Jose Kaneti

Accurate theoretical estimates of the electron affinities of AHn molecules by isogyric comparisons. Proton affinities of AHn− anions

Abstract Electron affinities of AH n molecules (A = Li to Cl) are estimated by ab initio molecular orbital theory, using isogyric comparisons with the hydrogen molecule. Results agree with experimental data to within 0.1 eV for first-row and most second-row compounds. Electron affinities are predicted for BH, BH 2 , AlH and AlH 2 . BH 2 − is predicted to have a singlet 1 A 1 ground state with the 3 B 1 state lying 0.16 eV higher in energy; the corresponding singlet-triplet difference for AlH 2 − is predicted to be larger, 0.73 eV. Theoretical proton affinities (PA) 0 for the AH n − anions are derived from the same data set.

Thorpe–Ingold effects in cyclizations to five-membered and six-membered rings containing planar segments. The rearrangement of N(1)-alkyl-substituted dihydroorotic acids to hydantoinacetic acids in base

While the gem-dimethyl effect (GDME) is quantitatively similar for cyclizations to cyclopentane and cyclohexane rings and their homomorphs, in systems containing planar segments the GDME is stronger for the formation of five-membered rings. Planar pentagons have smaller angles than planar hexagons and their formation is helped by the decrease in the potential internal bond angle caused by substituents, as suggested by Thorpe and Ingold for small rings. The phenomenon is illustrated with crystal structure data on five-membered hydantoins and six-membered dihydrouracils containing four-atom planar segments. Such a Thorpe-Ingold effect explains the rearrangement in base of N-alkyl substituted dihydroorotic acids 1 to hydantoinacetic acids 3. The reaction involves initial hydrolysis to N-(N-alkylcarbamoyl)aspartic acids 2 and their subsequent cyclization. The unsubstituted N-carbamoylaspartic acid 2a is stable in 1 M KOH, the N(1)-methyl and ethyl compounds 2b and 2c are in equilibrium with the hydantoinacetic acids 3, while the cyclization of the N(1)-isopropyl and cyclohexyl derivatives 2d and 2e is irreversible. Experimental data on equilibria and pK(a)s for ionization of the carboxy and NH groups allow equilibria and rates involving the N-unsubstituted compounds to be estimated and compared with those for the N-alkyl derivatives. The strongest effect is observed on the equilibrium [3(2-)]/2[(2-)], where substitution of H by methyl increases K 600-fold. In vitro the kinetic regioselectivity for acid catalyzed cyclization of N-carbamoylaspartic to hydantoinacetic acid against dihydroorotic acid is only 10:1. This, together with the weaker acidity of the remote carboxyl group, favours cyclization to dihydroorotic acid under biological conditions.

The structure of phenol-ammonia clusters before and after proton transfer. A theoretical investigation

Abstract Equilibrium structures are reported for clusters of phenol with up to five ammonia molecules at the Hartree-Fock level of theory with checks based on Moller-Plesset and density-functional methods. A systematic build-up of solvent shells is observed. For the clusters with five ammonias a metastable ion-pair structure is obtained. The results are related to the observed proton-transfer dynamics in electronically excited clusters.

Infrared spectra and structure of carbanions—XIII1: A study of the carbanions generated from acetonitrile, acetonitrile-d3 and acetonitrile-15n

Abstract The study of the IR spectral data for metallated acetonitrile (counter ions Li+, Na+, K+ in solvents tetrahydrofuran, THF, and hexamethylphosphotriamide, HMPT) and its D3− and 15N- derivatives together with CNDO 2 and normal coordinate calculations showed that the mesomeric ion H2CCN has a favoured planar structure and the carbon-metal bond has a pronounced ionic character.

The preference of 1-methylallyl polar organometallics and carbanions for cis rather than for trans geometries

In agreement with experimental equilibrium studies in solution, ab initio calculations, using diffuse function augmented split valence basis sets, favor cis geometries of 1-methylallyl lithium, sodium, potassium, and rubidium by 0.7, 1.1, 2.5, and 2.6 kcal/mol, respectively. These results, and the even greater cis preference of the 1-methylallyl anion over trans, 4.7 kcal/mol (MP2/6-31+G//3-21G) show the contrary conclusions from an early ICR study and from semiempirical theory to be incorrect. Later gas phase experiments demonstrated that stereointegrity was not maintained under the ICR conditions. Semiempirical theory is shown here to be inherently deficient for the examination of alkyl-substituted carbanions. The surprisingly large cis preferences of the 1-methyl-allyl anion systems are attributed to the largely electrostatic attraction between the positively charged methyl hydrogens and the remote allyl anion terminus. This attraction is largest in the free anion and attenuated to an increasingly greater extent the smaller the alkali metal gegenions become. The allyl moieties in the metal 1-methylallyl derivatives are calculated to be quite unsymmetric, in agreement with NMR observations.

Computational probes into the conceptual basis of silver ion chromatography: I. Silver(I) ion complexes of unsaturated fatty acids and esters

Silver ion chromatography of unsaturated fatty acid derivatives can be described satisfactorily by the suggestion of Ag(I) complexation with more than a single bonding site of esters of the 18:1, 18:2, etc. series (that is, one, two, etc. methyleneinterrupted double bonds in the C18 hydrocarbon chain) and by the formation of chelate-like complexes. Ab initio RHF effective core potential CEP-31G calculations of model C6 to C18 complexes in the gas phase and in solution indicate that the stability of such complexes depends on the position of CyC double bonds in ligand molecules, while a significant part of the complexation energy is contributed by the interaction of Ag(I) and alkoxy- (or aryloxy-)carbonyl groups of fatty acid esters. Factors important to the stability of Ag(I) fatty acid chelates are the ring sizes and strain, particularly manifested in unsaturated acids with the CyC double bond closer to the carboxyl group, i.e. with smaller chelate rings, as well as conformations of larger rings and hydrocarbon chains. Calculated theoretical solvent effects on chelation are more significant for larger Ag(I) containing rings and increase with increasing ring size. q 2002 Elsevier Science B.V. All rights reserved.

Electronic structure of the rotational isomers of furfural

Abstract The electronic structures of the two planar rotational isomers of furfural are discussed. Calculations by PPP-CI and Del Re methods demonstrate that definite differences should be observed between the properties of the possible retainers. On the basis of theoretical calculations and observed properties of both isomers it is possible to establish their relative content in equilibrium mixtures.

Electronic substituent effects and carbonyl stretching frequencies of 2-arylmethylene-1,3-indandiones

Abstract The CO stretching frequencies of 2-arylidene-1.3-indandiones are highly sensitive to electronic substituent effects. The slopes of the correlations - σ + found are comparable with those for benzaldehydes. A linear correlation between the electronic substituent effects and the value of the CO splitting is observed, the latter becoming larger with increasing electron donor properties of the substituent. The existence of such a correlation is an indication of an electronic mechanism of interaction between the carbonyl groups. Thus the existence of coupling of unconjugated groups in a π-electronic system is emphasised.

Semi-empirical AM1 calculation of the solvent effect on the fluorescence spectra of some dihydroquinolinones

The COnductor-like Screening MOdel of solvent-solute interactions of Klamt and Schüürmann, COSMO, at the semiempirical AM1 level of MO calculations, augmented by limited singles and doubles configuration interaction, proves useful for the study of solvent induced shifts of fluorescence spectra. Optimization of geometry of ground S0 and excited S1 states for each solvent separately provides estimates of the changes of solvation energy accompanying the electron transition process and helps the understanding of the related solvent-solute reorganization and fluorescence mechanisms.

Theoretical study of ketenimine: Geometry, electronic properties, force constants and barriers to inversion and rotation

Abstract Non-empirical calculations of the structure and properties of ketenimine have been performed using nine Gaussian basis sets. Values for the bond lengths and angles, HOMO and LUMO energies, atomic charges, overlap populations, dipole moments, bond energies, force constants and barriers to nitrogen inversion and internal rotation are predicted.