Sason Shaik | The Hebrew University of Jerusalem (original) (raw)

Papers by Sason Shaik

Journal of Organometallic Chemistry, Jun 1, 1975

Abstract A general mass spectrophotometric method for the identification of tellurium-containing ... more Abstract A general mass spectrophotometric method for the identification of tellurium-containing compounds is described. The method is based on the analysis of the typical pattern of cluster peaks containing tellurium due to -Te-, -Te 2 - or (X = Cl, Br). A comparison of the computer calculated and experimental mass spectra of some of the compounds containing tellurium is given.

International Journal of Mass Spectrometry and Ion Processes, May 1, 1993

The role of tropylium ion Cl'r +) versus benzylium ion (Bz +) formation by H loss from toluene ra... more The role of tropylium ion Cl'r +) versus benzylium ion (Bz +) formation by H loss from toluene radical cation (TOL +) was studied by time-resolved photoionization mass spectrometry, by ab initio calculations and by RRKM-QET. Tr + is not formed at its thermochemical threshold owing to the presence of a reverse activation energy for the reaction leading from the cycloheptatriene radical cation (CHT +) to Tr + + H. The critical energies required to form Bz + and Tr + from TOL + are very close, being 2.18 and 2.11 eV, respectively. The computational results are in excellent agreement with experiments regarding appearance energies, heats of formation and Tr+/Bz + abundance ratios as a function of energy.

The control of chemical reactivity at the single-molecule scale offers a unique opportunity for t... more The control of chemical reactivity at the single-molecule scale offers a unique opportunity for the design and fabrication of advanced nanodevices.1-3 As a prototypical reaction, tautomerization has been extensively explored as a promising tool for manipulating the single-molecule conductance of tetrapyrrole macrocycles absorbed on a surface.4-8 However, the resulting molecular devices were determined to undergo dynamic, spontaneous interconversions even under cryogenic conditions, so that control of tautomeric switching between well-de ned and speci c states remains challenging. Here, we report the design of a reversible single-molecule switch based on an enol-keto tautomerization reaction, which demonstrates controllable bistability and inhibits spontaneous interconversion even at room temperature. Such control is achieved by modulating the potential energy surface (PES) through a bias-triggered charge injection process. Our results reveal that the device operates through switching between two distinct redox-related PESs with opposite thermodynamic driving forces, i.e., one exhibits strong preference for the conducting enol form, while the other exhibits a preference for the insulating keto form. The described switching mechanism constitutes a promising approach to achieve robust switching devices at the single-molecule scale. Main Text The ability to modulate the electrical signal passing through devices by switching between two or more well-de ned states is essential for the design of logical circuits in data storage and processing. 9 Control of chemical reactions via inherent electronic triggers offers a promising switching mechanism at the single-molecule scale, thus opening an avenue for molecular-scale devices with bistability. 2 As a

Canadian Journal of Chemistry, 1986

An intrinsic selectivity is defined for identity SN2 reactions (X− + RX → XR + X−). This selectiv... more An intrinsic selectivity is defined for identity SN2 reactions (X− + RX → XR + X−). This selectivity parameter is shown to yield information about: (a) the average looseness of the TS geometry in a reaction series; and (b) the sensitivity of the reaction series to geometric loosening.

Physical review, Jul 15, 1996

We calculate that in cyano-substituted poly͑phenylene vinylene͒, CN-PPV, and poly͓2-methoxy, 5-͑2... more We calculate that in cyano-substituted poly͑phenylene vinylene͒, CN-PPV, and poly͓2-methoxy, 5-͑2Ј-ethyl-hexyloxy͒phenylene vinylene͔, MEH-PPV, the lowest-energy arrangement has the chains parallel, with the perpendicular distance between chains 3.4 Å in the former case, 4.1 Å in the latter. Although there is evidence that most of the excitations created by above-band-gap light are polaron pairs or excimers in both cases, only those in CN-PPV luminesce because the smaller interchain distance results in a much larger matrix element for emission. ͓S0163-1829͑96͒50528-X͔ PHYSICAL REVIEW B

Journal of Molecular Modeling, Sep 24, 2022

Journal of the Chemical Society, 1997

Journal of the Chemical Society, 1988

Journal of the American Chemical Society, Feb 1, 1988

Journal of the American Chemical Society, 1980

One can account for the data by assuming that ISC is not rate determining for la,b, but is rate d... more One can account for the data by assuming that ISC is not rate determining for la,b, but is rate determining for 8: siou h b l-'[la,bl-+ product ' 181 % '[8] product This appears unlikely as no a priori reason for this change in mechanism is immediately apparent. Another interpretation of the data is that triplets la,b cyclize to the excited triplet state of the acenaphthene.

Journal of the American Chemical Society, May 1, 1979

C2H2, 14), calcd for C&~oOsi 126.0501, obsd 126.0498. The formation of 9 is easily rationalized w... more C2H2, 14), calcd for C&~oOsi 126.0501, obsd 126.0498. The formation of 9 is easily rationalized when it is recognized that the methoxyl oxygen of 6 is in close proximity to an undoubtedly polar silicon-carbon double bond. Silicon-oxygen bond formation to produce zwitterion 8 would render the methoxyl methyl labile with regard to involvement in the elimination of tetramethylsilane.6 Oxasilin 9 is the second example of this ring system to be reported. Weber obtained 2-methoxy-2,3,6-trimethyl-l-oxo-2-silacyclohexa-3,5-diene (10) in yields of ''. ,. never better than a few percent. , ." from the reaction of methoxymeth-M e 'si: t M e M e n M e

Chemistry: A European Journal, Dec 5, 2016

Organometallics, Nov 30, 2005

... Mårten Ahlquist, † Sebastian Kozuch, ‡ Sason Shaik, ‡ David Tanner, † and Per-Ola Norrby* †. ... more ... Mårten Ahlquist, † Sebastian Kozuch, ‡ Sason Shaik, ‡ David Tanner, † and Per-Ola Norrby* †. ... Ilaria Ambrogio, Giancarlo Fabrizi, Sandro Cacchi, Signe Teuber Henriksen, Peter Fristrup, David Tanner and Per-Ola Norrby. Organometallics 2008 27 (13), 3187-3195. ...

Pure and Applied Chemistry, 1991

The whole may be understood by its reconstruction from building block components: this is a LEG0 ... more The whole may be understood by its reconstruction from building block components: this is a LEG0 principle. A curve crossing diagram is a model for the reconstruction of energy profiles from Valence Bond (VB) building blocks, and may be considered as the quantum chemical analog of Robinson's curved arrow mnemonic. There are two archetypal diagrams which describe chemical transformations: (a) a twc-curve diagram in which the spine is the crossing of the two spin-paired covalent forms of the bonds which interchange during the transformation, and (b) a three-curve diagram in which the principal curves (in (a)) are horizontally crossed by an intermediate curve. The two archetypal diagrams constitute a unifying framework for conceptualization and prediction of reactivity patterns, reaction mechanisms and structural trends related to "hypervalency" and "hypercoordination". Examples are discussed.

Journal of the American Chemical Society, Mar 1, 1978

Journal of Solid State Chemistry, Sep 1, 1980

The electronic structures of monomeric NbSed6-, dimeric Nb,Se a'-, and hypothetical and real one-... more The electronic structures of monomeric NbSed6-, dimeric Nb,Se a'-, and hypothetical and real one-, two-, and three-dimensional structures composed of NbSe, chains are examined in order to probe the nature of intra-and interchain interactions in this material. The geometrical deformation from perfect trigonal prismatic coordination is crucial, leading to an oxidation state formalism Nbt"(Se*-)(Se**-). The dimensionality of the conduction bands of NbSes is explored.

Acta Chemica Scandinavica, 1990

Chemistry: A European Journal, Jun 21, 2004

Inorganic Chemistry, Apr 1, 1986

Plots of kET vs. Tare shown in Figure 8. At high temperatures a more or less exponential relation... more Plots of kET vs. Tare shown in Figure 8. At high temperatures a more or less exponential relationship is observed. The values for kET (Tm) and AE, determined from this region, are contained in Table IV. They have to be considered as a lower limit to the true activation energy. The reason for the deviations from an Arrhenius type behavior at low temperatures lies in the dominance of alternative processes with very low activation barriers. One likely possibility are transfer processes to the killer traps from their near Ni2+ neighbors. The overall transfer rate kET from the nickel system to the quenching traps may be limited by the individual Ni2+-Ni2+ steps, i.e. diffusion within the Ni2+ system, or the trapping step ma) be limiting. Our data do not allow a clear assignment to one of these limiting cases, because the nature of the killer traps is not known. However, an activation barrier is expected for the Ni2+-Ni2+ step on several grounds. Resonant energy transfer depends on the spectral overlap between the donor emission and acceptor absorption bands. As a result of the geometrical expansion in the relaxed excited state and the concomitant Stokes shift, only the very weak electronic origins contribute to the spectral overlap below 10 K. This would be the most obvious reason for a thermal activation of the Ni2+-Ni2+ transfer. In antiferromagnetically ordered materials, and there are antiferromagnetic correlations along the chains in CsNiX, type compounds up to high temperature^,,^ there is an additional 25. 1201-1209 1201 barrier. Excitation transfer between nearest neighbors is doubly spin-forbidden. This can be overcome by magnon-assisted processes, which require finite magnon populations and thus lead to a thermal activation. Finally, the trigonal ,E, (T2J component may be highly more efficient to promote purely excitonic energy transfer than ,A,, (TtJ, which again would lead to a thermally activated process. This mechanism was recently postulated to be mainly responsible for the energy-transfer behavior in several antiferromagnetic Mn2+ compounds.35 The observed activation energies in CsNiC1, and CsNiBr, are similar in magnitude to those determined in [(CH,),N] MnC13,36 RbMnCI,, and C S M~B~, .~~ Relaxation in the excited state and transfer via a high-energy exciton are both likely mechanisms for the observed thermal barriers in the Ni2+ systems.