Instrumental Analytical Chemistry - Nuclear Magnetic Resonance

Comprehensive flashcards covering the basics of NMR spectroscopy, including history, tube maintenance, solvents, interpretation of 1D and 2D spectra, and applications.

What is the definition of Nuclear magnetic resonance (NMR) spectroscopy?

The study of molecules by recording the interaction of radiofrequency electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field.

Which two researchers were jointly awarded the 1952 Nobel Prize in physics for their research of NMR spectroscopy?

Felix Bloch (Stanford University) and Edward Purcell (Harvard University).

Why is chromic acid prohibited during the cleaning and maintenance of NMR tubes?

It leaves a paramagnetic residue.

At what temperature should NMR tubes be dried to prevent distortion?

In a low-temperature oven below $60\text{ }^{\circ}\text{C}$.

What volume of liquid sample is typically held in an NMR tube?

Typically $0.6 - 0.8\,\text{mL}$, requiring $2 - 10\,\text{mg}$ of analyte in $0.6\,\text{mL}$ of solvent.

What is the residual peak position of Chloroform-d ($CDCl_3$) in ${}^{1}H$ NMR?

$7.26\,\text{ppm}$.

Which NMR solvent is considered the standard for water-soluble samples?

Deuterium oxide ($D_2O$).

What are the common types of 2 Dimensional (2D) NMR mentioned in the notes?

HSQC, ${}^{1}H$-${}^{1}H$ COSY, and HMBC.

In ${}^{1}H$ NMR spectroscopy, what does peak integration represent?

The area under each signal, which is directly proportional to the relative number of hydrogen atoms (protons) causing that signal.

What compound is used as an internal standard and set to $0\,\text{ppm}$ in NMR spectroscopy?

Tetramethylsilane (TMS), with the chemical formula $(CH_3)_4Si$.

What are the four key pieces of information obtained from a ${}^{1}H$ NMR spectrum?

1. Position of signals (chemical shift), 2. Relative areas under signals (integration), 3. Splitting pattern, and 4. Coupling constant ($J$).

What is the typical chemical shift range for aldehyde protons in ${}^{1}H$ NMR?

$9.0 - 10\,\text{ppm}$.

What is the typical chemical shift range for aromatic protons in ${}^{1}H$ NMR?

$6.5 - 8.0\,\text{ppm}$.

Why is Tetramethylsilane (TMS) suitable as an NMR standard?

It does not interfere with the compound's spectrum, is easy to remove, consists of a single singlet peak, does not react with samples, and is soluble in nearly all typical solvents except $D_2O$.

What rule determines the number of neighboring protons based on the splitting pattern?

The $n + 1$ rule, where $n$ is the number of neighboring hydrogens.

What does a coupling constant ($J$) measure in NMR?

The frequency separation in Hertz ($Hz$) between split peaks in a multiplet.

What specific information is provided by HSQC (Heteronuclear single quantum coherence spectroscopy)?

It shows which hydrogens are directly attached to which carbon atoms.

What correlation is shown by ${}^{1}H$-${}^{1}H$ COSY (Correlation spectroscopy)?

The correlation between hydrogens that are coupled to each other within $2$ or $3$ bonds.

What does HMBC (Heteronuclear multiple bond correlation spectroscopy) show?

The correlations between protons and carbons across $3$ or $4$ bonds.

In ${}^{13}C$ NMR, what is the chemical shift of Chloroform-d ($CDCl_3$)?

$77.16\,\text{ppm}$.