J. K. S. Wan
Is There a Neutral Ammonium Radical?
Queen's University Kingston, Ontario, Canada
Forty years ago Sidgwick (1) recognized that the first row elements such as nitrogen are rigorously restricted to an octet of valence electrons and thus assigned a "maximum covalency" of four to the firstrow elements. Apparently, this is a consequence of the electronic configurations of these first-row atoms, which can accommodate a maximum of eight electrons in their 2s and 2 p orbitals. The well-known stable ammonium cation, NH4+, probably owes its stability to the valence shell octet. While it is generally accepted that ammonium compounds are similar in properties to those of alkali compounds, the question as to whether ammonium can exist in a neutral state as a radical or a metal has been a controversial subject in chemistry for many years. Some Theoretical Considerations
Two interesting theoretical approaches have been made in recent years in order to give some insight into the existence of NH,. The first approach was given by Bernstein (8) who related the stability of NHp to the question as to whether saturated molecules such as NH3 in the gas phase have an affinity for a hydrogen atom, since hydrogen bond formation between NH3 and ROH in gas phase has been detected by infrared spectroscopy. With the simple thermochemical cycle constrncted below, Bernstein (8) obtained the relation D(X-H)
+ D(XH-A)
=
+ D(X-HA)
D(H-A)
(1)
X + H + A D(X-H)/ XH D(XH-A)\
+
A
\
DW-A) X HA ~ ( H A - x )
+
XHA
.where A = saturated molecule, NH3, HX = hydrogen donor molecule, and D = bond dissociation energy. Since D(XH-A) equals -AH,, the enthalpy of hydrogen bond formation, one readily obtains the bond dissociation energy of the HA radical in the form of D(H-A) = D(X-H) - D(X-HA) - AH, (2) By treating X H and X-HA as diatoms so that an approximation of D(X-H) and D(X-HA) may be obtained from a Morse function, Bernstein (2) made an estimate t,hat NH, in the gas phase could he stable by about 7 to 33 kcal/mole. In a more recent and modified treatment (3) the same author estimated the stability to be less than 10 kcal/mole. In the other approach, Bishop (4) carried out a quantum mechanical calculation in which the neutral NH4 was described by a wave function composed of Slater orbitals centered on the nucleus of the nitrogen 40
/
Journal of
Chemicol Education
atom. The wave functions were of the same type as those used in a similar calculation for methane (5) except that the extra electron was placed in a 3s obital. The result of the calculation suggested that NH, is stable by approximately 4.4 kcal/mole and its ionization potential is about 3.94 ev. Although the calculated ionization energy seems to be slightly lower than the value of 4.2 ev, which could be calculated from a BornHaber cycle of ammonium halide crystals, nevertheless, Bishop's work is probably more reliable than a previous calculation by Horvath (6),who, using a set of less satisfactory wavefunctions, predicted NH, to be unstable. If one accepts the arguments proposed by Bernstein (2) and Bishop (4) that NH4 is a stable molecule, the next formidable task is to find direct experimental support. Some Experimental Approaches
The task of producing a neutral NH4 molecule for experimental study is by no means an easy one. Bernal and Massey (7) claimed that the reaction does not occur at low pressures and their calculations show that at O°K, a transition should occur from a mixture of crystalline NH3and Hz to metallic NH, at a pressure between 60,000 and 140,000 atmospheres. Such conditions of temperature and pressure would challenge and defy the effort by any experimentalist of the present day. However, Melton and co-workers (8, 9) seemed to have found an easier experimental approach and obtained some promising results in their study of decomposition of NH, on a platinum wire a t temperatures ranging from 75 to 1300°C. Using a mass spectrometer, they observed a transient ion, N&+; evolved from the hot surface of the catalyst. They suggested that reaction of NH3 with Hz, which was formed in the decomposition, led to formation of some species of empirical composition NH4 which was then ionized by the hot platinum surface to give N&+. A similar experiment of catalytic decomposition of HzO also leads to observation and conclusion of the presence of H30 (10). In a separate investigation, Martin (11 ) studied the room temperature emission of transient ions from NHs and Hz0 evolving from a palladium surface. In the case of NH,, Martin obtained a complex emission spectnm ranging from NHn+ to N+ ions. His result suggested complete atom scrambling and redistribution of the NH3 molecules on the catalyst surface. In the case of H20, he found H40+to be the major transient ion instead of the expected H30+. The apparent difference of Martin's and Melton's results could be partly due to the use of a different catalyst.
Although the evidence of NH4 from these studies is not direct,, it argues strongly in favor of the existence of NH,. Perhaps similar experiments can he carried out in an epr cavity with NH3flowingthrough aplatinum catalyst and the radical NH4 could be detected on the catalyst surface before it is ionized and desorbed. Historically two other major types of attempt have been made to search for experimental evidence of NHp. The first involved investigations of the reaction of a H atom with NHa and the second group studied the neutralization of NH4+by an electron.
Both H2Cl and HC1, were.treated as transition states with linear structure and their vibrational frequencies were calculated from kinetic analyses (19, 20). While such examples suggest reaction (9) can he treated so as to produce NH4 in a transition state, direct experimental detection of NH4 was lacking up to that time. Flash photolysis experiments involving reaction of NHa with hydrogen failed to produce NH4 (3). An esr study (21) of H atoms, generated by microwave discharge, reacting with NH8 in a flow system in which products were trapped at 77'I
