Ab initio spectrum

Figure 6.1. Matrix IR spectra of a deposit obtained by FVT of (F3Si)2S (13) [20] A 500-1100 cm B 150-650 cm on top predicted ab initio spectrum of 11 SiF4 peaks are marked by an asterisk and peaks due to contamination by FsSiSH with a filled rhomboid.

Theoretical determination of force fields may frequently be of assistance. The calculated frequencies are, however, dependent on the level of theory employed. Calculated ab initio harmonic frequencies for furan are sufficiently accurate to help in the assignment of experimental frequencies <88JPC1739> as well as the vibrational overtones <91JPC7659>. In a reinvestigation of the vibrational spectrum of tetrahydrofuran all the normal modes have been identified and assigned in terms of symmetry coordinates with the guidance of the computed ab initio spectrum <93JPC7844>. 

We have alluded to the comrection between the molecular PES and the spectroscopic Hamiltonian. These are two very different representations of the molecular Hamiltonian, yet both are supposed to describe the same molecular dynamics. Furthemrore, the PES often is obtained via ab initio quairtum mechanical calculations while the spectroscopic Hamiltonian is most often obtained by an empirical fit to an experimental spectrum. Is there a direct link between these two seemingly very different ways of apprehending the molecular Hamiltonian and dynamics And if so, how consistent are these two distinct ways of viewing the molecule ... 

Figure A3.7.6. Photoelectron spectrum of. Here the F is complexed to para-R - Solid curve experimental results. Dashed curve simulated spectrum from scattering calculation on ab initio surface.

We do not wish to go into the details of Figure 10. As an illustration of the reliability of the present results we compare, however, in Figure 11, the structure of the measured HCCS spectrum published by Tang and Saito (Fig. 3 of [139]) with the results of the theoretical study. Taking into account the very complex and unusual structure of this kind of spectra, we find the agreement between our ab initio theoretical results and those following from the interpretation of experimental spectra more than satisfactory. While strongly... 

Use the Electronic Spectrum dialog box to display and analyze the UV-visible spectrum produced by a singly excited Cl calculation. This dialog box is available from the Compute menu only after you do a single point Cl semi-empirical or ab initio calculation. Bectronic Dectrum is then activated on the Compute menu. 

In HyperChem, you can now compute the energy difference between the ground electronic state and the first few excited electronic states of a molecular system by using the ab initio method or any of the semi-empirical methods except for the Extended Hiickel. To generate a UV-vis spectrum, you must perform a singly excited Cl method with the ab initio method or semi-empirical method you choose. 

Since the authors did not succeed in obtaining an ESR spectrum, they were unable to decide whether the IV-pyrazolyl radical is of the a (112a) or the v (112b) type. Ab initio calculations indicate that the radical has Bi (rr) symmetry (76T1555). However, the radical is formed from (111) as a cr radical and is able to react as such in its lifetime. This is in agreement with the experimental results (75JOC915), no C-phenylated pyrazoles being detected. 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

The tetramethylammonium salt [Me4N][NSO] is obtained by cation exchange between M[NSO] (M = Rb, Cs) and tetramethylammonium chloride in liquid ammonia. An X-ray structural determination reveals approximately equal bond lengths of 1.43 and 1.44 A for the S-N and S-O bonds, respectively, and a bond angle characteristic bands in the IR spectrum at ca. 1270-1280, 985-1000 and 505-530 cm , corresponding to o(S-N), o(S-O) and (5(NSO), respectively. Ab initio molecular orbital calculations, including a correlation energy correction, indicate that the [NSO] anion is more stable than the isomer [SNO] by at least 9.1 kcal mol . ... 

At the other end of the spectrum are the ab initio ( from first principles ) methods, such as the calculations already discussed for H2 in Chapter 4. I am not trying to imply that these calculations are correct in any strict sense, although we would hope that the results would bear some relation to reality. An ab initio HF calculation of the potential energy curve for a diatomic Aj will generally give incorrect dissociation products, and so cannot possibly be right in the absolute sense. The phrase ab initio simply means that we have started with a certain Hamiltonian and a set of basis functions, and then done all the intermediate calculations with full rigour and no appeal to experiment. 

H = di(Z—iy di are the potential parameters I is the orbital quantum number 3 characterizes the spin direction Z is the nuclear charge). Our experience has show / that such a model potential is convenient to use for calculating physical characteristics of metals with a well know electronic structure. In this case, by fitting the parameters di, one reconstructs the electron spectrum estimated ab initio with is used for further calculations. 

The room temperature Raman spectrum excited in pre-resonance conditions [351 indeed shows bands at 169 cm-1 and 306 cm, which are in agreement with the modes observed in the fluorescence spectrum and that have been assigned by ab initio calculations to totally symmetric vibrations jl3). 

Tabic 6-5. Comparison of (he aK vibrational modes in the ground and excited states. The totally symmetric vibrations of the ground stale measured in tire Raman spectrum excited in pre-resonance conditions 3S] and in the fluorescence spectrum ]62 ate compared with the results of ab initio calculations [131- The corresponding vibrations in the excited stale arc measured in die absorption spectrum. 

The electronic structure and the UV-Vis spectrum of 83 have been studied by high-level ab initio MO calculations [139]. 

The IR bands of carbenes [2], [5], and [6] have also been observed in the spectrum after vacuum UV photolysis of matrix-isolated methylacetylene (Huang and Graham, 1990). It was found that a fourth carbene -propendiylidene [8] - was formed in this reaction as well. In accord with ab initio calculations, the first of two absorptions (3292 cm and 1960 cm ) has been assigned to V2 ( ) of cw-[8] and the second one to (a ) of trans-[8]. 

An informative IR spectrum of the t-butyl radical, containing 18 bands, has been recorded after freezing of the products of vacuum pyrolysis of azoisobutane [110] and 2-nitrosoisobutane [111] in an argon matrix at 10 K (Pacansky and Chang, 1981). This spectrum is in agreement with a pyramidal structure of the radical (CH3)3C (symmetry C3v) which has elongated CH bonds in positions trans to the radical centre. On the basis of experimental vibrational frequencies and ab initio calculations of the radical geometry the enthalpy value [// (300)] of its formation has been calculated as 44 kJ moP. ... 

The rotational spectrum has been calculated accuratly by ab-initio methods [2], and has been measured in the laboratory with high precision [3,4], so that the radio detection of C3H2can be done without ambiguity, encouraging its search in different environments as dense dark clouds [5], diffuse interstellar medium [6] or Hll regions [7]. 

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