and D atoms, respectively. For details of reactive C6H6 on the surface of amorphous solid C H , see the text and ref. 8. the diffusion processes of the reactants, as in aerosol chemistry 6 6 (40, 41), and enzyme catalysis (42, 43). The present findings indicate that tunneling should be considered in the study of chemical reactions involving hydrogen and deuterium on condensed phases, KIE for reaction R1 ðktunnel H=ktunnel D J 100Þ at 10–50 K in the gas phase (11). This suggests that the observed KIEs at 30–50 K even when an anomalously large reaction efficiency is observed are not the high-temperature limit of the KIE expressed in Eq. 5, alongside a small KIE. More complete understanding of the con- but are affected by less isotopically sensitive surface diffusion as tribution of tunneling to heterogeneous reaction dynamics can described below. The high-temperature limit of the KIE would improve the prediction of chemical kinetics (e.g., temperature not be observable in the present study, because κ becomes too dependence) and H/D isotope fractionation in astrochemistry, geochemistry, and biochemistry. small when (k k and k ) (Eq. 2), making P and P tunnel diff des H D The practical implications of this work are associated with the zero at high temperatures. reactions of interstellar aromatic and aliphatic hydrocarbons, At low temperatures of 15–25 K, the KIEs became even = – two of the main components of interstellar and circumstellar smaller (PH/PD 1.0 1.5) (Fig. 2). Next, we consider the reverse dust (7). C H is a precursor of interstellar polycyclic aromatic 6 6 case, kdiff and kdes ktunnel. This situation would require a hydrocarbons (PAHs) and hydrogenated amorphous carbon sufficiently low temperature, because kdes and kdiff drastically grains (aromatic/aliphatic mixture) (44–46). Its structure is rep- decrease with surface cooling. An H or D atom can interact with resentative of the peripheral sites of a PAH. In comparison with τ τ τ C6H6 for a longer period than tunnel ( tunnel ads), which in- C6H6, PAHs tend to have lower activation barriers to H or D creases the values of κH and κD. Finally, they are close to unity: addition owing to the higher flexibility (11, 47, 48). In fact, quantum calculations have shown that H and D addition by tunneling on the peripheral sites of pyrene (C16H10) occurs at κ = ktunnel H ≈ [6] H 1, faster rates than on C6H6 (11, 47). We suggest that interstellar ktunnel H + kdiff H + kdes H aromatic hydrocarbons including C6H6 can be hydrogenated or deuterated by the tunneling of H or D atoms at low tempera- κ = ktunnel D ≈ [7] tures. The deuteration of interstellar aromatic hydrocarbons to D 1. form deuterated aliphatic structures is of particular interest (12– ktunnel D + kdiff D + kdes D 15, 19), because such materials could represent a major carrier of −4 −2 3 deuterium enrichment (D/H = 10 to 10 ) beyond levels Therefore, Eq. can be approximated by only the surface diffu- × −5 sion and the number density of atoms, expected from the elemental D/H ratio in space (1.5 10 ) observed in carbonaceous meteorites and interplanetary dust k ½H particles (17, 20). They may carry signatures of the survival of ≈ diff H [8] interstellar materials within the solar system, because the deu- KIE ½ . kdiff D D terium enrichment is most noticeable in cold, dense interstellar regions (e.g., molecular clouds), where deuterated species are Eq. 8 suggests that the surface diffusion of atoms before they thermodynamically more stable than their hydrogenated coun- encounter reactive C6H6 predominantly controls the kinetics in terparts owing to the zero-point energy difference of several tens tunneling reaction R1 at low temperatures, and the KIE can be of kelvin (18). Because the gaseous atomic D/H ratio in mo- much smaller than that in Eq. 5 owing to the disappearance of lecular clouds can also be strongly enhanced from elemental −5 −2 −1 the tunneling KIE, ktunnel H=ktunnel D (Fig. 5). The very small ratios of 1.5 × 10 to 10 to 10 (18, 49), our results suggest that KIE observed at 15–25 K can be explained by Eq. 8. The present the deuterium enrichment of interstellar aromatic and aliphatic

Hama et al. PNAS Early Edition | 5of6 Downloaded by guest on October 1, 2021 hydrocarbons may occur at low temperatures via the tunneling of transferred to the solid C6H6 samples through an Al pipe cooled to 120 K by D atoms. Tunneling might represent a major deuteration mech- another He refrigerator to reduce the kinetic temperature of the H or D −4 anism for interstellar aromatic hydrocarbons, in addition to en- atoms. The pressure in the main chamber was increased to 3–4 × 10 Pa during the atom exposure. The fluxes of H and D atoms were estimated to ergetic deuteration processes (e.g., photolysis with solid D2Oand − − be (4–5) × 1014 and (3–4) × 1014 atoms·cm 2·s 1, respectively. For reference, hot D-atom irradiation) (50–52), because surface tunneling is fa- − − the typical flux of H atoms in molecular clouds at 10 K is 104 cm 2·s 1. Hence, vored in the cold, dense interstellar environment (2, 7). The the 120-min exposure in our experiments is roughly comparable to an ex- − present study indicates that the tunneling KIE would not strongly posure of about 107 y(3× 1018 atoms·cm 2), which is the typical lifetime of inhibit the deuteration of interstellar aromatic hydrocarbons, and molecular clouds (2). After the atom exposure, the sample was heated to thus it possibly links reactions in the interstellar medium and our 300 K at 2 K·min−1 for temperature-programmed desorption mass spectrom-

observation of bodies in the solar system. etry. For each experiment, a fresh amorphous solid C6H6 sample was prepared. In the additional H2 or D2 codeposition experiments, H2 or D2 molecules at Methods room temperature were introduced into the chamber by background vapor – × −3 Experiments were conducted in an ultrahigh vacuum chamber (base pressure deposition at (1 2) 10 Pa during the atom exposure. The fluxes of both H2 − – × 16 −2· −1 of 10 8 Pa) equipped with an atomic source, an aluminum (Al) substrate and D2 molecules were estimated to be (1 2) 10 cm s . mounted on the cold head of a closed-cycle helium (He) refrigerator, a quadrupole mass spectrometer, and an FTIR spectrometer (Supporting In- ACKNOWLEDGMENTS. We thank Drs. Hiroshi Hidaka and Yasuhiro Oba formation and Fig. S1). Samples of amorphous solid C H were formed on (Institute of Low Temperature Science, Hokkaido University). We also thank 6 6 Dr. Yoichi Nakai of RIKEN for useful suggestions regarding the kinetic isotope the Al substrate at 10 K by the background vapor deposition of C H . The 6 6 effect. This work was supported by Japan Society for the Promotion of Sci- – compositions of the samples were measured by in situ reflection absorption ence Grants-in-Aid for Scientific Research 26410001 and 24224012 and Min- spectroscopy with the FTIR spectrometer. H or D atoms were produced by a istry of Education, Culture, Sports, Science and Technology Grant-in-Aid for microwave-induced plasma in a Pyrex tube within the atomic source and Scientific Research 25108002.

1. Bell RP (1980) The Tunnel Effect in Chemistry (Chapman & Hall, New York). 28. Vidali G (2013) Cosmic low temperature physics: Making molecules on stardust. J Low 2. Hama T, Watanabe N (2013) Surface processes on interstellar amorphous solid water: Temp Phys 170(1–2):1–30. Adsorption, diffusion, tunneling reactions, and nuclear-spin conversion. Chem Rev 29. Zheng CZ, Yeung CK, Loy MM, Xiao X (2006) Quantum diffusion of H on Pt(111): Step 113(12):8783–8839. effects. Phys Rev Lett 97(16):166101. 3. Kohen A, Limbach H-H, eds (2006) Isotope Effects in Chemistry and Biology (CRC, Boca 30. Herbst E, Millar TJ (2008) The chemistry of cold interstellar cloud cores. Low Tempera- Raton, FL). tures and Cold Molecules, ed Smith IWM (Imperial College Press, London), pp 1–54. 4. Miyazaki T, ed (2004) Atom Tunneling Phenomena in Physics, Chemistry and Biology 31. Buch V, Zhang Q (1991) Sticking probability of H and D atoms on amorphous ice: A (Springer, Berlin). computational study. Astrophys J 379:647–652. 5. Enami S, Stewart LA, Hoffmann MR, Colussi AJ (2010) Superacid chemistry on mildly 32. Veeraghattam VK, Manrodt K, Lewis SP, Stancil PC (2014) The sticking of atomic acidic water. J Phys Chem Lett 1(24):3488–3493. hydrogen on amorphous water ice. Astrophys J 790(1):4. 6. Allemann RK (2009) Quantum Tunnelling in Enzyme-Catalysed Reactions,edScruttonNS 33. Ma J, Michaelides A, Alfè D (2011) Binding of hydrogen on benzene, coronene, and (Royal Society of Chemistry, Cambridge, UK). graphene from quantum Monte Carlo calculations. J Chem Phys 134(13):134701. 7. Bromley ST, Goumans TPM, Herbst E, Jones AP, Slater B (2014) Challenges in modelling 34. Bonfanti M, Martinazzo R, Tantardini GF, Ponti A (2007) Physisorption and diffusion the reaction chemistry of interstellar dust. Phys Chem Chem Phys 16(35):18623–18643. of hydrogen atoms on graphite from correlated calculations on the H-coronene 8. Hama T, Ueta H, Kouchi A, Watanabe N, Tachikawa H (2014) Quantum tunneling model system. J Phys Chem C 111(16):5825–5829. hydrogenation of solid benzene and its control via surface structure. J Phys Chem Lett 35. Ghio E, Mattera L, Salvo C, Tommasini F, Valbusa U (1980) Vibrational spectrum of H and 5(21):3843–3848. D on the (0001) graphite surface from scattering experiments. J Chem Phys 73(1):556–561.

9. Hoyermann K, Preuss AW, Wagner HG (1975) Die reaktionen von atomarem was- 36. Zhang Q, Sabelli N, Buch V (1991) Potential energy surface of H···H2O. J Chem Phys serstoff mit cyclohexen, cyclohexadien-1,3 und benzol. Berichte der Bunsengesell- 95(2):1080–1085. schaft für Phys Chemie 79(2):156–165. 37. Al-Halabi A, Kleyn AW, van Dishoeck EF, Kroes GJ (2002) Sticking of hydrogen atoms 10. Gao Y, et al. (2009) Enthalpy of formation of the cyclohexadienyl radical and the C-H to crystalline ice surfaces: Dependence on incidence energy and surface temperature. bond enthalpy of 1,4-cyclohexadiene: An experimental and computational re-evalu- J Phys Chem B 106(25):6515–6522. ation. J Phys Chem A 113(25):6955–6963. 38. Hidaka H, Kouchi A, Watanabe N (2007) Temperature, composition, and hydrogen

11. Goumans TPM, Kästner J (2010) Hydrogen-atom tunneling could contribute to H2 isotope effect in the hydrogenation of CO on amorphous ice surface at 10-20 K. formation in space. Angew Chem Int Ed Engl 49(40):7350–7352. J Chem Phys 126(20):204707. 12. Tielens AGGM (2008) Interstellar polycyclic aromatic hydrocarbon molecules. Annu 39. Andersson S, Goumans TPM, Arnaldsson A (2011) Tunneling in hydrogen and deu- Rev Astron Astrophys 46(1):289–337. terium atom addition to CO at low temperatures. Chem Phys Lett 513(1–3):31–36.

13. Peeters E, et al. (2004) Deuterated interstellar polycyclic aromatic hydrocarbons. As- 40. Hu JH, et al. (1995) Reactive uptake of Cl2(g) and Br2(g) by aqueous surfaces as a trophys J 604(1):252–257. function of Br- and I- ion concentration: The effect of chemical reaction at the in- 14. Onaka T, et al. (2014) Search for the infrared emission features from deuterated interface. J Phys Chem 99(21):8768–8776. terstellar polycyclic aromatic hydrocarbons. Astrophys J 780(2):114. 41. George IJ, Abbatt JPD (2010) Heterogeneous oxidation of atmospheric aerosol par- 15. Kerridge JF, Chang S, Shipp R (1987) Isotopic characterisation of kerogen-like material ticles by gas-phase radicals. Nat Chem 2(9):713–722. in the Murchison carbonaceous chondrite. Geochim Cosmochim Acta 51(9):2527–2540. 42. Alberty RA, Hammes GG (1958) Application of the theory of diffusion-controlled 16. Busemann H, et al. (2006) Interstellar chemistry recorded in organic matter from reactions to enzyme kinetics. J Phys Chem 62(2):154–159. primitive meteorites. Science 312(5774):727–730. 43. Chou K-C, Zhou G-P (1982) Role of the protein outside active site on the diffusion- 17. Messenger S (2000) Identification of molecular-cloud material in interplanetary dust controlled reaction of enzyme. J Am Chem Soc 104(5):1409–1413. particles. Nature 404(6781):968–971. 44. Jones BM, et al. (2011) Formation of benzene in the interstellar medium. Proc Natl 18. Tielens AGGM (2013) The molecular universe. Rev Mod Phys 85(3):1021–1081. Acad Sci USA 108(2):452–457. 19. Sandford SA, Bernstein MP, Dworkin JP (2001) Assessment of the interstellar processes 45. Parker DSN, et al. (2012) Low temperature formation of naphthalene and its role in leading to deuterium enrichment in meteoritic organics. Meteorit Planet Sci 36(8): the synthesis of PAHs (polycyclic aromatic hydrocarbons) in the interstellar medium. 1117–1133. Proc Natl Acad Sci USA 109(1):53–58. 20. Caselli P, Ceccarelli C (2012) Our astrochemical heritage. Astron Astrophys Rev 20(1):1–68. 46. Cherchneff I (2011) The formation of polycyclic aromatic hydrocarbons in evolved 21. Jakob P, Menzel D (1996) Initial stages of multilayer growth and structural phase circumstellar environments. EAS Publ Ser 46:177–189. transitions of physisorbed benzene on Ru (001). J Chem Phys 105(9):3838–3848. 47. Goumans TPM (2011) Hydrogen chemisorption on polycyclic aromatic hydrocarbons

22. Smoluchowski R (1979) Formation of H2 on amorphous ice grains and their impor- via tunnelling. Mon Not R Astron Soc 415(4):3129–3134. tance for planetary atmospheres. Astrophys Space Sci 65(1):29–38. 48. Rauls E, Hornekær L (2008) Catalyzed routes to molecular hydrogen formation and

23. Smoluchowski R (1981) Rate of H2 formation on amorphous grains. Astrophys Space hydrogen addition reactions on neutral polycyclic aromatic hydrocarbons under in- Sci 75(2):353–363. terstellar conditions. Astrophys J 679(1):531–536. 24. Smoluchowski R (1983) Adsorption and mobility on amorphous surfaces. Application 49. Roberts H, Herbst E, Millar TJ (2003) Enhanced deuterium fractionation in dense in- + to astrophysical problems. J Phys Chem 87(21):4229–4233. terstellar cores resulting from multiply deuterated H3 . Astrophys J 591(1):L41–L44. 25. Hama T, et al. (2012) The mechanism of surface diffusion of H and D atoms on 50. Sandford SA, Bernstein MP, Allamandola LJ, Gillette JS, Zare RN (2000) Deuterium amorphous solid water: Existence of various potential sites. Astrophys J 757(2):185. enrichment of polycyclic aromatic hydrocarbons by photochemically induced ex- 26. Pirronello V, Liu C, Roser JE, Vidali G (1999) Measurements of molecular hydrogen change with deuterium-rich cosmic ices. Astrophys J 538(2):691–697. formation on carbonaceous grains. Astron Astrophys 344:681–686. 51. Mennella V, Hornekær L, Thrower J, Accolla M (2012) The catalytic role of coronene

27. Pirronello V, Manicó G, Roser J, Vidali G (2004) H2 formation on dust grains. Astro- for molecular hydrogen formation. Astrophys J 745(1):L2. physics of Dust, eds Witt AN, Clayton GC, Draine BT (Astronomical Society of the 52. Skov AL, Thrower JD, Hornekaer L (2014) Polycyclic aromatic hydrocarbons—catalysts Pacific, San Francisco), pp 529–544. for molecular hydrogen formation. Faraday Discuss 168:223–234.

6of6 | www.pnas.org/cgi/doi/10.1073/pnas.1501328112 Hama et al. Downloaded by guest on October 1, 2021