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International Symposium on Molecular Spectroscopy, June 22-26, 2015 MATRIX ISOLATION IR SPECTROSCOPY AND QUANTUM CHEMISTRY STUDY OF 1:1 HYDROGEN BONDED COMPLEXES OF BENZENE WITH A SERIES OF FLUOROPHENOLS Pujarini Banerjee & Tapas Chakraborty Indian Association for the Cultivation of Science Kolkata, India O-H hydrogen bonded complexes In these complexes, phenolic O-H groups are made to interact with electron clouds of bound -molecular orbitals rather than with a discrete dipolar species. Specific systems: O-H donor(Phenol) -HB acceptor (Benzene) Motivation: O-H--- interactions are known to play important role in structural stability of molecular crystals and conformational preferences of functional structures of biological macromolecules Infrared spectroscopy is extensively used to identify OH--- hydrogen bonded complexes, and also to suggest relative strength of this interaction. Our aim here is to investigate whether any correlation can be established between measured IR spectral shifts and any of the energetic parameters as described in my previous talk. Recent reports on the nature of O-H --- interactions: This study suggests that spectral shift of O–H stretching fundamental is the outcome of Stark interaction between O–H dipole and electric field of benzene electrons. Thus, the interaction is suggested to be purely of electrostatic type. JACS, 2011, 133, 17414 Book excerpt.. Some of the much referred books express opposite views, for example.. The Weak Hydrogen Bond, Oxford University Press, 1999 -by Desiraju and Steiner Page 17, 4th para “....Electrostatics is dominant in strong hydrogen bonds, where it contributes 60-70 per cent of the attractive terms. In weak hydrogen bonds, the relative contribution of electrostatics is smaller, and in the weakest C-H..O bonds,....., the electrostatic term can be of the same magnitude or even smaller than the dispersion term.......” Our experimental strategy We have measured infrared spectra of 1:1 complexes of a series of fluorophenols with benzene. Thus, the phenolic O–H dipoles of all the complexes are subjected to a constant electric field of benzene pi-electrons. Phenols pKa 10.0 9.9 9.3 9.1 8.7 8.4 If local electrostatics is indeed the dominant factor, then O–H shifts of these fluorophenols must bear a correlation with their local O–H dipole moments. 8.2 Phenol-benzene complexes were synthesized in argon matrixes Temperature 8 K * Intensity Monomer Phenol-benzene complex O-H segment of the IR spectrum of phenol-benzene complex recorded under a matrix isolation condition # Spectral shift for OH is 78 cm−1 3500 On the other hand, the shift revealed in an IR-UV double resonance measurement under a Jet-cooling condition is also of the same value, 78 cm−1. C Phenol + Benzene B Phenol Benzene A 3400 3450 3550 3600 O-H (cm-1) 3650 3700 The same value of spectral shift measured under two completely different cryogenic conditions implies that the matrix medium hardly affects OH- hydrogen bonding In CCl4 at room temperature, the value of the shift measured is only 50 cm-1. Thus, OH- hydrogen bonds are largely distorted by thermal influence. Banerjee and Chakraborty, J. Phys. Chem. A 2014, 118, 7074−7084 IR spectrum bears signature for the shape of the complex C-H of benzene -1 683.1 00 cm Intensity 675 680 685 690 695 C-H of benzene in 1:1 complex with phenol Optimized geometry of the phenol-benzene complex (MP2/6-311++G(d,p) level) C-C of phenol c b a 650 660 670 cm-1 680 690 700 Out-of-plane C−H wagging (C−H) fundamental of benzene is blue-shifted due to T-shaped geometry of the complex Banerjee and Chakraborty, J. Phys. Chem. A 2014, 118, 7074−7084 Shifts on O–H of different phenols exerted by a benzene molecule 3,4,5-TFPh-B 3,4-DFPh-B Intensity 4-FPh-B 3,5-DFPh-B 3-FPh-B Ph-B 3400 3450 3500 3550 O-H (cm-1) 3600 3650 Phenols pKa 10.0 9.9 9.3 9.1 8.7 8.4 8.2 Banerjee and Chakraborty, J. Phys. Chem. A 2014, 118, 7074−7084 Correlation of O–H shifts with bulk acid dissociation constant A remarkable feature of the correlation is that unlike phenol-water complexes, although here the acceptor is benzene pi-electrons, spectral shifts still display good correlation with the bulk acidity parameter. The deviation of 2-fluorophenol from linearity is likely to be due to geometric factor. % change in spectral shift=27% O-H(cm-1) 140 120 3,5-DFPh-B 100 3,4,5-TFPh-B 80 3,4-DFPh-B 3-FPh-B Ph-B 2-FPh-B 4-FPh-B 60 8.0 8.4 8.8 9.2 pKa 9.6 10.0 Geometric constraint and spectral shift of 2-F-phenol complex with benzene OH– hydrogen bonding becomes less efficient due to ortho F atom resulting in lowering of O-H shifting. Binding energy (kcal/mol) M [B97D/6-311++G(d,p)] 3.9 * 3400 3500 3600 cm-1 3700 3800 IR spectrum of matrix isolated 2-FPh-benzene complex 6.2 Correlation of O–H shifts with binding energies It is notable that unlike phenol-water complexes, when benzene pi-electrons are the HB acceptor, the induced spectral shifts bear a somewhat linear correlation with the binding energies of the complexes, and the correlation is almost quantitative. O-H(cm-1) % change in spectral shift=27% 120 3,5-DFPh-B 100 3,4,5-DFPh-B 3-FPh-B 3,4-DFPh-B 80 2-FPh-B Ph-B 4-FPh-B 60 5.6 6.0 6.4 6.8 7.2 Binding energy (kcal/mol) % change=25% Calculations at B97D/6-311++G(d,p) Natural Charge (+) on Phenolic H of different phenol monomers and benzene complexes In contrary to a recent suggestion mentioned before, local electrostatics seems to have no contribution to the observed variation of spectral shifts. Calculation does not predict changes of dipole moment on ring fluorine substitution. Natural charge on phenolic H 0.8 -ve charge on O 0.6 +ve charge on H 4-FPh-W Ph-W 3,4-DFPh-W 3,-FPh-W 0.4 3,5-DFPh-W 3,4,5-TFPh-W 0.5 2-FPh-W Natural Charges 0.7 3530 3535 3540 3545 3550 3555 3560 3565 νOH (cm-1) Correlation of O–H shifts with hyperconjugative charge transfer % change in spectral shift=27% O-H(cm-1) 120 100 3,5-DFPh-B 3,4,5-TFPh-B Ph-B 80 3-FPh-B 3,4-DFPh-B 2-FPh-B 4-FPh-B 60 2.1 2.2 2.3 2.4 2.5 2.6 (benzene) → *(O-Hph) Hyperconjugation energy (kcal/mol) % change=19.3 Hyperconjugation and its increase across the fluorophenol series are the likely factors for spectral shifts and observed variation. The other major contributor to binding energy is dispersion interaction. Correlation of O–H shifts with transfer of total charges from benzene 100 O-H(cm-1) % change in spectral shift=27 120 3,4,5-TFPh-B 3,5-DFPh-B 3-FPh-B 80 3,4-DFPh-B Ph-B 2-FPh-B 4-FPh-B 60 0.006 0.007 0.008 0.009 0.010 Total charge transfer(e) % change =36.9 0.011 0.012 Correlation of O–H shifts with charge density depletion on O–H bond 140 O-H(cm-1) % change in spectral shift=27 120 100 3,4,5-TFPh-B 3,5-DFPh-B 3-FPh-B 3,4-DFPh-B 80 Ph-B 4-FPh-B 60 0.0044 0.0048 0.0052 O-H(a.u) % change=24.4 0.0056 0.0060 J. Phys. Chem. A 2014, 118, 7074−7084 Summary: In phenol-benzene complexes, the spectral shifts of phenolic ∆νOH increase with successive ring fluorine substitution, and the sequence follows aqueous phase acidity of the fluorophenols. This behavior is similar to what has been observed in the case of phenol-water complexes. The major contributions to binding interactions of phenol-benzene and phenol-water complexes are very different. In the former case, dispersion is likely to play a major role. Nevertheless, the factor that contribute to the spectral shifting effects, i.e., hyperconjugation, is effective in both types of complexes. Unlike phenol-water complexes, the spectral shifts of phenol-benzene complexes correlate linearly with the calculated total binding energies of the complexes. In the latter case, extended size of the acceptor and dominance of dispersion interaction could be origin for this difference.