PAPER No. 12: ORGANIC SPECTROSCOPY. Module 19: NMR Spectroscopy of N, P and F-atoms

Transcription

1 Subject Chemistry Paper No and Title Module No and Title Module Tag Paper 12: Organic Spectroscopy CHE_P12_M19_e-Text

2 TABLE OF CONTENTS 1. Learning Outcomes N NMR spectroscopy F NMR spectroscopy P NMR spectroscopy 5. Summary

3 1. Learning Outcomes After studying this module, you shall be able to To understand the basic concept of nitrogen, fluorine and phosphorous NMR To know about the internal and external standard to record these spectra To know about the heteronuclear coupling between nitrogen, fluorine and phosphorous atoms N NMR spectroscopy 15 N NMR spectroscopy is employed to determine the structure of nitrogen containing compounds. Nitrogen has two isotopes i.e. 14 N and 15 N and both are NMR active. 14 N has a nuclear spin of 1, whereas 15 N has a value of ½. The natural abundance of 14 N (99.63%) is much higher than the 15 N (0.37%). Since 14 N has an integer nuclear spin (I = 1) and thus it possess quadrupole moment and its signals are usually significantly broadened by quadrupole relaxation mechanism. Sometimes the broadening of the signal is to the extent that they are not observable on a high resolution NMR spectrometer. The other isotope, 15 N has a spin of 1/2 and no quadrupole moment, but it is only 0.37 percent abundant and hence is very insensitive. Sensitivity is made worse by its low gyromagnetic ratio (γ T 1 s 1 ), which is 10.14% that of 1 H. The signal to noise ratio for 1 H is about 300 fold greater than 15 N at the same magnetic field. Figure 1: Comparison of 14 N and 15 N spectra of urea. The 14 N signal is broad due to quadrupolar moment. The extent of broadening depends on the molecule structure. Larger the molecule and the more asymmetric the nitrogen's environment, broader is the signal. Hence, the small and highly symmetric aqueous ammonium ion gives a very sharp line (< 1 Hz widening). Liquid ammonia, being less symmetric, is 16 Hz wide. Urea is larger and asymmetric so the line width is approximately 1 KHz. Molecules that are significantly larger than urea yield signals too broad to be observed.

4 Figure 2: Comparison of line widths of 14 N signals. Larger and less symmetric molecule gives wider signal. With highly sensitive instruments and making maximum use of various signal enhancement methods such as DEPT, NOE, natural abundance spectra can be recorded for 15 N. The advent of new techniques, such as pulse sequences and polarization transfer, in conjunction with the use of high-field magnets and large-sample probe heads largely solved the detection problem. Chemical shifts of nitrogen nuclei cover a wide spectral range of about 900 ppm for most organic and inorganic compounds. There is no measurable isotopic substitution effect on the chemical shift value, hence 15 N and 14 N shifts may be used interchangeably. Various standards, internal or external, have so far been employed as references for nitrogen chemical shifts. There are two different chemical shift scales in 15 N NMR. IUPAC (international union of pure and applied chemistry) recommends nitromethane (CH 3NO 2) for the chemical shift reference (0 ppm). However some people use liquid ammonia. For 15 N, chemical shifts referenced with liquid NH 3 are ppm upfield from CH 3NO 2 (δ: NH 3 = δ: CH 3NO ppm). The heteronuclear coupling with 14 N is rarely observed because of its quadrupolar broadening. It can be observed only when the nitrogen atom is quaternary (ammonium ions) at room temperature and liquid ammonia at low temperatures. Two bond coupling ( 14 N-CH 2-) are not observed even when the nitrogen is quaternary, presumably because the coupling is very small.

5 Figure 3: 1H spectrum of natural abundance NH 4Cl (1.5 M) in 1 M HCl/H 2O showing coupling to 14 N (I = 1) as a triplet. Figure 4: 14 N spectrum of NH 4Cl (1.5 M) in 1 M HCl/H 2O showing coupling to 1 H as a quintet (four protons present at nitrogen atom) F NMR spectroscopy 19 F NMR is used to analyze the structure of fluorine containing compounds. 19 F is the naturally occurring isotope of fluorine with 100% abundance. It has a nuclear spin of 1/2 and a high gyromagnetic ratio, which means that this isotope is highly sensitive to NMR spectroscopy. Because of its favorable nuclear properties and high abundance, 19 F NMR spectroscopy is highly sensitive and fast as compared to 1 H NMR spectroscopy. Indeed the 19 F nucleus is the second most receptive NMR nucleus after the 1 H nucleus. The 19 F NMR chemical shift range is very wide ppm as compared to 1 H NMR (0-15 ppm). The reference compound for 19 F NMR is trichlorofluoromethane CFCl 3. It is generally inert, volatile and gives rise to a single 19 F peak at δ value of zero. When measurements are done at the same radio frequency, the signals from 19 F absorptions occur at considerably different magnetic field strength than that of proton. Thus one does not see the peaks due to 19 F absorptions in 1H NMR spectrum. However the peaks due to proton can show splitting due to spin-spin couplings between protons and 19 F nuclei.

6 The figure given below shows the chemical shift range for the fluoro organic compounds. The most commonly encountered signals arising from organofluorine compounds lie between 50 to 70 ppm (for CF 3 groups) and between 200 to 220 ppm (for CH 2F groups). The very wide spectral range can cause problems in recording spectra, such as poor data resolution and inaccurate integration. The multiplicity of the peaks gives the information about the neighbouring atoms to the fluorine. The coupling constants between fluorine nuclei are generally larger than those for H. Geminal F- F coupling ranges from 40 to 370 Hz and vicinal F-F from 0-45 Hz. The cis vicinal coupling ranges from 0-60 Hz and trans vicinal coupling ranges from Hz. Long range coupling more than five bonds is having very low value about 0-18 Hz.

7 Coupling between H and F nuclei is also strong. Geminal coupling ranges from Hz and vicinal coupling from Hz. The cis H-F coupling constant ranges from 0-20 Hz and trans coupling constant value range is from Hz. Fluorine attached to benzene also couples with protons on the ring. The coupling constant values for ortho, meta and para ranges from Hz, Hz and Hz, respectively. The figure 5 given below shows the 1 H and 19 F NMR spectrum of 1-bromo-1-fluoroethane (CH 3CHFBr). Each spectrum is recorded using a 1.4 T magnet. In the 1 H spectrum, the H-H coupling between CH 3 and CH protons should give a quartet and doublet but the H-F coupling complicates the splitting pattern. The CH 3 signal is further split by 19 F, each line of the doublet is split into two by the large vicinal H-F coupling (J = 22 Hz), the CH 3 signal is therefore appears as a double doublet. The CH signal is split into a quartet by CH 3 (J = 6 Hz) and then each line is split into two by the large germinal H-F coupling (J = 50 Hz), the CH signal is therefore appears as a doublet of quartets. In the 19 F spectrum the fluorine signal is split into a doublet by the germinal methine proton (J = 50 Hz) and each line is further split into four by the vicinal methyl protons (J = 22 Hz). Thus the peak due to fluorine is an overlapping doublet of quartets.

8 Figure 5: 1 H and 19 F NMR spectra of 1-Bromo-1-Fluoroethane The figure 6 given below shows the 19 F NMR spectrum of 1-bromo-3,4,5-trifluorobenzene. There are two sets of fluorine atoms. The fluorine atoms at the 3 and 5 positions are chemically different from the fluorine atom at 4-position. Consider either F-3 or F-5, both the nuclei couple with the adjacent F-4 nucleus to give a doublet which further split into a doublet by the neighbouring proton to give a doublet of doublets. The fluorine, F-4 will couple with the two equivalent fluorine nuclei (F3 and F5) to give a triplet, and also with the two chemically equivalent protons therefore signal due to this fluorine appears as a triplet of triplets. Figure 6: 19 F NMR spectrum of 1-bromo-3,4,5-trifluorobenzene P NMR spectroscopy Phosphorus-31 NMR spectroscopy is used to identify the phosphorous containing organic and inorganic compounds. It is commonly found in organic compounds and coordination complexes

9 (as phosphines, phosphites, phosphonium salts), nucleic acids (phosphate esters), and also other important bio-molecules such as ADP, ATP etc. It is one of the more routine NMR techniques as 31 P has an isotopic abundance of 100% and a relatively high gyromagnetic ratio (40.5% of that for 1 H). Like 1 H and 19 F, it has a spin of 1/2 thus making spectra relatively easy to interpret. 85% phosphoric acid (H 3PO 4) is conventionally used as an external standard, which is assigned the chemical shift of 0. The chemical shifts range for phosphorous (0-250 ppm) is much wider than typical for 1 H NMR. 31 P shows an interesting pattern in coupling with the neighbouring atoms. The size of the coupling normally decreases dramatically with the number of intervening bonds. A proton directly attached to phosphorous (PH 3) is split by a large coupling constant (one bond coupling) of about 600 Hz. Thus the two components of a doublet will be far apart from each other that it would normally not be accepted as a coupling. The two bond coupling Ph 3P-CH 2- are comparatively smaller ( Hz), while three bond coupling Ph 3P-CH 2-CH 2- (5-10 Hz) and four bond coupling Ph 3P-CH 2-CH 2-CH 2- are very small (<1 Hz) and rarely observed. Like the 13 C spectra, 31 P spectra are recorded with proton decoupling. In 31 P NMR spectrum of PPh 3 shows a sharp singlet (figure 7). If the decoupler is turned off, the 31 P peak will be observed as multiplet or broad due to the coupling with the ortho, meta and para protons of the phenyl rings. Figure 7: 31 P NMR spectrum of PPh 3 Figure 8 represents the 31 P NMR spectrum of diethyl phosphonate with 1 H decoupling and showing one-bond and three bond coupling between 31 P and 1 H.

10 Figure 8: 31 P NMR spectrum of diethyl phosphonate; A) with 1 H decoupling; B) with onebond and three bond coupling to 1 H Figure 9 shows the 31 P NMR and 1 H NMR spectrum of a square planar complex [MH(PPh 3) 3]. There are two different sets of triphenylphosphine group in this complex. The two PPh 3 groups shown in blue colour are equivalent and the one shown in red colour is different. Hence in the 31 P NMR spectrum two sets of peaks will be observed. The red phosphorous nucleus will couple to the two equivalent blue coloured phosphorous nuclei and will give a 1:2:1 triplet. Similarly the blue coloured phosphorous will couple to the single red phosphorous to give a doublet. The coupling constant, J P-P will be same in both the cases. It is important to note here that the coupling between P and H in this spectrum has been removed by the decoupler and all the coupling is P-P coupling only. Figure 9: A) 31 P NMR B) B) 1H NMR spectrum of square planar complex (signals due to aromatic protons have been removed in the 1 H spectrum)

11 Now consider the 1 H NMR spectrum. The hydrogen (in actual hydride attached to metal) will couple to the red PPh 3 to give a doublet. This hydride will also couple to the two equivalent blue PPh 3 to give a 1:2:1 triplet. The first (doublet) splitting would be larger than the second (triplet) splitting (coupling between trans-oriented nuclei is usually larger than the cis-ones). Thus the signal for the proton will be observed as a doublet of triplets. The phenyl protons in the molecule are far away from the hydride atom, hence coupling will not be observed with the aromatic protons. Note: The hydride atoms attached to metals appear at the extreme right i.e. negative ppm values in the proton spectrum (in the present case from 4.76 to 5.24 ppm). There will, of course, be some signals for the phenyl protons near 7 ppm in the proton spectrum, with an integral of 45 protons (three PPh 3 = 9 phenyl rings each with 5 protons). 5. Summary Nitrogen has two isotopes i.e. 14 N and 15 N and both are NMR active. There is no measurable isotopic substitution effect on the chemical shift value, hence 15 N and 14 N shifts may be used interchangeably. 19 F is the naturally occurring isotope of fluorine with 100% abundance. Phosphorus-31 NMR spectroscopy is used to identify the phosphorous containing organic and inorganic compounds. 31 P has an isotopic abundance of 100% and a relatively high gyromagnetic ratio (40.5% of that for 1 H). Like 1 H and 19 F, it has a spin of 1/2 thus making spectra relatively easy to interpret.