A DFT‐GIAO and DFT‐NBO study of polar substituent effects on NMR 17O chemical shifts in some rigid polycyclic alkanes

¹⁷O NMR shieldings of 3‐substituted(X)bicyclo[1.1.1]pentan‐1‐ols ( 1 , Y = OH), 4‐substituted(X)bicyclo[2.2.2]octan‐1‐ols ( 2 , Y = OH), 4‐substituted(X)‐bicyclo[2.2.1]heptan‐1‐ols ( 3 , Y = OH), 4‐substituted(X)‐cuban‐1‐ols ( 4 , Y = OH) and exo‐ and endo‐ 6‐substituted(X)exo‐bicyclo[2.2.1]heptan‐2‐ols ( 5 and 6 , Y = OH, respectively), as well as their conjugate bases ( 1 – 6 , Y = O^?), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH, F, Cl, OH, NH₂, CH₃, SiMe₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model at the B3LYP/6‐311 + G(2d, p) level of theory. By means of natural bond orbital (NBO) analysis various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁷O SCS and polar field and group electronegativity substituent constants (σ_F and σ_χ, respectively) and also the NBO derived molecular parameters (oxygen natural charge, Q_n, occupation numbers of the oxygen lone pairs, n_o, and occupancy of the C? O antibonding orbital, σ*_CO(occup)). In the case of the alcohols ( 1 – 6 , Y = OH) the ¹⁷O SCS appear to be governed predominantly by the σ_χ effect of the substituent. Furthermore, the key determining NBO parameters appear to be n_o and σ*_CO(occup). Unlike the alcohols, the calculated ¹⁷O SCS of the conjugate bases ( 1 – 6 , Y = O^?), except for system 1 , do not respond systematically to the electronic effects of the substituents. An analysis of the SCS of 1 (Y = O^?) raises a significant conundrum with respect to their origin. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparative studies on CH⋯F− hydrogen bond formation in benzene and exocyclically substituted pentafulvene derivatives

Optimization of CH?F^? complexes of exo‐substituted pentafulvene and meta‐substituted and para‐substituted benzene (substituents: NMe₂, NHMe, NH₂, NHOH, OH, OMe, Br, Cl, F, Me, CCH, CF₃, CONH₂, COMe, CHO, NO₂, NO, and CN) have been performed at the density functional theory level by using Becke hybrid B3LYP functional with 6‐311++G(d,p) basis set. The acidity of the ring CH bond in benzene and fulvene are of similar magnitude, whereas the acidity of the fulvene exocyclic CH₂ group is significantly higher. Various properties based on the H?F^? hydrogen bond (bond length, electron density at BCP, and bond dissociation energy), and the whole molecule (HOMA, sEDA, pEDA, substituent active region, and substituent effect stabilization energy) were analyzed and compared between the fulvene and benzene systems. Sensitivity of the ring CH?F^? hydrogen bond and other substituent dependent properties to substituent effect is substantially greater in fulvene than that of benzene derivatives. In fulvene, the 3‐position is more sensitive than the 4‐position. The sEDA and pEDA parameters used to measure sigma‐electron and pi‐electron excess/deficiency of the ring are mutually correlated for the studied systems. Copyright © 2013 John Wiley & Sons, Ltd.     

Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (ΔE_{HOMO ? LUMO}), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization, and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1_CH2) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ?‐SiMe₂, ?‐NMe, ?‐PMe, ?‐O, ?‐S, ?‐CH₂, ?‐cyclopropyl, and ?‐CMe₂ (2_SiMe2, 2_NMe, 2_PMe, 2_O, 2_S, 2_CH2, 2_cyclopropyl, and 2_CMe2, respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant p_π orbital of the carbene center (Walsh_cyclopropyl → p_{π carbene}). Among them, the singlet closed shell 2_NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (ΔE_{s ? t} = 27.1 kcal mol^?1). In contrast, the least stable derivative is triplet 2_SiMe2, which exhibits the lowest relative ΔE_{s ? t} of ?5.5 kcal mol^?1. The overall trend of ΔE_s‐t is 2_NMe > 2_PMe > 2_S > 2_O > 2_cyclopropyl > 2_CMe2 > 2_CH2 > 1_CH2 > 2_SiMe2. With one negative force constant, the unsubstituted 1_CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.     

Substituent effects on the 13C NMR chemical shifts of the imine carbon in N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines

Long‐range electronic substituent effects were targeted using the substituent dependence of δ_C(C═N), and specific cross‐interactions were explored extendedly. A wide set of N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines, p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y (X = NMe₂, OMe, Me, H, Cl, F, CN, or NO₂; Y = NMe₂, OMe, Me, H, Cl, or CN) were prepared for this study, and their ¹³C NMR chemical shifts δ_C(C═N) of C═N bonds were measured. The results show that both the inductive and resonance effects of the substituents Y on the δ_C(C═N) of p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y are less than those of the substituents Y in p‐X–C₆H₄CH═NC₆H₄–p‐Y. Moreover, the sensitivity of the electronic character of the C═N function to electron donation/electron withdrawal by the substituent X or Y attenuates as the length of the conjugated chain is elongated. It was confirmed that the substituent cross‐interaction is an important factor influencing δ_C(C═N), not only when both X and Y are varied but also when either X or Y is fixed. The long‐range transmission of the specific cross‐interaction effects on δ_C(C═N) decreases with increasing conjugated distance between X and Y. The results of this study suggest that there is a long‐range transmission of the substituent effects in p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y. Copyright © 2012 John Wiley & Sons, Ltd.     

Long distance structural consequences of H‐bonding: the case of complexes of para‐substituted phenol derivatives

H‐bonded complexes of p‐X‐PhOH/p‐X‐PhO^? with fluoride and hydrofluoric acid (X = OH, H, NO₂) were subject of optimization (by means of B3LYP/6‐311+G**) for gradually changed O···F distance from d_O···F = 4.0 Å down to (i) the distance of the proton transfer from the hydroxyl group to fluoride leading to O^?···HF interaction and (ii) fully optimized system (O^?···HF type). In this way, we simulate gradual changes of H‐bond strength estimating simultaneously the energy of interaction, E_int, energy of deformation, E_def, and the binding energy, E_tot. The obtained geometrical parameters allow us to show that H‐bond formation causes substantial changes in geometry, even at so distant parts of the system as the ring and bond length in para‐substituents (OH and NO₂). All these changes are monotonically dependent on interaction and deformation energies. Copyright © 2009 John Wiley & Sons, Ltd.     

Transmission of polar substituent effects across the cubane ring system: 19F substituent chemical shifts (SCS) of 4‐substituted (X) cub‐1‐yl fluorides revisited

¹⁹F NMR shieldings of 4‐substituted (X) cub‐1‐yl fluorides ( 4 ) for a set of substituents (X?H, NO₂, CN, NC, CF₃, COOH, F, Cl, HO, NH₂, CH₃, Si(CH₃)₃, Li, O^? and NH) covering a wide range of electronic substituent effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6‐311+G(2d,p), provided ¹⁹F substituent chemical shifts (SCS) in good agreement with experimental values where known. By means of NBO analysis, various molecular parameters were obtained from the optimized geometries. Linear regression analysis was employed to explore the relationship between the calculated ¹⁹F SCS and polar field, resonance and group electronegativity substituent constants (σ_F, σ_R and σ_x, respectively) and also the NBO derived molecular parameters (fluorine natural charges (Q_n), electron occupancies on fluorine of lone pairs (n_F) and occupation number of the C? F antibonding orbital (σ_CF*)). The key determining parameters appear to be n_F and σ_CF*(occup). Both factors are a function of the electrostatic field influence of the substituent (σ_F effect) but are counteractive in their influence on the shifts. No evidence for a significant resonance effect influence on the shifts could be identified. Copyright © 2009 John Wiley & Sons, Ltd.     

The electronic delocalization in para‐substituted β‐nitrostyrenes probed by resonance Raman spectroscopy and quantum‐chemical calculations

The electronic (UV‐vis) and resonance Raman (RR) spectra of a series of para‐substituted trans‐β‐nitrostyrenes were investigated to determine the influence of the electron donating properties of the substituent (X = H, NO₂, COOH, Cl, OCH₃, OH, N(CH₃)₂, and O⁻) on the extent of the charge transfer to the electron‐withdrawing NO₂ group directly linked to the ethylenic (C = C) unit. The Raman spectra and quantum chemical calculations show clearly the correlation of the electron donating power of the X group with the wavenumbers of the ν_s(NO₂) and ν (C = C)_sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH₃)₂ that the symmetric stretching of the NO₂, ν_s(NO₂), is the most substantially enhanced mode, whereas for X = O⁻, the chromophore is extended over the whole molecule, with substantial enhancement of several carbon backbone modes. Copyright © 2008 John Wiley & Sons, Ltd.     

Remote substituent effects on homolytic Fe?C bond energies of p‐G‐C6H4CH2Fe(CO)2(η5‐C5H5) and p‐G‐C6H4(H)(CN)CFe(CO)2(η5‐C5H5) studied by Hartree–Fock and density functional theory methods

In general, both stoichiometric and catalytic reactions of organometallic complexes involve breaking and forming metal–ligand bonds. Therefore, an evaluation of the thermodynamics of such reactions requires the knowledge of metal–ligand bond energies (BDEs). The homolytic Fe? C bond dissociation energies [i.e., ΔH_homo(Fe? C)s] of 12 para‐substituted benzyldicarbonyl(η⁵‐cyclopentadienyl)iron, p‐G‐C₆H₄CH₂Fp [1,G = NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO, NMe₂; Fp = (η⁵‐C₅H₅)(CO)₂Fe] and 12 para‐substituted α‐cyanobenzyldicarbonyl (η⁵‐cyclopentadienyl)iron, p‐G‐PANFp [2,PAN = C₆H₄CH(CN)] were studied using Hartree–Fock (HF) and density functional theory (DFT) methods with large basis sets. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔH_homo(Fe? C)s. The B3LYP method satisfactorily predicts the α and remote substituent effects on ΔH_homo(Fe? C)s [ΔΔH_homo(Fe? C)s]. The fair correlations [r = 0.97 (g, 1), 0.99(g, 2)] of ΔΔH_homo(Fe? C)s of series 1 and 2 with the substituent σ constants imply that the para substituent effects on ΔH_homo(Fe? C)s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. The molecule stabilization effects (MEs) causes that not only the magnitude of ΔΔH_homo(Fe? C)s(1) varies significantly but also the direction changes from S‐pattern to O‐pattern. ΔΔH_homo(Fe? C)s(2) were found to conform to the Capto‐dative Principle. The detailed knowledge of the factors that determine the Fp? C bond strengths would greatly aid in understanding reactivity patterns in many processes. Copyright © 2010 John Wiley & Sons, Ltd.     

A natural bond orbital analysis of aryl‐substituted polyfluorinated carbanions: negative hyperconjugation

Aryl‐substituted polyfluorinated carbanions, ArCHR_f^? where R_f = CF₃ ( 1 ), C₂F₅ ( 2 ), i‐C₃F₇ ( 3 ), and t‐C₄F₉ ( 4 ), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6‐311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα‐C1) and σ*(Cβ‐F) or σ*(Cβ‐Cγ) were observed for 1 , 2 , 3a (strong electron‐withdrawing substituent, from p‐CF₃ to p‐NO₂), and 4 . Their second‐order donor/acceptor perturbation interaction energy, E(2), values decreased with the increase of the stability of carbanions. A larger E(2) value corresponds to longer Cβ‐F and Cβ‐Cγ bonds and a shorter Cα‐Cβ bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the Cβ‐F and Cβ‐Cγ bonds. In accordance with this, the E(2) values for π(Cα‐C1) → σ*(Cβ‐F) were linearly correlated with the ΔG^o_β‐F values (an empirical measure of β‐fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron‐withdrawing substituents, from H to m‐NO₂) very large E(2) values for LP(Fβ) → π*(Cα‐Cβ) were obtained. This was attributed to the Cβ‐F bond cleavage and the Cα‐Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ‐F bond.     

Hartree–Fock and density functional theory study of remote substituent effects on heterolytic Fe―C bond energies of p‐G‐C6H4CH2Fe(CO)2(η5‐C5H5) and p‐G‐C6H4(H)(CN)CFe(CO)2(η5‐C5H5)

Knowledge of the strength of the metal–ligand bond breaking and formation is fundamental for an understanding of the thermodynamics underlying many important stoichiometric and catalytic organometallic reactions. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe―C bond energies of para‐substituted benzyldicarbonyl(η⁵‐cyclopentadienyl)iron, p‐G‐C₆H₄CH₂Fp [1, G = NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO, NMe₂; Fp = (η⁵‐C₅H₅)(CO)₂Fe], and para‐substituted α‐cyanobenzyldicarbonyl(η⁵‐cyclopentadienyl)iron, p‐G‐PANFp [2, PAN = C₆H₄CH(CN)]. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔH_het(Fe―C)'s. The good linear correlations [r = 0.98 (g, 1a), 0.99 (g, 2b)] between the substituent effects of heterolytic Fe―C bond energies [ΔΔH_het(Fe―C)'s] of series 1 and 2 and the differences of acidic dissociation constants (ΔpK_a) of C―H bonds of p‐G‐C₆H₄CH₃ and p‐G‐C₆H₄CH₂CN imply that the governing structural factors for these bond scissions are similar. And the excellent linear correlations [r = ?1.00 (g, 1c), ?0.99 (g, 2d)] between ΔΔH_het(Fe―C)'s and the substituent σ_p^? constants show that these correlations are in accordance with Hammett linear free energy relationships. The polar effects of these substituents and the basis set effects influence the accuracy of ΔH_het(Fe―C)'s. ΔΔH_het(Fe―C)'s(1, 2) follow the Capto‐dative Principle. The detailed knowledge of the factors that determine the Fp―C bond strengths would greatly aid in understanding reactivity patterns in many processes. Copyright © 2011 John Wiley & Sons, Ltd.     

Computer simulations and analysis of structural and energetic features of crystalline cage energetic compound: 2, 4, 6, 8, 12‐pentanitro‐10‐(3, 5, 6‐trinitro (2‐pyridyl))‐2, 4, 6, 8, 12‐hexaazatetracyclo [5.5.0.03,11.05,9]dodecane

First principles molecular orbital and plane‐wave ab initio calculations have been used to investigate the structural and energetic properties of a new cage compound 2, 4, 6, 8, 12‐pentanitro‐10‐(3, 5, 6‐trinitro (2‐pyridyl))‐2, 4, 6, 8, 12‐hexaazatetracyclo [5.5.0.0^3,11.0^5,9]dodecane (PNTNPHATCD) in both the gas and solid phases. The molecular orbital calculations using the density functional theory methods at the B3LYP/6‐31G(d,p) level indicate that both the heat of formation and strain energy of PNTNPHATCD are larger than those of 2, 4, 6, 8, 10, 12‐hexanitro‐2, 4, 6, 8, 10, 12‐hexaazatetracyclo [5.5.0.0.0] dodecane (CL‐20). The infrared spectra and the thermodynamic property in gas phase were predicted and discussed. The calculated detonation characteristics of PNTNPHATCD estimated using the Kamlet–Jacobs equation equally matched with those of CL‐20. Bond‐breaking results on the basis of natural bond orbital analysis imply that C–C bond in cage skeleton, C–N bond in pyridine, and N–NO₂ bond in the side chain of cage may be the trigger bonds in the pyrolysis. The structural properties of PNTNPHATCD crystal have been studied by a plane‐wave density functional theory method in the framework of the generalized gradient approximation. The crystal packing predicted using the Condensed‐phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force fields belongs to the Pbca space group, with the lattice parameters a = 20.87 Å, b = 24.95 Å, c = 7.48 Å, and Z = 8, respectively. The results of the band gap and density of state suggest that the N–NO₂ bond in PNTNPHATCD may be the initial breaking bond in the pyrolysis step. As the temperature increases, the heat capacity, enthalpy, and entropy of PNTNPHATCD crystal all increase, whereas the free energy decreases. Considering that the cage compound has the better detonation performances and stability, it may be a superior high energy density compound. Copyright © 2015 John Wiley & Sons, Ltd.     

Substituent effect investigation of 3‐(2, 4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one. Part 1. Correlation analysis of 13C NMR chemical shifts

A series of substituted chlorinated chalcones namely, 3‐(2,4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one, have been synthesized, X being H, NH₂, OMe, Me, F, Cl, CO₂Et, CN, and NO₂. Dual substituent parameter (DSP) models of ¹³C NMR chemical shift (CS) have revealed that π‐polarization concept could be utilized to explain the reverse field effect at CO, the enhanced substituent field effect at CO, C‐2, and C‐5, and the decreased sensitivity of substituent field effect at C‐6. Chlorine atoms dipole direction at the benzylidene ring either enhances or reduces substituent effect depending on how they couple with the substituent dipole at the probe site. The correlation of ¹³C NMR CS of C‐2, C‐5, and C‐6 with σ and σ indicates that chlorine atoms in the benzylidine ring deplete the ring from charges. Both MSP of Hammett and DSP of Taft ¹³C NMR CS models give similar trends of substituent effects at C‐2, C‐5, and C‐6. However, the former fail to give a significant correlation for CO and C‐6 ¹³C NMR CS. MSP of σ_q and DSP of Taft and Reynolds models significantly correlated ¹³C NMR CS of C_β. MSP of σ_q fails to correlate C‐1′ ¹³C NMR CS. Investigation of ¹³C NMR CS of non‐chlorinated chalcones series: 3‐phenyl‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one has revealed similar trends of substituent effects as in the chlorinated chalcones series for C‐1′, CO, C_α, and C_β. In contrast, the substituent effect of the non‐chlorinated chalcone series at C‐2, C‐5, and C‐6 did not correlate with any substituent constant. Copyright © 2010 John Wiley & Sons, Ltd.     

Substituent effects on gas‐phase homolytic Fe–N bond energies of m‐G‐C6H4NHFe(CO)2(η5‐C5H5) and m‐G‐C6H4N(COMe)Fe(CO)2(η5‐C5H5) studied using density functional theory methods

One of the most fundamental properties in chemistry is the bond dissociation energy, the energy required to break a specific bond of a molecule. In this paper, the Fe–N homolytic bond dissociation energies [ΔH_homo(Fe–N)'s] of 2 series of (meta‐substituted anilinyl)dicarbonyl(η⁵‐cyclopentadienyl) iron [m‐G‐C₆H₄NHFp ( 1 )] and (meta‐substituted α‐acetylanilinyl)dicarbonyl(η⁵‐cyclopentadienyl) iron [m‐G‐C₆H₄N(COMe)Fp ( 2 )] were studied using density functional theory methods with large basis sets. In this study, Fp is (η⁵‐C₅H₅)Fe(CO)₂, and G is NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO, and NMe₂. The results show that Tao‐Perdew‐Staroverov‐Scuseria, Minnesota 2006, and Becke's power‐series ansatz from 1997 with dispersion corrections functionals can provide the best price/performance ratio and accurate predictions of ΔH_homo(Fe–N)'s. The ΔΔH_homo(Fe–N)'s ( 1 and 2 ) conform to the captodative principle. The polar effects of the meta‐substituents show the dominant role to the magnitudes of ΔΔH_homo(Fe–N)'s. σ_α· and σ_c· values for meta‐substituents are all related to polar effects. Spin‐delocalization effects of the meta‐substituents in ΔΔH_homo(Fe–N)'s are small but not necessarily zero. RE plays an important role in determining the net substituent effects on ΔH_homo(Fe–N)'s. Insight from this work may help the design of more effective catalytic processes.     

Thermolysis kinetics of diethyl 2,3‐dicyano‐2,3‐di(p‐substituted phenyl)succinates

An experimental approach was developed to determine the intrinsic thermolysis rate constants of the central carbon–carbon bond during the dl/meso isomerization of diethyl 2,3‐dicyano‐2,3‐di(p‐substituted phenyl)succinates (G=H, Me, OMe, Cl, and NO₂) at temperatures ranging from 80 to 120 °C. The obtained rate constants are significantly affected by the polarity of the para substituents, in sharp contrast to their negligible effects on the dl/meso isomerization equilibrium constants. Moreover, the substituent effects on the activation enthalpies can be linearly correlated with the Hammett substituent resonance constants and the homolytic dissociation enthalpies (bond dissociation energies) of the benzylic C–H bonds of ethyl 2‐cyano‐2‐(p‐substituted phenyl)acetates. Copyright © 2011 John Wiley & Sons, Ltd.     

The quest for a better system for evaluating pi‐electron substituent constant: a comparison of benzoic,acrylic and tria‐, penta‐, heptafulvene‐based carboxylic acids. A computational study

Carboxylic acids based on exo‐substituted tria‐, penta‐, heptafulvenes and ethylene (acrylic acids) were examined in order to determine if they are more sensitive to the substituent effect than benzoic acid – the system originally employed by Hammett. In order to accomplish this task, all possible structural isomers of benzoic acid, tria‐, penta‐ and heptafulvene‐based carboxylic acids, acrylic and methacrylic acids substituted by 13 substiuents (BH₂, CHO, CN, COCN, NO₂, CF₃, Me, Cl, F, OH, OMe, NH₂ and NMe₂) were optimized at the B3LYP/6‐311++G(d,p) level of theory, and Gibbs free energies of carboxylic group dissociation (ΔG_dis) were calculated. These energies were subsequently intercorrelated, and from the slopes of linear regressions, it was estimated which system is associated with greatest changes of ΔG_dis due to substitution and thus is most sensitive to the substituent effect. It was found that all fulvene‐based carboxylic acids have greater range of ΔG_dis change than benzoic acid, but the largest range of change was observed in the case of acrylic and methacrylic acids. The acrylic acid as the most sensitive system to substitution could replace benzoic acid for an improved version of substituent constant used to measure pi‐electron substituent effect. Copyright © 2013 John Wiley & Sons, Ltd.     

Substituent effects on conformational preference in α‐substituted α‐fluorophenylacetic acid methyl ester model systems for chiral derivatizing agents

In connection with study of chiral derivatizing agents (CDAs) for NMR determination of absolute configuration of organic compounds, factors controlling the conformational preference between syn‐ and anti‐forms in α‐substituted α‐fluorophenylacetic acid methyl ester (FC(X)(Ph)COOMe) model systems were theoretically investigated. Substituents X at the stereogenic carbon atom were X = H, C?CH and CH₃, the electronic and steric properties of which were significantly different from each other. The model system with X = C?CH and that with X = CH₃ were found to be possible candidates for fluorine‐containing CDAs. The syn conformation is stable compared with the anti one by 0.7 kcal mol^?1 for the ester with X = C?CH. On the other hand, the anti conformation is stable compared with the syn one by 0.5 kcal mol^?1 for the ester with X = CH₃. Both natural bond orbital (NBO) analysis and deletion of selected orbitals based on the donor–acceptor NBO scheme were adopted for semi‐quantitative estimation of factors responsible for the conformational preference as well as a qualitative inspection of occupied canonical molecular orbitals (MOs). It was shown that [σ–(σ* + π*)(C?O)] and [σ–σ*(Ph) and π(Ph)–σ*] hyperconjugations are the main factors controlling the conformational preferences between the syn and anti conformations. Other types of effects such as electrostatic effects were also investigated. The role of the fluorine atom was also clarified. Copyright © 2009 John Wiley & Sons, Ltd.     

Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Sers measurement of oxidation-reduction reactions of adsorbates on gold microstructures

The use of surface enhanced Raman scattering (SERS) to study oxidation-reduction and complexation chemical reactions on Au surfaces is illustrated by: (1) the reaction of Au(CN)₂^? adsorbed on a Au colloid (2140 cm^?) to form Au(CN)₃^2? (2131 cm^?1) on the surface in excess CN^?; (2) the oxidation of Au(CN)₂^? by HNO₃, Cl₂, or Br₂ solutions to form Au(CN)₄^? (2190 cm^?) on a Au colloid; and (3) the dissolution of Au in excess CN^? with O₂. Unlike with Ag surfaces, no SERS is observed when Au powder is exposed to NO, NO₂, SO₂, CO, or CO₂ gases. The surface chemistry of Au is discussed in the light of these reactions.