Vibration umbrella

Figure 6.13 Illustration of (a) rocking, (b) twisting, (c) scissoring and (d) wagging vibrations in a CH2 group. Also shown are (e) the torsional vibration in ethylene, (f) the ring-breathing vibration in benzene and (g) the inversion, or umbrella, vibration in ammonia...

In the photodissociation of CH3I, the inelastic force —dVi/dr partly compensates the strong restoring force —dvBc/dr with the result that, relative to the pure FC limit, CH3 is generated with comparatively weak vibrational excitation during the fragmentation the final state interaction de-excites rather than excites the umbrella vibration of CH3. 

A much more critical case is that of the umbrella vibration of ammonia NH3, which is very soft. Based on density-functional calculations, Aquino et a/.35 calculated recently the frequency of this and obtained 0.837 cm-1 compared with an experimental value of 0.793 cm-1. Previous theoretical values ranged... 

The displacement of the P atom of the pyramidal PH3 molecule through the H3 plane (related to the Vg umbrella vibration) leads to an inverted configuration. The potential energy as a function of the distance of the P atom from the H3 plane is characterized by a double minimum (inversion barrier B). 

Fig. 5. CH3 umbrella vibrational distribution for the I (black boxes) and I (white boxes) channels following the photodissociation of CH3I in the A continuum for three different wavelengths. Reprinted, with permission of the American Institute of Physics, from Ref. 38.

Significant spectral changes occur in the 1378-cm symmetric methyl deformation mode (umbrella vibration) in structures which contain more than a single methyl group attached to the same carbon (or even in some cases where the methyl is attached to the adjacent or vicinal carbon atom). The most notable examples are in the case of isopropyl [—CH(CH3)2], gem-dimethyl [ C(CH3)2 ], and tertiary butyl f-C(C 113)3] groups. 

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