Molecular jets

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). 

The effect of successive introductions of chlorine substituents on the IE of 1,4-dibenzodioxin was evaluated using the method of resonance-enhanced two-color two-photon ionization (REMPI) in a cold molecular jet combined with time-of-flight (TOE) mass spectrometry and comparison with other dioxins and theoretical values < 1995IJM97, 1994IJM101>. 

To observe the transient spatiotemporal structure of the WP interference, we have performed the fs pump-fs probe experiment [37], The sample gas was prepared by molecular jet expansion of the mixture of iodine vapor and Ar buffer gas into a vacuum chamber. A continuous gas jet is preferable when we use a high-repetition-rate laser system. The estimated vibrational temperature was 170K[37]. 

Requires a molecular jet separator to operate with packed columns... 

Figure 5-1. Mass resolved excitation spectrum of bare aniline in a molecular jet. Several of the more intense vibronic transitions are assigned. 

Figure 2.19. Temporal profile of fluorescence trains from the S2(0°) level via the S2(0°) <— TxfS1) <— So(00) OODR probe as functions of time delay and spatial distance between the pump-probe lasers, showing the long trail contour of 4 ps indicates a long-lived Ti state due to coupling to higher energy levels in the S0 state. Inset Pump-probe laser configuration including a molecular jet. (Reprinted with permission from Ref. [30].)...

The spectra can further be interpreted to describe the excited-state dynamics [28b, d The advantage of our cold molecular jet experiment is that inhomogeneous effects are minimized. In absorption, all lines are lifetime broadened because of the efficient photochemistry of OCIO. The magnitude of this broadening does not depend on the rotational quantum numbers, but is strongly dependent on the... 

Although the idea behind these femtosecond experiments is simple, the experimental setup to carry out the experiments is not at all trivial. In brief, it consists of a pulsed laser source capable of generating two femtosecond pulses of various duration (at the moment the world s shortest pulse is 6 femtosecond) a high vacuum chamber where a beam of molecules is flowing, and a sophisticated detector. A start laser flash called the pump pulse intercepts an isolated molecule in a molecular jet and sets the experimental clock at zero. A second flash called the probe pulse suitably delayed with respect to the pump pulse hits the same molecule and captures a photograph of the reaction in progress at that particular instant. Like the cameras in Muybridge s experiment, a femtosecond camera takes snapshots at different delay times to... 

Fig. Schematic diagram of the femtosecond apparatus used by-Ahmed H. Zewail and his co-workers. The femtosecond pump and probe pulses delayed suitably in time with respect to each other are combined and focused on a molecular jet inside a high vacuum chamber. The signal from tire interaction region is detected in another direction which can distinguish between events starting at different times.

A molecular jet separator connected to a high vacuum pump. 

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.

The molecular jet of molecules is crossed with a tunable dye laser and the laser-induced fluorescence is collected with a lens and focused on a PMT detector (Fig. 5). In the original experiments, a standard pulsed dye laser was used to match the 10-Hz duty cycle of the pulsed valve and the pulsed Nd YAG vaporization laser. Although this approach provides a high S/N ratio and wide spectral coverage, the resolution is limited by the laser line width of typically 0.5cm-1 (no etalon) to 0.05cm-1 with an etalon. 

For LC-MS to become a reality an interface had to be designed which was capable of providing a vapour sample feed consistent with the vacuum requirements of the mass spectrometer ion source and of volatilising the sample without decomposition. Various enrichment interfaces have been developed such as the molecular jet, vacuum nebulising, the direct liquid introduction inlet and thermospray systems. 

Figure 7.7 GC-MS interfaces, (a) Direct injection from capillary columns, (b) Open slit interface. (c) Molecular jet separator, (d) Effusion separator, (e) Diffusion separator.

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... 

The main advantages of the molecular-jet technique are its capacity to observe conformations separated by very low energy barriers and the posssibility to identify the conformations by arguments based on symmetry and substitution patterns. The technique has also been used to demonstrate the existence of cis and trans isomers in aromatic hydroxy compounds (Oikawa et al., 1984 Dunn et al., 1985). Observations of six different conformers of tryptophan by the FE technique has been reported (Phillips et al., 1988). 

Fig. 3 Two-photon TOFMS from a molecular jet study of m-propyltoluene. The origins at 36 982, 37 040 and 37 060 cm correspond to three different propyl group conformations. Other features are ascribed to methyl torsion (Breen et al., 1987c).

Common interfaces include the molecular jet and flow spHtters. The first uses the difference in momentum between the low-molecular weight carrier gas and the high-molecular-weight analytes. The column effluent passes into the separator inlet line, which is enclosed in a glass chamber that is under vacuum. A small gap separates this line from the outlet hne. Sample molecules move preferentially from one line to the other by inertia, while much of the carrier gas is removed tangentially by the vacuum. The enrichment also increases the sensitivity of GC-MS. 

The molecular jet and both types of sphtter suffer from sample discrimination. 

Norman F. Ramsey (b. 1915), American physicist and professor at the University of Illinois and Columbia University, and then from 1947 on, at Harvard University. He is, first of all, an outstanding experimentalist in the domain of NMR measurements in molecular jets, but his hobby is theoretical physics. Ramsey carried out the first accurate measurement of the neutron magnetic moment and gave a lower-bound theoretical estimation to its dipole moment. In 1989 he received the Nobel... 

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. 

P Esherick, A Owyoung, C Patterson. Stimulated Raman spectrum of the fundamental of CD4 in a molecular jet. J Phys Chem 87 602-608, 1983,... 

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