1 Chemical ShiftChemical Shift Chemical shift for a given molecule:Chemical shift for a given molecule: •• Number of signals = nonequivalent nucleiNumber of signals = nonequivalent nuclei molecular symmetrymolecular symmetry •• Relative intensity = number of nucleiRelative intensity = number of nuclei •• Position in the spectrumPosition in the spectrum = shielding/chemical shift= shielding/chemical shift electronic structureelectronic structure •• Multiplicity = connectivity of atoms and groupsMultiplicity = connectivity of atoms and groups 2 Nuclear Magnetic ShieldingNuclear Magnetic Shielding σσ Basic physical phenomenon: Nuclear Magnetic ShieldingBasic physical phenomenon: Nuclear Magnetic Shielding σσ For diamagnetic samples, the nuclear magnetic shielding canFor diamagnetic samples, the nuclear magnetic shielding can be expressed as correction to the Zeeman splitting:be expressed as correction to the Zeeman splitting: ΔΔE =E = γγ ħħ BB00(1(1 –– σσ) =) = ϖϖ ħħ shielding constantshielding constant σσ •• In solutionIn solution, the nuclear magnetic shielding constant, the nuclear magnetic shielding constant σσ is ais a scalarscalar quantityquantity •• In solidsIn solids,, σσ is ais a tensortensor (3 x 3 = 9, only 6 measurable)(3 x 3 = 9, only 6 measurable) 3 FaradayFaraday's's LawLaw Changes in the magnetic flux through a coil of wire induce a voltage (emf) in the coil 4 LenzLenz's's LawLaw •A voltage is generated by a change in magnetic flux •The polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it •The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. 5 Nuclear Magnetic ShieldingNuclear Magnetic Shielding 6 Nuclear Magnetic ShieldingNuclear Magnetic Shielding σσ s-electrons spherically symmetric precess in the applied magnetic field = circulating electron is an electric current, producing a magnetic field at the nucleus which opposes the external field the resonant condition - the applied field must be increased - diamagnetic shift (shielding) all atoms have diamagnetic shifts p,d,f-electrons no spherical symmetry, and produce large magnetic fields at the nucleus - paramagnetic shifts (deshielding) 7 BBnuclnucl = B= B00 –– BBshieldshield BBshieldshield = B= B00 σσ BBnuclnucl = = B= = B00 –– BB00 σσ == BB00(1(1 –– σσ)) ϖϖ == γγ BBnuclnucl == γγ BB00(1(1 –– σσ)) LenzLenz’’s rules rule B =B = the magnetic flux density orthe magnetic flux density or magnetic inductionmagnetic induction (T)(T) H =H = thethe magneticmagnetic fieldfield ((strengthstrength)) (A/m)(A/m) B =B = μμ HH Two different nucleiTwo different nuclei Nuclear Magnetic ShieldingNuclear Magnetic Shielding σσ 8 Nuclear Magnetic ShieldingNuclear Magnetic Shielding σσ Bo Bo(1-σa) Bo(1-σb) Bo ϖϖ == γγ BBnuclnucl == γγ BB00(1(1 –– σσ)) 9 Absolute Magnetic ShieldingAbsolute Magnetic Shielding the absolute value of the nuclear magnetic shielding constantthe absolute value of the nuclear magnetic shielding constant cannot becannot be measured experimentally bymeasured experimentally by NMR, dNMR, difficultifficult toto measure, but can be done for atoms or small moleculesmeasure, but can be done for atoms or small molecules MHz vs. Hz 1:10MHz vs. Hz 1:1066 Relative Magnetic ShieldingRelative Magnetic Shielding Requires measurement of differences of resonance frequenciesRequires measurement of differences of resonance frequencies between a sample and abetween a sample and a standardstandard (much more easily done)(much more easily done) IInternalnternal standardstandard -- a referencea reference compoundcompound is in the same sampleis in the same sample preferred from spectroscopic point of view,preferred from spectroscopic point of view, may causemay cause chemical problemschemical problems EExternalxternal standardstandard -- in a different samplein a different sample tubetube 10 Absolute Chemical Shieldings 11 ChemicalChemical ShiftShift 2.70 ppm2.70 ppmδ value: 270 Hz162 HzShift From TMS: 100 MHz60 MHzOperating Frequency, ν0: B0 = 2.35 TB0 = 1.41 TStrength of Field: [ ] [ ]MHz Hzrefs 0υ υυ δ − = TheThe δδ scale (orscale (or ppmppm scale) is independent of the instrumentscale) is independent of the instrument used to obtain the spectrumused to obtain the spectrum 12 ChemicalChemical ShiftShift 13 The relative shielding of the sample can be expressed as:The relative shielding of the sample can be expressed as: AbsoluteAbsolute MagneticMagnetic ShieldingShielding ((σσ--scalescale):): σσ = 10= 1066 ((ννnuclnucl –– ννss ) /) / ννnuclnucl ννnuclnucl = absolute resonance frequency of the atom= absolute resonance frequency of the atom ννss = absolute resonance frequency of the signal= absolute resonance frequency of the signal ChemicalChemical Shift (Shift (δδ--scalescale):): δδ = 10= 1066 ((ννss –– ννrefref ) /) / ννrefref ννrefref = resonance frequency of the standard= resonance frequency of the standard ConversionConversion betweenbetween bothboth scalesscales:: δδ = (= (σσrefref –– σσss ) / (1) / (1 –– σσrefref )) ~~ ((σσrefref –– σσss )) σσrefref = absolute magnetic shielding value of the standard= absolute magnetic shielding value of the standard δδ (H(H33POPO44) = 0) = 0 σσ (H(H33POPO44) = 320) = 320 14 MagneticMagnetic fieldfield MagneticMagnetic ShieldingShielding ChemicalChemical shiftshift FrequencyFrequency NucleiNuclei inin electronelectron poorpoor environmentsenvironments NucleiNuclei inin electronelectron richrich environmentsenvironments DeshieldedDeshielded ShieldedShielded DownfieldDownfield UpfieldUpfield LowLow frequencyfrequencyHighHigh frequencyfrequency PositivePositive NegativeNegative Convention:Convention: HighHigh frequencyfrequency positivepositive 15 ChemicalChemical ShiftShift ReferencesReferences 11HH ppmppm SiMeSiMe44 00 DSSDSS 00 MeMe33SiSi--CHCH22--CHCH22--CHCH22--SOSO33NaNa TSPTSP 00 MeMe33SiSi--CDCD22--CDCD22--COONaCOONa 1919FF ppmppm CFClCFCl33 00 CFCF33COOHCOOH −−78.578.5 CC66FF66 −−162.9162.9 HFHF 198.4198.4 FF22 422.9422.9 129129Xe (I =Xe (I = ½½ , 26.4 %), 26.4 %) 131131Xe (I = 3/2 , 21.1 %)Xe (I = 3/2 , 21.1 %) Xenon in freonXenon in freon LiquidLiquid XeOFXeOF44 16 ChemicalChemical ShiftShift ReferencesReferences 1919FF,, ppmppm BeBe carefulcareful withwith literatureliterature datadata SometimesSometimes CC66FF66 = 0= 0 17 FactorsFactors IInfluencingnfluencing CChemicalhemical SShiftshifts (1) The physical state of the sample (solid, liquid, solution or gas) (2) For solutions, the solvent and the concentration of solute (3) The nature of the reference procedure, e.g. internal, external (coaxial tubes or substitution), absolute frequency (4) The reference compound and, if used internal to a solution, its concentration (5) The temperature and pressure of the sample (6) Whether oxygen and other gases have been removed from the sample (7) Any chemical present in the sample, in addition to the compound under investigation and any reference compound 18 FactorsFactors IInfluencingnfluencing CChemicalhemical SShiftshifts (1)(1) IntramolecularIntramolecular factorsfactors Diamagnetic contributionDiamagnetic contribution Paramagnetic contributionParamagnetic contribution MagneticMagnetic anisotropyanisotropy Ring currentsRing currents vanvan derder Waals repulsionWaals repulsion (2) Intermolecular factorsIntermolecular factors Volume susceptibilityVolume susceptibility vanvan derder Waals forcesWaals forces Induced electric fieldInduced electric field Collision complexesCollision complexes 19 Fundamental ContributionsFundamental Contributions to the Magnetic Shielding Constantto the Magnetic Shielding Constant σσ == σσdiadia ++ σσparapara ++ ΣΣ σσnonlocnonloc σσdiadia Interaction of electrons with the external magnetic field BInteraction of electrons with the external magnetic field B00 induces ainduces a diamagnetic current density. This produces an induced field at tdiamagnetic current density. This produces an induced field at the nucleus which ishe nucleus which is proportional to Bproportional to B00 andand opposite in signopposite in sign SHIELDING CONTRIBUTIONSHIELDING CONTRIBUTION σσparapara Interaction of BInteraction of B00 with electrons with nonwith electrons with non--vanishing orbital momentsvanishing orbital moments induces ainduces a polarisationpolarisation of the electron distribution. This produces an additionalof the electron distribution. This produces an additional induced field at the nucleus which is proportional to Binduced field at the nucleus which is proportional to B00 andand equal in signequal in sign DESHIELDING CONTRIBUTIONDESHIELDING CONTRIBUTION ΣΣ σσnonlocnonloc Electrons localized at distant nuclei may contribute to the shieElectrons localized at distant nuclei may contribute to the shieldinglding (ring currents in aromatic molecules, solvent influences, shield(ring currents in aromatic molecules, solvent influences, shielding anisotropy ofing anisotropy of carbonyl groups)carbonyl groups) SHIELDINGSHIELDING oror DESHIELDING CONTRIBUTIONDESHIELDING CONTRIBUTION Generally lower in magnitude thanGenerally lower in magnitude than σσdiadia oror σσparapara.. 20 Magnetic ShieldingMagnetic Shielding Which Electrons contribute to and ?σdia σpara σdia σpara Core Electrons Valence s-Electrons Valence p,d,f-Electrons Total orbital magnetic moment for closed shells : = 0l = 0l = 1,2,3l + + + – – + σpara = 0 for spherical closed-shell atoms or ions (F )– 21 Magnetic ShieldingMagnetic Shielding Magnetic Shielding Contributions for Different Elements s-Block Elements valence p-orbitals absent (H) or hardly occupied (group 1, 2 metals) diamagnetic term dominates large non-local contributions (up to 20% for H)1 ⇒ ⇒ p,d-Block Elements valence p,d-orbitals involved in bonding term dominates non-local contributions mostly not important (but may become important for nuclei with lone pairs) paramagnetic⇒ ⇒ 22 The Diamagnetic Contribution to theThe Diamagnetic Contribution to the Magnetic Shielding ConstantMagnetic Shielding Constant BBnuclnucl = B= B00 –– BBinducedinduced = B= B00 –– BB00 σσ BBnuclnucl = B= B00(1(1 –– σσ)) ϖϖ == γγ BBnuclnucl == γγ BB00(1(1 –– σσ)) 23 The Diamagnetic Contribution to theThe Diamagnetic Contribution to the Magnetic Shielding ConstantMagnetic Shielding Constant σσ == σσdd,,isis ++ σσdd σσdd,,isis Shielding Constant for anShielding Constant for an isolated atomisolated atom (LAMB, easily computed from first principles(LAMB, easily computed from first principles,, electronelectron in ain a sphericalspherical orbitorbit)) 010 2 0, || 4 ΨΨ= − r m e e isd π μ σ ΨΨ00 == vawefunctionvawefunction ofof thethe groundground statestate μμ00 == 44ππ 1010−−77 N AN A−−22 permeabilitypermeability ofof freefree spacespace mmee == electronelectron massmass r =r = electronelectron radiusradius 24 The Diamagnetic Contribution to theThe Diamagnetic Contribution to the Magnetic Shielding ConstantMagnetic Shielding Constant σσ == σσdd,,isis ++ σσdd σσdd Correction for Atoms in MoleculesCorrection for Atoms in Molecules (Approximation by FLYGARE)(Approximation by FLYGARE) Shielding increases whenShielding increases when ••element numberelement number ZZii of theof the ligandsligands increasesincreases ••coordination number of the observed atom increasescoordination number of the observed atom increases ••bond distancebond distance rrii decreasesdecreases ∑∑ == lignads i i lignads i i e d r Z k r Z m e π μ σ 4 2 0 25 DiamagneticDiamagnetic SShiftshifts forfor IIsolatedsolated AAtomstoms σσdiadia ppmppm 11 HH 1818 1313 CC 261261 14/1514/15 NN 325325 1717 OO 395395 1919 FF 471471 2121 NeNe 552552 3131 PP 961961 3333 SS 10501050 8383 KrKr 32463246 127127 II 55025502 129129 XeXe 56425642 195195 PtPt 93969396 207207 PbPb 1006110061 AlkalidesAlkalides MM−− 2323NaNa −−6262 3939KK −−105105 8787RbRb −−185185 113113CsCs −−280280 Shielding increases when element numberShielding increases when element number ZZ of the observed atom increasesof the observed atom increases σσdd ~ 0.319 10~ 0.319 10−−44 ZZ 4/34/3 Large and heavy atoms have largeLarge and heavy atoms have large ddiamagneticiamagnetic shieldingshielding 26 Influence ofInfluence of EElectronegativitylectronegativity CompoundCompound CHClCHCl33 CHCH22ClCl22 CHCH33ClCl CHCH44 δδ ((11H)H) // ppmppm 7.277.27 5.305.30 3.053.05 0.230.23 Influence of electronegativeInfluence of electronegative substituentssubstituents :: •• increases with their increasingincreases with their increasing numbernumber •• decreases with increasingdecreases with increasing distancedistance 27 CompoundCompound, CH, CH33XX CHCH33FF CHCH33OHOH CHCH33ClCl CHCH33BrBr CHCH33II CHCH44 (CH(CH33))44SiSi XX FF OO ClCl BrBr II HH SiSi ElnegatElnegat ofof XX 4.04.0 3.53.5 3.13.1 2.82.8 2.52.5 2.12.1 1.81.8 ChemicalChemical shiftshift,, δδ // ppmppm 4.264.26 3.43.4 3.053.05 2.682.68 2.162.16 0.230.23 00 δδ ((11H)H) // ppmppm 28 Influence ofInfluence of EElectronegativitylectronegativity H3C C H3C H CH3 H2C C CH3 H C C HPh pKpKaa ~55~55 4444 28.828.8 (in DMSO)(in DMSO) acidacidityity of C increasesof C increases % C% C--H sH s--charactercharacter 2525 3333 5050 electronegativityelectronegativity of C increasesof C increases 29 Influence ofInfluence of EElectronegativitylectronegativity 30 AromaticAromatic PProtonroton SShiftshifts Electrophilic substitution Meta directing Strongly deactivating Ortho, para directing Strongly activating 31 AromaticAromatic CCarbonarbon SShiftshifts 32 AromaticAromatic CCarbonarbon SShiftshifts Number of π electrons per C δδ ((1313C)C) // ppmppm 33 The Paramagnetic Contribution toThe Paramagnetic Contribution to the Magnetic Shielding Constantthe Magnetic Shielding Constant Quantum chemical approach by RAMSEY: The electron polarization lQuantum chemical approach by RAMSEY: The electron polarization leading toeading to σσparapara isis described in terms of mixing of the wave functions of the molecudescribed in terms of mixing of the wave functions of the molecular ground statelar ground state with excited states under the influence of the magnetic field.with excited states under the influence of the magnetic field. ApAppproximatroximativive expressions fore expressions for σσparapara were given forwere given for mainmain--group elements by KARPLUS and POPLE:group elements by KARPLUS and POPLE: σpara 2π Qibonds ΔE –1 –3 r 2 ~~ Σ – and for transition metals by GRIFFIN and ORGEL:and for transition metals by GRIFFIN and ORGEL: σpara 2π ΔE –1 –3 r 2 ~~ <(n-1)d> – < 0 L 0 > 2 | | 34 The Paramagnetic Contribution toThe Paramagnetic Contribution to the Magnetic Shielding Constantthe Magnetic Shielding Constant NonsphericalNonspherical circulation of electrons under influence of Bcirculation of electrons under influence of B00 px py dxy dx2-y2 35 Paramagnetic Contribution to theParamagnetic Contribution to the Magnetic ShieldingMagnetic Shielding 3 p imbalance para rE P const ×Δ Δ −=σ AverageAverage energyenergy approximationapproximation 36 The Paramagnetic Contribution toThe Paramagnetic Contribution to the Magnetic Shielding Constantthe Magnetic Shielding Constant CharacteristicsCharacteristics ofof σσparapara :: Magnitude ofMagnitude of σσparapara (=(= deshieldingdeshielding) increases when) increases when •• the mean electronic excitation energy decreases (the mean electronic excitation energy decreases (σσparapara ~~ ΔΔ EE––11 )) HOMOHOMO--LUMO gap,LUMO gap, ΔΔOO shielding is most susceptible to changes inshielding is most susceptible to changes in ΔΔ EE––11 (1(1 eVeV = 30= 30 ppmppm)) least precisely knownleast precisely known •• the effective radius of the valence shell decreases (the effective radius of the valence shell decreases (σσparapara ~ r~ r––33 )) more electrons = moremore electrons = more ee--ee repulsion = larger rrepulsion = larger r •• the imbalance of valence electrons increases (the imbalance of valence electrons increases (σσparapara == f(Qf(Qii /L/L22) )) ) increasing symmetry = decreasing imbalanceincreasing symmetry = decreasing imbalance higher bond order = shieldinghigher bond order = shielding 3 p imbalance para rE P const ×Δ Δ −=σ 37 The Paramagnetic Contribution toThe Paramagnetic Contribution to the Magnetic Shielding Constantthe Magnetic Shielding Constant Contributions toContributions to σσparapara by individual valence electron pairs areby individual valence electron pairs are anisotropic and may cancel out because of symmetry reasons !anisotropic and may cancel out because of symmetry reasons ! Ag + NH3Ag AgH3N NH3 symm. unsymm. symm. σσparapara = 0 for spherical symmetry, closed shell atoms ( e.g. F= 0 for spherical symmetry, closed shell atoms ( e.g. F−−)) 38 PatternsPatterns ofof ChemicalChemical ShiftsShifts For pFor p-- and dand d--block elements, chemical shifts are dominated by theblock elements, chemical shifts are dominated by the paramagnetic contribution to the magnetic shielding,paramagnetic contribution to the magnetic shielding, σσparapara.. TheThe KarplusKarplus--PoplePople approach proves useful to rationalize someapproach proves useful to rationalize some important general patterns of chemical shifts in terms of variatimportant general patterns of chemical shifts in terms of variations ofions of Different total chemical shift ranges of different elementsDifferent total chemical shift ranges of different elements Correlation between chemical shifts and electronic transitionsCorrelation between chemical shifts and electronic transitions Comparable chemical shift patterns for electronically similarComparable chemical shift patterns for electronically similar compounds of different elementscompounds of different elements –3 “radial term” ΔE–1 “energy term” “orbital term” Q (L )i 2 39 PatternsPatterns ofof ChemicalChemical ShiftsShifts –3 “radial term” NNHH44 ++ NNHH33 δδ ((1515N)N) −−325.9325.9 −−380.2380.2 PositivePositive chargecharge == p orbital contraction, lessp orbital contraction, less ee--ee repulsion,repulsion, rradiusadius decreasesdecreases == deshieldingdeshielding CCMeMe33 ++ HHCCMeMe33 δδ ((1313C)C) 335.7335.7 5050 3 p imbalance para rE P const ×Δ Δ −=σ 40 PatternsPatterns ofof ChemicalChemical ShiftsShifts –3 “radial term” 129129 XeXe Chemical Shift dependence on the oxidation stateChemical Shift dependence on the oxidation state −−53315331−−1592159225325321721720772077 Xe(OTeFXe(OTeF55))22Xe(OTeFXe(OTeF55))44XeOFXeOF44 −−23792379−−63763700 XeXeXeFXeF22XeFXeF44XeOXeO33XeOXeO66 22−− Xe(0)Xe(0)Xe(IIXe(II))Xe(IVXe(IV))Xe(VIXe(VI))Xe(VIIIXe(VIII)) Higher oxidation state = more positive charge = smaller > == deshieldingdeshielding 41 PatternsPatterns ofof ChemicalChemical ShiftsShifts 109109 Ag NMR Chemical Shift dependence on the oxidation stateAg NMR Chemical Shift dependence on the oxidation state 42 PatternsPatterns ofof ChemicalChemical ShiftsShifts Chemists are interested in correlations between NMR chemical shiChemists are interested in correlations between NMR chemical shifts and otherfts and other molecular properties related to changes in molecular structuresmolecular properties related to changes in molecular structures oror reactivitiesreactivities.. Some useful relations are found in particular for transition metSome useful relations are found in particular for transition metal compounds:al compounds: Chemical Shifts and theChemical Shifts and the SpectrochemicalSpectrochemical Series ofSeries of LigandsLigands ((ΔΔoo)) SpectrochemicalSpectrochemical Series = increase inSeries = increase in ΔΔE(dE(d--dd) = energy term decreases) = energy term decreases WeakWeak ligandsligands == deshieldingdeshielding weak bondsweak bonds σσparapara largelarge StrongStrong ligandsligands = shielding= shielding strong bondsstrong bonds σσparapara smallsmall II−− < Br< Br−− < S< S22−− < NCS* << NCS* < ClCl−− < NO< NO33 −− < F< F−− < OH< OH−− < RCOO< RCOO−− < ox < ONO* < H< ox < ONO* < H22O < SCN* increases = radial term decreases increases = radial term decreases –3 “radial term” FF−− < H< H22O < NHO < NH33 < en < ox< en < ox < SCN*< SCN* << ClCl−− < CN< CN−− < Br< Br−− < I< I−− NephelauxeticNephelauxetic effect increaseseffect increases == increasesincreases ElectronegativityElectronegativity increasesincreases Ionic bondingIonic bonding Covalent bondingCovalent bonding dedecreasescreases increasesincreases DeshieldingDeshielding ShieldingShielding 3 p imbalance para rE P const ×Δ Δ −=σ 48 PatternsPatterns ofof ChemicalChemical ShiftsShifts Change ofChange of ligandsligands induces usually changes of both the energy and radial termsinduces usually changes of both the energy and radial terms –3 “radial term” NephelauxeticNephelauxetic effecteffect NormalNormal halogen dependencehalogen dependence FF ClCl Br IBr I ΔE–1 “energy term” SpectrochemicalSpectrochemical SeriesSeries Inverse halogen dependenceInverse halogen dependence I BrI Br ClCl FF DeshieldingDeshielding withwith increasingincreasing electronegativityelectronegativity (similar to diamagnetic(similar to diamagnetic termterm)) ShieldingShielding withwith increasingincreasing electronegativityelectronegativity Depending on which effect dominates, the variation ofDepending on which effect dominates, the variation of σσ(M(M) with) with thethe electronegativityelectronegativity of X may follow completely different patterns.of X may follow completely different patterns. 49 PatternsPatterns ofof ChemicalChemical ShiftsShifts I.I. NormalNormal HalogenHalogen ((LigandLigand) Dependence) Dependence δδ(M(M) follows the) follows the nephelauxeticnephelauxetic series ofseries of ligandsligands,, increases in the series I < Br –3 “radial term” ΔE–1 “energy term” 3 p imbalance para rE P const ×Δ Δ −=σ 50 PatternsPatterns ofof ChemicalChemical ShiftsShifts II. Inverse Halogen(Ligand) Dependence I. Normal Halogen(Ligand) Dependence -17,4 -36,1 -83 F Cl Br I 0 -20 -40 -60 -80 δ(29Si)δ(91Zr) 0 50 100 150 -50 -100 -150 -120 -66 2 Br 2 ICl, Br2 Cl 126 0 -49 δ( Si) in H SiX 29 3 δ( Zr) in Cp ZrXY 91 2 51 5151 V NMR ofV NMR of VanadylVanadyl derivativesderivatives VOBrVOBr33 VOClVOCl33 VOFVOF33 432 (neat)432 (neat) 0 (neat)0 (neat) −−786 (CH786 (CH33CN)CN) Inverse HalogenInverse Halogen ((LigandLigand) Dependence) Dependence σpara 2π ΔE –1 –3 r 2 ~~ <(n-1)d> – < 0 L 0 > 2 | | 3 p imbalance para rE P const ×Δ Δ −=σ 52 5151 V NMR ofV NMR of VanadatranesVanadatranes N V R N NR RN E 53 PatternsPatterns ofof ChemicalChemical ShiftsShifts III. Complicated PatternsIII. Complicated Patterns Influence on and> and ΔΔEE--11 of similar magnitudeof similar magnitude NonNon--monotonous trend formonotonous trend for δδ(M(M)) ObservedObserved forfor some compounds of psome compounds of p--group elementsgroup elements III. Complicated Patterns δ(31P) 0 50 100 150 200 250 F Cl Br I 97 220 227,4 178 δ( P) in P31 X3 54 SymmetrySymmetry andand ChemicalChemical ShiftsShifts −−281281−−81819696220220δδ ((3131P)P) ppmppm PClPCl66 −−PClPCl55PClPCl44 ++PClPCl33 Increasing symmetry = lower imbalance = shieldingIncreasing symmetry = lower imbalance = shielding “orbital term” Q (L )i 2 55 CoordinationCoordination NNumberumber andand ChemicalChemical ShiftsShifts Main group elementsMain group elements Higher CN = shieldingHigher CN = shielding Transition metalsTransition metals Higher CN =Higher CN = deshieldingdeshielding MoreMore ππ bonding = shieldingbonding = shielding MoreMore σσ bonding =bonding = deshieldingdeshielding 56 CoordinationCoordination NNumberumber andand ChemicalChemical ShiftsShifts Higher CN = shieldingHigher CN = shielding AlO4 AlO5 AlO6 δδ ((2727Al)Al) 57 CoordinationCoordination NNumberumber andand ChemicalChemical ShiftsShifts Higher CN = shieldingHigher CN = shielding δδ ((2727Al)Al)