Adjacent CC Bonds Research Articles

Fuel molecular structure influences the polycyclic aromatic hydrocarbons formation of butanol/butane isomers: A ReaxFF molecular dynamics study

This work explores the influence differences in the molecular structure of butanol/butane isomers, particularly the hydroxyl (OH) group binding site, on polycyclic aromatic hydrocarbon (PAH) formation by using ReaxFF MD. Visualization of the fuel molecule decomposition and PAH formation processes revealed that OH bonded to interior carbon atoms could lead to earlier PAH formation onset in t-butanol and s-butanol. When OH groups were bonded to interior carbon atoms, these groups were easily abscised via simple fission, and the adjacent CC bonds showed less cleavage. Thus, abundant i-C4Hx (2-methylallyl) species were detected from t-butanol with a branched carbon skeleton structure and specific OH binding sites. s-Butanol produced more n-C4Hx species than t-butanol. Visualization of the formation of mono-aromatic ring molecules directly revealed that dehydrodimerization and dehydroaromatization by i-C4Hx lead to the earliest onset of mono-aromatic ring formation in t-butanol. Mono-aromatic ring formation via cyclization and condensation by polyyne radicals occurred later in i-butanol than the upper pathway and dominated straight fuels at a higher temperature of 2500 K. The drastic decay of C2H2 and visualization of HACA-like pathways in the simulation trajectories for n-butanol revealed the close concentration of the larger PAHs for straight-chain butanol isomers with the branched-chain isomers.

Resolving a puzzling anomaly in the spin-coupled generalized valence bond description of benzene.

In an earlier study of benzene, Small and Head-Gordon found that the spin-coupled generalized valence bond (SCGVB) wave function for the π system predicted a distorted (non-D6h ) geometry, one with alternating CC bond lengths. However, the variations in the energy were very small and the predictions were made using a very small basis set (STO-3G). We re-examined this prediction using a much larger basis set (aug-cc-pVTZ) to determine the dependence of the energy of benzene on the distortion angle, ΔθCXC (ΔθCXC = 0° corresponds to the D6h structure). We also found a distorted geometry with the optimum ΔθCXC being 0.31° with an energy 0.040 kcal mol-1 lower than that for the D6h structure. In the optimum geometry, adjacent CC bond lengths are 1.3861 Å and 1.4004 Å. Analysis of the SCGVB wave function led us to conclude that the cause of the unusual non-D6h geometry predicted by the SCGVB calculations seems to be a result of the interaction between the Kekulé and Dewar components of the full SCGVB wave function. The addition of doubly ionic configurations to the SCGVB wave function leads to the prediction of a D6h geometry for benzene and a dependence on ΔθCXC essentially the same as that predicted by the complete active space self-consistent field wave function.

Benzenoid rings resonance energies and local aromaticity of benzenoid hydrocarbons

In this article, we consider partitioning of the analytical expression for resonance energy (RE) in smaller benzenoid hydrocarbons, to individual benzenoid rings of polycyclic molecules. The analytical expression for molecular RE, available since 1976, is given by the count of all linearly independent conjugated circuit in all Kekulé structures in a molecule. Analytical expression for local ring RE (RRE) is given by counting all linearly independent conjugated circuits involving single benzenoid ring in all Kekulé structures, which when added, gives the molecular RE. If for benzene ring the RRE is taken to be 1.000, rings in polycyclic benzenoid hydrocarbons have their ring RRE, which give the degree of their local aromaticity, smaller than 1.000. The difference to 1.000 is a measure of the similarity of a ring to benzene in this one‐dimensional (1‐D) representation of local aromaticities of benzenoid hydrocarbons. The plot of RRE against the distance of the same ring from benzene in the Local Aromaticity Map, in which benzenoid rings are characterized ring bond orders and average variations of adjacent CC bonds, shows linear correlation (with r = 0.91), reducing the local aromaticity in benzenoid hydrocarbons to 1‐D molecular property. © 2018 Wiley Periodicals, Inc.

Biradicaloid and Polyenic Character of Quinoidal Oligothiophenes Revealed by the Presence of a Low-Lying Double-Exciton State

Evidence of the biradicaloid and polyenic character of quinoidal oligothiophenes is reported by proving at the CASSCF//CASPT2 computational level the presence of a low-lying double exciton state responsible for the weak features observed in the NIR absorption region of the longest members of this class of molecules. The energy lowering of this state, accompanying the length increase in the oligomers, causes a displacement of the ground-state equilibrium geometry toward more biradicaloid structures because of the more efficient S0-S1 state mixing. Furthermore, it is shown that the doubly excited state is strongly coupled to the ground electronic state, and the coupling is mediated by a collective mode dominated by the out-of-phase stretching of adjacent CC bonds, recently shown to govern the Raman activity. All together, this evidence offers a unified view of the low-lying electronic states for quinoidal oligothiophenes and polyenes.

Spectrochemical, ab initio and density functional studies on the conversion of 2-hydroxybenzonitrile ( o-cyanophenol) into the oxyanion

The spectral and structural changes caused by the conversion of 2-hydroxybenzonitrile ( o-cyanophenol) into the corresponding oxyanion have been followed by IR spectra, ab initio and density functional force field calculations. In agreement between theory and experiment, the conversion is accompanied by a 29 cm −1 frequency decrease of the cyano stretching band, 2.7-fold increase in its integrated intensity, 5.8-fold (total value) intensification of the aromatic skeletal bands of Wilson’s 8 and 19 types, and other essential spectral changes. According to the calculations, the strongest structural changes are the shortening of the PhO bond with 0.10 Å, lengthenings of the adjacent CC bonds in the phenylene ring with 0.06 Å and bond angle variations near the oxyanionic center. All these changes are connected with the formation of a quasi- ortho-quinonoidal structure of the o-phenylene ring in the oxyanion. According to the electronic density analysis, 0.41 e − (Mulliken) or 0.56 e − (natural bond orbital, NBO) of the anionic charge remain localized at the oxyanionic center. Conformations and hydrogen bonds have also been discussed on the basis of experimental and theoretical data.

The conversion of 3- and 4-hydroxybenzonitriles ( m- and p-cyanophenols) into oxyanions, followed by IR spectra, ab initio and density functional calculations

The conversion of 3- and 4-hydroxybenzonitriles ( m- and p-cyanophenols) into oxyanions has been followed by IR spectra, ab initio HF, MP2 6-31G(d) and density functional BLYP, B3LYP 6-31G(d) force field calculations. In a general agreement between theory and experiment, the conversion causes a 34 cm −1 frequency decrease in the cyano stretching band, 3.2-fold increase in its integrated intensity and other essential spectral changes in the para-isomer; for the meta-one the corresponding values are 13 cm −1 and 2.3-fold, respectively. According to the calculations, both molecule and anion of 4-hydroxybenzonitrile are more stable (due to the polar resonance) than the corresponding meta-substituted species. The strongest structural changes, caused by deprotonation, are the 0.105 Å shortenings of the Ph–O bonds, 0.054 Å lengthenings (mean values) of the adjacent CC bonds and essential bond angle changes in the phenylene rings near the oxyanionic centers. In the case of para-isomer these changes are related to the formation of a quasi- para-quinonoidal structure of the phenylene ring in the oxyanion. According to the electronic density analysis, a bit less (Mulliken) or a bit more (natural bond orbital) than halves of the anionic charges remain localized at the oxyanionic centers. Hydrogen bonds have also been discussed.

Supramolecular Structures of Low-Molecular-Weight Polybutadienes, as Studied by Dynamic Light Scattering, NMR and Infrared Spectroscopy

Three types of low-molecular-weight polybutadienes (bearing no or primary or secondary hydroxy groups on both chain ends), as well as their hydrogenated analogues, were studied by 1H NMR and IR spectroscopies and by dynamic light scattering. In bulk and at room temperature, hydroxylated polymers form hydrophilic microdomains (knots) based on hydrogen bonds between two or more OH end groups; these knots gradually disintegrate with increasing temperature and above 100 °C, only OH/OH pairs can be present, as is seen from IR spectra. Fixation of more than two chain ends in a single knot leads to the spatial restriction of the segmental motion of the chain links which manifests itself by the broadening of 1H NMR bands. With unsaturated, hydroxylated polymers, an intramolecular interaction between an OH end group and the adjacent CC bond of the terminal monomer unit is possible which makes the formation of the intermolecular OH/OH bonding less probable. The dynamic light scattering method revealed the presence ...

Simulation and interpretation of the vibrational spectra of heterofullerenes

This paper reports on a computer simulation of the vibrational spectra of C59B, C59N, C59BN, and C59HN. The modes whose frequencies are most sensitive to carbon substitution by heteroatoms are revealed. It is shown that the new bands at 818, 839, and 845 cm−1 in the experimental IR spectrum of C59HN arise from changes in selection rules and by removal of degeneracies due to a reduction in the symmetry of the molecule and force field perturbation. In the Raman spectrum of C59HN, the shifts of the 1460 and 491 cm−1 bands to the low-frequency region by 7 and 3 cm−1, respectively, relative to two Ag modes of C60 are explained by the perturbation induced by the N and C(H) atoms in the force field of adjacent CC bonds not containing these atoms.

Theoretical Study of the Effects of Protonation and Deprotonation on Bond Dissociation Energies of Second-Row Elements: Comparison with First-Row Elements

Ab initio MO calculations indicate that protonation of XHn to XHn+1+ increases bond dissociation energies (BDEs) for homolytic cleavage of CX bonds in CH3XHn, CH3−CH2−XHn, CH2CH−XHn, and CH⋮C−XHn compounds (X = N, O, F, P, S, and Cl). Deprotonation of XHn to XHn-1- (X = C, N, O, Si, P, and S) in saturated species generally results in small decreases in CX BDEs; in unsaturated substances, as a result of resonance, deprotonation yields large increases in the CX BDEs, X = Si being the exception. For adjacent CC bonds, protonation of XHn increases the CC BDEs because it produces larger electronegativity differences between bonded groups; deprotonation decreases the BDEs as a result of resonance effects. Two types of correlation between bond lengths and homolytic BDEs are observed, with second-row elements exhibiting bond length changes to a much lesser extent than first-row ones. Firstly, bond lengths of adjacent CC bonds increase as their BDEs decrease. Secondly, and apparently anomalously, CX bond lengths and homolytic BDEs both increase with protonation of XHn, except with X = P. The increase in BDE together with the increase in bond length is, in part, a result of the focus on homolytic BDEs: heterolytic cleavage of most protonated, and many deprotonated, species is actually preferred, in which case bond lengths generally increase as bond strengths decrease in accordance with the normally accepted trend.

Synthesis, Structure, and Properties of {(tBu2PC2H4PtBu2)Ni}2(μ-η2:η2-C6H6) and (tBu2PC2H4PtBu2)Ni(η2-C6F6)1

Thermolysis of (tBu2PC2H4PtBu2)NiMe2 in benzene or reduction of (tBu2PC2H4PtBu2)NiCl2 with Mg* in THF/benzene affords a solution of mononuclear (tBu2PC2H4PtBu2)Ni(η2-C6H6) (1) and dinuclear {(tBu2PC2H4PtBu2)Ni}2(μ-η2:η2-C6H6) (2) in equilibrium. Complex 2 has been isolated; the X-ray structure analysis reveals an antifacial coordination of the [(tBu2PC2H4PtBu2)Ni0] moieties to adjacent CC bonds of a formal cyclohexatriene ligand. According to solid-state and solution NMR the structure of 2 is static in the solid and fluxional in solution. Displacement of the benzene ligand in 1 or 2 by C6F6 affords mononuclear (tBu2PC2H4PtBu2)Ni(η2-C6F6) (5) for which the molecular structure is also reported.

ChemInform Abstract: TRANSFERABILITY OF INFRARED INTENSITY PARAMETERS. APPLICATION TO PROPYLENE

AbstractDie IR‐Absorptionsintensitäten von Propylen können durch Verwendung von elektrooptischen Parametern für die C?a‐Gruppe, die durch Interpolation von Werten für n‐Alkane und Propin erhalten wurden, vorausgesagt werden.

Transferability of infrared intensity parameters. Application to propylene

The IR absorption intensities of propylene can be reasonably predicted by using electrooptical parameters for the CH3 group interpolated from the values obtained in studies of n-alkanes and propyne. The results confirm the transferability of electrooptical parameters and offer additional support to the existence of a linear relation between the electrooptical parameters of the CH bond and the bond order of the adjacent CC bond.