Ab initio study of the unimolecular pyrolysis mechanisms of monothioformic acid

The mechanisms of the possible unimolecular reactions occurring during the pyrolysis of the four tautomers (two conformers of thiol- and two conformers of thiono-) of monothioformic acid have been proposed and investigated by ab initio methods with STO-3G and 6-31G⁷ basis sets. The effects of valence electron correlation were included by Møller-Plesset (MP) perturbation theory to the fourth order at the 6-31G⁷ level. Our best results of the activation energies are given by MP4/6-31G⁷//HF/ 6-31G⁷ plus scaled zero-point energy. The barrier heights of the dehydrogenation (via a four-centre transition state) and dehydrogensuphidation (via a three-centre transition state) of thiolformic acid pyrolysis are 67.47 and 67.09 kcal mol⁻¹ respectively. The s-cis thionformic acid is dehydrated via a three-centre transition state. The activation energy of the process (81.18 kcal mol⁻¹) is much higher than the activation energy of the dehydrogenation of the s-trans form (68.83 kcal mol⁻¹) which is dehydrogenated via a four-centre transition state. These results suggest that in thionformic acid pyrolysis, the dehyrogenation of the s-cis form is more favourable than the dehydration of the s-trans form.     

The role of internal H-bonding in the conformational preferences of some molecules of the formula CH3CHYCH2X

The conformational energies of 1-amino-2-propanol, 2-amino-1-propanol and 1,2-diaminopropane are studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. Calculations at both levels of theory generally favor conformations stabilized by internal H-bonding for all molecules considered. Results are first presented for conformations employing assumed geometries. Since the conformational energy differences as found by the initial set of calculations are in some cases rather small it then becomes necessary to introduce geometry optimizations into the study at the minimal STO-3G level. In addition, to get a better estimate of the energy differences of the various conformations 4-31G calculations are performed on the STO-3G optimized structures. These latter results indicate the following, (a) For 1-amino-2-propanol only one conformation that is stabilized by intramolecular H-bonding is low in energy; this has the methyl and amino groups anti. The other H-bonded conformer, where the methyl and amino groups are gauche, is predicted to be ca. 1.2 kcal mol^?1 less stable. Similar findings for this molecule have recently been provided by micro-wave spectroscopy. (b) For 2-amino-1-propanol the two H-bonded conformers are only separated by about 0.5 kcal mol^?1, with the anti conformer being more stable. Micro-wave spectroscopy again supports these calculations. (c) For 1,2-diaminopropane the gauche conformer is predicted to be of rather high energy (ca. 2.5 kcal mol^?1) compared to the corresponding anti H-bonded conformer. The value of 2.5 kcal mol^?1should be taken as an upper limit, since the geometry optimization of the gauche conformer of 1,2-diaminopropane is incomplete compared to the optimization carried out for the anti conformer.     

A theoretical study of the Si-O bond in disiloxane and related molecules

Summary A comparison of semi-empirical (MNDO) and ab initio (GAUSSIAN) calculations for disiloxane and related molecules is given. The STO-3G^* basis set well reproduced the observed geometries of disiloxane (*), DZP, TZVP) gave much poorer agreement with the observed geometries.Comparison of the STO-3G^* and the STO-3G basis sets demonstrates the necessity of including d-orbitals on the silicon. However, the semi-empirical MNDO program gave, despite the absence of d-orbitals, a better approximation to the molecular geometry than the complex ab initio basis sets.Force field parameters have been calculated for k_SiOSi, k_OSiO, 0.089 and 0.73 mdyneÅ/rad², and the SiOSiO torsion which has a V₁ potential of –0.68 kcal/mol. In addition, the HSiOH torsion is shown to have a three-fold potential of 0.78 kcal/mol. These are profoundly different from the analogous carbon-oxygen force constants, demonstrating that C-O parameters cannot be transferred to the corresponding Si-O systems.     

Predicted vibrational frequencies and assignment of the spectra of uracil and its n,n-dideutero derivative by quantum chemical methods

A recent theoretical study on the vibrational spectrum and force field of maleimide (ref.1) lead to scale factors transferable to molecules of similar structure (ref.2). The theoretical vibrational spectrum of uracil (refs.2–3) calculated from scaled CNDO/2 force constants using the scale factors of ref.1 agrees better with the experimental results (ref.4) than former calculations by $\text{ab initio}$ STO-3G (ref.5) or MINDO/3 (ref.6) methods. The scaled CNDO/2 force field of uracil has also been used to predict the normal frequencies of a set of its deuterated derivatives.     

Vapor-phase Beckmann rearrangement of oxime molecules over H-Faujasite zeolite.

The Beckmann rearrangement (BR) plays an important role in a variety of industries. The mechanism of this reaction rearrangement of oximes with different molecular sizes, specifically, the oximes of formaldehyde (H₂C?NOH), Z‐acetaldehyde (CH₃HC?NOH), E‐acetaldehyde (CH₃HC?NOH) and acetone (CH₃)₂C?NOH, catalyzed by the Faujasite zeolite is investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6‐31G (d,p) basis set. To enhance the energetic properties, single point calculations are undertaken at MP2/6‐311G(d,p). The rearrangement step, using the bare cluster model, is the rate determining step of the entire reaction of these oxime molecules of which the energy barrier is between 50–70 kcal mol^?1. The more accurate embedded cluster model, in which the effect of the zeolitic framework is included, yields as the rate determining step, the formaldehyde oxime reaction rearrangement with an energy barrier of 50.4 kcal mol^?1. With the inclusion of the methyl substitution at the carbon‐end of formaldehyde oxime, the rate determining step of the reaction becomes the 1,2 H‐shift step for Z‐acetaldehyde oxime (30.5 kcal mol^?1) and acetone oxime (31.2 kcal mol^?1), while, in the E‐acetaldehyde oxime, the rate determining step is either the 1,2 H‐shift (26.2 kcal mol^?1) or the rearrangement step (26.6 kcal mol^?1). These results signify the important role that the effect of the zeolite framework plays in lowering the activation energy by stabilizing all of the ionic species in the process. It should, however, be noted that the sizeable turnover of a reaction catalyzed by the Brønsted acid site might be delayed by the quantitatively high desorption energy of the product and readsorption of the reactant at the active center.     

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 M?ller-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.     

A conformational analysis of the six isomers of thiobispyridine

The conformational behaviour of the six isomers of thiobispyridine has been investigated using ab initio STO-3G*//rigid-roto, STO-3G*//STO-3G* and 6–31G**//STO-3G* molecular orbital models. The analysis reveals both the importance of optimising critical structure parameters and the basis set dependence of calculated rotational barrier heights. The most reliable model (6–31G**//STO-3G*) clearly indicates that the minimum energy conformers are not planar and that energy barriers between 30–100 kJ mol^?1 restrict inter-conversion to planar structures, thereby preventing conjugation between the p-electrons of the sulfur atom and the π system of both pyridine rings. From the calculated barrier heights, two mechanisms can be employed to explain conformer interconversion about the C? S bond: a disrotatory one-ring flip or a conrotatory two-ring flip mechanism. Where comparisons can be made (eg. 2,2′-thiobispyridine), dipole moment calculations are shown to be in good agreement with experiment. Finally, of the six isomers, appropriately substituted 2,2′, 2,3′- and 2,4′-thiobispyridines are most prone to a Smiles rearrangment.     

Gaussian-3 and related methods for accurate thermochemistry

In this paper, we present an overview of Gaussian-3 (G3) theory, a composite technique that employs a sequence of ab initio molecular orbital calculations to derive a total energy of a given molecular species. This method provides accurate energies of molecular systems for the calculation of enthalpies of formation, ionization potentials, electron affinities, proton affinities, etc. Also covered in this review are several variants of G3 theory including one based on scale factors (G3S) and an extended version (G3X) that uses improved geometries and larger Hartree-Fock basis sets. Finally, the G3/99 test set of accurate experimental data that is used for critical assessment is described. Overall, G3 theory has a mean absolute deviation from experiment of 1.07 kcal mol⁻¹ for the G3/99 test set and G3S theory has a similar accuracy of 1.08 kcal mol⁻¹. G3X theory is significantly more accurate with the mean absolute deviation from experiment decreasing from 1.07 kcal mol⁻¹ (G3) to 0.95 kcal mol⁻¹ (G3X). The scaled version of G3X theory shows a similar improvement. Received: 23 January 2002 / Accepted: 7 April 2002 / Published online: 4 July 2002     

Ab initio SCF calculations on hydrogen bonded cresol isomers

Ab initio GAUSSIAN 80 calculations with two different basis sets (STO-3G and 4–31 G*) were performed on hydrogen bonded cresol isomers for comparison with experimental data from free jet fluorescence excitation spectroscopy. Form-cresol, the calculated barriers for hindered internal rotation of the OH-group and the CH₃-group are in good agreement with experiment. The calculations show the trans-linear configuration ofp-cresol·B-clusters (B = H₂O, CH₃OH) to be more stable than the all-planar configuration. This agrees with CI calculations and microwave spectroscopic investigations of the water dimer. Calculations of both the intermolecular stretch and bend frequencies ofp-cresol·B-clusters show little dependence on the all-planar or trans-linear configuration but a strong dependence on the choice of the basis set. With the minimal basis set STO-3G, the vibrational energies are generally too high. The agreement between the calculated vibrational frequencies from the 4–31 G* basis set and the experimental values is fair.     

Ab initio quantum mechanical calculations of energy,geometry, vibrational frequencies and IR intensities of tetraphosphorus tetrasulphide, α-P4S4(D2d), and vibrational analysis of As4S4 and As4Se4

Geometry, vibrational frequencies and IR intensities are calculated for α-P₄S₄ by scaled quantum mechanical calculations at the 6-31G*/SCF and STO-3G*/SCF levels. For both basis sets the frequencies are scaled with factors close to or equal to those found for P₄S₃, and based on these results a revised assignment is proposed. The α-P₄S₄ force field is transferred to the isostructural As₄S₄ and As₄Se₄ molecules and rescaled, and based on a good fit to experimental frequencies a new assignment is also proposed for these compounds.     

Ab initio molecular orbital calculations on the associated complexes of lithium cyanide with ammonia

Ab initio molecular orbital (MO ) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of NH₃ with Li⁺, C?N^?, LiCN, and its isomer LiNC. The BSSE -corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated (C_3v) structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia. © 1995 John Wiley & Sons, Inc.     

Ab initio study of possible intermediates in the no(4Π) catalyzed geometric isomerizations of olefins

The geometries of acyclic and three-membered ring (nitroxide) H₄C₂NO radicals in their ground ²Π electronic states have been optimized completely at ab initio UHF and ROHF theoretical levels with the STO-3G and the 6-31G** basis sets. The optimizations favour the cyclic nitroxide structure energetically. However ΔE(acyclic - cyclic) at the UHF and ROHF/6-31G** levels are only 3.2 and 1.9 kcal mol^-1, respectively. Incomplete MP2/6-311G** optimizations support these results. The zero-point energy computed at the ROHF/6-31G** level for the nitroxide radical is 2.5 kcal mol^-1 higher than that for the acyclic structure, thus reversing the relative energies by 0.6 kcal mol^-1. The energies of the two radical structures, relative to the sum of those for ethylene and NO, are very close to literature values of the activation energies for the thermal, NO catalyzed geometrical isomerizations of olefins. Thus cyclic nitroxide intermediates may play a role not only in the Hg 6(³P₁) photosensitized, but also in the thermal, NO-catalyzed geometric isomerizations of olefins. Paper dedicated to Professor Otto P. Strausz; presented in part at the 75th Canadian Chemical Congress and Exhibition, Edmonton, May 31 – June 4, 1992.     

In-plane vibrational modes of cytosine from an ab initio MO calculation

Harmonic force constants, in-plane vibrational frequencies, and in-plane vibrational modes of cytosine were calculated by an ab initio Hartree—Fock SCF MO method. The force contants were calculated by the use of an energy gardient method with the STO-3G basis set, and then they were corrected into “4-31G force constants” by the scaling factors given by us previously for the case of uracil. The corrected set of force constants can produce a calculated vibrational spectra of cytosine and cytosine-1,amino-d₃, that can be well corrected with the observed Raman and infrared spectra of these compounds, with little ambiguity. Thus, the assignments of all the in-plane vibrations are now practically established. The calculated vibrational modes, in addition, can account for the recently published resonance Raman effects of cytosine residue.     

Quantum-chemical calculation of a spillover model on a graphite support

The simplest quantum-chemical models of hydrogen spillover over a graphite-like surface as a proton or radical have been considered. The condensed planar C₂₄H₁₂ molecule was used as a model surface. Theab initio calculations of the interaction of hydrogen with the model surface were carried out by the restricted Hartree-Fock (HF) method in the STO-3G and 6-31 G^* basis sets. The radical hydrogen can not bind to such a surface, whereas the proton binds to it with an energy release of 186 kcal mol⁻¹. The activation energy of the transfer of the proton between two neighboring carbon atoms (10 kcal mol⁻¹) has been determined. The simplest model of the hydrogen migration as a proton over the model surface can be used for describing the spillover of hydrogen over the graphite surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 428–430, March, 1997.     

A theoretical study of C4H5N isomers: Comparison of cyano and isocyano as substituents

Ab initio molecular orbital calculations using a 3-21G basis set have been used to optimize geometries for pyrrole, CH₃(X)CCH₂, CH₃(H)CCHX (both cis and trans), c-C₃H₅X, and CH₂CHCH₂X, where X is CN and NC. In all the alkenyl derivatives methyl groups are found to adopt the conformation in which the methyl hydrogen eclipses the double bond. 6-31G*∥3-21G level calculations show the alkenyl cyanides to be of similar energy to pyrrole, but the isocyanides are ～20 kcal mol^?1 higher in energy. For both substituents the cyclopropyl derivatives are higher in energy by ～10 kcal mol^?1. At the 6-31G* level ring strain is 27.7 kcal mol^?1 for the cyanide and 30.6 kcal mol^?1 for the isocyanide. Data on the relative energies of RCN and RNC are compared when R is (i) a saturated hydrocarbon, (ii) an unsaturated hydrocarbon, (iii) an α-carbenium ion, (iv) an allyl cation, and (v) an α-carbanion.     

Scaled quantum chemical studies of the structural and vibrational spectra of acetyl coumarin

The solid phase FT-IR and FR-Raman spectra of acetyl coumarin have been recorded in the regions 4000–50 cm⁻¹. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimization and force field calculations based on density functional theory (DFT) and Hartree-Fock (HF) at 6–31G* and 6–311++G** basis sets. The resulting force fields were transformed to internal coordinates, the calculated vibrational frequencies and normal modes were utilized in the assignment of the observed vibrational fundamentals. The measured spectral data were used to refine the vibrational force constants by means of a small number of scaling factors.     

Assessment of density functional methods for reaction energetics: Iridium‐catalyzed water oxidation as case study

We investigate basis set convergence for a series of density functional theory (DFT) functionals (both hybrid and nonhybrid) and compare to coupled‐cluster with single and double excitations and perturbative triples [CCSD(T)] benchmark calculations. The case studied is the energetics of the water oxidation reaction by an iridium‐oxo complex. Complexation energies for the reactants and products complexes as well as the transition state (TS) energy are considered. Contrary to the expectation of relatively weak basis set dependence for DFT, the basis set effects are large, for example, more than 10 kcal mol^?1 difference from converged basis for the activation energy with “small” basis sets (DZ/6‐31G** for Ir/other atoms, or SVP) and still more than 6 kcal mol^?1 for def2‐TZVPP/6‐31G**. Inclusion of the dispersion correction in DFT‐D3 schemes affects the energies of reactant complex (RC), TS, and product complex (PC) by almost the same amount; it significantly improves the complexation energy (the formation of RC), but has little effect on the activation energy with respect to RC. With converged basis, some pure GGAs (PBE‐D3, BP86‐D3) as well as the hybrid functional B3LYP‐D3 are very accurate compared to benchmark CCSD(T) calculations. © 2012 Wiley Periodicals, Inc.     

Acetaldehyde photochemistry: Testing the additivity assumption and polarized basis set effects

Selected portions of the S₀ and T₁ potential energy surfaces of acetaldehyde surveyed in our earlier studies have been reexamined. The assumption of the additivity of basis-set polarization and of electron correlation effects used extensively in our earlier work on acetaldehyde has been tested through explicit polarized basis-set electron-correlation calculations. The “additivity assumption” introduces average absolute errors in energy differences of only 1.9 (MP3) to 3.4 (MP2) kcal mol^?1 in seven comparisons. The effects of using 6?31G^** SCF optimized geometries as opposed to single-point calculations on 3?21G SCF structures (6–31G^**//3–21G) as in our previous papers were examined. In six comparisons, the average absolute error in relative SCF energies introduced by the use of the 3–21G geometries rather than the fully consistent 6–31G^– ones was only 0.3 kcal mole^?1. After a uniform scaling procedure, comparisons of the 6–31G^** and 3–21G calculated vibrational frequencies with experiment for CH₃CHO (S₀), CH₄, and CO (20 comparisons) yielded absolute differences of 41 cm^?1 (6–31G^**) and 57 cm^?1 (3–21G). All these more elaborate calculations support for the specific case of acetaldehyde and various minima and transition states of relevance to its photochemistry, the commonly used and practically important approximations (e.g., additivity) made in our earlier studies.     

Non-empirical calculations on the conformation and hyperfine structure of the silyl radical influence of vibrational effects

An ab-initio spin-restricted SCF and perturbative configuration interaction study of the silyl radical is presented. The vibrational dependence of isotropic coupling constants is investigated using double-zeta and double-zeta plus polarization basis sets. The calculations predict a nearly tetrahedral geometry for the radical with an inversion barrier of 5.85 kcal/mol. The vibrational treatment leads to coupling constants (α_Si = ?190.20 G;α_H = +5.03 G) in excellent agreement with experiment.     

Theoretical study of the changes in the electronic structure of benzonitrile accompanying its conversion into a radical anion

The electronic structure of benzonitrile and its radical anion have been investigated at different levels of ab initio MO theory: STO-3G, 6-31G and 6-31G^**. The changes in the electronic structure of the neutral molecule accompanying its conversion into the corresponding radical anion have been estimated. It was established that the radicalization leads to significant changes in the bond lengths with double and triple bond character expressed in the conjugated system. The distribution of the total atomic charges on transition from the neutral molecule to the corresponding radical anion have been investigated using the Mulliken population analysis. The distribution of the odd electron density in the radical anion was estimated at the different basis sets: STO-3G, 6-31G and 6-31G^**. The ab initio calculations suggest that the quinoid structure is preferred for the radical anion.