Nuclear magnetic resonance Spectroscopy Basic Concept

X-ray Bond breaking Ultraviolet/visible Electronic Infrared Vibration Microwave Rotational Radiofrequencies Nuclear Spin (Nuclear Magnetic Resonance) NMR 100MHz E=hν ν= c / λ= x cm/sec/λ ΔE = 6.6 x erg-sec x 100MHz x 1,000,000 sec -1 /MHz = 6.6 x erg x (1Joule/1 x 10 7 ergs) = 6.6 x Joule x (1 cal/4.184 Joules) x (1 kcal/1000cal) = 1.58 x kcal/molecule ΔE (kcal/mol) = 1.58 x kcal/molecule x x molecules/mol = 9.5 x kcal/mol

Nuclear Spin States A. Magnetic Properties of Nuclei ․ All nuclei carry a charge ․ this charge “spin” on the nuclear axis, 產生 a magnetic dipole along the axis (charge spin → magnetic dipole along the axis) ․ angular momentum of the spinning charge 稱為 quantum spin number I. I = 0, 1/2, 1, 3/2…… 若為 0 表示 no spin (angular momentum → quantum spin number I)

․ spin number I of some common Nuclei ․由 nuclear spin quantum number I 可知其 allowed spin states 是 2I+1

Nuclear Magnetic Moments

∆E=(h  /2  )B 0 這樣 radiofrequency  1 is given in megahertz (MHz) 100 MHz → 2.35 tesla (T)B 0 ∆E = h  可計算 the difference energy between the two spin state 3.98x10 -5 KJ/mole ( 太小， r.t. 足以產生此 energy)

The mechanism of Absorption (Resonance) radiation → nucleus → spin charge (resonance) 將 incoming radiation transferred to nucleus 產生 spin charge, 這樣稱為 resonance, 亦可寫成 ( 稱為 )nucleus have resonance with the incoming electromagnetic wave.

「甘魯生教授說 300MHz 的 1ppm 變化，就好像人在月球上要分辨 台北市相鄰的兩根電線桿」

The Chemical Shift and Shielding 想要得知 ”H” 在固定磁場下所得到的 frequency  1 = (r/2  )B 0  1 : applied frequency (chemical shifts) r/2  : constant B 0 : stationary magnetic field 但事情沒這麼單純 A proton in a molecule is shielded to a very small extent by its electron cloud. 這樣的 small different (shielded) 會改變 chemical shifts position ∴修 改上述方程式  eff = (r/2  )B 0 (1-s) s: shielding constant The degree of shielding depends on the density of the circulating electrons

某一個 proton 因其產生 shielding 進而得到不同的 absorption position, 再以 一個 reference proton 所得 absorption position 相比較, 則可得到某一個 proton 的 chemical shift ↓ proton in different chemical environment have different chemical shifts proton in same chemical environment have same chemical shifts

何謂 different? Same? applied magnetic field→ local diamagnetic current (diamagnetic shielding)→ magnetic field (opposed the applied) 外加磁場都一樣，但若 ”H” 的 electron density 不同則其所 induce 的第二個小磁場亦不同 shielded― right deshielded― left 一般所使用的 reference compound: organic D-solvent TMS (CH 3 ) 4 Si 0ppm D 2 O DSS (CH 3 ) 3 Si(CH 2 ) 3 SO 3 Na 0.015ppm

Chemical Equivalence—A Brief Overview Nuclei are chemical shift equivalence if they are interchangeable through any symmetry operation or by a rapid process. A. Symmetry operations Axis symmetry C n ; plane of symmetry s; inversion through a center of symmetry i 將這三種對稱粗分成 two heading: C n and S n S 1 ≡s ; S 2 ≡i higher S n rare 只要 Operation 前與後的化合物是 superposable 則表示 symmetry ․ interchange by rotation around a simple axis of symmetry (C n ) eg. CH 2 Cl 2 C 2 (2 protons are homotopic) S (plane of symmetry) ∴ 2 protons are chemical-shift equivalent ․ interchange by reflection through a plane of symmetry (s) ClCFH 2 只有 S 對稱面

Chemical Environment and Chemical Shift 由出現的 ppm 可判斷是哪一類官能基旁的 proton

Integrals and Integration

Distinguish: How many different types protons (Chemical Shift), is independent of the operating frequency but dependent upon the solvent How many of each different types (Integral) Solvent selection 理想中的 Solvent 1. no protons 2. inert 3. low boiling 4. inexpensive 現代的機器一定要用 deuterated solvent ∵ D signal to lock or stabilize the B 0 field of the magnet Internal Standard Tetramethylsilane (TMS) 1. a single sharp line 2. chemically inert 3. easily be removed 4. not involve in intermolecular association 5. resonance position is far removed from most proton

Local Diamagnetic Shielding Electronegativity Effects Electronegativity of the attachelement ↑, Chemical Shift ↑ 越多 Electronegative element 其  ↑, 且離 Electronegative element 越 遠其  ↓( 一般超過 3 個碳以上，幾乎沒影響 )

Hybridization Effects sp 3 More p character → better shielding (toward H nucleus) sp 2 More s character than sp 3 sp C≡C-H 2-3 ppm 一般會以為 Vinyl hydrogen up field than acetylenic hydrogen ， 事實上相反 because of anisotropy effect

Acidic and exchangeable protons; hydrogen bonding The more hydrogen bonding, the more deshielding a proton ․ concentration 會影響產生的 H-bonding 數目 (different heteroatom has the similar tendencies) Dilute ppm Conc. 4-5 ppm

Magnetic Anisotropy 磁異方位效應 Molecule contain  bonds → electron circulation → induced by applied magnetic field → magnetic field → anisotropy effect 很多情形並不能單用 electronegativity 來考慮，有另外一重要 effect 稱 為 ”magnetic anisotropy 磁異方位 ” magnetic fields associated with the electron circulation induced by 外加 磁場, in molecule containing  bonds, are anisotropic 此種現象稱為 anisotropic effect

擁有  electron 的化合物，在外 加磁場下因為 ”ring-current effect” 又產生第二個小磁場， 若與外加磁場同向 deshielded (left) ；與外加磁場反向 shielded (right)

事實上 benzene 共有三個 magnetic field: 1.Applied by the electromagnetic of the NMR ( 機器本身 ) 2.Usual shielding by the valence electrons around the proton ( 共價鍵產生 ) 3.Anisotropy generate by the system electrons

與外加磁場同向 deshielded (left) ；與外加磁場反向 shielded (right) inside H shielded to -3.0 ppm outside H deshielded to 9.3 ppm ortho H further deshielded to ~7.85 meta, para H further deshielded to ~7.40

Spin-spin Splitting (n+1) rule 1.Integral 2.Chemical shift 3.Splitting pattern

The Coupling Constant The distance between the peaks in a simple multiplet is called the coupling constant “J” (unit: Hz)

何時用到 Coupling constant? 1. 分辨誰是你鄰居 different magnitudes of J are found for different types of protons 2. Can tell geometrical isomer

Conclusion To explain 1 H-NMR is based on four informations: Integration Integration: ratio of H atoms Chemical shift Chemical shift: chemical environment Spin-spin coupling Spin-spin coupling: information about neighboring nuclei Coupling constant Coupling constant: isomers (cis or trans)

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