Medium-Resolution Studies

Ion+ (GeCl3)2CCl2 (Relative abundance) GeCl3CCl3 (Relative abundance) 

Molecular beam mass spectrometry has been used to study penta-fluorides ofNb, Ta, Mo, Re, Os,Ir, Ru, Rh, Pt, Sb, and Bi (139), indicating dimeric, trimeric, and even tetrameric ions (139). Simple fragmentations of the type 

Ion abundances are consistent with increasing B-N and decreasing B-X bond strength as X varies from F to I. 

The mass spectra of main group organometallic compounds have been discussed in many of the reviews listed in the Introduction, and here we only summarize some recent examples. 

Cragg and Weston (11) in their review of the mass spectra of boron compounds have shown the great amount of work being done with cyclic boron compounds. This trend has continued. Straight-chain trialkyl-borates fragment by C—0 cleavage reactions accompanied by hydrogen transfers. 

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Despite considerable biochemical work, high-resolution crystal structure determination of native RNase A and S, and some medium-resolution studies of RNase A-inhibitor complexes, a number of questions existed concerning the details of the catalytic mechanism and the role of specific amino acids. Study of the low-temperature kinetics and three-dimensional structures of the significant steps of the ribonuclease reaction was designed to address the following questions. 

B. Factors Affecting Spectra Low- and Medium-Resolution Studies A. Fragmentations... 

Medium-resolution absorption spectrometer emission spectrometer with red-sensitive photomultiplier or CCD detector laser excitation source such as listed in Table 1 (or medium pressure mercury arc such as described in earlier editions of this text) neon calibration lamp and power supply (available from, e.g.. Oriel Corp., Stratford, CT) reagent-grade iodine 100-mm glass cell with Teflon stoppers for absorption studies heating tape with controlling Variac 50-mm cell for emission studies vacuum system, preferably with a diffusion pump and cold trap, for pumping down emission cell. 

The study of the rotation-vibration spectra of polyatomic molecules in the gas phase can provide extensive information about the molecular structure, the force field and vibration-rotation interaction parameters. Such IR-spectra are sources of rotational information, in particular for molecules with no permanent dipole moment, since for these cases a pure rotational spectrum does not exist. Vibrational frequencies from gas phase spectra are desirable, because the molecular force field is not affected by intermolecular interactions. Besides, valuable support for the assignment of vibrational transitions can be obtained from the rotational fine structure of the vibrational bands. Even spectra recorded with medium resolution can contain a wealth of information hot bands , for instance, provide insight into the anharmonicity of vibrational potentials. Spectral contributions of isotopic molecules, certainly dependent on their abundance, may also be resolved. 

There have been some other studies where the PARS technique has been applied, e.g. in a medium-resolution experiment, in which a direct comparison between CARS and PARS spectra of the bands of glyoxal and of methane at pressures of 4 and 6.7 kPa, respectively, excited under similar conditions could be made (Duval et al., 1986). The CARS profiles showed a broader appearance which the authors attribute to contributions of the real part of the nonlinear susceptibility, however, this assumption should be proved by calculation of both profiles. 

The ultimate molecular level characterization of a pharmaceutical material is performed on the level of individual chemical environments of each atom in the compound, and this information is best obtained using nuclear magnetic resonance (NMR) spectroscopy. Advances in instrumentation and computer pulse sequences currently allow these studies to be carried out routinely in the solid state.2 Although any nucleus that can be studied in the solution phase also can be studied in the solid state, most work has focused on studies. H-NMR remains an extremely difficult measurement in the solid state, and the data obtained from such work can be obtained only at medium resolution. The main problem is that H-NMR has one of the smallest isotropic chemical shift ranges (12 ppm), but has peak broadening effects that can span several parts per million in magnitude. 

The accuracy of protein structures determined by NMR is very dependent on the quantity and quality of data that can be obtained. It has been known that the highest quality NMR structures have accuracies comparable to the medium-resolution X-ray structures (2.0-2.5A) for protein backbone atomic coordinates.3 To improve the structural qualities and utilize them for biological system studies, it is important to re-emphasize factors having a critical impact on quality as summarized below. 

We have used only medium-resolution GC thus far in our studies and have adopted the following criteria for distinguishing biogenic and petroleum hydrocarbons ... 

The spontaneous Raman spectra of spherical top molecules of the type XF were recorded by Claasen et al. [248] and Bos worth et al. [249]. Medium-resolution spectra of sulfur hexafluoride [250-252], uranium hexafluoride [252], and iridium hexafluoride [253] were studied in more detail. Recently, Rotger et al. [254] reexamined the spontaneous Raman spectra of M0F6, WFg (see Fig. 26), ReFg, OsFg, IrF, and IF7. The Q branch of the P] band of SFe was resolved by Raman gain spectroscopy [45,255]. 

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