Investigation of crown ether cation- systems using electrophoretic NMR Fredrik Petersson physical Chemistry royal institute of technology (KTH) Stockholm Sweden Supervisor Marianne Giesecke Examiner prof Istvan´ Furo´ Stockholm June 29, 2012 Abstract The purpose of this thesis was to investigate how crown ethers behave and interact with different cations and to optimise the setup of the electrophoretic NMR. To get a good electrophoretic NMR measurement the electrophoretic phase shift needs to be big. To increase the phase shift some parameters needed to be adjusted, parameters such as the concentration of crown ether and cation, the duration of magnetic field gradient pulse δ, the magnetic field gradient strength g,the diffusion time Δ and the applied voltage V. The main focus then put on crown ethers 15-crown-5 and 18-crown-6. The cations used were lithium (Li), sodium (Na), potassium (K), caesium (Cs), calcium (Ca) and barium(Ba). The effective charge was obtained by using pulsed gradient NMR to derive the diffusion coefficient and electrophoretic NMR to get the electrophoretic mobility. These data were used to calculate the equilibrium constant of the formed complex. The outcome of the investigation: the affinity for 18-crown-6 was in the following order barium > potassium > caesium > sodium > calcium > lithium and for 15-crown-5 barium > sodium > calcium > caesium > potassium > lithium. Sammanfattning Syftet med denna avhandling var att unders¨oka hur kronetrar beter sig och inter- agerar med olika katjoner och optimera den elektroforetiska NMR upps¨attningen, F¨or att f˚a en bra elektroforetiska NMR m¨atning m˚aste fasskiftet vara stort. F¨or att ¨oka fasskiftet beh¨ovs n˚agra parametrar st¨allas in s˚a som koncentration av kroneter och katjon, l¨angden av magnetf¨altsgradientspulsen δ, den gradi- entstyrkan g, diffusionstiden Δ och den applicerade sp¨anningen V. Fokus har lagts p˚a kronetrarna 15-kron-5 och 18-krona-6. De anv¨anda katjoner var litium (Li), natrium (Na), kalium (K), cesium (Cs), kalcium (Ca) och bar- ium (Ba). De olika systemen unders¨oktes med hj¨alp av diffusions NMR f¨or att m¨ata diffu- sionskoefficienten och elektroforetisk NMR f¨or att f˚a fram elektroforetiska mo- biliteten. Dessa uppm¨atta data anv¨andes f¨or att ber¨akna j¨amviktskonstanten av det bil- dade komplexet. Utfallet av studien blev: affiniteten f¨or f¨or 18-kron-6 barium > kalium > cesium > natrium > kalcium > litium i och f¨or 15-kron-5 barium > natrium > kalcium > cesium > kalium > litium. ii Contents Abstract................................... i Sammanfattning.............................. i Contents................................... iii Introduction................................. iv 1 Background 1 1.1Crownethers............................. 1 1.2Acetatesalts............................. 2 1.3Electrophoresis............................ 2 1.4DifferenttypesofNMRtechniques................. 3 1.4.1 ConventionalNMR...................... 3 1.4.2 PulsedfieldgradientNMR................. 6 1.4.3 ElectrophoreticNMR.................... 10 2 Summary of research 14 2.1Assemblingoftheelectrophoreticcell................ 14 2.2Calibration.............................. 14 2.2.1 Calibrationoftheelectrophoreticcell........... 14 2.2.2 Calibration of the diffusion measurement . 15 2.2.3 Calibrationofthegradient................. 16 2.3Samplepreparation.......................... 16 3 Results and discussion 19 3.1Diffusionmeasurements....................... 19 3.2ElectrophoreticNMR........................ 19 3.3Summaryofresults.......................... 23 3.4Sourcesoferrors........................... 24 4 Conclusions 25 Acknowledgements 26 Bibliography 28 Appendix 29 List of figures 43 iii Introduction Crown ethers interaction with cations are fairly well known, but not so many studies have been using electrophoretic NMR. Crown ethers are of interest be- cause their properties are useful in applications such as catalysts for chemical reactions [1], phase transfer reagents, increasing solubility of salts in organic liquids [19], hosts for transport across membranes [3] and separation processes [25]. To investigate the interaction between crown ethers and cations, three different NMR techniques were used: conventional NMR, pulsed field gradient NMR and electrophoretic NMR. Conventional NMR uses the magnetic moment of nuclei, to derive information about their surroundings. In pulsed gradient NMR magnetic field gradients are applied to achieve a loss in signal strength of the peaks in the spectra. The behaviour of the decaying signal can be used to derive the diffusion coefficient Electrophoretic NMR is a combination between electrophoresis and pulsed gra- dient NMR and measures phase shift in the spectra under increasing electric field and constant magnetic field gradient. If the diffusion coefficient is known then the technique makes it possible to derive information like: electrophoretic mobility, effective charge and equilibrium constant. iv Chapter 1 Background 1.1 Crown ethers The first crown ether was synthesized in 1967 by Charles J Pedersen [18]. This discovery later gave him the Nobel prize in 1987 together with Jean-Marie Lehn and Donald J Cram for their development and use of molecules with structure- specific interactions of high selectivity.[24] Crown ethers are ethers with a closed structure. To optimise the molecular dipole moment the chain folds into something that reminds of a crown, hence the name. The closed structure gives rise to a cavity and this is the origin of its interesting properties, such as binding to different cations. These phenomena can exist thanks to the interaction between the oxygen atoms in the crown ether and the cation in the cavity, this lowers the free energy for the complex con- stituents. Properties that influence the free energy are the charge of the cation, rigidity of the crown ether, entropic effects, solvation shells surrounding the complex and cation, size of the cation and the crown ether. The size selectivity tends to decrease as the ring size of the crown ether increases, since it is easier for a larger ether to achieve a folded configuration to optimise its interaction with the cation, because of the high flexibility.[24, 11] The sum of the thermodynamic effects gets reflected in the equilibrium constant [crown ether cation complex] K = . (1.1) [freecation][unoccupied crown ether] The equilibrium constant varies with the cation, which makes it possible to sep- arate cations from each other by using a crown ether that is selective to one of the cations in the system [25]. Other applications for crown ethers are catalysts for chemical reactions [1], phase transfer reagents, they can also increase the solubility of salts in organic liquids [19] The crown ethers used in this thesis are 15-crown-5 and 18-crown-6 with cavity sizes of approximately 1.3-1.7 A˚ respectively 0.9-1.1 A,˚ see Figure 1.1. 1 O O O O O O O O O O O (a) (b) 15crown5 18crown6 Figure 1.1: Crown ethers used in this thesis. 1.2 Acetate salts The advantage with acetate salts is that the acetate anion has protons and can be easily detected by 1H NMR. The cations to acetate explored here are listed in Table 1.1 together with their respective ionic radius [16] Cation Ionic radius [A]˚ Li+ 0.69 Na+ 1.02 K+ 1.38 Cs+ 1.70 Ca2+ 1.00 Ba2+ 1.36 Table 1.1: Cations used in this work [16] 1.3 Electrophoresis In electrophoresis the behaviour of charged spices in field is used. According to fundamental physics a charged particle in an electric field is influ- enced by a force [9] qU Fe = , (1.2) l where Fe is the force in [N], q is charge of the particle in [C], U is the electric potential difference in [V] and l is the distance between the electrodes in [m]. This phenomenon is explored in electrophoresis. If the particle is not moving in vacuum, its movement interferes with the surrounding medium which corre- sponds to a friction force that is acting to restrict motion. The friction force is dependent of the speed and the interaction between the particle and the medium and is given by the equation: Ff = fv (1.3) 2 −1 where Ff is the friction force in [N], f the friction coefficient in [Nsm ]andv the velocity in [ms−1]. When the two forces are equal then the acceleration stops and the particle travels at a constant speed which gives the following expression. qU v = (1.4) lf The self motion of a system in a medium is called diffusion and it depends on the thermal energy of the systems and the resistance against movement. The diffusion is described by the Einstein-Sutherland equation: kB T D = , (1.5) f 2 −1 where D is the diffusion coefficent in [m s ], kB is the Boltzmann factor in [JK−1]andT is the temperature in [K]. By combining eq 1.4 and eq 1.5 the following expression can be derived. vkB Tl D = (1.6) qU The definition of electrophoretic mobility is vl μ = (1.7) U which makes it possible to express the diffusion coefficient in the following way [7] μkB T D = . (1.8) q 1.4 Different types of NMR techniques Different types of modified setups of Nuclear Magnetic Resonance spectroscopy (NMR) have been proven to be powerful for deriving information about struc- ture, diffusion properties and electrophoretic mobility for different NMR active substances. [21, 7, 10] 1.4.1 Conventional NMR NMR exploit the nuclear spin properties of different atoms, which is useful to determine the local environment surrounding every NMR active nuclei. In order not to violate basic quantum mechanic, the spin of the nuclei, denoted by I, has to be quantized, as described by the magnetic quantum number mI .Ina magnetic field the different magnetic quantum numbers correspond to different energies [10] mI γhB E = (1.9) 2π where E is energy in [J], B the magnetic field strengthen at the site of the active nuclei in [Tesla(T )], γ is the gyromagnetic ratio [s−1T −1]andh is Planck constant [Js]. 3 Figure 1.2: The 90o pulse and the subsequent FID [7] Because of the energy difference the system gets a net magnetisation in the orientation of the static magnetic field (called thermal equilibrium).