Gas phase structure determination

The two techniques for gas phase structure determination are spectroscopy and electron diffraction. The following codes are used to indicate the method used for each set of data ... 

This section covers ab initio and density functional theory (DFT), semi-empirical and empirical, and molecular mechanics and molecular dynamics methods. For gas-phase structure determinations, a refinement to the use of ab initio calculations the SARACEN (Structure Analysis Restrained by Ab initio Calculations for Electron diffractioN) method, and other relevant theoretical and computational chemistry techniques, including quantitative structure-activity/property relationship (QSAR/QSPR) models for prediction of biological activity and physicochemical properties, are also covered. 

In the introduction to last year s review we commented on the now almost ubiquitous use of computational methods in gas-phase structure determinations. In the very large majority of cases, the results of these calculations are used in some way in the structural analysis, perhaps as computed amplitudes of vibration (sometimes scaled to reproduce experimental frequencies). Sometimes calculated differences between related parameters are used as hxed parameters in the rehnements, but there are still relatively few uses of the computed data as flexible restraints. In this present review we describe two cases where there have been independent studies of the same molecules, and the results include some parameters that do not agree, within the expected levels of precision. We have tried to identify the sources of the discrepancies, and it seems that the imphcit assumption that fixed parameters are absolutely correct may be a problem. It is certainly an issue that requires care. 

How can you assess the quality of a gas-phase structure determined by electron diffraction reported in the literature What are the important criteria to look at ... 

Oxotrifluorosulfur imide anions RNS(0)F3" are isoelectronic with the tetrafluorosulfiiriraides RN=SF4 Similar structures are expected with the substituent R in an axial position and inequivalent axial bonded fluorine substituents Fa, Fa (see Table H). This inequivalence of Fa and Fa has been established, e.g., for CH3N=SF4 (44) and FN=SF4 (57) by NMR in solution and by gas phase structure determinations. In tibe NMR experiments of RNS(0)F3 only one signal is found for Fa and Fa due to rapid exchange (either inversion at the nitrogen or rotation around the NS-bond). 

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]

The gas-phase structure of 1,3-dithietane 1-oxide (189) has been determined from its microwave spectrum and the spectra of eight isotopic modifications192. The ring is puckered, the angle between the two CSC planes being 39.3° with the oxygen equatorial. 

Historically, AuF has been one of the most elusive of all metal halides. At one time it was believed to be impossible to prepare, and theoretical papers speculating on how it might be observed or predicting spectroscopic and structural properties have been published until recently.3075- 1 The existence of AuF has been confirmed by microwave spectroscopy, the sample has been prepared by laser ablation of Au metal in the presence of a F precursor.3082 The gas-phase structure of Aul has also been determined by microwave spectroscopy.3083... 

The gas-phase structure of f-BuLi has also been probed by McLean and coworkers using photo ionization mass spectrometry". At room temperature, they detected only tetramers and determined the ionization potential of (f-BuLi)4 to be 6.2 eV. Other ions were detected above 8 eV with general formulae R Li4+(n = 1-3) and RLi2+(R = f-Bu). Both R3Li4+ and RLi2 had also been observed by Brown and coworkers in the H spectra of f-BuLi vapor, while R2Li4 and RLi4+ had not . 

In Table 2 are listed the hydroxylamines, oximes and hydroxamic acids for which we have determined the gas phase structures. We tried to select a representative group in each category. There are two types of oximes, as indicated, aldoximes and ketoximes. Due to restricted rotation around the C=N double bond, these can exist in two isomeric forms (except when R = H for an aldoxime and R = R" for a ketoxime). We have investigated both isomers in nearly every instance. For aldoximes, they are generally labeled syn when the H and OH are on the same side of the double bond and anti when on opposite sides. Note that the ketoximes in Table 2 contain one pair of isomers in which the >C=NOH group is not bonded to two carbons instead one bond is to a chlorine. One of these isomers wiU be of interest in Section B.D in the context of hydrogen bonding vi lone pair—lone pair repulsion. 

Experimental gas-phase structures of charged species are virtually non-existent. It is impossible to establish sufficiently high concentrations for conventional spectroscopy. Also, microwave spectroscopy, the principal technique for accurate structure determinations, cannot be applied to charged species. 

In conclusion, the dominant resonance structures for the ylides are as follows for non-stabilized ylides, A for stabilized ylides, 16b and 16c for P=C=P ylides, 22 (although further study on this class is advised to determine if the gas-phase structure may in fact be linear) and for P=C=C ylides, 28b and 28c. 

---