SUPERSONIC MOLECULAR-BEAM MASS-SPECTROMETRIC STUDY OF HYDROGEN BONDING IN AMMONIA

A supersonic molecular beam expanding from ammonia vapor at various pressures and temperatures is ionized by vacuum ultraviolet radiation dispersed from an electron storage ring. The resulting photoions are detected with a quadrupole mass spectrometer. Ions belonging to the series (NH3)nH+ for 1 ≤ n ≤ 9 are detected in addition to monomer molecular and fragment ions, indicating the presence of several clusters larger than the dimer. Examination of the dependence of ion intensities on source temperature, on pressure, and on nozzle-skimmer distance, indicates complex interaction among these variables, especially at high source pressures (≥200 torr). The temperature dependence of ion intensities at 100 torr provides estimates of the hydrogen bond energies for stepwise formation of ammonia clusters. These estimates (-4.61, -3.88, -5.22, and -3.84 ± 0.5 kcal mol-1 for dimer through pentamer, respectively) indicate cooperative enhancement of the stability of the neutral ammonia tetramer. This result is discussed in terms of a general model for hydrogen bonding, and is compared with results of other theoretical and experimental studies. © 1979.