HDO

HDO refers to a specific isotopologue of water in which one hydrogen atom is replaced by deuterium, giving the chemical formula HDO. It is one of three main isotopologues of water, alongside H2O and D2O (heavy water). In natural waters, deuterium is present at very low abundance, but through isotopic exchange with hydrogen-containing compounds, small, detectable amounts of HDO and D2O coexist with ordinary water. The relative concentrations depend on the D/H ratio of the environment and on chemical exchange processes.

Chemically, HDO behaves much like H2O in terms of polarity and hydrogen bonding, but the substitution of hydrogen with deuterium changes certain physical properties. The molecular weight of HDO is about 19 g/mol, and its vibrational spectrum differs from that of H2O due to the heavier isotope in the O–H bond; the O–D stretch appears at lower frequencies than the O–H stretch. Because of these shifts, HDO is commonly used in spectroscopic experiments to help deconvolute water spectra and to study hydrogen-bond networks without the spectral overlap that occurs with H2O.

Applications of HDO include its use as a tracer in isotope labeling and environmental studies, where controlled deuterium content helps track water movement and chemical reactions. In physical chemistry and spectroscopy, HDO measurements aid in understanding vibrational coupling and dynamics in liquid water. HDO is also used in conjunction with D2O in neutron scattering and infrared spectroscopy to separate contributions from hydrogen and deuterium atoms.

See also: isotopologues of water, deuterium oxide (D2O).