Hydrogen heavy atom motions

The oscillator mass Hv is 3.2 amu. This must be compared to the mass of the hydrogen atoms in the librational motion of the ammonium ion, 4 amu. Of importance here is not the precise value of the oscillator mass but that it is light, no assignment of this band to heavy atom motions is tenable. 

Therefore, in the first part of this section, intramolecular hydrogen transfers or intermolecular hydrogen transfers in preformed hydrogen bonded complexes in the solid state which are coupled only to minor heavy atom motions are discussed. H-transfers coupled to major heavy atom motions will then be treated in the second part they include pre-equilibria, hydrogen bond switches, conformational changes, solvent motions etc. 

The double hydrogen bond in principle facilitates two mechanisms of double proton transfer stepwise and concerted. Studying the transfer rates within the semi-classical tunneling approximation of the variational transition state theory, Kim found that the two protons are transferred synchronously across the transition state with D2h symmetry [194], The actual proton-tunneling distance is considerably reduced due to the contraction of the hydrogen bonds, i.e., heavy atom motion promotes the proton transfer. On the other hand, the ab initio path-integral Car-Parinello calculations predict that the motion of the two protons in the vicinity of the potential minima is... 

Finally, we note that in all cases besides the heavy atom motions leading to hydrogen bond compression as well as the minor heavy atom motions during proton tunneling no major reorganization of the environment takes place. 

Limbach, 11.11., Schowen, K.B, and Schowen, R.L. (2010) Heavy atom motions and tunneling in hydrogen transfer reactions the importance of the pre—tunneling state. J. Phys. Org. Chem., 23, 586-605. 

Eor biomolecules, such as proteins, the fastest motions are the stretching vibrations of the bonds connecting hydrogen atoms to heavy atoms (X—H stretching). The frequency of these motions is in the vicinity of 3000 cm , which means periods of about 10 fs (1 X lO s). Thus, an appropriate time step for simulating biomolecules would be At =... 

Heavy atom tunneling of entire alkyl groups between the different sides of the hydrogen bonded ring plane is much less likely even for methanol, in contrast to the analogous but fighter water case [108]. These motions correspond to hindered rotation of the monomer and are slowed down considerably in the complex due to the librational constraints. 

In a typical (classical) molecular system, the fastest motion is bond vibration which, for a heavy-atom-hydrogen bond has a period of about 10 " s. Thus, for a system containing such bonds, an integration time step At should not much exceed 0.1 fs. This rather short time... 

R. A. Marcus In the case of the reaction Klippenstein and I studied, which showed two transition states [1], the motion was that largely of heavy atoms rather than hydrogen atoms. We assumed incoherent motion between the two, though in some systems, such as the one you treated, coherence can certainly be important. In your ketene system was tunneling involved in passage through the two barriers ... 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

Let the chain obey the periodical boundary conditions and let protons be strongly connected with heavy atoms, so that tunnel proton transitions along the hydrogen bond are negligible in comparison with the reorientation motion of A—H groups. This means that only one proton is localized near each heavy atom in the chain that is, equality + /z/ / i = 1 holds, which makes it possible to introduce the following pseudo-spin operators ... 

---