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Chemical Shift (Δ) Chapter 9 ● Nuclear Magnetic Resonance Ch. 9 - 1 1. Introduction Classic methods for organic structure determination ● Boiling point ● Refractive index ● Solubility tests ● Functional group tests ● Derivative preparation ● Sodium fusion (to identify N, Cl, Br, I & S) ● Mixture melting point ● Combustion analysis ● Degradation Ch. 9 - 2 Classic methods for organic structure determination ● Require large quantities of sample and are time consuming Ch. 9 - 3 Spectroscopic methods for organic structure determination a) Mass Spectroscopy (MS) ● Molecular Mass & characteristic fragmentation pattern b) Infrared Spectroscopy (IR) ● Characteristic functional groups c) Ultraviolet Spectroscopy (UV) ● Characteristic chromophore d) Nuclear Magnetic Resonance (NMR) Ch. 9 - 4 Spectroscopic methods for organic structure determination ● Combination of these spectroscopic techniques provides a rapid, accurate and powerful tool for Identification and Structure Elucidation of organic compounds ● Rapid ● Effective in mg and microgram quantities Ch. 9 - 5 General steps for structure elucidation 1. Elemental analysis ● Empirical formula ● e.g. C2H4O 2. Mass spectroscopy ● Molecular weight ● Molecular formula ● e.g. C4H8O2, C6H12O3 … etc. ● Characteristic fragmentation pattern for certain functional groups Ch. 9 - 6 General steps for structure elucidation 3. From molecular formula ● Double bond equivalent (DBE) 4. Infrared spectroscopy (IR) ● Identify some specific functional groups ● e.g. C=O, C–O, O–H, COOH, NH2 … etc. Ch. 9 - 7 General steps for structure elucidation 5. UV ● Sometimes useful especially for conjugated systems ● e.g. dienes, aromatics, enones 6. 1H, 13C NMR and other advanced NMR techniques ● Full structure determination Ch. 9 - 8 Electromagnetic spectrum cosmic X-rays ultraviolet visible infrared micro- radio- & γ-rays wave wave λ: 0.1nm 200nm 400nm 800nm 50µm IR X-Ray UV NMR Crystallography 1Å = 10-10m 1nm = 10-9m 1µm = 10-6m Ch. 9 - 9 2. Nuclear Magnetic Resonance (NMR) Spectroscopy A graph that shows the characteristic energy absorption frequencies and intensities for a sample in a magnetic field is called a nuclear magnetic resonance (NMR) spectrum Ch. 9 - 10 Ch. 9 - 11 1. The number of signals in the spectrum tells us how many different sets of protons there are in the molecule 2. The position of the signals in the spectrum along the x-axis tells us about the magnetic environment of each set of protons arising largely from the electron density in their environment Ch. 9 - 12 3. The area under the signal tells us about how many protons there are in the set being measured 4. The multiplicity (or splitting pattern) of each signal tells us about the number of protons on atoms adjacent to the one whose signal is being measured Ch. 9 - 13 Typical 1H NMR spectrum ● Chemical Shift (δ) ● Integration (areas of peaks ⇒ no. of H) ● Multiplicity (spin-spin splitting) and coupling constant Ch. 9 - 14 Typical 1H NMR spectrum 1 Record as: H NMR (300 MHz, CDCl3): δ 4.35 (2H, t, J = 7.2 Hz, Hc) 2.05 (2H, sextet, J = 7.2 Hz, Hb) 1.02 (3H, t, J = 7.2 Hz, Ha) chemical coupling shift (δ) no. of H constant in ppm (integration) multiplicity in Hz Ch. 9 - 15 2A. Chemical Shift The position of a signal along the x-axis of an NMR spectrum is called its chemical shift The chemical shift of each signal gives information about the structural environment of the nuclei producing that signal Counting the number of signals in a 1H NMR spectrum indicates, at a first approximation, the number of distinct proton environments in a molecule Ch. 9 - 16 Ch. 9 - 17 Ch. 9 - 18 Normal range of 1H NMR "upfield" (more shielded) "downfield" (deshielded) 15 -10 (low field δ ppm (high field strength) strength) Ch. 9 - 19 Reference compound Me ● TMS = tetramethylsilane Me Si Me Me as a reference standard (0 ppm) ● Reasons for the choice of TMS as reference Resonance position at higher field than other organic compounds Unreactive and stable, not toxic Volatile and easily removed o (B.P. = 28 C) Ch. 9 - 20 NMR solvent ● Normal NMR solvents should not contain hydrogen ● Common solvents CDCl3 C6D6 CD3OD CD3COCD3 (d6-acetone) Ch. 9 - 21 The 300-MHz 1H NMR spectrum of 1,4-dimethylbenzene Ch. 9 - 22 2B. Integration of Signal Areas Integral Step Heights The area under each signal in a 1H NMR spectrum is proportional to the number of hydrogen atoms producing that signal It is signal area (integration), not signal height, that gives information about the number of hydrogen atoms Ch. 9 - 23 Ha Ha Hb R O Hb b H 3 Hb 2 Ha Hb Ha Ch. 9 - 24 2C. Coupling (Signal Splitting) Coupling is caused by the magnetic effect of nonequivalent hydrogen atoms that are within 2 or 3 bonds of the hydrogens producing the signal The n+1 rule ● Rule of Multiplicity: If a proton (or a set of magnetically equivalent nuclei) has n neighbors of magnetically equivalent protons. It’s multiplicity is n + 1 Ch. 9 - 25 Examples (1) b a H H Ha: multiplicity = 3 + 1 = 4 (a quartet) Hb C C Cl Hb: multiplicity = 2 + 1 = 3 (a triplet) Hb Ha (2) a b H H Ha: multiplicity = 2 + 1 = 3 (a triplet) Cl C C Cl Hb: multiplicity = 1 + 1 = 2 (a doublet) Cl Hb Ch. 9 - 26 Ch. 9 - 27 Examples (3) Hb Ha Ha: multiplicity = 6 + 1 = 7 (a septet) Hb C C Br Hb Hb Hb: multiplicity = 1 + 1 = 2 (a doublet) Hb Hb Note: All Hb’s are chemically and magnetically equivalent. Ch. 9 - 28 Pascal’s Triangle ● Use to predict relative intensity of various peaks in multiplet ● Given by the coefficient of binomial expansion (a + b)n singlet (s) 1 doublet (d) 1 1 triplet (t) 1 2 1 quartet (q) 1 3 3 1 quintet 1 4 6 4 1 sextet 1 5 10 10 5 1 Ch. 9 - 29 Pascal’s Triangle Ha Hb Due to symmetry, Ha ● For Br C C Br and Hb are Cl Cl identical ⇒ a singlet Ha Hb Ha ≠ Hb Cl C C Br ● For ⇒ two doublets Cl Br Ch. 9 - 30 3. How to Interpret Proton NMR Spectra 1. Count the number of signals to determine how many distinct proton environments are in the molecule (neglecting, for the time being, the possibility of overlapping signals) 2. Use chemical shift tables or charts to correlate chemical shifts with possible structural environments Ch. 9 - 31 3. Determine the relative area of each signal, as compared with the area of other signals, as an indication of the relative number of protons producing the signal 4. Interpret the splitting pattern for each signal to determine how many hydrogen atoms are present on carbon atoms adjacent to those producing the signal and sketch possible molecular fragments 5. Join the fragments to make a molecule in a fashion that is consistent with the data Ch. 9 - 32 Example: 1H NMR (300 MHz) of an unknown compound with molecular formula C3H7Br Ch. 9 - 33 Three distinct signals at ~ δ3.4, 1.8 and 1.1 ppm ⇒ δ3.4 ppm: likely to be near an electronegative group (Br) Ch. 9 - 34 δ (ppm): 3.4 1.8 1.1 Integral: 2 2 3 Ch. 9 - 35 δ (ppm): 3.4 1.8 1.1 Multiplicity: triplet sextet triplet 2 H's on 5 H's on 2 H's on adjacent C adjacent C adjacent C Ch. 9 - 36 Complete structure: most upfield signal most downfield signal CH2 CH3 Br CH2 • 2 H's from • 2 H's from • 3 H's from integration integration integration • triplet • sextet • triplet Ch. 9 - 37 4. Nuclear Spin: The Origin of the Signal The magnetic The spinning field associated proton with a spinning resembles a tiny proton bar magnet Ch. 9 - 38 Ch. 9 - 39 Ch. 9 - 40 Spin quantum number (I) 1H: I = ½ (two spin states: +½ or -½) ⇒ (similar for 13C, 19F, 31P) 12C, 16O, 32S: I = 0 ⇒ These nuclei do not give an NMR spectrum Ch. 9 - 41 5. Detecting the Signal: Fourier Transform NMR Spectrometers Ch. 9 - 42 Ch. 9 - 43 6. Shielding & Deshielding of Protons All protons do not absorb energy at the same frequency in a given external magnetic field Lower chemical shift values correspond with lower frequency Higher chemical shift values correspond with higher frequency "upfield" (more shielded) "downfield" (deshielded) 15 -10 (low field δ ppm (high field strength) strength) Ch. 9 - 44 Ch. 9 - 45 Deshielding by electronegative groups CH3X X = F OH Cl Br I H Electro- 4.0 3.5 3.1 2.8 2.5 2.1 negativity δ (ppm) 4.26 3.40 3.05 2.68 2.16 0.23 ● Greater electronegativity Deshielding of the proton Larger δ Ch. 9 - 46 Shielding and deshielding by circulation of π electrons ● If we were to consider only the relative electronegativities of carbon in its three hybridization states, we might expect the following order of protons attached to each type of carbon: (higher (lower sp < sp2 < sp3 frequency) frequency) Ch. 9 - 47 ● In fact, protons of terminal alkynes absorb between δ 2.0 and δ 3.0, and the order is (higher (lower sp2 < sp < sp3 frequency) frequency) Ch. 9 - 48 ● This upfield shift (lower frequency) of the absorption of protons of terminal alkynes is a result of shielding produced by the circulating π electrons of the triple bond H Shielded (δ 2 – 3 ppm) Ch. 9 - 49 ● Aromatic system Shielded region Deshielded region Ch. 9 - 50 ● e.g. Hc Hd Hb Ha δ (ppm) Ha & Hb: 7.9 & 7.4 (deshielded) Hc & Hd: 0.91 – 1.2 (shielded) Ch. 9 - 51 ● Alkenes H Deshielded (δ 4.5 – 7 ppm) Ch. 9 - 52 ● Aldehydes R O H Electronegativity effect + Anisotropy effect ⇒ δ = 8.5 – 10 ppm (deshielded) Ch.
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