Lecture 33 Rotational spectroscopy: energies. Rotational spectroscopy In this lecture, we will consider the rotational energy levels. In the next lecture,

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Presentation transcript:

Lecture 33 Rotational spectroscopy: energies

Rotational spectroscopy In this lecture, we will consider the rotational energy levels. In the next lecture, we will focus more on selection rules and intensities.

Diatomic molecule A rigid linear rotor in 3D = the particle on a sphere Moment of inertia J = 0 Energy

Moments of inertia Principal axis of rotation Principal moment of inertia

Linear rotors Two equal nonzero moments of inertia (I a and I b ) and the third (I c ) equal to zero

Spherical rotors Three equal moments of inertia

Symmetric rotors Two equal moments of inertia

The rotational energy levels (in cm –1 ) = 1 / λ = v / c = hv / hc = E / hc The rotational constant B is given in cm –1 20B 12B 6B6B 2B2B 0B0B Energy 2B2B 4B4B 6B6B 8B8B

Quantum in nature How could chemists know HF bond length is 0.917Å? Microwave spectroscopy or computational quantum chemistry Wikipedia

Spherical & linear rotors In units of cm –1 : 20B 12B 6B6B 2B2B 0B0B Energy 2B2B 4B4B 6B6B 8B8B

Symmetric rotors Classically, Quantum-mechanical rotational terms are

Symmetric rotors K acts just like M J. The only distinction is that K refers to rotation around the principal axis, whereas M J to externally fixed axis

Degeneracy Linear rotors (2J+1)-fold degeneracy (M J = 0,±1,…, ±J) Symmetric rotors (2J+1)-fold degeneracy (M J = 0,±1,…, ±J) another 2-fold degeneracy (K = ±1,…, ±J) [(A – B) ≠ 0 and K and –K give the same energy] Spherical rotors (2J+1)-fold degeneracy (M J = 0,±1,…, ±J) another (2J+1)-fold degeneracy (K = 0,±1,…, ±J) [(A – B) = 0 and energy does not depend on K] K for shape, M J for orientation.

Zeeman effect M J -degeneracy can be lifted by applying a static magnetic field. −3 −2 − MJMJ field on field off First-order perturbation theory

Zeeman effect −3 −2 − MJMJ field on field off

Stark effect M J -degeneracy can also be lifted by applying a static electric field. ±1 0 MJMJ field on field off ±2 ±3 ±4 Second-order perturbation theory

Stark effect ±1 0 MJMJ field on field off ±2 ±3 ±4

Linear rotors No degeneracy 2J+1 degenerate Stark effect Zeeman effect No rotation around the axis 2 ND order PT 1 ST order PT

Spherical rotors 2J+1 degenerate Stark effect Zeeman effect 2 ND order PT 1 ST order PT

Symmetric rotors Doubly degenerate 2J+1 degenerate Stark effect Zeeman effect 2 ND order PT 1 ST order PT

Willis Flygare From UIUC Department of Chemistry website “Willis Flygare earned his bachelor's degree from St. Olaf College in 1958 and his doctorate from the University of California at Berkeley in He was a professor of chemistry at Illinois from 1961 until his death. During that period he developed a new experimental method involving the molecular Zeeman effect, and with it, he measured most of the known molecular quadrupole moments and magnetic susceptibility anisotropies. He developed a highly sensitive microwave spectrometer by combining molecular beams with Fourier transform techniques.”

Summary We have learned about the rotational energy levels of molecules in the rigid-rotor approximation. We have classified rigid rotors into linear rotors, spherical rotors, symmetric rotors, and the rest. We have discussed the energy levels and their degeneracy of these rotors. We have learned about the Zeeman and Stark effects.