N-pair model

Fig. 1 Comparison of the experimentally determined geometries of the hydrogen-bonded complex H3N-- -HC1 and its halogen-bonded analogue H3N- C1F (both drawn to scale) with a non-bonding electron-pair (n-pair) model of NH3. Here, and in other figures, the n-pair electron distribution is drawn in the exaggerated style favoured by chemists. The key to the colour coding of atoms used in this and similar figures is also displayed...

Fig. 4 The experimentally determined geometries of H2S- -HC1 and H2S- -C1F drawn to scale. The n-pair model of H2S, as discussed in the text, is shown for comparison. See Fig. 1 for key to the colour coding of atoms...

It is clear from Fig. 8 and Table 1 that the angle 0 does indeed decrease as expected if the n-pair models and rule 1 were applicable. Moreover, the hydrogen bond non-linearity 9 decreases along the series B = oxirane, oxetane, 2,5-dihydrofuran. On the other hand, the values of 9 for oxirane- -ClF and 2,5-dihydrofuran- -ClF (included in Fig. 8) reveal that the halogen bond shows little propensity to be non-linear. 

Fig. 8 The n-pair models of 2,5-dihydrofuran, oxetane and oxirane (first column) and the experimental geometries of their complexes with HC1 (second column) and C1F (third column), each drawn to scale. The angle 0 is almost identical in B- HC1 and B- ClF for a given B but increases from 2,5-dihydrofuran, through oxetane, to oxirane, as expected from the model (see text). The non-linearity of the hydrogen bond increases monotoni-cally from 2,5-dihydrofuran to oxirane. See Fig. 1 for key to the colour coding of atoms...

The values of 0 are both close to 90°, which suggests an n-pair model of thiirane (see Fig. 9) similar to that described for H2S earlier. A reason for the... 

Sulfur dioxide is an example of a simple Lewis base that carries two sets of inequivalent n-pairs, one set on each O atom. The n-pair model (in which the tt bonding pairs are not drawn and are ignored here) is shown in Fig. 10. The geometries of S02 HF [126,127], S02 HC1 [28,126] and S02- C1F [70] have all been determined from investigations of their rotational spectra. Each molecule is planar and belongs to the Cs point group. Scale drawings for S02 HC1 and S02- C1F are displayed in Fig. 10. 

When the n-pairs carried by the acceptor atom are inequivalent, as for the oxygen atoms of SOj, the rules are non-committal. In fact, the observed angular geometry of SOj HF indicates that the HF molecule lies in the plane of the SO2 molecule and along a direction which almost coincides with the direction of the axis of the n-pair that is cis to the S=0 double bond. This may be confirmed by examination of the n-pair model of SOj shown in the final column of Table 2.1. 

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