International Symposium on Molecular Spectroscopy, June 22-26, 2015
MATRIX ISOLATION IR SPECTROSCOPY OF 1:1 COMPLEXES
OF ACETIC ACID AND TRIHALOACETIC ACIDS WITH WATER
AND BENZENE
Pujarini Banerjee & Tapas Chakraborty
Indian Association for the Cultivation of Science
Kolkata, India
Carboxylic acids as hydrogen bond donors
IR spectra recorded under the matrix isolation condition of acetic acid and
two of its tri-halo analogues with benzene and water
Typical optimized
structures
Binding energies
(relative)
3.2
7.9
(kcal/mol)
[MP2/6-311++G(d,p)]
Note: Binding energy values depend on the level of calculation used. At the
B97D/6-311++G(d,p) level, the magnitudes are higher. Therefore, we focus on
relative values in a series.
Carboxylic acids as hydrogen bond donors: Objective of the study
We have previously seen , in case of phenol as HB donor, that irrespective of the nature of
HB acceptor (water or benzene), spectral red-shifting of donor O-H maintains nice
correlations with aqueous phase acidity of the phenols. Also, in absence of HB acceptors,
O-H of all the phenol monomers have almost the same value.
We now discuss the spectral behavior observed for a different class of HB donor species,
acetic acid and its tri-halo derivatives, which have the same donor site, -OH group.
Aqueous phase acidity of CH3COOH (pKa = 4.76) is somewhat comparable with that of
F5PhOH (pKa = 5.8). Thus, the system is expected to allow investigating whether the
correlation between the bulk solvation and molecular level behavior, discussed so far, goes
beyond the domain of homologous systems.
Carboxylic acids as hydrogen bond donors
Temperature  8 K
Matrix isolation IR spectra of three carboxylic acids used as HB donors
Trichloro-AA
Trifluoro-AA
AA
3200
3300
3400
3500
3600
3700
3800
cm-1
pKa
AA
Trifluoro-AA
Trichloro-AA
4.76
0.52(±0.1)
0.3(±0.1)
IR spectra of benzene complexes of the three carboxylic acids (matrix isolated)
O-H
B.E.=4.8 kcal/mol
*
B.E.=4.6 kcal/mol
*
*
2800
3000
3200
3400
B.E.=3.2 kcal/mol)
3600
3800
cm-1
pKa
AA
Trifluoro-AA
Trichloro-AA
4.76
0.52(±0.1)
0.3(±0.1)
IR spectra of matrix isolated AA-benzene complexes
Special features of the spectra
•Red-shifting effects exerted by benzene pi-electrons on O-H of carboxylic acids
correlate qualitatively with the aqueous phase acidity of the acids. Thus, O-H of
AA-benzene binary complex is nearly two-thirds of that for F3AA-benzene
complex.
• The
magnitude of the shift observed for AA-benzene complex is comparable
with those of phenol-benzene complexes, although the donors in the two cases
belong to completely different class of molecules.
O-H is a measure of local interactions..?
Chemical substitution effects on correlation of bulk acidity parameter (pKa) with O-H of
OH∙∙∙ H-bonded isolated binary complexes of the two types of –OH donors.
We attempted to express this in terms of the slope, ∆ν/∆pKa
100
∆ν/∆pKa=14
140
120
∆ν/∆pKa=11
95
∆νOH (cm-1)
∆νOH (cm-1)
160
90
85
80
100
75
8.0
80
0
1
2
3
4
5
pKa
8.4
8.8
9.2
9.6
10.0
pKa
This implies that local interactions are dominant.
The slope, ∆ν/∆pKa, is a little higher for carboxylic acid, because here halogen substitutions are
made only on  carbon atom of AA, but those in the case of phenols are at larger distances.
O-H is a measure of local interactions..?
For the two classes of –OH donors, the correlation of O-H with the total binding
energy appears rather poor.
The slope ∆ν/∆E binding for two binary complexes are very different.
100
150
∆νOH (cm-1)
130
∆ν/∆BE=43
120
∆νOH (cm-1)
95
140
90
85
∆ν/∆BE=18
110
80
100
90
3.0
75
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
Binding energy (kcal/mol)
4.8
4.4
4.6
4.8
5.0
5.2
5.4
5.6
Binding energy (kcal/mol)
Total binding energy considerations give the impression that O-H of carboxylic acids
is more prone to be affected by benzene pi-electrons compared to that of phenols.
O-H is a measure of local interactions..?
It is remarkable to note that the local CT component correlates nicely with
O-H in OH∙∙∙ H-bonded binary complexes.
The slopes, ∆ν/∆E hyperconjugation, for two binary complexes appear almost same.
100
150
95
130
∆ν/∆HE=46
120
110
100
∆νOH (cm-1)
∆νOH (cm-1)
140
90
∆ν/∆HE=40
85
80
90
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Hyperconjugation energy (kcal/mol)
3.4
75
1.3
1.4
1.5
1.6
1.7
1.8
1.9
Hyperconjugation energy (kcal/mol)
This indicates again that ∆ν is an outcome of local interaction, not the overall binding
interaction.
O-H is a measure of local interactions..?
Although the sensitivity for changes of O-H with local CT interaction is
independent on the type of OH donor used, the same does not hold with respect
to overall electrostatics.
100
150
140
∆ν/∆E=43
120
110
∆νOH (cm-1)
∆νOH (cm-1)
95
130
90
∆ν/∆E=15
85
100
80
90
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
Electrostatic energy (kcal/mol)
6.0
75
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
Electrostatic energy (kcal/mol)
7.2
Problems in studying acetic-acid water HB in Matrix isolation
Acetic acids form strong O-H···O hydrogen bonded complexes with water. Thus
no distinct feature for the complex at ∆νO-H region was identified. Therefore,
systematic study with the measured O-H data of water complexes were not
successful.
F3AA
AA
Indistinguishable broad bands
throughout entire νOH region,
confusing assignments of dimer
and water-complexes.
3200
3300
3400
3500
cm-1
3600
3700
3800
Recent reports
Polarizable molecule CCl4 also perturbs νOH
Binding energies (kcal/mol)
[MP2/6-311++G(d,p)]
*
3.64
*
3300
3400
3500
4.52
cm-1
3600
3700
Summary:
 We have shown here that halogen substitutions on IR O-H of acetic acid have
almost no effect as long the acids are isolated.
 The substitution effects on O-H mode of the acids are revealed when the acids
are complexed with a polarizable group like benzene or carbon tetrachloride.
 The effect of bulk acidity enhancement of the donor moiety on red shifting effect
of O-H frequencies, occurring due to formation of binary complexes with benzene
and expressed as ∆ν/∆pKa, appears to be a robust parameter, and does not depend
on actual type of the –OH donor species.
 Similar robustness is noticed also with respect to local CT interaction energy at
the H-bonding site of the binary complexes, i.e., ∆ν/∆ECT is independent of the type
of OH donor.
 Binary complexes having the same type of H-bonding and  acceptor, ∆ν/∆Ebinding
and ∆ν/∆Eelectrostatics are not robust, and depend on actual nature of the donor
molecules.
Acknowledgements
Prof. T.Chakraborty
Aparajeo
Shreetama
Deb Pratim
Piyali