A theoretical approach to substituent effects: Stabilities and conformational preferences in β-substituted ethyl radicals

Ab initio molecular orbital theory is used to study substituent effects in a series of β-substituted Et radicals XCH₂CH₂. For X = BH₂ (plan.), CH₃, NH₂, OH and F, only slight conformational preferences and weak stabilizations are indicated. Such behaviour may be rationalized, using a PMO model, in terms of opposing changes, accompanying variation in X, in positive and negative hyperconjugation between the XCH₂ group and the CH₂ centre. On the other hand, for groups containing an appropriately oriented, low-lying vacant orbital, viz. X = Li, BeH and BH₂ (perp.), there is a pronounced preference for the perpendicular conformation of the radical. This is attributed to 1,3-interaction between the singly-occupied 2p(C) orbital and the vacant 2p(X) orbital.     

An ab initio molecular orbital study of the deprotonation of amines

The geometries of the amines NH₂X and amido anions NHX^?, where X = H, CH₃, NH₂, OH, F, C₂H, CHO, and CN have been optimized using ab initio molecular orbital calculations with a 4-31G basis set. The profiles to rotation about the N? X bonds in CH₃NH^?, NH₂NH^?, and HONH^? are very similar to those for the isoprotic and isoelectronic neutral compounds CH₃OH, NH₂OH, and HOOH. The amines with unsaturated bonds adjacent to the nitrogen atoms undergo considerable skeletal rearrangement on deprotonation such that most of the negative charge of the anion is on the substituent. The computed order of acidity for the amines NH₂X is X = CN > HCO > F ≈ C₂H > OH > NH₂ > CH₃ > H and for the reaction NHX^? + H⁺ → NH₂X the computed energies vary over the range 373–438 kcal/mol.     

Theoretical influence of third molecule on reaction channels of weakly bound complex CO2… HF systems

In this investigation, reaction channels of weakly bound complexes CO₂…HF, CO₂…HF…NH₃, CO₂…HF…H₂O and CO₂…HF…CH₃OH systems were established at the B3LYP/6‐311++G(3df,2pd) level, using the Gaussian 98 program. The conformers of syn‐fluoroformic acid or syn‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH) were found to be more stable than the conformers of the related anti‐fluoroformic acid or anti‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH). However, the weakly bound complexes were found to be more stable than either the related syn‐ and anti‐type fluoroformic acid or the acid plus third molecule (NH₃, H₂O, or CH₃OH) conformers. They decomposed into CO₂ + HF, CO₂ + NH₄F, CO₂ + H₃OF or CO₂ + (CH₃)OH₂F combined molecular systems. The weakly bound complexes have four reaction channels, each of which includes weakly bound complexes and related systems. Moreover, each reaction channel includes two transition state structures. The transition state between the weakly bound complex and anti‐fluoroformic acid type structure (T₁₃) is significantly larger than that of internal rotation (T₂₃) between the syn‐ and anti‐FCO₂H (or FCO₂H…NH₃, FCO₂H…H₂O, or FCO₂H…CH₃OH) structures. However, adding the third molecule NH₃, H₂O, or CH₃OH can significantly reduce the activation energy of T₁₃. The catalytic strengths of the third molecules are predicted to follow the order H₂O < NH₃ < CH₃OH. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Distonic radical cations : Guidelines for the assessment of their stability

Ab initio molecular orbital calculations on the distonic radical cations CH₂(CH₂)_nN⁺H₃ and their conventional isomers CH₃(CH₂)_nNH₂+ (n = 0,1, 2 and 3) indicate a preference in each case for the distonic isomer. The energy difference appears to converge with increasing n towards a limit which is close to the energy difference between the component systems CH₃·H₂+CH₃^+NH₃ (representing the distonic isomer) and CH₃CH₃+CH₃NH₂+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH₃·Y+CH₃X⁺H and CH₃YH+CH₃X^+. (or CH₃YH^+. + CH₃X) to predict the relative energies of the distonic ions ·Y(CH₂)_nX⁺H and their conventional isomers HY(CH₂)nX^+. (X = NH₂, OH, F, PH₂, SH, Cl; Y = CH₂, NH, O) and testing the predictions through explicit calculations for systems with n = 0,1 and 2. Although the predictions based on component systems are often close to the results of direct calculations, there are substantial discrepancies in a number of cases; the reasons for such discrepancies are discussed. Caution must be exercised in applying this and related predictive schemes. For the systems examined in the present study, the conventional radical cation is predicted in most cases to lie lower in energy than its distonic isomer. It is found that the more important factors contributing to a preference for distonic over conventional radical cations are the presence in the system of a group(X) with high proton affinity and the absence of a group (X, Y or perturbed (C—C) with low ionization energy.     

Mono- and bis-alkoxides of tris(3,5-dimethylpyrazolyl)borato molybdenum and tungsten nitrosyls

The complexes [ML^*(NO)Cl(OR)] {L^* = HB(3,5-Me₂C₃HN₂)₃; M= Mo, R = CH₂CH₂X, X = Cl, OMe or OEt; (CH₂)_nOH, n = 2, 5, 6; M = W, R = CH₂CH₂X, X = Cl, OMe or OEt; (CH₂)_nOH, n = 2–6; CH₂(CF₂)₃CH₂OH; CHMeCH₂CMe₂OH} and [ML^*(NO)(OR)₂] {M = Mo, R = CH₂CH₂X, X = Cl, OMe or OEt; (CH₂)_nOH, n = 2–6; M = W,R = CH₂CH₂X, X= Cl, OMe or OEt; (CH₂)_nOH, n = 2,4–6; CH₂(CF₂)₃CH₂OH} have been prepared from [ML^*(NO)Cl₂] and the appropriate alcohol in the presence of NEt₃ or NaCO₃, and have been characterized by IR, ¹H NMR and mass spectroscopy.     

Microwave spectrum,conformational preference,barrier to internal rotation,and centrifugal distortion of 1-amino-2-propanol

The microwave spectrum of CH₃CH(OH)CH₂NH₂ has been investigated in the 26.5–39.7 GHz region. One rotamer with an intramolecular hydrogen bond formed between hydroxyl and ammo groups was assigned. This conformation is also characterized by having the methyl group anti to the amino group. Other forms, if they exist, must be at least 1 kcal mole^?1 less stable. Four vibrationally excited states belonging to three different normal modes were assigned and the barrier to internal rotation of the methyl group was found to be 3173 ± 100 cal mole^?1.     

The nucleophilic reaction of polyfluoroaromatic compounds: II. The nucleophilic cyclization of chloropentafluorobenzene

The results of cyclization reaction between chloropentafluorobenzene and bifunctional nucleophiles such as XCH₂CH₂OH(X=OH, NHMe, NHEt) and HN (CH₂CH₂OH)₂, NH₂CH₂CH₂NH₂ were reported. It was found that the NaH/DMF system was a better reagent for the preparation of sodium alcoholates and the yields of cyclic products ranged from 50–80% which were much higher than that reported by previous workers.³     

Ab initio study of a purine nucleoside: Adenosine

The ab initio SCF LCAO-MO method is used to compute the main electronic properties of a purine nucleoside, adenosine, in two specific conformational arrangements (3′-endo conformation of the ribose, gt orientation of the extracyclic CH₂OH group, anti orientation of the base with respect to the sugar and 3′-endo conformation of the ribose, gg orientation of the extra-cyclic CH₂OH group, syn orientation of the base with respect to the sugar). The results are compared with those performed for the isolated component fragments, adenine and 3′-endo riboses.     

Substituent effects in second row molecules : Silicon-containing compounds

Substituent interaction energies are calculated by ab initio molecular orbital methods for the two series SiH₂X^- and SiH₃X for the directly bound substituents X = BH₂, CH₃, NH₂, OH, F and the results compared with those for the corresponding first row species. Interactions with the groups NH₂, OH, F are as large in the neutral as in the anionic series and this is attributed to the presence of important π-bonding interactions, supplementing the effects of inductive withdrawal of σ-electrons. The restoration of charge neutrality by π-donation to silicon is more important in the neutral molecules, σ-electron transfer from silicon in the anions. π-Bonding with the π-acceptor substitutent BH₂ is favourable, as it is in the CH₃X and CH₂X^- systems, but with π-donor substituents the interactions are always destabilizing.     

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

Although it has been generally assumed that electron attachment to disulfide derivatives leads to a systematic and significant activation of the S? S bond, we show, by using [CH₃SSX] (X=CH₃, NH₂, OH, F) derivatives as model compounds, that this is the case only when the X substituents have low electronegativity. Through the use of MP2, QCI and CASPT2 molecular orbital (MO) methods, we elucidate, for the first time, the mechanisms that lead to unimolecular fragmentation of disulfide derivatives after electron attachment. Our theoretical scrutiny indicates that these mechanisms are more intricate than assumed in previous studies. The most stable products, from a thermodynamic viewpoint, correspond to the release of neutral molecules; CH₄, NH₃, H₂O, and HF. However, the barriers to reach these products depend strongly on the electronegativity of the X substituents. Only for very electronegative substituents, such as OH or F, the loss of H₂O or HF is the most favorable process, and likely the only one observed. This is possible because of two concomitant factors, 1) the extra electron for [CH₃SSX]^? (X=OH, F) occupies a σ*(S? X) MO, which favors the cleavage of the S? X bond, and 2) the activation barriers associated with the hydrogen transfer process to produce H₂O and HF are rather low. Only when the substituents are less electronegative (X=H, CH₃, NH₂) the extra electron is located in a σ*(S? S) orbital and the cleavage of the disulfide bridge becomes the most favorable process. The intimate mechanism associated with the S? S bond dissociation process also depends strongly on the nature of the substituent. For X=H or CH₃ the process is strictly adiabatic, while for X=NH₂ it proceeds through a conical intersection ( CI ) associated with the charge reorganization necessary to obtain, from a molecular anion with the extra electron delocalized in a σ*(S? S) antibonding orbital, two fragments with the proper charge localization.     

Molecular orbital study of gas phase basicities of carbofunctional derivatives of group IVB elements; stereochemical control

The gas phase basicities of a series of carbofunctional derivatives H₃M(CH₂)_nX (M = C, Si, Ge; X = NH₂, OH, F; n = 1–3) were investigated by the semiempirical CNDO/2 method. The calculations indicate that the electronic effects of silyl- and germyl-substituents differ greatly from the effect of simple alkyls. Moreover, in contrast to simple carbon derivatives, the overall nature of the electronic effects of siliyl- and germyl-substituents is crucially influenced by the molecular conformation.     

A theoretical approach to substituent effects: Interaction between directly bonded groups in the isoelectronic series XNH3+, XCH3, and XBH3−

Ab initio molecular orbital theory including full geometry optimization at the 4-31G level is used to examine the interactions between substitutents X(X = Li, BeH, BH₂, CH₃, NH₂, OH and F) and substrates Y(Y = NH₃⁺, CH₃, BH₃^?) in the isoelectronic series XNH₃⁺, XCH₃ and XBH₃^?. The results indicate that the interaction energies are dominated by σ-effects. NH₃⁺ is found to interact favorably with the σ-donors (e.g. Li, BeH and BH₂) and unfavorably with the σ-acceptors (e.g. F, OH, NH₂). The reverse pattern a observed for XBH₃^?. The range of interaction energies for XCH₃ is considerably smaller than for XNH₃⁺ and XBH₃^?.     

含砜功能基的交联聚苯乙烯的红外光谱特性

 本文合成了一系列聚苯乙烯基支载的含砜基的化合物X=Cl,O(CH₂)₉CH₃,NH₂,NHNH₂,Na,CH₂CH₃,OH)并较系统地研究了这些聚合物中的=SO₂基团的红外光谱特性,比较了聚合物和低分子类似物的特征吸收频率的变化规律,考察了聚合物骨架对S=O基团的虹外吸收峰的影响。     

Why Are B ions stable species in peptide spectra?

Protonated amino acids and derivatives RCH(NH₂)C(+O)X · H⁺ (X = OH, NH₂, OCH₃) do not form stable acylium ions on loss of HX, but rather the acylium ion eliminates CO to form the immonium ion RCH = NH ₂ ⁺ . By contrast, protonated dipeptide derivatives H₂NCH(R)C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B₂ ions by elimination of HX. These B₂ ions fragment on the metastable ion time scale by elimination of CO with substantial kinetic energy release (T _1/2 = 0.3–0.5 eV). Similarly, protonated N-acetyl amino acid derivatives CH₃C(+O)NHCH(R′)C(+O)X · H⁺ [X = OH, OCH₃, NH₂, NHCH(R″)COOH] form stable B ions by loss of HX. These B ions also fragment unimolecularly by loss of CO with T _1/2 values of ～ 0.5 eV. These large kinetic energy releases indicate that a stable configuration of the B ions fragments by way of activation to a reacting configuration that is higher in energy than the products, and some of the fragmentation exothermicity of the final step is partitioned into kinetic energy of the separating fragments. We conclude that the stable configuration is a protonated oxazolone, which is formed by interaction of the developing charge (as HX is lost) with the N-terminus carbonyl group and that the reacting configuration is the acyclic acylium ion. This conclusion is supported by the similar fragmentation behavior of protonated 2-phenyl-5-oxazolone and the B ion derived by loss of H-Gly-OH from protonated C₆H₅C(+O)-Gly-Gly-OH. In addition, ab initio calculations on the simplest B ion, nominally HC(+O)NHCH₂CO⁺, show that the lowest energy structure is the protonated oxazolone. The acyclic acylium isomer is 1.49 eV higher in energy than the protonated oxazolone and 0.88 eV higher in energy than the fragmentation products, HC(+O)N⁺H = CH₂ + CO, which is consistent with the kinetic energy releases measured.     

The vibrational spectra of Pd(II) complexes with planar monosubstituted oxamides

The vibrational spectra, both i.r. and Raman, of some N-monosubstituted oxamide complexes with Pd(II) and their deuterated derivatives are reported, using ligands which have the general form NH₂COCONHR where R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, NH₂, CH₂CH₂NH₂ or CH₂CH₂OH. The article also reports on the thermal analysis of the compounds, mainly based on thermogravimetric measurements.     

A DFT study on the origin of the fluorine gauche effect in substituted fluoroethanes

DFT derived conformational energy profiles of a series of β-substituted α-fluoroethanes (F-CH₂CH₂-X) have been explored where the substituent X was varied as NH₃⁺, OCOH, NCO, NO₂, NHCHO, F, N₃, CHNH, NCS, CHCCH₂, CH₃, CHCH₂, 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.     

Designing new donor materials based on functionalized DCCnT with different electron-donating groups: A density functional theory (DFT) and time dependent density functional theory (TDDFT)-based study

Finding a promising donor/acceptor material of organic solar cells is one of the most important ways to improve their power conversion efficiency. Extensive studies have focused on designing and synthesizing new and suitable materials. Small organic molecule materials, different from polymers, have many merits, such as easy synthesis and modification, less by-products, and crystallinity. In the present work, we theoretically design a series of new donor materials based on 1-(1,1-dicyanomethylene)-cyclohex-2-ene-substituted oligothiophenes, that is, DCCnT (n = 1-4) series. Furthermore, we model and predict photoelectric properties of functionalized DCCnT with different electron-donating groups (─CH₃/─CHCH₂/─OCH₃/─NH₂/─OH). The calculated results, based on density functional theory and time-dependent functional theory, show that DCCnT-X (X = OH, NH₂, and OCH₃) series show odd-even effect of dipole moments when n varies from 1 to 4, whereas DCCnT-CH₃ and DCCnT-CHCH₂ do not. Finally, we find that DCC3T-X (X = OH, OCH₃, and NH₂) may be better candidates of donor materials because of their larger dipole moments, stronger electron donating ability, and smaller exciton binding energy with respect to prototype DCCnT molecules.     

Oemo analysis of ab inttio scf—mo computations. conformational preferences and asymmetries in methyl derivatives

A procedure is described which allows analysis of ab initio SCF—MO results in the framework of an OEMO model, in terms of total energy and a PMO quantitative approach. The procedure is applied to the analysis of the effects of non-bonded interactions upon the conformational preferences of molecules of the type CH₃—X (with X = CH₃, NH₂, OH). The use of a basis of fragment localized MO's allows discussion of the energy effects in terms of bond—bond and bond—lone pair repulsions and conjugative stabilizations. It is found that the non-bonded interactions are responsible for the conformational preference and the rotational barrier in these molecules: in all cases the optimum conformation is characterized by the smallest bond—bond repulsion and the largest conjugative stabilization. The factors determining the asymmetries of the methyl groups in methylamine and methanol have also been investigated.     

New catecholate complexes of triphenylantimony(V) based on 6-iminomethyl-3,5-di-<Emphasis Type="Italic">tert</Emphasis>-butylpyrocatechols N-functionalized by the aniline or phenol group

The following new triphenylantimony(V) catecholate complexes bearing the protonated imine group are synthesized from the new sterically hindered 3,5-di-tert-butylpyrocatechols (6-(CH=N-o-(C₆H₄–NH₂))-3,5-Cat)H₂ (H₂L¹) and (6-(CH=N-o-(C₆H₄–OH))-3,5-Cat)H₂ (H₂L²) containing in position 6 the iminomethyl group bonded to the aniline or phenol substituent: (6-(CH=NH⁺-o-(C₆H₄–NH₂))-3,5-Cat)SbPh₃X (X = Br (I), OMe (III)) and (6-(CH=NH⁺-o-(C₆H₄–OH))-3,5-Cat)SbPh₃X (X = Br (II), OMe (IV)). The molecular structure of complex III · CH ₃ OH in the crystalline state is determined by X-ray diffraction analysis (CIF file CCDC no. 1554694). The electrochemical properties of complexes III and IV are studied by cyclic voltammetry.     

An ab initio study of atomic interactions in monosubstituted benzenes

Molecules of monosubstituted benzenes XC₆H₅ (X = F, Cl, Br, OH, NH₂, CH₃, CH₂CH₃) were studied by the RHF/6-311G(d) method with full geometry optimization. Analysis of the molecular orbitals and contributions made to them by atomic orbitals, and also of the populations of the valence p orbitals of atoms in substituents X directly bonded to the aromatic ring showed that the features of the electron distribution in such molecules should not be attributed to the capability of the lone electron pairs of the heteroatoms in these substituents for p,π conjugation with the π-electron system of the molecule.