Structure and Infrared Spectrum of the Ag+− Phenol Ionic Complex

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Structure and Infrared Spectrum of the Ag+-Phenol Ionic Complex† Anita Lagutschenkov,‡ Rajeev K. Sinha,§ Philippe Maitre,§ and Otto Dopfer*,‡ Institut fu¨r Optik und Atomare Physik, Technische UniVersita¨t Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany, and Laboratoire de Chimie Physique, Faculte´ des Sciences, UniVersite´ Paris-Sud 11, UMR8000 CNRS, Baˆt. 350, 91405 Orsay Cedex, France ReceiVed: January 28, 2010; ReVised Manuscript ReceiVed: March 5, 2010

The structure and infrared (IR) spectrum of the Ag+-phenol cationic complex are characterized in the gas phase by photodissociation spectroscopy and quantum chemical calculations in order to determine the preferred metal ion binding site. The IR multiple photon dissociation (IRMPD) spectrum has been obtained in the 1100-1700 cm-1 fingerprint range by coupling the IR free electron laser at the Centre Laser Infrarouge d’Orsay (CLIO) with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an electrospray ionization source. The spectroscopic efforts are complemented by quantum chemical calculations at the MP2 and B3LYP levels using the aug-cc-pVTZ basis set. Analysis of the IRMPD spectrum is consistent with a π complex, in which the Ag+ ion binds to the aromatic ring in an η1 (B3LYP) or η2 (MP2) fashion to carbon atoms in the para position of the OH group. Ag+ bonding to the hydroxyl group in the form of a σ complex is calculated to be less favorable. Comparison of the structural and vibrational properties of phenol, Ag+-phenol, and phenol+ suggests partial charge transfer upon formation of the π complex. 1. Introduction Cation-π interactions are among the most important intermolecular binding forces relevant for aromatic recognition in chemical and biological processes, including biochemistry, organometallic chemistry, and catalysis.1-3 To understand the interaction of metal cations with aromatic π electron systems at the molecular level, a plethora of mass spectrometric, spectroscopic, and quantum chemical studies have aimed at the characterization of isolated complexes composed of metal cations and aromatic hydrocarbon molecules. For example, the knowledge about the fundamental interaction between transitionmetal cations and benzene has been reviewed recently.4 The interesting parameters of these cation-π interactions include the magnitude of the various contributions to the interaction energy, such as electrostatic forces (mainly charge-quadrupole), induction, dispersion, and charge transfer.5,6 The latter may be particularly relevant for transition-metal ions.4,7,8 In addition to the interaction strength, the favored structural binding motif is an important property of the interaction. For example, the metal ion can bind to the center or more toward one or several carbon atoms of the aromatic hydrocarbon carbon ring.5,6,8-10 The corresponding configurations are denoted as ηx, whereby x means the number of carbon atoms interacting most strongly with the metal ion (x ) 1-6). Substituted aromatic molecules with regions of high electron density at the functional group (e.g., lone pairs) offer additional attractive binding sites for metal cations, which may efficiently compete with the cation-π bonding to the aromatic ring. In addition, the substitution of functional groups changes the electron density in the aromatic ring, leading to an asymmetric potential for π binding above the ring and thus to variations in †

Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * To whom correspondence should be addressed. E-mail: dopfer@ physik.tu-berlin.de. Fax: (+49) 30-31423018. ‡ Technische Universita¨t Berlin. § Universite´ Paris-Sud 11.

Figure 1. Structures of phenol (a) and various Ag+-phenol isomers evaluated at the B3LYP (c,d) and MP2 levels (b) of theory. Bond distances are given in Å.

the relative bond strength for the various possible ηx types of cation-π interaction.8,11 The present work reports the results of an IR spectroscopic and quantum chemical study of the Ag+-phenol cation complex. Phenol offers two principal competing binding motifs for Ag+, namely, π bonding to the aromatic ring and σ bonding to the lone pairs of the oxygen atom of the hydroxyl group (Figure 1). While the π binding site is of relevance to cation-π interactions, the σ interaction is of interest to guest-host chemistry and molecular recognition phenomena involving metal-oxygen bonding.11 As phenol is the residue of the amino acid tyrosine, it is a possible target for metal ions binding to peptide chains. Ag+ is a closed-shell transition-metal ion with [Kr]4d105s0 configuration, and thus, predominantly electrostatic character of the cation-π interaction may be expected. However, orbital interactions involving

10.1021/jp100853m  2010 American Chemical Society Published on Web 03/23/2010

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electron transfer of the type π f Ag+ (σ donation) and Ag+ f π* (π back-donation) are conceivable, which influence both the energetic and structural parameters of the interaction.4,5,8-10 To the best of our knowledge, no previous experimental and theoretical data have been reported so far for Ag+-phenol. Thus, the present spectroscopic and quantum chemical study provides a first impression of the Ag+-phenol interaction. The potential energy surface of the related Ag+-benzene complex has extensively been characterized by quantum chemical calculations.5,6,9 The potential energy surface is found to be flat above the aromatic ring, and details of the structures obtained are sensitive to the theoretical approach. All calculations agree that the cation-π bond is mainly based on electrostatic interactions. Charge transfer is found to be small due to the large difference in the ionization energies of Ag (7.58 eV) and benzene (9.24 eV).12 A recent high-level calculation yields C6V symmetry for Ag+-benzene with a binding energy of 168 kJ/mol at the CCSD(T)/CBS//MP2/aug-cc-pVDZ level.6 This value is in good agreement with the experimental bond dissociation energies derived from collision-induced dissociation (156.5 ( 7 kJ/mol)13 and radiative association kinetics (162 ( 19 kJ/mol).14 The Ag+-benzene bond energy is relatively low in comparison to related transition-metal ion-benzene complexes. This observation is attributed to a large Ag+ radius and modest electron donation into the π* orbital of benzene.5 As the ionization energy of phenol is significantly lower than that of benzene (8.51 versus 9.24 eV),12 the orbital interactions may increase upon substitution with the OH group, leading to enhanced bonding in Ag+-phenol. A large variety of metal cation-phenol complexes have previously been studied by quantum chemical calculations and mass spectrometric methods.8,10,11,15-19 For nearly all metal ions, π bonding was predicted to be more stable than σ bonding.11 Moreover, the binding energies to phenol derived from mass spectrometry are larger than those to benzene, consistent with the theoretical predictions.11,15 The electrostatic and induction interactions of metal ions with phenol are stronger than those with benzene due to the additional dipole moment of phenol and its larger quadrupole moment and electronic polarizability.17 The lower ionization energy of phenol as compared to that of benzene may also increase the cation-π bonding with metal ions other than alkali ions via enhanced orbital interactions and charge transfer. Although mass spectrometric techniques are convenient tools to derive binding energies of complexes, they are not sensitive to their geometry. In contrast, IR spectroscopy in the O-H stretch range has proven to be a useful tool to determine the binding motif of metal ion-phenol complexes. So far, this approach has been applied for complexes of Na+, K+, and Fe+ with phenol.20,21 Ag+ is quite similar in size to Na+ 22 and thus offers an interesting comparison to the alkali metal ion complexes with phenol.20 The σ and π complexes of K+-phenol and Na+-phenol are calculated to be close in energy (to within a few kJ/mol),11,17 and the interpretation of their IR spectra suggests the π complex to be indeed the slightly more stable configuration for both alkali ions.20 Other related cation-phenol complexes characterized recently by IR spectroscopy include Fe+-phenol21 and H+-phenol.23-26 In both cases, clear preference of cation binding to the aromatic ring is theoretically predicted and experimentally observed. Recently, the Sc+-phenol complex has been characterized by highresolution photoelectron spectroscopy, revealing low-frequency vibrational modes assigned to two rotational isomers of a π-bonded η6 complex with nearly the same energy.27

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Figure 2. Comparison of IRMPD spectra of Ag+-phenol without (a) and with (b) the CO2 laser monitored in the Ag+ fragment channel. The addition of the CO2 laser radiation increases the IRMPD efficiency by a factor of 20.

In general, resonant IR photodissociation is the most widely applied technique to record IR spectra of mass-selected ions and cluster ions.4,28-36 In most cases, pulsed and tunable IR optical parametric oscillator (OPO) lasers have been employed in the 2000-4000 cm-1 spectral range. The present work employs the variant of IR multiple photon dissociation (IRMPD),37-42 which has become a popular technique for IR spectroscopy of strongly bound ions since the successful coupling of tandem mass spectrometers and ion traps with highpower IR free electron lasers (IR-FEL) providing tunable IR photons in the 50-2500 cm-1 spectral range.43,44 The IRMPD approach has recently been applied to a variety of complexes involving transition-metal ions,40,45-48 including complexes of Ag+ with aromatic amino acids22,49 and simple protonated aromatic molecules.39,50-57 While the previous IR spectra of metal ion-phenol complexes have been obtained in the O-H stretch range using OPO lasers,20,21 the IRMPD spectrum of Ag+-phenol recorded in the present work represents the first IR spectrum of a metal ion-phenol complex in the fingerprint range. 2. Experimental and Theoretical Techniques IRMPD spectra of Ag+-phenol were obtained with the tunable IR-FEL at CLIO coupled to a modified Bruker APEXQe FT-ICR mass spectrometer.47 The FEL was operated at 42 MeV to record IRMPD spectra in the 1100-1700 cm-1 range. The FEL output consists of 8 µs long macropulses at a repetition rate of 25 Hz. Each macropulse is composed of 500 micropulses, each a few picoseconds long and separated by 16 ns. For a typical IR average power of 500 mW used in the present work, the corresponding micropulse and macropulse energies are 40 µJ and 20 mJ, respectively. The laser wavelength was monitored with a monochromator coupled to a pyroelectric detector array. The spectral width (fwhm) was less than 0.5% of the central wavelength, which corresponds to 5 cm-1 at 1000 cm-1. The laser power varied between 400 and 600 mW in the frequency range of 800-1600 cm-1, as measured before and after the scan. The laser intensity above 1400 cm-1 was also affected by narrow spectral absorptions due to rovibrational transitions of the bend fundamental of residual water in the FEL laser path. To enhance the fragmentation yield, additional pulsed radiation of a CO2 laser (10 W at cw operation, λ ) 10.6 µm, ν ) 943 cm-1) was employed. The CO2 laser pulses (25 Hz) were synchronized to the IR-FEL macropulses with a delay of a few µs. The width of the CO2 laser pulse was adjusted to 5 ms in order to avoid IRMPD by the CO2 laser radiation alone. Figure 2 shows part

Structure and Infrared Spectrum of Ag+-Phenol of the IRMPD spectrum of Ag+-phenol with and without the CO2 laser, demonstrating the substantial improvement in the IRMPD yield by a factor of ∼20 upon addition of the CO2 laser radiation. The fragment yield using both the FEL and the CO2 laser was around 40% on the strongest resonance at ∼1270 cm-1 observed in the spectral range investigated. The FT-ICR mass spectrometer was equipped with an Apollo II ESI ion source, a quadrupole mass filter, a hexapole accumulation/collision cell, and a 7 T magnet. The solution was prepared by dissolving phenol and AgNO3 in a methanol/water mixture. Mass spectra of ions generated by ESI of this solution demonstrated the successful production of Ag+-phenol complexes. The characteristic isotopic pattern of Ag (m ) 107/109 u) confirmed the proper ion identification and selection of Ag+-phenol (m ) 201/203 u). To obtain the IRMPD spectra of Ag+-phenol, ions were first mass-selected in the quadrupole mass filter and then accumulated for 1 s in the collision cell filled with Ar. Subsequently, they were injected into the ICR trap, where they were irradiated for 5 s by the IR-FEL and the CO2 laser. With the FEL tuned to a vibrational transition, the ions absorb multiple photons of the FEL and the CO2 laser in a stepwise process until the dissociation threshold is reached. By monitoring the intensities of parent (Iparent) and resulting fragment ions (Ifragment) as a function of the laser wavenumber, the IRMPD spectrum was obtained as R ) -ln(Iparent/[Iparent + ∑ Ifragment]). The only fragmentation process observed upon IRMPD generated Ag+ ions and phenol. Despite its multiple photonic nature, the IRMPD spectrum predominantly reflects the absorption of the first IR photon (see ref 37 for a recent review of the IRMPD mechanism). This observation justifies a comparison of the experimental IRMPD spectrum with a calculated linear one-photon IR absorption spectrum.37-42 The IRMPD line width intrinsically depends on the finite laser bandwidth of 0.5% (corresponding to ∆ν ) 2.5-7.5 cm-1 for ν )500-1500 cm-1) and spectral broadening arising from hot bands and the multiphotonic character of the IRMPD process and the resulting heating of the ions during the long irradiation time.37,44,58 Using similar experimental conditions, when ions were thermally cooled through multiple collisions with argon in the hexapole accumulation ion trap at room temperature, IRMPD bands with a width on the order of 15 cm-1 could be observed.47 In the present case, the IRMPD bands have widths between 30 and 65 cm-1 depending on the spectral congestion arising from overlapping transitions. As the laser power is roughly constant in the frequency range of 800-1600 cm-1, with the exception of the power dips arising from the bending fundamental of water, the IRMPD spectra are not normalized. Quantum chemical calculations were performed at the B3LYP and MP2 levels of theory.59 The employed basis sets are a combination of Dunning’s aug-cc-pVDZ or aug-cc-pVTZ basis sets60 for H, C, and O and the Stuttgart effective core potentials for Ag, Cu, and Au (multielectron fit quasi-relativistic ECPs).61-63 As the results obtained with the aug-cc-pVDZ basis are similar to those with the aug-cc-pVTZ basis, only the latter ones are reported here. Calculations were performed for the Ag+-phenol complex to evaluate its geometry, IR spectral features, as well as its lowest-energy fragments Ag+ and phenol in order to determine binding energies and the complexation-induced effects on the structural and vibrational properties of phenol. Similar calculations were performed for Cu+ and Au+ interacting with phenol to facilitate comparison between the three coinage metal ions. The charge distribution was analyzed using the natural bond orbital (NBO) population analysis. Reported binding energies were corrected for basis set superposition error, scaled

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Figure 3. Linear IR absorption spectra of phenol calculated at the B3LYP (a) and MP2 (b) levels are compared to the experimental absorption spectrum (c) taken from the NIST database.12 The IRMPD spectrum of Ag+-phenol (d) is compared to linear IR absorption spectra of various isomers obtained at the MP2 (e) and B3LYP (f,g) levels. See Table 2 for line positions, intensities, and assignments.

harmonic zero-point vibrational energies, and fragment relaxation terms.64,65 Harmonic vibrational frequencies were scaled by a factor of 0.98 at the B3LYP and MP2 levels to establish agreement between the calculated and experimental IR spectra of phenol in the fingerprint range (Figure 3). The experimental IR spectrum of phenol was taken from the NIST database,12 and the assignments were taken from the literature,66,67 using the Wilson notation68 adapted for normal modes of monosubstituted benzene molecules with C2V symmetry. For the comparison of calculated and experimental IR spectra, a convolution width of 20 cm-1 was applied in the simulated IR spectra. 3. Results and Discussion Searches at the B3LYP level yielded two minima on the Ag+-phenol potential (Figure 1). In the global minimum, denoted π4-Ag+-Ph, Ag+ binds above the aromatic ring directly to the C4 atom of phenol in an η1 configuration, with a binding energy of 174 kJ/mol and a Ag-C4 separation of 2.275 Å (Figure 1d). The phenol ring remains nearly planar in this π-bonded structure, and only the C4-H bond points slightly out of the aromatic plane. In the second, about 43 kJ/mol less stable local minimum, denoted σ-Ag+-Ph, Ag+ binds to one of the lone pairs of the oxygen atom of phenol, with a binding energy of 131 kJ/mol and a Ag-O separation of 2.218 Å (Figure 1c). This hydronium-type Ag+-phenol isomer with σ bonding is similar in structure to the related oxonium isomer of protonated phenol.23,24 Interestingly, the calculations at the MP2

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TABLE 1: Selected Bond Distances (in Å) and Angles (in degrees) in Phenol, Phenol+, and Various Ag+-Phenol Isomers Calculated at the B3LYP and MP2 Levels ROH RC1O θCOH RC1C2 RC2C3 RC3C4 RC4C5 RC5C6 RC6C1

Ph MP2

π34-Ag+-Ph MP2

Ph B3LYP

π4-Ag+-Ph B3LYP

σ-Ag+-Ph B3LYP

Ph+ B3LYP

0.9641 1.3697 108.7 1.3933 1.3918 1.3946 1.3929 1.3937 1.3940

0.9666 1.3451 111.0 1.4005 1.4026 1.4194 1.4077 1.3921 1.4023

0.9622 1.3683 109.8 1.3922 1.3882 1.3917 1.3897 1.3902 1.3927

0.9656 1.3358 112.4 1.4044 1.3770 1.4237 1.4210 1.3793 1.4042

0.9650 1.4127 111.3 1.3818 1.3903 1.3902 1.3893 1.3921 1.3824

0.9718 1.3088 113.9 1.4316 1.3652 1.4127 1.4191 1.3641 1.4276

TABLE 2: Experimental Vibrational Frequencies of Phenol, Phenol+, and Ag+-Phenol Isomers Compared to Values Calculated at the B3LYP and MP2 Levelsa modeb

Ph MP2c

Ph B3LYPc

8b (σCC) 8a (σCC) 19a (σCC) 19b (σCC) 14 (σCC) 3 (βCH) 7a (σCO) δOH 9a (βCH) 9b (βCH) σOH

1618 (28) 1608 (46) 1493 (56) 1460 (20) 1440 (8) 1325 (18) 1259 (77) 1171 (138) 1159 (11) 1145 (4) 3745 (69)

1613 (33) 1603 (52) 1501 (53) 1475 (23) 1321 (6) 1348 (23) 1253 (89) 1167 (76) 1168 (50) 1152 (30) 3739 (61)

Ph expd 1603 (F) 1498 (D2) 1465 (D1) 1338 (C) 1255 (B) 1185 (A)

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Ph expe

Ag+-Ph expf

π34-Ag+-Ph MP2c

π4-Ag+-Ph B3LYPc

σ-Ag+-Ph B3LYPc

Ph+ B3LYPc

1610 1603 1505 1472 1349 ? 1261 1174 1168 1150 3656

1542 (E) 1578 (F) 1454 (D)

1568 (48) 1600 (116) 1477 (65) 1449 (2) 1428 (32) 1315 (31) 1294 (79) 1169 (136) 1164 (32) 1142 (0) 3714 (161)

1549 (29) 1595 (282) 1491 (70) 1468 (29) 1355 (35) 1345 (8) 1311 (137) 1166 (167) 1176 (23) 1152 (18) 3700 (182)

1623 (6) 1590 (39) 1494 (44) 1474 (23) 1315 (5) 1346 (20) 1168 (44) 1195 (103) 1176 (22) 1162 (7) 3707 (132)

1514 (21) 1621 (2) 1475 (227) 1422 (30) 1371 (53) 1347 (27) 1366 (30) 1122 (70) 1181 (0) 1167 (49) 3624 (264)

1454 (D) 1319 (C) 1270 (B) 1179 (A)

a

Calculations employ the aug-cc-pVTZ basis for H, O, and C and the Stuttgart ECP basis for Ag. b All modes are in-plane vibrations with a1 or b2 symmetry according to the Wilson notation68 adapted for monosubstituted benzene molecules with C2V symmetry. c Harmonic frequencies scaled by 0.98. IR intensities in km/mol are given in parentheses. d Peak positions from the spectrum in Figure 3c taken from the NIST database.12 e Vibrational frequencies and assignments taken from refs 66 and 67. f Peak positions taken from the IRMPD spectrum in Figure 3d.

level revealed only a single minimum on the Ag+-phenol potential, namely, the π-type isomer denoted π34-Ag+-Ph (Figure 1b). No stable σ isomer could be located at the MP2 level, indicative of a low barrier for σ f π isomerization. The π34-Ag+-Ph structure found at the MP2 level is similar to the π4 isomer obtained at the B3LYP level, with the notable difference that in the former, the Ag+ ion is located directly above the center of the C3-C4 bond (η2), with a Ag-C3/C4 separation of 2.285 Å and a binding energy of 142 kJ/mol. Consequently, both the C3-H and C4-H bonds point slightly out of the aromatic plane in π34-Ag+-Ph. A similar switch of the most stable binding motif from the π4 to the π34 isomer when going from the B3LYP to the MP2 level, as observed here for Ag+-phenol, was also found for the related Cu+-phenol complex.10 The ionization energy of Ag (7.58 eV)12 is around 1 eV lower than that of phenol (8.51 eV),69,70 suggesting that the positive charge in [Ag-phenol]+ is indeed largely localized on the Ag atom. However, as the binding energy of the complex is of the same order (174 kJ/mol ) 1.8 eV), substantial charge transfer upon complex formation appears feasible. Phenol is a closedshell molecule, and the highest occupied molecular orbital (HOMO) in its ground electronic state is a filled π orbital delocalized over the aromatic ring. Ag+ has also a closed-shell configuration, [Kr]4d105s0. Charge transfer may then involve transfer of electron population (σ donation) from the HOMO of phenol into the empty 5s orbital of Ag+ (π f Ag+). In addition, π back-donation may occur via electron transfer from the occupied 4d orbitals of Ag+ into the vacant lowest unoccupied molecular orbital (LUMO) of phenol (Ag+ f π*). From a detailed analysis of the bonding contributions in the related Ag+-C2H4 complex,71 it is expected that σ donation dominates the covalent contribution of the interaction, whereas π back-donation is less important. Indeed, this view is supported

by the NBO analysis for the π4-Ag+-Ph isomer at the B3LYP level, which reveals a net partial charge transfer from Ag+ to phenol of 0.17 e due to significant overlap between the HOMO of phenol and the 5s orbital of Ag+. On the other hand, charge transfer is predicted to be small for the σ-Ag+-Ph isomer (0.05 e) as the overlap between these two interacting orbitals is unfavorable. In contrast to the B3LYP level, the NBO analysis at the MP2 level yields less electron transfer for the π-bonded structure (0.02 versus 0.17 e), which is also correlated with a lower binding energy (142 vs 174 kJ/mol). As the B3LYP method tends to overestimate charge transfer, the magnitude of the contribution of orbital interactions is difficult to quantify at the present stage. As expected, the various binding sites of Ag+ have different impact on the geometry of phenol and the corresponding IR spectrum. The relevant geometrical parameters are listed in Table 1, whereas vibrational frequencies and IR intensities of the modes occurring in the investigated fingerprint range are summarized in Table 2. The simulated IR spectra are compared in Figure 3 with the corresponding experimental spectra. All fundamentals occurring in this spectral range are in-plane vibrations with a1 or b2 symmetry according to the Wilson notation68 adapted for monosubstituted benzene molecules with C2V symmetry. These include C-C stretch modes (σCC ) ν8a, ν8b, ν19a, ν19b, ν14), the C-O stretch (σCO ) ν7a), C-H bends (βCH ) ν3, ν9a, ν9b), and the C-O-H bend (δOH). For comparison, also the properties of the O-H bond are included in the tables, although the O-H stretch mode (σOH) has not been investigated experimentally yet. Before discussing the structural and spectroscopic properties of the Ag+-phenol isomers, the corresponding parameters of bare phenol are considered in order to test the performance of the theoretical procedures. As is evident from Tables 1 and 2, the structural and IR spectral properties of phenol determined

Structure and Infrared Spectrum of Ag+-Phenol at the MP2 and B3LYP levels are in reasonable agreement with each other (Figure 3a,b). In the fingerprint range, all theoretical frequencies (except ν14) agree to within 20 cm-1 with the available experimental values (Table 2). The major difference between both theoretical levels lies in the prediction of ν14, which shows a surprisingly large difference in frequency (1440 vs 1321 cm-1). As the IR intensity of this mode is low, comparison with the experimental spectrum in Figure 3c does not help to decide which level is closer to experiment. The experimental spectrum in Figure 3c is taken from the NIST database,12 and the measured band positions labeled A-F are listed in Table 2. The widths of isolated transitions in this spectrum taken at room temperature are typically 30 cm-1 (e.g., band B assigned to σCO ) ν7a at 1255 cm-1). For the π4-Ag+-Ph isomer, the NBO analysis yields a positive charge of +0.83 e on Ag+, implying that significant electron density of the HOMO of phenol is transferred to Ag+. As a consequence of this reduction of electron population in the bonding HOMO orbital of π character, the aromatic ring shows an overall expansion. For example, the two C-C bonds adjacent to the Ag-C4 bond, C3-C4 and C4-C5, expand by 0.03 Å. Similarly, the C1-C2 and C1-C6 bonds expand by 0.01 Å, and only the C2-C3 and C5-C6 bonds contract slightly (by 0.01 Å). As a consequence, the frequencies of the C-C stretch modes (σCC) tend to be reduced upon formation of the π4-Ag+-Ph complex. The largest red shift is observed for ν8b (-64 cm-1), featuring a considerable stretching contribution of the C3-C4 and C4-C5 bonds, which both are largely elongated upon Ag+ complexation. Interestingly, the charge transfer in π4-Ag+-Ph causes a substantial contraction of the C-O bond (0.025 Å), which in turn increases the C-O stretch frequency (σCO ) ν7a) by 58 cm-1 from 1253 to 1311 cm-1. In contrast to the C-O and C-C stretch modes, the C-H and O-H bend frequencies (βCH, δOH) are nearly unaffected. The O-H stretch frequency decreases slightly by 39 cm-1, in line with the modest elongation of the O-H bond upon Ag+ complexation (∆ROH ) +0.0034 Å). The effects upon Ag+ complexation for the π34Ag+-Ph isomer evaluated at the MP2 level are qualitatively similar to those obtained for the π4-Ag+-Ph isomer at the B3LYP level (see Tables 1 and 2 and Figure 3). There are subtle differences due to the slightly different binding sites (π34 versus π4), but the overall trends are comparable. In contrast to π4-Ag+-Ph, charge transfer from Ag+ to phenol is substantially smaller for the σ-Ag+-Ph isomer (0.05 e at the B3LYP level). Consequently, σ bonding of Ag+ to phenol has only a minor effect on the C-C bond lengths (|∆RC-C| e 0.01 Å) and corresponding C-C stretch frequencies (|∆σC-C| e 13 cm-1). Similar arguments apply to the βCH modes. However, the C-O bond drastically elongates (by 0.044 Å), giving rise to a substantial reduction of 85 cm-1 in σCO ) ν7a. As expected, δOH is also sensitive to σ bonding of Ag+ to the OH group, with an increase of 28 cm-1. Surprisingly, the effects on the O-H bond properties are modest, with ∆ROH ) +0.0028 Å and ∆σOH ) -32 cm-1. These changes are smaller than those induced by π bonding. Thus, in contrast to the naive expectation, IR spectroscopy in the O-H stretch range is rather insensitive to the binding site, π versus σ.21 Comparison between the simulated IR spectra of π4-Ag+-Ph and σ-Ag+-Ph at the B3LYP level reveals that there are essentially two isolated transitions, which can be used to distinguish between both isomers in the fingerprint range at the employed spectral resolution, namely, σCO ) ν7a and σCC ) ν8b. The differences in other modes are either smaller than the spectral resolution or blurred by spectral overlap (e.g., δOH).

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11057 The IRMPD spectrum of Ag+-phenol monitored in the Ag+ channel (Figure 3d) exhibits six distinct absorption features in the 1100-1700 cm-1 fingerprint range denoted A-F, with band centers (fwhm widths) of 1179 (65), 1270 (45), 1319 (40), 1454 (45), 1542 (30), and 1578 (30) cm-1, respectively (Table 2). The widths are essentially comparable to those of the absorption spectrum of phenol at room temperature (30 cm-1), suggesting that the dominant contribution to the widths of the IRMPD bands arises from unresolved rotational substructure. The effects of the limited laser bandwidth (∼5 cm-1), the broadening of the multiple photon absorption process, and spectral congestion provide further contributions to the widths. Comparison of the IRMPD spectra of Ag+-phenol with those simulated at the B3LYP and MP2 levels clearly reveals that the dominant carrier of the experimental spectrum is the π isomer. The agreement of the IRMPD spectrum is best for the MP2 spectrum of the π34-Ag+-Ph isomer (although the agreement with the B3LYP spectrum of the π4-Ag+-Ph isomer is also acceptable). Consequently, this is the favored assignment and is discussed in the following in some detail. In general, the overall appearance of the IRMPD spectrum of Ag+-phenol and the linear absorption spectrum of phenol is comparable. Two effects may contribute to differences and have to be considered when comparing both experimental spectra. First, Ag+ complexation may change frequencies and IR intensities of certain vibrational transitions, and these effects can be evaluated by comparing the complexation effects in the simulated spectra. Second, due to the multiple photon absorption process, the IRMPD mechanism may shift transitions to lower frequency by some 10-30 cm-1 and also affect, to a certain extent, IR intensities. This red shift is essentially connected to vibrational cross anharmonicities and may therefore be sensitive to the type of normal mode considered. For a detailed discussion of these IRMPD effects, the reader is referred to ref 44. Band A at 1179 cm-1 in the IRMPD spectrum of Ag+-phenol is largely attributed to δOH of π34-Ag+-Ph. There is essentially no shift with respect to the corresponding transition of bare phenol observed in the NIST spectrum (1185 cm-1), in agreement with the theoretical prediction. Band B at 1270 cm-1 in the IRMPD spectrum is cleary assigned to σCO ) ν7a of π34Ag+-Ph. This band displays a blue shift of ∼15 cm-1 compared to the phenol transition and is thus a clear-cut signature of the π isomer. The blue shift predicted at the MP2 level of 35 cm-1 somewhat overestimates the experimental shift, possibly due to the effects of the IRMPD mechanism. Band C at 1319 cm-1 in the IRMPD spectrum is associated with ν3 of π34-Ag+-Ph. It displays a red shift of 18 cm-1 compared to the corresponding band in the phenol NIST spectrum, which is slightly larger than the predicted complexation shift of 10 cm-1. Band D centered at 1454 cm-1 in the IRMPD spectrum is largely attributed to ν19a of π34-Ag+-Ph, possibly with overlapping weaker contributions of ν14 and ν19b. The substantial red shift of ν19a of 44 cm-1 from the corresponding phenol transition (band D2) is compatible with the predicted value (16 cm-1). Bands E and F at 1542 and 1578 cm-1 in the IRMPD spectrum are assigned to ν8b and ν8a of π34-Ag+-Ph, respectively. Their relative intensities are significantly reduced due to atmospheric water absorptions12 along the IR laser path in this spectral range. Both transitions are largely red-shifted by 61 and 25 cm-1 from the corresponding transitions in bare phenol, which occur as a single unresolved band F at 1603 cm-1. These red shifts are in good agreement with those predicted at the MP2 level (50 and 8 cm-1). In particular, the observation of band E at 1542 cm-1 (ν8b) is a unique spectral fingerprint of the π isomer. In

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conclusion, the comparison of the IRMPD spectrum of Ag+-phenol with those predicted for the π and σ isomers clearly indicates that the π isomer is the dominant carrier of the experimental spectrum, although minor contributions of the σ isomer cannot be completely ruled out. In particular, the high relative intensity of band B attributed to the isolated ν7a band of only the π34-Ag+-Ph isomer supports this conclusion. Moreover, the effects of Ag+ complexation at the π binding site on the structure and vibrational properties are wellreproduced by the experimental spectra. As the NBO analysis for the π isomer of Ag+-phenol suggests significant partial charge transfer (0.17 e at the B3LYP level), it is illustrative to compare its structural and vibrational properties not only with those of neutral phenol but also with those of the phenol+ cation. The phenol cation and its complexes were thoroughly studied, for example, by high-resolution photoelectron spectroscopy69,70,72-74 and IR spectroscopy.35,75,76 Ionization of phenol is achieved by removal of one electron from the HOMO with b1 symmetry, leading to the 2B1 radical cation electronic ground state.77 This delocalized π orbital has a large oxygen contribution mixed in. Removal of one electron of this bonding π orbital leads to the expansion of the aromatic ring and, at the same time, to a contraction of the C-O bond and an elongation of the O-H bond (Table 1). The latter effect is due to the strong antibonding character of the HOMO across the C-O linkage, which is reduced upon ionization. Thus, ionization leads to reduction of the C-C and O-H stretch frequencies, whereas the C-O stretch frequency is largely increased. Also, the C-O-H bond angle, θCOH, opens up upon ionization (Table 1). In general, the structural and vibrational parameters of the π isomer of Ag+-phenol are between those of phenol and phenol+, supporting the prediction of partial charge transfer from Ag+ to phenol at the B3LYP level. For example, the measured values for ν7a increase from 1261 to 1395 cm-1 upon ionization (i.e., ∆ν7a ) +134 cm-1),77 whereas σOH decreases from 3656 to 3534 cm-1 (∆σOH ) -122 cm-1).35,78 The corresponding changes predicted for Ag+ complexation of phenol at the B3LYP level are substantially smaller for the π4-Ag+-Ph isomer (∆ν7a ) +58 cm-1, ∆σOH ) -39 cm-1), indicating that much of the positive charge still remains on the Ag atom. It is interesting to compare the cation-π interaction for benzene and phenol complexes with the three coinage metal ions M ) Cu, Ag, and Au. Extensive studies of M+-benzene established that the cation-π interaction increases in the order Ag+ < Cu+ < Au+.4-6,9 For example, the bond energies for M+-benzene are predicted to be 168, 234, and 269 kJ/mol for Ag, Cu, and Au at the CCSD(T)/CBS//MP2/aug-cc-pVDZ level,6 in good accordance with the experimental values of 156 ( 7, 218 ( 21, and 289 ( 29 kJ/mol, respectively. This order of binding energies, Ag+ < Cu+ < Au+, is also observed for π bonding of coinage metal ions to nonaromatic hydrocarbon molecules, such as C2H4.71 Comparison of the ionization energies12 of M ) Cu (7.73 eV), Ag (7.58 eV), and Au (9.23 eV) with that of benzene (9.24 eV) suggests that the positive charge in these M+-benzene complexes is largely localized on the metal atom for M ) Cu and Ag. Strong covalent bond contributions via electron transfer are predicted for [Au-benzene]+ due to their almost identical ionization energies.6,9 For example, the NBO analysis at the MP2/aug-cc-pVTZ level yields charge transfer of -0.04, 0.02, and 0.21 e for M+-benzene with M ) Cu, Ag, and Au, respectively.6 Comparison between M+-phenol and M+-benzene reveals that introduction of the hydroxyl group enhances the cation-π

Lagutschenkov et al. interaction.11,17 For alkali metal ions, this effect is mainly ascribed to enhanced electrostatic and induction interactions, due to the additional dipole moment of phenol and its larger quadrupole moment and electronic polarizability.17 Moreover, the lower ionization energy of phenol as compared to that of benzene (8.51 versus 9.24 eV)12 increases the possibility for orbital interactions and charge transfer, in particular, in cation-π interactions with transition-metal ions. The present MP2 calculations yield binding energies of 142, 213, and 304 kJ/mol for M+-phenol with M ) Ag, Cu, and Au for dissociation into M+ and phenol, respectively. The corresponding numbers at the B3LYP level are 174, 233, and 283 kJ/mol, respectively. It is noted that for [Au-phenol]+, the lowest-energy dissociation channel corresponds in fact to Au and phenol+ due to the lower ionization energy of phenol. The Ag+-benzene bond energy of 159 kJ/mol at the B3LYP level is indeed slightly smaller than that for Ag+-phenol (174 kJ/mol). Moreover, it compares very well with the most reliable experimental values of 156.5 ( 713 and 162 ( 19 kJ/mol14 for the Ag+-benzene bond energy, suggesting that this theoretical level reliably reproduces the cation-π interaction in Ag+ complexes with aromatic hydrocarbon molecules. The corresponding MP2 values of 138 and 142 kJ/mol for Ag+-benzene and Ag+-phenol are somewhat lower than the B3LYP energies and the available experimental data, implying that the MP2 level underestimates the cation-π interaction using the aug-cc-pVTZ basis. The lowest-energy structures localized at the MP2 level are π34(η2) for Ag+phenol, π(η6) for Cu+-phenol, and π4(η1) for Au+-phenol, whereas at the B3LYP level, π4(η1), π(η6), and π4(η1) are obtained, respectively. The preference for η1 structures at the B3LYP level and η2 structures at the MP2 level was also observed previously for Cu+-phenol.10 Whereas for Ag+phenol and Au+-phenol no previous data appear to be available in the literature, the Cu+-phenol complex has been studied by both quantum chemical calculations10,11 and mass spectrometry.8 For example, the calculated binding energies for Cu+-phenol are 213 kJ/mol at the B3LYP/6-311G(d,p) level8 and 218 kJ/ mol at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level,10 both in good agreement with the experimental value of 210.3 ( 11.6 kJ/mol.8 The latter value is in line with both the MP2 and B3LYP values obtained in the present work, 213 and 233 kJ/mol, again adding confidence to the theoretical approaches employed. The NBO charges on the metal atom in M+-phenol are 0.83, 0.82, and 0.59 e for M ) Ag, Cu, and Au (B3LYP level), in line with the trend in the binding energy. The charge on Ag is slightly lower in Ag+-phenol as compared to that in Ag+-benzene (0.83 versus 0.88 e), again consistent with the stronger bonding in the former complex (174 versus 159 kJ/mol). 4. Conclusions The interaction between Ag+ and phenol has been characterized in some detail by IR spectroscopy and quantum chemical calculations. Comparison between the IRMPD spectrum recorded in the fingerprint range and IR spectra calculated at the MP2 and B3LYP levels indicates that the π isomer is the dominant carrier of the experimental spectrum, although minor contributions of the less stable σ isomer cannot be completely ruled out. Clearly, at all levels of theory considered, the cation-π interaction is energetically more favorable than the σ interaction with the OH substituent. This observation is in agreement with previous theoretical results obtained for a variety of related M+-phenol complexes11,17 and the limited spectroscopic data available for phenol complexes with Na+, K+, and

Structure and Infrared Spectrum of Ag+-Phenol Fe+.20,21 The cation-π bond in Ag+-phenol is calculated to be stronger than that of Ag+-benzene, in line with available data for other M+-phenol and M+-benzene.11 On the basis of the present theoretical results for Ag+-phenol/benzene and the measured binding energy of Ag+-benzene (156.5 ( 7 kJ/mol),13 the strength of the cation-π bond in Ag+-phenol is estimated to be ∼170 ( 15 kJ/mol. Comparison of the structural and vibrational properties of phenol, Ag+-phenol, and phenol+ suggests partial charge transfer upon formation of the π complex. However, in line with previous conclusions for Ag+-benzene, the contribution of charge transfer to the binding energy of Ag+-phenol is concluded to be small but detectable via frequency shifts of phenol vibrations upon Ag+ complexation (e.g., ∆ν7a). This is in contrast to the interaction of phenol with the other coinage metal ions, Cu+ and Au+, which exhibits much stronger bonding due to significant orbital interactions. Acknowledgment. This study was supported by the Deutsche Forschungsgemeinschaft (DO 729/2), the Fonds der Chemischen Industrie, and the European Union. We thank M. Schlangen (TU Berlin), A. Springer (FU Berlin), and the CLIO team (J. M. Ortega, C. Six, G. Perilhous, and J. P. Berthet) for their support during the experimental campaign. O.D. appreciates the fruitful discussions about phenol complexes with his scientific mentor, long-standing collaborator, and friend K. Müller-Detlefs. References and Notes (1) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210. (2) Ma, J. C.; Dougherty, D. A. Chem. ReV. 1997, 97, 1303. (3) Dougherty, D. A. Science 1996, 271, 163. (4) Duncan, M. A. Int. J. Mass Spectrom. 2008, 272, 99. (5) Bauschlicher, C. W.; Partridge, H.; Langhoff, S. R. J. Phys. Chem. 1992, 96, 3273. (6) Yi, H. B.; Lee, H. M.; Kim, K. S. J. Chem. Theory Comput. 2009, 5, 1709. (7) Elschenbroich, C. Organometallics, 3rd ed.; Wiley: Weinheim, Germany, 2006. (8) Ruan, C. H.; Yang, Z. B.; Rodgers, M. T. Phys. Chem. Chem. Phys. 2007, 9, 5902. (9) Dargel, T. K.; Hertwig, R. H.; Koch, W. Mol. Phys. 1999, 96, 583. (10) Corral, I.; Mo, O.; Yanez, M. Int. J. Mass Spectrom. 2006, 255, 20. (11) Dunbar, R. C. J. Phys. Chem. A 2002, 106, 7328. (12) Linstrom, P. J.; Mallard, W. G. NIST Chemistry WebBook; NIST Standards and Technology: Gaithersburg MD, 2001 (http://webbook.nist. gov). (13) Chen, Y. M.; Armentrout, P. B. Chem. Phys. Lett. 1993, 210, 123. (14) Ho, Y. P.; Yang, Y. C.; Klippenstein, S. J.; Dunbar, R. C. J. Phys. Chem. A 1997, 101, 3338. (15) Ryzhov, V.; Dunbar, R. C. J. Am. Chem. Soc. 1999, 121, 2259. (16) Hoyau, S.; Norrman, K.; McMahon, T. B.; Ohanessian, G. J. Am. Chem. Soc. 1999, 121, 8864. (17) Amunugama, R.; Rodgers, M. T. J. Phys. Chem. A 2002, 106, 9718. (18) Zierkiewicz, W.; Michalska, D.; Cerny, J.; Hobza, P. Mol. Phys. 2006, 104, 2317–2325. (19) Milko, P.; Roithova, J.; Schroder, D.; Lemaire, J.; Schwarz, H.; Holthausen, M. C. Chem.sEur. J. 2008, 14, 4318. (20) Vaden, T. D.; Lisy, J. M. J. Chem. Phys. 2004, 120, 721. (21) Altinay, G.; Metz, R. B. J. Am. Soc. Mass. Spectrom. 2010, DOI: 10.1016/j.jasms.2010.01.006. (22) Polfer, N. C.; Oomens, J.; Moore, D. T.; von Helden, G.; Meijer, G.; Dunbar, R. C. J. Am. Chem. Soc. 2006, 128, 517. (23) Solca`, N.; Dopfer, O. Chem. Phys. Lett. 2001, 342, 191. (24) Solca`, N.; Dopfer, O. J. Am. Chem. Soc. 2004, 126, 1716. (25) Solca`, N.; Dopfer, O. J. Chem. Phys. 2004, 120, 10470. (26) Solca`, N.; Dopfer, O. J. Chem. Phys. 2004, 121, 769. (27) Zhang, C. H.; Krasnokutski, S. A.; Zhang, B.; Yang, D. S. J. Chem. Phys. 2009, 131, 9. (28) Okumura, M.; Yeh, L. I.; Lee, Y. T. J. Chem. Phys. 1985, 83, 3705. (29) Lisy, J. M. J. Chem. Phys. 2006, 125, 19. (30) Duncan, M. A. Int. J. Mass Spectrom. 2000, 200, 545. (31) Duncan, M. A. Int. ReV. Phys. Chem. 2003, 22, 407. (32) Robertson, W. H.; Johnson, M. A. Annu. ReV. Phys. Chem. 2003, 54, 173.

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