O2 Binding to Heme is Strongly Facilitated by Near‐Degeneracy of Electronic States

This paper reports the computed O₂ binding to heme, which for the first time explains experimental enthalpies for this process of central importance to bioinorganic chemistry. All four spin states along the relaxed Fe? O₂‐binding curves were optimized using the full heme system with dispersion, thermodynamic, and scalar‐relativistic corrections, applying several density functionals. When including all these physical terms, the experimental enthalpy of O₂ binding (?59 kJ mol^?1) is closely reproduced by TPSSh‐D3 (?66 kJ mol^?1). Dispersion changes the potential energy surfaces and leads to the correct electronic singlet and heptet states for bound and dissociated O₂. The experimental activation enthalpy of dissociation (～82 kJ mol^?1) was also accurately computed (～75 kJ mol^?1) with an actual barrier height of ～60 kJ mol^?1 plus a vibrational component of ～10 and ～5 kJ mol^?1 due to the spin‐forbidden nature of the process, explaining the experimentally observed difference of ～20 kJ mol^?1 in enthalpies of binding and activation. Most importantly, the work shows how the nearly degenerate singlet and triplet states increase crossover probability up to ～0.5 and accelerate binding by ～100 times, explaining why the spin‐forbidden binding of O₂ to heme, so fundamental to higher life forms, is fast and reversible.     

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.     

Molecular vibrations,electronic structure and conformation of diprotonated thiocarbohydrazide

The infrared spectra of diprotonated species of thiocarbohydrazide and its perdeuterated derivative have been examined in the crystalline state. A complete vibrational assignment with a full normal coordinate treatment based on a Urey—Bradley type intramolecular potential Function supplemented with a valence force function for the out of plane and torsional modes is proposed and the origin of the amide II band splittings is explained. A CNDO/2 study of diprotonated thiocarbohydrazide and its neutral molecule is undertaken and the changes in the molecular electronic structures and conformations consequent to protonation are determined and briefly discussed. The magnitude of the N—N⁺H₃ torsional barrier is estimated to be 21 kJ mol^? (5.0 kcal mol^?1) whereas the barrier for the C—N group is found to be 92 kJ mol^?1 (22.0 kcal mol^?1).     

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.     

An investigation on the reaction mechanism of the F2+Cl2→2ClF using the B3LYP method

The reaction mechanism of F₂+Cl₂→2ClF has been investigated with the density functional theory at the B3LYP/6‐311G* level. Six transition states have been found for the three possible reaction paths and verified by the normal mode vibrational and IRC analyses. Ab initio MP2/6‐311G* geometry optimizations and CCSD(T)/6‐311G(2df)//MP2/6‐311G* single‐point energy calculations have been performed for comparison. It is found that when the F₂ (or Cl₂) molecule decomposes into atoms first and then the F (or Cl) atom reacts with the molecule Cl₂ (or F₂) nearly along the molecular axis, the energy barrier is very low. The calculated energy barrier of F attacking Cl₂ is zero and that of Cl attacking F₂ is only 15.57 kJ?mol^?1 at the B3LYP level. However, the calculated dissociation energies of F₂ and Cl₂ are as high as 145.40 and 192.48 kJ?mol^?1, respectively. When the reaction proceeds through a bimolecular reaction mechanism, two four‐center transition states are obtained and the lower energy barrier is 218.69 kJ?mol^?1. Therefore, the title reaction F₂+Cl₂→2ClF is most probably initiated from the atomization of the F₂ molecule and terminated by the reaction of F attacking Cl₂ nearly along the Cl? Cl bond. MP2 calculations lead to the same conclusion, but the geometry of TS and the energy barrier are somewhat different. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

The barrier to internal rotation of the methoxy group in anisoles: evidence from the microwave spectrum of p-fluoroanisole

The microwave spectrum of the excited states of the torsional vibration of the methoxy group in p-fluoroanisole has been investigated. The results are consistent with the gas-phase infrared torsional frequency of 70 cm^?1 and show that at least six pairs of levels lie within the potential well containing the rotamer with a planar heavy-atom skeleton. This places a lower limit of 4.6 kJ mol^?1 on the barrier to internal rotation. The microwave and gas-phase infrared spectra are not consistent with the existence of a rotamer with a non-planar heavy-atom skeleton of appreciable stability.     

Modeling the hydration and proton transport in solid electrolytes based on phenolsulfonic acids

Geometrical and energetic characteristics of crystal hydrates of individual aromatic sulfonic acids and their complexes with poly(vinyl alcohol) as well as the paths for the proton transport in them are calculated in the framework of the density functional theory (version B3LYP) employing the 6-31G^** basis set. The energy of attachment of water to ortho-substituted aromatic sulfonic acids is demonstrated to diminish from 74.4 to 54.8 kJ mol^?1 in the following series of substituents: -OH,-F,-CH₃,-H,-Cl, and -COOH. For the dimers that comprise individual phenolsulfonic acids, the energy of attachment of one water molecule to the SO₃H group is estimated to be equal to 92–105 kJ mol^?1. In the dimers comprising individual phenolsulfonic acids, the specific energy of intermolecular bonds (bond energy per monomer molecule) is found to be equal to 49.3 and 58.5 kJ mol^?1 for, respectively, phenol-2,4-disulfo and phenol-2-sulfo acids. During the formation of polymer membranes based on poly(vinyl alcohol) and phenolsulfonic acids, it is energetically favorable that at least one water molecule should remain in the vicinity of the SO₃H fragment. According to the calculations, the proton migration along the SO₃H group in anhydrous environment is hampered by a barrier of 125–132 kJ mol^?1. In the presence of water, the proton conductivity is of a relay character, with an activation barrier equal to 21–33 kJ mol^?1. The latter value is close to experimental data (17–25 kJ mol^?1).     

Reinterpretation of Dynamic Vibrational Spectroscopy to Determine the Molecular Structure and Dynamics of Ferrocene

Molecular distortion of dynamic molecules gives a clear signature in the vibrational spectra, which can be modeled to give estimates of the energy barrier and the sensitivity of the frequencies of the vibrational modes to the reaction coordinate. The reaction coordinate method (RCM) utilizes ab initio‐calculated spectra of the molecule in its ground and transition states together with their relative energies to predict the temperature dependence of the vibrational spectra. DFT‐calculated spectra of the eclipsed (D_5h) and staggered (D_5d) forms of ferrocene (Fc), and its deuterated analogue, within RCM explain the IR spectra of Fc in gas (350 K), solution (300 K), solid solution (7–300 K), and solid (7–300 K) states. In each case the D_5h rotamer is lowest in energy but with the barrier to interconversion between rotamers higher for solution‐phase samples (ca. 6 kJ mol^?1) than for the gas‐phase species (1–3 kJ mol^?1). The generality of the approach is demonstrated with application to tricarbonyl(η⁴‐norbornadiene)iron(0), Fe(NBD)(CO)₃. The temperature‐dependent coalescence of the ν(CO) bands of Fe(NBD)(CO)₃ is well explained by the RCM without recourse to NMR‐like rapid exchange. The RCM establishes a clear link between the calculated ground and transition states of dynamic molecules and the temperature‐dependence of their vibrational spectra.     

Kinetische untersuchung der reversiblen dimerisierung von o-phenylendioxidimethylsilan

The reversible dimerisation of o-phenylenedioxydimethylsilane (2,2-dimethyl-1,3,2-benzodioxasilole) has been studied by ¹H NMR spectroscopy. The kinetics of this reaction can be described quantitatively by a bimolecular 10-ring formulation reaction and a monomolecular backreaction. The thermodynamic and kinetic parameters are: ΔH⁰ = ?43 kJ mol^?1; ΔS⁰ = ?112 J mol^?1 K^?1; ΔG⁰₂₉₈ = ?9.6 kJ mol^?1; ΔH³₂₉₈ = 57 kJ mol^?1; ΔS³₂₉₈ = ?129 J mol^?1 K^?1; ΔG³₂₉₈ = 96 kJ mol^?1; E_a = 60 kJ mol^?1; A = 3.17 × 10⁶ l mol^?1 s^?1. Remarkable is the low activation energy of formation of the ten-membered ring, considering that two SiO bonds have to be cleaved during the reaction. Transition states and possible structures of the ten-membered heterocycle are discussed.     

Microwave spectra of cis and trans 2-furylisocyanate conformers

The microwave spectrum of 2-furylisocyanate has been obtained in the frequency region from 8 to 40 GHz. This spectrum is attributed to the ground state of the cis and trans configurations. The rotational constants for both ground vibrational states have been determined. Two sets of vibrational satellites are observed and assigned to the modes of C-N torsion and CNC bending. The microwave results show and MINDO/3 calculations confirm that the barrier between the cis and trans conformers is high and that the cis conformer is more stable than the trans by 3 kcal mol^?1.     

Microwave spectrum of propionyl chloride

The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the ³⁵Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the ³⁷Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm^?1. The CH₃ internal rotation frequency was found to be 197 cm^?1. Nuclear quadrupole coupling constants were determined for the ground state of the ³⁵Cl and ³⁷Cl species. From observed A-E splittings of ^bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V₃ = 2480 ± 40 cal mol^?1 (867 ± 14 cm^?1).     

The mechanism of the retro-Diels-Alder reaction in 4-vinylcyclohexene cation radical

Butadiene cation radicals are produced symmetrically from the ring and side-chain of the vinylcyclohexene cation radical near the onset of the fragmentation. The appearance energies of C₄H₆^+? and C₄H₂D₄^+? from (3,3,6,6-D₄)vinylcyclohex ene were measured as 11.07 ± 0.05 and 11.06 ± 0.06 eV, respectively. This sets the barrier to retro-Diels-Alder decomposition at 1140 kJ mol^?1 above the energy of 1 and 44 kJ mol^?1 above the thermochemical threshold corresponding to C₄H₆^+? + C₄H₆. Topological molecular orbital calculations indicate that this lowest-energy path involves a sequential rupture of the C₃C₄ and C₅C₆ bonds, with a calculated barrier of 211 kJ mol^?1. The second, two-step reaction channel proceeds by subsequent fission of the C₅C₆ and C₃C₄ bonds with a barrier of 299 kJ mol^?1. This channel is found experimentally as a break on the ionization efficiency curve at 12.1 eV. Both the supra-supra and the supra-antara pericyclic reactions go through energy maxima and are therefore forbidden. The supra-supra process is the most favorable route for decomposition from the first excited state, the activation energy being 333 kJ mol^?1. The preference for the two-step mechanism is due to hyperconjugative stabilization of intermediate molecular configurations.     

Far-IR spectrum,conformation stability,barriers to internal rotation,vibrational assignment,and ab initio calculations of 3-chloro-2-methylpropene

The far-IR spectrum from 375 to 30 cm⁻¹ of gaseous 3-chloro-2-methylpropene, CH₂=C(CH₃)CH₂Cl, has been recorded at a resolution of 0.10 cm⁻¹. The fundamental asymmetric torsional mode for the gauche conformer is observed at 84.3 cm⁻¹ with three excited states falling to lower frequency. For the higher energy s-cis conformer, where the chlorine atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm⁻¹ with two excited states falling to lower frequency. Utilizing the s-cis and gauche torsional frequencies, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in reciprocal centimeters): V₁=189±12, V₂=−358±11, V₃=886±2 and V₄=−12±2 with an enthalpy difference between the more stable gauche and s-cis conformers of 150 ±25 cm⁻¹ (430 ± 71 cal mol⁻¹). This function gives values of 661 cm⁻¹ (1.89 kcal mol⁻¹), 1226 cm⁻¹ (3.51 kcal mol⁻¹) and 812 cm⁻¹ (2.32 kcal mol⁻¹), for the s-cis to gauche, gauche to gauche, and gauche to s-cis barriers, respectively. From the methyl torsional frequency of 170 cm⁻¹ for the gauche conformer, the threefold barrier of 678 cm⁻¹ (1.94 kcal mol⁻¹) has been calculated. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3–21G^* and 6–31G^* basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G^* basis set. These data are compared with the corresponding data for some similar molecules.     

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⁺.     

Microwave spectrum,conformation, dipole moment and centrifugal distortion of glyoxylic acid

Microwave spectra of CHO-COOH and CHO-COOD are reported. The molecule has a planar equilibrium conformation with the two carbonyl groups trans to each other. A weak five-member intramolecular hydrogen bond is formed between the hydroxyl proton of the carboxyl group and the oxygen atom of the carbonyl group thus stabilizing the trans planar form. Other conformations having a statistical weight of 1 (cis and trans) are at least 1.3 kcal mol^?1 less stable, and rotamers with a statistical weight of 2 (e.g., gauche and skew) have at least 1.7 kcal mol^?1 higher energy. Four vibrationally excited states of CHO-COOH have been analyzed and relative intensity measurements yielded 167 ± 12 cm^?1 for the C-C torsional mode and 288 ± 26 cm^?1 for the lowest in-plane bending mode. The dipole moment was determined to be μ_a = 1.85 ± 0.03 D, μ_b = 0.20 ± 0.10 D, and μ_tot = 1.86 ± 0.04 D. A seven-parameter centrifugal distortion analysis has been carried out for the ground vibrational state of CHO-COOD and for the ground and three vibrationally excited states of CHO-COOH.     

Immobilization of Aspergillus nidulans SU04 cellulase on modified activated carbon

The present study deals with the immobilization of Aspergillus nidulans SU04 cellulase onto modified activated carbon (MAC). The effect of contact time, cellulase concentration, MAC dosage, and temperature for maximum immobilization percentage and immobilization capacity is investigated. The equilibrium nature of immobilization is described by Langmuir and Freundlich isotherms. The kinetic data were tested using the pseudo first order. The activation energy of immobilization was evaluated to be 11.78?J?mol^?1. Results of the thermodynamic investigation indicate the spontaneity (?G <0), slightly endothermic (?H >0), and irreversible (?S >0) nature of the sorption process. Entropy and enthalpy were found to be 41.32 J?mol^?1?mg^?1 and 10.99?kJ?mol^?1, respectively. The Gibbs free energy was found to be ?22.79?kJ?mol^?1. At 80?rpm, 323?K, 2?h, 5?mg of MAC, immobilization capacity was 4.935?mg cellulase per mg of MAC from an initial cellulase concentration of 16?mg?ml^?1 with retention of 70% of native cellulase activity up to 10 cycles of batch hydrolysis experiments. The diffusion studies that were carried out revealed the reaction rate as ??mol?min^?1. At optimized conditions, immobilized cellulase had a higher Michaelis?CMenten constant, K _m of 1.52?mmol and a lower reaction rate, V _max of 42.2???mol?min^?1, compared with the free cellulase, the K _m and V _max values of which were 0.52?mmol and 18.9???mol?min^?1, respectively, indicating the affinity of cellulase for MAC matrix.     

Erratum: Thermodynamic perturbation theory of a dense fluid of polar and polarizable spheres

A method is described for determination of the dissociation energy D₀ for hydrogen bonded dimers B…H-A using only measurements of rotational transition intensities at a single temperature. Application in the particular case HCN…HF gives D₀ = 18.5 ± 1.1 kJ mol^?1. By taking account of the vibrational modes of HCN…HF in the harmonic oscillator approximation, D_e is estimated as 25.6 ± 1.6 kJ mol^?1.     

[3]Ferrocenophane bridge reversal barriers : I. Sulphur,selenium and tellurium bridging atoms

Dynamic NMR studies have yielded accurate energy data for the bridge reversal fluxion of [3]ferrocenophanes with Group VI bridging atoms. This process, whilst appearing very analogous to the chair-to-chair reversal of corresponding 6-membered heterocyclic rings, appears to be a much higher energy process, its associated ΔG^≠ values being in the range 59 to 81 kJ mol^?1 depending on the types of Group VI bridging atoms. These data allow estimates to be made for the first time of the relative magnitudes of torsional barriers about single bonds involving like and unlike Group VI atoms. For example, the SS torsion is shown to be 3.9 kJ mol^?1 higher in energy than the SSe torsion and 5.8 kJ mol^?1 higher than the SeSe torsion. The probable mechanism of the bridge reversal process is discussed.     

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.     

Molecular motion in some radiolabelled dipeptides: a muon spin relaxation study

Activation parameters were determined for the dynamics of radicals formed by muonium addition to glycylglycine (GlyGly; H₃N⁺CH₂CONHCH₂CO₂^?) and the doubly protected alanylalanine derivative [Boc‐AlaAla‐Bz; Bu^tOCONHCH(Me)CONHCH(Me)CO—O—CH₂Ph]. GlyGly forms an adduct by muonium addition to the amide carbonyl group which isomerizes by flipping the muon between opposite sides of the molecule, requiring an activation energy of 20.4 kJ mol^?1. In Boc‐AlaAla‐Bz, muonium addition to the benzene ring of the benzyl (—CH₂Ph) group occurs, exhibiting an activation energy of 9.4 kJ mol^?1, believed to be from torsion about the C—Ph bond. Copyright © 2002 John Wiley & Sons, Ltd.