Butane rotation profile

C4H10  ·  rigid rotation about C2–C3, ideal sp3 geometry at every angle ⌂ Home

Skeletal structure

C1C2C3C4
The slider turns the C3 half of the molecule about the C2–C3 bond as a rigid body. Bond lengths and bond angles never change; only the dihedral does.

Dihedral angle C1–C2–C3–C4

180°anti

Energy along the rotation

Relative energy0.0 kcal/mol Strain termsnone C1···C4 distance Closest H···H between the methyls
Stationary-point energies are the textbook additive values (McMurry): H/H eclipsing 1.0, H/CH3 eclipsing 1.4, CH3/CH3 eclipsing 2.5, gauche 0.9 kcal/mol. The curve between them is a smooth interpolation, not a calculation. Two hydrogens closer than 2.4 Å (their van der Waals contact distance) are shown in red.

Newman projection (live)

front C2, back C3
anti (180°)0
gauche (60°, 300°)0.9
eclipsed H/CH3 (120°, 240°)3.8
eclipsed CH3/CH3 (0°)4.5
kcal/mol relative to anti

Let it relax

At 180°, 60°, 120° or 0°, Optimize moves the model to its quantum-chemical geometry (ORCA, r2SCAN-3c, started from the ideal structure; the two eclipsed conformers with the C1–C2–C3–C4 dihedral held). Notice how the methyls lean apart at 0° and how the gauche dihedral opens beyond 60°. Back to ideal restores the construction.

Exercise: count the gauche interactions

Ideal geometry used in the model

C–C bond length1.54 Å
C–H bond length1.09 Å
Every bond angle109.47°
Dihedrals not on the C2–C3 axis±60° / 180°
At 180° the whole molecule lies on the diamond lattice. The header badge re-checks every bond, angle, and dihedral each time the slider moves.

honestmolecules.com · © 2026 Thomas R. Hoye and Daniel Lee. All rights reserved. Free for classroom and personal study with attribution; no redistribution, modification, or commercial use without written permission.