Article pubs.acs.org/JPCA
C−H···Y Hydrogen Bonds in the Complexes of p‑Cresol and p‑Cyanophenol with Fluoroform and Chloroform Pranav R. Shirhatti,†,§ Dilip K. Maity,‡ and Sanjay Wategaonkar*,† †
Department of Chemical Sciences, Tata Institute of Fundamental Research, Mumbai 400005, India Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India
‡
ABSTRACT: In this work, we present spectroscopic investigations of hydrogen bonded complexes of CHF3 and CHCl3 with p-cresol and p-cyanophenol. The systems were chosen as the potential candidates bound by C−H···Y type hydrogen bonds that are known to exhibit unconventional blue shifts in the C−H stretching frequency. The two phenol derivatives chosen offer multiple hydrogen bonding acceptor sites. They also differ from each other in regard to the electron-donating/ withdrawing ability of the para substituents which could dictate the global minimum structure in each case. The complexes were formed using the supersonic jet expansion method and were investigated using a variant of the IR-UV double resonance technique, namely fluorescence depletion IR (FDIR) spectroscopy. It was found that in the case of p-cresol the complexes were C−H···π bound in which the C−H stretch was blue-shifted. In the case of p-cyanophenol the complexes were C−H···N bound. In its fluoroform complex the C−H frequency was blue-shifted by 27 cm−1, whereas the chloroform complex gave an example of zero-shifted hydrogen bond. The ab initio computational studies indicated that for the CHCl3 complexes it is necessary to optimize the structures on the BSSE-corrected PES using the counterpoise method to correctly predict the magnitudes of the C−H frequency shift.
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INTRODUCTION The phenomenon of hydrogen bonding has been studied extensively over decades and continues to attract the attention of a large number of researchers to date. The conventional definition of the hydrogen bond (H-bond) has also evolved as a number of examples of novel H-bonds have been reported in recent times. An exhaustive review of this evolution can be found in the recent report which also gives a modern definition of the hydrogen bond.1 Among the novel H-bonds, the C−H···Y [Y = H-bond acceptor] hydrogen bonding interaction has caught the attention of several researchers, as it exhibited an unusual blue shift in the stretching frequency of the H-bond-donating C−H bond. Hobza and co-workers2 performed the first systematic investigation on such interactions and were able to identify the blue-shifted hydrogen bond in the chloroform−fluorobenzene complex in the gas phase. They reported the interaction to be of the C−H···π type with a blue shift of 14 cm−1 along the C−H stretch mode. The halomethanes are a special class of molecules capable of donating H-bonds in which the C−H bond is slightly activated in comparison to that of simple alkanes. Intense activity followed thereafter with numerous reports of such unusual blueshifted hydrogen bonding interactions, where such C−H···Y complexes were investigated by ab initio computational chemistry methods3−5 and vibrational spectroscopy experiments in cryosolutions.6−8 From a theoretical standpoint, the nature of the C−H H-bond donor is not yet clearly understood. Hobza and co-workers have proposed that in the case of the blue-shifted H-bonds, unlike in conventional H-bonds, the charge transfer from the electron rich © 2013 American Chemical Society
H-bond acceptor is not limited to the X−H antibonding orbital (where X−H is the H-bond donor). It was claimed that the electron density also gets redistributed in the remote parts of the H-bond donor molecule which manifests in the blue shift in the X-H stretching frequency in a two-step process.6 There has also been a counterview that there is nothing special about the blueshifted H-bonds in regard to the charge transfer to various parts of the donor molecule. It was shown that in all (blue- as well as red-shifted) types of C−H H-bond donating systems there was no direct correlation between the shift in the C−H stretching frequency and the electron density shifts into the various parts of the donor molecule.9 Apart from the blue-shifted H-bonds, the cold matrix experiments have identified that the C−H moiety of halomethanes can also form red-shifted H-bonds.8 It must be remembered, however, that the magnitudes of frequency shifts in the C−H stretching vibrations in such complexes are usually small and of the same order as those of the matrix effects. Gasphase vibrational spectroscopic methods, especially in sizeselected molecular clusters prepared under collisionless conditions, offer several advantages in regard to the investigations of weak intermolecular interactions. It provides spectroscopic information which is free from all the perturbations and complexities induced by solvent−solute interactions that exist in the condensed-phase measurements. The data obtained in Received: November 25, 2012 Revised: January 15, 2013 Published: February 4, 2013 2307
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spatially overlapped in a counter propagating manner. With the help of electronic delays, the lasers were synchronized such that the IR pulse preceded the UV (probe) pulse by approximately 100 ns. The frequency calibration of the IR−OPO in the O−H stretch region (3500 to 3900 cm−1) and the C−H stretch region (2900 to 3150 cm−1) was performed by recording the photoacoustic spectrum of water and the absorption spectrum of methane, respectively. The standard spectra available in the HITRAN spectroscopy database15 were used for calibrating the OPO. The uncertainties in the IR frequencies were found to be of the order of 0.5 cm−1. The phenols PCR and PCNP, being solid at room temperature, were heated to 60 °C and 120 °C, respectively, to generate sufficient vapor for an optimum signal-to-noise ratio. The complexes were prepared by coexpanding the phenol vapors along with a mixture of 1−5% CHX3 in helium. The stagnation pressure in the present experiments typically ranged from 1 to 2 kg·cm−2. Computational Methods. An important characteristic of the H-bonded systems involving the C−H oscillator as the Hbond donor is the small magnitudes of either blue or red shifts in the C−H stretching frequency. The other characteristic is the large extent of the dispersion contribution in stabilizing these complexes. Therefore, to identify the acceptable common method of choice, ab initio electronic structure calculations (geometry optimization and harmonic frequency analysis) were carried out using DFT and MP2 methods and two different fairly large basis sets, viz., 6-311++G** and aug-cc-pVDZ. In the DFT calculations many functionals that appear in the literature were tried. Some of these have been reported to recover part of the electronic correlation energy at relatively smaller computational cost compared to the MP2 calculations. The PCR−CHCl3 complex was used as the test system. The results from a few select functionals such as B3LYP, LC-ωPBE, and PW1PW91 are presented. Based on the comparison of the results that include the frequency shifts, the number of common conformers that converged, and their relative binding energies, the best possible computational method was chosen for the remaining complexes. Our conclusion (see Results and Discussion for details) was, that among the methods mentioned above, the MP2 method was the most suitable for all the complexes. For the CHCl3 complexes, however, this was still not satisfactory and the MP2 computation using the BSSE-corrected (using the counterpoise method) potential energy surface was necessary to predict the magnitude of the C−H frequency shifts correctly; for the CHF3 complexes, it did not improve the C−H shifts significantly. Because the optimization using the BSSE-corrected PES was computationally much more expensive (more than five times that of the standard gradient methods), it was used only in specific cases where a large discrepancy was observed between the experimentally determined values of the C−H shift and their computed value. In the other cases the MP2 method with standard gradients was used. In each of the cases, the geometry optimization was accompanied by the harmonic frequency analysis to ensure that the true minimum energy structures were obtained. The binding energies for each of these complexes were evaluated after accounting for the zero point energy (ZPE) and the BSSE. All the computations were performed using Gaussian09 suite of programs.16 In addition, the analysis of the spatial distribution of the electron density was carried out to identify the bond critical points corresponding to the H-bond formation. These calculations were performed by the atoms-in-molecules (AIM)17 approach using the AIM2000 suite of programs.18 To gain
such measurements can be directly compared with the ab initio computations. However, despite the wide interest in the unusual blue-shifted H-bonds, the number of gas-phase investigations of such systems has remained dismally small.2,10−13 Preliminary computational studies reported in the literature5 and by our group14 have shown that the complexes of chloroform with substituted phenols are potential candidates for observing blue-shifted H-bonded interactions. These molecules also offer an interesting option of multiple H-bond acceptor sites. In this work we report the UV and IR-UV double resonance spectroscopic investigations of the complexes of CHX3 (X = F, Cl) with p-cresol (PCR) and p-cyanophenol (PCNP) with the aim of identifying the nature and preferred site of hydrogen bonding interaction. While in PCR the acceptor site could be the oxygen atom or the aromatic π electron density, in PCNP the π subsystem of the CN group as well as the nitrogen atom provides additional acceptor sites. In addition, the para substituents in these two derivatives differ from each other in regard to their electron-donating/withdrawing ability, which could also have an influence in directing the acceptor site. On the basis of our combined experimental and ab initio studies, we show that in all the complexes the C−H bond of CHX3 moiety was acting as the H-bond donor. While in the complexes with PCR the aromatic π electron cloud was the acceptor site, in the case of PCNP the complexes were N-atom bound. In all the complexes except the PCNP−CHCl3 complex the C−H stretching frequency was blue-shifted. In the PCNP−CHCl3 complex the C−H group, though acting as the H-bond donor, shows no shift. We have assigned this as a hydrogen bond with a zero shift.
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METHODS Experimental Methods. A detailed description of the experimental apparatus can be found in our previous works,13 and only a brief description is presented here. The supersonic jet expansion method was used to form the complexes of phenols (PCR, PCNP) with CHX3 in the gas phase. These complexes were probed by laser-induced fluorescence (LIF) and two-color resonantly enhanced multiphoton ionization (2c2p-REMPI) to obtain their electronic excitation spectra. For the LIF and REMPI measurements, the frequency-doubled output of Nd3+:YAG (fwhm ∼6 ns, 10 Hz repetition rate) pumped dye lasers working in the appropriate UV wavelength range were employed as the excitation and the ionization source. In the case of LIF measurements, the fluorescence emission was collected by a PMT/filter combination (Hamamatsu, 1P28). In the case of REMPI measurements, the ions were detected using a single channel electron multiplier (diam 25 mm, Dr. Sjuts Optotechnik, GmbH; KBL25RS). Considering the weak REMPI signals due to the haloform complexes of PCR and PCNP, for achieving respectable signalto-noise ratio, the fluorescence depletion IR (FDIR) spectroscopic method was preferred over RIDIRS (resonant ion depletion IR spectroscopy) for recording the vibrational spectra in this work. In our experimental setup, it was observed that probing closer to the nozzle (∼15 mm) using the LIF method yielded a more steady signal compared to that of the REMPIbased detection method where the probe volume was further away from the nozzle (∼100 mm). To generate tunable IR for the vibrational spectroscopy measurements, a LiNbO3-based optical parametric oscillator (OPO) with a line-narrowing etalon (LaserSpec, 2.6−4.0 μm, bandwidth ∼0.5 cm−1), pumped by a seeded Nd3+:YAG laser (Spectra Physics, Quanta-Ray Pro230− 10, 10 Hz, ∼10 ns fwhm), was used. The IR and UV beams were 2308
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further insight into the nature of C−H···Y hydrogen bonds especially in terms of the localized pairwise orbital interaction picture, natural bond orbital analysis was carried out using the NBO 5.0 suite of programs.19
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RESULTS Experimental Results. PCR−CHX3 Complexes. Figure 1 shows the LIF spectra of the PCR monomer as well as the PCR−
Figure 2. FDIR spectrum of the (a) PCR monomer, (b) PCR−CHF3 complex, and (c) PCR−CHCl3 complex with the probe at peak A, (d) PCR−CHCl3 with the probe at peak B in the C−H and the O−H stretch region. The feature observed at 3052 cm−1 (trace b) was assigned as the C−H stretch of the CHF3 moiety. The feature observed at 3044 cm−1 was identified as the C−H stretch of the CHCl3 moiety in the complex. The O−H stretch of the PCR moiety in both the complexes was found to undergo a very small red shift.
mode of the CHF3 moiety in the PCR−CHF3 complex. The C− H stretching frequency for the free CHF3 is 3035 cm−1,22 which means that the C−H stretch mode of the CHF3 in the complex is blue-shifted by 17 cm−1. Similar measurements in the O−H stretch region showed that the O−H stretch mode of the PCR− CHF3 complex undergoes a small red shift of 3 cm−1 (trace 2b), suggesting that the O−H site does not take part in the complex formation. The LIF excitation spectrum for the PCR−CHCl3 complex is also shown in Figure 1 (trace c). Two new features were observed at 45 and 66 cm−1 (marked as A and B, respectively) with respect to the monomer band origin. Unlike the CHF3 case, the observed features were not sharp and not as strong. The new features, however, indeed belonged to the PCR−CHCl3 complex and was confirmed by recording the REMPI spectrum in the time-offlight mass spectrometer, as shown in trace d of Figure 1. Along with the two strong features at 45 and 66 cm−1, a few more weak features were also observed in the REMPI spectrum. The break in the REMPI spectrum (trace d) at the PCR monomer band origin was an experimental artifact. Because of the huge monomer ion flux at this wavelength the ion detector momentarily becomes unresponsive due to saturation. The transitions appearing at 45 and 66 cm−1 could possibly be the band origin transitions corresponding to two different conformers of the complex or the members of a progression corresponding to the same structure. This issue was resolved by probing each of the bands while recording the FDIR spectra. Figure 2c,d shows the FDIR spectra of the PCR−CHCl3 complex, recorded by probing the electronic transitions labeled as A and B, respectively. Both spectra were identical over the entire C−H and O−H stretch regions. This indicates that the transitions A and B correspond to the same conformer of the PCR−CHCl3 complex, and these were assigned as the members of a small progression in a low frequency mode of ∼21 cm−1 with the third member of the progression appearing as
Figure 1. Electronic excitation spectra of PCR and its complexes with CHF3 and CHCl3; LIF spectra using (a) pure He, (b) CHF3/He mixture, and (c) CHCl3/He mixture as the buffer gas plotted with respect to the monomer band origin observed at 35331 cm−1; (d) the 2c2p-REMPI spectrum of the PCR−CHCl3 complex. The new feature observed at 101 cm−1 in trace b was assigned as the band origin transition for the PCR−CHF3 complex. Two strong features marked by A and B in traces c and d were observed at 45 and 66 cm−1 with respect to the monomer band origin, respectively.
CHX3 complexes. The PCR monomer band origin transition was observed at 35331 cm−1 (trace a). The new feature appearing (trace b) at 101 cm−1 on the blue side with respect to the PCR band origin when the CHF3/He mixture was used as buffer gas was assigned as the band origin transition for the PCR−CHF3 complex. The IR spectra for the monomer as well as the complex were recorded by the FDIR method using the respective band origin transitions as probe. Figure 2a shows the IR spectrum of the PCR monomer. The O−H stretch mode was observed at 3657 cm−1, which is in good agreement with the previously reported values.20,21 In the C−H stretch region, many features were observed as expected for a molecule such as PCR that has multiple C−H oscillators. Trace b in Figure 2 shows the IR spectrum of the complex. In the C−H stretch region, the spectrum of the complex was essentially similar to that of the monomer except that one additional sharp transition appeared at 3052 cm−1. This new feature was assigned as the C−H stretch 2309
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a weak transition. The most important feature to be noted in the IR spectrum was that a new intense transition was observed at 3044 cm−1. This feature was assigned to the C−H stretch mode of the CHCl3 moiety in the PCR−CHCl3 complex. Compared to the C−H stretch frequency in free CHCl3, viz., 3033 cm−1,22,23 the C−H stretch mode in the complex was blue-shifted by 11 cm−1. Apart from this new feature, the spectrum in the C−H region is essentially similar for the monomer and the complex. The O−H mode in the complex was red-shifted by 4 cm−1 with respect to that in the monomer. PCNP−CHX3 Complexes. The LIF excitation spectra for the PCNP and its complexes are shown in Figure 3. Because the LIF
Figure 4. FDIR spectra of (a) PCNP, (b) PCNP−CHCl3, (c) PCNP− CDCl3, and (d) PCNP−CHF3 complexes in the C−H and O−H stretch region. The feature appearing at 3033 cm−1 in trace b is assigned as the C−H stretch of CHCl3 moiety in the PCNP−CHCl3 complex. The feature appearing at 3062 cm−1 in trace d was assigned as the C−H stretch of CHF3 moiety in the PCNP−CHF3 complex.
uncharacteristic of a molecule with four C−H oscillators, and this point will be addressed in Discussion. The fact that only one transition was observed corresponding to the monomer makes the spectra for the complexes very simple to interpret and assign. In the case of the PCNP−CHCl3 complex, two features were observed at 3052 and 3033 cm−1. The IR spectrum of the CDCl3 complex (Figure 4c) shows only one transition at 3052 cm−1. This clearly indicates the transition at 3033 cm−1 as the C−H stretch due to the bound CHCl3 and that at 3052 cm−1 due to the PCNP moiety. This also means that in the bound form the C−H stretch mode of the CHCl3 moiety has remained unchanged. The relative intensity of this transition however indicates that the IR transition probability of the C−H stretch mode (in CHCl3) has become significantly enhanced upon complex formation. Figure 3 (trace d) shows the LIF spectrum obtained upon using a CHF3/He mixture as the buffer gas. A new feature was observed at 110 cm−1 on the blue side of the PCNP monomer band origin transition. This feature was assigned as the band origin transition of the PCNP−CHF3 complex. The vibrational spectrum for the PCNP−CHF3 complex is shown in Figure 4 (trace d). Once again only two features were observed in the C− H stretch region at 3054 and 3062 cm−1. The transition at 3054 cm−1 was assigned as the one arising from the PCNP moiety, and the second transition at 3062 cm−1 was assigned as the C−H stretch of the CHF3 moiety. With respect to the C−H mode in free CHF3 at 3035 cm−1, this mode in the complex is blue-shifted by 27 cm−1. The vibrational spectra in the O−H stretch region for both complexes, i.e., CHCl3 and CHF3, show very small shifts (4 cm−1 red shift and zero shift, respectively), indicating that the O−H group of the PCNP moiety does not participate in the complex formation in either of the cases. Computational Results. PCR−CHX3 Complexes. Table 1 lists the C−H frequency shifts obtained using the three functionals listed in Experimental Section as well as those for the MP2 calculations for the PCR−CHCl3 complex. All the DFT-based methods gave two structures, in which the C−H
Figure 3. LIF excitation spectra obtained when (a) pure He, (b) CHCl3/He mixture, (c) CDCl3/He mixture, and (d) CHF3/He mixture were used as the buffer gas. All the spectra are plotted with respect to the monomer band origin observed at 35549 cm−1. The new feature appearing at 112 cm−1 in traces b and c was assigned as the band origin transition for the PCR−CH(D)Cl3 complex. The new feature appearing at 110 cm−1 in trace d was assigned as the band origin transition for the PCR−CHF3 complex. The features appearing on the red side of the band origin transition of the monomer were due to the PCNP−H2O complex..
spectra gave clear signatures corresponding to the band origin transition of the complexes in this case, the REMPI measurements for further confirmation were not needed. The LIF spectra have been plotted with respect to the PCNP monomer band origin transition observed at 35549 cm−1. This value of the band origin transition is consistent with the values previously reported in the literature.24 The LIF spectra of the PCNP monomer and its complexes with CHCl3 and CDCl3 are shown in Figure 3, parts a, b, and c, respectively. The new feature observed at 112 cm−1 on the blue side of the monomer band origin transition was assigned as the band origin transition of the complex. As expected, both the CHCl3 and CDCl3 spectra look similar. The vibrational spectra of the PCNP and its complexes with CHCl3 and CDCl3 in the C−H stretch region were measured by probing the respective band origin transitions (Figure 4). In the IR spectrum of the PCNP monomer (trace a) only one transition was observed at 3051 cm−1 in the C−H stretch region. This is 2310
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Table 1. Computed Values of the Shifts in Hydrogen-Bonded C−H Stretch Mode of the CHX3 Moiety in Their Complexes with PCR and PCNP PCR−chloroforma method B3LYP/6-311+ +(d,p) MP2/6-311+ +G(d,p) B3LYP/aug-ccpVDZ LC-ωPBE/augcc-pVDZ PW1PW91/ aug-cc-pVDZ MP2/aug-ccpVDZ MP2(CP)/augcc-pVDZ
CH···O
PCR−fluoroformb
CH···π OH···Cl
B3LYP/6-311+ +G(d,p) MP2/aug-ccpVDZ MP2(CP)/augcc-pVDZ
CH···O (bent)
4.1
7.7
3.6
23.9
31.5
36.2
19.9
55.8
e
26.6
35.3
e
−3.2
−3.3
e
f
f
f
−9.4
−0.8
e
f
f
f
−4.6
−5.0
e
f
f
f
e
57.3
e
e
24.8
e
f
19.6
f
f
f
f
PCNP−chloroformc method
CH···O CH···π
Figure 5. Minimum energy structures of the PCR−CHF3 (top row) and PCR−CHCl3 complex (bottom row) as computed by the MP2/aug-ccpVDZ method (the PCR−CHCl3 structure shown here was optimized on a BSSE-corrected PES). In both the cases the structures were found to be stabilized by C−H···π interactions. (SV = side view; TV = top view).
PCNP−fluoroformd
CH···O
CH···π
CH···N
CH···O
CH···π
CH···N
11.1
e
−8.0
27.5
e
27.6
23.2
65.0
16.8
18.7
17.2
22.1
−0.7
11.3
−2.1
f
f
20.2
blue shift in this case was found to be 19.6 cm−1 (Table 1). The only misgiving about the outcome is that, despite repeated attempts, the CCl3 torsional frequency about the H-bond axis always showed an imaginary frequency of ∼3 cm−1. The magnitude of the frequency is so small that it indicates a very shallow potential along this normal coordinate. Nonetheless, the key point that emerged from this analysis was that the MP2 method with the BSSE-corrected optimization gives the vibrational frequency shift that was in better agreement with the observed value. As a compromise between the magnitude of the computational effort and the accuracy, the subsequent calculations were performed using the MP2/aug-cc-PVDZ level, and the BSSE-corrected optimization was used only wherever such unusually large discrepancies in the shifts were noted. The geometry optimization of different initial structures of the PCR−CHF3 complex gave three different conformers at the B3LYP/6-311++G** level. These are listed in Table 1 along with the predicted frequency shifts. The MP2 level computation using the 6-311++G** basis set gave only the C−H···O and C−H···π bound structures. At the MP2/aug-cc-pVDZ level, however, for all the initial starting geometries, the optimization converged on only the C−H···π structure (Figure 5). Therefore, only the C− H···π structure was considered for further analysis. The frequency shift in the H-bond donor C−H stretch was predicted to be +24.8 cm−1. This was in reasonable agreement with the experimentally observed blue shift of 17 cm−1. The BSSE- and ZPE-corrected binding energies for the C− H···π bound structures for the CHF3 and CHCl3 complexes of PCR, along with the other relevant parameters, are given in Table 2. These were calculated at the MP2/aug-cc-pVDZ level. The binding energies for the CHF3 and CHCl3 complexes were 3.5 and 5.9 kcal/mol, respectively. For the CHCl3 complex, the binding energy was refined to 6.8 kcal/mol using the BSSEcorrected gradients during the optimization process. Table 2 also lists the magnitudes of the BSSE, the change in the C−H bond length, IR intensity enhancement upon complex formation, and the electron density at the bond critical points (BCPs) obtained using the AIM analysis. The AIM analysis shows the presence of a BCP corresponding to the C−H···π H-bond with the electron density at the BCP being 0.0104 and 0.0152 au, respectively, for the CHF3 and CHCl3 complexes. It also shows an increase in the electron densities at the H-bond donor C−H BCPs which is consistent with the observed blue shifts as well as the decrease in
a Experimentally observed CH shift = 11 cm−1. bExperimentally observed CH shift = 17 cm−1. cExperimentally observed CH shift = 0 cm−1. dExperimentally observed CH shift = 27 cm−1. eThese structures did not converge at that particular level. fThese calculations were not performed (see text for detailed explanation regarding the choice of the appropriate computational method).
group of the CHCl3 moiety was acting as the hydrogen bond donor with the acceptor site being either the O atom or the π electron cloud of the PCR moiety. These are referred to as the C−H···O and C−H···π structures based on the type of the interaction present in each of the complex. Only the B3LYP/6311++G** calculation gave an additional O−H···Cl bound minimum energy structure. Using the aug-cc-pVDZ basis set, however, even the B3LYP method, resulted in only the former two structures getting converged. At the B3LYP level, the calculations predicted 4.1 and 7.7 cm−1 blue shift with the 6-311+ +G** basis set and 3.2 and 3.3 cm−1 red shift with the aug-ccpVDZ basis set for the C−H···O and C−H···π conformers, respectively. The LC-ωPBE and PW1PW91 functionals gave red shifts of 9.4, 0.8 and 4.6, 5.0 cm−1 for the two respective conformers, respectively. It should be noted that a blue shift of 11 cm−1 was observed for the experimentally detected conformer. Clearly, the above-mentioned DFT-based calculations are inconsistent with each other and also in this case have failed to predict the direction of the frequency shift correctly. In contrast, the geometry optimization of various initial structures using the MP2/aug-cc-pVDZ method converged on only the C−H···π type structure (Figure 5) with the C−H of the CHCl3 moiety pointing toward the six-membered ring of the PCR moiety. The direction of the shift in the H-bonded C−H stretch was predicted correctly as a blue shift, albeit with a large deviation (predicted blue shift being 57.5 cm−1) from the experimentally observed value of 11 cm−1. A considerable improvement in the agreement between the computed and the observed shift was seen when the geometry optimization was performed using the BSSE-corrected gradient followed by harmonic frequency analysis. The calculated 2311
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Table 2. Summary of the ab Initio Calculations and AIM Analysis for the CHX3 Complexes with PCR and PCNP Molecules (the following results are for the calculations at the MP2/aug-cc-pVDZ level using the standard gradient method)a p-cresol
p-cyanophenol
CHF3
CHCl3
propertyb
CH···π
CH···π
CH···N
CH···π
CH···O
CH···N
CH···π
CH···O
binding energy, D0 binding energy, De BSSE Δν C−H (cm−1) Δr C−H (mÅ) IR intensity enhancement ratio ρ (BCP, H···Y) (au) Δρ (BCP, C−H) (au)
3.5 4.2 4.0 24.8 −1.7 0.1 0.0104 0.0032
5.9 (6.8) 6.5 (7.1) 6.6 (5.4) 57.3 (19.6) −2.9 70.5 0.0152 0.0041
3.5 (3.7) 3.9 (4.0) 1.3 (1.2) 22.1 (20.2) −1.4 1.2 0.0135 0.0032
1.7 2.3 3.8 17.2 −1.2 0.3 0.0090, 0.0088c 0.0026
1.8 2.3 1.8 18.7 −1.2 0.1 0.012 0.0027
3.9 (5.4) 4.5 (5.8) 2.2 (2.8) 16.8 (−2.1) −0.5 231.8 (25.60) 0.0185 0.0027
4.2 (5.2) 4.9 (5.6) 6.4 (4.9) 65.0 (11.3) −3.7 44.3 (22.3) 0.0139 0.0044
3.7 (4.2) 4.1 (5.5) 4.5 (3.7) 23.2 (−0.7) −1.2 16.5 (10.6) 0.0109, 0.0107c 0.0021
CHF3
CHCl3
a
The numbers in parentheses in each row corresponds to the results for the calculations performed using the BSSE-corrected gradients using the counterpoise method. bBinding energy (D0 and De) and BSSE in kcal/mol; Δν (C−H) = vibrational frequency shift in the H-bond donor C−H stretching frequency in cm−1; Δr (C−H) = change in the C−H bond length with regard to the free monomer upon H-bond formation; ρ(BCP, H···Y) (au) = electron density at the BCP corresponding to the H···Y hydrogen bond in atomic units; (BCP, C−H) (au) = change in the electron density at the hydrogen bond donor C−H BCP upon C−H···Y hydrogen bond formation. cTwo BCPs were observed for these structures.
C−H···Y type of hydrogen bond exist with the magnitudes of the electron densities in the range expected for such types of hydrogen bonds.25 The optimizations for PCNP−CHF3 at the MP2/aug-ccpVDZ level also gave three structures, viz., the C−H···O, C− H···π, and C−H···N bound structures (Figure 7). Among these, the C−H···N bound structure was found to be the most stable with its binding energy being 3.5 kcal/mol (Table 2). The binding energies for the C−H···O and C−H···π bound structures were calculated to be 1.8 and 1.7 kcal/mol, respectively. For the C−H···N type of structure, which was found to be the most stable, additional calculations were performed using the BSSEcorrected PES. Using this method, we recalculated the binding energy to be 3.7 kcal/mol, and the shift in the H-bond donor C− H stretch was found to be 20.2 cm−1. In the case of the CHF3 complexes, computations using the BSSE-corrected gradient do not improve the values of the binding energy nor the frequency shifts in any significant manner over those calculated using the standard gradient method. Although the computed C−H frequency blue shifts were in the range of 17 to 22 cm−1 in all three cases, the observed complex was assigned the C−H···N type structure, which was the global minimum. The AIM analysis of the electron density shows the presence of BCPs characteristic of the C−H···Y (Y = O, N, and π electron cloud) type of hydrogen bond in each of these structures.
the C−H bond lengths predicted computationally in both the cases. PCNP−CHX3 Complexes. The situation regarding the PCNP−CHCl3 complex is somewhat complicated. Unlike the PCR−CHX3 complexes, geometry optimization at the MP2/ aug-cc-pVDZ level resulted in three types of structures (Figure 6a). These structures are termed as the C−H···O, C−H···π, and C−H···N type. The computed vibrational frequency shifts in the H-bond donor C−H stretch mode were 23.2, 65.0, and 16.8 cm−1 for the C−H···O, C−H···π, and C−H···N bound structures, respectively (Table 1). On the other hand, experimentally only a single conformer was observed with a zero shift in the C−H stretching frequency. In view of this large discrepancy in the calculated and the observed vibrational frequency shifts, further computations were performed for these complexes using the BSSE-corrected PES. The resulting structures for the C−H···O and the C−H···π type remained similar as in the previous case; however, the C−H···N structure showed a large change in the C−H···N H-bond angle, i.e., it changed from near 180° to 137° (Figure 6b), vide infra. The computed values of the shifts showed a remarkable improvement over those obtained from the BSSE-uncorrected optimization, and now they were found to be in the vicinity of zero shift for the C−H···O and C−H···N bound structures, viz., red shift of 0.7 and 2.1 cm−1, respectively, and a blue shift of 11.3 cm−1 for the C−H···π structure consistent with the other C−H···π bound cases. Table 2 lists the BSSE- and ZPE-corrected binding energies, the changes in the C−H stretching frequencies and the C−H bond length, the IR intensity changes upon complex formation, and the electron densities at the BCPs for all the three structures. The binding energies for the C−H···O, C−H···π, and C−H···N bound structures were evaluated to be 4.2, 5.2, and 5.4 kcal/mol, respectively. The C−H···N bound complex emerged as the most stable structure when the optimization was carried out using the BSSEcorrected PES. This stability order is clearly different compared to that predicted using the standard gradient method. The C− H···π bound structure, which is the second lowest energy structure, was ruled out on the basis of the disagreement between computed and observed C−H vibrational shifts. The C−H···O bound structure was the least stable structure. The AIM analysis shows that for all these structures, BCPs corresponding to the
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DISCUSSION PCR−CHX3 Complexes. Although there are two potential sites in PCR at which the CHX3 (X = F, Cl) could bind as Hbond donor, on the basis of UV and IR data it was inferred that only a single conformer was formed in the jet. The ab initio computations at the MP2/aug-cc-pVDZ level also resulted in only one type of structure, i.e., the C−H···π bound structure, consistent with the observation of only one type of complex. In the case of the PCR−CHF3 complex the computed frequency shift was in good agreement with the experimentally observed shift of 17 cm−1. For the PCR−CHCl3 complex, however, the computed C−H frequency shift had to be refined using the BSSE-corrected gradient to give a better agreement with the experimentally observed blue shift. The AIM analysis of the electron density reveals the presence of appropriate BCPs corresponding to the C−H···π type of interaction. The binding energies were calculated as 3.5 kcal mol−1 for the CHF3 complex 2312
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Figure 6. The optimized structures of the PCNP−CHCl3 complexes computed at MP2/aug-cc-pVDZ level using the BSSE-uncorrected (a) and -corrected (b) gradients. (SV = side view; TV = top view).
and 6.8 kcal mol−1 for the CHCl3 complex (using the BSSEcorrected gradient). The PCR−CHCl3 binding energy compares favorably with the experimentally measured binding energy of 5.2 kcal/mol for the benzene−CHCl3 C−H···π type complex.26 On the basis of these results, we conclude that the PCR−CHF3 and
PCR−CHCl3 complexes are stabilized by the blue-shifted C− H···π type of hydrogen bonding interaction. The above two examples of blue-shifted hydrogen bonds with C−H···π bound structures are very similar to that of the fluorobenzene−CHF3 complex reported previously by Reimann and co-workers10 where a blue shift of 21 cm−1 in the C−H 2313
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the C−H frequency shift as −2.1 cm−1 in good agreement with the observed zero shift. Therefore, in this case also we assign the observed conformer to the C−H···N bound structure. In a closely related system such as the benzonitrile−CHCl3 complex, it has been reported earlier that the complex was of the C−H···N type with the C−H showing a small blue shift of 2 cm−1.11 The binding energy of the C−H···π bound complex is only slightly lower (0.2 kcal/mol) than that of the C−H···N structure, and one would have expected to observe it in our experiments. It is possible that during the course of supersonic jet expansion, the higher energy structures get converted into the global minimum energy structure. In such cases it would be useful to search for these structures via alternative methods such as matrix isolation spectroscopy. A salient feature of the results reported in this work may be highlighted. The CHX3 molecule is hydrogen bonded to the π aromatic electron density in PCR where as in the case of PCNP it is bound to the nitrogen end of the CN group. The electronwithdrawing CN substituent in PCNP depletes the electron density in the aromatic ring whereas in the case of PCR the para methyl substituent enriches the aromatic electron density by hyperconjugation. The influence of para substituents on the preferred acceptor site for the C−H H-bond donor is clearly manifested in these two complexes, i.e., while it is a C−H···π type H-bond in the case of PCR−CHX3 complexes, it is a CH···N type H-bond in the case of PCNP−CHX3 complexes. In the case of CHCl3 complexes with PCR and PCNP, there was considerable difference in the C−H blue shifts calculated using the standard gradient optimization method and those observed experimentally. The calculations performed using the BSSE-corrected PES gave better agreement between the experiment and theory. This has also been reported in the case of the fluorobenzene−CHCl3 complex. In these calculations, using standard gradient and harmonic analysis, a large blue shift of 66 cm−1 was predicted, whereas a blue shift of 14 cm−1 was observed in their experiments. Considerable reduction in the calculated vibrational frequency shift and consequently better agreement with the observed shift resulted when the optimization was performed using the BSSE-corrected potential energy surface.2,10 For the sake of completeness, it must also be stated that a similar exercise for the CHF3 complexes did not produce any significant differences in the results obtained by the standard gradient method. In the present case of the PCNP− CHCl3 complex, although this exercise improved the agreement in the C−H frequency shift, the equilibrium geometry changed from the linear H-bonded structure to the bent H-bonded structure (H-bond angle = 137°). The bent structure is slightly counterintuitive, as one expects a linear structure in view of the sp-hybridized lone pair orbital on the nitrogen atom. The NBO analysis, however, indicated that not only the lone pair orbital on the nitrogen atom but also the π orbitals of the CN bond donate electron density to the antibonding orbital of C−H. Another general observation in the H-bonded complexes involving halomethanes is that the IR transition probability of the C−H stretching mode does not necessarily increase upon complex formation as in the cases of conventional H-bonded complexes. Typically in the case of CHF3 complexes the IR transition becomes weaker than that in its corresponding monomer whereas in the case of CHCl3 complexes the IR transition probability increases by at least one order of magnitude. Also, the blue shift has always been greater in the case CHF3 complexes compared to those in the case of CHCl3 complexes. It can be seen from the computational data given in
Figure 7. The optimized structures of the PCNP−CHF3 complexes computed at MP2/aug-cc-pVDZ level using the standard gradient method. *Figure in the last row shows the C−H···N bound structure (with the binding energy) as obtained by the optimization using the BSSE-corrected gradients. (SV = side view; TV = top view).
stretch mode was reported. In the case of the fluorobenzene− CHCl3 complex2 a blue shift of 14 cm−1 has been reported in the C−H stretch mode of the CHCl3 moiety. The CHX3 and fluorobenzene units were found to be interacting via the C−H···π type of hydrogen bond with the C−H group of the CHX3 moiety acting as a hydrogen bond donor. The relative shifts presented in this work are in good agreement with the earlier reports, both in regard to the magnitudes and to the trend. PCNP−CHX3 Complexes. Interestingly, the FDIR spectrum of the PCNP monomer is very simple. In the C−H stretch region, only one feature was observed at 3052 cm−1, which is unusual for a molecule with four C−H oscillators. However, the computed IR spectrum revealed that out of the four possible fundamental C−H stretch normal modes only one of the antisymmetric stretching modes has a transition probability greater by one order of magnitude compared with the other three modes. This made the interpretation of the IR spectra of the PCNP−CHX3 complexes very easy. Ab initio computations predicted three stable conformers for the PCNP−CHX3 complexes as expected, the details of which are given in Tables 1 and 2. In the case of the PCNP−CHF3 complex the C−H···N bound structure was the most stable one which gave the blue shift in the C−H stretching frequency as 20.2 cm−1. The excitation spectrum for the CHF3 complex indicated the presence of only single conformer in which the C−H stretching frequency of the CHF3 moiety was observed to be blue-shifted by 27 cm−1 (Figure 4d). This is in good agreement with the computed shift for the C−H···N bound structure. Therefore, the observed complex was assigned as the C−H···N bound structure. In the case of the CHCl3 complex the C−H stretch of the CHCl3 moiety was observed at 3033 cm−1 (Figure 4), indicating zero shift in the CH stretching frequency. At the MP2 level using the BSSE-corrected PES, the calculation predicted the C−H···N bound complex to be the most stable structure. It also predicted 2314
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with previously reported C−H···N complex in a closely related system (benzonitrile−CHCl3 complex), and the computed binding energies suggest that the structure of the PCNP− CHCl3 complex is of the C−H···N type. The presence of an electron-withdrawing CN group on the aromatic ring had a strong effect in directing the H-bonding site from the π cloud to the N atom. It was also noted that in the case of complexes involving CHCl3 the computation at the MP2 level using the BSSE-corrected potential energy surfaces resulted in a significant improvement in the computed frequency shifts compared to those from the standard gradient methods.
Table 2 that in the case of PCR−haloform complexes the C−H IR transition probability decreased by one order of magnitude for the fluoroform complex whereas in the case of the chloroform complex there was 70-fold increase. In the case of PCNP− haloform complexes for the fluoroform complex it pretty much remained the same (20% increase) whereas in the case of the chloroform complex it increased by 2 orders of magnitude. The experimentally observed data was in line with these predictions. A similar trend has also been reported by van der Veken et al. in the case of complexes of dimethyl ether with mixed fluorochloromethanes [CHClnF(3−n); n = 0 for CHF3], where as n increases the blue shift slowly decreases but the relative IR intensity increases and eventually the C−H stretch becomes red-shifted in the CHCl3 complex.8 The change in the direction of the C−H shift as well as the IR transition intensity variation was explained to be due to the electric field effect of the electron donor involved in the complexation. This has also been observed in the case of 3methylindole−halofrom complexes.13 In the case of PCNP−CHCl3 the following question may be asked. When the X−H donor, upon association with an acceptor group, shows no shift in the stretching frequency, can it be considered as hydrogen bonded at all? The simple answer is that mere shifts in the vibrational frequency need no longer to be considered as the sole signature of a H-bond. It has been realized that the hydrogen bond formation can lead to many characteristic signatures. The very fact that the C−H group is known to form both red-shifted and blue-shifted hydrogen bonds is a clear indicator that a hydrogen bond with a zero shift could very well exist. In situations where the competing effects causing the red and blue shifts cancel each other, such hydrogen bonds are possible. Joseph and Jemmis27 have shown that it is possible to provide a unified explanation of the vibrational frequency shifts associated with hydrogen bonds showing a red shift, blue shift, or zero shift. In such situations the indicators, such as the intensity change of the H-bond-donating X−H stretching frequency, existence of a bond critical point corresponding to the H···Y hydrogen bond with an electron density above a certain threshold, the minimum energy structure of the complex, and the magnitude of the binding energy, can be used to establish the hydrogen bond formation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
Institute of Physical Chemistry, Georg-August University of Goettingen, D-37077 Goettingen, Germany. Notes
The authors declare no competing financial interest.
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REFERENCES
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CONCLUDING REMARKS Combining the experimental results and ab initio computations, we conclude that the PCR−CHF3 and PCR−CHCl3 complexes are bound by blue-shifted C−H···π type hydrogen bonds. The magnitudes of the shifts observed in the hydrogen bonded C−H stretching frequency were 17 and 11 cm−1 in the case of the CHF3 and CHCl3 complex, respectively. In the case of the PCNP−CHF3 complex, multiple structures were predicted but the C−H···N bound structure emerged to be the most stable. This result is different from that of the PCR− CHF3 complex, which was found to be C−H···π bound. This difference is attributed to the presence of the electronwithdrawing C−N group, which makes the π bound structure less favored. A relatively larger blue shift of 27 cm−1 in the hydrogen bonded C−H stretching frequency was observed in this case. For the PCNP−CHCl3 complex, however, a zero shift was observed in the C−H stretch of the CHCl3 moiety. The computations predict close-lying minimum energy structures which differ in the hydrogen bond acceptor site, namely the N atom and the π electron cloud with the C−H···N type structure being more stable and having close to zero C−H frequency shift. Similarity with the PCNP−CHF3 LIF spectrum, comparison 2315
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