Symmetry determinations by electron diffraction

The structures of [2]ladderane and [3]ladderane were determined by electron diffraction.22,23 Each cyclobutane ring of bicyclo[2.2.0]hexane has a folded structure with fold angle 11.5°, and the molecule has C2 symmetry.22 Anti- and yy/7-tricyclo[4.2.0.02,5]octanes also have folded cyclobutane rings with fold angles of 8.0 and 9.0°, respectively.23 MM2 calculations on... 

The molecular structure of gaseous tetrahydropyran has been determined by electron diffraction the dimensions are presented in Figure 7 (79ACS(A)225). The molecule exists in the chair form with Cs symmetry consistent with the conclusions based on NMR, rotational and vibrational spectra. The torsional angles suggest that the heteroatom causes no flattening of the ring relative to that of cyclohexane. 

Herrmann and co-workers synthesized [Os(0)(Me)4] from 0s04 and dimethylzinc or methyltris(isopropoxy)titanium (180). An alternative route is by methylation of the glycolate osmium(VI) complex [0=0s(0CH2CH26)2] with dimethylzinc (180). The thermally labile ethyl derivative [Os(0)(Et)4] has also been prepared (180). [Os(0)(Me)4] is an orange, air-stable, volatile, crystalline compound that melts at 74°C without decomposition. The gas-phase average molecular structure of [Os(0)(Me)4], determined by electron diffraction techniques, is consistent with a theoretical model of C4 symmetry with d(Os—C) = 2.096(3) A, d(0s=0) = 1.681(4) A, and ZO—Os—C = 112.2(5)° (180). Cyclic voltammetric studies showed that [Os(0)(Me)4] undergoes reversible reduction at - 1.58 V and an irreversible oxidation at -f 2.2 V vs Ag/AgCl in MeCN. 

The molecule is pyramidal, having C3v symmetry with the nitrogen atom at the apex. The molecular dimensions have been determined by electron diffraction (266) and by microwave spectroscopy (161,271). The molecule with this symmetry will have four fundamental vibrations allowed, both in the infrared (IR) and the Raman spectra. The fundamental frequency assignments in the IR spectrum are 1031, vt 642, v2 (A ) 907, v3 (E) and 497 cm-1, v4 (E). The corresponding vibrations in the Raman spectrum appear at 1050, 667, 905, and 515 cm-1, respectively (8, 223, 293). The vacuum ultraviolet spectrum has also been studied (177). The 19F NMR spectrum of NF3 shows a triplet at 145 + 1 ppm relative to CC13F with JNF = 155 Hz (146, 216, 220,249, 280). 

The structures of gaseous tetrafluorohydridophosphane, HPF4, and trifluorodihydridophosphorane H2PF3, have been determined by electron diffraction. Both molecules adopt thp structures with the hydrogen atoms occupying equatorial sites in conformity with C2y symmetry. Theoretical... 

The i.r. and Raman spectra of trivinylborane, B(CH=CH2)3, may be assigned on the basis of the planar, C3h, structure determined by electron diffraction. The values of v(B—C) and v(C=C) are consistent with some B—C 7r-interaction.141 A second, independent, investigation of this vibrational spectrum agrees with these conclusions for the solid phase, but in the fluid phases an additional conformer, of Cs symmetry, was detected.142 This form may be obtained by twisting the vinyl groups out of the molecular olane. 

The existence of tellurium dichloride and dibromide has been shown in gaseous state and their structures have been determined by electron diffraction.9 9 Both molecules are expectedly bent with C2 symmetry. The Te-Cl and Te-Br bonds are 2.329(3) A and 2.51(2) A, respectively, and the respective bond angles are 97.0(6) and 98(3)°. 

SThe structure of tetrahydropyran in the gaseous state has been determined by electron diffraction and microwave spectroscopy. These show that the saturated oxygen six-membered ring exists in a chair conformation with Q symmetry (see Fig. 6.5), which is somewhat flattened when compared with cyclohexane ... 

Pure rotational spectroscopy in the microwave or far IR regions joins electron diffraction as one of the two principal methods for the accurate determination of structural parameters of molecules in the gas phase. The relative merits of the two techniques should therefore be summarised. Microwave spectroscopy usually requires sample partial pressures some two orders of magnitude greater than those needed for electron diffraction, which limits its applicability where substances of low volatility are under scrutiny. Compared with electron diffraction, microwave spectra yield fewer experimental parameters more parameters can be obtained by resort to isotopic substitution, because the replacement of, say, 160 by lsO will affect the rotational constants (unless the O atom is at the centre of the molecule, where the rotational axes coincide) without significantly changing the structural parameters. The microwave spectrum of a very complex molecule of low symmetry may defy complete analysis. But the microwave lines are much sharper than the peaks in the radial distribution function obtained by electron diffraction, so that for a fairly simple molecule whose structure can be determined completely, microwave spectroscopy yields more accurate parameters. Thus internuclear distances can often be measured with uncertainties of the order of 0.001 pm, compared with (at best) 0.1 pm with electron diffraction. If the sample is a mixture of gaseous species (perhaps two or more isomers in equilibrium), it may be possible to unravel the lines due to the different components in the microwave spectrum, but such resolution is more difficult to accomplish with electron diffraction. 

This colorless, volatile, and air-stable solid is the starting material for synthesis of many Cr derivatives. Commercially available, it is seldom synthesized however, Reductive Carbonylation (e.g. equation 5) gives higher yields and is more reliable than other methods. Cr(CO)6 melts at 130 °C, sublimes at room temperature in vacuo, and dissolves slightly in organic solvents. It exhibits perfect octahedral symmetry, as determined by electron and neutron diffraction and X-ray crystallography. The IR spectrum of this material in CCI4... 

The pRNA and the capsid between them sandwich a dodecameric connector, the structure of which has been determined crystallographi-cally (Simpson et al., 2000) (Fig. 2). The connector has 12-fold rotational symmetry and its structure determination was a beautiful application of hybrid methods. Orthogonal views of the connector reconstructed from two-dimensional arrays by electron diffraction analysis had been published previously (Valpuesta et al., 1999). These published images were used to generate a three-dimensional model that was positioned translation ally... 

The structure of pyrazine has been determined by X-ray analysis and by electron diffraction. Pyrazine is a planar hexagon with Z>2/, symmetry, the C-C bond lengths being very similar to... 

In some molecular crystals which contain only light elements bonding effects have been demonstrated using both x-ray and neutron diffraction. If there is any displacement of electrons relative to the nuclei the internuclear distances in the molecule are not necessarily given by the separation of the maxima in the charge density, and the nuclear positions need to be determined by neutron diffraction. If the problems associated with molecular vibrations can be overcome, a comparison of the measured electron distribution with that calculated for free atoms centered on the nuclear positions may reveal effects associated, for example, with bond formation and lone pairs. Figure 3 shows the experimental deviations from spherical symmetry for the charge density... 

In these molecules, as in most electron diffraction studies, the symmetry is assumed to be known from spectroscopic evidence. The number of independent structural parameters to be determined is thirteen for acrolein (point group C,), nine for butadiene (point group C21,), and five for glyoxal (point group Ca ). The simplest case is that of glyoxal, since the five structural parameters (chosen as the C=0, C-C, and C-H bond lengths and the C-C=0 and C-C-H angles) are all determined relatively well by electron diffraction alone. This may be seen in Table 6 in the column headed... 

Newman projection in Figure 26. The prevailing form in the vapour phase is the same as the one found in the crystal phase by X-ray diffraction (Jordan, Smith, Lohr and Lispcomb, 1963) addition to form I, a smaller amount of form II was also detected by electron diffraction. Both forms have 0 symmetry and their ratio is 4 1. The corresponding radial distributions are also shown in Figure 26. The geometrical parameters determined by the two cited investigations are collected in Table 19 ... 

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