Download 440400 - IDEALS @ Illinois

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.