SOLVENT EFFECTS ON IR MODES OF (R)-3-METHYLCYCLOPENTANONE CONFORMERS: A COMPUTATIONAL INVESTIGATION Watheq Al-Basheer Physics Department - King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia 68 th OSU International Symposium on Molecular Spectroscopy Jun. 20 th 2013

Solvent Effects on IR Spectra Generally, solute-solvent interactions can be classified into; 1- Short range: example (H-bond ) 2- Long range: example (Polarization) Observed solvent effects on IR spectra Frequency shift in the vibrational modes wavenumber. Alteration in IR Intensities. Half width of Bands may be greatly increased. In chiral Molecules, direct effect on conformers population, energies, and frequencies.

PCM & SCRF Solvent is treated as a polarizable continuum with a dielectric constant, , instead of explicit solvent molecules. The charge distribution of the solute polarizes the solvent producing a reaction potential. The reaction potential of solvent alters the solute due to electrostatic interactions. This interaction is represented by a solvent reaction potential introduced into the Hamiltonian. PCM is also know as self consistent reaction field (SCRF) methods due to Onsager’s seminal paper.

R-(+)-3-Methylcyclopentanone Conformers R3MCP (C 6 H 10 O) is a chiral ketone which can exist in as many as five conformers. Only 2 R3MCP conformers are dominant ; At room temperature, the average population of equatorial and axial methyl conformers 78:22, respectively. Equatorial Axial

DFT calculations of R3MCP conformers IR spectra in CCl 4 Conformer Markers

Equatorial R-(+)-3-Methylcyclopentanone IR marker in solvation W. Al-Basheer, J. Sol. Chem. 41, (2012)

Axial R-(+)-3-Methylcyclopentanone IR marker in solvation W. Al-Basheer, J. Sol. Chem. 41, (2012)

R3MCP conformers dipole moments in solvation solvent (μ x ) eq (μ y ) eq (μ z ) eq μ eq (D) (μ x ) ax (μ y ) ax (μ z ) ax μ ax (D) cyclohexane (C 6 H 12 ) carbon tetrachloride (CCl 4 ) toluene (C 7 H 8 ) benzene (C 6 H 6 ) chlorobenzene (C 6 H 5 Cl) chloroform (CHCl 3 ) dimethyl sulfoxide (C 2 H 6 OS) acetonitrile (C 2 H 3 N) ethanol (C 2 H 6 O) methanol (CH 4 O) water (H 2 O)

R3MCP conformers energy differences in solvation W. Al-Basheer, J. Sol. Chem. 41, (2012)

R3MCP conformers enthalpies in solvation W. Al-Basheer et al, J. Phys. Chem. A 111(12), (2007)

Summary & Conclusions DFT calculations had been carried out in 11 common solvents of wide polarity range to investigate R3MCP equatorial and axial conformers IR modes response and sensitivity to solvent polarity. Observed Hypsochromic (blue) shift in the equatorial conformer IR mode frequency, in comparison to Bathochromic (red) shift for the axial conformer as a function of solvent polarity increase. The opposite frequency shift is a reflection of the fact that equatorial and axial conformers dipole moments are opposite in direction. Axial conformer dipole moment magnitude (μ ax (D)) is higher than that of the equatorial (μ eq (D)) in each solvent medium due to relative stability and less vulnerability of the equatorial-methyl to environmental effects.

Acknowledgements Generous financial support of Deanship of Scientific Research at King Fahd University of Petroleum & Minerals. Computational resources at KFUPM chemistry Dept. and the University of Tennessee, Knoxville. Prof. Robert N. Compton of the University of Tennessee for many fruitful comments.