Spectroscopy, Microwave

Microwave spectra allow the determination of conformations. For example, 2-formylpyridine is shown to be planar with the carbonyl group trans to the nitrogen atom. 

Microwave spectra also provide precise values for dipole moments and values for pyridine and many substituted pyridines are available (Table 3 of CHEC 2.04). 

Microwave irradiation has been used to probe aromatic character in isoxazoles (74JA7394) 

The microwave spectra of phosphine and methyldifluorophosphine (170) have been recorded. The dipole moment, the P—C bond length, and the barrier to methyl rotation of (170) were estimated to be 2.056 D, 182 pm, 

The microwave spectrum of the parent 1,2,3-triazole was interpreted in terms of the l//-form 70SA(A)825 . The microwave spectrum of a highly enriched sample of V-deuterio-1,2,3-triazole was also assigned unambiguously, apparently to the 1-deuterio form 74CC605 . However, analysis of the microwave spectra of the parent molecule, the highly enriched species, and the V-deuterated 

The microwave spectra of 1//-benzotriazole and its N-D isotopomer have been studied in a heated cell. The molecule is planar. Due to the quadrupole coupling effects of the N nuclei, no hyperfine structures are observed. The dipole moment of benzotriazole obtained by microwave is 4.3 D, which is in agreement with the value determined in solution. The rotational spectrum is also assigned 93JSP(161)136 . 

The microwave spectra of ethylphosphine and its deuteriated analogues (112) and (113) show the presence of gauche- and fraws-conformers in a ratio of 45 55. Dipole moments, bond lengths, bond angles, and dihedral angles have been calculated for each conformer.138 

EH MO calculations on the phosphiran (109) are relevant to microwave studies of compounds in this series. - The calculations suggest that inversion involves all the atoms of the ring, including the hydrogens, and that although the 3c/-orbitals of phosphorus do not participate very much 

Bandoli, G. Bortolozzo, D. A. Clemente, U. Croatto, and C. Panattoni, Inorg. 

A model which takes into account the spin-rotation interaction has been found to satisfactorily explain the 0 rotation band of PHg. The millimetre-wave spectra of HCP and DCP have been compared with those of HCN and DCN. A method of estimating frequencies of bands in this region due to processes such as pseudorotation has been suggested. This new approach involves calculation of the rovibronic energy levels from the effects of quantum-mechanical tunnelling.  

Microwave spectra obtained from PHgD and PHDa in a magnetic field of about 25 kG showed Zeeman effects, from which molecular g values were calculated. They were 20 times smaller than those for ammonia. The molecular quadrupole moments of phosphine and ammonia were approximately the same. Magnetic susceptibilities and molecular quadrupole moments were also compared. 

The radiofrequency spectrum of phosphine has been measured in a molecular beam electric resonance spectrometer. The suspected inversion doubling was not observed its dipole moment (ju.) was 0.574 D. The calculated rotational barrier between the staggered and eclipsed conformers of methylphosphine is 1.83 and 1.71 kcal mol, in agreement with the experimental value of 1.96 from microwave measurements. An orbital-by-orbital analysis of the changes which occur upon rotation suggests a hydrogen-bond contribution when the phosphorus lone pair of electrons and a CH bond are appropriately orientated.The existence of a 1—2° tilt of a methyl group towards the phosphorus lone pair of electrons in methylphosphines (138) was a conclusion drawn from a microwave study 

The microwave spectrum of aminodifluorophosphine (141) indicates a planar PNHg group with a HNH bond angle of 117.2° and total jtx 2.58 The spectra of four phosphorus trihalides are also reported.  

Phosphabenzene (142) has been studied the spectrum is consistent with a CPC bond angle of 101—104°, and a planar ring with the phosphorus atom involved in 7r-conjugation. An interesting theoretical study of the 

The data show that the quadmpole hyperfine patterns of the rotational transitions are different between the two states, due to changes of the relative positions of some of the hyperfine components within the multiplet. The rotational spectrum of a pyrrole dimer is consistent with essentially a T-shaped structure, in which the planes of the two pyrrole monomers form an angle of 55.4(4)° and the nitrogen side of one ring is directed to the 7t-electron system of the other ring establishing a weak H bond 1997JCP504 . 

The rotational spectra of conformers of tryptamine and tryptophol have been determined 2004PCP2806 . Two conformers of tryptamine are stabilized by an intramolecular N-H- -n bridge, formed between the amino group of the lateral chain in position 3 and the 7t-system of the pyrrole moiety, whereas the most stable conformer of tryptophol is stabilized by a similar N-H--7t bridge, between the hydroxyl hydrogen and the 7t-system of the pyrrole unit. 

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Spectroscopy is the most important experimental source of infomiation on intemiolecular interactions. A wide range of spectroscopic teclmiques is being brought to bear on the problem of weakly bound or van der Waals complexes [94, 95]. Molecular beam microwave spectroscopy, pioneered by Klemperer and refined by Flygare, has been used to detemiine the microwave spectra of a large number of weakly bound complexes and obtain stmctiiral infomiation... 

Legon A 0, Millen D J and Mjdberg P J 1977 The hydrogen cyanide dimer identification and structure from microwave spectroscopy Chem. Phys. Lett. 47 589... 

Microwave spectroscopy began in 1934 with the observation of the -20 GHz absorption spectrum of ammonia by Cleeton and Williams. Here we will consider the microwave region of the electromagnetic... 

An alternative approach to obtaining microwave spectroscopy is Fourier transfonn microwave (FTMW) spectroscopy in a molecular beam [10], This may be considered as the microwave analogue of Fourier transfonn NMR spectroscopy. The molecular beam passes into a Fabry-Perot cavity, where it is subjected to a short microwave pulse (of a few milliseconds duration). This creates a macroscopic polarization of the molecules. After the microwave pulse, the time-domain signal due to coherent emission by the polarized molecules is detected and Fourier transfonned to obtain the microwave spectmm. 

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. 

Microwave spectroscopy is used to probe transitions between rotational energy levels in mol ecules... 

Townes, C. FI. and Schawlow, A. L. (1955) Microwave Spectroscopy, McGraw-FIill, New York. 

In considering the molecules in Table 5.2 it should be remembered that the method of detection filters out any molecules with zero dipole moment. There is known to be large quantities of FI2 and, no doubt, there are such molecules as C2, N2, O2, FI—C=C—FI and polyacetylenes to be found in the clouds, but these escape detection by radioffequency, millimetre wave or microwave spectroscopy. 

Carrington, A. (1974) Microwave Spectroscopy of Free Radicals, Academic Press, New York. Gordy, W. and Cook, R. L. (1984) Microwave Molecular Spectra, 3rd edn, Wiley-Interscience, New York. 

Sugden, T. M. and Kenney, C. N. (1965) Microwave Spectroscopy of Gases, Van Nostrand, London. Townes, C. FI. and Schawlow, A. L. (1975) Microwave Spectroscopy, Dover, New York. 

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 dielectric permittivity as a function of frequency may show resonance behavior in the case of gas molecules as studied in microwave spectroscopy (25) or more likely relaxation phenomena in soUds associated with the dissipative processes of polarization of molecules, be they nonpolar, dipolar, etc. There are exceptional circumstances of ferromagnetic resonance, electron magnetic resonance, or nmr. In most microwave treatments, the power dissipation or absorption process is described phenomenologically by equation 5, whatever the detailed molecular processes. 

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). 

C. H. Townes and A. L. Schawlow, Microwave Spectroscopy, McGraw-HiU Book Co., Inc., New York, 1955 corrected reprint, Dover, New York, 1975. 

G. W. Chantry, ed.. Modem Aspects of Microwave Spectroscopy, Academic Press, London, 1979. 

A large number of arsenin derivatives have also been studied. The potential aromaticity of this ring system has aroused considerable interest and has been investigated with the aid of nmr, uv, photoelectron, and microwave spectroscopy as well as by ab initio molecular orbital calculations (119). Arsenin does possess aromatic character. 

The stmctural parameters of ethylene oxide have been determined by microwave spectroscopy (34). Bond distances iu nm determined are as follows C—C, 0.1466 C—H, 0.1085 and C—O, 0.1431. The HCH bond angle is 116.6°, and the COC angle 61.64°. Recent ah initio studies usiug SCF, MP2, and CISD have predicted bond lengths that are very close to the experimental values (35,36). 

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... 

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