PhD papers (1997-2002)

Treitel, N. ; Shenhar, R. ; Aprahamian, I. ; Sheradsky, T. ; Rabinovitz, M. Calculations of PAH anions: When are diffuse functions necessary?. Physical Chemistry Chemical Physics 2004, 6 1113-1121.Abstract
The effect of including vs. excluding diffuse functions while calculating numerous parameters of PAH anions by various calculation methods is discussed. The omission of diffuse functions. appears to have a negligible effect while calculating geometry parameters or total energy; thus, acceptable results may be obtained without them. The conclusions for charge density appear to be the same; however, limited results make an unambiguous claim unachievable. Calculating H-1- and C-13-NMR shifts undoubtedly requires the use of these functions.
Shenhar, R. ; Beust, R. ; Hagen, S. ; Bronstein, H. E. ; Willner, I. ; Scott, L. T. ; Rabinovitz, M. From bifluorenylidene dianion to dibenzo[g,p]chrysene dianion: sensitivity of anisotropy changes to bonding structure. Journal of the Chemical Society-Perkin Transactions 2 2002, 449-454.Abstract

Five polycyclic aromatic hydrocarbons of the C-26 series having similar bonding structure yield dianions upon reduction with lithium metal. Anisotropy changes, revealed from an advanced charge distribution analysis performed on these dianions, show a correlation to the bonding structure of the dianions. Electron counting and orbital considerations rationalize this correlation in terms of aromatic/anti-aromatic behaviour that is mixed into the character of the aromatic PAH upon reduction. Predictions made regarding relative stability based on this correlation were successfully tested against calculation and experiment. The anisotropy change is suggested as a valid index for the reduction-induced change in the aromatic character of PAHs, which is applicable for both aromatic and anti-aromatic changes.

6. anisotropicity scale of polycyclic aromatic hydrocarbons

Shenhar, R. ; Willner, I. ; Rajca, A. ; Rabinovitz, M. Photoejection of electrons from cycloocta[1,2,3,4-def;5,6,7,8-d'e'f]bisbiphenylene radical-anion and dianion derivatives: Identification of a light-induced disproportionation. Journal of Physical Chemistry A 2002, 106, 6206-6212.Abstract

Photoejection of electrons from 2,5,8,11-tetra-tert-butylcycloocta[1,2,3,4-def;5,6,7,8-d'e'f']bisbiphe nylene radical anion, BPD.-, and the respective dianion, BPD2-, are described. Photoejection of an electron from BPD2- yields a [BPD.-...e(-)] cage complex and separated BPD.- and e(-)/Li+ species. Recombination of the cage photoproducts proceeds at room temperature with a rate constant of k(rec)(1) = (7.0 +/- 0.2) x 10(5) s(-1), and the separated photoproducts recombine by a diffusional back-electron-transfer rate constant of k(rec)(2) = (1.5 +/- 0.2) x 10(9). M-1.s(-1). Photoejection of the electron from BPD.- yields the neutral pi-system, BPD, and the ejected electron reduces an unreacted BPD.- to form BPD2-. The photoejection of the electron from BPD.- thus yields the disproportionation products, BPD and BPD2-. The disproportionation products recombine by a diffusional process, k(rec) = (1.0 +/- 0.2) x 10(10) M-1.s(-1) (at room temperature).

Shenhar, R. ; Wang, H. ; Hoffman, R. E. ; Frish, L. ; Avram, L. ; Willner, I. ; Rajca, A. ; Rabinovitz, M. Self-assembled, helically stacked anionic aggregates of 2,5,8,11-tetra-tert-butylcycloocta[1,2,3,4-def,5,6,7,8-d'e'f']bisbiphenylene, stabilized by electrostatic interactions. Journal of the American Chemical Society 2002, 124, 4685-4692.Abstract

Tetraanions of alkyl-substituted derivatives of cycloocta[1,2,3,4-def,5,6,7,8-d'e'f']bisbiphenylene (BPD) and their counter lithium cations self-assemble to form helically stacked assemblies, including a dimer, a trimer, and a tetramer. NMR self-diffusion measurements and unprecedented magnetic shielding effects for the sandwiched lithium cations support their aggregated nature. The D2-tetramer assembly is fully characterized by NMR spectroscopy, providing unequivocal evidence for a helix of four tetraanionic BPD layers with an estimated relative twist angle of about 45degrees and interlayer spacing of ca. 4 Angstrom. The barrier for racemization through the in-plane inter-deck rotation is DG200(double dagger) = 9.5 +/- 0.2 kcal mol-1 in the dimer compared to >15 kcal mol-1 in the tetramer.

7. Helical supramolecular tetramer held by electrostatic interactions

Shenhar, R. ; Beust, R. ; Hoffman, R. E. ; Willner, I. ; Bronstein, H. E. ; Scott, L. T. ; Rabinovitz, M. Lithium reduction of the bowl-shaped C-60 fragment diindeno[1,2,3,4-defg; 1 `,2 `,3 `,4 `-mnop]chrysene: An interplay between experiment and calculation. Journal of Organic Chemistry 2001, 66, 6004-6013.Abstract

Diindeno[1,2,3,4-defg;1',2',3',4'-mnop]chrysene (DIC) (one of the smallest symmetrical bowl-shaped fragments of C60) and its tetra-tert-butyl derivative are reduced with lithium metal to yield dianions and tetraanions. Due to the high degree of symmetry (C2v) of DIC and its derivative, their NMR spectra cannot be assigned using the standard two-dimensional NMR techniques. A novel carbon-edited NOESY method was used to complete the assignments of the neutral and dianion species, whereas the tetraanions are aided by DFT calculations for their assignment. Experimental charge-distribution patterns were obtained and match those of the calculations. An extension of the empirical approach for estimating the charge distribution from the 13C-NMR spectra enables a direct comparison between experimentally derived charge-distribution data and the computed electron density in each of the lowest unoccupied molecular orbitals. The overall picture evolving from the orbital structure of DIC is presented and reflects the surface reactivity of C60.

5. C-60 bowl fragment anions

Hoffman, R. E. ; Shenhar, R. ; Willner, I. ; Bronstein, H. E. ; Scott, L. T. ; Rajca, A. ; Rabinovitz, M. Heteronuclear editing method for detecting NOE between chemically equivalent protons: application to polycyclic aromatic hydrocarbons. Magnetic Resonance in Chemistry 2000, 38, 311-314.Abstract

The complete characterization of polycyclic aromatic hydrocarbons (PAHs) and tetrasubstituted ethylenes is critical to an understanding of their reactivity, for which NMR is an important tool. Chemical shifts can provide a direct measure of charge distribution and aromaticity. Unfortunately, COSY, NOESY and heteronuclear correlation cannot provide a complete assignment of the NMR spectra for some carbon-rich PAHs with symmetrical bay regions. The protons in the bay regions would yield NOE signals if they were not symmetrical. Natural substitution of C-12 with C-13 can be used to break the symmetry and yield these useful NOE signals. Using gradient-assisted editing, unequivocal assignments have been achieved for some previously problematic molecules. Copyright (C) 2000 John Wiley & Sons, Ltd.

Shenhar, R. ; Willner, I. ; Preda, D. V. ; Scott, L. T. ; Rabinovitz, M. Electron photoejection from corannulene dianion and Li+-mediated recombination of the photogenerated species. Journal of Physical Chemistry A 2000, 104, 10631-10636.Abstract

The antiaromatic corannulene dianion, Cor2-, undergoes photoejection of an electron to yield an intimate cage complex of Cor.- and the electron. Part of the complex species undergoes cage recombination, k(rec)(1) = 1.1 x 10(6) s(-1), while the other part of complex separates, k(s) = 2.3 x 10(6) s(-1), and yields Cor(.-) and a Li+/e(-) ion pair. Diffusional recombination of the later products proceeds with a bimolecular rate constant corresponding to k(rec)(2) = 1.3 x 10(9) M-1 s(-1). Time-resolved laser flash-photolysis and FT-EPR experiments are used to characterize the transient species.

Eshdat, L. ; Ayalon, A. ; Beust, R. ; Shenhar, R. ; Rabinovitz, M. Up to six units of charge and twist-boat benzene moieties: Alkali metal reduction of phenyl-perisubstituted benzenes. Journal of the American Chemical Society 2000, 122, 12637-12645.Abstract
Phenyl-perisubstituted benzenes, tetraphenylbenzene (1) and hexaphenylbenzene (2), were reduced by lithium and sodium metal in THF-d(8) under high vacuum. The reduction process and the nature of the reduction products were studied by NMR. Tetraphenylbenzene was reduced by both metals to yield the corresponding dianionic salt. It was found that the addition of extra charge into the system, restricted the free rotation of the four phenyl substituents about the sigma bond connecting them to the central ring (G(181)(double dagger) = 7.8 +/- 0.2 kcal mol(-1)). The reaction of the alkali metals with 2 yielded four diamagnetic species: the first three were assigned to the dianion, tetraanion, and the surprising hexaanion of 2. These species were calculated using density functional theory (DFT) and were found to have central benzene rings with an unusual twist-boat geometry. Computational and experimental evidences show that each phenyl ring and its attached carbon of the central ring behave like a benzyl anion. We therefore view the hexaanion of 2 as a cyclohexa(benzylanion). The fourth diamagnetic species was a product of a double-sided cyclization, which yielded the dianion of dihydro-9, 18-diphenylphenanthro[9,10-b]triphenylene (9,18-diphenyltetrabenz[a,c,h,j] anthracene dianion, 3(2-)). Reaction of the dianion with oxygen gave 3 in improved yields compared to literature preparations.
Shenhar, R. ; Rabinovitz, M. Aryl-substituted derivatives of trimethylenemethane dianion: a dynamic NMR study. Journal of the Chemical Society-Perkin Transactions 2 1997, 2463-2464.Abstract

Evidence is given for the structure of aryl-substituted trimethylenemethane dianions from NMR data, and the importance of p-pi conjugation is discussed.