Spin spin coupling and coupling constant
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Spin-spin coupling occurs between neighboring NMR-active nuclei and causes splitting of NMR spectra. The splitting pattern is related to the number of equivalent hydrogen atoms near the nuclei. The distance between peaks in a split signal is the coupling constant (J) measured in Hertz. Factors like number of bonds between nuclei, bond angles, and molecular rigidity can affect the coupling constant value. Complex coupling results when a set of hydrogen is coupled to two or more nonequivalent neighbors, producing more complex splitting patterns.
Spin spin coupling and coupling constant
- 1. Spin-Spin Coupling & Coupling Constant Presented by: Guided by: Himal Barakoti Dr. Raja Chakroborty M. Pharm, 1st Sem Associate Professor Department of Pharmacy Department of Pharmacy Assam Down Town University Assam Down Town University
- 2. Contents: Introduction Spin-Spin Coupling Coupling Constant Factors affecting coupling constant Complex coupling References 2
- 3. Introduction The simplest atom, Hydrogen is found in almost all organic compounds that is composed of single proton and single electron. For purpose of NMR, the key aspect of hydrogen nucleus is its angular momentum properties that resemble classical spinning particle. As the spinning hydrogen nucleus is positively charged, it generates magnetic field and possesses a magnetic moment μ, just as a charge moving in a circle. The NMR experiment exploits the magnetic properties of nuclei to provide information on molecular structure. 3
- 4. Spin-Spin Coupling The magnetic interaction between the spins of neighboring, non-equivalent NMR-active nuclei may cause splitting of NMR spectrum which is known as spin-spin coupling. The splitting pattern is related to the number of equivalent H-atom at the nearby nuclei. Example: 1,1,2-tribromoethane 4
- 5. By examining the structure of 1,1,2-tribromoethane, we might expect only two single peaks that correspond to two different types of hydrogen. However, what we see is slightly different. Instead of two singlet peaks, each peak consists of multiple lines. The signals for Hb consists of a doublet that for Ha consists of a triplet. The splitting of the peaks into multiple peaks is called spin-spin coupling which is the direct interaction between the neighboring hydrogen nuclei. The chemical shift of Ha is affected both by it’s own density and also by neighboring hydrogen nuclei. Each one of nuclei can spin either one of two ways: spin up (+1/2) or spin down (-1/2). Since there are two Hb nuclei, there are 4 possible spin combination around Ha atom. (↑↑), (↑↓), (↓↑) or (↓↓) Net magnetic field of Ha hydrogen can be modified by each one of the different combination. Two identical combination sums up to give higher intensity. 5
- 6. Pascal triangle & n+1 rule The Pascal’s triangle is a graphical device used to predict the ratio of heights of lines in a split NMR peak. The (n+1) Rule is used to predict the multiplicity and, in conjunction with pascal triangle, splitting pattern of peaks in 1H NMR spectra, states that if a given nucleus is coupled to n number of nuclei, the multiplicity of the peak is n+1. 6
- 8. Triclosan (antimicrobial agent) How many proton signal do we expect considering only three bond coupling? 8
- 9. Coupling Constant (J) Spin-Spin coupling causes the spectral lines to split and the distance between two adjacent sub-peaks in split signal is given by coupling constant (J) expressed in Hertz (Hz). The distance between two peaks for the resonance of one nucleus split by another is a measure of how strongly the nuclear spins influence each other. For our doublet in 1,1,2-tribromoethane spectrum, two sub peaks are separated by 6.1 Hz, thus we write 3J a-b = 6.1 Hz J value is same regardless of applied magnetic field strength of NMR magnet. This is because the strength of magnetic moment of a neighboring proton, which is the cause of spin-spin phenomena does not depend on the applied field 9
- 10. Factors affecting J value The number of bonds intervening between the coupling nuclei is important, since the coupling is transmitted via the electrons of these bonds. Geminal coupling involving protons on –CH2- groups, is strong, 2J being typically 10-18 Hz, but it will only be observed where the gem protons have different chemical shift positions. Vicinal coupling (three bonds separating the protons) varies from 3J= 0 to 12 Hz in rigid systems, but in freely rotating carbon chains (alkyl groups) it is usually around 8 Hz. Long-range coupling in alkane systems (extending over more than three bonds- i.e., 4J and longer) is usually vanishingly small, but is observed within rigid systems. Angle between atom; Trans coupling in alkene groups (3J, 11-19 Hz) is stronger than cis coupling (J, -14 Hz). 10
- 11. Complex coupling When a set of hydrogen is coupled to two or more sets of nonequivalent neighbors, the result is a phenomenon called complex coupling. Example: Methyl acrylate 11
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- 13. References: Joseph B. Lambert, Eugene P. Mazzola “Nuclear Magnetic Resonance Spectroscopy; An Introduction to principles, Applications and Experimental Methods” pg 22-27 https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/M ap%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)/Chapter_0 5%3A_Structure_Determination_II/5.5%3A_Spin-spin_coupling http://www.pharmatutor.org/pharma-analysis/spin-spin-splitting-and-factors- influencing-coupling-constant 13
- 14. Thank you 14