Molecular structure and absorption spectra can tell you a lot about what a molecule is made of. Different parts of the electromagnetic spectrum probe different kinds of molecular behavior, so each region tends to highlight different structural features.

Infrared region

Infrared absorption mainly comes from intramolecular vibrations and rotations. Many functional groups absorb at characteristic infrared frequencies, and the collection of peaks - especially in the fingerprint region - can be used to identify a compound. Electronic transitions can also produce infrared radiation.

Infrared frequencies lie just below red in the visible spectrum, which is why the region is called “infrared” (below red). At these relatively high frequencies, only atoms and molecules can vibrate rapidly enough to emit or absorb the radiation. Water molecules are especially active in the infrared. That’s why skin has an emissivity of about 0.97 in this region: night-vision scopes detect the infrared glow from warm objects and convert it to visible light.

The Sun behaves much like a blackbody at about 6000 K:

• Half of its energy arrives on Earth in the infrared
• Most of the rest arrives in the visible spectrum
• A smaller fraction arrives in the ultraviolet.

On average, half of this solar energy is absorbed by Earth. The Earth radiates primarily in the infrared, and much of that radiation is absorbed by $C{O}_2$ and $H_{2}O$ in the atmosphere and then re-radiated. This process - known as the greenhouse effect - keeps Earth’s surface about $40°$C warmer than it would otherwise be.

Visible region

When a molecule absorbs visible light, it often appears as the complementary color of the light it absorbs. For example, carotene absorbs blue light, so it appears orange. If a molecule’s structure changes, its visible absorption can shift; this is what happens with indicators.

Indicators are substances that “change color” when exposed to a certain pH or metal ions, or flouresce when exposed to certain bands of visible light.

Visible light usually spans 400 nm to 750 nm in wavelength. Red has the lowest frequencies and longest wavelengths, while violet has the highest frequencies and shortest wavelengths. Although the retina can detect some ultraviolet, these rays are typically blocked by the eye’s cornea and lens.

The Sun emits more intensely at red than violet, giving it a yellowish cast. Living organisms, such as plants, harness only a portion of the visible spectrum (e.g., in photosynthesis) for essential biological processes.

Ultraviolet region

Ultraviolet (UV) radiation lies just above violet in the electromagnetic spectrum, spanning from 400 nm down to about 10 nm. Ultraviolet absorption occurs when $\pi$-electrons and non-bonding electrons in a molecule transition to higher energy levels.

In conjugated systems, electrons are delocalized across multiple bonds. Because this delocalization lowers the energy gap between levels, the transitions occur at longer wavelengths, shifting absorption toward the UV-visible range.

Solar UV is split into UV-A (320-400 nm), UV-B (290-320 nm), and UV-C (220-290 nm). Most UV-B and all UV-C are absorbed by ozone in the upper atmosphere, leaving UV-A as the primary form reaching the Earth’s surface.

Even though only a small fraction of UV-B reaches the ground, it is the primary cause of skin cancer and sunburn. It damages DNA by distorting its helix. UV can also break down collagen fibers, which hastens skin aging and produces wrinkles. The tanning response helps protect living cells by placing pigments in inert skin layers.

Beyond its harmful effects, UV is used to treat infantile jaundice and some skin ailments, sterilize workspaces, and identify substances through fluorescence. Special black lights emit UV to make some objects glow visibly, while UV microscopes enhance resolution beyond what is possible with visible light.

Calculating UV dose

The UV dose (expressed in ${mJ/cm}^2$) is found by taking the power density (${mW/cm}^2$) and multiplying it by the duration of exposure (seconds).

Example. A black light has a power density of $12{mW/cm}^2$. If a surface is exposed for 6 minutes (360 seconds), what is the UV dose in ${mJ/cm}^2$?

(spoiler)

Multiplying power density ($12{mW/cm}^2$) by time ($360\mathrm{s}$) gives:

$$12{mW/cm}^2\times 360\mathrm{s}=4320{mJ/cm}^2$$

NMR spectroscopy

Nuclear magnetic resonance (NMR) is an absorption spectroscopy technique that studies certain atomic nuclei (such as hydrogen (protons)) in an externally applied magnetic field. These nuclei absorb energy in the radiofrequency range. Because the nuclei have a property called spin, they behave like tiny magnets and can “resonate,” or flip between energy levels, at specific frequencies.

In a laboratory setting, NMR helps determine molecular structure by analyzing the positions, intensities, and spin-spin splitting patterns of peaks in an NMR spectrum. This provides information about both molecular structure and dynamics.

Magnetic resonance imaging (MRI) is based on NMR and produces detailed two- or three-dimensional images of the body without using x-rays.