Pyrazine rotation

For heavy molecules with very small rotational state spacing, this limit on AJ puts severe upper limits on the amount of energy that can be taken up in the rotations of a heavy molecule during a collision. Despite these limitations, P(E, E ) distributions have been obtained by inverting data of the type described here for values of AE in the range -1500 cm > AE > -8000 cnD for the two donor molecules pyrazine and hexafluorobenzene with carbon dioxide as a bath acceptor molecule [15,16]. Figure C3.3.11 shows these experimentally derived... 

Mullin A S, Park J, Chou J Z, Flynn G W and Weston R E Jr 1993 Some rotations like it hot seleotive energy partitioning in the state resolved dynamios of oollisions between COj and highly vibrationally exoited pyrazine Chem. Phys. 175 53-70... 

Interatomic distances calculated from the detailed analysis of rotational fine structure of the UV spectrum of pyrazine are in close agreement with those observed in (7) and (8), with the calculated bond lengths for C—C of 1.395, C—N 1.341 and C—H 1.085 A (60DIS(20)4291). Thermochemical data have provided a figure of 75 kJ moP for the delocalization energy of the pyrazine ring (B-67MI21400). 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

Our understanding of the physicochemical properties of pyrazines has deepened. The internal rotation and IR spectrum of 2,5-pyrazinedicarboxamide were studied by quantum chemical calculations <05TC73>, and ab initio MO calculations at the MP2/6-31++G( ) level were used to explain the electronic and vibrational properties of complexes of pyrazine and HX linear acids <05JMS2822>. MM and MO calculations were used to investigate the conformational and electronic properties of odor-active pyrazines <05JMS169>, and NMR, IR, X-ray, and DFT methods were used to examine the structures of pyridol l,2-a pyrazinium bromide <05JMS7>. 

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.

The rotational dynamics of nitrogen and carbon dioxide were recorded by Akhmanov and Koroteev [7]. The transients look similar to the transients by Morgen et al. [8], recorded with time resolved Raman induced polarization spectroscopy [9]. A fs-DFWM experiment was performed by Frey et al. [10] on diatomics and linear polyatomics. To prevent collisional dephasing, they transferred the method into the expansion zone of a molecular beam. In succession, experiments on linear molecules and symmetric tops were performed on molecules like CHCI3 [11] and CgHf, [12], Transients of asymmetric tops like the near oblate pyrimidine, pyrazine and pyridine [13] and SO2 [11] were reported in the following years. 

Even reactions which are thermodynamically unfavoured, e.g. with Kaq as low as 5 x 10-6, may be electrocatalytically mediated. For example, rotating platinum electrodes covered with polymerized [Ru(4-vinyl-4 -methyl-2,2 -bipyridyl)3]2+wiil electrocatalytically mediate the oxidation96 of such species as [Ru(bipy)3]2+, [Ru(bipy)2(4,4 -bipy)2]2+, [Ru(bipy)2(py)(MeCN)]2+, [Ru(bipy)2-(MeCN)J2+ and [Ru(bipy)2(pyrazine)2]2+. 

In the X-ray analysis of a crystal structure the first step is the determination of the space group and the number of molecules in the unit cell. Occasionally it may be immediately apparent from such data that the molecule itself possesses certain elements of symmetry, and these may define or at least limit the possible molecular conformations. The elements most commonly found in aromatic molecules are centres of symmetry and twofold rotation axes. It might have been expected that the plane of symmetry would manifest itself in aromatic systems, but this is disappointingly rare. Indeed, amongst the structures reviewed here the only cases where a crystallographic symmetry plane coincides with that of a planar molecule occur in s-triazine and pyrazine (see Section V, A, 5). 

As discussed in Section III, TRPAD measurements are sensitive to molecular rotational motion by virtue of their geometric dependence on the molecular axis distribution in the LF. An elegant experimental demonstration of this has been performed by Suzuki and co-workers who measured the PAD temporal evolution from excited-state pyrazine [59]. In these experiments, the origin of the Si electronic state of pyrazine was excited by a pump pulse at 323 nm, and... 

Interatomic distances as calculated from the analysis of the rotational fine structure of the ultraviolet spectrum are C-C, 1.395 A C-N, 1.341A and C-H, 1.085 A.66 These are very similar to the bond lengths for pyridine which are C-2-C-3, 1.3945 A C-3-C-4, 1.3944 A and C-2-N, 1.3402 A. The C-N-C bond angle in pyrazine is 115° and the C-C-N bond angle 122.5°.56,67 A delocalization energy for pyrazine of ca. 18 kcal/mole is indicated from heats of combustion data.68 The C=N bond energy in 2,2,5,5-tetramethyl-2,5-dihydropyrazine has been calculated to be 130.3 kcal.58a... 

Picosecond study of pyrazine seeded in supersonic 271 expansions of He. Decay behaviour interpreted in terms of rapid dephasing of initially populated singlet state Decays of rotational states of the (0-0) transition 310 of pyrazine. When Fourier transformed, beating decays are obtained... 

The polarographic behavior of the 1-oxides and 1,4-dioxides of pyrazine, 2,5-dimethylpyrazine, and tetramethylpyrazine at various pH values has been investigated. It was assumed that at lower pH values, the A -oxide group was reduced in its protonated form. In acid media the 1-oxides exhibited double waves, the first of which is attributable to the reduction of jV-oxide groups and the second to that of the pyrazine nucleus (production of 1,4-dihydro compounds). Reduction of both A -oxide groups of pyrazine-1,4-dioxide proceeded simultaneously (588). Half-wave potentials of the voltammetric oxidation and reduction of pyrazine mono- and di-A -oxides have been measured in dimethylformamide, and in acetonitrile by the technique of a rotating platinum electrode (750). 

Tetrafluoropyrazine, cesium fluoride, octafluorobut-2-ene and sulpholan heated and rotated at 160° for 4 hours gave perfluoro-2,5-di-s-butylpyrazi . o (23) (494) and 2-carboxypyrazine with sulfur tetrafluoride in hydrogen fluoride at 150° gave 2-trifluoromethylpyrazine (759). 2-(co-Chloroacetyl)pyrazine has been prepared from 2-diazoacetylpyrazine and dry hydrogen chloride in ether (138) and... 

The free energy of the rotational barriers, AG, about the =CH-NMei bond in 2-(A7JV-dimethylaminomethyleneamino)pyrazine has been determined as 17.5kcal/mol, and in its 3,5-dibromo and 5[Pg.214]

Due to the their high coordination flexibility, originating from the rotation around the TV—TV bond, and the presence of possible additional donors either on R/R (e.g., pyridine, pyrazine, etc.) or X/X (e.g., NH2, OH, SH, etc.), these open-chain diazine (TV—TV) ligands are extremely... 

In Section V we will discuss the effect of rotations on the time behavior and the quantum yield of pyrazine. In Section VI we will discuss some less conventional experiments, and finally, in Section VII we will draw some conclusions and make some attempts to generalize to other relatively large molecules. 

Figure 3. The rotational excitation spectrum of the lBiu (0-0) transition of pyrazine.

As we outlined in the theoretical section, the time dependence of the fluorescence is critically dependent on the type of exciting source used. We therefore list the experiments as a function of increasing laser width. We will first limit ourselves to the photodynamics of the J = 0, K = 0 rotational state of the lB3u of pyrazine, since only of that state we know the ME spectrum. 

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