Tautomerism ionization potential

In this method, photons of an energy well in excess of the ionization potential are directed onto a molecule. The photoelectron spectrum which results allows assessment of the energies of filled orbitals in the molecule, and thus provides a characterization of a molecule. Comparisons between photoelectron spectra of related compounds give structural information, for example, on the tautomeric structure of a compound by comparison of its spectrum with those of models of each of the fixed forms. 

Benzo[b]furan, 2,3-dihydro-2-phenyl-synthesis, 4, 697 Benzo[b]furan, 2,3-dihydroxy-tautomerism, 4, 37 Benzo[6]furan, 4,6-dimethoxy-acylation, 4, 606 Benzo[6]furan, 2,3-dimethyl-acetylation, 4, 606 bromination, 4, 605 photooxygenation, 4, 642 Benzo[b]furan, 5,6-dimethyl-2,3-diphenyl-applications, 4, 709 Benzo[b]furan, 1,3-diphenyl-vertical ionization potential, 4, 587 Benzo[b]furan, 2-ethoxy-5-hydroxy-synthesis, 4, 127... 

Equilibrium constants and activation parameters have been determined [76ACS(B)101] [for a review see 82AHC(30)127]. Ionization potentials for tautomeric 2-hydroxyselenophenes have been analyzed by comparison with IP data for compounds derived from either tautomeric form. The enol form could not be detected [75ACS(B)647]. 

Ghosh and Almlof published many articles discussing the XPS (X-ray photoelectron spectroscopy) and UPS (ultraviolet photoelectron spectra) and the corresponding ionization potentials of porphyrins using high-level calculations. Tliese topics are indirectly related to the tautomerism of porphyrins (for an example see 94IC6057 and 95JA4691). 

The ionization potentials, using mass spectrometry, for both 2-hydroxy-and 3-hydroxythiophenes have been compared with data for compounds derived from either tautomeric form in order to analyze the tautomeric composition.124 125 In the 2-hydroxy-substituted system the enol isomer could not be detected. Of the two possible unsaturated lactones the oc,/l-unsaturated form was the major isomer. In the 3-hydroxy-substituted case both the oxo form and the enol form are important. The position of the equilibrium was compared with those of the corresponding furan and sele-nophene systems for both isomers. 

The potentially tautomeric side-chain thiol systems exist mainly in the thiol form in liquid solution and in the gas phase, as found by IR and NMR spectroscopy and by a study of ionization potentials.126 Upon alkylation using the ion-pair extraction method, only the S-alkylated compounds were obtained. The synthesis, reactions, and properties of some selenides of thiophene, furan, and selenophene have been reviewed.127... 

PE spectroscopy offers a method of investigating the tautomeric structure of pyridones and other related compounds in the gas phase by comparison of ionization potentials of the potentially tautomeric compound with those of fixed models. This method is discussed in CHEC 2.04. 

The reliability of semi-empirical methods (AMI, PM3, and MNDO) for the treatment of tautomeric equilibria has been tested for a series of five-membered nitrogen heterocycles, including 1,2,3-triazole and benzotriazole. The known tendency of MNDO to overestimate the stability of heterocycles with two or more adjacent pyridine-like lone pairs is also present in AMI and to a somewhat lesser extent in PM3. Tautomers with a different number of adjacent pyridine-like nitrogens cannot be adequately treated by these semi-empirical methods. Both AMI and PM3 represent major improvements over MNDO in the case of lactam-lactim tautomerism. The stability of N-oxides as compared to N-hydroxy tautomers is overestimated by PM3 method. All three methods give reliable ionization potentials and dipole moments (90ZN(A)1328). 

A2-1,3,4-Oxadiazolin-5-ones acylation, 6, 439 alkylation, 6, 439 biological activity, 6, 445 melting point, 6, 430 synthesis, 6, 441, 442 tautomerism, 6, 430 A3- l,3,4-Oxadiazolin-2-ones 5,5-disubstituted IR spectra, 6, 429 PE spectra, 6, 430 A3-1,3,4-Oxadiazolin-5-ones ionization potential, 6, 428... 

Concerning the other properties considered in Table XIV the ionization potential of xanthine has been found98 equal to 9.30 eV, a value intermediate, as usual, between those predicted by the PPP and the CNDO methods. The potential predicted for the two tautomeric forms being practically the same, the experimental value does not enable to fix their identity. 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

The theoretical calculations have been of undoubted value to the chemist in successfully predicting several physicochemical properties of purine molecules such as dipole moments and ionization potentials (see Tables 4 and 5) and providing information about the various tautomeric forms. 

Quantum-mechanical studies on the tautomerism of heterocyclic compounds involve, in general, two aspects. The first deals with the prediction of physicochemical properties of defined tautomeric forms (e.g., ultraviolet spectra, dipole moments, ionization potentials, etc.). This seems to be easy to handle. Using any semiempirical or nonempirical quantum-mechanical computational method, depending on approximations involved in the method, we are able to calculate properties that, more or less, agree with experimental values. Calculations of this type do not contribute to a direct estimation of the relative stability of the tautomers, however they are particularly important for cases in which a tautomeric form of a compound is so rare that it is not possible to measure it directly. 

The thiols themselves are best made by the lithiation sulfur method, followed by acidification.198-199 IR and NMR studies show that furan-2-thiols and furan-3-thiols are tautomeric but that the thiol, not the thione, is always by far the major isomer. This differs from the oxygen analogs. Evidence from the ionization potentials supports the thiol structure. Furylthioethers have IP values about 0.3 eV higher than do the parent thiols, just as in the benzene series.199... 

The acidity and basicity of 1,2,4-triazines have been calculated by the MOSP method. The pKa values correlate satisfactorily with the total (n + n) atomic charges of the proton accepting atom and the ionization potential.46 The proton affinities of 1,2,4-triazines have been calculated by the 3-21G, 6-31G, MINDO and AMI methods,38,47 the lithium-triazine binding energies by the 6-31 method,48 and the tautomeric equilibrium by the AM1 and MINDO-PM3 method.49... 

The core ionization potentials, more frequently called core electron binding energies (CEBEs) when molecular systems are studied, have been also recently calculated for the tautomeric structures of thio- and seleno-cytosine [35]. The role of relativistic effects in ly ionization have been studied for selenocytosine by the comparison of the nonrelativistic and relativistic SCE and MP2 results of the calculations. 

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