IV. Chemical Shifts - δ unit

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1 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 1 of 10. IV. Chemical Shifts - δ unit Each nucleus in a molecule has a different degree of electron surrounding it. igher the electron density is found surrounding the 1 nucleus, more the external magnetic energy is needed for the excitation of that 1 nucleus as the electron shields the nucleus. Where each 1 peak appears in the 1 NMR spectrum is the reflection of what kind of a chemical environment each 1 nucleus is in,* thus the name chemical shift. *This is manifested in the electron density surrounding each 1 nucleus. NMR spectra are obtained usually in CDCl 3 (99.8% D and 0.2% 1 ) with (C 3 ) 4 Si [tetramethylsilane or ] as internal reference in a certain operating magnetic field such as 200 Mz, 400 Mz, etc. 3 C C 3 (acetone) in CDCl 3 on a 200 Mz magnet NMR spectrometer: lower field or down-field residual CCl 3 in CDCl 3 acetone "The peak of acetone 1 's appears as one peak at 440 z down-filed from on a 200 Mz NMR spectrometer" 440 z higher field or up-field 3 C C 3 (acetone) in CDCl 3 on a 400 Mz magnet NMR spectrometer: residual CCl 3 in CDCl 3 acetone lower field or down-field 880 z higher field or up-field "The peak of acetone 1 's appears as one peak at 880 z down-filed from on a 200 Mz NMR spectrometer" note: Mz = 10 6 z Mz (mega ertz); z = cps Since these are too lengthy descriptions of 1 NMR data and the acetone peak appears at a down field position from proportionally to the strength of an operating magnetic field, the following dimension less ppm unit has been introduced. ( 3 C) 2 C= on a 200 Mz NMR spectrometer: 440 z / 200 x 10 6 z = 2.2 x ppm ( 3 C) 2 C= on a 400 Mz NMR spectrometer: 880 z / 400 x 10 6 z = 2.2 x ppm Now, these are independent of the strength of an operating magnetic field.

2 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 2 of 10. Conversely, 1 ppm on a 200 Mz NMR spectrometer corresponds to: 1 x 10-6 x 200 x 10 6 z = 200 z and 1 ppm on a 400 Mz NMR spectrometer corresponds to: 1 x 10-6 x 400 x 10 6 z = 400 z This ppm scale relative to and the increasing value to the lower magnetic field (i.e., to the left from ) is called the δ-scale. The 1 peaks of most of the organic compounds fall in between δ = 0 ~ 10 ppm (see Table 10.3 on p. 361 of Ege s book). The Factors That Affect Chemical Shifts (1) ybridization sp 3 C typically δ = 0.5 ~ 1.5 ppm sp 2 C typically δ = 5 ~ 6 ppm note: typically δ = 7 ~ 8 ppm alkenes owever, -C C - sp δ = ~ 2.2 ppm! aromatic 's (2) Electron Density on each 1 effective = 0 (1 σ) 0 : applied magnetic field σ = shielding constant, 10-2 ~ 10-5 (reflects chemical environments of a specific 1 ) increasing electron density on a 1 more shielding (larger σ) more external o to reach effective 1 peak at a higher field a smaller δ value for the peak. Examples: (i) tetramethylsilane (): C 3 3 C Si e.n.: 1.8 C 3 C electronegativity: 2.1 Consequently, the electron density of the 1 's of the C 3 increases --> 1 's of the C become highly shielded --> requires more external magnetic field energy. Therefore, ( 3 C) 4 Si s appear at a significantly higher field than s in most of the organic compounds; one of the reasons why is used as the internal reference compound. (ii) alomethanes [ 3 C-X] δ 4.30 C electronegativity: 4.0 F least shielded; most deshielded δ 3.05 C e.n.: 3.0 Cl δ 2.70 C e.n.: 2.8 Br δ 2.10 C I

3 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 3 of 10. (iii) 3 C---R 3 C---NR 2 3 C---SR δ 3.5 e.n.: 3.5 δ e.n.: 3.0 δ (iv) alkenes vs α,β-unsaturated ketones α δ 5.22 β deshielded + (3) Magnetic Anisotropy A through-space effect (i) Aromatic Ring-Current Effect See: Günther,. NMR Spectroscopy; Wiley: New York, 1998; p. 85. For a recent dispute on the origin of the aromatic ring-current effect, see: Wannere, C. S.; von Ragué Schleyer, P. rg. Lett. 2003, 5, 605. δ aromatic 's Δδ = ca. 2 ppm Induced magnetic field circulating π-electrons These aromatic delocalized 6 π electrons start circulating into one direction in an external magnetic field ( o ). This circulation of the 6 π-electrons results in the induction of a secondary magnetic field following the Fleming's rule. Induced magnetic field o induced magnetic field opposed to the applied magnetic field This type of differenct effect in magnetic properties at a different point in space is referred to as "magnetic anisotropy." flow of the 6 π- electrons in the ring induced magnetic field reinforcing the applied magnetic field o

4 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 4 of 10. (3) (i) Aromatic Ring-Current Effect (continued) ca. 55 shielding zone 's located inside this imaginary cone requires more external magnetic field power for their excitation --> highfield shifted; smaller δ values deshielding zone 's located outside the cone, including the 's on the benzene ring, requires less external magnetic field power (i.e., energy) for their excitation --> down-field shift; larger δ values Example: C C 2 C δ 2.63 Strongly deshielded Strongly shielded by the ring current effect of the benzene 6π-electrons C 2 C 2 C C C 2 C 2 C 2 The closer the is to the center of the benzene ring, the stronger the extent of the ring current effect is. (ii) Acetylene groups. R C C δ 2.2 R C C As this is quite close to the center of the C C bond,an extremely strong shielding effect is observed for this. R In a magnetic field, these π-electrons induce a secondary magnetic field V. Integration deshielding zone shielding zone The peak intensity is proportional to the number of s belonging to each peak in the case of 1 NMR. 3 C C 2 C3 chemical shifts integration of each of the peaks ppm These integrated areas are expressed in arbitrary numbers. nly their numerical ratio is important. These 12 and 36 represent the integrated areas of each peak and their ratios correspond to the ratios of the number of s belonging to each peak, i.e., 12 : 36 = 1 : 3. This could mean 1 and 3 s, 2 s and 6 s, or 3 s and 9 s, etc.

5 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 5 of 10. Integration (continued). If there is a 1 : 1 mixture (on a molar basis) of acetone and chlorform [CCl 3 ], the integrated ratio of each of the two peaks should be 1 : 6. chloroform [CCl 3 ] acetone [C 3 6 ] chemical shifts integration of each of the peaks ppm 2 12 Conversely, if the 1 NMR spectrum of a mixture of acetone and chloroform shows a 1 : 1 intensity by integration, what is the molar ratio of these two solvents in the solution? chloroform [CCl 3 ] integration trace acetone [C 3 6 ] chemical shifts integration value of each of the peaks ppm VI. Spin-Spin Couplings [Nuclear Spin-Nuclear Spin Interactions]-Through-Bond Interactions Acyclic Systems 3-bond interaction δ 5.8 δ C C 2 Cl Br Cl Br This 3-bond interaction is often referred to as a "vicinal coupling" and is expressed as 3 J 1,2 = 7.0 z through 3 bonds (i.e., 1 -C 1, C 1 -C 2, and C 2-2 ) The two spin states of 1 interact, through bonds, with the two spin states of 2, called "coupling," resulting in the formation of the spectral pattern consisting of a pair of doublets. These 's are called a 3-bond neighbor to each other.. between -1 and -2 3 J 1, δ (ppm)

6 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 6 of 10. V. Spin-Spin Coupling (continued) 3-Bond Neighbor Analysis: has 2 3-bond neighbors (i.e., neighboring 's) Cl have 2 3-bond neighbors Cl have 4 3-bond neighbors ============================================================ ethyl acetate [ 3 C-C 2 --C(=)-C 3 ] 3 chemically equivalent 's; have 2 3-bond neighbors C C C 3 2 chemically equiv. 's; have 3 3-bond neighbors 1 NMR spectrum of ethyl acetate CCl C 2 - quartet 4.1 singlet C 3 -C 3 triplet δ (ppm) : 3 : 3 integration values integration or intensity ratio For the C 3 's: there are two 3-bond neighbors. 6 z After the interaction with the first 3-bond neighbor (assuming J = 6 z) After the interation with the second 3-bond neighbor - each of the above doublet peaks splits to a doublet These middle two peaks overlap. verall, peak intensity ratio 1 : 2 : 1 6 z 6 z 6 z 6 z

7 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 7 of 10. V. Spin-Spin Coupling (continued) For the C 2 's: there are three 3-bond neighbors. After the interaction with the first 3-bond neighbor (assuming J = 6 z) After the interation with the second 3-bond neighbor - each of the above doublet peaks splits to a doublet 6 z These middle two peaks overlap. Thus, becoming a triplet with a 1 : 2 : 1 intensity ratio. 6 z 6 z 6 z 6 z Now, the interation with the third 3-bond neighbor - each of the 1 : 2 : 1 triplet peaks splits to a doublet These two peaks overlap. These two peaks overlap. 6 z 6 z 6 z verall, the C 2 peaks show up as a 1 : 3 : 3 : 1 quartet. 6 z 6 z 6 z In general, in an sp 3 acyclic system: If an has an n-number of 3-bond neighbor s, the NMR signal of the former splits into an (n + 1)-number of peaks. This is due to the fact that all of the vicinal couplings (i.e., 3 J) are virtually identical in a freely rotating acyclic system For example, have 1 3-bond neighbor (1 + 1) = 2 peaks: doublet C C C C has 6 3-bond neighbors have no 3-bond neighbors (6 + 1) = 7 peaks: septet (0 + 1) = 1 peak: singlet

8 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 8 of 10. V. Spin-Spin Coupling (continued) A 1-1 spin-spin coupling exists between any 2- and 3-bond neighboring s. owever, the only couplings that can be observed in the spectrum are those between chemically nonequivalent s.

9 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 9 of 10. V. Spin-Spin Coupling (continued) (6) vinyl acetate 4.88 (doublet of doublets) A B 4.57 (doublet of doublets) X δ 2.15 (singlet) C 3 down-field shifted due to the strong inductive effect by the oxygen atom 7.27 (doublet of doublets) These are high-field shifted due to the following resonance contribution: C 3 C 3 The coupling constants among these three 's are: 3 J AX = 14.2 z (trans-vicinal); 3 J BX = 6.5 z (cis-vicinal); 2 J AB = 1.2 z (geminal) For X : Upon interaction with A 3 J AX 14.2 z Then, upon interaction with B, each of the doublet splits into a doublet "a doublet of doublets" 3 J BX 3 J BX 6.5 z 6.5 z 3 J 3 AX J BX Unlike those in an acyclic system, these middle two peaks do not overlap, thus not producing a 1 : 2 : 1 triplet pattern. The situation is the same for both A and B. Therefore, A and B each shows a doublet of doublets.

10 Chem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II page 10 of 10. V. Spin-Spin Coupling (continued) (7) Aromatic ydrogens: typically 3 J ~ 8z (ortho); 4 J ~ <1 z (meta); 5 J ~ 0 z (para) (a) para-disubstituted Benzene Cl B B A A triplet (2 3-bond neighbors) at 1.2 ppm quartet (3 3-bond neighbors) at 2.8 ppm Two groups of chemically equivalent aromatic 's: each shows up as a doublet as a result of 3 J AB ~ 8 z with a 2 -intensity. (b) Three isomers of nitroaniline 6.82 ppm (d) D ortho meta para N 2 N (d) D N (s) A A N (d) A 7.34 (t) C B 6.67 (t) A 8.08 (d) 7.36 (t) C B N (d) B N 2 B 7.93 (d) Chemical shifts of each of these aromatic 's are explainable in terms of resonance contributions of the N 2 and N 2 groups. highest field-shift (smaller δ ppm) N N N N most e - density increase some e - density increase high-field shift (c) Long-range couplings doublet at 7.40 ppm C C 3 Br singlet at 2.5 ppm ~ 0 z a doublet of doublets at 8.04 ppm B N 2 A doublet at 8.38 ppm 3 J (ortho coupling) 8.6 z 4 J (meta coupling) C B 2.2 z C 3 N 2 Br A 5 J (para coupling)

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