Acetylene excited states

The properties of excited states are not easy to measure because of their generally short lifetimes and low concentrations, but enough work has been done for us to know that they often differ from the ground state in geometry, dipole moment, and acid or base strength. For example, acetylene, which is linear in the ground state, has a trans geometry... 

FuUerodendrimers also allow an evaluation of the accessibility of the Cgo core unit by studying bimolecular deactivation of its excited states by external quenchers. Recently Ito, Komatsu, and co-workers have used this approach to investigate a series of fuUerodendrimers (9-11) in which Frechet-type dendrons have been connected to a fullerene moiety via an acetylene linker (Fig. 5) [35]. 

Interestingly, after reaching the maximum at the 6-membered cycle, the yields drop again. This decrease in efficiency occurs despite the appreciable reduction in the distance between the terminal acetylenic carbons relative to the 6-membered analogue. Here, the efficiency may simply be a function of how photochemical excitation is distributed in the reactive excited state. Calculated enediyne geometries suggest the cyclization is more efficient for those enediynes where the terminal phenyl groups are rotated outside of the enediyne plane (Table 3). 

The heart of a traditional atomic absorption spectrometer is the burner, of which the most usual type is called a laminar flow burner. The stability of the flame is the most important factor in AAS. Typical working temperatures are 2200 2400°C for an air-acetylene flame, up to 2600-2800°C for acetylene-nitrous oxide. The fraction of species of a particular element that exist in the excited state can be calculated at these temperatures using the Boltzmann equation ... 

A similar situation arises in the ir-v excited states of ethylene and acetylene where the n interactions become antibonding the excited states are consequently twisted (11) and bent (12) respectively. Another example is provided by triatomic molecules formed by second row atoms ifthe total number of valence electrons exceeds 16, the molecules are bent 33) since there are now antibonding electrons present and bending reduces the resulting unfavourable 7r interaction by replacing p AOs by hybrid AOs (cf. ozone, 13). 

The only identified reaction product of laser-ablated Mg atoms and acetylene under matrix isolation conditions is MgC=CH It was suggested that this reaction involves the insertion of excited-state Mg into the H—C bond to form an excited complex (equation 7), which then decomposes via H atom loss (equation 8). 

Molecules such as C02, CSs, HCN and acetylene are linear in their ground state but bent in their lowest excited states. On the other hand. 

The degeneracy of the excited state is 2, whereas that of the ground state is 1. The fraction of Na in the excited state in an acetylene-air flame at 2 600 K is, from Equation 21-2,... 

Vacuum ultraviolet photolysis of acetylene results in formation of triplet C2, as evidenced by its characteristic emission.139 Presumably, triplet acetylene is first formed and decomposes to C2 and H2. Saturated hydrocarbons undergo radiolytic reactions, but the relative importance of excited states versus ionized states has not yet been established with any certainty. 

In the low temperature reaction of oxygen atoms with acetylene at pressures in the torr range, chemiluminescent emission is observed from one or more electronically excited states of CO, CH, C2, OH and CHO. In none of these cases is the mechanism well established, so we will only mention briefly some recent work. 

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