State Selection Experiments

The molecules in an ordinary molecular beam have both rotational and vibrational energy. By suitable modification of the experiment it is possible to produce beams of molecules in a particular vibrational state or with a particular orientation. Therefore one can directly study the effect of vibrational energy or of molecular orientation on chemical reactivity. 

Methyl iodide is a dipolar molecule it can therefore be aligned in an electric field. By passing a beam of CH3I molecules through such a field a beam of oriented molecules is produced. This technique was used in a study of the 

Beuhler, Jr., and R. B. Bernstein, /. Chem, Phys. 51,5305 (1969). The experiment is not designed to form a beam of parallel dipoles since, at the required field strengths, the molecules would ionize. Instead a field geometry is constructed to reject all molecules which are leading with their iodine (or methyl) end. 

In Section 8.3 we described the measurement of a reaction cross section. What measurements must be made to determine (a) the elastic cross section, (b) the inelastic (nonreactive) cross section  

A molecular beam source contains Cs(liq) in equilibrium with Cs(v) at 600 K. The vapor pressure of Cs is 4.5 x 10 atm. The aperture is a circular slit 1 mm in diameter other apertures maintain the beam as an atomic ribbon. Calculate  

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A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

Spin-state selective experiments have been an integral part in both the history and development of NMR spectroscopy. Experiments vary from the early simple double-... 

Most spin-state selective experiments have been used to measure coupling constants [5.228]. RNA research has often used chemical synthesized nucleosides as model compounds labelling the nucleosides with and investigating the IR and spin system to extract scalar and dipolar coupling constants, and correlation experiments have been a popular approach. Check it 5.9.2.3 illustrates a IR and correlation experiment ... 

More detailed and improved experiments have been made with NHj from CH3NH2. As in previous MPD studies [for instance, Refs. 10, 26, 29] NHj has been observed by laser excitation in the transition system, which exhibits a many-line spectrum reaching from the near-UV into the near-IR. Figure 4 shows in the lower part a small section around 493 nm of the whole NH2 excitation spectrum taken with a probe laser bandwidth of 1.5 GHz and a scanning speed of 4 GHz/min. The CH3NH2 beam was formed without carrier gas at a stagnation pressure of 500 torr. The section covers transitions from rotational levels N" = l to 6 in the t>" = (0,0,0) state of NH2. The MPD in beams with laser-induced fluorescence is obviously a powerful technique to produce and observe radicals with sufficient amounts for state-selective experiments of various kinds under rather refined conditions. 

Bouguet-Bonnet S, Mutzenhardt P, Canet D (2004) Measurement of 15 N csa/dipolar crosscorrelation rates by means of spin state selective experiments. J Biomol NMR 30 133-142... 

P. Permi, A spin-state-selective experiment for measuring heteronuclear one-bond and homonuclear two-bond couplings from an HSQC-type spectrum, J. Biomol. 

Theory and calculations on the chemical reactions of polyatomic molecules are very active areas of research, " There are several reasons for this. The most modem experimental techniques using lasers and molecular beams are being applied to study the microscopic details of such chemical reactions including how different vibrational modes of polyatomic molecules influence reactivity," and measurements of the lifetimes of reaction complexes. State-selected experiments of this type require detailed quantum reactive scattering theory in their interpretation. Furthermore, there is a need for the accurate calculation of kinetic data such as rate constants of polyatomic reactions that are sometimes difficult to study in the laboratory but are important in areas such as atmospheric, combustion, and interstellar chemistry. 

In a third step the S-matrix is related to state-selected reaction cross sections a., in principle observable in beam scattering experiments [28, 29, 30, 31, 32, 33, 34 and 35], by the fiindamental equation of scattering theory... 

This rate coefficient can be averaged in a fifth step over a translational energy distribution P (E ) appropriate for the bulk experiment. In principle, any distribution P (E ) as applicable in tire experiment can be introduced at this point. If this distribution is a thennal Maxwell-Boltzmann distribution one obtains a partially state-selected themial rate coefficient... 

Gerlioh D 1992 Inhomogeneous RF fields a versatile tool for the study of prooesses with slow ions State-Selected and State-to-State Ion-Molecule Reaction Dynamics Part 1. Experiment ed C Ng and M Baer (New York Wiley)... 

Flere qiiantum-mechanical vibrational state-to-state differential cross sections were calculated for a translational energy of = 20 eV and compared with experiments, with very good agreement between experiment and theory. In another application of this approach, state-selected integral cross sections were... 

By using this approach, it is possible to calculate vibrational state-selected cross-sections from minimal END trajectories obtained with a classical description of the nuclei. We have studied vibrationally excited H2(v) molecules produced in collisions with 30-eV protons [42,43]. The relevant experiments were performed by Toennies et al. [46] with comparisons to theoretical studies using the trajectory surface hopping model [11,47] fTSHM). This system has also stimulated a quantum mechanical study [48] using diatomics-in-molecule (DIM) surfaces [49] and invoicing the infinite-onler sudden approximation (lOSA). 

Nolls, P., Espinosa, J. F., Parella, T. Optimum spin-state selection for all multiplicities in the acquisition dimension of the HSQC experiment. 

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