MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 4d Spectroscopy Molecular Absorption

What this deck covers

This deck focuses on 4d Spectroscopy Molecular Absorption, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the relationship between frequency and wavelength for light in vacuum?

Answer: $c = \lambda\nu$. The speed of light equals the product of wavelength and frequency for electromagnetic waves in vacuum.

Flashcard 2: Which region of the electromagnetic spectrum is primarily used in $^1\text{H}$ NMR spectroscopy?

Answer: Radiofrequency (MHz range). Radiofrequency matches the energy differences between nuclear spin states in a magnetic field for proton NMR.

Flashcard 3: What IR wavenumber range is commonly called the fingerprint region?

Answer: About $1500\text{ cm}^{-1}$ to $500\text{ cm}^{-1}$. This range contains complex, molecule-specific bands useful for identifying unique molecular structures.

Flashcard 4: What is the typical IR absorption range for a carbonyl $\text{C}=\text{O}$ stretch?

Answer: About $1700\text{ cm}^{-1}$ (roughly $1650$–$1750\text{ cm}^{-1}$). Carbonyl stretches absorb in this range due to the bond's strength and reduced mass in the harmonic oscillator model.

Flashcard 5: What electronic transition is most commonly measured by UV-Vis spectroscopy in organic molecules?

Answer: $\pi \rightarrow \pi^*$ (and $n \rightarrow \pi^*$). These low-energy electronic promotions occur in conjugated systems, absorbing in the UV-Vis region.

Flashcard 6: Which region of the electromagnetic spectrum is primarily used in UV-Vis spectroscopy?

Answer: UV and visible light (about $200\text{ nm}$ to $800\text{ nm}$). UV-Vis light provides energy for electronic transitions in molecules, leading to absorption spectra in that range.

Flashcard 7: What type of molecular motion is primarily probed by IR spectroscopy?

Answer: Bond vibrations (stretching and bending). IR radiation energies align with vibrational transitions, causing bonds to stretch or bend upon absorption.

Flashcard 8: What is the selection rule for an IR-active vibrational mode?

Answer: Vibration must change the dipole moment. For IR absorption, the vibration must alter the molecule's dipole to interact with the electric field.

Flashcard 9: What is the Beer–Lambert law in terms of absorbance, path length, and concentration?

Answer: $A = \varepsilon \ell c$. Absorbance is proportional to molar absorptivity, path length, and concentration in dilute solutions.

Flashcard 10: Which region of the electromagnetic spectrum is primarily used in IR spectroscopy?

Answer: Mid-IR, about $4000\text{ cm}^{-1}$ to $400\text{ cm}^{-1}$. Mid-IR radiation matches the energy of molecular vibrations, enabling absorption in IR spectroscopy.

Flashcard 11: What is wavenumber and how is it related to wavelength?

Answer: $\tilde{\nu} = \frac{1}{\lambda}$. Wavenumber is the reciprocal of wavelength, often used in spectroscopy for its proportionality to energy.

Flashcard 12: What is absorbance in terms of incident and transmitted light intensities?

Answer: $A = \log_{10}\!\left(\frac{I_0}{I}\right)$. Absorbance quantifies light absorption as the logarithm of the ratio of incident to transmitted intensity.

Flashcard 13: What is transmittance in terms of transmitted and incident light intensities?

Answer: $T = \frac{I}{I_0}$. Transmittance measures the fraction of incident light that passes through the sample without absorption.

Flashcard 14: How are absorbance and transmittance related?

Answer: $A = -\log_{10}(T)$. Absorbance is the negative logarithm of transmittance, linking the two in spectroscopic measurements.

Flashcard 15: If concentration doubles with constant $\varepsilon$ and $\ell$, how does absorbance change?

Answer: Absorbance doubles. Per Beer-Lambert law, absorbance is directly proportional to concentration when other factors are constant.

Flashcard 16: If path length changes from $1.0\text{ cm}$ to $2.0\text{ cm}$, how does $A$ change (all else constant)?

Answer: $A$ doubles. Absorbance scales linearly with path length according to the Beer-Lambert law, assuming constant concentration and absorptivity.

Flashcard 17: Identify the quantity that equals $\frac{A}{\ell c}$ in Beer–Lambert law.

Answer: Molar absorptivity, $\varepsilon$. Molar absorptivity is the constant of proportionality in Beer-Lambert law, derived by rearranging the equation.

Flashcard 18: What is the typical unit for molar absorptivity $\varepsilon$ in Beer–Lambert law?

Answer: $\text{L}\,\text{mol}^{-1}\,\text{cm}^{-1}$. These units arise from absorbance (unitless), path length in cm, and concentration in mol/L.

Flashcard 19: What does $\lambda_{\max}$ represent on a UV-Vis absorption spectrum?

Answer: Wavelength of maximum absorbance. It indicates the wavelength where the molecule absorbs most strongly, corresponding to peak electronic transition efficiency.

Flashcard 20: What structural change typically causes a bathochromic (red) shift in UV-Vis absorption?

Answer: Increased conjugation (smaller $\Delta E$). Extended conjugation reduces the HOMO-LUMO energy gap, shifting absorption to longer wavelengths.

Flashcard 21: Which UV-Vis shift corresponds to absorption at a shorter wavelength (higher energy)?

Answer: Hypsochromic (blue) shift. Shorter wavelength absorption implies higher energy transitions, termed hypsochromic or blue shift.

Flashcard 22: What is the equation for resonance condition in NMR using gyromagnetic ratio $\gamma$ and field $B_0$?

Answer: $\omega_0 = \gamma B_0$. Resonance occurs when the applied frequency matches the Larmor (angular) frequency, given by gyromagnetic ratio times magnetic field.

Flashcard 23: What is the relationship between photon energy and frequency?

Answer: $E = h\nu$. Photon energy is directly proportional to its frequency, as described by Planck's relation with constant $h$.

Flashcard 24: What is the relationship between photon energy and wavelength?

Answer: $E = \frac{hc}{\lambda}$. Photon energy is inversely proportional to wavelength, derived from combining Planck's relation and the speed of light.

Flashcard 25: What does the IR spectrum x-axis usually report: wavelength, frequency, or wavenumber?

Answer: Wavenumber, $\text{cm}^{-1}$. IR spectra use wavenumber for its direct proportionality to vibrational energy and frequency.