Spectra and Theoretical Chemistry

Spectra and Theoretical Chemistry of Oxirans. - The use of conformational analysis, with the aid of torsion-angle notation, permits the interpretation and prediction of regioselective opening of epoxides/ Values for the molar refraction, electron polarization, and permanent electron dipole moments for oxiran and its simple derivatives have been calculated from literature data/ The molar Kerr constants for oxiran and its methyl derivatives in CCI4 were also calculated. The pericyclic reactivities of three-membered heterocycles have been rationalized, using the relaxation method.  

spectra for (112 X = O, NH, S, or SO) have been recorded and compared with that for (112 X = CH2). The annelation effects of the three-membered rings were determined. Using 350 MHz n.m.r. and dipole moments, (113 R = H) and (113 R = OMe) were shown to adopt a boat conformation, with the 0 of the oxiran ring in a pseudo-axial position. The absolute configuration of alliacolide (116) has been established by c.d. 

Henri-Rousseau, P. Pujol, and F. Texier, Bull. Soc. Chim. Fr., Part 2, 1980, 496. 

Kotkowska-Machnik, and J. Zakrzewski, Org. Magn. Reson., 1981, 16, 236. 

The molecular structure of the perfluoro-oxiran (117) has been determined, using gas-phase electron diffraction.  

Spectra and Theoretical Chemistry of Oxirans.—Electrophilic additions to a series of norcarene derivatives to yield the oxirans (84 R = H, = Me), (84 R = Me, R = H), and (85) have been described. Proton n.m.r, spectra and A -ray structures for these oxirans are given, and their conformations have been established. The chemical shifts and coupling constants for the H and n.m.r. spectra of a series of cis- and tran5-epoxystilbenes have been reported.  

Natural-abundance O n.m.r. spectra have been recorded for 21 oxirans, including those derived from the norbornene and benzonorbornene systems. The chemical shifts cover a range of 100 p.p.m. and have been interpreted in terms of the paramagnetic /3- and diamagnetic y-effects. 

The conformations of oxirans may be determined by studying dipole moments. For a number of substituted epoxy-styrenes, conjugation between the aromatic and oxiran rings is unimportant and, in the absence of steric or electrostatic hindrance, internal rotation is possible. Only one conformer is observed for (88 R = Me) and for (88 R = Ph) the carbonyl group is rotated some 40° out of the plane of the oxiran ring, with the oxygen atoms facing away from each other. 

Two conformations of (89) have been revealed by microwave spectroscopy. One form has the Cl cis to the O in the ring and the second gauche-2) has the Cl trans to the oxygen atom. 

The ionization and excitation energies of an isoelectronic series of three-membered rings, including oxiran, have been calculated by the semi-empirical HAM/3 method and compared with experimental values.  

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Spectra and Theoretical Chemistry of Oxirans. - Carbon-13 nmr has been used in the assignment of stereochemistry to the diastereomeric pairs of spiro epoxides (96)1( and a combination of and nmr was used in the determination of the structure of diacholestanes 20(R) and 20(S)... 

Spectra and Theoretical Chemistry. - 15N nmr shifts for variously substituted N-phenylaziridines have shown that the N lone pair in these compounds... 

A. Physical Properties and Theoretical Chemistry 1. Theoretical Chemistry, Ultraviolet and Visible Spectra... 

Magnetic resonance spectroscopies and theoretical chemistry have always been linked. On the one hand, the rich and detailed information hidden in ESR and NMR spectra has been a challenge for physicochemical interpretations and computational models. On the other hand, magnetic resonance spectroscopists have been looking for better tools to interpret the spectra. 

The term theoretical chemistry may be defined as the mathematical description of chemistry. The term computational chemistry is generally used when a mathematical method is sufficiently well developed that it can be automated for implementation on a computer. Note that the words exact and perfect do not appear in these definitions. Very few aspects of chemistry can be computed exactly, but almost every aspect of chemistry has been described in a qualitative or approximately quantitative computational scheme. The biggest mistake a computational chemist can make is to assume that any computed number is exact. However, just as not all spectra are perfectly resolved, often a qualitative or approximate computation can give useful insight into chemistry if the researcher understands what it does and does not predict. 

The approach to the evaluation of vibrational spectra described above is based on classical simulations for which quantum corrections are possible. The incorporation of quantum effects directly in simulations of large molecular systems is one of the most challenging areas in theoretical chemistry today. The development of quantum simulation methods is particularly important in the area of molecular spectroscopy for which quantum effects can be important and where the goal is to use simulations to help understand the structural and dynamical origins of changes in spectral lineshapes with environmental variables such as the temperature. The direct evaluation of quantum time- correlation functions for anharmonic systems is extremely difficult. Our initial approach to the evaluation of finite temperature anharmonic effects on vibrational lineshapes is derived from the fact that the moments of the vibrational lineshape spectrum can be expressed as functions of expectation values of positional and momentum operators. These expectation values can be evaluated using extremely efficient quantum Monte-Carlo techniques. The main points are summarized below. 

However, there is a critical lack of information on this system, mainly due to insufficient studies of its spectral signatures, which makes it difficult to insert this molecule with confidence in the astrochemical schemes. During these years, only a few experimental and theoretical studies were performed, aiming to the different spectra useful for interstellar identification and chemistry. Still a lot remains to do. 

To resolve hf and nuclear quadrupole interactions which are not accessible in the EPR spectra, George Feher introduced in 1956 a double resonance technique, in which the spin system is simultaneously irradiated by a microwave (MW) and a radio frequency (rf) field3. This electron nuclear double resonance (ENDOR) spectroscopy has widely been applied in physics, chemistry and biology during the last 25 years. Several monographs2,4 and review articles7 11 dealing with experimental and theoretical aspects of ENDOR have been published. 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

The present volume comprises 17 chapters, written by 27 authors from 11 countries, and deals with theoretical aspects and structural chemistry of peroxy compounds, with their thermochemistry, O NMR spectra and analysis, extensively with synthesis of cyclic peroxides and with the uses of peroxides in synthesis, and with peroxides in biological systems. Heterocyclic peroxides, containing silicon, germanium, sulfur and phosphorus, as well as transition metal peroxides are treated in several chapters. Special chapters deal with allylic peroxides, advances in the chemistry of dioxiranes and dioxetanes, and chemiluminescence of peroxide and with polar effects of their decomposition. A chapter on anti-malarial and anti-tumor peroxides, a hot topic in recent research of peroxides, closes the book. 

One of the main aims of quantum mechanical methods in chemistry is the calculation of energies of molecules as a function of their geometries. This requires the generation of potential energy hypersurfaces. If these surfaces can be calculated with sufficient accuracy, they may be employed to predict equilibrium geometries of molecules, relative energies of isomers, the rates of their interconversions, NMR chemical shifts, vibrational spectra, and other properties. Carbocations are ideally suited for calculations because relative energies of well-defined structural isomers are frequently not easily determined experimentally. It should, however, be kept in mind that theoretical calculations usually refer to isolated ion structures in the gas phase. 

Theoretical chemistry at UBC was further strengthened with the arrival of Delano Chong and Keith Mitchell in 1965 and 1966, respectively. Chong s interests in quantum chemistry have spanned the full range from semiempirical to ab initio molecular orbital methods. His long-standing interests in perturbation methods and constrained variations have figured prominently in his publications. He is probably best known for his attempts to calculate the X-ray and UV photoelectron spectra of molecules, often by means of perturbation corrections to Koopmans theorem.40 More recently he has shifted his focus to coupled pair functional methods and density functional methods, with a special interest in polarizabilities and hyperpolarizabilities.41... 

It is interesting to note that while both Harry Kroto and Robert Curl were primarily interested in microwave spectroscopy, they published papers in the field of theoretical chemistry in the 1960s. For example, the paper by R. F. Curl Jr. and C. A. Coulson [Proc. Phys. Soc., 78,831 (1965)], Coulomb Hole in the Ground State of Two-Electron Atoms, resulted from a sabbatical year at Oxford. See also, H. W. Kroto and D. P. Santry, J. Chem. Phys., 47, 792 (1967). CNDO Molecular-Orbital Theory of Molecular Spectra. I. The Virtual-Orbital Approximation to Excited States. 

A rational deduction of elemental abundance from solar and stellar spectra had to be based on quantum theory, and the necessary foundation was laid with the Indian physicist Meghnad Saha s theory of 1920. Saha, who as part of his postdoctoral work had stayed with Nernst in Berlin, combined Bohr s quantum theory of atoms with statistical thermodynamics and chemical equilibrium theory. Making an analogy between the thermal dissociation of molecules and the ionization of atoms, he carried the van t Hoff-Nernst theory of reaction-isochores over from the laboratory to the stars. Although his work clearly belonged to astrophysics, and not chemistry, it relied heavily on theoretical methods introduced by and associated with physical chemistry. This influence from physical chemistry, and probably from his stay with Nernst, is clear from his 1920 paper where he described ionization as a sort of chemical reaction, in which we have to substitute ionization for chemical decomposition. [81] The influence was even more evident in a second paper of 1922 where he extended his analysis. [82]... 

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