Supersonic molecular jet

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

Figure 4. Laser ablation fixture used to make a supersonic molecular jet. The pulsed valve is synchronized with the ablation laser and the gaseous products expand into a vacuum and cool.

General discussion of intra- and intermolecular interactions 3 van der Waals interactions 3 Coulombic interactions 5 Medium effects on conformational equilibria 5 Quantum mechanical interpretations of intramolecular interactions 7 Methods of study 8 Introduction 8 Nmr and esr spectroscopy 8 Microwave spectroscopy (MW) 12 Gas-phase electron diffraction (ED) 12 X-ray crystallographic methods 13 Circular-dichroism spectroscopy and optical rotation 14 Infrared and Raman spectroscopy 18 Supersonic molecular jet technique 20 Ultrasonic relaxation 22 Dipole moments and Kerr constants 22 Molecular mechanic calculations 23 Quantum mechanical calculations 25 Conformations with respect to rotation about sp —sp bonds 27 Carbon-carbon and carbon-silicon bonds 28 Carbon-nitrogen and carbon-phosphorus bonds 42 Carbon-oxygen and carbon-sulphur bonds 48 Conformations with respect to rotation about sp —sp bonds Alkenes and carbonyl derivatives 53 Aromatic and heteroaromatic compounds 60 Amides, thioamides and analogues 75 Conclusions 83 References 84... 

T. Haber, U. Schmitt, and M. A. Suhm, FTIR spectroscopy of molecular clusters in pulsed supersonic slit jet expansions. Phys. Chem. Chem. Phys. 1, 5573 5582 (1999). 

In order to produce supported samples for STM or FEM study, clusters formed on the centerline of the condensation reactor are extracted through a 1 mm diameter capillary into a vacuum chamber typically kept at 10-5 Torr. The resulting supersonic free jet flow is collimated to form a molecular beam of metal clusters, uncondensed metal atoms and inert gas atoms. 

The cited papers -deal with the dimers of rare gas atoms but extension to other vdW molecules can be expected in the near future. The importance of combining the PES technique with supersonic molecular beams has been pointed out in Ref. 25 jn Xe... Xe production a jet with a 35 pm aperture was used. No peak was observed at about 11.1-11.2 eV (the 1st adiabatic potential of Xe. .. Xe is 11.14 eV ) but a small gradual intensity increase culminates at about 11.7eV (a small, flat maximum) which was tentatively attributed to a transition to an excited vibrational level of the ionic ground state (Xe Further Xe. .. Xe... 

In the experiments described herein, the environment chosen for the chemical intermediates is a supersonic free jet expansion. We have found the jet in many ways an ideal device for studying the spectroscopy of a variety of chemical intermediates, including isolated molecular ions, ionic clusters, and both small and moderately large organic and inorganic neutral free radicals. 

One example is intracavity Raman spectroscopy of molecules in a supersonic jet, demonstrated by van Helvoort et al. [327]. If the intracavity beam waist of an argon-ion laser is shifted to different locations of the molecular jet (Fig. 3.12), the vibrational and rotational temperatures of the molecules (Sect. 4.2) and their local variations can be derived from the Raman spectra. 

The structure of molecular complexes in their electronic ground state can be obtained from direct IR laser absorption spectroscopy in pulsed supersonic-slit jet ex-... 

The structure of molecular complexes in their electronic ground state can be obtained from direct IR laser absorption spectroscopy in pulsed supersonic-slit jet expansions [9.47]. This allows one to follow the formation rate of clusters and complexes during the adiabatic expansion [9.48]. Selective photodissociation of van der Waals clusters by infrared lasers may be used for isotope separation [9.49]. 

Gas-phase spectroscopy of neutral molecules, as opposed to ions, usually involves the use of supersonic molecular beams [1 ]. For smaller compounds this can be achieved by seeding in the inert drive gas. This limitation excludes the study of neutral nucleosides or larger compounds while even some of the bare nucleobases, such as guanine, cannot be sufficiently heated without thermal degradation. Some work with bases and base mimics has been done in seeded beams [5-10]. Larger compounds can now be vaporized successfully by pulsed laser desorption, followed by entrainment in a supersonic jet [11-14]. This experimental advance has opened up the field of study of nucleobases and nucleosides in isolation in the gas phase, especially by IR spectroscopy. The cooling in molecular beams makes this approach particularly attractive for spectroscopy. Although temperatures are not as low as in ion traps or helium droplets, molecular beams can achieve internal temperatures typically of the order of 10-20 K, which provides very useful optical resolution. 

The first gas-phase spectroscopy of nudeobases was reported by Levy and coworkers in 1988 [23]. They measured the electronic spectra of the pyrimidine bases uradl and thymine in a supersonic molecular beam both by LIF and REM PI. The observed spectra were very broad and featureless, in spite of eflSdent jet cooling. 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

If a particularly parallel beam is required in the chamber into which it is flowing the beam may be skimmed in the region of hydrodynamic flow. A skimmer is a collimator which is specially constructed in order to avoid shockwaves travelling back into the gas and increasing 7). The gas that has been skimmed away may be pumped off in a separate vacuum chamber. Further collimation may be carried out in the region of molecular flow and a so-called supersonic beam results. When a skimmer is not used, a supersonic jet results this may or may not be collimated. 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

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. 

Pyrolysis method involves thermal decomposition of suitable precursors to produce free radicals. Pyrolysis sources based on continuous molecular beam nozzles are well developed (for example, methyl6 8 and benzyl9). Recently, Chen and co-workers have pioneered a flash pyrolysis/supersonic jet technique to produce free radical beams (Fig. I).10 In this radical... 

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