Article pubs.acs.org/JPCA
Interaction of Aromatic Compounds with Xenon: Spectroscopic and Computational Characterization for the Cases of p-Cresol and Toluene Qian Cao,†,‡ Natalya Andrijchenko,§ Alexander Ermilov,§ Markku Ras̈ an̈ en,† Alexander Nemukhin,§ and Leonid Khriachtchev*,† †
Department of Chemistry, University of Helsinki, P.O. Box 55, Helsinki FI-00014, Finland School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China § Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory 1/3, Moscow 119991, Russia ‡
S Supporting Information *
ABSTRACT: We have investigated noncovalent interactions of two aromatic compounds (toluene and p-cresol) with Xe atoms by using infrared spectroscopy in a Ne matrix and quantum chemical calculations. The present results show that the methyl group of these molecules is a sensitive probe of the interaction with Xe. We have used the molecules with the deuterated methyl group, possessing a relatively simple spectrum, which allows us to detect characteristic vibrational shifts in the complexes, in which a Xe atom interacts with the aromatic π electron system (π structure). For the p-cresol···Xe complex, we also observed evidence of the 1:1 Hbonded structure. The amount of the H-bonded structure of the cresol···Xe complex is relatively small, which agrees with the calculated interaction energies (stronger interaction for the π structure). The bands of the 1:1 complexes of p-cresol and toluene with Xe appear at low Xe concentration and their intensities relative to the monomer bands are nearly proportional to the Xe/Ne concentration ratio. For the p-cresol−Xe system, additional OH stretching bands appear at higher Xe concentrations, which are suitable for the complexes with several Xe atoms. The π structures studied in this work can probably be formed in the case of aromatic amino acids, for which these simple aromatic compounds are useful models.
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INTRODUCTION
Matrix-isolation infrared (IR) spectroscopy has been extensively used to study noncovalent interactions.22−27 The red shift of the OH stretching mode is characteristic of interaction with this group, and this effect has been particularly demonstrated for phenol complexes with H2O and N2.25,28 In contrast, the formation of the phenol complexes with the π structure may have no clear fingerprints in the vibrational spectra. The phenol···Xen clusters in a Ne matrix have recently been investigated by our group, and indirect but significant spectroscopic evidence supports the formation of the π structure of the 1:1 phenol···Xe complex.28 In another work, we studied complexes of three ACs (phenol, toluene, and pcresol) with N2O in a Ne matrix and found direct indications of the formation of the 1:1 complexes essentially with the π structures.29 Matrix-isolation studies of a few other toluene complexes and chemical reactions of toluene are available.30−32 The main objective of the present work is to study ACs model systems that can provide direct spectroscopic evidence
Many physical, chemical, and biological phenomena such as the formation of molecular crystals and biopolymers, solvation dynamics, protein folding, and molecular recognition are attributed to noncovalent interactions. Supramolecular systems of relatively simple aromatic compounds (ACs) are often used as prototypes of large biological molecules. Phenol is one of these popular models because it offers two principal binding sites for neutral ligands: interaction with the OH group (“Hbonded structure”) and with the aromatic π electron system (“π structure”). A number of gas-phase works supported by quantum chemical calculations have been devoted to the phenol···Rg complexes (Rg = Ar, Kr, and Xe).1−8 These studies have suggested that the π structure is the most probable in the neutral system. Phenol in small argon clusters has been studied experimentally and theoretically and several structures were found for this supramolecular system.7 Other small ACs, such as toluene and p-cresol, have also been studied with respect to intermolecular interactions although in a lesser extent than phenol. A number of complexes of these molecules have been investigated in supersonic jets.9−16 The effect of van der Waals interactions on the dynamics of toluene and p-cresol in an excited state has been investigated.17−21 © XXXX American Chemical Society
Special Issue: Markku Räsänen Festschrift Received: September 16, 2014 Revised: October 31, 2014
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of the intermolecular interaction between the ACs and Xe atoms. This work is particularly motivated by the attempts to understand the molecular-level mechanism of Xe anesthesia where simple ACs can be useful models for aromatic amino acids. Comparing the interaction between Xe atoms and ACs with different functional groups is important in this respect. The knowledge of structural and vibrational properties of weakly bound complexes of ACs with Xe is needed for theoretical studies of molecular mechanisms of Xe anesthesia and related processes. However, the relevant information is lacking so far. In the present work, we investigate the interactions of two ACs (toluene and p-cresol) with Xe atoms by using IR spectroscopy in a Ne matrix. These ACs have a methyl group that may be an effective indicator of the formation of the π structures, in contrast to the phenol−Xe system studied by us previously.28 To support the vibrational analysis, the deuterated ACs (toluene-d3 and p-cresol-d3) are mainly studied. In fact, these deuterated molecules have simpler spectra in the methyl stretching region compared to the spectra for “normal” ACs, leading to distinguishable spectral shifts in the AC···Xe complexes and providing direct spectroscopic fingerprints of the π structures. We use a Ne matrix because this environment is known to have a minimal effect on embedded species.33 The experimental studies are supported by quantum chemical calculations in the MP2 approximation.
Figure 1. 1:1 p-cresol···Xe complex: (a) H-bonded structure and (b) π structure. The characteristic distances are given in Å.
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CALCULATIONS Computational Details. The geometries, interaction energies, and vibrational spectra of the p-cresol···Xe and toluene···Xe complexes were calculated by the second-order Møller−Plesset perturbation theory (MP2) with the aug-ccpVTZ basis set using the Firefly program package.34 The Stuttgart-Koeln MCDHF RSC ECP was used for Xe.35 The interactions energies for the complexes were calculated with the basis-set superposition error (BSSE) correction.36 The vibrational analysis was performed in the harmonic approximation. As discussed elsewhere,37 the combination of the MP2 method with the aug-cc-pVTZ basis set provides a reasonable compromise for computer simulations of intermolecular complexes containing ACs. This approach provided a good interpretation of matrix-isolation experimental data in our previous study of the phenol···Xen complexes.28 To support the vibrational analysis, partially deuterated ACs (p-cresol-d3 and toluene-d3) were simulated, in addition to the molecules without deuteration. p-Cresol···Xe Complex. For the p-cresol···Xe complex, two true energy minima have been found on the potential energy surface. In the first structure, the Xe atom is located in the plane of the aromatic ring nearly in the line of the OH group (Hbonded structure, Figure 1a). In the more stable complex, the Xe atom is located near the aromatic ring with a small displacement toward the methyl group (π structure, Figure 1b). The interaction energies for the H-bonded and π structures are −1.7 (−1.1) and −3.4 (−2.6) kcal mol−1, respectively (the BSSE-corrected values in parentheses). The calculated vibrational frequencies (>500 cm−1) and assignments of the p-cresol-d3 monomer and the p-cresol-d3 complexes are presented in Table S1 (Supporting Information) (for normal p-cresol, see Table S2, Supporting Information). For the more stable π structure, the frequency shifts are relatively small, and the largest shifts are found for two stretching modes of the methyl group (about −3 cm−1 for p-
cresol-d3) whereas other spectral shifts are relatively small. For the H-bonded structure, the red shift of the OH stretching mode is large (−23.3 cm−1), which is a fingerprint of this structure, and the strong deformation mode also shows a significant blue shift (+6.9 cm−1). Toluene···Xe Complex. Only the π structure was found to be stable for the toluene···Xe complex (Figure 2) with an
Figure 2. Toluene···Xe complex (π structure). The characteristic distances are given in Å.
interaction energy of −3.3 kcal mol−1 (BSSE-corrected −2.6 kcal mol−1), and this value is close to that of the corresponding structures of the p-cresol···Xe and phenol···Xe complexes. The appearance of only one structure is a result of the absence of the OH group in toluene. The vibrational spectra of the monomer and the Xe complex of toluene-d3 (deuterated methyl group) are presented in Table S3 (Supporting Information) (for normal toluene, see Table S4, Supporting Information). The largest shifts are found for two stretching B
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modes of the methyl group (about −3 cm−1 for toluene-d3) whereas other spectral shifts are relatively small, similar to those for the π structure of p-cresol-d3.
should be mentioned that practically no formation of p-cresol dimer is observed at such low cresol concentration. It is clearly seen that the spectrum changes in the presence of Xe displaying red-shifted bands for the OH and CD stretching modes and blue-shifted bands for the deformation mode. By subtracting the p-cresol monomer bands, we obtain the Xe-induced bands (trace 2 in Figure 3), which are quite distinguishable from the monomer bands. The analysis is complicated by the band splitting, which may be due to the matrix-site effect, CD3 group rotation, and/or Fermi resonances. This splitting also makes the assignment of the bands in the CD3 stretching region somewhat uncertain. Nevertheless, the OH stretching bands at 3660.5 and 3656.8 cm−1 and the CD stretching bands at 2232.8, 2138.8, 2074.0, and 2062.2 cm−1 are suitable for the π structure of the 1:1 p-cresol···Xe complex (Table 1), providing direct evidence for the formation of this stronger structure. A disagreement between the experimental and calculated shifts is observed for the higher-frequency CD3 stretching mode, and this is discussed later. Other characteristic modes of this structure display only negligible shifts and/or broadening. In addition to the π structure, our calculations predict the existence of the H-bonded structure with characteristic bands in the OH stretching and deformation regions (Table S1, Supporting Information). The bands suitable for this structure are observed at 3636.0 and 1179/1175/1165 (and possibly 1198) cm−1, respectively. The Xe-induced frequency shift is ca. −23 cm−1 for the OH stretching mode and ca. +9 cm−1 (in average) for the deformation mode, which is consistent with the calculated results (−23.3 and +6.9 cm−1, respectively) for the H-bonded structure (Table 1). The 1198 cm−1 band may correspond to the weaker monomer band at 1176 cm−1, meaning a shift of +20 cm−1. For normal p-cresol, new bands also appear at 3636 and 1176 cm−1 as Xe is added. The experimental shifts (−20.4 and +3.4 cm−1) agree with the calculations for the H-bonded structure (−23.3 and +6.9 cm−1), providing additional evidence that the H-bonded structure is formed in some amounts. It is interesting that the H-bonded structure was not found for the phenol···Xe complex,28 and the reason for this is unclear based on the available experiments and calculations. A series of p-cresol-d3/Xe/Ne matrixes with different Xe concentrations were investigated to support the formation of the 1:1 complexes. All bands mentioned above appear at quite small Xe concentrations and their relative intensities to the cresol monomer bands are nearly proportional to the Xe/Ne concentration ratio (Figure 4a), which indicates that these bands originate from the 1:1 complexes. It should be noted that the H-bonded structure is formed in much smaller amounts than the π structure (∼1/6 as judged by the band intensities), which is consistent with the calculated interaction energies. At higher Xe concentrations, two additional bands in the OH stretching region at 3645 and 3630 cm−1 are observed for both normal p-cresol and p-cresol-d3. The intensities of these bands show a nonlinear dependence on the Xe concentration (Figure 4b,c). A similar behavior was previously observed in the case of the phenol···Xen complexes (n > 1) under similar experimental conditions (Figure 4d).28 The substantial Xe-induced red shifts (for p-cresol ca. −15 and −30 cm−1) are also comparable to those in the phenol···Xen complexes (n = 2−4). On the basis of these results, we explain these two bands at 3645 and 3630 cm−1 by the formation of the p-cresol complexes with several Xe atoms. No bands applicable to the higher-order complexes
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EXPERIMENT Experimental Details. Solid p-cresol CH3C6H4OH (Merck Schuchardt OHG, >98%), liquid toluene C6H5CH3 (SigmaAldrich, 99.9%), and their deuterated analogues with the deuterated methyl groups CD3C6H4OH (CDN Isotopes, 99.6 atom % D) and C6H5CD3 (Cambridge Isotope Laboratories, 98 atom % D) were purified by several freeze−pump−thaw cycles. The gases Ne (AGA, 99.9999%) and Xe (AGA, 99.997%) were used as supplied. The AC/Ne concentration ratios were ∼1/4200 for p-cresol and ∼1/2000 for toluene, and the Xe/AC concentration ratios were up to 30. The relatively low concentration of p-cresol in matrixes is caused by its low vapor pressure (∼0.10 Torr) at room temperature. The gaseous mixtures were deposited onto a CsI substrate typically at 4.3 and 8 K in a closed-cycle helium cryostat (Sumitomo RDK 408D). The IR absorption spectra in the 4000−600 cm−1 spectral range were measured at 4.3 K with a Bruker VERTEX 80 FTIR spectrometer by coadding 200 interferograms using 0.25 cm−1 resolution. Experimental Results and Assignment. Figure 3 (traces 1) shows the FTIR spectra of p-cresol-d3/Ne (1/4200) and pcresol-d3/Xe/Ne (1/2/4200) matrixes deposited at 8 K. It
Figure 3. FTIR spectra of p-cresol-d3/Ne (1/4200) and p-cresol-d3/ Xe/Ne (1/2.5/4200) matrixes recorded after deposition at 8 K in the OH stretching, CD stretching, and deformation regions. Presented are (1) the normal spectra and (2) the spectra showing the Xe-induced bands by subtraction of the monomer bands with an appropriate multiplication factor. The spectra were measured at 4.3 K. C
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Table 1. Characteristic Frequencies and Shifts (cm−1) of the p-Cresol-d3···Xe and Toluene-d3···Xe Complexes calculation
assignment
ω
experiment
π structure
monomer ω
H-bonded structure Δω
ω
monomer
complex
Δω
ω
ω
Δω
−23.2
3661.5 3657.8 2239.5 2226.0 2218.0 2144.8 2075.3 2065.2 1164.2 1178.0
3660.5 3656.8 3636.0 2232.8 2215.0
−1.0 −1.0 −23.6 −6.7 −7.0a
2138.8 2074.0 2062.2 1179.0 1175.0 1165.0
−6.0 −1.3 −3.0 +9.0
2236.5 2215.3 2136.5 2086.0 2064.0
−4.5 −2.0 −1.5 −1.0 −2.0
ν(OH)
3824.6
3823.5
p-Cresol-d3 −1.1 3801.4
ν(CD)asym
2342.2
2342.9
+0.7
2341.7
−0.5
ν(CD)asym ν(CD)sym
2325.9 2203.8
2322.4 2201.4
−3.5 −2.4
2325.7 2203.6
−0.2 −0.2
δ(COH) + δ(CCH)ring
1193.9
1193.9
0.0
1200.8
+6.9
ν(CD)asym
2343.9
2343.9
0.0
ν(CD)asym ν(CD)sym
2325.8 2203.8
2322.0 2201.1
−3.8 −2.7
Toluene-d3
a
2241.0 2217.3 2138.0 2087.0 2066.0
Average value.
Figure 4. Intensities of the complex bands (Ic) relative to the monomer bands (Im) of ACs (p-cresol-d3, normal p-cresol, and phenol) as a function of the Xe/Ne concentration ratio. The ACs/Ne matrix ratio was 1/4200 and the spectra were recorded at 4.3 K after deposition at 8 K. The data for phenol are from ref 28. The lines for the 1:1 complexes are linear fits.
subtraction of the toluene monomer spectrum, the Xe-induced bands are clearly seen at 2236.5, 2215.3, 2136.5, 2086.0, and 2064.0 cm−1, displaying small but distinguishable red shifts relative to the monomer bands, which is consistent with the calculations (Table 1). Other relatively strong modes display only negligible shifts and/or broadening. Figure 5c shows that these Xe-induced bands appear at very low Xe concentration (the Xe/Ne concentration ratio of 1/2000) and rise nearly linearly with the Xe concentration. Thus, we assign these CD stretching bands to the 1:1 toluene···Xe complex.
are observed in other spectral regions, which is also similar to the case of phenol···Xen complexes.28 The sensitivity of the methyl group to the formation of the π structures is fully supported by the experiments with the toluene···Xe complexes. In this case, the H-bonded structure is excluded due to the absence of the OH group and only the π structure is predicted by the calculations. Traces 1 in Figure 5a present the FTIR spectra of toluene-d3/Ne (1/2000) and toluene-d3/Xe/Ne (1/2/2000) matrixes in the CD stretching region. The spectrum in this region changes considerably as Xe is added to the mixture, similarly to the case of p-cresol-d3. After D
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Figure 6. (a) FTIR spectra of p-cresol-d3/Ne (1/4000) and cresol-d3/ N2/Ne (1/5/4000) matrixes in the OH stretching region. The matrixes were deposited at 4.3 K and annealed to 10 K. The spectra were recorded at 4.3 K. (b) Intensities of the p-cresol-d3···N2 complex bands (Ic) relative to the monomer bands (Im) as a function the N2/ Ne concentration ratio. The p-cresol-d3/Ne matrix ratio is 1/4200.
Figure 5. (a, b) FTIR spectra of toluene-d3/Xe/Ne matrixes with different Xe concentrations in the CD stretching region (spectra 1 and 3) and the spectra showing the Xe-induced bands by subtracting the monomer bands with an appropriate multiplication factor (spectra 2 and 4). The spectra were measured at 4.3 K after deposition at 4.3 K and annealing to 10 K. (c) Intensities of the complex bands (Ic) relative to the monomer bands (Im) as a function the Xe/Ne concentration ratio. The toluene-d3/Ne matrix ratio is 1/2000. The line for the 1:1 complexes is a linear fit.
trations and their intensities relative to the cresol monomer bands are nearly proportional to the N2/Ne concentration ratio (Figure 6b). All these data provide strong evidence that the Hbonded structure of the 1:1 p-cresol···N2 complex is formed for low N2 concentrations similarly to the case of phenol.28 The experiments with nitrogen indirectly support our earlier conclusions. First, a linear concentration dependence is a fingerprint of a 1:1 complex. It follows that the p-cresol···Xe and toluene···Xe complexes with similar concentration dependences in the spectrum are the 1:1 structures. Second, the addition of nitrogen to a p-cresol/Ne matrix does not change the spectrum in the methyl stretching region. This shows that the interaction with the OH group does not affect the methyl group significantly, in contrast to the case of Xe, for which the π structure is formed.
At higher concentrations, the bands assigned to the 1:1 toluene···Xe complex are still pronounced but a set of new absorptions (2234.0, 2206.8, 2128.5, and 2057.5 cm−1) appear at lower frequencies in the CD stretching region (spectra 3 and 4 in Figure 5b) as well as in the CH stretching, ring stretching, and bending regions (not shown). These additional bands are observed only at relatively high Xe concentrations (Xe/Ne concentration ratio >1/200). Such delayed formation is demonstrated in Figure 5c, indicating that these bands presumably originate from the complex of toluene with several Xe atoms. We also studied the p-cresol···N2 complex in a Ne matrix (Figure 6a). From the methodological point of view, these experiments are needed because nitrogen is a common impurity in matrix isolation. The addition of a small amount of nitrogen produces a new band in the OH stretching region centered at 3648.5 cm−1, displaying a red shift of about −9 cm−1 relative to the monomer. A similar spectral shift of the OH stretching mode was reported for the 1:1 phenol···N2 complex (−9.2 cm−1), and the band was assigned to the 1:1 H-bonded structure.28 Other characteristic absorptions of the p-cresol···N2 complex (1334.5, 1262.5, and 1185.0 cm−1) are also observed, shifted from the p-cresol monomer bands by ca. +4, +4, and +20 cm−1, respectively. These spectral shifts are quite comparable to the values for the H-bonded structure of the 1:1 phenol···N2 complex (Table 2).28 We did not calculate the p-cresol···N2 complex; however, we expect similar complexation effects on the characteristic vibrational modes of phenol and pcresol. These N2-induced bands appear at low N2 concen-
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CONCLUDING REMARKS The phenol−Xe system previously studied in a Ne matrix was found to be challenging for IR spectroscopy because the most stable 1:1 complex has the π structure without clear spectroscopic fingerprints.28 In this situation, only indirect evidence of the formation of the 1:1 complex was obtained by analyzing the Xe concentration dependence of the vibrational spectra. At low Xe concentrations, practically no changes in the spectrum were observed, which was connected to the formation of the π structure with hidden spectroscopic features. The cases of p-cresol and toluene are principally different from that of phenol due to the presence of the methyl group. The present results show that the methyl group is a sensitive probe of the AC···Xe complexes with the π structure. We used the molecules with the deuterated methyl group possessing a relatively simple spectrum, which allowed us to detect characteristic vibrational shifts in the AC···Xe complexes with E
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Table 2. Experimental Characteristic Frequencies and Shifts (ω and Δω, cm−1) of the p-Cresol-d3···N2 Complex Compared with the Previous Results for the 1:1 Phenol···N2 Complex phenola
p-cresol-d3 experiment
a
calculation, Δω
experiment
assignment
ω
Δω
ω
Δω
H-bonded structure
π structure
ν(OH) δ(CCH)ring + δ(COH) ν(CC) ring + ν(CO) δ(CCH)ring + δ(COH)
3648.5 1334.5 1262.5 1185.0
−9.2 +4.5 +3.7 +20.8
3646.2 1347.7 1265.4 1190.8
−9.2 +3.3 +3.2 +13.8
−12.1 +1.2 +3.1 +19.4
−1.1 +4.9 +0.8 +0.3
From ref 28.
the π structure. In this case, the π structure can be reliably characterized by combination of quantum chemistry calculations and experimental IR spectroscopy in cryogenic matrixes. The bands of the π structure appear at low Xe concentration and their relative intensity is a nearly linear function of the Xe concentration, which is characteristic of the 1:1 complexes. In the case of the p-cresol···Xe complex, the OH stretching bands with small red shifts (∼1 cm−1) were observed also for the π structure, and this is different from the case of phenol, for which similar bands were not detected. In fact, this distinction is consistent with the calculations because the calculated shift of the π structure for this mode of p-cresol-d3 (−1.1 cm−1) is somewhat larger compared to that of phenol (−0.4 cm−1) although these small spectral differences cannot be confidently predicted by the present computational method. In the p-cresol−Xe system, we also observed indications of the 1:1 complex with the H-bonded structure (bands at 1175 and 3636 cm−1). These bands appear at low Xe concentration and correlate with the bands assigned to the π structure. The amount of the H-bonded 1:1 complex is relatively small (∼1/6), which is consistent with the calculated interaction energies (stronger interaction for the π structure). The corresponding complex was not observed in the phenol−Xe system, despite similar interaction energies in these two cases. It is possible that the stabilization barrier of the H-bonded structure is lower in the case of phenol. As an indication of this, the Xe atom in the π structure in the case of phenol is displaced toward the oxygen atom, which is different from the case of pcresol where it is displaced toward the methyl group. In the phenol−Xe system, the new OH stretching bands appeared only at relatively high Xe concentration, and they were explained by the interaction of phenol with several Xe atoms.28 The formation of these higher-order complexes were justified by calculations. For the p-cresol−Xe system, the same behavior is observed for the OH stretching bands at 3630 and 3645 cm−1 and these bands are analogously assigned to the 1:n complexes (n > 1). In contrast, for the N2 complexes of phenol and p-cresol, the well-shifted OH stretching bands appear at low N2 concentration and their relative intensity is a nearly linear function of the N2 concentration, which shows the formation of the 1:1 complex with the H-bonded structure. The cases of p-cresol and toluene are worth discussing with respect to the computational results. The experimental methyl stretching modes in the complex of these ACs with Xe are redshifted, which is in agreement with the calculations for the two lower-frequency transitions of the π structures but disagrees for the highest frequency mode. This mismatch probably indicates the lack of the description at the present computational level. It should also be noted that the splitting of this band makes the comparison of the theory and experiment less certain. Despite
this small disagreement, the similarity for p-cresol and toluene suggests the formation of the π structure in both cases. Indeed, in the case of toluene, the formation of other structures is improbable due to the absence of the OH group. The shifts of the methyl bands predicted for the H-bonded structure of the p-cresol···Xe complex, which is a weaker complex, are much smaller than the experimental values. A partial disagreement between experiment and calculations was observed for the methyl group in the π structure of the toluene···N2O complex, with three experimental blue shifts whereas the calculations predict two blue shifts and one red shift.29 The formation of the π structure of the 1:1 complexes of Xe with phenol, p-cresol, and toluene suggests a general conclusion that these and similar ACs can form the π structures with Xe atoms and the presence of the hydroxyl and/or methyl groups on the aromatic ring does not change the character of the dominating interaction. Similar structures can be presumably formed for aromatic amino acids, for which these ACs are useful models.
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ASSOCIATED CONTENT
S Supporting Information *
The experimental spectra of the monomers in Ne matrixes and the calculated vibrational spectra of the monomers and the Xe complexes of p-cresol-d3 (Table S1), p-cresol (Table S2), toluene-d3 (Table S3), and toluene (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*L. Khriachtchev. E-mail: leonid.khriachtchev@helsinki.fi. Telephone number: +358 294150310. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Q. C. thanks the Academy of Finland for a postdoctoral grant (No. 1139425). This work is also a part of the Project KUMURA of the Academy of Finland (No. 1277993). We acknowledge the use of computer facilities of the M. V. Lomonosov Moscow State University and of the Joint Supercomputer Center of the Russian Academy of Sciences.
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REFERENCES
(1) Cerny, J.; Tong, X.; Hobza, P.; Mueller-Dethlefs, K. State of the Art Theoretical Study and Comparison to Experiment for the Phenol···Argon Complex. J. Chem. Phys. 2008, 128, 114319. (2) Dessent, C. E. H.; Haines, S. R.; Muller-Dethlefs, K. A New Detection Scheme for Synchronous, High Resolution ZEKE and
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MATI Spectroscopy Demonstrated on the Phenol···Ar Complex. Chem. Phys. Lett. 1999, 315, 103−108. (3) Dessent, C. E. H.; Muller-Dethlefs, K. Hydrogen-Bonding and van der Waals Complexes Studied by ZEKE and REMPI Spectroscopy. Chem. Rev. 2000, 100, 3999−4021. (4) Fujii, A.; Sawamura, T.; Tanabe, S.; Ebata, T.; Mikami, N. Infrared Dissociation Spectroscopy of the OH Stretching Vibration of Phenol-Rare Gas van der Waals Cluster Ions. Chem. Phys. Lett. 1994, 225, 104−107. (5) Haines, S. R.; Dessent, C. E. H.; Muller-Dethlefs, K. Is the Phenol···Ar Complex van der Waals or Hydrogen-Bonded? A REMPI and ZEKE Spectroscopic Study. J. Electron. Spectrosc. Relat. Phenom. 2000, 108, 1−11. (6) Mons, M.; Le Calve, J.; Piuzzi, F.; Dimicoli, I. Resonant TwoPhoton Ionization Spectra of the External Vibrational Modes of the Chlorobenzene-, Phenol-, and Toluene-Rare Gas (Ne, Ar, Kr, Xe) van der Waals Complexes. J. Chem. Phys. 1990, 92, 2155−2165. (7) Schmies, M.; Patzer, A.; Fujii, M.; Dopfer, O. Structures and IR/ UV Spectra of Neutral and Ionic Phenol-Arn Cluster Isomers (n ≤ 4): Competition Between Hydrogen Bonding and Stacking. Phys. Chem. Chem. Phys. 2011, 13, 13926−13941. (8) Ullrich, S.; Tarczay, G.; Muller-Dethlefs, K. A ResonanceEnhanced Multiphoton Ionization and Zero Kinetic Energy Photoelectron Study of the Phenol···Kr and Phenol···Xe van der Waals Complexes. J. Phys. Chem. A 2002, 106, 1496−1503. (9) Pohl, M.; Schmitt, M.; Kleinermanns, K. Microscopic Shifts of Size-Assigned p-Cresol/H2O-Cluster Spectra. J. Chem. Phys. 1991, 94, 1717−1723. (10) Gerhards, M.; Kimpfel, B.; Pohl, M.; Schmitt, M.; Kleinermanns, K. Vibronic Spectroscopy of Jet-Cooled Hydrogen-Bonded Clusters. J. Mol. Struct. 1992, 270, 301−324. (11) Ishikawa, S.; Ebata, T.; Ishikawa, H.; Inoue, T.; Mikami, N. Hole-Burning and Stimulated Raman−UV Double Resonance Spectroscopies of Jet-Cooled Toluene Dimer. J. Phys. Chem. 1996, 100, 10531−10535. (12) Joireman, P. W.; Ohline, S. M.; Felker, P. M. Structural Characterization of Aromatic−Aromatic Complexes by Rotational Coherence Spectroscopy. J. Phys. Chem. A 1998, 102, 4481−4494. (13) Biswal, H. S.; Chakraborty, S.; Wategaonkar, S. Experimental Evidence of O-H−S Hydrogen Bonding in Supersonic Jet. J. Chem. Phys. 2008, 129, 184311. (14) Biswal, H. S.; Shirhatti, P. R.; Wategaonkar, S. O-H···O versus O-H···S Hydrogen Bonding I: Experimental and Computational Studies on the p-Cresol·H2O and p-Cresol·H2S Complexes. J. Phys. Chem. A 2009, 113, 5633−5643. (15) Biswal, H. S.; Shirhatti, P. R.; Wategaonkar, S. O−H···O versus O−H···S Hydrogen Bonding. 2. Alcohols and Thiols as Hydrogen Bond Acceptors. J. Phys. Chem. A 2010, 114, 6944−6955. (16) Shirhatti, P. R.; Maity, D. K.; Wategaonkar, S. C−H···Y Hydrogen Bonds in the Complexes of p-Cresol and p-Cyanophenol with Fluoroform and Chloroform. J. Phys. Chem. A 2013, 117, 2307− 2316. (17) Doyle, R. J.; Love, E. S. J.; Da Campo, R.; Mackenzie, S. R. Electronic Spectroscopy of Toluene-Rare-Gas Clusters: The External Heavy Atom Effect and Vibrational Predissociation. J. Chem. Phys. 2005, 122, 194315. (18) Zhang, X.; Knee, J. L. Dynamics of Large Molecule van der Waals Complexes Studied with ZEKE Spectroscopy. Faraday Discuss. 1994, 97, 299−313. (19) Da Campo, R.; Mackenzie, S. R. Vibrational Predissociation in Weakly-Bound Clusters: A Dispersed Fluorescence Study of Toluene Rare-Gas Complexes. Chem. Phys. Lett. 2008, 452, 6−13. (20) Hernandez, F. J.; Capello, M. C.; Oldani, A. N.; Ferrero, J. C.; Maitre, P.; Pino, G. A. H-Bonded Network Rearrangements in the S0, S1 and D0 States of Neutral and Cationic p-cresol(H2O)(NH3) Complexes. Phys. Chem. Chem. Phys. 2012, 14, 8945−8955. (21) Biswal, H. S.; Bhattacharyya, S.; Wategaonkar, S. MolecularLevel Understanding of Ground- and Excited-State O-H···O Hydrogen Bonding Involving the Tyrosine Side Chain: A Combined High-
Resolution Laser Spectroscopy and Quantum Chemistry Study. ChemPhysChem 2013, 14, 4165−4176. (22) Barnes, A. J.; Mielke, Z. Matrix Effects on Hydrogen-Bonded Complexes Trapped in Low-Temperature Matrices. J. Mol. Struct. 2012, 1023, 216−221. (23) Lignell, A.; Khriachtchev, L. Intermolecular Interactions Involving Noble-Gas Hydrides: Where the Blue Shift of Vibrational Frequency Is a Normal Effect. J. Mol. Struct. 2008, 889, 1−11. (24) Marushkevich, K.; Khriachtchev, L.; Lundell, J.; Räsänen, M. cistrans Formic Acid Dimer: Experimental Observation and Improved Stability against Proton Tunneling. J. Am. Chem. Soc. 2006, 128, 12060−12061. (25) Gor, G. Y.; Tapio, S.; Domanskaya, A. V.; Räsänen, M.; Nemukhin, A. V.; Khriachtchev, L. Matrix-Isolation Study of the Phenol-Water Complex and Phenol Dimer. Chem. Phys. Lett. 2011, 517, 9−15. (26) Gebicki, J.; Krantz, A. Interactions between Phenol and Carbon Monoxide under Cryogenic Conditions. Evidence for a PhenolCarbon Monoxide Complex. J. Am. Chem. Soc. 1984, 106, 8093−8097. (27) Young, N. A. Main Group Coordination Chemistry at Low Temperatures: A Review of Matrix-Isolated Group 12 to Group 18 Complexes. Coord. Chem. Rev. 2013, 257, 956−1010. (28) Cao, Q.; Andrijchenko, N.; Ahola, A. E.; Domanskaya, A.; Räsänen, M.; Ermilov, A.; Nemukhin, A.; Khriachtchev, L. Interaction of Phenol with Xenon and Nitrogen: Spectroscopic and Computational Characterization. J. Chem. Phys. 2012, 137, 134305. (29) Cao, Q.; Gor, G. Y.; Krogh-Jespersen, K.; Khriachtchev, L. NonCovalent Interactions of Nitrous Oxide with Aromatic Compounds: Spectroscopic and Computational Evidence for the Formation of 1:1 Complexes. J. Chem. Phys. 2014, 140, 144304. (30) Bai, H.; Ault, B. S. Infrared Spectroscopic Characterization of Complexes of Nitrosyl Chloride with Lone Pair and π Electron Donors in Argon Matrices. J. Phys. Chem. 1990, 94, 606−610. (31) Parker, J. K.; Davis, S. R. Photochemical Reactions of Oxygen Atoms with Toluene, m-Xylene, p-Xylene, and Mesitylene: An Infrared Matrix Isolation Investigation. J. Phys. Chem. A 2000, 104, 4108−4114. (32) Hoops, M. D.; Ault, B. S. Matrix Isolation Investigation of the Photochemical Reaction of Methyl-Substituted Benzenes with CrCl2O2. J. Phys. Chem. A 2005, 110, 892−900. (33) Jacox, M. E. The Spectroscopy of Molecular Reaction Intermediates Trapped in the Solid Rare Gases. Chem. Soc. Rev. 2002, 31, 108−115. (34) Granovsky, A. A. Firefly version 8.0, http://classic.chem.msu.su/ gran/firefly/index.html. (35) Peterson, K. A. Systematically Convergent Basis Sets with Relativistic Pseudopotentials. I. Correlation Consistent Basis Sets for the Post-d Group 13−15 Elements. J. Chem. Phys. 2003, 119, 11099− 11112. (36) Boys, S. F.; Bernardi, F. Calculation of Small Molecular Interactions by Differences of Separate Total Energies - Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553−566. (37) Riley, K. E.; Hobza, P. Assessment of the MP2Method, along with Several Basis Sets, for the Computation of Interaction Energies of Biologically Relevant Hydrogen Bonded and Dispersion Bound Complexes. J. Phys. Chem. A 2007, 111, 8257−8263.
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Interaction of Aromatic Compounds with Xenon: Spectroscopic and Computational Characterization for the Cases of p-Cresol and Toluene Qian Cao,†,‡ Natalya Andrijchenko,§ Alexander Ermilov,§ Markku Ras̈ an̈ en,† Alexander Nemukhin,§ and Leonid Khriachtchev*,† †
Department of Chemistry, University of Helsinki, P.O. Box 55, Helsinki FI-00014, Finland School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China § Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory 1/3, Moscow 119991, Russia ‡
S Supporting Information *
ABSTRACT: We have investigated noncovalent interactions of two aromatic compounds (toluene and p-cresol) with Xe atoms by using infrared spectroscopy in a Ne matrix and quantum chemical calculations. The present results show that the methyl group of these molecules is a sensitive probe of the interaction with Xe. We have used the molecules with the deuterated methyl group, possessing a relatively simple spectrum, which allows us to detect characteristic vibrational shifts in the complexes, in which a Xe atom interacts with the aromatic π electron system (π structure). For the p-cresol···Xe complex, we also observed evidence of the 1:1 Hbonded structure. The amount of the H-bonded structure of the cresol···Xe complex is relatively small, which agrees with the calculated interaction energies (stronger interaction for the π structure). The bands of the 1:1 complexes of p-cresol and toluene with Xe appear at low Xe concentration and their intensities relative to the monomer bands are nearly proportional to the Xe/Ne concentration ratio. For the p-cresol−Xe system, additional OH stretching bands appear at higher Xe concentrations, which are suitable for the complexes with several Xe atoms. The π structures studied in this work can probably be formed in the case of aromatic amino acids, for which these simple aromatic compounds are useful models.
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INTRODUCTION
Matrix-isolation infrared (IR) spectroscopy has been extensively used to study noncovalent interactions.22−27 The red shift of the OH stretching mode is characteristic of interaction with this group, and this effect has been particularly demonstrated for phenol complexes with H2O and N2.25,28 In contrast, the formation of the phenol complexes with the π structure may have no clear fingerprints in the vibrational spectra. The phenol···Xen clusters in a Ne matrix have recently been investigated by our group, and indirect but significant spectroscopic evidence supports the formation of the π structure of the 1:1 phenol···Xe complex.28 In another work, we studied complexes of three ACs (phenol, toluene, and pcresol) with N2O in a Ne matrix and found direct indications of the formation of the 1:1 complexes essentially with the π structures.29 Matrix-isolation studies of a few other toluene complexes and chemical reactions of toluene are available.30−32 The main objective of the present work is to study ACs model systems that can provide direct spectroscopic evidence
Many physical, chemical, and biological phenomena such as the formation of molecular crystals and biopolymers, solvation dynamics, protein folding, and molecular recognition are attributed to noncovalent interactions. Supramolecular systems of relatively simple aromatic compounds (ACs) are often used as prototypes of large biological molecules. Phenol is one of these popular models because it offers two principal binding sites for neutral ligands: interaction with the OH group (“Hbonded structure”) and with the aromatic π electron system (“π structure”). A number of gas-phase works supported by quantum chemical calculations have been devoted to the phenol···Rg complexes (Rg = Ar, Kr, and Xe).1−8 These studies have suggested that the π structure is the most probable in the neutral system. Phenol in small argon clusters has been studied experimentally and theoretically and several structures were found for this supramolecular system.7 Other small ACs, such as toluene and p-cresol, have also been studied with respect to intermolecular interactions although in a lesser extent than phenol. A number of complexes of these molecules have been investigated in supersonic jets.9−16 The effect of van der Waals interactions on the dynamics of toluene and p-cresol in an excited state has been investigated.17−21 © XXXX American Chemical Society
Special Issue: Markku Räsänen Festschrift Received: September 16, 2014 Revised: October 31, 2014
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of the intermolecular interaction between the ACs and Xe atoms. This work is particularly motivated by the attempts to understand the molecular-level mechanism of Xe anesthesia where simple ACs can be useful models for aromatic amino acids. Comparing the interaction between Xe atoms and ACs with different functional groups is important in this respect. The knowledge of structural and vibrational properties of weakly bound complexes of ACs with Xe is needed for theoretical studies of molecular mechanisms of Xe anesthesia and related processes. However, the relevant information is lacking so far. In the present work, we investigate the interactions of two ACs (toluene and p-cresol) with Xe atoms by using IR spectroscopy in a Ne matrix. These ACs have a methyl group that may be an effective indicator of the formation of the π structures, in contrast to the phenol−Xe system studied by us previously.28 To support the vibrational analysis, the deuterated ACs (toluene-d3 and p-cresol-d3) are mainly studied. In fact, these deuterated molecules have simpler spectra in the methyl stretching region compared to the spectra for “normal” ACs, leading to distinguishable spectral shifts in the AC···Xe complexes and providing direct spectroscopic fingerprints of the π structures. We use a Ne matrix because this environment is known to have a minimal effect on embedded species.33 The experimental studies are supported by quantum chemical calculations in the MP2 approximation.
Figure 1. 1:1 p-cresol···Xe complex: (a) H-bonded structure and (b) π structure. The characteristic distances are given in Å.
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CALCULATIONS Computational Details. The geometries, interaction energies, and vibrational spectra of the p-cresol···Xe and toluene···Xe complexes were calculated by the second-order Møller−Plesset perturbation theory (MP2) with the aug-ccpVTZ basis set using the Firefly program package.34 The Stuttgart-Koeln MCDHF RSC ECP was used for Xe.35 The interactions energies for the complexes were calculated with the basis-set superposition error (BSSE) correction.36 The vibrational analysis was performed in the harmonic approximation. As discussed elsewhere,37 the combination of the MP2 method with the aug-cc-pVTZ basis set provides a reasonable compromise for computer simulations of intermolecular complexes containing ACs. This approach provided a good interpretation of matrix-isolation experimental data in our previous study of the phenol···Xen complexes.28 To support the vibrational analysis, partially deuterated ACs (p-cresol-d3 and toluene-d3) were simulated, in addition to the molecules without deuteration. p-Cresol···Xe Complex. For the p-cresol···Xe complex, two true energy minima have been found on the potential energy surface. In the first structure, the Xe atom is located in the plane of the aromatic ring nearly in the line of the OH group (Hbonded structure, Figure 1a). In the more stable complex, the Xe atom is located near the aromatic ring with a small displacement toward the methyl group (π structure, Figure 1b). The interaction energies for the H-bonded and π structures are −1.7 (−1.1) and −3.4 (−2.6) kcal mol−1, respectively (the BSSE-corrected values in parentheses). The calculated vibrational frequencies (>500 cm−1) and assignments of the p-cresol-d3 monomer and the p-cresol-d3 complexes are presented in Table S1 (Supporting Information) (for normal p-cresol, see Table S2, Supporting Information). For the more stable π structure, the frequency shifts are relatively small, and the largest shifts are found for two stretching modes of the methyl group (about −3 cm−1 for p-
cresol-d3) whereas other spectral shifts are relatively small. For the H-bonded structure, the red shift of the OH stretching mode is large (−23.3 cm−1), which is a fingerprint of this structure, and the strong deformation mode also shows a significant blue shift (+6.9 cm−1). Toluene···Xe Complex. Only the π structure was found to be stable for the toluene···Xe complex (Figure 2) with an
Figure 2. Toluene···Xe complex (π structure). The characteristic distances are given in Å.
interaction energy of −3.3 kcal mol−1 (BSSE-corrected −2.6 kcal mol−1), and this value is close to that of the corresponding structures of the p-cresol···Xe and phenol···Xe complexes. The appearance of only one structure is a result of the absence of the OH group in toluene. The vibrational spectra of the monomer and the Xe complex of toluene-d3 (deuterated methyl group) are presented in Table S3 (Supporting Information) (for normal toluene, see Table S4, Supporting Information). The largest shifts are found for two stretching B
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modes of the methyl group (about −3 cm−1 for toluene-d3) whereas other spectral shifts are relatively small, similar to those for the π structure of p-cresol-d3.
should be mentioned that practically no formation of p-cresol dimer is observed at such low cresol concentration. It is clearly seen that the spectrum changes in the presence of Xe displaying red-shifted bands for the OH and CD stretching modes and blue-shifted bands for the deformation mode. By subtracting the p-cresol monomer bands, we obtain the Xe-induced bands (trace 2 in Figure 3), which are quite distinguishable from the monomer bands. The analysis is complicated by the band splitting, which may be due to the matrix-site effect, CD3 group rotation, and/or Fermi resonances. This splitting also makes the assignment of the bands in the CD3 stretching region somewhat uncertain. Nevertheless, the OH stretching bands at 3660.5 and 3656.8 cm−1 and the CD stretching bands at 2232.8, 2138.8, 2074.0, and 2062.2 cm−1 are suitable for the π structure of the 1:1 p-cresol···Xe complex (Table 1), providing direct evidence for the formation of this stronger structure. A disagreement between the experimental and calculated shifts is observed for the higher-frequency CD3 stretching mode, and this is discussed later. Other characteristic modes of this structure display only negligible shifts and/or broadening. In addition to the π structure, our calculations predict the existence of the H-bonded structure with characteristic bands in the OH stretching and deformation regions (Table S1, Supporting Information). The bands suitable for this structure are observed at 3636.0 and 1179/1175/1165 (and possibly 1198) cm−1, respectively. The Xe-induced frequency shift is ca. −23 cm−1 for the OH stretching mode and ca. +9 cm−1 (in average) for the deformation mode, which is consistent with the calculated results (−23.3 and +6.9 cm−1, respectively) for the H-bonded structure (Table 1). The 1198 cm−1 band may correspond to the weaker monomer band at 1176 cm−1, meaning a shift of +20 cm−1. For normal p-cresol, new bands also appear at 3636 and 1176 cm−1 as Xe is added. The experimental shifts (−20.4 and +3.4 cm−1) agree with the calculations for the H-bonded structure (−23.3 and +6.9 cm−1), providing additional evidence that the H-bonded structure is formed in some amounts. It is interesting that the H-bonded structure was not found for the phenol···Xe complex,28 and the reason for this is unclear based on the available experiments and calculations. A series of p-cresol-d3/Xe/Ne matrixes with different Xe concentrations were investigated to support the formation of the 1:1 complexes. All bands mentioned above appear at quite small Xe concentrations and their relative intensities to the cresol monomer bands are nearly proportional to the Xe/Ne concentration ratio (Figure 4a), which indicates that these bands originate from the 1:1 complexes. It should be noted that the H-bonded structure is formed in much smaller amounts than the π structure (∼1/6 as judged by the band intensities), which is consistent with the calculated interaction energies. At higher Xe concentrations, two additional bands in the OH stretching region at 3645 and 3630 cm−1 are observed for both normal p-cresol and p-cresol-d3. The intensities of these bands show a nonlinear dependence on the Xe concentration (Figure 4b,c). A similar behavior was previously observed in the case of the phenol···Xen complexes (n > 1) under similar experimental conditions (Figure 4d).28 The substantial Xe-induced red shifts (for p-cresol ca. −15 and −30 cm−1) are also comparable to those in the phenol···Xen complexes (n = 2−4). On the basis of these results, we explain these two bands at 3645 and 3630 cm−1 by the formation of the p-cresol complexes with several Xe atoms. No bands applicable to the higher-order complexes
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EXPERIMENT Experimental Details. Solid p-cresol CH3C6H4OH (Merck Schuchardt OHG, >98%), liquid toluene C6H5CH3 (SigmaAldrich, 99.9%), and their deuterated analogues with the deuterated methyl groups CD3C6H4OH (CDN Isotopes, 99.6 atom % D) and C6H5CD3 (Cambridge Isotope Laboratories, 98 atom % D) were purified by several freeze−pump−thaw cycles. The gases Ne (AGA, 99.9999%) and Xe (AGA, 99.997%) were used as supplied. The AC/Ne concentration ratios were ∼1/4200 for p-cresol and ∼1/2000 for toluene, and the Xe/AC concentration ratios were up to 30. The relatively low concentration of p-cresol in matrixes is caused by its low vapor pressure (∼0.10 Torr) at room temperature. The gaseous mixtures were deposited onto a CsI substrate typically at 4.3 and 8 K in a closed-cycle helium cryostat (Sumitomo RDK 408D). The IR absorption spectra in the 4000−600 cm−1 spectral range were measured at 4.3 K with a Bruker VERTEX 80 FTIR spectrometer by coadding 200 interferograms using 0.25 cm−1 resolution. Experimental Results and Assignment. Figure 3 (traces 1) shows the FTIR spectra of p-cresol-d3/Ne (1/4200) and pcresol-d3/Xe/Ne (1/2/4200) matrixes deposited at 8 K. It
Figure 3. FTIR spectra of p-cresol-d3/Ne (1/4200) and p-cresol-d3/ Xe/Ne (1/2.5/4200) matrixes recorded after deposition at 8 K in the OH stretching, CD stretching, and deformation regions. Presented are (1) the normal spectra and (2) the spectra showing the Xe-induced bands by subtraction of the monomer bands with an appropriate multiplication factor. The spectra were measured at 4.3 K. C
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Table 1. Characteristic Frequencies and Shifts (cm−1) of the p-Cresol-d3···Xe and Toluene-d3···Xe Complexes calculation
assignment
ω
experiment
π structure
monomer ω
H-bonded structure Δω
ω
monomer
complex
Δω
ω
ω
Δω
−23.2
3661.5 3657.8 2239.5 2226.0 2218.0 2144.8 2075.3 2065.2 1164.2 1178.0
3660.5 3656.8 3636.0 2232.8 2215.0
−1.0 −1.0 −23.6 −6.7 −7.0a
2138.8 2074.0 2062.2 1179.0 1175.0 1165.0
−6.0 −1.3 −3.0 +9.0
2236.5 2215.3 2136.5 2086.0 2064.0
−4.5 −2.0 −1.5 −1.0 −2.0
ν(OH)
3824.6
3823.5
p-Cresol-d3 −1.1 3801.4
ν(CD)asym
2342.2
2342.9
+0.7
2341.7
−0.5
ν(CD)asym ν(CD)sym
2325.9 2203.8
2322.4 2201.4
−3.5 −2.4
2325.7 2203.6
−0.2 −0.2
δ(COH) + δ(CCH)ring
1193.9
1193.9
0.0
1200.8
+6.9
ν(CD)asym
2343.9
2343.9
0.0
ν(CD)asym ν(CD)sym
2325.8 2203.8
2322.0 2201.1
−3.8 −2.7
Toluene-d3
a
2241.0 2217.3 2138.0 2087.0 2066.0
Average value.
Figure 4. Intensities of the complex bands (Ic) relative to the monomer bands (Im) of ACs (p-cresol-d3, normal p-cresol, and phenol) as a function of the Xe/Ne concentration ratio. The ACs/Ne matrix ratio was 1/4200 and the spectra were recorded at 4.3 K after deposition at 8 K. The data for phenol are from ref 28. The lines for the 1:1 complexes are linear fits.
subtraction of the toluene monomer spectrum, the Xe-induced bands are clearly seen at 2236.5, 2215.3, 2136.5, 2086.0, and 2064.0 cm−1, displaying small but distinguishable red shifts relative to the monomer bands, which is consistent with the calculations (Table 1). Other relatively strong modes display only negligible shifts and/or broadening. Figure 5c shows that these Xe-induced bands appear at very low Xe concentration (the Xe/Ne concentration ratio of 1/2000) and rise nearly linearly with the Xe concentration. Thus, we assign these CD stretching bands to the 1:1 toluene···Xe complex.
are observed in other spectral regions, which is also similar to the case of phenol···Xen complexes.28 The sensitivity of the methyl group to the formation of the π structures is fully supported by the experiments with the toluene···Xe complexes. In this case, the H-bonded structure is excluded due to the absence of the OH group and only the π structure is predicted by the calculations. Traces 1 in Figure 5a present the FTIR spectra of toluene-d3/Ne (1/2000) and toluene-d3/Xe/Ne (1/2/2000) matrixes in the CD stretching region. The spectrum in this region changes considerably as Xe is added to the mixture, similarly to the case of p-cresol-d3. After D
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Figure 6. (a) FTIR spectra of p-cresol-d3/Ne (1/4000) and cresol-d3/ N2/Ne (1/5/4000) matrixes in the OH stretching region. The matrixes were deposited at 4.3 K and annealed to 10 K. The spectra were recorded at 4.3 K. (b) Intensities of the p-cresol-d3···N2 complex bands (Ic) relative to the monomer bands (Im) as a function the N2/ Ne concentration ratio. The p-cresol-d3/Ne matrix ratio is 1/4200.
Figure 5. (a, b) FTIR spectra of toluene-d3/Xe/Ne matrixes with different Xe concentrations in the CD stretching region (spectra 1 and 3) and the spectra showing the Xe-induced bands by subtracting the monomer bands with an appropriate multiplication factor (spectra 2 and 4). The spectra were measured at 4.3 K after deposition at 4.3 K and annealing to 10 K. (c) Intensities of the complex bands (Ic) relative to the monomer bands (Im) as a function the Xe/Ne concentration ratio. The toluene-d3/Ne matrix ratio is 1/2000. The line for the 1:1 complexes is a linear fit.
trations and their intensities relative to the cresol monomer bands are nearly proportional to the N2/Ne concentration ratio (Figure 6b). All these data provide strong evidence that the Hbonded structure of the 1:1 p-cresol···N2 complex is formed for low N2 concentrations similarly to the case of phenol.28 The experiments with nitrogen indirectly support our earlier conclusions. First, a linear concentration dependence is a fingerprint of a 1:1 complex. It follows that the p-cresol···Xe and toluene···Xe complexes with similar concentration dependences in the spectrum are the 1:1 structures. Second, the addition of nitrogen to a p-cresol/Ne matrix does not change the spectrum in the methyl stretching region. This shows that the interaction with the OH group does not affect the methyl group significantly, in contrast to the case of Xe, for which the π structure is formed.
At higher concentrations, the bands assigned to the 1:1 toluene···Xe complex are still pronounced but a set of new absorptions (2234.0, 2206.8, 2128.5, and 2057.5 cm−1) appear at lower frequencies in the CD stretching region (spectra 3 and 4 in Figure 5b) as well as in the CH stretching, ring stretching, and bending regions (not shown). These additional bands are observed only at relatively high Xe concentrations (Xe/Ne concentration ratio >1/200). Such delayed formation is demonstrated in Figure 5c, indicating that these bands presumably originate from the complex of toluene with several Xe atoms. We also studied the p-cresol···N2 complex in a Ne matrix (Figure 6a). From the methodological point of view, these experiments are needed because nitrogen is a common impurity in matrix isolation. The addition of a small amount of nitrogen produces a new band in the OH stretching region centered at 3648.5 cm−1, displaying a red shift of about −9 cm−1 relative to the monomer. A similar spectral shift of the OH stretching mode was reported for the 1:1 phenol···N2 complex (−9.2 cm−1), and the band was assigned to the 1:1 H-bonded structure.28 Other characteristic absorptions of the p-cresol···N2 complex (1334.5, 1262.5, and 1185.0 cm−1) are also observed, shifted from the p-cresol monomer bands by ca. +4, +4, and +20 cm−1, respectively. These spectral shifts are quite comparable to the values for the H-bonded structure of the 1:1 phenol···N2 complex (Table 2).28 We did not calculate the p-cresol···N2 complex; however, we expect similar complexation effects on the characteristic vibrational modes of phenol and pcresol. These N2-induced bands appear at low N2 concen-
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CONCLUDING REMARKS The phenol−Xe system previously studied in a Ne matrix was found to be challenging for IR spectroscopy because the most stable 1:1 complex has the π structure without clear spectroscopic fingerprints.28 In this situation, only indirect evidence of the formation of the 1:1 complex was obtained by analyzing the Xe concentration dependence of the vibrational spectra. At low Xe concentrations, practically no changes in the spectrum were observed, which was connected to the formation of the π structure with hidden spectroscopic features. The cases of p-cresol and toluene are principally different from that of phenol due to the presence of the methyl group. The present results show that the methyl group is a sensitive probe of the AC···Xe complexes with the π structure. We used the molecules with the deuterated methyl group possessing a relatively simple spectrum, which allowed us to detect characteristic vibrational shifts in the AC···Xe complexes with E
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Table 2. Experimental Characteristic Frequencies and Shifts (ω and Δω, cm−1) of the p-Cresol-d3···N2 Complex Compared with the Previous Results for the 1:1 Phenol···N2 Complex phenola
p-cresol-d3 experiment
a
calculation, Δω
experiment
assignment
ω
Δω
ω
Δω
H-bonded structure
π structure
ν(OH) δ(CCH)ring + δ(COH) ν(CC) ring + ν(CO) δ(CCH)ring + δ(COH)
3648.5 1334.5 1262.5 1185.0
−9.2 +4.5 +3.7 +20.8
3646.2 1347.7 1265.4 1190.8
−9.2 +3.3 +3.2 +13.8
−12.1 +1.2 +3.1 +19.4
−1.1 +4.9 +0.8 +0.3
From ref 28.
the π structure. In this case, the π structure can be reliably characterized by combination of quantum chemistry calculations and experimental IR spectroscopy in cryogenic matrixes. The bands of the π structure appear at low Xe concentration and their relative intensity is a nearly linear function of the Xe concentration, which is characteristic of the 1:1 complexes. In the case of the p-cresol···Xe complex, the OH stretching bands with small red shifts (∼1 cm−1) were observed also for the π structure, and this is different from the case of phenol, for which similar bands were not detected. In fact, this distinction is consistent with the calculations because the calculated shift of the π structure for this mode of p-cresol-d3 (−1.1 cm−1) is somewhat larger compared to that of phenol (−0.4 cm−1) although these small spectral differences cannot be confidently predicted by the present computational method. In the p-cresol−Xe system, we also observed indications of the 1:1 complex with the H-bonded structure (bands at 1175 and 3636 cm−1). These bands appear at low Xe concentration and correlate with the bands assigned to the π structure. The amount of the H-bonded 1:1 complex is relatively small (∼1/6), which is consistent with the calculated interaction energies (stronger interaction for the π structure). The corresponding complex was not observed in the phenol−Xe system, despite similar interaction energies in these two cases. It is possible that the stabilization barrier of the H-bonded structure is lower in the case of phenol. As an indication of this, the Xe atom in the π structure in the case of phenol is displaced toward the oxygen atom, which is different from the case of pcresol where it is displaced toward the methyl group. In the phenol−Xe system, the new OH stretching bands appeared only at relatively high Xe concentration, and they were explained by the interaction of phenol with several Xe atoms.28 The formation of these higher-order complexes were justified by calculations. For the p-cresol−Xe system, the same behavior is observed for the OH stretching bands at 3630 and 3645 cm−1 and these bands are analogously assigned to the 1:n complexes (n > 1). In contrast, for the N2 complexes of phenol and p-cresol, the well-shifted OH stretching bands appear at low N2 concentration and their relative intensity is a nearly linear function of the N2 concentration, which shows the formation of the 1:1 complex with the H-bonded structure. The cases of p-cresol and toluene are worth discussing with respect to the computational results. The experimental methyl stretching modes in the complex of these ACs with Xe are redshifted, which is in agreement with the calculations for the two lower-frequency transitions of the π structures but disagrees for the highest frequency mode. This mismatch probably indicates the lack of the description at the present computational level. It should also be noted that the splitting of this band makes the comparison of the theory and experiment less certain. Despite
this small disagreement, the similarity for p-cresol and toluene suggests the formation of the π structure in both cases. Indeed, in the case of toluene, the formation of other structures is improbable due to the absence of the OH group. The shifts of the methyl bands predicted for the H-bonded structure of the p-cresol···Xe complex, which is a weaker complex, are much smaller than the experimental values. A partial disagreement between experiment and calculations was observed for the methyl group in the π structure of the toluene···N2O complex, with three experimental blue shifts whereas the calculations predict two blue shifts and one red shift.29 The formation of the π structure of the 1:1 complexes of Xe with phenol, p-cresol, and toluene suggests a general conclusion that these and similar ACs can form the π structures with Xe atoms and the presence of the hydroxyl and/or methyl groups on the aromatic ring does not change the character of the dominating interaction. Similar structures can be presumably formed for aromatic amino acids, for which these ACs are useful models.
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ASSOCIATED CONTENT
S Supporting Information *
The experimental spectra of the monomers in Ne matrixes and the calculated vibrational spectra of the monomers and the Xe complexes of p-cresol-d3 (Table S1), p-cresol (Table S2), toluene-d3 (Table S3), and toluene (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*L. Khriachtchev. E-mail: leonid.khriachtchev@helsinki.fi. Telephone number: +358 294150310. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Q. C. thanks the Academy of Finland for a postdoctoral grant (No. 1139425). This work is also a part of the Project KUMURA of the Academy of Finland (No. 1277993). We acknowledge the use of computer facilities of the M. V. Lomonosov Moscow State University and of the Joint Supercomputer Center of the Russian Academy of Sciences.
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REFERENCES
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