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

Carboxylic acids as hydrogen bond donors [MP2/ G(d,p)] 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/ 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 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.

Temperature  8 K Carboxylic acids as hydrogen bond donors

Matrix isolation IR spectra of three carboxylic acids used as HB donors cm -1 Trichloro-AA Trifluoro-AA AA pKapKa (±0.1) 0.3(±0.1) Trichloro-AA Trifluoro-AA AA

IR spectra of benzene complexes of the three carboxylic acids (matrix isolated) pKapKa (±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

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

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.

 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.

∆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.

∆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.

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

Recent reports

Polarizable molecule CCl 4 also perturbs ν OH * * cm -1 Binding energies (kcal/mol) [MP2/ G(d,p)]

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.

Acknowledgements Prof. T.ChakrabortyShreetama Aparajeo Deb Pratim Piyali