Coupled motion, transition state

Fig. 8. Scattering the transition state from the surface. Measured vibrational distribution of NO resulting from scattering of laser-prepared NO(v = 15) from Au (111) at incidence = 5 kJ mol-1. Only a small fraction of the laser-prepared population of v = 15 remains in the initial vibrational state. The most probable scattered vibrational level is more than 150 kJ mol-1 lower in energy than the initial state. Vibrational states below v = 5 could not be detected due to background problems. These experiments provide direct evidence that the remarkable coupling of vibrational motion to metallic electrons postulated by Luntz et al. can in fact occur. (See Refs. 44 and 59.)...

Fig. 17 Plot of the calculated secondary deuterium KIE versus the extent of O—H bond formation for the model elimination reaction at 45°C Models 1 and 2 have different imaginary frequencies and no coupling of the Ca—D bending vibrational motion with the C0—H stretching motion in the transition state. Models 3,4 and 5 have increasing extents of coupling between the Ca—D bending and C —H stretching motion in the transition state. Reproduced, with permission, from Saunders (1997).

The observation of exalted secondary isotope effects, i.e., those that are substantially beyond the semiclassical limits of unity and the equilibrium isotope effect. These observations require coupling between the motion at the primary center and motion at the secondary center in the transition-state reaction coordinate, and in addition that tunneling is occurring along the reaction coordinate. 

Kinetic complexity definition, 43 Klinman s approach, 46 Kinetic isotope effects, 28 for 2,4,6-collidine, 31 a-secondary, 35 and coupled motion, 35, 40 in enzyme-catalyzed reactions, 35 as indicators of quantum tunneling, 70 in multistep enzymatic reactions, 44-45 normal temperature dependence, 37 Northrop notation, 45 Northrop s method of calculation, 55 rule of geometric mean, 36 secondary effects and transition state, 37 semiclassical treatment for hydrogen transfer,... 

The well-known Born-Oppenheimer approximation (BOA) assumes all couplings Kpa between the PES are identically zero. In this case, the dynamics is described simply as nuclear motion on a single adiabatic PES and is the fundamental basis for most traditional descriptions of chemistry, e.g., transition state theory (TST). Because the nuclear system remains on a single adiabatic PES, this is also often referred to as the adiabatic approximation. 

A third reason is that a sufficient length of polypeptide is needed to enforce the shape of a protein, and particularly the precise stereoelectronic architecture of its active site. In enzymes, for example, this allows coupled vibrations and energized motions that contribute to catalytic mechanisms and an exquisitely fine-tuned stabilization of transition states (Kraut, 1988 Havsteen, 1989 Retey, 1990 Knowles, 1991 Tonge and Carey, 1992 Williams, 1993). 

Figure 8.3) will move the transition state to an earlier point. Since strengthening acid HA facilitates motion Rx (transfer of proton from HA to carbonyl oxygen coupled with attack of nucleophile), the transition state will tend to come earlier with respect to this motion that is, it will be shifted in the direction indicated by the arrow R2 in Figure 8.3. But proton motion is also involved in the vibration designated by i and J 2 in Figure 8.3 reacting bond Rule 2 states that change in structure will shift the transition state in the direction indicated by the change. Here, strengthening acid HA aids motion i-...

The orientation dependence of the PES couples the rotational states of the molecules, consequently the scattered flux should show strong rotational excitation and de-excitation. For late barrier systems this also couples to vibrational motion, i.e. there are combined vibrational and rotational transitions. We shall return to this topic in Sections 5 and 6. 

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