Rotating bond approximation

Four-atom reactions came into focus with the development by Clary of the Rotating Bond Approximation (RBA)[10. 11] and Bowman s reduced-dimensionality adiabatic bend (RD-AB) calculations of four-atom reactions. In the latter three stretching vibrational motions are treated explicitly quantum dynamically while the bending degrees of freedom are treated adiabatically and one diatom is assumed to be a spectator[12, 13, 14, 15, 16], The RBA may be seen as an extension... 

Quantum reactive scattering calculations on the state-to-state dynamics of gas phase chemical reactions involving four or more atoms can now be carried out. This article describes how the Rotating Bond Approximation (RBA) can be applied to the four-atom reactions OH +... 

Rotating bond approximation calculations as applied to the CH4 + OH CH3 + H2O reaction have been discussed. The scattering calculations are performed within a model which treats CH3 as a pseudo-atom and CH4 as a pseudo-diatom. The rotation of OH, a reactive C-H stretch of CH4, two vibrations of H2O and relative translation are treated explicitly in the scattering calculations. An adiabatic approach is used to account for all other degrees of freedom. The reduced dimensionality potential energy surface RDPl [17] was employed in the calculations. 

Results of reduced dimensionality quantum calculations are displayed in the right panel of Fig. 5.1 the adiabatic bend approximation (ABA) of Bowman and coworkers [42] and the rotating bond approximation (RBA) of Clary [30]. Both approximations employ a three-dimensional description of the reaction process. For the H2+OH reaction the ABA approximation is in good agreement with the accurate result. In contrast, the RBA rate constants are too small at low temperatures (but, due to fortuitous cancelation of errors, agree well with experiment). However, this finding can not be generalized. For the H2+CN reaction, for example, RBA yields a better description than ABA (see Fig. 5.3). 

DVR = discrete variable representation LEPS = Lon-don-Eyring-Polanyi-Sato NIP = negative imaginary potential PODVR = potential optimized DVR QCT = quasiclassi-cal trajectories RBA = rotating bond approximation TST = transition state theory. 

The rotating bond approximation (RBA) was used. This method goes beyond the reduced dimensionality approximation described above in that the bending mode of the H2O and the rotational states of OH are included explicitly in a closecoupling expansion. In its fullest form, the technique can treat the quantum states in a reaction ... 

After removing duplicates, 4905 actives remained with percent inhibition >40%. Of these, 992 compounds with 175 > MW > 600, clogP > 6, donors > 5, acceptors > 10, rotatable bonds > 10 or TPSA > 140 were flagged and subject to visual assessment in Spotfire by a medicinal chemist. Approximately half of these were retained, leaving 4458 actives from this set for confirmation assays. 

As a consequence, using the Bosons description and performing the rotating wave approximation lead us to write the effective Hamiltonians (102) and (103) describing the H-bond bridge coupled to the thermal bath as follows ... 

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