Molecular conformational structures of 2-fluorobenzoyl chloride, 2-chlorobenzoyl chloride, and 2-bromobenzoyl chloride by gas electron diffraction and normal coordinate analysis aided by quantum chemical calculations

The gas phase molecular structures and conformational compositions of 2-fluorobenzoyl chloride, 2-chlorobenzoyl chloride, and 2-bromobenzoyl chloride have been investigated using gas electron diffraction data obtained from experiments performed in the laboratories of the University of Oslo and Oregon State University. The refinements on the experimental data have been aided by normal coordinate calculations as well as extensive ab initio molecular orbital and density functional theory calculations up to the levels of MP4(SDQ) and B3LYP with larger basis sets up to the level of 6-311 + G(2d,p) for the computed molecular geometries, electronic energies, vibrational zero-point energies and entropy corrections, gas mixture conformational compositions, and MP2(fc) quantum mechanical force fields. The three title molecules each exist in the gas phase as two stable non-planar conformers anti and gauche with respect to the halogen atom positions with anti the lower energy conformer in each case. Among the three title molecules there have been found considerable experimental and theoretical support for several trends in molecular or conformational behavior with increasing ortho halogen atomic size: An increasing although disputable trend in the C=O bond distance values; an increasing trend in the average phenyl ring C–C bond distance values; an increasing trend in the contribution of the gauche conformer to the gaseous mixture lowering the standard free energy difference values (ΔG ^o) correspondingly; and an increasing deviation from full planarity (C _s symmetry) in both the anti and the gauche conformers of the title molecules with increasing ortho halogen atomic size. Only in the anti conformer of 2-fluorobenzoyl chloride does the experimental data refinements suggest close to full planarity for these 2-halobenzoyl chloride molecules.     

The gauche-trans transformation of monoethanolamine in gas phase

Monoethanolamine is a model molecule for biological systems and widespread ligand in coordination chemistry; it has a large number of isomers, of which gauche and trans are the most stable in gas and crystalline phases, respectively. The work is devoted to non-empirical study of conformational transformations of different gauche and trans isomers of individual monoethanolamine molecule. These transformations was shown to be determined by the features of formation and cleavage of intramolecular hydrogen bonds O-H?N and N-H?O and the internal rotation of substituted methyl groups. Assessments were obtained for the free energies of conformers and activation energies of their mutual transformations.     

Dynamic structure of organic compounds in solution according to NMR data and quantum mechanical calculations: I. Soman

Dynamic structure of Soman diastereoisomers has been studied with the goal of obtaining accurate information to simulate molecular mechanisms of its action on living systems. The potential energy surface for internal rotation about the single P–O and O–C bonds has been constructed in terms of the Møller–Plesset second-order perturbation theory using 6-311G(d,p) basis set. The relative contributions of different conformers have been estimated by solving the vibrational problem according to the large-amplitude vibration model. The conformational dependences of the ⁴J_CF and ³J_CP coupling constants for the S,S and S,R diastereoisomers of Soman have been calculated at the FPT DFT B3LYP/6-311++G(2df,2p) level of theory. The calculated vibrationally averaged coupling constants have been compared with the available experimental data to determine the structure of the most toxic Soman stereoisomer.     

A mathematical expression for the enantioselectivity and thermodynamic factors in the conformational equilibrium of catalysts in the addition of diethylzinc to benzaldehyde

A mathematical expression for the enantioselectivity and thermodynamic factors is presented in the conformational equilibrium of a flexible chiral catalyst for the asymmetric addition of diethylzinc to aldehydes. The results show that the total enantioselectivity of the catalyzed reaction is not only governed by the free energy difference of two ground-state conformers, as well as the free energies of activation for the major enantiomeric product formed from each of the catalyst’s conformations, but also by the difference in the transition state energies of the formation of the (S)- and (R)-enantiomers from the individual conformers of the catalyst.     

An experimental and ab inttio molecular orbital study of benzaldehyde and isomeric phthalaldehydes

Analysis of the dipole moments of phthalaldehyde, isophthalaldehyde and terephthalaldehyde in benzene at 13.7 and 54.3°C indicates that the first two molecules exist mainly in a cis-trans conformation, and that the third is present as a nearly equimolecular mixture of cis and trans conformera. For isophthalaldehyde and terephthalaldehyde, these results are supported by the total molecular energies of the conformers obtained using STO-3G ab initio molecular orbital calculations. The π-electron distribution in benzaldehyde and all planar conformers of the three isomeric phthalaldehydes, the dipole moments and ^πc-co Mulliken overlap populations have also been calculated and are briefly discussed.     

Ionization Energies and Dyson Orbials of Allyl Alcohol and Allyl Mercaptan Conformers

The conformers of allyl alcohol and allyl mercaptan were studied with B3LYP/aug-cc-pVTZ method. Their relative energies were calculated at MP3, MP4(SDQ), and CCSD(T) levels. The most stable conformers for these two molecules are Gauche-gauche' (Gg'). The theo-retical photoelectron spectra simulated with the calculated ionization energies demonstrate that there are at least four conformers in allyl alcohol and four conformers in allyl mercaptan in the gas-phase experiments. The Dyson orbitals of the highest occupied molecular orbital (HOMO) and the next HOMO (HOMO-1) of allyl mercaptan Gg' conformer show strongly mixing n_S and π_C=C characteristics, which may be due to the resonance and inductive effects between π_C=C and n_S in HOMO-1 and HOMO.     

Structure and conformational dynamics of the dicyclopropyl ketone in the ground electronic state

Geometric parameters, harmonic and anharmonic vibrational frequencies, conformer energy differences and barriers to internal rotation were obtained for dicyclopropyl ketone (DCPK) in the ground electronic state through MP2, DFT, CCSD and CCSD(T) calculations. VFPA was used to improve the estimations of conformer energy differences and heights of barriers to internal rotation. The ab initio calculations demonstrated that there are three stable conformations of DCPK: the cis–cis, the cis–trans and the gauche–gauche. The energy of the gauche–gauche conformer is noticeably higher than the energy of the two other conformers, so this conformer was not found experimentally. To study the conformational dynamics of the DCPK molecule, one- and two-dimensional sections of the potential energy surface corresponding to the rotations of the cyclopropyl groups were calculated. These sections were used to calculate torsion transition energies and vibrational wave functions in anharmonic approach. It was found that there is a strong coupling of large-amplitude torsion motions in the area of the cis–cis and gauche–gauche conformers.     

A force-field study of the conformational characteristics of the iduronate ring

Methods of molecular mechanics were applied to investigate the conformation of the (methyl 2-O-sulfate-4-methyl-α-L-idopyranose) uronic acid (DMIS), in order to correlate the peculiar vicinal proton coupling constants observed in polysaccharides containing the iduronate ring to the conformational characteristics of this sugar ring. We found three conformers with comparable energies, namely the two chair forms ¹C₄ and ⁴C₁ and the skew-boat form ²S₀(L); the latter is separated from each chair form by a barrier of about 9 kcal/mol. Along the pseudorotational path three additional minima (³S₁, ¹S₃, and ¹S₅) were found, yet at least 4 kcal/mol higher than ²S₀. The results obtained for the relative energies of the three conformers and the conformation of the side groups were affected by the inclusion of the electrostatic term and, in particular, by the charge assigned to the ionic groups of DMIS. However, the conformational properties of the idopyranosidic ring in DMIS (and in related compounds) should still be interpreted in terms of equilibrium among these three conformers only.     

An ab initio study of some structural features and the rotational barriers in performic acid,formic acid and hydrogen peroxide

Calculations on performic acid at the 4-31G level, with and without bond functions with complete geometry optimization, and at the (9, 5) level, with and without polarization functions and rigid rotation, all give no sign of a well in the potential energy curve for rotation about the O/O bond axis in the region of 50° – 90° ; and all but the unaugmented 4-31G basis set find the cis-cis planar conformer to be the most stable form. Calculations at the (9,5) level with rigid rotation find the energies of the other planar conformers, relative to the cis-cis conformer, to be 0.94, 1.50 and 14.80 kcal mol^?1 for the trans-trans, cis-trans and trans-cis structures respectively. These energies and also that for the barrier separating the cis-cis and cis-trans conformers, 1–2 kcal mol^?1, are discussed in relation to corresponding data for formic acid, hydrogen peroxide and several planar four heavy-atom molecules. Dipole moment calculations using the same basis sets would seem to favor a skew conformation as the most stable for performic acid, but comparisons between calculated and experimental values for formic acid and for hydrogen peroxide cast doubt on the validity of such results.     

Ab initio- and density-functional studies of conformational behaviour of N-formylmethionine in gaseous phase

The current work is a study of the conformational space of the non-ionic N-formylmethionine molecule around its seven structurally significant internal backbone torsional angles at B3LYP/6-31++G(d,p) levels of theory in the gaseous phase. The potential energy surface exploration reveals that a total of 432 different conformers would result if all the possible combinations of the internal rotations were to be considered. A set of twelve conformers of the N-formylmethionine molecule are then further analysed in terms of their relative stabilities, theoretically predicted harmonic vibrational frequencies, HOMO-LUMO energy gaps, ESP charges, rotational constants and dipole moments calculated using MP2/6-31++G(d,p) and B3LYP/6-311++G(d,p) levels. The calculated relative energy-range of the conformers at the MP2 level is 11.08 kcal mol^?1 (1 kcal = 4.1868 kJ), whereas the same obtained at the B3LYP level is 10.02 kcal mol^?1. The results of this study provide a good account of the role of four types of intramolecular H-bonds, namely O…H—O, O…H—N, O…H—C and N…H—C, in influencing the energies of the conformers as well as their conformational and vibrational spectroscopic aspects. The relative stability order of the conformers appears to depend on the level of theory used while the vibrational frequencies calculated at the B3LYP level are in better agreement with the experimental values.     

An ab initio study of the geometry and energy of the four planar conformers of glyoxylic acid and the glyoxylate ion

The structures and energies of the four planar conformers of glyoxylic acid and the glyoxylate ion have been studied ab initio using the unscaled 4—31G basis set with full geometry optimization. Changes in the CO, OH and CO bond lengths in the conversion of the cc conformer into the ct and tt conformers, and into the tc conformer, are consistent with the formation of four-membered and five-membered hydrogen-bonded ring structures, respectively. Changes in the distances between the nearest non-bonded atoms around each C atom reveal that the internal geometry of the CHO and COOH groups is significantly affected by cis—trans isomerization with respect to the OCCO backbone, and that the geometry of the CHO group is affected by proton dissociation from the COOH group. Furthermore, the movement of the component atoms in each functional group, characterized as clockwise or anticlockwise about the C atom, results in some cases in a rotation of the functional group as a whole. Whereas experiment shows the tc conformer to be more stable than the tt by 1.2 ± 0.5 kcal mol^?1, the calculations find the tt conformer to be the most stable, separated in energy from the ct, tc and cc conformers by 0.4, 1.4 and 10.7 kcal mol^?1, respectively. Augmentation of the 4—31G basis set in several forms, and use of (9,5/4) and (9,5/4,1) basis sets, only served to decrease slightly the tt/tc energy difference, not change the sign. The calculated proton affinity of the glyoxylate ion with respect to the tt conformer is 342.7 kcal mol^?1, compared to 357.7 kcal mol^?1 for the formate ion. The expectation energy differences Δ V_nn, Δ V_ee and Δ V_en for the cis—trans isomerization of the ct and cc conformers are opposite in sign to those for the glyoxal reaction, and in magnitude they all far exceed the ΔE_T values, which shows that hydroxyl group substitution has a much greater influence than a comparison of only the ΔE_T values would suggest.     

Potential functions of internal rotation in <Emphasis Type="Italic">n</Emphasis>-alkanes and its contribution to thermodynamic properties

The potential functions of braked internal rotation V(?) in n-alkanes (ethane, propane, butane, n-pentane, n-hexane, n-heptane) were calculated by ab initio and DFT methods with the 6-311++G(3df,3pd) basis set. The functions were approximated as a series of six cosines. The dependences of V(?) on the length of the hydrocarbon chain in n-alkanes were analyzed. The heights of the trans-cis and trans-gauche barriers and the differences between the energies of the trans and gauche conformers were calculated and compared with the experimental data. From the calculated geometric parameters and V(?), the contributions of the braked internal rotation to the enthalpy, entropy, heat capacity, and Gibbs free energy at 298 K were determined. The contributions of internal rotations are transferable within the framework of additive approaches. The generalized function V _av(?) for n-alkanes and averaged contributions of internal rotation of the C-C bonds and CH₃- and -CH₂- tops to the thermodynamic properties were suggested.     

Structure and conformational dynamics of the cyclopropanecarbaldehyde molecule in the ground (S<Superscript>b0</Superscript>) and lowest excited (T<Superscript>b1</Superscript> and S<Superscript>b1</Superscript>) electronic states

The structure and conformational dynamics of nonrigid cyclopropanecarbaldehyde (CPCA) molecule in the ground (S^b0) and lowest excited triplet (T^b1) and singlet (S^b1) electronic states were calculated using the MP2, DFT, CASSCF, CASPT2, and CCSD quantum chemical methods. According to ab initio calculations, in the S^b0 electronic state there are two symmetrical (cis and trans) conformers of the CPCA molecule. Excitation of the CPCA molecule to the ?1 and S1 electronic states causes significant structural changes: carbonyl CCHO fragment becomes nonplanar, cyclopropane fragment rotates around the C–C bond, thus changing the relative positions of the formyl and cyclopropane fragments. Using sections of the potential energy surfaces (PES) of the CPCA molecule in the T^b1 and S^b1 states, we calculated the torsion and inversion wave functions and vibrational transition energies. The results obtained suggest a strong coupling of the torsion and inversion motions in the T^b1 and S^b1 excited states.     

S1←S0 vibronic spectrum and structure of fluoral in the S1 state

The vibronic absorption spectrum of fluoral vapor was studied in the region of the S₁←S₀ electronic transition (313–360 nm). The origin O₀ ⁰⁺) of the transition (29419 cm⁻¹) and a number of fundamental frequencies in the S₀ and S₁ states were determined. The character of intensity distribution in the spectral bands indicates that the electronic excitation leads to significant change of the CF₃ group orientation relative to the molecular frame. Moreover, it was found that the carbonyl fragment of the molecule in the S₁ state has pyramidal structure (in contrast, the carbonyl fragment of the fluoral molecule in the S₀ state is planar). The experimental torsion and inversion energy levels were used for the calculation of internal rotation and inversion potential functions of fluoral molecule in the S₁ state. The potential barriers to internal rotation and inversion were found to be 1270 cm⁻¹ (15.2 kJ mol⁻¹) and 550 cm⁻¹ (6.6 kJ mol⁻¹), respectively. The conformational changes caused by S₁←S₀ electronic excitation in the fluoral molecule are similar to those observed in acetaldehyde and biacetyl molecules and differ from the conformational behavior of hexafluorobiacetyl molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 294–299, February, 1998.     

A quantum-chemical study of the structure and conformational dynamics of the acrolein molecule in the ground electronic state

The geometric parameters (including vibrationally averaged parameters), energy differences (ΔE) between the s-cis and s-trans conformers, and barrier to internal rotation (V _t ) were calculated for the acrolein molecule CH₂=CHCHO by various quantum-chemical methods (MP2, DFT, CASSCF, QCISD, CCSD(T), and MR-AQCC). The MP2 and B3LYP methods were used to calculate internal rotation potential functions and vibrational frequencies; the calculations were performed in various anharmonic approximations. To refine the ΔE and V _t values, two-dimensional (using a basis set of atomic orbitals) VFPA extrapolation procedure was applied, which allowed the results to be estimated in the FCI/CBS approximation taking into account nonadiabaticity, core correlation effects, and changes in the difference between zero point energies.     

A scheme estimating the energy of intramolecular hydrogen bonds in diols

The relative energies of conformers of 1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol are split into a sum of five different terms including the intramolecular OH?O interaction. This scheme allows to estimate the energy of the O-H?O intramolecular hydrogen bond of the tGG′g and gGG′g conformers of 1,3-propanediol, the g′GG′Gt and g′GG′Gg conformers of 1,4-butanediol, and the energy of the non-bonded O-H?O interaction in the g′Gt, g′Gg and g′Gg′ conformers of 1,2-ethanediol. This scheme provides pure hydrogen bond energies without assuming the geometry and/or electronic features to be constant between the conformation having a IHB and a reference conformation. The fitted energies show a perfect linear correlation with the corresponding r(H?O)⁻¹ values. QTAIM atomic electron population and energies of the donor hydrogen calculated along the H-O-C-C internal rotation are found to be linearly correlated. These linear correlations display small changes at the BCP formation in 1,3-propanediol.     

Torsional transitions and barrier to internal rotation of 1,2-butadiene

The far i.r. spectrum of 1,2-butadiene (methyl allene) has been recorded in the gas phase from 370 to 40 cm⁻¹ with a resolution of 0.1 cm⁻¹. The methyl torsional fundamental has been observed for the first time at 154.3 cm⁻¹, along with some accompanying torsional hot bands. From these data the barrier to internal rotation has been calculated to be 556 cm⁻¹ (1.59 kcal/mol). Detailed K-structure has also been observed for both A—A and E—E torsional transitions and considered in the analysis. SCF calculations have been made for the structure and energies of conformers, so that both kinetic and potential constants for internal rotation have been obtained. The a′ skeletal fundamental is observed at 201.8 cm⁻¹ as a much stronger band than the torsional mode, and the a″ skeletal fundamental gives rise to an even stronger band at 319.8 cm⁻¹.     

Theoretical conformational analysis of unsaturated phospines and phosphinechalcogenides

A theoretical conformational analysis was performed for vinylphosphine CH₂=CHPH₂ and three vinylphosphinechalcogenides CH₂=CHPXH₂ (X = O, S, Se). According to the quantum-chemical calculations on the level of the second order perturbation theory MP2/6-311G** the prevailing conformations, their molar ratios, and relative energies were established for each compound. The analysis of the angular distribution of the probability density for the population of the rotational conformations calculated proceeding from the potential curves of the internal rotation made it possible to establish that each compound from this series existed as a mixture of two conformers, planar s-cis and twice degenerate orthogonal. The conformers transform into each other through the corresponding transition states whose nature was established as a result of the harmonic vibration analysis carried out in every case.     

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid–difluoroacetic acid adduct (CH₃COOH–CHF₂COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ?CHF₂ group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V₃(trans)=99.8(3) and V₃(gauche)=90.5(9) cm^?1 (where V₃ is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the ?CHF₂ group between its two equivalent conformations. The corresponding B₂ barrier is estimated to be 108(2) cm^?1. The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.     

Stereochemistry of seven-membered heterocycles: XLVI. Synthesis and dynamic <Superscript>13</Superscript>C NMR spectroscopy of spiro[cyclohexane-1,4′-[3,5]dioxabicyclo[5.1.0]octanes]. DFT calculations of structurally related formaldehyde and acetone acetals

Spiro[cyclohexane-1,4′-[3,5]dioxabicyclo[5.1.0]octanes] were synthesized, and their conformational behavior was studied by dynamic ¹³C NMR spectroscopy. Anancomeric displacement of conformational equilibrium toward two nonequivalent twist conformers with close energies was revealed. The relative Gibbs energies ΔG ^o and enthalpies of formation ΔH ^o of twist and chair-like conformers with endo and exo orientation of the three-membered ring of structurally related formaldehyde and acetone acetals were calculated in terms of the density functional theory at the B3LYP/6-31G(d,p) level. Like spiro-cyclohexane analogs, they were shown to have a non-chair conformation.