Proton Nuclear Magnetic Resonance Spectroscopy ('H-NMR) PDF

# III) Proton Nuclear Magnetic Resonance Spectroscopy ('H-NMR) NMR or Nuclear Magnetic Resonance spectroscopy is a technique used to determine a compound's unique structure. It identifies the carbon-hydrogen framework of an organic compound. Using this method and other instrumental methods includin...

# III) Proton Nuclear Magnetic Resonance Spectroscopy ('H-NMR) NMR or Nuclear Magnetic Resonance spectroscopy is a technique used to determine a compound's unique structure. It identifies the carbon-hydrogen framework of an organic compound. Using this method and other instrumental methods including infrared and mass spectrometry, scientists are able to determine the entire structure of a molecule. It gives information about the number and type of hydrogen atoms in the molecule. ## Magnetic and nonmagnetic nuclei: | Mass number | Atomic number | Magnetic/Nonmagnetic | Examples | |---|---|---|---| | Odd | Odd | Magnetic nucleus, H, 19F | | | Odd | Even | resulting in nuclear magnetic moment (μ), (has spin), | 13C | | Even | Odd | | 14N | | Even | Even | Nonmagnetic, has no spin, I = 0 | 12C, 16O | For the nucleus of the hydrogen atom (the proton 'H), is a spinning charged particle, it generates a magnetic field and has magnetic moment (u) along the axis of rotation. It has two spin states both have the same energy and are completely random in orientation. But, when an external magnetic field (Ho) is present, the nuclei align themselves either with (parallel) of lower energy state or against (antiparallel) of higher energy state to the field of the external magnet. ### a-spin state: Protons that align with the external magnetic field. They are in a lower energy state. ### ẞ-spin state: Protons that align against the external magnetic field. They are in a higher energy state. The AE is the energy difference between the a and ẞ spin states. When radiation, that has the same energy as the AE, is placed upon the sample, the spin flips from a to ẞ 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 AE as well. The 'HNMR spectrometer reads these signals and plots them on a graph of signal frequency versus intensity. The separation between the two nuclear energy levels (states) is given by the equation: $ΔE = hv = \frac{μHo}{I}$ Where: * µ = nuclear magnetic moment, * Ho = the applied magnetic field, * I = nuclear spin number, * hv = radio frequency region of electromagnetic radiation (emr). ## Structural information that the NMR spectrum tells us ### I. Number of Signals Each group of chemically equivalent protons gives rise to a signal. Chemically Equivalent Protons are protons that are in the same environment, and they must be identical in every way. Signals represent the No. of proton types not number of protons. For example, How many signals will the H'NMR spectrum produce for this molecule? There are three signals because there are three sets of equivalent protons. ### II. Position of Signals The positions of the signals in an NMR spectrum are based on how far they are from the signal of the reference compound. This information tells us the kind of proton or protons that are responsible for the signal. ### Reference compound: Tetramethylsilane (TMS) is usually used as the reference compound because: 1) It is inert, volatile and soluble in most organic solvents. 2) TMS gives intense single peak because it contains 12 equivalent protons. 3) It is at a lower frequency (highly shielded) than most other signals because its methyl protons are in a more electron dense environment than most protons are because silicon is less electronegative than carbon (which is a significant component of organic molecules). The position of the signals depends on the chemical shift. ### Chemical Shift: It is a measure of how far the signal produced from the proton is from the reference compound signal, and it usually measured using the δ (delta) scale. The TMS or reference compound is at the zero position on the right of the spectrum, and as it moves toward the left, the ppm values become larger. "ppm" stands for parts per million, and it is the unit used to measure chemical shifts. Proton chemical shifts range from 0 ppm to 15 ppm. The chemical shift of a particular proton depends on: 1. **Inductive effect of nearby atoms:** When the nuclei of protons in the molecule are found within a cloud of electrons that shields the protons from the applied magnetic field. So, the proton peaks are found in Upfield (shielded) Zone, i.e. farther to the right hand side of the spectrum. Protons in electron poor environments are less shielded due to fewer electrons, and therefore, they are located in Downfield (deshielded) zone i.e. farther to the left side of the spectrum. **How does electronegativity affect chemical shift?** The electron cloud shields the nucleus from the applied magnetic field, andelectronegativity is defined as the tendency of an atom to pull electrons toward itself. Therefore, electronegative atoms remove electron density from the proton. This causes the proton to have less electron density, and this leads to less shielding. Protons that are closer to the electronegative atom are in a less electron dense environment, which means that they're chemical shifts will be larger. 2. **Anisotropic effect:** Magnetic anisotropy is the magnetic field created by pi (π) electrons or rings. This describes an environment where different magnetic fields are found at different points in space. π electrons are held less strongly than sigma (σ) electrons, so π electrons are more able to move in response to the magnetic field. How this affects the chemical shift depends on the direction of the induced magnetic field relative to the direction of the applied magnetic field. In π electrons found in the benzene ring and an alkene, the magnetic field induced is in the same direction as the applied magnetic field, so the protons feel a stronger effective magnetic field. Therefore, the protons undergo resonance at a higher frequency due to the π electrons. If the magnetic fieldinduced is opposite direction as the applied magnetic field, the protons will feel a smaller effective magnetic field. **Example:** Put the chemical shifts of the nonequivalent protons from the lowest frequency to the highest frequency. CH3CH2CH2NO2 **Answer:** CH3 has the lowest chemical shift because it is the furthest away from the nitro group, and the CH2 in between the CH3 and CH2has the middle chemical shift, and the CH2 attached to the NO2 has the largest chemical shift because it is the closest to the nitro group. 3. **Hydrogen bonding:** Hydrogen bonding causes deshielding of protons due to lowering of the electron density around the proton. (X-H--------O=CH-). **Proton Chemical shift ranges in PPM:** * a) C-Haliphatic: 1-4 ppm * b) N-H or O-H :1-5 [ D₂O exchangeable] * c) C- H aromatic: 6.5-9 ppm * d) H aldehydic: 9-10 ppm * e) H (COOH) : 10-12 [D₂O exchangeable ] ### III-Integration or intensity of NMR Signals Integration is the area measurement that tells us the relative number of protons that give rise to each signal. The numbers do not always correspond to the exact or absolute number of protons. Instead, it tells us the relative number or ratio of the amount of equivalent protons. **Problem:** Explain how this molecule produces this H'NMR spectra using number of signals, position of the signals, and the integration of the signals. The molecule in the image produces a spectrum with two signals. The first signal is at 7.0 ppm and integrates to 1.6. The second signal is at 1.6 ppm and integrates to 3. The relative number of protons in each group is 2:1. ### IV-Splitting of Signals Splitting of signals is caused by (and therefore tells us the number of) protons bonded to adjacent carbons. **N+1 Rule:** N is the number of equivalent protons that are bonded to adjacent carbons. So, the number of splitting that occurs is one more than the number of equivalent protons bonded to the adjacent carbons. Using the N+1 rule, the signal for a proton with N neighbors is split into N+1 lines. If aproton has no neighbors, it is a singlet(s). If it has one neighbor, it is a doublet (d). If it has two neighbors, it is a triplet (t). If it has three neighbors, it is a quartet (q). If it has four neighbors, it is a pentet. **The rules for proton-proton spin-spin coupling** 1. **Only non equivalent protons couple.** Equivalent protons are in the same environment, and their signals overlap, so only non equivalent protons can split signals. 2. **Protons that are separated by more than three single bonds usually do not couple because they are not close enough to each other to be influenced by each other magnetic fields.** Benzene rings have a "coupling club" in which they only couple with each other. Nonequivalent benzene ring protons can couple with each other, but the coupling constants may be too small to be significant. This usually causes complicated splitting patterns. 3. **Signals for O-H and N-H protons are usually singlets.** **Multiplicity:** The number of peaks in a signal. The number of protons bonded to the immediately adjacent carbon determines multiplicity. **Example 1: CH3-CH2-OH** The spectrum shows three signals: * **a) signal for H of OH:** singlet, * **b) signal for H of CH2:** quartet * **c) signal for H of CH3:** triplet **Example 2: CH3CH2CH2 Br** The middle methylene appears as 6 lines due to 5 neighbours, 3+2. - The CH3 group appears as a triplet because it has two adjacent protons. - The CH2 group appears as a triplet because it has two adjacent protons.