Coupling To Other Nmractive Nuclei

Proton signals will also be split by (J coupled to) other NMR-active nuclei that are nearby in the bonding network. Two commonly encountered spin-/ nuclei are 19F and 31P (both 100% abundance). Common NMR-active nuclei that must be introduced by isotopic labeling include deuterium (2H, spin 1), and 13C and 15N (both spin /).

Couplings to 31P are similar to 1H-1H couplings in magnitude for two- and three-bond relationships. For example, dimethyl methylphosphonate (CH3P(O)(OCH3)2) gives two doublets in the 1H NMR spectrum: CH3P at 1.481 ppm (integral 3H) and CH3O at 3.741 ppm (integral 6H). The CH3O doublet is 11.0 Hz wide (3JHP = 11.0) and the CH3P doublet is 17.4 Hz wide (2JHP = 17.4). In the 13C spectrum we would expect two peaks because there are only two distinct 13C chemical shifts: CH3P and CH3O. But instead we see four peaks: a doublet at 52.19 ppm (CH3O, 2JCP = 6 Hz) and another doublet at 9.83 ppm (CH3P,1 Jcp = 144 Hz). We do not expect to see splitting in 13C spectra because they are 1H decoupled, but we have to remember that 1H is the only NMR-active nucleus that is decoupled: all other splittings will show up in routine 13 C spectra. In particular, the one-bond 13C-31P coupling (144 Hz) is large enough that, especially on lower field instruments, the doublet is easily mistaken for two different carbon chemical shifts. As with 1H-1H couplings, long-range (>3 bond) couplings are observed in conjugated systems. Triphenylphosphine oxide ((C6H5)3P=O) shows coupling from 31P to the ipso carbon (1JCP = 103 Hz), to the ortho carbon (2JCP = 10 Hz), to the meta carbon (3JCP = 12.5 Hz), and to the para carbon (4JCP = 3.5 Hz).

Coupling to 19F is also similar to 1H-1H couplings, but there are some unusually large couplings as well. Geminal 1H-19F couplings on saturated (sp3-hybridized) carbons are around 50 Hz, and on unsaturated (sp2-hybridized) carbons they can be around 80 Hz. Vicinal 1H-19F couplings in rigid saturated systems with an anti relationship (180° dihedral angle) are around 40 Hz, and in a fluoro-olefin the 3 J values are around 20 for cis and 50 for trans. In flexible saturated systems, vicinal couplings are similar to 1H-1H couplings. Long-range couplings can be significant: in aromatic rings the 4-bond or meta coupling (6-8 Hz) is similar to the vicinal or ortho coupling (8-10 Hz), and the 5-bond or para coupling is significant (~2 Hz). Coupling of 19F is significant in 13C spectra, again because only coupling to 1H is removed by decoupling. In fluorobenzene, for example, the JCF couplings are 245, 21, 8, and 3 Hz for 1J, 2J, 3J, and 4J, respectively. A CF3 group will be split into a quartet (1:3:3:1) with very wide coupling. Trifluoroacetic acid, for example, gives two quartets in the 13C spectrum: 1Jcf = 282 Hz (CF3) and 2Jcf = 44 Hz (CO2H). This splitting and the loss of the heteronuclear NOE can cause a fluorinated carbon to "disappear" into the noise. It is possible to decouple 19F, but most spectrometers do not have the capability to decouple both 1H and 19F simultaneously.

Coupling to deuterium, 2H, is observed for deuterated solvents and their residual peaks (from solvent molecules with one 2H replaced by1H).1H-2H coupling constants are proportional to the corresponding 1H-1H J value, reduced by a factor of about 7 (yH/yD = 6.51) due to the weaker nuclear magnet of deuterium. Because deuterium has a spin of 1, it has three spins states almost equally populated, and so it splits the 1 H signal into three equal peaks centered on the 1H chemical shift position. Multiple 2H splittings can be built up by diagramming just like with splittings, as long as each 2H results in a split into three peaks of intensity ratio 1:1:1. In 13 C spectra we see the splitting by the directly bound 2H nuclei, reduced by a factor of 6.5 from the corresponding 1H-13C couplings. For example, without 1H decoupling CHCl3 gives a doublet in the 13C spectrum with 1JCH = 209 Hz, and CDCl3 gives a 1:1:1 triplet in the 13C spectrum with 1JCD = 32 Hz (209/6.5 = 32.15). Exchange of 2H between the sample molecule and deuterated solvent can lead to loss of 1H peak intensity, for example, at an activated position next to a ketone functional group in CD3OD solvent. The carbon peak may disappear entirely from the 13C spectrum due to splitting by 2H into many smaller peaks that sink into the noise. In addition, carbons are much slower to relax when bound to 2H (relative to 1H), again due to the smaller nuclear magnet of deuterium, further reducing their intensity in the13 C spectrum.

Coupling to 13 C is observed as weak satellites on either side of each 1H resonance. Because the natural abundance of 13C is only 1.1%, we have to consider two separate molecules and add together their contributions to the 1H spectrum: one with our 1H bound to 12C (98.9% of the sample) and another with 1H bound to 13C (1.1% of the sample). In the 12C case, we see the normal 1H resonance with its splitting pattern from coupling to other protons (triplet, double-doublet, singlet, etc.). In the 13C case, we see this same splitting pattern but divided into two identical patterns, one about 75 Hz downfield of the 1H-(12C) pattern and one about 75 Hz upfield of this pattern (Fig. 2.17). The exact distance is one half of 1JCH, which is typically in the range of 150 Hz for a one-bond coupling between 13C and 1H. This doublet is 1.1% of the intensity of the central 1H-12C resonance, so each component of the doublet (each side) is 0.55% of the intensity of the central peak. For isotopically enriched ("labeled") compounds, the satellites are larger due to the greater abundance of 13C. For example, a sample of 13C-enriched methyl iodide (CH3I) with 50% of 13C will show a 12CH3I singlet (50% of total intensity) and a 13CH3I doublet centered on the same chemical shift (each side 25% of total intensity). This will look like a triplet with spacing of 1JCH and intensity ratio 1:2:1, but it is not a triplet and it is important to realize that this pattern is the superposition of two separate spectra from two different species (isotopomers) in solution.

Long-range (2 or 3 bond) coupling to 13 C is difficult to observe unless the sample is enriched. A sample of ethyl acetate (CH3-C*O-OCH2CH3) with 100% 13C label at the carbonyl carbon would give a 1H spectrum with a doublet for the acetate methyl group (2Jch), a double-quartet for the methylene group (3JCH), and the normal triplet for the

Triplet Coupling

ethyl CH3 group (4JCH = 0). These long-range couplings (2JCH and 3 JCH) are similar in magnitude to1H-1 H couplings (0-10 Hz), whereas couplings over more than three bonds are extremely rare. If the enrichment was less than 100%, this spectrum would be superimposed on the normal spectrum of ethyl acetate (singlet, quartet, triplet) with intensities proportional to the quantity of each isotopic species (12C or 13C). Figure 2.18 shows the 1H spectrum of alachlor herbicide with 99% 13C at the methyl group: N-CH2-O-13CH3. The methoxy group singlet is split into a doublet (1JCH = 142 Hz) and the N-CH2-O methylene singlet is also split into a doublet (3JCH = 5.3 Hz). The 1% 1H-12C peak for the methoxy group can be seen at the center of the 1H-13C doublet. It is important to understand that we are observing 1H here, not carbon nuclei. You cannot observe 12C by NMR because it has spin 0 (no magnetic properties of the nucleus), but you can observe the protons attached to 12C.

All heteronuclear couplings (couplings between two different kinds of nuclei—two different isotopes) are first order, without any distortion of peak intensities. The chemical shift difference between two different isotopes is usually on the order of megahertz to hundreds of megahertz, so even with large J couplings (hundreds of hertz), the shift difference is much, much larger than the J coupling, and there are no second-order (strong coupling) effects.

2.8 NON-FIRST-ORDER SPLITTING PATTERNS: STRONG COUPLING

The simple splitting patterns discussed above appear only when we have weak coupling: when the chemical shift difference between two nuclei (expressed in hertz) is much greater than the J coupling between them. In this case, the coupling pattern is symmetric with a maximum of 2n peaks (for n coupled spin V nuclei), and the chemical shift is at the exact center of the pattern. When the chemical shift difference in hertz is on the same order

AB System House

Figure 2.19

of magnitude as the J coupling, the quantum mechanical situation is more complex and we see distortions of peak intensity (nonsymmetric patterns), and in some cases new lines arise in the pattern. These second-order patterns are often observed as simple "leaning" of the classical (doublet, triplet, quartet, ...) patterns: the peaks on the "inside" (nearest to the chemical shift of the coupled nucleus) are taller (more intense) and the peaks on the "outside" (away from the chemical shift of the coupled nucleus) are shorter (weaker). For the simplest case of two protons strongly coupled to each other and no other protons, we call this an AB system. We use A and B because they are next to each other in the alphabet and indicate that the chemical shifts are very close together (Fig. 2.19). If we think of the sloping doublets as rooflines, we can use the analog to a cartoon house to remember the effect. The tall side of the leaning doublet always "points" toward the other spin that is splitting it. More complicated patterns can also "lean" when the J-coupled resonances get close to each other. In Figure 2.20 we see the simplest possible pattern from two adjacent methylene groups: X-CH2-CH2-Y. The two triplets "lean" toward each other so that the outer lines of the triplets are less than 1 in relative area and the inner lines are more than 1; the center lines still have relative area 2. Three methine (CH) groups in a row (Fig. 2.21) lead to doublets for the outer protons (Ha and Hc) and a double doublet for the middle proton (Hb). The two doublets will "lean" toward their coupling partner in the center, and the Hb pattern will lean both ways: the large coupling (Jab) "leans" toward Ha and the small coupling (Jbc) "leans" toward Hc. Another example is the case of three protons in a row on an aromatic ring, with

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