Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful and versatile techniques available to determine the arrangement of atoms in the structure of a molecule. Organic molecules composed principally of hydrogen and carbon atoms can be analyzed using techniques of proton (1H) and carbon-13 (13C) NMR spectroscopy. The basic interpretation of the NMR spectra for a molecule observes several properties including the strength and intensity of the magnetic field used to produce a peak, the interaction of the atom with atoms around it and the effects of neighboring atoms on spectra themselves. Analysis of the relative location of a peak in an NMR spectrum (called the chemical shift) and the splitting of that peak can provide significant information about the structure of a molecule. More detailed information about molecular structure can be determined through 2-dimensional techniques where the NMR pulses producing a magnetic field are combined and can show if two atomic nuclei on a molecule are directly coupled or separated by multiple atoms in the molecule.
The foundation of NMR spectroscopy, or nuclear spin, was first postulated by Wolfgang Pauli (Pauli, 1940) who predicted that certain atoms should have both spin and a magnetic moment and that a magnetic field should be able to split the energy levels of those atoms. Atoms have an atomic number and a mass number which can be used to calculate a value for their nuclear spin. Elements and their isotopes that have a spin with a non-zero value for example 1H and 13C can absorb energy in a magnetic field to transition from one spin state to another. Significant advances in the application of precise magnetic fields and the development of equipment for pulsing and interpreting those fields allows the capture of detailed spectra for molecules that contain atoms of carbon and hydrogen. A basic understanding of NMR magnetic field strength, field intensity, splitting and coupling allows the interpretation and assignment of structure to most organic compounds. More advanced techniques combining fields and even coupling different nuclei allow for sophisticated interpretation of molecules.
Proton (1H) NMR Spectra
The most common NMR spectra used for characterization of organic compounds is by far the 1H NMR Spectrum. For a given hydrocarbon, if all of the protons in the structure were in an identical environment, for example a molecule such as methane (CH4), the resulting 1H NMR spectrum obtained for the molecule in a pulsed magnetic field would be a single peak where every single proton absorbed the same energy. Most molecules are neither as symmetrical nor as simple in structure as methane and therefore most protons exist in dramatically different environments from each other despite fundamentally similar atomic structure. A sample of an organic molecule such as 1-nitropropane (CH3CH2CH2NO2) can be placed in the coils of a NMR spectrometer and spun while subjected to a pulse of a magnetic field. As the sample absorbs energy and relaxes the detector records and interprets changes in the magnetic field which is then plotted as a spectrum shown in figure 1 below:
Figure 1: 1H NMR Spectrum of 1-nitropropane from Roberts, Gilbert, Rodewald and Wingrove (1985)
Figure 1 shows the 1H NMR Spectrum for 1-nitropropane. The first important observation is the x-axis in the plot which is the strength of the magnetic field. The convention for field strength is termed downfield to the left where energy is lower and upfield to the right where energy is greater (Carey, Giuliano 2008). Since all protons have identical mass and identical spin, the requirements of more or less energy to produce a transition in a magnetic field are caused by the immediate environment where the protons sit. It is helpful to imagine an example such as a breeze blowing through a tree branch with leaves that are all the same size and weight. Leaves protected by a large neighbor such as a bird nest would require a stronger breeze to vibrate while those leaves sitting alone at the tip of a branch would require a weaker breeze to shake and vibrate. Although the physical forces for 1H are NMR dramatically different than the breeze blowing through a tree, the concept for why peaks appear at different points in the spectra can be understood from this analogy. Protons that experience shielding of their magnetic dipole by neighbors with larger magnetic fields as the leaves are shielded by the presence of a bird nest require more energy in the strength of the magnetic field and they appear upfield in the spectrum. The spectrum of 1-nitropropane shows a total of four general peaks or groups of peaks in the field. The unit used to designate downfield or upfield on the x-axis is part per million (ppm) which is not the unit for concentration but a unit that shows relative deviation from a reference peak used in NMR spectroscopy. The conventional reference standard is tetramethylsilane (TMS) which has been arbitrarily assigned a unit of zero because of the intense shielding the protons are subjected to in the molecule (Spectroscopy reference). In the 1-nitropropane spectrum the strong single peak at ? 0 at the far upfield (right) is TMS. The three remaining groups of peaks at ? =1.00, ? = 2.05, and ? = 4.38 are the three sets of protons on the molecule that are shifted downfield from the highly shielded TMS. The shift of the signal is typically related to the electron density and electron circulation near the proton as the rotating electrons generate a tiny magnetic field. In Figure 1, the spectrum for 1-nitropropane correlates with the number of different types of protons present in the spectrum; the assignment of the peaks to the carbon atoms can first be generally made by estimating which protons are more affected by induced magnetic fields and would be shielded from the magnetic field in the spectrometer. Substituents connected to a carbon atom with a proton have a strong effect on the chemical shift. A nitrate acts as an electron withdrawing group pulling electron density away from the sigma character of a carbon-hydrogen bond. The withdrawal of electron density results in a deshielding effect on the protons moving the signal for like protons on the carbon atom downfield from TMS. A preliminary assignment of the peaks present in Figure 1 places the methyl group connected to the nitro functional group downfield near 4 ppm, the bridging methylene near 2 ppm and the terminal methyl group closer to TMS at 1 ppm. Tables exist for functional group assignment and their effects on chemical shift for most common functional groups (Silverstein, Bassler, Morrill 1974)Assignment of structure using chemical shift provides a rough means of determination but is limited when more complicated molecules are studied.
Signal Splitting and Spin-Spin Coupling
The three peaks of 1-nitropropane have been roughly assigned on the basis of their chemical shift. The peaks contain further detail with splitting occurring in each group of peaks. These peaks are caused by a phenomenon called spin-spin coupling. Coupling arises from protons on adjacent sigma (single) bonded atoms. Signal splitting does not occur for protons that are in the same chemical environment with the same chemical shift (Solomons 1988). Signal splitting arises from protons that are coupled and do not have the same chemical shift. The phenomenon arises because the protons on adjacent atoms affect the magnetic field either aligning with the field or against it. This subtle shift in alignment and affect on the magnetic field causes the appearance of a smaller peak upfield and a second peak downfield. A single neighboring proton will split a signal into two peaks of roughly equal intensity. A pair of neighboring protons can have the following effects: both aligned with the magnetic field, one with and one against, both aligned against the magnetic field. The resulting splitting pattern for a signal with two adjacent equivalent protons is a set of three peaks with a ratio of 1:2:1. Continued analysis of three or more adjacent equivalent protons produces a predictable splitting pattern of n protons produces n + 1 peaks. Review of figure 1 predicts that the splitting pattern should result in a triplet for the methylene directly bound to the nitro group split by two non-adjacent protons on the bridging methylene. This splitting pattern should result in a 1:2:1 ratio of peak intensity. The terminal methyl group with the least shielding should also be split into a 1:2:1 triplet. The bridging methylene is slightly more complicated having five adjacent protons resulting in a sextet. Splitting patterns can become complex, however the predictive power of chemical shift and splitting can quickly resolve many spectra. The 1H NMR spectrum for ethyl chloroacetate consists of a singlet and quartet centered at approximately 4 ppm and a triplet at approximately 1.3 ppm. The formula ClCH2CO-OCH2CH3 allows quick resolution with the triplet quartet of the ethyl group and the singlet of the chloro-keto-methylene unsplit by any adjacent protons.
Carbon 13 (13C NMR)
Nearly all naturally occurring carbon is the isotope carbon-12…