Spectra and structure of organophosphorus compounds: XVII. Infrared and Raman spectra. vibrational assignment and barriers to internal rotation for t-butylphosphine

The infrared spectra of gaseous and solid tertiary-butylphosphine, [(CH₃)₃CPH₂], have been recorded from 50 cm^?1 to 3500 cm^?1. The Raman spectra of gaseous, liquid and solid (CH₃)₃CPH₂ have been recorded from 10 to 3500 cm^?1. A vibrational assignment of the 42 normal modes has been made. A harmonic approximation of the methyl torsional barrier from observed transitions in the solid state gave a result of 4.22 kcal mol^?1 and 3.81 kcal mol^?1 in the gaseous state. Hot band transitions for the phosphino torsional mode have been observed. The potential function for internal rotation about the C-P bond has been calculated. The two potential constants were determined to be: V₃ = 2.79 ± 0.01 kcal mol^?1 and V₆ = 0.07 ± 0.01 kcal mol^?1.     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

1H and 13C NMR studies of some aromatic dilactones (precursors of paracyclophanes)

Carbon-13 and proton NMR data of macrocyclic diaromatic dilactones are presented. The observed behaviour of the spectra as a function of temperature shows that the energy barrier for the re-orientation of the side chains is lower than 49 kJ mol^?1 (12 kcal mol^?1) and that the energy barrier for the rotation of the aromatic rings is larger than 99 kJ mol^?1 (24 kcal mol^?1). Hence, chiral substituted dilactones of this type will be resolvable, and the enantiomers can be easily handled at room temperature.     

An ab initio molecular orbital study of the potential energy surface of the N2H → N2 + H system

The N₂H potential energy surface has been examined by ab initio molecular orbital theory using the 6-31G^** basis set with correlation energy evaluated by Møller—Plesset perturbation theory to fourth order. The ΔE for N₂H → N₂ + H is ?14.4 kcal mol^?1 and the barrier to dissociation is 10.5 kcal mol^?1. Inclusion of zero-point vibrational energies reduces the barrier to 5.8 kcal mol^?1.     

The methylamine radical cation [CH3NH2]+˙ and its stable isomer the methylenammonium radical cation [CH2NH3]+˙

The potential energy surface for the [CH₅N]^+˙ system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence-electron correlation. Two [CH₅N]^+˙ isomers can be distinguished: the well known methylamine radical cation, [CH₃NH₂]^+˙, and the less familiar methylenammonium radical cation, [CH₂NH₃]^+˙. The latter is calculated to lie 8 kJ mol^?1 lower in energy. A substantial barrier (176 kJ mol^?1) is predicted for rearrangement of [CH₂NH₃]^+˙ to [CH₃NH₂]^+˙. In addition, a large barrier (202 kJ mol^?1) is found for loss of a hydrogen radical from [CH₂NH₃]^+˙ via direct N—H bond cleavage to give the aminomethyl cation [CH₂NH₂]⁺. These results are consistent with the existence of the methylenammonium ion [CH₂NH₃]^+˙ as a stable observable species. The barrier to loss of a hydrogen radical from [CH₃NH₂]^+˙ is calculated to be 140 kJ mol^?1.     

A CNDO/2 study of the OH torsion in phenol and its hydrogen bonded complex with pyridine

The CNDO/2 molecular orbital method has been applied to the study of the OH torsion, in phenol and phenol—pyridine hydrogen bonded complex. The calculated torsional barrier (13.58 kJ mol^?1) and force constant (5.4 × 10^?20 J rad^?2) of phenol agree well with the experimental quantities. The calculated force constant of the corresponding vibration in phenol—pyridine is increased sixfold, reproducing closely the rise in the torsional frequency observed when phenol is complexed to strong acceptors. It is shown that according to CNDO theory, most of the increase can be attributed to the influence of the intermolecular force field and not to a major change in the torsional force constant.     

Incoherent inelastic neutron scattering studies of the torsional modes of hexachloroethane

From incoherent inelastic neutron scattering studies of solid C₂Cl₆ the in- and out-of-phase torsions about the C-C axis are assigned at 56 and 95 cm^?1, respectively. Using a model for the potential barriers in the solid the torsional frequency in the gas has been calculated to be 76.7 cm^?1 and the internal barrier to be 67.8 kJ mol^?1.     

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.     

A structural and theoretical study of the intermolecular interactions in 8‐hydroxyquinolinium‐7‐carboxylate monohydrate

In the title compound, C₁₀H₇NO₃·H₂O, the zwitterionic organic molecules and the water molecules are connected by N—H...O and O—H...O hydrogen bonds to form ribbons, and π–π stacking interactions expand these ribbons into a three‐dimensional net. The energies of these hydrogen bonds adopt values typical for mildly weak interactions (3.33–7.75 kcal mol⁻¹; 1 kcal mol⁻¹ = 4.184 kJ mol⁻¹). The total π–π stacking interactions between aromatic molecules can be classified as mildly strong (energies of 15.3 and 33.9 kcal mol⁻¹), and they are made up of multiple constituent π–π interactions between six‐membered rings. The short intermolecular C—H...O contact between two zwitterionic molecules is nonbonding in character.     

The vibrational analysis and torsional study of trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane

The infrared spectra of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane have been recorded between 4000 and 200 cm^?1 in the polycrystalline solid phase, and 4000 to 80 cm^?1 in the gas phase. The Raman spectra of these two compounds in the gaseous and liquid phases were also recorded between 3100 and 10 cm^?1. An assignment of the thirty-nine fundamental vibrations for both cis- and trans-1,2-dimethylcyclopropane is proposed, and comparisons are made with the vibrations of other similar molecules. Additionally, ten torsional transitions were observed in the far infrared and Raman spectra of cis-1,2-dimethylcyclopropane, and four transitions were observed in the spectra of the trans compound. From these spectral data, torsional barriers were determined. The effective barriers to methyl torsion are 2.92 kcal mol^?1 (12.20 kJ mol^?1) for cis-1,2-dimethylcyclopropane and 2.61 kcal mol^?1 (11.14 kJ mol^?1) for trans-1,2-dimethylcyclopronane.     

Conformational analysis of N-methyl-m-fluoroaniline by microwave spectroscopy

Conformational analysis of N-methyl-m-fluoroaniline has been performed by low resolution microwave spectroscopy. Two rotational isomers, corresponding in a near-planar configuration to the m-fluorine being either cis or trans with respect to the amino hydrogen, have been detected. The energy difference is found to be 270 ± 70 cal mol^?1, the cis isomer being the more stable. Ab initio calculations indicate a barrier height for the internal rotation of the HNCH₃ group around the C_ph—N bond of 9.04 kcal mol^?1.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

The microwave spectrum and large-amplitude vibrations of N-methylaniline

Microwave spectra of the normal and amine-deuterated isotopic species of N-methyl-aniline in the ground and some low-lying excited vibrational states have been observed. The inertial defect indicates that the dihedral angle of the N-CH₃, bond with respect to the ring plane is somewhat less than that for the N—H bonds in aniline. The position of the amino-hydrogen atom is very poorly determined by the isotopic substitution method because of large zero-point effects. The excited vibrational states are consistent with a double-minimum potential for the inversion of the HNCH₃ group and there is some evidence for a lower barrier than in aniline. The excited states of the HNCH₃ torsion indicate a barrier in the range 8 < V₂ < 25 kJ mol^?1 to the internal rotation of this group. No splitting of the ground-state lines attributable to the torsion of the methyl group has been observed, which implies a barrier of V₃ > 8 kJ mol^?1.     

Strain energy minimization study of the mechanism of,and the barrier to,conformational interconversion in five-membered diamine chelate rings

The method of Lagrangian multipliers is used to constrain torsion angles during molecular mechanics refinement for the purpose of plotting strain energy against a reaction coordinate. A complete two-dimensional analysis of the conformational interconversion from δ- to λ-[Co(ethane-1,2-diamine) (NH₃)₄]³⁺ reveals a mechanism in which the transition state geometry has an envelope conformation and an inversion barrier of 15.7 kJ mol^?1. Substitution at the carbon atoms, variation of the metal-nitrogen distance, and replacement of the amine ligands with bidentate amines only slightly alters the inversion barrier. Substitution at the nitrogen atoms of the bidentate ligand increases the inversion barrier significantly to 24.6 kJ mol^?1 for (N,N,N′,N′-tetramethylethane-1,2-diamine) [(NH₃)₄]³⁺.     

A density functional theory investigation on the mechanism and kinetics of dimethyl carbonate formation on Cu2O catalyst

A theoretical analysis about the mechanism and kinetics of dimethyl carbonate (DMC) formation via oxidative carbonylation of methanol on Cu₂O catalyst is explored using periodic density functional calculations, both in gas phase and in solvent. The effect of solvent is taken into account using the conductor‐like screening model. The calculated results show that CO insertion to methoxide species to produce monomethyl carbonate species is the rate‐determining step, the corresponding activation barrier is 161.9 kJ mol^?1. Then, monomethyl carbonate species reacts with additional methoxide to form DMC with an activation barrier of 98.8 kJ mol^?1, above reaction pathway mainly contributes to the formation of DMC. CO insertion to dimethoxide species to form DMC is also considered and analyzed, the corresponding activation barrier is 308.5 kJ mol^?1, suggesting that CO insertion to dimethoxide species is not competitive in dynamics in comparison with CO insertion to methoxide species. The solvent effects on CO insertion to methoxide species involving the activation barriers suggest that the rate‐determining step can be significantly affected by the solvent, 70.2 kJ mol^?1 in methanol and 63.9 kJ mol^?1 in water, which means that solvent effect can reduce the activation barrier of CO insertion to methoxide species and make the reaction of CO insertion to methoxide in solvents much easier than that in gas phase. Above calculated results can provide good theoretical guidance for the mechanism and kinetics of DMC formation and suggest that solvent effect can well improve the performance of DMC formation on Cu₂O catalyst in a liquid‐phase slurry. © 2012 Wiley Periodicals, Inc.     

First Principles (DFT) Characterization of RhI/dppp‐Catalyzed CH Activation by Tandem 1,2‐Addition/1,4‐Rh Shift Reactions of Norbornene to Phenylboronic Acid

The C?H activation in the tandem, “merry‐go‐round”, [(dppp)Rh]‐catalyzed (dppp=1,3‐bis(diphenylphosphino)propane), four‐fold addition of norborene to PhB(OH)₂ has been postulated to occur by a C(alkyl)?H oxidative addition to square‐pyramidal Rh^III?H species, which in turn undergoes a C(aryl)?H reductive elimination. Our DFT calculations confirm the Rh^I/Rh^III mechanism. At the IEFPCM(toluene, 373.15 K)/PBE0/DGDZVP level of theory, the oxidative addition barrier was calculated to be 12.9 kcal mol^?1, and that of reductive elimination was 5.0 kcal mol^?1. The observed selectivity of the reaction correlates well with the relative energy barriers of the cycle steps. The higher barrier (20.9 kcal mol^?1) for norbornyl–Rh protonation ensures that the reaction is steered towards the 1,4‐shift (total barrier of 16.3 kcal mol^?1), acting as an equilibration shuttle. The carborhodation (13.2 kcal mol^?1) proceeds through a lower barrier than the protonation (16.7 kcal mol^?1) of the rearranged aryl–Rh species in the absence of o‐ or m‐substituents, ensuring multiple carborhodations take place. However, for 2,5‐dimethylphenyl, which was used as a model substrate, the barrier for carborhodation is increased to 19.4 kcal mol^?1, explaining the observed termination of the reaction at 1,2,3,4‐tetra(exo‐norborn‐2‐yl)benzene. Finally, calculations with (Z)‐2‐butene gave a carborhodation barrier of 20.2 kcal mol^?1, suggesting that carborhodation of non‐strained, open‐chain substrates would be disfavored relative to protonation.     

Fluorescence à 77 K,rotation interne et structure electronique du styréne

A vibronic analysis of the fluorescence spectrum of styrene in crystalline solution at 77 K corroborates the assignment of the O.O band at 34758 cm^?1 for gas-phase spectra. The torsional frequency of the vinyl group with respect to the phenyl ring is used to evaluate the internal rotation barrier of styrene (5.7 kcal mol^?1). Configuration interaction MO calculations including all the mono and di-excilations within the π-electron system of styrene suggest that the first π—π^* transition has a moment tilted by 10° with respect to the long axis of the molecule; this is in agreement with the rotational structure of the gas-phase band at 34758 cm^?1, the analysis of which results in an A-type band.     

Estimation of the torsional potential for perfluorodi-methyl ether from electron-diffraction data

A structural model is obtained for perfluorodimethyl ether, based on the assumption of an equilibrium geometry with the effects of internal rotation appearing in the observed vibrational amplitudes. By separation of frame vibrations obtained from a spectroscopie force field, explicit formulae for calculating the torsional contributions from single and double rotors indicate a three-fold barrier height of 6.0 ± 1.5 kcal mol^?1. With this barrier height, the observed torsional displacement away from the COC equilibrium plane is found to be a significant feature of the model.     

The synthesis and conformational analysis of tetrahydro-1,2,4-oxadiazines

The synthesis and variable temperature ¹H and ¹³C NMR spectra of three tetrahydro-1,2,4-oxadiazines are reported. The N(4)-Me inversion barriers are 6.8–7.0 (ax→ts) and 7.4–7.9 kcal mol^?1 (eq→ts) with ΔG° 0.6–0.9 kcal mol^?1. The N(2)-Me inversion barriers are 10.4–11.4 (ax→ts) and 11.6–13.1 kcal mol^?1 (eq→ts) with ΔGδ 1.2–1.7 kcal mol^?1. The barrier to ring inversion is ca. 12.7 kcal mol^?1. “R value” analysis shows the ring to have a 56.5±2δ dihedral angle about the C(5)-(6) bond, indicative of the expected chair conformation.     

Remarks on the analysis of torsional energy surfaces of cycloheptane and cyclooctane by molecular mechanics

A modified version (MM 2′) of the Allinger's 1977 force field is checked against cycloheptane and cyclooctane. Cycloheptane is characterized by two pseudorotating itineraries, chair/twist-chair and boat/twist-boat, separated by a barrier of 8.5 kcal mol^?1. The activation energy in the C/TC pseudorotation is estimated to be 0.96 kcal mol^?1, while B and TB transform into each other freely at an energy level 3.8 kcal mol^?1 above the global energy minimum (TC). With cyclooctane the lowest energy is calculated for the boat-chair form which participates in a pseudorotational process with TBC through a saddle point lying 3.5 kcal mol^?1 above BC. The chair/chair and boat/boat families contain only one local minimum, crown and BB, respectively, on the MM 2′ surface. The results are presented as an illustration for quick coverage of torsional energy surface by two-bond driver calculation with the block-diagonal Newton–Raphson minimization, followed by the force search of stationary points by full-matrix Newton–Raphson optimization.