Highly excited hydrocarbon states

It is possible, but has by no means been proved, that the lower state of the hydrocarbon flame bands is the same as that of the absorption bands just described. If the two systems do have a common lower state, the upper state of the flame bands would represent a third, more highly excited, electronic state of HCO. 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

Hydrogen. The reaction of O ( D2) with H2 takes place on the ground state potential surface of water, HiOf Ai). On the basis of trajectory calculations, (Whitlock et al., 1982) it has been suggested that, as is true for the hydrocarbons, parallel mechanisms involving insertion/elimination and direct abstraction govern the course of this reaction. The observation using laser induced fluorescence spectroscopy (Luntz et al., 1979 Smith and Butler, 1980) of a highly excited, non-Boltzmann rotational distribution and a nearly statistical vibrational distribution for v" = 1 and v" = 0 is consistent with the insertion/elimination... 

But such a reaction pathway would give a high activation barrier, because the excited triplet states of even unsaturated molecules are typically 40-70 kcal/mol less stable than the ground state, and those of saturated hydrocarbons are much higher. ... 

Fluorescence quenching of fluorene, dibenzofuran, and dibenzothiophens by aromatic nitriles and aliphatic amines is the result of electron transfer with exciplex formation,177 and ion-pair formation in pyromellitic dianhydride-ethylbenzene is followed by dissociation into separated ion pairs in their highly excited states.178 Photochemical iodination of aromatic hydrocarbons may proceed by way of an electronically excited Ia-aromatic charge-transfer complex.179 Modulation excitation spectrophotometry has been used to analyse the nature of some complexes between polycyclic aromatic hydrocarbons and chloranil.180... 

Equation 15.14 shows that application of the high dose rates in conditions favorable for radiation generation of the long-living excited molecnlar states allows observation of intense chain cracking reactions in hydrocarbons at lowered temperatnres without any thermal activation. This effect of dose rate on the efficiency of oil processing was used in the PetroBeam process. 

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