International Symposium on Molecular Spectroscopy June 22 26

International Symposium on Molecular Spectroscopy, June 22 -26, 2015 MATRIX ISOLATION INFRARED SPECTROSCOPY OF A SERIES OF 1: 1 PHENOL-WATER COMPLEXES Pujarini Banerjee & Tapas Chakraborty Indian Association for the Cultivation of Science Kolkata, India

Phenol-water complexes considered for studies Phenol 2 -fluorophenol 4 -fluorophenol 2, 4 -difluorophenol 3 -fluorophenol 3, 5 -difluorophenol 2, 6 -difluorophenol pentafluorophenol

Objective of study… § The key experimental descriptor of such HB formation is the weakening of the donor phenolic O-H bond, manifested as spectral red-shifting of its stretching frequency. §Conventionally, energy decomposition schemes based on Morokuma analysis or SAPT methods are used to decipher the contributions of electrostatic, dispersion and charge-transfer interactions towards total binding energies of HB complexes. §Our aim is to readdress origin of spectral shifting and decipher the intermolecular parameter that describes experimentally observed spectral shifts best, rather than overall energy considerations.

Conventional views of H-bonded spectral shifts Spectral shifts are known to conventionally relate with interaction energies, as stated in the Badger-Bauer rules and acclaimed widely in literature. “the overall correlations of H-bond strength with standard experimental RAH, AH or H metrics are reasonably robust, consistent with widespread usage of these properties as reliable indicators of H-bonding. ” -Weinhold and Klein, Molecular Physics, 2012, 110, 565 Our approach: Keeping the O-H O binding site and acceptor moiety the same, interaction strengths at the binding sites are altered by incorporating chemical substitutions, here only F, at remote sites of the HB (Phenolic O-H) donor.

Molecular complexes are synthesized in argon matrixes Temperature ~ 25 K Frozen most stable equilibrium orientations are thus obtained

Method: Molecules are isolated in the matrixes, then annealed to get the complexes Temperature ~ 7 K

Matrix isolation FTIR spectroscopy Temperature ~ 7 K Temperature sensor KBr Window window Needle IR Beam of an FTIR spectrometer

Matrix isolation infrared spectrum of phenol O-H of Phenol 3636 cm -1 } O-H of Water (cm-1) Gor et al, Chemical Physics Letters 517 (2011) 9– 15

Matrix isolation infrared spectrum of phenol 3460 cm-1 * (cm-1) 3636 cm-1

Matrix isolation infrared spectrum of phenol in presence of added water 3460 cm-1 (cm-1) 3636 cm-1

IR spectra of different phenol-water complexes: O–H Stretching range PFPh-W * 2, 6 -DFPh-W * 2 -FPh-W * 3, 5 -DFPh-W * 3 -FPh-W * cm-1

OH∙∙∙F hydrogen bonding effect on O-H of o-fluorophenols 20 cm-1 (cm-1)

O-H of phenols exerted by single water molecule bears a linear correlation with p. Ka of phenols PFPh-W O-H(cm-1) 2, 6 -DFPh-W 2, 4 -DFPh-W 2 -FPh-W 3, 5 -DFPh-W 4 -FPh-W 3 -FPh-W p. Ka

Shifts in phenolic O-H bear no correlation with overall binding energies O-H(cm-1) PFPh-W 2, 6 -DFPh-W 2, 4 -DFPh-W 2 -FPh-W 3, 5 -DFPh-W Overall binding energies calculated at the MP 2/6 -311++G(d, p) level 3 -FPh-W 4 -FPh-W Binding energy(kcal/mol) In electronic structure theory methods, binding energies are calculated for frozen geometries of the complexes, and the same is realized in measurements performed under a matrix isolation condition. The apparent lack of correlation between the two energetic parameters implies that some of the constituent energetic components of the overall binding energy do not contribute to spectral shifting effects, making total binding energy a poor descriptor of IR spectral shifting.

2 -Fluorophenol∙∙∙water: Spectra and structure +2. 92 kcal/mol 6. 8 Conformer 1(calc) 5. 5 kcal/mol 6. 1 2. 92 kcal/mol Binding energy (kcal/mol) C-C-O-H Conformer 2(calc) M Experiment cm-1

Significance of p. Ka: § In aqueous solutions, p. Ka is a measure of the free energy change ∆G associated with ionic dissociation of a weak acid under thermal equilibrium, where solvation plays a key role. AH A- + H+ PFPh-W Classical idea of solvation: O-H(cm-1) 2, 6 -DFPh-W 2, 4 -DFPh-W 2 -FPh-W 3, 5 -DFPh-W 4 -FPh-W 3 -FPh-W Layers of oriented water dipoles forming different solvation spheres Ph-W p. Ka The experimental spectral shifts here have been obtained for a frozen geometry. But p. Ka, from geometric viewpoint is an orientationally averaged energetic parameter. The observed correlation between and p. Ka might imply that under thermal equilibrium in a liquid, only the local interactions dominate, and some other long range components are averaged out. Therefore, we have explored whether the sequence of spectral shifting can be explained in terms of only components of local interactions.

Correlation with local electrostatic parameter: Natural Charge ( +) on Phenolic H q(H)= Natural charge on phenolic H PFPh-W O-H(cm-1) PFPh 2, 6 -DFPh-W 2, 6 -DFPh 2, 4 -DFPh 2 -FPh 3, 5 -DFPh 3 -FPh 4 -FPh Ph 2, 4 -DFPh-W 3, 5 -DFPh-W 4 -FPh-W 3 -FPh-W Natural charge on H 2 -FPh-W While natural charges on H vary only by ~5%, spectral shifts from Ph. OH to F 5 -Ph. OH differ by ~90%.

Energy Hyperconjugation and charge transfer * ( almost vacant) (filled) Hyperconjugative stabilization Charge transfer from lone pair orbital on water (n) to phenolic σ*(OH) orbital

Correlation of spectral shifting with charge transfer %change in Hyperconjugation energy (kcal/mol) 60 PFPh-W 320 O-H (cm-1) 3 -FPh-W 40 20 3, 5 -DFPh-W 4 -FPh-W 10 lp(Ow) 12 0 14 16 18 %change in *pop(O-Hph) 0 20 *(O-Hph) Hyperconjugation energy (kcal/mol) 40 20 60 2, 6 -DFPh-W 2 -FPh-W 240 160 100 80 200 80 PFPh-W 320 O-H (cm-1) 200 Natural Bond Orbital analysis 60 2, 4 -DFPh-W 2 -FPh-W 240 100 80 2, 6 -DFPh-W 280 160 80 % change in O-H (cm-1) 40 20 % change in O-H (cm-1) 0 3 -FPh-W 60 2, 4 -DFPh-W 40 20 3, 5 -DFPh-W 4 -FPh-W 0. 012 0. 014 0 0. 016 0. 018 *pop(O-Hph) 0. 020 0. 022

More correlations with spectral shifting… % change in total charge transfer 0 20 40 60 (III) 80 2, 4 -DFPh-W 2 -FPh-W 240 40 20 3 -FPh-W 160 0. 012 60 2, 6 -DFPh-W 0. 014 3, 5 -DFPh-W 4 -FPh-W 0. 016 0. 018 AIM analysis 0 0. 022 %change in ∆ρBCP OH Total Charge Transfer (e) PFPh-W 2, 6 -DFPh-W 2, 4 -DFPh-W 2 -FPh-W 3, 5 -DFPh-W 3 -FPh-W 4 -FPh-W ∆ρBCP OH (a. u) %change in O-H 280 O-H (cm-1) 320 200 100 %change in O-H PFPh-W

Poor quantitative correspondence of overall electrostatics with spectral shifting Energy Decomposition Analysis at MP 2/6 -311++G(d, p) Overall electrostatic contribution increases as a result of increasing flurorine substitutions, but cannot account quantitatively for spectral shifting variations.

Inferences § Spectral shifts of different complexes studied here do not correlate with overall binding energy parameters, against much used Badger-Bauer rule. § Variation of spectral shifting with charge-transfer parameters is linear and quantitative. Thus, a local interaction is the chief intermolecular interaction component responsible for spectral shifting effect. § Electrostatics is certainly an important major contributor to overall binding energies of O-H…O HB complexes, but its contribution to spectral shifting is insignificant.