Methane, structural parameters

Case58 investigated the effect of ring currents on NMR shielding constants by means of the DFT calculations. The studied rings included the ones commonly found in proteins and nucleic acids. The shielding constants were calculated for methane molecule placed in several positions relative to the ring. The calculations provided data needed to derive structural parameters from measured chemical shifts in proteins and nucleic acids. 

The current high level of interest in binuclear metal complexes arises from the expectation that the metal centers in these complexes will exhibit reactivity patterns that differ from the well-established modes of reactivity of mononuclear metal complexes. The diphosphine, bis(diphenylphosphino)methane (dpm), has proved to be a versatile ligand for linking two metals while allowing for considerable flexibility in the distance between the two metal ions involved (1). This chapter presents an overview of the reaction chemistry and structural parameters of some palladium complexes of dpm that display the unique properties found in some binuclear complexes. Palladium complexes of dpm are known for three different oxidation states. Palladium(O) is present in Pd2(dpm)3 (2). Although the structure of this molecule is unknown, it exhibits a single P-31 NMR reso-... 

Other Substituted Methanes, The structures of several methane derivatives have been determined by a variety of techniques. " From an analysis of the microwave spectra of ground-state and excited torsional states of a number of isotopic species of CHaSeH (1), the structural parameters of this... 

Table 2. Structural Parameters for Bis(dipheny/arsino)methane (dam)...

Petrov et al. have presented a variable temperature solid-state NMR investigation of cryptophane-Exhloroform and cryptophane-Erdichlorometh-ane inclusion complexes.The line shapes and nuclear spin relaxation rates were analysed in terms of the distribution of C-D bond orientations and the time scale of the guest dynamics. It was found that encaged chloroform produces broad spectra, and that its reorientation is relatively slow with a correlation time of 0.17 ps at 292 K. In contrast, the line shapes of encaged dichloromethane were narrow and the motion of this guest molecule was fast with a correlation time of 1.4 ps at 283 K. The NMR data were complemented by an X-ray diffraction study of the cryptophane-E dichloro-methane structure, which was utihsed in the analysis of the NMR parameters. 

These differences have been attributed to various factors caused by the introduction of new structural features. Thus isopentane has a tertiary carbon whose C—H bond does not have exactly the same amount of s character as the C—H bond in pentane, which for that matter contains secondary carbons not possessed by methane. It is known that D values, which can be measured, are not the same for primary, secondary, and tertiary C—H bonds (see Table 5.3). There is also the steric factor. Hence, it is certainly not correct to use the value of 99.5 kcal mol (416 kJ mol ) from methane as the E value for all C—H bonds. Several empirical equations have been devised that account for these factors the total energy can be computed if the proper set of parameters (one for each structural feature) is inserted. Of course these parameters are originally calculated from the known total energies of some molecules that contain the structural feature. 

The logical next step in this sequence of calculations was to perform simulations that included the extraframework cations Na and Ca in the zeolite A structure (56). The cations were situated near the hexagonal faces of the sodalite units in zeolite A such that the geometrical hindrance to methane molecules passing through the 8-ring windows was small. Interactions between the methane molecule and the cations were derived from the data of Ruthven and Derrah (58), and the same two sets of parameters were used (A and B). Long simulations were performed, 25 ns with a time step of 10 fs. 

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