Origin of the Argon Nanocoating Shift in the OH Stretching

Jun 1, 2009 - Supersonic jet Raman spectroscopy reveals an increasing bathochromic shift of the OH stretching vibration in the most stable conformatio...
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J. Phys. Chem. C 2009, 113, 10929–10938

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Origin of the Argon Nanocoating Shift in the OH Stretching Fundamental of n-Propanol: A Combined Experimental and Quantum Chemical Study Juhyon J. Lee,†,‡ Sebastian Ho¨fener,† Wim Klopper,*,† Tobias N. Wassermann,‡ and Martin A. Suhm*,‡ Lehrstuhl fu¨r Theoretische Chemie, Institut fu¨r Physikalische Chemie, UniVersita¨t Karlsruhe (TH), 76128 Karlsruhe, Germany and Institut fu¨r Physikalische Chemie, UniVersita¨t Go¨ttingen, Tammannstrasse 6, 37077 Go¨ttingen, Germany ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: April 23, 2009

Supersonic jet Raman spectroscopy reveals an increasing bathochromic shift of the OH stretching vibration in the most stable conformation of propanol with increasing extent of argon nanocoating. It falls short of the bulk matrix limit of 17 cm-1 even at large nozzle distances. Quantum chemical harmonic frequency calculations up to the CCSD(T) level show that this shift cannot be accounted for by individual Ar atoms or even a first solvation layer but instead requires several layers of Ar atoms around the molecule to be explained. It is shown that the stability of Ar-propanol clusters correlates with the number of close O and C contacts to the Ar and that bathochromic shifts are largely caused by backbone solvation. Hydrogen-bonding OH solvation tends to slightly increase the OH stretching frequency but is very sensitive to the computational level. 1. Introduction Matrix isolation is a well-established, powerful method to study the infrared spectra of molecules at low temperatures. It offers enormous sensitivity and a wide range of control parameters, ranging from the choice of the matrix gas1 over simple temperature cycling protocols to laser-induced isomerization.2 Raman matrix studies are also becoming possible but require thick and carefully prepared samples.3 When it comes to a comparison with quantum chemical predictions of the molecular spectra, the only drawback of the matrix isolation technique is the induction of matrix shifts and site splittings, which are difficult to predict.4 Quite often, these effects are small compared to other error sources, but occasionally, they can be large, and in the case of subtle spectral effects due to conformational isomerism, they may be too large to allow for a unique correlation between gas-phase theory and matrix isolation experiment. From another perspective, these matrix-induced shifts are interesting by themselves. Being the simplest conceivable solvents, rare gas matrices teach us how certain parts of a molecule interact with its environment. The modeling of this subtle interaction is very challenging, as it includes dispersive interactions, which are only captured at relatively sophisticated levels of electronic structure theory5-7 or empirically modified approaches.8 Another challenge is the modeling of packing effects, which arise from the incorporation of the molecule into a substitution site of the infinite (or rather ∼50 nm)9 crystal. This modeling problem can possibly be alleviated on the experimental side by generating amorphous nanometer-sized matrices, where the molecule dictates the matrix structure to a larger extent than in the crystalline packing.10 Obviously, the issue of solvation shells has to be addressed in this context. The matrix shift may be influenced not only by the first layer * To whom correspondence should be addressed. E-mail: [email protected] (W.K.) and [email protected] (M.A.S.). † Universita¨t Karlsruhe (TH). ‡ Universita¨t Go¨ttingen.

of rare gas atoms around the molecule but also by secondary layers, which may then become successively more ordered. This is a field in which many aspects remain poorly characterized on the nanometer scale, and the goal of the present work is to initiate a deeper understanding of the rare gas solvation process for aliphatic molecules. In this context, liquid rare gas studies should also be mentioned.11 While they have the benefits of working at thermal equilibrium and avoiding the problem of site splittings, they are only possible at intermediate temperatures, where subtle conformational isomerism may be washed out by thermal broadening. A notable exception is liquid helium in the form of nanodroplets, but the delicate quantum-dynamical effects in this particularly interesting nanosolvent are beyond the scope of the present study.12 We have chosen the OH stretching vibration of n-propanol as a test case for the ability to model matrix-induced spectral shifts, using argon as the nanosolvating agent. Ar is still one of the most widely used matrix gases.2 In contrast to He and Ne, the shifts are sufficiently large to be systematically studied at moderate resolution. In contrast to Kr or Xe, Ar is easier to treat by quantum chemical methods.5 Similar to HF,13,14 OH stretching vibrations are particularly sensitive to the environment of a molecule because of hydrogen-bond-like interactions with the polarizable rare gas atom.15 Other than for HF,14 quantumdynamical effects should be less pronounced in the heavier propanol case. Propanol offers a sufficiently large variety of docking sites for the Ar atoms, thus allowing for a study of regional preferences. It features a delicate conformational equilibrium among five spectroscopically distinguishable structures,16 which may be affected by the matrix environment. Nevertheless, the global minimum structure can be prepared in sizable excess over the others in a supersonic jet expansion by efficient relaxation over the conformational barriers.15 Therefore, the dominant spectral features can be attributed to this global minimum structure. Propanol is too large for rotationally resolved IR spectra of its Ar complexes,13 which would be able to provide detailed structural insight into the Ar coating

10.1021/jp902194h CCC: $40.75  2009 American Chemical Society Published on Web 06/01/2009

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process.17 However, we will show that the spectral evolution with increasing Ar attachment still allows for quantitative conclusions. On the theoretical side, a step-by-step approach of adding increasing numbers of Ar atoms to the propanol host appears appropriate. For a few Ar atoms, this can be done in a systematic way, whereas larger clusters18 require more or less sophisticated statistical sampling of the combinatorial explosion. Various issues such as pairwise additivity,14,19 mode mixing,20 basis set superposition error,21 and coordination site5 have to be addressed. The goal of this study is to go beyond empirical pair potentials, which would allow for detailed quantum Monte Carlo studies14,22 but are difficult to construct for larger organic molecules embedded in argon.5 The present work starts with an outline of the experimental techniques and theoretical approaches and their validation (section 2), followed by a presentation of the spectroscopic results, the quantum chemical calculations, and their critical comparison (section 3). It concludes with the key insights into the Ar coating effect on the OH stretching vibration and an outlook on future directions in the simulation of Ar matrix effects on functional organic molecules. 2. Methods 2.1. Experimental Technique. Raman spectra were measured using the newly developed curry-jet setup.23,24 This technique, which exploits spontaneous Stokes scattering in a supersonic jet expansion,25 is particularly suitable for the study of alcohol conformations.15,26,27 Gas mixtures were prepared by flowing a rare gas (He, Ar) through thermostatted propanol (Carl Roth, g99.5%, either at 0 or -7.5 °C), loaded into a 67 L stainless steel reservoir, and then expanded into the vacuum chamber via a 8 × 0.05 mm2 slit nozzle. Thermostating the propanol has the function to regulate the concentration of the alcohol in the rare gas carrier. By the time the gas mixture reaches the nozzle, the normal laboratory temperature is recovered. The 60 × 60 × 40 cm3 anodized Al vacuum chamber was evacuated by a 250 m3/h roots pump followed by a 100 m3/h rotary vane pump. The average stagnation pressure in the course of the experiments was 0.7 bar for He and 1.0 bar for Ar expansions. The background pressure in the jet chamber may reach ∼4 mbar, thus limiting the zone of silence to 4-5 mm in length. The zone of silence of the supersonic jet expansion was crossed perpendicularly by the mildly focused beam (beam waist ≈ 29 µm) of a frequency doubled continuous Nd:YVO4 laser (532 nm, Coherent Verdi V18, 18 W photon power). The distance between the nozzle exit and the laser beam was systematically varied to follow dynamical processes like the onset of Ar coating of propanol monomer molecules. The scattered light was collected at an angle of 90° and collimated by a Nikkor f/1.2 50 mm camera lens. It was then focused on the entrance slit of a single monochromator (McPherson 2051, f ) 1000 mm, f/8.7, 1200 gr/mm grating) by an achromatic planoconvex lens (Edmund Optics, Ø ) 50 mm, f/7). The Rayleigh scattering and residual stray light was attenuated by a Raman edge filter (L.O.T., Ø ) 25 mm, OD 6.0, >90% transmission, 535.4-1200 nm). Signals were detected by a backilluminated, liquid-nitrogen-cooled CCD camera (PI Acton, Spec-10: 400B/LN, 1340 × 400 pixels). Iterative comparison of block-averaged spectra allows for the removal of cosmic ray signals. 2.2. Computational Approach. 2.2.1. Quantum Chemical Methods. The quantum chemical calculations were carried out using a hierarchical approach. The potential-energy surface of

Lee et al. n-propanol with zero, one, and more Ar atoms was explored at the (frozen core) RI-MP2/def2-TZVPP level,28-30 using TURBOMOLE31 (modules DSCF,32 RIMP2,28,33,34 RICC2,35,36 RIDFT,37 and RELAX38). This level will be abbreviated RIMP2 in the following. Local minima were optimized until 10-8Eh maximum total energy change and 10-5Eh/a0 maximum Cartesian gradient norm were achieved. The optimized structures were characterized by a harmonic force field analysis using numerical second derivatives (NUMFORCE39 using the central and ecnomic option, a differentiation increment of 0.02a0, a threshold of 0.01Eh/a0 for the remaining gradient at the stationary reference point, and the RI-JK approximation). At these local minima, single-point MP2-F12/def2-QZVPP29,30 energy calculations were carried out to check the basis set convergence of the relative energies (denoted MP2-F12 in the following6,7,40-46). The SCF iterations were considered to be converged if the change of energy was less than 10-9Eh. The convergence criterion for the density matrix norm was 10-9, the CABS approximation47 with C-def2-SVP28 and the fixed amplitudes approximation48 were used, eigenvectors of the overlap matrix with eigenvalues less than 10-8 times the largest eigenvalue of the overlap matrix were abandoned.47 Furthermore, CCSD(T)/aug-cc-pVTZ energy calculations49-51 were carried out using the MOLPRO package52 with default settings53 to check for higher order electron correlation effects (abbreviated CCSD(T)). Unless indicated otherwise, the energies calculated in this way were corrected for basis set superposition error (BSSE) using the standard counterpoise correction.54-56 Structures were prepared and visualized with the ECCE builder57 and Molden,58 respectively. CODATA 2002 basic constants and conversion factors were used.59 2.2.2. Anharmonicity Considerations. The goal of this work is to predict Ar-induced wavenumber shifts within (20% or (1 cm-1, whichever is larger. With this goal in mind, anharmonic effects can be safely neglected, because the large anharmonicity of the OH oscillator will be only moderately affected by the presence of Ar. Three subtle anharmonic effects may be discussed: The softening of the OH oscillator due to the polarizability of the matrix environment, the stiffening or softening of the OH oscillator due to matrix packing effects, and the stiffening of the alcoholic OH torsional potential if the positive hydrogen is trapped by weak hydrogen bonding to Ar, thus generating extra zero-point energy. The latter effect may be viewed as a nondiagonal anharmonicity contribution and can be quite important in strong hydrogen bonds60 and for light molecules trapped in Ar matrices.14 However, it tends to cancel the softening effect, and either of the three contributions is most likely well within the formulated accuracy goal, because we are looking at a small perturbation to the isolated molecule. This should also apply for step-size effects in the numerical differentiation of the potential-energy surface and for BSSE corrections to the harmonic wavenumbers61 as well as other basis set insufficiencies if one uses at least triple-ζ basis sets and concentrates on the analysis of wavenumber differences or ratios.61-63 2.2.3. 1-Dimensional Approximation. A further advantage of testing the present approach with alcohols is that the OH stretching mode is energetically well separated from the other normal modes.63 This invites reduced 1D approximations to its harmonic wavenumber,20,61 which open up a venue to very high level treatments and could easily be extended to anharmonic analysis, if this turned out to be essential.63 To check the influence of higher order electron correlation on wavenumber shifts at moderate computational cost, such a

Origin of the Argon Nanocoating Shift

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TABLE 1: Ratios r ) ωAr/ω between the Wavenumbers ωAr with Ar and ω without Ar for Two Different Methanol-Ar Structures f (Front Coordination) and b (Back Coordination) within the 1D Approximationa structure optimization

single point

RI

nDp

ωAr/ω(f)

ωAr/ω(b)

MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVQZ MP2/aug-cc-pVQZ MP2/aug-cc-pVQZ

MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/def2-TZVPP MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ MP2/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ MP2/aug-cc-pVQZ MP2/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ

y y y n n n n n n n n n n n n n n n

harm. 1D5 1D7 1D7 1D5 1D7 1Deq 7 1D5 1D7 harm. 1D5 1D7 1D5 1D7 1Deq 7 harm. 1D7 1D7

1.0005 1.0006 1.0006 1.0010 0.9995 0.9995 1.0003 0.9999 0.9999 0.9999 1.0000 1.0001 1.0006 1.0006 1.0010 0.9996 0.9999 1.0004

0.9995 0.9996 0.9996 0.9997 0.9994 0.9995 0.9997 0.9997 0.9997 0.9994 0.9993 0.9993 0.9996 0.9996 0.9991 0.9992 0.9994 0.9997

a For the def2 basis set, the RI approximation28,33,37 was typically used (y), whereas for the aug-cc basis sets, a full calculation was performed (n). In the column nDp, harmonic calculations are abbreviated as harm. The index p denotes the number of points used for the 1D calculations, whereas the superscript eq indicates that the ratio has been calculated at the equilibrium OH distance from the structure optimization.

1-dimensional local approximation to the harmonic OH stretching wavenumber shift was established.20 For this purpose, the OH bond was elongated and compressed relative to the minimum structure with and without Ar by moving only the hydrogen in steps of 0.01a0 along the bond vector. Seven points on the local OH potential were thus generated. They were fitted to a third-order polynomial in the elongation coordinate using Gnuplot,64 carefully avoiding numerical cancellations in the fitting procedure and checking the significance of the parameters by using only the innermost 5 points. We tested two ways to evaluate the curvature of the 1D potential in cases where the method used for geometry optimization did not agree with the method used for the 1D scan. Evaluating the curvature at the minimum of the fitted polynomial turned out to give more robust results than at the equilibrium structure (eq) obtained in the geometry optimization, as expected. Thus, the 1D approach can also compensate in part for incomplete geometry optimization or for different methods used in minimization and 1D scan. To obtain consistent 1D results, the standard SCF convergence criteria of TURBOMOLE65 had to be changed to 10-10 for the change in the density matrix norm and 10-9Eh for the change in total energy. The 1D approximation is expected to introduce only minor errors.20 In particular, freezing of the Ar frame should have little influence on the harmonic OH stretching potential. Indeed, the Ar amplitude of the OH stretching normal coordinate was less than 0.4% of the H amplitude for the investigated structures. To obtain approximate harmonic wavenumbers from these 1-dimensional potential curves, a mass of 1 u was assumed for H and 16 u for O. However, the relevant quantity for this work is the ratio of the wavenumber with and without Ar, which does not depend on the particular choice of the reduced mass, as long as Ar is not included in the dynamics. Such ratios r ) ωAr/ω will be discussed extensively in the text and are believed to be numerically accurate to within at least 0.0001 (corresponding to 0.4 cm-1) in the ratio. The r values are obviously smaller than 1 for bathochromic matrix shifts and were generated for a variety of methodological combinations, as elaborated below. 2.2.4. Validation for Methanol-Ar. Table 1 serves to validate the essential part of our quantum chemical approach,

Figure 1. Four minimum structures of the methanol-Ar dimer at the RI-MP2/def2-TZVPP level with relative electronic energies (in kJ/mol) at the counterpoise-corrected CCSD(T)/aug-cc-pVTZ level. Front coordination (f) of the OH group is most stable, followed by back (b) and side-on (s) coordination of the OH group and finally coordination of the CH3 end group (e).

using two dimers of methanol with Ar. In the first case, the Ar atom complexes the methanol molecule from the front (f) with a short distance to the OH hydrogen. In the second case, it coordinates methanol from the back (b) with a short distance to the oxygen lone electron pairs (see Figure 1). In both cases, the shortest nonbonded heavy atom distance is Ar-O, whereas the Ar distance to the OH hydrogen is shortest in structure f and longest in structure b. While structure b leads to a red shift of the OH stretching vibration relative to the free alcohol (r ) ωAr/ω < 1), the front coordination is much less red or even slightly blue shifted. We note that in addition to the three methanol-Ar dimer structures which were recently reported as local minima,5 we found a fourth one where the Ar is not located in the plane defined by

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the COH group (lowest vibrational wavenumbers 46 (MP2/augcc-pVTZ) and 44 cm-1 (MP2/aug-cc-pVQZ)). It is denoted s in Figure 1 and comparable in energy to the second most stable structure reported before.5 The entries in Table 1 support the local (1D) approximation for the OH stretching harmonic oscillator. Where it may be compared to a full harmonic approximation (namely, at the RIMP2 level with TURBOMOLE and at the MP2/aug-cc-pVTZ level using MOLPRO), the local and full treatments deviate by less than 2 × 10-4 in r. Different 1D implementations using 5 or 7 data points also differ by 1 × 10-4 or less. The effect of moving from the MP2 level to the CCSD(T) level for the same basis set is fairly systematic. It increases the r value by (4-6) × 10-4 in the f structure and (2-3) × 10-4 in the b structure. Thus, the CCSD(T) contribution beyond MP2 appears to reduce the hydrogen-bond-induced red shift somewhat. It certainly does not increase the red shift in these examples. Strictly speaking, structure optimizations at the CCSD(T) level followed by the 1D analysis would have to be carried out to further corroborate our finding. One might expect that small offsets from the minimum structure like in the difference between different basis set optimizations should not change the 1D harmonic frequencies substantially if the force constant is evaluated at the minimum of the 1D curve. This is the case for structure b, where the difference in the MP2/ or CCSD(T)/aug-cc-pVTZ ratio between MP2/def2-TZVPP and MP2/aug-cc-pVTZ structures does not exceed 2 × 10-4. For structure f, this difference is larger (up to 7 × 10-4) and systematic. The MP2/aug-cc-pVTZ structure leads to a higher complex frequency than the MP2/def2-TZVPP structure for a given basis set. This may be related to a 0.13a0 shorter H · · · Ar distance. Somewhat surprisingly, the consistent RI-MP2 calculation also leads to a higher complex wavenumber. The RI approximation28,33,37 is not the reason for this, as a test calculation without RI (and frozen core65) approximation shows (Table 1). The OH stretching shift appears to be particularly sensitive to small structural changes if the Ar is close to the H atom. This is confirmed by a MP2/aug-cc-pVQZ optimization and force field calculation which yields a larger red shift and a larger Ar · · · O distance than the MP2/aug-cc-pVTZ calculation. If the 1D force constant is evaluated at the equilibrium distance of a preceding structure optimization (superscript eq in Table 1) the predictions are less robust than if the 1D minimum geometry is used. Overall, the test results in Table 1 indicate that the MP2 treatment overestimates red shifts compared to a more complete (CCSD(T)) calculation, whereas the def2-TZVPP basis underestimates red shifts with respect to a more complete (aug-cc-pVQZ) basis, leading to a partial error compensation in the MP2/def2-TZVPP approach. In summary, we expect that the proposed approach is robust for all Ar-alcohol interactions except for the O-H · · · Ar coordinating one. 3. Results and Discussion 3.1. Supersonic Jet Data. Table 2 summarizes the band positions of the most stable propanol conformation Gt15 (see also the next section) obtained in different experiments and their shifts from the cryogenic gas-phase reference value. With respect to this reference at 3682.4 ( 0.5 cm-1, which is very close to the room-temperature gas-phase value, the bulk matrix shift of the Gt conformation of propanol in Ar has been determined to be ca. -17 cm-1 1,66 and verified to be close to -17 cm-1 in a reinvestigation.67 Comparison with the gas-phase and He jet experiments15 rules out uncoated conformers of propanol as the origin of these bands.

Lee et al. TABLE 2: Experimental OH Stretching Wavenumbers ν˜ OH and Ar-Induced Wavenumber Red Shifts -∆ν˜ OH of the Gt Conformer of Propanol (in cm-1) method

ν˜ OH/cm-1

Raman, jet (this work) Raman, gas phase, 298 K IR, jet15 Raman, jet, first coating phase Raman, jet, second coating phase IR, jet, coating limit15 IR, matrix1,66 IR, matrix67

3682.4 ( 0.5 3682.1 ( 0.5 3681.5 3678 3671 3670 3665 3665.6

-∆ν˜ OH/cm-1 0.0 4.5 ( 1.0 11.4 ( 1.0 12.5 ( 1.0 17 17

In a recent supersonic FTIR study, a limiting shift for Arcoated Gt propanol of -12.5 cm-1 was obtained.15 The discrepancy to the bulk value may be due to an insufficiently thick or inhomogeneous coating of the alcohol molecule in the supersonic jet expansion, which was probed and averaged over a fairly wide range of nozzle distances in the jet experiment. Furthermore, the effective temperature of the clusters is somewhat higher than that achievable in the bulk matrix. However, the major difference to the bulk matrix shift is most likely due to a different nature of the Ar environment, which is largely crystalline in the bulk experiment and presumably amorphous in the jet expansion. This could lead to a compression of the O-H · · · Ar distance in the former, which in turn could induce a lowering of the O-H stretching wavenumber. We will argue below that this is a qualitatively reasonable assumption. To investigate the nanocoating phenomenon in more detail, we initiated a jet-Raman study, which offers two major advantages: It provides a much higher spatial resolution25 so that the Ar coating process can be followed as a function of flight time or nozzle displacement. It also offers a higher spectral resolution, because the Raman scattering spectra of near-prolate top molecules are often dominated by narrow Q-branch transitions and less affected by finite temperature rovibrational structure involving a change in the overall J quantum number. This allows for a better separation of conformational isomers and Ar aggregates of propanol. Figure 2 shows a series of spectra, where both the propanol concentration (controlled via the temperature of the liquid propanol reservoir) and the nozzle distance were varied to enhance the extent of Ar nanocoating from the top to the bottom. It is crucial to include the first stages of the expansion into this analysis, i.e., a distance from the nozzle corresponding to 10-50 nozzle slit diameters. This is where efficient cooling and sticking collisions are expected and where dispersion- and polarizationdriven Ar coating competes with propanol self-aggregation. The latter releases significant thermal energy from the hydrogenbond formation, which may stop or even reverse the coating process. Therefore, dilution and nozzle separation are measures of similar effectiveness for the Ar coating of monomers, and both are explored in Figure 2. We note that dimers and larger clusters of propanol are more easily coated by Ar, because these aggregates are more polarizable. However, this interesting effect15 will not be at the focus of the present study. As shown in Figure 2, the onset of coating of propanol monomers can best be followed by concentrating on its most stable rotamer Gt,15,44 which shows the highest intensity already at room temperature. However, our experimental results indicate that the less stable isomers show a coating behavior similar to that of Gt. Focusing now on the evolution of the dominant Gt band at 3682.4 cm-1 (Figure 2, top trace (a)), an equivalent band

Origin of the Argon Nanocoating Shift

Figure 2. Stepwise Ar coating of n-propanol (symbolized by @), increasing from top to bottom. The spectra were recorded in (a) the gas phase (divided by 50 to approximately match the others in integral scattering intensity), where the Gt conformation already dominates, (b) a He jet 3 mm away from the nozzle with the n-propanol saturator at 0 °C (divided by 2), (c-h) an Ar jet with the following nozzle distances, concentration governing saturator temperatures and scaling factors: (c) 1 mm, 0 °C (×1), (d) 1 mm, -7.5 °C (×1), (e) 2 mm, 0 °C (×3.5), (f) 2 mm, -7.5 °C (×3), (g) 3 mm, 0 °C (×5), and (h) 3 mm, - 7.5 °C (×4). One observes at least two steps of Ar coating, symbolized by Gt@ and Gt@@. The bulk Ar limit is marked by a thick bar (|). Small dimer acceptor contributions are marked by a solid star (f).

develops under the harshest coating conditions (3 mm nozzle distance, highest dilution, trace h) at about 3671 cm- 1, whereas the original band has almost disappeared. This corresponds to a nearly complete removal of uncoated Gt propanol monomers in favor of partially or fully coated ones. The resulting shift of -11.4 cm-1 is already quite close to the IR jet value of -12.5 cm-1 from our previous investigations,15 although the average nozzle distance in the IR experiment is much larger. Both values fall short of the Ar bulk matrix value of -17 cm-1 1,66,67 (see the thick bar in the lowest trace), for reasons which will be discussed below. In the focus of this study was the question whether one could identify intermediately shifted bands in the jet spectra corresponding to earlier coating stages. This is indeed the case in the intermediate traces of Figure 2. Close to the nozzle exit (traces c and d), the spectrum is still dominated by the free OH oscillator at 3682.4 cm-1, but the band structure is significantly broadened and becomes asymmetric. This asymmetric shoulder develops into a relatively broad secondary band (traces d and e) at about 3678 cm-1. When the nozzle distance is further increased, the intensity of this intermediate band follows that of the free monomer and falls back. Instead, a second, further red-shifted band develops in trace d, with an initial peak position of 3672 cm-1. It further shifts and grows into the dominant band at 3671 cm-1 in trace h. The fact that the bulk matrix band position is further red shifted by ∼5-6

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10933 cm-1 shows that the jet-coated species still represent intermediates on the way to the crystalline solid state. Further red shifted to the dominant peak, we observe some additional weakly structured absorptions. Most likely, these are due to other conformations of propanol and their Ar-coated derivatives and shall not be discussed in detail, because a unique identification is out of reach. An exception may be the band near 3650 cm-1 marked by f@, which could correspond to a hydrogen-bondacceptor band of a propanol dimer band after Ar coating. The corresponding free dimer acceptor bands are marked with f in trace b. 3.2. Quantum Chemical Results. 3.2.1. Propanol. We will again concentrate on the most stable conformation of propanol. This has been shown to be the Gt structure.15,16,44,68 Here, G stands for the gauche or synclinal arrangement of the γC-βC-RC-O heavy atom frame, where it is spectroscopically irrelevant whether the four atoms form an approximately +60° or -60° angle.69,70 The reason is that the second torsional angle, between the OH group and the RC-βC group, is trans or antiperiplanar (approximately 180°), designated as t. Therefore, the combination of the two torsional angles does not give rise to diastereoisomerism. The origin of the stability of the Gt structure is a weak hydrogen-bond-like contact between the γC-H and a lone electron pair of the oxygen atom,15 which helps in blue shifting the O-H stretching vibration such that it is reasonably well separated from the absorptions of other conformations. 3.2.2. Propanol-Ar. An Ar atom has the possibility to form a close intermolecular contact with either one, two, or three of the heavy atoms of the propanol frame (denoted o for oxygen, R, β, γ for the three carbon atoms), and we will designate the arising isomers by this sequence of contacts. The most intimate contact is possible in the GtoRγ and Gtoβγ structures, as well as in a carbon-only GtRβγ coordination. A somewhat weaker interaction involves two heavy atoms, namely, GtoR, GtRβ, and Gtβγ. Finally, there is the possibility for the Ar atom to interact only with the terminal C in Gtγ. These are indeed the propanol-argon dimer structures which were found in a search at the RI-MP2 level. They are summarized in Figure 3, and the shortest Ar-frame contacts are given in Table 3. Their CP-corrected electronic binding energies relative to the isolated fragments at the MP2-F12 and CCSD(T) levels are listed in Table 4. Monomer relaxation contributions are included in the correction scheme, although they are only on the order of 0.01 kJ/mol.71,72 The binding energy of the Ar atom depends on the number of close-by heavy atoms, as one would predict also from a simple dispersion-dominated model. Triple coordination is on average 20% more stable than double coordination and much more stable than single coordination. Within the same coordination number, oxygen coordination is about 20% more stable than carbon coordination, but OH coordination does not appear to provide a large extra binding energy. Therefore, OH coordination does not correspond to the global minimum structure for the complex of n-propanol with Ar. This indicates a dominance of dispersion over induction forces. Note that in the case of methanol, the situation is different.5 Here, OH coordination is most favorable, because the backbone for dispersion interactions is shorter. The difference between MP2F12 and CCSD(T) energies for propanol-Ar is not very large, thus suggesting that triple excitations are relatively unimportant and double excitations are captured well by the MP2 approximation. The maximum binding energy of Ar to propanol

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Lee et al.

Figure 3. RI-MP2-optimized structures involving the Gt propanol conformation and a single Ar atom. The site of the Ar atom is given by the closest backbone contacts to oxygen (o) and the three C atoms (R, β, γ) adjacent to it.

TABLE 3: Contact Distances in a0 of the Ar Atom in Gt-Ar Dimersa structure

Ar-O

Ar-RC

Ar-βC

Ar-γC

GtoRγ Gtoβγ GtRβγ GtoR GtRβ Gtβγ Gtγ

6.68 6.87 9.81 6.93 8.99 11.13 11.02

7.22 8.27 7.46 6.97 7.45 9.76 11.19

8.46 7.28 7.17 9.81 7.05 7.02 9.84

7.33 7.17 7.44 11.27 9.81 7.45 6.97

a

Close contacts 2. We also explored such clusters of Gt propanol with more than two Ar atoms. Figure 4 shows a few representative RI-MP2-optimized structures together with the calculated harmonic OH stretching fundamental shifts. The

Figure 4. RI-MP2-optimized structures involving the Gt propanol conformation solvated by 3-10 Ar atoms. The negative numbers indicate the calculated OH stretching shifts induced by the Ar atoms.

upper row gives examples for 3-fold coordination of the propanol molecule, concentrating around the C-O bond. This leads to the largest red shifts, which are seen to be on the order of 3 cm-1. With four and five atoms (second row), the shift increases up to 5 cm-1. However, it can vary substantially, depending on the selective coordination of the OH group and the alkyl end of the alcohol, as exemplified by two structures with six Ar atoms (third row). This is a particularly extreme example in which the simple transfer of one Ar atom from the right to the left end leads to a 5-fold shift enhancement after

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Figure 5. Compression of the Ar · · · O distance ∆d(Ar · · · O) in the GtoRγ structure by 0.5a0 or 80 cm-1 (right scale) leads to an increase of the 1D red shift (left scale) by ∼15%.

structural relaxation, illustrating the regiosensitivity of the vibrational red shift. More complete first solvation shells involving 8-10 Ar atoms (bottom row) tend to yield a red shift on the order of 5 cm-1. This appears to be a first threshold representative for a molecule solvated by one layer of Ar. As the test calculations for single Ar solvation have shown little change from the MP2 to the CCSD(T) predictions, it is gratifying to observe a similar limiting bathochromic shift of 4.5 ( 1.0 cm-1 in the experimental spectra close to the nozzle (see Figure 2 and Table 2). We therefore conclude that the additional red shift, which is observed at larger Raman nozzle distances, in the IR experiments and in bulk matrices must be due to the influence of further solvation shells. Possibly, the Ar-Ar attraction starts to play a more important role in such structures and increases the constraints on the propanol site. 3.2.5. Compression Effects. To explore matrix compression effects, we carried out 1D MP2/aug-cc-pVTZ calculations on compressed Gt-Ar dimers. Figure 5 shows how the GtoRγ dimer energy goes up with the compression of the O · · · Ar distance (right scale) and how the OH stretching mode is softened at the same time (left scale). It appears that the softening, expressed as the fundamental wavenumber shift relative to the uncompressed dimer, is 2 orders of magnitude smaller than the compression energy. With an Ar-Ar interaction energy on the order of 70 cm-1, one can therefore expect cumulative packing shifts on the order of a few cm-1, when the Ar around the alcohol molecule starts to form a crystal lattice.82-84 While this effect will have to be quantified more systematically, it has the potential to close part of the gap between predicted first solvation shell shifts of -5 cm-1 and the experimentally observed bulk matrix shift of -17 cm-1. Argon nanocoating, which shows shifts between -5 and -12 cm-1, can close this gap on the experimental side. It should be noted, however, that the H-atom coordination in GtoR behaves differently. Here, compression causes a very strong positive shift of the OH stretching fundamental, which is of the same order of magnitude as the compression energy itself. Clearly, this predicted effect is not reflected in the experimental spectra, which show unusual red shifts. This reemphasizes that direct coordination of the alcoholic H atom by Ar remains poorly understood. 3.2.6. Butanol-Ar. Analogous calculations for n-butanol85 have also been carried out and confirm the picture which has emerged for n-propanol. The Ar prefers a backbone solvation, keeping contact with the oxygen. This induces a red shift, whereas the less stable O-H coordination causes a slight blue

Lee et al.

Figure 6. Red shift -∆ν˜ (in cm-1) of the n-propanol OH stretching fundamental as a function of the inverse coordination number 1/nAr, as obtained at the harmonic RI-MP2 level. For comparison, the experimental jet Raman results (assuming nAr ≈ 8, corresponding to a first solvation shell, and nAr ≈ 8 + 16, including a second solvation shell) and the experimental jet FTIR limit (nAr . 10) as well as the bulk Ar matrix limit (nAr ≈ ∞) are shown.

shift. A belt of Ar atoms around the polar end of the alcohol backbone predicts shifts of about 5 cm-1, similar to the n-propanol case. 4. Conclusions Figure 6 merges the experimental and theoretical results of the present work on the OH stretching red shift ∆ν˜ as a function of the reciprocal Ar coordination number 1/nAr. While there is a substantial site-dependent scatter in the calculated shifts, the underlying trend is that of a progressive red shift with increasing nAr. Overlap between experiment and theory is reached at about the first solvation shell. To the best of our knowledge, this is the first highly correlated study of the effect of extensive Ar solvation on the vibrational spectra of aliphatic organic molecules. It clearly goes beyond the addition of a single Ar atom5,86,87 and attempts to bridge the gap to the bulk matrix. Argon solvation is predicted to shift the OH stretching frequency of n-propanol in characteristic ways, depending on the coordination site. Hydrogen-bond-like coordination of the OH group may lead to slight blue shifts which are sensitive to the detailed model and remain poorly understood. In contrast, coordination of the backbone near the OH group leads to more robust red shifts. The effects of a few Ar atoms are qualitatively additive. Build up of a first solvation shell is predicted to lead to a red shift of about 5 cm-1. This matches the first solvation step observed in a supersonic expansion, when probed by Raman scattering. Further solvation more than doubles the red shift.15 We attribute this further evolution of the spectrum to the build up of additional solvation shells. Bulk matrices feature a roughly tripled red shift, which we explain at least in part by packing constraints of the now crystalline matrix environment on the trapped molecule. We conclude that spectral effects in nanomatrix environments, as they can be prepared in supersonic jet expansions, may be easier to model ab initio than bulk matrix embeddings.88 The results of this combined theoretical and Raman study invite microwave investigations68,89 of isolated propanol-Ar complexes. They should be able to unravel the detailed structures of the complexes involving one or more Ar atoms. Such investigations are rare for aliphatic systems,5,86,87 but they could provide valuable constraints for an accurate description of the

Origin of the Argon Nanocoating Shift rare gas solvation process. On the theoretical side, the front solvation of the OH group by Ar will require further investigations, because it shows a strong basis set and compression dependence of the OH stretching fundamental, which is not apparent in the experimental spectra. Our work also sets the stage for calculations on more extensively Ar-solvated alcohol molecules, up to the simulation of doped crystal segments, possibly including periodic boundaries. As we have shown, the MP2 level of theory should be quite sufficient for such an endeavor. Acknowledgment. The present work was supported by the Fonds der Chemischen Industrie and the DFG (Su 121/2 and research training group 782 (www.pcgg.de)). The development of the MP2-F12 method used in the present work has been supported by the DFG Priority Programme SPP 1145 (Kl 721/ 2-3). S.H. gratefully acknowledges support from the Deutsche Telekom Stiftung. T.N.W. received a Chemiefonds scholarship from the Fonds der Chemischen Industrie, and J.J.L. was supported through an undergraduate student scholarship by the German National Academic Foundation (Studienstiftung des deutschen Volkes). We thank Florian A. Bischoff and David P. Tew for helpful discussions. References and Notes (1) Lotta, T.; Murto, J.; Ra¨sa¨nen, M.; Aspiala, A. IR-induced rotamerization of 1-propanol in low-temperature matrices and ab initio calculations. Chem. Phys. 1984, 86, 105–114. (2) Coussan, S.; Alikhani, M. E.; Perchard, J. P.; Zheng, W. Q. Infraredinduced isomerization of ethanol dimers trapped in argon and nitrogen matrices: Monochromatic irradiation experiments and DFT calculations. J. Phys. Chem. A 2000, 104, 5475–5483. (3) Olbert-Majkut, A.; Ahokas, J.; Lundell, J.; Pettersson, M. Raman spectroscopy of formic acid and its dimers isolated in low temperature argon matrices. Chem. Phys. Lett. 2009, 468, 176–183. (4) Bochenkova, A. V.; Suhm, M. A.; Granovsky, A. A.; Nemukhin, A. V. Hybrid diatomics-in-molecules-based quantum mechanical/molecular mechanical approach applied to the modeling of structures and spectra of mixed molecular clusters Arn(HCl)m and Arn(HF)m. J. Chem. Phys. 2004, 120, 3732–3743. (5) Tasic´, U.; Alexeev, Y.; Vayner, G.; Crawford, T. D.; Windus, T. L.; Hase, W. L. Ab initio and analytic intermolecular potentials for Ar-CH3OH. Phys. Chem. Chem. Phys. 2006, 8, 4678–4684. (6) Klopper, W.; Manby, F. R.; Ten-no, S.; Valeev, E. F. R12 methods in explicitly correlated molecular electronic structure theory. Int. ReV. Phys. Chem. 2006, 25, 427–468. (7) Bachorz, R. A.; Bischoff, F. A.; Ho¨fener, S.; Klopper, W.; Ottiger, P.; Leist, R.; Frey, J. A.; Leutwyler, S. Scope and limitations of the SCSMP2 method for stacking and hydrogen bonding interactions. Phys. Chem. Chem. Phys. 2008, 10, 2758–2766. (8) Schwabe, T.; Grimme, S. Double-hybrid density functionals with long-range dispersion corrections: higher accuracy and extended applicability. Phys. Chem. Chem. Phys. 2007, 9, 3397–3406. (9) Kno¨zinger, E.; Babka, E.; Hallamasek, D. Cage structure and longrange order in solid rare gas matrixes: A combined FTIR and XRD study. J. Phys. Chem. A 2001, 105, 8176–8182. (10) Ha¨ber, T.; Schmitt, U.; Suhm, M. A. FTIR-spectroscopy of molecular clusters in pulsed supersonic slit-jet expansions. Phys. Chem. Chem. Phys. 1999, 1, 5573–5582. (11) Rutkowski, K. S.; Melikova, S. M.; Rodziewicz, P.; Herrebout, W. A.; van der Veken, B. J.; Koll, A. Solvent effect on the blue shifted weakly H-bound F3CH · · · FCD3 complex. J. Mol. Struct. 2008, 880, 64– 68. (12) Choi, M. Y.; Douberly, G. E.; Falconer, T. M.; Lewis, W. K.; Lindsay, C. M.; Merritt, J. M.; Stiles, P. L.; Miller, R. E. Infrared spectroscopy of helium nanodroplets: novel methods for physics and chemistry. Int. ReV. Phys. Chem. 2006, 25, 15–75. (13) McIlroy, A.; Lascola, R.; Lovejoy, C. M.; Nesbitt, D. J. Structural dependence of hydrogen fluoride vibrational red shifts in argon-hydrogen fluoride (ArnHF, n ) 1-4), via high-resolution slit jet infrared spectroscopy. J. Phys. Chem. 1991, 95, 2636–2644. (14) Jiang, H.; Xu, M.; Hutson, J. M.; Bacˇic´, Z. ArnHF van der Waals clusters revisited: II. Energetics and HF vibrational frequency shifts from diffusion Monte Carlo calculations on additive and nonadditive potentialenergy surfaces for n ) 1-12. J. Chem. Phys. 2005, 123, 054305.

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