Instrumental analysis notes NMR spect. -3Modified PDF

Instrumental analysis notes CHEMSTRY DEPARTMENT GRADE FOUR Prof. Dr. Samy Abuelwafa Dr. Fouz Omar Instrumental analysis notes Lecture 3: NMR Spectroscopy of Chemical Compounds Lectur 3 contents: Introduction to NMR Spectroscopy. The instruments used to analysis based on NMR sp...

Instrumental analysis notes CHEMSTRY DEPARTMENT GRADE FOUR Prof. Dr. Samy Abuelwafa Dr. Fouz Omar Instrumental analysis notes Lecture 3: NMR Spectroscopy of Chemical Compounds Lectur 3 contents: Introduction to NMR Spectroscopy. The instruments used to analysis based on NMR spectroscopy. Application in coordination chemistry and Examples. Questions References and Videos.  Introduction to NMR Spectroscopy NMR spectroscopy is an effective tool used for examining the electronic structure of a molecule and properties of various chemical species. It is based on the absorption of electromagnetic radiation in the radio-frequency (RF) region. The frequencies of NMR transitions are typically in the range of 10→1000 MHz, and nuclei of atoms are detected by this radiation. Concept of Quantum Number  Nuclear magnetic resonance is suitable for studying compounds containing elements with magnetic nuclei, especially hydrogen. Nuclear magnetic resonance (NMR) is the most powerful and widely used spectroscopic method for the determination of molecular structures in solution and pure liquids. There are few elements that contain nucleus possessing powerful magnetic properties, So they can be analyzed by NMR technique such as: 1H, 19F, and 31P. These elements have odd number of protons or neutrons and their nuclear spin quantum number =1/2. Where, magnetic quantum number = +½ or – ½. By applying external magnetic field Βo , the directions of magnetic moments leads to splitting the energy of spin motion into two energy levels calculated from the equation: E=-mµΒo /l E1= –mµΒo / ½ Where: m= + ½ , E= -µΒo E2= -mµΒo / ½ Where: m= - ½ , E= +µΒo ΔE = E1 - E2 = +µΒo – (-µΒo) =+µΒo +µΒo =2µΒo A nucleus of spin I can take up 2I + 1 orientations relative to the direction of an applied magnetic field.  Nuclei with spin quantum number of 0 have no angular momentum and are therefore not observable by NMR ( Examples: 12C, 16O, 32S).  Almost every atom has an isotope that can be studied by NMR.  Nuclei with spin > ½ have poor magnetic properties and are not commonly studied. Examples: 2H, 14N (I=1 integer no.).  The most commonly studies nuclei, especially for biological samples, are: 1H, 13C, 15N, 19F, and 31P.  13C and 15N are stable isotopes, but have low natural abundance ( 13C = 1.1% , 15N = 0.36%)  NMR uses electromagnetic radiation in the radio frequency range Long wavelength, very low energy. Low energy has significant consequences: Sharp signals (Good) Poor sensitivity (Bad) Longer experiment time (Bad)  The sensitivity of NMR depends on several factors, including the abundance of the isotope and the size of its nuclear magnetic moment. For example, 1H, with 99.98 per cent natural abundance and a large magnetic moment, is easier to observe than 13C, which has a smaller magnetic moment and only 1.1 per cent natural abundance. With modern multinuclear NMR techniques it is particularly easy to observe spectra for 1H, 19 F, and 31 P, and useful spectra can also be obtained for many other isotopes. Notice that nuclei with even atomic numbers and even mass numbers (such as 12C and 16O) have zero spin and are invisible in NMR spectum. Nuclear spin characteristics of common nuclei used in NMR Nuclear spin characteristics of common nuclei used in NMR 1-Chemical shift  Chemical shifts in NMR (Nuclear Magnetic Resonance) spectroscopy refer to the phenomenon where the resonant frequency of a nucleus in a magnetic field is influenced by its chemical environment. This effect arises from the shielding or deshielding of the nucleus by the surrounding electron cloud. TMS acts as a standard because it is chemically inert, symmetrical, volatile (bp = 27 C), and soluble in most organic solvents. Most importantly, it gives a single absorption peak and its protons are more shielded than almost all organic protons. ( more shielded means more toward the right of the spectrum and a lower chemical shift).  Tetramethylsilane (TMS) is often used as a reference compound. The Si atom is electropositive with respect to carbon, therefore the electron density on the methyl groups is higher than would be found on the equivalent hydrocarbon. The high electron density shields the methyl carbon and protons, leading to a lower effective field at the nucleus and a lower resonance frequency. EXAMPLE: For CHCl3, there is a peak at 1451 Hz in a H1 NMR spectra from a 400 MHz spectrometer. Convert to δ (ppm) units. Solution δ = ((1451 Hz - 0 Hz)/400,000,000 Hz) x 106 ppm = 3.627 ppm Remember: TMS is our standard and is set to 0 Hz (or 0 ppm). The following discussion is about protons, but applies to any NMR active nuclei. Since all protons have the same magnetic moment it might be expected that all hydrogen atoms will resonate at the same frequency and only give rise to a single, but different protons will give rise to slightly (ppm) different frequencies. The reason is that the effective magnetic field Beff that a nuclei experiences is equal to the very large static magnetic field (B0) plus a much smaller shielding magnetic field which arises from electrons surrounding the nucleus.  A common standard for 1H, 13C, or 29Si spectra is tetramethylsilane Si(CH3) 4 (TMS). When δ < 0 the nucleus is said to be shielded (with a resonance that is referred to as occurring at ‘low frequency’) relative to the standard; δ > 0 corresponds to a nucleus that is deshielded (with a resonance that is said to occur at ‘high frequency’) with respect to the reference. An H atom bound to a closed-shell, low-oxidation-state, d-block element from Groups 6 to 10 (such as [HCo(CO)4]) is generally found to be highly shielded where as an oxoacid (such as H2SO4) is deshielded. The H chemical shift in CH4 is only 0.1 because the H nuclei are in an environment similar to that in the standard, tetramethylsilane, but the H chemical shift in GeH4 is δ = 3.1 2-Area Under the Peaks(Integral Line) However when more than one signal is observed, this integral line will appear and help determine the structure of certain groups. Spin--Spin Splitting 3-The Origin of Spin-Spin Splitting Different numbers of equivalent adjacent nuclei will cause different splitting patterns and the pattern follows Pascal’s triangle Notes: Notes Instrumentation NMR Spectrometer constituents: 1.sample holder Glass tube with 8.5 cm long, 0.3 cm in diameter. 2.Permanent magnet It provides a homogeneous magnetic field at 60-100 MHZ 3.Magnetic coils These coils induce a magnetic field when current flows through them 4.Sweep generator To produce an equal amount of magnetic field pass through the sample 5.Radio frequency transmitter A radio transmitter coil transmitter that produces a short powerful pulse of radio waves 6-Radio frequency receiver A radio receiver coil that detects radio frequencies emitted as nuclei relax to a lower energy level 7-Read out systems A computer that analyses and records the data.  NMR Spectrometer includes a radio station and recording studio. RF pulses at specific frequencies are pulsed at high energy into the sample (Radio station) which sits inside the magnet. Tiny currents are then picked up by the receiver coil, amplified, and digitized into a signal (Recording studio) and collecting to line spectrum.  The coupling of the nuclear spins of different isotopes is called heteronuclear coupling; the GeH4 coupling just discussed is an example. Homonuclear coupling between nuclei of the same isotope is detectable when the nuclei are in chemically inequivalent locations. A multiplet of 2I + 1 lines is obtained when a nucleus of spin 1/2 (or a set of symmetry-related I = 1/2 nuclei) is coupled to a nucleus of spin I. In the spectrum of GeH4 shown in the following Fig., the single central line arises from the four equivalent H nuclei in GeH4 molecule that contain Ge isotopes with I = 0. This central line is surrounded by 10 evenly spaced but less intense lines that arise from a small fraction of GeH4 that contains 73Ge, for which I = 𝟗Τ𝟐; the four 1H nuclei are coupled to the 73Ge nucleus to yield a 10-line multiplet (2 *9Τ2 +1 = 10).  The 1H-NMR spectrum of GeH4. The main resonance is at δ = 3.1 with satellites given by J(1H- 73Ge) spin–spin oupling  The 19F NMR spectrum of ClF3 is shown as above, The signal ascribed to the two axial F nuclei (each with I = 1/2 ) is split into a doublet by the single equatorial 19F nucleus, and the signal from the latter is split into a triplet by the two axial 19F nuclei (19F is in 100 per cent abundance). Thus, the pattern of 19F resonances readily distinguishes this unsymmetrical structure from trigonal-planar and trigonal pyramidal structures, both of which would have equivalent F nuclei and hence a single 19F resonance.  Usually these protons can be exchanged with D2O. To verify that a particular peak is due to O—H or N—H, shake the sample with D2O to exchange the H for a D. If the deuterium is invisible in the proton NMR so the original signal for the OH will disappear. Although most NMR measurements are conducted on diamagnetic compounds, paramagnetic samples are also amenable to analysis and give rise to special effects indicated by a wide chemical shift range and broadened signals. Paramagnetism diminishes the resolution of an NMR spectrum to the extent that coupling is rarely resolved.  The NMR spectra of solids rarely show the high resolution that can be obtained from solution NMR. This difference is mainly due to anisotropic interactions such as dipolar magnetic couplings between nuclei, These effects mean that in the solid state chemically equivalent nuclei might be in different magnetic environments and so have different resonance frequencies. A typical result of these additional couplings is to produce very broad resonances, often more than 10 kHz wide. The broadening of signals can sometimes be so great that signal widths are comparable to the chemical shift range for some nuclei. This broadening is a particular problem for 1H, which typically has a chemical shift range of Δ δ = 10. Broad signals are less of a problem for nuclei such as 195Pt, for which the range in chemical shifts is Δ δ = 16 000, although this large range can be reflected in large anisotropic linewidths. References: 1. Basics of NMR Spectroscopy, Mark Maciejewski,[email protected], Nov 29, 2016 2. Inorganic chemistry P.W. Atkins, T.L. Overton, J.P. Rourke, M.T. Weller, and F.A. Armstrong, Oxford University Press,2014 3. Nuclear Magnetic Resonance Spectroscopy, Teresa Lehmann, USA; tlehmann, 2018 4. Paramagnetic Complexes in Solution: The NMR Approach, Köhler, Frank H. "". , John Wiley & Sons, (2011). 5. Physical Methods in Chemistry (2nd ed.), Drago, Russell S., Philadelphia: W. B. Saunders, (1977). 6. file:///C:/Users/it/Desktop/Instrumental%20%20analytical%20chemistry(4th%20grade)/Spectroscopy/NMR%2 0Spectroscopy/nmr_ref_notes_2011.pdf 7. 5.5: Chemical Shift - Chemistry LibreTexts Videos : 1-NMR Instrumentation & Spectrum https://youtu.be/0Rj7Mg0ZLfM?si=vvrOQeTjamXxXHzo 2-How NMR Spectroscopy worInorganic NMR Spectroscopy (Part-1) | Number of Signals & Multiplicity https://youtu.be/XTE-SG8AjgU?si=308ZDeNFHuEHul2X NMR spectroscopy in easy way - Part 1 https://youtu.be/RqBAW-uFHK0?si=rhgt00wV8-X6g3kI 3-What's Nuclear Magnetic Resonance (NMR)? How Does It Work? What's It Used For? A Brief Introduction. https://youtu.be/Sn3dNMv-67k?si=Jn2QkUBwx8tc_1VK 4-What is Nuclear Magnetic Resonance (NMR)? // HSC Chemistry https://youtu.be/XXoc12uaB6M?si=Nek2RsMSSxn2uekx