Ring puckering dynamics

A time-resolved ion yield study of the adenine excited-state dynamics yielded an excited-state lifetime of 1 ps and seemed to support the model of internal conversion via the nn state along a coordinate involving six-membered ring puckering [187]. In order to determine the global importance of the tict channel, a comparison of the primary photophysics of adenine with 9-methyl adenine will be useful, as the latter lacks a tict channel at the excitation energies of concern here. The first study of this type revealed no apparent changes in excited-state lifetime upon methylation at the N9 position [188] a lifetime of 1 ps was observed for both adenine and 9-methyl adenine. This was interpreted as evidence that the tict is not involved in adenine electronic relaxation. 

Let X be the ring-puckering coordinate and y the PH inversion coordinate in 3-phospholene (see Fig. 33). The dynamics of isomerization in this molecule is assumed to be adequately described by these coupled two DOF s without need for consideration of other DOFs. Under this assumption, the Harthcock-Laane potential energy surface consists of a fourth-order polynomial in x, a fourth-order polynomial in y, and a sixth-order polynomial coupling x and y. With molecular rotation neglected, the model Hamiltonian for 3-phospholene is given by... 

We shall also not deal in our paper with the large amplitude ring puckering and pseudorotation in small ring molecules. Molecular dynamics of these motions and the determination of the barriers of pseudorotation from microwave and infrared spectra have been the subject of a great number of papers this problem has been recently reviewed in the monography ... 

In the interpretation of the spectral data it is usually the constants A and B in Eq. (3.27) or some other set of reduced potential constants that are evaluated. The barrier height, AB2/4 for B < 0, is thus determined directly. However, if one wants to relate the value of the dimensionless parameter Z I at the minimum of a double-minimum potential function to the absolute geometry of the puckered ring, the reduced mass must be known in order to find x from IZI. One is then required to introduce assumptions about the dynamical model of the ring-puckering motion. 

A COMPUTER SIMULATION OF THE RING PUCKERING AND OXYGEN WAGGING DYNAMICS IN THE So STATE OF CYCLOBUTANONE. 

A Computer Simulation of the Ring Puckering and Oxygen Wagging Dynamics 347... 

The differing conformations of the ribose have been named with respect to that ring atom which puckers out of the plane given by the other ring atoms. The most prominent conformations are Cy-endo in B-helices and Cy-endo in A-helices. At room temperature both conformers are in a dynamic equilibrium. Intermediates between Cy-endo and Cy-endo are found in several (time-averaged) structures obtained from X-ray crystallography or NMR. When changing the pucker phase... 

Energy profiles in Figure 14 also reveal that planar furanose forms are often of lower energy than puckered conformers. For example, relative conformational energies determined for 7 with the 3-21G basis set indicate that the planar conformer is more stable than the Eq conformer in 8, the planar conformer is calculated to be more stable than. These observations suggest that the conformational dynamics of some furanose rings may not be completely described by pseudorotation in these cases,conformer interconversion may occur by both inversion and pseudorotational pathways, with the latter being the more preferred route. 

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