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Bsc Chemistry Subject Chemistry Paper No and Title Paper 12: Organic Spectroscopy Module No and Title Module 17: First order and second order spectra Module Tag CHE_P12_M17_e-Text CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY Module 17: First order and second order spectra TABLE OF CONTENTS 1. Learning Outcomes 2. Simple Splitting 3. Complex Splitting 4. First Order Spectra 5. Second Order Spectra 6. Nomenclature of spin system 6.1 Two-Spin Systems (A2, AB, AX) 6.2 Three-Spin Systems (A3, AB2, A2B, AX2, A2X, ABC, AMX, ABX) 7. Magnetic equivalent protons 8. Summary CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY Module 17: First order and second order spectra 1. Learning Outcomes After studying this module, you shall be able to To understand simple and complex splitting pattern of various compounds. To understand the first and second order spectra Nomenclature of various spin systems To know about the magnetically equivalent protons in aliphatic and aromatic systems 2. Simple Splitting In a simple spin-spin coupling we observed that a particular proton is coupled to a set of equivalent protons and the multiplicity is decided by the total number of the similar types of protons in the molecule as shown in the figure. The intensity of the peaks is according to the Pascal’s triangle. 3. Complex Splitting There are various factors which may lead to the complexity of the NMR spectra. 1. If the n protons are non-equivalent, the (n+1) rule is not obeyed. 2. The chemical shifts of the coupled protons are very close. 3. The presence of the chiral centre (s) in the molecule. The presence of chiral centre makes the CH2 group distereotopic and thus non-equivalent protons, which may have different coupling constant with neighbouring protons. CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY Module 17: First order and second order spectra If a proton is spin-coupled to two or more different sets of neighbouring protons (non-equivalent protons with different J values), say Na number of protons of Ha type, Nb number of protons of Hb, Nc number of protons of Hc and so on, the n+1 rule does not predict the entire splitting pattern. In such cases, the number of neighbouring protons does not add up simply as in first order splitting, but the multiplicity of the signal is given by (Na + 1) (Nb + 1) (Nc + 1) and so on. It is important to note that there may be fortuitous coincidence of some lines if a smaller J is a factor of a larger J. This phenomenon is called complex coupling/splitting. The following compound is a good example of showing complex splitting. The chemical shift of Hc hydrogens is in the most downfield region and it shows a triplet due to coupling (Jbc) with 2 equivalent protons i.e. Hb. Similarly, hydrogens Ha of the methyl group display a triplet in the upfield region due to coupling (Jab) with 2 equivalent protons Hb. The two coupling constants are not equal. The chemical shift of Hb protons is in between. One can expect a sextet for Hb. But in actual the signal of Hb appears as quartet of triplets due to the non- equivalent nature of the coupled protons Ha and Hc and hence unequal coupling. The signal for Hb is first split into a quartet due to Ha protons, and each of the four lines in quartet is further split into a triplet because of the Hc protons. The signal will therefore have 12 peaks in the form of a quartet of triplets. CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY Module 17: First order and second order spectra The appearance of the signal depends greatly on the J values. If Jab is much greater than Jbc, then the signal will appear as a quartet of triplets as shown in the above example. If, however, Jbc is much greater than Jab, then the signal will appear as a triplet of quartets. In most cases, the J values are not much different, and we will observe neither a clean quartet of triplets nor a clean triplet of quartets. Many of the peaks overlap producing a multiplet and it is difficult to extract the J values from this. Similarly the splitting pattern (second order splitting) for the methyl acrylate can be explained. CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY Module 17: First order and second order spectra The proton Hc has two types of neighbour protons Ha and Hb and hence it couples to both Ha and Hb, but with two different coupling constants. Hc is trans to Ha across the double bond, and splits 3 the Hc signal into a doublet with a coupling constant of Jac = 17.4 Hz. In addition, each of these lines in doublet is further split by Hb (geminal coupling) into two more doublets, each with a 2 small coupling constant of Jbc = 1.5 Hz. This type of pattern is referred as a doublet of doublets, abbreviated ‘dd’. 3 Similarly the signal for Hb at 5.64 ppm is split into a doublet by Ha with coupling constant Jab = 10.4 Hz (cis coupling). Each of the resulting lines is split again by Hc, with the same geminal 2 coupling constant Jbc = 1.5 Hz. The overall result is again a doublet of doublets. 3 The signal for Ha at 5.95 ppm is also a doublet of doublets, with coupling constants Jac= 17.4 Hz 3 (trans coupling) and Jab = 10.5 Hz (cis coupling). Note: The signal shown above look like a quartet but it is not a quartet because in a quartet the line intensities are 1:3:3:1 and the individual peaks are equally spaced. In doublet of doublets, all the lines are of equal intensity and the individual peaks are not necessarily equally spaced. Hence for complete identification of the splitting pattern we must look carefully at the number of lines, their shape and the intensity. In some cases, Jab and Jac will be almost identical. For example, consider the 1H NMR spectrum of 1-nitropropane. Look carefully at the splitting pattern of the Hb protons (δ ≈ 2 ppm). This signal looks like a sextet, because the two coupling constants Jab and Jbc are very similar. In such a case, it appears “as if” there are five equivalent neighbours, even though all five protons are not equivalent. In case both the coupling constants are not similar, it would have shown 12 lines in total in the form of a quartet of triplets via the successive coupling first with Ha protons and then with Hc protons. Due to the very close value of both the coupling constants several lines coincide to give a deceptively simple multiplet i.e., a sextet. One, however, may expect 12 peaks at high resolution. Similarly in 1-bromo-3-chloropropane, the CH2Cl and CH2Br protons are not chemically equivalent. Inspite of this the middle CH2 appears as a quintet. This is again due to the fact that the magnitude of coupling to both types of protons is very similar. Therefore it seems to obey n+1 rule. CHEMISTRY PAPER No. 12: ORGANIC SPECTROSCOPY Module 17: First order and second order spectra 4. First Order Spectra If the splitting pattern of a multiplet is given by the n+1 rule, then the spectra is called as ideal or “first order” spectra. This is usually observed if the spin-coupled nuclei have very different chemical shifts (i.e. Δν is large compared to J). The chemical shift value and the coupling constant J can be measured directly from the spectrum. The first order spectra can be analysed by the following features 1. The multiplicity of a peak is given by the expression 2nI+1, where I is the nuclear spin of the concerned nucleus and n is the total number of coupled nuclei. 2. The relative intensity of the component peaks of a multiplet is given by the coefficients of the terms in binomial expansion of (x+1)n for nuclei with I = 1/2. 3. Centre of the multiplet gives the chemical shift of the concerned nucleus. 4. The separations of peaks in two coupled multiplets are exactly equal and correspond to the coupling constant. It is well known that the chemical shift measured in Hz depends on the applied magnetic field and the value of ∆ν increases as the field strength increases. The value of coupling constant J remains constant and hence the value of ∆ν/J will increase on increasing the field strength. Therefore at high field strength, most 1H NMR spectra become first order. It is important to notice that high field NMR instruments give better resolution and relatively easily interpretable spectra. 5. Second Order Spectra If the coupled nuclei have approximately same chemical shifts and their chemical shift difference is comparable to the coupling constant between them, the first order splitting rule (2nI+1) is not applicable. The splitting pattern in a multiplet is distorted and the bands are no longer symmetrical. In fact, signal splitting disappears if the chemical shifts of the coupled nuclei are the same. Such types of spectra are known as second order spectra. It is difficult to measure the chemical shift and coupling constants values simply by just analysing the spectra. As the value of Δν/J decreases, the coupled multiplets approach each other and the inner lines increase in intensity and the outer peaks decrease. In some extreme cases the outer lines may become too weak to be observed and hence the resulting multiplet is wrongly interpreted. Since there is leaning of coupled multiplets towards one another and hence the chemical shift values CHEMISTRY PAPER No.
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