Chapter 14 Infrared Spectroscopy and Mass Spectrometry - Practice Flashcards

A comprehensive set of Q&A flashcards covering IR spectroscopy fundamentals, IR peak interpretation, mass spectrometry principles, isotope patterns, fragmentation, and related topics from the notes.

What is spectroscopy?

The interaction between matter and light (electromagnetic radiation).

How can light be described in spectroscopy?

As waves of energy or as photons; key properties are wavelength and frequency; energy is inversely proportional to wavelength and directly proportional to frequency.

Which region of the electromagnetic spectrum is used for Nuclear Magnetic Resonance (NMR) spectroscopy?

Radio waves.

Which region corresponds to IR spectroscopy and what information does it provide?

Infrared region; provides information about the functional groups present in a compound.

Which region is used in UV-Vis spectroscopy and what information does it reveal?

Visible and ultraviolet region; reveals conjugated π systems.

What quantum behavior do molecules exhibit on the molecular scale?

Discrete energy levels for rotation and vibration (quantized) in contrast to macroscopic continuous behavior.

What happens during vibrational excitation in IR spectroscopy?

A photon with the exact energy required is absorbed, promoting a vibrational transition; the molecule absorbs energy and vibrates.

Which type of molecular motion does IR light primarily induce?

Vibrational motion (stretching and bending) of bonds.

What is the main focus of Chapter 14.2 in IR spectroscopy?

Bond stretching frequencies.

What are common samples preparation methods for IR spectroscopy?

Depositing sample on a NaCl plate, dissolving in a solvent, or embedding in a KBr pellet.

What does an IR absorption spectrum plot, and what are its peaks called?

% Transmittance versus frequency; peaks are absorption bands.

What are the units of frequency used in IR spectroscopy and their typical range?

Wavenumbers (cm−1); typically 400–4000 cm−1.

What three characteristics define an IR peak?

Wavenumber, intensity, and shape.

What two factors determine the wavenumber of a bond-stretching vibration?

Bond strength and the mass difference of the bonded atoms.

How does bond strength affect stretching frequency?

Stronger bonds have higher stretching frequencies.

How does mass difference between bonded atoms affect stretching frequency?

A larger mass difference leads to a higher stretching frequency.

What are the diagnostic and fingerprint regions in IR spectroscopy?

Diagnostic region: above ~1500 cm−1; Fingerprint region: below ~1500 cm−1.

Why can IR spectra of isomeric alcohols be difficult to distinguish?

They contain the same types of covalent bonds, leading to very similar absorption patterns.

How do C–H stretching frequencies vary with hybridization?

Alkyl C–H bonds are typically

Can an alkene or alkyne show signals above 3000 cm−1 in IR?

It is possible that they do not show signals above 3000 cm−1 in some cases.

What effect does conjugation have on C=O stretching frequencies?

Conjugation lowers the C=O stretching frequency due to resonance and delocalization.

What is a typical C=O stretching range for common carbonyl-containing groups?

Approximate values: aldehyde ~1730 cm−1; ketone ~1720 cm−1; ester ~1735 cm−1; carboxylic acid ~1715 cm−1; amide ~1650 cm−1; anhydride ~1820 cm−1 and ~1760 cm−1.

What signals are emphasized in the diagnostic region for IR analysis?

Signals related to double bonds (1600–1850 cm−1), triple bonds (2100–2300 cm−1), and X–H bonds (2700–4000 cm−1).

What is the base principle behind identifying functional groups using IR?

Irradiate with IR light and observe which frequencies are absorbed to infer which bonds/functional groups are present.

What is the Mass Spectrum used to determine?

Molar mass and molecular formula of a compound.

What happens to a molecule in a mass spectrometer?

It is vaporized, ionized, and fragmented; detected ions are analyzed by their mass-to-charge ratio (m/z).

What is the molecular ion (M+•) in mass spectrometry?

The radical cation that corresponds to the intact parent molecule’s mass (if it remains intact).

What does the base peak represent in a mass spectrum?

The most abundant ion peak in the spectrum.

How can you determine if a molecule contains nitrogen using the M+• peak?

An odd-massed M+• peak suggests an odd number of nitrogen atoms; even-massed M+• suggests no nitrogen or an even number of nitrogens.

What is the origin of the M+1 peak and what does its intensity indicate?

Originates from 13C isotopes; about 1% of carbon is 13C; the M+1 peak intensity correlates with the number of carbons.

What do the M+ and M+2 peaks indicate for chlorine-containing compounds?

A characteristic 3:1 ratio of M+ to M+2 due to 35Cl (75.8%) and 37Cl (24.2%).

What pattern do bromine-containing compounds typically show in mass spectra?

Two isotopes 79Br and 81Br with nearly equal abundance, giving similar M+ and M+2 peak intensities.

What determines the most abundant fragment in a mass spectrum?

The most stable cation formed during fragmentation.

What are common fragmentation patterns for alcohols and amines?

Alcohols: alpha cleavage and dehydration to form oxonium ions; amines: alpha cleavage.

What is McLafferty rearrangement and when does it occur?

A rearrangement common for ketones and aldehydes during fragmentation.

What does HDI stand for and what does it measure?

Degree of Unsaturation; a count of rings and pi bonds in a molecule.

How does each degree of unsaturation affect hydrogen count in a formula?

Each degree reduces the number of hydrogens by 2.

How should halogens, oxygen, and nitrogen be treated in the HDI calculation?

Halogens count as hydrogen (X treated as H); oxygen has no effect; nitrogen increases the hydrogen count by one.

What can HDI tell you about a molecule when its value is zero?

The molecule cannot have any rings or multiple bonds (fully saturated).

What is high-resolution mass spectrometry (HRMS) capable of measuring?

Masses with up to four decimal places; masses are not restricted to integers.

How does ESI differ from EI in mass spectrometry?

EI (electron impact) is a hard ionization causing extensive fragmentation; ESI (electrospray ionization) is a softer method that often preserves large biomolecules.

What is GC-MS and what data does it provide?

Gas chromatography-mass spectrometry; provides retention time (GC) and mass spectrum (MS) for identifying compounds in mixtures.

Why is mass spectrometry useful for large biomolecules?

Because techniques like ESI-MS are gentle and provide molecular-weight information with limited fragmentation for large molecules.