Microwave data, equilibrium

The case of a, -unsaturated caAonyl compounds is analogous to that of 1,3-dienes, in that stereoelectronic factors favor coplanaiity of the C=C—C=0 system. The rotamers that are important are the s-trans and s-cis conformations. Microwave data indicate that the s-trans form is the only conformation present in detectable amounts in acrolein (2-propenal). The equilibrium distribution of s-trans and s-cis conformations of a,fi-unsatuiated ketones depends on the extent of van der Waals interaction between substituents. Methyl vinyl ketone has minimal unfavorable van der Waals repulsions between substituents and exists predominantly as the s-trans conformer ... 

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 equilibrium molecular geometry of thiophene has been determined from a combination of gas-phase electron diffraction (ED) vibrational and microwave data and ab initio and DFT calculations (Table 19) <2001JST(567)29>. 

Additionally, the final equilibrium geometries for the tautomeric pair have been computed and are shown below. In agreement with the microwave data (c/. Section 4.11.3.2.2) both minimal and double zeta bases predict the 2H-tautomer to be more stable than the l//-form (81ZN(A)1246). 

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]. 

Van der Waals complexes can be observed spectroscopically by a variety of different teclmiques, including microwave, infrared and ultraviolet/visible spectroscopy. Their existence is perhaps the simplest and most direct demonstration that there are attractive forces between stable molecules. Indeed the spectroscopic properties of Van der Waals complexes provide one of the most detailed sources of infonnation available on intennolecular forces, especially in the region around the potential minimum. The measured rotational constants of Van der Waals complexes provide infonnation on intennolecular distances and orientations, and the frequencies of bending and stretching vibrations provide infonnation on how easily the complex can be distorted from its equilibrium confonnation. In favourable cases, the whole of the potential well can be mapped out from spectroscopic data. 

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. 

Butene exists as an equilibrium mixture of two conformations, Me-skew and Me-syn (21). The most reliable composition to date is 83 17% according to combined ED, microwave (MW), and ab initio MO analysis (133). This study includes the MM (CFF)-ED-MW analysis of this molecule for comparison, which gave a final skew/syn ratio of 80 20. The molecular orbital constrained electron diffraction (MOCED) results appear to agree better with the observed data than does the MM constrained analysis, the R value of the least-squares analysis of the latter being 20% higher than that of MOCED. However, one may ask whether such a small difference in R values justifies the enormous difference in computer time between the ab initio (about 200 hr on an IBM 370/155) and MM (less than a minute) methods used in this work. 

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. ... 

Equilibrium geometries for upwards of four thousand small molecules have been determined experimentally in the gas phase, primarily by microwave spectroscopy and electron diffraction. In the best cases, the experimental techniques are able to provide bond lengths and angles to within a few thousandths of an A and a few tenths of a degree, respectively. For larger systems, lack of data usually prohibits complete stmcture determination, and some geometrical variables may have been assumed in the reported stmcture. 

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are included in Table 5. In 1,3-dioxolane the bent form is more stable than the twisted, and pseudorotation occurs. In 1,2,4-trioxocyclopentane the equilibrium conformation is twisted, and there is a barrier of 6.3 kJ mol 1 opposing pseudorotation (74PMH(6)53). 

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