orthohydrogenn.
/ˌɔːθəʊˈhʌɪdrədʒ(ə)n/, /ˈɔːθəʊˌhʌɪdrədʒ(ə)n/, U.S. /ˈˌɔrθoʊˈˌhaɪdrədʒ(ə)n/ Chemistry.
The form of molecular hydrogen whose spectrum exhibits a hyperfine structure of triplets owing to the two nuclei in the molecule having parallel spins. Cf. parahydrogen n.Orthohydrogen differs slightly in physical properties from the other form, parahydrogen, and forms about 75 per cent of hydrogen in equilibrium at room temperature.
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orthohydrogen1929
parahydrogen1929
1929 Chem. Abstr. 23 2614 (heading) Experiments on para- and ortho-hydrogen.
1935 A. Farkas Orthohydrogen, Parahydrogen & Heavy Hydrogen ii. 4 The parahydrogen molecules have antiparallel nuclear spins and even rotational quantum numbers, while the orthohydrogen molecules possess parallel nuclear spins and odd rotational quantum numbers.
1962 P. J. Durrant & B. Durrant Introd. Adv. Inorg. Chem. xiii. 368 Ordinary hydrogen gas is a tautomeric mixture of orthohydrogen and parahydrogen.
1966 D. H. Whiffen Spectrosc. ix. 114 In the absence of magnetic materials non-equilibrium mixtures of ortho and para hydrogen are stable for many months.
1999 Proc. National Acad. Sci. U.S.A. 96 12269/1 The equilibrium concentration of ortho-hydrogen drops from 75% at room temperature, to 61% at 100 K.
This entry has been updated (OED Third Edition, September 2004; most recently modified version published online March 2022).
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n.1929