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Hyperconjugative Interactions (hyperconjugative + interaction)
Selected AbstractsThe effect of carbonyl group in the asymmetry of 3, 4JCH coupling constants in norbornanonesMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2008Francisco P. dos Santos Abstract A rationalization of the known difference between the 3, 4J and 3, 4J couplings transmitted mainly through the 7-bridge in norbornanone is presented in terms of the effects of hyperconjugative interactions involving the carbonyl group. Theoretical and experimental studies of 3, 4JCH couplings were carried out in 3- endo - and 3- exo -X-2-norbornanone derivatives (X = Cl, Br) and in exo - and endo -2-noborneol compounds. Hyperconjugative interactions were studied with the natural bond orbital (NBO) method. Hyperconjugative interactions involving the carbonyl ,* and ,* antibonding orbitals produce a decrease of three-bond contribution to both 3, 4J and 3, 4J couplings. However, the latter antibonding orbital also undergoes a strong , , ,* interaction, which defines an additional coupling pathway for 3, 4J but not for 3, 4J. This pathway is similar to that known for homoallylic couplings, the only difference being the nature of the intermediate antibonding orbital; i.e. for 3, 4J it is of ,*-type, while in homoallylic couplings it is of ,*-type. Copyright © 2007 John Wiley & Sons, Ltd. [source] AIM and NBO analysisMAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2010In the series of diaminoenones, large high-frequency shifts of the 1H NMR of the NH group in the cis -position relative to the carbonyl group suggests strong NH···O intramolecular hydrogen bonding comprising a six-membered chelate ring. The NH···O hydrogen bond causes an increase of the 1J(N,H) coupling constant by 2,4 Hz and high-frequency shift of the 15N signal by 9,10 ppm despite of the lengthening of the relevant NH bond. These experimental trends are substantiated by gauge-independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3-bis(isopropylamino)-1-(aryl)prop-2-en-1-one (12) for conformations with the Z - and E -orientations of the carbonyl group relative to the NH group. The effects of the NH···O hydrogen-bond on the NMR parameters are analyzed with the atoms-in-molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the NH···O hydrogen bond as compared with that of 1,1-di(pyrrol-2-yl)-2-formylethene (13) where NH···O hydrogen bridge establishes a seven-membered chelate ring, and the corresponding 1J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) ,,*NH hyperconjugative interaction is weakened on going from the six-membered chelate ring to the seven-membered one due to a more bent hydrogen bond in the former case. A dominating effect of the NH bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the 1J(N,H) value as a consequence of the NH···O hydrogen bonding in the studied diaminoenones. Copyright © 2010 John Wiley & Sons, Ltd. [source] ,-Carbonyl Substituent Effect on the Lifetimes of Triplet 1,4-Biradicals from Norrish-Type-II ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2006Xichen Cai Dr. Abstract Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from ,-heteroatom-substituted ,-branched butyrophenones 1,6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2,4 (R,=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 ,s). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 ,s. Photolyses of the ,-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid CO bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3,LYP/6,31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing ,-oxy-substituted over ,-amino-substituted radicals. [source] On the Reactivity of C(sp3),H ,-Bonds: Oxygenation with Methyl(trifluoromethyl)dioxiraneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 3 2008Rossella Mello Abstract The reactivity of C,H ,-bonds of a series of 2-substituted adamantanes 2 towards methyl(trifluoromethyl)dioxirane (1) shows a consistent dependence on the electron-withdrawing ability, either inductive or by resonance, of the substituent. The results are interpreted in terms of the ability of the substrate molecule to delocalize the electronic perturbation of the reacting center at the beginning of the reaction path. The model shows that the electronic demand from the reacting C,H ,-bond is transmitted along the substrate through a chain of hyperconjugative interactions, the relative intensities of which depend on the ,-bonds involved. The substrate molecule simultaneously provides positive and negative stabilizing hyperconjugative interactions to the reacting center, their balance defining the geometry of the system at the beginning of the reaction path. The model constitutes a new experimental approach to measurement of the perturbation induced by substituents with significant resonance contributions on an adjacent C,H ,-bond. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Relative energies of conformations and sulfinyl oxygen-induced pentacoordination at silicon in 4-bromo- and 4,4-dibromo-4-silathiacyclohexane 1-oxide: A computational studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005Fillmore Freeman Abstract The equilibrium geometries and relative energies of the chair, twist, and boat conformations of cis - and trans -4-bromo-4-silathiacyclohexane 1-oxide and 4,4-dibromo-4-silathiacyclohexane 1-oxide have been calculated at the B3LYP/6-311G(d,p) and MP2/6-311+G(d,p) theoretical levels. The axial (SO) chair conformers of the sulfoxides are of lower energy than the chair conformers of the corresponding equatorial (SO) sulfoxides. The chair conformer of the axial (SO) trans -4-bromo-4-silathiacyclohexane 1-oxide is only 0.10 kcal/mol more stable than the corresponding 1,4-boat conformer which is stabilized by a transannular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The 1,4-boat structure of equatorial (SO) trans -4-bromo-4-silathiacyclohexane 1-oxide is a transition state and is 5.77 kcal/mol higher in energy than the respective chair conformer. The 1,4-boat conformer of axial (SO) 4,4-dibromo-4-silathiacyclohexane 1-oxide is also stabilized by transannular coordination of the sulfinyl oxygen and silicon, but it is 4.31 kcal/mol higher in energy than the corresponding chair conformer. The relatively lower stability of the 1,4-boat conformer of 4,4-dibromo-4-silathiacyclohexane 1-oxide may be due to repulsive interactions of the axial halogen and sulfinyl oxygen atom. The relative energies of the conformers and transition states are discussed in terms of hyperconjugative interactions, orbital interactions, nonbonded interactions, and transannular sulfinyl oxygen-silicon coordination. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] A computational study of conformational interconversions in 1,4-dithiacyclohexane (1,4-dithiane)JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2003Fillmore Freeman Abstract Ab initio molecular orbital theory with the 6-31G(d), 6-31G(d,p), 6-31+G(d), 6-31+G(d,p), 6-31+G(2d,p), 6-311G(d), 6-311G(d,p), and 6-311+G(2d,p) basis sets and density functional theory (BLYP, B3LYP, B3P86, B3PW91) have been used to locate transition states involved in the conformational interconversions of 1,4-dithiacyclohexane (1,4-dithiane) and to calculate the geometry optimized structures, relative energies, enthalpies, entropies, and free energies of the chair and twist conformers. In the chair and 1,4-twist conformers the CHax and CHeq bond lengths are equal at each carbon, which suggest an absence of stereoelectronic hyperconjugative interactions involving carbon,hydrogen bonds. The 1,4-boat transition state structure was 9.53 to 10.5 kcal/mol higher in energy than the chair conformer and 4.75 to 5.82 kcal/mol higher in energy than the 1,4-twist conformer. Intrinsic reaction coordinate (IRC) calculations showed that the 1,4-boat transition state structure was the energy maximum in the interconversion of the enantiomers of the 1,4-twist conformer. The energy difference between the chair conformer and the 1,4-twist conformer was 4.85 kcal/mol and the chair-1,4-twist free energy difference (,G°c-t) was 4.93 kcal/mol at 298.15 K. Intrinsic reaction coordinate (IRC) calculations connected the transition state between the chair conformer and the 1,4-twist conformer. This transition state is 11.7 kcal/mol higher in energy than the chair conformer. The effects of basis sets on the 1,4-dithiane calculations and the relative energies of saturated and unsaturated six-membered dithianes and dioxanes are also discussed. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 909,919, 2003 [source] Computational study of the chair,chair interconversion and stereoelectronic interactions in 1,2,3-trithiacyclo-hexane (1,2,3-trithiane)JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2004Fillmore Freeman Abstract Ab initio theory, density functional theory (DFT) and Møller,Plesset perturbation theory (MP2) with the 6,31G(d), 6,31++G(d), 6,31G(d,p), 6,31+G(d,p), 6,31++G(d,p), 6,311G(d,p) and 6,311+G(d,p) basis sets were used to study stereoelectronic hyperconjugative interactions and the mechanism of the chair,chair conformational interconversion in 1,2,3-trithiacyclohexane (1,2,3-trithiane). The relative energies, enthalpies, entropies, free energies and structural parameters of the chair, 1,4-twist and 2,5-twist conformers, a distorted 1,4-boat transition state and a 2,5-boat transition state were calculated. The HF calculated energy difference (,E) between the chair conformer of 1,2,3-trithiane and the distorted 1,4-boat transition state was 10.59,kcal,mol,1 (1 kcal=4.184,kJ). The 1,4-twist conformer and the 2,5-boat transition state are close in energy, as are the 2,5-twist conformer and the distorted 1,4-boat transition state. B3LYP/6,311+G(d,p) calculated the chair conformer of 1,2,3-trithiane to be 5.83, 10.09, and 5.96,kcal,mol,1, respectively, lower in energy than the 1,4-twist conformer, 2,5-twist conformer and 2,5-boat transition state. Intrinsic reaction coordinate (IRC) calculations were used to connect the transition state between the chair conformer and the 1,4-twist conformer. B3LYP/6,31+G(d,p) and B3LYP/6,311+G(d,p) calculated this transition state to be 14.25,kcal,mol,1 higher in energy than the chair conformer. In the chair conformer, the respective C4,H and C6,H bond lengths are equal, but the C5,Heq bond is longer than the C5,Hax bond. In the 1,4-twist conformer, the C4,Hiso bond lengths are equal, the C5,H,eq bond is longer than the C5,H,ax bond and the C6,H bond lengths are equal. In the 2,5-twist conformer, equal C,H bond lengths are found at C4 and at C5, but the C6,H,eq bond is longer than the C6,H,ax bond. Copyright © 2003 John Wiley & Sons, Ltd. Additional material for this paper is available in Wiley Intersciene [source] NMR spin-spin couplings involving nuclei in the neighborhood of a carbonyl group.MAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2008-substituted acetamides, 3JCH couplings in Abstract In this work 3JCH spin-spin coupling constants (SSCCs) for the cis- and trans- conformers for ,-X-acetamides (X = F, Cl, Br and CN) (1,4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis -conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3JCH SSCCs and its potential as a probe to study the stereochemical properties of the XH2C group are discussed. Copyright © 2008 John Wiley & Sons, Ltd. [source] The effect of carbonyl group in the asymmetry of 3, 4JCH coupling constants in norbornanonesMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2008Francisco P. dos Santos Abstract A rationalization of the known difference between the 3, 4J and 3, 4J couplings transmitted mainly through the 7-bridge in norbornanone is presented in terms of the effects of hyperconjugative interactions involving the carbonyl group. Theoretical and experimental studies of 3, 4JCH couplings were carried out in 3- endo - and 3- exo -X-2-norbornanone derivatives (X = Cl, Br) and in exo - and endo -2-noborneol compounds. Hyperconjugative interactions were studied with the natural bond orbital (NBO) method. Hyperconjugative interactions involving the carbonyl ,* and ,* antibonding orbitals produce a decrease of three-bond contribution to both 3, 4J and 3, 4J couplings. However, the latter antibonding orbital also undergoes a strong , , ,* interaction, which defines an additional coupling pathway for 3, 4J but not for 3, 4J. This pathway is similar to that known for homoallylic couplings, the only difference being the nature of the intermediate antibonding orbital; i.e. for 3, 4J it is of ,*-type, while in homoallylic couplings it is of ,*-type. Copyright © 2007 John Wiley & Sons, Ltd. [source] Non-empirical calculations of NMR indirect carbon,carbon coupling constants.MAGNETIC RESONANCE IN CHEMISTRY, Issue 8 2004Monocycloalkanes, Part Abstract Carbon,carbon and carbon,hydrogen spin,spin coupling constants were calculated in the series of the first six monocycloalkanes using SOPPA and SOPPA(CCSD) methods, and very good agreement with the available experimental data was achieved, with the latter method showing slightly better results in most cases, at least in those involving calculations of J(C,C). Benchmark calculations of all possible 21 coupling constants J(C,C), J(C,H) and J(H,H) in chair cyclohexane revealed the importance of using the appropriate level of theory and adequate quality of the basis sets. Many unknown couplings in this series were predicted with high confidence and several interesting structural trends (hybridization effects, multipath coupling transmission mechanisms, hyperconjugative interactions) were elucidated and are discussed based on the present calculations of spin,spin couplings. Copyright © 2004 John Wiley & Sons, Ltd. [source] Computation and analysis of 19F substituent chemical shifts of some bridgehead-substituted polycyclic alkyl fluoridesMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2003William Adcock Abstract The 19F NMR shieldings for several remotely substituted rigid polycyclic alkyl fluorides with common sets of substituents covering a wide range of electronic effects were calculated using the DFT-GIAO theoretical model. The level of theory, B3LYP/6,311+G(2d,p), was chosen based on trial calculations which gave good agreement with experimental values where known. The optimized geometries were used to obtain various molecular parameters (fluorine natural charges, electron occupancies on fluorine of lone pairs and of the CF bond, and hybridization states) by means of natural bond orbital (NBO) analysis which could help in understanding electronic transmission mechanisms underlying 19F substituent chemical shifts (SCS) in these systems. Linear regression analysis was employed to explore the relationship between the calculated 19F SCS and polar substituent constants and also the NBO derived molecular parameters. The 19F SCS are best described by an electronegativity parameter. The most pertinent molecular parameters appear to be the occupation number of the NBO p-type fluorine lone pair and the occupation number of the CF antibonding orbital. This trend suggests that in these types of rigid saturated systems hyperconjugative interactions play a key role in determining the 19F SCS. Electrostatic field effects appear to be relatively unimportant. Copyright © 2003 John Wiley & Sons, Ltd. [source] | ||||||||||