X-ray crystal structures of 141 halogen-bonded complexes Y-X.B formed between homo- and heteronuclear dihalogens Cl(2), Br(2), I(2), IBr and ICl with O, S, Se, N, P and As Lewis bases show remarkable and constant geometrical features. The metrics of the halogen bond found in the gas phase for simple complexes [Legon (1999a). Angew Chem. Int. Ed. Eng. 38, 2686-2714] is supported (i). in the solid state, (ii). for new Lewis acids (I(2) and IBr), (iii). for new basic centers (Se, As and =N-) and (iv). for more complicated bases. The Y-X...B arrangement is more linear than the corresponding Y-H...B hydrogen bond and the axis of the Y-X molecule lies in the plane of the B lone pair(s), with a preference for the putative lone-pair direction within that plane. However, exceptions to this lone-pair rule are found for sterically hindered thiocarbonyl and selenocarbonyl bases. A bond-order model of the halogen bond correctly predicts the observed correlation between the shortening of the X...B distance and the lengthening, deltad(Y-X), of the Y-X bond. The expectation that the solid-state geometric parameters d(X...B) and deltad(Y-X) reflect the strength of the interaction is supported by their significant relationships with the solution thermodynamic parameters of Lewis acidity and basicity strength, such as the Gibbs energy of 1:1 complexation of Lewis bases with diiodine. This analysis of halogen-bonded complexes in the solid state reinforces the similarities already known to exist between hydrogen and halogen bonding.

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Family-independent relationships between computed molecular surface properties and solute hydrogen bond aciditylbasicity and solute-induced methanol 0-H infrared frequency shifts are presented. The molecular surface quantities, computed at the ab initio HFl6-31G* level, are primarily related to the electrostatic potential of the molecule, but also include the average local ionization energy on its surface.

The new diiodine basicity scale pK(BI2) is quasi-orthogonal to most known Lewis basicity scales (hydrogen-bond, dative-bond and cation basicity scales). The diiodine basicity falls in the sequence N>P≈Se>S>I≈O>Br>Cl>F for the iodine-bond acceptor atomic site and SbO≈NO≈AsO>SeO>PO>SO>C=O>-O->SO(2) or PS≫-S->C=S≫N=C=S for the functionality of oxygen or sulfur bases. Substituent effects are quantified through linear free energy relationships, which allow the calculation of individual complexation constants for each site of polybases and thus the classification of aromatic ethers as carbon π bases and of aromatic amines, thioethers and selenoethers as N, S and Se bases, respectively. The pK(BI2) values of nBu(3)N(+)-N(-)C≡N, 2-aminopyridine and 1,10-phenanthroline reveal a superbasic nitrile, a hydrogen-bond-assisted iodine bond and a two-centre iodine bond, respectively. The diiodine basicity scale is a general inorganic but family-dependent organic halogen-bond basicity scale because organic halogen-bond donors such as IC≡N and ICF(3) have a stronger electrostatic character than I(2). The family independence can be restored by the addition of an electrostatic parameter, either the experimental pK(BHX) hydrogen-bond basicity scale or the computed minimum electrostatic potential.

The gas-phase basicities (GBs) of nornicotine, nicotine, and model pyrrolidines have been measured by FT-ICR. These experimental GBs are compared with those calculated (for the two sites of protonation in the case of nicotine and nornicotine) at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level, or those estimated from substituent effects on the GBs of 2-substituted pyrrolidines, 2-substituted N-methylpyrrolidines, and 3-substituted pyridines. It is found that, in contrast to the Nsp(3) protonation in water, in the gas phase nornicotine is protonated on the pyridine nitrogen, because the effects of an intramolecular CH.Nsp(3) hydrogen bond and of the polarizability of the 3-(pyrrolidin-2-yl) substituent add up on the Nsp(2) basicity, while the polarizability effect of the 2-(3-pyridyl) substituent on the Nsp(3) basicity is canceled by its field/inductive electron-withdrawing effect. The same structural effects operate on the Nsp(3) and Nsp(2) basicities of nicotine, but here, the polarizability effect of the methyl group puts the pyrrolidine nitrogen basicity very close to that of pyridine. Consequently, protonated nicotine is a mixture of the Nsp(3) and Nsp(2) protonated forms.

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The thermodynamics of the O-H...B hydrogen bond (HB) has been determined in CCl(4) by FTIR spectrometry for a wide variety of carbon pi bases, oxygen bases, and miscellaneous first- to fourth-row bases, using 4-fluorophenol as a reference hydrogen-bond donor (HBD). After inclusion of previously studied nitrogen, sulfur, and halogen bases, this 4-fluorophenol affinity scale contains 314 varied organic bases and ranges over 40 kJ mol(-1). The 4-fluorophenol affinity scale in CCl(4) is shown to be applicable to most HBDs in most media, provided a small family dependence is taken into account. The HB affinity orders are quantitatively established according to the atomic acceptor site or to its bearing functional group. A comprehensive survey of the influence of substituents on these affinity orders is then achieved, considering electronic and steric effects, as well as effects of vinylogy or iminology. Iminology is found to be more efficient than vinylogy for transmitting resonance effects. Steric effects are shown to be less important in HB affinity than in HB basicity since they mainly act on the HB entropy. The spatial proximity of two acceptor sites can favor complexation through three-center hydrogen bonds, leading to superhydrogen-bond bases on the affinity scale.

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