Transition perpendicular

The di-tert. butyl substituted calicene 230 was calculated to possess considerable non-bonding (steric) interactions in the planar geometry184. The relief of strain when going to a perpendicular transition state is reflected by the low coalescence temperature of tert. butyl signals found on temperature-dependent H-NMR spectroscopy187. ... 

The hypothesis that 128 is twisted in solution and that the transition state to E-Z isomerization has perpendicular fluorene groups was supported by a study of 2,3,2, 3 -dibenzo-annelated 128 (159). In this, the steric situation around the double bond is similar to that in 128a, but an improved stabilization of the diradical perpendicular transition state should lead to a lower barrier to E-Z isomerization, as is found (AG 53 = 23.5 kcal/mol). A close approach of the 1 and 1 (8 and 8 ) positions in 128 is indicated by a through-space H- F coupling of 7 Hz in ( )-l, 1 -difluoro-128 (160). In this compound the barrier to E-Z exchange is >25.6 kcal, because of the small contribution of the fluorine atoms to the ground state strain. 

For the case of a perpendicular transition when the transition dipole is in the body-fixed y direction (perpendicular to molecular plane), we obtain the following ... 

For a perpendicular transition, evaluation of the relevant direction-cosine integrals gives... 

The dipole operator d is a vector defined in the body-fixed frame of the molecule. Consequently, the transition dipole moment /a defined in (2.35) is a vector field with three components each depending — like the potential — on R, r, and 7. For a parallel transition the transition dipole lies in the plane defined by the three atoms and for a perpendicular transition it is perpendicular to this plane. Following Balint-Kurti and Shapiro, the projection of /z, which is normally calculated in the body-fixed coordinate system, on the space-fixed z-axis, which is assumed to be parallel to the polarization of the electric field, can be written as... 

Fig. 11.10. Schematic illustration of the alignment of a diatomic molecule owing to a parallel and a perpendicular transition. The polarization of the electric field vector, Eq, is parallel to the laboratory z-axis.

The evolution of the wavepacket through the curve crossing is nicely reflected in the polarization of the photons emitted to the electronic ground state during dissociation (Lao, Person, Xayariboun, and Butler, 1990). The transition dipole moments of the two excited states, 3Qq and 1Q1, with the ground state are parallel and perpendicular to the C-I bond, respectively. The initial excitation is due to a parallel transition. The subsequent emission, however, involves both parallel and perpendicular transitions because the 1Qi state becomes populated during the breakup. 

Fig. 15.4. The fraction of photons emitted by the dissociating CH3I molecule via a perpendicular transition moment, plotted versus the C-I stretching level to which emission occurred. Reproduced from Lao, Person, Xayariboun, and Butler (1990).

As = 0 and Ap = dj. Vibrations that are perpendicular to the surface are more difficult to detect by ATR (usually we use grazing angle spectroscopy for this purpose). This is because of the fact that in the ATR mode djl < dellx. For example, the Ap values predicted for transitions perpendicular to the surface are smaller than those predicted for parallel transitions of same intrinsic intensity (transition moment dipoles) by factors of 2.22 (Ge), 2.10 (Si), and 1.67 (ZnSe). Therefore, it is recommended to use ZnSe when possible for analysis of molecules with both parallel and perpendicular transitions, and where grazing angle spectroscopy is not available. 

Fig. 1.5. Direction of the dipole moment d of an optical transition in a diatomic molecule (a), (c) parallel transition (b), (d) perpendicular transition (e) arbitrary orientation of the angular momentum J.

In the case of perpendicular transitions at A 0, accounting for the finite value of the angular momentum J does not lead to the appearance of new transitions, since all types of dipole transitions - P, Q and R -are already permitted for A = 0. 

In the case of the tetrasilyldisilene 96, / ,Z-isomerization is rapid even at 0°C, and dynamic NMR experiments were employed to estimate a value of A 62 kJmol-1 for the interconversion of ( )- and (Z)-96 at 30 °C134. The considerably lower barrier to ,Z-isomerization of this compound compared to 92-95 was ascribed to hyperconjugative stabilization of the perpendicular transition state by the trialkylsilyl substituents134 144. 

Since Bv < B" and since as J increases the quadratic term overtakes the linear term 23) (they are inversely proportional to r2) there is a bandhead in the R branch that is at the high frequency side of v0. For A 4= 0 there is a third, the Q branch for AJ = 0 which is absent or weak for parallel transitions (AA = 0) and strong for perpendicular transitions. (AA = 1). 

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