page_title: Hydrogen chloride
section_title: Structure and properties
hierarchical_section_title: Hydrogen chloride / Chemistry / Structure and properties
caption_attribution_description: HCl spectrum eigen werk
context_page_description: The compound hydrogen chloride has the chemical formula HCl and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride gas and hydrochloric acid are important in technology and industry. Hydrochloric acid, the aqueous solution of hydrogen chloride, is also commonly given the formula HCl.
context_section_description: Frozen HCl undergoes phase transition at 98.4 K. X-ray powder diffraction of the frozen material shows that the material changes from an orthorhombic structure to a cubic one during this transition.  In both structures the chlorine atoms are in a face-centered array.  However, the hydrogen atoms could not be located.  Analysis of spectroscopic and dielectric data, and determination of the structure of DCl (deuterium chloride) indicates that HCl forms zigzag chains in the solid, as does HF (see figure on right).
  The infrared spectrum of gaseous hydrogen chloride, shown on the left, consists of a number of sharp absorption lines grouped around 2886 cm⁻¹ (wavelength ~3.47 µm). At room temperature, almost all molecules are in the ground vibrational state v = 0. Including anharmonicity the vibrational energy can be written as.
  To promote an HCl molecule from the v = 0 to the v = 1 state, we would expect to see an infrared absorption about νₒ = νₑ + 2xₑνₑ = 2880 cm⁻¹. However, this absorption corresponding to the Q-branch is not observed due to it being forbidden by symmetry. Instead, two sets of signals (P- and R-branches) are seen owing to a simultaneous change in the rotational state of the molecules. Because of quantum mechanical selection rules, only certain rotational transitions are permitted. The states are characterized by the rotational quantum number J = 0, 1, 2, 3, ... selection rules state that ΔJ is only able to take values of ±1.
  The value of the rotational constant B is much smaller than the vibrational one νₒ, such that a much smaller amount of energy is required to rotate the molecule; for a typical molecule, this lies within the microwave region. However, the vibrational energy of HCl molecule places its absorptions within the infrared region, allowing a spectrum showing the rovibrational transitions of this molecule to be easily collected using an infrared spectrometer with a gas cell. The latter can even be made of quartz as the HCl absorption lies in a window of transparency for this material.
  Naturally abundant chlorine consists of two isotopes, ³⁵Cl and ³⁷Cl, in a ratio of approximately 3:1. While the spring constants are identical within experimental error, the reduced masses are different causing measurable differences in the rotational energy, thus doublets are observed on close inspection of each absorption line, weighted in the same ratio of 3:1.
caption: Infrared (IR) absorption spectrum
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