Gas-phase conformations

Optimized structures for the analysis of NMR spectra. Gas phase conformations of nonpolar molecules are generally very similar to solution conformations, and the importance of using the fully optimized structure is being more fully recognized. 

Deficiencies in intensities, which occur in x-ray powder dififiaction as well as in single crystal electron diffiaction, may cause problems even in early stages of ab initio structure analysis. Nevertheless, examples for successful use of the tangent formula or Sayre equation for structure determination from ED data have been worked out [14]. Other direct methods, like maximum entropy can provide us with an envelope of the molecules in the cell, which delivers an idea of its orientation [15]. An alternative approach to ab initio structure determination is the calculation of the gas phase conformation of an initial model for subsequent refinement by energy minimization [16]. 

M. T. Application of ion mobility to the gas-phase conformational analysis of polyhedral oligomeric silsesquioxanes (POSS). Int J Mass Spectrom 2003, 222, 63-73. 

A significant issue widi modem force fields is that it can be difficult to simultaneously address both generality and suitability for use in condensed-phase simulations. For example, the MMFF94 force field is reasonably robust for gas-phase conformational analysis over a broad range of chemical functional groups, but erroneously fails to predict a periodic box of n-butane to be a liquid at —0.5 °C (Kaminski and Jorgensen 1996). The OPLS force field, on the other hand, is very accurate for condensed-phase simulations of molecules over which it is defined, but it is an example of a force field whose parameterization is limited primarily to functionality of particular relevance to biomolecules, so it is not obvious how to include arbitrary solutes in the modeling endeavor. 

Changes in the conformations of the sorbed alkane molecules were found to be far more pronounced for hexane than for butane when compared with their gas-phase conformations. In the case of butane, only slightly... 

It has been pointed out6 that trans-1,4-dihalocyclohexanes are ideally suited for determination of changes in free energy, enthalpy, and entropy by electron diffraction, since the ratio between the conformers may be found quite accurately by comparing areas of the appropriate peaks in the experimental RD curve (see Fig. 17). Accurate thermodynamic parameters of gas phase conformational equilibria are rather scarce and these molecules may perhaps be able to provide some much-wanted high-quality data with the aid of the modern electron-diffraction technique. 

Five of the eight theoretically possible 1,2,3,4,5,6-hexachlorocyclohexanes have been studied and their preferred gas phase conformations determined325. ... 

Rice and Chabalowski, [62] Harris and Lammertsma [63], and Vladimiroff and Rice [64] have reported density functional theory (DFT) and second-order Moeller-Plesset (MP2) results that greatly clarify many of the structural details of the RDX gas-phase conformations, the energy barriers between them, and provide predictions of the vibrational frequencies. 

Figure 15). The optimized gas-phase conformation most likely also represents the solution structure (Scheme 8b) in aprotic solvents. 

PE spectroscopy has been used to identify gas phase conformations of 1,3,4-oxadiazolidines. The ring adopts a half-chair conformation (80CB221). Inversion at nitrogen in 3,4-disubstituted 1,3,4-oxadiazolidines has been studied using variable temperature NMR spectroscopy <75JCS(P2)1191). 

Figure 5.5 Ao represents the gas-phase conformation with the optimized Au-Au distance of 8.842 A. Ai depicts the molecule with the stretched Au-Au distance of 10.061 A along the y-axis. Ai shows the molecule with the stretched Au-Au distance of 10.061 A placed between electrodes that are separated by 8.824 A.

B. D. Ross and N. S. True, NMR spectroscopy of cyclohexane. Gas-phase conformational kinetics. 

Wyttenbach T, Bowers MT (2003) Gas Phase Conformations The Ion Mobility/Ion Chromatography Method. In Topics in Current Chemistry, Schalley CA (ed) Springer, Berlin, Top Curr Chem 225 207-232. 

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