Development very weakly bound complexes

Many researchers still consider mass spectrometry [1] to be a rather destructive method. The soft ionization methods - despite the revolution they caused in the life sciences - are often not appropriately highlighted in lecture courses on mass spectrometry, so prejudice has it that mass spectrometry almost unavoidably causes fragmentation even of covalent bonds. Consequently, the examination of noncovalent complexes is too often considered to be futile and successful only in some very special cases. However, the soft ionizaton methods developed in the 1980s reduce fragmentation to a minimum and even noncovalent, weakly-bound complexes can be ionized without complete destruction. Technically, the problem of intact ionization of weakly-bound complexes can be solved in many cases [2]. 

As indicated above, the study of the vibrational predissociation dynamics of weakly bound complexes is in a very fruitful period owing to the overlap which now exists between experiment and theory. Future developments in theory most certainly will be in the area of polyatomic systems where the role of open vibrational channels are likely to be important. 

This definition is still valid today. However, what is considered an independent system today is very different than in 1960. Through the development of a wide range of sophisticated synthetic techniques and sensitive measurement technologies, experimental physical chemists have prepared and studied a wide range of weakly bound molecular complexes, ranging from hydrogen-bonded dimers that are bound by a few kcal mol to weakly bound complexes such as Ar-HX (X = F, Cl) that are bound by a few tenths of a kcal mol , to, recently, the most weakly bound system of all, He2. 

The chiral cavitands 3.109 have been developed by combining the amino acid residue L-alanine with macrocyclic cavitands (calixarenes - Section 3.14).51 These ammonium ion receptors are able to complex a range of amino acids and their methylester hydrochloride salts, all of which contain an -NH3+ functionality capable of interaction with the carboxylate residues of the host. In general amino acids are bound only very weakly in aqueous solution, while association constants with the chiral methyl esters range from 620 M-1 for L-tryptophan methylester to 110 M-1 for L-alanine methylester. The methylester of glycine is not bound at all. Receptors related to 3.109 with variable four peptide loops arrayed around a central calixarene core have been used to bind to the surfaces of proteins. The... 

In recent years our understanding of the vibrational predissociation dynamics of weakly bound molecular complexes has improved greatly owing to both the experimental and theoretical advances that have been made in this area. We are now in the very fortunate position where both theory and experiment often can be brought to bare on the same system. The developments made in our laboratory involve the use of the opto-thermal detection technique to measure the infrared spectra and state-to-state dissociation rates for a number of these complexes. In the present report we examine the affects of molecular orientation on the rate of vibrational relaxation. This is done by measuring the predissociation lifetimes associated with several vibrational modes of different isomeric forms of a binary complex. The vibrational relaxation rates are found to be highly anisotropic and several interesting correlations can be made between this data an l the collisional relaxation results already available in the literature. 

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