Fragment interaction analysis in the framework of ab-initio UHF-MO computations

Bernardi, Fernando; Bottoni, Andreà; Fossey, Jacques; Sorba, J.

doi:10.1016/s0040-4020(01)88161-8

Cited by 19 publications

(11 citation statements)

References 37 publications

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“…An early INDO calculation 43 showed that CH 2 ClCH 2 has the asymmetrically bridged structure. However, later ab initio studies [31][32][33][34][35][36][37][38][39][40][41][42] showed no evidence of asymmetric bridging. increase with the torsion angle to minimize the repulsion rather than decrease as observed in Figure 4.…”

Section: Results For β-Substitutedmentioning

confidence: 99%

“…Since the experimental results hardly distinguish the possibilities, many theoretical calculations have been performed to determine the most stable structures. Most theoretical studies have been devoted to the β-fluoroethyl radical (CH 2 FCH 2 • ) [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] and to the β-chloroethyl radical (CH 2 ClCH 2 • ) [31][32][33][34][35][36][37][38][39][40][41][42][43] with very little attention to the β-bromoethyl radical (CH 2 BrCH 2 • ) 35 and β-iodoethyl radical (CH 2 ICH 2 • ).…”

Section: Methodsmentioning

confidence: 99%

“…However, later ab initio studies [31][32][33][34][35][36][37][38][39][40][41][42] showed no evidence of asymmetric bridging. Figure 4 displays the changes in the structural parameters as a function of the torsion angle.…”

Section: Calculationsmentioning

confidence: 99%

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Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

1999

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Radicals such as CH 2 XCH 2• , where X is a halogen, play an important role in the stereochemical control observed in many chemical reactions. To elucidate the origin of the stereoselectivity, we calculated the structures and potential energy surfaces of the haloethyl radicals (CH 2 XCH 2 • , X ) F, Cl, Br, I) using ab initio quantum mechanics [HF, local MP2, DFT (both B3PW91 and B3LYP)]. We find that the CH 2 BrCH 2• and CH 2 ICH 2• radicals strongly favor the symmetrically bridged structures while the CH 2 ClCH 2 • radical leads to similar energy for symmetric bridging and classical structures. (In contrast, X ) H and F leads to dramatically different structures). This confirms the Skell hypothesis of symmetric bridging to explain the stereochemical control of the CH 2 BrCH 2• and CH 2 ICH 2 • radicals, indicating that such bridged structures play an important role in the dissociation processes involving CH 2 XCH 2• with X ) Cl, Br, and I. The trends in the rotational barriers and structural parameters are consistent with hyperconjugation between the singly occupied carbon 2p orbital and the σ*(C-X) MO. We find that the rotational barrier, bridged structure, and dissociation of the radicals are described much more accurately using DFT (with GGA) than with HF or LMP2. IntroductionThe class of halo radicals such as CH 2 XCH 2 • is important in a number of chemical processes [1][2][3][4] and determines the stereoselectivity of the reaction products from halogenation 1,2,5-9 of alkenes and alkanes. Their role in stereoselective control is determined by whether the radical is classical (Figure 1c) or bridged and if bridged whether the structure is symmetric ( Figure 1a) or asymmetric (Figure 1b). The possible minima and transition structures for rotation around the C-C bond are schematically represented in Figure 2. Anti (I) and gauche (III) rotamers are candidates for local energy minima on the rotational energy surface because Pauli repulsion between bonding pairs would be minimized at these two conformations. The other two structures (II and IV) possessing eclipsed bonds are also possible transition states on the rotational energy surface.If the structure of the radical is bridged, we expect retention of the stereochemistry. However, if the radical prefers a classical asymmetric conformation, additional conditions must be fulfilled to exert stereochemical control. Namely, a strong preference for high population of the anti conformer and nonplanarity of the radical center are required. Experimental 10-20 and theoretical 21-43 studies have led to a good understanding of the structure and energetics for the cases of X ) H, F, and Cl; however, little is known about X ) Br and I. The motivation for our studies was to elucidate the X ) Br and I systems. But we also studied X ) H, F, Cl in cases to compare to previous experiments and theory.In this paper we use four techniques of first principles quantum mechanics (QM) methods to examine the potential surface for CH 2 XCH 2 • with X ) H, F, Cl, Br, and I. These QM met...

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Section: Results For β-Substitutedmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

1999

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“…However, the CH2CF2H radical has two conformations, one in which the P hydrogen nearly eclipses with the SOMO and the other in which one of the fluorine atoms is in the plane of the SOMO. The strong geometrical preference observed in both the CH2CF2C1 and CH2CH2C1 radicals is a result of the strong two-center one-electron hyperconjugative interaction between the SOMO and the cr*CC1 antibonding orbital (20). Comparison of the equilibrium structures reveals that the C-Cl bond distance in the CH2CF2Cl radicals is 0.039 A shorter than in the CH2CH2Cl radical, and that the C-F bond distance in…”

Section: -Chloro-i -Difluoroethyl Radical Ch2cf2cl (2)mentioning

confidence: 91%

An ab initio study of the structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the hydrochlorofluorocarbon CH₃CF₂Cl and the corresponding radical CH₂CF₂Cl

Paddison¹,

Chen²,

Tschukow-Roux³

1994

Can. J. Chem.

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Ab initio molecular orbital calculations were performed using the GAUSSIAN 90 system of programs at the HF/6-31G* level of theory, on the hydrochlorofluorocarbon (HCFC) 1-chloro-1,1-difluoroethane and the 1-chloro-1,1-difluoroethyl radical. Equilibrium geometries, barriers for internal rotation, and harmonic vibrational frequencies were thus calculated. A single conformational minimum in the potential energy surface was located for both the radical and the parent molecule. The radical center in CH2CF2Cl was found to be nonplanar. Transition structures for internal rotation about the C—C bond were located for both the molecule and the radical using analytical methods. The rotation barriers, evaluated at the fourth-order Møller–Plesset perturbation theory ((U)MP4/6-311G**/6-31G*). were calculated after inclusion of zero-point vibrational energy differences to be 1.11 and 4.12 kcal/mol for the radical and the parent molecule, respectively. Computed thermodynamic properties including heat capacity, entropy, enthalpy, and free energy functions are reported as a function of temperature. Using an experimentally measured heat of formation of CH3CF2Cl at 298 K, the heat of formation of CH2CF2Cl was calculated to be −74.3 ± 1.7 kcal/mol. Tabulations of ΔH0f,T, ΔG0f,T, and log10Kf,T over the temperature range of 0–1500 K are also reported for both species.

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“…32 ' 96 As a result of this, the electronegativity of X has only a weak influence on the relative stabilities of the (E) and (Z)-conformations. If X includes Second period atoms, then virtually free rotation about the C(l)-C(2) bond is observed in radicals, 60 ' 97 " 99 but if X contains Third Period atoms 60 the (Z)-conformation is more stable owing to the greater polarisability of the C-X bonds. Analysis showed that under these conditions an increase in the electronegativity of the group X entails a decrease in the contribution of the θςχ-pt interaction to the stabilisation of the (Z)-conformation, while the contribution of the * * .…”

Section: Structural and Energetic Manifestations Of Hyperconjugamentioning

confidence: 99%

Stereoelectronic effects in silicon, phosphorus, and sulfur molecules. Quantum-chemical calculations and qualitative orbital models

Korkin¹

1992

Russ. Chem. Rev.

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Fragment interaction analysis in the framework of ab-initio UHF-MO computations

Cited by 19 publications

References 37 publications

Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

An ab initio study of the structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the hydrochlorofluorocarbon CH₃CF₂Cl and the corresponding radical CH₂CF₂Cl

Stereoelectronic effects in silicon, phosphorus, and sulfur molecules. Quantum-chemical calculations and qualitative orbital models

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Fragment interaction analysis in the framework of ab-initio UHF-MO computations

Cited by 19 publications

References 37 publications

Conformations and Barriers of Haloethyl Radicals (CH2XCH2, X = F, Cl, Br, I): Ab Initio Studies

Conformations and Barriers of Haloethyl Radicals (CH2XCH2, X = F, Cl, Br, I): Ab Initio Studies

An ab initio study of the structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the hydrochlorofluorocarbon CH3CF2Cl and the corresponding radical CH2CF2Cl

Stereoelectronic effects in silicon, phosphorus, and sulfur molecules. Quantum-chemical calculations and qualitative orbital models

Contact Info

Product

Resources

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Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

Conformations and Barriers of Haloethyl Radicals (CH₂XCH₂, X = F, Cl, Br, I): Ab Initio Studies

An ab initio study of the structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the hydrochlorofluorocarbon CH₃CF₂Cl and the corresponding radical CH₂CF₂Cl