Introduction:
The origins and interpretation of 13C nuclear magnetic resonance spectra are identical to proton magnetic resonance spectra. Though, they are generally simpler and thus simpler to interpret. This is because of the two major factors: (1) The low natural abundance of the isotope and (2) The mode of operation of the spectrometer. Both factors simplify the spectrum via preventing the splitting of signals through spin-spin coupling between nuclei.
The natural abundance of 13C isotopes is 1.1%, which is; roughly one carbon atom in each and every hundred is a 13C isotope. This signifies that it is highly improbable that adjacent carbon atoms in a molecule will be 13C isotopes. As 13C nuclei don't absorb radio frequency radiation whenever put in a strong magnetic field, most of the carbon atoms in a molecule will be incapable of spin-spin coupling with any 13C nuclei present. Though, there will be enough 13C isotopes in all the molecules found in the sample to get an average absorption spectrum of all the molecules in that sample. This average spectrum is in effect that of a theoretically not possible molecule whose carbon atoms are all non-coupled to the 13C isotopes.
13C-NMR Spectra:
13C resonance takes place at a frequency of ~ 25.1 MHz whenever proton resonance is occurring at ~ 100 MHz. Therefore it is at lower energy than proton resonance and the spread of resonances for 13C is over ca 180 ppm; therefore there is less likelihood of overlapping lines in 13C NMR. The figure below illustrates the chemical shifts of some 13C signals. This is only an estimated guide.
Fig: Typical chemical shifts of 13C Atoms
A 13C atom will couple to any protons linked to it example a carbon having one proton attached will appear as a double. Though to get the most information from the weak carbon spectrum, it is better if this coupling is eliminated.
Carbon-13 spectra cover a much broader range of chemical shifts as compare to proton spectra; however the positions of resonances are usually determined via the similar factors. Though, for simplicity of interpretation, they are often recorded as decoupled spectra to remove the effects of coupling to adjacent protons that would otherwise split the carbon-13 resonances according to the n + 1 rule and Pascal's triangle. Decoupled spectra comprises of a single peak for each chemically dissimilar carbon in the molecule and spectra interpretation is confined to the correlation of their chemical shifts with structure, augmented through reference to chemical shift data and the spectra of known compounds. Proton coupling can be observed under suitable experimental conditions.
Interpretation of 13C-NMR Spectra:
The given is a general approach:
1) Note the presence or absence of saturated structures; generally provide resonances between 0 and 90 ppm.
2) Note the presence or absence of unsaturated structures in the region between around 100 and 160 ppm. (The alkyne carbons are exceptions appearing between 70 and 100 ppm).
3) Note any low field resonances (160 to 220 ppm) that are related by carbonyl and ether carbons. Carboxylic acids, anhydrides, esters, amides, acylhalides and ethers are all found in the range 160 to 180 ppm, whilst aldehydes and ketones lie between 180 and around 220 ppm.
Examples of 13C-NMR Spectra Interpretation:
Fig: 13C Spectrum of Alanine
The three carbons exhibit a very broad range of chemical shifts. The lowest field resonance corresponds to the carbonyl carbon that is highly deshielded by the double-bonded oxygen. The nitrogen deshields the CH carbon much less, and the CH3 carbon is the least deshielded of the three.
Tutorsglobe: A way to secure high grade in your curriculum (Online Tutoring)
Expand your confidence, grow study skills and improve your grades.
Since 2009, Tutorsglobe has proactively helped millions of students to get better grades in school, college or university and score well in competitive tests with live, one-on-one online tutoring.
Using an advanced developed tutoring system providing little or no wait time, the students are connected on-demand with a tutor at www.tutorsglobe.com. Students work one-on-one, in real-time with a tutor, communicating and studying using a virtual whiteboard technology. Scientific and mathematical notation, symbols, geometric figures, graphing and freehand drawing can be rendered quickly and easily in the advanced whiteboard.
Free to know our price and packages for online Chemistry tutoring. Chat with us or submit request at [email protected]
History and Principles of Chemotherapy tutorial all along with the key concepts of History of Chemotherapy, Principles of chemotherapy, Process of Cell Division, Principles of Chemotherapy
tutorsglobe.com description of a flower assignment help-homework help by online flower-a metamorphosed shoot tutors
tutorsglobe.com optometry assignment help-homework help by online errors of refraction tutors
Charcoal Analysis and Percentage of Water in a Hydrate tutorial all along with the key concepts of Moisture Content, Volatile Combustible Matter, Ash and Fixed Carbon, Percentage of water in Hydrate and Theory of percentage of water in hydrate
Theory and lecture notes of Fiscal Policy-Automatic Stabilizers all along with the key concepts of Rules vs. Authorities, Competence and Objectives, Political Business Cycle , Central Bank Independence. Tutorsglobe offers homework help, assignment help and tutor’s assistance on Fiscal Policy-Automatic Stabilizers.
Synthetic Dyes-Fibres tutorial all along with the key concepts of Advantages and Disadvantages of Synthetic Fibre, properties of synthetic substances, tensile strengths
tutorsglobe.com meiosis-i assignment help-homework help by online meiosis tutors
tutorsglobe.com bioinformatics assignment help-homework help by online modern genetics tutors
The advantage of diagrammatic presentation of cost data is - Diagrams and charts create the costing data simple and simply intelligible. One gets a clear picture in short time of anything is being presented by them.
Directors’ share option schemes have been a popular method of rewarding the directors of large listed companies and several arguments have been put forward to support their use.
Homeostasis tutorial all along with the key concepts of Principles of Homeostasis in Animals, Regulating body temperature, Regulating blood glucose, Organs used for Homeostasis, Homeostatic Control System and Homeostatic imbalance
Transfer of Heat and Heat Capacities tutorial all along with the key concepts of Conduction of Heat, Convection of Heat through Liquids and Gases, Radiation of Heat, Heat capacity, Specific Heat Capacity, Heat capacity of a substance and Newton's Law of Cooling
For transformer oil the specifications are completely covered by IS: 335-1993. For instructions for keeping the insulating oil IS Code of Practice No. 1866 may be considered.
Crystal Diffraction-I tutorial all along with the key concepts of Bragg formulation of diffraction by a crystal, Statement of Bragg's law, Von Laue formulation of diffraction by a crystal, Diffraction of crystals by electrons and neutrons
tutorsglobe.com hypersensitivity-cytotoxic reactions assignment help-homework help by online classification of hypersensitivity reactions tutors
1963178
Questions Asked
3689
Tutors
1455028
Questions Answered
Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!