Conformational stability and barriers to internal rotation of methacrolein (CHO and CDO) from far infrared spectral data, ab initio calculations and the microwave spectrum of methacrolein-d1

Conformational energies of reference organic molecules: benchmarking of common efficient computational methods against coupled cluster theory

We selected 145 reference organic molecules that include model fragments used in computer-aided drug design. We calculated 158 conformational energies and barriers using force fields, with wide applicability in commercial and free softwares and extensive application on the calculation of conformational energies of organic molecules, e.g. the UFF and DREIDING force fields, the Allinger's force fields MM3-96, MM3-00, MM4-8, the MM2-91 clones MMX and MM+, the MMFF94 force field, MM4, ab initio Hartree-Fock (HF) theory with different basis sets, the standard density functional theory B3LYP, the second-order post-HF MP2 theory and the Domain-based Local Pair Natural Orbital Coupled Cluster DLPNO-CCSD(T) theory, with the latter used for accurate reference values. The data set of the organic molecules includes hydrocarbons, haloalkanes, conjugated compounds, and oxygen-, nitrogen-, phosphorus- and sulphur-containing compounds. We reviewed in detail the conformational aspects of these model organic molecules providing the current understanding of the steric and electronic factors that determine the stability of low energy conformers and the literature including previous experimental observations and calculated findings. While progress on the computer hardware allows the calculations of thousands of conformations for later use in drug design projects, this study is an update from previous classical studies that used, as reference values, experimental ones using a variety of methods and different environments. The lowest mean error against the DLPNO-CCSD(T) reference was calculated for MP2 (0.35 kcal mol-1), followed by B3LYP (0.69 kcal mol-1) and the HF theories (0.81-1.0 kcal mol-1). As regards the force fields, the lowest errors were observed for the Allinger's force fields MM3-00 (1.28 kcal mol-1), ΜΜ3-96 (1.40 kcal mol-1) and the Halgren's MMFF94 force field (1.30 kcal mol-1) and then for the MM2-91 clones MMX (1.77 kcal mol-1) and MM+ (2.01 kcal mol-1) and MM4 (2.05 kcal mol-1). The DREIDING (3.63 kcal mol-1) and UFF (3.77 kcal mol-1) force fields have the lowest performance. These model organic molecules we used are often present as fragments in drug-like molecules. The values calculated using DLPNO-CCSD(T) make up a valuable data set for further comparisons and for improved force field parameterization.

Spectroscopic and thermodynamic investigations of clathrate hydrates of methacrolein

This study characterized new structure II (sII) clathrate hydrates, consisting of 136 H₂O molecules with 8 large 5¹²6⁴ cages and 16 small 5¹² cages, with methacrolein for the first time.

Theoretical investigation of the oxidation pathways of HO•-initiated reactions of acrolein, methacrolein, and trans-crotonaldehyde

Ab initio molecular-orbital calculations have been performed to investigate the reaction mechanisms of the HO•-initiated reactions of the α,β-unsaturated aldehydes: acrolein (ACR), methacrolein (MACR), and trans-crotonaldehyde (CROT). All geometries were fully optimized at the MP2(Full)/6–31G(d,p) level. The correlation energy corrections were introduced by carrying out single-point calculations using both spin-projected second-order Møller–Plesset perturbation theory (PMP2) and coupled-cluster theory (CCSD(T)) using basis sets of different sizes. All reaction pathways studied proceed via a barrierless formation of a loosely bound pre-reactive complex in the entrance channel. The transition-state structures of the HO• additions to the terminal (β) and to the central (α) carbon atoms of the C=C double bond are found to be reactant-like structures. The lowest-energy barrier pathways are found to be the aldehydic H-atom abstraction. The β-addition pathways are found to be energetically more favored than the α-addition pathways. The HO• addition and aldehydic H-atom abstraction pathways are found to be highly exoergic, with the H-atom abstraction pathway being more exoergic than the addition pathways. Although the methyl substitution at the C=C double bond of methacrolein and crotonaldehyde lowers the energies of the transition-state structures of both α- and β-addition pathways, it destabilizes the energies of the transition-state structures of the corresponding aldehydic H-atom abstraction pathways, compared with that of acrolein.

Methyl Vinyl Ketone+OH and Methacrolein+OH Oxidation Reactions: A Master Equation Analysis of the Pressure- and Temperature-Dependent Rate Constants

High-level electronic structure calculations and master equation analyses were carried out to obtain the pressure- and temperature-dependent rate constants of the methyl vinyl ketone+OH and methacrolein+OH reactions. The balance between the OH addition reactions at the high-pressure limit, the OH addition reactions in the fall-off region, and the pressure-independent hydrogen abstractions involved in these multiwell and multichannel systems, has been shown to be crucial to understand the pressure and temperature dependence of each global reaction. In particular, the fall-off region of the OH addition reactions contributes to the inverse temperature dependence of the rate constants in the Arrhenius plots, leading to pressure-dependent negative activation energies. The pressure dependence of the methyl vinyl ketone+OH reaction is clearly more important than in the case of the methacrolein+OH reaction owing to the weight of the hydrogen abstraction process in this second system. Comparison of the theoretical rate constants and the experimental measurements shows quite good agreement.

Pressure dependence in the methyl vinyl ketone + OH and methacrolein + OH oxidation reactions: an electronic structure study

High-level electronic structure calculations were carried out for the study of the reaction pathways in the OH-initiated oxidations of methyl vinyl ketone (MVK) and methacrolein (MACR). For the two conformers of MVK (called synperiplanar and antiperiplanar), the addition channels of OH to the terminal and central carbon atom of the double bond dominate the overall rate constant, whereas the abstraction of the methyl hydrogen atoms has no significant kinetic role. In the case of MACR, only the antiperiplanar conformer is important in its reactivity. In addition, the lower Gibbs free energy barrier for MACR corresponds to the aldehydic hydrogen abstraction reaction, which will be somewhat more favorable than the addition processes. The subtle balance between the different pathways (additions versus abstractions) serves to give an understanding of the pressure dependence of the rate constants of these tropospheric oxidation processes.

Conformational stability, optimized geometries, vibrational and electronic spectra of methacryloyl bromide in ground and excited electronic states

In order to understand conformational isomerism in methacryloyl bromide (MABR) in the ground (S(0)) and the first excited (S(1)) electronic states and to interpret the vibrational and electronic spectra of its conformers in the S(0) state, quantum mechanical calculations using Density Functional Theory (DFT) and RHF methods with extended basis sets 6-31G, 6-31G** and 6-311+G(d,p) have been conducted. In RHF calculations, electron correlation effects have been included at the Møller-Plesset MP2 level. It is inferred that in both the electronic states the molecule may exist in two isomeric forms-s-trans and s-cis; the former being more stable than the later by about 1.629 kcal mol(-1) in the S(0) state and by about 2.218 kcal mol(-1) in the S(1) state. Electronic transition tends to increase the s-trans/s-cis and s-cis/s-trans, rotational barriers from 7.059 kcal mol(-1) (2468.1 cm(-1)) and 5.428 kcal mol(-1) (1897.8 cm(-1)) in S(0) state to 23.594 kcal mol(-1) (8249.4 cm(-1)) and 21.376 kcal mol(-1) (7473.9 cm(-1)) in the S(1) state. Completely optimized geometries of the two conformers in S(0) state reveal that while there is no significant difference in their bond lengths, some of the bond angles associated with COBr group are appreciably different. Electronic excitation tends to change both the bond lengths and bond angles. Based on suitably scaled DFT and RHF results obtained from the use of 6-31G** and 6-311+G(d,p) basis sets, a complete assignment is provided to the fundamental vibrational bands of both the s-trans and s-cis conformers in terms of frequency, form and intensity of vibrations and potential distribution across the symmetry coordinates in the S(0) state and a comparison has been made with experimental assignments. A theoretical prediction of the electronic transitions in the near UV-region in the two conformers and their tentative assignment has been provided on the basis of CI level calculations using 6-31G basis set.

Generation, Microwave Spectrum, Barrier to Internal Rotation of Methyl Group, and ab Initio MO Calculation of syn-2-Nitrosopropene, syn-CH(2)&dbond;C(CH(3))&bond;N&dbond;O

syn-2-Nitrosopropene was generated, in the gas phase, by chemical reaction of 1-chloro-2-(hydroxyimino)propane with K(2)CO(3) and identified by microwave spectroscopy. The microwave spectrum of the reaction product was observed in the frequency range from 8.0 to 40.0 GHz. The rotational constants (MHz) were determined as A = 8744.09(6), B = 4846.07(2), and C = 3177.84(3) for CH(2)&dbond;C(CH(3))&bond;(14)NO (normal species) and A = 8664.36(5), B = 4822.15(3), and C = 3157.04(3) for CH(2)&dbond;C(CH(3))&bond;(15)NO ((15)N species) in the ground vibrational state. The values of the planar moment (P(cc) = (I(a) + I(b) - I(c))/2) obtained for the normal and (15)N species were 1.525(1) and 1.526(1) u Å(2), respectively. This suggests that the nitrogen atom lies in or is close to the ab inertial plane of the molecule and shows also that only two hydrogen atoms are located symmetrically out of the symmetry plane. The reaction product was determined to be syn-2-nitrosopropene by comparing the observed and calculated rotational constants, kappa (Ray's asymmetry parameter) and r(s) coordinates of the nitrogen atom. The dipole moments (D) were determined to be µ(a) = 2.43(5), µ(b) = 1.12(7), and µ(total) = 2.67(7). The barrier heights of the internal rotation owing to the methyl group of the normal species in the ground and first excited torsional states were determined to be 1750(50) and 1740(50) cal/mol (1 cal/mol = 4.184 J/mol), respectively. The (14)N nuclear quadrupole coupling constants (MHz) were determined to be chi(aa) = 0.25(21), chi(bb) = -7.11(40), and chi(cc) = 6.85(61). Two vibrational excited states were observed and the vibrational frequencies (cm(-1)) of the C-N and C-C torsional modes were determined to be 160(40) and 175(40), respectively. The lifetime of syn-2-nitrosopropene was found to be ca. 2 min in the waveguide cell. Copyright 2000 Academic Press.