rotasjonskonstanter

Rotasjonskonstanter, also known as rotational constants, are fundamental parameters that describe the rotational energy levels of a rigid rotor, which is an idealized model of a rotating molecule. In quantum mechanics, the energy of a rigid rotor is given by E_J = B J (J + 1), where J is the rotational quantum number and B is the rotational constant. This constant directly depends on the moment of inertia (I) of the molecule through the relationship B = h / (8π²cI), with h representing Planck’s constant and c the speed of light.

For linear molecules, only a single rotational constant is needed, whereas symmetric and asymmetric tops require multiple constants (A, B, and C) corresponding to rotations about different principal axes. The magnitude of a rotational constant inversely correlates with the moment of inertia; lighter molecules with smaller moments of inertia exhibit larger rotational constants and thus more widely spaced rotational energy levels.

Rotational constants are typically measured through spectroscopic techniques such as microwave spectroscopy, far‑infrared spectroscopy, or Raman spectroscopy. These methods analyze transitions between rotational levels, allowing precise determination of B, and in multi‑constant molecules, A, B, and C. Experimental data are compared with theoretical predictions from quantum chemical calculations to validate molecular models and to examine subtle effects such as centrifugal distortion or electron–vibration coupling.

In astrophysics, rotational constants help identify molecular species in interstellar space. Observations of rotational spectral lines in radio astronomy provide insights into the composition, temperature, and dynamics of molecular clouds. Similarly, planetary science uses rotational constants to infer physical properties of atmospheric molecules on planets and moons.

Understanding rotasjonskonstanter is essential for disciplines ranging from physical chemistry and spectroscopy to astrophysics and molecular physics, as they provide a direct link between a molecule’s structure and its observable rotational behavior.