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The review focusses on the fundamental importance of non-covalent interactions in nature by illustrating specific examples from chemistry, physics and the biosciences. Laser spectroscopic methods and both ab initio and molecular modelling procedures used for the study of non-covalent interactions in molecular clusters are briefly outlined. The role of structure and geometry, stabilization energy, potential and free energy surfaces for molecular clusters is extensively discussed in the light of the most advanced ab initio computational results for the CCSD(T) method, extrapolated to the CBS limit. The most important types of non-covalent complexes are classified and several small and medium size non-covalent systems, including H-bonded and improper H-bonded complexes, nucleic acid base pairs, and peptides and proteins are discussed with some detail. Finally, we evaluate the interpretation of experimental results in comparison with state of the art theoretical models: this is illustrated for phenol...Ar, the benzene dimer and nucleic acid base pairs. A review with 270 references.

The complete set of vibrational frequencies and absolute infrared intensities has been determined for o-benzyne and two of its isotopomers: C6D4 and 1,2J3C2C4H4. In addition, for the majority of the transitions symmetries were assigned from infrared linear dichroism of the matrix-isolated samples, photooriented with polarized light during several photochemical transformations. Thermal relaxation of the high static pressure created by the initial photofragmentation causes dramatic changes of the fine site structure of each band of o-benzyne and results in a singlesite infrared absorption spectra. A high-rmlution, single-site vibrational spectrum was also obtained independently from laser hole-burning experiments. Band-shape analysis in different inert gas matrices (Ne, Ar, Xe, N2, and CO) greatly facilitates the correlation of isotopomer bands with those of unlabeled o-benzyne. The triple bond stretching vibration appears at 1846 cm-' in a Ne matrix, with an experimental absolute intensity of 2.0 f 0.4 km/mol in the unlabeled o-benzyne and is polarized along the symmetry axis. It is red-shifted by 2 cm-I in the perdeutero-ebenzyne and by 53 cm-l in the doubly IT-labeled compound, in very good agreement with our theoretical prediction (MP2/6-3 1G**) and previous gas-phase data for o-benzyne.

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Until now the study of organic compounds in which the n-electron system is excited by absorbed light has been mainly concentrated on the ultraviolet and visible regions of the electromagnetic spectrum. Various new applications, such as the use of conjugated organic compounds as dye lasers or as materials for storing information with the help of diode lasers, led to the synthesis of new compounds which absorb light in the near infrared (NIR). It is possible to use structure-color relationships to predict the properties of such new compounds when they belong to dyestuff classes which have already been studied in detail; in this case the approach involves decreasing the energy difference between the ground state and the first excited state. A less conventional starting point is provided by molecular structures in which from the outset there is only a very small energy difference between the lowest-energy electronic states; such diradicaloid molecules occupy a special position among the various types of organic compounds. It is possible by means of suitable structural modification to stabilize such molecules in a singlet form which absorbs light at very long wavelengths (i.e. at small wave numbers).