A molecular orbital study of trans-polyenes to 18 double bonds

Calculations involving molecular orbitals have appeared in the chemical literature over the past few years, but all have used smalltrans-polyenes. We now report extended Huckel molecular orbital calculations ontrans-polyenes of up to 18 double bonds (36 carbons and 38 hydrogen atoms). Information obtained from these calculations include total energies, bond gaps, and net charges on each atom. Also found is that the band gap approaches 1.41 eV as the number of double bonds approaches infinity. This value is quite close to reported experimental values. Data are also presented for polyenes assuming equal C-C bond lengths.     

X-ray spectral and quantum chemical analyses of the electronic structure of poly(monofluorocarbon)

The electronic structure of poly(monofluorocarbon) has been studied by X-ray spectral and quantum chemical methods. Calculations were performed in terms of the MNDO method, with the fluorographite layer modeled by clusters of different sizes. The high-resolution CK_a and FK_a spectra have been obtained; the calculated spectra are consistent with the experimental ones. It has been shown that carbon and fluorine are bonded mainly through the σ bonds. The p orbitals of fluorine atoms that are perpendicular to the C-F bond are not involved in the chemical bond, while the transitions from the molecular orbitals consisting of these p orbitals are responsible for the main maximum in the FK_a spectrum. Deceased. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 630–635, July–August, 1995. Translated by I. Izvekova     

[La(CCl3COO)3·dipy·H2O]2配合物的电子结构和化学键

作者曾系统研究[Ln(CCl₃COO)₃·dipy·H₂O]₂配合物的合成和性质,并测定了[La(CCl₃COO)₃·dipy·H₂O]₂的晶体结构(待发表)。     

Non-alkane behavior of cyclopropane and its derivatives: characterization of unconventional hydrogen bond interactions

Eight cyclopropane derivatives (Δ − R) have been modeled, with R = −H, −CH₃, −NH₂, −C ≡ CH, −C ≡ CCH₃, −OH, −F and −C ≡ N. All geometries have been fully optimized at the MP2/ AUG-cc-pVTZ level of calculations. Natural bond orbital analyses reveal extra p character (sp^λ, λ > 3) in the C-C bonds of the cyclopropyl rings. The banana-like σ _CC bonds in the rings are described in detail. Alkene-like complexes between Δ − R molecules and hydrogen fluoride are identified. These weakly bonded complexes are formed through unconventional hydrogen bond interactions between the hydrogen atom in the HF molecule and the carbon–carbon bonds in the cyclopropane ring. A topological analysis of the electronic charge density and its Laplacian has been used to characterize the interactions. The possible relevance of such complexes in the modeling of substrate–receptor interactions in some anti-AIDS drugs is discussed. Contribution to the Serafin Fraga Memorial Issue.     

The structural effects of fluorine substitution in pyridine,pyrimidine, and s-triazine: An ab initio study

The structures of 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, pentafluoropyridine, 2-fluoropyrimidine, 5-fluoropyrimidine, and fluoro-s-triazine have been evaluated by the ab initio gradient method with a 4-21 basis set augmented as needed with polarization functions on nitrogen. The structural effects of fluorination on the parent heterocycle are very similar to the effects of fluorination on benzene studied earlier. The ring angle is enlarged by about 2° at the point of fluorination and the adjacent ring bonds are shortened, much more for an adjacent C-N than for C-C. Fluorination of pyridine at the C₂ position causes an in-crease of bond localization in the ring. Rotational constants calculated from the structural parameters corrected with standard offset values are in exceptionally close agreement with experimental constants where these are known.     

The role of quantum-mechanical interference and quasi-classical effects in conjugated hydrocarbons

The nature of the chemical bond in conjugated hydrocarbons is analyzed through the generalized product function energy partitioning (GPF-EP) method, which allows the calculation of the quantum-mechanical interference and quasi-classical contributions to the energy. The method is applied to investigate the differences between the chemical bonding in conjugated and non-conjugated hydrocarbon isomers and to evaluate the contribution from the energy components to the stabilization of the molecules. It is shown that in all cases quantum-mechanical interference has the effect of concentrating π electron density between the two carbon atoms directly involved in the (C-C)π bonds. For the conjugated isomers, this effect is accompanied by a substantial reduction of electron density in the π space of the neighbouring (C-C)σ bond. On the other hand, quasi-classical effects are shown to be responsible for the extra stabilization of the conjugated isomers, in which a decrease of the π space kinetic reference energy seems to play an important role. Finally, it is shown that the polarization of p-like orbitals in compounds with alternating single and double bonds ultimately increases electron density in the π space of the neighbouring (C-C)σ bond. Therefore, quasi-classical effects, rather than covalent ones, seem to be responsible for several properties of conjugated molecules, such as thermodynamic stability, planarity and (C-C)σ bond shortening. The shortcomings of the delocalization concept are discussed.     

Partitioning of methyl internal rotational barrier energy of thioacetaldehyde

The nature of methyl internal rotational barrier in thioacetaldehyde has been investigated by relaxation effect, natural bond orbital (NBO) analysis and Pauling exchange interactions. The true experimental barrier can be obtained by considering fully relaxed rotation. Nuclear-electron attraction term is a barrier forming term in the fully relaxed rotation, but it appears as an antibarrier for rigid rotation. It is seen that during methyl rotation, the torsional mode is coupled with the aldehydic hydrogen out-of-plane wagging motion. Natural bond orbital analysis shows that the principal barrier forming term originates from the C-C bond. The lengthening of the C-C bond is explained by considering charge transfer interaction between several bonding and antibonding orbitals in the C-C bond region, which leads to higher bonding overlap for the eclipsed conformer compared to the staggered conformer. S-C(σ)/C^me-Hp and C-H_ald/C_me-H_op interactions appear to be the main barrier-forming Pauling exchange terms but have less contribution to make to the barrier compared to the C-C bond interaction.     

Anomeric effects in the symmetrical and asymmetrical structures of triethylamine. Blue-shifts of the C-h stretching vibrations in complexed and protonated triethylamine

Quantum mechanical calculations using density functional theory with the hybrid B3LYP functional and the 6-31++G(d,p) basis set are performed on isolated triethylamine (TEA), its hydrogen-bond complex with phenol, and protonated TEA. The calculations include the optimized geometries and the results of a natural bond orbital (NBO) analysis (occupation of sigma* orbitals, hyperconjugative energies, and atomic charges). The harmonic frequencies of the C-H stretching vibrations of TEA are predicted at the same level of theory. Two stable structures are found for isolated TEA. In the most stable symmetrical structure (TEA-S), the three C-C bond lengths are equal and one of the C-H bond of each of the three CH2 groups is more elongated than the three other ones. In the asymmetrical structure (TEA-AS), one of the C-C bonds and two C-H bonds of two different CH2 groups are more elongated than the other ones. These structures result from the hyperconjugation of the N lone pair to the considered sigma*(C-H) orbitals (TEA-S) or to the sigma*(C-C) and sigma*(C-H) orbitals of the CH2 groups (TEA-AS). The formation of a OH...N hydrogen bond with phenol results in a decrease of the hyperconjugation, a contraction of the C-H bonds, and blue-shifts of 28-33 cm-1 (TEA-S) or 40-48 cm-1 (TEA-AS) of the nus(CH2) vibrations. The nu(CH3) vibrations are found to shift to a lesser extent. Cancellation of the lone pair reorganization in protonated TEA-S and TEA-AS results in large blue-shifts of the nu(CH2) vibrations, between 170 and 190 cm-1. Most importantly, in contrast with the blue-shifting hydrogen bonds involving C-H groups, the blue-shifts occurring at C-H groups not participating in hydrogen bond formation is mainly due to a reduction of the hyperconjugation and the resulting decrease in the occupation of the corresponding sigma*(C-H) orbitals. A linear correlation is established between the C-H distances and the occupation of the corresponding sigma*(C-H) orbitals in the CH2 groups.     

Understanding glycine conformation through molecular orbitals

The four most stable C(s) conformers of glycine have been investigated using a variety of quantum-mechanical methods based on Hartree-Fock theory, density-functional theory (B3LYP and statistical average of orbital potential), and electron propagation (OVGF) treatments. Information obtained from these models were analyzed in coordinate and momentum spaces using dual space analysis to provide insight based on orbitals into the bonding mechanisms of glycine conformers, which are generated by rotation of C-O(H) (II), C-C (III), and C-N (IV) bonds from the global minimum structure (I). Wave functions generated from the B3LYP/TZVP model revealed that each rotation produced a unique set of fingerprint orbitals that correspond to a specific group of outer valence orbitals, generally of a' symmetry. Orbitals 14a', 13a', 12a', and 11a' are identified as the fingerprint orbitals for the C-O(H) (II) rotation, whereas fingerprint orbitals for the C-C (III) bond rotation are located as 16a' [highest occupied molecular orbital (HOMO)], 15a' [next highest molecular occupied molecular orbital (NHOMO)], 14a', and 12a' orbitals. Fingerprint orbitals for IV generated by the combined rotations around the C-C, C-O(H), and C-N bonds are found as 16a', 15a', 14a', 13a', and 11a', as well as in orbitals 2a" and 1a". Orbital 14a' is identified as the fingerprint orbital for all three conformational processes, as it is the only orbital in the outer valence region which is significantly affected by the conformational processes regardless rotation of which bond. Binding energies, molecular geometries, and other molecular properties such as dipole moments calculated based on the specified treatments agree well with available experimental measurements and with previous theoretical calculation.     

The behavior of transition metal nitrido bonds towards protonation rationalized by means of localized bonding schemes and their weights

A new computational scheme is applied to rationalize the different protonation behaviors of the nitrido complexes [L'Mn(V)(N)(acac)](+), [LCr(V)(N)(acac)](+), and [LV(V)(N)(acac)](+). L and L' represent the macrocycles 1,4,7-triazacyclononane and its N-methylated derivative, respectively, and acac is the bidentate monoanion pentane-2,4-dionate. The bonds of the complexes are partitioned into bonds to be investigated and bonds of lesser interest. The investigated bonds are the transition metal nitrido bonds M(V)[triple chemical bond]N| (M = Mn, Cr, and V) and the bonds of lesser interest are located in the ligands. The ligand bonds are described by means of the strongly occupied natural bond orbitals. The electrons in the M(V)[triple chemical bond]N| nitrido bonds, however, are treated more accurately. A full configuration interaction procedure is applied in the space spanned by the strongly occupied natural bond orbitals and their corresponding antibonding orbitals. Localized bonding schemes and their weights are obtained for the d(pi)-p(pi) bonds of interest. This is achieved by representing the two-center natural bond orbitals for a d(pi)-p(pi) bond by the one-center natural hybrid orbitals localized at the bond atoms. The obtained bonding schemes are close to orthogonal valence bond structures. Their weights indicate that the nitrido nitrogen in [LV(V)(N)(acac)](+) is more easily protonated than the nitrido nitrogens in [L'Mn(V)(N)(acac)](+) and [LCr(V)(N)(acac)](+). This result is in good accord with experiment.     

Sigma-antiaromaticity in cyclobutane,cubane, and other molecules with saturated four-membered rings

Dissected nucleus-independent chemical shift (NICS) analyses of cycloalkanes and cage hydrocarbons reveal contrasting ring current effects, diatropic in three- and five-membered and paratropic in four-membered ring systems. The large shielding effects of the C-C bonds of the archetypal sigma-aromatic, cyclopropane, are magnified in tetrahedrane and related structures. The remarkable deshielding effect of the cyclobutane C-C(sigma) bonds is general: cubane and cages with four-membered rings are strongly deshielding (i.e., sigma-antiaromatic).[structure--see text]     

Transient bonds and chemical reactivity of molecules

Electron delocalization between the reagent and reactant molecules is the principal driving force of chemical reactions. It brings about the formation of new bonds and the cleavage of old bonds. By taking the aromatic substitution reaction as an example, we have shown the orbitals participating in electron delocalization. The interacting orbitals obtained are localized around the reaction sites, showing the chemical bonds that should be generated and broken transiently along the reaction path. By projecting a reference orbital function that has been chosen to specify the bond being formed on to the MOs of the reactant molecules, the reactive orbitals that are very similar to the interacting orbital have been obtained. The local potential of the reaction site for electron donation estimated for substituted benzene molecules by using these projected orbitals shows a fair correlation with the experimental scale of the electron-donating and -withdrawing strength of substituent groups. The reactivity is shown to be governed by local electronegativity and local chemical hardness and also by the localizability of interaction on the reaction site. © 1996 John Wiley & Sons, Inc.     

Electron pairing and chemical bonds: Chemical bonds from the condition of minimum fluctuation of electron pair

The role of electron pairing in chemical bonding stressed by the Lewis electron-pair model of the chemical bond is analyzed and discussed from the point of view of the proposal that chemical bonds are the regions of space populated roughly by two electrons and which at the same time exhibit low fluctuation of an electron pair. Based on this assumption, we have been able to introduce a new localization procedure, the output of which are just the orbitals (chemical bonds) satisfying the criterion of minimum pair fluctuation. It has been shown that these orbitals remarkably well display the most important attributes of chemical bonds, namely, the localization in the regions where classical bonds are expected and there is very high transferability from one molecule to another. The applicability of this procedure as a new means of the analysis and the visualization of the molecular structure is also discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 193–200, 1998     

The para‐didehydropyridine,para‐didehydropyridinium,and related biradicals—a contribution to the chemistry of enediyne antitumor drugs

Structure and stability of seven singlet (S) biradicals formed by Bergman cyclization from enediynes are investigated with unrestricted DFT using B3LYP/6‐31G(d,p) and B3LYP/6‐311+G(3df,3pd). The corresponding triplets (T) are also calculated and compared with their S states utilizing the on‐top pair density and the S‐T difference on‐top pair density. A relationship between the geometry of a S biradical, its stability, and its biradical character is established using the on‐top pair density and calculated S‐T splittings. Through‐bond coupling between the single electrons of the S biradical can be enhanced by the incorporation of a N atom into para‐didehydrobenzene 1 due to lowering of antibonding orbitals, shortening of ring bonds by anomeric effect, and increased overlap between the interacting orbitals. Strong through‐bond interactions lead to a stabilization of the S state and an increase of the S‐T splitting. Because through‐bond interactions also determine the degree of coupling between the single electrons, stabilization of the S biradical, and an increase of the S‐T splitting always means a lowering of the biradical character and the H abstraction ability, which is relevant for the use of N‐containing enediynes and their biradicals in connection with the design of new antitumor drugs. The S para‐didehydropyridine biradical 2 is strongly stabilized and, therefore, has only reduced biradical character. However, the latter can be enhanced by protonation, because this always leads to a lengthening of ring bonds and a reduction of the overlap between interacting orbitals. In the weakly acidic medium of a tumor cell, S biradicals containing an amidine group can be protonated to yield S biradicals with high biradical character (low S‐T splittings, small changes in bond alternation relative to the T state), which will abstract H atoms from the DNA of a tumor cell. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 216–229, 2001     

Transition metal-catalyzed carbon-carbon bond activation

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained.     

Mechanisms of oxygen- and argon-RF-plasma-induced surface chemistry of cellulose

Pure cellulose samples were Ar- and O₂-RF-plasma treated under various external plasma parameter conditions. Plasma induced macromolecular chain and pyranosidic ring cleavage mechanisms are discussed based on survey and high resolution ESCA and ATR-FTIR analysis of cellulose, discharge-exposed cellulose, and discharge-exposed and TFAA and PFPH-derivatized cellulose samples. Analyses have also been made of bothin situ andex situ post plasma oxidation reactions. The new plasma created functionalities were identified and their relative ratios were related to plasma parameters. It was found that Ar plasma treatments initiate reactions mainly associated with the cleavage of C1–C2 linkages leading to the formation of C=O and O-CO-O groups, while O₂-plasma treatments are associated with more intense pyranosidic ring (C-O-C bonds) splitting mechanisms. As a result of our detailed investigation of the high resolution C1s spectra of cellulose and carbohydrates we have reassigned the nonequivalent carbon bond affiliation (C-OH, C-O-C, and C-C) at 285–287 eV. Paper based on the results presented during the workshop of the Engineering Research Center for Plasma-Aided Manufacturing held in Madison, Wisconsin, in Spring 1996.     

Thermal decomposition mechanism of Ba(DPM)2

We have investigated the thermal decomposition behavior of Ba(DPM)₂ using thermogravimetry (TG), mass spectrometry (MS), ultraviolet (UV) absorption and in-situ Fourier transform infrared (FTIR) spectroscopy. FTIR has been used particularly for direct monitoring of the bond dissociation order in the metal complex by thermal treatment in either N₂ or O₂. TG analysis shows that the ambient gas has a significant effect on the weight loss patterns of Ba(DPM)₂. The chemical bonds of Ba(DPM)₂ begin to decompose at low temperatures below 50 °C and are sequentially dissociated when the temperature is raised. The C-C(CH₃)₃ and the Ba-O bonds are decomposed most easily at low temperatures, followed by the C-H bond, but the stable C-C and C-O bonds do not dissociate until the total complex is gasified. The decomposition sequence of the chemical bonds in Ba(DPM)₂ is similar to that of Sr(DPM)₂ but differs from that of Ti(O-iPr)₂(DPM)₂ which is decomposed in the sequence of C(CH₃)₃ > C-H and C-O > Ti-O. The major difference in the decomposition sequence between Ba and Ti complexes can be seen to derive from the intrinsic character of the individual metal-oxygen bond as observed by UV spectroscopy.     

Photochromism in the arylaroylaziridine system

Photochromism of a number of arylaroylaziridines has been observed in the solid state or in the rigid matrix. The photochromic behavior is dependent on the intensity of the incident radiation, the reaction medium, the wavelength of light used and on the relationship of the substituents on the three-membered ring. From the visible absorption spectra it is clear that cis-arylaroylaziridines give different colored species than do the trans isomers. These results suggest that the coloration is due to an extensive electrical interaction between the bent bonds of the aziridine ring and the π orbitals of the benzoyl and phenyl groups. Attempts to trap the colored species by co-irradiation with substrates containing a multiple bond failed. A reaction has been found to occur upon heating various arylaroylaziridines with dimethylacetylene dicarboxylate in inert solvents.