Editing Rotational spectroscopy (section)

{{Use American English|date=January 2019}}{{Short description|Spectroscopy of quantized rotational states of gases}}
[[File:CF3I spectrum2.png|thumb|right|400px|Part of the rotational spectrum of [[trifluoroiodomethane]], {{chem|CF|3|I}}.<ref group=notes>The spectrum was measured over a couple of hours with the aid of a chirped-pulse Fourier transform microwave spectrometer at the University of Bristol.</ref> Each rotational transition is labeled with the quantum numbers, ''J'', of the final and initial states, and is extensively split by the effects of [[Nuclear quadrupole resonance|nuclear quadrupole coupling]] with the <sup>127</sup>I nucleus.]]

'''Rotational spectroscopy''' is concerned with the measurement of the energies of transitions between quantized rotational states of [[molecule]]s in the [[gas phase]]. The '''rotational spectrum''' ([[power spectral density]] vs. [[rotational frequency]]) of [[chemical polarity|polar]] molecules can be measured in [[Absorption (optics)|absorption]] or [[Emission (electromagnetic radiation)|emission]] by [[microwave]] spectroscopy<ref>{{cite book|last=Gordy|first=W.|title=Microwave Molecular Spectra in Technique of Organic Chemistry|volume=IX|editor=A. Weissberger|date=1970|publisher=Interscience|location=New York}}</ref> or by [[far infrared]] spectroscopy. The rotational spectra of non-polar molecules cannot be observed by those methods, but can be observed and measured by [[Raman spectroscopy]]. Rotational spectroscopy is sometimes referred to as ''pure'' rotational spectroscopy to distinguish it from [[rotational-vibrational spectroscopy]] where changes in rotational energy occur together with changes in vibrational energy, and also from ro-vibronic spectroscopy (or just [[vibronic spectroscopy]]) where rotational, vibrational and electronic energy changes occur simultaneously.

For rotational spectroscopy, molecules are classified according to symmetry into spherical tops, linear molecules, and symmetric tops; analytical expressions can be derived for the rotational energy terms of these molecules. Analytical expressions can be derived for the fourth category, asymmetric top, for rotational levels up to J=3, but higher [[energy level]]s need to be determined using numerical methods. The rotational energies are derived theoretically by considering the molecules to be [[rigid rotor]]s and then applying extra terms to account for [[centrifugal force|centrifugal distortion]], [[fine structure]], [[hyperfine structure]] and [[Coriolis force|Coriolis]] [[Rotational–vibrational coupling|coupling]]. Fitting the spectra to the theoretical expressions gives numerical values of the angular [[moment of inertia|moments of inertia]] from which very precise values of molecular bond lengths and angles can be derived in favorable cases. In the presence of an electrostatic field there is [[Stark effect|Stark splitting]] which allows molecular [[electric dipole moment]]s to be determined.

An important application of rotational spectroscopy is in exploration of the chemical composition of the [[interstellar medium]] using [[radio telescope]]s.