Utente:Grasso Luigi/sanbox1/Composti dell'elio

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L'elio è l'elemento meno reattivo, quindi si credeva comunemente che i composti dell'elio non esistessero affatto.[1]La prima energia di ionizzazione è di 24.57 eV che è la più alta tra gli elementi chimici.[2] L'elio ha una shell elettronica completa, e in questa condizione l'atomo non facilmente accetta un elettrone in più o crea una coppia elettronica per formare un composto covalente. L'affinità elettronica è di 0.080 eV, che è molto vicina a zero.[2] L'atomo è piccolo con un raggio della shell elettronica esterna di 0.29 Å.[2] Inoltre è molto duro con durezza di Pearson di 12,3   eV.[3] Possiede la più bassa polarizzazione di tutti gli atomi conosciuti. Tuttavia esistono delle forze molto deboli tra l'elio e altri atomi. Questa forza può superare le forze repulsive. Quindi a temperature estremamente basse l'elio può formare molecole.

Le forze repulsive tra l'elio e altri atomi possono essere superate dall'alta pressione. L'elio può formare un composto cristallino con il sodio a tali pressioni. Delle pressioni adatte per forzare l'elio in combinazioni solide si possono trovare all'interno dei pianeti. Si formano i clatrati con elio sotto pressione nel ghiaccio, e altre piccole molecole con l'azoto.

Altri modi per rendere reattivo l'elio sono: la conversione a ione, o l'eccitazione di un elettrone a un livello superiore, permettendogli la formazione di eccimeri. L'elio ionizzato (He+), anche detto He II, è un materiale ad altissima energia in grado di estrarre un elettrone da qualsiasi altro atomo. He+ ha configurazione elettronica simile all'idrogeno, quindi oltre ad essere ionico può formare legami covalenti. Gli eccimeri non durano per molto tempo, poiché la molecola contenente l'atomo di elio di livello energetico superiore può decadere rapidamente in uno stato fondamentale repulsivo, dove due degli atomi che compongono il legame si respingono. Tuttavia, in alcuni casi come l'elio presente nelle stelle nane bianche, le condizioni possono essere adatte per formare rapidamente atomi di elio eccitati. L'atomo di elio eccitato ha un elettrone 1s promosso a 2s. Ciò richiede una energia di 1 900 chilojoule (450 kcal) per grammo di elio, che si può ottenere per impatto elettronico, o per scarica elettronica.[4] Lo stato dell'elettrone eccitato 2s assomiglia a quello del litio.

Fasi solide conosciute[modifica | modifica wikitesto]

La maggior parte delle combinazioni solide di elio con altre sostanze richiedono alta pressione. L'elio non forma legami con gli altri atomi, ma le sostanze possono avere una struttura cristallina ben definita.

Eliuro disodio[modifica | modifica wikitesto]

Lo stesso argomento in dettaglio: Disodium helide.

L'eliuro disodio (Na2He) è un composto di elio e sodio stabile a pressione alte di circa 113 gigapascal (1 130 000 bar). Il composto era stato previsto dalla teoria[5] utilizzando il software USPEX ed è stato sintetizzato nel 2016.[2][6] La teoria stabiliva una stabilità termodinamica a circa 160 GPa e una stabilità dinamica dopo 100 GPa. Na2He ha una struttura cristallina cubica, simile alla fluorite. Alla pressione di 300 GPa il lato della cella unitaria del cristallo è a = 8.062 Å. Ciascuna cella unitaria contiene quattro atomi di elio al centro delle faccie cubiche e agli angoli, e otto atomi di sodio alle posizioni di un quarto di cella da ogni faccia. Due elettroni (2e−) stanno su ogni bordo e al centro della cella unitaria.[postille 1] Ogni coppia di elettroni ha spin accoppiati. La presenza di elettroni isolati lo rende un elettride. Gli atomi di elio non partecipano ai legami. Tuttavia le coppie di elettroni possono essere considerate un legame a due elettroni con otto centri.[2] Il composto è stato previsto essere un isolante e senza colore.[2]

Silicati[modifica | modifica wikitesto]

L'elio nel 2007 è stato osservato dentro un silicato. Il minerale melanoflogite è un silicoclatrato naturale (clatrasile) che di solito contiene anidride carbonica, metano o azoto. Se compresso con elio, si forma un nuovo clatrato. Questo ha un valore molto più alto del modulo di massa ed è resistente all'amorfizzazione. L'elio è stato assorbito intorno ai 17 GPa, allargando la cella unitaria, e restituito di nuovo quando la pressione è scesa a 11 GPa.[7]

La Cristobalite He II (SiO2He) è stabile nel campo di pressioni 1.7 a 6.4 GPa. Ha un gruppo spaziale esagonale denominato R-3c con dimensioni della cella unitaria di a = 9.080 Å, α = 31.809° e V = 184.77 Å3 alla pressione di 4 GPa.[8]

La Cristobalite He I (SiO2He) si forma sotto pressioni di elio più elevate di 6.4 GPa. Ha un gruppo spaziale monoclino denominato P21/C con dimensioni della cella unitaria di a = 8.062 Å, b = 4.797 Å, c = 9.491 Å, β = 120.43° e V = 316.47 Å3 alla pressione di 10 GPa.[8]

L'elio penetra nella silice fusa ad alta pressione, reducendone la compressibilità.[9]

La chibaite, un'altro silicoclatrato naturale ha la sua struttura con elio a pressioni superiori di 2.5 GPa. La presenza degli idrocarburi ospiti non impedisce tale presenza. Il Neon richiede una pressione maggiore, circa 4.5 GPa, per stare nella struttura e, a differenza dell'elio, mostra l'isteresi.[10]

Le zeoliti Linde-tipo A sono meno comprimibili quando s'inserisce l'elio tra i 2 e i 7 GPa.[11]

Arsenolite con elio[modifica | modifica wikitesto]

L'arsenolite con elio As4O6·2He è un composto osservato nel campo di pressioni tra 3  GPa e fino ad almeno 30  GPa[12] L'arsenolite è uno dei minerali più morbidi e comprimibili.[13] L'elio impedisce un'amorfizzazione che altrimenti si verifica nell'arsenolite sotto pressione.[13]

Il solido contenente elio è più forte e più duro, con una velocità del suono più elevata rispetto alla semplice arsenolite.[14]L'elio che è incluso nel cristallo provoca uno stress più uniforme sulle molecole As4O6.

Nessun legame effettivo si forma tra l'arsenico e l'elio nonostante vi siano coppie solitarie di elettroni disponibili.[15] La diffusione dell'elio nell'arsenolite è un processo lento che richiede giorni ad una pressione di circa 3 GPa. Tuttavia, se la pressione nel cristallo continua a salire (13 GPa) la penetrazione dell'elio non avviene, poiché gli spazi tra le molecole di arsenolite diventano troppo piccoli.[15]

Il neon non si diffonde nell'arsenolite.[15]

Perovskiti[modifica | modifica wikitesto]

L'elio può essere inserito nei siti A durante l'espansione termica negativa delle perovskiti a colmare i difetti che si creano nel sito. A temperatura ambiente e pressione 350 Mpa l'elio è presente in CaZrF6 per espandere la sua cella unitaria producendo HeCaZrF6. Circa la metà dei siti A sono riempiti da atomi di elio. Questa sostanza perde elio per diversi minuti durante la depressurizzazione a temperatura ambiente, ma sotto i 130 gradi Kelvin lo trattiene.[16]

Formiati[modifica | modifica wikitesto]

Sotto pressione l'elio è stato osservato nel formiato di ferro dimetilammonio (CH3)2NH2Fe(HCOO)3. Influenza il composto provocando un cambiamento in uno stato ordinato monoclino presente a una pressione inferiore (circa 4 GPa) rispetto allo stato senza l'elio.[17]

Molecole piccole van der Waals[modifica | modifica wikitesto]

He(N2)11 è un composto van der Waals che si presenta a cristalli esagonali. A 10 GPa la cella unitaria composta dai 22 atomi di azoto presenta un volume di 558 Å3, e diventa 512 Å3 a una pressione di 15 GPa. Queste dimensioni sono 10 Å3 più piccole della quantità equivalente di azoto solido δ-N2 a tali pressioni. La sostanza viene prodotta comprimendo l'azoto e l'elio in una cella ad incudine di diamante.[18]

NeHe2 ha la stessa struttura cristallina esagonale del tipo MgZn2 a 13.7 GPa. La cella unitaria ha dimensioni a = 4.066 Å, c = 6.616 Å; e a 21.8 GPa, a = 3.885 Å, c = 6.328 Å. Ci sono quattro atomi in ogni cella unitaria. Fonde a 12,8  GPa e 296  K,[19] mentre diventa un solido stabile a pressioni superiori 90 GPa.[20]

Clatrati[modifica | modifica wikitesto]

I clatrati di elio si formano solo sotto pressione. Con il ghiaccio II a pressioni tra 280 e 480 MPa esiste un solido di elio idrato con un rapporto tra He:H2O di 1:6.[21] Un altro clatrato con un rapporto acqua/elio di 2.833 è stato realizzato nella struttura del clatrato SII. Essa ha due diverse gabbie nel ghiaccio, quella piccola può contenere un atomo di elio, e la grande ne può contenere quattro. È stato prodotto dal clatrato di neon che perde l'atomo di neon, e viene sostituito con elio a 141  K e 150  MPa[22] Sono stati previsti dalla teoria idrati di elio con il rapporto elio/acqua nel ghiaccio-Ih, ghiaccio-Ic di 1:1, e nel ghiaccio-Ic di 2:1.[21] Questi composti possono esistere in pianeti come Nettuno o Urano.[22] Il clatrato idrato di elio dovrebbe essere simile al clatrato di idrogeno essendo le dimensioni simili alla molecola di idrogeno.[22]

L'elio può entrare nei cristalli di altri solidi molecolari sotto pressione alterandone la struttura e le proprietà. Per esempio la chlorpropamide a 0.3 GPa in elio diventa una struttura monoclina, mentre produce un'altra forma a 1.0 GPa.[23]

Fulleriti[modifica | modifica wikitesto]

L'elio può formare composti di intercalazione con le fulleriti, ad esempio con il buckminsterfullerene C60 e C70. Nel solido C60 vi sono spazi tra le sfere del composto, sia di forma tetraedrica che ottaedrica. L'elio può diffondersi nella fullerite solida anche a pressione atmosferica. L'elio entra nel reticolo in due fasi. La prima fase rapida richiede un paio di giorni, ed espande il reticolo dello 0,16% (cioè 2,2pm) riempiendo i siti ottaedrici più grandi. La seconda fase richiede migliaia di ore per assorbire più elio ed espandere nuovamente il reticolo due volte tanto (0,32%) riempiendo i siti tetraedrici. Tuttavia il solido C60·3He non è stabile e perde l'elio in circa 340 ore da quando non è sotto atmosfera di elio. Quando l'elio intercalato nella fullerite viene raffreddato avviene una transizione di fase orientativa che è 10K superiore a quella del solido C60 puro. L'effettiva variazione discontinua del volume a quel punto è più piccola, ma ci sono cambiamenti più rapidi vicino alla temperatura di transizione, forse a causa della diversa occupazione dei vuoti dall'elio.[24][25]

Endofullereni[modifica | modifica wikitesto]

Gli atomi di elio possono essere intrappolati all'interno di gabbie molecolari come nei fullereni. He@C60, He@C70, He2@C60 e He2@C70 sono stati tutti realizzati utilizzando elio compresso e fullereni.[26]

Usando pressione e temperatura possibili in maniera esaustiva, la resa è sempre bassa, inferiore all'1%. Tuttavia, rompendo e riformando le sfere di carbonio, si ottiene una maggiore concentrazione dei prodotti He@C60 or He@C70. La cromatografia liquida ad alte prestazioni può concentrare il materiale contenente elio. Con tale metodo sono stati ottenuti HeN@C60 e HeN@C70. Questi hanno una simmetria inferiore a causa dei due atomi intrappolati insieme nella stessa cavità. Ciò causa una linea ESR ampliata.[27]

Il Dodecaedrano può intrappolare l'elio da un fascio di ioni di elio per dare He@C20H20. Nella minuscola sfera interna la pressione è 4×1026 atmosfere.[28]

Anche altre gabbie come molecole inorganiche o organiche possono intrappolare l'elio, esempi sono C8He, dove gli atomi He sono dentro dei cubi, [29] e He@Mo6Cl8F6.[30]

Condensati di elio con impurezze[modifica | modifica wikitesto]

I Condensati di elio con impurezze (IHCs) (o gel di elio impuro)[31] vengono depositati sotto forma di mantello come gel in elio liquido così vari atomi o molecole vengono assorbiti sulla superficie di elio superfluido. Gli atomi possono essere H, N, Na, Ne, Ar, Kr, Xe, alcalini o terre alcaline. Le impurità formano ammassi di nanoparticelle ricoperti di elio localizzato trattenuto dalla forza di van der Waals. Gli atomi di elio non sono in grado di muoversi rispetto l'impurità, ma potrebbero farlo perpendicolarmente al loro strato.[32] La copertura nel solido è strutturata come un aerogel. Quando atomi liberi sono inclusi nella condensa è possibile ottenere un'elevata densità di energia superiore a 860 J cm−1 o 5 kJ g−1.[33] Questi condensati sono stati studiati per la prima volta come possibile carburante per i missili.[34] Alle miscele viene data una notazione a parentesi quadre, ad esempio [N]/[He] rappresenta un condensato di atomi azoto nell'elio.

[N]/[He] atomic nitrogen impurity helium is produced when a radio frequency discharge in a nitrogen helium mixture is absorbed into superfluid helium, it can have up to 4% nitrogen atoms included.[35] The substance resembles crumbly snow and condenses and settles from the liquid helium.[35] It also contains variable proportions of N2 molecules.[35] This substance is a high energy solid, with as much power as conventional explosives. When it is heated above 2.19 K (the lambda point of helium), the solid decomposes and explodes.[35] This substance is not a true compound, but more like a solid solution.[32] E. B. Gordon et al. suggested that this material may exist in 1974.[35] The localised helium shells around an individual atom are termed van der Waals spheres.[35] However the idea that the nitrogen atoms are dispersed in the helium has been replaced by the concept of nitrogen atoms attached to the surface of clusters of nitrogen molecules. The energy density of the solid can be increased by pressing it.[36]

Other inert gas impurity helium condensates can also be made from a gas beam into superfluid helium.[37] [Ne]/[He] decomposes at 8.5 K with release of heat and formation of solid neon. Its composition approximates NeHe16.

[Ar]/[He] contains 40–60 helium atoms per argon atom.[38]

[Kr]/[He] contains 40–60 helium atoms per krypton atom[38] and is stable up to 20 K.[33]

[Xe]/[He] contains 40–60 helium atoms per xenon atom.[38]

[N2]/[He] contains 12—17 He atoms per N2 molecule.[38] It is stable up to 13 K[33]

[N]/[Ne]/[He] Formed from a gas beam generated from a radio-frequency electric discharge in mixtures of neon, nitrogen and helium blown into superfluid He. Additional inert gas stabilises more nitrogen atoms. It decomposes around 7 K with a blue green light flash.[37] Excited nitrogen atoms in the N(2D) state can be relative long lasting, up to hours, and give off a green luminescence.[37]

[H2]/[He], or [D2]/[He] when dihydrogen or dideuterium is absorbed into superfluid helium, filaments are formed. When enough of these form, the solid resembles cotton, rather than snow.[39] Using H2 results in the product floating and stopping further production, but with deuterium, or a half-half mixture, it can sink and accumulate.[33] Atomic hydrogen in impurity helium decays fairly rapidly due to quantum tunneling (H + H → H2). Atomic deuterium dimerises slower (D + D → D2), but reacts very quickly with any diprotium present. (D + H2 → HD + H).[33] Atomic hydrogen solids are further stabilised by other noble gases such as krypton.[40][41][42] Lowering temperatures into the millikelvin range can prolong the lifetime of atomic hydrogen condensates.[34] Condensates containing heavy water or deuterium are under investigation for the production of ultracold neutrons.[31] Other impurity gels have been investigated for producing ultracold neutrons include CD4 (deuterated methane) and C2D5OD. (deuterated ethanol)[43]

The water-helium condensate [H2O]/[He] contains water clusters of several nanometers in diameter, and pores from 8 to 800 nm.[44]

Oxygen O2 impurity helium contains solid oxygen clusters from 1 to 100 nm.[45]

Elio sodio con impurezze[modifica | modifica wikitesto]

Introducing impurities into solid helium yields a blue solid that melts at a higher temperature than pure He.[46] For cesium the absorption has a peak at 750 nm, and for rubidium, maximal absorption is at 640 nm. These are due to metal clusters with diameters of 10 nm or so. However the low concentration of clusters in this substance should not be sufficient to solidify helium as the amount of metal in the solid is less than billionth that of the impurity helium condensate solids, and liquid helium does not "wet" cesium metal. The solid is possibly due to helium snowballs attached to Cs+ (or Rb+) ions.[46] The snowball is a shell that contains helium atoms solidified in particular positions around the ion. The helium atoms are immobilized in the snowball by polarization. Neutral metallic atoms in liquid helium are also surrounded by a bubble caused by electron repulsion. They have typical sizes ranging from 10 to 14 Å diameter.[47] Free electrons in liquid helium are enclosed in a bubble 17 Å in diameter. Under 25 atmosphere pressure an electron bubble reduces to 11 Å.[48]

Soluzione solida[modifica | modifica wikitesto]

Helium can dissolve to a limited extent in hot metal, with concentration proportional to pressure. At atmospheric pressure, 500 °C bismuth can absorb 1 part in a billion; at 649 °C lithium can take 5 parts per billion; and at 482 °C potassium can take 2.9 parts per million (all atom fractions).[49] In nickel there can be 1 in 1010 atoms, and in gold 1 in 107. The supposition is that the higher the melting point the less helium can be dissolved. However, when a liquid metal is quenched, higher concentrations of helium can be left dissolved. So cooled liquid steel can have one part per million of helium. In order to get a helium atom into a metal lattice, a hole has to be formed. The energy to make that hole in the metal is basically the heat of solution.[50]

Nanofili[modifica | modifica wikitesto]

Gold, copper, rubidium, caesium, or barium atoms evaporated into liquid helium form spider web like structures.[51] Rhenium produces nano flakes. Molybdenum, tungsten, and niobium produce thin nanowires with diameters of 20, 25 and 40 Å.[52] When platinum, molybdenum or tungsten is evaporated into liquid helium, nanoclusters are first formed, accompanied by high temperature thermal emission pulse, above the melting point of the metals. In superfluid helium, these clusters migrate to the vortices and weld together to yield nanowires once the clusters are mostly solid. In higher temperature liquid helium, larger clusters of metal are formed instead of wires. The metal vapours can only penetrate about 0.5 mm into liquid helium.[53] Indium, tin, lead and nickel produce nanowires about 80 Å in diameter.[54] These same four metals also produce smooth spheres about 2 μm across that explode when examined with an electron microscope.[55] Copper, permalloy, and bismuth also make nanowires.[56]

Cristallo ionico bidimensionale[modifica | modifica wikitesto]

Helium II ions (He+) in liquid helium when attracted by an electric field can form a two-dimensional crystal at temperatures below 100 mK. There are about half a trillion ions per square meter just below the surface of the helium. Free electrons float above the helium surface.[57]

Molecole di van der Waals[modifica | modifica wikitesto]

  • LiHe[58]
  • Dihelium
  • Trihelium
  • Ag3He[59]
  • HeCO is weakly bound by van der Waals forces. It is potentially important in cold interstellar media as both CO and He are common.[60]
  • CF4He and CCl4He both exist.[61]
  • HeI2 can be formed by supersonic expansion of high pressure helium with a trace of iodine into a vacuum. It was the first known triatomic helium van der Waals molecule. It can be detected by fluorescence. HeI2 has a similar optical spectrum to I2, except that the bands and lines are shifted to form two extra series. One series is blueshifted by between 2.4 and 4.0 cm−1, and the other between 9.4 and 9.9 cm−1. The two series may be due to different amounts of vibration in the He–I bond. The lines are narrow indicating that the molecules in their excited vibrational state have a long lifetime.[62]
  • Na2He molecules can form on the surface of helium nanodroplets.[63]

Ioni[modifica | modifica wikitesto]

Helium has the highest ionisation energy, so a He+ ion will strip electrons off any other neutral atom or molecule. However it can also then bind to the ion produced. The He+ ion can be studied in gas, or in liquid helium. Its chemistry is not completely trivial. For example, He+ can react with SF6 to yield SF+6 or SF+5 and atomic fluorine.[64]

Complessi ionici[modifica | modifica wikitesto]

He+2 was predicted to exist by Linus Pauling in 1933. It was discovered when doing mass spectroscopy on ionised helium. The dihelium cation is formed by an ionised helium atom combining with a helium atom: He+ + He → He+2.[65]

The diionised dihelium He2+2 (1Σ+g) is in a singlet state. It breaks up He2+2 → He+ + He+ releasing 200 kcal/mol of energy. It has a barrier to decomposition of 35 kcal/mol and a bond length of 0.70 Å.[65]

The trihelium cation He+3[66] is in equilibrium with He+2 between 135 and 200K[67]

Elionio[modifica | modifica wikitesto]

The helium hydride ion HeH+ has been known since 1925.[65] The protonated dihelium ion He2H+ can be formed when the dihelium cation reacts with dihydrogen: He+2 + H2 → He2H+ + H. This is believed to be a linear molecule.[65] Larger protonated helium cluster ions exist HenH+ with n from 3 to 14. He6H+ and He13H+ appear to be more common. These can be made by reacting the H+2 or the H+3 with gaseous helium.[65]

HeH2+ is unstable in its ground state. But when it is excited to the 2pσ state the molecule is bound with an energy of 20 lcalmol−1. This doubly charged ion has been made by accelerating the helium hydride ion to 900 keV, and firing it into argon. It only has a short life of 4 ns.[65]

H2He+ has been made and could occur in nature via H2 + He+ → H2He+.[65]

H3He+n exists for n from 1 to over 30, and there are also clusters with more hydrogen atoms and helium.[68]

Gas nobili[modifica | modifica wikitesto]

Noble gas cluster ions exist for different noble gases. Singly charged cluster ions containing xenon exist with the formula HenXe+m, where n and m ≥ 1.[69]

Many different HenKr+ exist with n=1 to 17 at least. HenKr+2 and HenKr+3 also exist for many values of n. He12Kr+2 and He12Kr+3 ions are commons. These singly charged cluster ions can be made from krypton in helium nanodroplets subject to vacuum ultraviolet radiation.[69]

The Ar+ argon ion can form many different sized clusters with helium ranging from HeAr+ to He50Ar+, but the most common clusters are He12Ar+ and smaller. These clusters are made by capturing an argon atom in a liquid helium nanodroplet, and then ionising with high speed electrons. He+ is formed, which can transfer charge to argon and then form a cluster ion when the rest of the droplet evaporates.[70]

NeHe+n can be made by ultraviolet photoionisation. Clusters only contain one neon atom. The number of helium atoms n can vary from 1 to 23, but NeHe+4 and NeHe+8 are more likely to be observed.[69]

Doubly charged ions of helium with noble gas atoms also exist including ArHe2+, KrHe2+, and XeHe2+.[71]

Metalli[modifica | modifica wikitesto]

Various metal-helium ions are known.

Alkali metal helide ions are known for all the alkalis. The molecule ground state for the diatomic ions is in the X1Σ+ state. The bond length gets bigger as the periodic table is descended with lengths of 1.96, 2.41, 2.90, 3.10, and 3.38 Å for Li+He, Na+He, K+He, Rb+He, and Cs+He. The dissociation energies are 1.9, 0.9, 0.5, 0.4 and 0.3 kcal/mol, showing bond energy decreases. When the molecule breaks up the positive charge is never on the helium atom.[65]

When there are many helium atoms around, alkali metal ions can attract shells of helium atoms. Clusters can be formed from absorbing metal into helium droplets. The doped droplets are ionised with high speed electrons. For sodium clusters appear with the formula Na+Hen with n from 1 to 26. Na+He is the most common, but Na+He2 is very close in abundance. Na+He8 is much more abundant than clusters with more helium. Na+2Hen with n from 1 to 20 also appears. Na+3Hen with small n is also made. For potassium, K+Hen with n up to 28, and K+2Hen for n from 1 to 20 is formed. K+He and K+He2 are both common, and K+He12 is a bit more commonly formed than other similar sized clusters.[72] Cesium and rubidium cations also form clusters with helium.[72]

Other known metal-helium ions include Cr+He, Co+He, Co+He3, Ni+He, and Ni+He3.[65] PtHe2+;[73][74] formed by high electric field off platinum surface in helium,[71] VHe2+,[71] HeRh2+ is decomposed in high strength electric field,[75][76] Ta2+He, Mo2+He, W2+He, Re2+He, Ir2+He, Pt2+He2, W3+He2, W3+He3, and W3+He4.[65]

Non metalli[modifica | modifica wikitesto]

HeN+2 can form at around 4 K from an ion beam of N+2 into cold helium gas.[77] The energy needed to break up the molecule is 140 cm−1 which is quite a bit stronger than the van der Waals neutral molecules. HeN+2 is tough enough to have several vibrational, bending and rotational states.[78] HenN+2 with n from 2 to 6 have been made by shooting electrons at a supersonically expanding mix of nitrogen and helium.[65]

C60He+ is formed by irradiating C60 with 50eV electrons and then steering ions into cold helium gas. C60He+2 is also known.[79]

He(OH)+ has been detected, although it is not produced when HTO (tritiated water) decays.[65]

Hen(CO)+ has been detected for values of n from 1 to 12. Also CH3He+, OCHHe+ and NH2He+ have been detected.[65]

Young and Coggiola claimed to make HeC+ by an electric discharge off graphite into helium.[80]

When tritium substituted methane (CH3T) decays, CH3He+ is produced in a very small amount.[81]

The helium formyl cation, HeHCO+ is a linear molecule. It has a vibrational frequency red shifted 12.4 cm−1 compared to HCO+. It can be considered as a deenergized protonation reaction intermediate for the HeH+ + CO → HCO+ + He.[78] HeHCO+ can be produced by a supersonic expansion of a gas mixture of He, CO, and H2, which is hit by a cross beam of electrons. CO and H2 are only supplied at 1% of the helium.[78]

The HeHN+2 molecule is linear. The He-H bondlength is 1.72 Å. It has an infrared band, due to B-H stretching, with a base at 3158.42 cm−1.[78][82] The binding energy is 378 cm−1 in the 000 vibrational state, and 431 cm−1 in the 100 vibrational state.[83] He2HN+2 is also known. One helium atom is linked to a hydrogen, and the other is less tightly bound.[83]

Eccimeri[modifica | modifica wikitesto]

The He*2 excimer is responsible for the Hopfield continuum. Helium also forms an excimer with barium, Ba+He*.[84]

Composti teorici[modifica | modifica wikitesto]

Solidi[modifica | modifica wikitesto]

Crystal structure of the hypothetical compound MgF2He. Helium in white, magnesium in orange and fluorine in blue

He(H2O)2 is predicted to form a solid with orthorhomic structure Ibam.[85]

Iron helide (FeHe) was early on claimed to have been found,[86] but the discovery was classified as an alloy.[49] Early studies predicted the FeHe exists as an interstitial compound under high pressure,[87] perhaps in dense planetary cores,[88] or, as suggested by Freeman Dyson, in neutron star crust material.[89] Recent density functional theory calculations predict the formation of FeHe compounds at pressures above about 4 TPa,[90] suggesting indeed that these compounds could be found inside giant planets, white dwarf stars, or neutron stars.

Na2HeO is predicted to have a similar structure to Na2He, but with oxygen atoms in the same position as the electron pair, so that it becomes O2−. It would be stable from 13 to 106 GPa.[2] This substance could be a way to store helium in a solid.[91]

La2/3-xLi3xTiO3He is a porous lithium ion conduction perovskite that can contain helium like a clathrate.[29]

Helium is predicted to be included under pressure in ionic compounds of the form A2B or AB2. These compounds could include Na2OHe, MgF2He (over 107 GPa) and CaF2He (30-110 GPa). Stabilisation occurs by the helium atom positioning itself between the two like charged ions, and partially shielding them from each other.[92]

Molecole di van der Waals[modifica | modifica wikitesto]

The beryllium oxide helium adduct, HeBeO is believed to be bonded much more strongly than a normal van der Waals molecule with about 5 kcal/mol of binding energy. The bond is enhanced by a dipole induced positive charge on beryllium, and a vacancy in the σ orbital on beryllium where it faces the helium.[93][94]

Variations on the beryllium oxide adduct include HeBe2O2,[94] RNBeHe including HNBeHe, CH3NBeHe,[94] CH4−xNBeHex, SiH4−xNBeHex, NH3−xNBeHex, PH3−xNBeHex, OH2−xNBeHex, SH2−xNBeHex,[95] and HeBe(C5H5)+.[96]

Hydridohelium fluoride HHeF is predicted to have a Template:Clarify.[97] The lifetime of the deuterium isotopomer is predicted to be much longer due to a greater difficulty of tunneling for deuterium.[98] This molecule's metastability is slated due to electrostatic attraction between HHe+ and F− which increases the barrier to an exothermic breakup.[93] Under pressures over 23 GPa HHeF should be stable.[99]

Calculations for coinage metal fluorides include HeCuF as stable,[97] HeAgF is unstable,[97] HeAuF is predicted,[97] and Ag3He with binding energy 1.4 cm−1,[100] Ag4He binding energy 1.85 cm−1, Au3He binding energy 4.91 cm−1,[100] and Au4He binding energy 5.87 cm−1[100]

HeNaO is predicted.

Calculation for binary van der Waals helium molecules include HeNe, Li4He binding energy 0.008 cm−1, the Li3He is not stable.[100] Na4He binding energy 0.03 cm−1, the Na3He is not stable.[100] Cu3He binding energy 0.90 cm−1,[100] O4He binding energy 5.83 cm−1,[100] S4He binding energy 6.34 cm−1,[100] Se4He binding energy 6.50 cm−1,[100] F4He binding energy 3.85 cm−1,[100] Cl4He binding energy 7.48 cm−1,[100] Br4He binding energy 7.75 cm−1,[100] I4He binding energy 8.40 cm−1,[100] N4He binding energy 2.85 cm−1,[100] P4He binding energy 3.42 cm−1,[100] As4He binding energy 3.49 cm−1,[100] Bi4He binding energy 33.26 cm−1,[100] Si4He binding energy 1.95 cm−1,[100] Ge4He binding energy 2.08 cm−1,[100] CaH4He binding energy 0.96 cm−1,[100] NH4He binding energy 4.42 cm−1,[100] MnH4He binding energy 1.01 cm−1,[100] YbF4He binding energy 5.57 cm−1[100] I42He or I32He,[101]

Bonds are predicted to form to nickel with helium as a weak ligand in HeNiCO and HeNiN2.[93]

(HeO)(LiF)2 is predicted to form a planar metastable molecule.[102] 1-Tris(pyrazolyl)borate beryllium and 1-tris(pyrazolyl)borate magnesium are predicted to bind helium at low temperatures.[103] There is also a prediction of a He-O bond in a molecule with caesium fluoride or tetramethyl ammonium fluoride.[104]

LiHe2 is predicted to be in an Efimov state when excited.[105]

Ioni[modifica | modifica wikitesto]

Fluoroheliate ion

Many ions have been investigated theoretically to see if they could exist. Just about every diatomic cation with helium has been studied. For the diatomic dications, for stability the second ionisation level of the partner atom has to be below the first ionisation level of helium, 24.6 eV. For Li, F, and Ne the ground state is repulsive, so molecules will not form. For N and O the molecule would break up to release He+. However HeBe2+, HeB2+ and HeC2+ are predicted to be stable. Also second row elements from Na to Cl are predicted to have a stable HeX2+ ion.[65]

HeY3+ is predicted to be the lightest stable diatomic triply charged ion.[106] Other possibly thermochemically stable ions include HeZr3+, HeHf3+, HeLa3+, HeNd3+, HeCe3+, HePr3+, HePm3+, HeSm3+, HeGa3+, HeTb3+, HeDy3+, HeHo3+, HeEr3+, HeTm3+, and HeLu3+ where the third ionisation point is below that of helium.[65]

The positronium helide ion PsHe+ should be formed when positrons encounter helium.[107]

The Fluoroheliate FHeO− ion should be stable but salts like LiFHeO are not stable.[108][66]

  • HHeCO+ theoretical[109]
  • FHeS- is predicted to be stable.[110]
  • FHeBN−
  • HHeN2+ is unlikely to exist.[111]
  • (HHe+)(OH2) is probably unstable.[112]

The lithium hydrohelide cation HLiHe+ is linear in theory. This molecular ion could exist with big bang nucleosynthesis elements.[113] Other hydrohelide cations that exist in theory are HNaHe+ sodium hydrohelide cation, HKHe+ potassium hydrohelide cation, HBeHe2+ beryllium hydrohelide cation, HMgHe2+ magnesium hydrohelide cation, and HCaHe2+ calcium hydrohelide cation.[113]

HeBeO+ is predicted to have a relatively high binding energy of 25 kcal mol−1.[114]

For negative ions the adduct is very weakly bound.[65] Those studied include HeCl−, HeBr−, HeF−, HeO− and HeS−.[66]

HHeNH+3 is predicted to have a C3v symmetry and a H-He bond length of 0.768 Å and He-N 1.830. The energy barrier against decomposition to ammonium is 19.1 kJ/mol with an energy release of 563.4 kJ/mol. Decomposition to hydrohelium ion and ammonium releases 126.2 kJ/mol.[66]

Osservazioni screditate o improbabili[modifica | modifica wikitesto]

Numerous researchers attempted to create chemical compounds of helium in the early part of the twentieth century.[115] In 1895 L. Troost and L. Ouvrard believed they had witnessed a reaction between magnesium vapour and helium (and also argon) due to the spectrum of helium disappearing from the tube they were passing it through.[116] In 1906, W. Ternant Cooke claimed to have noticed a reaction of helium with cadmium or mercury vapour by observing an increase in the density of the vapour. Zinc vapour did not react with helium.[117]

J. J. Manley claimed to have found gaseous mercury helide HeHg in 1925[118][119][120] HgHe10;[121][122] publishing the results in Nature, but then had trouble finding a stable composition, and eventually gave up.

Between 1925 and 1940 in Buenos Aires, Horacio Damianovich studied various metal-helium combinations including beryllium (BeHe), iron (FeHe), palladium (PdHe), platinum (Pt3He), bismuth, and uranium.[123][86] To make these substances, electrical discharges impacted helium into the surface of the metal.[4] Later these were demoted from the status of compounds, to that of alloys.[49]

Platinum helide, Pt3He was discredited by J. G. Waller in 1960.[124]

Palladium helide, PdHe is formed from tritium decay in palladium tritide, the helium (3He) is retained in the solid as a solution.

Boomer claimed the discovery of tungsten helide WHe2 as a black solid.[125] It is formed by way of an electric discharge in helium with a heated tungsten filament. When dissolved in nitric acid or potassium hydroxide, tungstic acid forms and helium escapes in bubbles. The electric discharge had a current of 5 mA and 1000 V at a pressure between 0.05 and 0.5 mm Hg for the helium. Functional electrolysis currents are from 2-20 mA, and 5-10 mA works best. The process works slowly at 200 V. and 0.02 mm Hg of mercury vapour accelerates tungsten evaporation by five times. The search for this was suggested by Ernest Rutherford. It was discredited by J. G. Waller in 1960.[124] Boomer also studied mercury, iodine, sulfur, and phosphorus combinations with helium. Mercury and iodine helium combinations decomposed around −70 °C[126] Sulfur and phosphorus helium combinations decomposed around −120 °C[126]

H. Krefft and R. Rompe claimed reactions between helium and sodium, potassium, zinc, rubidium, indium, and thallium.[130]

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