Sherif A. Kafafi

Intramolecular interaction energies in model alanine and glycine tetrapeptides. Evaluation of anisotropy, polarization, and correlation effects. A parallel ab initio HF/MP2, DFT, and polarizable molecular mechanics study

An extension of the SIBFA polarizable molecular mechanics procedure to flexible oligopeptides is reported. The procedure is evaluated by computing the relative conformational energies, δEconf, of the alanine tetrapeptide in 10 representative conformations, which were originally derived by Beachy et al. (J Am Chem Soc 1997, 119, 5908) to benchmark molecular mechanics procedures with respect to ab initio computations. In the present study, a particular emphasis is on the separable nature of the components of the energy and the particular impact of the polarization energy component on δEconf. We perform comparisons with respect to single‐point HF, DFT, LMP2, and MP2 computations done at the SIBFA‐derived energy minima. Such comparisons are made first for the 10 conformers derived from ϕ/ψ torsional angle energy‐minimization (the rigid rotor approach), and, in a second step, after allowing additional relaxation of the Cα centered valence angles. In both series of energy‐minimization, the SIBFA δEconf compared best with the LMP2 results using the 6‐311G** basis set, the rms being 1.3 kcal/mol. In the absence of the polarization component, the rms is 3.5 kcal/mol. In both series of minimizations, the magnitudes of δEconf, computed as differences with respect to the most stable conformer taken as energy zero, decrease along the series: HF > DFT > LMP2 > SIBFA > MP2, indicative of increasing stabilization of the most highly folded conformers.

Carbon acidities of aromatic compounds. 2. Ionic probes of aromaticity in annelated rings

Ionization by the deprotonation of benzene and pyridine, and by the protonation of pyridine, involves lone pairs in the u plane without significant a effects. In these cases annelation by a benzene ring increases the acidity or proton affinty by a constant 6 & 1 kcal/mol, ascribed to increased polarizability. In comparison, protonation of benzene and deprotonation of cyclopentadiene disrupts or creates a 6-electron aromatic system, respectively, and in their annelated derivatives, naphthalene and indene, a secondary 4-electron conjugated K system. These cases involving a electrons show annelation effects that are substantially larger (13.4 kcal/mol) or smaller (1 .O kcal/mol), respectively, than just the electrostatic effect. Analysis of these data suggests that the stability of secondary 4-electron systems in the annelated rings is smaller by 6 f 1 kcal/mol than the aromatic 6-electron systems, in fair agreement with Herndon’s structure-resonance values for these species. Annelation effects are reproduced well by Dewar’s AM1 semiempirical method. Aromaticity is a primary concept in the understanding of the structure and energetics of organic compounds. Extensive literature exists on the subject,’J and many experimental and theoretical approaches have been offered to evaluate the magnitude and origin of this phenomenon. We will not attempt to review what is known, but rather present a simple and suggestive application of gas-phase ion energetics to this problem based mostly on the present measurements. We offer below a measure of resonance stabilization in annelated compounds as applies to the present data. This paper exploits the observation that, in some aromatic compounds and their annelated derivatives, protonation or deprotonation involves lone pairs in the c plane of the molecule, without significant direct a interactions. In contrast, there are cases where protonation or deprotonation destroys or creates the aromatic 6-electron system (Le. Clar’s “aromatic sextet”’ in the parent compound or a Celectron system, conjugated to a 6-electron one, in the annelated derivatives. These points are illustrated by structures 1-12. It may be expected that increasing the molecule’s

The ionization potential, electron affinity and energy gap of polyimide

Abstract The ionization potential (IP), electron affinity (EA) and energy gap ( E g ) of polyimide are estimated from molecular orbital (MO) computations using the AM1 method on model compounds. Their computed values are 8.5, 1.4 and 7.1 eV, respectively. Interestingly, it was found that the EA of polyimide is comparable to that of N-phenylphthalimide.

The proton affinities of styrene, trsns-?-methylstyrene, and indene: A theoretical and experimental study

We report experimental and theoretical AM1 proton affinities of styrene,β-methylstyrenes, and indene. The computed AM1 proton affinities for the species of interest were in good agreement with the experimental values.trans-β-Methylstyrene was found to have a proton affinity slightly lower than that of styrene. This is an unusual result since methyl substitution in most classes of compounds increases the proton affinity by 2–4 kcal mol−1. The lower basicity oftrans-β-methylstyrene compared to styrene is due to the greater stabilizing effect of the methyl group in the neutral species compared to the cation.

The electronic structure and conformation of dimethylaminobenzonitrile

Ab initio molecular orbital (MO) computations on the closed shell singlet ground state ofN, N-dimethylaminobenzonitrile (DMABN) are reported. Fully optimized structures of several conformers of DMABN were calculated at the HF/6-31G level of theory. Our results indicate that for each of these conformations the minimum energy structure has a trigonal (sp2 hybridized] amino nitrogen. The most stable DMABN conformer was found to be planar with its methyl groups eclipsed. The Koopmans ionization potentials and dipole moments of the various ground state conformers are compared. The implications for dynamical models of twisted-intramolecular charge transfer (TICT) are discussed. Moreover, the use of qualitative MO theory arguments provides an interpretation of the computational results in a simple orbital interaction framework.