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The nature of acids in an aqueous environment is fundamental to many aspects of chemistry. The defining feature of an acid is its ability to transfer a proton to water, but this seemingly simple process can be complex and highly dependent on solvation. This review describes studies aimed at obtaining molecular-level descriptions of acid-water interactions through the study of neutral acid-water complexes. Current understanding of the nature of hydrated protons is first summarized, and a parallel discussion of structural aspects of acid-water complexes is then presented. The nature of proton transfer, and its evolution with cluster size, is described. The review concludes with several examples that lie at the intersection between fundamental work on hydrated acids and important physiochemical processes in the atmosphere.

Rotational spectra of the complex HCN-BF3 have been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectra have been observed for the l l B and loB species with both HC14N and HCISN, and the hyperfine structure has been analyzed. The complex has the expected C3" structure, with the nitrogen end of the H C N toward the boron, but the observed BN bond length of 2.473(29) A is notably shorter than that in the related weakly bound systems NGN-BF~ and NGC-CEN-BF~.The out-of-plane distortion of the BF3 cannot be determined accurately but is probably less than 3 O . We compare the structure, force constants, quadrupole coupling constants, and binding energy of the complex with those of other species formed from BF3 and a variety of nitrogen donors. Despite the rather short B-N bond length, the force constant for the intermolecular bond, as well as the boron and nitrogen nuclear quadrupole coupling constants, is comparable to those of a truly weakly bound system.

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