1. 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

2. 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.

3. 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

4. Some of the much referred books express opposite views, for example..
Book excerpt..
Some of the much referred books express opposite views, for example..
The Weak Hydrogen Bond, Oxford University Press, by Desiraju and Steiner Page 17, 4th para “....Electrostatics is dominant in strong hydrogen bonds, where it contributes 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 ”

5. 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.
pKa
10.0
9.9
9.3
8.4
8.7
9.1
8.2
Phenols
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.

6. Phenol-benzene complexes were synthesized in argon matrixes
Temperature  8 K

7. O-H segment of the IR spectrum of phenol-benzene complex recorded under a matrix isolation condition * #
Monomer
Phenol-benzene complex C B A
O-H (cm-1)
Intensity
Spectral shift for OH is 78 cm−1
Phenol + Benzene
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.
Phenol
Benzene
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

8. IR spectrum bears signature for the shape of the complex
Optimized geometry of the
phenol-benzene complex
(MP2/ G(d,p) level)
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

9. Shifts on O–H of different phenols exerted by a benzene molecule
pKa
10.0
9.9
9.3
8.4
8.7
9.1
8.2
Phenols
Banerjee and Chakraborty, J. Phys. Chem. A 2014, 118, 7074−7084

10. 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.
O-H(cm-1)
pKa
% change in spectral shift=27%
Ph-B
4-FPh-B
3-FPh-B
3,4-DFPh-B
2-FPh-B
3,5-DFPh-B
3,4,5-TFPh-B

11. 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. M *
cm-1
Binding energy (kcal/mol)
[B97D/ G(d,p)]
3.9
6.2
IR spectrum of matrix isolated 2-FPh-benzene complex

12. 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%
% change=25%
Binding energy (kcal/mol)
Ph-B
4-FPh-B
2-FPh-B
3-FPh-B
3,4-DFPh-B
3,5-DFPh-B
3,4,5-DFPh-B
Calculations at B97D/ G(d,p)

13. changes of dipole moment on ring fluorine substitution.
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
2-FPh-W
3,5-DFPh-W
3,4,5-TFPh-W
3,-FPh-W
3,4-DFPh-W
Ph-W
4-FPh-W
+ve charge on H
-ve charge on O
νOH (cm-1)
Natural Charges

14. Correlation of O–H shifts with hyperconjugative charge transfer
O-H(cm-1)
% change in spectral shift=27%
% change=19.3
Ph-B
4-FPh-B
3-FPh-B
2-FPh-B
3,4-DFPh-B
3,5-DFPh-B
3,4,5-TFPh-B
(benzene) → *(O-Hph) Hyperconjugation energy (kcal/mol)
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.

15. Correlation of O–H shifts with transfer of total charges from benzene
% change =36.9
O-H(cm-1)
% change in spectral shift=27
Total charge transfer(e)
2-FPh-B
Ph-B
3-FPh-B
4-FPh-B
3,4-DFPh-B
3,5-DFPh-B
3,4,5-TFPh-B

16. Correlation of O–H shifts with charge density depletion on O–H bond
Ph-B
4-FPh-B
3-FPh-B
3,5-DFPh-B
3,4-DFPh-B
3,4,5-TFPh-B
O-H(cm-1)
O-H(a.u)
% change in spectral shift=27
% change=24.4

17. J. Phys. Chem. A 2014, 118, 7074−7084

18. 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.