July 2015

Monodeuterated Water

The first detection of interstellar monodeuterated water (HDO) was reported in 1975 by Turner et al. toward the Orion KL nebula on the basis of one rotational transition at 80.6 GHz. A second transition, at 241.6 GHz, was detected in Orion KL in 1982 by Beckman et al. Later detections of additional lines of HDO toward Orion KL and other sources include reports by Olofsson (1984), Henkel et al. (1987, NGC7538), Petuchowski & Bennett (1988, Orion A) Helmich et al. (1996, W3(OH)), and Pardo et al. (2001). The most complete observation of HDO to date is the 2013 report of Neill et al. involved 37 transitions that were detected with the Herschel HIFI instrument.

HDO was detected in comet Hayakutake by Bockelée-Morvan et al. in 1998. It was detected in icy grain mantles in 1999 by Teixeira et al.

As shown below (with experimental frequencies taken from the NIST Webbook's entries for water and HDO), the vibrational energy of an OD stretch is about 1000 cm-1 less than that of an OH stretch. That means that substituting one H with one D reduces the zero point energy of a compound by about 1.4 kcal/mol, which is enough of a difference to enhance certain chemical reactions to favor reactions involving the deteurated species over the non-deuterated one. This is the basis of deuterium fractionation.

H2O (3657 cm-1) H2O (3756 cm-1) HDO (2727 cm-1) HDO (3707 cm-1)

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