Microwave and Photoelectron Spectroscopy

The electronic structure and gas-phase thermolysis of 4-substituted 3,3,5,5-tetramethyl-3,5-dihydro-4//-pyrazoles has been studied by photoelectron spectroscopy and the first evidence for an alkylideneselenirane was obtained 1996T1965 . The 351.1 nm photoelectron spectrum of the 1-pyrazolide anion has been measured 2006PCA8457 . The 1-pyrazolide ion 29 is produced by hydroxide deprotonation of pyrazole in a flowing afterglow ion source and a small amount of the 5-pyrazolide ion 30 was also detected and studied by photoelectron spectroscopy. 

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Conformational analysis of 1,4,7-trithiacyclononane 10 in the gas phase was done using ab initio molecular orbital calculations at the HF and MP2 levels as well as microwave and photoelectron spectroscopies. The photoelectron spectroscopic data showed evidence for at least two conformations with different ionization energies. Using the calculated photoelectron spectra, the observed sulfur 3p-ionization peaks can be assigned to Cj and U2 conformations. Forty of the observed microwave transitions can be assigned to a Cj symmetry, while additional microwave lines are believed to be due to a nonrigid U2-symmetry conformation <1997PCA9180>. 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

Theoretical methods are surveyed, followed by data on molecular dimensions obtained from X-ray diffraction or microwave spectroscopy. The various types of NMR spectroscopic characteristics are then surveyed, including H, 13C and nitrogen NMR spectroscopy. This is followed by a discussion of UV and visible and then IR, mass and photoelectron spectroscopy. Each of the spectroscopic sections deals with both the various parent rings and the effect of substituents. 

The special types of bonding in three-membered ethyleneimine rings (41—43) have been studied using microwave spectroscopy (44—47), electron diffraction (48), and photoelectron spectroscopy (49—51), and have occupied theoretical chemists up to the present day (52). These studies reveal that ethyleneimine has a distincdy shortened C—C bond of 0.148 nm (as compared to 0.154 nm in open-chain compounds) and a noticeably lengthened C—N bond of 0.149 nm (compared to 0.146 nm). Because of the high s character of the free electron pair on the nitrogen, ethyleneimine also shows a lower basicity (p Ka = 7.98) than noncyclic aliphatic amines such as dimethyl amine (p Ka = 10.7) (53). 

Structure determinations of alkyl-substituted cyclopropanes have been carried out by microwave spectroscopy, theoretical methods , Raman and photoelectron spectroscopy Rotational barriers around the C-C bond from the ring to... 

The simplicity of the structure of phosgene has precipitated many structural and spectroscopic studies. This Chapter summarizes the results of the studies by electron diffraction, microwave and vibrational spectroscopy, nuclear magnetic resonance and nuclear quadrupole resonance spectroscopy, and mass spectrometry. Studies by electronic absorption and emission spectroscopy, and photoelectron spectroscopy, are discussed in Chapter 17. 

This volume is made of four subvolume, each containing the geometric parameters determined in the gas phase either by an analysis of the rotational constants (and sometimes the vibrational constants) observed in microwave, infrared, Raman, electronic and photoelectron spectroscopy or by an analysis of electron diffraction intensities. Most of the stmctures listed in the tables are for molecules in the electronic ground state, but stmctures for electronically excited states have also been included as far as they are available (see 1.5). 

All experimental methods for the determination of quantitative structural data of free molecules have been considered microwave, infrared, Raman, electronic and photoelectron spectroscopy and related spectroscopic methods as well as electron diffraction. All data obtained by these methods have been critically evaluated and compiled. The data are presented separately for each molecule, together with original references and in many cases with computer-drawn figure(s) carefully prepared by Dr. N. Vogt. 

It is well established that disulfur difluoride (S2F2) exists in two isomeric forms, the nonplanar disulfane FSSF and the branched thiosulfoxide form p2S=S, with the latter found to be the more stable isomer. Both isomers have been characterized by microwave spectroscopy, mass spectrometry, infrared and Raman spectroscopy as well as photoelectron spectra [6] (and refer-... 

Infra-red, microwave, and X-ray photoelectron spectroscopy Infra-red and ultra-violet spectroscopy has been widely used for investigating the structure of intermolecularly hydrogen-bonded complexes in the solid state (Novak, 1974) and in solution (Zundel, 1976, 1978 Clements et al., 1971a,b,c Pawlak et al., 1984). By analysing the infra-red spectra of equimolar liquid mixtures of amines with formic or acetic acid, the relative importance of structures [10] and [11] was estimated (Lindemann and Zundel, 1977). It was proposed that [10] and [11] make equal contributions to the observed structure of the complex when the p -value of the carboxylic acid is approximately two units lower than that of the protonated amine. 

A review of the Journal of Physical Chemistry A, volume 110, issues 6 and 7, reveals that computational chemistry plays a major or supporting role in the majority of papers. Computational tools include use of large Gaussian basis sets and density functional theory, molecular mechanics, and molecular dynamics. There were quantum chemistry studies of complex reaction schemes to create detailed reaction potential energy surfaces/maps, molecular mechanics and molecular dynamics studies of larger chemical systems, and conformational analysis studies. Spectroscopic methods included photoelectron spectroscopy, microwave spectroscopy circular dichroism, IR, UV-vis, EPR, ENDOR, and ENDOR induced EPR. The kinetics papers focused on elucidation of complex mechanisms and potential energy reaction coordinate surfaces. 

Methods covered include X-ray, neutron and electron diffraction, microwave spectroscopy, and their results in terms of molecular dimensions. NMR spectroscopy is treated in some detail as befits its importance, not only proton, but particularly 13C, 15N, and, where appropriate, other nuclei. The section on mass spectrometry briefly covers fragmentation patterns. UV/Fluorescence, IR/Raman, photoelectron spectroscopy, ESR, and dipole moments are covered as appropriate. 

Spectroscopy covers a very wide area which has been widened further since the mid-1960s by the development of lasers and such techniques as photoelectron spectroscopy and other closely related spectroscopies. The importance of spectroscopy in the physical and chemical processes going on in planets, stars, comets and the interstellar medium has continued to grow as a result of the use of satellites and the building of radiotelescopes for the microwave and millimetre wave regions. 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

This interpretation of the microwave spectrum stimulated a flurry of activity to unearth the correct equilibrium geometry. It was suggested, for example, that photoelectron spectroscopy was not inconsistent with a cyclic structure whereas iirfrared photodissociation and matrix infrared measurements suggested the two molecules are not equivalent and supported the microwave equilibrium geometry. Another set of measurements led to the notion that a tunneling motion, similar to that in the HE dimer, which interchanges the roles of proton donor and acceptor, was responsible for the two IR bands observed in the gas phase. State selection in a hexapole electric field indicated that the dimer has a small dipole moment and that it is not a symmetric top structure. ... 

Structure and reactivity of neutral pyrazole, its anion and its cation. 2.2 Structure and reactivity of substituted pyrazoles. 2.3 Structure and reactivity of indazoles. 2.4 Dipole moments Structural Methods. 3.1 X-Ray diffraction. 3.2 Microwave spectroscopy. 3.3 H NMR spectroscopy. 3.4 C NMR spectroscopy. 3.5 Nitrogen NMR spectroscopy 3.6 UV spectroscopy. 3.7 JR spectroscopy. 3.8 Mass spectrometry. 3.9 Photoelectron spectroscopy. 3.10 Electron spin resonance spectroscopy Thermodynamic Aspects. 4.1 Intermolecular forces. 4.2 Stability and stabilization. 4.3 Conformation and configuration Tautomerism... 

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