Showing papers by "Paul von Ragué Schleyer published in 1980"

Stable carbocations. 225. Proton and carbon-13 NMR spectroscopic study of 9-fluorenyl cations

Abstract: A series of 9-substituted 9-fluorenyl cation were prepared and characterized by 1 H and 13 C NMR spectroscopy Unsuccessful attempts were made to observe intramolecular interconversion of 9-methyl-9-fluorenyl cation via capped pyramidal ions with ring deuterated and methylated analogues MINDO/3 calculations on isomeric structures of cyclopentadienyl, indenyl, and fluorenyl cations indicated strongly decreasing relative stabilities of the pyramidal forms due to benzoannulation In deuterated fluorosulfonic acid solution, the 9-methyl-9-fluorenyl (1-CH 3 ) and 39-dimethyl-9-fluorenyl cations (14) underwent hydrogen-deuterium exchange consistent with a protonation-deprotonation mechanism

Small elemental clusters. I. The structures of Be2, Be3, Be4, and Be5

Abstract: The geometries and energies of beryllium clusters up to Be5 are examined using ab initio molecular orbital theory. Allowances are made for electron correlation with Moller—Plesset perturbation theory to fourth order. Correlation is found to have a dramatic effect on the relative energies of the several structures examined for Be4 and Be5. Furthermore, the effect of d-type basis functions on the correlation energy results in an increased binding energy for the clusters. Be2 is only weakly bound. For Be3, the best estimate of the binding energy is 6 kcal/mole for the singlet equilateral triangle. Be4 is tetrahedral in its ground state and the estimated binding is 56 kcal/mole. The best structure for Be5 is a singlet trigonal bipyramid, and the binding energy is 88 kcal/mole at the highest level of theory used.

Geometries and Energies of Dilithioethylene Isomers and of Vinyl Lithium. An Ab Initio Study

Abstract: The structures (4-31G optimization) and energies (6-31G*) of vinyl lithium (5) and of the 1,1 (3 and 4), cis-1,2 (6), trans-1,2 (7) and doubly bridged (8) dilithioethylenes have been calculated using standard ab initio methods. Vinyl lithium (5), indicated to have a classical geometry, is about 11 kcal/mole more stable than ethyl lithium in an hydride exhange reaction (Eq. (2)). Singlet trans-1,2-dilithioethylene, the most stable C2H2Li2 isomer (see also addendum), has a highly distorted, partially bridged structure (7a) with < CCLi = 87°. Symmetrically bridged 8 and the cis-isomer 6 are considerably less stable, 54 and 19 kcal/mole, respectively. The partial Li-H bonding in 7a contributes significantly to this behavior. 1,1-Dilithioethylene is found to prefer a perpendicular (3) over the usual planar (4) geometry both in singlet and in triplet states. Triplet symmetrically bridged 8 and triplet cis dilithioethylene (6) are indicated to be local minima, but are less stable than singlet trans 7a. Cis-dilithioethylene (6) is indicated to be thermodynamically unstable towards dissociation into Li2 and acetylene. This factor, and the low lying triplet state, may contribute to the experimental difficulties in preparing 6. The nature of the carbon–lithium bonding in these molecules is discussed. We find that an electrostatic model cannot account for the structural details or for the relative energies of the C2H2Li2 isomers. The importance of the multicenter covalent nature of bonding to lithium is emphasized.

The structure of carbon trioxide

Abstract: Open (1) and cyclic (2) singlet forms of CO3 were investigated by means of ab initio calculations. At the highest level of theory employed, MP2/6-31G* (which includes the effects of electron correlation), 2 was indicated to be much more stable than 1 and thermodynamically stable toward dissociation into CO2 and O(3P). The open form 1 has a long OO bond and can be regarded as a weak dative complex between CO2 and a singlet oxygen atom.

Dissoziative Ringöffnung halogensubstituierter Methylcyclopropan‐Radikalkationen in der Gasphase

Abstract: Die unimolekularen Zerfalle einiger isomerer C4H7Br+˙-Verbindungen werden in der Gasphase studiert. Die Br˙-Abspaltung aus ionisiertem Cyclobutylbromid (6) und 2- bzw. 3-Methylallylbromid (9 bzw. 10) erfolgt ohne Isomerisierung des entstehenden C4H7+-Ions. Bei allen ubrigen Verbindungen ist die dissoziative Ionisation mit einer Isomerisierung des Kations verbunden. Diese Umlagerung, die vor der eigentlichen Br˙-Eliminierung eintritt, manifestiert sich in einer Linienverbreiterung der Ubergangssignale („kinetic energy release”). Die resultierenden C4H7+-Kationen lassen sich anhand ihrer Stosaktivierungsspektren charakterisieren. Aus den Radikalkationen der 1- und 2-Methylbromcyclopropane entsteht bei der Br˙-Abspaltung das 1-Methylallylkation und nicht das 2-Methylallylkation. MNDO-Rechnungen fur die analogen C4H7Cl+˙-Radikalkationen unterstutzen die bei den Bromiden gemachten experimentellen Befunde; auserdem enthullen diese Rechnungen mechanistische Details der unimolekularen Reaktionen. Dissociative Ring Opening of Halogen Substituted Methylcyclopropane Cation Radicals in the Gas Phase The unimolecular gas phase reactions of several C4H7Br+˙ isomers are investigated. Rearrangement of the incipient C4H7+ ion is observed prior to Br˙ loss from all C4H7Br+˙ isomers except ionised 2- (9) and 3-methylallyl- (10) and cyclobutylbromides (6). This is evidenced by the increased average kinetic energy release associated with Br˙ elimination. In addition, the structure of the C4H7+ ions may be established using collisional activation spectroscopy. Ionised 1- and 2-methylcyclopropylbromides both yield 1-methylallyl cation rather than 2-methylallyl cation. MNDO calculations on the analogous C4H7Cl+˙ isomers lead to similar conclusions; moreover, these calculations shed light on the probable mechanism for both isomerization and decomposition of the molecular ions.

Hydrogenolysis of alkyl‐substituted adamantanes, diamantanes, and triamantanes in the gas phase on a nickel‐alumina catalyst

Abstract: Dealkylation of several alkyladamantanes, diamantanes, and triamantanes has been observed in the gas phase with hydrogen at atmospheric pressure on a 30% nickel-alumina catalyst. Yield-temperature profiles show that the optimum temperature for obtaining the pure parent compound in high yield can be as low as 215 °C. The ease of removal of an alkyl substituent depends on whether it is secondary or tertiary on the diamondoid nucleus. Rupture of the diamondoid skeleton requires much higher temperatures (over 280 °C) than dealkylation. The hydrogenolytic degradation of diamantane to adamantane was examined with the aid of molecular mechanics calculations; likely pathways for CC bond cleavage were deduced which are consistent with the several intermediates detected experimentally. The mechanism of dealkylation is discussed in terms of steric and thermodynamic factors and the nature of possible surface intermediates. Hydrogenolyse von alkylsubstituierten Adamantanen, Diamantanen and Triamantanen mit Nickel-Aluminiumoxid-Katalysatoren in der Gasphase Bei der Gasphasen-Hydrogenolyse verschiedener Alkyladamantane, Diamantane und Triamantane unter Normaldruck mit Wasserstoff an einem Nickel-Aluminiumoxid-Katalysator wurde glatte Dealkylierung beobachtet. Die optimale Temperatur zur Erzeugung des reinen Grundkorpers in guten Ausbeuten kann den Temperaturprofilen entnommen werden. Die optimale Dealkylierungstemperatur zeigt starke Abhangigkeit von der Position (sekundar oder tertiar) der Alkylgruppe am Kafigmolekul. Die Zerstorung des Kafig-Grundkorpers erfolgt erst bei wesentlich hoheren Temperaturen als die Dealkylierung. Der hydrogenolytische Abbau des Diamantans zu Adamantan wurde untersucht, und der wahrscheinlichste Reaktionsweg wird mit der Hilfe von Kraft-Feld-Rechnungen diskutiert. Unter Berucksichtigung von sterischen und thermodynamischen Faktoren sowie der Natur moglicher Zwischenstufen an der Katalysatoroberflache wird ein Dealkylierungs-Mechanismus vorgeschlagen.

Theoretical study of the isomeric cyclopropylidenemethyl and 1-cyclobutenyl cations, unusually stable vinyl cations

Abstract: Ab initio and MIND0/3 calculations were employed to explore the geometries and energies of unusual vinyl cations. In agreement with experiment, both the cyclopropylidenemethyl (8) and the cyclobutenyl (4) cations were found to be highly stabilized species. The former benefits from effective hyperconjugation involving the cyclopropyl ring while the latter is indicated to be a nonclassical ion with C 3 bridging almost equidistantly between C 1 and C 2 . This 1-cyclobutenyl cation (4) is found to be significantly more stable than 8. This difference is very much reduced in the methyl-substituted isomers 10 and 11. The homopropargyl ion (9), not a minimum on the potential energy surface, should rearrange directly to the much more stable 4.

Pseudorotational isomers of Wittig intermediates. Structures and relative energies

Abstract: Ab initio molecular orbital calculations indicate the pseudorotational oxaphosphetan Witting intermediates to have unexpectedly small energy differences.

Isomeric structures of protonated ozone: A theoretical study

Abstract: Ab initio molecular orbital calculations have been used to determine the structure of protonated ozone. Four stable minima were found on the O3H+ singlet potential energy surface. Three forms correspond to ozone protonated at the central oxygen (C2v) or at the terminal oxygen (two Cs isomers, E and Z). The fourth isomer (Cs) is a derivative of trioxirane. The most stable structure is the planar E form I. The proton affinity of ozone (to give I) is given as 123.6 kcal/mole (MP2/6-31G*//4-31G). The energy difference between I and protonated trioxirane VI is greater than that between ozone and trioxirane.

An Experimental and Theoretical Study of the Degenerate Carbon Skeleton Isomerization of the Cyclopentyl Cation in the Gas Phase

Abstract: The non-classical pyramidal cation(4) provides a rational explanation of all the phenomena observed on mass-spectrometic degradation of the cyclopentyl cation (1). On generation from (2a), (2b), (3a), or (3b) (• = 13 C), (1) always gives 13 C 2 H 4 , 13 C1 2 CH 4 , 12 C 2 H 4 in the ratio of 10:60:30, i.e. all the C atoms must have become statistically equivalent prior to elimination of ethylene from (1). MINDO/3 calculations show degradation to proceed via the cations (4) and (5).

7 Li–13C n.m.r. coupling constants and the nature of the carbon–lithium bond: INDO MO calculations

Abstract: INDO molecular orbital calculations suggest that the 7Li–13C n.m.r. coupling constant for the methyl-lithium monomer should be very large; this indicates the predominantly covalent character of the C–Li bond.

Deuterium isotope effect on the e.s.r. Spectrum of annulene radicals: a reappraisal

Abstract: The deuterium isotope effect on the e.s.r. spectrum of the cyclopentadienyl radical is suggested to be the result of deuterium perturbation of the equilibrium between the positional isomers of the Jahn–Teller distorted radical and not of vibrational perturbation of the Huckel resonance integral, β.