So now that we have Case 1 (i.e., I = ½, 100% abundant) under our belts, we’re ready to tackle Case 2 (i.e., I = ½, <100% abundance)!
This is probably easiest to conceptualize by highlighting a specific example, so for sake of simplicity we look at a basic hydrocarbon: iso-propanol ((CH3)2CH(OH)). The proton NMR will have three signals: (i) –OH; (ii) –CH; and (iii) –CH3 (as the methyl groups are equivalent). This data would be reported as:
1H NMR (60.16 MHz, 10% (v/v), D2O, 8 scans, 4 sec per scan): d 4.75 (broad s, 1H, -OH), 3.96 (septet, 1H, -CH), 1.15 (d, 6H, -CH3) ppm.
In terms of homonuclear coupling this looks exactly like what we’re used to seeing! By this I mean that the spectra can be explained with standard 3JH-H splitting:
1) A broad, uncoupled singlet for the alcohol;
2) A septet for the methine (as the methyl groups are equivalent ‘n + 1’ = ‘6 + 1’ = 7);
3) A large doublet at 1.15 ppm for the methyl groups (as ‘n + 1’ = ‘1 + 1’ = 2).
Okay – pretty standard – when do we learn about carbon-13 induced coupling? Well, look closely at the base of this large doublet. See those equally spaced doublets?!?! Well – there it is! Finally, some evidence of the elusive ‘Case 2’ coupling!
Due to the isotopic ratio of carbon, 98.9% of the time the protons in question will be bonded to a carbon-12, which, of course, is the NMR silent isotope. If this is the case, the nuclear spins will not communicate and no heteronuclear coupling will occur. Subsequently, 98.9% of the time we only observe the homonuclear coupling – that is, we ONLY see the 3JH-H splitting of the methine and that big central doublet. HOWEVER, 1.07% of the time, these methyl groups will ‘see’ a carbon-13 atom (NMR active, I = ½). This will result in that itty-bitty doublet of doublets that surround the central peak. This is due to both heteronuclear and homonuclear coupling (i.e., 1J13C-H = 125 Hz and 3JH-H = 6 Hz).
So, the cliff notes for understanding 13C satellites – for any given proton resonance – 98.9% of the peak area will not be split and 1.1% will be. This is why we call this type of coupling satellites – these heteronuclear multiplicities are small peaks that ‘hover’ around the big peak.
Practically carbon-13 satellites can be difficult to spot. They contain a total of only 1.1% of the area of any given peak and they are split so they contain a maximum of only 1.1%/2 = 0.55% of the intensity of the main peak. I selected the two-methyl group resonance in iPrOH to illustrate this coupling because: (i) the large peak makes these easier to spot; and (ii) the other signals are considerably downfield so the satellites are well resolved. For peaks that are less intense and closer to other signals, you can’t see the carbon-13 satellites (e.g., the methine group). On a low-field NMR, this is, of course, more noticeable due to the inherently lower sensitivityand resolution.
Typical 1J13C-1H coupling constants fall within the 115 – 270 Hz range and are dependent on bond strength, bond angle, and amount of s character in the bond (where s character progresses as sp3 < sp2 < sp). Long-range 13C-1H coupling constants are rarely observed. Although these satellites are rarely reported, they do have implications for (i) extracting structural information, and (ii) qNMR and generating very accurate integrals.
I have taken the time to illustrate 13C coupling. Although less ubiquitous, other heteronuclear couplings can also be observed. For example, tin is an interesting case because it has more than one NMR active isotopes. In fact it has three:
1) 115Sn (I = ½, natural abundance = 0.34%)
2) 117Sn (I = ½, natural abundance = 7.68%)
3) 119Sn (I = ½, natural abundance = 8.59%)
115Sn is typically too low in natural abundance, to be routinely observed, but both 117Sn and 119Sn satellites are frequently observed. These coupling constants are useful in understanding structure and bonding in organotin and/or stannane compounds and both 13C-117/119Sn couplings and 1H-117/119Sn couplings can be exploited. It’s pretty cool!
Interested in learning more about tin couplings and low field NMR?? More to come……
Silverstein, R. M.; Webster, F. X.; Kiemle, D. J.; “Spectrometer Identification of Organic Compounds” 7th Ed. John Wiley & Sons Inc.: USA