Spectroscopy and Mass Spectrometry Quiz

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Electromagnetic Radiation and Molecules (Part 1)

• Electromagnetic spectrum: Shows the range of electromagnetic radiation from increasing energy (gamma rays) to decreasing energy (radio waves). Includes: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.
• Photons: Particles of energy that travel through space with wave-like properties
• Photon energy (E): Calculated as E = hv, where h is Planck's constant and v is the frequency of the associated wave.
• Energy status of matter: Atoms and molecules exist in discrete energy levels (ground state & excited states). Energy transitions (absorption/emission) occur between these levels.
• Quantum nature of matter limitations: Specific energy levels available to atoms/molecules. These energy levels are unique to each species, acting as a 'fingerprint'.
• Absorption and emission spectroscopy: Uses the concept of energy level transitions to identify unknown compounds by measuring transitions between these levels.

Electronic, Vibrational, and Rotational Energy Levels

• Electronic energy levels (E): Correspond to different electron orbitals with varying potential energies. Electron orbital changes result in absorption/emission of photons with specific energy.
• Vibrational energy levels (V): Associated with the vibrational motions of atoms within a molecule.
• Rotational energy levels (R): Associated with the rotational motions of the molecule as a whole

Energy Level Transitions in Spectroscopy

• Absorption: When electromagnetic radiation interacts with a molecule, the molecule absorbs the energy of the photon if the energy corresponds to an available transition in one of the energy levels.
• Internal energy increase: This increase occurs with the absorption of light.
• Ground State --> Excited state: An excited state is higher in energy than the ground state.

UV-Visible Spectroscopy (Part I)

• UV-Visible range: 200-800 nm
• Atmospheric gases absorption: Light below 200nm is absorbed by atmospheric gases. Measurements must be performed under vacuum in this range.
• Electronic transitions: UV-Vis absorption involves transitions between electronic energy levels in a molecule, often called electronic spectroscopy..
• Molecular orbitals (MO): Combination of atomic orbitals that describe the entire molecule.
• Bonding orbitals: Low energy orbitals
• Antibonding orbitals: High energy orbitals
• Non-bonding orbitals: Neither bonding nor antibonding, intermediate energy. -Molecules absorb UV radiation or visible light causing transitions between various electronic energy levels

Spectroscopic Techniques (Part II)

• General: Analysis methods quantify absorption/emission of radiation from a sample. All methods depend on the fact that energy transitions require fixed amounts of energy. (Quantization of energy).
• UV-Vis: Specific peaks related to electron transitions within the molecule resulting from the absorption of UV-Vis radiation.
• Chromophores: Groups of atoms containing electrons responsible for absorption (Eg: C=C, C=O)
• Auxochromes: Substituents that do not absorb but shift the absorption maximum to longer wavelengths (Eg: methyl, hydroxyl).
• Terminology (shifts): Bathochromic (longer wavelength), Hypsochromic (shorter wavelength), Hyperchromic (greater absorbance), Hypochromic (lower absorbance).
• Absorption of UV/Vis radiation: Electromagnetic radiation. Absorption results in transition among electronic energy levels, involving energy transfer to an absorbing molecule.

Infrared (IR) Spectroscopy

• Range: 0.8- 100 micrometers, divided into near-IR (0.8-2.5 μm), mid-IR (2.5-15.4 μm), and far-IR.
• Molecular vibrations: IR radiation can be absorbed if the molecule's charge distribution changes during vibration, leading to a change in dipole moment.
• Vibrational transitions (allowed): Certain vibrations are IR-active because they result in changes in dipole moment.
• Stretching & Bending vibrations: Stretching (symmetric/asymmetric) and bending (scissoring, rocking, twisting, wagging) are important IR-active vibrations.
• Band shapes: Some bonds have bands that are narrow and pointed, and some are wide and smoother.
• IR-active bonds: Polar bonds or molecules with changing dipole moments can absorb IR radiation.
• Functional group identification: Specific functional groups display characteristic absorption bands in the IR spectrum

Mass Spectroscopy

• Ionization: Vaporization of sample and production of ions (often cations), typically through electron impact (EI) or chemical ionization (CI).
• Mass analysis: Separation of ions by their mass-to-charge ratio from the ion source.
• Detection: Measurement of the abundance of each ion to form a mass spectrum; this spectrum displays the relative abundance of fragment ions versus their mass-to-charge ration (m/z).
• Fragment ions: Result when the molecular ions produced from ionization break down into smaller ions.
• Base peak: The most abundant ion peak in the mass spectrum, which is used as a standard of comparison.
• Molecular ion: The largest ion peak; in the mass spectrum; it represents the original molecule's mass.

NMR Spectroscopy (Part 1)

• Nuclear spin: Some atomic nuclei possess a nuclear spin (quantum property), acting like a tiny bar magnet.
• Energy levels (no/with external field): In absence of an external magnetic field, nuclear configurations have equal energy. When a magnetic field is applied, the nuclear configurations split into distinct energy levels.
• Chemical shift (ppm): Variation in the resonant frequency of a nucleus compared to a standard (often TMS), which is affected by its chemical environment (electron density).
• Shielding: The effect of electrons around a nucleus that reduces the strength of the applied magnetic field felt by the nucleus.
• Deshielding: Occurs when electron density around a nucleus is reduced, leading to a stronger applied magnetic field required to achieve resonance (higher chemical shift).

NMR Spectroscopy (Part 2)

• Spin-spin coupling (splitting): The presence of neighboring protons (or other nuclei with spin) causes separation of signals into multiple peaks. The specific pattern is determined by the number of neighboring protons.
• Integration: Integration reflects the relative number of nuclei that produce each peak. Ratio of integration values correlates with ratio of atom counts.
• Coupling constant (J): The constant distance between peaks in a splitting pattern. It is a measure of the proximity and interaction between coupled nuclei.
• Chemical shifts as identifiers: Different types of protons (or other nuclei) exhibit different chemical shifts due to variations in their chemical environment. These shifts can serve to identify specific protons or groups in a molecule.