Gas-phase structure

The mean value for C(sp2)—F distances in monofluorinated groups (substructure VIII) of 134.0 pm is only slightly shorter than the gas-phase distance in fluoroethylene [134.8 (4) pm]. (C—F)m in monofluorinated and in meta- or/ zra-substituted difluorinated six-mem-bered aromatic rings [substructure IX, (C—F)m = 136.3 pm] is about 1 pm longer than the values determined for the gas-phase structures of fluorobenzene [135.6(4) and 135.4(5) pm, Table 17] and those of 1,3-and 1,4-difluorobenzene [135.61(18) and 135.2(2) pm]. On the other hand, the mean of 167 observations for orr/zo-difluorinated rings (134.0 pm) is slightly shorter than the MW result for 1,2-difluorobenzene [134.94(12) pm]. 

Although the number of compounds containing C—Cl bonds, whose structures were determined in the gas phase, is considerably smaller than that for carbon-fluorine com- 

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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]

An SCRF geometry optimization, beginning at the optimized gas phase structure, for each model. 

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. 

The lower members of the polysulfane series with n=2-6 have been prepared as pure compounds, while all members with n values up to 35 have been detected by H-NMR spectroscopy in so-called crude sulfane [31]. The gas-phase structures of the first three members of the polysulfane series are well established from either microwave spectra (H2S2 [32], H2S3 [33]) or high-level ab initio MO calculations (H2S4 [34]). Systematic ab initio studies of the structures, vibrational frequencies and heats of formation of H2S with i up to 6 have also been reported [4, 16]. 

Two wider ranging, more systematic investigations of conformational dependence have since been performed to establish whether the conformational sensitivity noted in the above PECD smdies may generally provide a means for identifying and distinguishing gas-phase structure of suitable chiral species. The B-spline method has been applied to the model system (l/f,2f )-l,2-dibromo-l,2-dichloro-l,2-difluoroethane [60]. Rotation around the C C bond creates three stable conformational possibilities for this molecule to adopt. The results for both core and valence shell ionizations reaffirm an earlier conclusion a and p are almost unaffected by the rotational conformation adopted, whereas the PECD varies significantly. Eor the C Ij ionization to show any sensitivity at aU to the relative disposition of the halogen atoms further reinforces the point made previously in connection with the core level PECD phenomenon. 

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 solid-state structures of (PhS)3N and (CgF5S)3N and the gas-phase structure of (CF3S)3N all show nearly planar S3N units with the S-N bond lengths longer than typical single-bond values. Tribenzenesulfenamide... 

The weakest point of our approach is the treatment of the bulk solvent. The energies derived from an implicit solvent model like IPCM are mainly based on energy calculations on gas-phase structures and effects of explicit solvent molecules are not included. 

This solid state structure is comparable with the first stannylene gas-phase structure reported475 by Lappert. In [(Me3Si)2CH]2Sn, the Sn-C distances were 222 pm and the CSnC angle was 97(2)°, essentially equivalent to the above. 

Figure 22. (a) Two tetra-chromophoric systems 24. (b) Profiles of the (U)HF/ 3-21G gas phase structures for the "collapsed" ground state and giant CS state of syn,syn-24. The "normal" ground state structure was located using the AM1 semi-empirical method. 

Two of the worst outliers were N,N-dimethylformamide and N,N-dimethyl-acetamide. For both of these, solubility in water was greatly underestimated. This may illustrate a situation in which conformation does assume importance. In the gas phase structures used to compute the surface properties, the nitrogens are planar. There is reason to believe, however, that interaction with water molecules will cause the nitrogens to become pyramidal,48 since that produces more localized lone pairs that better attract water hydrogens. Thus, analysis involving planar nitrogens would not indicate the true strength of the interaction. 

Hamish, D. Holmes, J.L. Ion-Radical Complexes in the Gas Phase Structure and Mechanism in the Fragmentation of Ionized Alkyl Phenyl Ethers. J. Am. Chem. Soc. 1991,113,9729-9734. 

These figures are based on published and submitted papers. Among others, carboranes, donor-acceptor complexes of boron, ring molecules with silicon, phosphoranes, sul-fones have received much attention. The lack of gas-phase structural studied of other classes of compounds, e. g., sulfuranes, is also noteworthy. The above elements were selected as most typical. Much interest is concentrated however on other elements as well, which may have been somewhat neglected in the past. The increase of the amount of structural data is also facilitating demand for further extension of the circle of compounds studied. 

X-Ray studies on the phosphorus oxides led to similar observations (82-86) P4O7, for example, cannot be considered as made up of fragments taken from P4O8 and P40jq (85). They also reveal the effect of packing, as well as significant differences between the solid and gas phase structures (84,86). 

Six examples are selected to illustrate various facets of semiempirically calculated hypersurfaces for medium-sized molecules and their correlation with experiment. They comprise Example I The gas phase structure of H3CS-C3C-SCH3 and its ionization pattern. 

Example I The Gas Phase Structure and the Ionization Pattern of Pi(thiomethyl) Acetylene H2CS-CSC-SCH3. Unstable substituted acetylenes can be generated in the gas phase by thermal... 

Silyl cations like 3 and 7 in which the positively charged silicon is part of a n-conjugated system attracted particular interest. The marginally stable silatropylium ion 7, is characterized by a Si NMR resonance at 8 Si = 149 in CD2CI2 at —50 °C, downfield-shifted by 192 ppm compared to the precursor silane.This experimental value is in fair agreement with the calculated silicon NMR chemical shift for the optimized gas phase structure of 7 (8 Si = 159.9, at GIAO/HF/6-311 + G(2df,p)(Si), 6-31G(d) (C,H)). This indicates only small interactions between the cation and dichloromethane, the solvent used for the NMR investigations. 

They are derived from x-ray and neutron diffraction of crystals, and from electron diffraction and spectroscopic measurements with microwave, infrared and Raman techniques on the gaseous phase. For optimization of PEFs on small molecules, gas-phase structures are used. They are rather numerous, and they are all calculated... 

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