Steric effects of the alkyl groups: evaluation on isolated molecules by means of isodesmic reactions

Several possible scales of steric effects of the alkyl groups were suggested on the basis of isodesmic model reactions, in which a sterically crowded compound is formally synthesized from simpler derivatives. The reaction energies were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d.p)//B3LYP/6-311+G(d.p) for 6 model systems and 7 various alkyl groups. The most important systems were cis-1,2-dialkylcyclopropanes 1 synthesized from two mono derivatives and sterically crowded derivatives of bicyclo[2.2.2]octane 2 with C(3) symmetry. The scales of steric effects evaluated from the two models were rather different: the first scale depended in effect only on the C atoms in the alpha and beta positions and the effects were almost equal for all primary alkyls. The second scale depended also on the gamma position and the effect of the CH(2)-t-Bu group was much greater than that of the ethyl group. Any relationship between various systems was found rarely, only in the case of very similar reaction series; even in such cases the relationship was sometimes linear, sometimes distinctly curvilinear. It is concluded that any universal scale of steric effects is in principle not possible since these effects depend specifically on the surroundings of the substituent in a particular reaction. Nevertheless, there is a similarity between various scales; a bulky group appears as bulky in any scale. Therefore, very rough correlations of steric effects are possible.     

Enthalpies of formation of monoderivatives of hydrocarbons: Interaction of polar groups with an alkyl group

Energies of hydrocarbon monoderivatives CH(3)X, C(2)H(5)X, n-C(4)H(9)X, and n-C(5)H(11)X with 16 different substituents X were calculated at the levels B3LYP/6-311+G(d,p) and B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p). The results were used to test the validity of the additive rule that has served commonly for estimating the enthalpies of formation Delta(f)H(T). The exact additivity corresponds to zero reaction energy DeltaE of the isodesmic reaction, in which the substituent X is transferred from one alkyl group R to another. Additivity is approximately fulfilled for butyl and pentyl derivatives with the differences less than 0.3 kJ mol(-1) (except charged groups X). Methyl derivatives deviated from the additive rule up to 22 kJ mol(-1) for dipolar groups X and 45 kJ mol(-1) for charged group, in agreement with the available experiments and with the anticipation of all suggested empirical schemes. In addition, smaller deviations of ethyl derivatives (3 or 20 kJ mol(-1), respectively) were observed here for the first time. There is no correlation between the deviations of methyl and ethyl derivatives; they are also not related to steric effects, and only partly to polarization. Deviations of methyl derivatives are proportional to the electronegativity of the first atom of the substituent; even when the definition of electronegativity is somewhat questionable, one can say in any case that it is controlled by the first atom.     

Intramolecular hydrogen bond in the push–pull CF3-aminoenones: DFT and FTIR study,NBO analysis

Postulated conformers of trifluoromethylated β-aminoenones stabilized by intramolecular NH?O and N?HO bonds were studied by IR and NMR spectroscopy and evaluated with quantum chemical calculations (B3LYP/6-311+G(d,p), MP2/6-311+G(d,p)//B3LYP/6-311+G(d,p) and MP2/6-31G(d,p)) and NBO analysis. The influence of the nature of EWG, substituents at the nitrogen atom and double bond, and of orbital interactions of heteroatoms and double bonds in these structures on the proton affinity of basic and acid centers, strength of hydrogen bonds, and the energy of tautomeric transfers is discussed. The theoretical results agree satisfactorily with the experimental observations.     

Parquet compounds on the basis of eight- and twelve-membered structure blocks: Quantum-chemical study

Annelated structures of cyclooctatetraene and cyclododecahexene and their expanded lowmolecular derivatives were studied using DFT B3LYP method with 6-311G(d,p), 6-311(df,p), 6-311+G(3df) basis sets. Monomer systems including planar antiaromatic cycles are stable in the singlet state, whereas the corresponding triplet states are significantly destabilized. The stability of nonstandard planar forms with antiaromatic cycles is provided by steric factors and the effects of π-electron interaction between the central and peripheral fragments.     

Inductive effects in radicals calculated from DFT energies; substituted bicyclo[2.2.2]octan-1-yloxy radicals

Energies of a series of 4-substituted 1-oxybicyclo[2.2.2]octan-1-yloxy radicals with 18 various substituents were calculated within the framework of the DFT theory at the levels UB3LYP/6-311+G(d,p)//UB3LYP/6-311+G(d,p) and UB3LYP/6-311++G(2df,p)//UB3LYP/6-311+G(d,p) and compared with similar series of the parent alcohols, their deprotonated and protonated forms calculated at the levels B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) and B3LYP/6-311++G(2df,p)//B3LYP/6-311+G(d,p). The two levels are of the same performance and both are sufficient for molecules of this type according to comparison with scarce experimental gas-phase acidities and basicities. The substituent effects were analyzed in terms of isodesmic equations. In addition to strong dependence on the substituent inductive effect, a slight dependence on the electronegativity of the first atom of the substituent was proven in certain cases. In all aspects, there is no qualitative difference between the effects on radicals and on similar closed shell species. Radicals behave as slightly electron deficient; the substituent effect is weaker than that on the ions but stronger than on neutral molecules.     

Protonated nitro group: structure, energy and conjugation

Structure of protonated nitro compounds was investigated by calculations at the levels MP2(FC)/6-311++G(2d,2p)//MP2(FC)/6-311++G(2d,2p)(nitromethane and reference compounds) or B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)(nitrobenzene and its 18 meta- and para-substituted derivatives). The group NO2H+ reveals many similarities with the isoelectronic group CO2H as the preferred conformation, conformational equilibrium, and stabilization by interaction (resonance) within the group quantified by means of isodesmic reactions. However, there is a difference in the interaction with donor groups (for instance in 4-nitroaniline) that is much stronger with NO2H+ than with CO2H. This interaction may be called resonance and may be described by standard resonance formulas, but these formulas predict only partially the geometry and cannot explain the great interaction energy.     

Theoretical investigation on the GaCl3-catalyzed ring-closing metathesis reaction of N-2,3-butadienyl-2-propynyl-1-amine: three-membered ring versus four-membered ring mechanism

The gallium chloride (GaCl(3))-catalyzed ring-closing metathesis reaction mechanism of N-2,3-butadienyl-2-propynyl-1-amine has been studied at the Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (B3LYP)/6-31G(d), B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p)//B3LYP/ 6-31G(d) and the second-order M?ller-Plesset perturbation (MP2)/6-311++G(d,p)//B3LYP/6-31+G(d,p) levels. It was found that the final metathesis product can be yielded via a three-membered or four-membered ring mechanism. The three-membered ring pathway is favorable due to its low energy barrier at the rate determining step. The whole reaction is stepwise and strongly exothermic.     

Theoretical Studies on the Aminolysis of Ester: Effects of the Catalysis and Substituent to the Reaction of Methyl Indole-3-acetate with Ammonia

A comprehensive exploration of the aminolysis mechanism for methyl indole-3-acetate with ammonia is carried out by employing the B3 LYP/6-311++G(d,p), M06-2 X/6-311++G(d,p) and MP2/6-311++G(d,p)//M06-2 X/6-311++G(d,p) levels. Two alterative reaction channels of the concerted and addition/elimination stepwise processes including the uncatalyzed, base-catalyzed reactions are taken into consideration. Subsequently, the substituent effects and solvent effects in methanol are also evaluated at the M06-2 X/6-311++G(d,p) level. The calculated results indicate that the calculated values of M06-2 X level are quite close to those of MP2, the stepwise pathway has more advantages to the concerted one for all of the reaction processes and the catalyst facilitates the proton migration and decreases the energy barriers as well. It is shown that the most preferred mechanism is the based-catalyzed stepwise process, the substituent of NH2 group slightly accelerates all the aminolysis reaction processes, and the solvent effect does not remarkably change the mechanism of the reaction.     

Electrostatic potential minimum of the aromatic ring as a measure of substituent constant

The molecular electrostatic potential minimum (Vmin) observed for the arene pi system of a substituted benzene derivatives is found to correlate linearly with the substituent constant sigma(p) degrees . The use of Vmin as a measure of substituent effect is further confirmed by obtaining a linear correlation between Vmin and a thermodynamic measure of the substituent effect obtained from an isodesmic reaction scheme involving benzene derivatives. Vmin and the recently proposed electrostatic potential value at the nucleus of the para carbon atom (Vc) show a nearly identical trend toward quantification of substituent effects. Both quantities have been compared at three different density functional theory methods, viz. B3LYP/6-311+G(2d,2p), BPW91/6-311G(d,p), and B3LYP/aug-cc-pvtz, as well as the at the MP2/6-31+G(d,p) level of theory, showing remarkable consistency among them.     

Mechanistic insights into triterpene synthesis from quantum mechanical calculations. Detection of systematic errors in B3LYP cyclization energies

Most quantum mechanical studies of triterpene synthesis have been done on small models. We calculated mPW1PW91/6-311+G(2d,p)//B3LYP/6-31G* energies for many C30H51O+ intermediates to establish the first comprehensive energy profiles for the cationic cyclization of oxidosqualene to lanosterol, lupeol, and hopen-3beta-ol. Differences among these 3 profiles were attributed to ring strain, steric effects, and proton affinity. Modest activation energy barriers and the ample exothermicity of most annulations indicated that the cationic intermediates rarely need enzymatic stabilization. The course of reaction is guided by hyperconjugation of the carbocationic 2p orbital with parallel C-C and C-H bonds. Hyperconjugation for cations with a horizontal 2p orbital (in the plane of the ABCD ring system) leads to annulation and ring expansion. If the 2p orbital becomes vertical, hyperconjugation fosters 1,2-methyl and hydride shifts. Transition states leading to rings D and E were bridged cyclopropane/carbonium ions, which allow ring expansion/annulation to bypass formation of undesirable anti-Markovnikov cations. Similar bridged species are also involved in many cation rearrangements. Our calculations revealed systematic errors in DFT cyclization energies. A spectacular example was the B3LYP/6-311+G(2d,p)//B3LYP/6-31G* prediction of endothermicity for the strongly exothermic cyclization of squalene to hopene. DFT cyclization energies for the 6-311+G(2d,p) basis set ranged from reasonable accuracy (mPW1PW91, TPSSh with 25% HF exchange) to underestimation (B3LYP, HCTH, TPSS, O3LYP) or overestimation (MP2, MPW1K, PBE1PBE). Despite minor inaccuracies, B3LYP/6-31G* geometries usually gave credible mPW1PW91 single-point energies. Nevertheless, DFT energies should be used cautiously until broadly reliable methods are established.     

已烯自由基阳离子的密度泛函理论研究

使用密度泛函理论B3LYP方法和6-31G(d,p),6-31+G(d,p),6-311G(d,p)及6- 311+G(d,p)基组,分别对1-C_6H_(12)~+,2-C_6H_(12)~+和3-C_6H_(12)~+的各种构 象进行了几何构型优化,并在B3LYP/6-311G(d,p)水平上进行了频率分析计算,在 各优化构型上,使用B3LYP和MP2(full)方法进行了超精细结构的计算。计算的3- C_6H_(12)~+的超精细偶合常数比以往的计算结果更好;1-C_6H_(12)~+和2-C_6H_ (12)~+的超精细偶合常数目前尚无实验数据报道,本计算预言了它们的超精细偶合 常数和最稳定构型。     

CH_3SH+CN微观反应机理的理论研究

采用BMC-CCSD//B3LYP/6-311G(d,p)方法对CH3SH+CN反应机理进行了详细的理论研究.反应中涉及的各稳定点的构型、振动频率和零点能在B3LYP/6-311G(d,p)水平下计算得到,计算结果表明,该反应存在两种反应机理,5条可能的反应通道.SN2机理由于能垒太高,与直接氢抽提机理相比可以忽略.该反应的最可行通道为CN中的C原子进攻SH中的H原子经由一个前期和一个后期分子络合物生成产物CH3S和HCN.计算得到的反应焓变与已有实验值非常吻合.     

戊烯自由基阳离子的密度泛函理论研究

使用密度泛函理论B3LYP方法和6-31G(d,p)、6-31＋G(d,p)、6-311G(d,p)及6-311＋G(d,p)基组，分别对2-C5H10＋和1-C5H10＋的各种构象进行了几何构型优化，并用B3LYP/6-311G(d,p)进行了频率分析计算.计算预言1-C5H10＋具有非平面构型，与以往报导的从头算计算结论相反.在两个自由基阳离子的各种构象的B3LYP几何构型上，进行了B3LYP和UMP2(full)方法的超精细偶合常数计算，得到了比以往更好的结果.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

A computational study of the reactions of thiiranes with ammonia and amines

The relative rates of reaction of thiirane and thiirane derivatives with NH3, a series of secondary amines including aziridine, and trimethylamine were determined in the gas phase by means of B3LYP/6-31+G(d)//HF/6-31+G(d) computations and transition state theory. Convergence of the results was selectively tested using the 6-311++G(d,p) basis set. Comparison with MP2/6-31 + G(d)//MP2/6-31G(d) computations was made in model cases. These results are significant in that they supplement the only reported gas-phase experimental study of this type of reaction. The reaction rates of thiirane with secondary amines can best be rationalized by means of an interplay of steric and polarizability effects. While beta-halo substituents retard S(N)2 reactions in solution, both 2-fluorothiirane and its acyclic model react more than l0(6) times faster with NH3 than the unsubstituted compounds in the gas phase. 2-Fluorothiirane was calculated to react with NH3 at C2 by a factor of 0.142 with respect to thiirane itself; attack at C3 was found to be 3.42 x 10(6) times faster than the parent compound. 2-Methylthirane reacts with NH3 at 0. 230 the rate of thiirane with a 12.8-fold regioselectivity for C3. In the reaction of 2,2-dimethylthirane and NH3, this preference for C3 increases to a factor of 124. Ground-state destabilization of cis-2,3-dimethylthiirane is sufficient to account for its calculated rate acceleration with respect to the trans isomer.     

OH-与CH2ClF反应的阴离子产物通道

理论研究了羟基负离子(OH^-)与氟氯代甲烷(CH₂ClF)反应的阴离子产物通道. 分别在B3LYP/6-31+G(d,p)和B3LYP/6-311++G(2d,p)水平上得到反应势能面上各关键物种的优化构型, 进而计算得到谐振频率和零点能. 基于CCSD(T)/6-311+G(3df,3dp)水平的相对能量, 描述了由质子转移和双分子亲核取代(S_N2)过程生成各阴离子产物的途径. 各阴离子产物途径势垒的计算结果表明质子转移过程是实验中的主要产物通道, 与以往实验测量的结论相符. 此外, 计算还显示双分子亲核取代过程得到了非典型的阴离子产物, 其中动力学效应可能会导致F^-的生成.     

丁烯和丁烷自由基阳离子的分子结构和超精细结构的 理论研究

用密度函数理论B3LYP方法和6-31G(d,p),6-311G(d,p)及6-311+G(d,p)基组，分别对1-C4H^+~8,2-C4H^+~8和C4H^+~10进行了构型优化和频率分析计算，预言1-C4H^+~8具有非平面构型，与以往报道的从头算和密度函数理论计算结果不同。在各自由基阳离子的B3LYP构型上，进行了B3LYP、MP2及MRSDCI方法的超精细偶合常数计算，得到了比以往更好的结果，特别是MP2/B3LYP计算值是至今与实验值符合得最好的理论计算结果。     

Neutral homoaromaticity in some heterocyclic systems

[reaction: see text] Neutral homoaromaticity has been evaluated in heterocyclic systems related to the bicyclo[3.2.1]octane skeleton with replacement of CH(2) at C-2 in bicyclo[3.2.1]octa-3,6-diene with X = BH, AlH, Be, Mg, O, S, PH, NH (12); replacement of CH at C-3 in bicyclo[3.2.1]octa-3,6-dien-2-yl anion with PH, S, NH, O (13); and replacement at C-2 and C-3 with N and O (14). Stabilization energies (SE) are evaluated using density functional theory and homodesmotic equations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level for series 12-14. Stabilization energies are compared with diamagnetic susceptibility exaltations, Lambda, CSGT-B3LYP/6-31G(d)//B3LYP/6-31G(d), and nucleus-independent chemical shifts (NICS), GIAO-B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d). Analysis of frontier orbitals and geometries, B3LYP/6-31G(d)//B3LYP/6-31G(d), and proton affinities of 2-azabicyclo[3.2.1]octa-3,6-diene, pyrrole, and related model systems, B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d), provide complementary evidence supporting the division of the substrates evaluated into antihomoaromatic (12, X = BH, AlH, and Be), nonhomoaromatic (12, X = O, S, NH, PH), and homoaromatic (13, X = S, PH, NH, O and 14 X = ON), with 12 (X = Mg) appearing as transitional between anti- and nonhomoaromatic.     

3,5,7,9-Substituted hexaazaacridines: toward structures with nearly degenerate singlet-triplet energy separations

Toward the goal of preparing stable, neutral open-shell systems, we synthesized a novel series of p-phenyl-substituted 3,5,7,9-hexaazaacridine and 3,5,7,9-hexaazaanthracene derivatives. The effects of substitution on the molecular electronic properties were probed both experimentally and computationally [B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p)]. While the experimentally prepared structures already have small (20-25 kcal/mol) singlet-triplet energy gaps, systems with even smaller (<9 kcal/mol) singlet-triplet energy separations can be realized through systematic variation of the substituent numbers, types, and patterns. Hexaazaanthracenes show generally smaller singlet-triplet energy gaps than hexaazaacridines. Nitrogen-bonded sigma- and pi-acceptor substituents that cause positive inductive and mesomeric effects as well as carbon-bonded sigma-donor substituents make substituted hexaazaanthracenes promising candidates for purely organic high-spin systems.