Microwave spectroscopy, for

Similar operational definitions have to be taken into account for every experimental tool of structural chemistry to define the meaning of the observables that it provides6. In microwave spectroscopy, for example, structural information is obtained from the rotational constants... 

The primary significance of microwave spectroscopy for chemistry is in determination of molecular structure. Assignment of microwave spectral lines to transitions between specific rotational levels allows determination of the rotational constants A0, B0, and C0, and the corresponding moments of inertia. The moments of inertia are dependent on the molecular bond distances and bond angles. 

The use of microwave spectroscopy for chemical analysis has been suggested. A commercially available microwave spectrometer gives accurate intensity measurements, and applications to chemical analysis should increase. 

CQ and C . Aluminum-27 quadrupolar coupling constants have been measured by microwave spectroscopy for A1H (37), A1F (52), A1C1 (35, 74) and A1NC (36). 

A geometry similar to 37 was found experimentally by microwave spectroscopy for c-PrSMe . As yet a comparison of the efficiency of overlap of oxygen and sulfur lone pairs with the 4e LUMO of cyclopropane has not appeared. 

Further evidence for the 77--allyl intermediate is given by the work of Naito et al. who used the more sensitive technique of microwave spectroscopy for gas-phase deuteropropene analysis. In the presence of D2 at room temperature... 

Naito et al. studied hydrogenation with use of adsorption measurements, mass spectrometry, and microwave spectroscopy for product analysis. In the room temperature deuteriation of propene, butene, and 1,3-butadiene, the main products were [ H2]-propane, [ H2]-butane, and l,2-[ H2]-but-l-ene, respectively. They showed, using mixtures of H2 and D2, that deuterium was added in the molecular form and at a rate proportional to the partial pressure of D2, as opposed to D surface coverage the reaction rates were zero order in hydrocarbon. They proposed, therefore, in contrast to the model of Dent and Kokes for ethene (but note in this case that reaction rate was 0.5 order in hydrogen pressure and proportional to ethene surface coverage), that hydrogenation proceeded by interaction of adsorbed hydrocarbon with gas-phase D2, that is by an Eley-Rideal mechanism. 

In an excellent and lucid article (ideal for teaching purposes), phosgene has been used as a case study for the complementary use of electron diffraction and microwave spectroscopy for precise structure determination [1476]. Full details of this study have also appeared elsewhere [1479,1481], and these data provide the most precise structural details yet determined for phosgene. 

Rotational constants and centrifugal distortion constants of the upper vibrational state 2 vg of H2B-NH2 have also been determined by microwave spectroscopy for details, see [3]. Also, the He(I) photoelectron spectrum of H2B-NH2 (produced by controlled thermal decomposition of H3N-BH3) has been measured [4]. The five ionization potentials observed up to 21.2 eV have been correlated with those of ethene. A good correspondence of the observed values was obtained with data from Koopmans theorem calculations for the ground state molecule (semiempirical MNDO and SCF ab initio calculations with 3-21G and 6-31G bases). Experimental ionization potentials (IP) and calculated orbital energies are given in Table 4/24, p. 222 [4]. A correlation of the IP data of H2B-NH2 and H2CCH2 is given for the five uppermost filled levels in Fig. 4-47, p. 222. 

For three isotopic species in their vibrational ground states, the following rotational constants (in MHz) were derived from the microwave spectrum [1], see also the tables [2] of molecular constants from microwave spectroscopy (for axis system, see Fig. 1 [1]). 

The chief disadvantage of microwave spectroscopy for gas-phase analytical applications is that its sensitivity is not as high as for some other methods (such as laser fluorescence or mass spectrometry). For low molecular weight polar species such as SO2, NH3 and NO2, analytical detection sensitivities using FT-MWS instruments certainly extend into the parts per billion (ppb) range. However, as the molecular size and mass increase or the polarity decreases the sensitivities may fall more typically into the ppm range. Naturally, as with all spectroscopic methods, appropriate preconcentration or preselection schemes may lead to effectively improved detection limits. 

It is almost a certainty that you are familiar with microwave radiation—have you ever cooked or heated food in a microwave oven Microwave radiation plays a prominent role in a branch of science called rotational (or microwave) spectroscopy. For a discussion of rotational spectroscopy and how it is used not only to make precise measurements of bond lengths but also to detect molecules in interstellar space, go to the Focus On feature for Chapter 10, Molecules in Space Measuring Bond Lengths, on the MasteringChemistry site. 

Forbes M D E, Peterson J and Breivogel C S 1991 Simple modification of Varian E-line microwave bridges for fast time-resolved EPR spectroscopy Rev. Sc/. Instrum. 66 2662-5... 

Microwave studies in molecular beams are usually limited to studying the ground vibrational state of the complex. For complexes made up of two molecules (as opposed to atoms), the intennolecular vibrations are usually of relatively low amplitude (though there are some notable exceptions to this, such as the ammonia dimer). Under these circumstances, the methods of classical microwave spectroscopy can be used to detennine the stmcture of the complex. The principal quantities obtained from a microwave spectmm are the rotational constants of the complex, which are conventionally designated A, B and C in decreasing order of magnitude there is one rotational constant 5 for a linear complex, two constants (A and B or B and C) for a complex that is a symmetric top and tliree constants (A, B and C) for an... 

As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

Fluoroacetic acid [144-49-OJ, FCH2COOH, is noted for its high, toxicity to animals, including humans. It is sold in the form of its sodium salt as a rodenticide and general mammalian pest control agent. The acid has mp, 33°C bp, 165°C heat of combustion, —715.8 kJ/mol( —171.08 kcal/mol) (1) enthalpy of vaporization, 83.89 kJ /mol (20.05 kcal/mol) (2). Some thermodynamic and transport properties of its aqueous solutions have been pubHshed (3), as has the molecular stmcture of the acid as deterrnined by microwave spectroscopy (4). Although first prepared in 1896 (5), its unusual toxicity was not pubhshed until 50 years later (6). The acid is the toxic constituent of a South African plant Dichapetalum i mosum better known as gifirlaar (7). At least 24 other poisonous plant species are known to contain it (8). 

The longest wavelengths of the electromagnetic spectmm are sensitive probes of molecular rotation and hyperfine stmcture. An important appHcation is radio astronomy (23—26), which uses both radio and microwaves for chemical analysis on galactic and extragalactic scales. Herein the terrestrial uses of microwave spectroscopy are emphasized (27—29). 

Applications. Molecules couple to an electromagnetic field through their electric dipoles, so only those having a permanent dipole moment exhibit significant rotational spectra. For such species, microwave spectroscopy yields highly precise moments of inertia and details of centrifugal... 

Microwave spectroscopy is used for studyiag free radicals and ia gas analysis (30). Much laboratory work has been devoted to molecules of astrophysical iaterest (31). The technique is highly sensitive 10 mole may suffice for a spectmm. At microwave resolution, frequencies are so specific that a single line can unambiguously identify a component of a gas mixture. Tabulations of microwave transitions are available (32,33). Remote atmospheric sensing (34) is illustrated by the analysis of trace CIO, O, HO2, HCN, and N2O at the part per trillion level ia the stratosphere, usiag a ground-based millimeter-wave superheterodyne receiver at 260—280 GH2 (35). 

In Figure 2 the bond lengths and internal bond angles are given for some of the simple azines. Gas-phase electron diffraction, microwave spectroscopy, or the two techniques in combination, provided the results on compounds which were sufficiently volatile but with insufficient tendency to crystallize at accessible temperatures X-ray diffraction provided the remainder. 

High accuracy molecular dimensions for the planar parent heterocycles in the gas phase have been obtained by microwave spectroscopy and are recorded in Table 2. These values have been corroborated for furan by a low-temperature X-ray crystallographic study... 

The precise geometrical data obtained by microwave spectroscopy allow conclusions regarding bond delocalization and hence aromaticity. For example, the microwave spectrum of thiazole has shown that the structure is very close to the average of the structures of thiophene and 1,3,4-thiadiazole, which indicates a similar trend in aromaticity. However, different methods have frequently given inconsistent results. 

Conformations A and B are of the eclipsed type, whereas C and D are bisected. It has been determined by microwave spectroscopy that the eclipsed conformations are more stable than the bisected ones and that B is about 0.15 kcal more stable than A. MO calculations at the 6-31G level have found relative energies of 0.00, —0.25, 1.75, and 1.74kcal/mol, respectively, for A-D. ... 

The Cs structure and dimensions (Fig. 17.26b) were established by microwave spectroscopy which also yielded a value for the molecular dipole moment p. 1.72D. Other physical properties of this colourless gas are mp -115° (or -123°), bp -6°, A//f(g,298K) —34 10kJmol [or — 273kJmol when corrected for A//f(HF, g) ]. FCIO2 is thermally stable at room temperature in dry passivated metal containers and quartz. Thermal decomposition of the gas (first-order kinetics) only becomes measurable above 300° in quartz and above 200° in Monel metal ... 

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