ARTICLE pubs.acs.org/JPCA
Interaction of Alcohols with 2-Fluoro- and 4-Fluorophenylacetylenes: Infrared-Optical Double Resonance Spectroscopic and Computational Investigation Surajit Maity,† Dilip K. Maity,‡ and G. Naresh Patwari*,† † ‡
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India ABSTRACT: Alcohol complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Methanol forms a cyclic complex with both the fluorophenylacetylenes incorporating C H 3 3 3 O and O H 3 3 3 π hydrogen bonds, the structure of which is similar to that of the corresponding water complex but different from that of a phenylacetylene methanol complex. The anti conformer of ethanol also binds in a similar fashion to both the fluorophenylacetylenes. Additionally, the gauche conformer of ethanol binds to 2-fluorophenylacetylene in a distinctly different structural motif that incorporates C H 3 3 3 F and O H 3 3 3 π hydrogen bonds. The OH group of trifluoroethanol interacts primarily with the π electron density of the CtC bond. The π electron density of the CtC bond is the principal point of interaction between the alcohols and both the fluorophenylacetylenes. The present results are indicative of the fact that fluorine substitution on the phenyl ring is sufficient to eliminate the subtle hydrogen bonding behavior of phenylacetylene.
’ INTRODUCTION The majority of hydrogen bonding observed both in the gas and the condensed phases can be summarized using Etter rules.1 In addition, based on the geometries of several mixed dimers obtained using rotationally resolved spectroscopy Legon and Millen laid down rules for formation of X H 3 3 3 B hydrogen bonding.2 Both the Etter and the Legon Millen rules govern the hierarchy of hydrogen bond formation. The Etter rules, in particular, have been consistently used to predict the formation of supra-molecular synthons and thus form guidelines for crystal engineering. However, exceptions to Etter rules have been reported, and these exceptions mostly pertain to hydrogen-bonding patterns observed in multifunctional molecules. This is even more vital if the functional groups present cannot be graded into known hierarchal patterns governed either by the Etter or the Legon Millen rules.1,2 In the case of crystal structures of imidazole-4,5-dicarboxylic acid derivatives, it has been shown that a substitution of a single methyl group can significantly alter the crystal structure, as a result of changes in the intermolecular hydrogen bonding pattern.3 Such behavior was attributed to the electronic effects. However, the data size of such deviations from the Etter’s rules is too small to make any unambiguous predictions. In the crystal structures it not always possible to separate out the electronic and steric factors. On the other hand binary gas phase complexes offer a much more convenient way to investigate hydrogen bonding in multifunctional molecules, wherein the electronic effects can be tuned without affecting (almost!) the steric factors. We had previously investigated hydrogen-bonded complexes of phenylacetylene (PHA) with several solvent molecules such as water, methanol (MeOH), ethanol r 2011 American Chemical Society
(EtOH), ammonia, alcohols, and amines,4 in an attempt to understand the hydrogen bonding behavior of multifunctional molecules. PHA forms a wide variety of hydrogen-bonded complexes with diverse intermolecular structures. These variations can be attributed to the suitable balance of intermolecular forces in various possible intermolecular structures. For instance, PHA forms a quasi-planar cyclic complex with water, in which one of the O H groups of water interacts with the π electron density of the CtC bond. Additionally, the C H group on the phenyl ring in the ortho position interacts with the oxygen atom of the water molecule.4a On the other hand MeOH and EtOH interact with the π electron density of the phenyl ring leading to the formation of an O H 3 3 3 π hydrogen-bonded complex.4b Similarly, it was found that ammonia interacts with PHA to form a linear C H 3 3 3 N ‘σ’ hydrogen-bonded complex, while the complex with methylamine is characterized by the presence of a N H 3 3 3 π hydrogen bond.4c The changes in the intermolecular structures are influenced by very subtle changes in the interacting molecules. In the case of hydrogen-bonded complexes of PHA, even substitution with a methyl group can bring about this change.4 One of the most interesting aspects of the hydrogen-bonded complexes of PHA is the difference in the intermolecular structures of the water and MeOH complexes, which is rather unique. Further, in the case of alcohol complexes of PHA, which Special Issue: Pavel Hobza Festschrift Received: May 9, 2011 Revised: August 11, 2011 Published: September 01, 2011 11229
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The Journal of Physical Chemistry A include water, MeOH, EtOH, and trifluoroethanol (TFEtOH), it was found that water and TFEtOH interact with π electron density of the acetylenic CtC bond, while MeOH and EtOH interact with the π electron density of the phenyl ring in PHA. This switching of the interaction site was attributed to very subtle changes in the nature of the approaching OH group, which PHA is able to sense.4b Substitution of fluorine on the phenyl ring of PHA increases the number of possible hydrogen-bonding sites in the form of lone pair electrons of fluorine atom. Further, it also alters (decreases) the π electron density of phenyl ring and possibly the π electron density acetylenic CtC bond in addition to its effect on the acidity of the acetylenic C H group.5 Investigations on hydrogen-bonded complexes of water with 2-fluorophenylacetylene (2FPHA) and 4-fluorophenylacetylne (4FPHA) revealed that the intermolecular structure of the two complexes is identical to that of PHA water complex, which incorporates O H 3 3 3 π and C H 3 3 3 O hydrogen bonds.6 Interestingly, unlike benzene fluorobenzene and styrene fluorostyrene pairs,7,8 substitution of fluorine atom on PHA does not lead to change in the intermolecular structure. This indicates that, in the case of the PHA water complex, substitution of fluorine does not affect the structure of in-plane complexes with water, and the formation of O H 3 3 3 π hydrogen bonding is favored over the formation of O H 3 3 3 F hydrogen bonding, which is in contradiction with the Legon Millen rules.2 In this article we present electronic and vibrational spectroscopic investigation on hydrogen-bonded complexes of 4FPHA and 2FPHA with MeOH, EtOH, and TFEtOH. The motivation for taking up such an investigation is to probe the effect of fluorine atom substitution on the differences in the intermolecular structures of various alcohol complexes as observed in the case of PHA. In other words, will the intermolecular structures of water and MeOH complexes of fluorophenylacetylenes be similar or different?
’ EXPERIMENTAL AND COMPUTATIONAL METHODS The details of the experimental setup have been described elsewhere.9 Briefly, helium buffer gas at 4 atm is bubbled through a mixture of 4FPHA (Aldrich)/2FPHA (Aldrich) and alcohols (MeOH, EtOH, and TFEtOH) kept at room temperature and expanded through a 0.5 mm diameter pulsed nozzle (Series 9, Iota One; General Valve Corporation). The electronic excitation was achieved using a frequency-doubled output of a tunable dye laser (Narrow Scan GR; Radiant Dyes) pumped with second harmonic of a Nd:YAG laser (Surelite I-10; Continuum). The fluorescence excitation spectra were recorded by monitoring the total fluorescence with a photomultiplier tube (9780SB+1252 5F; Electron Tubes Limited) and a filter (WG-320) combination, while scanning UV laser frequency. The IR spectra in the hydride (X H) stretching region were obtained using the fluorescence dip infrared (FDIR) spectroscopic method.10 In this method, the population of a target species is monitored by the fluorescence intensity following its electronic excitation to the S1rS0 origin band with an UV laser pulse. A tunable IR laser pulse is introduced 100 ns prior to the UV laser pulse. When the IR frequency is resonant with the vibrational transition of the target species, the ground state population decreases, resulting in the depletion of the fluorescence signal. In our experiments, the source of tunable IR light is an idler component of a LiNbO3 OPO (Custom IR OPO; Euroscan Instruments) pumped with an injection-seeded Nd:YAG laser (Brilliant-B; Quantel). The IR OPO was calibrated
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Figure 1. Fluorescence excitation spectrum of 4FPHA in the presence of (A) 4FPHA MeOH, (B) 4FPHA EtOH, (C) 4FPHA TFEtOH, and (D) 4FPHA H2O . The peaks marked with a and b correspond to the band-origin transitions of 4FPHA and 2FPHA (impurity in 4FPHA) monomers, respectively. The transitions marked 1 3 are due to complexes of 4FPHA with MeOH, EtOH, and TFEtOH, respectively. The transition marked with w is the band-origin transition of the 4FPHA water cluster.
by recording the photoacoustic spectrum of ambient water vapor. The typical bandwidth of both UV and IR lasers is about 1 cm 1, and the absolute frequency calibration is within (2 cm 1. To supplement the experimental observations, we carried out density functional theory-based calculations using the GAUSSIAN-03 suite of programs.11 The equilibrium structures of the monomers and various binary complexes were calculated at DFT-MPW3LYP and MP2 levels of theory using the aug-ccpVDZ basis set. The nature of the stationary points obtained was verified by calculating the vibrational frequencies at the same level of theory. The stabilization energies were corrected for the zero point vibrational energy (ZPVE). For medium sized basis sets, 100% of BSSE correction is believed to often underestimate the interaction energy, and 50% correction is a good empirical approximation.12 Therefore, we report the stabilization energies with 50% BSSE correction. It is well known that the MP2 level of theory overestimates the dispersion interaction.13,14 To improve upon the energies, SCS-MP2/aug-cc-pVDZ single-point calculations were carried out on the MP2/aug-cc-pVDZ optimized structures.15 We had earlier seen in the case of PHA complexes with N-hetrocyclic aromatic rings the SCS-MP2 level calculation considerably improves the stabilization energies.4g,h The vibratonal frequencies in the O H stretching regions were scaled with factors of 0.9625 and 0.96 for the DFT-MPW3LYP and MP2 levels of theory, respectively, based on the ratio of the average of experimental to the average of calculated O H stretching frequencies for the water molecule.
’ RESULTS AND DISCUSSION (a). Spectra. The fluorescence excitation spectra of 4FPHA and 2FPHA were recorded in the presence of MeOH, EtOH, and TFEtOH and are shown in Figures 1 and 2, respectively. The transition at 35601 cm 1, marked a in Figure 1 is the band-origin transition for 4FPHA as reported earlier.6 The addition of MeOH, 11230
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Figure 2. Fluorescence excitation spectrum of 2FPHA in the presence of of (A) MeOH, (B) EtOH, and (C) TFEtOH. The peak marked with b corresponds to the band-origin transition 2FPHA monomers. The transitions marked 4, 5-6, and 7 are due to complexes of 2FPHA with MeOH, EtOH and TFEtOH, respectively.
EtOH, and TFEtOH to 4FPHA leads to the appearance of single new bands in each case at 35766 (1), 35763 (2), and 35778 cm 1 (3), and are are shifted to the blue by 165, 162, and 177 cm 1, respectively, in comparison with the band-origin transition of bare 4FPHA. The fluorescence excitation spectrum of 4FPHA recorded in the presence of water is shown in Figure 1D for the sake of comparison. All the newly appearing bands for the alcohol complexes are almost lined up with the band corresponding to the water complex.6 It can be inferred from the electronic transitions that the intermolecular structure of 4FPHA complexes with MeOH, EtOH, and TFEtOH might be similar to the water complex. Figure 2 depicts the fluorescence excitation spectra of 2FPHA in the presence of MeOH (Trace A), EtOH (Trace B) and TFEtOH (Trace C). The strong band at 35588 cm 1, marked as b, is the band-origin transition of 2FPHA. In the presence of MeOH, a new transition appears at in the fluorescence excitation spectrum 35556 cm 1 marked as 4. On the other hand, two new bands appear at 35557 and 35521 cm 1 in the presence of EtOH, which are marked 5 and 6. These transitions are red-shifted by 31 and 67 cm 1 relative to the bandorigin transition of bare 2FPHA. The position of bands 4 and 5 are almost identical, which indicates that the intermolecular structures of these two complexes might be similar. The addition of TFEtOH gives rise to several transitions in the 35620 35650 cm 1 region, as can be seen in Figure 2C. The most intense transition among them is at 35646 cm 1. For structural characterization of various complexes, the FDIR spectra of all the complexes were recorded in the acetylenic C H and the O H stretching region. The FDIR spectra of bare 4FPHA and its three alcohol complexes are shown in Figure 3. Figure 3A represents the FDIR spectra of the 4FPHA, which shows three prominent bands at 3323, 3332, and 3347 cm 1. The appearance of this spectrum is very similar to the corresponding spectrum of PHA and is indicative of the presence of Fermi resonance.16 The FDIR spectrum of the 4FPHA MeOH complex (band 1), depicted in Figure 3B, shows an intense band at 3317 cm 1 and a weak band at 3336 cm 1. The FDIR spectrum of the 4FPHA-EtOH complex (band 2), shown in Figure 3C,
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Figure 3. FDIR spectrum of (A) 4FPHA (band a), (B) 4FPHA-MeOH (band 1), (C) 4FPHA-EtOH (band 2), and (D) 4FPHA-TFEtOH (band 3). The arrows indicate the position of the O H oscillators of the alcohol monomers.
consists of two bands at 3312 and 3335 cm 1. The FDIR spectrum of the 4FPHA TFEtOH complex (band 3), depicted in Figure 3D, also shows two bands at 3319 and 3338 cm 1. The FDIR spectra in the acetylenic C H stretching region for all the 4FPHA alcohol complexes are almost identical and are distinctly different from that of bare 4FPHA. The appearance of Fermi resonance bands in 4FPHA involves coupling of the CtC and C H oscillators, any perturbation of these two oscillators will lead to changes in the characteristics of the IR spectrum in the acetylenic C H region.4 The changes in the Fermi resonance coupling observed for the MeOH, EtOH, and TFEtOH complexes clearly signify that all three alcohols interact with the acetylenic moiety of 4FPHA. The FDIR spectra were recorded in the O H stretching region as well and are depicted in Figure 3. The FDIR spectrum of the MeOH complex (Figure 3B) shows a single band at 3620 cm 1 with a red shift of 66 cm 1 from the corresponding band of free MeOH at 3686 cm 1.17 The FDIR spectra of the EtOH (Figure 3C) and TFEtOH (Figure 3D) complexes also show single bands at 3620 and 3582 cm 1, respectively. These spectra indicate a redshift by 58 cm 1 for the anti conformer of EtOH (41 cm 1 for the gauche conformer) and 75 cm 1 for the gauche conformer of TFEtOH.18,19 The FDIR spectra in the O H stretching region clearly indicate that the OH groups of all three alcohols act as hydrogen bond donors to 4FPHA. Figure 4 shows the FDIR spectra in the acetylenic C H stretching region for 2FPHA and its complexes with MeOH, EtOH, and TFEtOH. The FDIR spectrum of 2FPHA (Figure 4A) shows an intense transition at 3334 cm 1, accompanied by two weak transitions at 3318 cm 1 and 3341 cm 1. It is therefore reasonable to assume that the strong transition observed at 3334 cm 1 predominantly has the C H stretching character and therefore can be assigned to the acetylenic C H stretching frequency.6 Figure 4B shows the FDIR spectrum of 2FPHA MeOH (band 4), which shows a strong transition at 3326 cm 1 and a weak transition at 3338 cm 1. FDIR spectra of both the EtOH complexes (bands 5 and 6) are presented in Figure 4C,D. Both spectra are similar in appearance to the corresponding spectrum of the 2FPHA MeOH complex (Figure 4B). 11231
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The Journal of Physical Chemistry A The FDIR spectrum of band 5 consists of a strong transition at 3326 cm 1 and a weak transition at 3342 cm 1 (Figure 4C) and is similar in appearance to the corresponding. On the other hand, the FDIR spectrum of band 6 (Figure 4D) shows a strong transition at 3331 cm 1 along with two weak transitions at 3338 and 3344 cm 1. Depicted in Figure 4E is the FDIR spectrum of the TFEtOH complex (band 7), which once again is very similar to the FDIR spectra of the MeOH and EtOH complexes. The FDIR spectra of all four alcohol complexes show the resurgence in Fermi resonance
Figure 4. FDIR spectra of (A) 2FPHA (band b), (B) 2FPHA MeOH (band 4), (C) 2FPHA EtOH (band 5), (D) 2FPHA EtOH (band 6), and (E) 2FPHA TFEtOH (band 7). The arrows indicate the position of the O H oscillators of the alcohol monomers.
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coupling, albeit weak, which is almost absent in bare 2FPHA. This reappearance of Fermi resonance coupling is an indicator of the fact that the alcohols are interacting with the acetylenic moiety of 2FPHA, similar to that of the 2FPHA H2O complex.6 The FDIR spectra of 2FPHA complexes in the O H stretching region are also shown in Figure 4. The FDIR spectrum of the 2FPHA MeOH complex (band 4) shows a single band at 3622 cm 1, which can be assigned to the O H stretching vibration of the complex. Similarly, the FDIR spectra of 2FPHA EtOH complexes (bands 5 and 6), also show single transition at 3621 and at 3617 cm 1, respectively. In the case of the TFEtOH complex (band 7), a strong transition at 3586 cm 1 was observed along with a couple of weak transitions at 3594 and 3601 cm 1, which can be assigned to combination bands of intermolecular vibrations on the O H stretching vibration. In all four cases, the O H stretching shifted to a lower frequency, indicating that the OH groups of the alcohols act as hydrogen bond donors. Further, IR-UV hole-burning spectroscopy was carried out to determine the origin of several transitions appearing in the fluorescence excitation spectrum of 2FPHA in the presence of TFEtOH (see Figure 2C) in the 35620 35650 cm 1 region (figure not shown). These investigations reveal that the presence of a single isomer of 2FPHA TFEtOH complex exists in the present experimental conditions. (b). Structures. Geometry optimizations were carried out to get detailed information about all the possible minima on the potential energy surface using MPW3LYP and MP2 levels of theory using the aug-cc-pVDZ basis set. Optimized structures of the MeOH complexes of 4FPHA and 2FPHA are shown in Figure 5. Four stable minima were found for the 4FPHA MeOH complex (A4FM, B4FM, C4FM, and D4FM) and five minima were found for the 2FPHA MeOH complex (A2FM, B2FM, C2FM, D2FM, and E4FM). The first four structures have one-to-one correspondence. Structures A4FM and A2FM are stabilized by the linear C H 3 3 3 O hydrogen bonding, where the acetylenic
Figure 5. Optimized structures of MeOH with 4FPH and 2FPHA. 11232
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Figure 6. Optimized structures of EtOH with 4FPH and 2FPHA.
group acts as a donor. Structures B4FM and B2FM are stabilized by the O H 3 3 3 π hydrogen bonding wherein the OH group of MeOH interacts with the π electron density of the benzene ring. The structures C4FM and C2FM are characterized by the presence of C H 3 3 3 O and O H 3 3 3 π (acetylenic) hydrogen bonds leading to the formation of a quasi-planar cyclic complex. D4FM and D2FM represent in-plane complexes incorporating C H 3 3 3 O and O H 3 3 3 F hydrogen bonds. An additional structure E2FM was observed for the 2FPHA MeOH complex, in which the OH group of MeOH forms a pair of bifurcated hydrogen bonds with the π electron density of the CtC bond and the electron lone-pair present on the fluorine atom, leading to the formation of O H 3 3 3 π and O H 3 3 3 F hydrogen bonds. This structure is feasible due to the spatial proximity of the two hydrogen bond acceptor groups present in 2FPHA.
EtOH has two low lying conformations: the anti and gauche forms. The energy difference between the two conformers is 0.6 kJ mol 1, with the anti conformation being more stable.20 EtOH complexes with 4FPHA and 2FPHA were optimized for both the anti and gauche conformations and are depicted in Figure 6. The anti conformation of EtOH forms four structures with 4FPHA (A4FEA, B4FEA, C4FEA, and D4FEA) and five structures with 2FPHA (A2FEA, B2FEA, C2FEA, D2FEA, and E2FEA). The gauche conformation of EtOH forms four structures with 4FPHA (A4FEG, B4FEG, C4FEG, and D4FEG) and six structures each with 2FPHA (A2FEG, B2FEG, C2FEG, D2FEG, E2FEG, and F2FEG). One-to-one correspondence was observed for anti- and gauche-ETOH complexes with MeOH complexes for both 4FPHA and 2FPHA. For the gauche conformation of EtOH, an additional structure was observed with 11233
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Figure 7. Optimized structures of TFEtOH with 4FPH and 2FPHA.
2FPHA (F2FEG) in which EtOH is hydrogen-bonded in a double donor fashion forming O H 3 3 3 π (acetylenic) and C H 3 3 3 F hydrogen bonds. In the case of TFEtOH, only the gauche conformer was considered for calculating the structures (Figure 7). In the case of 4FPHA, five structures were obtained (A4FTG, B4FTG, C4FTG, D4FTG, and F4FTG) and six structures were obtained for 2FPHA (A2FTG, B2FTG, C2FTG, D2FTG, E2FTG, and F2FTG). The first four structures of both the 4FPHA and 2FPHA are similar to MeOH complexes. The fifth structure of the 4FPHA TFEtOH complex (F4FTG) and the sixth structure of the 2FPHA TFEtOH complex (F4FTG) are hydrogenbonded structures incorporating O H 3 3 3 π (acetylenic) and C H 3 3 3 F hydrogen bonds. On the other hand, the fifth structure of the 2FPHA TFEtOH complex (E4FTG) forms a pair of bifurcated hydrogen bonds wherein the OH group interacts with the π electron density of the CtC bond and the electron lone-pairs of the fluorine atom. (c). Structural Assignment. Infrared spectroscopy is an excellent tool to establish the intermolecular structures as it probes the local interaction present in molecules and molecular complexes. The IR spectra in the hydride stretching region (C H, N H, and O H) are particularly useful since these vibrations are directly involved in hydrogen bond formation. The calculated vibrational frequencies were compared with the experimentally observed frequencies, the agreement between these two sets, in most of the cases, served as a benchmark for the structural assignment of various complexes. The FDIR spectrum of 4FPHA (see Figure 3A) shows three bands, which indicates the presence of Fermi resonance coupling. In PHA and its substituted analogues, Fermi resonance bands observed in the acetylenic C H stretching region are due to coupling of the acetylenic C H stretching vibration with the combination band arising out of one quantum of CtC stretching vibration and two quanta of C H out-of-plane bending vibration.14 The Fermi resonance coupling is localized on the acetylenic moiety and any interaction that perturbs the CtC oscillator or the C H oscillator or both will lead to drastic changes in the coupling pattern. The FDIR spectrum of bare 2FPHA (see Figure 4A) shows an intense transition at 3334 cm 1, accompanied by two weak transitions at 3318 cm 1 and 3341 cm 1. This spectrum indicates that the Fermi
resonance coupling in 2FPHA is much weaker in comparison with PHA and 4FPHA.6 The FDIR spectrum of the 4FPHA MeOH complex in the acetylenic C H stretching region (see Figure 3B) is indicative of the fact that MeOH interacts with the acetylenic moiety. However, the formation of the linear C H 3 3 3 O ‘σ’ hydrogenbonded structure A4FM can be ruled out. Only the structure C4FM incorporates the interaction between MeOH and the acetylenic moiety of 4FPHA. The FDIR spectrum in the O H stretching region (see Figure 3B) shows a band at 3620 cm 1 with ΔνOH of 66 cm 1 relative to bare MeOH. Comparison of experimental and calculated O H stretching frequencies listed in Table 1 for the MeOH complexes. The calculated O H stretching frequency and the ΔνOH for the C4FM complex at both DFT-MPW3LYP and MP2 levels of theory are in good agreement with the experimental values. Further, the structure C4FM is also the global minimum on the 4FPHA MeOH potential energy surface at all three levels of theory. Therefore, the structure of 4FPHA MeOH can be assigned to C4FM. In the case of the 2FPHA MeOH complex, the FDIR spectrum in the acetylenic C H stretching region (see Figure 3B) shows the resurgence of Fermi resonance bands, which were dormant in bare 2FPHA, which is indicative of the fact that MeOH interacts with the acetylenic moiety of 2FPHA. The structures A2FM, C2FM, and E2FM account for such interaction. However, based on the position of the acetylenic C H stretching vibration, the linear C H 3 3 3 O ‘σ’ hydrogen-bonded structure A2FM can be ruled out once again. The stabilization energies at the MP2 and SCS-MP2 level favor the formation of O H 3 3 3 π (phenyl) hydrogen-bonded structure B2FM. The overestimation of the dispersion component of energy is well-known for the MP2 level.13,14 In the present case, even though the SCS-MP2 level does lower the stabilization energy, the lowering is not enough, and the O H 3 3 3 π (phenyl) hydrogen-bonded structure is still the lowest energy structure. However, a comparison of the experimental O H stretching frequency shift with the calculated values (see Table 1) clearly favors the assignment of C2FM structure to the observed 2FPHA MeOH complex. MeOH forms a quasi-planar cyclic complex with both 4FPHA and 2PFHA incorporating O H 3 3 3 π (acetylenic) and C H 3 3 3 O 11234
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Table 1. Binding Energies (kJ mol 1) and Scaled Vibrational Frequencies (cm 1) and Their Shifts along with Experimental Frequencies (cm 1) for the MeOH Complexes of 4FPHA and 2FPHA structures
a
νOH (expt)
MeOH
3686
4FPHA MeOH 2FPHA MeOH
3620 3622
ΔνOH (expt)
νOH (calc)a
ΔνOH (calc)a
3686
νOH (calc)b
ΔνOH (calc)b
ΔEa
ΔEb
ΔEc
3687
66 64
A4FM
3685
1
3684
3
9.9
12.7
10.5
B4FM
3661
25
3650
37
4.8
18.0
12.1
C4FM
3608
78
3612
75
10.8
19.0
13.0
D4FM
3662
24
3662
25
10.2
13.6
11.2
A2FM
3683
3
3685
2
10.3
13.4
11.0
B2FM
3658
28
3648
39
5.7
20.0
13.7
C2FM D2FM
3610 3662
76 24
3611 3662
76 25
10.5 10.6
15.7 14.0
12.0 11.6
E2FM
3650
36
3653
34
10.5
15.6
12.7
DFT-MPW3LYP level of theory. b MP2 level of theory. c SCS-MP2 level of theory.
and hydrogen bonds. The intermolecular structure of the MeOH complexes of both 4FPHA and 2FPHA are similar to the corresponding water complexes.6 The present scenario is drastically different from that of PHA, wherein the intermolecular structures of water and MeOH complexes are different.4b The FDIR spectrum of the 4FPHA EtOH complex in the acetylenic C H stretching region (see Figure 3C) shows weakening of Fermi resonance coupling as compared to the bare 4FPHA. This observation is similar to the MeOH complex and suggests that EtOH interacts with the acetylenic moiety in 4FPHA. Among the eight optimized structures of 4FPHA EtOH complexes (includes both anti and gauche conformations of EtOH), four structures (A4FEA, A4FEG, C4FEA, and C4FEG) account for the interaction of EtOH with acetylenic moiety. On the basis of the appearance of the FDIR spectra in the acetylenic C H stretching region, structures A4FEA and A4FEG can be ruled out as they are characterized by the presence of linear C H 3 3 3 O ‘σ’ hydrogen-bonds. It must be noted that the O H stretching frequency of the gauche conformer is about 16 cm 1 further lower than the anti conformer.17,18 In the event of a gauche conformer of EtOH forming complex with 4FPHA, the O H stretching frequency of the complex would have appeared at much lower frequency (about 15 cm 1) in comparison with the corresponding MeOH complex. Therefore, based on the position and the shifts in the O H stretching frequency, the structure of the 4FPHA EtOH complex can be assigned to C4FEA. The fluorescence excitation spectrum of 2FPHA in the presence of EtOH, depicted in Figure 2C, indicates the formation of two isomers. The characteristics of the FDIR spectra of both the bands (5 and 6) of the 2FPHA EtOH complex in the acetylenic C H stretching region (see Figure 4C,D) are similar to that of the 2FPHA MeOH complex (Figure 4B). However, the FDIR spectra in the O H stretching region for bands 4 and 5 are marginally different. Among the 11 optimized structures of the 2FPHA EtOH complex, which includes both anti and gauche conformations of EtOH, seven structures (A2FEA, A2FEG, C2FEA, C2FEG, E2FEA, E2FEG, and F2FEG) are possible structures. Once again the two structures A2FEA and A2FEG can be ruled out in a straightforward manner. The position of band 5 almost exactly coincides with the position of band 4 corresponding to the 2FPHA MeOH complex (see Figure 2), hence the formation of similar intermolecular structures can be
expected. Further, comparison of the observed and calculated vibrational frequencies in the O H stretching region leads to assignment of the C2FEA structure to band 5 (Table 2). Since the structure C2FEA has already been assigned to band 5, the FDIR spectrum of band 6 in the acetylenic C H stretching region (Figure 4D) suggests that C2FEG, E2FEA, E2FEG, and F2FEG are possible structures. In the event of C2FEG being the structure, the shift in the O H for both bands 5 and 6 would have been almost identical, which is not the case; therefore we can rule out this structure. Of the remaining structures, E2FEA, E2FEG, and F2FEG, the calculations at the MP2 level indicate that the shift in the O H stretching frequencies for all three structures would be same. However, it can be noted that both at the MP2 and SCS-MP2 level, F2FEG is the most stable structure. Therefore band 6 is assigned to the structure F2FEG. This assignment is further supported by the fact that the electronic transition of bands 5 and 6 are distinctly different. Interestingly, the two conformers of EtOH interact differently with 2FPHA. The FDIR spectra of TFEtOH complexes with 4FPHA and 2FPHA are shown in Figures 3D and 4D. In the case of the 4FPHA complex, the appearance of the spectrum complex suggests that TFEtOH interacts with the acetylenic due to modulation of the Fermi resonance coupling. On the other hand, in the case of the 2FPHA complex, the appearance of the spectrum in the acetylenic C H stretching region is similar to the monomer; however, all the bands of the complex are shifted by 4 cm 1 to a lower frequency in comparison with the 2FPHA monomer. This also indicates the interaction of TFEtOH with the acetylenic group of 2FPHA. The geometry optimizations converge onto five structures for the 4FPHA TFEtOH system and six structures for the 2FPHA TFEtOH system. In the case of the 4FPHA TFEtOH complex, only the structures A4FTG, C4FTG, and F4FTG, wherein the TFEtOH molecule interacts with the acetylenic moiety, can be considered for possible assignment; however, A4FTG can be ruled out. In C4FTG, the TFEtOH molecule approaches 4FPHA in the plane of the molecule and forms a quasi planar cyclic complex with O H 3 3 3 π and C H 3 3 3 O and hydrogen bonds. On the other hand, in F4FTG, the TFEtOH molecule approaches 4FPHA above the plane of the phenyl ring to form O H 3 3 3 π (acetylenic) and C H 3 3 3 F hydrogen bonds. The stabilization energy data reveals that at the DFT-MPW3LYP level ,the structure C4FTG (13.9 kJ mol 1) is 11235
dx.doi.org/10.1021/jp204286b |J. Phys. Chem. A 2011, 115, 11229–11237
The Journal of Physical Chemistry A
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Table 2. Binding Energies (kJ mol 1) and Scaled Vibrational Frequencies (cm 1) and Their Shifts along with Experimental Frequencies (cm 1) for the EtOH Complexes of 4FPHA and 2FPHA structures
a
νOH (expt)
ΔνOH (expt)
νOH (calc)a
ΔνOH (calc)a
νOH (calc)b
EtOH (anti)
3678
3686
3678
EtOH (gauche) 4FPHA EtOH
3661 3620
3667
3666
ΔνOH (calc)b
ΔEa
ΔEb
ΔEc
58 (anti)
2FPHA EtOH(1)
3621
57 (anti)
2FPHA EtOH(2)
3617
44 (gauche)
A4FEA
3684
2
3676
2
10.1
14.1
B4FEA
3664
22
3642
36
5.0
20.1
11.7 13.6
C4FEA
3614
72
3601
77
10.9
16.8
12.9
D4FEA
3663
23
3652
26
10.8
14.5
12.0
A4FEG B4FEG
3668 3649
1 18
3662 3627
4 39
10.4 5.2
14.0 22.2
11.5 14.7
C4FEG
3601
66
3590
76
10.4
17.3
13.1
D4FEG
3646
21
3640
26
10.6
14.9
12.3
A2FEA
3682
4
3675
3
10.0
15.4
11.6
B2FEA
3659
27
3638
40
6.0
22.7
15.4
C2FEA
3615
71
3601
77
10.5
16.9
12.8
D2FEA
3662
24
3652
26
10.8
15.3
12.4
E2FEA A2FEG
3650 3665
36 2
3648 3664
30 2
10.6 10.3
13.8 15.5
10.3 11.8
B2FEG
3644
23
3638
28
5.6
25.2
17.1
C2FEG
3603
64
3601
65
10.6
17.2
13.3
D2FEG
3646
21
3640
26
10.7
19.0
12.6
E2FEG
3639
28
3635
31
10.5
16.7
12.9
F2FEG
3618
49
3624
32
9.1
24.5
16.7
DFT-MPW3LYP level of theory. b MP2 level of theory. c SCS-MP2 level of theory.
Table 3. Binding Energies (kJ mol 1) and Scaled Vibrational Frequencies (cm 1) and Their Shifts along with Experimental Frequencies (cm 1) for the TFEtOH Complexes of 4FPHA and 2FPHA structures
ΔνOH (expt)
TFEtOH (gauche)
3657
4FPHA
TFEtOH
3582
75
2FPHA TFEtOH A4FTG
3586
71
B4FTG
a
νOH (expt)
νOH (calc)a
ΔνOH (calc)a
3661
νOH (calc)b
ΔνOH (calc)b
ΔEa
ΔEb
ΔEc
3659
3660
1
3655
4
7.4
18.4
12.1
3634
27
3606
53
8.2
25.5
18.3 20.8
C4FTG
3533
128
3594
65
13.9
29.2
D4FTG
3621
40
3618
41
11.4
19.5
14.7
F4FTG
3557
104
3594
65
13.0
29.2
20.8
A2FTG
3658
4
3657
2
6.9
20.1
13.6
B2FTG
3613
48
3614
45
8.2
26.4
18.8
C2FTG D2FTG
3535 3622
126 39
3601 3624
58 35
14.3 11.6
28.3 21.1
19.7 15.1
E2FTG
3618
43
3615
44
14.7
26.8
19.3
F2FTG
3568
93
3604
55
12.6
28.4
20.0
DFT-MPW3LYP level of theory. b MP2 level of theory. c SCS-MP2 level of theory.
marginally more stable than E4FTG (13.0 kJ mol 1). However, both the structures are iso-energetic at both the MP2 and SCSMP2 levels of theory. Further, the vibrational frequency calculations at the MP2 level also show that shifts are the same for both of the structures (Table 3). On the other hand, the MPW3LYP level overestimates the shifts in the O H stretching frequencies. From the experimental and the calculated data, it is not possible
to unambiguously assign the structure of the observed of 4FPHA TFEtOH to either C4FTG or F4FTG. However, both the structures C4FTG or F4FTG the OH group of TFEtOH interacts with the π electron density of the acetylenic CtC bond, but the orientation differs. On the basis of similar reasoning, the structure of the 2FPHA TFEtOH complex can be assigned to either C4FTG or F2FTG, which once again point to the fact that 11236
dx.doi.org/10.1021/jp204286b |J. Phys. Chem. A 2011, 115, 11229–11237
The Journal of Physical Chemistry A the OH group of TFEtOH interacts with the π electron density of the acetylenic CtC bond.
’ SUMMARY Intermolecular structures of the alcohol complexes with singly substituted fluorine PHA, viz., 4FPHA and 2FPHA, were investigated using IR-UV double resonance spectroscopic technique in combination with DFT-MPW3LYP and MP2 levels of theory using the aug-cc-pVDZ basis set. The structures of the MeOH complexes with 4FPHA and 2FPHA incorporate O H 3 3 3 π and C H 3 3 3 O hydrogen bonds, leading to the formation of quasi-planar cyclic complexes. These structures are similar to the corresponding structure of the water complex. This behavior is significantly different from that of PHA, wherein the intermolecular structures of the water and MeOH complexes are different. The structure of the 4FPHA EtOH complex is similar to that of the corresponding water and MeOH complexes. EtOH forms two complexes with 2FPHA, the first of which is similar to that of the corresponding MeOH complex. However, in the second complex, the conformation of EtOH and the intermolecular structures are different. This is an example of conformation-dependent hydrogen bond preferences. The structures of the TFEtOH complexes could not be unambiguously assigned, but point to the fact that the OH group interacts with the π electron density of the CtC bond. All three alcohols interact primarily with the π electron density of the CtC bond. Unlike MeOH and EtOH complexes of PHA, none of the alcohol interacts with 4FPHA and 2FPHA via the π electron density of the phenyl ring. Furthermore, substitution of a single fluorine atom on PHA is sufficient to eliminate the subtle hydrogen bonding behavior of PHA. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This material is based upon work supported by the Department of Science and Technology (Grant No.SR/S1/PC/23/2008), the Board of Research in Nuclear Sciences (Grant No. 2004/37/5/ BRNS/398), and the Council of Scientific and Industrial Research (Grant No. 01(2268)/08/EMR-II). The authors wish to thank Mr. Sohidul Islam Mondal for his help with some calculations. S.M. thanks UGC for the research fellowship.
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dx.doi.org/10.1021/jp204286b |J. Phys. Chem. A 2011, 115, 11229–11237
