2-Pyridinethiol/2-pyridinethione tautomeric equilibrium. A comparative experimental and computational study

The gas phase and solvent dependent preference of the tautomerization between 2-pyridinethiol (2SH) and 2-pyridinethione (2S) has been assessed using variable temperature Fourier transform infrared (FTIR) experiments, as well as ab initio and density functional theory computations. No spectroscopic evidence (nu(S)(-)(H) stretch) for 2SH was observed in toluene, C(6)D(6), heptane, or methylene chloride solutions. Although, C(s)() 2SH is 2.61 kcal/mol more stable than C(s)() 2S (CCSD(T)/cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPE), cyclohexane solvent-field relative energies (IPCM-MP2/6-311+G(3df,2p)) favor 2S by 1.96 kcal/mol. This is in accord with the FTIR observations and in quantitative agreement with the -2.6 kcal/mol solution (toluene or C(6)D(6)) calorimetric enthalpy for the 2S/2SH tautomerization favoring the thione. As the intramolecular transition state for the 2S, 2SH tautomerization (2TS) lies 25 (CBS-Q) to 30 kcal/mol (CCSD/cc-pVTZ) higher in energy than either tautomer, tautomerization probably occurs in the hydrogen bonded dimer. The B3LYP/6-311+G(3df,2p) optimized C(2) 2SH dimer is 10.23 kcal/mol + ZPE higher in energy than the C(2)(h)() 2S dimer and is only 2.95 kcal/mol + ZPE lower in energy than the C(2) 2TS dimer transition state. Dimerization equilibrium measurements (FTIR, C(6)D(6)) over the temperature range 22-63 degrees C agree: K(eq)(298) = 165 +/- 40 M(-)(1), DeltaH = -7.0 +/- 0.7 kcal/mol, and DeltaS = -13.4 +/- 3.0 cal/(mol deg). The difference between experimental and B3LYP/6-311+G(3df,2p) [-34.62 cal/(mol deg)] entropy changes is due to solvent effects. The B3LYP/6-311+G(3df,2p) nucleus independent chemical shifts (NICS) are -8.8 and -3.5 ppm 1 A above the 2SH and 2S ring centers, respectively, and the thiol is aromatic. Although the thione is not aromatic, it is stabilized by the thioamide resonance. In solvent, the large 2S dipole, 2-3 times greater than 2SH, favors the thione tautomer and, in conclusion, 2S is thermodynamically more stable than 2SH in solution.     

A CNDO/2 study of the ethylene-chlorine reaction

CNDO/2 calculations have been carried out of the energies of reaction and changes in charge distribution for some reactions involving the ethylene chlorine complex. The addition of chlorine to ethylene may involve the attack of a chloride ion on the ethylene chlorine complex.     

Solvent effects on the tautomeric equilibria of 4,4/5,5-disubstituted 4H/5H-imidazol-5/4-ones

The tautomerism of 4,4/5,5-disubstituted 4H/5H-imidazol-5/4-ones has been investigated by ir, uv, and pK studies. In nonpolar solvents the non-conjugated tautomer predominates, but in polar solvents varying amounts of the conjugated tautomer may also be observed. The proportion of conjugated tautomer is increased by the presence of electron-releasing groups at the 2-position.     

顺铂与DNA键合机制的CNDO/2研究

本文用CNDO/2法研究了顺铂与DNA键合的机制.计算结果指出,在所考虑的几个模型配合物中,从Pt-N_7间重叠集居和双原子能分析,以顺铂与两个鸟嘌呤的N_7键合形成[(NH_3)_2PtG_2]~(2 )的可能性最大,因而支持了顺铂和DNA的相邻两个鸟嘌呤的N_7键合的链内交联机制.但是,螯合机理不能完全排除,在一定条件下,顺铂可能与一个鸟嘌呤的N_7和O键合形成一定量的螯合物.然而,由于它较[(NH_3)_2PtG_2]~(2 )不稳定,因此,不能成为顺铂伤害癌细胞的主要原因.至于链内交联机理如何造成DNA复制障碍尚待进一步研究.     

Solvent effect on tautomeric equilibria and dipole moments of some alpha substituted benzoylacetanilides

Based on dipole moment measurements, a study was made of the keto-enol equilibria of five alpha substituted benzoylacetanilide derivatives in two non-polar and one polar solvents at 30°C. The enol mole fractions of the compounds investigated were determined by analysis of their ultraviolet spectra. The same type of study was extended to salicylanilide for its resemblance to the enol tautomer of these compounds. A deduction of the most probable configurations of the keto and enol forms was tentatively made through a comparison between the dipole moments of some suggested configurations for these forms of benzoylacetanilide and those measured in benzene for the alpha methyl benzoylacetanilide and salicylanilide, respectively. These inferred configurations were used to calculate the dipole moments of both tautomers of the other compounds, and consequently, the dipole moment values of their tautomeric mixtures in dioxane and chloroform. A comparison between the dipole moment values of their tautomeric mixtures in dioxane and chloroform. A comparison between the dipole moment values calculated in this way and those measured indicated a solute-solvent interaction with dioxane, which chloroform behaved as a non-interacting solvent.     

A CNDO/2 calculation on stilbene

The equilibrium conformations of trans- and cis-stilbene and the rotational barrier between the two isomers were calculated using the CNDO/2 method. The results for the equilibrium angles and distances are in good agreement with experimental data. However, according to the calculations the phenyl rings would have a nearly free rotation which is only limited by steric hindrance. This result obviously deviates from the current view. The results are discussed in relation to previous theoretical and experimental evidence.

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Zusammenfassung Die Gleichgewichtskonformation von trans- und cis-Stilben und die Rotationsbarriere zwischen zwei Isomeren wurde nach der CNDO/2-Methode berechnet. Die Ergebnisse in bezug auf die Geometrie stimmen mit experimentellen Daten gut überein, die Rotationsbarriere für die Drehung der Phenylringe dagegen ist praktisch Null.

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Résumé La méthode CNDO/2 a été employée pour calculer les conformations d'équilibre du trans- et du cis-stilbène ainsi que la barrière de rotation entre ces deux isomères. Angles d'équilibre et distances calculés sont en bon accord avec les données expérimentales. Cependant les calculs indiquent que les cycles phényles devraient posséder une rotation libre soumise seulement à l'empêchement stérique. Ce résultat s'écarte de l'opinion courante. Les résultats sont discutés en relation avec les données théoriques et expérimentales antérieures.

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This work was supported by grant No 2741-4 from The Swedish Natural Science Research Council and grant No 69-927/U 697 from The Swedish Board for Technical Development.     

CNDO/2 study of benzene-chlorine complex

We have carried out CNDO/2 calculations for the benzene-chlorine complex in a number of configurations. In order to obtain reasonable complexation energies, it was necessary to exclude chlorine 3d-orbitals from the basis set. When onlys- andp-orbitals were included, we found maximum stability for the complex for an unsymmetrical geometry. The calculated complexation energy was ?3.5 kcal/mole. Configuration-interaction calculations predicted a wavelength of about 270 nm for the charge-transfer band. Most, but not all, geometries were found to produce stable complexes.     

A further study on the CNDO/2 structures of water dimer

The structure of water dimer has been further studied by the CNDO/2 method. Some results have been found to be different from those in Thiel's paper.     

4-Azaindane-1,3-dione derivatives i. study of the tautomeric and prototropic equilibria of some analogs of 4-azaindane-1,3-diones

The acidity constants of 2,6,6-trimethyl- and 6,6-dimethyl-2-phenyl-9-ethoxycarbonyl-5,6,7,8-tetrahydro-4-azabenz[f]indane-1,3,8-triones were determined by a spectrophotometric method, and the position of the tautomeric equilibria was established for various pH values.     

The effect of solvent on the internal rotation of formamide: A CNDO/2-solvaton method study

The effect of solvent on the barrier to internal rotation of formamide had been studied using a solvaton method within the CNDO/2 parameterization. The experimental trend of increasing rotational barrier with increasing solvent dielectric constant has been reproduced. A critical examination of the manner in which interactions between solute and solvent were allowed to modify solute energies and the polarization of the solute wavefunction was performed.Work performed while A.J.D. was a visting scientist at the Polymer Institute of the Slovak Academy of Sciences sponsored under the exchange program between the U.S. National Academy of Sciences and the Czechoslovak Academy of Sciences during the summer of 1980.     

A CNDO/2 study of the polymerization of beryllium chloride

The LCGO-CNDO/2 method has been applied in a study of polymer molecules of the type (BeCl₂) _n for values of n between 2 and 6. The calculations predict chain stretching and increasing. covalent character of the Be-Cl ring bonds as polymerization proceeds.     

Synthesis and study of tautomeric properties of 2-(5-R-2-furyl)thiazolin-4-ones

Compounds were synthesized by reaction of 5-R-2-cyanofurans with thioglycolic acid, which, according to IR, PMR, and mass spectrometric data, exist in the form of two tautomers, 4-hydroxythiazole and thiazolin-4-one.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 404–409, March, 1987.     

由4-羟基苯甲醛合成香兰素的CNDO/2研究

用CNDO/2法研究了从4-羟基苯甲醛合成香兰素的化学反应，并作了理论解析。结果表明，合成路线是可行的，总收率为85％。     

由4-羟基苯甲醛合成香兰素的CNDO/2研究

用CNDO/2法研究了从4-羟基苯甲醛合成香兰素的化学反应,并作了理论解析。结果表明,合成路线是可行的,总收率为85%。     

A CNDO/S study on the alkylbenzene-TCNE complexes

The calculations based on the semiempirical CNDO/S Hamiltonian appeared useful in predicting the electronic structures of electron donor-acceptor (EDA) complexes of tetracyanoethylene (TCNE) and various alkylbenzenes. The results are in satisfactory agreement with the experimental data. The calculated complex stabilization energies are related to the experimental gas phase enthalpies of formation by linear regression (τ = 0.941). A fairly good correlation (τ = 0.985) between the measured and calculated energies of the charge transfer (CT) electronic transitions is established. The problem of double CT absorption bands is discussed by referring to the theoretical results. The ionization potentials of alkylbenzenes calculated by CNDO/S and MNDO methods are considered as well.     

CNDO/2 study on tautomerism in monosubstituted pyridines and pyrimidines

A molecular orbital calculation in the CNDO/2 approximation has been used to discuss the tautomeric stability of pyridine and pyrimidine substituted by amino, mercapto and hydroxy groups, respectively. The results of the calculations predict the stability of the amino and mercapto compounds, while the results for hydroxy substituted molecules are less satisfactory. A tentative calculation has been performed for the barrier of the proton motion in the intramolecular hydrogen bond of 2-substituted pyridines.     

Comprehensive study of the effects of methylation on tautomeric equilibria of nucleic acid bases

Minor tautomers of nucleic acid bases can result by intramolecular proton transfer. These rare tautomers could be stabilized through the addition of methyl groups to DNA bases. A comprehensive theoretical study of tautomers of methylated derivatives of guanine, adenine, cytosine, thymine, and uracil was performed. Molecular geometries of all tautomers were obtained at the density functional theory and MP2 levels with the 6-31G(d,p) basis set, and single-point calculations were performed at the CCSD(T)/6-311G(d,p) level. Tautomers obtained by protonation at the preferred protonation site for methylated isolated bases were compared to their nonmethylated counterparts. The effects of methylation on the relative stabilities of nucleic acid base tautomers are also studied and discussed in this work. The results suggest that some sites on the bases may not be mutagenic and may even stabilize the canonical Watson-Crick form. The results also indicate that a number of methylation sites can stabilize the tautomers, suggesting possible mechanisms for mutagenic changes.     

A CNDO/2 localised orbital study of dipole moments of sila- and germa-ethylenes

CNDO/2 calculations have been performed to predict dipole moments for silaethylenes, with either Slater of Burns atomic orbitals, and for germaethylenes with Burns orbitals only. Trends on replacement of two hydrogen atoms at either end of the double bond by fluorine are similar in both sets for the silaethylenes and in accordance with expectation, but an anomalous trend is observed for fluorination of the carbon in germaethylenes.Localised orbitals have been examined to seek an explanation for this anomaly. In all cases the trends are in the unexpected sense for the π orbitals but in the expected sense for the others taken together and for most of these individually. The anomaly with the germaethylenes thus appears to be associated with the relative magnitudes of these opposing trends, where donation of electrons via the π bond to the germanium atom outweighs the accumulation of charge around the fluorine atoms attached to the carbon.