Proton affinity correlations for alkyl chlorides

Ab initio SCF calculations with minimal STO-3G and extended 44-31G basis sets have been performed on the simple alkyl chlorides, HCl to t-BuCl, and their protonated analogs. MINDO/3 calculations are also reported for these species and a variety of cyclic and bicyclic chlorides. The much closer agreement with experiment for STO-3G proton affinities than for 44-31G values is in sharp contrast to the results for first-row bases. An excellent correlation is found between both the STO-3G and MINDO/3 proton affinities and the charge on the CIH fragment in RCIH⁺. For the acyclic chlorides, correlations of PA's with the polar substituent constant, σ*, and IP's are also reasonable. In addition, the calculated carbonium ion-HCl interaction energy for t-BuClH⁺ indicates that protonated tertiary chlorides are no more than marginally stable in the gas phase.     

Geometrical preferences of the crotyl anion,radical and cation

ab initio MO calculations have been performed on the cis and trans isomers of the crotyl cation, free radical and anion in each of two orientations of the Me rotor about the allylic framework. In agreement with available experimental data, both the crotyl cation and free radical prefer trans skeletal geometries. On the other hand, the cis isomer of the crotyl anion is found to be more stable than the trans, the same preference as has been noted for alkali metal allyl organometallics in solution, but opposite to that recently reported for the free (gas phase) anion. The Me groups are predicted to eclipse the partial double bond for the trans isomers of all three systems and for the cis cation. These results are rationalized with the aid of perturbation MO theory.     

An ab initio study of the proton affinities of some heteroatomic rings: Imidazole,oxazole, and thiazole

The proton affinities of imidazole, oxazole, and thiazole rings, relevant to the binding of lexitropsins that contain these rings to the minor groove of DNA, are calculated using ab initio (Hartree–Fock) calculations. It is found that the proton affinities decrease in the order imidazole, oxazole, thiazole and that a methyl group substituent increases the proton affinity of imidazole, while a peptidic group decreases it.     

MNDO study of the proton affinity of fluorinated formaldehydes and acetones

The MNDO molecular orbital method is employed to calculate the proton affinities of fluorinated formaldehydes and acetones. Agreement with experimentally reported proton affinities is good. In the acetone series a decrease in proton affinity is calculated for each successive fluorine substituent. The calculated strength of the intramolecular hydrogen bond in the protonated fluoro-formaldehydes and acetones is 0.6—2.7 kcal mol⁻¹, in good agreement with the experimental value of 2—3 kcal mol⁻¹ in the protonated fluoroacetones. Examination of the calculated charge distribution shows that the trends in proton affinity can be understood qualitatively both in terms of initial-state and final-state effects caused by the fluorine substituents. Protonation at the fluorine atom is less stable by about 25 kcal mol⁻¹ than protonation at the oxygen atom for monofluoroacetone.     

Enhanced anion binding of N-(anilino)thioureas. Contribution of the N-anilino-NH proton acidity

We report here that N-anilino-N'-phenythioureas in general function as a new family of thiourea-based efficient anion receptors superior to classical N-alkyl(aryl)thioureas, when the N-anilino-NH proton is acidic enough; that is, the N-phenyl substituent is not less electron-withdrawing than m-Cl. Changes due to anion binding in the absorption spectra of these N-anilinothioureas are much more substantial than those of N-alkyl(aryl)thioureas, and anion binding constants in MeCN, at 10(6)-10(7) mol(-1) L order of magnitude for AcO(-) for example, are much higher despite a similar acidity of the thioureido-NH protons. Crystal structure and (1)H NMR data show that the N-aniline chromophore is electronically decoupled from the thiourea anion binding site by the N-N bond, and an intramolecular hydrogen bond exists in MeCN but not in DMSO between the N-anilino-NH nitrogen atom and the other thioureido-NH proton. Conformation changes in the N-anilinothioureas upon anion binding were assumed to occur and lead to a much higher increment in the electron-donating ability in the N-aniline chromophore that the charge transfer (CT) is enhanced or switched on, compared to not switching on a CT in the case of N-phenylthioureas. The anion binding constant shows a stronger dependence on the N-phenyl substituent than on the N'-phenyl substituent, opposite to that observed with N-benzamidothioureas, and the CT band position of the anion binding complex depends much more on the N-phenyl substituent than that of the anion binding complexes of N-benzamidothioureas. The implications of these findings for new anion-receptors design and thiourea-based organocatalysts development are discussed.     

5,5-Diaryldipyrromethanes: syntheses and anion binding properties

A two-step synthesis of 5,5-diaryldipyrromethanes in good yields is described. The adopted synthetic strategy can be used to tune the substituent at the meso-carbon very easily by choosing the Grignard reagent of interest. Further, the influence of the incorporation of various diaryl units at the meso-carbon atom in the inherent anion binding affinities of the dipyrromethanes through hydrogen bonding was discussed.     

Carbamate-stabilized anions of 2-azabicyclo[2.1.1]hexanes

The regiochemical outcomes for s-BuLi/TMEDA deprotonations of N-Boc-2-azabicyclo[2.1.1]hexanes had been shown to be temperature dependent. Computational methods have been applied to advance understanding of the complexes that the reagents form, the character of the deprotonations, and hence the experimentally observed regiochemical biases. The tertiary anion is formed more readily than the secondary anion and is also the more stable anion. Computations for the enthalpy of proton abstraction from the analogous N-methoxycarbonyl structure also indicate greater stability for the tertiary carbamate anion.     

Theoretical study of substituent effects on the acidity of the methyl group: Structure of anions CH2X−

Geometries have been optimized using molecular-orbital calculations (a) with a 4-31G Gaussian basis set for carbanions CH₂X^? where X = H, CH₃, NH₂, OH, F, C?CH, CH?CH₂, CHO, COCH₃, CN, and NO₂; and (b) with an STO -3G basis set for methyl acetate and acetyl deprotonated methyl acetate. All the carbanions containing unsaturated substituents are planar, with a considerable shortening of the C? X bond. Carbanions containing saturated substituents are pyramidal with the out-of-plane angle α increasing with the electronegativity of the substituent. Double-zeta basis set calculations give proton affinities over the range 449 (for CH₃CH₂^?) to 355 kcal/mol (for CH₂NO₂^?), with all unsaturated anions having smaller affinities than saturated anions. The correlation of proton affinities with 1s binding energies, and with charges on both the carbon of the anion and on the acidic proton of the neutral molecule are examined.     

Computational study of the thermal decomposition and the thermochemistry of allyl ethers and allyl sulfides

In spite of the several experimental and computational studies on the thermal decomposition of allyl ethers and allyl sulfides, there are still disagreements on aspects of the reaction mechanism, such as the true nature of the transition states and the grade of synchronicity of the reactions. This work presents a computational study of the gas-phase thermolysis reaction of allyl ethers and allyl sulfides substituted at α-carbon, at the M05-2X/6-31+G(d,p) level of theory and a temperature range from 586.15 to 673.15 K. The substituent groups were methyl, ethyl, n-propyl, i-propyl, allyl, benzyl and acetonyl. It was found that the sulfides react faster than the homologous ethers and that the substituent groups with the capacity of delocalize charge increase the reaction rate. Through natural bond orbital calculations, the transition states were characterized. The synchronicities and atomic charges of the studied reactions were determined. A computational study at the G3 level of theory on the thermochemistry of allyl ethers and sulfides was also carried out.     

Factors affecting relative stabilities and proton affinities of oxazolidinone and its N,C5-formyl derivatives

The proton affinities of all the potential sites of oxazolidinone (OXA) and formyl substituted OXA have been evaluated using ab initio and DFT methods. N4- and C5-formyl oxazolidinone isomers and their protonated structures have been analyzed for relative stabilities. The proton affinity (PA) of carbonyl oxygen of oxazolidinone is observed to be highest in un-substituted and formyl substituted OXA molecules. The PA values decrease for the potential sites in the range 0.5–15.51 kcal/mol as a result of the presence of the formyl substituent. Atomic charges and electron delocalization of neutral and protonated species have been analyzed with the application of NBO. The various factors such as variation in geometrical parameters, atomic charge redistribution, alterations in conjugative interactions, effect of formyl substituent, the presence of intramolecular hydrogen bonding and electronic effects have been explored to rationalize the relative stabilities and proton affinities of OXA and its N,C5 formyl derivatives.     

The effect of charge on hydroxyl loss from ortho-substituted nitrobenzene ions

An abundant loss of hydroxyl in decompositions of ortho-substituted nitroarene cations is commonly observed when the substituent contains one or more labile hydrogen atoms. The major loss of hydroxyl also takes place from many but not all of the corresponding molecular anions. Data are reported for the collisionally activated decompositions of the cations and anions of o-nitrotoiuene, o-nitrophenol and o-nitroaniline. Data for some dinitro ions are also reported. The results can be rationalized on the basis of a greater degree of charge developed at the substituent in the transition state of the anions that leads to a rearranged ion. It is from this structure tint hydroxyl is lost via simple bond cleavage. This can be viewed most simply as a proton transfer from the substituent to the nitro group in the anion as opposed to hydrogen transfer in the analogous step for the cation. The degree to which hydroxyl loss occurs is therefore largely determined by the tendency for hydrogen (cations), or proton (anions), transfer from the substituent to the nitro group.     

A forbidden rearrangement

[reaction: see text] A barrelene derivative fragments to afford benzene and trappable 1,2,3-tricyanocyclopropene. The barrelene anion fragments more easily to liberate benzene and the 1,2,3-tricyanocyclopropenyl anion, which is not trappable or stable in solution. However, the major thermal product from the barrelene anion is a rearranged allyl anion that is formed by disrotatory cleavage of the cyclopropyl ring, a formally Woodward-Hoffmann-forbidden process. Several proposals are offered to rationalize this forbidden rearrangement.     

Semiempirical evaluation for proton affinities of phosphorus compounds

We here report the AM 1 and PM 3 evaluation of proton affinities for phosphorus compounds. The substituent effects, the preferred site of protonation, and the geometry changes produced upon protonation are discussed in terms of the present semiempirical calculations that are compared with experimental data and previous ab initio results. © 1993 John Wiley & Sons, Inc.     

A reevaluation of computed proton affinities for the common α-amino acids

The proton affinities of the 20 common amino acids have been computed at the G3MP2 level using structures derived from broad conformational searches at a variety of levels including G3MP2. In some cases, the conformational surveys identified more stable species than had been used in previous studies of proton affinities, though the differences in energy are sometimes rather small. The present values are likely the most reliable measure of amino acid proton affinities in the gas phase. An analysis of differences between these values and those obtained experimentally via the kinetic method indicates that the extraction of proton affinities from kinetic method data can potentially lead to large errors linked to the estimation of relative protonation entropies.     

Correlation of 2-, 3-, 4- and disubstituted pyridine gas-phase proton affinities with ab initio calculated energies at the sto-3g basis set level

Total energies of 2-, 3-, 4- and disubstituted pyridines were calculated for the salt and the free base using ab initio molecular orbital calculations at the STO-3G basis set level [2]. In each set, the difference in energy, ΔE_H, between the salt and the free base was calculated and plotted against experimentally derived gas-phase proton affinities. The correlation was very good for each of the substituent categories listed. All of the energies and proton affinities were then plotted together on the same graph. The result was an excellent correlation with r = 0.97. The linear equation for gas phase proton affinity, PA_B = 28.51 + 435.45ΔE_H kcal/mole, was derived from this plot and was used to calculate proton affinities for all of the thirty-one compounds used in this study as well as for a series of dicyanopyridines for which values of proton affinity are not available at this time.     

Efficient diffuse function-augmented basis sets for anion calculations. III. The 3-21+G basis set for first-row elements,Li–F

The relatively small diffuse function-augmented basis set, 3-21+G, is shown to describe anion geometries and proton affinities adequately. The diffuse sp orbital exponents are recommended for general use to augment larger basis sets.     

Polybenzimidazolium hydroxides - Structure, stability and degradation

Polybenzimidazolium hydroxides are of potential interest for the use in alkaline anion exchange membrane fuel cells (AAEMFC). Introduction of an ether group in para-position of the 2-phenyl substituent improves the mesomeric stabilization of imidazolium cations, and O-PBI seems to be more stable than meta-PBI under alkaline conditions. NMR, IR and XPS analysis show a structural change when the iodide form is exchanged into the hydroxide form. Surprisingly, this structural change is subject to a pH sensitive equilibrium and the pure imidazolium form is retained immediately when the pH is lowered. The molecular structure at high pH is either the 2-carbinol or the amine-amide. Further hydrolysis necessitates excess hydroxide.     

Interconversion of 3-Acylaminobenzisoxazoles and 3-(2-Hydroxyphenyl)-1,2,4-oxadiazoles

Interconversion of 3-(2-hydroxyphenyl)-1,2,4-oxadiazoles (1) and 3-acylaminobenzisoxazoles (2) was observed in the presence of base carboxylate anion, triethylamine, alkali hydroxide, alcoholate. With proton transferring reagents (carboxylate, triethylamine) the equilibrium 1?2 is dependent on the substituent R; with anionic reagents (hydroxy anion, ethoxyl anion) the less basic anion of 1 is preferred. Alcohol effects further transformation of this anion and the alcohol adduct anion (6) is subject both to hydrolysis and alcoholysis (7) to yield 3-amino-benzisoxazole (3).     

Quantum-chemical study of nitrosonium complexes derived from nitrogen-containing heterocycles

The affinities of a series of azines for nitrosonium ion (A _NO ⁺) were calculated by quantum-chemical methods, AM1 and ab initio. The A _NO ⁺ values were found to increase as the number of nitrogen atoms in the ring decreases and the donor power of substituents increases; ring fusion also increases A _NO ⁺. The best agreement between the calculated and experimental A _NO ⁺ values was attained with the use of AM1 calculations and ab initio methods with split polarization or diffuse functions. Substituted pyridines showed linear correlations between the calculated A _NO ⁺ values and substituent constants σ or σ⁺ or overall charges on the NO group in the complex. Linear correlations were also found between the calculated A _NO ⁺ values of azines and their experimental proton affinities.     

Reactivite des carbanions benzyliques : VIII. Etude de la structure d'alkyl-9 lithio-10 dihydro-9,10 anthracenes par rmn du proton et spectrometrie d'absorption electronique; influence du substituant,du solvant et de la temperature

The structure of the 9,10-dihydroanthracenyl anion and of a series of 9-alkyl-10-lithio-9-10-dihydroanthracenes (9-R-10-LiDHA, I–V where R = H, Me, Et, i-Pr, t-Bu) was studied in solution by electronic absortion spectrometry and proton magnetic resonance. Our electronic absorption results, in addition to those of other authors, show that the contact ion pairs (c.i.p.) have an absorption at λ_max}- 400 nm (I–III) and 415 nm (V) whereas the loose ion pairs (l.i.p.) absorb at λmax}- 450 nm (I–V). In the NMR the chemical shift of the proton para with respect to the carbanionic center was examined as a function of solvent (THF, THF/HMPA, and in some cases ether or pure HMPA) and temperature (+20 to ?40°C). The para proton is shielded significantly with regard to the aromatic protons of the hydrocarbon (Δδ(H_para) ca. 1–1.7 ppm). The weakest shielding was observed in ether, in agreement with the existence of c.i.p. The largest shielding (THF/HMPA or pure HMPA) is in connection with the presence of l.i.p. where the negative charge is less localised at position 10. Moreover, in the same solvent, and at the same temperature, Δδ(H_para) was observed to increase with the substituent bulk, up to the point that there are only l.i.p. present. As found previously (namely for the fluorenyl anion) the l.i.p./c.i.p. ratio increases when temperature decreases. The results of this structural study allow to rationalize the protonation stereochemistry of 9-alkyl-10-lithio-9,10-dihydroanthracenes in the above-mentioned solvents.