Review of Concepts and Vocabulary: Spectroscopy & Mass Spectrometry (Sections 14.1–14.16)

Vocabulary flashcards covering spectroscopy, IR, mass spectrometry, GC-MS, ESI, and related concepts from Sections 14.1–14.16.

Spectroscopy

The study of the interaction between light and matter.

Wavelength

Distance between adjacent peaks of an oscillating field.

Frequency

Number of wavelengths passing a fixed point per unit time.

Photon energy

Energy of a photon, determined by its frequency (E = hν).

Electromagnetic spectrum

The full range of possible frequencies of electromagnetic radiation.

Quantized energy

Energy exists in discrete levels; transitions occur between levels.

ΔE (vibrational energy gap)

Energy difference between vibrational levels, set by bond nature.

Vibrational excitation

Promotion of a molecule to a higher vibrational level; includes stretching and bending.

IR spectroscopy

Technique used to identify functional groups by IR absorption.

Absorption spectrum

Plot of percent transmittance vs frequency (or wavenumber).

Wavenumber

Frequency-related unit used in IR (cm⁻¹) to locate signals.

Bond strength

Factor that largely determines IR wavenumber along with atom masses.

Conjugated C=O

C=O in conjugation lowers the energy (lowers wavenumber) of the signal.

Diagnostic region

IR region containing signals from double/triple bonds and Z–H bonds.

Fingerprint region

IR region where most single bonds vibrate; signals are highly compound-specific.

C═O bond

Carbonyl bond; typically produces strong IR signals.

C═C bond

Carbon–carbon double bond; often yields weaker IR signals.

Symmetrical C═C bond

Does not produce an IR signal.

Z–H bonds

Bonds of the form Z–H (e.g., vinylic) with characteristic signals.

Concentrated alcohols

Concentrated alcohols give broad signals; dilute alcohols give narrow signals.

Primary amines

Exhibit two signals from symmetric and asymmetric N–H stretching.

IR signal characteristics

Every IR signal has wavenumber, intensity, and shape.

3000 cm⁻¹ region

Signal region left of 3000 cm⁻¹ indicates Z–H bonds.

Mass spectrometry

Technique to determine molecular weight and molecular formula.

Electron impact ionization (EI)

Ionizes by bombarding with high-energy electrons to form a molecular ion (M)+ radical cation.

Molecular ion (M)+ radical cation

The unfragmented molecular ion in mass spectrometry.

Fragmentation

Breakdown of the molecular ion into cationic fragments.

m/z

Mass-to-charge ratio; used to separate ions in a mass spectrum.

Base peak

Tallest peak in a mass spectrum; assigned 100% intensity.

M+ peak

Peak corresponding to the molecular ion (molecular weight).

Nitrogen rule

Odd molecular weight suggests an odd number of N atoms; even MW suggests no N or an even number of N.

M and M+1 peaks

Relative heights indicate the number of carbon atoms (M+1 from 13C).

M+2 peak

Helps identify halogens: Cl shows ~1:3 ratio with M, Br shows similar heights to M.

M-15

Fragment ion indicating loss of a methyl group.

M-29

Fragment ion indicating loss of an ethyl group.

Carbocation stability

More stable carbocation formed → greater likelihood of fragmentation at that site.

High-resolution mass spectrometry

MS with high resolving power to determine exact molecular formula.

Atomic mass unit (amu)

1 g divided by Avogadro’s number; unit of atomic/molecular mass.

Standard atomic weight

Weighted average of isotopic masses based on natural abundance.

GC–MS

Gas chromatography–mass spectrometry; separates mixture and analyzes each compound.

Retention time

Time a compound takes to elute from the GC column; shown on a chromatogram.

Electrospray ionization (ESI)

Ionization method used for large biomolecules (proteins, nucleic acids).

Saturated alkanes

Molecular formula CnH2n+2.

Pi bond

A pi bond indicates unsaturation; its presence means the molecule is unsaturated.

Degree of unsaturation

Each double bond or ring adds one degree of unsaturation; total is the HDI.

Hydrogen deficiency index (HDI)

Measure of the number of degrees of unsaturation in a molecule.