NH3 Stretching Vibrations

Gold (I) cyanide [506-65-0] M 223.0, m dec on heating. The lemon yellow powder is sparingly soluble in H2O and EtOH but soluble in aqueous NH3. It is obtained by heating H[Au(CN)2] at 110°. Wash well with H2O and EtOH and dry at 110°. It has an IR band at v 2239cm typical fo C=N stretching vibration. [Handbook of Preparative Inorg anic Chemistry (Ed. Brauer) Vol II 1064 7965.] CARE may evolve HCN. 

It is more convenient to start with the triflate ion [Rh(NH3)5(CF3S03)]2+ since triflate is a much better leaving group than chloride and is immediately replaced by liquid ammonia [87]. A third route involves acid hydrolysis of the cyanate complex [Rh(NH3)5(NCO)]2+, which proceeds quantitatively (probably via a carbamic acid complex). Vibrational studies on Rh(NH3) + assign stretching vibrations as i(Alg) at 514cm-1, i/2(Eg) at 483 cm-1 and i/j(T,u) at 472 cm-1 [88],... 

Further changes in C-O stretching vibrations occur when other ligands are present. For example, the CO stretching band in Cr(CO)6 is found at 2100cm-1, whereas that in Cr(NH3)3(CO)3 is found at approximately 1900 cm-1. The structure of the latter compound is... 

Inelastic tunnelling electrons can also be used to selectively induce either the translation over a metallic surface or desorption from the metallic surface of individual molecules, as has been shown for NH3 on Cu(lOO) surfaces (Pascual et al, 2003). Activation of either the stretching vibration of ammonia ( 408 meV) leading to lateral translation on the surface, or the inversion of its pyramidal structure (umbrella mode s(NH3) 139 meV) leading to desorption, can be achieved by adjusting 7t and Vt. 

The four observed frequencies and the pictorial representations of the normal modes for NH3 are displayed in Fig. 7.3.1. From this figure, we can see that vi and V3 are stretching modes, while i 2 and m are bending modes. Note that the number of stretching vibrations is equal to the number of bonds. Also, the stretching modes have higher frequencies than the bending ones. 

For the amines they found NH3 < MeNH2 < Me2NH < Me3N. For the chloroform-amine complexes the Av are 12,26,45 and 53 cm-1 in this order. No subbands were resolved. This is, no doubt, due to the low frequency of the bridge stretching vibration v for these very weakly H-bonded systems and to the weakness of the anharmonic coupling between them. The existence of the molecules in more than one conformation broadens the bands and makes the observation of the subbands even more difficult. 

---