Three-membered Ring Structure Research Articles

Theoretical study on the addition reactions of silylenoids H 2SiLiX (X = F, Cl) to formaldehyde

The addition reactions of silylenoids H 2SiLiX (X = F, Cl) to formaldehyde were studied using ab initio and DFT methods. The three-membered ring structures and the p-complex structures of H 2SiLiX were adopted. The structures of reactants, transition states and products were fully optimized at the B3LYP/6-311+G(d,p) and MP2 (full)/6-31G(d) levels. G3(MP2) theory was used for calculations of molecular energies. The reaction paths were investigated and confirmed by intrinsic reaction coordinate calculations. The difference between the asymmetric addition reactions of H 2SiLiX to formaldehyde and the symmetric addition reactions of H 2SiLiX to ethylene and acetylene was also discussed. At the B3LYP/6-31G(d) level, calculations about solvent effects on the addition reactions of H 2SiLiX to formaldehyde show that the additions are easier to occur in the polar solvent, and the higher the solvent polarity is, the lower the barriers of the addition reactions are.

Theoretical study on the beryllium chlorogermylenoid H 2GeClBeCl

The beryllium chlorogermylenoid H 2GeClBeCl was studied for the first time by using the DFT B3LYP and QCISD methods. Geometry optimization calculations indicate that H 2GeClBeCl has three equilibrium configurations, in which the p-complex is the lowest in energy and is the most stable structure. Three transition states for isomerization reactions of H 2GeClBeCl are located and the energy barriers are calculated. The p-complex and the three-membered ring structure are suggested to be experimentally detectable ones. The infrared spectrums of the p-complex and the three-membered ring structure have been simulated.

Reaction Pathways for the Additions of Triplet Carbene, Silylene, Germylene and Nitrene onto the Diamond (100) Surface: A Theoretical Investigation

The additions of triplet carbene (CH2), silylene (SiH2), germylene (GeH2), and nitrene (NH) onto the diamond (100) surface have been investigated by means of density functional theory in conjunction with effective cluster models. The calculated reaction energies and reaction pathways have demonstrated that the reaction profiles for the additions of triplet species onto the diamond (100) surface is clearly different from those for singlet ones. Namely, triplet CH2, SiH2, GeH2, and NH are inclined to form the one-end-sided species by attaching to only one carbon of the surface dimer of diamond (100). Hence, the other carbon of the surface dimer will be open, which can be potentially used for anchoring other functional groups. This is in distinct contrast to the profile for the additions of singlet ones onto diamond (100) in which the as-formed surface species feature a three-membered ring structure. The results illustrate that the addition profiles of CH2s, SiH2s, GeH2s, and NHs onto the diamond (100) surface could be appropriately tuned depending on their spin states.

Femtosecond time-resolved investigation of the vibrational modes of vitreous GeO2

The vibrational response of vitreous GeO2 is investigated using an impulsive stimulated Raman scattering technique in the femtosecond regime. The results yield evidence for a weak vibrational mode ascribed to oxygen motion in three-membered planar ring structures. Its frequency and damping are determined and compared to theoretical predictions.

Theoretical study of solvent effects on the structures and isomerization of silylenoid H2SiLiF

The structures and isomerization of silylenoid H2SiLiF are investigated by density functional theory (DFT) at B3LYP/6-311+G(d,p) level. The solvent effects are modeled using the self-consistent reaction field (SCRF) method with Tomasi’s polarized continuum model (PCM). Five representative solvents, i.e. benzene, ether, THF, acetone, and DMSO, are chosen for this work. Special attention is paid to THF solvent. The results indicate that the polarity of solvents has played an important role on the structures and relative stabilities of different isomers. Contrary to the three-membered ring structure 1, the relative stability of the “classical” tetrahedron structure 4 increases with increasing dielectric constants (e) of solvents. The σ-complex isomer 3 is most unstable structure. Although the relative energies are reduced with increasing dielectric constants (e) of solvents, the p-complex structure 2 still has the lowest energy. The effects of solvents on the dipole moments and charge distributions are also discussed.

Theoretical study on the isomeric structures and the stability of silylenoid (Tsi)Cl2SiLi (Tsi = C(SiMe3)3)

The structures and isomerization of silylenoid (Tsi)Cl(2)SiLi (Tsi = C(SiMe(3))(3)) were studied by density functional theory (DFT) at the B3LYP/6-31G(d) level. Four equilibrium structures and three isomeric transition states were located. The three-membered ring and p-complex structures, 1 and 2, are the two most stable forms. Two other local minima, the sigma-complex 3 and tetrahedron structure 4, should rearrange to 1 with very low barriers, and then to the most stable isomer 2. To exploit further the stability of silylenoid (Tsi)Cl(2)SiLi, the insertion reactions of 2 and silylene (Tsi)ClSi into the HF molecule have been investigated at the B3LYP/6-31G(d) level, respectively. The results show that the insertion of 2 into HF is very similar to that of (Tsi)ClSi into HF, but the latter is more favorable. To probe the influence of the substituent Tsi on the stability of silylenoid (Tsi)Cl(2)SiLi, the isomers and insertion reaction of silylenoid CH(3)Cl(2)SiLi were investigated in a similar way of those with (Tsi)Cl(2)SiLi. The results indicate that silylenoid containing very bulky group Tsi exhibits unusual stability because of the severe steric hindrance produced by Tsi at the center to which it is attached.

Stoichiometric Hydrosilylation of Nitriles with Hydrido(hydrosilylene)tungsten Complexes: Formation of W−Si−N Three-Membered Ring Complexes and Their Unique Thermal Behaviors

Reactions of hydrido(hydrosilylene)tungsten complexes, Cp'(CO)2(H)W=Si(H)[C(SiMe3)3], with nitriles (MeCN, tBuCN) at 60 degrees C gave hydrosilylation products, Cp'(CO)2W[kappa2(N,Si)-Si(H)(N=CHR'){C(SiMe3)3}] (R' = Me, tBu), with a novel W-Si-N three-membered ring structure. The product of the hydrosilylation of tBuCN underwent reversible rearrangement at 70 degrees C to a silylene complex, Cp'(CO)2(H)W=Si(N=CHtBu)[C(SiMe3)3], which was a major component in equilibrium. A reaction mechanism for the hydrosilylation involving coordination of nitriles to the silylene ligand and subsequent migration of the hydrido ligand to the nitrile carbon was proposed.

A Combined Density Functional Theory and Coupled Cluster Method Investigation of the Structural Properties and Stabilities of Radical CH2CP and Its Isomers

The doublet potential energy surface of radical system [C(2), H(2), P] is investigated at the UB3LYP/6-311++G(d,p) and UCCSD(T)/6-311++G(2df,2p) (single-point) levels. Eight chainlike and three-membered ring structures are located as energy minima connected by 10 interconversion transition states. At the final UCCSD(T)/6-311++G(2df,2p)//UB3LYP/6-311++G(d,p) level with zero-point vibrational energy correction, species CH(2)CP is found to be thermodynamically the most stable isomer followed by HCCPH, H-cCPC-H, cPCC-H(H), H-cCCP-H, cis-CC(H)PH, trans-CC(H)PH, and CCPH(2) at 11.01, 12.57, 40.07, 43.63, 50.25, 56.82, and 65.36 kcal/mol, respectively. The computed results indicate that the chainlike isomers CH(2)CP and HCCPH and cyclic radical H-cCPC-H possess considerable kinetic stability at extra low pressures and temperatures. Interestingly, radical CCPH(2), whose energy is the highest in all predicted CH(2)CP isomers, can be also regarded as a kinetically stable species with the smallest isomerization barrier of 22.26 kcal/mol at extra low pressures and temperatures. Therefore, considering higher kinetic stability, in addition to the microwave spectroscopy characterized isomer CH(2)CP in previous experiments, the species HCCPH, H-cCPC-H, and CCPH(2) should be considered as excellent candidates for possible experimental observation. Furthermore, the structural nature of stable radical isomers is discussed based on bonding characteristics, single electron spin distribution, and comparison with their analogues.

Study on Synthesis of Thebaine Derivatives

The reaction of 7α-acetyl-6,14-endoethano-6,7,8,14-tetrahydrothebaine with 2-(thien-2-yl)ethylmagnesium bromide was investigated. The tertiary alcohol derivative 7α-[R-1-hydroxyl-1-methyl-3-(thien-2-yl)propyl])-6,14-endoethano-6,7,8,14-tetrahydrothebaine (3) and a by-product 4 were isolated. The structure of 4 was elucidated by X-ray analysis. The Grignard reaction shows high degree of stereoselectivity according with Cram rule. The crystal structure of 4 indicates that dihydrofuran ring was opened to form a phenolic hydroxyl group and a three-membered ring structure. It maintains the main rigid structure of morphine and contains a C(6)-C(14) enthano bridge. The 1-hydroxyl-1-methyl-3-(thien-2-yl)propyl group at C(7) position adopted S-configuration.

Theoretical Study of Addition Reactions of Heavy Carbenes to Carbon and Boron Nitride Nanotubes

In an effort to gain insight into the activation energies and reaction enthalpies of the chemical functionalization of carbon and boron nitride nanotubes, calculations using density functional theory have been carried out for the cycloaddition of a heavy carbene to a single-walled carbon (SWCNT; C(130)H(20)) and a boron nitride (SWBNNT; B(65)N(65)H(20)) nanotube. The (CH(3))(2)X + SWCNT and (CH(3))(2)X + SWBNNT (X = C, Si, Ge, Sn, and Pb) reactions are the subject of the present study. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. The major conclusions that can be drawn from this work are as follows: (i) Considering both the activation barrier and reaction enthalpy based on the model calculations presented here, it is found that the order of (CH(3))(2)X reactivity is X = C > Si >> Ge > Sn > Pb, irrespective of whether cycloaddition is to a SWCNT or a SWBNNT sidewall. That is to say, (CH(3))(2)C and (CH(3))(2)Si can readily add to the sidewalls of SWCNT and SWBNNT, whereas (CH(3))(2)Ge, (CH(3))(2)Sn, and (CH(3))(2)Pb are unreactive. (ii) Since the chemical reactivities of SWCNT and SWBNNT sidewalls closely resemble those of the small C(16)H(10) and B(8)N(8)H(10) molecules, at least in a qualitative sense, the use of the above small molecules as models is sufficient to provide qualitatively correct results. (iii) Our theoretical observations indicate that all the (5,5) SWCNT and SWBNNT cycloadducts favor opened rather than closed three-membered ring structures. (iv) The theoretical investigations demonstrate that the singlet-triplet splitting of the carbene species (R(2)X) as well as that of the small model molecules can be used as a diagnostic tool to predict the addition reactivities of carbene analogues and sidewalls of various nanotubes, respectively. Moreover, the results obtained in this work allow a number of predictions to be made.

An ab initio study on structures and isomerization of magnesium chlorosilylenoid H 2SiClMgCl

The structures and isomerization of magnesium chlorosilylenoid H 2SiClMgCl were investigated by ab initio molecular orbital theory for the first time. Four equilibrium structures and three isomeric transition states were located and fully optimized at the G3MP2B3 level. For more accurate structural analysis, the B3LYP/cc-pVTZ and QCISD/6-311+G* calculations were performed, respectively. Based on the B3LYP/6-31G(d) optimized geometries, 29Si chemical shifts and harmonic frequencies of various structures were obtained. Isomerization paths for isomers were confirmed by the intrinsic reaction coordinate (IRC) calculations. Tetrahedral, three-membered ring and p-complex structures are suggested to be experimentally detectable ones. σ-Complex structure has the highest energy and will not exist. The solvent effects were considered by means of the polarizable continuum model (PCM) using THF as a solvent.

Structural evolution of [2+1] cycloaddition derivatives of single-wall carbon nanotubes: From open structure to closed three-membered ring structure with increasing tube diameter

By using density functional theory calculations, it is found that the [2+1] cycloaddition derivatives of armchair ( m, m) single-wall carbon nanotube (SWNT) evolve from open structure to closed three-membered ring structure as m>11 for NH addition, m>10 for O and CH 2 additions, and m>5 for SiH 2 addition. The diameter upper limit of the opening of the sidewall of SWNTs upon [2+1] cycloaddition is predicted to be about 15 Å.

Optical defects produced in fused silica during laser-induced breakdown

Fused silica irradiated with ∼3-ns 1064-, 355-, and 266-nm laser pulses as well as with ∼120-fs 825-nm pulses is studied by a combination of photoluminescence (PL) and Raman scattering spectroscopies. Results show that, for laser fluences above the laser-induced breakdown threshold, in all the cases studied, irradiation results in the formation of four defect-related PL bands centered on ∼1.9 (655), 2.2 (565), 2.7 (460), and 4.3 eV (290 nm). Bands centered on 1.9, 2.7, and 4.3 eV are attributed to nonbridging oxygen hole centers (1.9 eV) and oxygen-deficiency defects (2.7 and 4.3 eV). However, defects giving rise to a broad band at ∼2.2 eV are unknown. For all the laser-modified samples studied, Raman spectroscopy reveals a dramatic increase in the intensity of D1 and D2 lines, associated with in-phase breathing motions of oxygen atoms in puckered four- and planar three-membered ring structures, respectively. This indicates laser-induced material densification. Based on these results, we discuss physical processes occurring during the catastrophic laser-induced material breakdown, leading to material densification and the formation of point defects.

Ab initio studies of the C 2H 2BN potential energy surface

The C 2H 2BN potential energy surface has been investigated at the MP2-level using the 6-311G** basis set. The potential surface proved to be very complex permitting only partial characterization. Sixty-six stationary points, including 42 energy minima and 24 transition states, were found. The linear ( C ∞ v ) isomer HCC–B −N +–H, 9, was the most stable species identified. The most stable cyclic structure was the non-planar isomer 46, consisting of two fused three-membered rings. Isomer 46 is 12.9 kcal/mol more stable than the planar four-membered ▪ ring structure, 4. The planar isomer 4 has a short cross-ring CB distance and could be considered as an example of a long sought after system with a planar four-coordinated carbon. Planar four-membered ring isomer 5 has shorter CN and CB (and longer CC and BN) lengths than 4 and was only 6.7 kcal/mol less stable. Planar four-membered ring, 7, ▪ was 18.7 kcal/mol less stable than 4 and it exhibits alternating single and double bonds. The C 2 v isomer 18, H 2CBCN (CBCN portion linear) was found to be a local minimum, lying 17.3 kcal/mol above the global minimum 9 and is proposed to be observable under proper conditions. Isomer 18 can be represented by the valence bond structure, 3, H 2CB–CN. In addition to 9, 46, 4, and 18, other candidates for experimental detection are the linear isomers HCC–NBH, 10, HCNBCH, 11, and HNCCBH, 12, the acyclic C 2 v structures H 2CBNC, 19, H 2CCNB, 21, and H 2CCBN, 22, and the three-membered ring structures ▪, and ▪, 62. 10, 11, 12, 19, 21, 22, 60 and 62 are less stable than 9 by 17.5, 61.6, 35.2, 27.5, 59.6, 77.5, 73.2 and 78.4 kcal/mol, respectively.

Stable 2H-azasilirene and 2H-phosphasilirene: Addition reaction of an overcrowded silylene to a nitrile and a phosphaalkyne

In this paper, we present the reaction of an overcrowded silylene bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group with pivalonitrile and t-butylphosphaalkyne to give the corresponding [1+2] cycloadducts, 2H-azasilirene and 2H-phosphasilirene derivatives. This is the first exampleof the isolation of a stable 2H-azasilirene derivative, and the X-ray crystallographic analysis unambiguously revealed its three-membered ring structure in the solid state. In addition, DFT calculations supported three-membered ring character in the structures of the 2H-azasilirene and 2H-phosphasilirene.

Raman spectra of fluorine-doped silica glasses with various fictive temperatures

We have demonstrated the change of the Raman spectra in F-doped silica glasses with various fictive temperatures, and have also presented a useful peak-deconvolution technique for evaluating the relative concentration of D1 and D2 bands that could be attributed to four- and three-membered ring structures. The intensities of D1 and D2 bands are mainly dominated by the fictive temperature, and they are not almost affected by the F content at the same fictive temperature. It has been made clear that the intensity of D1 band as well as D2 band has a linear relationship with the IR peak position around 2260 cm−1 that is correlated with the average Si–O–Si bond angle, irrespective of the F content. The “activation energies” of D1 and D2, estimated from the Arrhenius plot, are 0.26 and 0.41 eV, respectively.

Theoretical Study on Potential Energy Surface of the Interstellar Molecule SiC2N

The potential energy surface of the interstellar molecule SiC2N is explored at the B3LYP/6-311G(d), QCISD/6-311G(d), and CCSD(T)/6-311G(2d) (single-point) levels. Eleven species including the chainlike, three-membered ring and four-membered ring structures are located as energy minima connected by twenty interconversion transition states. At the CCSD(T)/6-311G(2d) level, the lowest-lying isomer is linear SiCCN 1 with 2Π state followed by linear SiCNC 2 with 2Π state at 23.2 kcal/mol and two close-energy isomers, i.e., cyclic SiCCN with SiC cross-bonding 9 with 2A‘ ‘ state at 28.0 kcal/mol and bent SiNCC 5 with 2A‘ state at 29.5 kcal/mol. The potential energy surface of SiC2N indicates that only the three chainlike isomers 1, 2, and 5 are kinetically stable, whereas the low-lying cyclic isomer 9 and other isomers are kinetically unstable toward conversion to more stable isomers. The results are compared with those of the well-known analogous interstellar C3N radical. The calculated bond lengths, harmonic v...

Theoretical studies on the structures and isomerization of methylenelithoflurosilylenoid H 2CSiLiF

The main characters of the potential energy surface of the methylenelithoflurosilylenoid (H 2CSiLiF) are studied by ab initio calculations at the G2(MP2) level. Four equilibrium structures, a p-complex, a three-membered ring, a σ-complex and a silene, and three isomerization transition states are located, and isomerization reaction paths are investigated and confirmed by the intrinsic reaction coordinate (IRC) calculations. The calculations have shown that nonplanar p-complex structure has the lowest energy and should be experimentally detectable among four equilibrium structures of the H 2CSiLiF. The silene and σ-complex structures have high energies and are easy to isomerize to the three-membered ring structure with lower energy, but in fact, they will not exist. Also, the structural characteristics and bonding properties of various structures are analyzed in this Letter.

C 4 N : The first CnN radical with stable cyclic isomers

The potential-energy surface of the interstellar molecule C4N is explored at the B3LYP/6-311G(d) level of theory. Thirteen isomers including the linear, three-membered ring, four-membered ring, A-like, Y-like, and cage-like structures are located as minima connected by 23 interconversion transition states. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by single-point energy calculations at the MP4SDTQ, CCSD(T), and QCISD(T) levels with the 6-311G(2df) basis set. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d) level, the lowest-lying isomer is a linear structure CCCCN 1 followed by a CCC three-membered ring structure 4 with exocyclic CCN bonding that lies only 2.8 kcal/mol higher. The third and fourth low-lying isomers possess a CCC three-membered ring structure 5 with exocyclic CNC bonding at 21.4 kcal/mol and a linear structure CCCNC 2 at 23.4 kcal/mol, respectively. All the four isomers 1, 2, 4, and 5 and another high-lying isomer 3 with a linear CCNCC structure at 62.5 kcal/mol are shown to have considerable kinetic stability towards isomerization and dissociation. Thus, all the five isomers may be experimentally observable. Possible formation of these five stable C4N isomers in interstellar space is discussed. Finally, our calculations show that the Møller–Plesset methods fail to predict even qualitatively the energetic properties between the four low-lying isomers 1, 2, 4, and 5, in comparison with the QCISD and CCSD results. This paper indicates that C4N may be the first interstellar molecule with stable low-lying cyclic isomers among the CnN radical series to be detectable in near future. The results presented in this paper may provide useful information for future laboratory and interstellar identification of various C4N isomers.

A Theoretical Study of Neighboring-Group Participation in Thione-to-Thiol Rearrangement of Xanthates. Molecular Orbital Calculation Using a Conductor-Like Screening Model (COSMO) Approach.

The transition structures of the thione-to-thiol rearrangement of O-(2-dimethylaminoethyl and 2-methylthio-) S-methyl xanthates in several solvents were located by semiempirical molecular orbital method (PM3) using the conductor-like screening model (COSMO) approach. Each transition state transforms into the ion-pair intermediate with a three-membered ring structure (aziridinium or thiiranium), indicating that the rearrangement proceeds through an ionic intermediate with the anchimeric assistance of the neighboring group. The intermediary structures in gas phase are also analyzed by ab initio and density functional theory calculations.