Infrared-Optical Double Resonance Spectroscopic Investigation of

Article pubs.acs.org/JPCA

Infrared-Optical Double Resonance Spectroscopic Investigation of Trifluoromethylphenols and Their Water Complexes Arghya Dey and G. Naresh Patwari* Department of Chemistry, Indian Institute of Technology, Bombay Powai, Mumbai 400076, India ABSTRACT: The hydrogen bonding behavior of trifluoromethylphenols and their water complexes were investigated using IR-UV double resonance spectroscopy. Both ortho- and meta-trifluoromethylphenols exist in the syn conformer, which is the global minimum in both the cases. The IR spectrum in the O−H stretching region reveals the absence of an intramolecular O−H···F hydrogen bond in the syn-o-trifluoromethylphenol, which is in contrast to the results reported in the literature. The water complexes of both o-trifluoromethylphenol and m-trifluoromethylphenol are characterized by formation of O−H···O hydrogen bonds between the donor phenolic OH group and the acceptor water molecule. In addition, the o-trifluoromethylphenol−(water)2 complex was also observed.

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INTRODUCTION Hydrogen bonding is known to play a well-defined role in modulating the conformational preferences of molecules and clusters. For example, in the case 2-fluorophenylacetylene, the intermolecular structures with anti and gauche forms of ethanol are distinctly different.1 On the other hand ortho and metacresol exist in two different conformers, syn and anti, depending on the orientation of the OH group relative to the methyl group. The existence of the two rotational isomers of neutral ortho and meta-cresol was established using a variety of laser spectroscopic techniques under jet-cooled conditions.2 In the case of o-cresol the close proximity of the OH and CH3 groups renders a possibility of interaction between the two groups. The conformer specific infrared spectra, recorded using ion-dip infrared spectroscopic technique, show that the O−H stretching frequencies for both the syn and anti conformers were identical.3 Furthermore, the infrared spectra in the O−H stretching region of the syn and the anti conformers of o-cresol were also identical to the syn and the anti conformers of mcresol. These results clearly suggest the absence of any hydrogen-bonded interaction between the OH and the CH3 groups in the syn conformer of o-cresol. Substitution of the methyl group in cresol with a trifluoromethyl group is expected to bring about changes in the electron density of the aromatic ring. Additionally, the fluorine atoms in the trifluoromethyl group can also act as a hydrogen bond acceptor promoting the formation of intramolecular hydrogen bonding with the OH group in otrifluoromethylphenol. Quantum chemical calculations on conformers of both ortho- and meta-trifluoromethylphenol indicate that the syn conformer of o-trifluormethylphenol is more stable by 7.2 kJ mol−1 relative to the anti conformer.4 On the other hand, in the case of m-trifluormethylphenol the syn conformer is marginally stable than the anti conformer by 0.9 kJ mol−1.5 © 2012 American Chemical Society

The presence of the syn conformer of o-trifluoromethylphenol was established both in the gas-phase using rotational spectroscopy and in the crystal structures with the aid of electron diffraction studies.6,7 The rotational spectroscopic and the electron diffraction measurements clearly suggest that the O−H bond is symmetrically placed between two C−F bonds, leading to the formation of a pair of bifurcated intramolecular O−H···F hydrogen bonds. However, the formation of O−H···F hydrogen bonding in the syn conformer of o-trifluoromethylphenol is a conjecture based on the geometrical positioning of the OH group relative to the CF3 group. The possibility of such interaction is debatable based on the fact that organic fluorine (as C−F bonds) is known to be poor hydrogen bond acceptor.8 It is well-known that the vibrational spectroscopy in the hydride (X−H) stretching region is the most important spectroscopic tool for the identification of hydrogen bonding. This is due to the fact that the X−H group being directly involved in hydrogen bonding shows a characteristic shift to a lower frequency upon hydrogen bonding.9 It is, therefore, expected that the IR spectra in the O−H stretching region will provide unambiguous evidence for the presence or absence of intramolecular hydrogen bonding in the syn conformer of ortho-trifluoromethylphenol. In this article, we present, the IR spectroscopic investigation in the O−H stretching region for ortho- and meta-trifluoromethylphenol. Also presented are the IR spectra and the structures of their water complexes.

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EXPERIMENTAL AND COMPUTATIONAL DETAILS The details of the experimental setup have been described elsewhere.10 Briefly, helium buffer gas at 4 atm was bubbled Received: February 7, 2012 Revised: May 19, 2012 Published: May 21, 2012 6996

dx.doi.org/10.1021/jp301208z | J. Phys. Chem. A 2012, 116, 6996−7003

The Journal of Physical Chemistry A

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through o- and m-trifluoromethylphenol (Aldrich, 97%) 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) operating with Rhodamine-19 dye, pumped with the second harmonic of a Nd:YAG laser (Surelite I-10; Continuum). The LIF excitation spectra were recorded by monitoring the total fluorescence with a photomultiplier tube (9780SB+1252-5F; Electron Tubes Limited) and a filter (WG320) combination, The IR spectra were obtained using a fluorescence dip infrared (FDIR) spectroscopic method.11 Further, in order to separate out the transitions belonging to various species present in the LIF spectrum, IR-UV holeburning spectra were also recorded. In our experiments, the source of tunable IR light was an idler component of a LiNbO3 OPO (Custom IR OPO; Euroscan Instruments) and KTP OPO (Custom IR OPO; Laserspec) pumped with an injection seeded Nd:YAG laser (Brilliant-B; Quantel). The bandwidth of UV laser, LiNbO3 OPO, and KTP OPO are about 1, 1, and 3 cm−1, respectively, and the absolute frequency calibration of all of the lasers is within ±2 cm−1. The LiNbO3 OPO has a hole in the tuning curve in the region 3450−3520 cm−1; therefore, KTP OPO was used record the spectra in this frequency region. A detailed conformational search was followed by a complete geometry optimization at the MP2(FC) level of theory using the aug-cc-pVDZ basis set. The nature of the equilibrium structure obtained was verified by performing vibrational frequency calculations at the same level of theory. All of the structures reported here are true minima with real vibrational frequencies. The symmetric and antisymmetric stretching of the water molecule at MP2(FC)/aug-cc-pVDZ level were 3937 and 3803 cm−1, whereas the corresponding experimental values are 3756 and 3657 cm−1, respectively. Thus, a scaling factor of 0.9576 was devised from the ratio of the average of experimental frequencies (3706 cm−1) to the average of calculated frequencies (3870 cm−1). The scaling factor was intended to correct for basis set truncation, partial neglect of electronic correlation, and harmonic approximation. The stabilization energies were corrected for vibrational zero-point energy (ZPE) and the basis set superposition error (BSSE). The BSSE correction was made after geometry optimization. For medium sized basis sets the 100% BSSE correction is often considered to underestimate the interaction energy and 50% BSSE correction is considered to more accurate;12 therefore, the stabilization energy values are reported with 0, 50, and 100% BSSE correction. For the structural assignment, the IR spectra in the O−H stretching region for all the equilibrium structures was generated by convoluting a Lorentzian function of width (fwhm) of 2 cm−1 to the calculated stick spectrum and compared with the experimental spectra.13 All of the calculations reported here were carried out using Gaussian 09 suite of programs.14

Figure 1. Potential energy curve drawn following geometry optimizations at various dihedral angles obtained through rotation of the O−H bond for o-TFMP and m-TFMP calculated at MP2(FC)/ aug-cc-pVDZ level of theory. The barriers for rotation for o-TFMP and m-TFMP are 17.5 and 14.7 kJ mol−1, respectively.

1 lists relative energies of the syn and the anti conformers of both o-TFMP and m-TFMP calculated at various levels of Table 1. Relative Energies (kJ mol−1) of o-TFMP and mTFMP Calculated at Various Levels of Theorya

syn-o-TFMP anti-o-TFMP syn-m-TFMP anti-m-TFMP

MPWLYP

MP2(FC)

G3MP2 (0 K)

G3B3 (0 K)

νOH

0.0 5.8 0.0 0.6

0.0 6.4 0.0 0.7

0.0 5.2 0.0 0.7

0.0 4.1 0.0 0.8

3637 3642 3642 3645

a The phenolic O−H stretching frequencies (cm−1) were calculated at the MP2(FC)/aug-cc-pVDZ level of theory.

theory, and in all cases, the syn conformer is more stable than the anti conformer. The stabilization of the syn conformer of oTFMP relative to the anti conformer can possibly be attributed to the interaction of the OH group with the CF3 group, which also manifests in higher rotational barrier of 17.5 kJ mol−1 in the case of o-TFMP. The corresponding rotational barrier for m-TFMP is 14.7 kJ mol−1. The laser induced fluorescence (LIF) excitation spectra of oTFMP and m-TFMP are depicted in Figure 2. The LIF excitation spectrum of o-TFMP is crowded with several transitions, whereas the corresponding spectrum of m-TFMP has relatively sparse transitions. Since both o-TFMP and mTFMP are highly hygroscopic, similar to phenol, there is a possibility that some of the transitions observed in both spectra might be due to water complexes. In both cases, the most intense transitions at 35 783 and 35 738 cm−1 are assigned to the band-origin transition of the syn conformer as the syn conformer is more stable in both cases (see Table 1). The difference in the energies of syn and anti conformers of oTFMP is 6.4 kJ mol−1, which is much higher than the corresponding difference of 2.2 kJ mol−1 for the syn and anti conformers of o-cresol.3b Such a large energy difference between the syn and the anti conformers of o-TFMP suggests

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RESULTS AND DISCUSSION Figure 1 shows the potential energy scan for orthotrifluoromethylphenol (o-TFMP) and meta-trifluoromethylphenol (m-TFMP) along the C−O bond rotation. In the case of oTFMP the two minima correspond to the syn and the anti conformers, of which the syn conformer is relatively more stable by 6.4 kJ mol−1. On the other hand the syn and the anti conformers of m-TFMP are almost isoenergetic, with the syn conformer being marginally more stable by 0.7 kJ mol−1. Table 6997



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substituted phenols such as cresol and p-cyanophenol, wherein the O−H stretching vibration appears at 3657 cm−1.3,16,17 Comparison of IR spectra of the syn conformers of o-TFMP and m-TFMP clearly rules out the formation of O−H···F hydrogen bonding in syn conformer of o-TFMP. This result is contrary to what has been proposed based on the electrondiffraction data and the rotational spectra.6,7 In view of the fact that the intramolecular of the O−H···F hydrogen bond does not exist in the syn conformer of o-TFMP, the stability of syn-oTFMP relative to anti-o-TFMP can therefore be attributed to the gauche effect, which minimizes the repulsion between the lone-pair electrons on the oxygen and fluorine atoms. In addition to the transitions from the bare molecules, the LIF excitation spectra of o-TFMP and m-TFMP also show transitions arising out of their water complexes. This is due to the fact both o-TFMP and m-TFMP are hygroscopic in nature and also accompanied by the fact that the gas lines have some residual water vapor. The FDIR spectra of the o-TFMP−water complex in the O−H stretching region are depicted in Figure 4. Figure 2. Laser induced fluorescence (LIF) excitation spectra of (A) oTFMP and (B) m-TFMP. The bands at 35 783 and 35 738 cm−1 correspond to S0 ← S1 band origin transitions of o-TFMP and mTFMP, respectively, whereas the bands at 35 540 and 35 572 cm−1 correspond to their water complexes.

that only the syn conformer would be populated. On the other hand for m-TFMP, the energy difference between the of syn and anti conformers is only 0.7 kJ mol−1, which implies that both the conformers could be populated. The IR spectra of o-TFMP and m-TFMP in the O−H stretching region, recorded using FDIR technique, are depicted in Figure 3. Surprisingly, both the IR spectra are almost identical with the O−H stretching vibration appearing at 3655 cm−1. Further, it can be noted that a shoulder appears in the O−H stretching vibration, which is a common phenomenon for the IR spectra of phenols and has been attributed to the power fluctuation on account of ambient water vapor absorption.15 These two spectra are also very similar to the phenol and other

Figure 4. FDIR spectrum of o-TFMP-H2O complex recorded by probing the band at 35 540 cm−1 with (A) LiNbO3 OPO and (B) KTP OPO. Also presented are the simulated IR spectra for the structures OTPW1−OTPW4.

Trace A shows the IR spectrum in the O−H stretching region, which shows a single band at 3747 cm−1 corresponding to the asymmetric stretching vibration of a bound water molecule. Evidently, the phenolic O−H band does not appear in this spectrum, which is due to the hole in the region 3450−3520 cm−1 for the LiNbO3 OPO. Therefore the IR spectrum of the o-TFMP−water complex was once again recorded using KTP OPO and is depicted in Figure 4B. Two bands at 3747 and 3438 cm−1 can be clearly seen in the spectrum, which are assigned to the asymmetric stretching vibration of the bound water molecule and the hydrogen-bonded O−H stretching

Figure 3. Fluorescence dip infrared (FDIR) spectra of (A) o-TFMP and (B) m-TFMP. The two spectra were recorded by monitoring the fluorescence while probing the respective S0 ← S1 band origin transitions at 35 738 and 35 738 cm−1. 6998



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vibration of the o-TFMP moiety, respectively. From the appearance of this spectrum it can be inferred that o-TFMP forms an O−H···O hydrogen-bonded complex with the water molecule, similar to that of phenol and other substituted phenols.16−18 The FDIR spectra of the water complex of m-TFMP, recorded using LiNbO3 and KTP OPOs are shown in Figure 5.

lower energy compared to the anti conformer. The stabilization energies along with O−H stretching frequencies and their shifts relative to the bare syn-o-TFMP are listed in Table 2. The first Table 2. ZPVE Corrected Stabilization Energies (kJ mol−1) and the O−H Stretching Frequencies along with Their Shifts (cm−1) for Various Water Complexes of o-TFMP Calculated at the MP2(FC)/aug-cc-pVDZ Level of Theory

a

structure

ΔEa

ΔEb

ΔEc

νO−H

OTPW1

−24.8

−20.3

−15.8

OTPW2

−16.5

−13.5

−10.4

OTPW3

−15.6

−10.9

−6.2

OTPW4

−10.9

−8.0

−5.2

3405 3754 3630 3584 3743 3624 3631 3743 3620 3638 3765 3637

ΔνO−H −250 −2 −27 −71 −13 −33 −24 −13 −37 −17 +9 −20

No BSSE correction. b50% BSSE correction. c100% BSSE correction.

structure (OTPW1) is characterized by the presence of the O− H···O hydrogen bond between the phenolic OH group of syn-oTFMP and the oxygen atom of the water molecule. In the second structure OTPW2 has one of the OH groups of the water molecule hydrogen-bonded to the phenolic oxygen atom of syn-o-TFMP. The OPTW3 structure is characterized by the presence of O−H···π hydrogen bonding between one of the OH groups of a water molecule and the π electron density of the phenyl ring. Finally in the case of the OTPW4 structure, the water molecule forms a cyclic hydrogen-bonded complex with o-TFMP, wherein the water molecule acts as both hydrogen bond donor and acceptor. In this structure, the OH group of water is hydrogen-bonded to the fluorine atom of the CF3 group, while the oxygen atom of water interacts with the aromatic CH group in the ortho position. The simulated IR spectra in the O−H stretching region for various structures of o-TFMP−water complex are also presented in Figure 4. Comparison of the experimental spectrum with the simulated spectra reveals an excellent agreement with structure OTPW1. Therefore, the observed water complex of o-TFMP is assigned to the structures OTPW1, wherein the OH group of o-TFMP forms a hydrogen bond with the oxygen atom of the water molecule. The formation of O−H···O hydrogen bonding leads to a red shift of 217 cm−1 in the phenolic O−H stretching frequency, much larger than the corresponding shift of 143 cm−1 in the phenol−water complex.16 The larger shift in the

Figure 5. FDIR spectrum of the m-TFMP−H2O complex recorded by probing the band at 35 572 cm−1 with (A) LiNbO3 OPO and (B) KTP OPO. Also presented are the simulated IR spectra for the structures MTPW1−MTPW10.

The appearance of FDIR spectrum for water complex of mTFMP is very similar to that of o-TFMP water complex, with two bands at 3751 and 3461 cm−1, corresponding to the asymmetric stretching vibration of a bound water molecule and the hydrogen-bonded O−H stretching vibration of m-TFMP moiety, respectively. Comparison with the IR spectra of the bare molecules (see Figure 3) indicates that the phenolic O−H stretching vibrations shift to low frequency by 217 and 194 cm−1 in the water complexes of o-TFMP and m-TFMP, respectively. Geometry optimization calculations converged on to four minima for the single water complex of o-TFMP, structures of which are depicted in Figure 6. In these calculations only the syn conformer of o-TFMP was considered due to its much

Figure 6. Optimized structures of the o-TFMP−H2O complex calculated at the MP2(FC)/aug-cc-pVDZ level of theory. 6999



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Table 3. ZPVE Corrected Stabilization Energies (kJ mol−1) and the O−H Stretching Frequencies along with Their Shifts (cm−1) for Various Water Complexes of m-TFMP Calculated at the MP2(FC)/aug-cc-pVDZ Level of Theory

present case, in comparison with that of the phenol−water complex, can be attributed to the electron withdrawing nature of the CF3 group. In the case of p-cyanophenol the corresponding shift was 174 cm−1, which can also be attributed to the withdrawing nature of the CN group.17 In the case of a water complex of m-TFMP, the geometry optimization calculations converged to ten structures, five each for the syn and the anti conformers, which are depicted in Figure 7. Both the syn and the anti conformers were considered

a

structure

ΔEa

ΔEb

ΔEc

νO−H

MTPW1

−29.5

−26.0

−22.6

MTPW2

−15.9

−13.0

−10.1

MTPW3

−16.2

−11.4

−6.7

MTPW4

−13.5

−10.1

−6.8

MTPW5

−18.1

−14.6

−11.1

MTPW6

−28.4

−25.1

−21.9

MTPW7

−15.0

−11.8

−8.6

MTPW8

−16.6

−11.8

−7.0

MTPW9

−14.4

−10.7

−7.4

MTPW10

−13.2

−9.8

−6.3

3450 3756 3632 3592 3746 3643 3640 3737 3620 3643 3756 3631 3643 3752 3631 3452 3753 3629 3596 3747 3645 3643 3741 3618 3644 3756 3631 3643 3757 3632

ΔνO−H −205 0 −25 −63 −10 −14 −15 −19 −37 −12 0 −26 −12 −4 −26 −203 −3 −28 −59 −9 −12 −12 −15 −39 −11 0 −25 −12 0 −24


and have similar intermolecular structures to MTPW1, MTPW2, MTPW3, MTPW4, and MTPW5, respectively. The simulated spectra in the O−H stretching region for various structures of m-TFMP water complex are also presented in Figure 5. Comparison of the experimental spectrum with the simulated spectra indicates that MTPW1 and MTPW6 are possible structures. In the case of m-TFMP the syn conformer is marginally (about 0.7 kJ mol−1) more stable than the anti conformer. Further, the MTPW1 structure is more stable by 0.9 kJ mol−1 than the MTPW6 structure from corresponding bare molecules. On the absolute energy scale MTPW1 structure would be more stable than MTPW6 by 1.6 kJ mol−1. Therefore the observed m-TFMP-H2O complex is assigned to MTPW1 structure. In an effort to sort out various transitions in the LIF excitation spectra, IR-UV hole burning spectroscopy was carried out, and the results for o-TFMP are presented in Figure 8. Trace A of Figure 8 shows the LIF excitation spectrum of o-TFMP, whereas trace 8B is the IR-UV hole-burnt spectrum recorded by pumping the IR transition at 3655 cm−1, which corresponds to the O−H stretching vibration of the oTFMP monomer. Trace 8C is the difference spectrum, which was obtained by subtracting the LIF spectrum from the holeburnt spectrum. Transitions corresponding to syn-o-TFMP show negative intensities in the difference spectrum. Trace 8D

Figure 7. Optimized structures of m-TFMP−H2O complex calculated at the MP2(FC)/aug-cc-pVDZ level of theory.

for calculation as the difference in the energies is only marginal. Table 3 lists the stabilization energies along with O−H stretching frequencies and their shifts relative to the bare molecules. The structure MTPW1 is characterized by hydrogen bonding between the phenolic OH group and the oxygen atom of the water molecule for the syn-m-TFMP. In the structure MTPW2, one of the OH groups of the water molecule forms a hydrogen bond with the oxygen atom of the phenolic OH group. The next structure MTPW3 is characterized by the presence of the O−H···π hydrogen bond to the π electron density of the phenyl ring. Structures MTPW4 and MTPW5 are cyclic complexes, characterized by the presence of O−H···F and C−H···O hydrogen bond. These two structures differ in the orientation of water molecule relative to the phenolic OH group. The structures MTPW6, MTPW7, MTPW8, MTPW9, and MTPW10 correspond to the anti conformer of m-TFMP 7000



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Figure 9. (A) LIF excitation spectrum of m-TFMP in the presence of water. (B) IR-UV hole-burnt spectrum recorded by pumping the phenolic O−H stretching vibration of m-TFMP monomer at 3655 cm−1. (C) Difference spectrum B-A. (D) IR-UV hole-burnt spectrum recorded by pumping asymmetric O−H stretching vibration of water moiety in m-TFMP−H2O complex at 3751 cm−1. (E) Difference spectrum D-A. The transitions marked with “*” and “+” correspond to m-TFMP and the m-TFMP−H2O complex, respectively.

Figure 8. (A) LIF excitation spectrum of o-TFMP in the presence of water. (B) IR-UV hole-burnt spectrum recorded by pumping phenolic O−H stretching vibration of o-TFMP monomer at 3655 cm−1. (C) Difference spectrum B-A. (D) IR-UV hole-burnt spectrum recorded by pumping asymmetric O−H stretching vibration of water moiety in oTFMP−H2O complex at 3747 cm−1. (E) Difference spectrum D-A. (F) IR-UV hole-burnt spectrum recorded by pumping asymmetric O− H stretching vibration of water moiety in o-TFMP−H2O complex at 3585 cm−1. (G) Difference spectrum F-A. The transitions marked with *”, “+” and “−” correspond to o-TFMP, o-TFMP−H2O, and oTFMP−(H2O)2, respectively.

spectrum from the hole-burnt spectrum. Similarly, the IR-UV hole-burnt spectrum was recorded for the m-TFMP−H2O complex by pumping the asymmetric O−H stretching vibration of water moiety at 3751 cm−1 (trace 9D). Trace 9E is the difference spectrum (traces 9D−9A). With these two holeburnt spectra, all of the transitions appearing in the LIF excitation spectrum of m-TFMP (Figure 2B) can be assigned to be originating from either bare m-TFMP or m-TFMP−H2O complex. It is surprising that the IR-UV hole-burning spectroscopic measurements could identify only a single conformer of m-TFMP, even though the energy difference between the syn and the anti conformers of m-TFMP is only about 0.7 kJ mol−1. This can be attributed to the fact that the IR-UV hole burning spectroscopy would bunch both of the conformers together if the phenolic O−H stretching frequencies of both of the conformers are identical (within the bandwidth of the laser). Therefore the absence of the anti conformer of m-TFMP cannot be ruled out unambiguously. In order to assign the two bands observed at 35 567 and 35 871 cm−1, FDIR spectra probing this band were recorded, and the results are presented in Figure 10. The FDIR spectrum recorded using the LiNbO3 OPO (Figure 10A) shows four distinct transitions, two sharp closely spaced transitions at 3734 and 3721 cm−1, a strong transition at 3585 cm−1, and a broad transition at 3331 cm−1. The corresponding FDIR spectrum recorded using KTP OPO (Figure 10B) shows an additional band was observed at 3462 cm−1 which was absent in spectrum obtained using the LiNbO3 OPO (LiNbO3 OPO has hole in the tuning region ∼3450−3520 cm−1). The appearance of five bands in the O−H stretching region is a clear indication of the fact that the two bands appearing at 35 567 and 35 871 cm−1 in

is the IR-UV hole-burnt spectrum of the o-TFMP−H2O complex recorded by pumping the asymmetric O−H stretching of water moiety at 3747 cm−1, and the difference spectrum is depicted in trace 8E (traces 8D−8A). The difference spectrum shows two transitions with negative intensities at 35 540 and 35 840 cm−1, which correspond to the o-TFMP−H2O complex. Trace 8F is the IR-UV hole-burnt spectrum of o-TFMP− (H2O)2 complex, vide infra, recorded by pumping the O−H stretching vibration of water moiety at 3585 cm−1, and the difference spectrum is depicted in trace 8G (traces 8F−8A). Two negative transitions appearing at 35 567 and 35 871 cm−1 correspond to the o-TFMP-(H2O)2 complex. With this all of the transitions appearing in the LIF excitation spectrum of oTFMP have been accounted for. The difference spectra of the o-TFMP−H2O and o-TFMP−(H2O)2 complexes (Figure 8E,G) show some activity for the bands corresponding to the bare o-TFMP. A closer examination reveals that most of the bands have first derivative shape due to the error in the reproducibility of the dye laser tuning and also due to small intensity fluctuations. Results of IR-UV hole burning spectroscopic measurements for m-TFMP are presented in Figure 9. Trace 9A is the LIF excitation spectrum, whereas trace 9B is the hole-burnt spectrum recorded by exciting the O−H stretching vibration of m-TFMP at 3655 cm−1. Trace 9C is the difference spectrum, which was obtained by subtracting the LIF 7001



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Figure 11. Optimized structures of o-TFMP−(H2O)2 complexes calculated at the MP2(FC)/aug-cc-pVDZ level of theory. Figure 10. FDIR spectrum of the o-TFMP−(H2O)2 complex recorded by probing the band at 35 567 cm−1 with (A) LiNbO3 OPO and (B) KTP OPO. Also presented are the simulated IR spectra for the structures OTP2W1−OTP2W4.

Table 4. ZPVE Corrected Stabilization Energies (kJ mol−1) and the O−H Stretching Frequencies along with Their Shifts (cm−1) for Various Water Complexes of o-TFMP-(H2O)2 Complexes Calculated at the MP2(FC)/aug-cc-pVDZ Level of Theory

the LIF excitation spectrum of o-TFMP (Figure 2) correspond to the o-TFMP−(H2O)2 complex. The optimized structures of o-TFMP−(H2O)2 are depicted in Figure 11 and Table 4 lists the stabilization energies along with O−H stretching frequencies and their shifts relative to the bare molecules. In the view of the fact that the syn conformer of o-TFMP is much lower in energy compared to the anti conformer, only the syn-o-TFMP was considered for optimizing the structures of the o-TFMP−(H2O)2 complex, similar to the o-TFMP−H2O complex . In the structure OTP2W1, both the water molecules act as acceptors of hydrogen bonding. One of the water molecules binds with the phenolic OH group, whereas the second water molecule interacts with the aromatic CH group present in the ortho position to the trifluoromethyl group. In the structures OTP2W2 and OTP2W3, the phenolic OH acts both as a hydrogen bond donor and acceptor. The hydrogen of phenolic OH forms hydrogen bonding with one water molecule while the hydrogen of the other water molecule forms a hydrogen bond with the oxygen of phenolic OH group. However, OTP2W2 is a cyclic structure with hydrogen bonding interaction between both the water molecules while in OTP2W3 is an acyclic structure and is devoid of any interaction between the two water molecules. In the structure OTP2W4, the two water molecules bridge the donor (phenolic OH group) and the acceptor (fluorine atom of CF3 group) sites of o-TFMP. The experimental FDIR spectrum recorded using LiNbO3 and KTP OPO along with the simulated IR spectra obtained for the o-TFMP−(H2O)2 complexes are presented in Figure 10. Comparison of the simulated IR spectra and the energetics of the observed o-TFMP−(H2O)2 complex is assigned the OTP2W2 structure, wherein the two water

a

structure

ΔEa

ΔEb

ΔEc

νO−H

OTP2W1

−32.6

−25.6

−18.7

OTP2W2

−53.3

−44.9

−36.5

OTP2W3

−44.0

−36.2

−28.5

OTP2W4

−50.5

−42.2

−33.9

3407 3759 3745 3628 3635 3234 3731 3719 3541 3493 3368 3751 3734 3630 3555 3308 3748 3719 3624 3458

ΔνO−H −248 +3 −11 −29 −22 −421 −25 −37 −116 −164 −287 −5 −22 −27 −102 −347 −8 −37 −33 −199


molecules along with the phenolic OH group form a sixmembered cyclic structure, similar to the phenol−(H2O)2 complex.18 It must be pointed out here that the single and two water complexes of anti-o-TFMP were also calculated (data not presented) and compared with the corresponding syn-oTFMP complexes. The structures of the anti conformer for 7002



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both o-TFMP−H2O and o-TFMP−(H2O)2 complexes had higher stabilization energies and better agreement with experimental spectra. However, formation of hydrogen bonded complexes of anti-o-TFMP was ruled out due to the fact that the population of anti-o-TFMP, to begin with, is negligible in comparison to that of syn-o-TFMP.

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CONCLUSIONS The electronic and vibrational spectroscopic measurements were carried out on o-TFMP and m-TFMP. The band origin transitions for the S1 ← S0 electronic excitation of syn conformers of o-TFMP and m-TFMP occur at 35 783 and 35 378 cm−1, respectively. The FDIR spectra in the O−H stretching region of both on o-TFMP and m-TFMP were identical with the band appearing at 3655 cm−1. This signifies that the trifluoromethyl group does not interact with the OH group in syn-o-TFMP, thereby ruling out the possibility of formation of an intramolecular O−H···F hydrogen bond. The present result is in complete contrast to the finding reported by electron diffraction studies on the single crystals and rotational spectra in the gas phase. The water complexes of both o-TFMP and m-TFMP are characterized by the O−H···O hydrogen bond, wherein the phenolic OH group interacts with the oxygen atom of the water molecule. The shifts in the O−H stretching frequencies for o-TFMP and m-TFMP were 217 and 194 cm−1, respectively. The present O−H stretching frequency shifts are much larger than in the case of the phenol−water complex, which can be attributed to the electron withdrawing nature of the CF3 group. The water dimer complex of o-TFMP was also observed, the structure of which has been assigned to a cyclic complex, wherein the water molecules bridge the phenolic OH group. The intermolecular structures of oTFMP−H2O, m-TFMP−H2O, and o-TFMP−(H2O)2 complexes are very similar to the corresponding phenol complexes. The structural assignments clearly indicate that in the case of oTFMP the CF3 group does not directly participate in hydrogen bonding but modulates the hydrogen bond strength due to its electron withdrawing nature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.D. thanks CSIR for the research fellowship. This material is based upon the work supported by Department of Science and Technology (Grant No. SR/S1/PC/23/2008) and Council of Scientific and Industrial Research (Grant No. 01(2268)/08/ EMR-II).

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REFERENCES

(1) Maity, S.; Maity, D. K.; Patwari, G. N. J. Phys. Chem. A 2011, 115, 11229. (2) (a) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1984, 88, 5180. (b) Oikawa, A.; Ito, M. J. Mol. Struct. 1985, 126, 133. (c) Muzini, H.; Okuyama, K.; Ebata, T.; Ito, M. J. Phys. Chem. 1987, 91, 5589. (d) Lipert, R. J.; Colson, S. D. J. Phys. Chem. 1989, 93, 3894. (e) Huang, J.; Lin, J. L.; Tzeng, W. B. Spectrochem. Acta. A 2007, 67, 989. (f) Aota, T.; Ebata, T.; Ito, M. J. Phys. Chem. 1989, 93, 3519. (g) Ito, M.; Yamamoto, S.; Aota, T.; Ebata, T. J. Mol. Struct. 1990, 237, 105. 7003


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