Lowest Isomers Research Articles

Spectroscopy and characterization of AlNX (X = O and S): Triatomic circumstellar molecules.

Three isomers of the triatomic [Al, N, O] molecular system have been observed in a solid argon matrix by infrared absorption spectroscopy using 15N and 18O isotopic substitution. The present work provides high-level quantum chemical predictions of their spectroscopic parameters to observe this system in the interstellar medium. The spectroscopic parameters, stability, and geometries of the lowest stable isomers of its isoelectronic system [Al, N, S] were characterized using coupled-cluster CCSD(T), explicitly correlated coupled-cluster CCSD(T)-F12, and multireference configuration interaction. The three-dimensional potential energy surfaces of all isomers were computed at the CCSD(T)-F12/aug-cc-pV5Z level, and a set of spectroscopic parameters were calculated. In both systems, the most stable isomer is linear with an X3Σ- electronic ground state, and all linear isomers are characterized by small bending modes of less than 200 cm-1. Due to their large dipole moments, the high intensities of such modes, and the nonexistence of anharmonic resonance complicating their spectra, our results facilitate the detection of AlNO and AlNS in the laboratory or in the interstellar medium.

Submillimeter spectroscopy and astronomical searches of vinyl mercaptan, C2H3SH

Context. New laboratory investigations of the rotational spectrum of postulated astronomical species are essential to support the assignment and analysis of current astronomical surveys. In particular, considerable interest surrounds sulfur analogs of oxygen-containing interstellar molecules and their isomers. Aims. To enable reliable interstellar searches of vinyl mercaptan, the sulfur-containing analog to the astronomical species vinyl alcohol, we investigated its pure rotational spectrum at millimeter wavelengths. Methods. We extended the pure rotational investigation of the two isomers syn and anti vinyl mercaptan to the millimeter domain using a frequency-multiplication spectrometer. The species were produced by a radiofrequency discharge in 1,2-ethanedithiol. Additional transitions were remeasured in the centimeter band using Fourier-transform microwave spectroscopy to better determine rest frequencies of transitions with low-J and low-Ka values. Experimental investigations were supported by quantum chemical calculations on the energetics of both the [C2,H4,S] and [C2,H4,O] isomeric families. Interstellar searches for both syn and anti vinyl mercaptan as well as vinyl alcohol were performed in the EMoCA spectral line survey carried out toward Sgr B2(N2) with ALMA. Results. Highly accurate experimental frequencies (to better than 100 kHz accuracy) for both syn and anti isomers of vinyl mercaptan are measured up to 250 GHz; these deviate considerably from predictions based on extrapolation of previous microwave measurements. Reliable frequency predictions of the astronomically most interesting millimeter-wave lines for these two species can now be derived from the best-fit spectroscopic constants. From the energetic investigations, the four lowest singlet isomers of the [C2,H4,S] family are calculated to be nearly isoenergetic, which makes this family a fairly unique test bed for assessing possible reaction pathways. Upper limits for the column density of syn and anti vinyl mercaptan are derived toward the extremely molecule-rich star-forming region Sgr B2(N2) enabling comparison with selected complex organic molecules.

Ab initio investigation of possible lower-energy candidate structure for cationic water cluster (H2O) 12+ via particle swarm optimization method

Detecting the underlying performance of hydrated electrons and hydroxyl radicals in the cationic water cluster can greatly help to understand the inter reaction mechanism in the liquid water and aqueous solutions. Based on our previous (H2O)10+ research, we have paid attention to more problems of larger cationic clusters in this work, including the existence of hemibonded type, long-range correction functions, and hydrogen-bonded site analyses. The lower-energy structures of the cationic water cluster (H2O)12+ have been comprehensively explored, and more experienced functions are introduced to check the ground state and vibration spectrum. Unlike the configuration regularity of neutral (H2O)12 clusters and small cationic water clusters, those new-found structures for (H2O)12+ are inclined to adopt three dimension (3D) cage-like structures and the H2O-OH2 structure appears in the higher energy isomer. The calculation reveals that the lowest stable isomer is the 3D cage structure W14 predicted at MP2 level, which has not been reported yet. In the thermal simulation, structure transition from the cage-like to the ring-like occurs at T > ≈256 K, and the two dimension (2D) ring-like structure occupies a dominant position at high temperature range. The infrared spectra explain that the difference of the spectra between the 2D net structures and 3D cage-structures is mainly caused by the weight fluctuation of single acceptor-single donor (AD), double acceptor-single donor (AAD), and single acceptor-double donor (ADD) sites in these isomers. This further gives a similarity relation between (H2O)12+ and H+(H2O)12 clusters in the shape of the network and spectral characteristics. By molecular orbitals and topological analysis, we find that the lone pair orbital on hydroxyl radical dominates the reactivity and stability of cationic system. The present research may be helpful for exploring the evolution law of the larger cationic water clusters in the future.

Ab initio study of structural and electronic properties of SinC5-nH8 (n = 0–5) series: Probing the 2D to 3D structural transition

Abstract In this study, we studied the structural and electronic properties in the Si n C 5 - n H 8 (n = 0–5) series using ab initio calculations. We performed global minimum and low-lying isomers search for the C5H8, Si C 4 H 8 , Si 2 C 3 H 8 , Si 3 C 2 H 8 , Si 4 CH 8 and Si 5 H 8 stoichiometries at the B3LYP/3-21G level of theory. The lowest isomers were recalculated at the CCSD(T)/CBS//B3LYP/6-311++G∗∗ level of theory. It was shown that Si 2 C 3 H 8 structure is more stable than the other structures with high ionization potential and HOMO-LUMO energy gap. We also showed the 2D-3D that transition, which occurs only at n = 3, is due to stronger C–C σ bonds compared to weaker C–Si and Si–Si σ bonds. The results were found in good agreement with available experimental findings.

First-principles study of MoS2 and MoSe2 nanoclusters in the framework of evolutionary algorithm and density functional theory

Evolutionary algorithm is combined with full-potential ab initio calculations to investigate conformational space of (MoS2)n and (MoSe2)n (n = 1–10) nanoclusters and to identify the lowest energy structural isomers of these systems. It is argued that within both BLYP and PBE functionals, these nanoclusters favor sandwiched planar configurations, similar to their ideal planar sheets. The second order difference in total energy (Δ2E) of the lowest energy isomers is computed to estimate the abundance of the clusters at different sizes and to determine the magic sizes of (MoS2)n and (MoSe2)n nanoclusters. In order to investigate the electronic properties of nanoclusters, their energy gap is calculated by several methods, including hybrid functionals (B3LYP and PBE0), GW approach, and Δscf method. At the end, the vibrational modes of the lowest lying isomers are calculated by using the force constants method and the IR active modes of the systems are identified. The vibrational spectra are used to calculate the Helmholtz free energy of the systems and then to investigate abundance of the nanoclusters at finite temperatures.

"Star" morphologies of charged nanodrops comprised of conformational isomers.

We study the spatial distribution of conformational isomers surrounding a central macroion in a charged droplet with linear dimensions in the nanometer range. Dimethyl carbonate and formic acid are selected as typical solvents that undergo isomerization and a charged buckyball (C60) is selected as a representative example of a macroion. The study is performed by atomistic molecular dynamics simulations. We find that when the charge of the buckyball is above a threshold value, it induces the formation of concentric shells of different conformational isomers surrounding the macroion. The presence of layers with different dielectric properties necessitates the use of different state equations for the solvent polarization in each layer. We find that at a high charge state of the buckyball, the nearest layer to the macroion comprises the conformers with the highest dipole moment. The interface of the outer layers of conformers is characterized by "ray"-forming structures of the higher dielectric constant isomers penetrating into the layer of the lowest dielectric constant isomers. For high values of the solvent dielectric constant, the charged droplet acquires a "star"-like global shape. We demonstrate that these distinct droplet structures are a manifestation of charge-induced instability. We describe this simulation-based phenomenology by an analytical theory that supports this conclusion. The findings suggest new experimental research venues that may explore the reactivity and assembly of molecules within regions of different dielectric properties in droplets.

Ab initio molecular dynamics studies of formic acid dimer colliding with liquid water.

Ab initio molecular dynamics simulations of formic acid (FA) dimer colliding with liquid water at 300 K have been performed using density functional theory. The two energetically lowest FA dimer isomers were collided with a water slab at thermal and high kinetic energies up to 68kBT. Our simulations agree with recent experimental observations of nearly a complete uptake of gas-phase FA dimer: the calculated average kinetic energy of the dimers immediately after collision is 5 ± 4% of the incoming kinetic energy, which compares well with the experimental value of 10%. Simulations support the experimental observation of no delayed desorption of FA dimers following initial adsorption. Our analysis shows that the FA dimer forms hydrogen bonds with surface water molecules, where the hydrogen bond order depends on the dimer structure, such that the most stable isomer possesses fewer FA-water hydrogen bonds than the higher energy isomer. Nevertheless, even the most stable isomer can attach to the surface through one hydrogen bond despite its reduced hydrophilicity. Our simulations further show that the probability of FA dimer dissociation is increased by high collision energies, the dimer undergoes isomerization from the higher energy to the lowest energy isomer, and concerted double-proton transfer occurs between the FA monomers. Interestingly, proton transfer appears to be driven by the release of energy arising from such isomerization, which stimulates those internal vibrational degrees of freedom that overcome the barrier of a proton transfer.

Thermodynamic investigation of tautomerism in triazine oximes

The geometric and energetic preference of the possible isomers of four tautomeric triazine derivatives has been investigated by density functional theory and topological calculations and their thermodynamic stability was assessed. For the monoxime series the amino keto tautomer is favored by 2.6 kcal/mol (10.9 kJ/mol) as compared to the enol form, while the lowest energy imino isomer is 4.1 kcal/mol (17.1 kJ/mol) higher in energy. However, for the dioxime tautomer series the imino keto tautomer is 4.2 kcal/mol (17.6 kJ/mol) lower in energy as compared to the most stable amino keto tautomer. The computed excitation energies for the first electron transitions agree with the experimental observations.

C4H4 radical cation isomers: Generation, structure, stability, and isomerization reactions

The B3LYP, M06-2X and MP2 methods are used to study the generation, relative stability, structure, charge and spin distributions, along with isomerization reactions, of C4H4+ radical cations in gas phase. The highly endothermic generation of C4H4+ species by ionization of neutral C4H4 precursors is for 7 cases. Stability orders for 10 C4H4+ isomers as derived from the three methods correlate well with one another, where methylenecyclopropene radical cation is the global minimum. Available computational data also agrees well with these orders, and experimental data fairly well. The charge and spin distributions describe one isomer as distonic (with charge and spin separated). Concerted isomerization reactions for the methylenecyclopropene, butatriene, vinylacetylene and methyleneallene radical cations and 2 hypothetical higher energy species proceed via well-defined transition states described with regard to their relative positions along the reaction coordinate in accordance with Hammond’s postulate. These one-step isomerizations are combined to yield five multi-step conversions of various C4H4+ species to the lowest energy methylenecyclopropene isomer. The application of transition state theory and computation of rate coefficients leads to the estimation of activation energies for the concerted isomerizations.

Sulfur and carbon as heteroatoms in ferrathiacarboranes

The complete series of ferrathiacarboranes CpFeCHSBn−3Hn−3 (n=8, 9, 10, 11, 12) related to the experimentally known CpFeC[B(OH)2]SB8H8 and CpFeC(NHCMe3)SB9H9 have been investigated by density functional theory. Deltahedral structures having the iron atoms located at degree 5 or 6 vertices, the carbon and sulfur atoms located at degree 4 vertices, and Fe–S edges are found to be energetically preferred. In general the most spherical closo deltahedra found in the deltahedral borane anions BnHn2− are energetically preferred as central FeCSBn−3 polyhedra in these CpFeCHSBn−3Hn−3 structures. However, for the 9-vertex CpFeCHSB6H6 system, structures having a central FeCSB6isocloso deltahedron with the iron atom located at the unique degree 6 vertex and the sulfur and carbon atoms located at adjacent degree 4 vertices are comparable in energy with closo structures having a central FeCSB6closo tricapped trigonal prism. For the 11-vertex CpFeCHSB8H8 system the unique closo deltahedral structure having the iron atom located at the unique degree 6 vertex and the carbon and sulfur atoms located at the two degree 4 vertices adjacent to the iron atom is particularly favorable since it lies ∼14kcal/mol in energy below the next lowest energy CpFeCHSB8H8 isomer. The central FeCSBn−3 deltahedra found experimentally by X-ray crystallography for the ferrathiacarboranes CpFeC[B(OH)2]SB8H8 and CpFeC(NHCMe3)SB9H9 correspond to the lowest energy CpFeCHSB8H8 and CpFeCHSB9H9 structures, respectively, with similar Fe–S, Fe–C, and Fe–B edge lengths.

About the detection of urea in the interstellar medium: the energetic aspect

Context. The results of an observational search for gas phase urea, (NH2 )2 CO toward the Sgr B2-LMH region, have been reported recently. In spite of strong presumptions, whether it is urea or another species (for example an isomer) seems to remain a pending question.Aims. In this note, we consider the energetic aspect of this would-be detection by addressing the relative stabilities of the 22 isomers that can possibly be formed with the CH4 N2 O set of atoms. By extension, we also consider the 22 sulphur analogues of CH4 N2 S chemical formula. Methods. The question was first addressed by means of quantum density functional theory (DFT) simulations. The hybrid B3LYP functional was used throughout. The geometries of the 44 molecules part of this survey were fully optimized and verified to be real minima by vibrational analysis. The lowest isomers found this way were then reconsidered in higher level post Hartree-Fock MP2 and coupled cluster CCSD and CCSD(T) calculations to derive more accurate energy differences and dipole moments, whose knowledge is crucial for interpreting micro- and millimetre-wave spectra. Results. We found that urea and thio-urea are the most stable compounds in their respective families. The closest isomers on the energy scale are the iminol tautomeric forms, HN=COH-NH2 and HN=CSH-NH2 , whose rotational constants and dipole moments have also been determined. Conclusions. That urea is the lowest energy isomer possibly formed is a strong argument making the detection of this species more than probable. After formamide and acetamide, this result confirms the greatest stability of the -[NH-C=O]- linkage, underlining the interest of the minimum energy criterion as a tool for the primary search of target molecules. Additionally, thio-urea should the analogue to search for.

Polyhedral dinickelaboranes as analogues of the dicarbaboranes

The lowest energy structures of the dinickelaboranes Cp2Ni2Bn−2Hn−2 (n=8–12) are found by density functional theory to be the most spherical closo deltahedra similar to those of the isoelectronic dicarbaboranes C2Bn−2Hn. The nickel atoms in such dinickelaboranes do not prefer higher degree vertices in contrast to metallaboranes containing earlier transition metals. Similarly, Cp2Ni2Bn−2Hn−2 structures having adjacent nickel atoms are of higher energy than isomeric structures having non-adjacent nickel atoms. Thus the brown icosahedral ortho Cp2Ni2B10H10 isomer with adjacent nickel atoms obtained by Hawthorne and co-workers from B10H102− and Cp2Ni, [Cp3Ni2][BF4], or Cp2Ni2(CO)2 and structurally characterized by X-ray crystallography is not the lowest energy isomer but instead a kinetically favored product. However, the structure with non-adjacent nickel atoms suggested by 11B NMR for the Cp2Ni2B8H8 isomer isolated in small quantities by Grimes and co-workers from a B5H8−/C5H5−/NiBr2 reaction mixture is found to be the lowest energy isomer by ∼6kcal/mol. This latter reaction also gives small quantities of a yellow Cp2Ni2B10H10 isomer suggested by its 11B NMR spectrum to be the lowest energy meta isomer with non-adjacent non-antipodal nickel atoms.

Is C50 a superaromat? Evidence from electronic structure and ring current calculations.

The fullerene-50 is a 'magic number' cage according to the 2(N + 1)(2) rule. For the three lowest isomers of C50 with trigonal and pentagonal symmetries, we calculate the sphericity index, the spherical parentage of the occupied π-orbitals, and the current density in an applied magnetic field. The minimal energy isomer, with D3 symmetry, comes closest to a spherical aromat or a superaromat. In the D5h bond-stretch isomers the electronic structure shows larger deviations from the ideal spherical shells, with hybridisation or even reversal of spherical parentages. It is shown that relative stabilities of fullerene cages do not correlate well with aromaticity, unlike the magnetic properties which are very sensitive indicators of spherical aromaticity. Superaromatic diamagnetism in the D3 cage is characterized by global diatropic currents, which encircle the whole cage. The breakdown of sphericity in the D5h cages gives rise to local paratropic countercurrents.

Bonding, aromaticity, and planar tetracoordinated carbon in Si2CH 2 and Ge 2CH 2.

Natural bond orbital (NBO) analyses and dissected nucleus-independent chemical shifts (NICS π z z ) were computed to evaluate the bonding (bond type, electron occupation, hybridization) and aromatic character of the three lowest-lying Si2CH2 (1-Si, 2-Si, 3-Si) and Ge2CH2 (1-Ge, 2-Ge, 3-Ge) isomers. While their carbon C3H2 analogs favor classical alkene, allene, and alkyne type bonding, these Si and Ge derivatives are more polarizable and can favor "highly electron delocalized"? and "non-classical"? structures. The lowest energy Si 2CH2 and Ge 2CH2 isomers, 1-Si and 1-Ge, exhibit two sets of 3-center 2-electron (3c-2e) bonding; a π-3c-2e bond involving the heavy atoms (C-Si-Si and C-Ge-Ge), and a σ-3c-2e bond (Si-H-Si, Ge-H-Ge). Both 3-Si and 3-Ge exhibit π and σ-3c-2e bonding involving a planar tetracoordinated carbon (ptC) center. Despite their highly electron delocalized nature, all of the Si2CH2 and Ge2CH2 isomers considered display only modest two π electron aromatic character (NICS(0) π z z =--6.2 to -8.9 ppm, computed at the heavy atom ring center) compared to the cyclic-C 3H2 (-13.3 ppm). Graphical Abstract The three lowest Si2CH2 and Ge2CH2 isomers.

Structural and electronic properties of sodium clusters under confinement

Using real-space density functional theory, electronic structure and equilibrium geometries of sodium clusters in the size range of 2--20 atoms have been calculated as a function of confinement. We have examined the evolution of the five lowest isomers as a function of volume for six different compressions. The minimum volume considered is about $1/15$ to $1/10$ of the free-space box volume. We observe a strong tendency for isomeric transitions in many cases, with the higher isomers evolving into the ground state under confinement. In general the clusters tend to become more spherical. The changes in the total energies and the geometries are not significant until the volume gets reduced beyond the 1/3 of the original volume. In this sense, the clusters are not easy to compress. Once the critical volume is reached, the changes in the total energies and geometries are rapid. It turns out that the increase in the total energy is mainly due to the ion-ion and Hartree energies of electrons. We also address how anisotropic confinement affects the geometry of clusters. We further show that geometries obtained with anisotropic confinement are strongly supported by the simulation of clusters inside a carbon nanotube using a hybrid quantum-mechanical and molecular-mechanics approach.

Separation and analysis of charged isomers of monoclonal immunoglobulin G by ceramic hydroxyapatite chromatography

ABSTRACTAnalysis of monoclonal antibody (MAb) heterogeneity caused by posttranslational modifications is important for pharmaceutical quality assurance of antibody drugs. In this study, by employing small ceramic hydroxyapatite (HAp) particles that were self-manufactured to have a 2.5-µm diameter, we attempted to separate and analyze MAb isomers. The MAb without N-linked oligosaccharides could be separated by 2.5-µm HAp chromatography as well as MAb with N-linked oligosaccharides. Hence, a variety of N-linked oligosaccharides do not appear to be involved in the separation mechanism of HAp chromatography. However, there is a difference in retention time between MAb with and without N-linked oligosaccharides, meaning that the presence of N-glycan could influence the retention time of HAp chromatography. Subsequently, the MAb fractions separated by 2.5-µm HAp chromatography were analyzed by isoelectric focusing, and seven isomers of the MAb having different isoelectric points (pI) were identified. The MAb isomers were eluted in order of lower pI isomers with sodium phosphate buffer, and enzyme-linked immunosorbent assay indicated the immunoreactivity of the fraction including the lowest pI isomers to be remarkably reduced. This study yielded details of the separation behavior of HAp chromatography owing to 2.5-µm HAp particles.

Thermodynamic stability versus kinetic stability: is the planar hexacoordinate carbon species D(3h) CN₃Mg₃⁺ viable?

The experimentally observed planar hypercoordinate carbon species were detected in gas phase experiments and characterized by photoelectron spectroscopy. According to the Boltzmann distribution law, the thermodynamically favorable isomers, especially global minima, were relatively easier to detect than other isomers in such an experimental process. Here, we reported a thermodynamically unfavorable case, i.e., D3h CN3Mg3(+) (1a), which we think is experimentally viable because all isomers that are energetically lower than 1a show bimolecular assembly type structures consisting of an N2 unit and various types of CNMg3(+) units. The natural bond orbital (NBO) analysis suggests that the bonding between N2 and CNMg3(+) is rather weak, and we think it is very hard to retain their basic structures when kinetic factors are considered. Consistently, the four lowest isomers in the second group show dissociation (to free N2 molecule and CNMg3(+) cations) during Born-Oppenheimer molecular dynamic (BOMD) simulations. In contrast, the structure of 1a can be maintained under temperatures up to 2000 K during the BOMD simulation, and ring-opening reaction studies suggest the barrier to be very high, 46.75 kcal/mol. We think the excellent kinetic stability of 1a will compensate for its thermodynamic instability and it will own its existence in the gas phase synthesis. Although many isomers in the second group are energetically more favorable than 1a, they will be dissociated by the kinetic process. In the magnetic field, the positively charged CNMg3(+) units will be separated quickly from N2 molecules in the general gas phase synthesis, and they are therefore undetectable.

Decay of the high-Kisomeric state to a rotational band in257Rf

The 257Rf isotope has been populated via the 208Pb(50Ti, n) fusion-evaporation reaction and delayed gamma-ray and electron decay spectroscopy has been performed. The existence of a high-K isomeric state in 257Rf has been confirmed. The isomeric state decays into a rotational band based on the 11/2(-)[725] excitation, which was observed up to spin of (23/2(-)). Three multipolarity-E1 gamma transitions depopulating the isomeric state have been observed, which fixes the spin for that state to (21/2(+)). This assignment agrees with theoretical predictions calculated with the microscopic-macroscopic approach, which suggest the isomeric state to be formed by coupling an unpaired 11/2(-)[725] quasineutron to the (1/2(-)[521] circle times 9/2(+)[624])(5)- two-quasiproton state. The same two-quasiproton excitation is possible for the lowest isomer in 256Rf. (Less)

UV-Irradiation Effect on the Photodegradation of Plasticized Poly (Ortho, Meta, and Para-Fluorostyrene) Isomers in Solid Films

The photostability of thin films of poly (Fluorostyrene) isomers was studied by irradiation with UV-light, in presence of air at room temperature. The irradiated polymer films were exposed to different intervals of time and the degradation process was investigated with absorption, fluorescence and FT-IR spectroscopic methods. The influence of phthalate and terephthalate plasticizers on photo-oxidative degradation was also investigated. Blending with these plasticizers was found to decrease the stability of the irradiated polymers. The same observation was noticed in the photodegradation of other substituted polystyrenes films. The intensity of absorption was also found to increase with time of irradiation and in the intensity of a new absorption band at longer wavelength. In addition the formation on new fluorescence band at longer wavelength for the irradiated film is an evidence of photodegradation of the irradiated polymer films. The FT-IR spectra of irradiated polymers and for blended polymer films with phthalates and terephthalate, showed an increase in the absorption bands of these isomers indicating the possibility of degradation. The mechanism of photodegradation of these isomers was found to be similar to that of polystyrene. The order of photostability of these isomers was found that poly (p-fluorostyrene) is the most stable isomer and, poly (o-fluorostyrene) is the lowest stable isomer towards irradiation effect.

Si6–nCnH6 (n = 0–6) Series: When Do Silabenzenes Become Planar and Global Minima?

In the current work we studied a structural transition from nonplanar three-dimensional structures to planar benzene-like structures in the Si(6-n)C(n)H(6) (n = 0-6) series. We performed unbiased Coalescence-Kick global minimum and low-lying isomers search for the Si(6)H(6), Si(5)CH(6), Si(4)C(2)H(6), Si(3)C(3)H(6), Si(2)C(4)H(6), and SiC(5)H(6) stoichiometries at the B3LYP/6-31G** level of theory. The lowest isomers were recalculated at the CCSD(T)/CBS//B3LYP/6-311++G** level of theory. It was shown that the pseudo-Jahn-Teller effect, which is responsible for the deformation of planar Si(6)H(6), Si(5)CH(6), and Si(4)C(2)H(6) structures, is suppressed at n = 3 (the planar structure of 1,3,5-trisilabenzene). We also showed that the 3D-2D transition, which occurs only at n = 5, is due to the aromaticity of monosilabenzene (SiC(5)H(6)) along with other factors, such as stronger C-C σ bonds compared to weaker C-Si and Si-Si σ bonds.