Search results for "SPECTRA"
showing 10 items of 3542 documents
Rotational spectra of isotopic species of silyl fluoride. Part II: theoretical and empirical equilibrium structure
2010
Abstract The equilibrium structure of silyl fluoride, SiH 3 F, has been reinvestigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, and inclusion of core correlation as well as relativistic corrections ( r (Si–F) = 1.5911 A, r (Si–H) = 1.4695 A, and ∠FSiH = 108.30°). A semi-experimental equilibrium structure has been determined based on the available rotational constants for the various isotopic species of silyl fluoride ( 28 SiH 3 F, 28 SiD 3 F, 29 SiH 3 F, 29 SiD 3 F, 30 S…
<strong>QUANTITATIVE QUANTUM MECHANICAL NMR ANALYSIS: THE SUPERIOR TOOL FOR ANALYSIS OF BIOFLUIDS</strong>
2016
Almost automatic quantitative analysis of biofluids is now behind only a few clicks from sample to EXCEL table after minimal sample preparation (move 0.3 ml sample into NMR tube and add buffer), without separations, calibration and reference materials, even for unknown compounds! Each organic compound with protons gives a highly diagnostic and unique NMR spectrum which is practically identical with any spectrometer operating at certain field. A distinctive feature of high-resolution 1D NMR spectra is that even the most complex spectrum of a compound can be described by a few spectral parameters within experimental accuracy, employing a quantum mechanical theory. The NMR spectral parameters…
13C nuclear magnetic resonance spectra of several podocarpane and cassane diterpenoids
1990
The 13C NMR spectra of several diterpenic derivatives having the podocarpane and cassane skeleton were recorded and interpreted. The most significant effects due to substituent orientation, B/C ring junction stereochemistry and conformational changes are briefly discussed.
Theoretical characterization of the absorption spectra of phenanthrene and its radical cation
2003
The vertical absorption spectra of phenanthrene and its radical cation have been studied theoretically by means of a multiconfigurational second-order perturbation approach. Singlet-singlet transition energies and oscillator strengths, and singlet-triplet excitation energies have been studied in the absorption spectrum of phenanthrene up to 6 eV. The absorption spectrum of the pehnanthrene radical cation has been computed up to 3.4 eV. The results obtained confirm previous assignments and also lead to new interpretations of the main features of the spectra of these systems.
Electronic Spectra of 2,2‘-Bithiophene and 2,2‘:5‘,2‘ ‘-Terthiophene Radical Cations: A Theoretical Analysis
2001
Analysis of the electronic spectra of 2,2‘-bithiophene and 2,2‘:5‘,2‘ ‘-terthiophene radical cations has been performed by using multiconfigurational second-order perturbation theory (CASPT2). Atomic natural orbital (ANO) type basis sets of split valence quality, including polarization functions on all heavy atoms, have been used. In agreement with experimental data, in the energy range below the lowest optically allowed transition of the respective neutral system, theoretical results predict two main absorption bands for both cations. The 22Au and 32Au states computed at 1.94 and 2.80 eV, respectively, are related to the corresponding band maxima recorded for bithiophene cation. The 12B1 s…
Interpretation of the electronic absorption spectrum of free base porphin by using multiconfigurational second-order perturbation theory
1998
Abstract Multiconfigurational second-order perturbation (CASPT2) calculations have been performed on the low-lying optically allowed valence excited states of the free base porphin molecule in order to assign the four lowest bands of the spectrum. The low-lying triplet states have also been characterized. A basis set of the atomic natural orbital type of split-valence plus polarization quality for first-row atoms has been employed. Polarization functions are important for an accurate description of the transitions. These CASPT2 results provide a consistent picture of the experimental spectrum. Each band of the spectrum up to 4.5 eV is composed of a pair of states, which become degenerate in…
On the vertical excitation energy of cyclopentadiene
2004
The vertical excitation energy for the lowest valence pi--pi(*) transition of cyclopentadiene is investigated. Using a combination of high-level theoretical methods and spectroscopic simulations, the vertical separation at the ground state geometry is estimated to be 5.43+/-0.05 eV. This value is intermediate between those calculated with coupled-cluster and multireference perturbation theory methods and is about 0.13 eV higher than the observed maximum in the absorption profile.
Synthesis and Electronic Spectra of Substitutedp-Distyrylbenzenes for the Use in Light-Emitting Diodes
2000
The influence of substitution on the absorption and Luminescence spectra of oligo(phenylenevinylene)s has been studied using distyrylbenzene (DSB) as a model compound. The degree, character, and pattern of substitution was varied systematically, altering the electronic properties of the DSB, the wavelength of the emitted light could be tuned over a range of 100 nm. The syntheses of 6b—h were performed by twofold Wittig Horner-olefinations of bisphoshonates 1a, b with substituted benzaldehydes 2a—i, 6ivia Heck-reaction of the dibromosulfonylbenzene 3, 6k by Siegrist-reaction of 4 with N-phenylbenzaldimine and the Knoevenagel-reaction of benzyl cyanide with 5 led to 6l.
Comparison of the molecular structure and spectra of benzene and borazine
1970
Ab initio SCF MO and CI calculations for two different gaussian basis sets are carried out for the isoelectronic molecules benzene C6H6 and borazine B3N3H3 in order to investigate the effect of increasing the flexibility in the representation of their respective π systems. In the process it is found from comparison of orbital charge density contour diagrams and inner shell orbital energies of borazine with analogous data for other systems that the BN bonds of this compound are considerably less polar (B+N−) than that of ammonia borane BNH6 (B−N+). CI calculations employing the larger basis set produce generally better agreement with the experimental transition energies of benzene than do th…
Mass spectrometry of 3,4-dihydroquinazolin-4-ones of pharmaceutical interest. Part3. Electron ionization mass spectra of 2-substituted-3-(5′-pyrazoly…
1992
The fragmentation reactions induced by electron impact of eighteen title compounds has been investigated with the aid of low beam energy spectra (14 eV, nom. value), metastable ion detection, high resolution measurements and labelling experiments. The loss of the 4-carbonyl oxygen together with the 3-substituent, which constitutes a characteristic fragmentation route of 3-aryl and 3-heteroaromatic substituted-4(3H)-quinazolinones, is again observed, but the presence of a carboxyethyl group at the 4′-position of the pyrazole ring is responsible of an anomalous loss of 47 daltons from the molecular ion. Lastly, a comparison with the previously described behaviour of 3-(5′-isoxazolyl) derivati…