Final answer:
To check for ionizable hydrogens in a molecule using NMR, one should look for their characteristic chemical shift values in the ¹H-NMR spectrum, which can be influenced by the hydroxide ion concentration. Peaks specific to ionizable hydrogens often disappear when the solvent is changed to D₂O due to the exchange with deuterium. Chemical shifts are usually referenced against Me4Si as an external standard in NMR spectrometers.
Step-by-step explanation:
To check for ionizable hydrogens using NMR (Nuclear Magnetic Resonance), we focus on the ¹H-NMR spectrum. In ¹H-NMR spectroscopy, each hydrogen atom in a molecule will absorb electromagnetic radiation at a different frequency depending on its chemical environment. Protons that are bonded to heteroatoms like nitrogen or oxygen, which are more electronegative than carbon, often show up at different frequencies (usually downfield/higher ppm) compared to those attached to carbons due to the electronegativity difference.
Ionizable hydrogens, which are usually part of acidic functional groups such as carboxylic acids or phenols, can be identified in the spectrum as they often display characteristic chemical shift values. For instance, the hydroxide ion concentration can influence the chemical shift of these hydrogens. An ionizable hydrogen attached to an oxygen on a carboxylic acid group, for example, typically resonates downfield (higher ppm) compared to non-ionizable hydrogen atoms bonded to carbon, making it distinguishable on the ¹H-NMR spectrum. To identify these, we look for a peak with the appropriate chemical shift and also consider its exchangeability with deuterium when D₂O is used as a solvent, which can cause such ionizable proton signals to disappear.
Chemical shift referencing is usually done against Me4Si (tetramethylsilane) as an external standard, as mentioned in NMR spectrometers specifications like those of a Bruker Avance device. Information like chemical shifts given in ppm, and the coupling constants in Hz, are essential for identifying different hydrogen environments in a molecule, including those that can ionize.