7? system resonances

Gnanakaran S, Haran R, Kumble R and Hochstrasser R M 1999 Energy transfer and localization application to photosynthetic systems Resonance Energy Transfer ed D L Andrews and A A Demidov (New York Wiley) pp 308-65... 

The COSY-45° spectrum of vasicinone displays two distinct spin systems, as indicated by square brackets. Cross-peaks A-H represent coupling between five relatively upfield protons cross-peaks I-K are due to couplings between the aromatic protons. We start from the most down-field proton of the upfield spin system, resonating at 8 5.10, and trace all the other coupling interactions. For instance, the peak at 8 5.10... 

In a molecule such as formaldehyde, the bonding and non-bonding orbitals are localized (like the bonds) between pairs of atoms. Such a picture of localized orbitals is valid for the a orbitals of single bonds and for the n orbitals of isolated double bonds, but it is no longer adequate in the case of alternate single and double carbon-carbon bonds (in so-called conjugated systems). In fact, overlap of the n orbitals allows the electrons to be delocalized over the whole system (resonance... 

In order to understand any chemical process, you need to remember that stability is the key. In the case of aromatic systems, resonance is important to stability. To find out more about stability and resonance, we begin by examining the resonance in each of the different sigma complexes that may form, listed below and shown in Figure 7-15. The entering group can attack in one of three relative positions ... 

Extended p-p n bonding between the basic site and an adjacent tt system (resonance) delocalizes the electronic charge. 

Why is the excited state of a conjugated system of double bonds stabilized more, relative to the ground state, than for a nonconjugated system Resonance theory provides an explanation (see Section 6-5). Of the several conventional valence-bond structures that can be written for 1,3-butadiene, four of which are shown here, 2a-2d, only structure 2a has a low enough energy to be dominant for the ground state of 1,3-butadiene ... 

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442... 

Fig. 10. Modifying the resonances in an electrode system can be useful when two cell types have only small differences in passive electrical properties and in dielectrophoretic force spectra. The best frequency to achieve a separation lies near the crossover frequencies when one cell is showing positive DEP and the other negative. But at such a frequency the forces are small. By suitable adjustment of capacitive and inductive elements at each electrode, it is possible to make a system resonate at the desired frequency, thereby increasing the drive voltage (and force) many fold without the need for expensive high voltage signal generators. The real dielectrophoretic force spectra (a) can be transformed into effective spectra (b)...

As stated above, a new scale is desirable to describe quantitatively the global aromaticity in the VB model. Since the global aromaticity measures the average benzene character of a conjugated system, resonance energy per hexagon (REPH), as defined by Eq. (48), may be a natural choice for BHs [22],... 

The DMTA operates at fixed frequencies over a broad temperature range. Sixteen discrete frequencies from 0.01 Hz to 200 Hz are available. The very low frequencies, below about 0.1 Hz, require a long time to complete, while frequencies above 30 Hz are often near or above the system resonance and require special consideration. Though the system is capable of a... 

A transducer or converted) to convert eleotrioal power into meohanical vibrations. This is a tuned system resonating at the operating frequency. 

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