Spectroscopy Lecture Review

Vocabulary practice flashcards covering key concepts, equations, shifts, transitions, and instrumentation in UV-Visible, IR, and NMR Spectroscopy.

Spectroscopy

The study of interaction between matter and electromagnetic radiation, serving as a tool for analyzing atomic and molecular structures.

Atomic Spectroscopy

The study of the interaction of electromagnetic radiation with atoms, resulting in a line spectrum due to limited atomic energy levels.

Molecular Spectroscopy

The study of the interaction of electromagnetic radiation with molecules, resulting in a broad band spectrum due to transitions among electronic, vibrational, and rotational energy levels.

Beer-Lambert's Law

States that when monochromatic radiation passes through a homogeneous solution, the absorbance ($A$) is proportional to the product of concentration ($c$) and optical path length ($l$), given by $A = \frac{\text{log} I_0}{I} = \beta c l$.

Molar Absorption Coefficient ($\varepsilon$)

The absorbance of a $1\,\text{M}$ solution in a $1\,\text{cm}$ cell, expressed in units of $\text{M}^{-1}\,\text{cm}^{-1}$ or $\text{mol}^{-1}\,\text{L}\,\text{cm}^{-1}$.

HOMO and LUMO

Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO); the most favored electronic transition occurs from HOMO to LUMO.

$\sigma \rightarrow \sigma^*$ Transition

An electronic transition where an electron is excited from a bonding $\sigma$ orbital to an antibonding $\sigma^*$ orbital, occurring in saturated hydrocarbons like methane ($\text{CH}_4$).

$n \rightarrow \sigma^*$ Transition

An electronic transition occurring in compounds containing non-bonding electrons on atoms such as $\text{O}$, $\text{N}$, $\text{S}$, and halogens ($\text{F}$, $\text{Cl}$, $\text{Br}$, $\text{I}$).

$\pi \rightarrow \pi^*$ Transition

An electronic transition occurring in molecules containing $\pi$ electrons, such as compounds with double bonds, triple bonds, or aromatic rings.

$n \rightarrow \pi^*$ Transition

An electronic transition in unsaturated compounds containing heteroatoms (such as $\text{O}$, $\text{N}$, $\text{S}$) that requires the least energy and occurs at longer wavelengths ($270\text{--}300\,\text{m}\mu$ in aldehydes and ketones).

Chromophore

Any isolated covalently bonded group that shows characteristic absorption in the UV-visible region.

Auxochrome

A color-enhancing group (such as $-\text{OH}$, $-\text{OR}$, $-\text{NH}_2$) that shifts the absorption band toward longer wavelengths.

Bathochromic Shift

Also known as a red shift, it is the shift of absorption maximum ($\lambda_{\text{max}}$) toward longer wavelength or lower energy, caused by auxochromes or solvent polarity changes.

Hypsochromic Shift

Also known as a blue shift, it is the shift of absorption maximum toward shorter wavelength or higher energy, caused by removal of conjugation or solvent polarity changes.

Hyperchromic Effect

An effect that increases the intensity of the absorption maximum ($\varepsilon_{\text{max}}$), usually caused by introducing an auxochrome.

Hypochromic Shift

An effect that decreases the intensity of the absorption maximum ($\varepsilon_{\text{max}}$), caused by introducing a group that distorts the chromophore.

Functional Group Region

The region of an IR spectrum from $4000\,\text{cm}^{-1}$ to $1400\,\text{cm}^{-1}$ where most functional groups absorb radiation.

Fingerprint Region

The region of an IR spectrum from $1400\,\text{cm}^{-1}$ to $667\,\text{cm}^{-1}$ that produces a unique absorption pattern characteristic of a specific compound.

IR Absorption Conditions

Requires that the natural vibrational frequency of the molecule equals the frequency of incident IR radiation and that the dipole moment changes during vibration.

Hooke's Law in IR Spectroscopy

An equation describing vibrational frequency $\bar{\nu} = \frac{1}{2\pi c}\sqrt{\frac{K}{\mu}}$, where wave number depends directly on bond force constant ($K$) and inversely on reduced mass ($\mu$).

Zero Point Energy

The vibrational energy level at ground state ($v = 0$), given by $E = \frac{1}{2}h\nu_0$, showing that molecules vibrate even at absolute zero to satisfy Heisenberg's Uncertainty Principle.

Nuclear Magnetic Resonance (NMR) Spectroscopy

A technique based on the absorption of radiofrequency radiation ($4\text{ to }900\,\text{MHz}$) by atomic nuclei with net magnetic spin under an applied magnetic field.

Tetramethylsilane (TMS)

The standard reference compound in NMR spectroscopy because it is inert, volatile, miscible, and produces a single sharp reference line at the extreme right.

Chemical Shift ($\delta$)

The proportional difference (in ppm) between the resonance frequency of an observed proton and the reference standard TMS.

Shielding in NMR

High electron density surrounding a nucleus that opposes the external magnetic field, shifting the resonance signal upfield.

Deshielding in NMR

Lower electron density around a nucleus caused by electronegative groups, shifting the resonance signal downfield.

Spin-Spin Splitting

The phenomenon where an NMR proton signal splits into sub-peaks due to magnetic coupling with neighboring non-equivalent protons, predicted by the $n+1$ rule.

Coupling Constant ($J$)

The distance between adjacent sub-peaks in a split NMR signal, expressed in Hertz ($\text{Hz}$) or cycles per second ($\text{cps}$).

Magnetic Resonance Imaging (MRI)

A diagnostic radiology technique based on NMR principles that visualizes internal body structures by analyzing radiofrequency signals emitted mostly by water hydrogen nuclei.

Pascal's Triangle (NMR)

A mathematical triangle used to determine the theoretical relative intensities of split sub-peaks in an NMR spectrum.

UV-Visible Spectrophotometer Components

The essential instrumentation layout for UV-Vis spectroscopy, consisting of a source, monochromator, sample and reference cells, detector, amplifier, and recorder.

Absorption Band Shifts Diagram

A diagram depicting the directional shifts in UV-Vis absorption spectra: Bathochromic (red), Hypsochromic (blue), Hyperchromic, and Hypochromic.