The NonThe Non--local Contribution to thelocal Contribution to the Magnetic Shielding ConstantMagnetic Shielding Constant σσ == σσdiadia ++ σσparapara ++ ΣΣ σσnonlocnonloc magnetic anisotropy of neighboring groupsmagnetic anisotropy of neighboring groups temperaturetemperature isotope shiftisotope shift solvent effects ASIS, SIISsolvent effects ASIS, SIIS HH--bondingbonding concentration effectsconcentration effects locallocal MagneticMagnetic AAnisotropynisotropy ofof NNeighboringeighboring GGroupsroups Magnetic anisotropy of neighboring groupsMagnetic anisotropy of neighboring groups Remote shielding effects by electrons of nonRemote shielding effects by electrons of non--sphericallyspherically symmetric groupssymmetric groups –– (nearly all groups, but some strong)(nearly all groups, but some strong) McConnell formula (cylindrical symmetry)McConnell formula (cylindrical symmetry) σσgroupgroup = (= (χχ|||| −− χχ⊥⊥)) ((11 −− 3 cos3 cos22 θθ)/(3r)/(3r33)) χχ|||| ,, χχ⊥⊥ < 0< 0 11 −− 3 cos3 cos22 θθ = 0= 0 forfor θθ = 54.7= 54.7ºº In a magnetic field, valence electrons are induced to circulate. This generates a secondary magnetic field that opposes/enhances the applied field near the nucleus A higher/lower field is needed to achieve resonance = shielding/deshielding effect MagneticMagnetic AAnisotropynisotropy θθ is the angle between the vector r and the symmetry axis ((χχ|||| -- χχ⊥⊥))the molar anisotropy of the bond χχ|||| -- χχ⊥⊥ the susceptibilities parallel and perpendicular to the symmetry axis H = measured nucleusH = measured nucleus Z = anisotropic neighboring groupsZ = anisotropic neighboring groups McConnelMcConnel formulaformula (cylindrical symmetry, group Z approximated as a magnetic dipole(cylindrical symmetry, group Z approximated as a magnetic dipole σσgroupgroup = (= (χχ|||| −− χχ⊥⊥)) ((11 −− 3 cos3 cos22 θθ)/(3r)/(3r33)) GroupsGroups withwith MagneticMagnetic AAnisotropynisotropy RingRing CCurrenturrent inin AAromaticromatic RRingsings π electrons in aromatic rings are induced to circulate in a magnetic field Diatropic ring current •induces magnetic field aligned with the applied field in the vicinity of the aryl protons (causing deshielding = downfield shift) •opposes the applied field at protons above and below the ring (causing shielding = upfield shift) RingRing CCurrenturrent inin AAromaticromatic RRingsings shielding surfaces 0.1 ppm in yellow, at 0.5 ppm in green, at 1 ppm in green-blue, at 2 ppm in cyan, and 5 ppm in blue deshielding surface at 0.1 ppm in red Ring current = measure of cyclic delocalization of π electrons in aromatic rings Shielding strong Deshielding weak MagneticMagnetic AAnisotropynisotropy multmult 77..11 ppmppm singletsinglet 4.24.2 ppmppm Octamethyl-[2, 2]-metacyclophane 8 Me groups on C-C bridges not shown RingRing CCurrenturrent inin AAromaticromatic RRingsings H2C CH2 H2C H2C H2 C H2 C CH2 CH2 2.6 ppm 0.3 ppm1 H NMR RingRing CCurrenturrent inin AAromaticromatic RRingsings 1,6-methano[10]annulene 1H NMR MagneticMagnetic AAnisotropynisotropy N Me Me N Me Me Me Me N Me Me N H H H H 11.7511.75 −−14.2614.26 1H NMR MagneticMagnetic AAnisotropynisotropy S Si F S S Si F 3 δ (19 F) 160.6 δ (19 F) 5.3 RingRing CCurrenturrent inin AntiaromaticAntiaromatic RRingsings Ring systems ofRing systems of antiaromaticantiaromatic character with [4n]character with [4n] ππ--electronselectrons exhibit a reversed anisotropy effect of decreased intensityexhibit a reversed anisotropy effect of decreased intensity –– paratropicparatropic ring currentring current ••aa deshieldingdeshielding area above and below the plane of the ring systemarea above and below the plane of the ring system ••a shielding area in the plane of the ring systema shielding area in the plane of the ring system pentalenepentalene shielding surfaces 0.1 ppm in yellow 0.5 ppm in green 1 ppm in green-blue 2 ppm in cyan 5 ppm in blue deshielding surface at 0.1 ppm in red RingRing CCurrenturrent inin Aromatic/Aromatic/AntiaromaticAntiaromatic RRingsings NICSNICS Nucleus independent chemical shiftNucleus independent chemical shift •• absolute shielding calculated inabsolute shielding calculated in the center of a moleculethe center of a molecule •• measuresmeasures aromaticityaromaticity Negative NICS = aromaticNegative NICS = aromatic Positive NICS =Positive NICS = antiaromaticantiaromatic Aromatic/Aromatic/AntiaromaticAntiaromatic RRingsings Trans-15,16-dimethyl-15,16-dihydropyrene aromaticaromatic [4n+2][4n+2] ππ--electronselectrons Trans-15,16-dimethyl-15,16-dihydropyrene dianion antiaromaticantiaromatic [4n][4n] ππ--electronselectrons CH3 CH3 CH3 CH3 2-4.25 ppm 21.0 ppm 1H NMR Aromatic/Aromatic/AntiaromaticAntiaromatic RRingsings [18] annulene aromaticaromatic [4n+2][4n+2] ππ--electronselectrons DiatropicDiatropic ring currentring current [18] annulene dianions antiaromaticantiaromatic [4n][4n] ππ--electronselectrons ParatropicParatropic ring currentring current Low temp. 1H NMR H H 2- 9.28 ppm -3.0 ppm H H-1.1 ppm 20.8 29.5 ppm 2- K Kekulene Kekulene is extremely insoluble. 1H NMR spectrum taken at 200° C in deuterated tetrachlorobenzene 2 annulenes or 6 benzene rings [4n+2][4n+2] ππ--electronselectrons MagneticMagnetic AAnisotropynisotropy AcetylenicAcetylenic HH MagneticMagnetic AAnisotropynisotropy of Ethyleneof Ethylene EthylenicEthylenic HH MagneticMagnetic AAnisotropynisotropy of Ethyleneof Ethylene C = grey, H = black) 0.1 ppm deshielding isosurface = yellow 0.1 ppm shielding isosurface = magenta MagneticMagnetic AAnisotropynisotropy Hax Heq The equatorial protons areThe equatorial protons are deshieldeddeshielded by 0.48by 0.48 ppmppm wrtwrt the axialthe axial Magnetic Anisotropy of CMagnetic Anisotropy of C6060 Diatropic ring current −7.0 ppm Paratropic ring current +4.5 ppm antiaromatic aromatic MagneticMagnetic AAnisotropynisotropy 3He @ C60 3He @ C70 3He + C60/ C70 650 ºC 3000 bar δ (3He) −6.3 ppm δ (3He) −28.8 ppm MagneticMagnetic AAnisotropynisotropy 3He @ C60 δ (3He) −6.3 ppm δ (3He) −49.2 ppm 3He @ C60 6shifted to high field = higher aromatic character 6-MRs and 5-MRs of the fullerene cage of C60 6- show diamagnetic ring currents MagneticMagnetic AAnisotropynisotropy δ (3He) −28.8 ppm δ (3He) +8.2 ppm shifted to low field = a reduction in aromaticity 3He @ C70 3He @ C70 6- MagneticMagnetic AAnisotropynisotropy 1H NMR spectra H2 in liquids ∼4 ppm H2@C60 in 1,2-dichlorobenzene-d4 −1.5 ppm OrthoOrtho-- andand ParahydrogenParahydrogen MagneticMagnetic AAnisotropynisotropy The 1H NMR spectrum of 2 in pyridine-d5 - A singlet at δ -32.18 (16 H) characteristic of a C8H8 ligand bound to uranium(IV) - Two signals at δ +4.49 (8 H) and +1.96 (12 H) due to a single NEt4 + group Paramagnetic compoundsParamagnetic compounds •Organic radicals, transition metal complexes •Unpaired electron = large fluctuating magnetic field •Chemical shift – 1H NMR range 200 ppm •Relaxation – unpaired electron reduces T1 and T2 = broad lines •Coupling of nuclear and electron spins Isotropic shift (diamagnetic vs. paramagnetic) Contact shift – delocalized e = through bond Pseudocontact – dipolar = through space paramagdiamagiso ννν Δ−Δ=Δ pseudocontcontiso ννν Δ+Δ=Δ Pseudocontact Shift The anisotropic magnetic susceptibility affects the Larmor frequencies of nearby nuclei the throughspace “dipolar” or “pseudocontact” shift 9 H along the Fe-C bond vector are shifted downfield (the addition of the internal field to the applied field causes them to resonate at a low applied field) H along the yz plane (perpendicular to the Fe-C bond vector) are shifted upfield An analogy is the diamagnetic “ring current” in aromatics, which gives downfield shifts of protons in the plane of the electron circulation and upfield shifts of protons normal to the plane of the electron circulation. Pseudocontact Shift The paramagnetic current in the iron compounds shifts H in the yz plane upfield those normal to the yz plane downfield The dominance of the pseudocontact shift is anomalous for paramagnetic complexes, for which the chemical shifts typically are dominated by the through-bond “contact” shift. Pseudocontact Shift Solvent EffectsSolvent Effects •Chemical shift – considerable •Coupling constants – small •Relaxation – higher viscosity reduces T1 and T2 of small molecules Van der Waals forces 0.1 – 0.2 ppm in 1H NMR Magnetic anisotropy of solvent – benzene, aromatics (solvent/solute orientation not averaged to zero) Hydrogen bonding 11 H Chemical Shifts of Methanol inH Chemical Shifts of Methanol in Selected SolventsSelected Solvents Solvent CDCl3 CD3COCD3 CD3SOCD3 CD3C≡N CH3 3.40 3.31 3.16 3.28 O–H 1.10 3.12 4.01 2.16 HydrogenHydrogen BBondingonding Increasing concentration More extensive H-bonding Deshielding of OH signal HydrogenHydrogen BBondingonding δδ ((1717O) waterO) water liquidliquid 0.00.0 ppmppm gasgas −−36.136.1 ppmppm HydrogenHydrogen BBondingonding Ar = mesityl CH3 H3C CH3 B N CH3 CH3 CH3 Ar Ar Ha Hb F The methylene hydrogens are diastereotopic – steric congestion two H signals at 3.69 and 4.81 ppm B N CH3 CH3 CH3 Ar Ar O3S-CF3 Ha Hb H-F hydrogen bonding Ha 6.50 ppm – deshielding coupling to F nucleus doublet of doublets 1JH-F = 9.2 Hz 2JH-H = 12.9 Hz The peaks marked by *correspond to mesityl CH resonances TemperatureTemperature EEffectsffects AnharmonicAnharmonic potentialpotential Occupation ofOccupation of vibrationalvibrational levels changes with temperaturelevels changes with temperature Changes in effective distance between atomsChanges in effective distance between atoms Chemical shift is a weighted average of the individualChemical shift is a weighted average of the individual vibrationalvibrational statesstates Temperature in NMRTemperature in NMR Temperature dependent NMR parametersTemperature dependent NMR parameters ••Chemical shiftChemical shift ••Number of signalsNumber of signals –– dynamic NMR spectroscopydynamic NMR spectroscopy ••Kinetics of exchange processesKinetics of exchange processes ••EquilibriumEquilibrium –– reaction,reaction, tautomerstautomers, conformers, conformers ••RelaxationRelaxation –– TT11 and Tand T22 depend on molecular tumblingdepend on molecular tumbling ••Dipolar and scalar couplingDipolar and scalar coupling –– exchangeexchange ••Molecular diffusion coefficient DMolecular diffusion coefficient D –– StokesStokes--EinsteinEinstein ••Equilibrium magnetization MEquilibrium magnetization M00 Thermocouple positionThermocouple position wrtwrt samplesample Temperature gradients within the sampleTemperature gradients within the sample Sample heating by decoupling powerSample heating by decoupling power MethanolMethanol ThermometerThermometer Methanol (neat) Temperature range: 178 – 330 K Peaks used: -CH3 and -OH Equation: T [K] = 409.0 - 36.54 Δδ - 21.85 (Δδ)2 C. Amman, P. Meier and A. E. Merbach, J. Magn. Reson. 1982, 46, 319-321. Ethylene glycol (neat) Temperature range: 273 – 416 K Peaks used: -CH2- and -OH Equation: T [K] = 466.5 - 102.00 Δδ C. Amman, P. Meier and A. E. Merbach, J. Magn. Reson. 1982, 46, 319-321. CCl4 and (CD3)2CO (50/50 vol% mixture) Temperature range: 190 – 360 K Peaks used: CD3-CO-CD3 and CCl4 Equation: T [K] = 5802.3 - 50.73 Δδ J. J. Led, S. B. Petersen, J. Magn. Reson. 1978, 32, 1-17. TeMe2 (neat) Temperature range: Peaks used: 125Te high field shift 0.128 ppm K-1 IdealIdeal ThermometerThermometer Nonreactive and stable/ internal thermometer Intramolecular effect / one compound added, no concentration, solvent dependency Wide range of temperatures Linear response Strong response ∆δ/ ∆T Solvent independent Chemical Shift ThermometerChemical Shift Thermometer H 13C SiMe3 SiMe3 SiMe3 13C Enriched