Investigation of the 2050 Å system of the nitric oxide dimer

Abstract

The far ultra-violet continuum of nitric oxide was assigned to the dimer by obtaining the following experimental evidence : (a) the intensity of the spectrum is proportional to [NO]²; (b) the possibility that the carrier may be some other known oxide of nitrogen was eliminated; (c) from the temperature dependence the enthalpy change accompanying dissociation was 2.24 (±0.1 ) kcal/mol (100–140 K), in good agreement with 2.4 (±0.2) kcal/mol deduced from the dimer infra-red intensities⁴; (d) approximate upper and lower limits to the dissociation constant, and the temperature dependence, were consistent with Guggenheim's⁵ method of analyzing the second virial coefficients by means of the principle of corresponding states.

An absolute scale of dissociation constants was fixed from the excess of the reduced second virial coefficient at its normal boiling point, and the temperature dependence found spectroscopically. The dissociation energy of the dimer is 1.6 (±0.1) kcal/mol. Evidence for substantial population of excited states of the dimer at 300 K was discussed. The dimer continuum exhibits a maximum at 2050 Å, and the oscillator strength of the system was recorded as 0.26.