Hydrogen nonclassical

As an explanation of the preferred formation of pyrrolidines as compared to lower and higher membered heterocyclic rings, the necessity of a nearly linear arrangement of the involved centers in the hydrogen transfer step and a minimum of nonclassical strain in a cyclic 6-membered chair-like intermediate was postulated although the experimental evidence is not conclusive. 

The complex OsHCl(CO)(P Pr3)2 reacts with HX (X = H, SiEt3, Cl) molecules to give derivatives of the type OsXCl(r)2-H2)(CO)(P Pr3)2 (X = H, SiEt3, Cl), where the hydrogen atoms bonded to the osmium atom undergo nonclassical interaction (Scheme 16). 

Goldman has studied the effect of nonclassical behavior on the solubility of helium, hydrogen, and neon in simple solvents and benzene (2J3). He has discussed the Henry s constant of water as a solute 24). So far his results do not apply to water as a solvent. 

Isotope (H (deuterium), discovered by Urey et al. (1932), is usually denoted by symbol D. The large relative mass difference between H and D induces significant fractionation ascribable to equilibrium, kinetic, and diffusional effects. The main difference in the calculation of equilibrium isotopic fractionation effects in hydrogen molecules with respect to oxygen arises from the fact that the rotational partition function of hydrogen is nonclassical. Rotational contributions to the isotopic fractionation do not cancel out at high T, as in the classical approximation, and must be accounted for in the estimates of the partition function ratio /. 

In a different approach, Bottinga (1969a) evaluated the nonclassical rotational partition function for water vapor, molecular hydrogen, and methane through the asymptotic expansion of Strip and Kirkwood (1951) ... 

The concept of hydrogen bonding is constantly evolving from classical hydrogen bonds to nonclassical (or nonconventional) hydrogen-bonded complexes. Here, on the basis of new experimental and theoretical data and new approaches to this problem, the nature of a proton-donor component and a proton-acceptor site is reformulated completely. In addition, experimental criteria that have been used successfully earlier for the detection of hydrogen bonds are also changed. 

Hydrogen bonds [3] can be classified as nonconventional (or nonclassical), when they are formed with the participation of nonconventional proton donors and/or unconventional proton acceptors ... 

Noyori, R., Yamakawa, M. and Hashiquchi, S. Metal-Ligand Bifunctional Catalysis A Nonclassical Mechanism for Asymmetric Hydrogen Transfer between Alcohols and Carbonyl Compounds. J. Org. Chem. 2001, 66, 7931-7944. 

The difficulties encountered in firmly establishing the structure of the first di-hydrogen complex still have not been entirely overcome. The H—D coupling constant experiment described above offers a method of distinguishing between the classical and nonclassical forms, but it is not applicable if the system is rapidly fluxional. 

---