Theoretical study of the mechanism of cycloaddition reaction between dichloro-germylidene and acetaldehyde

The mechanism of the cycloadditional reaction between singlet dichloro-germylidene(R1) and (acetaldehyde(R2) has been investigated with MP2/6-31G* method, including geometry optimization, vibrational analysis and energies for the involved stationary points on the potential energy surface. From the potential energy profile, we predict that the cycloaddition reaction between singlet dichloro-germylidene and acetaldehyde has two competitive dominant reaction pathways. Going with the formation of two side products (INT3 and INT4), simultaneously. The two competitive reactions both consist of two steps: (1) two reactants firstly form a three-membered ring intermediate (INT1) and a twisted four-membered ring intermediate (INT2), respectively, both of which are barrier-free exothermic reactions of 44.5 and 63.0 kJ/mol; (2) then INT1 and INT2 further isomerize to a four-membered ring product (P1) and a chlorine-transfer product (P2) via transitions (TS1 and TS2), respectively, with the barriers of 9.3 and 1.0 kJ/mol; simultaneously, P1 and INT2 react further with acetaldehyde(R2) to give two side products (INT3 and INT4), respectively, which are also barrier-free exothermic reaction of 65.4 and 102.7 kJ/mol.     

Theoretical survey of the reaction between osmium and acetaldehyde

The mechanism of the reaction of osmium atom with acetaldehyde has been investigated with a DFT approach. All the stationary points are determined at the UB3LYP/sdd/6-311++G** level of the theory. Both ground and excited state potential energy surfaces are investigated in detail. The present results show that the title reaction start with the formation of a CH₃CHO-metal complex followed by C-C, aldehyde C-H, C-O, and methyl C-H activation. These reactions can lead to four different products (HOsCH₃ + CO, OsCO + CH₄, OsCOCH₃ + H, and OsO + C₂H₄). The minimum energy reaction path is found to involve the spin inversion in the initial reaction step. This potential energy curve-crossing dramatically affects reaction exothermic. The present results may be helpful in understanding the mechanism of the title reaction and further experimental investigation of the reaction.     

Theoretical study on the gas-phase reaction of acetaldehyde with methoxy radical

Structural Chemistry - The reaction of acetaldehyde with methoxy radical has been investigated theoretically by means of quantum chemistry methods at the BMC-QCISD//B3LYP/6-311+G(d,p) level. The...     

Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms

The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S(1) state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S(0) state or undergoes intersystem crossing to the first excited triplet T(1) state, from where the molecule can dissociate over two main channels: the radical (CH(3) + HCO) and the molecular (CO + CH(4)) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T(1) barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ～308 nm. Towards the blue edge, at 230 nm, an upper limit of ～50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.     

Elimination of methane from protonated acetaldehyde: The Ab initio transition state

The transition state (TS) for loss of CH₄ from protonated acetaldehyde has been located at the second-order Moller-Plesset (MP2)/6-31G(d,p) level of theory. The activation energy is predicted to be 263.9 kJ/mol starting from the more stable form (methyl and hydrogen E) and 261.6 kJ/mol starting from the less stable form (methyl and hydrogen Z) that is required for reaction. The products (methane and the formyl ion) are predicted to lie 136.6 kJ/mol below the TS for their formation. MP2 methods underestimate the heats of formation of both the TS and the reaction products by about 40 kJ/mol when compared with experiment. Restricted Hartree-Fock (RHF) calculations give much more accurate relative energies. The MP2 TS leads directly to fragmentation and is described as a protonation of the methyl group by the acidic proton on oxygen. Under RHF theory the reaction is stepwise. An RHF TS similar to the MP2 TS leads to a nonclassical intermediate (which is stable at this level of theory) that has one of the C---H bonds protonated. This mechanism (protonation of an alkyl group) appears to be a general one for high energy 1,2 eliminations from organic cations. (J Am Soc Mass Spectrom 1994, 5, 1102-1106)     

Acetaldehyde solid state reactivity at low temperature: formation of the acetaldehyde ammonia trimer

We focus on low temperature reactivity from 25 to 300 K, in ice containing acetaldehyde, ammonia, and formic acid. We show that the warming of this ice mixture forms the acetaldehyde ammonia trimer (2,4,6-trimethyl-1,3,5-hexahydrotriazine, C(6)H(15)N(3)) after five steps. The reaction is monitored by FTIR spectroscopy and mass spectrometry. We propose a mechanism for its formation that differs from the one proposed in the liquid phase. The reaction intermediates, α-aminoethanol (from 80 K) and ethanimine (formed at 180 K), have been identified by a mechanistic approach: each step of the reaction has been treated separately. The chemical implications and the astrophysical relevance of the study are also discussed.     

A study of the gas-phase reaction between protonated acetaldehyde and methanol

Protonated acetaldehyde is methylated on the oxygen during interaction with methanol in the gas phase. The ionic product of the ion/molecule reaction between methanol and protonated acetaldehyde is identical with C-protonated methylvinyl ether (high-pressure ionization), and with the (M − C₂H₅)⁺ fragment ion of sec-butyl methyl ether (following electron ionization), and also with the (M − OCH₃)⁺ fragment ion of acetaldehyde dimethylacetal (following electron ionization). The structures of these ions and the mechanism of their formation were established by isotope-labeling experiments and collision-induced dissociation mass spectra of model compounds obtained with three different types of tandem mass spectrometers (BEQQ, triple-quadrupole, and a penta-quadrupole instrument). Gas phase synthesis of the product ion from [²H₃]-methanol or [²H₄]-acetaldehyde provided insight into its mode of formation and collision-induced dissociation.     

Theoretical study of three predominant tautomers of 2-oxo-6-methylpurine and their two transition state structures

The three predominantly stable tautomers of 2-oxo-6-methylpurine were studied in gas phase and aqueous solution by means of quantum mechanical calculations. Two transition state structures connecting these three tautomeric forms on the free energy surface were determined. The activation free energy for the intramolecular proton transfer in gas phase was calculated to be considerably smaller than the bond energy of either N-H or O-H: 59.01 and 30.37 kcal/mol for N9→N3 and N1→O2, respectively, obtained at the QCISD(T)/6-31G+(d)//MP2(full)/6-31G(d) level of theory.     

Theoretical study of the first transition row oxides and sulfides

Summary The first transition row oxides and sulfides are studied using several different levels of theory. The calculations show the bonding mechanism in the sulfides and oxides to be very similar. For the oxides, accurate experimental data allow the theoretical methods to be calibrated. The same level of theory is used to study the sulfides where there is far less experimental information. For ScO through MnO and CuO the coupled cluster singles and doubles technique including a perturbational estimate of the connected triple excitations [CCSD(T)] yields spectroscopic constants (_e, _e, andD ₀) in good agreement with experiment. The triple excitations are found to be very important in achieving this accuracy. For FeO to NiO, the self-consistent-field (SCF) approach yields orbitals that are localized on the metal or oxygen. This appears to cause problems for the single reference techniques; this is discussed in detail for NiO. The complete-active-space SCF/internally contracted averaged coupled pair functional approach (CASSCF/ICACPF) works well for FeO to NiO. The calculation of accurate dipole moments is found to be very difficult.     

Dynamic NMR study of the oxaphosphetane complexation with lithium during the Wittig reaction

Dynamic NMR spectroscopy at very low temperatures (148–182 K) reveal the dynamic behavior of the (2‐tri(3‐furyl)‐3‐methyl‐4‐cyclopropenyl‐oxaphosphetane) generated during a Wittig reaction between tri(3‐furyl)ethylphosphonium iodine and cyclopropylaldehyde. The possibility of formation of different adducts between Li⁺ ions and oxaphosphetane or betainic intermediates was checked calculating the formation enthalpies using the MNDO, AM1, and PM3 semiempirical MO methods. The observed species are interpreted as oxaphosphetane complexes with lithium ions present in solution. Quantum mechanical calculations confirm the spectroscopic results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Theoretical study on the aromaticity of transition states in pericyclic reactions

The aromaticity of transition states in pericyclic reactions such as electrocyclic reactions, cycloaddition reactions, and sigmatropic shifts was studied by the IDA (index of deviation from aromaticity) on the basis of a CASSCF wave function. The aromaticity defined by the IDA classified the allowed and forbidden transition states of pericyclic reactions treated here. The order of the aromaticity levels corresponds to that of the energy barriers of some reactions. The difference between the aromaticity defined by the IDA and that by the magnetic properties as a NICS is also discussed.     

Reaction of acetaldehyde with Ni+: an extended theoretical study of the decarbonylation mechanism of acetaldehyde by first-row transition metal ions

We report herein a theoretical study of the reaction of acetaldehyde with Ni+ as an extension of our two recent papers on the decarbonylation of acetaldehyde by late first-row transition metal ions [Zhao, Zhang, Guo, Wu, Lu Chem. Phys. Lett. 2005, 414, 28; Zhao, Guo, Zhang, Wu, Lu ChemPhysChem 2006, 7, 1345]. Geometries of all the stationary points involved in the reaction have been fully optimized at the B3LYP/6-311+G(2df,2pd) level and the decarbonylation mechanism is analyzed in terms of the topology of potential energy surface. Combining with the previous studies, it is found that for the Cr+, Co+, and 4Fe+ mediated systems decarbonylation of CH3CHO only takes place via C-C activation, and aldehyde C-H activation is unlikely to be important, whereas both C-C and aldehyde C-H activations by Ni+ and 6Fe+ could result in the decarbonylation of CH3CHO, where hydride-containing species M+(H)(CO)(CH3) is found to be a common minimum along the reaction pathways.     

Exit routes from the transition state: Angular momentum constraints on the formation of products

We have analyzed experimental data from a number of exothermic processes in which molecules in well-defined initial states are deactivated by inelastic, dissociative, or reactive collisions. Further, we analyze deactivation processes that do not occur in molecules despite their containing high levels of excitation. Significant common elements are found among these forms of deactivation. The initial step consists of transition to a product state involving minimum rotation state change (Delta j) consistent with energy conservation. Frequently, this process is near-energy-resonant. More critically, it may frequently require substantial angular momentum (AM) change. Analysis of experimental data indicates that constraints act upon on the formation of products in processes that involve release of excess energy. These constraints are associated with the magnitude of AM that must be generated for the initial transition to occur and this AM "load" increases with the amount of energy to be released. In general, the probability of generating rotational AM falls rapidly as Delta j increases, and this effectively limits the size of energy gap that may be bridged by a given reactant pair and at some point the constraint is sufficient to constitute a barrier that prevents the process from taking place. The choice of reactant species strongly affects the probability of each process that increases (i) when molecules efficiently interconvert momentum and (ii) when many product states are available in the critical near-resonant region. These factors increase the proportion of initial trajectories that possess the energy and momentum necessary to open a "product" channel. Evidence is presented showing that AM load-reduction strategies lead to marked enhancement of rates of collision-induced processes, suggesting that reduction of constraints in the exit channels from the transition state may constitute a previously unrecognized form of catalysis.     

Ab initio torsional potential and transition frequencies of acetaldehyde

High-level ab initio electronic structure calculations, including extrapolations to the complete basis set limit as well as relativistic and diagonal Born-Oppenheimer corrections, resulted in a torsional potential of acetaldehyde in its electronic ground state. This benchmark-quality potential fully reflects the symmetry and internal rotation dynamics of this molecule in the energy range probed by spectroscopic experiments in the infrared and microwave regions. The torsional transition frequencies calculated from this potential and the ab initio torsional inverse effective mass function are within 2 cm(-1) of the available experimental values. Furthermore, the computed contortional parameter rho of the rho-axis system Hamiltonian is also in excellent agreement with that obtained from spectral analyses of acetaldehyde.     

Theoretical study on the singlet excited state of pterin and its deactivation pathway

The excited-state properties and related photophysical processes of the acidic and basic forms of pterin have been investigated by the density functional theory and ab initio methodologies. The solvent effects on the low-lying states have been estimated by the polarized continuum model and combined QM/MM calculations. Calculations reveal that the observed two strong absorptions arise from the strong pi --> pi* transitions to 1(pipi*L(a)) and 1(pipi*L(b)) in the acidic and basic forms of pterin. The first 1(pipi*L(a)) excited state is exclusively responsible for the experimental emission band. The vertical 1(n(N)pi*) state with a small oscillator strength, slightly higher in energy than the 1(pipi*L(a)) state, is less accessible by the direct electronic transition. The 1(n(N)pi*) state may be involved in the photophysical process of the excited pterin via the 1(pipi*L(a)/n(N)pi*) conical intersection. The radiationless decay of the excited PT to the ground state experiences a barrier of 13.8 kcal/mol for the acidic form to reach the (S(1)/S(0)) conical intersection. Such internal conversion can be enhanced with the increase in excitation energy, which will reduce the fluorescence intensity as observed experimentally.     

Carbonylation of acetaldehyde in the presence of transition metal compounds

The carbonylation of acetaldehyde to give butyl lactate has been carried out in the presence of rhodium and cobalt compounds atP _CO=5–9 MPa andT=383–483 K.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 570–571, March, 1995.     

Theoretical study of photodissociation dynamics on the lowest-lying Rydberg state of ketene

In the present study, an attempt is made to reveal the main mechanism of photodissociation on the lowest-lying Rydberg state (1)B(1) of ketene, referred to as the second singlet excited state S(2), by means of the complete active space self-consistent field and the second-order multiconfigurational perturbation theory methods. The located S(2)S(1)T(1) three-surface intersection plays an important role in the dissociation process. It is shown that the intersection permits an efficient internal conversion from S(2) to S(1) state, but prohibits the intersystem crossing from S(2) to T(1) state because of the small spin-orbital coupling value of 0.136 cm(-1). The main photodissociation process could be described as follows: after one photon absorption to the S(2) state, ketene preferentially relaxes to the minimum S(2)C(2v), and undergoes a transition state S(2)TS with small potential barrier along the C(s)-I (out-of-plane bent) symmetry, and passes through the S(2)S(1)T(1) intersection to reach S(1) surface, then arrives at the transition state S(1)TS along the minimum energy path. As is well known, S(1)-->S(0) internal conversion around the Franck-Condon region is expected to be very efficient, and eventually the hot S(0) molecule has accumulated enough energy to yield the CH(2) (a (1)A(1)) and CO (X (1)Sigma(+)) products.