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J. Phys. Chem. C 2008, 112, 1524-1532
Influence of Molecular and Supramolecular Structure on the Gas-Liquid Interfacial Reactivity of Hydrocarbon Liquids with O(3P) Atoms Mhairi Allan, Paul A. J. Bagot, Robin E. Westacott, Matthew L. Costen, and Kenneth G. McKendrick* School of Engineering and Physical Sciences, Heriot-Watt UniVersity, Edinburgh, EH14 4AS, United Kingdom ReceiVed: August 10, 2007; In Final Form: NoVember 3, 2007
We have investigated the interfacial reactivity of gas-phase O(3P) atoms with a representative range of longchain liquid hydrocarbons. These consisted of two branched molecules, squalane (C30H62, 2,6,10,15,19,23hexamethyltetracosane) and pristane (C19H40, 2,6,10,14-tetramethylpentadecane), and three linear ones, n-docosane (C22H46), n-tetracosane (C24H50) and n-octacosane (C28H58). This represents the first systematic investigation of reactions of this type for molecules other than squalane. The O(3P) atoms were generated by 355-nm laser photolysis of a low pressure of NO2 above the liquid surface. The nascent gas-phase OH radical products were detected by laser-induced fluorescence (LIF). Measurements for the linear hydrocarbons were constrained by their vapor pressures to single temperatures slightly (∼1 K) above their respective melting points. Pristane was studied at the lowest temperature practically achievable. Squalane was compared as a reference at the full set of temperatures. Appearance profiles for all of the liquids showed similar characteristic differences between OH V′)0 and 1. LIF excitation spectra were obtained for each of the vibrational levels at both the rising edge and peak of the appearance profiles. We conclude that the observed variations in rotational temperatures are consistent with dual contributions to the reaction mechanism for all the liquids, involving both direct escape and trapping-desorption components of the observed OH, as has previously been proposed for squalane. The relative yields of OH showed some surprising dependences on the liquid, including an unexpectedly strong variation with linear hydrocarbon chain length. These cannot all be explained by the relative reactivity of primary, secondary, and tertiary H-C units. We discuss the possibility that the known “surface freezing” phenomenon for linear hydrocarbons may play a role.
Introduction Compared to the vast accumulation of knowledge on the dynamics of elementary chemical reactions in the homogeneous gas phase, reactions at the gas-liquid interface have thus far received much less attention. Despite this, such reactions have a relevance spanning a diversity of fields involving biological (respiration), industrial (distillation), and atmospheric (reactions at the sea surface and of aerosol particles) processes. Motivated by this widespread importance, a number of dynamical studies into gas-liquid interfacial collisions have now been carried out, although the majority so far have examined inelastic rather than reactive scattering. The most common experimental approach has been based on molecular beam scattering, most notably in the work of Nathanson and co-workers.1-7 From these studies, mainly of rare-gas atoms, it was found that the scattered projectiles leave the surface with a bimodal kinetic energy distribution. The two components are conveniently labeled as either “direct” or “trapping-desorption”. The direct component has a non-thermal kinetic energy related to that of the impinging atoms. Scattering is concentrated near specular angles. In contrast, the trapping-desorption component has a kinetic energy distribution well described by a Boltzmann distribution at the surface temperature, the angular scattering follows a cosine distribution around the surface normal. In a subsequent development, Minton, initially in collaboration with Casavecchia and then with a number of co-workers, performed the first molecular beam-based dynamical investiga* Corresponding author. E-mail:
[email protected].
tions into reactive scattering at the gas-liquid interface. The system selected was reaction of O(3P) atoms at a liquid squalane surface.8-12 This was motivated in part by the oxidation of hydrocarbon polymers on the outer surfaces of spacecraft in low Earth orbit (LEO), in the range 200-700 km. As a result of the orbital motion, the (effectively stationary) atmospheric constituents collide with the spacecraft with collision energies as high as 5 eV (∼480 kJ mol-1), leading to severe irreversible damage. In a different atmospheric context, the oxidation of hydrocarbon surfaces plays an important role in the processing of atmospheric aerosols at lower altitudes. The accumulation of even modest amounts of organic constituents by the aqueous aerosol particles results in the formation of a micelle-like structure with a hydrophobic outer layer.13,14 Upon oxidation, predominantly by the hydroxyl radical but in some cases by other oxidants present in the atmosphere, hydrophilic sites are created on the outer layer. This in turn enables them to capture water and act as cloud condensation nuclei. Minton and co-workers’ approach was based on a rotatable quadrupole mass filter, which was used to measure the velocity and angular distribution of the scattered products.8-12 The main species detected were inelastically scattered oxygen atoms and the reaction products hydroxyl radicals and water. All were found to have a bimodal kinetic energy distribution, ascribed to direct and trapping-desorption components of the type by then well-established for inelastic scattering. This set the scene for our own series of experiments, where we have used spectroscopic detection of the hydroxyl radicals, formed after reaction of O(3P) atoms with a liquid squalane
10.1021/jp076441n CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008
Hydrocarbon/O(3P) Atom Interfacial Reactivity surface, to yield new information on their internal energy distributions.15-18 We discovered the existence of vibrationally excited products, which had naturally not been detected in the molecular beam experiments. By measuring rotational distributions as a function of liquid surface temperature, we were able to demonstrate that the two-component mechanism inferred from the translational distributions is also reflected in the rotational distributions. Furthermore, although we cannot rival the velocity resolution of the molecular-beam scattering experiments, we were able to deconvolute time-dependent appearance profiles in terms of a corresponding distinct pair of velocity distributions. It was therefore possible to identify correlations between the internal and translational energy distributions. The internal energy distribution of the faster component was isolated by exploiting its shorter time-of-flight. It was shown that the measured OH rotational distributions at these early delay times were consistent with the OH being formed via direct abstraction, strengthening our earlier mechanistic conclusions. A similar conclusion was reached subsequently in the work of Nesbitt and co-workers on the related but much more exothermic reaction of fluorine atoms with a squalane surface.19 They used an alternative spectroscopic approach, IR-absorption, to investigate the HF reaction product. This was also found to have a bimodal rotational distribution, once again attributed to a hyperthermal abstraction mechanism and a trapping-desorption mechanism. Most recently, we have shown that the O(3P) + squalane system shows some interesting, subtle differences in the effect of liquid temperature on the yields of OH V′)0 and 1.20 One possible interpretation is that the escape probability of vibrationally excited products is affected by the temperaturedependent structure of the liquid. An important aspect for the interpretation of all previous experiments is an understanding of the atoms or segments of the squalane molecules with which the incoming atoms collide. Squalane contains all three H-C bond typessprimary, secondary, and tertiarysin a ratio of 24:32:6. The nature of the reactive site will influence the reactivity because of the well-established decrease in barrier heights in the sequence primary > secondary > tertiary.21 It may also affect the internal and translational energy distributions due to the increase in exothermicity in the same sequence. Harris22 used molecular dynamics (MD) simulations to investigate the structure of the liquid-vapor interface of the linear molecules n-decane and n-eicosane (C10H22 and C20H42). A slight preference for the methyl groups to protrude into the vacuum was found. More recently, Siepmann and coworkers have investigated the liquid-vapor interface of squalane itself using a Monte Carlo method. Although no explicit preference for any of the groups to protrude into the vacuum was identified, this was not the main focus of the work. We have subsequently carried out our own MD simulations to further investigate the structure of the liquid squalane interface, with particular focus on the accessibility of different groups to incoming atoms.23 Once again a slight preference for methyl groups to protrude into the vacuum was found, with this effect diminishing at higher temperatures. Information on the likelihood of hitting different segments of the squalane molecules was obtained using Monte Carlo simulations of the trajectories of incoming atoms. It was shown that all three C-H bond types were attacked with probabilities that did not differ markedly from their statistical ratios in squalane. Until very recently, the theoretical studies perhaps most closely related to the gas-liquid reactive scattering experiments were the simulations performed by Troya and Schatz,24 and independently by Hase and co-workers,25,26 on the reaction
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1525 between O(3P) atoms and a model self-assembled monolayer (SAM) surface. Both studies used a mixed quantum mechanics/ molecular mechanics (QM/MM) model. The O(3P) atom and the outer hydrocarbon segments of the SAM were treated quantum-mechanically, whereas the remainder of the SAM surface was treated by classical molecular mechanics. In their calculations, Troya and Schatz used the high collision energies (∼5 eV) relevant to those in LEO. They found that, in addition to the OH abstraction mechanism, hydrogen elimination, C-C bond breakage, and the formation of water were present as competing pathways. In agreement with previous work, a thermal and a hyperthermal OH translational component were found to contribute to the overall abstraction mechanism. In contrast, Hase and co-workers used much lower O(3P) atom collision energies. From their work, they concluded that there are three distinct types of O(3P) + SAM inelastic scattering trajectories: direct scattering from the top of the SAM, physisorption on the top of the SAM followed by desorption, and penetration into the SAM. They found the dominant trajectory type was dependent on the collision energy and the incident angle. Most recently, the Schatz group have presented the first dynamical calculations of reaction at a liquid surface, having extended the QM/MM methodology to treat high-energy collisions of O(3P) with liquid squalane.27 The more diffuse nature of the gas-liquid interface required a dynamic partitioning of atoms between the QM and MM regions. In a number of respects, the results are similar to O(3P) + SAMs, but there are some notable differences. These are also primarily traceable to the lower density of liquid squalane, resulting in lower overall energy transfer and corresponding differences in the trapping time of the O(3P) atoms or the nascent products within the surface. These dynamical differences are reflected in product attributes, notably the OH rotational energy distributions. From this existing body of experimental work, it can be seen that squalane has been established as something of a lone benchmark molecule for gas-liquid dynamics studies. In this paper, we set out to expand current knowledge by studying for the first time a range of other liquid hydrocarbons. We have conducted a spectroscopically based dynamical investigation into the reaction between O(3P) atoms and a series of different longchain hydrocarbons. The aim is to identify any systematic variations in interfacial reactivity with molecular structural features such as chain length and the degree of branching. In particular, we present measurements of the yield of OH in both the ground and first vibrationally excited states from each hydrocarbon. In addition, we have determined the influence of the hydrocarbon on the internal (rotational and fine-structure) energy distributions in both OH vibrational states. We consider what mechanistic conclusions may be inferred, drawing on previous realistic MD simulations of the “supramolecular” structure of the surface of liquid squalane and further MD calculations on a simplified model of linear hydrocarbons. Experimental Section The experimental apparatus has been described in detail previously.16 In summary, it consists of a stainless steel wheel, 50 mm in diameter, mounted on a long (∼30 cm) axle. The wheel rotates at 0.5 Hz through a copper bath, which holds the hydrocarbon of interest, providing a continuously refreshed liquid surface. At any instant, just under half of the wheel surface area is submerged within the bath, which maintains the liquid at a set temperature between 263 and 333 K ((1 K). Five different hydrocarbons were investigated. These consisted of two branched molecules, squalane (99%) (C30H62, 2,6,10,15,-
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Figure 1. Structures of the branched molecules (a) squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and (b) pristane (C19H40, 2,6,10,14-tetramethylpentadecane) used in these experiments.
19,23-hexamethyltetracosane) and pristane (g98%) (C19H40, 2,6,10,14-tetramethylpentadecane), along with three linear ones, n-docosane (99%) (C22H46), n-tetracosane (g99%) (C24H50), and n-octacosane (99%) (C28H58). All hydrocarbons were supplied by Sigma-Aldrich, and were used without further purification. The structures of squalane and pristane are shown in Figure 1. Part of the reason squalane has been so widely used in this field is its conveniently low vapor pressure (< ∼10-8 Torr) over a wide range of practical temperatures for which it remains a liquid. In extending the set of hydrocarbons in this work, we were constrained to operate within the much tighter restrictions imposed by their vapor pressures and melting points. Pristane (C19H40) could only be used when the temperature was reduced to e263 K, close to the lowest value achievable with the current apparatus, as above this it rapidly evaporated in the vacuum chamber. Conversely, the linear hydrocarbons are all solids at room temperature. They were therefore heated to just above (∼1 K) their respective literature melting points (in vacuum). The temperatures used were 318 K (docosane, mp 315-318 K), 323 K (tetracosane, mp 322-325 K) and 333 K (octacosane, mp 330-335 K). We verified by visual inspection that the material on the surface of the wheel was liquid in all cases at the set temperature. Under these conditions, our own direct measurements confirmed that the vapor pressures of all the liquids were well below the 1 mTorr total pressure that we maintain during the reactive experiments (see below). This is in agreement with the literature vapor pressures for these molecules.28,29 Attempts to study these liquids at higher temperatures proved impractical because of unacceptably rapid evaporation and were therefore not pursued. To ensure reproducibility of results, experiments were repeated with each hydrocarbon on at least 3 different days, paying very careful attention to keeping all parameters as constant as possible (especially laser energies, gas pressure, and distance of the wheel from the laser axis). When changing liquids, the bath assembly and wheel were cleaned thoroughly, initially with acetone and then with methanol in an ultrasonic bath. Additionally, each set of measurements on any 1 day using either pristane, docosane, tetracosane, or octacosane was followed by a measurement of squalane reactivity at the same temperature. This allowed a means to check signal consistency across different measurement days. In the case of OH V′)1, it provided a common reference to correct for day-to-day variations in the raw signals (see below). O(3P) atoms were generated at a mean, but precisely measured, distance of 5 mm from the wheel surface by photolyzing a carefully controlled pressure (nominally 1 mTorr) of NO2 (BOC, 98.3%). This was achieved using the third harmonic of a Nd:YAG laser (Continuum Surelite II-10), supplying 355 nm light pulses of width 4-6 ns at 10 Hz. The laser energy was maintained at a constant value of around 70 mJ per pulse, measured upon entry to the vacuum chamber. The spatial distribution of the O(3P) produced in this manner is
Figure 2. Measured appearance profiles of OH V′)0 LIF signal after reaction of O(3P) atoms with different liquid hydrocarbon surfaces, as indicated. Profiles were recorded on the Q1(1) line of the OH A-X (1,0) band. Signals are normalized to the peak intensity for squalane at a temperature of 333 K. Error bars represent 2σ variations on repeated measurements. Bath temperatures ) 333 K (squalane and octacosane), 323 K (tetracosane), 318 K (docosane), 263 K (pristane); p(NO2) ∼ 1 mTorr; distance surface-probe laser ) 5 mm.
described by an anisotropy parameter of +0.7.30 The photolysis laser was horizontally polarized, so roughly half of the O(3P) was directed toward the liquid surface. The collision energy of the O(3P) atoms is broadly distributed around an average value of 15.8 kJ mol-1 (with a fwhm of 26 kJ mol-1)16 in the laboratory frame, corresponding to an average speed of 1340 ms-1. Upon impact with the liquid hydrocarbon, some of the O(3P) atoms extract hydrogen atoms, generating OH X2Π radicals, at least some of which in turn escape from the surface. These were probed on the OH A2Σ+-X2Π (1,0) and (1,1) bands using a Nd:YAG (Continuum Surelite II-10) pumped dye laser (Sirah Cobra Stretch). This supplied ca. 1 mJ, 4-6 ns pulses, again measured at the entrance to the vacuum chamber. The returning OH A-X fluorescence was collected by a liquid light guide (Ultrafine Technology, Ltd.) mounted 1 cm directly above the common laser axis. The fluorescence passed through custom interference filters before being converted into a signal by a photomultiplier tube (PMT, Electron Tubes Ltd.). This signal was in turn digitized and passed to a PC, which collected data and controlled the wavelength and timing of the lasers using custom-written LabVIEW programs. Results Appearance Profiles Figure 2 shows OH V′)0 appearance profiles for each of the five hydrocarbons under investigation. The relative OH laserinduced fluorescence (LIF) intensity on the Q1(1) (1,0) line is plotted against the time delay between the photolysis and probe lasers. As indicated above, all liquids other than squalane could only be studied at a single specific temperature, which were 263 K (pristane), 333 K (octacosane), 323 K (tetracosane), and 318 K (docosane). In a previous publication20 we demonstrated that the absolute yield of OH V′)0 from squalane has only a very slight temperature dependence in the relevant range 263333 K. In this work we (arbitrarily) chose a liquid temperature of 333 K for squalane as the unit signal to which the other peak signals are referenced. Each profile shown in Figure 2 is the average of between 6 and 12 individual scans, repeated on different days to ensure consistency. The error bars indicate the statistical variations (2σ) at each delay time. For V′)0, no further normalization was applied.
Hydrocarbon/O(3P) Atom Interfacial Reactivity
Figure 3. Measured appearance profiles of OH V′)1 LIF signal after reaction of O(3P) atoms with different liquid hydrocarbon surfaces. Profiles were recorded on the Q1(1) line of the OH A-X (1,1) band. Signals have been scaled to squalane reference measurements at each of the liquid temperatures, and then corrected for the known variation of squalane yield with temperature [ref 20], as described in the text. Dotted lines indicate squalane reference, which were carried out at the same temperature as the corresponding hydrocarbon: 333 K (octacosane), 323 K (tetracosane), 318 K (docosane), 263 K (pristane); p(NO2) ∼ 1 mTorr; distance surface-probe laser ) 5 mm.
The profiles are clearly qualitatively similar, with a characteristic initial “dead time” that confirms the observed OH comes from reaction at the liquid surface. In no case was there a significant contribution from any homogeneous gas-phase reaction, which is as expected because of the very low partial pressures of hydrocarbon vapor. To a reasonable degree of reproducibility, the peak of the profiles always arrives at approximately the same time, regardless of the hydrocarbon used. The slight differences are most likely due to experimental errors in repositioning the wheel at exactly the same distance from the common laser axis following a hydrocarbon change, which requires removal and disassembly of the liquid surface apparatus from the vacuum chamber. When the profiles are normalized to equal height, we see in most cases no significant change in their shape. Probably the only exception is tetracosane, where the peak arrival time is perceptibly later. We suspect this is simply the result of a slightly larger than average laser-beamto-surface distance. If so, this would also have resulted in correspondingly lower signal intensities. Therefore the tetracosane peak intensity may be slightly underestimated relative to the other molecules in Figure 2. It is clear from Figure 2 that squalane is the most reactive liquid, followed next by the other branched hydrocarbon pristane. All three linear molecules yield significantly less OH V′)0 than squalane. Most notably, the linear chain length markedly affects the OH yield; they are progressively more reactive as the chain length increases, at least within the range of 22-28 carbon atoms explored. Note that all appearance profiles were measured on the Q1(1) line, as for previous studies.20 We have confirmed that any correction of the peak intensities to reflect total populations over all rotational levels because of different measured OH rotational temperatures (see below) results in insignificant changes. It does not materially alter the relative sequence of signal strengths. A similar insensitivity was demonstrated previously for squalane at different liquid temperatures.20 Figure 3 shows the corresponding OH yields for the first vibrationally excited state, V′)1. For OH V′)0, we had found highly consistent signal intensities across all days of measure-
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1527 ments for any one hydrocarbon, so no correction of the raw signals was required other than a final normalization of the averaged profiles to the selected unit height for squalane at 333 K. However, for OH V′)1 there was sufficient day-to-day variation in the absolute intensities, on the order of 50% maximum variance, for there to be unacceptably large uncertainties if the data were simply averaged. The reasons for this remain unclear. To overcome this problem, when the V′)1 profiles for each of the hydrocarbons were recorded, a reference measurement of V′)1 from squalane, at the same temperature as the liquid in question, was taken on the same day. The ratios of signals from squalane and each of the other molecules did remain consistent from day-to-day. The daily squalane profiles were therefore used to normalize all the raw signal intensities for the other molecules to a single, consistent relative intensity scale. We have previously shown the OH V′)1 yield from squalane to be dependent on liquid temperature, much more so than for OH V′)0.20 This variation was taken into account when scaling the profiles for the other hydrocarbons. The normalized signals therefore represent the relative signal sizes that would have been obtained in a “perfect” experiment (i.e., not subject to day-today fluctuations), with the peak signal from squalane at 333 K scaled to unity. These corrected yields for pristane, octacosane, tetracosane, and docosane are shown as solid lines in Figure 3. The corresponding signal from squalane at each of the respective temperatures is shown as a dotted line in the same color. For clarity, error bars have been omitted from the squalane references, but they are comparable in magnitude to those shown for the other hydrocarbons. The peak yields for each vibrational level from each liquid are summarized for convenience in Table 1. Reflecting Figures 2 and 3, they are shown in two forms: on a single fixed scale taken to be unity for squalane at 333 K, and relative to squalane at the same temperature, Tliq, as the liquid in question. Note that the absolute branching to V′)1 has been found previously for squalane to be 0.07 ( 0.02 at 298 K16, measured at the peak of the V′)0 signal. OH V′)1 therefore remains a minority channel for all the liquids so far investigated. The overall trends in reactivity for V′)1 are very similar to those for OH V′)0, with branched hydrocarbons producing more OH V′)1 than the linear ones, and with the yield for the linear molecules increasing with chain length. The largest statistically significant difference between the two vibrational levels is for pristane versus squalane. For OH V′)0, pristane yielded approximately 25% less than squalane, while for V′)1 their yields are essentially equal at the same temperature of 263 K (noting that the yield from squalane drops significantly as the temperature is reduced). Comparison of Figures 2 and 3 shows that all the hydrocarbons reproduce the effect that we had found previously for squalane,20 namely, that the peak arrival time for V′)1 is distinctly later than V′)0. The much smaller shift to later times in OH V′)1 as the temperature is reduced for squalane, at least for the large step to the lowest temperature of 263 K, is also reproduced here. Otherwise, we do not consider that any slight differences between the shapes of the V′)1 profiles for the different hydrocarbons in Figure 3 are significant. Rotational Temperatures. We have in previous publications reported measurements of product rotational distributions of OD and OH in both V′)0 and V′)1 from perdeuterated and normal squalane liquid surfaces.16 For OH, these were carried out at two different delay times: originally at the peak,16 and subsequently at the rising
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TABLE 1: Relative Peak Signal Sizes for OH W′)0 and 1 from Different Hydrocarbon Liquids relative to squalane at 333 K
relative to squalane at Tliq
molecule
Tliq/K
OH V′)0
OH V′)1
OH V′)0
OH V′)1
pristane octacosane tetracosane docosane
263 333 323 318
0.70 ( 0.05 0.64 ( 0.04 0.41 ( 0.04 0.26 ( 0.03
0.53 ( 0.08 0.56 ( 0.04 0.43 ( 0.04 0.33 ( 0.08
0.78 ( 0.06 0.64 ( 0.04 0.41 ( 0.04 0.26 ( 0.03
0.94 ( 0.22 0.56 ( 0.04 0.47 ( 0.07 0.36 ( 0.11
TABLE 2: Rotational Temperatures of Nascent OH W′)0 and W′)1 Averaged over F1 and F2 Manifolds, Measured at the Rising Edge and Peak of the Appearance Profilesa rotational temperature at rising edge/K molecule b
squalane
pristane octacosane docosane a
rotational temperature at peak/K
Tliq/K
V′)0
V′)1
V′)0
V′)1
273 343 263 333 318
380 ( 7 371 ( 7 385 ( 13 341 ( 8 354 ( 12
265 ( 9 281 ( 6 262 ( 8 286 ( 7 271 ( 8
298 ( 4 329 ( 7 346 ( 3 321 ( 3 324 ( 4
257 ( 7 293 ( 6 258 ( 1 292 ( 2 273 ( 4
Error limits indicate standard least-squares errors calculated from linear fits to Boltzmann plots. b Data for squalane are from ref 17.
edge of the appearance profiles.17 We have extended this work in a similar manner here for pristane, octacosane, and docosane. This provides OH rotational distributions from two branched and two (the longest and shortest) linear molecules. For completeness, we have measured the distributions for both V′)0 and 1 at both the rising edge (6 ( 1 µs for V′)0, 9 ( 1 µs for V′)1) and the peak (11 ( 1 µs for V′)0, 14 ( 1 µs for V′)1), although, on the basis of our previous work on squalane,20 we did not anticipate significant differences between the two delay times for V′)1. The resulting best-fit rotational temperatures to these distributions are collected in Table 2. Each temperature and associated error shown in Table 2 is the average of eight individual measured Boltzmann temperatures. These were collected from four complete LIF excitation spectra, examples of which are shown for OH V′)0 from pristane in Figure 4. This illustrates the quality of the signal-to-noise achievable in the experiments. It also demonstrates that the rising-edge spectrum is, to the experienced eye, visually slightly hotter than the peak. Each of such spectra was calibrated against a thermal OH spectrum, as described previously.15 Great care was taken throughout the duration of each scan (around 3 h) to closely maintain NO2 pressure, laser energies and liquid surface coverage on the wheel. From a single experiment, we determined individual Boltzmann temperatures for each of the two spin-orbit manifolds, at the two time delays chosen, to give four values. The values at a common time delay from different spin-orbit manifolds were then averaged, correctly accounting for population weighting. Figure 5 shows representative Boltzmann plots of the F1 manifold for OH V′)0, generated from the spectra from pristane shown in Figure 4, illustrating the statistical significance of differences in individual measurements of the temperature. To help visualize the differences between vibrational levels and delay times, and any correlations with liquid temperatures, the data from Table 2 are also presented in graphical form in Figure 6. Considering the rising edge first, the most striking trend is that OH V′)1 (Figure 6, lower panel) always has a significantly lower rotational temperature than OH V′)0 (upper panel). This is unmistakable for all four liquids. Turning to the peaks of the appearance profiles, the principal difference from the rising edge is a distinct reduction in the rotational temperatures for OH V′)0. For the “hot” squalane (343 K) and for octacosane, there is actually a slight overcooling of the OH V′)0 molecules at the peak, overshooting the liquid temperature; this is unphysical and most likely just reflects the combined true
systematic uncertainties in both temperatures. Such errors do not mask the statistically significant trends when there is a large difference between the temperature at the rising edge and the liquid temperature, as is the case for the “cold” squalane (liquid temperature 273 K) and pristane (263 K). Interestingly, while these two liquids yield very similar temperatures at the rising edge, and have similar liquid surface temperatures, at the peak of the appearance profiles the OH V′)0 leaving the squalane surface is significantly rotationally colder (298 ( 4 K) than that from the pristane surface (346 ( 3 K). Turning to OH V′)1, as already noted, the molecules have substantially less rotational energy at the rising edge than for V′)0. Upon moving to the peak, there is little change in any of these temperatures, again in contrast to V′)0 but as we had
Figure 4. Representative OH A-X (1,0) LIF excitation spectra from reaction of O(3P) with pristane at 263 K. p(NO2) ) 1 mTorr; distance surface-probe laser ) 5 mm. Photolysis probe delays ) 6 µs, upper panel (rising edge); 11 µs, lower panel (peak).
Hydrocarbon/O(3P) Atom Interfacial Reactivity
Figure 5. Representative Boltzmann plots (natural logarithm of relative population P, divided by rotational degeneracy, g, against rotational energy E) of the rotational distributions in the F1 manifold of OH V′)0 from pristane. Black symbols: rising edge (photolysis-probe delay of 6 µs). Red symbols: peak (photolysis-probe delay of 11 µs). Liquid temperature ) 263 K, p(NO2) ) 1 mTorr, distance surface-probe laser ) 5 mm.
Figure 6. Measured rotational temperatures of OH at two points in the appearance profiles, as indicated, and applied liquid surface temperatures for pristane, squalane (two surface temperatures), docosane, and octacosane. Upper panel: OH V′)0, lower panel: OH V′)1.
anticipated on the basis of previous results for squalane.20 There is, however, a clear correlation between either of these two V′)1 temperatures and that of the liquid for which they were measured. Discussion There are two interesting principal features of the results that we seek to explain: the sensitivity of the vibrational-leveldependent relative yields of OH to the nature of the hydrocarbon
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1529 molecule, and the variations in the OH rotational temperatures measured at the rising edges and peaks of the appearance profiles. We begin with the relative yields of OH from the different molecules. It is apparent from Figures 2 and 3 that there is a significant dependence of the yields of both V′)0 and 1 on the molecule. The most reactive molecule, squalane, yields around 4 times more OH than the least reactive, docosane. One possibility is that these variations do not, in fact, reflect different reactivities of the hydrocarbons as such. Some of the variations between the molecules could, in principle, simply result from the measurements necessarily being made at different temperatures, for the practical reasons discussed above. This does not apply, of course, to the differences between each of the other molecules and squalane, because we have shown explicitly here that all the other molecules yield less OH than squalane even when it is measured at the same temperature as each of them. For the similarly structured linear molecules, though, it may be the case that the increasing OH yields with rising chain length are in fact a consequence of the necessarily higher liquid temperatures. However, we do not believe this to be the case. This conclusion is based on our previous detailed investigation of the temperature-dependence of the yield from squalane.20 We established clearly that, for squalane, the yield of OH V′)0 is virtually temperature-independent. For V′)1, we did find a temperature dependence, as reproduced in Figure 3. However, for the corresponding range of temperatures (318-333 K) over which the measurements for the three linear molecules are made, there is less than 15% variation in the yield of OH V′)1 from squalane. The yields here of V′)1 for the linear molecules themselves vary much more significantly, by around a factor of 2. This is similar to, although slightly less than, the equivalent variation for V′)0. These observations lead us to our conclusion that it is unlikely, unless the variations with temperature for squalane are completely unrepresentative of those for the linear molecules, that much of the observed differences in OH yield between the molecules could be the result of temperature differences alone. Therefore, perhaps the next most obvious explanation is that the differences in reactivity are due to changes in molecular structure. As is well-established and has been widely discussed previously,21 primary, secondary, and tertiary H-C bonds have progressively lower barrier heights for H-atom abstraction by O(3P). It is not at all straightforward, though, to convert this into a quantitative prediction for the different hydrocarbons in this study. There is some discrepancy among the measured values of the barrier heights for different bond types,21 although the situation may be clarified by very recent theoretical work.31 Above these thresholds, the (known) distribution of collision energies resulting from NO2 photolysis30 would need to be convoluted with the (at best, sparsely known32) excitation functions. Nevertheless, it is clear that there will be a progressive increase in reactivity in the sequence primary < secondary < tertiary. For the two branched molecules, squalane and pristane, the normalized primary:secondary:tertiary ratios are 0.387:0.516: 0.097 and 0.45:0.45:0.10, respectively. It would therefore be expected that, all other factors being equal, squalane would have a higher reactivity than pristane because of its lower primary/ secondary ratio and essentially equal proportion of tertiary H-C bonds. This is at least qualitatively consistent with our observations, at least for OH V′)0 in Figure 2, where the pristane yield is ∼70% of that of squalane. For the minority channel OH V′)1,
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Figure 7. MD simulations of 507 bead-like, united-atom model molecules of a C10H22 linear hydrocarbon, based on the work of Yamamoto et al. (ref 45). Interfaces with vacuum are shown at the top and bottom of the liquid slab. Left view: full equilibration at 400 K; right: following quenching to 240 K and equilibration for 600 ps.
which represents around 7% of the total yield for squalane,16 the yields from squalane and pristane measured at the same temperature are more nearly equal. We return to the discussion of this point below. It is not so straightforward to make a reliable prediction for the relative reactivities of the branched versus any of the linear molecules. Upon transformation from a linear to a branched structure, there are opposing effects that cannot be resolved by purely qualitative arguments. Nearly half of the moderately reactive secondary atoms in the linear hydrocarbons are converted mostly to significantly less reactive primary atoms but also to a smaller number of more reactive tertiary sites. However, a detailed analysis of these effects is in any case probably pointless, because a far more striking feature of the observations, and perhaps the most surprising aspect altogether of our results, is that the different linear molecules have distinctly different reactivities. The yield of the majority product OH V′)0 increases by more than a factor of 2 over the range of chain lengths C22 to C28. The trend is similar, or slightly stronger, for V′)1. These variations cannot sensibly be caused by the very modest decrease in the primary to secondary ratio along the same series, from 0.130:0.870 to 0.103:0.897. This observation therefore opens to question the assumption that reactivity is determined by the intramolecular structure of individual hydrocarbon molecules. An alternative possibility is that they result from collective, “supramolecular” order at the liquid surface. Our own previous MD simulations for squalane23 suggest that the surface is highly disordered, with only a modest preference for methyl groups to dominate the extreme outer layers. We calculated that there was a near-statistical chance of an incoming O(3P) atom colliding with each type of H-C group. Corresponding simulations for pristane we have subsequently carried out suggest its surface is similarly disordered.33 For linear hydrocarbons, Harris22 carried out the first systematic MD study of decane (C10H22) at 300 and 400 K, and eicosane (C20H42) at 400 K. Some subtle ordering at the surface was found, with a modest preference for terminal methyl groups to dominate the outer layers. In all cases, these temperatures are well above the respective melting points, though, so they are potentially not directly comparable to the present work. There is, however, quite an extensive collection of literature
on experimental evidence for a “surface freezing” phenomenon in linear hydrocarbons, across a wide range of molecular chain lengths, at temperatures just above the melting point. We note again that this is exactly the region in which we are constrained to work by the vapor pressures. A number of independent measurements, including X-ray and surface tension measurements,34,35 differential scanning calorimetry,36-38 differential thermal analysis,39 ellipsometry,40 and sum frequency spectroscopy,41,42 indicate that the molecules tend to align with their axes perpendicular to the liquid-vapor interface. This phenomenon has been investigated in model calculations by Yamamoto.43-45 In his earlier work,45 a simplified “bead” model was used to represent a generic (C10) linear hydrocarbon. The neglect of the potential for bond angle or dihedral angle distortions of the molecules has the advantage that there is much greater flexibility of the chain. This greatly accelerates the conformational changes necessary to get cooperative organization of multiple molecules. We have repeated for illustrative purposes some of these calculations for a 10-atom bead model, using the program DL_POLY46 and a potential defined by
1 Ub ) kb(r - r0)2 2
(1)
where kb ) 350 kJ mol-1Å-2, and the average bond length, r0 ) 1.54 Å. This controls only interactions between united atoms that are bonded together. All other non-connected atoms are governed by van der Waals interactions:
UvdW(r) ) 4
[(σr ) - (σr ) ] 12
6
(2)
where ) 598.64 J mol-1, σ ) 3.923 Å, and interactions are cut off beyond 9.8 Å. As shown in Figure 7, which displays randomly selected snapshots of a slab of 507 bead-like molecules with two liquidvacuum interfaces, we reproduce the spontaneous surfaceordering phenomenon found by Yamamoto.45 To illustrate the type of structures that emerge, the system was initially randomized at 400 K (Figure 7, left), well above the nominal melting point of 298 K (as determined from the phase behavior revealed by variation of the total interaction energy with temperature).
Hydrocarbon/O(3P) Atom Interfacial Reactivity The system spontaneously develops order at the interfaces, shown in Figure 7 (right), upon being quenched to 240 K and allowed to evolve towards equilibrium for a period of 600 ps. Yamamoto has gone on to show43,44 that essentially similar behavior can be induced in a more sophisticated model that includes a more realistic force-field, but only by “flash heating” an initially pre-ordered sample to a temperature of 425 K, which is above the melting point, for a brief (unspecified) period. Upon rapidly reducing the temperature and allowing the system to re-equilibrate, stable ordered structures emerged at the surfaces of disordered bulk phases. This surface order persists in fully equilibrated samples around the melting point, over a 25 K range (385-410 K). Below this, a pure crystalline structure was observed.43 In further MD simulations of our own using more realistic potentials,47 we have confirmed Yamamoto’s observation that surface order is not developed on a realistic time scale (in practical terms limited to several tens of nanoseconds of simulation time) from a fully equilibrated sample, which he has ascribed to a kinetic effect. Although further work is clearly needed to fully characterize this surface freezing phenomenon, a very interesting possibility is that different degrees of ordering at the surfaces of the liquids of different chain lengths could explain the distinct differences in reactivity with O(3P) atoms that we observe here. It is beyond doubt that the terminal primary H-C bonds are less reactive toward O(3P) than the secondary atoms along the backbone. The reactivity would therefore be quite sensitive to the extent of predominance of end-groups at the surface and the penetrability between chains aligned along the surface normal. It is plausible, though it remains to be rigorously established, that this might vary with chain length. The empirical trend in our results is that the longer chains are more reactive. Probably the most straightforward interpretation is that this would be consistent with the outer layers of the longer chains being less well-ordered. This may be intuitively reasonable because they were necessarily studied at higher surface temperatures. All other effects of chain length being neglected, this could conceivably lead to more thermal randomization of the outermost segments of the chain. However, it should be balanced by the fact that the overall attractive forces are, of course, stronger for larger molecules, resulting in their having higher melting points in the first place. Although there has been successful modeling of the effect of chain length on the width of the temperature interval within which surface freezing takes place,35 we are not aware that chain-length-dependent variations of the degree of order within these intervals has been addressed. In any case, as we have discussed previously,20 the mechanistic connection between surface order and the observed yield of the OH product is subtle. There are opposing factors in the penetrability of the surface, giving better access to more reactive secondary groups, and the escape probability of the OH formed, which may reduce if the OH is formed at greater depth as a result of a higher probability of secondary reaction to form H2O. (H2O is unobserved in our experiments, but known to be produced from the complementary molecular beam measurements of Minton.8,11) Probably as much as we can say with certainty is that the outer surfaces of the linear hydrocarbon liquids must appear significantly different to the incoming O(3P) atoms to cause such substantial differences in the OH yield, but the very interesting question as to exactly what these differences are remains to be established through further experiments and complementary theoretical modeling. Turning now to the rotational temperatures, perhaps the most striking feature of all the measurements is the large difference
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1531 between the hotter distributions in V′)0 and the much colder ones in V′)1, particularly at the rising edge. This strongly suggests that there is a direct component for all liquids, although the extent to which this is established separately for either V′)0 or V′)1 for any particular molecule depends on the accidental relationship between the rotational temperature and the liquid temperature. As we have discussed at length previously,17 for V′)0, the rising edge effectively consists entirely of the fastest molecules, presumed to be formed in a direct mechanism. For “cold” squalane and pristane, there is a very large difference between the observed rising-edge rotational temperatures and the liquid temperature, consistent with these molecules being formed rotationally hot in a direct process. For the other molecules, and “hot” squalane, this is less clear-cut. However, the complement is then true that the V′)1 rotational temperatures are substantially lower than the liquid temperatures. This indicates strongly that these molecules are formed directly under a constraint of fixed total energy, which for V′)1 must necessarily be selectively channeled to vibration at the expense of rotation (and translation). Evidence for a thermal, trapping-desorption component would come from a partial accommodation toward the liquid temperature of the rotational temperature measured at the peak. For V′)0, this is also very obvious for “cold” squalane and pristane, although there is an interesting quantitative distinction between them, to which we return shortly below. For “hot” squalane and the two linear molecules, docosane and octacosane, there is also a consistent drop from the rising edge to the peak, although it is more marginally statistically significant and in some cases represents a slight, non-physical overcooling as noted above. For V′)1, as noted above and as we have discussed in detail in a recent publication,20 it would not be expected that there would be a clean separation of direct and trappingdesorption components on the basis of velocity. This is simply because the much lower translational energy available to a vibrationally excited molecule produced in an energy-constrained direct process is comparable to thermal kinetic energies, resulting in similar flight times for both direct and thermally desorbed components. This explains why there is essentially no significant difference between the rising edge and peak rotational temperatures for V′)1 in any of the molecules. Nevertheless, closer examination shows a definite correlation between either of these temperatures and the temperature of the liquid. This suggests that there is also some component of trapping-desorption for V′)1. A principal conclusion is therefore that it appears, on the basis of the rotational temperatures, that the combination of direct and trapping-desorption components is a general phenomenon for reaction of O(3P) with liquid hydrocarbons. It does not appear to be confined to squalane, the only molecule to have been investigated prior to this work. This conclusion is supported by the generally very similar nature of the appearance profiles for all the molecules. There is relatively little visible variation within the linear molecules, or significant differences between branched and linear. On a final, more speculative note, perhaps the only significant differences that we have detected throughout are in the rotational temperatures and vibrational-level-dependent yields of squalane and pristane. There is a substantially greater cooling between the rising edge and the peak for squalane than there is for pristane, well beyond the limits of the uncertainties in the measurements. This could indicate a higher proportion of the trapped component for squalane than for pristane. This may, in fact be corroborated by the other observed difference that
1532 J. Phys. Chem. C, Vol. 112, No. 5, 2008 pristane gains relative to squalane in V′)1 compared to V′)0: as is apparent from Figures 2 and 3 for V′)0, the yield from pristane is only about 70% of that from squalane, whereas in Figure 3 their yields of V′)1, measured at the same temperature, are essentially equal. In our recent work on the yields from squalane,20 we had speculated that the temperature dependence of the OH V′)1 yield could be the result of the effect of liquid temperature, through changes in liquid structure, on the competition between escape and loss via vibrational relaxation. This would then be consistent with squalane having a lower V′)1/ V′)0 ratio than pristane at the same temperature, if pristane has an overall lower trapping probability. No doubt there are other potential explanations for these observations, but they do serve to illustrate the level of microscopic mechanistic information that is now becoming accessible through experiments of this type. They would provide a sensitive test of the predictions of realistic theoretical simulations of gas-liquid interfacial reactions so far only attempted at higher collision energies.27 Conclusions We have successfully measured appearance profiles and rotational temperatures for gas-phase OH V′)0 and 1 produced in the reaction of O(3P) at the surfaces of a range of representative branched and linear long-chain liquid hydrocarbons. All the molecules investigated show evidence for the dual component mechanism, involving both direct escape and trapping-desorption of the product OH, previously only established for the “benchmark” molecule, squalane. The relative yields of OH show a surprising dependence on the identity of the liquid hydrocarbon. Branched molecules are generally more reactive than linear ones, which may possibly be explained by the known differences in reactivity of primary, secondary, and tertiary H-C units. However, this cannot account for the unexpectedly strong variation in the reactivity of linear hydrocarbons with chain length, by more than a factor of 2 from C22 to C28. We have suggested a possible explanation based on the known “surface freezing” phenomenon of linear hydrocarbon liquids at temperatures just above their melting points. Acknowledgment. We thank the EPSRC for a research grant, studentship funding for M.A., and access to the resources of the National Centre for Computational Chemistry Software. M.L.C. is grateful to Research Councils U.K. for an Academic Fellowship. References and Notes (1) Saecker, M. E.; Govoni, S. T.; Kowalski, D. V.; King, M. E.; Nathanson, G. M. Science 1991, 252, 1421-1424. (2) King, M. E.; Nathanson, G. M.; Hanning-Lee, M. A.; Minton, T. K. Phys. ReV. Lett. 1993, 70, 1026-1029. (3) Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1993, 99, 70567075. (4) Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1994, 100, 39994005. (5) Lipkin, N.; Gerber, R. B.; Moiseyev, N.; Nathanson, G. M. J. Chem. Phys. 1994, 100, 8408-8417. (6) King, M. E.; Saecker, M. E.; Nathanson, G. M. J. Chem. Phys. 1994, 101, 2539-2547. (7) King, M. E.; Fiehrer, K. M.; Nathanson, G. M.; Minton, T. K. J. Phys. Chem. A 1997, 101, 6556-6561.
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