Unsymmetric Triatomic Molecules

Linear Unsymmetric Triatomic Molecules.—Reducing the symmetry from Daoh to Coot, as in NaO, OCS, and HCN, increases the number of parameters in the general quartic force field to 2re + 4/2 + 6f3 + 9/i Table 7 shows their relationship to the primary spectroscopic observables. It is clear that problems of insufficient data to determine the general force field are already on the horizon for example, data from at least two different isotopic species must be combined in order to determine frrr, frrit, fruit, and fium from the observed values of a and a . In practice, of course, substitutions like 14N for 15N tend to change the spectroscopic constants by only a small fraction, and conversely the observed data on the constants of such isotopic species tend to give nearly parallel information on the force field to that obtained from the parent species. For these reasons the anharmonic force field of molecules like N20 is much less well determined than that of C02. These effects are apparent in the uncertainties obtained on the force constants in the refinement calculations referred to in Table 4. 

For HCN the situation is somewhat better, because the data on DCN are much more effectively independent of the HCN data. This molecule has also been the subject of much high-resolution spectroscopic study, so that the vibration-rotation energy levels are particularly well known and its vibrational spectrum is free of accidental resonances. Table 8 compares the results of three quite different calculations. The calculation by Strey and Mills is the most recent, and was based on the latest spectroscopic data the refinement was made to a and x values rather than to the vibrational levels and rotational constants as used by both the earlier workers. Strey and Mills also constrained 3 of the quartic interaction constants to zero, and refined to cubic and quartic force constants in a separate calculation to the quadratic refinement. The level of agreement between the calculations leads to conclusions rather similar to those made above for C02 in particular, standard errors should be multiplied by at 

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The molecule nitrous oxide NgO, contains just as many electrons as CO2. Hence, one would at first be inclined to expect that it too is rectilinear and symmetrical. The investigations of Plyler and Barker(i39), however, have shown conclusively that the molecule is rectilinear and unsymmetrical, very probably of the type NNO, although an unsymmetrical configuration NON cannot be excluded in principle. The rectilinear character is confirmed again by the simple type of rotational structure of all the vibration-rotation bands, already discussed in the case of diatomic molecules like HOI (see section 23 and fig. 33) and also found for CO2 (see previous section, fig. 43). That the molecule is unsymmetrical follows in the first place from the fact that all three normal frequencies ct>2, CO3 are active and, more generally, that the selection rule of Dennison for symmetrical triatomic molecules, according to which for the observable combination bands AtJg + A 3 must be odd, is not obeyed, in contrast to CO2. In addition, if... 

In the more general case of an unsymmetrical linear triatomic molecule, the slopes of the dashed lines in Figure 2 are no longer + 1 but -l-c and — 1/c with 0 < c < 1. Combining the modified expressions for V Vq now gives ... 

Nitrous oxide is a moderately unreactive gas comprised of linear unsymmetrical molecules, as expected for a 16-electron triatomic species (p. 433). The symmetrical structure N-O-N is precluded on the basis of orbital energetics. Some physical properties are in Table 11.8 it will be seen that the N-N and N-O distances are... 

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