H3 Chemistry 9813 · Study focus: H3 Chemistry: Describe bending vibrations
H3 Chemistry: Describe bending vibrations
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Describe bending vibrations
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Watch an angle open and close
Start with H2O at 104.5°. Move both H atoms in the molecular plane so the angle becomes 110° while O–H lengths stay nearly fixed.
The changing internal coordinate is the H–O–H angle, so the motion is bending.
Bending families
Bending changes bond angle with little bond-length change. Scissoring and rocking are in-plane; wagging and twisting are out-of-plane descriptions for multi-atom groups.
For linear CO2 the bend is doubly degenerate: two perpendicular bending planes share one vibrational frequency near 667 cm⁻¹.
At equilibrium CO2 is linear and non-polar, but a bent instantaneous geometry has bond dipoles that no longer cancel.
The oscillating dipole makes the degenerate bending frequency IR active even though the equilibrium molecule has no permanent dipole.
Text alternative: Alt text names the reference plane, each atom's direction and whether bond angle or bond length changes.
Dipole change during a bend
At equilibrium CO2 is linear and non-polar, but a bent instantaneous geometry has bond dipoles that no longer cancel.
The oscillating dipole makes the degenerate bending frequency IR active even though the equilibrium molecule has no permanent dipole.
CO2 bending planes
One CO2 bend moves both O atoms above and below the molecular axis in the page; the equal-energy partner bends perpendicular to the page.
The two modes are distinct motions but degenerate, so they contribute one absorption frequency near 667 cm⁻¹.
- Explain why two CO2 bending motions give one IR band.
Open the feedback checkpoint after attempting
- Award perpendicular bending planes, equal energy/degeneracy and one shared absorption frequency.
Distinguish CH2 bends
In scissoring the two C–H bonds move toward and away from each other in the molecular plane.
In rocking both H atoms move in the same in-plane direction; neither description requires C–H stretching.
- Describe the arrow difference between CH2 scissoring and rocking.
Open the feedback checkpoint after attempting
- Scissoring uses opposing in-plane H motions; rocking uses same-direction in-plane motion.
Transfer to out-of-plane motion
For CH2 wagging, both H atoms move together above then below the reference plane.
For twisting, one H moves above as the other moves below.
- Sketch and label wagging and twisting of a CH2 group.
Open the feedback checkpoint after attempting
- The sketches must distinguish together versus opposite out-of-plane motion and keep C–H lengths approximately fixed.
Bending is not a weak stretch
Bending and stretching are different normal-coordinate patterns, not merely low- and high-energy versions of the same motion.
A bend is identified by angle change; a stretch by bond-length change.
- Repair: “The 667 cm⁻¹ CO2 band is a slow C=O stretch.”
Open the feedback checkpoint after attempting
- Replace slow stretch with the doubly degenerate angle-changing bend and state its changing dipole.
Check bending before mode counting
Complete twelve bending checks, then the unseen OCS case and the later different NH2Cl re-test.
Next objective: predict simple-molecule mode counts at /learning/h3-infrared-simple-molecule-modes-lesson.html.
- Use fixed arrows to distinguish CO2 bending from stretching.
Open the feedback checkpoint after attempting
- A complete response changes the O=C=O angle, keeps C=O lengths nearly fixed and links the bend to the 667 cm⁻¹ band.