Microwave data, equilibrium structures

Recent microwave data for the potential interstellar molecule Sis is used together with high-level coupled-cluster calculations to extract an accurate equilibrium structure. Observed rotational constants for several isotopomers have been corrected for effects of vibration-rotation interaction subsequent least-squares refinements of structural parameters provide the equilibrium structure. This combined experimental-theoretical approach yields the following parameters for this C2v molecule re(SiSi) = 2.173 0.002A and 0e(SiSiSi) = 78.1 O.2 ... 

The purpose of this report is to demonstrate the ease with which highly accurate equilibrium structures can be determined by combining laboratory microwave data with the results of ab initio calculations. In this procedure, the effects of vibration-rotation interaction are calculated and removed from the observed rotational constants, Aq, Bq and Cq. The resulting values correspond to approximate rigid-rotor constants and and are thus inversely... 

The data from microwave spectroscopy have been interpreted with a dihedral angle H—O—O—H = 120.0° for the gas-phase equilibrium structure of H202. The nonplanarity of the peroxide gives rise to a stereogenic 0—0 axis . The computed total parity violating energy shift of —1.9 x 10 kJmoH between the two enantiomers, however, is too small in order to be measured with contemporary devices. ... 

There have been many studies of the structure of thiophene over the years, using data from many experimental techniques. A new paper reports the equilibrium structure, determined from gas-phase electron diffraction, vibrational and microwave data. In addition, quadratic and cubic force constants were calculated theoretically (up to the B3LYP/6-311 -I- G level). Harmonic scale factors were included as refinable parameters in the analysis of the data. The outcome is a structure that is less precise than one determined earlier, which also made use of dipolar coupling constants, but the ra distances from the two studies agree well. The refined equilibrium parameters include distances C = C 137.2(3), C-C 142.1(4) and S-C 170.4(2) pm and angles CSC 92.4(2), SCC 111.6 and CCC 112.2°. 

The equilibrium structural parameters re(HN) = 1.03359(43) A and re(NN) = 1.092766(92) A were obtained from the rotational constants [20]. The substitution structural parameters s(HN) = 1.031426(56) A and rs(NN) = 1.095415(6) A, obtained from microwave data on several isotopomers [33], apparently supersede earlier r values which were determined in the same laboratory [17, 31, 32]. The geometric structure was also calculated from microwave data [32] using a mass-dependent scaling of the moments of inertia [34]. 

The physical data index summarizes the quantitative data given for specific compounds in the text, tables and figures in Volumes 1-7. It does not give any actual data but includes references both to the appropriate text page and to the original literature. The structural and spectroscopic methods covered include UV, IR, Raman, microwave, MS, PES, NMR, ORD, CD, X-ray, neutron and electron diffraction, together with such quantities as dipole moment, pX a, rate constant and activation energy, and equilibrium constant. 

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are included in Table 7. In 1,3-dioxolane the twist conformation (see Table 41) is more stable than the envelope conformation, and pseudorotation occurs. In 1,2,4-trioxolane the equilibrium conformation is the twist form, in which the peroxide bond straddles the plane of the other three atoms, and there is a barrier of 6.3 kj mol 1 opposing pseudorotation <1974PM H (6)53>. Preference for the twist conformation is also observed in the crystal structures of some 1,2,4-trioxolane derivatives (see Table 41, Section 2.4.4.4) . 

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