A theoretical study on the protonation of benzamide

We have performed an ab initio study of the protonation of benzamide, using a STO-3G minimal basis set. According to our results, benzamide is an oxygen-base in the gas-phase. Rotation of the -CONH₂ group respect to the aromatic ring does not affect the basicity of the molecule. If the presence of the solvent causes pyrimidization of the -NH₂ group, the intrinsic basicity of the O atom decreases, while that of the N atom increases and the interaction of this center with the solvent is stronger, favoring nitrogen protonation in solution.     

A theoretical study of the protonation of methylindole derivatives

Ab initio calculations on indole and all its mono-substituted methyl derivatives, using an STO-3G minimal basis set, show that the most basic site is C3. Protonation at the nitrogen atom cannot compete with protonation at C3; and C2 is the less basic site in all cases. The basicity increases with methyl substitution, with the only exception of 3-methyl indole. A good linear correlation exists between calculated and corresponding thermodynamics pK values. 2-Aminoindole is a much stronger base than methylindoles and its high pK value can be explained by the strong interactions with the solvent through tautomeric forms which accumulate positive charge at the NH₂ group. Intramolecular quenching of the fluorescence of some indole derivatives involves intramolecular proton transfer to C4 rather than C2. Reasons why ring nitrogens can behave as either π-acceptors or π-donors in this series are discussed.     

Regioselective hydrosilations. II. The synthesis of silicon–hydrogen functional compounds

The discovery that the rhodium-catalyzed hydrosilation of unsaturated epoxides with compounds containing two Si? H functional groups proceeds in a discrete, stepwise fashion, has made possible the synthesis of a series of interesting compounds containing both Si? H and epoxy groups in the same molecule. These compounds can be used both as an interesting new class of epoxy monomers as well as intermediates for the preparation of a wide variety of novel monomers which incorporate epoxide as well as other types of polymerizable functional groups © 1993 John Wiley & Sons, Inc.     

Ionization and protonation of MgC3: A theoretical study

A theoretical study of the MgC₃⁺ and MgC₃H⁺ species has been carried out. Predictions for their geometries and vibrational frequencies have been made at both second‐order Møller–Plesset (MP2) and B3LYP levels, whereas electronic energies have been computed at G2 and coupled cluster single and double excitation model augmented with a noniterative triple excitation conection (CCSD(T)) levels. The predicted global minimum for MgC₃⁺ is a rhombic structure (²A₁ electronic state), whereas a T‐shaped structure and an open‐chain isomer lie about 10 and 12 kcal/mol, respectively, higher in energy. In the case of MgC₃H⁺ the predicted global minimum is also a four‐membered ring obtained upon protonation of the most stable neutral isomer. Low ionization potentials and high proton affinities are generally obtained, especially for the most stable MgC₃⁺ isomer. The estimated values at the CCSD(T) level for the predicted global minimum are 7.20 eV [ionization potential (IP)] and 256.5 kcal/mol [proton affinities (PA)]. Therefore, if present in the interstellar medium, MgC₃ should be easily ionized and would react quite easily to give the protonated species. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

SQSQh: 1H‐detected SQ–SQ experiment for determination of signed silicon–carbon coupling constants

A new variant of SQ–SQ pulse sequence (SQSQh) for relative sign determination and detection of small silicon–carbon couplings over more than one bond is presented. In the SQSQh sequence, proton detection replaces carbon detection used in the original SQ–SQ pulse sequence (SQSQc). The theoretical gain in sensitivity was experimentally tested on two samples (trimethylsiloxyethane, 1, and 1,2,4‐tris(trimethylsiloxy)benzene, 2), the experimentally found gain provided by the SQSQh over the SQSQc method varied between 6 and 8. The method can be applied to linear spin systems, i.e. to systems where the silicon is coupled to the carbon in question and to any hydrogen not necessarily bonded to the carbon. Copyright © 2010 John Wiley & Sons, Ltd.     

A theoretical study on camptothecin–AM1 treatment

Camptothecin (irinotecan), a new cytotoxic agent having antitumor activity and especially used in the treatment of advanced colorectal cancers is considered for AM1–RHF type semiempirical quantum chemical treatment together with some analogs of camptothecin. The structural properties and energetics of these molecules have been compared. The structures arising from monoprotonation of camptothecin at various sites are analyzed from thermodynamic point of view.     

1,3‐Dipolar cycloadditions of Stone–Wales defective single‐walled carbon nanotubes: A theoretical study

The presence of Stone‐Wales defects in single‐walled carbon nanotubes (SWNTs) not only leads to new interesting properties, but also provides opportunities for tailoring physical and chemical properties, and expands their novel potential applications. With a two‐layered ONIOM method, 1,3‐dipolar cycloadditions (1,3‐DCs) of a series of 1,3‐dipoles (azomethine ylide, nitrone, nitrile imine, nitrile ylide, nitrile oxide, and methyl azide) with Stone‐Wales defective SWNTs have been investigated theoretically for the first time. The calculated results demonstrate that the bond c , rather than the previously focused central bond a , exhibits the highest chemical reactivity among the defective sites. More interestingly, bond c is even more reactive thermodynamically and kinetically than the perfect C? C bond in SWNTs, suggesting the feasibility of utilizing 1,3‐DC reactions to separate and purify perfect and defective SWNTs. The reactivity order for nonequivalent bonds in defective sites is different from that of [1+2] cycloaddition, indicating that the reactivity order for nonequivalent bonds depends on the kind of the chemical reactions. Except azomethine ylide, nitrile ylide and nitrile imine are found to be good candidates for 1,3‐DCs upon Stone‐Wales defective SWNTs. The SW‐ A and SW‐ B defective SWNTs show different chemical reactivity toward nitrile ylide, making it possible to purify and separate the SW‐ A and SW‐ B defective SWNTs. The SWNT diameters are found to moderately influence the 1,3‐DC reactivity of both perfect and Stone‐Wales defective SWNTs, implying that Stone‐Wales defective SWNTs with different diameter would be separated experimentally through 1,3‐DC chemistry. The above 1,3‐DC reactivity can be well understood in terms of the distortion/interaction theory, which means that instead of frontier molecular orbitals interaction energy, the distortion energy controls the chemical reactivity. © 2013 Wiley Periodicals, Inc.     

Studies of silicon–nitrogen ring formation from tetrachlorodisilanes

The addition of n equiv of MesNHLi (n = 1, 2, 3, 4) and m equiv of 12-crown-4 (m = 0.1, 1, 2, 3, 4) to (RsiCl₂)₂ (R = Mesityl; t-Butyl) in THF at −78°C resulted in higher yields and improved selectivities of cyclodisilazanes and azadisilacyclopropanes. The reaction of 2 equiv of MesNDLi with (MesSiCl₂)₂ ( I ) yielded cis-2-chloro-1,2,3,4-tetramestylcyclodisilazane ( 2a ) with 95% Si H deuteration. 2a was quantitatively chlorinated with retention of stereochemistry by N-chlorosuccinimide to give the cis dichlorocyclodisilazane ( 3 ). Variable-temperature ¹H NMR studies from −70°C to 25°C in d₈-THF were performed on 1 , 2a , 2b , and 3 . 1 exhibited little variation throughout the temperature range, whereas 2a , 2b , and 3 showed several rotational isomers. trans-2,3-Bis(mesitylamino)-1,2,3-trimesitylazadisilacyclopropane was shown to isomerize to the cis isomer in the presence of n-BuLi or MesNHLi. The molecular structures of 1 and 3 were determined by X-ray crystallography. Compound 1 crystallized in the monoclinic space group P2₁/c, with cell parameters a = 16.599(2) Å b = 14.633(2) Å, c = 16.945(2) Å, β = 92.044(13)°, V = 4113.1(8) ų, Z = 8, d (calcd) = 1.409 g/cm³, and R = 6.88%. Compound 3 ° C₆H₆ crystallized in the orthorhombic space group Pbca, with cell parameters a = 17.906(4) Å, b = 13.918(3) Å, c = 31.700(6) Å, V = 7900(3) ų, Z = 8, d (calcd) = 1.194 g/cm³, and R = 7.52%.     

Gas-phase protonation of unsaturated ethers: Experimental and theoretical study of 2,3-and 2,5-dihydrofuran and related compounds

The protonation of 2,3- and 2,5-dihydrofuran is examined in gas-phase equilibrium proton transfer reactions conducted in an ion cyclotron resonance spectrometer. The thermodynamically favoured site of protonation in the two compounds is seen to be different: whereas the first isomer forms a carbocation upon protonation, the second isomer protonates on the oxygen atom to form an oxonium ion. The results obtained with substituted derivatives and with linear analogues confirm these conclusions. Molecular orbital calculations on the various structures for protonated bases are performed at the 4–31G level with correction for configuration interaction effects and at the 4–31G* level. The latter basis set provides the best results owing to the introduction of d-type orbitals on the oxygen atom. The calculation results substantiate the experimental observations and provide details on the molecular structure of the protonated species.     

Quantum mechanical valence study of a bond-breaking–bond-forming process in triatomic systems

The recently introduced set of the quadratic, two-electron covalent and ionic valence indices is used to investigate the bond-breaking–bond-forming (BB-BF ) process in an atom exchange reaction between H₂ and X (X = H, F—I) as well as in the O₂—H system. Valence changes accompanying selected charge reorganizations are examined within the three-orbital model and valence diagrams for symmetric transition states (TS s) are given. The UHF valence data for Li₂O and CO₂ and the H—H—X, O—O—H, and O—H—O (ABC) TS s (collinear and angular) are reported and compared to valence data in the separated fragments limits (SFL ), AB and BC. The overall valence, ν(ABC), and the total (ionic plus covalent) diatomic valences, ν_AB and ν_BC, are used as measures of the overall bond-order in a concerted BB–BF reaction, to test the postulate of the bond-energy–bond-order (BEBO ) model. In collinear TS s of H₂X, ν ? ?1, i.e., one bonding electron pari, is found to be roughly preserved, whereas in the angular H₂X and in collinear O—H—O TS s, the effect of increased valence at the saddle-point is observed, relative to that of diatomic fragments (reactiants or products). For the angular O—O—H TS , a similar increase in | ν (ABC)| relative to both O₂ and OH SFL s is detected; smaller changes relative to the O₂ data are found in the collinear TS . This observation is in agreement with earlier predictions from the intersecting-state model. The relative diatomic valences, ν/ν and ν/ν, are shown to conserve the overall relative bond multiplicity around 1 in both collinear and angular TS s of the H₂—X systems. © 1994 John Wiley & Sons, Inc.     

A theoretical study of malononitrile addition to carbonyl compounds

The nucleophilic addition of the malononitrile anion (MN⁻) to formaldehyde was studied theoretically by the AM1 semiempirical MO method. The addition is found to be endothermic with a late productlike transition state on the reaction coordinate. Additions of MN⁻ to a series of carbonyl compounds were studied in order to investigate the substituent effect on the energetics of the title addition and the nucleophilic attack reactivity. The solvent effect was stimulated by hydrogen bonding a single molecule of water to the formaldehyde oxygen and/or to the MN⁻ anion. Its influence on the energetics and the transition-state geometry was estimated. The Hammond postulate was satisfied for the studied additions. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 419–426, 1997     

A theoretical study of the C–HN hydrogen bond in the methane–ammonia complex

The C–HN hydrogen bond in the methane–ammonia complex is studied by determining its bond dissociation energy (BDE) and the n(N)→σ^*(C–H) interaction. At the MP2(Full)/6-311++G(3df,2p) level of theory with basis set superposition error (BSSE) correction, the BDE was determined to be 2.5 kJ mol⁻¹. The n(N)→σ^*(C–H) interaction at this level of theory was found to be 3.7 kJ mol⁻¹ by natural bond orbital (NBO) analysis. It was also found that the NBO values are in general higher than the BDE values with BSSE correction when they are compared at the same level of theory.     

Theoretical study of silicon–sulfur clusters (SiS2)n− (n=1–6)

The possible geometrical structures and relative stability of silicon–sulfur clusters (SiS₂) (n=1–6) are explored by means of density functional theory (DFT) quantum chemical calculations. We also compare DFT with second‐order Møller–Plesset (MP2) and Hartree–Fock (HF) methods. The effects of polarization functions, diffuse functions, and electron correlation are included in MP2 and B3LYP quantum chemical calculations, and B3LYP is effective in larger cluster structure optimization, so we can conclude that the DFT approach is useful in establishing trends. The electronic structures and vibrational spectra of the most stable geometrical structures of (SiS₂)_n⁻ are analyzed by B3LYP. As a result, the regularity of the (SiS₂)_n⁻ cluster growing is obtained, and the calculation may predict the formation mechanism of the (SiS₂)_n⁻ cluster. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 280–290, 2001     

Rovibrational interactions in linear triatomic molecules: a theoretical study in curvilinear vibrational coordinates

Variational solution of the rovibrational problem in curvilinear vibrational coordinates has been implemented and used to investigate the nuclear motions in several linear triatomic molecules, like HCN, OCS, and HCP. The dependence of the rovibrational energy levels on the rotational quantum numbers and the l-doubling has been studied. Two approximations to the rovibrational Hamiltonian have been examined, depending on the level of truncation of the potential energy operator. It turns out that the truncation after the fifth order in the potential is sufficient to produce vibrational energies of high accuracy. An interesting feature of the present formulation of the problem in terms of the curvilinear vibrational coordinates is the explanation of the l-doubling of the rovibrational levels, which in this picture is interpreted as the result of the inequivalency of the average rotational constants in mutually perpendicular planes, rather than as the effect of the Coriolis-type interactions between the vibrational and rotational motions. The present theoretical results are compared with the available experimental data from high-resolution spectroscopy, as well as with other ab initio calculations.     

A comparative theoretical study on CO insertion into Rh–C bond

A DFT study on CO insertion into Rh–C(alkylethyl, ethenyl, 2-propenyl, trans-propenyl, cis-propenyl, and allyl) bonds were carried out comprehensively. All structures were optimized completely and the mechanism of the CO insertion was discussed in detail. The present results indicated that except for the CO insertion into Rh–C(allyl), other CO insertions were feasible from the thermodynamics point of view. The CO insertion into Rh–C(cis-propenyl) was the easiest process with 24.42 kJ/mol of the lowest activation free energy and the most difficult was the CO insertion into Rh–C(allyl). Also, entropy played a critical role in the CO insertion except for the CO insertion into Rh–C(ethenyl).     

A theoretical study for mechanical contact between carbon nanotubes

We have theoretically investigated motions of single-walled carbon nanotubes (SWNTs) which are mounted on a flat substrate layer of SWNTs by tight-binding molecular dynamics simulations. One of the most interesting motions is the conversion of force and torque, where the force and torque acting initially on the mounted tube finally results in the lateral motion and rolling of the supporting tubes in the substrate. This motion is well understood in terms of the total energy surface of the SWNT/SWNT system. It is suggested that an undulation of the total energy surface plays a role as an atomic-scale gear tooth in the field of nanomechanics, in spite of the atomically smooth surface of SWNT.