Nuclear magnetic resonance (NMR) is a critical tool for scientists undertaking structural elucidation or quantification of species in mixtures. Along with single crystal X-ray diffraction (SCXRD), no other technique provides as much information about a molecule’s conformation as NMR. This approach provides information about molecules in solution, while SCXRD gives insight into molecules in the solid state.
Upon acquiring a proton (1H) NMR spectrum, for example, a large amount of information about the molecule of interest is obtained. Namely, the chemical shift of the various resonances provides insight into the chemical environment of this specific resonance. In other words, the chemical shift will be characteristic of what type of proton this is (aromatic, aliphatic, allylic, etc.), and is very helpful in ultimately determining the structure of the overall molecule. Additionally, the integrated area of the signal of interest in NMR is directly proportional to the number of atoms in the molecule giving rise to this signal. As such, the relative number of protons in a molecule, for example, can be determined.
Finally, a phenomenon called spin-spin coupling (or J-coupling) provides crucial information about the groups surrounding the signal of interest. This type of coupling gives rises to two major pieces of information: splitting patterns (multiplicity) and coupling constants. Spin-spin coupling is transmitted through bonds within a molecule and arises from the interaction of two nuclear spins. Splitting patterns in NMR follow the general “2·I·n+1” rule, where I is the spin number of the appropriate nuclide and n is the number of spins coupling to the signal of interest. In 1H NMR (I = ½), a proton on a carbon neighboring a methyl group (n = 3) would be split into 4 (a quartet), with the intensities obeying Pascal’s triangle. Furthermore, if this same proton was on a carbon also neighboring a methylene group (n = 2), the quartet would be further split into 3 (a triplet), ultimately leading to a triplet of quartets. Even simple molecules can have complex splitting patterns. However, these typically prove to be very useful and can provide “NMR fingerprints” of various common functional groups.[1] For previous blog posts about spin-spin coupling, please see the following: Heteronuclear J-Coupling (Part 1 and Part 2), J-Coupling & Tin Can Phones and Heteronuclear Spin-Spin Coupling. This post will focus on a specific example, wherein coupling constants are shown to greatly help with structural elucidation and assignment of resonances within a 1H spectrum.
A very important consequence of signals being split by surrounding nuclear spins are the coupling constants that arise from this splitting, which are denoted as J (in Hz). In a quartet, for example, the distance (in Hz) between peaks will be the same, which helps with the assignment of different signals. Critically, this coupling constant will be the same for both spins involved in the interaction, which means that coupling partners can oftentimes easily be paired after a quick analysis of the NMR spectrum. The origins of J-coupling will be not be covered in this blog post, but rather, a brief analysis of a simple molecule will be used to illustrate how powerful NMR and J-coupling can be for structural elucidation.
Styrene, as the precursor to polystyrene and other copolymers, has been of crucial importance in a wide array of chemical processes for close to 100 years.[2] Issues with biodegradability have led to a shift in focus towards less deleterious polymers, but styrene nonetheless remains an extremely important building block for chemists across various industries. A relatively simple molecule, it consists of benzene derivatized with the simplest vinyl group (-CH=CH2). The structure is shown below in Figure 1, along with a breakdown of the J-coupling within the vinyl fragment, from an NMR point of view.