Transition, lowest electronic

For long (infinite) /am.v-polyacclylene chains, the treatment of quantum lattice fluctuations is very complicated, because many lattice degrees of freedom couple in a non-linear way to the lowest electronic transitions. We have recently shown that for chains of up to 70 CH units, the amount of relevant lattice degrees of freedom reduces to only one or two, which makes it possible to calculate the low-energy part of the absorption spectrum in an essentially exact way [681. It remains a challenge to study models in which both disorder and the lattice quantum dynamics are considered. 

In recent years, dynamic calculations of both the electronic and the molecular structure of complex molecular systems have started to become feasible. " These methods are based on the general idea that the electronic structure of the system is to be calculated on the fly as the nuclei move, while the nuclei respond to the forces determined from the dynamically calculated electronic structure. This assumes that the system moves on the lowest electronic state, and transitions between states are either ignored (because they are well separated in energy) or treated semiclassi-cally. 

HFS calculations, the two lowest electronic transitions of (CS2N3 ) anion are a HOMO-LUMO... 

Photochemistry. One might deduce that since the lowest electronic transition corresponds to transfer of an electron from an oxygen atom to a carbon atom, the nn state should have substantial diradical character and should react also by a McLafferty-type rearrangement or a cleavage, as in the mass spectrometer. This is indeed the case. The photochemical a cleavage is called the Norrish type I reaction, and the rearrangement is called the Norrish type II reaction. Both are discussed in Chapter 15. 

In an electronic transition, an electron from one molecular orbital moves to another orbital, with a concomitant increase or decrease in the energy of the molecule. The lowest-energy electronic transition of formaldehyde promotes a nonbonding (ri) electron to the antibonding pi orbital (it ).1 1 There are two possible transitions, depending on the spin quantum numbers in the excited state (Figure 18-11). The state in which the spins are opposed is called a singlet state. If the spins are parallel, we have a triplet state. 

Calculations of the electron density on the carbon atoms in the title compound (11) show that position 2 (and 7) has the lowest electron density (Table I) and therefore can be expected to have the lowest transition state energy for the charge-controlled addition of the amide ion.13141819 These calculations agree with experiment upon dissolving 11 in KNH,/NH3 at — 40 C, the H- and 13C-NMR spectra unequivocally show the presence of only one rr-adduct, 2-aminodihydro-l,8-naphthyridinide (12)15 (see Tables II and III). Increasing the temperature of this solution from —40 to 10 C does not change the H- and 13C-NMR spectra 12 is still the sole [Pg.105]

Quite interesting structural features are found for both compounds. The Si—Si bond distance in hepta-f-butylcyclotetrasilane with 254.2 pm is the longest reported so far in cyclotetrasilanes, while the dihedral angles near 16° are relatively small compared with those of other t-butyl-substituted cyclotetrasilanes. The hexa-t-butyl derivative, however, adopts a planar conformation with unexceptional Si—Si bond lengths of 238.7 pm. In the UV spectrum of hepta-f-butylcyclotetrasilane the longest-wavelength absorption maximum appears at 315 nm. This is the lowest electron transition energy of all alkyl-substituted cyclotetrasilanes reported so far. 

Adding up the 18 bond lengths around the ring, we obtain a value of 2510 pm. If we take this to be the circumference of the molecular ring, the radius R of the ring is approximately 400 pm. Thus the lowest electronic transition of this molecule requires an energy of... 

To determine the excitation energy of the lowest electronic level the contributions in the absorption spectrum from transitions to different vibrational modes of the excited state have to be separated. In the deconvolution of the absorption spectra Gaussian line shapes are assumed for the transitions to the different vibrational levels. The analysis leads to the transition wavelength A00 for the excitation from the ground state to the zero vibrational level of the excited state. 

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