A quantum chemical study of conformational and tautomeric preferences, intramolecular hydrogen bonding and π-electron delocalization on dinitrosomethane; in gas phase and water solution

HF, B3LYP, and MP2 methods with the standard basis set, 6-311++G(d,p), were employed to study various aspects of dinitrosomethane (DNM). These results are compared with the outcomes of G2, G2MP2, G3, and CBS-QB3 methods. In the present study, we first characterized the equilibrium conformations, especially global minima. In general, the nitroso-oxime (NO) tautomers of DNM are stabler than the dioxime and dinitroso ones. Furthermore, it was found that the stablest form of NO tautomer is global minima among the known local minima. Surprisingly, the chelated form of NO tautomer, with O–H···O intramolecular hydrogen bond (IMHB), is less stable than the global minimum. In spite of this instability, we comprehensively studied various aspects of IMHB to evaluate the effect of heteroatom’s (N). The results of open–close and related rotamer models predict that the heteroatoms weaken the hydrogen bond, whereas, the geometric, topologic, and natural bond orbital parameters emphasize on opposite conclusion. The HOMA of aromaticity aromaticity index clearly predicts that the π-electron delocalization of chelated form of NO tautomer is greater than the malonaldehyde. Finally, the solvent effects on the properties of DNM tautomers have been estimated by continuum (PCM, IPCM, and SCIPCM), discrete, and mixed models. Theoretical results clearly show that the potential energy surface of DNM, especially global minima, is strongly affected by the solvent.     

CNDO/2 Molecular orbital calculations on hydrogen bond and charge transfer interactions with π-electron donors

CNDO/2 calculations for hydrogen bond interaction of H₂0 and HF and the charge transfer interaction of chlorine with benzene have been carried out. While the dissociation energies for hydrogen bond interactions are satisfactory, they are large for charge transfer interactions. The axial model for the chlorinebenzene interaction has been found to be more stable.     

Hydrogen bond donors and acceptors are generally depolarized in α-helices as revealed by a molecular tailoring approach

Hydrogen-bond (H-bond) interaction energies in α-helices of short alanine peptides were systematically examined by precise density functional theory calculations, followed by a molecular tailoring approach. The contribution of each H-bond interaction in α-helices was estimated in detail from the entire conformation energies, and the results were compared with those in the minimal H-bond models, in which only H-bond donors and acceptors exist with the capping methyl groups. The former interaction energies were always significantly weaker than the latter energies, when the same geometries of the H-bond donors and acceptors were applied. The chemical origin of this phenomenon was investigated by analyzing the differences among the electronic structures of the local peptide backbones of the α-helices and those of the minimal H-bond models. Consequently, we found that the reduced H-bond energy originated from the depolarizations of both the H-bond donor and acceptor groups, due to the repulsive interactions with the neighboring polar peptide groups in the α-helix backbone. The classical force fields provide similar H-bond energies to those in the minimal H-bond models, which ignore the current depolarization effect, and thus they overestimate the actual H-bond energies in α-helices. © 2019 The Authors. Journal of Computational Chemistry published by Wiley Periodicals, Inc.     

Structural variation, π-charge transfer,and transmission of electronic substituent effects through the carbon-carbon triple bond in β-substituted phenylacetylenes: a quantum chemical study,and a comparison with (E)-β-substituted styrenes

Structural Chemistry - The transmission of electronic substituent effects through a carbon-carbon triple bond has been investigated from the small changes induced by a variable substituent X on the...     

Topology-driven physicochemical properties of pi-electron systems. 1. Does the Clar rule work in cyclic pi-electron systems with the intramolecular hydrogen or lithium bond?

The Clar model predicting stability and electron distribution in benzenoid hydrocarbons seems to be also a good predictor for related properties in lithium o-acylphenolates and to a lesser extent in the phenols themselves. This conclusion is based on analysis of geometry changes in the analogues of naphthalene, phenanthrene, anthracene, and triphenylene, where benzene rings are systematically replaced with a quasi-ring formed as a beta-ketoenol or beta-ketoenolate complex with a lithium cation, i.e., where CH-CH-CH fragments are replaced with O...Li...O or O...H...O fragments. These systems were optimized at the MP2/6-31G(2d,p) level of theory. The energy of bond separation reactions in line with aromaticity indices HOMA and NICSs supported the above statement.     

Structural variation, π-charge transfer, and transmission of electronic substituent effects in (E)-β-substituted styrenes: a quantum chemical study

The nature and transmission mechanism of substituent effects in (E)-β-substituted styrenes, C₆H₅–CH=CH–X, have been investigated from the structural changes induced by a variable substituent on the phenyl group. The molecular structures of 46 (E)-β-substituted styrenes were determined from MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by two orthogonal linear combinations of the internal ring angles, S _F ^STY and S _R ^STY . Regression analysis of S _F ^STY using appropriate explanatory variables reveals a composite field effect, the main component of which originates from the long-range effect of the substituent enhanced by field-induced π-polarization of the vinylene spacer and resonance-induced field effects. The electronegativity of the substituent also plays a role in determining the value of S _F ^STY . Comparison with coplanar 4-substituted biphenyls reveals that the components of the field effect in the two molecular systems are of the same nature (apart from the electronegativity contribution, which is not present in biphenyl derivatives). However, the structural variation of the phenyl probe is more pronounced in (E)-β-substituted styrenes due to the shorter distance between substituent and probe. Analysis of π-charge distribution shows that the aptitude of the substituents to exchange π-electrons with the styrene and coplanar biphenyl frames is nearly the same. Nevertheless, the π-charge variation on the phenyl probe of (E)-β-substituted styrenes is 57 % greater than the corresponding quantity in coplanar 4-substituted biphenyls. Thus, the vinylene spacer is more effective than the phenylene spacer in transmitting π-charges. The S _R ^STY parameter is related to the amount of π-charge transferred from the –CH=CH–X moiety into the π-system of the benzene ring, or vice versa, due to the resonance effect of the variable substituent and, to a lesser extent, to field-induced π-polarization.     

Electronic coupling through intramolecular π-π interactions in biscobaltocenes: a structural, spectroscopic, and magnetic study

The paramagnetic dinuclear complexes 1,8-bis(cobaltocenyl)naphthalene (2) and 1,8-bis[(pentamethyl-η(5)-cyclopentadienyl)(η(5)-cyclopentadiendiyl)cobalt(II)]naphthalene (4) were synthesized. The molecular structures were characterized by X-ray structure analysis and consisted of two cobaltocenes linked through a distorted naphthalene clamp. Electronic interactions between the two cobalt atoms were observed by cyclic voltammetric studies. Superconducting quantum interference device (SQUID) measurements of the pure compounds and diluted in their diamagnetic iron derivatives, as well as variable-temperature NMR spectroscopy experiments in solution are presented. Magnetic measurements revealed an antiferromagnetic coupling of the electrons in complexes 2 and 4. From NMR spectroscopy experiments, Curie behavior in the temperature range from -60 to +60 °C can be deduced. The electronic structure and magnetic behavior is supported by results of broken-symmetry DFT and multireference calculations along with UV/Vis spectroscopic data, which revealed an intramolecular through space π-π interaction between the cobaltocene units.     

Reactions of azulene and guaiazulene with all-trans-retinal and trans-cinnamaldehyde: comparative studies on spectroscopic, chemical, and electrochemical properties of monocarbenium-ions stabilized by expanded π-electron systems with an azulenyl or 3-guaiazulenyl group

Reaction of azulene (1) with all-trans-retinal in diethyl ether in the presence of hexafluorophosphoric acid at −10 °C for 1 h in a dark room gives the corresponding monocarbenium-ion compound, (2E,4E,6E,8E)-1-azulenyl-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraen-1-ylium hexafluorophosphate (3), in 74% isolated yield. The spectroscopic, chemical, and electrochemical properties of 3 compared with those of the previously-documented (2E,4E,6E,8E)-1-(3-guaiazulenyl)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraen-1-ylium hexafluorophosphate (4) are reported. Along with the above delocalized monocarbenium-ion compounds 3 and 4, stabilized by the expanded π-electron systems possessing an azulenyl (or 3-guaiazulenyl) group, an efficient preparation as well as the spectroscopic, chemical, and electrochemical properties of (2E)-1-azulenyl-3-phenyl-2-propen-1-ylium and (2E)-1-(3-guaiazulenyl)-3-phenyl-2-propen-1-ylium hexafluorophosphates (5 and 6) (90 and 96% isolated yields), having a similar partial structure [i.e., the (2E)-1-azulenyl-2-propen-1-ylium-ion or (2E)-1-(3-guaiazulenyl)-2-propen-1-ylium-ion part] to those of 3 and 4, is documented. Moreover, the crystal structure of 6, whose carbenium-ion framework is planar, is shown.     

Location of protons in N-H···N hydrogen-bonded systems: a theoretical study on intramolecular pyridine-dihydropyridine and pyridine-pyridinium pairs

Two-dimensional potential energy surfaces (PESs) were calculated for the degenerate intramolecular proton transfer (PT) in two N-H···N hydrogen-bonded systems, (Z)-2-(2-pyridylmethylidene)-1,2-dihydropyridine (1) and monoprotonated di(2-pyridyl) ether (2), at the MP2/cc-pVDZ level of theory. The calculated PES had two minima in both cases. The energy barrier in 1 was higher than the zero-point energy (ZPE) level, while that in 2 was close to the ZPE. Vibrational wavefunctions were obtained by solving time-independent Schr?dinger equations with the calculated PESs. The maximum points of the probability density were shifted from the energy minima towards the region where the covalent N-H bond was elongated and the N···N distance shortened. The effects of a polar solvent on the PES were investigated with the continuum or cluster models in such a way that the solute-solvent electrostatic interactions could be taken into account under non-equilibrated conditions. A solvated contact ion-pair was modelled by a cluster consisting of one cation 2, one chloride ion and 26 molecules of acetonitrile. The calculation with this model suggested that the bridging proton is localised in the deeper well due to the significant asymmetry of the PES and the high potential barrier.     

MO investigations on lignin model compounds: IX. A PCILO study of the hydrogen bond O-H…O and O-H…N type formed by phenol with some proton acceptors

A systematic PCILO study has been carried out on the intermolecular hydrogen bond formed by phenol with proton acceptors such as acetone, ether, p-benzoquinone, N,N-dimethylformamide, trimethylamine, pyridine, acetonitrile and acrylonitrile. The complexes studied form hydrogen bonds with energy intervals of 20 − 45 kJ mol⁻¹ at distances R_O…Y = 0.265 − 0.300 nm (Y=N,0). For the system phenol-pyridine both the 1:1 complex PhPy and the 2:1 complex Ph₂Py were also studied. The calculated hydrogen bond energies are compared and discussed with experimental data from the literature. The calculated hydrogen bond and O-H stretching force constants are in range expected.     

A comparative study of interplay effects between the cation-π and intramolecular hydrogen bond interactions in the various complexes of methyl salicylate with Mn+, Fe2+, Co+, Ni2+, Cu+, and Zn2+ cations

Structural Chemistry - The interplay among two important noncovalent interactions involving aromatic ring is studied by means of density functional theory (DFT) calculations on complexes of methyl...     

Reversible and irreversible higher-order cycloaddition reactions of polyolefins with a multiple-bonded heavier group 13 alkene analogue: contrasting the behavior of systems with π-π, π-π*, and π-n+ frontier molecular orbital symmetry

The heavier group 13 element alkene analogue, digallene Ar(iPr(4))GaGaAr(iPr(4)) (1) [Ar(iPr(4)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)], has been shown to react readily in [n + 2] (n = 6, 4, 2 + 2) cycloaddition reactions with norbornadiene and quadricyclane, 1,3,5,7-cyclooctatetraene, 1,3-cyclopentadiene, and 1,3,5-cycloheptatriene to afford the heavier element deltacyclane species Ar(iPr(4))Ga(C(7)H(8))GaAr(iPr(4)) (2), pseudoinverse sandwiches Ar(iPr(4))Ga(C(8)H(8))GaAr(iPr(4)) (3, 3(iso)), and polycyclic compounds Ar(iPr(4))Ga(C(5)H(6))GaAr(iPr(4)) (4) and Ar(iPr(4))Ga(C(7)H(8))GaAr(iPr(4)) (5, 5(iso)), respectively, under ambient conditions. These reactions are facile and may be contrasted with other all-carbon versions, which require transition-metal catalysis or forcing conditions (temperature, pressure), or with the reactions of the corresponding heavier group 14 species Ar(iPr(4))EEAr(iPr(4)) (E = Ge, Sn), which give very different product structures. We discuss several mechanistic possibilities, including radical- and non-radical-mediated cyclization pathways. These mechanisms are consistent with the improved energetic accessibility of the LUMO of the heavier group 13 element multiple bond in comparison with that of a simple alkene or alkyne. We show that the calculated frontier molecular orbitals (FMOs) of Ar(iPr(4))GaGaAr(iPr(4)) are of π-π symmetry, allowing this molecule to engage in a wider range of reactions than permitted by the usual π-π* FMOs of C-C π bonds or the π-n(+) FMOs of heavier group 14 alkyne analogues.     

Synthesis and spectroscopic studies on charge-transfer molecular complexes formed in the reaction of imidazole and 1-benzylimidazole with σ- and π-acceptors

The spectrophotometric characteristics of the solid charge-transfer molecular complexes (CT) formed in the reaction of the electron donors imidazole (IML) and 1-benzylimidazole (BIML) with the σ-acceptor iodine and π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoethylene (TCNE) and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL) have been studied in chloroform at 25 °C. These were investigated through electronic and infrared spectra as well as elemental analysis. The results show that the formed solid CT-complexes have the formulas [(IML)2 I]I3, [(IML)(DDQ)], [(IML)2(TCNE)5] and [(IML)(CHL)] for imidazole and [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2] for 1-benzylimidazole in full agreement with the known reaction stoichiometries in solution as well as the elemental measurements. The formation constant KCT, molar extinction coefficient ?CT, free energy change ΔG0, CT energy ECT and ionization potential Ip have been calculated for the CT-complexes [(IML)2 I]I3, [(IML)(DDQ)], [(IML)(CHL)], [(BIML) I]I3, [(BIML)(DDQ)2], [(BIML)(TCNE)2] and [(BIML)(CHL)2].