Module I: Spectroscopic Techniques Practice Flashcards

Comprehensive practice questions covering spectroscopic terminology, Beer-Lambert Law, UV-Visible, Fluorescence, Atomic, IR, and NMR spectroscopy based on Module I lecture notes.

What is the formal definition of spectroscopy as described in the lecture notes?

Spectroscopy is the branch of science that studies the interaction of electromagnetic radiation (EMR) with matter.

What is the constant value for the speed of light ($c$) in a vacuum?

$$c = 3 \times 10^8\,m/s$$

How are wavelength ($\lambda$) and energy related in electromagnetic radiation?

They are inversely proportional; longer wavelengths have lower energy, while shorter wavelengths have higher energy.

What is the mathematical formula for photon energy ($E$)?

$E = h\nu$, where $h$ is Planck's constant and $\nu$ is frequency.

State the wavelength range for the visible region of the electromagnetic spectrum.

$$400\text{--}700\,nm$$

Define wave number ($\bar{\nu}$) and provide its formula and common unit.

Wave number is the number of waves present in one centimetre. Formula: $\bar{\nu} = \frac{1}{\lambda}$. Unit: $cm^{-1}$ or Kaysers ($K$).

What does Lambert's Law state regarding the path length of a medium?

It states that the intensity of transmitted light decreases exponentially with an increase in the path length (thickness) of the medium.

What is the mathematical expression for the combined Beer-Lambert Law?

$A = \epsilon cl$, where $A$ is absorbance, $\epsilon$ is molar extinction coefficient, $c$ is concentration ($mol/L$), and $l$ is path length ($cm$).

Under what conditions is the Beer-Lambert Law considered valid?

It is valid when monochromatic light is used, the solution is dilute, the sample is homogeneous, there is no light scattering, temperature is constant, and no chemical reactions occur during analysis.

What is the definition and unit of the molar extinction coefficient ($\epsilon$)?

The molar extinction coefficient is the absorbance of a $1\,M$ solution measured in a $1\,cm$ path length cuvette at a specific wavelength. Its unit is $L\,mol^{-1}\,cm^{-1}$.

Which electronic transition requires the highest energy in UV-Visible spectroscopy?

The $\sigma \rightarrow \sigma^*$ transition.

What is a chromophore?

A chromophore is a light-absorbing group present in a molecule that contains $\pi$-electrons or non-bonding electrons and is responsible for the absorption of UV or visible radiation/colour.

Define 'Bathochromic Shift' (Red Shift).

The shift of the absorption maximum ($\lambda_{max}$) towards a longer wavelength.

Why must quartz cuvettes be used for analysis in the UV region ($200\text{--}400\,nm$)?

Quartz cuvettes are used because glass absorbs UV radiation, which would interfere with the sample analysis.

What is the primary operational difference between a single beam and a double beam spectrophotometer?

In a single beam instrument, the blank and sample are measured separately (one after another), while in a double beam instrument, the light is split so the blank and sample are measured simultaneously.

What is the core principle of Spectrofluorometry?

Certain molecules absorb UV or visible light and immediately emit light of a longer wavelength (fluorescence).

What is 'Stokes Shift' in fluorescence?

The difference between the excitation wavelength and the (longer) emission wavelength.

Why is the detector in a spectrofluorometer placed at a 90° angle to the excitation beam?

To avoid detecting the transmitted incident light and only measure the emitted fluorescence.

Name a common light source used in spectrofluorometry that provides continuous radiation.

Xenon Arc Lamp.

How does Atomic Absorption Spectroscopy (AAS) differ from Atomic Emission Spectroscopy (AES) in terms of the state of the atom measured?

AAS measures radiation absorbed by ground-state atoms, while AES measures radiation emitted by excited atoms returning to the ground state.

What is the function of a Hollow Cathode Lamp (HCL) in AAS?

It is the primary light source that emits the specific characteristic wavelength of the metal being analysed.

What molecular behavior is responsible for the absorption of Infrared (IR) radiation?

Molecular vibrations (stretching and bending of chemical bonds).

What condition must a molecule meet to be 'IR active'?

The molecular vibration must cause a change in the dipole moment of the molecule.

Identify the 'Fingerprint Region' in an IR spectrum and its purpose.

The Fingerprint Region is between $1500\text{--}400\,cm^{-1}$. It is unique for every compound and used to confirm the identity of unknown compounds.

What is the advantage of Fourier Transform Infrared Spectroscopy (FTIR) over conventional IR?

FTIR uses an interferometer to obtain the entire spectrum rapidly, providing faster analysis, higher sensitivity, and better resolution.

Which form of radiation is used in Nuclear Magnetic Resonance (NMR) spectroscopy?

Radio frequency (RF) radiation.

What is the standard reference compound used in NMR spectroscopy, and what is its assigned chemical shift?

Tetramethylsilane (TMS), which is assigned a chemical shift of $0\,ppm$.

Differentiate between 'Shielded' and 'Deshielded' protons in NMR.

Shielded protons are surrounded by high electron density and appear at lower chemical shifts (upfield/lower ppm). Deshielded protons have low electron density surroundings and appear at higher chemical shifts (downfield/higher ppm).

Why are deuterated solvents (like $CDCl_3$ or $D_2O$) required for proton ($H^1$) NMR?

They do not contain protons ($H^1$), so they do not produce signals that would interfere with the NMR spectrum of the sample.

What is 'Chemical Shift' ($\delta$) in the context of NMR?

Chemical shift is the difference in resonance frequency of a nucleus compared with a standard reference compound (TMS), measured in parts per million ($ppm$).