Ring puckering internal

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. 

Usually there are many possible interconversion movonents between these isomers, which can change one degenerate configuration into another one of same energy. These interconversion movements are internal rotations, inversions, ring puckerings, etc...They can be described by some operators, M,-, acting on X, which will leave the nuclear Hamiltonian operator (7) invariant. 

Gwinn, W. D., Gaylord, A. S. Spectroscopic Studies of Ring-Puckering Motions, in MTP International Reviews of Science, Physical Chemistry Series 2, Vol. 3 Spectroscopy, Ramsay, D. A., (ed.) London and Baltimore Butterworths and University Park Press 1976... 

Both the ring puckering and the carbonyl wagging motions are needed for the description the low frequency modes of cyclobutanone. Energy points on a grid defined by selected values of the internal coordinates were obtained from fully optimized Hartree-Fock calculations with Moller Plesset corrections for electron correlation. These data points were fitted to the power series expansion... 

Schaefer et al studied departures from the Karplus-type relationship ( Barfield effect ) to determine the ring pucker in 2-phenyl-1,3-dithiane. The dependence of the 7(H,H) Karplus equation on the internal C—C—H angles was given in mathematical form by Barfield et al., who also recalculated the... 

When internal motion is present, the ensemble averaging in equation (2) becomes more complicated. There are a variety of ways in which these motions can be described here 1 will outline a simple model, in which the spins are assumed to undergo instantaneous jumps between discrete conformations. Many aspects of real motion can be usefully described in this way. For example, the motion of methyl protons about their symmetry axis approximates jumps between three equivalent positions many other side chain motions can also be conveniently modeled as jumps between allowed rotamers or ring puckers. Even small amplitude motions that might not ordinarily be considered as jumps can often be usefully approximated by transitions between points that sample the endpoints of allowed motion. 

Analyses of this type reveal plausible and consistent internal motions for such diverse cases as proline ring-puckering, floppy peptides, proteins including the mobile binding of chloride ion, " nucleic acids, and even solid rubber. ... 

When the internal motion is not well represented as a small-amplitude motion, the analysis becomes more difficult and perturbation treatments are unsatisfactory. Over the last decade, considerable improvements and advances have been made to treat the complicated spectra of certain classes of molecules. These formulations provide more convenient and accurate treatments of large-amplitude motions for (i) molecules with internal rotation, ring puckering, inversion, or umbrella-Uke motions, (ii) quasilinear or quasisymmetric tops, (iii) floppy molecular complexes, and (iv) molecules with two internal rotators. Description of these treatments is beyond the scope of this presentation. For further information the reader is directed to the Bibliography. 

An Arrhenius-type plot of the correlation time of the overall motion (calculated with the use of Eq. 16) versus reciprocal temperature yields an activation energy of 20.4 kJ/mol. Among the various models examined to evaluate internal rotations, a two-state jump model56,143 was found satisfactory for interpreting the oscillatory and puckering motions of the flexible 5,6-O-isopropylidene ring. In this model the C—H vector jumps between two equivalent, stable states A and B characterized by correlation times ta and tb, respectively (Fig. 10). The angle (3 is defined by the... 

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