NMR Spectroscopy Basics

40 question-and-answer flashcards reviewing fundamental concepts, instrumentation, and interpretation of proton NMR spectroscopy.

What physical property of a proton causes it to act like a tiny bar magnet in NMR?

Its positive charge combined with its spin on its axis generates a magnetic field.

How are nuclear magnetic moments oriented when no external magnetic field is applied?

They are randomly oriented in all directions.

When an external magnetic field B0 is applied, in what two ways can nuclear spins orient?

Either with (parallel to) B0 or against (antiparallel to) B0.

Which spin state (α or β) is lower in energy for a proton in a magnetic field?

The α state, in which the nucleus is aligned with the external field.

What must the energy of radio-frequency radiation equal for a nucleus to undergo a spin flip?

It must match the energy difference ΔE between the α and β spin states (E = hν).

What two variables fundamentally characterize an NMR experiment?

The strength of the applied magnetic field B0 (tesla) and the frequency ν of the RF radiation (hertz).

Define 'resonance' in the context of NMR spectroscopy.

The condition in which a nucleus absorbs RF energy and transitions from the lower to the higher spin state.

How does increasing B0 affect the energy gap between spin states and the required resonance frequency?

A stronger B0 increases ΔE and therefore requires a higher resonance frequency.

What time-domain signal is detected immediately after an RF pulse in NMR?

The Free Induction Decay (FID).

Which mathematical process converts the FID into a frequency-domain spectrum?

A Fourier Transform.

Why is a sample usually dissolved in a deuterated solvent for 1H NMR?

To minimize background 1H signals from the solvent, allowing clearer observation of sample protons.

What compound is routinely added as an internal reference at 0 ppm in 1H NMR spectra?

Tetramethyl silane, (CH₃)₄Si (TMS).

Only nuclei with what characteristic give rise to NMR signals?

An odd mass number (and/or odd atomic number), e.g., ¹H, ¹³C, ¹⁹F, ³¹P.

Within what approximate ppm range do most ¹H NMR absorptions appear?

Between 0 and 12 ppm.

On an NMR spectrum, which direction is described as 'upfield'?

Toward the right, corresponding to higher effective magnetic field and lower ppm.

What four pieces of structural information can be obtained from a ¹H NMR spectrum?

Number of signals, chemical shift positions, relative intensities (integration), and spin-spin splitting patterns.

What does the number of distinct signals in a ¹H NMR spectrum indicate?

The number of chemically nonequivalent sets of protons in the molecule.

How does electron density around a proton influence its shielding?

Greater electron density increases shielding, reducing the field felt by the nucleus.

How does shielding affect the position of an NMR absorption peak?

Shielded nuclei absorb upfield (lower ppm, lower frequency).

What is the effect of deshielding on chemical shift?

Deshielded nuclei absorb downfield (higher ppm, higher frequency).

How do electronegative atoms near a proton influence its chemical shift?

They withdraw electron density, deshield the proton, and move its signal downfield.

In ethanol (CH₃CH₂OH), why do the methyl, methylene, and hydroxyl protons resonate at different frequencies?

Each set of protons experiences a different electronic environment, leading to different chemical shifts.

Give the formula used to calculate the chemical shift (δ) in ppm.

δ (ppm) = (observed shift from TMS in Hz) ÷ (operating frequency of the spectrometer in Hz) × 10⁶.

What unit is used for the applied magnetic field strength B0 in NMR?

Tesla (T).

At approximately what temperature are superconducting magnet coils kept to eliminate electrical resistance?

Around 4 K, maintained by liquid helium.

Which cryogenic liquids are typically used to cool an NMR superconducting magnet?

Liquid helium (≈4 K) and liquid nitrogen (≈77 K).

What effect does the inductive field created by circulating π electrons in benzene have on its protons?

It reinforces B0 near the protons, causing deshielding and downfield shifts.

Why does a shielded nucleus require a lower frequency to achieve resonance?

Because the effective magnetic field it experiences is reduced by the opposing induced field from surrounding electrons.

What term describes the signal splitting caused by interactions between nonequivalent neighboring protons?

Spin–spin coupling (or splitting).

If a proton is closer to multiple electronegative chlorine atoms than another proton, how will its chemical shift compare?

It will be more deshielded and appear further downfield.

How does the number of equivalent protons relate to peak integration in an NMR spectrum?

Peak area (integration) is proportional to the number of equivalent protons contributing to that signal.

What is meant by 'ring current' in aromatic systems?

The circulation of π electrons in a magnetic field, which induces its own magnetic field.

In NMR terminology, what does 'low field' correspond to on the chemical-shift axis?

Downfield (left side), higher ppm, higher resonance frequency.

Describe the relationship between chemical shift and electronegativity based on the provided ppm examples.

As electronegativity of the attached atom increases, the proton's chemical shift moves to higher ppm values.

What is the usual current supplied to NMR superconducting magnet coils?

About 50 amperes.

What happens to macroscopic magnetization after the RF pulse is turned off?

It decays toward equilibrium, producing the Free Induction Decay signal.

What is the meaning of 'ppm' in NMR chemical shifts?

Parts per million; it normalizes frequency differences to spectrometer field strength.

What is required for an NMR-active nucleus to 'spin flip' from α to β?

Absorption of radiofrequency energy matching the ΔE between the spin states.

Why is TMS particularly suitable as a chemical shift reference?

It is volatile, inert, and its ¹H nuclei are highly shielded, producing a single sharp upfield peak.

How does decreasing electron density around a nucleus affect the magnetic field it feels and the frequency required for resonance?

It feels a larger effective magnetic field, so a higher resonance frequency (downfield shift) is required.