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Dinitrogen trioxide is the chemical compound with the formula N₂O₃. This deep blue solid is one of the binary nitrogen oxides. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21 °C (−6 °F):

Dinitrogen trioxide is only isolable at low temperatures, i.e. in the liquid and solid phases. At higher temperatures the equilibrium favors the constituent gases, with K_diss = 193 kPa (25 °C).

Structure and bonding

Typically, N–N bonds are similar in length to that in hydrazine (145 pm). Dinitrogen trioxide, however, has an unusually long N–N bond at 186 pm. Some other nitrogen oxides do also possess long N–N bonds, including dinitrogen tetroxide (175 pm). The N₂O₃ molecule is planar and exhibits Cs symmetry. The dimensions displayed below come from microwave spectroscopy of low-temperature, gaseous N₂O₃:

It is the anhydride of the unstable nitrous acid (HNO₂), and produces it when mixed into water. An alternative structure might be anticipated for the true anhydride, i.e. O=N–O–N=O, but this isomer is not observed. If the nitrous acid is not then used up quickly, it decomposes into nitric oxide and nitric acid. Nitrite salts are sometimes produced by adding N₂O₃ to solutions of bases:

Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide, is the chemical compound N₂O₄. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide.

Dinitrogen tetroxide is a powerful oxidizer that is hypergolic (spontaneously reacts) upon contact with various forms of hydrazine, which makes the pair a popular bipropellant for rockets.

Structure and properties

Dinitrogen tetroxide forms an equilibrium mixture with nitrogen dioxide. The molecule is planar with an N-N bond distance of 1.78 Å and N-O distances of 1.19 Å. The N-N distance corresponds to a weak bond, since it is significantly longer than the average N-N single bond length of 1.45 Å.

Unlike NO₂, N₂O₄ is diamagnetic since it has no unpaired electrons. The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO₂ according to the following equilibrium:

Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some dinitrogen tetroxide is a component of smog containing nitrogen dioxide.

The Sŵn Festival (sŵn is Welsh for "sound, noise", Welsh pronunciation: [suːn]) is a music festival curated by BBC Radio 1 DJ Huw Stephens and Cardiff-based promoter and Chief Executive of the Welsh Music Foundation, John Rostron. The festival takes place annually in Stephens' hometown of Cardiff, Wales. The first Sŵn Festival took place in November 2007. Bands playing included The Cribs, Beirut, David Holmes, Edwyn Collins and Cherry Ghost.

Music is the festival's core medium, although arts and crafts, bakery masterclasses and 5-a-side football tournaments are incorporated into the schedule. Stephens' eclectic musical taste is reflected in the diversity of the artists performing, and Welsh language bands are well represented on the line-up.

The 2008 festival took place on 14–16 November 2008. Bands playing included Golden Silvers, Truckers of Husk, Micachu And The Shapes, Little Comets, Young Marble Giants, Euros Childs, Colorama, Sweet Baboo and Rob Da Bank.

The 2009 festival took place on 22-24 October 2009. Dananananaykroyd, Johnny Foreigner, Copy Haho, Munch Munch, the Drums, Longcut, Girls, Gaggle, the Twilight Sad, Cate Le Bon, Talons, Sweet Baboo, and Cardiff-based band Los Campesinos! played. Venues used in 2009 were Chapter Arts Centre, Barfly, City Arms, Clwb Ifor Bach, Dempseys, Y Fuwch Goch, The Model Inn, The Toucan, Cardiff University Students Union, The Gate, The Vulcan and the National Museum Cardiff.

Sine nomine (abbreviated s.n.) is a Latin expression, meaning "without a name". It is most commonly used in the contexts of publishing and bibliographical listings such as library catalogs, to signify that the publisher (or distributor, etc.) of a listed work is unknown, or not printed or specified on the work. Compare with sine loco (s.l.), "without a place", used where the place of publication of a work is unknown or unspecified. While it may sometimes be used to disclose unknown authorship, this is more commonly indicated as anon. or similar.

The phrase and its abbreviation have been deprecated in Anglophone cataloging with the adoption of the Resource Description and Access standard, which instead prescribes the unabbreviated English phrase "publisher not identified" (or "distributor not identified", etc.). Sine loco is likewise replaced by "place of publication not identified".

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In Norse mythology, Óðrerir, Óðrørir or Óðrœrir refers either to one of the vessels that contain the mead of poetry (along with Boðn and Són) or to the mead itself.

Attestations

Poetic Edda

Óðrerir is mentioned in two ambiguous passages of the Hávamál. In a first stanza (107), it is sometimes assumed that Óðrerir is synonymous with mead of poetry, but both interpretations are possible.

In another stanza (140), the meaning of Óðrerir depends on the translation.

In most translations, Óðrerir seems to refer to a vessel, but other interpretations of ausinn Óðreri are possible, which can lead to understand Óðrerir to be the mead itself.

Prose Edda

For Snorri Sturluson, Óðrerir is the name of the kettle in which Kvasir's blood was mixed with honey to create the mead:

Similarly, Snorri considers that "liquid of Óðrerir and Boðn and Són" (lögr Óðreris ok Boðnar ok Sónar) is a kenning for the mead of poetry (Skáldskaparmál, 3).

But in skaldic poetry, Óðrerir is a synonym of mead of poetry and it is therefore assumed that Óðrerir as a vessel is Snorri's invention. Moreover, the etymology of the name – which can be rendered into "stirrer of inspiration" or "stirrer of fury" – suggests that it rather refers to the mead. Boðn probably means "vessel" and Són signifies either "reconciliation" or "blood".

In software engineering, a freeze is a point in time in the development process after which the rules for making changes to the source code or related resources become more strict, or the period during which those rules are applied. A freeze helps move the project forward towards a release or the end of an iteration by reducing the scale or frequency of changes, and may be used to help meet a roadmap. The exact rules depend on the type of freeze and the particular development process in use; for example, they may include only allowing changes which fix bugs, or allowing changes only after thorough review by other members of the development team. They may also specify what happens if a change contrary to the rules is required, such as restarting the freeze period.

Two common types of freezes are: