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Reactivity of aldehydes at the air–water interface. Insights from molecular dynamics simulations and ab initio calculations Marilia T. C. Martins-Costa,a,b Francisco F. García-Prietoa,b,c and Manuel F. Ruiz-López*a,b Understanding the influence of solute–solvent interactions on chemical reactivity has been a subject of intense research in the last few decades. Theoretical studies have focused on bulk solvation phenomena and a variety of models and methods have been developed that are now widely used by both theoreticians and experimentalists. Much less attention has been paid, however, to processes that occur at liquid interfaces despite the important role such interfaces play in chemistry and biology. In this study, we have carried out sequential molecular dynamics simulations and quantum mechanical calculations to analyse the influence of the air–water interface on the reactivity of formaldehyde, acetaldehyde and benzaldehyde, three simple aldehydes of atmospheric interest. The calculated free-energy profiles exhibit a minimum at the interface, where the average reactivity indices may display large solvation effects. The study emphasizes the role of solvation dynamics, which are responsible for large fluctuations of some molecular properties.
Received 23rd September 2014, Accepted 25th November 2014 DOI: 10.1039/c4ob02029d www.rsc.org/obc
We also show that the photolysis rate constant of benzaldehyde in the range 290–308 nm increases by one order of magnitude at the surface of a water droplet, from 2.7 × 10−5 s−1 in the gas phase to 2.8 × 10−4 s−1 at the air–water interface, and we discuss the potential impact of this result on the chemistry of the troposphere. Experimental data in this domain are still scarce and computer simulations like those presented in this work may provide some insights that can be useful to design new experiments.
Introduction Aqueous interfaces are ubiquitous on earth. From the surface of the ocean to that of atmospheric aerosols and cloud droplets, the air–water interface provides a unique environment where neutral molecules, radicals and ions meet, interact and eventually react to form new compounds. Water–hydrophobic interfaces are involved in many other important domains such as the selective transport of chemical species across the semipermeable lipid bilayer of cell membranes, a phenomenon which is of paramount importance in biology. While significant progress has been made in the last few decades in the study of the physical properties of such interfaces and the uptake of molecules, unravelling the mechanism of chemical and photochemical reactions at aqueous interfaces remains a major scientific and technological challenge. In a recent work, we have shown that chemical processes can be significantly accelerated at the air–water interface due to unexpectedly large solvation effects. This conclusion has a
SRSMC, University of Lorraine, BP 70239, 54506 Vandoeuvre-lès-Nancy, France. E-mail: [email protected] b CNRS, UMR N° 7565, 54506 Vandoeuvre-lès-Nancy, France c University of Extremadura, Avda Elvas S-N, E-06071 Badajoz, Spain
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been reached through first-principles molecular dynamics simulations of small radicals1 and molecules2,3 of atmospheric interest. In particular, we have found that the production of OH radicals from ozone photolysis in the troposphere is about four orders of magnitude larger when the reaction takes place at the surface of cloud water droplets, compared to the same process in the gas phase.3 Indeed, water droplets and aqueous aerosols can be considered as micro-reactors that can contribute to the atmospheric chemistry on a global scale.4–9 Water mediated reactions in the troposphere play a significant role10,11 but the heterogeneous processes occurring at the air–water interface12,13 are usually neglected in current atmospheric models. Their impact in terms of air quality and global warming, for instance, remains unknown and clearly such a topic deserves further investigation. In the present paper, we have studied the chemical properties of some aldehydes at the air–water interface. Aldehydes are an important category of volatile organic compounds (VOC), which are released into the atmosphere by both natural and anthropogenic sources. They are emitted by plants and are produced by incomplete combustion processes. They can also be formed by oxidation of other VOCs. Their uptake by water surface has been experimentally14 and theoretically15 studied.
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The main tropospheric sinks are photolysis and reaction with radicals such as OH (daytime reactions) or nitrate radicals (night-time reactions).16 Our aim in this work is to get some insights on how solvation at the water surface affects their molecular properties. The study is carried out by combining classical molecular dynamics simulations and QM/MM calculations (QM/MM stands for a hybrid quantum mechanics and molecular mechanics approach). We first analyze reactivity indices of formaldehyde, acetaldehyde and benzaldehyde, then, we analyze the UV-Vis spectrum of benzaldehyde and discuss the atmospheric implications.
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lation. Reactivity indices (chemical potential, hardness and electrophilicity) are obtained using the conceptual density functional theory and the approximated formula:27 Chemical potential: 1 μ ¼ ðεHOMO þ εLUMO Þ 2 Chemical hardness: η ¼ ðεLUMO εHOMO Þ Electrophilicity: ω¼
Model and calculation method Molecular dynamics simulations have been carried out using the OPLS force field17 for the aldehydes and the TIP3P forcefield18 for water. The simulation box contains 499 water molecules and one aldehyde molecule. The box size is roughly (in Å) 24.7 × 24.7 × 130 (with small differences for the X and Y axis due to different aldehyde masses; in the case of formaldehyde, acetaldehyde and benzaldehyde, these axes are equal to 24.65 Å, 24.66 Å and 24.719 Å, respectively). Periodic boundary conditions are used along the X and Y directions. Simulations were carried out at T = 298 K using a Nosé–Hoover thermostat19,20 and a time step of 0.5 fs with the aldehyde placed initially at the interface. After equilibration, the simulation was carried out for 2 ns. During the simulation, 2000 snapshots were saved at regular intervals for further analysis. Free energy was calculated using the umbrella sampling21 and WHAM22 methods. The reaction coordinate is calculated as the distance between the centre of mass of the solute and the centre of mass of the solvent. This distance was varied by steps of 0.25 Å. The bias potential force constant is k = 10 kcal mol−1 Å−2. After thermalisation, the trajectory was carried out for 200 ps at each point of the reaction coordinate. The properties were computed using the combined quantum mechanics and molecular mechanics (QM/MM) approach23,24 developed by us: the solute is described quantum mechanically at the B3LYP/6-311+G(d) level25 while the water solvent is described classically using the TIP3P forcefield. The solute’s wave function is therefore computed using a Hamiltonian that accounts for electrostatic embedding (i.e. it includes the electrostatic interaction with the solvent). In this way, polarization effects on the electronic properties of the solute are accounted for. Electronic transitions for the first two singlet states (nπ* and ππ*) were calculated using the TD-DFT method.26 The cross-sections were convoluted for each snapshot using the expression: 2 vv 2:772 Δv 0 f 12 1=2 σðvÞ ¼ 0:811 10 e Δv1=2 where σ units are cm2 per molecule if Δν1/2 is expressed in cm−1. We used Δν1/2 = 3226 cm−1 (0.4 eV). The final spectrum is obtained by averaging over 2000 snapshots from the simu-
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μ2 η
where εHOMO/LUMO stands for the energy of the HOMO and LUMO orbitals, respectively. The simulations were done using Tinker 4.228 and the combined QM/MM calculations were done using Gaussian 0929 and the program interface that was developed by us.24 For comparison, we have also reported the calculation of properties for the isolated molecule in the gas phase at its equilibrium geometry. For consistency with the calculations at the air–water interface, the gas phase geometry was optimized with the force-field used in the simulations. The UV-Vis spectrum was convoluted using the same expression and the same width at half maximum, Δν1/2, than in the interface calculation.
Results and discussion Solvation at the interface Fig. 1 displays the calculated free energy profiles for water accommodation of the three aldehydes studied here. All the systems exhibit a free energy minimum at the air–water interface, which is significantly larger for benzaldehyde. This molecule exhibits a preference for the interface that is about 3 kcal mol−1 with respect to bulk water, not far from the value 2.8 kcal mol−1 reported earlier by Canneaux et al.15 The result for formaldehyde (1.5 kcal mol−1) reproduces the value reported before using QM/MM simulations.2 It is interesting to point out that the affinity for the air water interface increases in the order formaldehyde < acetaldehyde < benzaldehyde (compare the stabilisation at the interface with respect to the gas phase), which parallels the increasing hydrogen-bond acceptor character of the compounds. It also parallels the increasing polarity of the carbonyl group in the three aldehydes (see below). A key factor in solvation phenomena at liquid interfaces is the role of the orientation dynamics, since different orientations imply different induced polarizations. For instance, molecules of the same type displaying similar dipole moments and similar polarisabilities in the gas-phase, may interact with the interface in different orientations and undergo quite different polarisations.3 This is a specific property of the interface because in bulk solution, according to Onsager’s theory, the reaction field is roughly proportional to the dipole
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Fig. 1 Calculated free energy profiles at the air–water interface for the aldehydes studied in this work. The formal interface corresponds to R ≈ 12 Å.
moment, regardless of the solute orientation. For the systems under study here, the carbonyl oxygen atom is expected to form hydrogen bonds with water molecules, and intuitively one would deduce that such an interaction orientates the molecules with the CvO group being directed towards the water phase. Moreover, based on aldehyde–water interaction strength alone, one would expect a higher orientation effect of the interface on benzaldehyde followed by acetaldehyde and formaldehyde. Though hydrogen-bonding does certainly represent an important factor in determining the average orientation of the solute with respect to the interface, our results reveal that it is not the only factor. The nature and size of the R group in the aldehyde (RCHO) is also relevant because it governs the hydrophobic interactions with water and restrains the ability of the molecule to cross the interface. As a matter of fact, solvation at the air–water interface is characterized by continuous crossings and recrossings of the solute through the average plane defining the formal interface. These oscillations define an interface width, which in previous simulations of aminoacids at the water–hydrophobic interface30,31 or small molecules at the air–water interface,1,2 has been estimated to be roughly 1 nm. There are situations in which the solute is located in the air layer interacting weakly with the water layer; in that case the solute’s properties are close to gas phase values. In other cases, the solute is well inside the water layer and its properties are close to bulk solvation values. Hence, the time fluctuations of the molecular properties of a solute adsorbed at the air– water interface are expected to be quite large. Fig. 2 displays the statistical averages for the orientation of the aldehydes with respect to the interface. The Z-axis is
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Fig. 2 Angular distribution of aldehydes at the air–water interface from molecular dynamics simulations. The black line corresponds to the uniform distribution 21 sin θ.
defined as the axis perpendicular to the water surface and the angle θ as the angle formed between the Z-axis and the CvO bond of the solute. On an average, formaldehyde and benzaldehyde display a slight preferential orientation with respect to the interface, the interface favouring the situations in which the CvO bond is directed towards the water. In the case of acetaldehyde, the angular distribution is essentially uniform, suggesting that the dynamics in this system plays a more important role. Reactivity indices Solvation effects on reactivity indices in bulk solution have been discussed previously32–35 but to the best of our knowledge, no analyses have yet been reported in the literature for molecules adsorbed at the air–water interface. Indeed, as shown in our previous QM/MM simulations,1,2,23 a systematic energy shift of the molecular orbitals is usually obtained due to the electrostatic potential created by the polarized water layer. In the case of hydrogen-bond acceptors, like in the present case, the polarization of the water layer is such that a net positive potential is created. One then expects the whole set of the solute’s molecular orbitals to be stabilized through the interaction with the interface. If the shifts were strictly the same for all the orbitals, then no net effect of chemical hardness would happen, while a decrease of the chemical potential, in the algebraic value, had to be expected. Obviously, the electrostatic potential created by the polarised water layer is not strictly constant, and the effect of the associated electrostatic field has to be taken into account too. When rigorous calculations were carried out for the molecules considered in the present study, the following results were obtained (see Table 1). First, a large interface effect for
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Table 1 Calculated reactivity indices at the air–water interface from molecular dynamics simulations and QM/MM calculations. Comparison with gas phase values. The change with respect to the gas-phase values is indicated in parenthesis
Dipole (D) μ (eV) η (eV)
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ω (eV)
Gas Interface Gas Interface Gas Interface Gas Interface
Formaldehyde
Acetaldehyde
Benzaldehyde
2.63 3.10 (+0.47) −4.82 −4.92 (−0.10) 5.82 5.68 (−0.14) 2.00 2.16 (+0.16)
2.91 3.68 (+0.77) −4.42 −4.44 (−0.02) 5.92 6.08 (+0.16) 1.65 1.64 (−0.01)
3.56 3.96 (+0.40) −4.81 −4.73 (+0.08) 5.15 4.94 (−0.21) 2.24 2.29 (+0.05)
the hardness index was predicted. The effect can be positive (hardness increase) or negative (hardness decrease), depending on the molecule. Second, the effect on the chemical potential is significantly negative for formaldehyde, as expected for a proton acceptor, but it is significantly positive for benzaldehyde. Finally, the electrophilicity index change is large and positive for formaldehyde but relatively small for the other two molecules. These trends cannot be explained through a single parameter such as the hydrogen-bond strength, and more likely they result from a combination of different factors among which two are probably decisive. On the one hand, there are fundamental differences between benzaldehyde and the other two aldehydes in terms of the electronic structure. In the aromatic system, the π system of the carbonyl bond has some degree of delocalisation over the π orbitals of the phenyl ring. As a consequence, the LUMO is stabilized (as reflected by the smaller HOMO–LUMO gap compared to the other aldehydes), and its polarisability is enhanced. On the other hand, as noted above, the dynamics of the solutes present noticeable differences, with acetaldehyde being less structured than the
others in the sense that it does not display a clear preferential orientation in the angular distribution. The solute–interface orientation determines the characteristics of the water layer reaction field and therefore the polarisation of the aldehyde molecular orbitals. It is to be noted that although the first factor is relevant in bulk solvation too, the second one is specific to the interaction with the interface. The large magnitude of the fluctuations due to solvation effects is illustrated in Fig. 3 by the change in the chemical potential of benzaldehyde along the simulation. As shown, the instantaneous shift oscillates between ±1 eV, roughly, which represents about 20% of the average chemical potential value. Even more interesting is that the histogram displays a fairly symmetric shape, meaning that positive and negative solvation effects occur with comparable intensities and probabilities. UV-Vis spectrum of benzaldehyde Excitation energies and oscillator strengths for benzaldehyde in the gas phase are summarized in Table 2, where we compare our values with those reported by other authors using higher computational levels,36–38 and also with the available experimental data.39 The calculated cross-sections in the gas phase and at the air–water interface are reported in Fig. 4, which also reports recent experimental data in the gas phase.40 As shown in Table 2 and Fig. 4, our calculations in the gas phase agree reasonably well with calculations at higher levels Table 2 Calculated excitation energies (eV) of the first two electronic transitions of benzaldehyde in the gas phase. Oscillator strengths are indicated in parenthesis. Comparison with other high-level calculations and with experimental dataa
Calculations
Fig. 3 Histogram of the instantaneous chemical potential shift of benzaldehyde at the air–water interface with respect to the gas phase value. Results from molecular dynamics simulations and QM/MM calculations.
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Exp.
(a)
(b)
(c)
(d)
nπ*
3.61 (10−4)
3.92/4.02 (10−4)
ππ*
4.52 (0.0202)
4.55/4.50 (0.0206)
3.84 (1.9 10−4) 4.79 (0.012)
3.71 ( 290 nm). First, the main peak is slightly red-shifted by approximately 0.1 eV, from 274 nm (4.52 eV) to 282 nm (4.40 eV). Besides, the band is enlarged, especially around the symmetry-forbidden band corresponding to the nπ* transition; this is a usual effect because the interactions with water molecules break the planar symmetry around the carbonyl bond. Finally, it must be noted that, owing to these effects, the light absorption intensity for λ > 290 nm is always higher at the water surface than in the gas phase. Atmospheric implications Photolysis is an important sink of aldehydes in the troposphere. Absorption cross-sections and photodissociation quantum yields applicable to atmospheric conditions have been reported for formaldehyde and acetaldehyde41,42 and these properties have also been measured for benzaldehyde under different conditions.43 In the UV-Vis range, benzaldehyde can dissociate in the following ways: C6 H5 CHO þ hν ) C6 H5 þ HCO
ðλ < 292 nmÞ
C6 H5 CHO þ hν ) C6 H5 CO þ H
ðλ < 328 nmÞ
The thresholds shown above correspond to estimations made on the basis of formation enthalpies.43 However, Zhu and Cronin43 have reported that the HCO formation channel
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can be active beyond that threshold, at least until λ = 308 nm. The quantum yields were measured at different wavelengths (0.32 ± 0.05, 0.45 ± 0.05 and 0.29 ± 0.05, at wavelengths of 280 nm, 285 nm and 308 nm, respectively). Assuming a linear variation of both the cross-section and the HCO yield between 285 and 308 nm, these authors estimated the benzaldehyde photolysis rate constant to be between 1.5 × 10−6 s−1 and 1.5 × 10−5 s−1, depending on season (noontime, clear sky at sea level, latitude 40°N). Using the same HCO quantum yields, the previously reported actinic flux values44 (Earth’s surface, noontime, no surface Albedo), together with our calculated crosssection in the gas phase, we obtain 2.7 × 10−5 s−1 for the HCO photolysis rate constant in the range of 290–308 nm. Considering the different approximations made, the agreement with the experimental estimation is reasonably good. Using our calculated cross-sections at the air–water interface, a rate constant of 2.8 × 10−4 s−1 has been obtained. Hence, we have predicted that the aqueous interface effect enhances the rate constant of benzaldehyde photolysis by roughly one order of magnitude. The increase of the rate constant is not the only factor favouring a photolysis rate enhancement since, as shown in Fig. 1, benzaldehyde is significantly stabilized at the interface, where it can accumulate. To estimate the relative interface/gas phase concentration, one can use the free energy of adsorption reported before, which amounts to 7 kcal mol−1.15 It is to be noted that this value is quite consistent with the one obtained by adding the experimental free energy of solvation in water (ranging between −4.8 and −3.10 kcal mol−1, see ref. 45) and the solvation energy excess at the air–water interface calculated in our work (−3 kcal mol−1). Using this value, the concentration at the interface should be about 105 higher than in the gas phase at 298 K. Taking together the increase of the rate constant and the higher concentration at the interface, the photolysis rate should rise with respect to its value in the gas phase by a factor of roughly 106. The impact of this finding in the global tropospheric chemistry is difficult to quantify because one has to incorporate other factors such as altitude effects, UV penetration in clouds, size and distribution of water droplets and aqueous aerosols, etc, but our results point towards the necessity of taking into account chemical reactivity at aqueous interfaces in atmospheric and environmental models.
Conclusions There is increasing experimental evidence that effects on chemical reactions due to restricted solvation at aqueous interfaces may be significant and, in some cases, larger than bulk solvation effects (see for instance ref. 46–61). These findings have enormous impact in different areas of chemistry, although interpreting the experimental observations at the microscopic level is not always straightforward. The analysis of computer simulations such as those reported in the present work, can be helpful not only to get a better understanding of the measured effects but also to design new experiments.
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The results reported in this work highlight the difficulties to rationalize solvation effects on chemical reactivity at aqueous interfaces. Large fluctuations of the molecular properties are expected at the interface since the solute enters and leaves the water layer at the scale of a few picoseconds. Besides, the electrostatic potential due to the polarised interface is strongly dependent on the relative solute–interface orientation. These issues have been illustrated by the analysis of reactivity indices and photolysis rate constants of some aldehydes at the air–water interface. Despite their chemical similarity, formaldehyde, acetaldehyde and benzaldehyde undergo different effects that in some cases are in opposite directions. The most striking result is the increase of the photolysis rate of benzaldehyde by one order of magnitude, compared to the gas phase value. This effect, combined with the trend of aldehydes to accumulate at the water surface as a result of thermodynamic stabilisation, has led us to estimate a photolysis rate increase by six orders of magnitude. The potential impact of these results on environmental and atmospheric chemistry is therefore considerable. Even though further theoretical and experimental research will be necessary to assess the robustness of this conclusion, the present study corroborates the previous findings reported by us and others on the important role played by solvation at aqueous interfaces on chemical reactivity.
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49 H. Mishra, S. Enami, R. J. Nielsen, L. A. Stewart, M. R. Hoffmann, W. A. Goddard III and A. J. Colussi, Proc. Natl. Acad. Sci. U. S. A, 2012, 109, 18679–18683. 50 S. Enami, M. R. Hoffmann and A. J. Colussi, J. Phys. Chem. Lett., 2012, 3, 3102–3108. 51 E. C. Griffith and V. Vaida, J. Am. Chem. Soc., 2013, 135, 710–716. 52 E. A. Henderson and D. J. Donaldson, J. Phys. Chem. A, 2012, 116, 423–429. 53 A. M. Jubb, W. Hua and H. C. Allen, Annu. Rev. Phys. Chem., 2012, 63, 107–130. 54 P. Nissenson, C. J. H. Knox, B. J. Finlayson-Pitts, L. F. Phillips and D. Dabdub, Phys. Chem. Chem. Phys., 2006, 8, 4700–4710. 55 D. J. Donaldson and K. T. Valsaraj, Environ. Sci. Technol., 2010, 44, 865–873. 56 D. J. Donaldson and V. Vaida, Chem. Rev., 2006, 106, 1445– 1461. 57 R. S. Strekowski, R. Remorov and C. George, J. Phys. Chem. A, 2003, 107, 2497–2504. 58 B. T. Mmereki and D. J. Donaldson, J. Phys. Chem. A, 2003, 107, 11038–11042. 59 B. T. Mmereki, D. J. Donaldson, J. B. Gilman, T. L. Eliason and V. Vaida, Atmos. Environ., 2004, 38, 6091–6103. 60 C. E. Kolb, R. A. Cox, J. P. D. Abbatt, M. Ammann, E. J. Davis, D. J. Donaldson, B. C. Garrett, C. George, P. T. Griffiths, D. R. Hanson, M. Kulmala, G. McFiggans, U. Poschl, I. Riipinen, M. J. Rossi, Y. Rudich, P. E. Wagner, P. M. Winkler, D. R. Worsnop and C. D. O’ Dowd, Atmos. Chem. Phys., 2010, 10, 10561–10605. 61 S. Raja and K. T. Valsaraj, J. Air Waste Manage. Assoc., 2005, 55, 1345–1355.
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Cite this: DOI: 10.1039/c4ob02029d
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Reactivity of aldehydes at the air–water interface. Insights from molecular dynamics simulations and ab initio calculations Marilia T. C. Martins-Costa,a,b Francisco F. García-Prietoa,b,c and Manuel F. Ruiz-López*a,b Understanding the influence of solute–solvent interactions on chemical reactivity has been a subject of intense research in the last few decades. Theoretical studies have focused on bulk solvation phenomena and a variety of models and methods have been developed that are now widely used by both theoreticians and experimentalists. Much less attention has been paid, however, to processes that occur at liquid interfaces despite the important role such interfaces play in chemistry and biology. In this study, we have carried out sequential molecular dynamics simulations and quantum mechanical calculations to analyse the influence of the air–water interface on the reactivity of formaldehyde, acetaldehyde and benzaldehyde, three simple aldehydes of atmospheric interest. The calculated free-energy profiles exhibit a minimum at the interface, where the average reactivity indices may display large solvation effects. The study emphasizes the role of solvation dynamics, which are responsible for large fluctuations of some molecular properties.
Received 23rd September 2014, Accepted 25th November 2014 DOI: 10.1039/c4ob02029d www.rsc.org/obc
We also show that the photolysis rate constant of benzaldehyde in the range 290–308 nm increases by one order of magnitude at the surface of a water droplet, from 2.7 × 10−5 s−1 in the gas phase to 2.8 × 10−4 s−1 at the air–water interface, and we discuss the potential impact of this result on the chemistry of the troposphere. Experimental data in this domain are still scarce and computer simulations like those presented in this work may provide some insights that can be useful to design new experiments.
Introduction Aqueous interfaces are ubiquitous on earth. From the surface of the ocean to that of atmospheric aerosols and cloud droplets, the air–water interface provides a unique environment where neutral molecules, radicals and ions meet, interact and eventually react to form new compounds. Water–hydrophobic interfaces are involved in many other important domains such as the selective transport of chemical species across the semipermeable lipid bilayer of cell membranes, a phenomenon which is of paramount importance in biology. While significant progress has been made in the last few decades in the study of the physical properties of such interfaces and the uptake of molecules, unravelling the mechanism of chemical and photochemical reactions at aqueous interfaces remains a major scientific and technological challenge. In a recent work, we have shown that chemical processes can be significantly accelerated at the air–water interface due to unexpectedly large solvation effects. This conclusion has a
SRSMC, University of Lorraine, BP 70239, 54506 Vandoeuvre-lès-Nancy, France. E-mail: [email protected] b CNRS, UMR N° 7565, 54506 Vandoeuvre-lès-Nancy, France c University of Extremadura, Avda Elvas S-N, E-06071 Badajoz, Spain
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been reached through first-principles molecular dynamics simulations of small radicals1 and molecules2,3 of atmospheric interest. In particular, we have found that the production of OH radicals from ozone photolysis in the troposphere is about four orders of magnitude larger when the reaction takes place at the surface of cloud water droplets, compared to the same process in the gas phase.3 Indeed, water droplets and aqueous aerosols can be considered as micro-reactors that can contribute to the atmospheric chemistry on a global scale.4–9 Water mediated reactions in the troposphere play a significant role10,11 but the heterogeneous processes occurring at the air–water interface12,13 are usually neglected in current atmospheric models. Their impact in terms of air quality and global warming, for instance, remains unknown and clearly such a topic deserves further investigation. In the present paper, we have studied the chemical properties of some aldehydes at the air–water interface. Aldehydes are an important category of volatile organic compounds (VOC), which are released into the atmosphere by both natural and anthropogenic sources. They are emitted by plants and are produced by incomplete combustion processes. They can also be formed by oxidation of other VOCs. Their uptake by water surface has been experimentally14 and theoretically15 studied.
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The main tropospheric sinks are photolysis and reaction with radicals such as OH (daytime reactions) or nitrate radicals (night-time reactions).16 Our aim in this work is to get some insights on how solvation at the water surface affects their molecular properties. The study is carried out by combining classical molecular dynamics simulations and QM/MM calculations (QM/MM stands for a hybrid quantum mechanics and molecular mechanics approach). We first analyze reactivity indices of formaldehyde, acetaldehyde and benzaldehyde, then, we analyze the UV-Vis spectrum of benzaldehyde and discuss the atmospheric implications.
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lation. Reactivity indices (chemical potential, hardness and electrophilicity) are obtained using the conceptual density functional theory and the approximated formula:27 Chemical potential: 1 μ ¼ ðεHOMO þ εLUMO Þ 2 Chemical hardness: η ¼ ðεLUMO εHOMO Þ Electrophilicity: ω¼
Model and calculation method Molecular dynamics simulations have been carried out using the OPLS force field17 for the aldehydes and the TIP3P forcefield18 for water. The simulation box contains 499 water molecules and one aldehyde molecule. The box size is roughly (in Å) 24.7 × 24.7 × 130 (with small differences for the X and Y axis due to different aldehyde masses; in the case of formaldehyde, acetaldehyde and benzaldehyde, these axes are equal to 24.65 Å, 24.66 Å and 24.719 Å, respectively). Periodic boundary conditions are used along the X and Y directions. Simulations were carried out at T = 298 K using a Nosé–Hoover thermostat19,20 and a time step of 0.5 fs with the aldehyde placed initially at the interface. After equilibration, the simulation was carried out for 2 ns. During the simulation, 2000 snapshots were saved at regular intervals for further analysis. Free energy was calculated using the umbrella sampling21 and WHAM22 methods. The reaction coordinate is calculated as the distance between the centre of mass of the solute and the centre of mass of the solvent. This distance was varied by steps of 0.25 Å. The bias potential force constant is k = 10 kcal mol−1 Å−2. After thermalisation, the trajectory was carried out for 200 ps at each point of the reaction coordinate. The properties were computed using the combined quantum mechanics and molecular mechanics (QM/MM) approach23,24 developed by us: the solute is described quantum mechanically at the B3LYP/6-311+G(d) level25 while the water solvent is described classically using the TIP3P forcefield. The solute’s wave function is therefore computed using a Hamiltonian that accounts for electrostatic embedding (i.e. it includes the electrostatic interaction with the solvent). In this way, polarization effects on the electronic properties of the solute are accounted for. Electronic transitions for the first two singlet states (nπ* and ππ*) were calculated using the TD-DFT method.26 The cross-sections were convoluted for each snapshot using the expression: 2 vv 2:772 Δv 0 f 12 1=2 σðvÞ ¼ 0:811 10 e Δv1=2 where σ units are cm2 per molecule if Δν1/2 is expressed in cm−1. We used Δν1/2 = 3226 cm−1 (0.4 eV). The final spectrum is obtained by averaging over 2000 snapshots from the simu-
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μ2 η
where εHOMO/LUMO stands for the energy of the HOMO and LUMO orbitals, respectively. The simulations were done using Tinker 4.228 and the combined QM/MM calculations were done using Gaussian 0929 and the program interface that was developed by us.24 For comparison, we have also reported the calculation of properties for the isolated molecule in the gas phase at its equilibrium geometry. For consistency with the calculations at the air–water interface, the gas phase geometry was optimized with the force-field used in the simulations. The UV-Vis spectrum was convoluted using the same expression and the same width at half maximum, Δν1/2, than in the interface calculation.
Results and discussion Solvation at the interface Fig. 1 displays the calculated free energy profiles for water accommodation of the three aldehydes studied here. All the systems exhibit a free energy minimum at the air–water interface, which is significantly larger for benzaldehyde. This molecule exhibits a preference for the interface that is about 3 kcal mol−1 with respect to bulk water, not far from the value 2.8 kcal mol−1 reported earlier by Canneaux et al.15 The result for formaldehyde (1.5 kcal mol−1) reproduces the value reported before using QM/MM simulations.2 It is interesting to point out that the affinity for the air water interface increases in the order formaldehyde < acetaldehyde < benzaldehyde (compare the stabilisation at the interface with respect to the gas phase), which parallels the increasing hydrogen-bond acceptor character of the compounds. It also parallels the increasing polarity of the carbonyl group in the three aldehydes (see below). A key factor in solvation phenomena at liquid interfaces is the role of the orientation dynamics, since different orientations imply different induced polarizations. For instance, molecules of the same type displaying similar dipole moments and similar polarisabilities in the gas-phase, may interact with the interface in different orientations and undergo quite different polarisations.3 This is a specific property of the interface because in bulk solution, according to Onsager’s theory, the reaction field is roughly proportional to the dipole
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Fig. 1 Calculated free energy profiles at the air–water interface for the aldehydes studied in this work. The formal interface corresponds to R ≈ 12 Å.
moment, regardless of the solute orientation. For the systems under study here, the carbonyl oxygen atom is expected to form hydrogen bonds with water molecules, and intuitively one would deduce that such an interaction orientates the molecules with the CvO group being directed towards the water phase. Moreover, based on aldehyde–water interaction strength alone, one would expect a higher orientation effect of the interface on benzaldehyde followed by acetaldehyde and formaldehyde. Though hydrogen-bonding does certainly represent an important factor in determining the average orientation of the solute with respect to the interface, our results reveal that it is not the only factor. The nature and size of the R group in the aldehyde (RCHO) is also relevant because it governs the hydrophobic interactions with water and restrains the ability of the molecule to cross the interface. As a matter of fact, solvation at the air–water interface is characterized by continuous crossings and recrossings of the solute through the average plane defining the formal interface. These oscillations define an interface width, which in previous simulations of aminoacids at the water–hydrophobic interface30,31 or small molecules at the air–water interface,1,2 has been estimated to be roughly 1 nm. There are situations in which the solute is located in the air layer interacting weakly with the water layer; in that case the solute’s properties are close to gas phase values. In other cases, the solute is well inside the water layer and its properties are close to bulk solvation values. Hence, the time fluctuations of the molecular properties of a solute adsorbed at the air– water interface are expected to be quite large. Fig. 2 displays the statistical averages for the orientation of the aldehydes with respect to the interface. The Z-axis is
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Fig. 2 Angular distribution of aldehydes at the air–water interface from molecular dynamics simulations. The black line corresponds to the uniform distribution 21 sin θ.
defined as the axis perpendicular to the water surface and the angle θ as the angle formed between the Z-axis and the CvO bond of the solute. On an average, formaldehyde and benzaldehyde display a slight preferential orientation with respect to the interface, the interface favouring the situations in which the CvO bond is directed towards the water. In the case of acetaldehyde, the angular distribution is essentially uniform, suggesting that the dynamics in this system plays a more important role. Reactivity indices Solvation effects on reactivity indices in bulk solution have been discussed previously32–35 but to the best of our knowledge, no analyses have yet been reported in the literature for molecules adsorbed at the air–water interface. Indeed, as shown in our previous QM/MM simulations,1,2,23 a systematic energy shift of the molecular orbitals is usually obtained due to the electrostatic potential created by the polarized water layer. In the case of hydrogen-bond acceptors, like in the present case, the polarization of the water layer is such that a net positive potential is created. One then expects the whole set of the solute’s molecular orbitals to be stabilized through the interaction with the interface. If the shifts were strictly the same for all the orbitals, then no net effect of chemical hardness would happen, while a decrease of the chemical potential, in the algebraic value, had to be expected. Obviously, the electrostatic potential created by the polarised water layer is not strictly constant, and the effect of the associated electrostatic field has to be taken into account too. When rigorous calculations were carried out for the molecules considered in the present study, the following results were obtained (see Table 1). First, a large interface effect for
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Table 1 Calculated reactivity indices at the air–water interface from molecular dynamics simulations and QM/MM calculations. Comparison with gas phase values. The change with respect to the gas-phase values is indicated in parenthesis
Dipole (D) μ (eV) η (eV)
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ω (eV)
Gas Interface Gas Interface Gas Interface Gas Interface
Formaldehyde
Acetaldehyde
Benzaldehyde
2.63 3.10 (+0.47) −4.82 −4.92 (−0.10) 5.82 5.68 (−0.14) 2.00 2.16 (+0.16)
2.91 3.68 (+0.77) −4.42 −4.44 (−0.02) 5.92 6.08 (+0.16) 1.65 1.64 (−0.01)
3.56 3.96 (+0.40) −4.81 −4.73 (+0.08) 5.15 4.94 (−0.21) 2.24 2.29 (+0.05)
the hardness index was predicted. The effect can be positive (hardness increase) or negative (hardness decrease), depending on the molecule. Second, the effect on the chemical potential is significantly negative for formaldehyde, as expected for a proton acceptor, but it is significantly positive for benzaldehyde. Finally, the electrophilicity index change is large and positive for formaldehyde but relatively small for the other two molecules. These trends cannot be explained through a single parameter such as the hydrogen-bond strength, and more likely they result from a combination of different factors among which two are probably decisive. On the one hand, there are fundamental differences between benzaldehyde and the other two aldehydes in terms of the electronic structure. In the aromatic system, the π system of the carbonyl bond has some degree of delocalisation over the π orbitals of the phenyl ring. As a consequence, the LUMO is stabilized (as reflected by the smaller HOMO–LUMO gap compared to the other aldehydes), and its polarisability is enhanced. On the other hand, as noted above, the dynamics of the solutes present noticeable differences, with acetaldehyde being less structured than the
others in the sense that it does not display a clear preferential orientation in the angular distribution. The solute–interface orientation determines the characteristics of the water layer reaction field and therefore the polarisation of the aldehyde molecular orbitals. It is to be noted that although the first factor is relevant in bulk solvation too, the second one is specific to the interaction with the interface. The large magnitude of the fluctuations due to solvation effects is illustrated in Fig. 3 by the change in the chemical potential of benzaldehyde along the simulation. As shown, the instantaneous shift oscillates between ±1 eV, roughly, which represents about 20% of the average chemical potential value. Even more interesting is that the histogram displays a fairly symmetric shape, meaning that positive and negative solvation effects occur with comparable intensities and probabilities. UV-Vis spectrum of benzaldehyde Excitation energies and oscillator strengths for benzaldehyde in the gas phase are summarized in Table 2, where we compare our values with those reported by other authors using higher computational levels,36–38 and also with the available experimental data.39 The calculated cross-sections in the gas phase and at the air–water interface are reported in Fig. 4, which also reports recent experimental data in the gas phase.40 As shown in Table 2 and Fig. 4, our calculations in the gas phase agree reasonably well with calculations at higher levels Table 2 Calculated excitation energies (eV) of the first two electronic transitions of benzaldehyde in the gas phase. Oscillator strengths are indicated in parenthesis. Comparison with other high-level calculations and with experimental dataa
Calculations
Fig. 3 Histogram of the instantaneous chemical potential shift of benzaldehyde at the air–water interface with respect to the gas phase value. Results from molecular dynamics simulations and QM/MM calculations.
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Exp.
(a)
(b)
(c)
(d)
nπ*
3.61 (10−4)
3.92/4.02 (10−4)
ππ*
4.52 (0.0202)
4.55/4.50 (0.0206)
3.84 (1.9 10−4) 4.79 (0.012)
3.71 ( 290 nm). First, the main peak is slightly red-shifted by approximately 0.1 eV, from 274 nm (4.52 eV) to 282 nm (4.40 eV). Besides, the band is enlarged, especially around the symmetry-forbidden band corresponding to the nπ* transition; this is a usual effect because the interactions with water molecules break the planar symmetry around the carbonyl bond. Finally, it must be noted that, owing to these effects, the light absorption intensity for λ > 290 nm is always higher at the water surface than in the gas phase. Atmospheric implications Photolysis is an important sink of aldehydes in the troposphere. Absorption cross-sections and photodissociation quantum yields applicable to atmospheric conditions have been reported for formaldehyde and acetaldehyde41,42 and these properties have also been measured for benzaldehyde under different conditions.43 In the UV-Vis range, benzaldehyde can dissociate in the following ways: C6 H5 CHO þ hν ) C6 H5 þ HCO
ðλ < 292 nmÞ
C6 H5 CHO þ hν ) C6 H5 CO þ H
ðλ < 328 nmÞ
The thresholds shown above correspond to estimations made on the basis of formation enthalpies.43 However, Zhu and Cronin43 have reported that the HCO formation channel
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can be active beyond that threshold, at least until λ = 308 nm. The quantum yields were measured at different wavelengths (0.32 ± 0.05, 0.45 ± 0.05 and 0.29 ± 0.05, at wavelengths of 280 nm, 285 nm and 308 nm, respectively). Assuming a linear variation of both the cross-section and the HCO yield between 285 and 308 nm, these authors estimated the benzaldehyde photolysis rate constant to be between 1.5 × 10−6 s−1 and 1.5 × 10−5 s−1, depending on season (noontime, clear sky at sea level, latitude 40°N). Using the same HCO quantum yields, the previously reported actinic flux values44 (Earth’s surface, noontime, no surface Albedo), together with our calculated crosssection in the gas phase, we obtain 2.7 × 10−5 s−1 for the HCO photolysis rate constant in the range of 290–308 nm. Considering the different approximations made, the agreement with the experimental estimation is reasonably good. Using our calculated cross-sections at the air–water interface, a rate constant of 2.8 × 10−4 s−1 has been obtained. Hence, we have predicted that the aqueous interface effect enhances the rate constant of benzaldehyde photolysis by roughly one order of magnitude. The increase of the rate constant is not the only factor favouring a photolysis rate enhancement since, as shown in Fig. 1, benzaldehyde is significantly stabilized at the interface, where it can accumulate. To estimate the relative interface/gas phase concentration, one can use the free energy of adsorption reported before, which amounts to 7 kcal mol−1.15 It is to be noted that this value is quite consistent with the one obtained by adding the experimental free energy of solvation in water (ranging between −4.8 and −3.10 kcal mol−1, see ref. 45) and the solvation energy excess at the air–water interface calculated in our work (−3 kcal mol−1). Using this value, the concentration at the interface should be about 105 higher than in the gas phase at 298 K. Taking together the increase of the rate constant and the higher concentration at the interface, the photolysis rate should rise with respect to its value in the gas phase by a factor of roughly 106. The impact of this finding in the global tropospheric chemistry is difficult to quantify because one has to incorporate other factors such as altitude effects, UV penetration in clouds, size and distribution of water droplets and aqueous aerosols, etc, but our results point towards the necessity of taking into account chemical reactivity at aqueous interfaces in atmospheric and environmental models.
Conclusions There is increasing experimental evidence that effects on chemical reactions due to restricted solvation at aqueous interfaces may be significant and, in some cases, larger than bulk solvation effects (see for instance ref. 46–61). These findings have enormous impact in different areas of chemistry, although interpreting the experimental observations at the microscopic level is not always straightforward. The analysis of computer simulations such as those reported in the present work, can be helpful not only to get a better understanding of the measured effects but also to design new experiments.
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The results reported in this work highlight the difficulties to rationalize solvation effects on chemical reactivity at aqueous interfaces. Large fluctuations of the molecular properties are expected at the interface since the solute enters and leaves the water layer at the scale of a few picoseconds. Besides, the electrostatic potential due to the polarised interface is strongly dependent on the relative solute–interface orientation. These issues have been illustrated by the analysis of reactivity indices and photolysis rate constants of some aldehydes at the air–water interface. Despite their chemical similarity, formaldehyde, acetaldehyde and benzaldehyde undergo different effects that in some cases are in opposite directions. The most striking result is the increase of the photolysis rate of benzaldehyde by one order of magnitude, compared to the gas phase value. This effect, combined with the trend of aldehydes to accumulate at the water surface as a result of thermodynamic stabilisation, has led us to estimate a photolysis rate increase by six orders of magnitude. The potential impact of these results on environmental and atmospheric chemistry is therefore considerable. Even though further theoretical and experimental research will be necessary to assess the robustness of this conclusion, the present study corroborates the previous findings reported by us and others on the important role played by solvation at aqueous interfaces on chemical reactivity.
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