Supersonic jet expansion

If both alcohol monomers forming a dimer are on average achiral, one may still have chirality synchronization events, where the two monomers match their transient chiral conformations when they bind to each other. A particularly simple example is that of ethanol dimer, where the lowest-energy conformer involves two gauche monomers of the same helicity [80,91]. However, the energy difference to other conformers is so small that efficient isomerizing collisions in a supersonic jet expansion are required to favor the lowest-energy form over the others. A more... 

As became obvious in the preceding section, progress in understanding alcohol clusters very much depends on the ability to generate these clusters in supersonic jet expansions or in other variants of low temperature isolation and to detect their dynamics via spectroscopic methods. Therefore, some important spectroscopic tools employed in this field shall be summarized, with focus on the alcoholic systems that have been addressed by them. Solution [22, 26, 141, 142] and supercritical [24 26] state techniques will not be covered systematically. 

The preparation of single isomers for methanol dimer, trimer, and presumably tetramer [16] in a supersonic jet expansion contrasts the structural diversity that can be prepared and manipulated in cryogenic matrices [34]. It underscores the ability of supersonic jet expansions to funnel all intermolecular isomers down to the global minimum, if there are no major barriers to overcome on the way. 

D. Luckhaus, M. Quack, U. Schmitt, and M. A. Suhm, On FTIR spectroscopy in asynchro nously pulsed supersonic jet expansions and on the interpretation of stretching spectra of HF clusters. Ber. Bunsenges. Phys. Chem. 99, 457 468 (1995). 

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. 

Quantum Beats and Dephasing in Isolated Large Molecules Cooled by Supersonic Jet Expansion and Excited by Picosecond Pulses Anthracene, W. R. Lambert, P. M. Felker, and A. H. Zewail, J. Chem. Phys. 75, 5958 (1981). 

Quack, M., Schmitt, U., and Suhm, M. A., Evidence for the (HF) complex in the HF stretching FTIR absorption spectra of pulsed and continuous supersonic jet expansions of hydrogen fluoride, Chem. Phys. Lett. 208, 446-452 (1993). 

Fluorescence excitation, fluorescence, and MPI spectra of tolane (PhC=CPh) in a supersonic jet expansion t One- and two-photon excitation of diphenylbutadiene. Isomerization and/or other non-radiative processes become important for vibrational energies in excess of 1050 cm- ... 

Structure determination as a function of cluster size t Spectra and structures of. s-tetrazine dimers formed 915 in a supersonic jet expansion... 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Figure 9. Effect of increasing the excess excitation energy on the LIE spectra of II in a supersonic jet expansion. TOP mass spectra (right) indicate that the emission spectra (left, bottom) correspond to the monomer. The numbers indicate the wavelengths of excitation (in nm).

Table 1. Spectral position of the origin (Vvac in cm ) and the vibrational bands observed in the LIF excitation spectra of the monomers of CDMA (I, Figure 2), its o-methylated derivative (II) and 4-(dialkylamino)pyrimidines (VI, VII and IX) in the supersonic jet expansion" "...

Spectroscopists have shown many powerful variations on the fluorescence experiment, which can generate additional selectivity and structural information. These include two-photon excitation (55) supersonic jet expansion (56) constant-energy synchronous fluorescence, where the excitation and emission monochromators are scanned synchronously to maintain a constant energy difference (57) and pulsed excitation for the real-time measurement of fluorescence lifetimes (58). However, it is far from certain that these techniques will ever be useful for the practicing analyst. 

Later workers accepted Porter s assignment without reservation [88], The gas phase spectrum was reproduced by Berry using phenyl azide as the precursor [25]. Laser-induced fluorescence attributed to triplet phenyl nitrene was observed upon pumping the 368 nm transition in ordinary gas phase experiments and in supersonic jet expansions where very highly resolved spectra could be generated [89a]. 

Plate 3. Molecularfree radicals being created in DC discharges in a reactor flow oven (lefl) and a supersonic jet expansion (right). (See page 9 of color inserts.)... 

In the experimental studies of state specific NO2 unimolecular dissociation (Miy-awaki et al., 1993 Hunter et al., 1993 Reid et al., 1994, 1993), NO2 is first vibra-tionally/rotationally cooled to 1 K by supersonic jet expansion. Ultraviolet excitation is then used to excite a NO2 resonance state which is an admixture of the optically active and the ground electronic states. [It should be noted that in the subpicosecond experiments by Ionov et al. (1993a) discussed in section 6.2.3.1, a superposition of resonance states is prepared instead of a single resonance state.] The NO product states are detected by laser-induced fluorescence. Both lifetime and product energy distributions for individual resonances are measured in these experiments. A stepwise increase in the unimolecular rate constant is observed when a new product channel opens. Fluctuations in the product state distributions, depending on the resonance state excited, are observed. The origin of the dynamical results is not clearly understood, but it apparently does not arise from mode specificity, since analyses of... 

Mukherjee, Bandyopadhyay and Chakraborty " investigated the photophysics of 7-azaindole homodimer (Fig. 52) in Ar and N2 matrices and the souble-proton transfer reaction taken place upon UV excitation. The IR spectra revealed that a doubly hydrogen bonded dimer is predominantly produced upon annealing the seeded matrices at 28 K. The authors found that, unlike in hydrocarbon solutions or in a supersonic jet expansion, the matrix-isolated doubly hydrogen-bonded dimer emits exclusively in ultraviolet from the locally excited state, and the main photophysical process (excited state double proton transfer ESDTP) was slowed down dramatically. They interpreted these observations in terms of matrix effect on the double proton transfer potential, specifically the increase of the effective ESDPT barriers in the rigid inert gas matrices compared to those in the gas phase or hydrocarbon solution. 

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