The boiling point of has been extrapolated as being , approximately higher than water. In practice,<unk>  will undergo potentially explosive thermal decomposition if heated to this temperature. It may be safely distilled at lower temperatures under reduced pressure. Structure<unk>  () is a nonplanar molecule with (twisted) C2 symmetry; this was first shown by Paul-Antoine Giguère in 1950 using infrared spectroscopy. Although the O−O bond is a single bond, the molecule has a relatively high rotational barrier of 386 cm−1 (4.62 kJ/mol) for rotation between enantiomers via the trans configuration, and 2460 cm−1 (29.4 kJ/mol) via the cis configuration. These barriers are proposed to be due to repulsion between the lone pairs of the adjacent oxygen atoms and dipolar effects between the two O–H bonds. For comparison, the rotational barrier for ethane is 1040 cm−1 (12.4 kJ/mol). The approximately 100° dihedral angle between the two O–H bonds makes the molecule chiral. It is the smallest and simplest molecule to exhibit enantiomerism. It has been proposed that the enantiospecific interactions of one rather than the other may have led to amplification of one enantiomeric form of ribonucleic acids and therefore an origin of homochirality in an RNA world. The molecular structures of gaseous and crystalline are significantly different. This difference is attributed to the effects of<unk>  bonding, which is absent in the gaseous state. Crystals of are tetragonal with the space group DP4121. Aqueous solutions In aqueous solutions,<unk>  differs from the pure substance due to the effects of<unk>  bonding between water and<unk>  molecules.<unk>  and water form a eutectic mixture, exhibiting freezing-point depression down as low as -56 °C; pure water has a freezing point of 0 °C and pure<unk>  of -0.43 °C. The boiling point of the same mixtures is also depressed in relation with the mean of both boiling points (125.1 °C). It occurs at 114 °C. This boiling point is 14 °C greater than that of pure water and 36.2 °C less than that of pure<unk> . Phase diagram of and water: Area above blue line is liquid. Dotted lines separate solid–liquid phases from solid–solid phases. + Density of aqueous solution of<unk> <unk>  (w/w) Density(g/cm3) Temp.(°C) 3% 1.0095 15 27% 1.10 20 35% 1.13 20 50% 1.20 20 70% 1.29 20 75% 1.33 20 96% 1.42 20 98% 1.43 20 100% 1.45 20 Comparison with analogues<unk>  has several structural analogues with Hm−X−X−Hn bonding arrangements (water also shown for comparison). It has the highest (theoretical) boiling point of this series (X = O, N, S). Its melting point is also fairly high, being comparable to that of hydrazine and water, with only hydroxylamine crystallising significantly more readily, indicative of particularly strong<unk>  bonding. Diphosphane and<unk>  disulfide exhibit only weak<unk>  bonding and have little chemical similarity to<unk> . Structurally, the analogues all adopt similar skewed structures, due to repulsion between adjacent lone pairs. + Properties of<unk>  and its analoguesValues marked * are extrapolated Name Formula Molar mass(g/mol) Meltingpoint (°C) Boilingpoint (°C)<unk> <unk>  34.01 −0.43 150.2* Water HOH 18.02 0.00 99.98<unk>  disulfide HSSH 66.15 −89.6 70.7 Hydrazine H2NNH2 32.05 2 114 Hydroxylamine NH2OH 33.03 33 58* Diphosphane H2PPH2 65.98 −99 63.5*
Hydrogen peroxide
Carbon monoxide
Hydroxyl radical
Cyanide
Nitrosamine
Isothiocyanate
Nicosan
Hydrogen fluoride