Gas-phase NMR spectroscopy

The first conformational process for which pressure-dependent rate constants were obtained using gas-phase NMR spectroscopy was the syn-anti exchange process of methyl nitrite (MeONO). Figure 4 shows gas-phase H NMR spectra of pure MeONO as a function of pressure at 262 K. At low pressures the rate of the exchange process is slow on the NMR timescale and the spectrum is resolved into resonances of the syn and anti conformers. As the pressure increases, the exchange rate increases and causes the syn and anti resonances to ultimately collapse into a single line. 

Gas-phase NMR spectroscopy has been used to obtain equilibrium constants and rate constants for many low-energy molecular processes. These data have been used to address questions regarding the relative stability of conformers and tautomers in the gas phase, the kinetics of exchange processes in the gas phase, and the direction and magnitude of solvent effects on these equilibria and processes. Most of the studies have appeared in the last 10 years. Continued progress in NMR instrumentation and techniques as well as considerable recent developments in kinetic theory ensure that the next 10 years will see many novel applications of gas-phase NMR spectroscopy. 

Specific applications of gas phase NMR spectroscopy to research in heterogeneous catalysis, conformational dynamics and absorption processes in zeolites are described below. The application of gas-phase NMR spectroscopy to many other research areas is described in the Further reading section at the end of this article. 

Gas-phase NMR can be used to study the kinetics of reactions that consume or generate volatile species. In a recent study, gas-phase NMR spectroscopy was used to follow the kinetics of the hydrogenation of cis/trans mixtures of perfluoro-2-butenes (R=GF3) and perfiuoro-2-pentenes (R=G2Fs) over palladium... 

Variable-temperature NMR spectroscopy is a well-established technique in the study of conformational exchange kinetics in liquid amides. [7,58] Using DNMR spectroscopy, it is possible to measure the temperature dependence of interconversion rates in order to yield a complete set of kinetic parameters characterizing the internal rotation about the carbon-nitrogen bond. In the last several years, the development of gas-phase NMR methods has provided a valuable tool for the elucidation of the magnitude of intrinsic and environmental contributions associated with this conformational exchange. In order to characterize these contributions, a series of molecules with systematic variations in substituent size and polarity must be analyzed both in the liquid and gas phase. 

Both compounds were identified by means of multinuclear NMR spectroscopy ( H, C, F) and gas-phase IR spectroscopy. F3SiN(SiMc3)NMe2 was further characterized by and Si NMR and by mass spectrometry. 

The xenon fluorides, Xep2, Xep4, and XePg, were characterised by gas-phase Xe and P NMR spectroscopy, providing the first gas-phase NMR spectroscopic data for the xenon fluorides. Xenon difluoride was also characterised by Xe and P solid-state NMR spectroscopy, providing experimental values for the Xe (4260 10 ppm) and P (125 5 ppm) shielding anisotropies in Xep2-... 

Time-resolved spectroscopy has become an important field from x-rays to the far-IR. Both IR and Raman spectroscopies have been adapted to time-resolved studies. There have been a large number of studies using time-resolved Raman [39], time-resolved resonance Raman [7] and higher order two-dimensional Raman spectroscopy (which can provide coupling infonuation analogous to two-dimensional NMR studies) [40]. Time-resolved IR has probed neutrals and ions in solution [41, 42], gas phase kmetics [42] and vibrational dynamics of molecules chemisorbed and physisorbed to surfaces [44]- Since vibrational frequencies are very sensitive to the chemical enviromnent, pump-probe studies with IR probe pulses allow stmctiiral changes to... 

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

Organic thionylamines have planar, cis structures (9.9) in the solid state and in solution, as determined by X-ray crystallography and N NMR spectroscopy, respectively. The gas-phase structures of the parent compound HNSO and MeNSO have been determined by microwave spectroscopy. The S=N and S=0 double bond lengths are 1.51-1.52 and 1.45-1.47 A, respectively. The bond angle [Pg.168]

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