l17: intro to spec

What is spectroscopy, and what types of light–matter interactions can it measure?

A technique that uses light to obtain information about matter. It can measure absorption, emission, reflection, refraction, and scattering. [Slide 3]

How are wavelength, frequency, and photon energy related?

Shorter wavelength → higher frequency → higher photon energy. c = λν and E = hν. [Slide 5]

How do the wave and particle descriptions of electromagnetic radiation differ?

The wave description explains how radiation travels through space, including reflection, refraction, interference, and diffraction. The particle (photon) description explains energy exchange with matter, including absorption and emission. [Slide 6]

What are amplitude, period, frequency, and wavelength?

Amplitude = maximum electric-field strength; period = time between successive maxima/minima at a fixed point; frequency = oscillations per unit time; wavelength = distance between successive maxima/minima. [Slide 7]

What is wavenumber?

Wavenumber (ν̄) is the number of waves per centimeter. ν̄ = 1/λ and its common spectroscopy unit is cm^-1. [Slide 8]

What is the wavenumber of radiation with λ = 5.00 μm?

2000 cm^-1. [Slides 9–10]

What equations describe the energy of a photon?

Ephoton = hν = hc/λ = hcν̄. [Slide 11]

Which regions of electromagnetic radiation are typically used in optical spectroscopy?

Ultraviolet, visible, and infrared radiation. [Slide 12]

What is transmittance and how is percent transmittance calculated?

T = P/P0, where P0 is incident irradiance and P is transmitted irradiance. %T = (P/P0) × 100. T ranges from 0 to 1. [Slide 13]

What is attenuation of light?

A reduction in light intensity as it travels through a medium due to absorption or scattering of photons. [Slide 14]

What is the Beer–Lambert law?

A = εbc. [Slides 14–15]

How are absorbance and transmittance related?

A = -log T = -log(P/P0) = εbc. [Slide 15]

What do ε, b, and c represent in Beer–Lambert law?

ε = molar absorptivity (L mol^-1 cm^-1); b = path length (cm); c = concentration of absorbing species (mol L^-1). [Slide 15]

Besides absorption by the analyte, what other processes can reduce measured light intensity?

Reflection losses at interfaces and scattering losses in solution can also reduce the emergent beam intensity. [Slide 16]

In the benzene Beer–Lambert example, what molar absorptivity was calculated at 256 nm?

ε = 201.3 M^-1 cm^-1. [Slides 17–18]

For ε = 313 M^-1 cm^-1, b = 2 cm, and c = 0.0024 M, what are A and T?

A = 1.5 and T = 0.0316. [Slides 19–20]

What is the fundamental or real limitation of Beer’s law?

Beer’s law is a limiting law that describes dilute solutions. At analyte concentrations greater than about 0.1 M, non-linearity begins to occur. [Slide 21]

How can polychromatic radiation cause an instrumental deviation from Beer’s law?

Polychromatic radiation contains several wavelengths, while analytes have wavelength-specific responses, so the measured response can deviate from ideal Beer–Lambert behavior. [Slide 22]

What is stray light?

Radiation from the instrument that lies outside the nominal wavelength band chosen for detection. It often results from scattering or reflection off gratings, lenses, mirrors, or filters. [Slide 23]

How does stray light affect measured absorbance?

Stray light always causes the apparent absorbance to be lower than the true absorbance. [Slide 23]

How can a chemical equilibrium cause deviation from Beer’s law?

If the fraction of different absorbing species changes with concentration, the overall absorptivity changes because the species can have different ε values, causing non-linearity. [Slides 24–25]

Why can mismatched cuvettes cause measurement problems?

Different cells can have different path lengths, material composition/thickness, shapes, and curvature, changing light transmission between measurements. [Slide 26]

If the true absorbance is 2.00 and there is 1.0% stray light, what apparent absorbance is calculated in the lecture?

A' = 1.70, which is lower than the true absorbance. [Slides 27–28]

What is an absorption spectrum?

A plot of absorbance versus wavelength. [Slide 29]

What must happen for absorption to occur at the atomic level?

The photon energy must match the energy difference between the ground state and an excited state, causing an electronic transition. [Slide 31]

What is an electron volt (eV)?

The energy associated with moving an electron through a potential difference of 1 V. 1 eV = 1.6 × 10^-19 J. [Slide 32]

If the energy gap between the 3p and 3s orbitals is 2.107 eV, what wavelength is absorbed?

λ = 590 nm. [Slides 33–34]

What three types of molecular transitions are discussed in the lecture?

Electronic transitions, vibrational transitions, and rotational transitions. [Slides 36–37]

What happens during vibrational and rotational molecular transitions?

Vibrational transition: atoms move relative to each other. Rotational transition: the whole molecule changes orientation in space. [Slide 37]

How is the total molecular energy represented?

E = Eelectronic + Evibrational + Erotational. [Slide 37]

What type of transition is typically induced by UV–Vis radiation, and what wavelength ranges are given?

UV–Vis radiation is generally energetic enough to cause electronic transitions. UV: 190–380 nm; visible: 380–750 nm. [Slide 38]

What transitions can infrared radiation induce?

IR is generally not energetic enough to cause electronic transitions, but it can induce vibrational and rotational transitions. [Slide 39]

Rank electronic, vibrational, and rotational transitions by energy gap and state the typical EM region for each.

Electronic = largest energy gap → UV–visible; vibrational = intermediate → infrared; rotational = smallest → microwave. [Slide 40]

Why is a blank absorbance subtracted before applying Beer–Lambert law in the iodide example?

The blank contributes absorbance that is not due to the analyte, so the analyte absorbance is obtained from Asample - Ablank before using A = εbc. [Slides 41–44]

In the iodide coloured-complex example, what molar absorptivity is calculated after blank correction?

ε = 7.87 × 10^4 M^-1 cm^-1. [Slides 41–42]

In the iodide example, what concentration is calculated for an unknown with A = 0.175 and blank A = 0.019?

c = 1.98 × 10^-6 M. [Slides 43–44]