Infrared Signature of the Cation π Interaction …

1 Infrared Signature of the Cation Interaction between calcite andAromatic HydrocarbonsHaitao Wang, Daniel J. Grant, Peter C. Burns, , and Chongzheng Na*, Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, NotreDame, Indiana 46556, United States Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556,United States*SSupporting InformationABSTRACT:The Cation Interaction is proposed as animportant mechanism for the adsorption of aromatic hydro-carbons having non-zero quadrupole moments by mineralsurfaces.

2 Direct evidence supporting such a mechanism is,however, limited. Using the model mineral calcite , we probethe Cation Interaction with adsorbed benzene, toluene, andethylbenzene (BTE) molecules using attenuated totalreflectance Fourier transform Infrared spectroscopy. We showthat the presence of calcite increases the energy required toexcite the synchronized bending of aromatic C H bonds ofBTE molecules. The unique conformation of this vibrational mode indicates that the planar aromatic rings of BTE molecules areconstrained in a tilted face-down position by the Cation Interaction , as further confirmed by density functional theorycalculations. Our results suggest that the shift of the excitation energy of the aromatic C H bending may be used as an infraredsignature for the Cation Interaction occurring on mineral INTRODUCTIONThe Cation Interaction can arise from the electrostatic forcebetween a positively charged Cation and the quadrupolemoment of a neutral molecule with additional contributionsfrom polarization, induction, exchange, and dispersion ,2 Theoretical calculations suggest that the enthalpy of thecation Interaction can be as high as 80 kJ mol 1, which isapproximately1/5the enthalpy of a covalent bond and 5 timesthat of a hydrogen ,4 Given the strength of the Cation Interaction .

3 It is well-recognized as an important mechanism forcontrolling intermolecular processes, such as biomolecule self-assembly,5ligand protein recognition,6organic synthesis,7andselectivity of ion comparison, only limited effortshave been devoted to elucidating the mechanism of thecation Interaction in surface-related 13On a surface, the Cation Interaction can be envisioned tooccur over the positively charged surface cations and, thus, hasbeen proposed as an important mechanism responsible for theadsorption of aromatic hydrocarbons on mineral ,10 Direct evidence supporting the Cation Interaction on amineral surface is currently limited to measurements madeusing deuterium nuclear magnetic resonance (2H NMR) pioneering application of2H NMR showedthat the quadrupole Interaction of deuteriated aromatichydrocarbons with an external magneticfield decreased extent of the decrease varied with the changeof surface cations, following a trend consistent with that ofpartitioning coefficients deduced from sorption , complementary analytical techniques are in need tofurther advance the knowledge of the Cation interactioninvolving mineral transform Infrared spectroscopy (FTIR)

4 Is a sensitiveanalytical technique that has been used to study the interactionsof aromatic hydrocarbons with metals,14metal oxides,15aluminosilicates,16and probe molecules adsorbedon a solid surface, a sampling technique called attenuated totalreflection (ATR) is often used in conjunction with ,19In ATR FTIR,20,21a powder of the solid is pressed on a crystalwith a high reflective index serving as a conduit to direct theexcitation light by total internal reflection. The light creates anevanescent wave extending out of the crystal and into thepowder. The absorption of the excitation light by the powderprovides structural information pertaining to the moleculesadsorbed on the powder surface-adsorbedaromatic hydrocarbons, as compared to their counterparts ina liquid, interactions such as the Cation Interaction with thesurface can vary the energies required for exciting certainmodes of molecular , an Infrared (IR) Signature distinctively linking ATR FTIR measurement andthe Cation Interaction has not been :February 14, 2015 Revised:May 13, 2015 Published:May 14, 2015 American Chemical Society5820 DOI.

5 , 31, 5820 5826 Here, we report the characterization of Cation interactionsbetween benzene, toluene, and ethylbenzene (BTE) com-pounds and calcite (CaCO3) using ATR FTIR. calcite is amodel rhombohedral carbonate mineral that is the maincomponent of oil-trapping chalk formations and a majorcomponent of soils, sediments, and atmospheric dust compounds are important constituents of organic solventsand liquid fuels that are widely used in industrial processes andconsumer products. We show that the adsorption of BTEmolecules on calcite results in a significant increase of theenergy required to excite the out-of-plane aromatic C Hvibration ( C H), providing a prominent analytical Signature forprobing the Cation Interaction on mineral surfaces.

6 Theconnection between C Hvibration and the Cation Interaction is further supported by density functional theory(DFT) EXPERIMENTAL SECTIONHigh-quality calcite was purchased from Ward s Science (Rochester,NY). The structure of the calcite was verified using X-ray diffraction(Bruker Advance). The elemental impurities in calcite weredetermined to be less than (see Table S1 of the SupportingInformation) by inductively coupled plasma mass spectrometry(Thermo Scientific Element 2) after digestion in concentrated nitricacid. calcite was ground into a powder using a mortar and a powder was sifted using standard sieves (270 and 325) to selectfor particles having diameters from 45 to 53 m [nominal diameter of49 ( 4) m].

7 Reagent-grade liquids of BTE were purchased fromSigma-Aldrich (> ). The liquids were dried using a 4 molecular sieve (Fisher Scientific), which reduced the water contentbelow 5 ppb, as measured by the Karl Fischer titration (Aqua Counter300).Before mixing with BTE, the calcite powder was baked at 300 C for24 h to remove water from the surface of the particles withoutchanging their structure and dehydrated powder wascooled to room temperature in a vacuum desiccator (Bel-Art) to avoidinteractions with water vapor before mixing with BTE compounds. Tofacilitate the deconvolution of the IR spectrum, the dehydratedpowder was rehydrated in humid air overnight to saturate the calcitesurface with water molecules. This was performed by spreading thepowder evenly at the bottom of a sealed chamber that contained asmall beaker of deionized (DI) dehydrated calcite powder (400 mg) was mixed with 100 LofBTE liquid in a dry glass vial.

8 The vial was then sealed and placed in anoven at 60 C for h. The elevated temperature facilitated thedispersion of BTE molecules and created a uniform surface coverage ofBTE molecules on the calcite particles. The rehydrated powder wasmixed with the BTE liquid at room temperature to prevent drying ofthe hydrated probe the Cation Interaction at the calcite BTE interface,approximately 35 mg of the BTE-wetted powder was placed on thediamond crystal element of a FTIR spectrometer equipped with anFigure spectra of (a) calcite , (b, d, and f) liquids of BTE, and (c, e, and g) calcite surfaces absorbed with BTE molecules. Each spectrum ispresented with a full scan from 400 to 4000 cm 1on the left and an expanded view from 640 to 770 cm 1on the right.

9 The wavenumber range forthe right panels is highlighted on the left panels in gray. The bands of the aromatic C H out-of-plane bending of BTE molecules arefitted withLorentzian models. On the basis of the shifts, the Lorentzianfits are marked as I for thefirst layer of surface-bound molecules and E for the extendedlayers of surface-bound molecules, as compared to L for the molecules in liquids. The absorbance is scaled, so that the spectra could be plottedtogether. The inset structures represent the synchronized out-of-plane aromatic C H bending modes ( ,C; , H; and X = CH3for toluene andC2H5for ethylbenzene).LangmuirArticleDOI: , 31, 5820 58265821 ATR assembly (Bruker Tensor 27). The instrument was enclosed andoperated in a fume hood.

10 IR spectra were acquired immediately afterthe powder was transferred from the sealed vial onto the diamondcrystal. The acquisition continued until the signals for surface-boundBTE molecules completely disappeared as a result of ranging from 400 to 4000 cm 1were acquired with aresolution of 1 cm 1. To identify the IR absorption bands unique tosurface-bound molecules, spectra were also obtained for pure BTEliquids and the pristine calcite powder for comparison. All spectra werereferenced to the bare diamond optimizations and single-point calculations wereperformed using the Gaussian program do so, a BTEmolecule was placed on top of the 101 4 cleavage surface of a calciteslab consisting of 18 were performed withthe M06 functional and a 6-31G*basis set on all ,28 Fullgeometry optimizations of benzene, toluene, and ethylbenzene wereperformed inD6h,C1, andCssymmetries, respectively.