First principle computational study on the full conformational space of L-proline diamides

Ab initio molecular orbital computations were carried out at three levels of theory: RHF/3-21G, RHF/6-31G(d), and B3LYP/6-31G(d), on four model systems of the amino acid proline, HCO-Pro-NH2 [I], HCO-Pro-NH-Me [II], MeCO-Pro-NH2 [III], and MeCO-Pro-NH-Me [IV], representing a systematic variation in the protecting N- and C-terminal groups. Three previously located backbone conformations, gammaL, epsilonL, and alphaL, were characterized together with two ring-puckered forms syn (gauche+ = g+) or "DOWN" and anti (gauche- = g-) or "UP", as well as trans-trans, trans-cis, cis-trans, and cis-cis peptide bond isomers. The topologies of the conformational potential energy cross-sections (PECS) of the potential energy hypersurfaces (PEHS) for compounds [I]-[IV] were explored and analyzed in terms of potential energy curves (PEC), and HCO-Pro-NH2 [I] was also analyzed in terms of potential energy surfaces (PESs). Thermodynamic functions were also calculated for HCO-Pro-NH2 [I] at the CBS-4M and G3MP2 levels of theory. The study confirms that the use of the simplest model, compound [I] with P(N) = P(C) = H, along with the RHF/3-21G level of theory, is an acceptable practice for the analysis of peptide models because only minor differences in geometry and stability are observed.     

Comparison of ab initio and DFT electronic structure methods for peptides containing an aromatic ring: effect of dispersion and BSSE

We establish that routine B3LYP and MP2 methods give qualitatively wrong conformations for flexible organic systems containing pi systems and that recently developed methods can overcome the known inadequacies of these methods. This is illustrated for a molecule (a conformer of the Tyr-Gly dipeptide) for which B3LYP/6-31+G(d) and MP2/6-31+G(d) geometry optimizations yield strikingly different structures [Mol. Phys. 2006, 104, 559-570]: MP2 predicts a folded "closed-book" conformer with the glycine residue located above the tyrosine ring, whereas B3LYP predicts a more open conformation. By employing different levels of theory, including the local electron correlation methods LMP2 (local MP2) and LCCSD(T0) (local coupled cluster with single, double, and noniterative local triple excitations) and large basis sets (aug-cc-pVnZ, n=D, T, Q), it is shown that the folded MP2 minimum is an artifact caused by large intramolecular BSSE (basis set superposition error) effects in the MP2/6-31+G(d) calculations. The B3LYP functional gives the correct minimum, but the potential energy apparently rises too steeply when the glycine and tyrosine residues approach each other, presumably due to missing dispersion effects in the B3LYP calculations. The PWB6K and M05-2X functionals, designed to give good results for weak interactions, remedy this to some extent. The reduced BSSE in the LMP2 calculations leads to faster convergence with increasing basis set quality, and accurate results can be obtained with smaller basis sets as compared to canonical MP2. We propose LMP2 as a suitable method to study interactions with pi-electron clouds.     

Cyclic vinyl p-tolyl sulfilimines as chiral dienophiles: theoretical study on the stereoselectivity, Lewis acid catalysis, and solvent effects in their Diels-Alder reactions

The theoretical study reported in the present work deals with chiral cyclic vinyl sulfilimines and their reactivity as dienophiles in [4 + 2] cycloaddition reactions, using B3LYP/6-31G(d)//AM1 and B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistries. Consideration of Lewis acid catalysis, illustrated by BF(3), decreases the activation energies of the cycloaddition process while the charge transfer from the diene to the sulfilimine is augmented. The [4 + 2] cycloaddition reactions of sulfilimines with both furan and cyclopentadiene occur in the gas phase with endo stereoselectivity, which is more pronounced with the latter diene. Endo-exo energy differences in the gas phase with the B3LYP/6-31+G(d)//B3LYP/6-31+G(d), B3LYP/6-31G(d)//B3LYP/6-31G(d), and B3LYP/6-31G(d)//AM1 model chemistries are almost the same. Solvent effects are responsible for the inversion of the stereoselectivity in the reactions of sulfilimines with furan because of the great difference in the dipole moments in endo and exo approaches.     

Diazotization of the amino acid [closo-1-CB9H8-1-COOH-6-NH3] and reactivity of the [closo-1-CB9H8-1-COO-6-N2]- anion

A comparative study of the reactivity of dinitrogen acids [closo-1-CB(9)H(8)-1-COOH-10-N(2)] (3[10]) and [closo-1-CB(9)H(8)-1-COOH-6-N(2)] (3[6]) was conducted by diazotization of a mixture of amino acids [closo-1-CB(9)H(8)-1-COOH-6-NH(3)] (1[6]) and [closo-1-CB(9)H(8)-1-COOH-10-NH(3)] (1[10]) with NO(+)BF(4)(-) in the presence of a heterocyclic base (pyridine, 4-methoxypyridine, 2-picoline, or quinoline). The 10-amino acid 1[10] formed an isolable stable 10-dinitrogen acid 3[10], while the 6-dinitrogen carboxylate 3[6](-) reacted in situ, giving products of N-substitution at the B6 position with the heterocyclic solvent (4[6]). The molecular and crystal structures for pyridinium acid 4[6]a were determined by X-ray crystallography. The electronic structures and reactivity of the 6-dinitrogen derivatives of the {1-CB(9)} cluster were assessed computationally at the B3LYP/6-31G(d,p) and MP2/6-31G(d,p) levels of theory and compared to those of the 10-dinitrogen, 2-dinitrogen, and 1-dinitrogen analogues.     

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.     

（E）-1-[4-（2-hydroxy-5-methoxybenzylideneamino）-phenyl]ethanone and （E）-1-[4-（2-hydroxy-4-methoxybenzylideneamino） phenyl]ethanone：X-ray and DFT-calculated structures

The isomeric structures of(E)-1-[4-(2-hydroxy-5-methoxybenzylideneamino)-phenyl] ethanone(I) and(E)-1-[4-(2-hydroxy-4-methoxybenzylideneamino) phenyl]ethanone(Ⅱ) ,both C16H15NO3,have been determined using X-ray diffraction techniques and characterized by IR,and their molecular structures have also been optimized at the B3LYP/6-31G(d,p) level using density functional theory(DFT) . The energetic behaviors of the title compounds in solvent media have been examined using B3LYP method with the 6-31G(d,p) basis set by applying the polarizable continuum model(PCM) . The total energies of the title compounds decrease with the increasing polarity of the solvent. In addition,DFT calculations of the title compounds' molecular electrostatic potentials(MEP) were performed at the B3LYP/6-31G(d,p) level of theory. X-ray study shows that the title compounds both have strong intramolecular O-H…N hydrogen bonds. The molecules of Ⅰ are linked into a one-dimensional framework structure by C-H…π interactions,while in Ⅱ,intermolecular π···π interactions result in the formation of infinite chains running along the [010].     

Imidazo[1,2-a]pyrazine-3,6-diones derived from alpha-amino acids: a theoretical mechanistic study of their formation via pyrolysis and silica-catalyzed process

Imidazo[1,2-a]pyrazine-3,6-diones are unusual compounds composed of three alpha-amino acid fragments. These bicyclic amidines (BCAs) form under high temperatures or with the use of strong dehydrating reagents. We gave insight into the mechanisms of BCA formation via gas-phase pyrolytic and silica-catalyzed reactions of glycine (Gly) and alpha-aminoisobutyric acid (AIB) with related diketopiperazines (DKPs), using quantum chemical calculations. The entire process requires four steps: (1) O-acylation of DKP with free or silica-bonded amino acid, (2) acyl transfer from the oxygen to the nitrogen atom, (3) intramolecular condensation of the N-acyl DKP into a cyclol, and (4) elimination of water. To study step (1) at silica surface (modeled by H7Si8O12-OH cluster), we employed two-level ONIOM calculations (AM1:UFF, B3LYP/3-21G:UFF and B3LYP/6-31G(d):UFF); all gas-phase reactions were studied at the AM1, B3LYP/3-21G and B3LYP/6-31G(d) levels. The catalytic effect of silica was observed for both Gly and AIB: the activation energy in the O-acylation at the surface was lower by more than 9 kcal mol(-1) as compared to the gas-phase process. Contrary to the exothermic O-acylation, the gas-phase transfer reaction (step 2) was exothermic in both cases, but more favorable for Gly. The cyclocondensation of N-acylated DKPs into BCAs (steps 3 and 4) is endothermic for Gly and exothermic for AIB.     

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.     

Homoaromaticity in carbene intermediates

Analyses of isodesmic reactions for singlet 7-carbenanorbornene (4S), 8-carbena-endo-tricyclo[3.2.1.0(2,4)]octane (5S), 3-carbenabicyclo[3.1.0]hexane (3S), 2-carbenanorbornene 6S, and 2-carbenabicyclooctadiene 7S at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level provide stabilization energies of 13.83, 13. 50, 3.00, -2.22, and -3.01 kcal/mol, respectively. The C7 carbene in 4S and the C8 carbene center in 5S are strongly bent toward the double bond and cyclopropane ring, respectively, in contrast to their related triplets, 4T and 5T and parent hydrocarbons. The geometric change for 3S compared to 3T or to parent bicyclo[3.1. 0]hexane is minimal. Comparison of the stability of 6S with 2-carbenanorbornane and the geometry of 6S with that of 6T and also with the singlet and triplet 2-carbenanorbornane suggests very modest bridging. The stabilization energy and geometry of 2-carbenabicyclooctadiene 7 resemble antihomoaromatic bicyclooctadienyl cation 9 rather than the related homoaromatic bicyclooctadienyl anion 8. The diamagnetic susceptibility exhaltations (Deltachi) of 3, 4, 5, and 7, calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G level, are -0.7, 22.7, 26.0, and -10.3 cgs-ppm, respectively. The singlet-triplet energy differences, DeltaE(TS), for carbenacyclohexane, carbenacyclopentane, 3, 4, 5, 6, 7, and 2-carbenabicyclo[3.2.1]oct-3-ene at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level are 3.4, 10.3, 8.8, 27.1, 25.9, 12.7, -4.0, and -0.9 kcal/mol, respectively.     

Design of TTF-based phosphazenes combining a good electron-donor capacity and possible inclusion adduct formation

The physical properties of porous material can be modulated by intercalation of small molecules, whose size, in the case of tris-o-phenylenedioxy)cyclotriphosphazene (TPP)-based materials, vary with the different side groups. Starting from the TPP structure, a series of new derivatives were constructed through the core ring [(NP)(3)] substitution by [(CNH)(3)], [(CO)(3)], and [(CS)(3)] or/and the side group substitution by tetrathiafulvalene and a series of related fragments including bis(ethylenedithio)tetrathiafulvalene, 2-methylene-1, 3-dithiole, and 2-methylene-5,6-dihydro[1,3]dithiolo[4,5-b][1,4]dithiine. In the side fragment, such a substitution corresponds to the replacement of a ring heteroatom, an addition of substituents, or both. With use of theoretical methodologies based on DFT-B3LYP/6-31G* and HF/6-31G*//B3LYP/6-31G* approaches, molecular geometries and electronic properties including the LUMO and HOMO energies, the HOMO-LUMO gap, as well as the ionization potential (IP) were calculated. In comparison with the commonly used organic superconductors, most of the molecules investigated were predicted to show comparable or better electron-donor strength. Interestingly, a number of cyclophosphazene [(NP)(3)]-containing compounds were predicted to show the "paddle wheel" shape responsible for inclusion adducts formation, making these compounds to be potential candidates for organic superconductors with the ease of modulating their conducting properties by intercalation of suitable acceptors.     

A new scheme for determining the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of glycine and alanine peptides

In this paper a new scheme was proposed to calculate the intramolecular hydrogen-bonding energies in peptides and was applied to calculate the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of the glycine and alanine peptides. The density-functional theory B3LYP6-31G(d) and B3LYP6-311G(d,p) methods and the second-order Moller-Plesset perturbation theory MP26-31G(d) method were used to calculate the optimal geometries and frequencies of glycine and alanine peptides and related structures. MP26-311++G(d,p), MP26-311++G(3df,2p), and MP2/aug-cc-pVTZ methods were then used to evaluate the single-point energies. It was found that the B3LYP6-31G(d), MP26-31G(d), and B3LYP6-311G(d,p) methods yield almost similar structural parameters for the conformers of the glycine and alanine dipeptides. MP2/aug-cc-pVTZ predicts that the intramolecular seven-membered ring N-H...O=C hydrogen-bonding strength has a value of 5.54 kcal/mol in glycine dipeptide and 5.73 and 5.19 kcal/mol in alanine dipeptides, while the steric repulsive interactions of the seven-membered ring conformers are 4.13 kcal/mol in glycine dipeptide and 6.62 and 3.71 kcal/mol in alanine dipeptides. It was also found that MP26-311++G(3df,2p) gives as accurate intramolecular N-H...O=C hydrogen-bonding energies and steric repulsive interactions as the much more costly MP2/aug-cc-pVTZ does.     

Role of singlet diradicals in reactions of 2-carbenabicyclo[3.2.1]octa-3,6-diene

The generation of 2-carbenabicyclo[3.2.1]octa-3,6-diene (1) results in the formation of C(8)H(8) hydrocarbons endo-6-ethynylbicyclo[3.1.0]hex-2-ene (4), semibullvalene (5), and 5-ethynyl-1,3-cyclohexadiene (6), and C(8)H(10) hydrocarbons bicyclo[3.2.1]octa-2,6-diene (7), tricyclo[3.2.1.0(4,6)]oct-2-ene (8), and tetracyclo[3.3.0.0(2,8)0(4,6)]octane (9). Focus is placed on three mechanistic pathways for the formation of the C(8)H(10) hydrocarbon fraction: (a) abstraction of hydrogen by triplet carbene 1T to produce an equilibrating set of monoradicals, (b) interconversion of triplet carbene 1T into tricyclic triplet diradical 19T and tetracyclic triplet diradical 20T, and (c) interconversion of singlet 1S with analogous singlet diradical 19S and 20S. Ab initio calculations at the (U)B3LYP/6-311+G(3df,2p)//(U)B3LYP/6-31G(d,p) and broken spin symmetry UBS B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels rule out choices (a) and (b) and are consistent with the singlet diradical process.     

Conformational space of 4,4′-methoxypropylstilbene molecule

Quantum-chemical simulation of the ground state [the density function B3LYP/6-31G, B3LYP/6-31G(d), and B3LYP/6-31+G(d,p) and the perturbation theory MP2/6-31G(d) methods] and the transition states [the B3LYP/6-31G(d) method] of 4,4′-methoxypropylstilbene molecule has been performed. Using the Ellinger MM2 force field method, the potentials of internal rotation have been obtained for each rotational degree of freedom of the molecule. The B3LYP simulation has revealed the planarity of the conjugated system and the orthogonal position of the alkyl substituent, whereas the benzene rings have deviated by about 20° with respect to the double bond plane according to the MP2 data. Three transition states of the molecule corresponding to the saddle points of the first and the second orders have been revealed. The stationary points have been identified by means of vibrational analysis.     

Theoretical prediction of gas-phase infrared spectra of imidazo[1,2-a]pyrazinediones and imidazo[1,2-a]imidazo[1,2-d]pyrazinediones derived from glycine

Imidazo[1,2-a]pyrazine-3,6-diones and imidazo[1,2-a]imidazo[1,2-d]pyrazine-3,8-diones can be produced by pyrolysis of simple amino acids. While such bicyclic and tricyclic amidines were detected and characterized by IR spectroscopy for some alpha-substituted amino acids, the parent systems composed of glycine fragments are unknown up to now. IR spectra for five amidines derived from glycine were calculated by using different semi-empirical (PM3, AM1, MNDO and MINDO/3), HF, and hybrid DFT (B3LYP, B3P86 and B3PW91) methods in conjunction with 6-31G(d) basis set (for HF and DFT). Vibration frequencies in the experimental IR spectra were predicted based upon the B3LYP data, by correcting the calculated wavenumbers by a scaling factor of 0.959. The behavior of most characteristic bands (nu(CX), nu(NH), etc.) and their shifts with respect to such bands in the spectra of alanine and alpha-aminoisobutyric acid derivatives studied before, are discussed. Performance of the semi-empirical methods was tested, bearing in mind possible future needs for IR spectra predictions for larger molecular systems of similar chemical nature; the use of MINDO/3 and MNDO is recommended. A basis set effect on the B3LYP fundamental vibration frequencies for hexahydroimidazo[1,2-a]pyrazine-3,6-dione was studied by varying Pople basis sets from minimal STO-3G to 6-311++G(d, p). No significant improvements were found beyond the 6-31G(d) basis set, which thus can be recommended to predict IR spectra for the amidines and similar molecules.     

A computational study of tetrafluoro-[2.2]cyclophanes

A computational study of the isomers of tetrafluorinated [2.2]cyclophanes persubstituted in one ring, namely F4-[2.2]paracyclophane (4), F4-anti-[2.2]metacyclophane (5a), F4-syn-[2.2]metacyclophane (5b), and F4-[2.2]metaparacyclophane (6a and 6b), was carried out. The effects of fluorination on the geometries, relative energies, local and global aromaticity, and strain energies of the bridges and rings were investigated. An analysis of the electron density by B3PW91/6-31+G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) was carried out using the natural bond orbitals (NBO), natural steric analysis (NSA), and atoms in molecules (AIM) methods. The analysis of frontier molecular orbitals (MOs) was also employed. The results indicated that the molecular structure of [2.2]paracyclophane is the most affected by the fluorination. Isodesmic reactions showed that the fluorinated rings are more strained than the nonfluorinated ones. The NICS, HOMA, and PDI criteria evidenced that the fluorination affects the aromaticity of both the fluorinated and the nonfluorinated rings. The NBO and NSA analyses gave an indication that the fluorination increases not only the number of through-space interactions but also their magnitude. The AIM analysis suggested that the through-space interactions are restricted to the F4-[2.2]metacyclophanes. In addition, the atomic properties, computed over the atomic basins, gave evidence that not only the substitution, but also the position of the bridges could affect the atomic charges, the first atomic moments, and the atomic volumes.     

Inequivalence of substitution pairs in hydroxynaphthaldehyde: A theoretical measurement by intramolecular hydrogen bond strength,aromaticity, and excited‐state intramolecular proton transfer reaction

The inequivalence of substitution pair positions of naphthalene ring has been investigated by a theoretical measurement of hydrogen bond strength, aromaticity, and excited state intramolecular proton transfer (ESIPT) reaction as the tools in three substituted naphthalene compounds viz 1‐hydroxy‐2‐naphthaldehyde (HN12), 2‐hydroxy‐1‐naphthaldehyde (HN21), and 2‐hydroxy‐3‐naphthaldehyde (HN23). The difference in intramolecular hydrogen bond (IMHB) strength clearly reflects the inequivalence of substitution pairs where the calculated IMHB strength is found to be greater for HN12 and HN21 than HN23. The H‐bonding interactions have been explored by calculation of electron density ρ(r) and Laplacian ?²ρ(r) at the bond critical point using atoms in molecule method and by calculation of interaction between σ* of OH with lone pair of carbonyl oxygen atom using NBO analysis. The ground and excited state potential energy surfaces (PESs) for the proton transfer reaction at HF (6‐31G**) and DFT (B3LYP/6‐31G**) levels are similar for HN12, HN21 and different for HN23. The computed aromaticity of the two rings of naphthalene moiety at B3LYP/6‐31G** method also predicts similarity between HN12 and HN21, but different for HN23. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Basis set superposition error in MP2 and density-functional theory: a case of methane-nitric oxide association

A systematic study of basis set superposition error (BSSE) behavior in H(3)C-H[ellipsis (horizontal)][NO] complexes for both -H...N- and -H...O- orientations were carried out using MP2 and density-functional theory with Pople's [6-31G(d,p),6-311++G(nd,nd), where n=1,2,3, and 6-311++G(3df,3pd)] and Dunning's augmented correlation consistent basis sets [aug-cc-pVXZ (X=D and T)]. Corrected and uncorrected counterpoise potential-energy surfaces (PESs) were explored and differences obtained between them indicate that reliable optimizations of these molecular interactions must be carried out in a PES free of BSSE, even in the case of large basis sets and popularly used functionals such as B3LYP. Although all basis used could be always considered within a margin of approximation for representing molecular orbitals and show important values of BSSE, 6-311++G(2d,2p) basis set shows the best results in uncorrected PES with respect to the corrected ones. B3LYP functional produces erratic results: complexes appear repulsive and the intermolecular distances are always large, evidencing the lack of a correct dispersive forces treatment in the original parameterization. According to the MP2 results, the -H...N- interactions appear as slightly more stable than those of the -H...O- orientation.     

The role of chalcogen-chalcogen interactions in the intrinsic basicity acidity of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH

The intrinsic acidity and basicity of a series of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH (X=O, S; Y=Se, Te) were investigated by B3LYP/6-311+G(3df,2p) density functional and G2(MP2) calculations on geometries optimized at the B3LYP/6-31G(d) level for neutral molecules and at the B3LYP/6-31+G(d) level for anions. The results showed that selenovinylaldehyde and selenovinylthioaldehyde should behave as Se bases in the gas phase, because the most stable neutral conformer is stabilized by an X[bond]H...Se (X=O, S) intramolecular hydrogen bond (IHB). In contrast the Te-containing analogues behave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X...Y[bond]H chalcogen-chalcogen interactions. These compounds have a lower basicity than expected because either chalcogen-chalcogen interactions or IHBs become weaker upon protonation. Similarly, they are also weaker acids than expected because deprotonation results in a significantly destabilized anion. Loss of the proton from the X[bond]H or Y[bond]H groups is a much more favorable than from the C[bond]H groups. Therefore, for Se compounds the deprotonation process results in loss of the X[bond]H...Se (X=O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X...Y[bond]H chalcogen-chalcogen interaction present in the most stable neutral conformer becomes repulsive in the corresponding anion.     

Overcrowding motifs in large PAHs. An ab initio study.

Planar and overcrowded LPAHs C(34)H(18) anthra[9,1,2-cde]benzo[rst]penaphene (1), benzo[rst]phenanthro[10,1,2-cde]pentaphene (2), tetrabenzo[a,cd,j,lm]perylene (3), tetrabenzo[a,cd,lm,o]perylene (4), and LPAHs C(38)H(18) anthra[2,1,9,8-klmno]naphtho[3,2,1,8,7-vwxyz]hexaphene (5), dianthra[2,1,9,8-stuva;2',1',9',8'-hijkl]pentacene (6), dibenzo[jk,uv]dinaphtho[2,1,8,7-defg;2',1',8',7'-opqr]perylene (7), diphenanthro[5,4,3-abcd;3',4',5'-lmno]perylene (8), potential products of peri-peri reductive couplings of benzanthrone and of naphthanthrone, respectively, were subjected to an ab initio study with emphasis on overcrowding motifs. The HF and DFT B3LYP methods were employed to calculate energies and geometries of the minima conformations of these LPAHs. The most stable LPAHs in these series were found to be planar C(2)(v)()-1 and C(2)(v)()-5, respectively. Among overcrowded LPAHs, twisted-folded C(2)-3 and C(2)-7 with two cove regions were found to be more stable than their respective isomers twisted-folded C(2)-4 and C(2)-8 with one fjord region each, in contrast to the semiempirical predictions. The energy differences between the most stable planar isomer and the overcrowded isomers were significantly smaller in the C(38)H(18) series, than in the C(34)H(18) series. Overcrowded twisted-folded C(2)-7 with two coves was found to be more stable than planar C(2)(h)()-6 by 2.0 kJ/mol (at B3LYP/6-311G), indicating enhanced role of aromatic stabilization and decreased destabilization due to overcrowding, with increasing the number of aromatic rings. Heats of formation of LPAHs 1-8 were derived from the ab initio total energies (at B3LYP/6-31G). A search of the conformational spaces of 3 and 4 revealed an anti-folded local minimum C(i)()-3 and a syn-folded transition state C(s)()-4, 23.7 and 120.3 kJ/mol higher in energy than the twisted-folded C(2)-3 and C(2)-4, respectively (at B3LYP/6-31G). The cove and fjord torsion angles in the C(38)H(18) series were found to be smaller than in the C(34)H(18) series. The nonbonding distances between carbon atoms at cove and fjord regions of the overcrowded LPAHs were found to be smaller than the sum of the van der Waals radii of two carbon atoms