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Enhanced thermodynamic stability is a fundamental characteristic of aromatic molecules, yet most previous studies of aromatic stabilization energy (ASE) have been limited to small rings with up to 18 π-electrons. Here we demonstrate that ASE can be detected experimentally in π-conjugated porphyrin nanorings with Hückel circuits of 76-108 π-electrons. This conclusion is supported by analyzing redox potentials to calculate the energy change for isodesmic reactions that convert an aromatic ring to an antiaromatic ring or vice versa. It is also supported by analyzing the energy barriers to conformational equilibria that disrupt aromaticity in the transition state. Both types of experiment indicate that cationic porphyrin nanorings display ASEs of 1-5 kJ mol-1. Density functional theory calculations reproduce the results for both types of experiment and predict ASEs in the range of 1-16 kJ mol-1. The experimental ASEs in porphyrin nanorings are compared with an experimental ASE of [18]annulene of ∼11 kJ mol-1, deduced from analysis of the energy barriers to conformational equilibria in [16], [18], and [20]annulene. Calculated energies of isodesmic reactions give an ASE of ∼37 kJ mol-1 in [18]annulene. This work contributes to a fundamental understanding of aromaticity in large macrocycles.

The supramolecular polymerization of 2,11-dithia[3.3]paracyclophanes through self-complementary intermolecular and transannular amide hydrogen bonding is presented. An n → π* interaction between the amide hydrogen bonding units and the central bridging atom results from the single-point exchange of a carbon atom for a sulfur atom. This orbital donor-acceptor interaction can be strengthened by oxidizing the sulfide to a sulfone which acts to shorten the donor···acceptor distance and increase orbital overlap. Experimental signatures of the increased n → π* interaction include larger isodesmic polymerization elongation constants in solution, changes in characteristic bond stretching frequencies, and geometric/structural changes evaluated by X-ray crystallography. The experimental data are supported by extensive computational investigations of both assembling and nonassembling 2,11-dithia[3.3]paracyclophanes as well as a rationally designed model system to confirm the role of stereoelectronic effects on supramolecular polymer assembly.

Various rhodium(I) pincer complexes with different structural features have been prepared and found to display interesting self-assembly properties due to the extensive Rh(I)···Rh(I) interactions. The incorporation of electron-withdrawing −CF3 substituent has been found to improve the stability of the complexes and also facilitate the directed assembly of complex molecules, providing an opportunity for the systematic investigation of the various noncovalent interactions in their versatile self-assembly behaviors and insights into the structure–property relationship in governing the intermolecular interactions. An isodesmic growth mechanism is identified for the solvent-induced aggregation process. The complex molecules exhibit intense low-energy absorption bands corresponding to the absorptions of the dimers, trimers, and higher order oligomers upon aggregation, with energies related to the electronic properties of the tridentate N-donor ligand. Chiral auxiliaries have also been introduced into the rhodiu...

Pathway complexity in supramolecular polymerization has recently sparked interest as a method to generate complex material behavior. The response of these systems relies on the existence of a metastable, kinetically trapped state. In this work, we show that strong switch-like behavior in supramolecular polymers can also be achieved through the introduction of competing aggregation pathways. This behavior is illustrated with the supramolecular polymerization of a porphyrin-based monomer at various concentrations, solvent compositions, and temperatures. It is found that the monomers aggregate via an isodesmic mechanism in weakly coupled J-type aggregates at intermediate solvent quality and temperature, followed by nucleated H-aggregates at lower solvent qualities and temperatures. At further increased thermodynamic driving forces, such as high concentration and low temperature, the H-aggregates can form hierarchical superhelices. Our mathematical models show that, contrary to a single-pathway polymerization, the existence of the isodesmic aggregation pathway buffers the free monomer pool and renders the nucleation of the H-aggregates insensitive to concentration changes in the limit of high concentrations. We also show that, at a given temperature or solvent quality, the thermodynamically stable aggregate morphology can be selected by controlling the remaining free external parameter. As a result, the judicious application of pathway complexity allows us to synthesize a diverse set of materials from only a single monomer. We envision that the engineering of competing pathways can increase the robustness in a wide variety of supramolecular polymer materials and lead to increasingly versatile applications.

The potential advantages of using arylboronic esters as boron sources in C-H borylation are discussed. The concept is showcased using commercially available 2-mercaptopyridine as a metal-free catalyst for the transfer borylation of heteroarenes using arylboronates as borylation agents. The catalysis shows a unique functional group tolerance among C-H borylation reactions, tolerating notably terminal alkene and alkyne functional groups. The mechanistic investigation is also described.

The "alkane branching effect" denotes the fact that simple alkanes with more highly branched carbon skeletons, for example, isobutane and neopentane, are more stable than their normal isomers, for example, n-butane and n-pentane. Although n-alkanes have no branches, the "kinks" (or "protobranches") in their chains (defined as the composite of 1,3-alkyl-alkyl interactions-including methine, methylene, and methyl groups as alkyl entities-present in most linear, cyclic, and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-self-consistent field (SCF) treatments, which account for medium range (∼1.5-3.0 Å) electron correlation. Localized molecular orbital second-order Møller-Plesset (LMO-MP2) partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3-alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes.

Amino-acid functionalized naphthalenediimides self-assemble into hydrogen-bonded supramolecular helical nanotubes via a noncooperative, isodesmic process; the self-assembly of ordered helical systems is usually realized through a cooperative process. This unexpected behavior was rationalized as a manifestation of entropy-enthalpy compensation. Fundamental insights into the thermodynamics governing this self-assembly were obtained through the fitting of the isodesmic model to (1)H NMR spectrometry and circular dichroism spectroscopy measurements. Furthermore, we have extended the application of this mathematical model, for the first time, to quantitatively estimate the effect of guests, solvents, and side chains on the stability of the supramolecular nanotube; most significantly, we demonstrate that C(60) acts as a template to stabilize the nanotube assembly and thereby substantially increase the degree of polymerization.

The self-assembly of DNA dumbbell conjugates possessing hydrophobic perylenediimide (PDI) linkers separated by an eight-base pair A-tract has been investigated. Cryo-TEM images obtained from dilute solutions of the dumbbell in aqueous buffer containing 100 mM NaCl show the presence of structures corresponding to linear end-to-end assemblies of 10-30 dumbbell monomers. The formation of assemblies of this size is consistent with analysis of the UV-vis and fluorescence spectra of these solutions for the content of PDI monomer and dimer chromophores. Assembly size is dependent upon the concentration of dumbbell and salt as well as the temperature. Kinetic analysis of the assembly process by means of salt-jump stopped-flow measurements shows that it occurs by a salt-triggered isodesmic mechanism in which the rate constants for association and dissociation in 100 mM NaCl are 3.2 × 10(7) M(-1)s(-1) and 1.0 s(-1), respectively, faster than the typical rate constants for DNA hybridization. TEM and AFM images of samples deposited from solutions having higher concentrations of dumbbell and NaCl display branched assemblies with linear regions >1 μm in length and diameters indicative of the formation of small bundles of dumbbell end-to-end assemblies. These observations provide the first example of the use of hydrophobic association for the assembly of small DNA duplex conjugates into supramolecular polymers and larger branched aggregates.

This study examines the use of transition-metal hydride complexes that can be generated by the heterolytic cleavage of H(2) gas to form B-H bonds. Specifically, these studies are focused on providing a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to three-coordinate BX(3) (X = OR, SPh, F, H; R = Ph, p-C(6)H(4)OMe, C(6)F(5), (t)Bu, Si(Me)(3)) compounds will be favorable. This involves both experimental and theoretical determinations of hydride transfer abilities. Thermodynamic hydride donor abilities (DeltaG(o)(H(-))) were determined for HRh(dmpe)(2) and HRh(depe)(2), where dmpe = 1,2-bis(dimethylphosphinoethane) and depe = 1,2-bis(diethylphosphinoethane), on a previously established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability of [HBEt(3)](-) on this scale. Isodesmic reactions between [HBEt(3)](-) and selected BX(3) compounds to form BEt(3) and [HBX(3)](-) were examined computationally to determine their relative hydride affinities. The use of these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX(3) compounds is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings indicate that it is possible to form B-H bonds from B-X bonds, and the extent to which BX(3) compounds are reduced by transition-metal hydride complexes forming species containing multiple B-H bonds depends on the heterolytic B-X bond energy. An example is the reduction of B(SPh)(3) using HRh(dmpe)(2) in the presence of triethylamine to form Et(3)N-BH(3) in high yields.

Because of their great flexibility and strength resistance, both spider silks and silkworm silks are of increasing scientific and commercial interest. Despite numerous spectroscopic and theoretical studies, several structural properties at the atomic level have yet to be identified. The present theoretical investigation focuses on these issues by studying three silk-like model peptides: (AG)(64), [(AG)(4)EG](16), and [(AG)(4)PEG](16), using a Lego-type approach to construct these polypeptides. On the basis of these examples it is shown that thermoneutral isodesmic reactions and ab initio calculations provide a capable method to investigate structural properties of repetitive polypeptides. The most probable overall fold schema of these molecules with respect to the type of embedded hairpin structures were determined at the ab initio level of theory (RHF/6-311++G(d,p)//RHF/3-21G). Further on, analysis is carried out on the possible hairpin and turn regions and on their effect on the global fold. In the case of the (AG)(64) model peptide, the optimal beta-sheet/turn ratio was also determined, which provided good support for experimental observations. In addition, lateral shearing of a hairpin "folding unit" was investigated at the quantum chemical level to explain the mechanical properties of spider silk. The unique mechanical characteristics of silk bio-compounds are now investigated at the atomic level.

The usual assumption of the extra stability of icosahedral boranes (2) over pentagonal-bipyramidal boranes (1) is reversed by substitution of a vertex by a group 13 metal. This preference is a result of the geometrical requirements for optimum overlap between the five-membered face of the ligand and the metal fragment. Isodesmic equations calculated at the B3LYP/LANL2DZ level indicate that the extra stability of $1-M-2,4-C_2B_4H_7$ varies from 14.44 kcal/mol (M= ) Al) to 15.30 kcal/mol (M= ) Tl). Similarly, $M(2,4-C_2B_4H_6)_2^{1-}$ is more stable than $M(2,4-C_2B_9H_11)_2^{1-}$ by 9.26 kcal/mol (M= Al) and by 6.75 kcal/mol (M= Tl). The preference for $(MC_2B_4H_6)_2$ over $(MC_2B_9H_11)_2$ at the same level is 30.54 kcal/mol (M= Al), 33.16 kcal/ mol (M= Ga) and 37.77 kcal/mol (M= In). The metal-metal bonding here is comparable to those in CpZn-ZnCp and $H_2M-MH_2$ (M= Al, Ga, and In).

The mechanism of the C(6)H(5) + C(2)H(2) reaction has been investigated by various quantum chemical methods. Electrophilic addition to the CC triple bond is found to be the only important mode of phenyl radical attack on acetylene. The initially formed chemically activated C(6)H(5)C(2)H(2) adducts may follow several isomerization pathways in competition with collisional stabilization and H-elimination. Thermochemistry of various decomposition and isomerization channels is evaluated by the G2M method. For key intermediates, the following standard enthalpies of formation have been deduced from isodesmic reactions: 94.2 +/- 2.0 kcal/mol (C(6)H(5)CHCH), 86.4 +/- 2.0 kcal/mol (C(6)H(5)CCH(2)), and 95.5 +/- 1.8 kcal/ mol (o-C(6)H(4)C(2)H(3)). The accuracy of theoretical predictions was examined through extensive comparisons with available experimental and theoretical data. The kinetics and product branching of the C(6)H(5) + C(2)H(2) reaction have been evaluated by weak collision master equation/Rice-Ramsperger-Kassel-Marcus (RRKM) analysis of the truncated kinetic model including only kinetically important transformations of the isomeric C(8)H(7) radicals. Available experimental kinetic data can be quantitatively reproduced by calculation with a minor adjustment of the C(6)H(5) addition barrier from 3.7 to 4.1 kcal/mol. Our predicted total rate constant, k(R1) = (1.29 x 10(10))T(0.834) exp(-2320/T) cm(3) mol(-)(1) s(-)(1), is weakly dependent on P and corresponds to the phenylation process under combustion conditions (T1000 K).

The gradient corrected Perdew-Burke-Ernzerhof density functional in conjunction with a 3-21G basis set and periodic boundary conditions was employed to investigate the geometries and energies of C(2)F fluorinated armchair single wall carbon nanotubes (F-SWNT's) with diameters ranging from 16.4 to 4.2 A [(12,12) to (3,3)] as well as a C(2)F graphene sheet fluorinated on one side only. Using an isodesmic equation, we find that the thermodynamic stability of F-SWNT's increases with decreasing tube diameter. On the other hand, the mean bond dissociation energies of the C-F bonds increase as the tubes become thinner. The C-F bonds in the (5,5) F-SWNT's are about as strong as those in graphite fluoride (CF)(n)() and are also covalent albeit slightly (0.04 A) stretched. Whereas a fluorine atom is found not to bind covalently to the concave surface of [60]fullerene, endohedral covalent binding is possible inside a (5,5) SWNT despite a diameter similar to that of the C(60) cage.

Computations on 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarbene (1) using ab initio and density functional theory methods underscore the unusual stability of the triplet over the singlet state. At the B3LYP/6-311G(d,p) level, the triplet state had a slightly bent central C-C-C bond angle of 167 degrees, whereas this angle in the singlet was 134 degrees. The B3LYP singlet-triplet splitting (12.2 kcal/mol) was larger than that of the parent molecule (5.8 kcal/mol), diphenylcarbene (2), which also has a triplet ground state. The energy of a suitable isodesmic reaction showed the triplet and singlet states of (1) to be destabilized, by 6.3 and 12.5 kcal/mol, respectively, due to the combined effects of the CF3, Br, and alkyl substituents. The linear-coplanar form of (3)(1), which might facilitate dimerization or electrophilic attack at the more exposed diradical center, was prohibitively (35.9 kcal/mol) higher in energy. Our results confirm Tomioka's conclusion that the triplet diarylcarbene, ortho-substituted with bulky CF3 and Br substituents, is persistent due to steric protection of the diradical center. Dimerization and other possible reaction pathways are inhibited, not only by the bulky ortho substituents but also by the para alkyl groups. The increase in stability of the triplet ((3)(1)) state relative to the singlet ((1)(1)) state does not influence the reactivity directly.

Ab initio molecular orbital and density functional calculations on isodesmic disproportionation reactions of nitrated cubanes indicate that the repulsion between nitro groups on adjacent carbons in octanitrocubane (ONC) is surprisingly small and that the energy required for successive substitutions of nitro groups on the cubane frame diminishes progressively with increasing substitution. Mulliken population analyses show that, as a consequence of the inductive effects of the nitro groups on one another, the negative charges on the oxygens also decrease with increasing substitution reducing the magnitudes of the Coulomb repulsion between nearest-neighbor nitro groups. The computed D4 symmetric equilibrium conformation of ONC, which is essentially the same as the conformation found in the crystalline state, effectively interdigitates the nitro substituents, thereby avoiding close O···O contacts. The O···O interactions remain very small throughout 180° rotations about the C−NO2 bonds, a process that is compu...

Substituent effects on the mechanism of the Cope rearrangement were studied by computing C2h (Cs) cuts through the potential energy surface (PES) for the reaction of 1,5-hexadiene and its di-, tri-, and tetracyano derivatives at the (U)B3LYP/6-31G* and (U)BPW91/6-31G* levels. The stabilization of substituted structures along the cuts is discussed in terms of the energies of isodesmic formation from allyl radicals and acetonitrile molecules. Cyano groups at C1, C3, and C5 provide a nearly additive stabilization of each point along the Cs cut even though their influence on the geometry is competitive. Evaluation of the density of effectively unpaired electrons at various geometries indicates that the radical character of a transition state (TS) is not altered by radical stabilizing substituents as such but depends solely on the interallylic bond length. Although the UB3LYP diyl intermediate for the parent compound is plausible when compared to the lowest triplet PES, neither MRPT2 at the UB3LYP geometries n...

Formaldonitrone, CH2N(H)O (1), the hitherto elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2N(H)O+• (1+•). Ion 1+• was generated by dissociative ionization of 1,2-oxazolidine and characterized by collisionally activated dissociation mass spectra and augmented Gaussian 2(MP2) calculations. Nitrone 1 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime (2) and nitrosomethane (3). 1 was calculated to be more stable than its isomers CH2−O−NH (5) and oxaziridine (6). The enthalpy of formation of 1 was calculated from enthalpies of atomization and two isodesmic reactions as ΔHf,298(1) = 58 ± 1 kJ mol-1. The adiabatic and vertical ionization energies of 1 were calculated as IEa = 9.40 eV and IEv = 9.42 eV, the vertical recombination energy of 1+• was REv = 9.35 eV. Formation of 1 by collisional electron transfer was accompanied by negligible Franck−C...

Using ab initio MO theory, a series of isodesmic reactions was studied in which nitrogen-, oxygen-, and fluorine-containing species acted as proton donors and acceptors. Comparison of protonation and deprotonation energies suggests that approximately three-quarters of the enhanced acidity of acetic acid comes from electrostatic stabilization, while the remaining quarter results from π resonance. Similar logic shows that only one-third of the enhanced acidity of the nitrogen analogue acetamidine is electrostatic in nature, and that the remaining two-thirds derives from resonance. The primary importance of electrostatics for oxygen and of π resonance for nitrogen is further supported by the behavior of carbonic acid and guanidine. The contributions of hydroxy, amino, and fluorine substituents to the acidity and basicity of a series of alcohols and amines are well described by a simple electrostatic model with a single adjustable parameter. The model requires the electrostatic contributions of polar bonds to...

The energies of acetic acid and its CH3C(O)X derivatives and of the corresponding enols H2CC(OH)X, for X = OH, NH2, NMe2, OMe, OCHO, F, Cl, and Br, were calculated by the MO ab initio method MP2(full)/6-31G**, by single-point CCSD(T)(full)/6-311G**/MP2(full)/6-31G**), and by the hybrid density functional method B3LYP/6-31G**. The calculated pKEnol = −log KEnol values for the “keto”/enol equilibria are all high (17.5−24.0) and follow the order for X:H < alkyl < OCHO < Br∼Cl < F < NH2 < NMe2 < OH, OMe. By using the appropriate isodesmic reactions, it was shown that all substituents stabilize more the acid species than its enol (where Br and Cl are slightly destabilizing). All the computational methods display a similar trend. The structures and energies of the various conformations of both species are given. Comparison with the scarce experimental pKEnol data shows a reasonable agreement with the calculations.

Quantum mechanical ab initio calculations at the MP2/VDZ+P level of theory (for some molecules at MP2/VTZ+D+P) are reported for the cations AX3+ and AH2X+ (A = C, Si, Ge, Sn, Pb; X = F, Cl, Br, I) and the isoelectronic neutral Lewis acids YX3 and YH2X (Y = B, Al, Ga, In, Tl; X = F, Cl, Br, I). The π-donor ability of the halogens given by the p(π) population at atom A increases for all cations with F < Cl < Br < I, which is the opposite to what has been reported in a recent study of Olah and co-workers (J. Am. Chem. Soc. 1996, 118, 3580). The same trend is found at the central atom Y of most neutral compounds, but the increase is less pronounced than for the cations. The calculated order of the energetic stabilization of the cations and neutral compounds by the halogens is less uniform, because the σ- and π-donation show different trends. The energies of isodesmic reactions and the X3A+−OH2 complexation energies predict for the thermodynamic stabilization of the Y-conjugated cations the same trend as the π...

High-level ab initio computations were used to characterize the parent species and αC radicals for alanine, serine, and threonine, both as free neutral amino acids (AH) and as residues in model peptides (PH) intended to mimic the midchain environment in proteins. The ab initio energies were used in isodesmic reactions to predict bond dissociation energies (BDEs, DαCH) at 298 K, in kJ mol-1, to an estimated accuracy of ±10 kJ mol-1. For the fully optimized systems the values of DαCH are AH(Gly), 331; AH(Ala), 317; AH(Ser), 327; AH(Thr), 328; PH(Gly), 348; PH(Ala), 344; PH(Ser), 348; PH(Thr), 356. All of the DαCH values are less than the BDE of a typical SH bond (370 kJ mol-1), as in cysteine or glutathione (GSH), a result that suggests that oxidative damage at the αC site will not be repaired efficiently by the mechanism of H donation from GSH. Values of DαCH in typical peptide conformations, such as β-sheet and α-helical secondary structure, were estimated by constraining the Ramachandran dihedral angles,...

The stabilization afforded a vinyl cation by a β-(CH3)3Si substituent has been determined by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of three alkynes (RCCR‘) with (CH3)3Si+ and with the proton. The measured −ΔH° (kcal mol-1) and −ΔS° (in parentheses, cal K-1 mol-1) values for the reaction (CH3)3Si+ + RCCR‘ ⇄ (CH3)3Si·C(R)CR‘+ are as follows: 1-hexyne (R = H, R‘ = n-C4H9) 25.9 ± 1.5 (19.1 ± 0.2), 2-hexyne (R = CH3, R‘ = n-C3H7) 28.8 ± 1.4 (25.5 ± 0.3), and phenylacetylene (R = H, R‘ = C6H5) 28.2 ± 2.8 (16.5 ± 0.4). By comparison the values for 1-hexene which forms an alkyl cation are 38.2 ± 0.5 kcal mol-1 (48.2 ± 0.1 cal K-1 mol-1). The deduced stabilizations (A) for all the substituents (R, R‘ and (CH3)3Si) obtained from the isodesmic reaction (CH3)3Si·C(R)CR‘+ + CH2CH2 → (CH3)3Si·C(R)C(H)R‘ + CH2CH+ are (kcal mol-1) as follows: 1-hexyne 55, 2-hexyne 58, and phenylacetylene 58. The deduced stabilization for the (CH3)3Si+ adduct of 1-hexene relative to th...

The influence of the ratio between poor and good solvent on the stability and dynamics of supramolecular polymers is studied via a combination of experiments and simulations. Step-wise addition of good solvent to supramolecular polymers assembled via a cooperative (nucleated) growth mechanism results in complete disassembly at a critical good/poor solvent ratio. In contrast, gradual disassembly profiles upon addition of good solvent are observed for isodesmic (non-nucleated) systems. Due to the weak association of good solvent molecules to monomers, the solvent-dependent aggregate stability can be described by a linear free-energy relationship. With respect to dynamics, the depolymerization of p-conjugated oligo(p-phenylene vinylene) (OPV) assemblies in methylcyclohexane (MCH) upon addition of chloroform as a good solvent is shown to proceed with a minimum rate around a critical chloroform/MCH solvent ratio. This minimum disassembly rate bears an intriguing resemblance to phenomena observed in protein unfolding, where minimum rates are observed at the thermodynamic midpoint of a protein denaturation experiment. A kinetic nucleation–elongation model in which the rate constants explicitly depend on the good solvent fraction is developed to rationalize the kinetic traces and further extend the insights by simulation. It is shown that cooperativity, i.e., the nucleation of new aggregates, plays a key role in the minimum polymerization and depolymerization rate at the critical solvent composition. Importantly, this shows that the mixing protocol by which one-dimensional aggregates are prepared via solution-based processing using good/poor solvent mixtures is of major influence on self-assembly dynamics.

The presence of two (4n+2)-electron conjugated systems in perpendicular planes results in considerable aromatic stabilization. Despite having two fewer hydrogens, the 6 pi e-2 sigma e 3,5-dehydrophenyl cation (C6H3+, 1) is 32.7 (CCSD(T)/6-31G**) and 35.2 kcal/mol (RMP4sdtq/6-3iG*//RMP2(fu)/6-31G*) more stable than the phenyl cation (evaluated by an isodesmic reaction involving benzene and m-dehydrobenzene (4)). Cation 1, the global C6H3+ minimum, is 11.7,24.2, 11.8, and 30.4 kcal/mol lower in energy than the 2,6- (11) and 3,4-dehydrophenyl (12) cations as well as the open-chain isomers 13 and 14 (RMP4sdtq/6-31G*//RMP2(fu)/6-31G* + ZPE(RMP2(fu)/6-31G*)). The stability of 1 is increased hyperconjugatively by 2,4,6-trisilyl substitution. The double aromaticity of 1 is indicated by the computed magnetic susceptibility exaltations (IGLO/II//RMP2(fu)/6-31G*) of -5.2, -6.8, -15, and -23.2 relative to 11, 12, 13, and 14, respectively. Thus, 1 fulfills the geometric, energetic, and magnetic criteria of aromaticity. The double aromaticity of the D-6h cyclo[6]carbon is apparent from the same criteria

Two classical terms «Steric Hindrance to Resonance» and «Buttressing Effect» are revisited on the basis of the gas-phase acidities of nine methyl-substituted benzoic acids and of the gas-phase basicities of their methyl esters, measured using FT-ICR spectrometry. By combining these data with published heats of formation of the neutrals and by using the principle of isodesmic reactions, relative enthalpies of formation were evaluated separately for the acid molecules, their anions (deprotonated), and their protonated cations (substituted by the protonated forms of the corresponding methyl esters). Energies of all species were also calculated at the semiempirical level (AM1)

The structures, total energies, singlet-triplet splittings, and absolute heats of formation of o-, m-, and p-benzyne have been calculated with the use of a variety of multiconfigurational self-consistent field (MCSCF) and configuration-interaction (CI) methods. The performance of each method is evaluated by comparison of the calculated singlet-triplet energy difference and absolute heat of formation for o-benzyne with the experimentally-determined values. Correlation-consistent CI (CCCI) methods, when used in conjunction with relatively large basis sets and molecular structures derived from MCSCF-based geometry optimizations, are found to give the best agreement, although the performance of larger-scale CI (e.g., CISD) calculations is comparable. All three benzyne isomers are found to have singlet biradical ground states at each of the levels of theory used. The most probable values for the singlet-triplet splittings in o-, m-, and p-benzyne derived from the CI calculations are 36 [+-] 2, 17 [+-] 1, and 2.2 [+-] 0.5 kcal/mol, respectively. The energetics of the hypothetical isodesmic reaction between each of the benzynes and benzene to produce two phenyl radicals have been evaluated. These energy changes are discussed in terms of the stabilization or destabilization of the singlet and triplet states of each biradical with respect to simple bond-strength additivity models.more » 78 refs., 2 figs., 7 tabs.« less

The series NH 3 , H 2 NNH 2 , HN(NH 2 ) 2 (triazane), and N(NH 2 ) 3 (triaminoammonia) has been studied by computational methods (HF/6-31G * and MP2/6-31G * optimizations for all species, G2 (see paragraphoncomputations) energies for the most stable form of each molecule). Thermodynamic functions have been computed. Structural aspects have been discussed in terms of stabilizing and destabilizing factors. The importance of hydrogen bonding and hyperconjugation has been discussed on the basis of potential surface differences and vibrational spectra. Relative stabilities have been discussed from isodesmic reaction schemes, from which also the following estimates of N-N bond energy are obtained: hydrazine, 64 kcal/mol; triazane, 66 kcal/mol; triaminoammonia, 59 kcal/mol

The results of a uniform set of ab initio quantum-chemical calculations for 10 carbon/nitrogen cubanoids are presented. There are 22 possible C/N cubanoids with formulas (CH)[sub 8[minus]n]N[sub n], where 0 [le] n [le] 8. We give results for one cubanoid for each value of n; for n = 4 both the T[sub d] and C[sub 4v] structures are discussed. The geometries were optimized on the RHF/6-31G*//RHF/6-31G* energy hypersurfaces, and the stationary points so obtained were characterized by vibrational analyses. The effect of electron correlation on the energies of these structures is examined via MP2/6-31G*//RHF/6-31G* theory. All 10 structures are predicted to be stable molecules, and the vibrational analyses indicate that their geometries are well-defined by the RHF/6-31G*//energy hypersurfaces. Heats of formation are predicted via appropriate isodesmic reactions. All the structures are found to have large positive [delta]H[sub f]'s. Perhaps the most interesting result obtained is that the high-energy content of the nitrogen-rich cubanoids is not primarily due to bond strain, but rather to the high-energy content of the NN single bond. Comparisons of the ab initio [delta]H[sub f] values with semiempirical AM1, PM3, and MNDO predictions are made. If they can be synthesized and have reasonable kinetic stability, the C/Nmore » cubanoids containing four or more nitrogen atoms are likely to be important energetic materials. A brief discussion is given of the potential of these materials as propellants and explosives. 22 refs., 5 figs., 4 tabs.« less

Energies, electronic structures, gas-phase proton affinities, and isodesmic enthalpies (ΔH 0 iso ) of a series of methyl-substitutcd oxonium and sulfonium dications and carboxonium dications (di-O-protonated carbonyl compounds) were calculated using ab initio molecular orbital theory. On the basis of computed energetics, the proton affinities and decomposition barriers were also estimated. The role of dicationic superelectrophiles in a number of superacid-catalyzed reactions is discussed

Isodesmic energy comparisons of 6-31G*//6-31G* calculations for the fulvenes 1-3 and fulvenones 4-6 show significant destabilization for triafulvenone (4, 17.1 kcal) and heptafulvenone (6, -5.1 kcal/mol) and stabilization for pentafulvenone (5, 4.1 kcal/mol). These effects are attributed to enhanced antioaromatic destabilization in 4 and 6 compared to the corresponding fulvenes and enhanced aromatic stabilization in pentafulvenone (5) compared to pentafulvene (2). Analysis of dipole moments, atomic charges, and bond lengths provides further evidence for these aromaticity effects. Similar evidence is found for antiaromatic destabilization upon fluorine substitution in the exocyclic methylene group of triafulvene

Ab initio electronic structure calculations have been carried out on a series of heteropolar inorganic analogues of hydrocarbons in order to evaluate reaction energies for an isodesmic reduction by ethane and homodesmotic reactions as indicators of the relative aromaticities of the recently synthesized X 3 Y 3 R 6 rings where XY is BP and AIN. Calculations where XY is CC and BN are also reported to establish the scale. Optimized geometries and energies through RHF/6-31 G* are reported for both reactions, and energies at the SCF optimized geometries including fourth-order Moller-Plesset perturbation corrections are reported for the homodesmotic reaction

Amide bonds -NH-CO- preferentially exist in trans conformations, the cis conformation being thermodynamically unfavored with respect to the trans by about 2 kcal/mol. Yet, the main reason most proteins or peptides cannot be made from cis-peptide plaques only lies in that connecting them into open chains appears to be sterically impracticable. It is possible, however, to build all-cis cyclic peptides in which all cis-plaques are efficiently locked. The present work examines, through quantum calculations, the structural and energetic issues associated with these peculiar arrangements. Systematic exploration at DFT-B3LYP level of the potential-energy surfaces for all-cis cyclopolyglycines cG(n)(c) (n = 2-10,15), and to a lesser extent, for all-cis cyclopolyalanines and all-cis cyclopolyphenylalanines confirms that all these structures are true minima. Optimal ring size occurs around eight peptide units, resulting in planar cG7(c), cG8(c), and cG9(c). In smaller systems, the ring strain is relieved through nonplanar cup-like distortions, particularly in cG6(c). From 10 peptide units and beyond, the ring framework distorts into a saddle-edge shape. These molecules disclose some molecular flexibility, as combinatorial tilting of the plaques may give sets of minima close in energy. Indexes based on isodesmic reactions are used to estimate the energy for joining all-cis or all-trans plaques into cyclic peptides. One of them, the mean plaque-junction energy (MPJE) suggests that within sensible sizes from six peptide units and beyond, all-cis plaque association is almost equally favorable as all-trans one. The frame of radiating cis-amide bonds can be considered as defining a new kind of peptidic material, endowed with specific self-assembling properties.

Reported here is a theoretical study of possible stratospheric chlorine reservoir species including isomers of chemical formula ClNO(4) and ClNO(5), in addition to the well-known ClONO(2) reservoir species. Density functional theory (DFT) in conjunction with large one-particle basis sets has been used to determine equilibrium structures, dipole moments, rotational constants, harmonic vibrational frequencies, and infrared intensities. The B3LYP functional was used since it has previously been shown to perform well for similar compounds. The equilibrium geometry and vibrational spectra of ClONO(2) are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations reported previously. Three stable isomers have been identified for each ClNO(4) and ClNO(5). The vibrational spectrum of O(2)ClONO(2) has been compared with the available experimental data and found to be in good agreement. The relative energetics of the ClNO(4) and ClNO(5) isomers have been determined using large atomic natural orbital (ANO) basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational correction for triple excitations, denoted CCSD(T). Accurate heats of formation have been evaluated by computing energies for isodesmic reactions involving the ClNO(4) and ClNO(5) isomers. The stability of these molecules with respect to thermal dissociation is examined. The present study suggests that isomers of ClNO(4) and ClNO(5) may have no atmospheric chemical relevance because the atmospheric concentrations of the necessary reactants are insufficient, but it is also found that under laboratory conditions the formation of O(2)ClONO(2) cannot be ignored.

Unrestricted density functional theory (UB3LYP), CASSCF, and CASPT2 calculations have been employed to compute the relative energies of the C(s) and C(2v) geometries of several 1,5-disubstituted semibullvalenes. Substitution at these positions with R = F, -CH(2)-, or -O- affords semibullvalenes that are predicted to have C(2v) equilibrium geometries. Calculated singlet-triplet energy splittings and the energies of isodesmic reactions are used to assess the amount of bishomoaromatic character at these geometries. The results of these calculations show that employing strain to destabilize the C(s) geometries of semibullvalenes can lead to a significant decrease in the amount of bishomoaromatic stabilization of the C(2v) geometries, due to reduced through-space interaction between the two allyl groups. However, the C(2v) equilibrium geometries of the 1,5-disubstituted semibullvalenes with R = F and -RR- = -O- do benefit from stabilizing through-bond interactions between the two allyl groups. These interactions involve mixing of the bisallyl HOMO with the low-lying C-F or C-O sigma orbital combinations of the same symmetry. In contrast, for -RR- = -CH(2)-, through-bond interactions destabilize the bisallyl HOMO and are predicted to make the ground state of this semibullvalene a triplet.

The aromaticity of all possible cyclopenta-fused pyrene congeners has been investigated at various levels of theory. On the basis of the calculated resonance energies and magnetic properties (delta(1)H data, magnetic susceptibility anisotropies, and NICS values), the overall aromaticity of these compounds is found to decrease gradually with increasing number of externally fused five-membered rings. The relatively small differences (5 kcal/mol) in thermodynamic stability of the isomeric dicyclopentapyrenes (E(tot): dicyclopenta[cd,fg]-dicyclopenta[cd,jk]-dicyclopenta[cd,mn]pyrene), which differs from the aromaticity order based on the magnetic criteria (dicyclopenta[cd,mn]-dicyclopenta[cd,fg]-dicyclopenta[cd,jk]pyrene), is shown by model calculations to be dominated by sigma-strain imposed on the pyrene skeleton by sequential cyclopenta-fusion. This is supported by the computed homodesmotic reaction energies and aromatic stabilization energy (ASE(isom)) from isodesmic aromatic-nonaromatic isomerization, and by the model calculations on "distorted" cyclopenta[cd]pyrenes. The elusive tetracyclopenta[cd,fg,jk,mn]pyrene is computed to be bowl-shaped; its corresponding planar geometry is the transition state for bowl-bowl interconversion.

In this study, we calculate the stabilization of /3-substituted ethyl cations (R = H, Li, BeH, BH,. CH,, NH,, OH, F, Na, MgH, A1H2, SiH,, PH2, SH, CI) and a-substituted methyl cations (R = H, Li, CH,, NH,, OH, F, Na, SiH,, PH,, SH, CI) in order to obtain a relationship between the nature of the substituent and the degree of stabilization of the cation. Results show that the stabilization energy is related to the electronegativity of the @ substituents, but not the CY substituents. The rotational barrier of the @-substituted ethyl cation is linearly related to the Mulliken population of the 2p(C+) orbital. We found that the stabilization energy is linearly related to the ionization potential of the aand PLA @-substituted radicals, and the HOMO energies of the PLA /3-substituted radical are linearly related to the corresponding ionization potentials. Trends in the stabilization by secondand third-row substituents are discussed. A wide variety of theoretical results have been presented which examine substituent effects in carbocation sy~tems.I-’~ Simple theoretical models have been studied as a means of interpreting and a-acceptors. For the case of carbocations the a-acceptor function is of less importance than for radicals or carbanions.I2 experimental results obtained for larger, more complex-carbocations. As a result, a relatively clear qualitative picture has emerged of the factors that can affect the stability of carbocations.‘, 99. 1291. (1) Hoffmann, R.; Radom, L.; Pople, J. A,; Schleyer, P. v. R.; Hehre, W. (2) Apeloig, Y.; Schleyer, P. v. R.; Pople, J . A. J . Am. Chem. SOC. 1977, J , ; Salem, L. J , Am. Chem, sot, 1972, 94, 6221, One way of assessing the stability of a carbocation is to use an isodesmic reaction such as eq 1, where SE+ (SE+ is the SE+ RCH2CH2’ + HCH2CH3 RCHZCH, + HCH2CH2’ (1) “stabilization” energy of the cation) is defined as AE.IZ (Here we use a @-substituted ethyl cation as an example; an analogous equation would be used for methyl cations.) When SE+ is greater than zero the cation RCH2CH2+ is more stable than the corresponding unsubstituted one, HCH2CH2+. It has been determined that two distinct types of substituents are capable of stabilizing the cation center. The first consists of substituents more electropositive than H (e.g., BeH, BH,, AIH,) that can act as cr-donors *Author to whom correspondence should be addressed. (3) Apeloig, Y.; Schleyer, P. v. R.; Pople, J. A. J . Am. Chem. SOC. 1977, (4) Pross, A.; Radom, L. Aust. J . Chem. 1980, 33, 241. ( 5 ) Radom, L.; Pople, J . A,; Schleyer, P. v. R. J . Am. Chem. SOC. 1972, ( 6 ) Creary, X.; Merhrsheikh-Mohammadi, M. E.; Eggers, M. D. 1. Am. (7 ) Pople, J . A.; Apeloig, Y.; Schleyer, P. v. R. Chem. Phys. Lett. 1982, (8) Hopkinson, A. C.; Lien, M. H. Can. J . Chem. 1985, 63, 3582. (9) Lien, M. H.; Hopkinson, A. C. J . Mol. Struct. (Theochem) 1985, 121, (10) Dorado, M.; M6,O.; YBRez, M. J . Am. Chem. SOC. 1980, 102,947. ( I 1 ) Radom, L.; Poppinger, D.; Haddon, R. C. Carbonium Ions; Olah, G . (12) Hehre, W. J . ; Radom, L.; Schleyer, P. v. R.; Pople, J . A. Ab Initio ( 1 3) Rdriquez, C. F.; Hopkinson, A. C. J . Mol. Struct. (Theochem) 1987, 99, 5901.

The strain energy in four three-membered rings is calculated by using the appropriate isodesmic reactions. Calculations with and without d functions on the heavy atoms indicate that while these polarization functions tend to reduce ring strain, the addition of d orbitals has little effect on the predicted trends. While introduciton of unsaturation drastically increases the strain in the carbon system (cyclopropane---. cyclopropene), this is not the case for silicon (silacyclopropane--+ silacyclopropene). As a result cyclopropane and cyclopropene are predicted to be the least and most strained rings, respectively. Ring strain is a concept which is frequently used to rationalize instability and reactivity of small cyclic compounds. Recently, a novel quantitative measure of ring strain has been proposed. 1 This method makes use of isodesmic reactions in which the number of bonds of each type is formally conserved on going from reactants to products. The use of isodesmic reactions is particularly useful in quantum chemistry since the effect of correlation error is expected to be minimal in such reactions. As a result, one can use Hartree-Fock-level energy differences with some degree of confidence. In this paper the ring strain in cyclopropane, cyclopropene, silacyclopropane, and silacyclopropene is examined by using ab initio molecular orbital calculations. Cyclopropane and cyclopropene have been studied before1 and are included here for internal consistency. While semiempirical24 and ab initio5 calculations have been carried out on the silicon rings, no attempt has been made to evaluate the strain in these rings or, more importantly, their ring strain relative to the hydrocarbon analogues. In view of the apparent stability of the silicon-containing rings,6 such a comparison is clearly of interest. The methodology used in the calculations is summarized in the Methodology section, while the results of the ring strain calculations are presented and discussed in the Results and Discussion section. To aid the latter discussion approximate quadratic force fields have been generated for the four rings, and these are also presented in the Results and Discussion section.

Ab initio (6-31G*/ /3-21G) SCF calculations have been carried out on 15 isomers with the chemical formula C2SiH4• The isomers include structures with formal double and triple bonds to silicon, as well as carbenes and silylenes, so a direct comparison of these types of species is possible. The isomerization energies provide an opportunity to consider the relative strengths of analogous bonds to carbon and silicon. With the appropriate isodesmic reactions, the stabilizing or destabilizing effects of substituents and strain in three-membered rings both may be investigated.