Four Contributions to the Resonance Frequency of Nuclei in Matter (D = y B ALL TENSORS 1) Shielding, chemical shift electron distribution around nuclei, induced magnetic field 2) Dipolar interactions magnetic nuclei distribution, through space interactions, in solids 3) Electric field gradient distribution of nuclei (positive charge) and electrons (negative charge) quadrupolar nuclei 4) Scalar coupling dipolar interactions through electrons in molecules, bonds 1 Chemical Shift Information The presence of an element in the sample Number of signals = number of chemically different atoms Symmetry of the molecule Relative intensity = ratio of atoms - integration Position = chemical shielding — electronic environment Type of bonds, oxidation state, coordination number Multiplicity = connectivity of functional groups Chemical Shift Information The presence of an element in the sample PCI5 + CH3CN X-ray crystallography 1H NMR (6.3 ppm, dd 43 and 13 Hz) Chemical Shift A ^^^^^^^^^ XH NMR spectrum I Tfce tf*ree peaks of ethyl alcohol as first observed in 1951 at Stanford University. Chemical Shift Chemical shift for a given molecule: • Number of signals = nonequivalent nuclei molecular symmetry • Intensity = number of nuclei * Position in the spectrum = shielding electronic structure CH3CH2OH Multiplicity = connectivity of atoms and groups Methylene quartet Molecular Symmetry Number of signals = nonequivalent nuclei, molecular symmetry Flow chart for point group determination Symmetry Elements and Operations B Element Operation E Identity Identity No change, (— 1) • 1 Center of symmetry (inversion center) POINT Inversion through the central point every point x,y,z translated to -x,-y,-z Rotation axis LINE True (proper) rotation Rotation by an angle 360/n a Plane of symmetry, mirror PLANE Reflectio n Reflection through a mirror plane Improper axis Roto-reflection axis LINE Improper rotation Rotation by an angle 360/n followed by reflection through a mirror plane 7 Inversion Center NO inversion center Rotation Axis C Rotation by 360/n about Cn brings the object to an indistinguishable position from the original l 10 Rotation Axis C ft ^ ^ ^ ^4^ — E Plane of Symmetry CT Plane of Symmetry a Planar molecules - symmetry plane of the molecule ah = horizontal plane, perpendicular to principal axis gv = vertical plane, parallel to principal axis, bisects the most atoms ad= dihedral plane, colinear with principal axis, bisecting two C2' I Improper Axis Sn Rotation-reflection = a compound operation, rotation (Cn) followed by a reflection through a plane perpendicular to the Cn axis S, = C, x a = G S2 = C2x G = i (a) (2) Reflect^C^^ (1) Rotate ---- Rotate _ (2) Reflect 17 Symmetry Elements in a Molecule Equivalent atoms = exchanged by symmetry operations F4 — F5 F1 — F2 — F3 19 Figure 2. The Founding Fathers. R. M. Barrer (1910-1996) (right) and R. M. Milton (1920-2000) photographed 20 Chemical Shift Number of signals = nonequivalent nuclei Chemically different atoms 13C NMR 3 x t-Bu groups No C3 axis Geometrical difference = chemical difference 1 x Me group signal Co axis 23 Chemical Shift 25 26 28 Chemical Shift UB NMR Isomers of B10H10C2H2 Chemical Shift UB NMR Mono- and Disubstituted B^H^2" Molecules with Identical Substituents 31 32 Icosahedfon Truncated icosahedton 34 Chemical Shift a 150 14fl 14? 139 13hJ L50.07 L47.52 L46.82 L44.77 L30.28 Geminal Groups Geminal groups — paired ligands Symmetry 38 Chemical Shift Nonequivalence of Geminal Ligands in Prochiral Groups Chiral Group X Me' Q—CI H : i Prochiral Group Me Me Me C2 Group I Geminal groups —C—Me Me, Me Homotopic Equivalent (isochronous) Me" ►Me H I o I Me, Me Enantiotopic Equivalent (isochronous) in achiral media Nonequivalent (anisochronous) in chiral media No a present Me, Me Diastereotopic Nonequivalent (anisochronous) If X is chiral, the paired ligands in a prochiral group are always diastereotoppic replace Ha replace Hb| Hb7 D l same compound ^; Hj ^nd Hb ^rp hnnnnl-npir Hb' Substitution Test of Geminal Groups replace H< D Hb' \ C / replace Hb| Y-^^F /-cil D enantiomers Ha and Hb are enantiotopic diastereomers hU and Hb are diastereotopic 41 Chemical Shift Nonequivalence in Prochif al Groups mer-CI, trans-P fac-CI, cis-P Chemical Shift Nonequivalence in Prochiral Groups Chemical Shift Nonequivalence in Prochif al Groups Pyramidal N - Fast inversion on N 44 Chemical Shift Nonequivalence in Prochif al Groups I R= Et R= iPf 45 46 X ij 3 ... J-Ll-ll 62 6J A,HJiii 62 ■ i: 1 _J lie. t..l -64 fi.1 I 30 ■ 5 3 I I I I I I I I ; I (JURE 1 NMH spectra of CH cp) moiety of compound 2: a), b)1H NMR spec-;ra inCDCla and CDCla? DjjO respectively; c) aiP{1H} NMR spectrum in CDCla; I) CH COSY diagram in CDCIa, [4] triangulane 48 Prochiral Groups lH NMR spectrum 6 CH aromatic signals 6 CH3 mesityl signals 49 The methylene hydrogens are diastereotopic two signals at 3.69 and 4.81 ppm 6 CH aromatic signals 6 CH3 mesityl signals Chemical shift for a given molecule: • Number of signals = nonequivalent nuclei molecular symmetry •Intensity = number of nuclei • Position in the spectrum = shielding electronic structure • Multiplicity = connectivity of atoms and groups Integration I The area under each signal is proportional to the number of protons that give rise to that signal The height of each integration step is proportional to the area under a specific signal The integration tells us the relative number of protons that give rise to each signal, not absolute number Relative Signal Intensity 53 Polyphosphate Chain Length 54 Relative Signal Intensity *H NMR spectrum of a mixture of n-propanol and n-butanol Estimate their ratio r f 0.9918 2.0876 2.6420 3.0162 PPT1