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Supersonic jet studies

The fluorescence excitation spectrum of parent porphycene isolated in an ultracold supersonic jet consists of doublets, separated by 4.4 cm i (Fig. 8.6). This behavior contrasts with that of porphyrin, which exhibits normal behavior, with single peaks corresponding to particular vibronic transitions [73]. Upon replacing one or two inner protons with deuterons, the splitting disappears (which, given the experimental resolution, means that it becomes less than 0.1 cm i). Adding [Pg.253]

For alkyl-substituted porphycenes la and lb, with smaller N-N separation and thus stronger hydrogen bonds, one would expect a larger tuimeling splitting. The experiments in the supersonic jets reveal a quite complicated pattern, different from that of parent porphycene [76]. Two peaks, separated by 12.5 cm are observed in the 0-0 region of the fluorescence excitation spectrum (Fig. 8.8). This doublet structure does not disappear for singly and doubly deuterated compounds. Each of the peaks consists of doublets in undeuterated species and is associated with a different vibronic structure. Two low frequency modes (32 and [Pg.255]

38 cm i) are associated with the upper peak, whereas only one vibration (38 cnr ) related to the lower component is detected. Hole-burning experiments in the jets were performed to secure these assignments. These results can be interpreted as evidence for two different ground state structures, trans and cis tautomers. The calculations suggest that the two forms should have very similar energies (Table 8.1). The presence of two species in la and lb has also been detected, via biexponential fluorescence decay, in solutions, glasses, polymers and rare gas matrices [77]. [Pg.256]

The values of tunneling splitting were obtained for ground and excited state cis and trans forms of undeuterated la. The splittings, similar for cis and trans species, indicate a larger barrier to tautomerization in than in Sq. [Pg.256]

The computational results confirm the large differences between 1 and 2 observed experimentally. Although the trans forms are always predicted as the most stable species, the cis tautomers are much closer in energy in porphycenes. Actually, for la both forms are predicted to be practically isoenergetic, which agrees nicely with the experimentally observed presence of two forms in this molecule. [Pg.258]


Mechanisms of Intramolecular Electronic Energy Transfer in Bichromophoric Molecular Systems Solution and Supersonic Jet Studies, Chem. Rev. 96, 1953—1976. [Pg.272]

S. Speiser, Photophysics and mechanisms of intramolecular electronic energy transfer in bichromophoric molecular systems solution and supersonic jet studies, Chem. Rev., 96 (1996) 1953-1976. [Pg.497]

Several of these compounds have been studied without surrounding solvent in isolated gas phase conditions (supersonic jet studies) but evidence for TICT emission was found only in a few cases. More successful in terms of dual fluorescence in the jet were studies with respect to microsolvation and selfcluster formation. Both these processes enhance TICT formation. A beautiful example how pretwisting and microsolvation enhance the possibility for TICT formation is shown for a derivative of TriMAPYM in Fig. 4 [36],... [Pg.261]

Raman scattering is normally of such very low intensity that gas phase Raman spectroscopy is one of the more difficult techniques. This is particularly the case for vibration-rotation Raman spectroscopy since scattering involving vibrational transitions is much weaker than that involving rotational transitions, which were described in Sections 5.3.3 and 5.3.5. For this reason we shall consider here only the more easily studied infrared vibration-rotation spectroscopy which must also be investigated in the gas phase (or in a supersonic jet, see Section 9.3.8). [Pg.173]

Laser spectroscopy is such a wide subject, with many ingenious experiments using one or two CW or pulsed lasers to study atomic or molecular stmcture or dynamics, that it is difficult to do justice to it at the level at which Modern Spectroscopy is aimed. In this edition 1 have expanded the section on supersonic jet spectroscopy, which is an extremely important and wide-ranging field. [Pg.469]

New IR techniques introduced for the study of prototropic tautomerism include IR dicroism for the photoinduced double proton transfer in por-phine 71 (Scheme 24) [89CPH(136)165], and IR spectroscopy in a supersonic jet (less than 50 K) for demonstrating the presence, in these conditions, of 2//-benzotriazole (57b) [96CPL(262)689]. [Pg.47]

Despite the fact that relaxation of rotational energy in nitrogen has already been experimentally studied for nearly 30 years, a reliable value of the cross-section is still not well established. Experiments on absorption of ultrasonic sound give different values in the interval 7.7-12.2 A2 [242], As we have seen already, data obtained in supersonic jets are smaller by a factor two but should be rather carefully compared with bulk data as the velocity distribution in a jet differs from the Maxwellian one. In the contrast, the NMR estimation of a3 = 30 A2 in [81] brought the authors to the conclusion that o E = 40 A in the frame of classical /-diffusion. As the latter is purely nonadiabatic it is natural that the authors of [237] obtained a somewhat lower value by taking into account adiabaticity of collisions by non-zero parameter b in the fitting law. [Pg.191]

The simplest unsaturated alcohol with ip3-carbon O—H is allyl alcohol (propenol). The monomer occurs in two energetically similar conformations in the gas phase [145, 240], which are both stabilized by intramolecular O—H- -n interactions. The dimer has only been studied in matrix isolation [241]. Spectroscopic evidence for an intermolecular O—H ji hydrogen bond was found. A vibrational (IR and Raman) supersonic jet measurement would be able to unravel the different monomer and dimer conformations involved. [Pg.34]

In a supersonic jet, the fluorescence spectra are virtually free of environmental perturbations (in contrast to condensed-phase samples) and can thus provide information on isolated solute molecules. Moreover, van der Waals complexes with other solute molecules can be studied, which is of great fundamental interest. [Pg.70]

Van der Waals complexes of C2v symmetry between 1,2,4,5-tetrazine and a number of light gases (He, Ar, H2) have been observed and characterized by laser spectroscopic studies of free supersonic jet expansion of the tetrazine in the carrier gas (84CHEC-(3)53l). In these complexes, one equivalent of noble gas sits on top of the aromatic TT-system of the heterocycle. 1,2,4,5-Tetrazine, its 3-methyl, and 3,6-dimethyl derivative as well as aminotetrazine have all been used as heterocycles with noble gases, water, HC1, benzene, and acetylene, playing the role of the second partner. [Pg.184]

The self-quenching rate constant of 21 x 10 cm /molecule s that was measured in these studies is unusually large (69). Quenching by various gases and collision induced rotational transitions have also been measured (68,69). The quenching cross sections by He and Ar in a supersonic jet increase at very low temperature (70), indicating the formation of collision complexes. ... [Pg.20]

Fuke and Kaya [1989] studied the tautomerization of dimer (6.15) formed in a supersonic jet to investigate the vibrational selectivity of concerted two-proton transfer in excited electronic state. Tunneling in the NH N fragments leads to broadening of certain vibrational bands in the fluorescence excitation spectrum (Figure 6.16). Tunneling is promoted by the symmetric intermolecular vibration with frequency 120 cm-1. The widths of bands with n = 0, 1, and 2 are 5, 10, and... [Pg.171]

Among van der Waals complexes, the complexes of simple molecules with rare gas atoms has received a great deal of attention because the PES of these species can be reconstructed from spectroscopic data. Studies of isolated complexes in supersonic jets provide a unique opportunity to determine experimentally the parameters of complex multidimensional PES, which may have several minima, with accuracy approaching 0.01-0.001 cm-1 in some cases. This feat seemed to be a formidable challenge only a few years ago On the other hand, in view of their relative simplicity these complexes are reasonable subjects for quantum chemical calculations, so reliable comparisons of theory with experiment are possible. [Pg.302]

If one considers the van der Waals complexes as a way to study binary collisions, the possibility of the formation of clusters of given size is a way to probe the role of the environment of other molecules on the reactivity. It is well known that solvent effects play an important role, not only in the kinetics but also in the results of chemical reaction. The study of molecular clusters in supersonic jet experiments allows step-by-step solvation of reactants as will be shown in this chapter, most of the reactions which have been studied occur when a finite number of molecules is reached—this number being often small (less than ten molecules). [Pg.102]

Zuckermann et al. [119] studied IR multiphoton dissociation (IRMPD) of RDX and HMX in a supersonic jet. A C02 laser was used for dissociation, with a pulsed frequency-doubled dye laser tuned to 280nm to excite the A 22(v = 1) X2 (v = 0) transition... [Pg.307]


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