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Pentane, Hexane, Cyclopentane, Cyclohexane, 1-Hexene, 1-Pentene, cis-2-Pentene, Cyclohexene, and Cyclopentene at Vapor/r-Alumina and Liquid/r-Alumina Interfaces Studied by Broadband Sum Frequency Generation Avram M. Buchbinder, Eric Weitz, and Franz M. Geiger* Department of Chemistry and the Institute for Catalysis in Energy Processes, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: September 23, 2009; ReVised Manuscript ReceiVed: October 27, 2009
The molecular orientation and structure of adsorbates at oxide interfaces is driven by surface-molecule and molecule-molecule interactions and is useful for predicting reactivity and product selectivity in heterogeneous chemical reactions, including those important in catalytic processes. Using broadband vibrational sum-frequency generation spectroscopy, we probed cyclic and acyclic alkanes and olefins at buried vapor/solid and liquid/ solid interfaces of the R-alumina (0001) surface under ambient conditions. The spectroscopic signatures of the adsorbates were measured and compared with bulk phase spectra and spectra of model surfaces containing hydrocarbons covalently linked to glass slides using silane chemistry. By utilizing appropriate polarization combinations, the orientations of hydrocarbons adsorbed from the vapor were evaluated and compared to the hydrocarbon orientations at the liquid/solid interface. The data support a proposed orientation for n-alkanes and cycloalkanes at the liquid/solid interface in which the hydrocarbons lie with the planes of their carbon backbones parallel to the surface, whereas at the vapor/solid interface, the adsorbates are oriented with their carbon backbones in an orientation neither parallel nor perpendicular to the surface. The surface vibrational spectra of olefins at both types of interfaces indicate that their orientations differ from saturated counterparts, and their unique spectral signatures allow differentiation of adsorbed olefins from alkanes. 1. Introduction Reactions of inorganic and organic species on metals and metal oxides have received much attention in heterogeneous catalysis,1-41 culminating in the 2007 Nobel Prize in chemistry.42 One of the major recent advances in the field is that it is now understood that the molecular orientations of the adsorbed organic species are critical for controlling interfacial reactivity and kinetics in heterogeneous catalysis. Although much work has been carried out on understanding reactivity and kinetics on metal substrates relevant to catalysis,43-55 vibrational studies focusing on the interaction of catalytically relevant oxides56-62 with hydrocarbons are more sparse63-69 and tend not to focus on determining the molecular orientations of the hydrocarbon species at the catalytically relevant surface unless they are computational.70,71 Bridging this knowledge gap could improve many research fields, including catalyst performance and design,72-78 chromatographic separations,79,80 environmental chemistry,81-89 and tribology.90-92 Because molecular structure and orientation play crucial roles in interfacial chemistry, the interactions of organic molecules with surfaces can depend heavily on the nature of the hydrocarbons involved, thus posing a difficulty in many areas of science and technology. For instance, cyclic alkanes are often used to screen potential oxidative dehydration catalysts, but the results are often interpreted to be general for alkanes.93-95 Likewise, the atmospheric chemistry of biogenic emissions with oxide dust aerosols, which are of great relevance to climate change,81,83-89 involves cyclic, bicyclic and acyclic structures.96-98 Yet, most model systems do not address chemical diversity in their studies. Thus, there is a need to understand the similarities * Corresponding author. E-mail:
[email protected].
and differences between the interactions of linear versus cyclic hydrocarbons at oxide interfaces. Chromatography, temperature-programmed desorption, and batch techniques have been used to determine binding energies for the interactions between organic adsorbates and oxide surfaces.73,99,100 These studies are complemented by computational work101-103 that corroborate surface energies and interfacial structure determined from experiments. For instance, Hase and Schneider and co-workers have used molecular dynamics simulations to predict that n-octane molecules adopt a flat orientation at the vapor/R-alumina interface.101 In extending their work to liquid/R-alumina interfaces, they proposed that alkanes form an ordered layer consisting of hydrocarbon molecules arranged parallel to the surface under the disordered bulk.104 Experimental surface science approaches to studying hydrocarbons at interfaces include electron energy loss spectroscopy105,106 and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR),107-109 which are utilized to characterize surface species on flat oxide surfaces. Raman spectroscopy74,110,111 and transmission IR spectroscopy76,112-115 have overcome the transparency and reflectivity barrier and are used for investigating the adsorption of hydrocarbons on high-surface-area powders. Yet, both approaches tend to focus on topics other than determining the molecular orientations of the hydrocarbon species at the catalytically relevant surface. Whereas ATR-FTIR can provide information on adsorbate orientation,108 many studies have used vibrational sum-frequency generation (SFG) as a surfacespecific method with the unique access to structural information for molecules of all orientations at any total pressure on metal45,91,116-121 and, more recently, on oxide surfaces.81,83,88,92,122-124 For instance, Somorjai and co-workers reported that adsorbates undergo many types of interactions with metal surfaces,
10.1021/jp909172j 2010 American Chemical Society Published on Web 11/20/2009
Alkanes and Alkenes at Alumina Interfaces resulting in the softening of various vibrational modes and changes in the chemical identity of surface species as compared to the bulk.45,118-121 SFG has also been used to characterize adsorbate structures at insulating surfaces where metal-organic back-bonding interactions are negligible, such as the liquid/air interface,82,125-131 the aqueous/solid oxide interface,122,132-135 and recent work at the organic/solid oxide interface.92,123,124 Because SFG does not require the use of highly reflective substrates or high surface areas, and image dipoles do not form on oxides as they do on metals, SFG is a powerful tool for determining the orientations and structures of adsorbates at oxide interfaces. For instance, Sefler et al. collected SFG spectra of hexadecane liquid at the silica interface and found that the C16 n-alkane is oriented flat on the oxide surface.124 Similarly, Nanjundiah and Dhinojwala studied hexadecane at the liquid/ R-alumina interface92 and found that the organic molecules are oriented parallel to that interface. Recently, Brindza and Walker used SFG to determine the orientation of C8-C11 n-alkanes at the vapor/silica interface; their results are forthcoming.123 These results all stand in contrast to the organic liquid/air interface for which SFG experiments by Esenturk and Walker show that n-alkanes adopt an orientation perpendicular to the surface.125,126 To our knowledge, there are no surface spectroscopic studies in the literature that compare the orientation of cyclic, acyclic, saturated, and unsaturated small hydrocarbons at oxide/vapor and oxide/liquid interfaces. A direct comparison of these important classes of hydrocarbons at the liquid/solid and vapor/ solid interface is instructive to evaluate and predict the differences between molecular orientation and structure in complex interfacial environments, which may be driven by disparate surface energies for the vapor/oxide and liquid/oxide interactions.99,101,104 By studying both acyclic and cyclic alkanes and olefins, this work expands the current scientific knowledge of organic/oxide interfaces. We utilize the capabilities of SFG and obtain vibrational frequencies and orientation information. 2. Experimental Section 2.1. Materials. All chemicals in this work were used as received. Several of the experiments were repeated after passing the liquids over a bed of activated alumina but no significant differences were observed in the resulting SFG spectra. Pentane (>99%) was purchased from Fluka; cyclopentane (>99%), cyclohexane (>99.7%), and hexane (>99%), from SigmaAldrich; cyclohexene (99%), 1-pentene (99%), 1-hexene (97%), and cis-2-pentene (98%), from Aldrich; and cyclopentene (98%+), from Alfa Aesar. 2.2. Sum Frequency Generation. Details of the broadband vibrational sum-frequency generation (SFG) setup and methods used in these experiments have been presented in earlier work.83,122,136 The development of this technique137-139 and the theoretical basis for SFG at interfaces140-145 is detailed elsewhere. A brief description of our setup follows. We utilize a Spitfire Pro (Spectra-Physics) laser system that produces 120 fs pulses at 800 nm with a repetition rate of 1 kHz. Approximately 50% of the power is used to pump an Optical Parametric Amplifier (Spectra-Physics OPA-CF) that generates tunable infrared (IR) light between ∼3.2 and 3.6 µm. In these experiments, a notch filter compresses the bandwidth of the remainder of the 800 nm light to 1.6 nm, which is then used to up-convert the tunable IR light. The IR and the 800 nm light are incident on the outside of the sapphire window (vide infra) at angles of 60° and 45° from the surface normal, respectively, and result in incident angles of 23° and 30° at the sample surface, respectively, after accounting for refraction through the
J. Phys. Chem. C, Vol. 114, No. 1, 2010 555 SCHEME 1: Schematic of the Sample Cell and Geometry of the Incident Laser Beams and the SFG Signal
window (Scheme 1). The resulting sum-frequency signal is dispersed on a 0.5 m spectrograph (Acton Research, 1200 grooves/mm) and detected with a liquid-nitrogen-cooled, chargecoupled device (Roper Scientific, 1340 × 100 pixels). The two polarization combinations utilized in this work are SSP and SPS to interrogate the vibrational modes oriented mainly perpendicular and parallel to the surface, respectively. Polarizations are reported in the order SFG, up-converter, and IR. For instance, SSP entails detecting S-polarized SFG signal generated from S-polarized visible and P-polarized IR incident beams. The SSP spectra reported here are an average of 5-10 2-min spectral acquisitions, whereas SPS spectra are an average of 5-10 4-min spectral acquisitions. Spectra were calibrated using the 2955 cm-1 peak of poly(methyl methacrylate),146 and all spectra were collected with the IR centered at several frequencies and then normalized to the sum-frequency signal of gold as detailed elsewhere.83,126 All experiments were repeated in duplicate or triplicate. Spectra were fit using an iterative fitting procedure with overlapping Lorentzian peaks allowed to interfere with each other with fixed 0° or 180° phases. Nonresonant signal contributions were included in the fits as either a constant or a peak with Gaussian line-shape. More details regarding the fitting procedure can be found in the Supporting Information and in earlier work.83,84,136 Absolute intensities are not directly comparable between SSP and SPS spectra in our setup because S- and P-polarized light of the same intensities will not reach the sample surface with the same efficiencies due to the use of a flat window in an internal reflection geometry.145 SFG measurements were made at the (0001) surface of a single crystalline R-alumina window (ISP optics) in internal reflection geometry (Scheme 1). Laue X-ray diffraction backscattering determined that the R-alumina surface used as the experimental interface was the basal (0001) plane (5°. The R-alumina sample was mounted on a custom-built Teflon chamber with a small sample volume of ∼0.5 mL. All measurements were made under ambient conditions on the same window, with the window in the same position relative to the rotational axis normal to the surface. Experiments at the vapor/ solid interface were conducted using the equilibrium vapor pressure of a drop of hydrocarbon liquid that was injected into the sample cell (but not allowed to contact the R-alumina crystal). The vapor pressures of all of the hydrocarbons can be found in Table S1 of the Supporting Information. The surface coverages at the vapor/solid interfaces are not known. However, among the species studied, cyclohexene has the lowest equi-
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TABLE 1: SFG Peak Frequencies and Relative Amplitudes of Linear Alkanes CH3 a.s.
vapor liquid
CH2 a.s.
freq.
rel. amp.
SSP SPS SSP SPS
2962 2961 2965 2966
(1.0) (?) (1.0) (1.0)
SSP SPS SSP SPS
2955
(1.0)
freq.
CH3 f.r.
rel. amp.
freq. Hexane 2935 2917a
CH3 s.s.
CH2 s.s.
rel. amp.
freq.
rel. amp.
freq.
rel. amp.
(0.6)
2882
(0.3)
2866
(0.3)
(0.3)
2866 (0.2) broad feature
Pentane vapor liquid a
2962 2967
2941 (?) weak shoulder
(1.0) (0.7)
2885 2882
(0.4) (?)
2867
(1.0)
2849
(0.2)
CH2 f.r. or CH2 a.s.
TABLE 2: SFG Peak Frequencies and Relative Amplitudes of Linear Alkenes )CH
CH3 a.s.
CH2 a.s.
freq.
rel. amp.
freq.
rel. amp.
freq.
f.r.
rel. amp.
CH3 s.s.
CH2 s.s.
freq.
rel. amp.
freq.
rel. amp.
freq.
rel. amp.
2919
(0.1)
2882
(0.7)
2867 2855
(1.0) (0.6)
2918
(0.5)
2881 2878
(1.0) (0.1)
2858 2856
(0.8) (0.3)
2923
(0.7)
2882
(0.4)
2839
(0.5)
1-Hexene vapor liquid
SSP SSP
3038
(0.1)
2952 2955
(0.9) (1.0)
vapor liquid
SSP SSP
3052a 3070
(1.2) (0.2)
2950 2959
(0.8) (1.0)
vapor liquid
SSP SSP
(0.9)
2951 2965
(1.0) (1.0)
1-Pentene
cis-2-Pentene
a
3007
Broad peak.
TABLE 3: SFG Peak Frequencies and Relative Amplitudes of Cyclic Alkanes CH2 a.s.
CH2 a.s.
CH2 f.r.?
freq.
rel. amp.
freq.
rel. amp.
SSP SPS SSP SPS
2945 2943
(0.7) (0.4)
2933
(0.9)
2940
(1.0)
SSP SPS SSP SPS
2972
(0.1)
freq.
CH2 s.s.
rel. amp.
CH2 s.s.
freq.
rel. amp.
2862
(1.0)
freq.
rel. amp.
2833
(1.0)
2845
(0.2)
Cyclohexane vapor liquid
2914 2933
(0.8)
(1.0) 2857
(0.7)
2872
(0.6)
Cyclopentane vapor liquid
2943
(1.0)
2949 2950
(1.0) (1.0)
TABLE 4: SFG Peak Frequencies and Relative Amplitudes of Cyclic Alkenes )CH freq. vapor liquid
vapor liquidd liquidd
)CH
rel. amp.
SSP SSP SPS SSP SSP SPS
freq.
rel. amp.
small feature 3023 (0.5)
3070
a Asymmetric CH2 stretch. CH2 stretch. d Low signal.
(0.1)
b
3050
(0.6)
CH2 a.s. freq.
CH2 stretch
rel. amp.
Cyclohexene 2946 (1.0) 2956 (1.0) (0.3) 2942b Cyclopentene 2957 (0.7)
CH2 s.s.
CH2 s.s.
freq.
rel. amp.
freq.
rel. amp.
freq.
rel. amp.
2922a
(0.2)
2865 2869
(0.7) (0.1)
2845 2846
(1.0) (0.2)
2915b
(1.0)
2909c
(1.0)
2882
(0.1)
2858
(0.6)
Peak position uncertain because signal obscured by noise, but likely an asymmetric CH2 stretch. c Symmetric
librium vapor pressure (8 Torr) by a factor of 70, but also has the second-highest SFG signal intensity. This high SFG signal intensity for an experiment using a compound with low vapor pressure indicates that surface coverage is likely saturated, considering that some of the experiments using compounds with much higher vapor pressures result in SFG spectra with
significantly lower signal intensity. After sonication in methanol, then in Millipore water, and cleaning in oxygen plasma for 5 min, the window was checked by SFG for spectral signatures in the C-H stretching region prior to every vapor/solid experiment (see Figure S2 in the Supporting Information). Experiments at the liquid/solid interface were conducted using
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the same window and sample cell and directly followed the corresponding vapor/solid experiment to allow for direct comparisons. In all liquid/solid experiments, the sample chamber was filled with the appropriate neat hydrocarbon liquid. 3. Results and Discussion SFG spectra were collected for the vapor/solid and liquid/ solid interfaces at the R-alumina (0001) surface for 5C and 6C cyclic and acyclic alkanes and olefins. Power-normalized spectra in SSP polarization at the vapor/solid interface generally exhibited 50-250 counts/min, whereas the maximum signal intensity at the liquid/solid interface was generally 2-5 times weaker than the corresponding vapor/solid interface (with a few notable exceptions; vide infra). Generally, we assigned the vibrational mode origin of peaks in the spectra on the basis of the literature values for analogous molecules studied using surface spectroscopies, IR and Raman studies of the bulk phase, and theoretical work. The spectral fits as well as the component lineshapes resulting from the fits are overlaid on the spectra discussed below. The frequencies of the component peaks and the relative magnitudes of the amplitudes of each component peak (normalized to the highest peak below 3000 cm-1) are listed in Tables 1-4. On the basis of polarization selection rules, the presence or absence of various vibrational modes in spectra collected in different SFG polarization combinations indicate molecular orientation. For instance, the SSP polarization combination is sensitive only to IR transition moment components perpendicular to the interface. Therefore, the signal due to symmetric stretching modes of methyl groups (-CH3) is strongest in SSP polarization when the local C3 axis of the methyl group, which coincides with its IR transition moment, is aligned along the surface normal. Conversely, the signal is weakest when the local C3 axis of the methyl group is oriented close to parallel with the surface.126,147 The transition moments for the degenerate methyl asymmetric stretches are perpendicular to that of the symmetric stretch, within the local mode approximation. Thus, in the SSP polarization, a methyl group with its C3 axis along the surface normal will show a large signal for the symmetric stretch and small or no signal for the asymmetric stretch (and the opposite for a methyl group parallel to the surface).126,147 For methylene groups (-CH2-), the transition dipole for the symmetric stretch bisects the H-C-H bonds, whereas for the asymmetric stretch, it lies perpendicular to this bisector but in the H-C-H plane. Thus, a methylene group oriented with its H-C-H plane parallel to the surface will result in no signal in the SSP polarization combination, irrespective of whether the CH stretch mode is symmetric or asymmetric. This selection rule again assumes the local mode approximation is valid. Corresponding but orthogonal rules apply to all vibrational modes in the SPS polarization combination, which is sensitive to transition dipoles that lie parallel to the surface.126,143,147 Another consideration in interpreting SFG spectra arises when multiple methyl or methylene groups are present, because modes can constructively or destructively interfere. For instance, straight-chain hydrocarbons with even numbers of carbons often give rise to no methylene stretching signals because all of the transition dipole moments cancel if the molecule exhibits a pure trans configuration.148-151 Similarly, if the adsorbates at an interface assume mostly randomized orientations, the orientationally averaged hyperpolarizability, which is probed by SFG, is very small, and a SFG signal is not observed. 3.1. General Features of SFG Spectra. Given the complexity of the data discussed in this work, we begin with a qualitative
Figure 1. Amplitude magnitudes for peaks resulting from spectral fitting for the SSP spectra of (bottom to top) hexane, pentane, 1-hexene, 1-pentene, cyclohexane, cyclopentane, cyclohexene, and cyclopentene plotted as a function of peak frequency. The error bars represent one standard deviation in uncertainty for the amplitude (Y direction) and frequency (X direction) fitting parameters.
overview of our spectral library. A graphical representation of the SFG frequencies and magnitudes of peak amplitudes obtained through spectral fitting is displayed in Figure 1 (note that these amplitudes, which are taken from spectral fits, may differ from the square root of the observed intensities because of the line width or damping coefficient for each mode. See the Supporting Information for details). Here, we discuss some of the general features of SFG spectra at the vapor/solid and liquid/solid interfaces; a detailed description of the vibrational mode assignments and molecular orientation for each individual compound follows below. First, it is evident that the unsaturated hydrocarbons contain more peaks, as determined by spectral fitting, than their saturated counterparts. This is not surprising given that the unsaturated compounds generally possess lower molecular symmetry152 and more types of C-H bonds than the corresponding saturated compounds. In addition, for each individual compound, the number of peaks in the liquid/solid spectrum is less than or equal to the number of peaks in the corresponding vapor/solid spectrum. As explained above, the disappearance of modes in the liquid/solid spectrum that are present in the vapor/solid spectrum indicate that the molecular orientation likely changes in the liquid/solid spectrum when compared to the vapor/solid spectrum. To further indicate differences in orientation between the liquid/solid and vapor/solid interfaces, the amplitudes of peaks found below 2900 cm-1 (where symmetric stretching modes appear) are lower in the liquid/ solid spectrum than in the vapor/solid spectrum, but peaks found above 2900 cm-1 (where asymmetric stretching modes appear) do not consistently exhibit that trend. We observe that the maximum signal intensities for the liquid interfaces, relative to the corresponding vapor interfaces, generally decrease by approximately the amount expected based on
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Figure 2. Liquid/vapor intensity ratios of the highest peak in the SFG spectra for eight hydrocarbons. The intensity ratio is determined by dividing the maximum signal intensity for the liquid/solid interface by the maximum signal intensity for the vapor/solid interface within the indicated frequency ranges 2900-3000 cm-1 (A), and 2800-2900 cm-1 (B). The dashed lines indicate the intensity ratio expected if the variation in intensity were due to only refractive index differences between the two types of interfaces.
the change in refractive index of the organic medium.145 However, in some cases, differences in molecular orientation cause deviation from these expected signal intensities. This is demonstrated in Figure 2, where we divide the signal intensity of the highest peak of the liquid/solid spectrum by the maximum signal intensity of the vapor/solid spectrum. This ratio is displayed for each compound for two different frequency ranges: 2900-3000 cm-1 (where asymmetric C-H stretches are generally observed) and 2800-2900 cm-1 (associated with symmetric C-H stretches). Note that this particular analysis did not necessarily compare the same peak in both spectra, only the highest peak in either the symmetric or asymmetric stretching regions, and a detailed analysis that compares the same peaks is found below. With other factors, such as orientation, oscillator strength, and surface coverage being equal, the variation in intensity between the liquid/solid and vapor/solid spectra would be due strictly to differences in refractive index, and the ratio would be 0.3.145 This turns out to be the case in approximately half of the spectra. In the other half, the SFG signal intensity differs between the liquid/solid and vapor/solid interfaces for a reason other than the refractive index difference between liquid and vapor. Surface coverage is likely saturated in both cases (vide supra), and there is no reason to believe that oscillator strength would change significantly, so deviations from the predicted ratio are most likely dominated by differences in molecular orientation. We detail the specific orientation differences between liquid/vapor and solid/vapor interfaces below. 3.2. Acyclic Alkanes. n-Hexane and n-pentane served as model systems for acyclic alkanes interacting with the sapphire surface. SFG spectra were collected in both SSP and SPS polarization combinations for vapor/solid and liquid/solid interfaces (Figure 3 and Table 1). These results can be compared directly to SFG spectra of hexadecane liquid at the silica interface measured by Sefler et al.,124 those by Nanjundiah and Dhinojwala at the hexadecane liquid/sapphire interface,92 and those by Brindza and Walker at the vapor/silica interface.123
Buchbinder et al.
Figure 3. SSP and SPS polarized SFG spectra of n-hexane (left) and n-pentane (right) at the vapor/solid and liquid/solid interfaces, as indicated. Spectral fits (smooth blue and green lines) and component peaks (black lines) are indicated, as well. Dashed component peaks indicate a 180° phase; solid black lines indicate 0° phase.
Figure 4. (A) Integrated peak area for the 2966 cm-1 peak in the SSP polarized SFG spectrum of n-Hexane. The shaded regions indicate flow of hexane-saturated helium, and unshaded regions indicate flow of pure helium. The return of the signal to baseline after hexane flow is suspended indicates reversibility of binding of hexane vapor to R-alumina. B and C show regions of the same data in A, enlarged for clarity and to highlight the time scale of adsorption and desorption.
All of these groups have proposed that n-alkanes lie flat at oxide interfaces, although it is unclear if this would be the case for the shorter alkanes in the current work. We present our results here for the purpose of comparisons between liquid/oxide and vapor/oxide interfaces and for comparisons of n-alkanes with unsaturated and with cyclic hydrocarbons. In addition, we will determine if shorter alkanes behave differently from the longerchain alkanes studied in the literature. To determine the nature of the interaction between alkane vapor molecules and the R-alumina surface, we tested the reversibility of binding. The static setup, which was used for all other experiments, was modified to a flow setup in which room temperature helium gas could be directed to flow through a bubbler containing hexane or diverted to bypass the bubbler. As shown in Figure 4, we used SFG to monitor the presence of hexane on the R-alumina surface during two cycles of alternating hexane saturated- and pure-helium flow.
Alkanes and Alkenes at Alumina Interfaces The interaction between hexane and the surface is fully reversible, indicating that hexane physisorbs to the surface. In general, the n-alkanes exhibit strong peaks in the C-H stretching region for vapor/solid and liquid/solid interfaces with the SSP polarization combination and little nonresonant signal. Spectra collected with the SPS polarization combination at the vapor/solid interface exhibited large broad features that are likely due to nonresonant signal that overlaps with sharper peaks resulting from vibrational resonances. These spectra are difficult to fit unambiguously using spectral fitting procedures due to large nonresonant background signal with unknown phase. Thus, these fits were omitted from the data presented here. SPS polarized spectra at the liquid/solid interface contain sharp features that are likely due to vibrational resonances. Vibrational spectra of n-alkanes in the bulk condensed or gaseous phases are plentiful in the literature, and the frequencies of resonances are well-established.84,92,124,126,148,153-155 Methyl asymmetric stretching modes generally appear between 2945 and 2975 cm-1, methyl symmetric stretches appear in the 2870-2890 cm-1 range, and methyl Fermi resonances appear between 2915 and 2945 cm-1. Methylene asymmetric stretching modes appear in the range 2920-2940 cm-1, and symmetric stretches appear between 2840 and 2870 cm-1. We assign vibrational modes for acyclic alkanes using this strong literature precedent as well as the general practice that in the SSP polarization combination, symmetric stretches and Fermi resonances are 180° out of phase with corresponding asymmetric stretches.142,156 3.2.1. Hexane. Vapor. The spectrum of n-hexane at the vapor/solid interface for the SSP polarization combination exhibits two somewhat broad features, each of which consists of two unresolved peaks, as shown by spectral fitting. The peaks appear at frequencies of 2962, 2935, 2882, and 2866 cm-1 that we assign to a methyl asymmetric stretch, a methyl Fermi resonance, a methyl symmetric stretch, and a methylene symmetric stretch, respectively. The peak at 2962 cm -1 also appears in the SPS polarized spectrum. Liquid. Compared to the spectrum at the vapor/solid interface, the SSP spectrum of hexane at the liquid/solid interface exhibits features at similar frequencies, such as a mode at 2965, which we assign to a methyl asymmetric vibration, and a peak at 2866 cm-1 that we assign to a methylene symmetric stretch. To closely model the measured spectrum, a third peak, at 2917 cm-1, must be included in the spectral fit that is in phase with the peak at 2866 cm-1. This additional peak is at a frequency normally associated with a methylene asymmetric stretch. However, because it is in phase with the methylene symmetric stretch, it is more likely a Fermi resonance associated with the methylene groups, such as those observed by Watry and Richmond in alkane-containing surfactants157 at 2917 cm-1 and amino acids158 at 2912 cm-1, both at liquid interfaces. In comparing the spectrum of the liquid/solid interface to the vapor/ solid interface, it is important to note that the peak for the vibrational mode at 2882 cm-1 that we observed for the vapor is absent. Furthermore, the peak intensity of the mode at 2866 cm-1 is significantly smaller for the liquid interface than for the vapor interface, more than can be solely attributed to the difference in refractive index. These differences in comparison to the vapor/solid spectrum suggest that the methyl groups in liquid hexane are closer to lying with their C3 axes parallel to the interface than they are at the vapor/solid interface. The SPS polarized spectrum contains a peak at 2966 cm-1 that is assigned to the methyl asymmetric stretch and a broad feature at 2864 cm-1 that could be either a peak corresponding to the methylene
J. Phys. Chem. C, Vol. 114, No. 1, 2010 559 SCHEME 2: Schematic of the Proposed Orientation of Pentane at the Liquid-Solid Interfacea
a Molecules lie with the plane of the C-C backbone parallel to the interface (left), some with gauche defects (right). Arrows indicate the IR transition dipole for vibrational modes visible in SSP polarization (bold labels) and invisible in SSP polarization (italic labels).
symmetric stretch, two unresolved peaks, or signal due to a nonresonant response. 3.2.2. Pentane. Vapor. The SSP spectrum of n-pentane at the vapor/solid interface exhibits only two peaks, with frequencies of 2955 and 2885 cm-1. These peaks can easily be assigned to a methyl asymmetric stretching mode and a methyl symmetric stretching mode. Sharp features are observed at 2941 and 2882 cm-1 in the SPS spectrum, although spectral fitting procedures were not successfully applied due to the strong broad feature that is likely a nonresonant background. The lower frequency feature is assigned to a methyl symmetric stretch, although the assignment of the feature at 2941 cm-1 is not certain. Liquid. A peak corresponding to a methyl asymmetric stretch, at 2962 cm-1, is also observed in the SSP polarized spectrum of pentane at the liquid/solid interface. The methyl symmetric stretch is not present, but a small feature at 2849 cm-1 is observed that is indicative of a methylene symmetric stretch. A very small shoulder is found at 2925 cm-1 (although too small to fit), which indicates a low-intensity methylene asymmetric stretch. Much like the interfacial structure proposed by Sefler et al. for the liquid hexadecane/quartz interface124 and by Nanjundiah and Dhinojwala for the liquid hexadecane/sapphire interface,92 our data for pentane and hexane imply that the methyl symmetric stretching modes are oriented parallel to the surface at the liquid/ solid interface and is consistent with the arrangement of the long axis of pentane and the plane of the C-C bonds parallel to the surface but with the presence of some gauche defects, as illustrated in Scheme 2. Also consistent with this conclusion is the SPS polarized spectrum that contains a sharp peak at 2967 cm-1, assigned to the methyl asymmetric stretch, and a broad feature at 2867 cm-1 that could result from unresolved methyl and methylene symmetric stretches. 3.3. Acyclic Olefins. The SFG spectra for the SSP polarization combination for the vapor/solid and the liquid/solid interfaces of 1-hexene and 1-pentene are displayed in Figure 5. Spectra for the SPS polarization for these compounds appear in the Supporting Information and exhibit only broad features attributable to nonresonant signals. The frequencies and relative amplitudes resulting from spectral fitting procedures are detailed in Table 2. Generally, the acyclic olefins exhibit more features than their saturated counterparts. On the basis of the literature, assignments of spectral features to vibrational modes for these compounds are similar to the alkanes, with the addition of resonances attributable to C-H stretches of the olefin that appear above 3000 cm-1.83,153-155 3.3.1. 1-Hexene. Vapor. The SSP polarized spectrum of 1-hexene at the vapor/solid interface exhibits more features than
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Figure 5. SSP polarized SFG spectra of 1-hexane (left) and 1-pentane (right) at the vapor/solid and liquid/solid interfaces as indicated. Spectral fits (smooth blue and green lines) and component peaks (black lines) are indicated, as well. Dashed component peaks indicate a 180° phase; solid black lines indicate 0° phase.
the corresponding alkane. The spectrum is fit by five features: weak peaks at 3038 and 2919 cm-1 and strong peaks at 2952, 2882, and 2867 cm-1. We assign the peak at 3038 cm-1 to a stretching motion of the C-H bonds of the olefin, the peak at 2952 cm-1 to a methyl asymmetric stretch, the peak at 2882 cm-1 to a methyl symmetric stretch, and the peak at 2867 cm-1 to a methylene symmetric stretch. The peak at 2919 cm-1 is barely above the noise level in the spectrum, but its inclusion was necessary to obtain a proper spectral fit. It has the same phase as the methylene symmetric stretching peak and may therefore be tentatively assigned to a methylene Fermi resonance, although it could also easily be assigned to a small contribution from methylene asymmetric stretches that generally occur at that frequency, and the small size of the peak may cause uncertainty in the peak’s phase when the spectrum is modeled. Liquid. At the liquid/solid interface, the SSP polarized spectrum of 1-hexene contains a peak corresponding to the same methyl asymmetric stretch mode, at 2955 cm-1, as was observed at the vapor/solid interface. The only other feature observed in the spectrum is a peak at 2855 cm-1, which we assign to a methylene symmetric stretch, although it is at a lower frequency than that observed for the vapor/solid interface. This shift could indicate that the methylene stretching mode in the vapor/solid spectrum originates from a different methylene group, possibly the one adjacent to the olefin. We have observed this effect previously in a terminal alkene bound covalently to a silica substrate through its other end.81,83 The presence of these two features and no others is consistent with an orientation in which the long axis of the molecule lies parallel to the surface, much like the n-alkanes discussed above. 3.3.2. 1-Pentene. Vapor. 1-Pentene at the vapor/solid interface exhibits an SSP polarized spectrum similar to that of 1-hexene, although with a broader olefinic C-H stretching peak and no feature that is assigned to a Fermi resonance. Specifically, spectral fitting results in peaks at 3052, 2950, 2881, and 2858 cm-1, which are assigned to an olefinic C-H stretch, a methyl asymmetric stretch, a methyl symmetric stretch, and a methylene symmetric stretch, respectively.
Buchbinder et al. Liquid. In contrast to the spectrum of 1-hexene at the liquid/ solid interface, the spectrum of 1-pentene at the liquid/solid interface exhibits five peaks. We observe a small peak corresponding to olefinic C-H stretching vibrations at 3070 cm-1, a peak that we assign as a methyl asymmetric stretch at 2959 cm-1, a Fermi resonance at 2918 cm-1, a methyl symmetric stretch at 2878 cm-1, and a methylene symmetric stretch at 2856 cm-1. The presence of all expected modes except the methylene asymmetric stretch could indicate that the molecule adopts a predominately random orientation. However, because of the large signal intensity, a random orientation is unlikely. Alternatively, 1-pentene could adopt an orientation in which all of the observed modes have a component lying along the surface normal, such as a configuration in which the double bond lies parallel to the surface or an orientation in which the long axis of the molecule is parallel to the surface, but in which the local C2 axes of the methylene groups are oriented along the surface normal. These orientations would also direct the transition dipoles of the methylene asymmetric stretches parallel to the surface and silence these modes for the SSP polarization combination. 3.4. Cyclic Alkanes. The conformations of cyclic molecules are inherently more constrained than those of acyclic molecules. In addition, their geometries could lead to changes in interfacial energy.73,103 For these reasons, orientations of cyclic molecules at interfaces may drastically differ from acyclic analogues. Additionally, we expect differences in the spectra of cyclic molecules as compared to linear analogues because they do not contain methyl groups, because ring strain has been shown to shift the frequencies of methylene stretches to higher frequencies,159-161 and because the well-defined conformations of the molecules result in coupling among the vibrations of methylene groups, which in turn results in multiple nondegenerate methylene stretching modes.162-165 Before interpreting SFG spectra of cyclic molecules, vibrational frequency assignments must be determined. Vibrational spectra of cycloalkanes at oxide surfaces may differ from bulk spectra due to the distortion of molecular symmetry. Furthermore, the vibrational frequencies may not be the same as those observed on metal surfaces due to the presence of metal-organic interactions and the formation of image dipoles. Therefore, an investigation into assignments of vibrational modes for cyclic molecules at oxide interfaces is necessary. This is especially the case for the asymmetric C-H stretching region, where we observe peaks at frequencies generally associated with methyl asymmetric stretches, despite the absence of methyl groups (vide infra). Another complicating factor is that specific vibrational modes may be silenced by the molecular orientations adopted, which varies among compounds. To fix and standardize orientation, we utilized an anilinesilane linker to covalently attach cycloalkanes to silica surfaces. We have demonstrated in previous work that this silane chemistry directs the molecular orientation of linked molecules.83 To address the question of high frequency C-H stretching modes, we followed that work83 to link cyclohexyl, cyclopenteyl, cyclobutyl, and cyclopropyl groups to silica surfaces via anilinesilane linkers (Figure 6A). SSP polarized SFG spectra of these surfaces are found in Figure 6B. The highest frequency mode in the C-H stretching region blueshifts with increasing ring strain. To determine if the highest frequency signals are due to asymmetric methylene stretches rather than overtones or Fermi resonances, we compared them with the spectra of bulk cycloalkanes, for which assignments are made on the basis of evidence from theory, computations,
Alkanes and Alkenes at Alumina Interfaces
Figure 6. (A) Illustration of the surface reaction used to link hydrocarbons to the surface via an anilinesilane linker. (B) SSP polarized SFG spectra of C3-C6 cycloalkanes linked to silica substrates via an anilinesilane linker. Spectral fits (smooth blue and green lines) and component peaks (black lines) are indicated, as well. Dashed component peaks indicate a 180° phase; solid black lines indicate 0° phase. Component peaks shaded in green indicate the highest-frequency C-H stretching peak assigned to an asymmetric stretch; the peak shaded in violet indicates assignment to a symmetric stretch. (C) Vibrational frequencies from reference 160 for C3-C6 cycloalkanes observed in bulk IR spectra (black stars) and from this work observed for hydrocarbons linked to silica surfaces via anilinesilane linkers (Red dots), both as a function of the H-C-H bond angle (associated with ring strain).
and isotope substitution.155,159,160,162,163,166-177 There is a trend of vibrational modes that are blue-shifted with correlation to ring strain in spectra of bulk cycloalkanes, which is similar to the trend observed for silane linked cycloalkanes (Figure 6C). For four-, five-, and six-membered cycloalkanes, this stretching mode is assigned to an asymmetric C-H stretching motion on the basis of assignments made for bulk spectra,155,159,160,162,163,166-174 whereas for cylopropane, the blue-shift is so significant that the C-H stretch we observe on the silane surface is assigned as a symmetric stretch because the asymmetric stretch would occur at a frequency higher than the IR light probed in these experiments.159,160,175-177 Also notable in the silane spectra is the appearance of multiple peaks in the C-H asymmetric and symmetric stretching regions. The appearance of multiple peaks is not surprising, given that the frequencies correspond to those observed in spectra of bulk phases. In those spectra of liquid and gaseous cycloalkanes, the multiple peaks in the C-H symmetric and asymmetric stretching regions are assigned to coupled motions of the several methylene groups in the rings that lead to nondegenerate methylene stretches.159,162,163,166-177 With a better understanding of vibrational mode assignments for cyclic molecules, we examined the SSP and SPS polarized SFG spectra of cyclic alkanes at the vapor/solid and liquid/ solid interfaces (shown in Figure 7; resonant frequencies with relative amplitudes displayed in Table 3). Both cyclohexane and cyclopentane have sharp vibrational resonances for the SSP polarization combination at the vapor/solid and liquid/solid interfaces, but the spectra of the liquid/solid interfaces have
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Figure 7. SSP and SPS polarized SFG spectra of cyclohexane (left) and cyclopentane (right) at the vapor/solid and liquid/solid interfaces as indicated. Spectral fits (smooth blue and green lines) and component peaks (black lines) are indicated, as well. Dashed component peaks indicate a 180° phase; solid black lines indicate 0° phase.
much stronger signals at frequencies greater than 2900 cm-1, as compared to the signals at frequencies less than 2900 cm-1. For the SPS polarization combination, both broad and sharp features are present for vapor/solid and liquid/solid spectra. Where possible, these SPS spectra were fit with Lorentzian peaks for vibrational resonances interfering with a Gaussianshaped nonresonant background. 3.4.1. Cyclohexane. The lowest-energy confirmation of cyclohexane is the chair confirmation, which possesses D3d symmetry. There is an activation barrier of 45 kJ/mol to form the boat conformer, which is 23 kJ/mol less stable than the chair conformer.178 At room temperature, the amount of chair conformer is estimated at 99.9%.178,179 Since cyclohexane in the chair confirmation possesses an inversion center, there are no vibrational modes that are both IR- and Raman-active. Unless the confirmation of cyclohexane at the interface is not the chair, there must be net order at the interface, which breaks the molecular symmetry to produce the SFG signal. This situation is comparable to that of benzene, which is SFG-active at the liquid/air interface despite the presence of a center of inversion in the unperturbed molecule.180 A lowering of symmetry compared to the bulk has also been observed using reflection adsorption IR spectroscopy to probe cyclohexane on the Cu(111) surface.181 Alternatively, the equilibrium conformation of cyclohexane at the interface may be different from the conformation in bulk vapor and condensed phases. Vapor. Three peaks are observed in the spectrum of cyclohexane at the vapor/solid interface collected for the SSP polarization combination: at 2945, 2933, and 2862 cm-1. The highest two frequency modes, which both have the same phase, are nondegenerate asymmetric methylene stretches, similar to those observed in bulk phase, where they are assigned to coupled motions of multiple methylene groups.162,166,171 The higher of these modes is blue-shifted from frequencies between 2915 and 2935 cm-1, where methylene stretches are assigned for cyclohexane in bulk spectra.155,162,166,171 However, the observation of methylene stretching modes above 2940 cm-1 in cyclohexane is not unprecedented. Wiberg et al. observe a mode at 2950 cm-1 in the room temperature vapor Raman spectrum which
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they assign to a methylene stretching mode.166 Likewise, we have previously observed unusually high frequencies for C-H stretches in cyclohexane in the case of a cyclohexane-modified silane on glass, where symmetry is broken by immobilization on the surface.83 Similarly, Ma et al. observe modes at 2937 and 2948 cm-1 in the SFG spectra of neat piperidine (saturated pyridine) at the R-alumina interface.82 We assign the peak at 2862 cm-1 to a symmetric methylene stretch based on the wellestablished literature values.83,107,119,155,159,162,166,171 The corresponding SPS spectrum also exhibits broad peaks at 2943, 2913, and 2832 cm-1 which are consistent with the literature83,107,119,155,159,162,166,171 as well as a broad nonresonant background. We assign the peaks as an asymmetric methylene stretch, a Fermi resonance, and a methylene symmetric stretch, respectively. Liquid. The SSP polarized spectrum for cyclohexane at the liquid/solid interface has one peak at 2932 cm-1. The SPS spectrum exhibits two peaks: one at 2940 cm-1 and a weaker one at 2857 cm-1. These are assigned to methylene asymmetric and to methylene symmetric stretching motions, respectively. The absence of a peak in the symmetric stretching region of the SSP spectrum indicates that cyclohexane molecules have an orientation at the liquid/solid interface different from that at the vapor/solid interface. More specifically, cyclohexane may lie flat, especially when considering that the symmetric stretch peak is present in the vapor/solid spectrum. The stretches of the methylene groups, when perturbed by a surface, result in asymmetric stretching motions of axial C-H bonds with transition dipoles perpendicular to the surface, whereas symmetric stretches may sum to result in only a small component of the transition dipole along the surface normal.181 Consistent with this description of orientation is the presence of a signal in the symmetric stretching region in the SPS spectrum, which could easily result from symmetric stretches of uncoupled methylene groups or from coupled symmetric stretches of equatorial C-H bonds. 3.4.2. Cyclopentane. Vapor. The SSP polarized spectrum of cyclopentane at the vapor/solid interface contains two asymmetric methylene stretching peaks: a strong one at 2942 cm-1 and a weak one at 2971 cm-1. The existence of multiple peaks in the methylene asymmetric stretching region is consistent with nondegenerate methylene asymmetric stretches expected for cyclopentane in an envelope confirmation, which possesses Cs symmetry.163,167-170 In Cs symmetry, all modes are both Ramanand IR-active. The spectrum of cyclopentane vapor also exhibits a strong, sharp peak at 2871 cm-1, which we assign to a methylene symmetric stretch. In the SPS spectrum, a single peak interferes with the broad nonresonant signal, but because the phase between the resonant and nonresonant signals is unknown, and since the feature is not unambiguously either a dip or a peak, it is not possible to determine its frequency. Liquid. Spectra of liquid cyclopentane at the R-alumina interface for the SSP polarization combination consistently exhibit a very strong peak at 2949 cm-1, which we assign to methylene asymmetric stretches. Compared to the analogous peak observed for the vapor/solid interface, the intensity of this peak is more than 10 times the intensity that is expected due solely to the difference in refractive index between the vapor and the liquid (Figure 2)145 Furthermore, the only peak observed below 2900 cm-1 (at 2844 cm-1) is small. These observations indicate that there is likely a change in molecular ordering from vapor to liquid and that the molecules at the liquid/solid interface lie with the transition dipole of the coupled methylene asymmetric stretches perpendicular to the surface, requiring a nearly
Buchbinder et al.
Figure 8. SSP and SPS polarized SFG spectra of cyclohexene (left) and cyclopentene (right) at the vapor/solid and liquid/solid interfaces as indicated. Spectral fits (smooth blue and green lines) and component peaks (black lines) are indicated, as well. Dashed component peaks indicate a 180° phase; solid black lines indicate 0° phase. Inset: the olefinic stretch of cyclohexane collected with input IR power centered at ∼3010 cm-1.
flat arrangement of the cyclopentane ring on the R-alumina surface. A spectrum taken in the SPS polarization combination contains a broad feature that can be fit to a peak at 2950 cm-1 interfering with a strong nonresonant signal. The presence of this broad signal at the same frequency that we observed in the SSP spectrum indicates that it is also due to a methylene stretching motion; however, it is common for multiple methylene stretching peaks in cyclopentane to appear at very similar frequencies, even if they correspond to vibrational modes of different symmetry. For example, Schettino et al. assign a vibrational mode observed at 2960 cm-1 in the IR spectrum of cyclopentane liquid to unresolved stretching modes of A′ and A′′ symmetry resulting from coupled methylene asymmetric stretches.169 Therefore, it is possible that the SSP and SPS spectra sample different coupled methylene stretching motions. 3.5. Cyclic Olefins. The SSP polarized SFG spectra of vapor cyclic olefins at the R-alumina interface contain strong signatures of many vibrational modes. At the liquid/solid interface, these signatures drastically diminish in intensity, below the limit of detection in the case of cyclopentene. The SFG spectra of cyclopentene and cyclohexene are displayed in Figure 8; the frequencies and relative amplitudes are listed in Table 4. 3.5.1. Cyclohexene. Vapor. In the SSP spectrum of cyclohexene at the vapor/solid interface, two peaks are located in the asymmetric stretching region at 2945 and 2922 cm-1 and two in the symmetric stretching region at 2865 and 2844 cm-1. These peaks correspond well to literature values for nondegenerate asymmetric and symmetric methylene stretches predicted by vibrational mode analysis182,183 and observed in gas phase by IR and Raman spectroscopy.183,184 The SPS spectrum consists of a strong, broad peak with some small features that appear to correspond in frequency to the features in the SSP polarized spectrum. However, spectral fitting could not establish exact frequencies for the small features in the SPS spectrum. No large peaks were observed above 3000 cm-1 for either polarization combination, where peaks corresponding to C-H stretches of sp2 hybridized carbon atoms generally appear, although a very
Alkanes and Alkenes at Alumina Interfaces small feature can be seen when the input IR power is centered at ∼3010 cm-1 (Figure 8 inset). This spectrum should be compared to our previous work in which a strong signal was observed at 3035 cm-1 in the SSP polarized SFG spectrum for a cyclohexene moiety covalently linked to glass through a silane linkage, which we assigned to a C-H stretching mode of the olefin carbons.81,83 The extremely low strength of this mode for physisorbed cyclohexene at the vapor/solid interface may indicate that the plane of the olefin substituents lies parallel to the interface. Liquid. The spectrum of cyclohexene at the liquid/solid interface in the SSP polarization exhibits a peak at 3023 cm-1, which we assign to the C-H stretching vibrations of the sp2 hybridized carbons. The spectrum of the liquid/solid interface exhibits three additional peaks at 2956, 2869, and 2846 cm-1, the first of which is assigned to an asymmetric stretch and the latter two of which are assigned as symmetric methylene stretches (vide supra). The SPS polarized spectrum contains a moderate nonresonant background that interferes with a sharp peak at 2941 cm-1 and a broad peak at 2914 cm-1, which is out of phase with the higher-frequency peak. However, this spectrum has a large amount of noise, so definitive assignments are not made. In the literature, peaks have been observed at similar frequencies, where they are assigned to C-H stretching vibrations of nondegenerate methylene groups by Lespade et al.185 (based on vapor-phase FTIR and Raman spectra of partially deuterated cyclohexenes at room temperature) and are also observed by Somorjai and Rupprechter in SFG spectra of cyclohexene molecularly adsorbed on Pt(111) at 130 K.118 3.5.2. Cyclopentene. Vapor. At the vapor/solid interface, the SSP polarized spectrum of cyclopentene has many features. There is a sizable feature at about 3060 cm-1 that can be fit with two oppositely phased peaks at 3050 and 3069 cm-1, which is consistent with assignment to symmetric and asymmetric stretching modes of the two C-H bonds of the olefin.81,83,164,165 Unlike cyclohexene, the appearance of this vibrational mode indicates that the olefin plane is not oriented entirely parallel to the interface. Four other peaks were observed at 2957, 2909, 2882, and 2858 cm-1. We assign the peak at 2957 cm-1 to a methylene symmetric stretch that is blue-shifted due to the ring strain of cyclopentene,159-161 and the peak at 2909 cm-1 is assigned to a methylene symmetric stretch, like that observed in the IR and Raman bulk spectra and assigned to a symmetric stretch of the unique methylene group.165,186 Following literature precedent, the peaks at 2882 and 2858 cm-1 are assigned to methylene symmetric stretches.83,159,165,186 Liquid. The SFG signal generated at the liquid/solid interface for cyclopentene is below the detection limit. Because signal intensity at the vapor/solid interface is high and because we observe significant signal at the liquid/solid interface for other hydrocarbons with similar refractive indices, we can infer that cyclopentene at the liquid/solid interface has negligible net ordering. 3.6. cis-2-Pentene. To directly compare cyclic olefins (which are necessarily in a cis configuration) to an acyclic olefin, we collected SFG spectra of cis-2-pentene, which is an acyclic analogue of cyclopentene. cis-2-Pentene is expected to exhibit some spectroscopic signatures that differ from cyclopentene because it contains methyl groups and because its methylene group and adjacent methyl group will presumably be configured in a trans geometry, as opposed to cyclopentene, which contains permanent gauche-configured methylenes. The spectra at the vapor/solid and liquid/solid interface of cis-2-pentene are shown
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Figure 9. SSP and SPS polarized SFG spectra of cis-2-pentene at the vapor/solid and liquid/solid interfaces as indicated. Spectral fits (smooth blue and green lines) and component peaks (black lines) are indicated, as well. Dashed component peaks indicate a 180° phase; solid black lines indicate 0° phase.
in Figure 9; the vibrational resonant frequencies and relative amplitudes are summarized in Table 2. Vapor. Unlike the other acyclic olefins studied in this work, the SSP polarized spectrum of cis-2-pentene at the vapor solid interface does not exhibit a peak that is attributable to an olefin (>3000 cm-1). This finding could indicate that the molecule orients itself with the plane established by the olefin and its substituents parallel to the surface, and thus, the olefinic C-H stretches are silent in the SSP polarization combination, or the olefin interacts directly with the surface such that the intensity of the olefinic C-H stretch vibrations is diminished. The spectrum also contains peaks which, on the basis of spectral fitting and the same well-established literature precedent as used for other acyclic alkenes above, we assign to a methyl asymmetric stretch at 2951 cm-1, a methyl Fermi resonance at 2923 cm-1, a methyl symmetric stretch at 2882 cm-1, and a methylene symmetric stretch at 2839 cm-1.84,92,124,126,148,153-155 Liquid. The spectrum of cis-2-pentene at the liquid/solid interface exhibits a peak at 2965 cm-1, which we assign to a methyl symmetric stretch, with a shoulder at 3007 cm-1, which may be due to an olefinic stretching mode. Since there are two unique methyl groups in the molecule, it is possible that the change in frequency of the methyl asymmetric stretch when comparing vapor to liquid is due to different methyl groups being sampled in each case. The absence of a methyl symmetric stretching mode suggests that an orientation is adopted in which the entire molecule lies in one plane parallel to the surface. 4. Conclusions Sum-frequency generation, which is sensitive to molecular orientation, was used to determine that there is generally a significant difference in molecular orientation of hydrocarbons at organic/R-alumina interfaces when comparing the vapor/oxide interface to the liquid/oxide interface. Differences in molecular interfacial orientation were observed for all eight of the C5 and C6 linear and cyclic alkanes, 1-olefins, and cyclic olefins. In
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general, n-alkanes and cycloalkanes lie flat at liquid/oxide surfaces, n-alkanes with their molecular planes parallel to the interface, and cycloalkanes most likely lie with the plane of the ring parallel to the interface. This is in contrast to vapor/ solid interfaces, where more vibrational modes are observed, leading to the conclusion that n-alkanes and cycloalkanes at the vapor/solid interface adopt an orientation different from the flat orientation observed at the liquid/solid interface. The conclusions are not as general for olefins, which did not exhibit consistent trends in signal intensities, although at least one vibrational mode originating from the olefin was observed either at the liquid/solid or vapor/solid interface for each olefin studied. Nonetheless, in each individual case, orientations were proposed from a basic analysis. Further work is necessary to determine if the double bond plays a role in guiding orientation or if the differences observed between saturated and unsaturated species are simply due to differing molecular symmetries and numbers of methyl and methylene groups present in the respective molecules. To answer this question, correlations between adsorption energy, reactivity, and molecular orientation are necessary and are the subjects of ongoing studies. In either case, the differences observed in the spectra between saturated and the corresponding unsaturated species are significant. Hence, spectral markers detailed here indicate that SFG is a promising technique for future work to identify surface species before, during, or after reactions or sorption events at liquid/oxide or vapor/oxide interfaces and for benchmarking computer simulations of the orientations and conformations of hydrocarbons at such interfaces. Acknowledgment. We gratefully acknowledge fruitful discussions with James E. Rekoske and Thomas M. Mezza of UOP, LCC (Des Plaines, IL). We are grateful to Professor Robert Walker (Montana State University) and Dr. Michael Brindza for helpful conversations. We also thank Dr. Jerry Carsello of the Jerome B. Cohen X-ray Diffraction Facility at Northwestern University for performing and interpreting the Laue X-ray backscattering measurement to determine the exposed face of the R-alumina crystal. The authors also thank Spectra Physics, a Division of Newport Corporation, for equipment loans and donations as well as technical support. This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Grant DE-FG02-03-ER15457), at the Northwestern University Institute for Catalysis in Energy Processes; by the National Science Foundation division of chemical, bioengineering, environmental and transport systems, catalysis, and biocatalysis CBET program (Grant no. 0931701); and by the National Aeronautics and Space Administration Earth System Science Fellowship program (Grant 07-Earth07R-0084). F.M.G. gratefully acknowledges support from the Alfred P. Sloan Foundation. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Goodman, D. W. Chem. ReV. 1995, 95, 523. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735. (3) (4) (5) (6) (7)
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