Ethanol Monomers and Dimers Revisited: A Raman Study of

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Ethanol Monomers and Dimers Revisited: A Raman Study of Conformational Preferences and Argon Nanocoating Effects Tobias N. Wassermann† and Martin A. Suhm* Institut fu¨r Physikalische Chemie, UniVersita¨t Go¨ttingen, Tammannstrasse 6, 37077 Go¨ttingen, Germany ReceiVed: May 27, 2010; ReVised Manuscript ReceiVed: July 13, 2010

The gauche-trans conformational distribution in ethanol can be determined from the OH stretching Raman spectrum of seeded supersonic jet expansions, which thus provides a sensitive conformational thermometer. Depending on the rare gas mixture, one, two or four ethanol dimer conformations are abundant. Their conformational assignment is facilitated by the observation of hydrogen bond acceptor modes, which have similar Raman cross sections but much inferior infrared intensities than donor modes. Ethanol monomers and dimers can be progressively Ar-coated, and the resulting spectra may be compared with those in a bulk argon matrix. The low frequency range of torsional transitions provides some evidence for conformation-changing transitions in Raman jet spectra. 1. Introduction Ethanol lends itself particularly well to a study of the interplay between hydrogen bond formation and molecular conformation. As an isolated monomer, it favors the trans state (et),1 whereas the hydrogen bonded dimer has a global minimum structure consisting of gauche states (egeg), in which the two OH groups prefer the same side of the backbone.2 This represents a simple case of adaptive aggregation, an even simpler one being the mixed dimer of ethanol and water.3 In contrast to the latter, ethanol dimer may also be viewed as an example for chirality synchronization between the two transiently chiral alcohol units.4 This phenomenon is suppressed in a low-temperature Ar matrix, where both ethanol units adopt trans conformations,5 like the isolated monomer.1 The monomer conformational equilibrium in ethanol is rather subtle, with an energy separation of only ≈0.5 kJ mol-1,1,6,7 but the spectra are less congested than in the case of longer chain alcohols,8–10 and Raman spectroscopy has been shown to provide a particularly clear-cut access to the relative abundance of the two monomer conformations.11,12 Early infrared dimer studies provided evidence for up to four different conformations.13–15 Three of them have been structurally characterized by microwave spectroscopy.16 Raman spectroscopy was shown to provide complementary information for alcohol clusters in general,8,12,17,18 despite a lack of significant signal enhancement by the hydrogen bond. The dominance of the ∆J ) 0 Raman transitions allows for a better conformational resolution than typical rovibrational profiles present in infrared spectra at nonzero temperatures. The narrow Raman band profiles also help in the investigation of Ar-coated ethanol molecules and clusters. These provide a bridge between small complexes, such as the ethanol-Ar dimer,19 and ethanol embedded in bulk argon matrixes.5,20 The accurate modeling of the underlying Ar-alcohol interactions and their influence on the vibrational spectra remains a challenge.10,21 In the present Raman study, we set out to characterize the energy sequence of the various ethanol dimer conformations * Corresponding author. E-mail: [email protected]. † Present address: Laboratoire de Chimie Physique Moléculaire, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.

on the basis of a series of supersonic jet relaxation experiments. We explore the effects of partial Ar condensation on ethanol molecules and dimers. The sharp Raman signals are used as a conformational thermometer that probes the relatively facile interconversion between spectroscopic eg and et states by collisions in the supersonic expansion zone. 2. Methods 2.1. Raman Jet Measurements. Within the past few years, powerful and sensitive setups for the measurement of spontaneous Raman scattering have been developed22 with which the investigation and characterization of supersonic expansions became possible.23–25 The Raman jet spectra in this work have been recorded using the curry-jet setup (from classical unrestricted Raman spectroscopy in a jet),12 suitable for the study of molecular conformations and intermolecular interactions, with special emphasis on hydrogen bonding.3,10,26 Gas mixtures of ≈0.3, 0.5, or 0.9% ethanol (Carl Roth, G99.8%) or ethanolOD (CH3CH2OD, Cambridge Isotope Lab., 99 atom % D) in a carrier gas (He (Air Liquide, G99.996%), Ne (Air Liquide, G99.99%), ≈1% N2 (Air Liquide, G99.9999%) in He, or ≈8% Ar (Air Liquide, G99.998%) in He) were prepared by flowing the carrier gas through a thermostattable saturator filled with liquid ethanol. The gas mixtures were stored in a stainless steel reservoir with a volume of either 67 or 4.7 L. From there, they were expanded through a slit nozzle (4 × 0.15 mm2 or 8 × 0.05 mm2, corresponding to length/width ratios L/D of ≈27 and ≈160, respectively) into a 0.14 m3 vacuum chamber. The chamber was evacuated by Roots pumps (500 m3 h-1 and 250 m3 h-1), backed up by a rotary vane pump (100 m3 h-1). Typical stagnation pressures were pS ) 0.7-1.0 bar. Under these conditions, the background pressure in the chamber reaches values of up to ≈3 mbar. Inside the chamber, the supersonic jet expansion was crossed perpendicularly by the ≈30 µm beam waist of a frequency doubled cw Nd:YVO4 laser (λ ) 532 nm, Coherent Verdi V18, P ) 18 W), resulting in an irradiance of >2 MW cm-2. The scattered light was collected at a 90° angle and collimated using a high sensitivity camera lens (Nikon, L ) 50 mm, f/1.2). An achromatic planoconvex lens (Edmund Optics, L ) 50 mm, f/7) then focused it onto the entrance slit of a McPherson model

10.1021/jp104861q  2010 American Chemical Society Published on Web 07/27/2010

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2051 monochromator (f/8.6, f ) 1000 mm), which was equipped with a 1200 gr/mm grating (110 × 110 mm2). A spectral resolution of ≈1.3 cm-1 (estimated from the linewidths of rovibrational N2 transitions) was thus achieved. Rayleigh scattered photons were suppressed using a Raman edge filter (L.O.T., L ) 25 mm, OD 6.0, T > 90%, 535.4-1200 nm). Stokes scattered light was detected using a liquid-N2-cooled back-illuminated CCD camera (PI Acton, Spec-10: 400 B/LN, 1340 × 400 pixel) with high quantum efficiency. Cosmic ray signals were removed by a comparison of 6-10 block-averaged spectra, and the emission lines of a Ne I atomic light source were used for the wavelength calibration. Typical integration times in the measurements were between 200 and 900 s for each of the scans. Gas phase measurements were carried out at shorter integration times by filling the chamber with the gas mixture up to the desired pressure or by a continuous gas flow at reduced pumping speed. The slit nozzle was precisely positioned and varied in the direction of the expansion, relative to the fixed laser. This was used to characterize the expansion23 or to follow dynamical processes, such as the relaxation of conformers or isomers or the onset of Ar condensation onto molecules and aggregates.10,27 2.2. Quantum Chemical Predictions. Wavenumbers and Raman scattering activities of ethanol monomers and dimers were estimated using the B3LYP hybrid density functional or MP2 perturbation theory in combination with different basis sets (6-311+G(d) up to 6-311++G(2d,p)) with the Gaussian03 program package.28 These results were used to assist the vibrational assignments (see, e.g., Table S1 in the Supporting Information for a comparison of harmonic wavenumbers and scattering cross sections of the OH and OD stretching vibrations of ethanol and ethanol-OD at different levels of approximation). For energetic sequences of different dimer isomers, the more accurate results obtained in refs 2 and 29 were consulted. Raman scattering activities Ak, which are calculated from the polarizability derivative invariants R ′k and γ ′k for the corresponding vibration k by Gaussian03 according to Ak ) 45R ′2k + 7γ′2k , were converted to experimental scattering cross sections, σ ′. k For the present case of a photon counting measurement with a CCD detector and simultaneous detection of both polarization directions (⊥s + |s) at 90° detection geometry, eq 1 applies, when the polarization vector of the incident laser light is oriented perpendicularly to the plane of detection (⊥ i):

σk′(90o ;⊥s + ||s, ⊥i) )

2π2h · 45cν˜ k

(ν˜ 0 - ν˜ k)3ν˜ 0 · gkAk hcν˜ k 1 - exp kT (1)

(

)

Figure 1. Raman jet spectrum and et, e+ g , eg energy level scheme for the OH stretching vibration in ethanol together with g+, t ethanol structures.

nates from the summation over all populated levels at the temperature T.31 Thermal excitation of energetically higher lying states with quantum numbers υ > 0 leads to an enhancement of the scattering cross section due to the larger amplitudes of the vibrating nuclei in these states.31 In supersonic expansions, the exact temperature of each individual normal mode of vibration k is difficult to determine. It was assumed to be low; that is, hcν˜ k . kT. For high frequency vibrations, such as hydride stretching motions, this assumption is fulfilled even at room temperature. For low-frequency vibrations, where thermal excitation leads to a significant population of states with higher quantum number υ, the scattering cross sections may even be somewhat affected in jet experiments. A relaxed OH torsional scan for ethanol monomers was calculated in steps of 15°. In addition to a normal-mode analysis, the well-separated32 harmonic OH stretching frequency was evaluated in a one-dimensional local mode approximation.2,10 At each torsional angle, the OH bond length was varied in steps of 0.02 Å between 0.9 and 1.04 Å. A third order polynomial was fit to the eight points. Its derivative at the minimum was used together with effective masses of mH ) 1.67 × 10-27 kg and mO ) 26.6 × 10-27 kg to obtain the approximate harmonic wavenumber. The behavior of the OH stretching frequency as a function of the torsional angle can be used to predict whether hot torsional bands are expected to be red- or blue-shifted. 3. Results and Discussion

In a nomenclature based on laboratory Cartesian coordinates analogous to the one employed in ref 30, the incident laser light propagates along z with its polarization vector parallel to the y axis. Detection is carried out in the direction of the x axis, and both components with the polarization vector parallel to the y or the z axis are detected; thus, one can specify the scattering In the following, the symbol σ′ cross section as σ ′(z(y[yz])x). k is used without repeating the details. In eq 1, ν˜ k stands for the wavenumber of the corresponding vibration, gk is its degeneracy, ν˜ 0 is the wavenumber of the incident laser light (in this work, λ0 ) 532 nm; i.e., ν˜ 0 ≈ 18797 cm-1), h and k are the Planck and Boltzmann constants, c is the speed of light, and T is the absolute temperature. The factor (1 - exp(-hcν˜ k/kT))-1 origi-

Ethanol monomers can occur in the trans (t) or the two enantiomeric gauche forms (g() with the torsional angle τOH either at 180° or approximately (60°. The wave functions in the g+ and g- potential wells can interact with each other via quantum mechanical tunneling and split into two states we name eg+ and eg-, according to their parity. In the vibrational ground state, the two tunneling states eg+ and eg- are split by 3.3 cm-1, whereas the et state lies ≈39.5 cm-1 below the eg+ level.1,6,33–35 Excitation of the OH stretching vibration2,12 (see Figure 1) lowers the et - e+g energetic difference by ≈18 cm-1. Therefore, the two bands from et and eg ethanol are observed at 3678 and 3660 cm-1, respectively. The e+ g /eg splitting appears to be more or less unaffected by the OH stretching excitation. Even in

Ethanol Monomers and Dimers

J. Phys. Chem. A, Vol. 114, No. 32, 2010 8225 ambiguity in the reduced masses. Starting from each of the potential energy minima, the OH stretching wavenumber increases in the direction of the transition states. The highest wavenumber is predicted at the transition state between the t and g rotamers. These results confirm that hot vibrational transitions from an excited torsional state are expected to show a blue shift with respect to the fundamental position. Therefore, the peak at 3685 in Figure 2) is assigned to a hot OH cm-1 (labeled eτ(OH) t stretching transition in a thermally excited torsional state of t ethanol. According to Figure 3, the same applies for g ethanol. Therefore, at least parts of the unresolved structure between the et and eg band can be attributed to hot transitions of g ethanol. This is even more pronounced at elevated temperatures.11,37 At the reduced temperatures in a jet expansion, the hot band structures and rovibrational contributions are strongly reduced. Instead, contributions from dimer OH acceptor modes start to contribute (see Section 3.2). The intensity ratio of the et and eg monomer bands, Iet/Ieg, can still be used to estimate the population ratio of the conformers, Nt/Ng, from which a conformational temperature, Tconf, can be calculated according to eq 2 with the energetic difference between the et and eg states ∆E ≈ 0.49 kJ mol-1 (see Figure 1).

σ′egIet σ′etIeg Figure 2. Scaled monomer OH stretching Raman jet spectra of ethanol under different conditions. From bottom to top: in the gas phase (GP), at reduced temperature in a He jet at d ) 3 mm (distance from the nozzle downstream the expansion), in a Ne jet at d ) 3 mm, and in Ar jet expansions at increasing distances (d ) 1 and 3 mm). The position from bulk Ar matrix measurements20 is indicated by a thick bar, |, for comparison and was shown to correspond to et.

Raman spectra11,12 (see Figure 1), which are dominated by narrow ∆J ) 0 branches, no splitting or broadening of the eg OH stretching band can be resolved. The relatively low barrier36 of ≈4.8 kJ mol-1 from t to g ethanol and the small mass of the proton facilitate the interchange between the conformers at room temperature. The g form is favored by entropy; therefore, the eg band is more intense in room temperature gas phase measurements (see Figure 2). In supersonic jet expansions, the t form profits from the reduced temperatures and its larger scattering cross section (see Table S1). The intensity ratio can be further influenced by varying the distance from the nozzle or the nature of the carrier gas. This will be explored in the following section. 3.1. Monomer Relaxation. Figure 2 shows a room temperature gas phase spectrum and jet spectra of the monomer OH stretching vibration with different carrier gases and distances, d, from the nozzle. The spectra are scaled to comparable intensities. The gas phase spectrum has a rather complex appearance. The dominant ∆J ) 0 branches are surrounded by hot vibrational transitions.37 Most prominent is the highest wavenumber one at 3685 cm-1. It is expected to originate in an excited state of the OH torsional mode.37 To check this hypothesis, the torsional potential of the OH group was calculated (see Figure 3, left ordinate axis). It shows the typical profile with three minima for t and g( ethanol. Along the path, normal and local mode predictions for the OH stretching wavenumber were made (see Section 2.2). They behave in a parallel way, and the offset can be accounted for by the

)

( )

Nt gt ∆E ) · exp Ng gg kTconf

(2)

The ratio of the scattering cross sections σ e′ t/σ ′eg can be estimated from either quantum chemical calculations using eq 1 or from a reference measurement at known temperature. Table 1 gathers values for the ratio of the cross sections, σ e′ t/σ ′eg, from different approaches. The experimental ratio of 1.5 ( 0.1 was calculated from the integrated intensities of the et and eg bands in a 300 K gas phase measurement. The theoretical ratios are within this error margin. The experimental σ ′et/σ ′eg ratio was used to calculate conformational temperatures, Tconf (K), in the expansions under different conditions. Conformational temperatures as a function of the nozzle distance24 allow one to follow the adiabatic cooling25 of the molecules within the first few millimeters downstream from the nozzle exit as well as the reheating close to the Mach disk. Peak heights were used for this purpose, but peak areas gave similar results. Error bars were estimated by adding and subtracting the root-mean-square of the spectral noise. Figure 4 demonstrates the effects of slit nozzle geometry and pressure ratio on the progression of the conformational temperatures along the expansion direction.38,39 Initially, the cooling of the molecules proceeds similarly in the zone of silence downstream the expansion near the nozzle exit. Depending on the detailed expansion conditions, the proximity of the shock waves further downstream leads to a reheating of the molecules, which goes along with a discontinuous jump of other parameters (e.g. the molecular density40). Rotational temperatures show a fast increase when the shock waves are encountered. The ethanol conformational temperature shows a slower rise, which mirrors a slower cooling speed close to the nozzle. Isomerization involves a small barrier36 and requires more collisions than rotational excitation.23,25 Relaxation of excited torsional states, which does not involve any barrier band, proceeds and is reflected by the intensity of the eτ(OH) t faster than isomerization within the first few millimeters downstream the expansion.

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Figure 3. Relative electronic energies E, kJ mol-1, of ethanol (9, thick solid line, left ordinate axis) for the torsion of the OH group from a relaxed potential scan (B3LYP/6-311+G(d)) and harmonic OH stretching wavenumbers ν˜ OH (right ordinate axis) from normal-mode analysis (O, dashed line) and from a local mode approach (0, short dashed line; see text).

TABLE 1: Ratio of the Scattering Cross Sections σ′ of Ethanol t and g According to Different Theoretical Methods and Extracted from a Gas Phase Measurement at T ) 300 K, Together with the Known Energy Difference σ′et/σ′eg

B3LYP

MP2

6-311+G(d) 6-311+G(d,p) 6-311++G(d) 6-311++G(d,p) 6-311++G(2d,p) experimental:

1.52 1.49 1.51 1.48 1.48

1.50 1.47 1.49 1.47 1.47

1.5 ( 0.1

Isomerization can be assisted by adding traces of a heavier carrier gas to the carrier gas mixture2,41 or by performing expansions in pure Ne.41,42 Consequently, lower ethanol conformational temperatures were observed when using such carrier gases or carrier gas mixtures. Although the lowest conformational temperatures in He expansions range between ≈50 and 70 K, minimum temperatures of ≈40 and ≈20 K were reached when a few percent of Ar or N2 was added to the He carrier or pure Ne was used as the carrier gas, respectively. Higher concentrations of Ar led to clustering effects2 (see Section 3.4). The position of the Mach disk, dMD, behind a slit nozzle may be estimated using the empirical formula of Beylich:38

dMD ) 0.67 D

pS L pB D

0.47

()

(3)

Here, L and D are the length and width of the slit nozzle, 2 < (L/D) < 50, and pS and pB are the stagnation pressure and the background pressure in the vacuum chamber. For slit nozzles with L/D > 50, eq 3 does not apply.39 This is the case for the 8 × 0.05 mm2 nozzle (L/D ) 160), which does not match the L/D ratios investigated in ref 39, either. Indeed, the experimental rise of the conformational temperature for this nozzle at (pS/pB)1/2 ≈ 30 occurs much earlier (d ≈ 4-5 mm) than predicted by eq 3 (d ≈ 10 mm). This is also true for the rotational temperature in a 1% N2 in He expansion under similar

conditions (see Figure 4). Clearly, eq 3 cannot be extrapolated to L/D ) 160. In contrast, experiment and prediction match quite nicely in the case of the shorter nozzle with L/D ≈ 27. The onset of conformational heating is observed between 7 and 8 mm, whereas the Mach disk is predicted at 9.5 mm. The residual deviation may be due to imperfections of the nozzle or of the empirical equation. However, a large degree of turbulence and smearing out of the Mach disk may be expected under the chosen conditions. Figure 4 shows that a good compromise between efficient monomer relaxation and acceptable signal-to-noise ratio is achieved at distances around d ≈ 3 mm. In other spectral regions, the discrimination between ethanol et and eg bands is less straightforward than for the OH stretching fundamental. In the fingerprint region, the most intense Raman bands (Figure 5) at 879, 883, and 892 cm-1 were attributed to the symmetric CCO stretching vibration.43–45 The relaxation behavior in the jet expansion with different carrier gases shows that the signal with the highest wavenumber corresponds to et. The intermediate peak is reduced and partially shifted in the jet expansion, possibly due to overlap with cluster bands. The lowest peak survives better and must be due to eg+. One may thus speculate that eg- is responsible for the 883 cm-1 peak, corresponding to an enhancement of the eg+/eg- splitting by ≈4 cm-1. At the same time, the energy difference between et and eg+ in this state is reduced by ≈13 cm-1. The other vibrations in this range are more difficult to interpret because of couplings13,43 and cluster contributions.44 The methyl rocking vibrations were assigned around 1030 and 1090 cm-1, and the asymmetric CCO stretching vibration was around 1060 cm-1.13,43,45 In the jet spectra, sharp intense peaks are observed near 1028 (et), 1090 (et), 1113 (eg), and 1117 cm-1 (eg). Their conformational assignment derives from correlation with the symmetric CCO peaks. The broad features around 1058 and 1102 cm-1 seem to be due to clusters but also contain monomer contributions. A weak OH bending (δ) signal is observed around 1248 cm-1, and the corresponding cluster band is blue-shifted to ≈1276 cm-1. We refrain from a detailed


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Figure 4. Ethanol conformational temperature Tconf in supersonic expansions as a function of the nozzle distance. Thick, blue, dashed line: He expansion through a 4 × 0.15 mm2 slit nozzle. Black, solid line: He expansion through a 8 × 0.05 mm2 slit nozzle. For the latter nozzle, the conformational temperatures in an expansion upon addition of 1% N2 to the carrier gas (pS/pB)1/2 ≈ 30 and in an expansion in pure Ne (pS/pB)1/2 ≈ 16 are shown. Rotational temperatures23,25 in an expansion of 1% N2 in He (red dotted line) are plotted for comparison. dMD and dMD ′ mark the calculated positions of the Mach disk according to eq 338 (see text).

Figure 5. Raman gas phase (GP) and jet spectra (d ) 2 mm, He or 30% Ne in He as carrier gases) of ethanol in the fingerprint region of the vibrational spectrum. Bands marked C possibly originate in hydrogen bonded clusters and are in part close to earlier size-selected CO2 laser dissociation spectra.13

analysis of this spectral region, which would profit from higher resolution due to the spectral congestion.

The influence of aggregation is better studied in the range of the OH stretching vibration.2 Here, the clusters can easily be discriminated from the monomers, and the bands from the aggregates can thus be identified, as will be shown in the next section. 3.2. Dimer Relaxation. Figure 6 demonstrates the effects of collisional relaxation on the spectra of ethanol in the monomer and dimer OH stretching region. For this purpose, a gas phase and a He jet measurement are compared with jet measurements using pure Ne and ∼8% Ar in He as carrier gases. The dimer donor and acceptor vibrations are also shown at 5 × magnification. The He jet measurements show a close correspondence to former direct absorption FTIR measurements2,14 and infrared cavity ring-down laser absorption spectroscopy (IR-CRLAS).15 Compared with an earlier Raman jet measurement,12 the signalto-noise ratio could be improved, and the nozzle distance extended. The band positions from an IR-VUV doubleresonance study46 cannot be confirmed, for either the monomers or for the dimer donor or acceptor bands. Three dominant dimer donor peaks are observed at 3532, 3540, and 3548 cm-1.2,15 The width of the central band is indicative of band overlap. Therefore, at least four dimer isomers seem to be observable in He expansions. In an earlier jet microwave investigation, three different dimer isomers have been assigned.16 In the nomenclature of ref 2, in which the donor conformation is followed by the acceptor conformation and a particular lone electron pair is chosen for the hydrogen bond, they are denoted g+g+, g+t, and g-t (see Figure 7). The relaxation behavior upon Ar addition reproduces the earlier IR jet study.2 The most red-shifted donor band at 3532 cm-1 (marked egeg in Figure 6) survives, whereas the less red-shifted ones drop in intensity. At the same time, an

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Figure 6. Ethanol spectra in the gas phase (GP, intensity scaled ÷ 40) and in jet expansions with different carrier gases at d ) 3 mm (He, Ne) and d ) 2 mm (≈8% Ar in He, intensity scaled ÷ 3) in the region of the OH stretching vibrations of the monomers and dimers. Intensities in the inserted boxes were magnified by a factor of 5.

intensity drop at the t acceptor band (eet, 3672 cm-1) is observed, whereas the g acceptor band (eeg, 3654 cm-1) increases against the monomer trend. This confirms that the most red-shifted and presumably most stable dimer donor does not feature a t acceptor molecule. Previously, this assignment had to rest on a correlation of theoretical predictions.2 In a pure Ne expansion, the conformational monomer temperature is even lower; eg signals have almost vanished (see section 3.1, Tconf ≈ 20 K). Nevertheless, the egeg band and the eeg band are still present. Every second non-hydrogen bonded gauche OH group is now embedded in a dimer. In contrast, the fraction of embedded trans OH groups remains at a value of 5-10% in all measurements. This provides unambiguous evidence for conformational isomerization from t monomers to g monomers upon dimer formation in the Ne jet, wherever there is an energetic driving force. Therefore, it is rewarding to inspect the dimer band intensities in this expansion more closely. In the donor region, the broad central peak has now lost its red-shifted component. Like in the Ar/He expansion, it relaxes more strongly than the blue-shifted component. It has to be assigned to a rather unstable dimer isomer, which is stabilized only in pure He expansions. The band center of the blue-shifted component can now be identified at 3542 cm-1 (see Table 2). The most prominent dimer donor peak in Ne is at 3548 cm-1. It would consequently have to be assigned to the second-moststable ethanol dimer isomer. The assignment of its acceptor conformation is straightforward via inspection of the free OH range. The t acceptor eet band is enhanced as compared with the other expansions. The donor assignment is less rigorous. In

Wassermann and Suhm

Figure 7. B3LYP structures of the six most stable hydrogen bonded ethanol dimer isomers, according to theoretical predictions.2 Weaker secondary CH · · · O interactions are indicated by arrows when the distance is gt > other



isomers. It can be rationalized by looking at weak secondary interactions, mainly between the CβH’s of the acceptor and the oxygen lone pairs of the donor (see Figure 7 for B3LYP structures of the six most stable ethanol dimers, MP2 calculations2 give similar results). Such contacts cannot occur in dimers with a g- acceptor. The optimization of weak secondary CH · · · O interactions can also help to explain the higher dissociation energy of g+g+ and g-t as compared with g+t and tt. For g+g+ and g-t, CH · · · O interaction between the donor methyl group and the second lone pair of the acceptor oxygen is favored. In the case of the g+g+ dimer, dispersive interactions between the ethyl groups of the donor and acceptor molecules seem to further stabilize this arrangement. The observation that Ar, but not Ne, is able to convert the g-t conformer into the more stable g+g+ conformer tends to favor an associative mechanism in supersonic jets,42 rather than a purely impulsive mechanism. Transient binding of an Ar atom to the complex brings in sufficient internal energy to overcome isomerization barriers, whereas Ne is too weakly bound for this process. In the tunneling-dominated monomer case, impulsive isomerization by Ne is more efficient. 3.3. Deuterated Ethanol. Jet measurements of O-deuterated ethanol CH3CH2OD as a function of nozzle distance were carried out to assist the vibrational assignment and to estimate anharmonicity constants, ωexe, on the basis of an approximate Morse oscillator approach.17,47–49 Although overtone measurements50–52 provide more robust anharmonicities, these are not easily accessible for hydrogen bonded clusters.53 The Morse deuteration approach assumes that OH and OD have the same localized normal coordinate and, thus, the same force constant, k. The difference in the harmonic wavenumbers, ωe, of the OH and OD stretching vibrations thus stems only from the difference in the reduced masses µ. The anharmonicity constant, ωexe, in a Morse oscillator is related to the square of the harmonic wavenumber via the dissociation wavenumber, De, according to

ωexe )

ωe2 4De

(4)

For localized Morse oscillators XH and XD (e.g., OH and OD groups in ethanol and ethanol-OD), one thus obtains in terms of the transition wavenumbers ν˜ XH and ν˜ XD:

ωe )

ν˜ XD · r - ν˜ XH

√r - 1 ωe - ν˜ XH ωe xe ) 2

(5)

where r ) µXD/µXH is the ratio of the reduced masses of the XD and XH groups (µOH ≈ 0.9481 u, µOD ≈ 1.7889 u, r ) µOD/µOH ≈ 1.8868) and ωe and ωexe refer to the XH oscillator. Figure 8 compares jet spectra of ethanol-OD at different positions upstream (at d ) 3 mm) and downstream from the Mach disk (at d ) 9 mm) using He as the carrier gas. The similarities between the ethanol-OH (Figure 6) and ethanolOD spectra (Figure 8) invite a common assignment. As for ethanol-OH, ethanol-OD shows eDt and egD monomer bands at 2714 and 2700 cm-1, respectively, as well as dimer acceptor bands at 2709 and 2696 cm-1. The central peak of the three dominant dimer donor bands may feature two contributing isomers near 2615 and 2613 cm-1. The most red-shifted and

Figure 8. OD stretching spectra of ethanol-OD expanded in He carrier gas at different nozzle distances, d (mm). Intensities in the inserted box are magnified by a factor of 10.

the least red-shifted donor bands are observed at 2608 and 2619 cm-1, respectively. The OD stretching vibration is spectrally less isolated than the OH stretching mode. Signatures of the adjacent CH stretching fundamentals and bending overtones are visible mainly on the higher frequency side. At higher temperatures, the signal between the eDt and egD OD stretching band at ≈2706 cm-1 has been shown to originate from hot OD torsional bands.54 After passage through the Mach disk (see spectrum at d ) 9 mm in Figure 8), the band gains in strength, confirming this interpretation for lower temperature. Additional weak bands may originate from isotopic impurities. The eg/et intensity ratio changes less with jet cooling than for the OH species. Several effects may contribute to this. The ratio of the scattering cross sections σ′et/σ′eg is smaller for ethanolOD than for ethanol-OH (≈1.3 for OD as compared with ≈1.5 for OH, see Tables 1 and S1). Furthermore, according to harmonic predictions, the zero-point energy difference between the conformers may be affected to some degree by deuteration, but experiment36 suggests that this effect is only minor. Because the fraction of torsionally excited states is higher at room temperature, torsional cooling is likely to interfere more strongly with conformational cooling in the ethanol-OD case. As in the case of ethanol-OH, the σ e′ t/σ ′eg ratio is more or less conserved in the dimer transitions (see Tables S1-S3 in the Supporting Information). The donors show an enhancement of the cross section, whereas the acceptors possess similar or slightly lower σ′ values than the monomers. Also in analogy to ethanol-OH, the predictions for donor red shifts are overestimated, whereas the predictions for acceptor red shifts fit the experimental findings quite well. The OH anharmonicity constant estimates, ωexe, and the harmonic wavenumbers, ωe, for ethanol monomers and dimers from the Morse deuteration approach are reported in Table S4 in the Supporting Information. The monomer values slightly overestimate overtone-derived values,50 as in the methanol case.17 This can be remedied by using an effective r value of ≈1.8835, reproducing the overtone results almost within experimental error bar. Most importantly, the dimer acceptor and donor vibrations show identical anharmonicity constants within error bars based on wavenumber calibration accuracy. This provides further support to the donor and acceptor band assignments. 3.4. Argon Coating. Ar matrix isolation is a useful technique for the investigation of the IR spectra of alcohols and their

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Wassermann and Suhm TABLE 3: Experimental Raman OH Stretching Wavenumbers ν˜ OH and Ar-Induced Red Shifts, ∆ν˜ OH, of the Different Bands Observed in Ar Jet Expansionsa ν˜ OH/cm-1 et eet eg eeg

- ∆ν˜ OH/cm-1

Free Monomer and Dimer Acceptor Bands 3678.0 ( 0.5 3672.1 ( 0.5 3660.0 ( 0.5 3654.0 ( 0.5

R β IR: (etAr)b (eet)Ar (?)

(Partially) Ar-Coated Ethanol t 3673 3666 3663 3658

5 12 15 14

γ

(Partially) Ar-Coated Ethanol g 3651

9

Monomer Solid Matrix Positions ((et)Ar matrix)c 3655.6 ((et)N2 matrix)c 3652.9 ((eg)N2 matrix)c 3649.9 egeg (ee)Ar ((etet)Ar matrix)c

Free and Coated Dimer Bands 3532 ≈3522 3527.2

22.4 25.1 10.1

≈10 5

a Labels correspond to Figure 9. Band positions from the IR Ar jet and Ar and N2 matrix measurements were taken from refs 2, 20, and 5. b Ref 2. c Refs 5 and 20.

Figure 9. OH stretching signatures of stepwise Ar coating of ethanol monomers and dimers in Ar jet expansions at distances of d ) 1, 2, and 3 mm as compared with a He jet spectrum at d ) 3 mm (Figure 6). For the labels of the marked band positions, see the text and Table 3. The band positions from bulk matrix measurements (indicated by thick bars, |) were taken from refs 5 and 20.

clusters5,20 but suffers from matrix-induced wavenumber shifts. Investigations in Ar jet expansions can bridge this gap10,27 by generating nanomatrix environments for the molecules2,10 and their clusters2,27,55 in the gas phase.56 Similar effects can also be observed in N2 expansions2 in comparison to N2 matrixes.5,20 The technique differs from pick-up techniques in which molecules or clusters are attached to preformed Ar clusters13 and presumably stay at the surface. Raman spectroscopy allows for a detailed investigation of this coating phenomenon.10 The OH stretching vibration of small alcohols reacts sensitively to the interaction of the molecule with the Ar atoms.10 In addition to variation in concentration and stagnation pressure in FTIR studies,2,27 variation of the nozzle distance is instructive in the Raman jet experiments due to the high spatial resolution. Ethanol is well suited for such studies due to its limited number of different conformations and binding sites for the Ar atoms. This renders the identification of the coating phases more straightforward than in larger alcohols.8,10 The effect of Ar coating on the monomers was already demonstrated in Figure 2, together with the bulk limit of the solid Ar matrix. Figure 9 shows further details and includes the dimer region as well as a N2 matrix reference,5,20 which also stabilizes the g conformer.20 Different stages can be identified in the monomer coating process. At d ) 1 mm, both monomer bands are already broadened and develop shoulders on the low frequency slope. Due to relaxation, the eg peak is depleted. At larger nozzle distances, distinct new bands of partially coated t ethanol develop. At d ) 2 mm, a band labeled R in Figure 9 is observed at 3673 cm-1, that is, with a red shift of 5 cm-1. A second

coating stage in the Raman spectra can be observed around 3666 cm-1 (labeled β in Figure 9). In the IR jet study,2 in which the measurements were carried out at larger distances from the nozzle exit, coated ethanol t was observed as a single feature at 3663 cm-1. The limiting red shift of 12 (15) cm-1 in the Raman (IR) Ar nanomatrix measurements does not reach the bulk matrix20 red shift of 22 cm-1. Crystal forces probably account for the difference.10 The assignment of the γ band in Figure 9 is less straightforward. It may correspond to Ar-coated g ethanol with a reduced eg/et splitting upon nanocoating. Such reductions of conformational splittings have been observed before in bulk Ar and N2 matrixes of ethanol20 and 1-propanol.8,57,58 OH groups in the trans position apparently show larger red shifts than their gauche analogues. Dimer signals behave differently, as shown in Figure 9. Already at short distances downstream the expansion near the nozzle exit, the dimer band is unstructured apart from residual uncoated dimers, indicative of stronger dispersive interaction by the dimer than by the monomer. The band center of the coated dimers is close to 3522 cm-1, red-shifted by only 10 cm-1 as compared with the uncoated egeg band. The solid Ar matrix shows an even lower red shift of ≈5 cm-1.5 Together with a small IR shift of Ar nanomatrixes,2 the postulated conformational switch to a tt dimer in an Ar environment5 is supported. The expected coated et acceptor band may correspond to the signal at 3658 cm-1 in the measurements at larger distances. Its Ar-induced red shift of ≈14 cm-1 is comparable to the limiting shift for the free t monomers found in the Raman and IR jet2 measurements. The observed band positions and their corresponding Arinduced shifts (relative to the corresponding free monomer or dimer species) are summarized in Table 3 and compared with known values from the literature. Raman spectroscopy is found to provide important details on the Ar coating process, which



Figure 11. Energy level scheme with possible transitions for the OH torsion τOH (υτ ) 1 r υτ ) 0) according to ref 1.

TABLE 4: Observed Raman Band Positions ν˜ /cm-1 in the Low Frequency Region of Ethanol and Some Preliminary Assignments Based on the Comparison with Data from the Literature1,36,64 δC-C-O

Figure 10. A gas phase measurement (GP, divided by 30) obtained in a slow gas stream of ethanol in He and jet spectra at different nozzle distances d of the low frequency vibrations of ethanol. At d ) 1 mm from a to b and at d ) 3 mm from c to d, the relative ethanol concentration is augmented by a factor of ≈3.5, respectively. Spectra were scaled to comparable intensities (a × 1, b ÷ 2, c × 3, d × 2). Predicted OH torsion transitions, which are indicated by the bars below the experimental spectra, were derived from the energy level scheme in ref 1 (see text and Figure 11).

may be easier to simulate by theory than the bulk matrix environment.59 Our results invite a theoretical modeling of the stepwise Ar coating10 of ethanol. The minimum level required for this appears to be dispersion corrected density functional theory. However, preliminary harmonic results for the B97-D method60 using Gaussian0961 suggest a qualitatively wrong behavior with pronounced blue shift for the monomer. 3.5. Low Frequency Vibrations. The strong influence of OH torsional motion on the alcohol dynamics1 and its sensitivity to hydrogen bonding invite direct studies of the associated fundamental vibrations.36,45,62–64 Here, we show that Raman jet spectroscopy can provide valuable information in this frequency range, as well. In addition to OH torsion, τOH, near 200 cm-1, other ethanol vibrational modes are expected in the wavenumber range between 500 and 100 cm-1: the CH3 torsional mode τCH3 in close vicinity and a frame bending fundamental δC-C-O higher in frequency (see Figure 10). The δC-C-O vibration exhibits a sharp peak at 417 cm-1 with a shoulder near 420 cm-1. In the supersonic jet expansions, the shoulder is suppressed, indicating an eg origin affected by relaxation. Theoretical predictions (see Table S5 in the Supporting Information) confirm that eg is higher in wavenumber than et. On the high-frequency slope, cluster contributions appear. Because the dimer fraction under these expansion conditions (traces a and c in Figure 10) is

Ethanol Monomers and Dimers Revisited: A Raman Study of

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