Bonds that stretch and bend

Section 3 of 5

Carbon dioxide, water and methane absorb infrared, and nitrogen and oxygen let it pass. The reason is in the way their bonds move.

Bonds stretch and bend like springs, and each vibration absorbs infrared of its own wavenumber, the number of waves that fit into a centimetre. A vibration absorbs infrared only when it moves charge about, making one side of the molecule more δ− and the other more δ+, then swapping them back (lesson 23 shows these vibrations in infrared spectra).

Which movements let a molecule absorb infrared Four rows, each a molecule with two-headed arrows showing how its atoms move. Row 1: nitrogen, N₂, stretches its triple bond. Its two atoms are the same, so its charge stays evenly spread and it absorbs no infrared. Row 2: carbon dioxide stretches both bonds together. It stays balanced and absorbs none. Row 3: carbon dioxide bends, the carbon moving up as the oxygens move down. Its charge goes lopsided, and it absorbs infrared of 15,000 nanometres, and this row is shaded as the one that matters most for warming. Row 4: carbon dioxide stretches unevenly, the carbon moving toward one oxygen and then the other. Its charge goes lopsided, and it absorbs infrared of 4,300 nanometres. N₂ stretches stays even absorbs none CO₂ stretches stays balanced absorbs none CO₂ bends goes lopsided absorbs 15,000 nm CO₂ stretches unevenly: lopsided absorbs 4,300 nm
Two-headed arrows show how the atoms move. A vibration that keeps the charge evenly spread absorbs no infrared. A vibration that makes the charge lopsided absorbs infrared of its own wavelength. Carbon dioxide's bend absorbs where the ground gives off plenty of infrared, and its uneven stretch where the ground gives off very little.

Carbon dioxide

Carbon dioxide is straight, and its two polar C=O bonds pull in exactly opposite directions, so the molecule is non-polar overall. When both bonds stretch together, the two pulls still cancel, and that vibration absorbs nothing.

When the molecule bends, both δ− oxygens swing to one side of the carbon. The pulls stop cancelling, and for a moment the molecule has a δ− side and a δ+ side. As it bends back and forth, the charge swings with it. This bend absorbs infrared at about 15,000 nm, a wavenumber of 667, close to the middle of the infrared the ground gives off. An uneven stretch, with one bond long while the other is short, makes the charge lopsided too, and it absorbs at about 4,300 nm (2349).

Water, methane, nitrogen and oxygen

Water is bent and polar to start with, and every one of its vibrations shifts its charge, so it absorbs infrared over a wide spread of wavelengths. Methane's C–⁠H bonds share their electrons almost evenly, and their small charges are enough: when the bonds stretch and bend out of step, the charge goes lopsided, and methane absorbs infrared at about 3,300 nm (3019) and 7,700 nm (1306).

The ground gives off very little infrared shorter than about 5,000 nm, so the bands that matter for warming are carbon dioxide's bend at 15,000 nm and methane's bend at 7,700 nm. Methane's bend absorbs in a stretch of the infrared that few other gases in the air take up, which is one reason each methane molecule has such a large effect.

Nitrogen and oxygen each have two identical atoms, which share their electrons exactly evenly. The only way their bond can vibrate is to stretch, and a stretch keeps the sharing even, so they absorb almost no infrared. Argon's single atoms have no bonds at all. More than 99.9% of dry air lets the ground's infrared through, and the tiny amounts of the other gases absorb it.

Nitrogen, N₂Two nitrogen atoms joined by a triple bond.NN
Nitrogen, N₂: two identical atoms share their electrons evenly, so stretching never gives the molecule a charged end.
Methane, CH₄One carbon atom joined by single bonds to four hydrogen atoms.HHCHH
Methane, CH₄: when its bonds bend out of step, its small charges go lopsided, so it absorbs infrared.

Quick check

Why does nitrogen, N₂, absorb almost none of the infrared from the ground?