Nuggets of Knowledge for Chapter 5 – Nuclear Magnetic Resonance Spectroscopy (NMR)Chem 2310I. Introduction to Spectroscopy• What NMR does:o NMR spectroscopy exposes an organic compound to radio waves in the presence of a strong magnetic field. The radio waves cause transitions in the energy levels of the nuclei of the atoms. o The process by which this gives us a spectrum is quite complex; in this class we will focus on interpretation of spectra rather than understanding how the instrument works.• What information NMR gives us: o Because the change takes place in the nucleus, the spectrum gives us information about the atoms in the compound. o This information can then be used to deduce the structure of the compound. • Types of atoms that can be used:o Only atoms with odd (rather than even) mass numbers can be detected by NMR. o The most common atoms used are 1H, or proton NMR, and 13C NMR, since these atoms are the most common in organic compounds. o Of these two, proton NMR is by far the most common, and when no atom is specified, it is assumed to be proton NMR.• What a spectrum looks like:o The x-axis is written at the bottom of the spectrum, from right to left. In proton NMR, the range is usually 0-10 ppm (but is sometimes expanded to 0-13 ppm). In 13C NMR, the range is much larger, from 0-250 ppm.o Each signal on the NMR spectrum is called a peak. A peak may be a single spike, or a cluster of spikes. Each peak has three characteristics:  Where it falls on the x-axis, which is called the chemical shift. This tells us something about what other atoms are nearby. The area covered by the peak, which is called the integration. This tells us how many atoms are creating this peak. The number of spikes in the peak, which is called the splitting. This tells us how many hydrogen atoms are next to the ones creating the peak.o All three pieces of information for each peak must be used together. There are three levels of expertise that you will be expected to master. Initially, you will learn to assign hydrogens in a known compound to peaks in a spectrum. Then, you will learn to sketch the spectrum of a known compound by using its structure. Finally, you will learn to deduce the structure of an unknown compound from its spectrum. II. Distinguishing Equivalent Hydrogens• When considering the structure of a compound (either to assign its hydrogen atoms to the peaks on a spectrum, or to sketch the spectrum), the first thing to consider is which of the hydrogen atoms are equivalent.• When two (or more) hydrogen atoms are in the same environment – that is, they have the same nearby atoms and the same neighboring hydrogen atoms – they are said to be equivalent. o Equivalent hydrogen atoms generate identical NMR peaks which exactly overlap, giving just one peak with more area. The additional area is proportional to how many equivalent hydrogen atoms there are.• Rules of thumb:o Hydrogen atoms on the same carbon are nearly always equivalent (exceptions include hydrogen atoms on carbon-carbon double bonds).o Hydrogen atoms on different carbon atoms are usually not equivalent (exceptions occur when there are repeated elements in a compound).• More exact rule:o If two hydrogens could each be separately replaced by another atom (say, a chlorine), and the same new compound results, then they are equivalent.III. Chemical Shift• The “0” on the chemical shift scale is set by a reference compound, tetramethyl silane (usually referred to as TMS). Chemical shift refers to how far a peak is “shifted” from this value.• Toward 0 is defined as upfield, while away from 0 is define as downfield.• There are several factors which allow you to predict the relative positions of two peaks:o Number of hydrogen atoms on the same carbon : The more hydrogen atoms there are on the same carbon, the farther upfield the peak will be.  This is a relatively small effect, but can differentiate between a CH3, CH2, or CH that are otherwise similar.o Nearby electronegative atoms : If an electronegative atom such as nitrogen, oxygen, or a halogen is near the hydrogen atom, this will cause the peak to be shifted downfield.  This effect is quite strong next to the electronegative atom, but falls off quickly the farther away the hydrogen atom is. The more electronegative the atom, the further down the peak will be. Also, two electronegative atoms will shift the peak farther than one electronegative atom. This is most useful for identifying hydrogens next to alcohols, amines, and alkyl halides, but can also be helpful for hydrogens next to ketones, aldehydes, esters, amides, and carboxylic acids.o Hybridization of the atom the hydrogen is attached to : Hydrogen atoms attached to sp2 atoms are furthest downfield; hydrogens attached to sp3 atoms are furthest upfield. Hydrogen atoms attached to sp atoms are in the middle. This is most useful for identifying hydrogens attached to carbon-carbon double bonds.o Aromaticity : Hydrogens attached to aromatic rings are significantly further downfield than hydrogens attached to nonaromatic rings or chains. This is a huge effect, and is very useful for identifying aromatic compounds.o Hydrogen bonding : The degree of hydrogen bonding causes the hydrogen atoms involved to shift. This will be affected by the concentration of the sample as well as the identity of the compound, causing these hydrogen atoms to appear in quite a large range. This makes alcohol, amine, and amide hydrogens difficult to identify, as they can be in different places in different spectra of the same compound. This can also affect the width of the peak, sometime making these peaks wider than usual.• When two sets of nonequivalent hydrogen atoms have very similar chemical shifts, their peaks may overlap. When it is not possible to clearly distinguish one peak from another, they are considered together, even though they aren’t actually equivalent.o This most often occurs when several CH2 groups are next to each other, but not close to any electronegative atoms, aromatic rings, sp2 hybridized carbons, etc.• The following is a list of ranges where hydrogen atoms often appear in relationship to other atoms and bonds. These ranges are guidelines, not hard and fast rules.range description0.5-2.0 ppmH’s unaffected by other factors1.0-5.0 ppm H's on alcohols and amines (sometimes broad)2.0-2.5 ppm H's next to