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

Phenol-water complexes considered for studies Phenol3-fluorophenol3,5-difluorophenol 2-fluorophenol2,6-difluorophenol pentafluorophenol 4-fluorophenol 2,4-difluorophenol

 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. Objective of study…

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  R AH,  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 Conventional views of H-bonded spectral shifts 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

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

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

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

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

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

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

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

Shifts in phenolic O-H bear no correlation with overall binding energies Overall binding energies calculated at the MP2/ G(d,p) level 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.  O-H (cm -1 ) Binding energy(kcal/mol) 4-FPh-W 3-FPh-W 3,5-DFPh-W 2-FPh-W 2,4-DFPh-W 2,6-DFPh-W Ph-W PFPh-W

Conformer 1(calc) Conformer 2(calc) Experiment cm -1 2-Fluorophenol∙∙∙water: Spectra and structure Binding energy (kcal/mol) kcal/mol 2.92 kcal/mol 5.5 kcal/mol  C-C-O-H M

 In aqueous solutions, pK a 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 + Classical idea of solvation: Layers of oriented water dipoles forming different solvation spheres Significance of pK a : The experimental spectral shifts here have been obtained for a frozen geometry. But pK a, from geometric viewpoint is an orientationally averaged energetic parameter. The observed correlation between  and pK a might imply that under the 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. Ph-W 4-FPh-W 3-FPh-W 3,5-DFPh-W 2,4-DFPh-W 2-FPh-W 2,6-DFPh-W PFPh-W  O-H (cm -1 ) pKapKa

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

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

Hyperconjugation energy (kcal/mol )  O-H (cm -1 ) Ph-W 4-FPh-W 3-FPh-W 3,5-DFPh-W 2-FPh-W 2,4-DFPh-W 2,6-DFPh-W PFPh-W lp(O w )  *(O-H ph ) % change in  O-H (cm -1 ) %change in Hyperconjugation energy (kcal/mol )  * pop (O-H ph )  O-H (cm -1 ) Ph-W 4-FPh-W 3-FPh-W 3,5-DFPh-W 2-FPh-W 2,4-DFPh-W 2,6-DFPh-W PFPh-W % change in  O-H (cm -1 ) %change in  * pop (O-H ph ) Correlation of spectral shifting with charge transfer Natural Bond Orbital analysis

 O-H (cm -1 ) %change in  O-H ∆ρ BCP OH (a.u) %change in ∆ρ BCP OH Ph-W 4-FPh-W 3-FPh-W 3,5-DFPh-W 2-FPh-W 2,4-DFPh-W 2,6-DFPh-W PFPh-W More correlations with spectral shifting…  O-H (cm -1 ) Total Charge Transfer (e) Ph-W 4-FPh-W 3-FPh-W 3,5-DFPh-W 2,4-DFPh-W 2,6-DFPh-W 2-FPh-W PFPh-W (III) % change in total charge transfer %change in  O-H AIM analysis

Poor quantitative correspondence of overall electrostatics with spectral shifting Energy Decomposition Analysis at MP2/ 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.