A structural ab initio study of the T1 triplet state of acetaldehyde. The effects of electron correlation and additional functions in the basis set (original) (raw)
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Theoretica Chimica Acta, 1994
An ab initio study of the ground and the first singlet excited states of acetaldehyde has been performed to analyze the molecular properties as a function of the methyl torsion and the aldehydic hydrogen wagging angles. The structural characteristics and the conformational behaviour in both electronic states have been determined. The important structural changes between the two states have been analyzed by a decomposition of the total energy into its components. It was found that the methyl torsion barriers arise mainly from attractive interactions. Evidence is presented which shows that these barriers arise from in-plane and out-of-plane hyperconjugative effects involving the oxygen atom. It is also shown that the pyramidalization experienced by the carbonyl carbon in the first singlet excited state has two sources, namely, a decrease in the electronic repulsion and an increase in the electron-nucleus attraction.
Chemical Physics, 1994
The molecular structure and the potential energy hypersurfaccs of acetaWhyde for the Se state were determined by CISD/ IUIF/4_31G** ab initio molecular orbital c&ulations and CISDT/ROHF/4_31G** for the Sr state. The torsional-wagging energy levels were evaluated by the variational method using free rotor basis functions expressed as symmetrized double Fourier expansions. The relative strengths of the Her&erg-Teller vibronically induced transitions and the direct Franck-Condon electto@&y allowed transitions were calculated from the transition dipole momenta. The torsional-wagging spectrum simulated from the two-dimensional calcuIation was compared to the Sr + So, n + 5~~ jet-cooled fhr orescencc excitation spectrum.. 1. Inaodnction Electronic excitation in a molecule involves the promotion of an electron from an occupied molecular orbital to a virtual orbital that is to some extent antibonding. The introduction of antibonding density leads to excited states with greater molecular flexibility than the corresponding ground state. In the case of n + P* excitation in the carbonyl chromophore, the resulting upper singlet S, or triplet T, states experience a pyramidaliz.ation at the carbonyl carbon atom and the structure inverts with a large amplitude wagging motion between two equilibrium positions. When a methyl group is attached directly to the chromophore to form the acetaldehyde molecule, the internal rotation of the methyl group against the molecular frame give rise to a second large amplitude vibration. Thus, two large amplitude modes are present in the S, (n, Q*) and * Comsponding author.
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 ͓J. Chem. Phys. 117, 6489 ͑2002͔͒ 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 Ϫ1 of the available experimental values. Furthermore, the computed contortional parameter of the rho-axis system Hamiltonian is also in excellent agreement with that obtained from spectral analyses of acetaldehyde.
Hydrogen bonding in acetylacetaldehyde: Theoretical insights from the theory of atoms in molecules
International Journal of Quantum Chemistry, 2009
All the possible conformations of tautomeric structures (keto and enol) of acetylacetaldehyde (AAD) were fully optimized at HF, B3LYP, and MP2 levels with 6-31G(d,p) and 6-311ϩϩG(d,p) basis sets to determine the conformational equilibrium. Theoretical results show that two chelated enol forms have extra stability with respect to the other conformers, but identification of global minimum is very difficult. The high level ab initio calculations G2(MP2) and CBS-QB3) also support the HF conclusion. It seems that the chelated enol forms have equal stability, and the energy gap between them is probably lies in the computational error range. Finally, the analysis of hydrogen bond in these molecules by quantum theory of atoms in molecules (AIM) and natural bond orbital (NBO) methods fairly support the ab initio results.
ChemInform Abstract: The Ground Torsional State of Acetaldehyde
ChemInform, 1991
New microwave measurements on the ground state of acetaldehyde have been carried out using a Fourier transform spectrometer in the region from 7 to 26 GHz (typical measurement uncertainty 4 kHz), and a conventional Stark spectrometer in the region from 45 to 116 GHz (typical measurement uncertainty 40 kHz). These new ground state measurements and remeasurements have permitted a much better fit to two theoretical models of a data set containing far-infrared combination differences from the literature, microwave transitions from the literature, and the new microwave transitions. Root-mean-square residuals obtained here for all these data (which come from a large number of sources) are only slightly larger (for either model) than the estimated measurement uncertainties. The first theoretical model is essentially a high-barrier effective Hamiltonian for one vibrational state only, based on Fourier expansions in terms of the form cos( 2*n/ 3)(pK -6). The second model is based on calculations using the internal-rotation potential function, and is in principle much more powerful than the first. The present successful fits using either model indicate that earlier fitting difficulties using the second model and a combined infrared and microwave data set were caused by problems in the microwave data set, rather than problems in the model. It is hoped that similar success can be achieved with the more powerful second model when data from higher excited torsional states are considered. o
The ground and first torsional states of acetaldehyde
Journal of Molecular Spectroscopy, 1992
We have fitted to within experimental accuracy a data set for acetaldehyde consisting of 423 ut = 0 microwave lines, 270 u, = 1 microwave lines, and 2 14 u1 = 1 + 0 far-infrared fines, using a global model from the earlier literature. The v, = I microwave data set was extended and corrected by 76 new measurements from NET: the theoretical model was extended by inclusion of higher order terms from the literature. The final tit requires only 34 parameters to achieve a unitless weighted standard deviation for the whole fit of 1.15, demonstrating both the power of the model and the internal consistency of the data. 0 1992 Academic press, IZIC.
Molecular mechanics (MM3) calculations on aldehydes and ketones
Journal of the American Chemical Society, 1991
the hyperfine coupling constants measured in derivatives of C O Tmay be providing less information about the degree of bond alternation at the equilibrium geometries of these radical ions than about the ease with which geometries with more nearly equal bond lengths are accessed. On the basis of INDO calculations, Hammons, Bernstein, and MyersI3 have advanced explanations of the effects of substituents on the EPR spectra of COT'that are similar to those presented here. Other r e~e a r c h e r s~~.~~ have proposed an alternative model, which assumes bond alternation does not occur, so that the NBMOs in eq 1 are not mixed. Instead, this latter model postulates that there is a Boltzmann poDulation of the lowest excited methyl, and cyano derivatives. Our calculations suggest that these substituents have a relatively small effect on the extent of bond length alternation at the equilibrium geometry. However, both F, a A donor, and CN, a A acceptor, are found to reduce substantially the barrier to bond equalization. Acknowledgment. We thank the National Science Foundation for its support of this research and for providing funds that enabled the purchase of the Convex C-2 computer, on which some of the calculations reported here were performed. We also thank the San Diego Supercomputer Center for a generous allocation of time on the Cray YMP-8/864 computer at SDSC. electronic state in which one eledtron is thermally excited from Registry No. C O T , 34510-85-5; F-COT, 70741-95-6; CH3-COT, the lower energy of the two NBMOs to the upper. 3451 9-36-3; CN-COT, 70741-98-9. Our CI calculations indicate that the basic assumption of the latter model is incorrect, since we find that bond alternation is energetically favorable, not only in COT*-, but also in its fluoro, Supplementary Material Available: ROHF/3-21G-optimized geometries and ROHF and CI energies for bond-alternated (C h angle-alternated (C,,,), and midpoint geometries of fluorocvclo- .-,ocktetraene radical anion (2 pap&). OTdering information is given on any current masthead page.
High-Level ab Initio Calculations on the Intramolecular Hydrogen Bond in Thiomalonaldehyde
Journal of Physical Chemistry A, 1997
High-level ab initio calculations, in the framework of the G2(MP2) theory, have been carried out on the different tautomers of thiomalonaldehyde (TMA). These calculations are compared with those obtained using density functional theory methods, namely B3LYP, with extended basis sets. In general the enethiol tautomers of TMA are 5-10 kcal/mol more stable than the corresponding enol analogues, with the only exception being the Z-enol (E1) and the Z-enethiol (T1) hydrogen-bonded species, which are the global minima of both series. At the G2(MP2) level both tautomers are nearly degenerate, the enethiol T1 being 0.2 kcal/mol more stable than the enol E1. Electron correlation effects stabilize preferentially the enol form, while the ZPE corrections go in the opposite direction, due essentially to the differences between S-H and O-H stretching frequencies. As a consequence, when the hydrogen atom involved in the intramolecular hydrogen bond (IHB) of both tautomers is replaced by deuterium, the stability order is reversed and E1 is predicted to be more stable than T1. An analysis of these IHBs in terms of the topological characteristics of the electron charge density and of the shifts of the S-H and O-H vibrational frequencies reveals that the HB in E1 is much stronger than in T1. The existence of this IHB results in an increase of the electron delocalization which enhances the stability of tautomer E1. At the G2(MP2) level two open-chain rotamers, namely T4 and T7, are predicted to be within an energy gap smaller than 0.5 kcal/mol with respect to the global minimum. The use of continuum and discrete-continuum models indicates that both open-chain enethiols and enols are significantly stabilized by solute-solvent interactions, and they should predominate in aqueous solution. B3LYP/6-311+G(3df,2p) relative stabilities are in excellent agreement with G2(MP2) values.