Insights into photodissociation dynamics of benzamide and formanilide from ab initio calculations

In the present study, the five lowest electronic states that control the UV photodissociation of formanilide and benzamide have been characterized using the complete active space self-consistent field theory. The mechanisms for the initial relaxation and subsequent dissociation processes have been determined on the basis of the calculated potential energy surfaces and their intersections. It was found that the S(1)/T(1)/T(2) three-surface intersection plays an important role in the photodissociation processes of benzamide. However, the dissociation behavior of formanilide and benzamide was found to be quite different from that for aliphatic amides. The present study provides several insights into the photodissociation dynamics of formanilide and benzamide.     

Insights into dynamics of the S2 state of thiophosgene from ab initio calculations

The S2 potential energy surface for Cl2CS dissociation has been characterized with a combined complete active space self-consistent field and multireference configuration interaction method. The S3/S2 minimum-energy intersection has been determined with the state-averaged complete active space self-consistent field method. The S2 direct dissociation was found to have a barrier of 6.0 kcal/mol, leading to formation of Cl(X2P)+ClCS(A2A") in the excited electronic state. Dynamics of the S2 state of Cl2CS can be summarized as follows: (1) The S2-S0 fluorescence occurs with high quantum yield at low excess energies; (2) Both the S(2) dissociation and the S2-->S3 internal conversion cause the loss of the S2-S0 fluorescence upon photoexcitation at 235-253 nm; (3) The S2-->S3 internal conversion (IC) followed by the direct IC to the ground electronic state results in the fragments produced in the ground state, while the S2 dissociation leads to formation of the fragments in excited electronic states.     

Insights into the structure of cyclohexane from femtosecond degenerate four-wave mixing spectroscopy and ab initio calculations

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order M?ller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes.     

Insights into the photochemical processes of ClC(O)SCl from ab initio calculations

All possible unimolecular processes upon photolysis of ClC(O)SCl in the UV-visible region have been characterized in the present paper through the optimized stationary structures and computed potential-energy profiles of the S0, S1, T2, and S2 states with the MP2, B3LYP, CASSCF, and MR-CI methods in conjugation with the cc-pVDZ basis set. Upon photoexcitation in the range of 300-400 nm, the ClC(O)SCl molecules are excited to the S1 state. From this state, the dissociation into ClC(O)S + Cl takes place immediately and subsequently Cl2 and SCO are formed. The C-Cl and C-S bond fissions that start from the S2 state are the dominant channels upon photodecomposition of ClC(O)SCl in the gas and condensed phases in the wavelength range of 200-248 nm. The formed Cl, C(O)SCl, ClCO, and SCl radicals are very reactive, and the Cl2, SCO, CO, and SCl2 molecules are subsequently produced as stable products in the condensed phase.     

Insights into photodissociation dynamics of propionyl chloride from ab initio calculations and molecular dynamics simulations

The potential energy surfaces of isomerization, dissociation, and elimination reactions for CH3CH2COCl in the S0 and S1 states have been mapped with the different ab initio calculations. Mechanistic photodissociation of CH3CH2COCl at 266 nm has been characterized through the computed potential energy surfaces, the optimized surface crossing structure, intrinsic reaction coordinate, and ab initio molecular dynamics calculations. Photoexcitation at 266 nm leads to the CH3CH2COCl molecules in the S1 state. From this state, the C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase. The barrier to the C-Cl bond cleavage on the S1 surface is significantly increased by effects of the matrix and the internal conversion to the ground state prevails in the condensed phase. The HCl eliminations as a result of internal conversion to the ground state become the dominant channel upon photodissociation of CH3CH2COCl in the argon matrix at 10 K.     

An ab initio Study of the Rotamers and Rotations of Propane-1,3-dial by DFT and SCF Calculations

Summary.  Eight planar rotamers of the enol form of malonaldehyde were considered at the HF (Hartree-Fock) and DFT (density functional theory) levels with 6-311G^** and D95^** (Dunning/Huzinaga full double-ζ) basis sets with the aim to establish the most stable of them and to find the energy barriers of their conversions. The results show that the rotamer with an intramolecular hydrogen bond is the most stable one. High energy barriers were ascertained for the conversions including rotations around a CC double bond. Most of the reactions connected with breaking of the hydrogen bond display strongly asymmetric energy barriers. Their transition states were determined as first-order saddle points because of one imaginary frequency in the IR spectrum related with a negative energy gradient. Received July 6, 2000. Accepted (revised) September 7, 2000     

The Rydberg states of trans-1,3-5-hexatriene from ab initio and configuration interaction calculations

Self-consistent ab initio and configuration interaction (CI) calculations are presented for the Rydberg states of the trans-1,3,5-hexatriene molecule. Seven Rydberg series were identified, four optically allowed (ns, nd _{s ²}, nd ₂₉) and three optically forbidden (np _z, np _y, np _z). These present results plus previous calculations on the valence states are used to assign the transitions observed in the ultraviolet (UV), electron-impact (EI) and two-photon spectra of this molecule.     

Structure and Tautomerism of Cyclopentadiene Derivatives: X. 1,5-Sigmatropic Shifts of the p-Nitrobenzyl Group in Tetramethyl 5-Methylcyclopentadienetetracarboxylate

The reaction of thallium 5-methyl-1,2,3,4-tetrakis(methoxycarbonyl)cyclopentadienide with p-nitrobenzyl bromide gave a mixture of isomeric p-nitrobenzylcyclopentadienes. The isomers were separated, and the structure of each isomer was established by ¹H and ¹³C NMR spectroscopy. 1,5-Sigmatropic shifts of the p-nitrobenzyl group along the cyclopentadienyl ring were revealed, their Gibbs activation energies G 120°C ranging from 29.5 to 30.6 kcal/mol.     

Insights into mechanistic photodissociation of acetyl chloride by ab initio calculations and molecular dynamics simulations

The potential energy surfaces of dissociation and elimination reactions for CH(3)COCl in the ground (S0) and first excited singlet (S1) states have been mapped with the different ab inito calculations. Mechanistic photodissociation of CH(3)COCl has been characterized through the intrinsic reaction coordinate and ab initio molecular dynamics calculations. The alpha-C-C bond cleavage along the S1 pathway leads to the fragments of COCl((2)A' ') and CH(3) ((2)A') in an excited electronic state and a high barrier exists on the pathway. This channel is inaccessible in energy upon photoexcitation of the CH(3)COCl molecules at 236 nm. The S1 alpha-C-Cl bond cleavage yields the Cl((2)P) and CH(3)CO(X(2)A') fragments in the ground state and there is very small or no barrier on the pathway. The S1 alpha-C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase, followed by CH(3)CO decomposition to CH(3) and CO. The barrier to the C-Cl bond cleavage on the S1 surface is significantly increased by effects of the argon matrix. The S1 alpha-C-Cl bond cleavage in the argon matrix becomes inaccessible in energy upon photoexcitation of CH(3)COCl at 266 nm. In this case, the excited CH(3)COCl(S1) molecules cannot undergo the C-Cl bond cleavage in a short period. The internal conversion from S1 to S0 becomes the dominant process for the CH(3)COCl(S1) molecules in the condensed phase. As a result, the direct HCl elimination in the ground state becomes the exclusive channel upon 266 nm photodissociation of CH(3)COCl in the argon matrix at 11 K.     

Insights into the structures, energetics, and vibrations of aqua-rubidium(I) complexes: ab initio study

We have carried out ab initio and density functional theory calculations of hydrated rubidium cations. The calculations involve a detailed evaluation of the structures, thermodynamic properties, and IR spectra of several plausible conformers of Rb+ (H2O)(n=1-8) clusters. An extensive search was made to find out the most stable conformers. Since the water-water interactions are important in hydrated Rb+ complexes, we investigated the vibrational frequency shifts of the OH stretching modes depending on the number of water molecules and the presence/absence of outer-shell water molecules. The predicted harmonic and anharmonic vibrational frequencies of the aqua-Rb+ clusters reflect the H-bonding signature, and would be used in experimental identification of the hydrated structures of Rb+ cation.     

Electronic and Spatial Structure of Piperidine and Its Substituted Derivatives from the Results of ab initio Calculations

The results of non empirical quantum-chemical calculations using the RHF/6-31G(d) and MP2/6-31G(d) methods do not agree with proposals for the axial position of the H atom on the N atom in the piperidine molecule. According to RHF/6-31G(d) calculations for the N-methylpiperidine molecule and its chloro-substituted derivatives an equatorially placed methyl group is energetically more favored than an axial. The axial C-Cl and C-H bonds in these molecules are longer than the equatorial. The ³⁵ Cl NQR frequencies for the axial Cl atoms are lower than the equatorial. The ³⁵ Cl NQR frequency of the axial chlorine atom in 2-chloro-1-methylpiperidine is anomalously low. This is chiefly due to the high population density of its p σ-orbital and this is a result of the polarization of the C-Cl bond via the N atom unshared electron pair directly through the field. The effect of a similar unshared electron pair on the parameters of the C-Cl bond in the ClCH₂NH₂ molecule has been studied by the RHF/6-31(g) method for different angles of rotation of the ClCH₂ group around the C-N bond. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1044–1052, July, 2005.     

Molecular Structure of Triphenylphosphine by Gas-Phase Electron Diffraction and ab initio Calculations

The molecular structure of triphenylphosphine was determined by gas-phase electron diffraction and quantum-chemical calculations. The symmetry of the molecule is C ₃ with the L_pPCC torsion angle of 32.2(3.0)°. The main geometrical parameters are as follows: bond distances, Å: P-Cl 1.839(2), C-Cav 1.400(1), and C-H 1.098(3); bond angles, deg: CPC 102.2(3), PCC 115.3(5), and CC _ipso C 119.4(4).     

Special Features of the Geometry of the 2,2,2,4,4,4-Hexachloro-1,3-dimethyl-1,3-diaza-2,4-diphosphetane Molecule and Its Electron Distribution According to Data of ab initio Calculations

The results of a nonempirical calculation of the 2,2,2,4,4,4-hexachloro-1,3-dimethyl-1,3-diaza-2,4-diphosphetane (Cl₃PNCH₃)₂ molecule by the RHF 6-31G(d) method are in agreement with the data of X-ray structural analysis of this compound. Calculated ³⁵Cl NQR frequencies for axial and equatorial chlorine atoms are close to the experimental values. The population of the orbitals of the lone electron pairs and the p orbitals of the equatorial Cl atoms were significantly lower than those of the axial atoms. Among the MO there was no MO corresponding to a three-center bond involving a P atom and axial Cl and N atoms.     

An ab initio molecular orbital study of intramolecular H-bonding: 1,3 propanediol

An ab initio molecular orbital calculation has been carried out for three different conformations of 1,3 propanediol, one of which permits intramolecular H-bond studied by ab initio quantum mechanical methods. The ΔE for H-bonding formation is compated to be 0.9 kcal/mole and the charge redistributions and molecular orbital energy changes are compared to those found in intermolecular H-bonds.     

Conformational analysis of 1,3-dithiacyclobutane: An ab initio molecular orbital study

The force field governing the in-plane vibrations of pyridine was determined using a revised frequency assignment, and subsequently applied to pyridine-metal complexes. The results show that the frequency shifts of pyridine observed on coordination to a metal are mainly caused by the kinetic coupling between the pyridine molecule and a metal, and that the force field of pyridine remains virtually unchanged.     

Diels-Alder Reaction of phosphaethene with 1,3-dienes: an ab initio study

Computations on Diels-Alder (DA) reactions of phosphaethene with 1,3-butadiene and with isoprene reveal asynchronous transition structures. The DFT (B3LYP/6-311+G) activation energies of these reactions, 12-14 kcal/mol, are much lower than that of the parent ethene-butadiene reaction, 28 kcal/mol, even though the exothermicities of all lie in the same range, from -29 to -33 kcal/mol. The transition states (TSs) for the phosphethene-butadiene or isoprene DA reactions are earlier than the TSs of the butadiene-ethene cycloaddition. Due to the weakness of the C=P pi bond compared to the C=C pi bonds, the energies required to reach the phosphaethene TSs are much less than the carbocyclic cases. The computed (1)H NMR chemical shifts and nucleus independent chemical shifts (NICS) quantify the aromatic character of the transition states. Regioselectivities of the neutral phosphaethene-isoprene DA reactions are modest, at best. However, computations on radical cation DA reactions of phosphaethene with isoprene, which proceed stepwise with open chain intermediates, can account for the high regioselectivities that have been observed in some cases.     

35Cl NQR Spectra and ab initio Calculations of gem-Substituted 1-Chlorocyclohexane Derivatives

The ³⁵Cl NQR spectra of gem-substiutted 1-chlorocyclohexane derivatives C₆H₁₀XCl (X = H, CH₃, CH₃O, Cl) at 77 K were measured. Ab initio RHF/6-31G(d) calculations of their conformers, and also of conformers of monosubstituted cyclohexanes were performed. Based on the calculation results, the NQR lines were assigned and the conformational energies of the substituents were evaluated. The relative conformational energies of the Cl atom and CH₃O group disagree with previous data.     

Facile 1,3- and 1,5-Chlorine Migration

High-level ab initio molecular orbital calculations, using the G2(MP2,SVP) theory (and semiempirical methods) have been used to examine several 1,3- and 1,5-chlorine migrations. It is found that the interaction of chlorine lone pair electrons with a low-lying LUMO accelerates the Cl shift dramatically (lone pair-LUMO-mediated pericyclic reaction). The activation barriers for the 1,3-migration in chloro oxo ketene 1 (Cl(C=O)CH=C=O) and the 1,5-migration in (2-(chlorocarbonyl)vinyl)ketene 2 (Cl(C=O)CH=CHCH=C=O) are only 53 and 61 kJ mol(-)(1), respectively, compared to the 216 and 173 kJ mol(-)(1) barriers for the corresponding unassisted 1,3- and 1,5-sigmatropic shifts of Cl in 3-chloro-1-propene and 5-chloro-1,3-pentadiene. The transition structures for 1 and 2 reveal that migration of the chlorine atoms takes place in the molecular planes. The 1,5-chlorine shift in 6-chlorocyclohexa-2,4-dienone (3) has a significantly higher barrier due to a lack of appropriate orbital interaction. The related 1,3-shift in the (chlorocarbonyl)imine-alpha-chloro isocyanate system is also dramatically accelerated compared with conventional pericyclic 1,3-Cl migration.