NMR spectroscopy is a powerful analytical technique used by chemists to determine the molecular structure of organic compounds. It works by probing the magnetic properties of atomic nuclei, most commonly hydrogen nuclei (protons), within a molecule. The resulting NMR spectrum contains peaks that correspond to the different chemical environments of the nuclei. A feature called “integration” is automatically calculated and displayed, providing quantitative information. Integration allows for the direct measurement of the relative number of nuclei responsible for each signal.
Understanding the NMR Spectrum
The horizontal axis of an NMR spectrum is measured in parts per million (ppm) and represents the chemical shift. This value indicates the exact position of a signal and is directly related to the electronic environment surrounding the nucleus. Nuclei in different chemical environments, such as those near an electronegative atom, resonate at distinct positions on the spectrum.
The presence of multiple peaks signifies that the molecule contains several groups of chemically non-equivalent nuclei. For instance, protons on a methyl group are often in a different environment than protons on an adjacent methylene group. Each peak acts as a marker for a specific set of equivalent nuclei within the molecule. While chemical shift information helps identify functional groups, it does not specify the quantity of nuclei in that group.
The Calculation of Integration
Integration is a mathematical process that determines the total area under the peak signal. This calculation is performed automatically by the spectrometer software using numerical methods. The resulting value is based on the signal’s total intensity across its width, not the height of the signal.
The integral is typically represented visually as a step-like curve superimposed over the signals. The vertical rise, or height, of each step is directly proportional to the calculated area beneath the corresponding peak. This graphical representation allows a chemist to visually compare the relative areas of different signals. The software displays a numerical value next to the step, which is the calculated integral value for that specific signal.
The process requires the user to define the boundaries, or integral region, for each signal to ensure accurate measurement. The accuracy of the integration is typically within a few percentage points, which is sufficient for structural analysis. Since the total area includes the entire shape of the signal, it correctly accounts for signals split into multiple smaller peaks due to spin-spin coupling.
What the Integrated Ratio Represents
The fundamental significance of the integrated area is its direct proportionality to the number of equivalent protons that generated the signal. A larger area means a greater number of nuclei are resonating at that particular chemical shift. This relationship allows integration to function as a powerful quantitative tool.
Integration provides a relative ratio of the different types of protons, not the absolute count of protons in the molecule. For example, a 2:3 integral ratio indicates that the first signal is caused by two-thirds the number of protons as the second signal. These numbers could represent 2 and 3 protons, 4 and 6 protons, or any other whole-number multiple.
Chemists use the smallest integral value as a reference point, dividing all other integral values by it to find the simplest whole-number ratio. This simplified ratio reveals the relative count of chemically equivalent atoms in the molecule. For instance, a 1:2:3 ratio indicates one proton of one type, two of a second type, and three of a third. This ratio is then scaled up using the known molecular formula to determine the exact number of protons for each signal.
Using Integration for Structure Determination
Integration data is used in conjunction with chemical shift information to build a complete picture of the molecular structure. The chemical shift tells a chemist what kind of proton environment exists, such as an aromatic proton or a proton next to an oxygen atom. The integral ratio then specifies how many protons are in each of those identified environments.
For example, the ethyl group is composed of a methyl (CH\(_3\)) and a methylene (CH\(_2\)) group, which are two distinct proton environments. The methyl group has three equivalent protons, and the methylene group has two, meaning the integral ratio for those two signals must be 3:2. Identifying this specific ratio suggests the presence of an ethyl group in the molecule.
Chemists combine this relative count with other spectral information, such as signal splitting patterns, to confirm the compound’s structure. If a signal has a chemical shift consistent with a methyl group and an integral value corresponding to three protons, the data supports the presence of a CH\(_3\) unit. By assigning a specific number of protons to every signal, the molecular structure can be deduced.