If four atoms A, B, C, and D are connected linearly and there is rotation possible along bond BC, dihedral
angel on this bond is described as
1. the angel between AB and CD
2. the angel formed between plane ABD and plane ACD
3. the angel formed between plane ABC and plane BCD
4. the angel between ACand BD

Detailed Explanation:

Correct Answer:
3. the angle formed between plane ABC and plane BCD

What is a Dihedral Angle?

A dihedral angle refers to the angle between two planes formed by four atoms in a molecule. This angle is crucial in defining the molecular geometry and plays a role in determining the conformation of a molecule.

In the context of the question, we are considering four atoms, A, B, C, and D, connected linearly. There is rotation possible along the bond BC, and the dihedral angle is formed due to the relative positioning of the planes involving these atoms.

Explaining the Options:

• Option 1: the angle between AB and CD: This description is incorrect because it suggests the angle between two bonds or directions, not between two planes formed by atoms.

• Option 2: the angle formed between plane ABD and plane ACD: This option describes the angle between planes that do not involve the bond BC, so it is not correct in this context.

• Option 3: the angle formed between plane ABC and plane BCD: This is the correct option. When atoms A, B, C, and D are connected linearly, the dihedral angle is defined by the two planes: plane ABC (formed by atoms A, B, and C) and plane BCD (formed by atoms B, C, and D). The angle between these two planes is the dihedral angle formed by the rotation around bond BC.

• Option 4: the angle between AC and BD: This is also incorrect as it does not describe the dihedral angle formed by the rotation around the bond BC.

Why is the Dihedral Angle Important?

In molecules, dihedral angles are vital because they determine the conformation of the molecule, which in turn affects its physical properties and reactivity. The ability of molecules to rotate around bonds influences their shape, and thus the way they interact with other molecules, such as in enzyme binding or drug design.