Monocyclic Isomers Research Articles

Theoretical Investigation on C11H8 Bicyclic Carbene and Allene Isomers

Recent studies postulate that the presence of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM) could have been formed through resonance-stabilized arylcarbene intermediates. However, identifying most of these reactive intermediates is very challenging experimentally due to their metastability and other experimental constrains. Thus, computational studies that cover the thermodynamic versus kinetic stability of various possible structures would be beneficial for successfully identifying new molecules either in the laboratory and/or in the ISM. In this paper, more than four hundred C11H8 carbene isomers have been theoretically investigated employing density functional theory (DFT). Hybrid density functionals B3LYP and ωB97XD with 6-311 + G (d,p) basis set have been used for singlet electronic states, whereas, triplet spin states were optimized at the same level using an unrestricted Hartree-Fock wavefunction. Although the skeletal structures of C11H8 can be categorized into monocyclic, bicyclic, tricyclic, tetracyclic and acyclic isomers, bicyclic carbenes have shown better stability due to the presence of resonance stabilized azulenyl/naphthyl rings. In this category, some isomers (1-, 2-, 5- and 6-azulenylcarbenes and 1- and 2-naphthylcarbenes) have also been detected recently in the laboratory and simple aromatic carbenes such as cyclopropenylidene and its homologues are detected in the ISM. Thus, we have systematically investigated the energetic and spectroscopic properties of resonance stabilized 5-, 6-, 7- and 8-membered ring containing bicyclic isomers of C11H8 and the fingerprint regions of the infrared spectrum for each class of these bicyclic compounds.

Roles of Nanostructured Bimetallic Supported on Alumina-Zeolite (AZ) in Light Cycle Oil (LCO) Upgrading

Light cycle oil (LCO) is one of the major products in Fluid catalytic cracking (FCC) processes, and has drawbacks such as high aromatics, sulfur, and nitrogen contents, and low cetane number (CN). Hydro-upgrading is one of the most typical processes for LCO upgrading, and alumina-zeolite (AZ) is an effective hydrotreating catalyst support. This paper examined the effects of different bimetallic catalysts (CoMo/AZ, NiMo/AZ, and NiW/AZ) supported by AZ on hydro-upgrading of both model compounds and real LCO. CoMo/AZ preferred the direct desulfurization (DDS) route while the NiMo/AZ and NiW/AZ catalysts favored the desulfurization route through hydrogenation (HYD). The presence of nitrogen compounds in the feed introduced a competitive adsorption mechanism and reduced the number of available acid sites. Aromatics were partially hydrogenated into methyltetralines at first, and then further hydrogenated, cracked, and isomerized into methyldecalins, monocyclic, and methyltetralines isomers. CoMo/AZ is the best hydrodesulfurization (HDS) catalyst for the model compounds at low H2 pressure (550 psi) and for LCO at lower temperature (573 K), while NiMo/AZ performs the best for LCO at higher temperature (648 K). NiMo/AZ is the best hydrodenitrogenation (HDN) catalyst for LCO. The hydrodearomatization (HDA) performances of NiMo/AZ and NiW/AZ improved significantly and overwhelmingly higher than that of the CoMo/AZ when the H2 pressure was increased to 1100 psi.

Large-scale profiling of carotenoids by using non aqueous reversed phase liquid chromatography – photodiode array detection – triple quadrupole linear ion trap mass spectrometry: Application to some varieties of sweet pepper (Capsicum annuum L.)

Analysis of carotenoids is very complex and demanding in terms of both separation and detection. In this article, an analytical strategy relying on high-performance liquid chromatography-photodiode array detection-tandem mass spectrometry (HPLC-PAD-MS/MS) is presented for the large-scale screening of these phytochemicals. Separation was realized by means of Non-Aqueous Reversed Phase (NARP) chromatography on a triacontyl stationary phase kept at subambient temperature, using a mobile phase compatible with atmospheric pressure chemical ionization (APCI). The standards of 14 analytes were used to optimize the method and to predict the chromatographic behaviour of untargeted carotenoids. MS and MS/MS data, obtained during Information Dependent Acquisition (IDA) experiments, were utilized to set up a sensitive HPLC- selected reaction monitoring (SRM) method. Relative abundance between SRM ion currents (ion ratio) allowed the MS distinction of structural isomers (for example, bicyclic, monocyclic and acyclic isomers), while the identification of geometrical isomers was based on Qratio and fine structure, as calculated from UV–vis spectra. The comparison of LC-PAD- SRM chromatograms, acquired after applying two different extraction procedures (matrix solid-phase dispersion and overnight cold saponification), allowed verifying that sweet peppers are a good source of xanthophylls, prevalently occurring as esterified forms. The overall strategy could identify more than 40 carotenoids in some sweet pepper varieties (Jolly, horn and sweet chili pepper) available on the Italian and European food market.

Ultraslow isomerization in photoexcited gas-phase carbon cluster {{rm C}}_{10}^ -

Isomerization and carbon chemistry in the gas phase are key processes in many scientific studies. Here we report on the isomerization process from linear {{rm C}}_{10}^ - to its monocyclic isomer. {{rm C}}_{10}^ - ions were trapped in an electrostatic ion beam trap and then excited with a laser pulse of precise energy. The neutral products formed upon photoexcitation were measured as a function of time after the laser pulse. It was found using a statistical model that, although the system is excited above its isomerization barrier energy, the actual isomerization from linear to monocyclic conformation takes place on a very long time scale of up to hundreds of microseconds. This finding may indicate a general phenomenon that can affect the interstellar medium chemistry of large molecule formation as well as other gas phase processes.

Formation of 6-methyl-1,4-dihydronaphthalene in the reaction of the p-tolyl radical with 1,3-butadiene under single-collision conditions.

Crossed molecular beam reactions of p-tolyl (C7H7) plus 1,3-butadiene (C4H6), p-tolyl (C7H7) plus 1,3-butadiene-d6 (C4D6), and p-tolyl-d7 (C7D7) plus 1,3-butadiene (C4H6) were carried out under single-collision conditions at collision energies of about 55 kJ mol(-1). 6-Methyl-1,4-dihydronaphthalene was identified as the major reaction product formed at fractions of about 94% with the monocyclic isomer (trans-1-p-tolyl-1,3-butadiene) contributing only about 6%. The reaction is initiated by barrierless addition of the p-tolyl radical to the terminal carbon atom of the 1,3-butadiene via a van der Waals complex. The collision complex isomerizes via cyclization to a bicyclic intermediate, which then ejects a hydrogen atom from the bridging carbon to form 6-methyl-1,4-dihydronaphthalene through a tight exit transition state located about 27 kJ mol(-1) above the separated products. This is the dominant channel under the present experimental conditions. Alternatively, the collision complex can also undergo hydrogen ejection to form trans-1-p-tolyl-1,3-butadiene; this is a minor contributor to the present experiment. The de facto barrierless formation of a methyl-substituted aromatic hydrocarbons by dehydrogenation via a single event represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated analogues in combustion flames and the interstellar medium.

Theoretical study on the interconversion of silabenzenes and their monocyclic non-aromatic isomers via the [1,3]-substituent shift: Interplay of aromaticity and Bent's rule

Density functional theory (DFT) calculations were performed to examine the substituent effects on the interconversion of silabenzenes and their monocyclic non-aromatic isomers. A previous study suggested that aromaticity is the driving force for this process. Interestingly, our systematic calculations reveal that the contribution from aromaticity can be evaluated quantitatively (ca. 30 kcal mol−1). Thus it is the interplay of aromaticity and Bent's rule that determines their relative stabilities. In addition, strong correlations of reaction energies as well as reaction barriers have been identified against the s character of Si in the Si-X bonds, providing an important guide to the synthesis of silabenzenes.

The quinones of bicyclo[3.1.0]hexatriene: A computational study of their chemistry and thermochemistry

Quinones of bicyclo[3.1.0]hexa-1,3,5-triene were examined computationally. The six compounds considered were the five possible classical and one non-classical quinone: bicyclo[3.1.0]hexa-1(6),4-diene-2,3-dione (and its monocyclic isomer with a long trans-annular bond), bicyclo[3.1.0]hexa-1(5),3-diene-2,6-dione, bicyclo[3.1.0]hexa-1,4-diene-3,6-dione (and its monocyclic isomer with a long trans-annular bond), and bicyclo[3.1.0]hexa-1(5),4-diene-2,4-dione-3,6-diyl, a non-classical (non-Kekulé) zwitterion. The two long trans-annular bond structures are akin to that found for m-benzyne. Geometries were calculated (BLYP/6-31G∗, CASSCF(2,2)/6-31G∗, MP2/6-31G∗) and electronic structural inferences were made from the geometries. Also calculated were relative energies and heats of formation (CBS-QB3), singlet and triplet energies (BLYP/6-31G∗), and ionization energies and electron affinities (HF/6-311+G∗∗//BLYP/6-31G∗). The NICS(1) calculations were performed as a probe of the aromaticity of the diverse quinones.

DFT Studies on Configurations, Stabilities, and IR Spectra of Neutral Carbon Clusters

The linear chains and monocyclic rings with carbon atoms fewer than 26 were studied at B3LYP/6-311G ∗ level. The change trends that the stabilities of lin- ear chains and monocyclic rings increase with the enlargement of cluster sizes were obtained, respectively. The theoretical calculations for carbon clusters lead to the con- clusions that the small linear isomers of C3∼C9 are more stable than the cyclic isomers and the monocyclic isomers contrarily become more stable than the linear isomers as cluster size is larger than C10, which are consistent with those of early UPS and ion chromatography. Comparing with the stabilities of triplet and singlet states of lin- ear clusters, it was confirmed that the triplet states of even-numbered clusters have higher stabilities, and so do the singlet states of odd-numbered clusters contradicto- rily. The average energies per atom of even-numbered monocyclic rings indicate that (4m+2)-numbered series are aromatic and 4m-numbered series are anti-aromatic. The total energy comparisons give the conclusion that the 2D tetracyclic cluster and the cycloadduts with 24 carbon atoms are absolutely less stable than monocyclic isomer and three fullerenes. Several 3D polycyclic clusters and fullerene with D6d symme- try have middle stabilities. In addition, the theoretical IR spectra of linear chains and odd-numbered monocyclic rings were predicted.

Molecule of the month

Three new monocyclic C6H6 isomers which are highly strained have been made in recent years.

Theoretical study of the stable states of small carbon clustersCn(n=2–10)

Both even- and odd-numbered neutral carbon clusters ${\text{C}}_{n}$ $(n=2--10)$ are systematically studied using the energy minimization method and the modified Brenner potential for the carbon-carbon interactions. Many stable configurations were found, and several new isomers are predicted. For the lowest energy stable configurations we obtained their binding energies and bond lengths. We found that for $n\ensuremath{\le}5$ the linear isomer is the most stable one while for $ng5$ the monocyclic isomer becomes the most stable. The latter was found to be regular for all studied clusters. The dependence of the binding energy for linear and cyclic clusters versus the cluster size $n$ $(n=2--10)$ is found to be in good agreement with several previous calculations, in particular with ab initio calculations as well as with experimental data for $n=2--5$.

An approach toward the total synthesis of subergorgic acid

Abstract Ireland–Claisen rearrangement of a substituted alkenyl cyclopentanecarboxylate provided a monocyclic isomer containing the proper stereochemistry for four of the five stereogenic centers in subergorgic acid. Efforts to construct the additional rings in the molecule were thwarted as a result of steric and other factors. The results provide insights regarding the Ireland–Claisen rearrangement in five-membered ring systems. The formation of spiro-compounds from sterically and stereoelectronically demanding systems as reported herein has the potential to serve as a general strategy for the synthesis of such sub-units in both natural and unnatural products.

Carbo-[3]oxocarbon and its isomers: evaluation of the stability and of the electron delocalization

Various isomers of carbo-[3]oxocarbon C9O3 have been characterized on the singlet and triplet spin state potential energy surfaces. Despite its localized structure, the ring carbo-mer of [3]oxocarbon is thermodynamically protected from subsequent isomerization and stable versus dissociation into C3O. It therefore appears as a reasonable synthetic target. In contrast, the less stable tetracyclic isomer exhibits electron delocalization both in the central six-membered ring and in the external three-membered rings, as evidenced from structural, magnetic and electron localization function (ELF) analysis. Another monocyclic isomer may be considered as a planar C6(CO)3 "carbon-complex", a carbon version of the related iron carbonyl complex Fe(PH3)2(CO)3. C6 and Fe(PH3)2 are indeed isolobal on the basis of frontier orbital analysis.

Comprehensive Kinetic and Mechanistic Considerations for the Gas‐Phase Behaviour of Pinane‐Type Compounds

AbstractThe thermal behaviour of selected pinane‐type compounds, α‐pinene (1), β‐pinene (2), pinane (3) and nopinone (4), has been investigated. The conversion of the bicyclic starting materials to their acyclic and monocyclic isomers as well as the consecutive reactions of the acyclic main isomerisation products are discussed. The conversion of 1–4 in a reaction network is presented and the experimental evidence for the formation of pyrolysis products by a biradical pathway is discussed. In addition to these results a kinetic model describing the isomerisation of the bicyclic compounds to their acyclic and monocyclic isomers is presented. A good correlation between kinetic simulations and experimental data is revealed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Gas-phase electronic spectra of C18 and C22 rings

The electronic spectra of C(18) and C(22) in the 15 150-36 900 cm(-1) range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D(9 h) symmetry for C(18) and D(11 h) for C(22), based on time-dependent-density-functional calculations and reactivity with respect to H(2). The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the A(1)A(2) (")<--X(1)A(1) (') transition of C(18) at 592.89 nm shows a pair of 1 cm(-1) broadbands spaced by 1.5 cm(-1). The next electronic transitions exhibited much broader, approximately 30 (in the visible) to 200 cm(-1) (in ultraviolet range), features. The spectrum of C(22) exhibits an absorption pattern similar to C(18), except that the narrow features to the red are missing; the oscillator strength of the A<--X transition is predicted to be low.

Non‐Rusty [2]Catenanes with Huge Rings and Their Polymers

AbstractFor Abstract see ChemInform Abstract in Full Text.

Non‐Rusty [2]Catenanes with Huge Rings and Their Polymers

AbstractWe describe the design and synthetic realisation of [2]catenanes characterised by having huge rings that can rotate freely and shift laterally within the constraints of concatenation. The synthetic strategy, which is based on using a carbonate group as a covalent template and oxidative dimerization of alkynes to achieve ring formation, is versatile with respect to ring size and the presence of functional groups. The monocyclic constitutional isomer of the catenane is available by a simple exchange of steps in the sequence. EPR spectroscopy revealed that in solution the catenanes adopt all possible co‐conformations in equal abundances. The thermotropic liquid crystallinity of the [2]catenanes and their corresponding non‐intertwined macrocycles proves intermolecular ordering in the bulk phase. The [2]catenanes were polymerized through ester formation and through acyclic diene metathesis. All of the poly[2]catenanes have rather low degrees of polymerisation (&lt;Pn&gt; ≈︁ 10), possibly because cyclisation occurs. For the preparation of [n]catenanes having n &gt; 3, we propose fusing the rings of [2]catenanes so that the crucial steps of threading and cyclisation need only occur during the synthesis of the smallest catenane. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2003)

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics, structure and interconversion of dihydrotropylium ions.

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H((1))](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)([CHT + H((1))](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H((1))](+). The 1,4-dihydrotropylium ion, [CHT + H((3))](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H((1))](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H((1))](+) and [CHT + H((3))](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H((1))](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.

Vibrational Predissociation Spectroscopic and Ab Initio Theoretical Studies on Protonated Ethylenediamine−(Water)3 Complex

Structures, interactions, and vibrations of an intramolecular hydrogen bond (IHB)-containing protonated ion(water) 3 complex, g-enH + (H 2 O) 3 (g-enH + = gauche-protonated ethylenediamine) and t-enH + (H 2 O) 3 (t-enH + = trans-protonated ethylenediamine) with no IHB were investigated by combined vibrational predissociation spectroscopy and ab initio calculations. The observed vibrational feature at 3552 cm - 1 (signature of cyclic ion-(water) 3 structure), consistent with ab initio results on the stabilities of structural isomers, suggested the IHB-assisted bicyclic structure for g-enH + (H 2 O) 3 , Ab initio many-body analyses further indicated that the greater stability of bicyclic isomer versus monocyclic and bicyclic tripod isomers originates from the destabilization of the latter by the large relaxation energies and repulsive nonadditive interactions. The close correlation between the observed and calculated vibrational spectra for g-enH + (H 2 O) 3 revealed the coexistence of bicyclic isomers and monocyclic open isomers in our beam, consistent with the trend of the calculated Gibbs free energies at 150 K.

Unusual Reaction of Alkaloid Cotarnine with 1,3‐Dimethylbarbituric Acid.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Unusual Reaction of Alkaloid Cotarnine with 1,3-Dimethylbarbituric Acid

Cotarnine is a natural pseudobase of the isoquinoline series, which is capable of undergoing ring3chain transformations [1]. Its structure may be represented as two isomeric forms occurring in equilibrium: bicyclic Ia and monocyclic Ib [2]. Monocyclic isomer Ib is responsible for formation of derivatives at the aldehyde (e.g., oxime [3]) or amino group (amides [4]). In the other cases cotarnine behaves as cyclic semiaminal Ia, which is readily dehydrated to give iminium ion, and reacts with some CH and NH acids following the Mannich reaction pattern and yielding the corresponding tetrahydroisoquinoline bases [2, 5, 6]. The reaction of cotarnine with 1,3-dimethylbarbituric acid (II) on heating in chloroform leads to formation of 6-hydroxy-1,3-dimethyl-5-(4-methoxy-6methyl-5,6,7,8-tetrahydro[1,3]dioxolo[4,5-g]isoquinolin-5-yl)-1,2,3,4-tetrahydropyrimidine-2,4-dione (III) as inner salt [7]. When the same reaction was performed with 2 equiv of barbituric acid II under more severe conditions, we isolated an unusual product, spiropyrimidine derivative IV (Scheme 1). The structure of IV was derived from the data of mass spectrometry and NMR spectroscopy (H and C; the spectra were recorded in a standard mode and also using COSY, DEPT, and INADEQUATE techniques). Presumably, elevated temperature favors formation of cotarnine tautomer Ib which react with 1,3-dimethylbarbituric acid (II) to give adduct V. Such products are typical of reactions of barbituric acids with aromatic aldehydes [8]. Compound IV is likely