The Nitrogen Tetrahydride Anion, NH

The NH4 ion most probably exists both as a stable H NHg ion-molecule complex and as a tetrahedral species with a geometry very similar to that of NH4, i.e., as a so-called double-Rydberg anion 

The H NH3 complex has long been thought to occur in liquid-ammonia solutions as an intermediate in the proton-transfer reaction NHJ + H2 H + NH3 [1 to 4]. 

Gaseous NH4 ions were generated in a Fourier transform ion cyclotron resonance (ICR) spectrometer by the reaction of formaldehyde with NH2 [5] via the steps 

Deuterium-labeling experiments on the formation of NH4, its reaction with formaldehyde (NH4+CH20- NH3 + CH30 ) [5], and on the collision-induced dissociation and charge reversal of NH4 [6] indicated that the four H atoms were not equivalent and that the anion is best described as a H ion solvated by an ammonia molecule, i.e., H NH3. 

The structures and stabilities of the two NH4 isomers have been studied theoretically with various quantum-chemical methods. Geometry optimizations for the H NH3 species by the CEPA [9], MP2 and SDQ-CI [10], MP2 [11, 12], SD-CI [13], and MBPT(2) [14] methods all agreed on a structure of Cg symmetry in which the H ion is bound nearly linearly to one of the N-H bonds of the slightly distorted NH3 molecule with a small tilt (maximal 15 ) toward the other hydrogen atoms. The relatively large H-H distance (around 2 A) Indicates a bond arising from dipole-dipole attraction. Values between 25 and 63 kJ/mol 

---