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1.
A new model based on 1,3 repulsive steric interactions (geminal repulsion) is proposed for explaining the variation in the C-H bond strengths of the alkanes. The model builds from the assumption that 1,3 repulsive interactions are the major factor in determining the stability of a C-C or C-H bond in an alkane. From this simple premise, the model successfully reproduces the effect of branching on the stability of alkanes, alkyl radicals, and alkenes. The results suggest that geminal repulsion can provide a simple, unified explanation for these fundamental stability trends. Although previous explanations have been widely accepted, it is shown that the theoretical support for them is relatively shallow and that the current hyperconjugative stabilization model is inconsistent with several experimental and computational results concerning alkyl radicals. In contrast, an explanation based on geminal repulsion provides a general conceptual framework for rationalizing each of these stability trends and is based on a physical effect that is known to play a role in the stability of alkanes and related species. 相似文献
2.
Kinetic energy release (KER) was studied by experimental methods and semiempirical (MNDO and AM1) molecular orbital calculations in the case of various charge separation processes: loss of a methyl ion from [CH 3? C 4? CH 3] 2+, [CH 3? C 3? CH 3] 2+ and [ N,N-dimethyl- p-phenylenediamine] 2+. It was found that the KER corresponding to the width of a dish-topped peak at half-height is very close to the mean KER of the process. The calculated potential energy curves of these reactions show significant reverse critical energies, a large part of which was found to be due, in agreement with conventional assumptions, to electric repulsion between the two separating singly charged products. The bond order between the two separating ions is nearly zero in the transition state, so exchange of internal energy between them is unlikely. These explain the good agreement between the (calculated) reverse critical energy and the measured kinetic energy release. 相似文献
3.
The potential energy surface for the [CH 5N] +˙ system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence-electron correlation. Two [CH 5N] +˙ isomers can be distinguished: the well known methylamine radical cation, [CH 3NH 2] +˙, and the less familiar methylenammonium radical cation, [CH 2NH 3] +˙. The latter is calculated to lie 8 kJ mol ?1 lower in energy. A substantial barrier (176 kJ mol ?1) is predicted for rearrangement of [CH 2NH 3] +˙ to [CH 3NH 2] +˙. In addition, a large barrier (202 kJ mol ?1) is found for loss of a hydrogen radical from [CH 2NH 3] +˙ via direct N—H bond cleavage to give the aminomethyl cation [CH 2NH 2] +. These results are consistent with the existence of the methylenammonium ion [CH 2NH 3] +˙ as a stable observable species. The barrier to loss of a hydrogen radical from [CH 3NH 2] +˙ is calculated to be 140 kJ mol ?1. 相似文献
4.
We respond to a paper by Fernández, Frenking, and Uggerud (FFU: Chem. Eur. J. 2009 , 15, 2166) in which they conclude that not steric hindrance but reduced electrostatic attraction and reduced orbital interactions are responsible for the S N2 barrier, in particular in the case of more highly substituted substrates, for example, F ? + C(CH 3) 3F. We disagree with this conclusion, which we show is the result of neglecting geometry relaxation processes that are induced by increased Pauli repulsion in the sterically congested S N2 transition state. 相似文献
5.
Rate constants for the self- and cross-termination of the isopropylol radical [(CH 3) 2?OH] and its anion [(CH 3) 2?O ?] in aqueous solution are determined by kinetic electron spin resonance. Whereas the self-termination of the neutral radical occurs close to the diffusion-controlled limit, the cross- and self-terminations involving the anion are slower and reflect effects of charge repulsion and steric constraints by solvation. 相似文献
7.
Bimodal kinetic energy release distributions (KERDs) were obtained for the metastable ion reactions: CH 3C(OH) = *CH 2? + → CH 3 + *CH 3CO + and CH 3CH 2C(OH)=CH 2? + → C 2H 5 + CH 3CO +. The bimodality is attributed to incomplete intramolecular energy redistribution in the short-lived intermediate keto ions, CH 3COCH 3? + and CH 3CH 2COCH 3? +, respectivley, formed en route to the dissociation products. 相似文献
8.
B3LYP calculations in conjunction with natural bond orbital population analysis have been performed for a previtamin D model
and corresponding transition structures for the [1,7]-hydrogen migration. In addition the 19,19-difluoro, 19-methoxy and 19-fluoro
substituted analogs were investigated. The calculated activation barriers decrease in the following order: CHF 2>CH 3>CH 2OCH 3 (24.8, 23.5 and 20.1 kcal/mol). This is in qualitative agreement with experiments. It has been suggested that a decrease
of the barrier by a 19-methoxy substituent and its increase by a 19,19-difluoro substituent are phenomena of different origin.
In the case of 19-methoxy substitution, the effect is due to the charge redistribution in the triene system and the decrease
of the C(19)–H bond energy. The effect of two fluorine substituents at C-19 on the activation barrier is suggested to originate
from the combination and balance of several factors: electrostatic repulsion between the negative fluorine atom and the π-electron
cloud over the conjugated system, an increase of the HOMO–LUMO gap, and geminal difluoro substitution affecting C–F and C–C
bond energies.
Received: 17 May 2002 / Accepted: 11 September 2002 /
Published online: 14 February 2003 相似文献
9.
Protonated and deprotonated adipic acids (PAA: HOOC? (CH 2) 4? COOH 2+ and DAA: HOOC? (CH 2) 4? COO ?) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H 2O???H???OH 2) + Zundel‐like symmetric hydrogen bonding, whereas that of DAA has H 3O + Eigen‐like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel‐like ions for PAA and Eigen‐like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH???O short hydrogen‐bond stretching peaks are predicted in the range of 1000–1700 cm ?1 in the Car–Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen‐bond peaks. The O? H? O stretching peaks in the range of 1800–2700 cm ?1 become insignificant above around 150 K and are almost washed out at about 300 K. 相似文献
10.
The interaction potential index IPI(X) of 16 kinds of substituents X (X=OH, SH, NH 2, Br, Cl, I, NO 2, CN, CHO, COOH, CH 3, CH=CH 2, C≡CH, Ph, COCH 3, COOCH 3) were proposed, which are derived from the experimental enthalpies of formation Δ fH?(g) values of monosubstituted straight-chain alkanes. Based on the IPI(X) and polarizability effect index, a simple and effective model was constructed to estimate the Δ fH?(g) values of monosubstituted alkanes RX (including the branched derivatives). The present model takes into account not only the contributions of the alkyl R and the substituent X, but also the contribution of the interaction between R and X. Its stability and prediction ability was confirmed by the results of leave-one-out method. Compared with previous reported studies, the obtained equation can be used to estimate enthalpies of formation for much more kinds of monosubstituted alkanes with less parameters. Thus, it is recommended for the calculation of the Δ fH?(g) for the RX. 相似文献
11.
The anomeric effect of the functional groups X = C?N, C?CH, COOH, COO ?, O? CH 3, NH 2, and NH +3 has been studied with ab initio techniques. Geometry effects upon rotation around the central C? O bond in X? CH 2? O? CH 3 have been compared in the various compounds. The energy differences between the conformers with a gauche and trans ( X? C? O? C) arrangement were calculated at the 6-31G* level in the fully optimized 4-21G geometries. Energy differences calculated at the 4-21G level appeared not to be reliable, especially for the groups X that contain non- sp3 hybridized atoms. The 6-31G* energy differences indicate a normal anomeric effect for X = COO ?, O? CH 3, and NH 2(g +) (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, C?N, and C?CH (ca. 6 kJ/mol). For X = NH 2(t) and NH +3 a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X? C? O? C follows roughly the same trend. 相似文献
13.
The complexes of Li+, Na+, K+, Be2+, Mg2+, and Ca2+ metal cations with [N3P3R4O(CH2CH2O)4] (R?=?H(1), NMe2(2), NC(NMe2)2(3)) PNP-lariat ethers were systematically studied in the gas phase by using density functional theory (DFT) B3LYP-D3/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) method. The gas phase cation affinities were calculated to span the wide range between 64.2 and 496.1 kcal mol?1 in order K+?<?Na+?<?Li+?<?Ca2+?<?Mg2+?<?Be2+. The structural and electronic properties of 1–3 and their complexes were investigated and effects of electron-donor substituents were analyzed. The electron-donor substituents were found to promote the cation affinity. Sidearm coordinative interaction with the crown ether-complexed metal ion has been noticed. The nature of the metal–ligand interactions was investigated using Bader’s Quantum theory of atoms in molecule. It has been found that the Be2+–N bonds are partly covalent in nature while other coordinate bonds are of the electrostatic nature. The electron density at the bond critical points was found to be consistent with cation affinity. Natural bond orbital analysis was performed on the optimized geometries. The results showed that the stabilization interaction energies are caused by the donation of O/N lone pair electrons to the LP* orbitals of the metal cations. The amount of charge transfer follows the cation affinity order. The largest charge transfer and associated second-order perturbation stabilization energy were observed for Be2+ complexes. 相似文献
14.
The relative energies of 11 [C 3H 3O] + ions are calculated by different molecular orbital methods (MINDO/3, MNDO, ab initio with 3-21G and 4-31G* basis set and configuration interaction). The four most stable structures are: a ([CH 2?CH? CO] +), b c ([CH?C? CHOH] +) and d ([CH 2?C?COH] +); their relative energies at the CI/4-31G*//3-21G level are 0, 117, 171 and 218 kJ mol ?1, respectively. The isomerizations c→[CH?CH? CHO] +→[CH 2?C? CHO] +→ a and dissociations into [C 2H 3] ++CO and [HCO] ++C 2H 2 are explored. The calculated potential energy profile reveals that the energy-determining step is the 1,3-H migration c→[CH?CH? CHO] +. This explains the value of unity of the branching ratio and the spread of kinetic energy released for the two dissociation channels. 相似文献
15.
The mass spectra of deuterated species shows that both the isomeric ions [CH 2?SH] + and [CH 3? S] + are formed in the ratio 2:1 from CH 3SH; the ions [CH 3CH?SH] + and [CH 3CH 2S] + in the ratio 0·8:1 from CH 3CH 2SH; and [CH 2?OH] + and [CH 3? O] + in the ratio 6·7:1 from methanol. The heats of formation of [CH 3S] + and [C 2H 5S] + are of the order of 222 and 203 Kcal.mole ?1 respectively. The isomeric ions cannot be distinguished on thermodynamic grounds. 相似文献
16.
DFT derived conformational energy profiles of a series of β-substituted α-fluoroethanes (F-CH 2CH 2-X) have been explored where the substituent X was varied as NH 3+, OCOH, NCO, NO 2, NHCHO, F, N 3, CHNH, NCS, CHCCH 2, CH 3, CHCH 2, NC, CN, CHO, and CCH. Comparisons were correlated relative to 1,2-difluoroethane, a compound which exhibits a well known gauche preference. Only four of the compounds displayed an anti preference, with the large majority preferring a gauche conformation. In particular the influence of steric and electrostatic attraction/repulsion between the fluorine atom and the X-substituent was explored by evaluating rotational energy profiles for all compounds and separately NBO correlations were evaluated to assess the contribution of hyperconjugation to the minimised gauche and anti conformers. In the event the gauche preference for 1,2-difluoroethane was shown to have an origin due largely to σ(C-H)→σ*(C-F) hyperconjugative interactions, whereas the conformational preference for the remaining structures is rationalised by hyperconjugative as well as steric and electrostatic contributions. The anti preferred compounds 13, 14 and 16 possessed triple bonds and the preference arose due to fluorine/ p-orbital repulsion. 相似文献
17.
Ab initio and MNDO calculations of VSEPR model were carried out on CH 2, CH 2+, CH 2?, SiH 2, SiH 2+ and SiH 2?. Comparisons between the second row carbon and its third row silicon analogue as unshared electron pair contributors are considered. The repulsion effects as well as the volume requirement of the unshared electron pair on several structural and energetic properties arc investigated. 相似文献
18.
State-of-the-art ab initio studies demonstrate that the reaction Pd + + CH 3I → PdCH 2I + + H . is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy ( BDE) of ca. 83 kcal/mol for the Pd +? CH 2I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol < BDE(Pd +? CH 2I) < 92 kcal/mol. Based on these findings, the previously studied Pd +/CH 3I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH 2I + occurs stepwise via PdCH as a reactive intermediate. Further, ion/molecule reactions of PdCH 2I + with unsaturated hydrocarbons are studied, which reveal the formation of carbon–carbon bonds in the gas phase. 相似文献
19.
1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations,
elimination of CH 2=CH 2 and CH 3CH 3 from (CH 3) 2NH +CH 2CH 3 ( 1) and of CH 4 from (CH 3) 2NH 2+ are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from
methyl to the bridging position in ethyl and then to N to reach (CH 3) 2NH 2+ + CH 2=CH 2 from 1. CH 3CH 3 elimination by H-transfer to C 2H 5+ to form CH 3NH +=CH 2 + CH 3CH 3 also takes place. (CH 3) 2NH 2+ eliminates methane by CN bond extension followed by β-H-transfer to give CH 2=NH + + CH 4. Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type
of reaction. As in many complex-mediated reactions, these reactions transfer H between incipient fragments. They are distinguished
from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition
state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described
lower energy reactions, have 1,1-like transition states, or utilize highly asynchronous 1,2 transition states. All of these
avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry. 相似文献
20.
The resonance character of Cu/Ag/Au bonding is investigated in B???M?X (M=Cu, Ag, Au; X=F, Cl, Br, CH 3, CF 3; B=CO, H 2O, H 2S, C 2H 2, C 2H 4) complexes. The natural bond orbital/natural resonance theory results strongly support the general resonance‐type three‐center/four‐electron (3c/4e) picture of Cu/Ag/Au bonding, B:M?X?B +?M:X ?, which mainly arises from hyperconjugation interactions. On the basis of such resonance‐type bonding mechanisms, the ligand effects in the more strongly bound OC???M?X series are analyzed, and distinct competition between CO and the axial ligand X is observed. This competitive bonding picture directly explains why CO in OC???Au?CF 3 can be readily replaced by a number of other ligands. Additionally, conservation of the bond order indicates that the idealized relationship bB???M+ bMX=1 should be suitably generalized for intermolecular bonding, especially if there is additional partial multiple bonding at one end of the 3c/4e hyperbonded triad. 相似文献
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