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Pujarini Banerjee & Tapas Chakraborty Indian Association for the Cultivation of Science Kolkata, India 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
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Carboxylic acids as hydrogen bond donors [MP2/6-311++G(d,p)] 3.27.9 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) (kcal/mol) 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.
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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 CH 3 COOH (pK a = 4.76) is somewhat comparable with that of F 5 PhOH (pK a = 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.
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Temperature 8 K Carboxylic acids as hydrogen bond donors
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Matrix isolation IR spectra of three carboxylic acids used as HB donors cm -1 Trichloro-AA Trifluoro-AA AA pKapKa 4.76 0.52(±0.1) 0.3(±0.1) Trichloro-AA Trifluoro-AA AA
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IR spectra of benzene complexes of the three carboxylic acids (matrix isolated) pKapKa 4.76 0.52(±0.1) 0.3(±0.1) Trichloro-AA Trifluoro-AA AA * * * cm -1 B.E.=3.2 kcal/mol) B.E.=4.6 kcal/mol O-H B.E.=4.8 kcal/mol
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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 F 3 AA-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. IR spectra of matrix isolated AA-benzene complexes
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pKapKa ∆ν OH (cm -1 ) ∆ν/∆pK a =14 ∆ν OH (cm -1 ) pKapKa ∆ν/∆pK a =11 O-H is a measure of local interactions..? Chemical substitution effects on correlation of bulk acidity parameter (pK a ) 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/∆pK a The slope, ∆ν/∆pK a, 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. This implies that local interactions are dominant.
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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. ∆ν OH (cm -1 ) Binding energy (kcal/mol ) ∆ν/∆BE=43 ∆ν OH (cm -1 ) Binding energy (kcal/mol ) ∆ν/∆BE=18 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.
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∆ν OH (cm -1 ) Hyperconjugation energy (kcal/mol ) ∆ν/∆HE=46 ∆ν OH (cm -1 ) Hyperconjugation energy (kcal/mol ) ∆ν/∆HE=40 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. This indicates again that ∆ν is an outcome of local interaction, not the overall binding interaction.
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∆ν OH (cm -1 ) Electrostatic energy (kcal/mol) ∆ν/∆E=43 ∆ν OH (cm -1 ) Electrostatic energy (kcal/mol) ∆ν/∆E=15 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.
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Problems in studying acetic-acid water HB in Matrix isolation Indistinguishable broad bands throughout entire ν OH region, confusing assignments of dimer and water-complexes. Acetic acids form strong O-H···O hydrogen bonded complexes with water. Thus no distinct feature for the complex at ∆ν O-H region was identified. Therefore, systematic study with the measured O-H data of water complexes were not successful. cm -1 AA F 3 AA
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Recent reports
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Polarizable molecule CCl 4 also perturbs ν OH * * cm -1 Binding energies (kcal/mol) [MP2/6-311++G(d,p)] 3.64 4.52
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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/∆pK a, 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/∆E CT is independent of the type of OH donor. Binary complexes having the same type of H-bonding and acceptor, ∆ν/∆E binding and ∆ν/∆E electrostatics are not robust, and depend on actual nature of the donor molecules.
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Acknowledgements Prof. T.ChakrabortyShreetama Aparajeo Deb Pratim Piyali
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