The spectra of solutions of sodium nitrite in aqueous solutions of sulphuric and perchloric acids have been measured at a number of temperatures, and from these spectra the concentrations of the species HNO2 and NO+ have been calculated over the acid-concentration ranges 0-100% sulphuric acid (by wt.) and 0-72% perchloric acid (by wt.). The changes due to temperature were found to be less than the experimental error in the determination of the concentrations of nitrous acid and nitrosonium ion. The equilibria of importance in such systems are discussed and equilibrium constants have been calculated for the reactions H2SO4 + HNO2 <=> H2O + NO+ + HSO4- and HClO4 + HNO2 H2O + NO+ + ClO4- Attempts have been made to correlate the determined values for the concentration of nitrous acid and nitrosonium ion with the acidity functions J0, C0, and Hr for both solvent systems. The nitroacidium ion (H2NO2+) in these systems is found to be less important than previously thought.'
Data have been obtained relevant to the mechanism of formation of nitrite, oxygen and peroxynitrite by the photolysis of aqueous sodium nitrate solutions. In alkaline solutions, the origin of O2 as determined by the use of 18O-enriched water, and the lowering of nitrite yields by various solutes, are consistent with the dissociative process �������������� ����������������������������� (NO3-)*→ NO2-+Orather than with ����������������������������� (NO3-)*→ NO2+O- The effect of these solutes is attributed to reduction of (NO3-)*. It is proposed that peroxynitrite (OONO-) is formed by intramolecular rearrangement of the pyramidal (NO3-)* in the 3ππ state. Peroxynitrite is not a precursor to NO2-. The quantum yield depends on both wavelength and temperature, and the apparent energy of activation decreases with decreasing wavelength. These results are interpreted in terms of the different reactivities of (NO3-)* produced in the nπ* and the ππ* absorption bands. ��� Low initial yields at low pH can be explained by protonation of (NO3-)* followed by ��������������������������������� (HOONO)*→ OH+NO2and ���������������������������������� OH+NO2→ H++NO3 The self-inhibition process evident in acidic conditions is consistent with the formation of NO3 (from NO2 and O) which re-oxidizes NO2-. Sharp yield increases on the addition of some reagents are due to scavenging of the OH intermediate in the (HOONO)* → NO3- reversion.
The ultraviolet spectrum of sodium nitrite in aqueous sulphuric acid is essentially that of nitrous acid below 40 per cent, acid, and that of nitrosonium ion above 70 per cant. acid. In the intermediate range there is evidence that the nitroacidium ion (H2NO2+) is an important constituent of the solution. Equilibria involving the three species have been calculated using activity data for water and sulphuric acid. Similar results are obtained in aqueous phosphoric acid as a solvent. In hydrochloric acid of low water activity the high chloride activity causes what appears to be almost total conversion to nitrosyl chloride. The ultraviolet spectrum of the nitrosonium ion is a structureless transition with εmax.=3850 at about 46 kK (2200 Ǻ). The nitroacidium ion does not absorb appreciably within the accessible range.
The ultraviolet spectra of benzene, toluene, and chlorobenzene at 2600 and 2000 Ǻ have been measured in carbon tetrachloride, chloroform, cyclohexane, 1%-hexane, ethanol, and water. Compared with the gases the solution spectra are all displaced to the red by amounts that agree qualitatively with the predicted effect of the solvent refractive index and the transition intensity according to the theory of Bayliss (1950). Quantitative agreement with this theory can be obtained only by assuming the effective cavity occupied by the solute molecule to be considerably smaller than the actual molecular size. The significance of this effect is discussed. The intensities of the solution spectra vary with the solvent refractive index, but in a way that is incompatible with the classical theory of Chako (1934). A marked increase in the intensity (particularly in toluene) is found where the solute absorption is close to an absorption band of the solvent, that is, for the 2600 Ǻ transitions in carbon tetrachloride and to a less extent in chloroform. In the 2600 Ǻ transition of benzene, a band appears in water, chloroform, and carbon tetrachloride that is very close to the position of the (0,0) band that is forbidden in the gas spectrum. The nature of this band is discussed.
Frequency displacements and intensities are reported for the C=O stretching fundamental and first overtone in acetone, acetaldehyde, and diisopropyl ketone, for the chloroform C-H stretching fundamental, and for the acetonitrile C-C stretching fundamental, all in a variety of non-polar and polar solvents. The solvent displacement of C-C is very small (?1 K), for C-H and C=O it is to the red and of the order of 10-20 K, with C=O overtones being displaced about twice as much as the fundamentals. The Kirkwood relation between the frequency displacement and solvent dielectric constant is inadequate if the static D is used. The C=O results can be. interpreted in terms of two superimposed effects : (i) the electronic polarization of the solvent causes a frequency shift related to the solvent refractive index, and (ii) in polar solvents there is an additional shift due to solvent dipole orientation. Effect (ii) causes an added contribution to the intensity. The C-H results do not fit easily into this interpretation.
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