Infrared Spectroscopy of PhenolâTriethylsilane Dihydrogen-Bonded

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Infrared Spectroscopy of the Dihydrogen-Bonded PhenolTriethylsilane Cluster and its Cationic Analogues: Intrinsic Strength of the Si-H•••H-O Dihydrogen Bond Haruki Ishikawa, Takayuki Kawasaki, and Risa Inomata J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5097508 • Publication Date (Web): 05 Jan 2015 Downloaded from http://pubs.acs.org on January 12, 2015

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The Journal of Physical Chemistry

Infrared Spectroscopy of Phenol-Triethylsilane Dihydrogen-Bonded Cluster and its Cationic Analogues: Intrinsic Strength of the Si−H···H−O Dihydrogen Bond Haruki Ishikawa,* Takayuki Kawasaki, Risa Inomata Department of Chemistry, School of Science, Kitasato University, Minami-ku, Sagamihara 2520373, Japan

ACS Paragon Plus Environment

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ABSTRACT Spectroscopic and theoretical investigations have been carried out for neutral phenoltriethylsilane clusters to reveal an intrinsic nature of the Si−H···H−O type dihydrogen bond. Based on the laser-induced fluorescence and infrared spectra, four isomers are identified. Three of them have a structure in which the dihydrogen bond and the dispersion interaction are competing in the stabilization of the cluster. However, the other isomer is found to have a distinct structure in which the dihydrogen bond is much stronger than the other isomers. In addition to the neutral clusters, cationic phenol+-diethylmethylsilane and phenol+-triethylsilane clusters are investigated by infrared photodissociation spectroscopy. It is found that the dihydrogen bond is the dominant intermolecular interaction in these clusters. Based on the redshifts of the OH stretching bands, it is revealed that the strength of the Si−H···H−O dihydrogen bond is stronger than that of the π-type hydrogen bond. The proton affinities of triethylsilane and diethylmethylsilane estimated by the theoretical calculation are larger than those of benzene and ethylene. This results is consistent with our experimental observations.

KEYWORDS. Hydrogen bond, Laser Spectroscopy, Molecular Structure, Proton affinity, Intermolecular Interaction.


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I. INTRODUCTION Recent technical developments of infrared spectroscopy for gas-phase molecular clusters and quantum chemical calculations enable us to determine hydrogen-bonded structures of many systems.1-5 In addition to the well-known σ- and π-type hydrogen bonds, unconventional types of hydrogen bonds, such as CH···π interaction6, have been investigated both by experimentally and theoretically. Dihydrogen bond is known to be one of the unconventional hydrogen bonds. When a hydrogen atom is bonded to an electropositive atom, such as B, Al, and transition metal elements, the hydrogen atom has a partial negative charge.

Then, a hydrogen-bond type

interaction are formed between the oppositely charged two hydrogen atoms. This interaction is called “dihydrogen bond”. One of first reports suggesting the existence of the dihydrogen bond is a study on the crystal structure of cis-[IrH(OH)(PMe3)4]PF6.7,8

The short value of the

Ir−H···H−O distance indicates an attractive interaction between two hydrogen atoms. After this observation, the dihydrogen bonds in the system involving transition metals and boron in their crystal forms were extensively investigated.9-16 Theoretical investigations on these systems have also been carried out so far.17-22 The dihydrogen bond interaction is one of the active sub-class of hydrogen bond interactions.23-30 Spectroscopic measurements are important to understand the nature of intermolecular interaction. In 2000, the first spectroscopic evidence of the dihydrogen bond in the gas-phase molecular cluster was reported.31 Patwari and coworkers reported on the B−H···H−O and B−H···H−N dihydrogen bonds investigated by infrared spectroscopy and quantum chemical calculations.31-38 In addition to their studies on the dihydrogen-bonded systems involving the B−H group, Ishikawa and coworkers reported the spectroscopic study on the Si−H···H−O dihydrogen bond in the phenol(PhOH)-diethylmethylsilane (DEMS) cluster.39 This is the first


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report of the dihydrogen-bond involving the Si–H group.

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Following this work, an IR

spectroscopy on PhOH-triethylgermanium hydrate, which is a germanium analogue of the PhOH-DEMS cluster, was reported.40

Theoretical studies related on these system are also

reported, so far.41,42 The Si−H···H−O dihydrogen bond is considered to be an intermediate motif in the reaction such as H2O + SiH4 → SiH3OH + H2.43 To gain a better understanding of the nature of the Si−H···H−O dihydrogen bond, it is necessary to collect further spectroscopic information. In the PhOH-DEMS, however, it was suggested that the dihydrogen-bond interaction is competing with the dispersion interaction and that the cluster structure is determined by the balance of these competing interactions.39 Thus, the strength of the Si−H···H−O was considered to be weak and comparable to that of the dispersion interaction. To reveal intrinsic nature of the Si−H···H−O dihydrogen-bond, a system in which the dihydrogen bond is a dominant interaction compared to the other intermolecular interactions must be investigated. In the present study, we carried out Laser-induced fluorescence (LIF) and IR spectroscopies and theoretical calculations on jet-cooled PhOH-triethylsilane (TES) cluster. In the course of the study, we found an isomer of PhOH-TES that has a much stronger Si−H···H−O dihydrogen bond compared with the other isomers. This observation enabled us to discuss on the intrinsic strength of the Si−H···H−O dihydrogen-bond. In addition, we also carried out IR photodissociation spectroscopy and theoretical calculations on cationic PhOH+-DEMS and PhOH+-TES. Since PhOH+ is more acidic than neutral PhOH, an effect of the dihydrogen bond is expected to be emphasized. Such systems are good objects to investigate the character of the dihydrogen bond involving the Si−H group. In the present paper, we report the results of the spectroscopic


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measurement and the theoretical calculations of neutral PhOH-TES and cationic PhOH+-DEMS and PhOH+-TES and discuss the nature of the Si−H···H−O dihydrogen-bond. II. EXPRERIMENTAL AND THEORETICAL METHODS In the present study, spectroscopic measurements for neutral and cationic clusters were carried out. In the case of the neutral PhOH-TES cluster, a conventional supersonic jet apparatus was used. To produce the PhOH-TES clusters, a mixed vapor of PhOH and TES diluted by He was supersonically expanded through a pulsed nozzle into a vacuum chamber. The vapor pressure of the TES was controlled by keeping the temperature of liquid TES sample at around −15 °C. The PhOH sample was treated at the room temperature. A frequency-doubled output of the tunable dye laser (Lumonics HD500) pumped by a Nd:YAG laser (Continuum NY61) was used to excite clusters to their electronic excited S1 states. Ultraviolet (UV) laser beam was irradiated at the right angle to the supersonic jet. Fluorescence from the S1 state was collected by a lens system in a direction perpendicular to both the jet and the laser beam. LIF spectra were recorded by measuring fluorescence intensity against the UV laser wavenumbers. Infrared (IR) spectra of the neutral PhOH-TES clusters were measured by means of IR-UV double resonance technique.1,44 In this measurement, the UV laser wavenumbers was fixed to an electronic band of certain species. The IR laser pulse was introduced several 10 ns prior to the UV laser pulse. Since the fluorescence intensity reflects the population of the initial level of the UV excitation, the IR absorption is detected as a depletion of the fluorescence intensity. When we discriminated the vibronic bands of each isomers in the LIF spectrum from others, IR-UV hole-burning spectroscopy was carried out. In this measurement, the IR laser wavenumber was fixed to the IR band of a certain isomer, while the UV wavenumbers were scanned. Then, the hole-burning spectrum was obtained as a difference in the intensities of the LIF spectra with IR excitation


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from that without IR. An output of an opto-parametric oscillator/amplifier system (LaserVision) pumped by an injection-seeded nanosecond Nd:YAG laser (Continuum Powerlite 8000) was used for the IR light. In the case of the cationic PhOH+-DEMS/TES clusters, an IR photodissociation spectroscopy was carried out. In this measurement, we used a temperature-variable 22-pole ion trap apparatus45-47 as a simple tandem mass spectrometer, where the 22-pole ion trap was used as an ion-guide. The PhOH+-DEMS and PhOH+-TES clusters were produced as follows. The gaseous mixture of the PhOH and DEMS (or TES) diluted by He was supersonically expanded into the vacuum chamber. Just at the exit of the pulsed nozzle, the UV laser beam was irradiated to the jet to ionize PhOH monomer. The wavenumber of the UV laser light was fixed to that of the S1S0 origin band of the PhOH monomer. The PhOH+-DEMS/TES was produced by collisions among photoionized PhOH, DEMS (or TES) and He. The IR spectra of PhOH+-DEMS/TES were recorded by IR photodissociation spectroscopy. In the present measurements, the signal of PhOH+ was monitored as a photo-fragment. IR spectra of gaseous DEMS and TES at the room temperature were recorded by FT-IR spectrometer (JASCO FT/IR-4200) with the resolution of 2.0 cm-1. The wavenumbers of the visible output of the dye laser were calibrated by means of an optogalvanic spectroscopy of Ne atom. The IR laser wavenumbers were calibrated by the absorption of gaseous water in the ambient atmosphere. In the present study, local minimum structures and vibrational wavenumbers of both the neutral and cationic clusters were obtained by density functional theory (DFT) calculations using Gaussian 09 program package.48 In the previous study, it was revealed that the dispersion


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interaction must be taken into account to obtain appropriate structures of the PhOH-DEMS clusters.39 We confirmed that the DFT calculation with the M05-2X functional provided the same optimized structure of the PhOH-DEMS obtained by the MP2 calculation in the previous study. Therefore, we used M05-2X functional in the present study. In the optimization procedure, initial surveys of the local minima or isomers were carried out by using the 631++G(d, p) basis set. Then the final optimization and the vibrational analysis were performed using the 6-311++G(3d, 2p) basis set. All the optimizations were followed by the vibrational analysis to confirm that the obtained structures correspond to local minima. When relative energies among isomers for each clusters were evaluated, a zero-point energy correction was included. In addition, a basis-set superposition error was also corrected by the counterpoise method implemented in Gaussian 09. In the vibrational analysis, wavenumbers of the vibrational transitions were scaled by factors that determined by matching the experimental and theoretical νOH of the neutral PhOH monomer or cationic PhOH+ monomer, respectively. The values of the scale factors of 0.9362 and 0.9332 were used for the neutral and cationic clusters, respectively. In addition to the vibrational analysis, we performed a natural bond orbital (NBO) analysis in the present study. Natural charges of the atoms in the clusters were also obtained by the NBO (2)

analysis. The second-order perturbation energies, Ei→j* of the charge-transfer type interaction i j*

from orbitals i to j* is expressed as Εi→j* = −2 ε (2)

εi j*

is a Fock operator, ε denotes the , where F

(2)

orbital energy, respectively.49 The values of Ei→j* and NBOs were obtained by the programs implemented in Gaussian 09. In the case of the PhOH+-DEMS and PhOH+-TES clusters, the (2)

Ei→j* values of α and β electrons were summed and used for the discussion.


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III. RESULTS AND DISCUSSION LIF spectra of the PhOH-TES clusters Figure 1 shows the LIF spectra of the PhOH/TES system observed in the present study. The LIF spectrum recorded without TES in the sample is shown in Figure 1a for comparison. In the figure, the strong 0-0 band of the S1-S0 transition of the PhOH monomer appear at 36349 cm-1.50 By introducing TES into the sample, several new bands appear, as clearly seen in Figure 1b. The newly appeared bands are located both in the lower- and higher-frequency sides of the 0-0 band of the PhOH monomer. In the present study, we identified four band systems in the spectrum denoted as A to D in the figure. These band systems were verified by IR-UV hole-burning spectra shown in Figure S1 in the supporting information. The IR band of each isomer used in the IR-UV hole-burning spectroscopy will be shown later. We have assigned the spectral carriers for these band systems as the isomers of the PhOH-TES cluster. Hereafter, we use the labels A to D in referring to both the band systems and also their spectral carriers, which are isomers of the PhOH-TES. The wavenumbers of the 0-0 band, ν0, for each isomers and their shifts from that of the PhOH monomer, ∆ν0, are summarized in Table 1. For comparison, the corresponding values of PhOH and PhOH-DEMS clusters are also listed in Table 1. The isomers A to C exhibit similar values of ∆ν0 to those of the PhOH-DEMS clusters reported in the previous study.39 It indicates that the structures of these isomers A to C of the PhOH-TES are similar to those of the PhOH-DEMS clusters. A clear progression with an interval of ~6 cm-1 of the isomer C appears in the high-frequency side of the 0-0 band of the PhOH monomer. Their intensity is the strongest among the isomers observed in the spectrum.


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The isomer C also exhibits other progressions of smaller intervals with small intensity. These progressions can be assigned as intermolecular vibrations in the S1 state. The ∆ν 0 value of the isomer A is also close to that of the isomer A of the PhOH-DEMS cluster observed in the previous study.39 Although the band pattern of the isomer A of the PhOH-DEMS is much complicated, the isomer A of the PhOH-TES exhibits a clear and long progression with the interval of ~16 cm-1. The complicated band pattern of the isomer A of the PhOH-DEMS may arise from the internal rotation of the methyl group directly attached to the Si atom. On the contrary, the isomer B exhibits a short progression. In the present study, we have found another band denoted by D in the Figure 1b. The amount of ∆ν0 of −120 cm-1 is much larger compared with the other isomers. The isomer D is expected to have a distinct structure than the other isomers A to C.


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Figure 1. LIF spectra of (a) PhOH and (b) PhOH/TES systems. The band systems of the isomers of PhOH-TES clusters are denoted as A to D in the figure.

Table 1. Wavenumbers of the 0-0 bands of the S1-S0 transition, ν0, and the OH stretching frequencies, νOH, of the PhOH-TES clusters, PhOH monomer, and PhOH-DEMS clusters. Shifts of ν0 (∆ν0) and νOH (∆νOH) of PhOH-TES measured from those of the PhOH monomer are also listed in the table. All the values are in units of cm-1.

ν0

∆ν0

νOH

∆νOH

PhOH-TES (isomer A)

36257

−91

3626

−31

PhOH-TES (isomer B)

36336

−12

3635

−22

PhOH-TES (isomer C)

36355

+6

3633

−24

PhOH-TES (isomer D)

36229

−120

3579

−78

PhOH

36348.71a

---

3657b

---

PhOH-DEMS (isomer A)c

36270

−78

3628

−29

PhOH- DEMS (isomer B)c

36343

−5

3636

−21

PhOH- DEMS (isomer C)c

36352

+4

3633

-24

a

Reference 50. bReference 44. cReference 39.

IR spectra of the PhOH-TES clusters Using the above-mentioned electronic bands as probe transitions, the IR-UV double resonance spectroscopy was carried out. The IR spectra observed are shown in Figure 2. In


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Figure 2, the top trace displays the IR spectrum of the OH stretch band of the PhOH monomer for the reference. The traces A to D show the IR spectra of isomers A to D identified in the LIF spectrum, respectively. In each spectrum, a phenolic OH band clearly appears. All the isomers exhibit definitive red-shifts of the OH stretching wavenumbers compared to that of the PhOH monomer, ∆νOH. Since there is no proton accepting position in TES other than the Si–H group, these definitive red-shifts indicate the formation of the Si−H···H−O dihydrogen bond between PhOH and TES. The values of the OH stretching wavenumbers, νOH, and the ∆νOH, for each isomer are summarized in Table 1. The amount of ∆νOH as well as ∆ν0 of the isomers A to C are quite similar to those of the PhOH-DEMS dihydrogen-bonded clusters. On the other hand, the isomer D exhibits a much larger value of ∆νOH compared with the other isomers. The ∆νOH value of −78 cm-1 is similar to that of the PhOH-Benzene π-hydrogen bonded cluster (−78 cm1 51

).

This result indicates that the dihydrogen-bonding structure of the isomer D is much

different from those of the isomers A to C. To obtain information about the dihydrogen bonding structure, DFT calculations were carried out.


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Figure 2. Infrared spectra of PhOH and PhOH-TES clusters observed in the present study. Spectral carriers of each trace is indicated in the figure. The OH stretching wavenumber, νOH, of each isomer in units of cm-1 is indicated at each band. Values in parentheses indicate the shift of the OH stretching wavenumbers, ∆νOH, compared to that of the PhOH monomer.

DFT calculation of the PhOH-TES clusters Before discussing the structure of the PhOH-TES clusters, the conformations of TES molecule is briefly examined. TES molecule has three ethyl groups. Relative orientation of each ethyl group can be labeled based on an H-Si-C-C dihedral angle. According their values, the relative orientations are grouped into three. One is a trans (t) configuration where the dihedral angle is close to 180° and the other two are gauche configurations. There are two gauche


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configurations g1 and g2 having a value either ~+60° or ~−60°. Thus, there are 27 conformers due to their relative configurations. However, some sets of the conformers can be superimposed by the overall rotation of the molecule. In addition, other sets of the conformers are in the relation of mirror images. Then, there are only seven sets of independent configurations that have a distinct relative energies. Relative energies among the conformers are summarized in Table 2. In Table 2, conformers are labeled such as [t-t-t] or [g1-t-g1], according to the paper on triethylgermanium hydride.40 For example, the notation [A-B-C] means that ethyl groups A, B, and C are lined up counter-clockwise direction when H atom of the Si−H group is put in the opposite side from an observer. Taking the number of the isoenergetic configurations in each set and the differences in the free energy at 298 K, ∆G, into account, relative populations of conformers, P, are evaluated assuming the Boltzmann distribution and listed in Table 2. As a result, the values of P for [t-g2-g1] and [t-g1-g1] conformers are found to be much larger compared with others as indicated in Table 2.


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Table 2. Relative energies, ∆E0, and Gibbs free energies, ∆G, of conformers of triethylsilane obtained by the DFT calculation of M05-2X/6-311++G(3d, 2p) level. Relative populations, P, at 1 atm and 298 K are also listed. ∆E0 / cm-1

∆G / cm-1

P

[t-g2-g1], [g2-g1-t], [g1-t-g2]

0.0

0.0

0.362

[t-g1-g1], [g1-g1-t], [g1-t-g1], [t-g2-g2], [g2-g2-t], [g2-t-g2]

78.4

97.9

0.454

[g1-g1-g1], [g2-g2-g2]

177.9

374.9

0.040

[t-t-g1], [t-g1-t], [g1-t-t], [t-t-g2], [t-g2-t], [g2-t-t]

360.8

582.3

0.045

[t-g1-g2], [g1-g2-t], [g2-t-g1]

440.1

490.7

0.035

[g1-g2-g2], [g2-g2-g1], [g2-g1-g2], [g2-g1-g1], [g1-g1-g2], [g1-g2-g1]

135.4

510.5

0.064

[t-t-t]

744.3

1294.2

0.000

Conformation

Since these two conformers of TES are expected to exist at the room temperature before the supersonic expansion, clusters between these conformers of TES and PhOH are examined in the optimization. In the present calculation, more than ten stable structures are obtained. Among these structures, four structures having the lowest energies are shown in Figure 3. Energetic, structural, and vibrational parameters of all local minima obtained are summarized in Table S1 in the supporting information. The structures obtained in the present calculation are labelled from i to xvii in the order of the relative energy, ∆E0, compared to the most stable structure i. As seen


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in Figure 1, the band of the isomer C in the LIF spectrum is the strongest among the isomers of PhOH-TES. It suggests that the isomer C should be most abundant and therefore it should correspond to the most stable configuration. Since the structure i has the lowest energy among the structures obtained by the calculation, it is assigned to the isomer C. Since the relative energies of the other three structures in Figure 3 are similar to each other, the structures ii and iv are assigned to isomers A and B, according to the ∆νOH values. Of course, since the energy difference is not so large, the assignment carried out here is a tentative one.

Figure 3.

Optimized structures of PhOH-TES clusters.

For each isomer, a side view is

displayed in the right side. The values of ∆E0 and ∆νOH are in units of cm-1, whereas the values of binding energy (B.E.) are in units of kJ mol-1.

As shown in Figure 3, the structures i, ii, and iv have similar intermolecular orientation or dihydrogen-bond structures with each other. The natural charge of the OH and SiH hydrogens are around +0.49e and –0.23e. The distances between the two hydrogens in the dihydrogen bond


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are 2.220 – 2.341 Å and these values are smaller than the twice of the van der Waals radius of hydrogen atom, 2.4 Å.52 Thus, these theoretical results support the formation of the dihydrogenbond. The binding energy of these isomers are calculated to be 15.2 – 16.4 kJ mol–1. In these structure, a dihedral angle between the phenyl ring and the COH plain in the PhOH moiety is 15– 16 degrees. This means that the OH hydrogen of PhOH is pulled up by the TES from the phenyl plane through the dihydrogen bond. In these structures, a relatively large spatial overlap between the TES and the phenyl ring of the PhOH moiety is observed. It indicates that both the dihydrogen-bond and also the dispersion interactions contribute to the cluster formation as in the case of the PhOH-DEMS.39 We found a different type of stable structure that can be a candidate for the isomer D in our calculation. The structures of the candidates are shown in Figure 4. In these structures, the spatial overlap between the TES and the phenyl ring of the PhOH moiety is small compared to the structures i to iv. Thus, the balance of the dihydrogen bond compared to the dispersed interaction is expected to become larger. These structures are a little bit less stable compared to the structures i to iv, based on the relative and binding energies obtained in the calculations. However, the predicted ∆νOH values from –68 cm-1 to –82 cm-1 are in good agreement with that of the isomer D. Thus we assigned these structures are the possible candidates of the isomer D. In these structures, the distance between two hydrogen atoms in the dihydrogen bond is in the range from 1.932 to 1.972 Å that is much smaller than those of the possible structures for the isomer A to C. Due to the dihydrogen-bond formation, the Si–H bond is elongated from 1.491 Å (monomer) to 1.503 Å. The natures of the dihydrogen bond in these structures, v, x, xi, and xvi, are similar with each other according to the results of the calculation. Thus, in the following


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discussions, we refer to the structure v as a possible structure of the isomer D, since it is the most stable isomer among the candidates of the isomer D.

Figure 4. The candidates for the isomer D of PhOH-TES cluster. For each isomer, a side view is displayed in the right side. The values of ∆E0 and ∆νOH are in units of cm-1, whereas the values of binding energy (B.E.) are in units of kJ mol-1.

IR photodissociation spectroscopy of PhOH+-DEMS/TES clusters As mentioned above, the isomer D of the PhOH-TES cluster has a much stronger Si–H···H–O dihydrogen bond compared with the other isomers A to C. To reveal the intrinsic nature of the dihydrogen bond involving the Si–H group, we have explored another system that should exhibit much stronger dihydrogen bond. In the present study, we changed the proton donor from neutral PhOH to cationic PhOH, since the cationic PhOH is known to be more acidic than the neutral ones. In addition, since an electron is removed from the π orbital of the phenyl ring to form PhOH+, the dispersion interaction between the PhOH+ and DEMS/TES should be weakened.


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Thus, we carried out the IR photodissociation spectroscopy of PhOH+-DEMS and PhOH+-TES. The IR spectra obtained are shown in Figure 5. In the both clusters, a very strong and broad band with small structure appears below 3200 cm-1 region. The spectra of these clusters are quite similar with each other. In these spectra, several features, or structures can be seen. The νOH of the PhOH+ monomer is reported to be 3534 cm-1.53 Since there is no band in the vicinity of this position, it is clear that the νOH of these clusters are largely red-shifted due to the dihydrogen-bond. Overlapping to the broad profile, a strong peak appears at 2977 cm-1 and several peaks follow it in the low frequency side.

Figure 5. Infrared photodissociation spectra of (a) PhOH+-DEMS and (b) PhOH+-TES clusters. Simulated spectra for these clusters are also displayed. The simulated spectra are convoluted by Lorentzian function of 4 cm-1 HWHM.

To assign the spectral features, we measured IR spectra of DEMS and TES vapor as shown in Figure 6. The IR spectra of DEMS and TES monomers are quite similar with each other,


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especially in the CH stretch region. A small difference is observed in the Si−H stretch band, which is shown in the insets of Figure 6. Although the difference in their position is small, a clear difference is observed in their rotational envelopes. For DEMS and TES, there are three peaks in the CH stretch region; 2963, 2923, and 2888 cm-1. Since the band positions and intensity patterns of these bands are quite similar to the structure in the IR bands of the PhOH+DEMS and PhOH+-TES, they are assigned as the CH stretch band of the DEMS/TES moiety. Thus, the main strong and broad profile of the IR band of the PhOH+-DEMS and PhOH+-TES clusters is assigned as the phenolic OH stretching band. The center of the OH stretching band of both isomers is about 2860 cm-1. It corresponds to the red-shift of νOH of 674 cm-1. The strength of the dihydrogen-bond in these clusters are almost the same. The broad width of the band is interpreted by the anharmonicity of the OH stretch level and also contributions from isomers which having different conformation of the ethyl groups in the DEMS/TES moiety. Other small structures, for instance, a band at 2640 cm-1 and so on, are tentatively considered to arise due to the interaction between the OH stretch level and some combination or overtone levels.


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Figure 6. Infrared spectra of gaseous (a) DEMS and (b) TES. Insets are expanded portions in the vicinity of Si–H stretching band.

DFT calculation of the PhOH+-DEMS/TES clusters To obtain the structural information of the PhOH+-DMES/TES clusters, DFT calculations are carried out.

In the present calculation, about ten structures are obtained in the optimization.

Figure 7 shows the most stable structures obtained for each cluster. The relative orientation between PhOH+ and DEMS/TES is similar to that in the isomer D in the neutral cluster. Tables S2 and S3 in the supporting information summarize the results of the calculations for the PhOH+DMES and -TES clusters, respectively. The calculations provide somewhat stronger binding energy for the PhOH+-TES clusters (49.1 – 50.9 kJ mol-1) than the PhOH+-DEMS clusters (47.5 – 48.9 kJ mol-1). The distance between the two hydrogen atoms consisting of the dihydrogen-bond is in the range from 1.496 to 1.522 Å and much smaller compared to that in the neutral clusters. The energy difference among the


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isomers is not so large. This results indicate that the dihydrogen bond is the dominant interaction and that the TES and/or DEMS moiety can rotate along the dihydrogen bond axis with small barrier. The natural charge of the OH and the SiH hydrogens are about +0.54e and –0.29e, which are larger in magnitude than those of the neutral clusters. The νOHs of all the optimized structures for PhOH+-DMES/TES clusters are calculated to be in the range from 2905 to 2969 cm-1 and these values are in good agreement with the observation. As expected from the IR spectra, the structure of the PhOH+-DEMS and PhOH+-TES is nearly the same. Simulated IR spectra are also shown in Figure 5. Since the IR intensity of the OH stretch band is quite larger than the CH stretch bands of the DEMS or TES moiety, the simulated spectrum is seen as a single band. However, the CH bands of the DEMS/TES can be identified in the observed IR spectra of the PhOH+-DEMS and -TES clusters. Since these band patterns and positions are similar to those of the DEMS and TES vapors, the effect of the formation of the dihydrogen bond on the CH stretch modes is considered to be small. Thus, the large IR intensity of the OH stretch band is considered to be distributed to the energetically nearby levels by a vibrational anharmonicity and then the band width becomes very broad. The results of the calculation suggest that there exist several isomers in the present experimental condition. However, the range of the predicted νOH is much narrower than the width of the IR band observed. Thus, as mentioned above, the broad width of the νOH band should originate from the strong anharmonicity of the hydrogen-bonded OH stretch. The several band structures at lower than 2900 cm-1 are tentatively assigned as the combination bands that get IR intensity from the anharmonic interaction with the OH stretch band.


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Figure 7. Optimized structures of (a) PhOH+-DEMS and (b) -TES clusters. For each clusters, a side view is displayed in the right side. NBO analysis of the Si−H···H−O dihydrogen-bond The nature of the σ-type hydrogen bond is frequently discussed from a viewpoint of an electron donor-acceptor charge-transfer interaction in the NBO analysis.49 To examine whether the same analysis is valid for the dihydrogen-bond interaction or not, we carried out the NBO analysis. The charge-transfer interaction in the case of the Si−H···H−O dihydrogen bond, the value of Ei→j* between the σSi-H and σ*OH orbitals should be evaluated. First, we examined the (2)

most stable structure i and the strongly dihydrogen-bonded structure v of the neutral PhOH-TES (2)

clusters. Pairs of NBOs having appreciable Ei→j* values are picked up and displayed in Figure 8. In the case of structure i, four pairs of electron-donating and -accepting NBOs have similar values of Ei→j* . The charge-transfer from the σSi-H to σ*OH orbitals corresponds to the (2)

dihydrogen bond and its Ei→j* value is 1.8 kJ mol-1. Based on the Ei→j* values, the dihydrogen (2)

(2)

(2)

bond is not a dominant interaction in this structure. On the contrary, the values of Ei→j* for the


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dihydrogen bond is found to be 10.8 kJ mol-1 in the structure v, which is the candidate of the isomer D observed in the present experiment. In this isomer, it is clear that the dihydrogen-bond type interaction is significant among the charge-transfer type interactions.

(2)

Figure 8. Paris of natural bond orbitals having appreciable Ei→j* values for (a) the structure i and (2)

(b) the structure v of the PhOH-TES cluster. The concerning orbitals and Ei→j* values in units of kJ mol-1 are indicated figure.

Next, cationic PhOH+-DEMS/TES clusters were examined. In these cases, the NBO analysis (2)

was carried out for the most stable structure for each cluster. The Ei→j* values from the σSi-H to σ*OH orbitals become to 120 and 119 kJ mol-1 for the PhOH+-DEMS and -TES clusters, respectively. This interaction energy is more than ten times larger than that in the neutral case. In the case of the cationic cluster cases, a backward-like charge-transfer interaction from the σ*OH to σ*Si-H orbitals is found to have an appreciable interaction energy of 25 and 24 kJ mol-1 for the PhOH+-DEMS and -TES clusters, respectively. Whether this type of interaction is a


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feature of the dihydrogen bond or not cannot be concluded at the present stage. Detailed theoretical analysis on the nature of the Si−H···H−O dihydrogen bond is currently undergoing.

Competition between the dihydrogen bond and the dispersion interaction To discuss the nature of the Si−H···H−O dihydrogen-bond, it is necessary to estimate the contribution of the dispersion interaction in the cluster stabilization. Thus, to make a rough estimation of the contribution of the dispersion interaction, we carried out the structural optimization of the neutral PhOH-TES cluster by using the B3LYP functional. Since the B3LYP functional cannot treat the dispersion interaction, the optimization provides us with structures in which the dihydrogen bond is the main intermolecular interaction. Figure 9a shows the optimized structure obtained by using the B3LYP functional. The structure has a Cs symmetry with respect to the phenyl plane. The binding energy of the PhOH-TES is calculated to be 5.0 kJ mol-1. This value is smaller than that of the structure v of 13.7 kJ mol-1 obtained by the M05-2X calculation. This difference in the binding energy qualitatively indicates that the contribution of the dispersion interaction to the energetic stabilization is still large in the isomer D. However, the observed ∆νOH value of –78 cm-1 is comparable to the scaled value of –90 cm-1 obtained by (2)

the B3LYP calculation. In addition, the NBO analysis for the B3LYP structure gives the Ei→j* value of 11.6 kJ mol-1. This value is also close to that obtained for the structure v in the M05-2X calculation, 10.8 kJ mol-1. Thus, the nature of the dihydrogen bond in the isomer D is considered to be very close to its intrinsic one, though the dispersion interaction still largely contributes to the energetic stabilization of the whole system. We carried out the same estimation for the cationic PhOH+-TES cluster. The optimized structure obtained is shown in Figure 9b. The


24

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B3LYP calculation provides the binding energy of 47.5 kJ mol-1, which is close to the value of 50.0 kJ mol-1 obtained by the M05-2X calculation for the most stable isomer. The small difference suggests that the dihydrogen bond is the dominant intermolecular interaction in the case of the cationic clusters.

Figure 9. Optimized structures of (a) PhOH-TES and (b) PhOH+-TES clusters using the B3LYP functional in the DFT calculation. For each clusters, a side view is displayed in the right side. The values of ∆νOH are in units of cm-1, whereas the values of binding energy (B.E.) are in units of kJ mol-1.

Strength of the Si–H···H–O dihydrogen bond It is well known that there is a good correlation between the strength of the hydrogen-bond and ∆νOH in the 1:1 hydrogen-bonded complex. To examine the strength of the Si−H···H−O dihydrogen-bond, the values of ∆νOH are compared among several hydrogen-bonded clusters of PhOH and PhOH+ as listed in Table 3. Since the nature of dihydrogen bond in the isomer D of the PhOH-TES cluster is close to the intrinsic one as discussed above, the ∆νOH value of the isomer D is used for the Si−H···H−O dihydrogen-bond in the table. It is clear that the ∆νOH value of the PhOH-TES is close to that of the PhOH-benzene and PhOH-ethylene clusters. In


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the previous study, the strength of the Si−H···H−O dihydrogen bond was considered to be weak because of the small ∆νOH values. However, it is revealed in the present study that the strength of the Si−H···H−O dihydrogen bond is comparable to that of the π-hydrogen-bond. In the case of the cationic clusters, the ∆νOH value of the PhOH+-DEMS/TES is somewhat larger than those of the π-hydrogen bonded clusters. It means that the Si−H···H−O dihydrogen bond is stronger than that of the π···H−O dihydrogen bond. According to the ∆νOH values, the Si−H···H−O dihydrogen-bond strength is similar to that of the π-hydrogen bond in the neutral case, whereas it is stronger in the case of cationic clusters. This small discrepancy comes from the fact that the contribution of the dispersion interaction is still exist in the neutral isomer D as discussed in the previous section. It is known that there is a good correlation between the ∆νOH values in the 1:1 hydrogenbonded clusters and the proton affinities of the proton accepter in the cluster.51,55,56,58 Thus, our experimental results of the present study indicate that the proton affinity of the TES and DEMS should be larger than those of C6H6 and C2H4. The proton affinity at 0 K is estimated as a binding energy between the proton and the molecule concerned. The M05-2X/6-311++G(3d,2p) level calculation provides the proton affinities of C6H6 and C2H4 to be 177 and 162 kcal mol-1, respectively. These values are very close to the reported values, 179.3 and 162.6 kcal mol-1 for C6H6 and C2H4, respectively.59 By the same procedure, the proton affinities of TES and DEMS are estimated to be 193 and 188 kcal mol-1, respectively. These value are larger than those of C6H6 and C2H4 as expected based on the ∆νOH values. This results is consistent with the expectation based on our experimental observations.


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Table 3. Comparison of ∆νOH in the PhOH-X and PhOH+-X (X = proton acceptor) clusters. Values of ∆νOH are in unit of cm-1.

Acceptor X

PhOH

PhOH+

N2

−5 a

−159 a, −169 b

CO

−33 c

−211 c

TES

−79 d

~ −674 d

C2H4

−77 e

−514 e

C6H6

−78 e

−474 e

H2O

−133 f

−954 g

a

Reference 54. bReference 55. cReference 56. dPresent study. eReference 51. fReference 44. g Reference 57, calculated value.

IV. CONCLUSIONS In the present study, to reveal an intrinsic character or a strength of the Si−H···H−O dihydrogen bond, spectroscopic measurements were carried out for neutral PhOH-TES, cationic PhOH+DEMS and -TES clusters. Based on the red-shift of the phenolic OH stretch wavenumbers, the strength of the dihydrogen bond is examined. As a result, the present study revealed that the intrinsic strength of the Si−H···H−O dihydrogen bond is somewhat stronger than those of the π···HO hydrogen bond. The proton affinities of TES and DEMS are estimated to be larger than those of π-systems such as C6H6 and C2H4. The relatively large proton affinities of TES and DEMS may be related to the fact that the hydrogen atom bonded to the silicon atom is very reactive.


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ASSOCIATED CONTENT Supporting Information. Hole-burning spectra of the PhOH-TES clusters. Mass spectra of the PhOH/DEMS and PhOH/TES systems obtained in the present experiment. Summary of the DFT calculation on the neutral PhOH-TES clusters. Summary of the DFT calculation on the cationic PhOH+-DEMS clusters. Summary of the DFT calculation on the cationic PhOH+-TES clusters. The complete author list of ref 48. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *H. Ishikawa: e-mail, [email protected]. Notes The authors declare no competing financial conflict. ACKNOWLEDGMENT The authors acknowledge to Dr. Kota Daigoku for his helpful discussion.


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(49) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular Interaction from a Natural Bond Orbital, Donor-Acceptor Viewpoint. Chem. Rev. 1988, 88, 899-926. (50) Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K. High Resolution UV Spectroscopy of Phenol and the Hydrogen Bonded Phenol-Water Cluster. J. Chem. Phys. 1996, 104, 972-982. (51) Fujii, A.; Ebata, T.; Mikami, N.; An Infrared Study of π-Hydrogen Bonds in Microsolvated Phenol: OH Stretching Vibrations of Phenol−X (X = C6H6, C2H4, and C2H2) Clusters in the Neutral and Cationic Ground States. J. Phys. Chem. A 2002, 106, 8554-8560. (52) Bondi, A. van der Waals Volumes and Radii. J. Chem. Phys. 1964, 68, 441-451. (53) Fujii, A.; Iwasaki, A.; Ebata, T.; Mikami, N. Autoionization-Detected Infrared Spectroscopy of Molecular Ions. J. Phys. Chem. A 1997, 101, 5963-5965. (54) Fujii, A.; Miyazaki, M.; Ebata, T.; Mikami, N. Infrared Spectroscopy of the Phenol-N2 Cluster in S0 and D0: Direct Evidence of the In-plane Structure of the Cluster. J. Chem. Phys. 1999, 110, 11125-11128. (55) Solcà, N.; Dopfer, O. Microsolvation of the Phenol Cation (Ph+) in Nonpolar Environments: Infrared Spectra of Ph+−Ln (L = He, Ne, Ar, N2, CH4). J. Phys. Chem. A 2001, 105, 5637-5645. (56) Fujii, A.; Ebata, T.; Mikami, N.; Direct Observation of Weak Hydrogen Bonds in Microsolvated Phenol: Infrared Spectroscopy of OH Stretching Vibrations of Phenol−CO and – CO2 in S0 and D0. J. Phys. Chem. A 2002, 106, 10124-10129.


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(57) Gerhards, M.; Jansen, A.; Unterberg, C.; Kleinermanns, K. OH Stretching Vibration of the Phenol(H2O)1+ Cation. Chem. Phys. Lett. 2001, 344, 113-119. (58) Iwasaki, A.; Fujii, A.; Watanabe, T.; Ebata, T.; Mikami, N. Infrared Spectroscopy of Hydrogen-Bonded Phenol–Amine Clusters in Supersonic Jets. J. Phys. Chem. 1996, 100, 1605316057. (59) Hunter, E. P. L.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update. J. Phys. Chem. Ref. Data 1998, 27, 413-656.


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