PL1005 Pharmaceutical Chemistry: Organic Spectroscopy (Infrared & UV-Vis)

Comprehensive vocabulary flashcards covering the principles of infrared and UV-Vis spectroscopy, molecular transitions, group frequency and fingerprint regions, sample preparation methods, and pharmaceutical standards.

Ultraviolet (UV) Radiation Range

Electromagnetic radiation in the range of $180-400\,\text{nm}$ that causes electronic excitation from bonding to antibonding molecular orbitals.

Visible (VIS) Radiation Range

Electromagnetic radiation in the range of $400-780\,\text{nm}$ that interacts with molecules to induce electronic transitions.

Electronic Transition

The transition of an electron between different energy levels, such as from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO), observed in UV-Vis absorption and fluorescence.

Vibrational Transitions

Radiation-induced transitions between quantized vibrational energy levels stacked within each electronic level, associated with the stretching and bending of chemical bonds.

Wavenumber

The reciprocal of wavelength, expressed in units of $\text{cm}^{-1}$, used as the frequency-related scale on the x-axis of an infrared spectrum.

Dipole Requirement for IR Absorption

The criterion stating that a bond must be polarisable and undergo an asymmetrical vibration to absorb infrared radiation; fully symmetrical molecules that undergo no dipole change (e.g., $O_2$, $N_2$) do not display IR absorption.

Symmetric Stretching

A vibrational mode where bonded atoms move synchronously in the same relative directions along bond axes, producing no change in dipole moment in fully symmetrical molecules.

Asymmetric Stretching

A vibrational mode where bonded atoms move non-synchronously along bond axes (e.g., one bond extends while another compresses), creating a net change in dipole moment that actively absorbs IR radiation.

Bending Vibration

A vibrational mode involving a change in the bond angle between atoms, which requires less energy and generally occurs at lower frequencies than stretching vibrations.

Group Frequency Region

The area of the infrared spectrum between $4000-1500\,\text{cm}^{-1}$, primarily arising from stretching vibrations of specific diatomic units and used to identify functional groups.

Fingerprint Region

The complex spectral region between $1500-400\,\text{cm}^{-1}$ containing unique patterns of absorption bands characteristic of the overall molecular framework, used for qualitative identity verification.

Effect of Atomic Mass on IR Frequency

Bonds between lighter atoms vibrate and absorb at higher frequencies; for example, C-H absorbs at $3000\,\text{cm}^{-1}$, C-D at $2200\,\text{cm}^{-1}$, C-O at $1100\,\text{cm}^{-1}$, and C-Cl at $700\,\text{cm}^{-1}$.

Effect of Bond Strength on IR Frequency

Stronger chemical bonds require more energy to vibrate and absorb at higher frequencies; for example, $C \equiv C$ absorbs at $2143\,\text{cm}^{-1}$, $C = C$ at $1715\,\text{cm}^{-1}$, and $C - C$ at $1100\,\text{cm}^{-1}$.

Hydrogen Bond Stretching Frequencies and Strengths

Among single bonds to hydrogen, bond strength and IR frequency correlate directly: O-H ($500\,\text{kJ\,mol}^{-1}$) absorbs at $3500-3600\,\text{cm}^{-1}$, N-H ($450\,\text{kJ\,mol}^{-1}$) absorbs at $3300-3400\,\text{cm}^{-1}$, and C-H ($440\,\text{kJ\,mol}^{-1}$) absorbs at $2900-3200\,\text{cm}^{-1}$.

Liquid Film Method

An IR sampling technique where a drop of liquid sample is spread between two IR-transparent plates; this method is not suitable for volatile liquids.

Nujol Mull

A solid sample preparation method where a solid is ground into a paste with an oil mulling agent (Nujol) and placed between two IR plates.

Potassium Bromide (KBr) Disc Method

A solid sample preparation method where the solid sample is mixed with dry KBr in a mortar and subjected to high pressure in an evacuated die to produce a transparent disc.

Gas Cell Pathlength Requirement

Because gases have substantially lower densities than liquids and solids, gas sample cells require significantly longer pathlengths of usually $10\,\text{cm}$ or more.

British Pharmacopoeia (BP) IR Standards

Compendial reference IR spectra used to verify drug identity by requiring an exact match of all peak positions and relative intensities, and to detect impurities such as solvent residues or by-products.