- #1
steve_a
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I'm trying to understand the rotations of rigid diatomic molecules such as HCl. My understanding of the orbital angular momentum is that it is quantized with a total value equal to
$$E=\frac{\hbar^2}{2I}J(J+1)$$
where I is the rotational moment of inertia and J is the quantum number. Also, J can be 0, 1, 2, etc. According to this, the minimum rotational energy is 0, which is surprising but ok. Also, the total angular momentum is given by
$$L^2=\hbar^2 J(J+1)$$
This implies that the total angular momentum is 0 if J is 0. This is consistent with the rotational energy being 0.
This implies that I have complete knowledge of the angular momentum if J = 0 (i.e. it must be exactly 0). However, the Heisenberg uncertainty principle says that complete knowledge of the angular momentum would require complete lack of knowledge of the orientation. To the contrary, I know that the orientation is between 0 and 2π, which is not complete lack of knowledge and thus seems to violate the Heisenberg uncertainty principle.
If there were a zero-point energy for rotations, then everything would make sense, but it doesn't seem that there is one.
What am I doing wrong? Thanks for any replies!
-Steve
$$E=\frac{\hbar^2}{2I}J(J+1)$$
where I is the rotational moment of inertia and J is the quantum number. Also, J can be 0, 1, 2, etc. According to this, the minimum rotational energy is 0, which is surprising but ok. Also, the total angular momentum is given by
$$L^2=\hbar^2 J(J+1)$$
This implies that the total angular momentum is 0 if J is 0. This is consistent with the rotational energy being 0.
This implies that I have complete knowledge of the angular momentum if J = 0 (i.e. it must be exactly 0). However, the Heisenberg uncertainty principle says that complete knowledge of the angular momentum would require complete lack of knowledge of the orientation. To the contrary, I know that the orientation is between 0 and 2π, which is not complete lack of knowledge and thus seems to violate the Heisenberg uncertainty principle.
If there were a zero-point energy for rotations, then everything would make sense, but it doesn't seem that there is one.
What am I doing wrong? Thanks for any replies!
-Steve