# ¹H NMR (Proton NMR) Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy is an application of nuclear magnetic resonance that focuses on the hydrogen-1 nuclei within a substance. It is a powerful technique used to determine the structure of organic molecules. ### 18.8.1 Use of Deuterated Solvents To avoid interference from solvent protons in the analysis of a sample, special solvents are used where hydrogen (¹H) is replaced by its isotope, deuterium (²H or D). Deuterium is NMR-inactive at the frequencies used for proton NMR. *Common Deuterated Solvents:* - Deuterated water: $\mathrm{D_2O}$ - Deuterated acetone: $(\mathrm{CD_3})_2\mathrm{CO}$ - Deuterated chloroform: $\mathrm{CDCl_3}$ *Non-Hydrogen Solvents:* Solvents that contain no hydrogen atoms can also be used. - Carbon disulfide: $\mathrm{CS_2}$ - Carbon tetrachloride: $\mathrm{CCl_4}$ ### 18.8.2 Tetramethylsilane (TMS) as a Standard Tetramethylsilane, with the formula $\mathrm{Si(CH_3)_4}$, is used as the standard reference compound in ¹H NMR spectroscopy. *Structure:* TMS is a tetrahedral molecule where a central silicon atom is bonded to four methyl groups. *Why it is the Standard:* 1. **Chemical Equivalence:** All 12 protons in TMS are in the same chemical environment, producing a single, strong signal. 2. **High Shielding:** Silicon is less electronegative than carbon, causing the protons in TMS to be highly shielded by electrons. This means they resonate at a higher field (a lower frequency) than most protons in organic molecules. 3. **Reference Point:** The signal from TMS is used to define the zero point on the chemical shift scale, so its chemical shift ($\delta$) is set to **0 ppm**. 4. **Inert and Volatile:** TMS is chemically inert and has a low boiling point, making it easy to remove from the sample after analysis. Most organic compounds have proton signals that appear *downfield* from TMS, in the range of **-4 to +14 ppm**. ### 18.8.3 Chemical Shift ($\delta$) and Integration *Nuclear spin flipping* is the underlying principle of NMR. When atomic nuclei with a property called "spin" are placed in a strong magnetic field, they can exist in different energy states. By irradiating the sample with radiofrequency energy, the nuclei can absorb energy and "flip" to a higher energy state. The precise frequency required for this transition is called the **resonant frequency**. The resonant frequency of a proton is influenced by its local electronic environment. This gives rise to the **chemical shift ($\delta$)**, which is the difference between the resonant frequency of a specific proton and the reference standard (TMS). The key information obtained from a ¹H NMR spectrum includes: 1. **Number of Signals:** Indicates the number of different (non-equivalent) proton environments in the molecule. 2. **Chemical Shift ($\delta$):** Reveals the electronic environment of the protons. - ***Shielding:*** High electron density around a proton creates a small magnetic field that *opposes* the external field. The proton is "shielded" and requires a lower frequency to resonate. These signals appear **upfield** (closer to 0 ppm). - ***Deshielding:*** Low electron density around a proton (e.g., due to a nearby electronegative atom) means it experiences more of the external magnetic field. The proton is "deshielded" and requires a higher frequency to resonate. These signals appear **downfield** (further from 0 ppm). 3. **Integration:** The area under each peak is proportional to the number of protons in that specific environment. The ratio of the areas gives the ratio of the numbers of hydrogen atoms in each environment. **Example: Ethyl Chloride ($\mathrm{CH_3CH_2Cl}$)** The spectrum of ethyl chloride provides a clear illustration of these principles. - **Number of Signals:** There are two signals, indicating two distinct proton environments: the methyl ($\mathrm{CH_3}$) protons and the methylene ($\mathrm{CH_2}$) protons. - **Integration:** The ratio of the peak areas is 3:2, corresponding to the three $\mathrm{CH_3}$ protons and the two $\mathrm{CH_2}$ protons. - **Chemical Shift:** - A signal at **$\delta$ = 1.5 ppm** corresponds to the three protons of the methyl ($\mathrm{CH_3}$) group. - A signal at **$\delta$ = 3.5 ppm** corresponds to the two protons of the methylene ($\mathrm{CH_2}$) group. - The $\mathrm{CH_2}$ protons are more *deshielded* and appear further downfield because they are directly attached to the highly electronegative chlorine atom. **Example: Benzene ($\mathrm{C_6H_6}$)** The spectrum of benzene shows only a single peak at **$\delta$ = 7.2 ppm**. This is because all six protons are chemically equivalent. The downfield position is due to the *diamagnetic ring current* of the aromatic system, which strongly deshields the protons. ### 18.8.4 Multiplicity (Spin-Spin Splitting) The signal for a set of protons can be split into multiple smaller peaks (a *multiplet*) by the magnetic fields of protons on adjacent carbon atoms. This phenomenon is called **spin-spin splitting** or **coupling**. *The (n+1) Rule:* The multiplicity of a signal is given by the rule **(n + 1)**, where **n** is the number of equivalent protons on the adjacent carbon(s). | **n** (Adjacent Protons) | **Multiplicity** | **Peak Pattern** | | :----------------------: | :--------------: | :--------------: | | 0 | Singlet | 1 peak | | 1 | Doublet | 2 peaks | | 2 | Triplet | 3 peaks | | 3 | Quartet | 4 peaks | | 4 | Quintet | 5 peaks | **Revisiting Ethyl Chloride ($\mathrm{CH_3CH_2Cl}$):** - The **$\mathrm{CH_2}$** signal ($\delta$ = 3.5 ppm) is split by the 3 adjacent protons on the $\mathrm{CH_3}$ group. Following the (n+1) rule where n=3, the signal is a **quartet** (3+1=4 peaks). - The **$\mathrm{CH_3}$** signal ($\delta$ = 1.5 ppm) is split by the 2 adjacent protons on the $\mathrm{CH_2}$ group. Following the (n+1) rule where n=2, the signal is a **triplet** (2+1=3 peaks). ### 18.8.5 Proton Exchange by $\mathrm{D_2O}$ Protons attached to highly electronegative atoms like oxygen or nitrogen (e.g., in alcohols, O-H, or amines, N-H) are acidic and can be exchanged. - If a sample containing O-H or N-H protons is shaken with deuterium oxide ($\mathrm{D_2O}$), the hydrogen atoms are replaced by deuterium atoms: $$ \mathrm{R-O-H} + \mathrm{D_2O} \rightleftharpoons \mathrm{R-O-D} + \mathrm{DHO} $$ - Since deuterium is NMR inactive, the signal corresponding to the O-H or N-H proton will **disappear** from the spectrum. - This "$\mathrm{D_2O}$ shake" is a simple and effective test to confirm the presence of exchangeable protons like those in alcohols, carboxylic acids, and amines. ## Possible Questions and Answers **Q1:** How many peaks do you expect in the proton NMR of benzene? **A:** One. All six protons in benzene are chemically equivalent due to the molecule's symmetry, so they produce a single signal (a singlet). **Q2:** Why the peak of two protons attached with alpha carbon of ethyl alcohol have greater chemical shift than the ethylene group of ethyl chloride? **A:** This premise is generally incorrect. Chlorine is more electronegative than oxygen. Therefore, the protons on the alpha carbon ($\mathrm{CH_2}$ group) of **ethyl chloride** are more deshielded and typically have a *higher* chemical shift (e.g., ~3.5 ppm) than those in **ethyl alcohol** (e.g., ~3.6 ppm, though values can be very close). The chemical shift is primarily determined by the deshielding effect of the adjacent electronegative atom. **Q3:** Given is a H-NMR spectrum of an organic compound "x" with a molecular formula $\mathrm{C_4H_{10}O}$. Deduce its structural formula by using its H-NMR spectrum. **A:** 1. **Analyze the signals:** - A **quartet** around $\delta$ = 3.4 ppm. A quartet means it is adjacent to a group with 3 protons (a $\mathrm{CH_3}$ group). - A **triplet** around $\delta$ = 1.2 ppm. A triplet means it is adjacent to a group with 2 protons (a $\mathrm{CH_2}$ group). 2. **Identify the fragment:** A quartet and a triplet together strongly suggest the presence of an **ethyl group** ($\mathrm{CH_3CH_2-}$). 3. **Deduce the structure:** The molecular formula is $\mathrm{C_4H_{10}O}$. An ethyl group accounts for $\mathrm{C_2H_5}$. This means the molecule consists of two ethyl groups attached to an oxygen atom. 4. **Conclusion:** The structure is **diethyl ether**, $\mathrm{CH_3CH_2OCH_2CH_3}$. - The $\mathrm{CH_2}$ groups are equivalent and adjacent to $\mathrm{CH_3}$, hence the quartet at 3.4 ppm (deshielded by oxygen). - The $\mathrm{CH_3}$ groups are equivalent and adjacent to $\mathrm{CH_2}$, hence the triplet at 1.2 ppm. - The integration ratio of quartet:triplet is 4H:6H, or 2:3, which is consistent with the structure. **Q4:** How many peaks do you expect in the NMR spectrum of acetone? If it is a single peak describe the reason. **A:** One peak. In acetone, $(\mathrm{CH_3})_2\mathrm{CO}$, both methyl groups are chemically equivalent. All six protons are in the identical environment and have no adjacent, non-equivalent protons to couple with. Therefore, they produce a single signal (a singlet). **Q5:** What would be the difference between the proton NMR spectrum of ethyl alcohol and ethyl chloride? Why is the chemical shift of OH proton in alcohol greater? **A:** - **Ethyl Chloride ($\mathrm{CH_3CH_2Cl}$):** Shows two signals: a triplet for the $\mathrm{CH_3}$ group and a quartet for the $\mathrm{CH_2}$ group. - **Ethyl Alcohol ($\mathrm{CH_3CH_2OH}$):** Shows three signals: a triplet ($\mathrm{CH_3}$), a quartet ($\mathrm{CH_2}$), and a singlet for the hydroxyl (-OH) proton. (Note: The OH proton usually does not couple with adjacent protons due to rapid chemical exchange). - The chemical shift of the OH proton is significant because the proton is attached to a highly electronegative oxygen atom, which deshields it. Its chemical shift is also highly variable (typically $\delta$ = 1-5 ppm) as it is sensitive to concentration, solvent, and temperature due to hydrogen bonding.