Hyperconjugation in the 2-butyl cation

A comparison of the optimized geometries of the hydrogen-bridged ethyl and 2-butyl cations suggest the contribution of hyperconjugation to the stability of the latter to be 10 kcal/mole. An examination of the barrier to rotation of a methyl group is in accord with the phenomenon of hyperconjugation.     

Long-lived 9-cyano-9,10-dimethylphenanthrenyl cation. Migration of cyano group in carbocations

Long-lived 9-cyano-9,10-dimethylphenanthrenyl cation was generated in superacidic medium, and its structure was determined by ¹H and ¹³C NMR spectroscopy. The energy barrier to 1,2-shift of the cyano group in 9-cyano-9,10-dimethylphenanthrenyl cation was estimated by NMR and DFT calculations.     

Esr studies of crown complexed semiquinone ion pairs in solution

ESR spectra of alkyl-substituted p-benzosemiquinone anion radicals produced by sodium reduction in the presence of 18-crown-6 in tetrahy drofuran show, remarkable temperature and concentration dependences, not observed in the absence of the crown. The observed spectral changes are interpreted in terms of equilibria between structurally different ion pairs.     

On the usefulness of crown ethers in freezing out the intramolecular cation migration in radical ion pairs

The additon of stoichiometric amounts of dibenzo-18-crown-6 ether to solutions of the sodium ion pairs of pyrazine and therephthalonitrile has been found to reduce dramatically the rate constant for the intramolecular migration of the alkali cation. The proton and nitrogen hyperfine splitting in the slow exchange region, could be easily measured under these experimental conditions.     

Isotopic scrambling in Di-13C-labeled 2-butyl cation: evidence for a protonated cyclopropane intermediate

The (13)C NMR spectrum of 2-butyl-1,2-(13)C(2) cation (1) is unchanged on heating the sample to -78 degrees C, indicating no isomerization to another isotopomer. In contrast, the spectrum of 2-butyl-2,3-(13)C(2) cation (2) shows rapid formation of all of the other isotopomers except 1. These results are consistent with a protonated cyclopropane intermediate in the rearrangement process. In this mechanism, either C(1) and C(2) or C(3) and C(4) interchange. Only the bond between C(2) and C(3) breaks.     

Theoretical investigation of the structure of the ion pairs of 1-hydroxypyridinium halides

A theoretical investigation was undertaken into the structure of the intimate ion pairs of 1-hydroxy-pyridinium halides and their 4-methyl- and 4-fluorine-substituted derivatives. It was established that the salts are stereochemically flexible molecules, the most likely geometric configuration of which corresponds to the location of the anions above the plane of the pyridine ring in the region adjacent to the nitrogen atom. The effect of the substituent on the dissociation energy, geometric characteristics, and intramolecular dynamics of the ion pairs was studied.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 91–94, January–February, 1987.     

Theoretical studies on cycloaddition reactions between the 2-aza-1,3-butadiene cation and olefins

Density functional (B3LYP) calculations, using the 6-31G basis set, have been employed to study the title reactions. For the model reaction (H(2)C=C-NH(+)=CH(2) + H(2)C=CH(2)), a complex has been formed with 6.2 kcal/mol of stabilization energy and the transition state is 4.0 kcal/mol above this complex, but 2.1 kcal/mol below the reactants. However, the substituent effects are quite remarkable. When ethene is substituted by electron-withdrawing group CN, the reaction could also yield six-membered-ring products, but the energy barriers are all more than 7 kcal/mol, which shows that CN group unfavors the reaction. The other substituents, such as CH(3)O and CH(3) groups, have also been considered in the present work, and the results show that they are favorable for the formation of six-membered-ring adducts. The calculated results have been rationalized with frontier orbital interaction and topological analysis.     

Theoretical studies of molecular ions. Vertical ionization potentials of the nitrogen molecule

The ²Σ⁺_g, ²Π_u, and ²Σ⁺_u vertical ionization energies of nitrogen are obtained by using our theory of molecular electron affinities and ionization potentials, which permits the direct calculation of the ion-molecule energy differences. The contributions of charge redistribution and correlation energy change to the calculated ionization potentials are evaluated. The computational efficiency of the method is illustrated and comparisons are made with recent experimental results.     

Theoretical studies of photoexcitation and ionization in hydrogen sulfide

Computational studies are reported of photoexcitation and partial-channel photoionization processes in the valence shell of H₂S employing single- and coupled-channel, vertical-electronic, static-exchange approximations. Detailed assignments are provided on the basis of the calculations of all prominent features observed below the first ionization threshold in recent photoabsorption studies. Comparisons are also made with experimental partial-channel photoionization cross sections obtained from electron-impact measurements in the dipole limit, and with branching ratios obtained from synchrotron-radiation photoelectron spectroscopy studies. The calculated discrete valence-shell excitation series are in generally good agreement with recent high-resolution VUV spectral measurements, although some of the present assignments are not in accord with those suggested on basis of the experimental data alone, nor with those based on earlier computational studies. The origins of these discrepancies are indicated and clarified. Configuration mixing is seen to influence the absorption intensities in the H₂S valence-shell spectrum, particularly in ¹A₁ → ¹A₁ and ¹A₁ → ¹B₂ symmetries due to the presence of intravelence excitations in these cases. The calculated valence-shell partial-channel photoionization cross sections are in good accord with the recent electron-impact study, and with branching ratios obtained from synchrotron-radiation measurements. Agreement with previous computational studies is not entirely satisfactory, however. The discrepancies are attributed to use of Coulomb and local-potential approximations in the earlier calculations. As in previous reports in this series on molecular photoabsorption and photoionization in polyatomic molecules, the general natures of the excitation and ionization spectra in H₂S are clarified on basis of the presence of valence-like (3b₂(σ^*), 6a₁(γ^*)) and diffuse orbital contributions to both discrete and continuum states.     

Evidence for anchimeric assistance by the Cp2ZrCl-group in the homolytic cleavage of an OCH3 bond

Anchimeric assistance by a transition metal in the homolysis of a CO bond is discussed for the thermal rearrangement of methyl(zirconocenyidiphenylmethyl)ether $\text{1c}$.     

Theoretical investigation of the formation of the tropylium ion from the toluene radical cation

The formation of the tropylium ion, C₇H₇⁺, in the mass spectrum of toluene is a chemical process that has been extensively studied. There is, however, still debate as to the structure of the moieties and the reaction pathways involved. This work presents the first computationally complete reaction schemes for the formation of tropylium from toluene to be reported. The calculations were performed at the HF/6‐31G(d, p) and the DFT/B3LYP/6‐311++G(2d) levels of theory using Gaussian 03W. The previously unreported optimized structures and energies for a transition state and an intermediate in one scheme and a transition state in the other have been determined. These results are consistent with the previously reported literature and the available experimental data. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical study of lithium-fluoride and lithium-chloride ion pairs in aqueous solution. An SCF-CNDO/2 approach including continuum solvent effects

SCF-CNDO/2 calculations, including solvent effects via an extended version of the Generalized Born Formula (GBF), have been performed for LiX(H₂O) species (n = 1,2; X = F, Cl). Several minima in the free energy surface, representing intimate and solvent-separated ion pair structures, have been analyzed. Qualitative results show a preferential stabilization of the intimate forms with respect to the solvent-separated ones. The results are discussed on the basis of a convenient partition of the total solute-solvent free energy. The interaction of the ionic species with the bulk solvent neglected in previous studies appears to be responsible for the preferential stability of the intimate forms.     

Influence of the nucleobase and anchimeric assistance of the carboxyl acid groups in the hydrolysis of amino acid nucleoside phosphoramidates

Nucleoside phosphoramidates (NPs) are a class of nucleotide analogues that has been developed as potential antiviral/antitumor prodrugs. Recently, we have shown that some amino acid nucleoside phosphoramidates (aaNPs) can act as substrates for viral polymerases like HIV‐1 RT. Herein, we report the synthesis and hydrolysis of a series of new aaNPs, containing either natural or modified nucleobases to define the basis for their differential reactivity. Aqueous stability, kinetics, and hydrolysis pathways were studied by NMR spectroscopy at different solution pD values (5–7) and temperatures. It was observed that the kinetics and mechanism (P? N and/or P? O bond cleavage) of the hydrolysis reaction largely depend on the nature of the nucleobase and amino acid moieties. Aspartyl NPs were found to be more reactive than Gly or β‐Ala NPs. For aspartyl NPs, the order of reactivity of the nucleobase was 1‐deazaadenine>7‐deazaadenine>adenine>thymine≥3‐deazaadenine. Notably, neutral aqueous solutions of Asp‐1‐deaza‐dAMP degraded spontaneously even at 4 °C through exclusive P? O bond hydrolysis (a 50‐fold reactivity difference for Asp‐1‐deaza‐dAMP vs. Asp‐3‐deaza‐dAMP at pD 5 and 70 °C). Conformational studies by NMR spectroscopy and molecular modeling suggest the involvement of the protonated N3 atom in adenine and 1‐ and 7‐deazaadenine in the intramolecular catalysis of the hydrolysis reaction through the rare syn conformation.     

Spectroscopy of the UO+2 cation and the delayed ionization of UO2

Vibronically resolved spectra for the UO+2 cation have been recorded using the pulsed field ionization zero electron kinetic energy (PFI-ZEKE) technique. For the ground state, long progressions in both the bending and symmetric stretch vibrations were observed. Bend and stretch progressions of the first electronically excited state were also observed, and the origin was found at an energy of 2678 cm(-1) above the ground state zero-point level. This observation is consistent with a recent theoretical prediction [Infante et al., J. Chem. Phys. 127, 124308 (2007)]. The ionization energy for UO2, derived from the PFI-ZEKE spectrum, namely, 6.127(1) eV, is in excellent agreement with the value obtained from an earlier photoionization efficiency measurement. Delayed ionization of UO2 in the gas phase has been reported previously [Han et al., J. Chem. Phys. 120, 5155 (2004)]. Here, we extend the characterization of the delayed ionization process by performing a quantitative study of the ionization rate as a function of the energy above the ionization threshold. The ionization rate was found to be 5 x 10(6) s(-1) at threshold, and increased linearly with increasing energy in the range investigated (0-1200 cm(-1)).     

A novel chemical ionization reagent ion for organic analytes: the aquachloromanganese(II) cation [ClMn(H2O)+]

The reactivity of ClMn(H₂O)⁺ towards small organic compounds (L) was examined in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The organic compounds studied are aliphatic and aromatic alcohols, aliphatic amines, ketones, an epoxide, an ether, a thiol and a phosphine. All the reactions lead to the formation of the ClMn(H₂O)(L)⁺ complex, which dissociates by loss of the H₂O molecule. In general, the reactions were found to occur with high efficiencies (>85%), indicating them to be exothermic. Electron transfer was also observed between ClMn(H₂O)⁺ and compounds with low ionization energies (IE), to form the molecular ion (L^+?) of the analyte. Based on these observations, the IE of ClMn(H₂O)⁺ is approximated to be 8.1 ± 0.1 eV. Thus, the utility of ClMn(H₂O)⁺ as a chemical ionization reagent in mass spectrometry is expected to be limited to organic compounds with IEs greater than 8 eV. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical and experimental studies of ion imprinted polymer for nitrate detection

Molecular imprinting is an approach to synthesize receptors with specific molecular recognition properties. A computational method was carried out to study interaction between template and monomer in prepolymerization mixture. The functional monomer and template complexes were optimized, at the minimum energy confirmation using Austin Model 1 semi empirical method within Restricted Hartree Fock formalism. The theoretical results showed that allylthiourea (functional monomer) has the largest interaction energy towards template (sodium nitrate) with the mole ratio of 4 : 1; functional monomer : template. The resulting polymers were characterized using Fourier Transform infrared spectroscopy, thermogravimetry analysis and field emission scanning electron microscopy. Rebinding experiments were carried out to evaluate binding capacity of the polymer. The adsorption data of ion imprinted polymer (IIP) were fitted with Langmuir-Freundlich isotherm model. Pseudo-second order kinetic model was used to describe the kinetic adsorption behavior of IIP. The experimental binding result showed good agreement with theoretical computation and the IIP was further used for nitrate ion detection. The results of membrane optimization indicated that the sensor, which composed of 30% polyvinylchloride, 60% nitrophenyl octyl ether as a plasticizer, 2% sodium tetraphenyl borate, and 10% IIP as ionophore exhibited an almost Nernstian slope with the limit of detection 3.9 × 10^-6 M. The fabricated sensor had shown good potential in nitrate detection with wide linear range, low limit of detection and found to have good selectivity towards nitrate ion over other anion.     

Theoretical studies on the ionization potential of interacting atoms at large separations

The theory of the one-particle Green's function is applied to calculations of the ionization potential of interacting atoms which are at large separations. Equations for the ionization potential involve terms which relate to Van der Waals interactions between separated atoms and long-range interactions between an atom and an ion. Numerical calculations of the ionization potential of two hydrogen atoms and two helium atoms at large separations are performed. Applications to the ionization potentials of weakly-interacting Van der Waals molecules (NeAr, NeKr, NeXe) are also reported.     

Theoretical studies of diamagnetic properties of the hydrogen molecule ion. I. Approximate variation-perturbation calculation

A variation-perturbation method, analogous to a technique devised by Das and Bersohn for the study of the hydrogen molecule, is developed for calculation of the diamagnetic susceptibility χ and magnetic shielding σ and is applied to a study of the hydrogen molecule ion. Two approximations are investigated–in a first approximation, χ = ?0.37814α²a and σ = 1.112 × 10^?5 c.g.s. units and in a second approximation, χ = ?0.37569α²a and σ = 1.128 × 10^?5 c.g.s. units.     

Direct observation of the ionization step in solvolysis reactions: electrophilicity versus electrofugality of carbocations

Rates and equilibria of the reactions of highly stabilized amino-substituted benzhydrylium ions (Ar2CH+) with carboxylate ions have been determined photometrically in acetone and acetonitrile solutions. Treatment of covalent benzhydryl carboxylates (Ar2CH-O2CR) with aqueous acetone or acetonitrile leads to the regeneration of the colored amino-substituted benzhydrylium ions Ar2CH+, which do not undergo subsequent reactions with the solvent. One can, therefore, directly measure the first step of S(N)1 reactions. The electrofugality order, i.e., the relative ionization rates of benzhydryl esters Ar2CH-O2CR with the same anionic leaving group, does not correlate with the corresponding electrophilicity order, i.e., the relative reactivities of the corresponding benzhydrylium ions Ar2CH+ toward a common nucleophile. Thus, benzhydrylium ions which are produced with equal rates by ionization of the corresponding covalent esters may differ by more than 2 orders of magnitude in their reactivities toward nucleophiles, e.g., carboxylate ions. Variable intrinsic barriers account for the breakdown of the rate-equilibrium relationships. Complete free-energy profiles for the ionization of benzhydryl carboxylates Ar2CH-O2CR are constructed, which demonstrate that the transition states of these ionizations are not carbocation-like. As a consequence, variation of the solvent-ionizing power Y has only a small effect on the ionization rate constant (m = 0.35 to 0.55) indicating that small values of m in the Winstein-Grunwald equation do not necessarily imply an S(N)2 type mechanism.