Proton NuclearMagneticResonanceSpectroscopy Introduction:
The NMR Spectrum serves as a great resource in determining the structure of an organic compound by revealing the hydrogen and carbon skeleton. Historically, NMR was initially used to study the nuclei of Hydrogen atoms; however, any atom with an odd mass or atomic number has a nuclear spin that can be studied by NMR. Without the application of an applied magnetic field, protons are spinning in a randomly oriented manner and are generating a magnetic field (called the magnetic moment) 1 . However, once an external (applied) magnetic field is present the protons either align with (parallel) or against (anti parallel) it. The parallel orientation, called the alpha spin, has a lower energy than the anti parallel (beta) spin. The stronger the applied magnetic field the greater the energy difference (∆E) between the parallel and anti parallel states (Diagram 1) 2 . Therefore, the strength of the magnetic field determines the energy required to cause a nuclear spin flip. The energy difference (∆E) between the ground and excited states is approximately 0.02 cal/mol which correlates to radio wave photons. An NMR signal is created once the radio wave photons supplied match the (∆E) of the nucleus.
In this laboratory exercise we will learn how to use the Chemistry Department's NuclearMagneticResonance (NMR) spectrometer and how to interpret the spectra obtained using this spectrometer. NMR is one of the most powerful techniques available to the organic chemist for molecular structure determination. Therefore, knowing how to obtain and interpret NMR spectra is of critical importance.
NMR techniques are utilized in determining the changes on heat-treated foods. In this technique, change of water distribution and mobility in foods could be determined without damaging to food 9 . Also, it is one of the techniques used to determine the level of melamine in foods 10 . In addition, nuclearmagneticresonancespectroscopy is a good method to determine water holding capacity, intramuscular fat and total water content of meat 5 .
In order to quantitate the pathways by which liver glycogen is repleted, we administered [1- 13C]glucose by gavage into awake 24-h fasted rats and examined the labeling pattern of 13C in hepatic glycogen. Two doses of [1-13C]glucose, 1 and 6 mg/g body wt, were given to examine whether differences in the plasma glucose concentration altered the metabolic pathways via which liver glycogen was replenished. After 1 and 3 h (high-dose group) and after 1 and 2 h (low-dose group), the animals were anesthetized and the liver was quickly freeze-clamped. Liver glycogen was extracted and the purified glycogen hydrolyzed to glucose with amyloglucosidase. The distribution of the 13C-label was subsequently determined by 13C-nuclearmagneticresonancespectroscopy. The percent 13C
Nuclearmagneticresonancespectroscopy (NMR) is the great tool for elucidating the molecular structure of organic compounds and inorganic complexes. This technique gives the information regarding the number of magnetically distinct nuclei under investigation and gives the vital facts about the nature of the surrounding nuclei. The hydrogen and carbon nuclei are the major constituents of inorganic and organic complexes. Proton and carbon-13 NMR are the important tool used to investigate the structures in organic and inorganic complexes (organometallics). The chemical environment around each proton under investigation is different and it is possible to differentiate them. This characteristic feature is due to different chemical environments around each nuclei. The external magnetic field influences the valence electrons and is highly affected and these tend to generate a magnetic field, that too in opposite direction to the applied magnetic field. This characteristic is used for the application of lanthanide complexes as Shift Reagents (SR).
MagneticResonance Imaging (MRI)
MagneticResonance Imaging (MRI) is a diagnostic technique of enormous value to the medical community. MRI takes advantage of the magnetic properties of certain nuclei, typically hydrogen, and of the signals emitted when those nuclei are stimulated by radiofrequency energy. Signals detected by MRI vary with the density of hydrogen atoms and with the nature of their surroundings, allowing identification of different types of tissue and even allowing the visualization of motion.
Many good textbooks describe the theory of NMR spectroscopy in more detail, a selection of which are listed in Further reading at the end of this chapter.
Important NMR-active nuclei
Although all nuclei have at least one isotope that is, in principle, NMR active, most NMR spectra are based on just a few nuclear types. There are several reasons for this. One is that nuclei with I >½ have a property called a nuclear quadrupole moment that, in general, results in short lifetimes in the excited spin states and a rapid return to the low energy state, which gives very broad NMR lines. Secondly, many NMR-responsive nuclei exist at low natural abundances and so are difficult to detect without isotopic enrichment. Thirdly, the strength of the NMR response is related to the size of the nuclearmagnetic moment, for which many nuclei have rather small values and so have low detectability. Finally, some nuclei, once excited to the upper level, are slow to relax back to the ground state, which must occur before another scan can be added. This then incurs a time penalty for acquiring the summed scans necessary to improve detection limits. Sometimes these difficulties of low sensitivity, low natural abundance and long relaxation times come together.
Nuclei that have a nuclear spin such as 1 H, 13 C, 19 F, 31 P are considered as spinning charge and they create a magnetic moment while spinning, so these nuclei can be thought of as tiny magnets.
If we place these nuclei in a magnetic field, they can line up with or against the field by spinning clockwise or counter clockwise.
The vector sum of the magnetic moments of one type of nuclei is a measure of the number of protons of that type in the molecule.
The determination of the magnetic moment for protons in each type of electronic environment forms the essence of the NMR experiment.
• In a magnetic field, there are now two energy states for a proton: a
lower energy state with the nucleus aligned in the same direction as B 0 , and a higher energy state in which the nucleus aligned against B 0 .
• When an external energy source (h ) that matches the energy difference (E) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another.
a b s t r a c t
Cement is the ubiquitous material upon which modern civilisation is built, providing long-term strength, impermeability and durability for housing and infrastructure. The fundamental chemical interactions which control the structure and performance of cements have been the subject of intense research for decades, but the complex, crystallographically disordered nature of the key phases which form in hardened cements has raised difﬁculty in obtaining detailed information about local structure, reaction mechanisms and kinetics. Solid-state nuclearmagneticresonance (SS NMR) spectroscopy can resolve key atomic structural details within these materials and has emerged as a crucial tool in characterising cement structure and properties. This review provides a comprehensive overview of the application of multinuclear SS NMR spectroscopy to understand compositionestructureeproperty relationships in cements. This includes anhydrous and hydrated phases in Portland cement, calcium aluminate cements, calcium sulfoaluminate cements, magnesia-based cements, alkali-activated and geopolymer cements and synthetic model systems. Advanced and multidimensional experiments probe 1 H, 13 C, 17 O, 19 F, 23 Na, 25 Mg, 27 Al, 29 Si, 31 P, 33 S, 35 Cl, 39 K and 43 Ca nuclei, to study atomic structure, phase evolution, nanostructural development, reaction mechanisms and kinetics. Thus, the mechanisms controlling the physical properties of cements can now be resolved and understood at an unprecedented and essential level of detail.
Peptides present attractive, simplified models for the study of protein folding.
Short peptides however typically exhibit a large amount of conformational flexibility.
Unlike in full-length proteins, it is by consequence common that secondary structure elements are not well-defined in peptides. The use of non aqueous solvents is a popular way to induce secondary structure formation, albeit at the expense of being further removed from physiological conditions. Peptide sequences can also be designed to give rise to intrinsically higher structural stability by exploiting specific interactions. A set of peptides that adopt a remarkably stable -hairpin secondary structure in water has been introduced by Cochran et al. 53 These tryptophan zipper peptides are mutants of the B1 domain of Protein G. They contain several tryptophan residues that seem to confer stability through side chain – side chain interaction. Since their introduction, the stability and folding of trpzip peptides has been studied in various ways, including by circular dichroism (CD) and infrared spectroscopy. 53,132-134
Distribution of Particles between magnetic quantum states
In absence of magnetic field, E of 2 states are identical so # of nuclei in 2 states are equal
In magnetic field the nuclei want to be oriented with the magnetic field so they are in their lowest E state. When we were in UV we used the Boltzmann distribution to calculate the # of atoms in ground and excited states, and found that the number in the excited state was incredibly small. Even in the IR we found that the number in the excited state was usually < 1% of the molecules. What about the NMR, where we are using very low frequencies and Energies
The _E is the energy difference between the _ and _ spin states. This depends on the applied magnetic field. As shown by the graph above, the greater the strength of the applied magnetic field, the larger the energy difference between the two spin states. When radiation, that has the same energy as the _E, is placed upon the sample, the spin flips from _ to _ spin states. Then, the nuclei undergoes relaxation. Relaxation is when the nuclei return to their original state. In this process, they emit electromagnetic signals whose frequencies depend on _E as well. The H- NMR spectrometer reads these signals and plots them on a graph of signal frequency versus intensity. Resonance is when the nuclei flip back and forth between _ and _ spin states due to the radiation that is placed on them. To summarize, an NMR signal is observed when the radiation supplied matches the _E. And, the energy required to cause spin flip is dependent on the magnetic environment experienced by the nucleus.
through 7 likely represent a distribution of a variety of cyclic chlorophosphazenes containing different number of phosphorus atoms in the ring. Noteworthy is resonance 5 at 10.7 ppm, which can be also observed in the spectrum of solvent-cast PBFP. At this point, definite assignment of this peak remains elusive; however, based on the notion that increased size of the phosphazene ring causes the negative shift of the corresponding frequency, it can be suggested that peak 5 represents the hexamer species. Similarly, signals 7, 6 and 4 would correspond to the four-, five- and seven-member cyclic phosphazenes, respectively. Subsequently, this working model allows for the assignment of peak 8 (δ 18) to the hexachlorocyclotriphosphazene, which is consistent with its chemical shift determined previously. Origins of the intense narrow signal 9 at 21.2 ppm and the narrow line overlapped with a broad shoulder centered at 23.7 ppm, are yet to be determined. At this stage, their assignment is not supported by chemical shifts of the known signals, nor can their high relative intensities be explained. In congruence with the current understanding of the behavior of cyclic phosphazenes upon heteroatom substitution, it can be therefore hypothesized that signals 9 and 10 correspond to the partially trifluoroethoxy-substituted chlorocyclophosphazenes.