Describe the signals for the methylene hydrogens (underlined) of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHPh}(\mathbf{1})\) in its \({ }^{1} \mathrm{H} \mathrm{NMR}\) spectrum taken at low temperature.

Methylene hydrogens are hydrogen atoms attached to a carbon atom that is connected to two other carbon atoms. In this particular molecule, \(\text{CH}_3\text{\text{CH}_2}\text{NHPh}\), the methylene hydrogens are the two hydrogen atoms attached to the carbon that is adjacent to the nitrogen (NH) group. This specific configuration plays a significant role in determining the chemical environment of these hydrogens, thus affecting their NMR signals. When analyzing NMR spectroscopy data, recognizing these atoms helps predict their behavior in the spectrum. Step by step, identifying the position and environment of these methylene hydrogens is crucial in understanding the subsequent signals in the NMR spectrum.

Conducting \({ }^{1}\text{H}\) NMR spectroscopy at low temperatures improves the resolution of signals. At lower temperatures, molecular motion slows down, reducing conformational exchanges and providing more distinct peaks in the spectrum. This is especially useful for complex molecules, as it allows each hydrogen environment to be observed more clearly. For the molecule \(\text{CH}_3\text{CH}_2\text{NHPh}\), low temperature helps in distinguishing the methylene hydrogen signals, which might otherwise be equivalent and overlap. This makes the analysis more precise, revealing subtle interactions that occur in the molecular structure.

Spin-spin coupling is a phenomenon in \({ }^{1}\text{H}\) NMR spectroscopy where hydrogens influence each other’s magnetic environments, leading to splitting of NMR signals. For the methylene hydrogens in \(\text{CH}_3\text{CH}_2\text{NHPh}\), each hydrogen interacts with adjacent hydrogens, notably the three protons on the neighboring methyl group (CH3). This results in a splitting pattern known as a triplet. Moreover, if the NH proton is not exchanging rapidly, it could further couple with methylene hydrogens, complicating the splitting. Understanding spin-spin coupling helps predict and interpret the multiplets observed in NMR spectra, providing insight into molecular structures.

Deshielding occurs when electrons are pulled away from a hydrogen nucleus, increasing the local magnetic field experienced by the hydrogen. This causes its NMR signal to shift downfield (to a higher ppm value). In the molecule \(\text{CH}_3\text{CH}_2\text{NHPh}\), the methylene hydrogens experience deshielding due to the electronegativity of the adjacent nitrogen. The phenyl (Ph) group attached to the nitrogen further impacts the electronic environment. The result is that methylene hydrogen signals appear at different chemical shifts compared to those in a lesser electronegative environment. Recognizing deshielding effects is key in assigning chemical shifts in NMR spectra.

Molecular structure analysis using \({ }^{1}\text{H}\) NMR involves interpreting the spectrum to determine the arrangement of atoms in a molecule. By examining chemical shifts, splitting patterns, and the number of signals, one can deduce the positions of various hydrogens within the molecule. For \(\text{CH}_3\text{CH}_2\text{NHPh}\), the NMR spectrum provides insight into the environment of each hydrogen. For example, the methylene group’s hydrogens produce characteristic signals that help identify their connectivity and neighboring groups. Correlating these signals with known chemical behaviors aids in constructing an accurate molecular structure, which is fundamental in fields like organic chemistry and biochemistry.