Pyrazine molecule

Figure C3.3.8. A typical trajectory for a soft collision between a hot pyrazine molecule and a CO2 bath molecule in which the CO 2 becomes vibrationally excited.

While the simulations of the pyrazine system discussed in the previous sections of this chapter have employed a three-mode model (Model I), the semiclassical simulations we will present here are based on two different models a four-mode model and a model including all 24 normal modes of the pyrazine molecule. Let us first consider the four-mode model of the S1-S2 conical intersection in pyrazine which was developed by Domcke and coworkers [269]. In addition to the three modes considered in Model I, it takes into account another Condon-active mode (V9a). Figure 37 shows the modulus of the autocorrelation function [cf. Eq. (24)] of this model after photoexcitation to the S2 electronic state. The exact quantum results (full line) are compared to the... 

Figure 41 shows the absorption spectrum for the 24-mode model of pyrazine. As was done by Raab et al. [277], we have included a phenomenological dephasing time of T2 = 150 fs to model the experimental broadening due to hnite resolution and rotational motion. It can be seen that the inclusion of all 24 normal modes of the pyrazine molecule leads to a shape of the spectrum which is in good agreement with the experimental result (Fig. 38b). The semiclassical result is seen to be in fairly good agreement with the quantum result. The spurious structure in the semiclassical spectrum is presumably due to the statistical error.

Figure 8.12 Packing of distyryl-pyrazine molecules in the solid state (viewed down the h-axis) that favours facile photopolymerization. (Following Jones Thomas, 1979.)...

The (n - 71 )1 3 excited states of the pyrazine molecule are a well-known case of wave function symmetry breaking [27,55]. An accidental degeneracy arises when one considers the valence electronic excitations within the equivalent nitrogen lone pairs. The pairs are in opposite and equivalent positions and, when there are two singly occupied orbitals in different symmetries, we will have a pair of accidentally degenerate states as shown below ... 

Calculations by the self-consistent field LCAO-MO method for the ground state wave function of the pyrazine molecule indicate that the lone pairs are quite different. The lower lone pair is little delocalized (1.88 electrons on nitrogen), but the second lone pair is as delocalized as the lone pair in pyridine with 1.37 electrons on nitrogen, 0.22 electrons on hydrogen, and 0.40 electrons on carbon.63... 

Thus in xenon reaction (10) should be considered as electron attachment to a pyrazine molecule that already has a cluster of xenon atoms around it. 

The electron affinity normally used is that of the molecule in the gas phase. But in this case the volume change information shows that the electron reacts with a clustered pyrazine molecule, PyzXe, and the electron affinity of the clustered species should be used. Since there was evidence that the electron affinity of the analogous species in argon, PyzAr, increased with w by a few tenths of an electron volt, the measured values of AG/liq) were used with Eq. (13a) along with calculated values of P c and Fg to evaluate E.A. The results showed... 

Such large enhancement factors for localized and isolated hot spots from few atom Ag clusters arising from only the chemical enhancement under certain conditions are supported by calculations. Zhao working with Jensen and Schatz used time-dependent density functional theory (TDDFT) to investigate the adsorption and Raman response of pyrazine molecules [21]. Figure 10.6 shows the Raman response of (a) isolated pyrazine compared to that of pyrazine complexed to the vertex of a (b) one and (c) two 20 Ag atom clusters with enhancements of 10 and 10 predicted, respectively. Small clusters of Ag atoms have little or no plasmon response, suggesting that the chemical enhancement can be quite significant and certainly may allow for enhancement hot spots. 

Even though leucine did not appear to be incorporated into a pyrazine molecule, the diversion of n-keto isovalerate to form leucine decreases the availability of this compound to form valine... 

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast Internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example . The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge... 

Luminescent coordination compounds continue to attract considerable attention. Zink recently reported a new mixed-ligand copper(I) polymer that shows interesting photoluminescence (232). The complex [CuCl(L44)Ph3P] consists of a one-dimensional chain lattice of metal ions bridged by both Cl" ions and pyrazine molecules. The compound shows conductivity of less than 10-8 S cm 1. The absorption spectrum of the complex shows a band at 495 nm, which could be interpreted as the promotion of an electron from the valence band to the conduction band. On the basis of resonance Raman spectra, the lowest excited state in the polymer is assigned to the Cu(I)-to-pyrazine metal-to-ligand charge-transfer excited state. 

The Cl of the adiabatic PESs is a common phenomenon in molecules [11-13], The singular nonadiabatic coupling (NAC) associated with Cl is the origin of ultrafast non-Born-Oppenheimer transitions. For a number of years, the effects of Cl on IC (or other nonadiabatic processes) have been much discussed and numerous PESs with CIs have been obtained [11, 12] for qualitative discussion. Actual numerical calculations of IC rates are still missing. In this chapter, we shall calculate IC rate with 2-dependent nonadiabatic coupling for the pyrazine molecule as an example to show how to deal with the IC process with the effect of CL Recently, Suzuki et al. have researched the nn state lifetimes for pyrazine in the fs time-resolved pump-probe experiments [13]. The population and coherence dynamics are often involved in such fs photophysical processes. The density matrix method is ideal to describe these types of ultrafast processes and fs time-resolved pump-probe experiments [14-19]. 

This model can be commonly used to describe the Cl of tm and nn electronic states of the pyrazine molecule. Near the bottom of the two potential surfaces, the two electronic states in the diabatic approximation are described by j(njt ) and The adiabatic approximation and b wiU be employed to describe the electronic states in the Cl region. Thus,... 

In the fourth part, we study the effect of Cl on IC. It was applied to study the TtTt ->nTT transition of the pyrazine molecule. In this nonadiabatic process, the Cl of the TCK and nir PESs is believed to play a major role in the nonadiabatic fs transition. In fact, the Cl has been widely proposed to play the key factor in an IC, and quantum trajectory calculations have been used to calculate the IC rates [45]. However, this method cannot properly take into account of the initial conditions of the population and coherence of the system created by the fs pumping laser. In this chapter, we propose to develop a method to calculate the IC with conical intersections. It should be known that for the IC between Si and So in most molecules (in these cases, the energy gap between and So is of several eV), the surface crossings do not take place due to the anharmonic effect in the two PESs. Thus, the Cl should not play any role in these cases. We have proposed one method to calculate the IC rate of mt of the pyrazine molecule. The... 

Pure [PcRu(pyz)] is accessible by heating PcRu(pyz)2 to 300 °C. Only under these conditions one pyrazine molecule is eliminated and the resulting intermediate polymerizes to form [PcRu(pyz)]n [64]. 

It was possible to grow single crystals of the mononuclear TPPFe(pyz)2. Its structure was solved by X-ray diffraction analysis [85, 90]. Furthermore, the crystal structure [91] of the pyz-bridged (n-pyrazine)bis(dimethylglyoximato)-cobalt(II) demonstrates, that the pyrazine molecules within a chain are all arranged in a plane perpendicular to the plane of the planar bis(dimethyl-glyoximato)-cobalt(II) moiety (see Fig. 8). 

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