22 and Applications of 13 C NMR

Transcription

1 Subject Chemistry Paper No and Title Module No and Title Module Tag 12 and rganic Spectroscopy 22 and Applications of 13 C NMR CHE_P12_M22

2 TABLE F CNTENTS 1. Learning utcomes 2. Introduction 3. Structural Determination 4. Factors affecting 13 C NMR chemical shifts C NMR of selected molecules 6. Summary

3 1. Learning utcomes After studying this module, you shall be able to Know about structural determination using 13 C NMR Learn the factors affecting 13 C NMR chemical shifts 2. Introduction The first NMR signal for 1 H was observed in 1945 but the first 13 C NMR signal was detected in 1957 by Lauterbur. The first 13 C NMR of some organic compound was recorded in early This discovery made structural determination very simple and interesting. This is more helpful to those molecules which have very few C-H bonds. This is because, this type of molecules are not identified completely by 1 H NMR. Also, the complex molecules mainly biomolecules found this method of identification more efficient. The study of biosynthetic pathways of natural products and brain metabolism can easily be monitored with the help of 13 C NMR spectroscopy. The recording of 13 C NMR spectrum is usually done as 1H decoupled for simplicity. Each chemically unique carbon present in the molecule gives a single peak whereas chemically equivalent carbons contribute to the same peak. The presence of different signals or peaks indicates the number of different kinds of carbon present in different chemically and magnetically different environment. The following sections are going to discuss the pathway for structural determination of different molecules. 3. Structural Determination Similar to the 1 H NMR spectroscopy; 13 C NMR spectroscopy also helps in the determination of structure of the substances. This spectroscopy helps in finding the presence of carbon skeleton in study of natural products, synthetic molecules, polymers and other molecules. It is always difficult to find the complete structure of any compound with any individual spectroscopic data.

4 The reference compound for 13 C NMR is same as that of 1 H NMR which is Tetramethylsilane [TMS; (CH3)4Si). The chemical shifts are expressed in ppm. There is large difference between the chemical shift of 13 CNMR and 1 HNMR, in 13 C the range of chemical shift is ranges from ppm while in case of 1 HNMR it ranges from 0-15 ppm. CH 3 H 3 C Si CH 3 CH 3 Tetramethylsilane (1) Correlation Data for 13 C NMR spectra A 13 C spectrum is typically recorded from ppm with zero being the methyl carbon in TMS. This range is about 20 times that of routine PMR spectra (~12 δ). Due to this large spread of chemical shifts, relatively fewer peaks overlap in 13 C NMR spectra. These spectra are shielded or deshielded due to the same factors as for 1 H NMR, such as: Electron withdrawing group Hybridization Electron current effects Some functionality only appears in 13 C NMR not in 1 H NMR such as quaternary carbon, ipso carbons, carbonyl carbons etc. Several cations absorb at ~335 δ downfield and CI4 has been recorded at approximately -290 δ upfield from TMS. A general values for different types of carbon environment is given in figure 9. Some general rules for the correlation data is important to know:

5 aromatic sp 3 C-EWG sp 3 carbon carbonyl alkenes alkynes (ppm) Figure 9. (ppm) values for different carbon environments sp 3 hybridised carbons The sp 3 hybridised carbons are either unsubstituted or substituted with electron withdrawing groups (EWG). verall range of resonance observed in between 0 to 50 ppm for unsubstituted and between ppm for EWG substituted carbons. The influence of strong electronic and steric effect cannot be ruled out. For an extreme case like tetraiodomethane (CI4) where, carbon has been observed at 300 ppm lower than the TMS ( = -300). The 13 C NMR signal ranges for some substituted sp 3 carbons are: CH2Br ppm CH2NH ppm CH2H ppm sp 2 hybridised carbons The sp 2 hybridised carbons include alkene and aromatic carbons. These carbons give signals in overlapped regions. The range alkene and aromatic carbon signals are between 90 to 150 and 100 to 170 respectively. In this case, a diversity of other functional groups also falls. For example, >C=, >C=N etc. The shift in ppm differs for these cases depending upon the electronic and steric factors. The >C=N present in the Schiffs bases are generally observed in the range ppm. The 13 C NMR signal ranges of important >C= groups are:

6 ester C= ppm acid C= ppm aldehyde C= ppm ketone C= ppm sp hybridised carbons The sp hybridised carbons include alkynes, nitriles and isonitriles. The signals for these groups are generally present in very narrow range. The unsubstituted alkynes have been observed in the range of ppm. (2) Chemical Shift Equivalence for 13 C NMR spectra The chemical shift equivalence for carbon atoms is similar to the one applied to protons. The chemical equivalence or non-equivalence of carbon is mainly judged in the same way as required for 1 H NMR. It is important to recognize various kinds of carbon atoms present in the molecule. Some of the common molecules are discussed below: t-butyl alcohol [(CH3)3CH] 1CH 3 1 H 3 C C 2 H 1 CH 3 t-butyl alcohol In t-butyl alcohol, the carbon atoms of all the three methyl groups are equivalent whereas the carbon attached with H is in different environment. Hence, the 13 C NMR of this molecule gives only two signals for carbons 1 and 2.

7 Diethyl ether Diethyl ether has two kind of carbon atoms labeled as 1and 2. Hence, this molecule gives two signals in 13 C NMR spectra CH 3 CH 2 CH 2 CH 3 1,3-Dicholropropane The 1,3-dichloropropane has two types of carbon atoms labeled as 1and 2. Hence this molecule gives two signals in 13 C NMR spectra ClCH 2 CH 2 CH 2 Cl 1-Chloro-4-methylbenzene The 1-chloro-4-methylbenzene molecule has five types of carbon atoms labeled as 1-5. Hence this molecule gives five signals in 13 C NMR spectra Cl CH Butan-2-ol The butan-2-ol molecule has four types of carbon atoms labeled as 1-4. Hence this molecule gives four signals in 13 C NMR spectra CH 3 CHCH 2 CH 3 H

8 4. Factor affecting 13 C NMR chemical shifts Some of the important factors that affect 13 C NMR are discussed. (a) α-, - and γ-substituent Effects The chemical shifts for 13 C NMR spectra are affected by the presence and absence of substituents at α-, - and γ-positions. The α-substituents results from the replacement of a directly bonded H by an X group: C-H C-X The -substituents results from the replacement of a H on an adjacent atom by an X group: C-C-H C-C-X The -substituents results from the replacement of a H on an adjacent to adjacent atom by an X group: C-C-C-H C-C-C-X The chemical shift increases on going from primary to secondary to tertiary to quaternary carbon atoms. The average increase per carbon atom lies in the range of 7-10 ppm. The substituents at - and -position increases the value. In other words, the 13 C absorption position is shifted downfield. The case with -subsitution is opposite. In this case, the 13 C absorption position is shifted upfield i.e. the value is decreased. Based on the observations, a formula for calculating 13 C shifts is given as: C = n + 9.4n - 2.5n + 0.3n where, C is chemical shift for carbon with reference to TMS. n is number of carbon atoms one bond away n is number of carbon atoms two bond away n is number of carbon atoms three bond away n is number of carbon atoms four bond away from the carbon atom whose chemical shift is calculated.

9 There is very good agreement has been observed between the calculated and observed value. The other important factor influencing the substituent effects is the electronegativity of the attached atom. The attachment of electronegative atom shifts the value towards higher side. For example, in the 13 C NMR spectrum of ethanol (Figure 10), the signal for CH3 is observed at higher value in comparison to the CH2. This is due to the attachment of CH2 group with electron withdrawing group H. (b) Hybridization Effect The hybrid state of carbon also affects the chemical shift of carbon. The details of this effect are discussed in correlation data section. (c) Three-Membered Rings The three-membered ring has significant effect on the chemical shifts in 13 C NMR spectra. The compounds such as cyclopropanes, cyclopropenes, epoxides, aziridines and other 3-membered rings tend to show pronounced upfield shifts i.e. shift towards lower - values. For example, the C-2 carbon of propane is observed at 16 ppm in 13 C NMR spectra whereas the carbon in cyclopropane is observed at -3 ppm (Figure 11). Similar trend has also been observed for propene and dimethyl ether with respect to their cyclic counterpart carbon (Figure 11). (d) Conjugation to carbonyl group The conjugation to carbonyl carbon also affects the chemical shift. The conjugation may be due to a double bond or aromatic ring. This causes upfield shifts i.e. towards lower

10 value about 6-10 ppm for all types of carbonyl compounds. The effect is smaller for nitriles also. For example, the carbonyl carbon of 2-butanone appears at 206 ppm whereas, corresponding conjugated carbonyl carbon for but-3-ene-2-one appears at 197 ppm and acetophenone at 195 ppm (Figure 12). (e) Hydrogen Bonding Effects The intramolecular hydrogen bonding causes substantial downfield shifts in 13 C NMR spectra i.e. towards higher value. It has been observed that most carbon signals are resistant to solvent effects but the carbonyl groups are an exception. The chemical shifts for carbonyl carbon move to downfield in protic solvents. This may be attributed to hydrogen bonding. Figure C NMR spectrum of ethanol

11 Figure C NMR showing ring effect 5. Coupling and Decoupling in 13 C NMR 13 C NMR of selected molecules The examples of proton decoupled 13C NMR of few compounds is given below (Figure 13,14). The solvent and TMS peaks has not been shown. H 3 C C CH 3 C CH 2 C 206 C H2 H 3 C 197 C H H 3 C Figure C NMR chemical shifts of conjugated carbonyl carbon

12 Figure C NMR (proton decoupled) of conjugated carbonyl carbon, crotonaldehyde

13 Figure C NMR (proton decoupled) of cyclohexene 6. Summary Chemical shift range is normally 0 to 220 ppm For recording the spectra about mg of sample dissolved in ml of deuterated solvent are required, and a good spectrum would be obtained in scans. The 13 C NMR spectrum gives us the carbon backbone of a molecule with following informations: The number of signals gives information about the number of equivalent carbons. The position of chemical shift gives information about the environment of carbon atom. The integration of peak gives the ratios of equivalent carbons. The 13 C-NMR splitting suggests that the presence of NMR active nuclei in neighbourhood. In the spectrum, the horizontal scale is shown as (ppm). is called the chemical shift and is measured in parts per million - ppm. The zero is where you would find a peak due to the carbon-13 atoms in tetramethylsilane - usually called TMS. Factors affecting 13 C NMR chemical shifts are: o α-, - and γ-substituent Effects o Hybridization effects o Ring effects o Conjugation to carbonyl group o Hydrogen bond effect