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6, we shall indicate that a simple, essentially electrostatic model for the hydrogen bond is also appropriate for the halogen bond in outer complexes. 2 Properties of Isolated Complexes B· · · XY: How to Measure Them Rotational spectroscopy is a precise means by which the properties of molecules in effective isolation in the gas phase may be measured. Only for strong complexes (particularly those linked by a hydrogen bond) have rotational spectra been detected by using equilibrium gas mixtures of the two components at normal or slightly lower temperatures [23].

See Fig. 1 for key to the colour coding of atoms slightly smaller angle in the case of (CH2 )2 S· · ·ClF is discussed in [69]. 5(2)◦ ]. The hydrogen bonds in the complexes (CH2 )2 S· · ·HF [124] and (CH2 )2 S· · ·HBr [28, 125] are also significantly non-linear. 3 B Carries Two Inequivalent n-Pairs 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 π bonding pairs are not drawn and are ignored here) is shown in Fig.

The angle φ is almost identical in B· · ·HCl 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 monotonically from 2,5-dihydrofuran to oxirane. See Fig. 1 for key to the colour coding of atoms Table 1 The angles φ and θ (in degrees; see Fig. 0(2) because the axis a and c lie in the molecular symmetry plane. Thus both φ and θ can be determined for each complex, in contrast to the position for the H2 S analogues, for which only in H2 S· · ·ClF was it possible to establish the collinearity of the S· · ·Cl – F nuclei.