Intramolecular hydrogen bond, molecular structure and vibrational assignment of tetra-acetylethane. A density functional study

The intramolecular hydrogen bond, molecular structure and vibrational frequencies of tetra-acetylethane have been investigated by means of high-level density functional theory (DFT) methods with most popular basis sets. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were recorded in the regions 400-4000 cm(-1) and 40-4000 cm(-1), respectively. The calculated geometrical parameters of tetra-acetylethane were compared to the experimental results of this compound and its parent molecule (acetylacetone), obtained from X-ray diffraction. The O...O distance in tetra-acetylethane, about 2.424A, suggests that the hydrogen bond in this compound is stronger than acetylacetone. This conclusion is well supported by the NMR proton chemical shifts and O-H stretching mode at 2626 cm(-1). Furthermore, the calculated hydrogen bond energy in the title compound is 17.22 kcal/mol, which is greater than the acetylacetone value. On the other hand, the results of theoretical calculations show that the bulky substitution in alpha-position of acetylacetone results in an increase of the conjugation of pi electrons in the chelate ring. Finally, we applied the atoms in molecules (AIM) theory and natural bond orbital method (NBO) for detail analyzing the hydrogen bond in tetra-acetylethane and acetylacetone. These results are in agreement with the vibrational spectra interpretation and quantum chemical calculation results. Also, the conformations of methyl groups with respect to the plane of the molecule and with respect to each other were investigated.     

Structure and vibrational assignment of the enol form of 1,1,1-trifluoro-2,4-pentanedione

Molecular structure of 1,1,1-trifluoro-pentane-2,4-dione, known as trifluoro-acetylacetone (TFAA), has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of acetylacetone (AA) and hexafluoro-acetylacetone (HFAA). The harmonic vibrational frequencies of both stable cis-enol forms were calculated at B3LYP level of theory using 6-31G** and 6-311++G** basis sets. We also calculated the anharmonic frequencies at B3LYP/6-31G** level of theory for both stable cis-enol isomers. The calculated frequencies, Raman and IR intensities, and depolarization ratios were compared with the experimental results. The energy difference between the two stable cis-enol forms, calculated at B3LYP/6-311++G**, is only 5.89 kJ/mol. The observed vibrational frequencies and Raman and IR intensities are in excellent agreement with the corresponding values calculated for the most stable conformation, 2TFAA. According to the theoretical calculations, the hydrogen bond strength for the most stable conformer is 57 kJ/mol, about 9.5kJ/mol less than that of AA and about 14.5 kJ/mol more than that of HFAA. These hydrogen bond strengths are consistent with the frequency shifts for OH/OD stretching and OH/OD out-of-plane bending modes upon substitution of CH(3) groups with CF(3) groups. By comparing the vibrational spectra of both theoretical and experimental data, it was concluded that 2TFAA is the dominant isomer.     

An Ab Initio Study of N_2O Decomposition on MgO Catalyst

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Vibrational assignment and structure of dibenzoylmethane. A density functional theoretical study

Molecular structure and vibrational frequencies of 1,3-diphenyl-1,3-propanedione, known as dibenzoylmethane (DBM), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA) and acetylacetone (AA), the parent molecule. IR and Raman spectra of DBM and its deuterated analogue were clearly assigned.The calculated hydrogen bond energy of DBM is 16.15 kcal/mol, calculated at B3LYP/6-311++G** level of theory, which is 0.28 kcal/mol more than that of AA. This result is in agreement with the vibrational and NMR spectroscopy results. The molecular stability and the hydrogen bond strength were investigated by applying the Natural Bond Orbital analysis (NBO) and geometry calculations. The theoretical calculations indicate that the hydrogen bond in DBM is relatively stronger than that in BA and AA.     

AB initio SCF LCAO MO study of the HOH…Cl− hydrogen bond

Ab initio SCF LCAO MO calculations for the [H₂O…Cl]^? complex have been performed. The energy of the linear hydrogen bond has been found to be lower than the energy of the bifurcated one. The difference of the energies is about 3 kcal/mole. The calculated equilibrium distance between the oxygen and chlorine atoms equals 5.75 au. The interaction energy of the chlorine anion and the rigid water molecule amounts to ?19 kcal/mole. The optimization of the OH bond length in the complex (linear hydrogen bond) leads to an interaction energy of ?19.5 kcal/mole (the experimental value equals ?13.1 kcal/mole). As a result of the hydrogen bond formation the OH bond length increases by 0.08 au.     

Density functional calculations of ATP systems. 2. ATP hydrolysis at the active site of actin

The hydrolysis of adenosine 5'-triphosphate (ATP) at the active site of actin has been studied using density functional calculations. The active site is modeled by the triphosphate tail of ATP, an Mg cation, surrounding water molecules, and the nearby protein residues. Four reaction paths have been followed by constraining coordinates that allow phosphate stretching, nucleophilic attack of the catalytic water, and OH(-) formation via water deprotonation. The lowest-energy barrier (21.0 kcal/mol) is obtained for a dissociative reaction where the terminal phosphate breaks on approaching the catalytic water, followed by proton release via a proton wire mechanism. A higher barrier (39.6 kcal/mol) results for an associative reaction path where OH(-) is formed first, with a pentacoordinated phosphorus atom (P-O distances 2.1 A). Stretching the terminal bridging P-O bond results in bond rupture at 2.8 A with an energy barrier of 28.8 kcal/mol. The residues Gln137 and His161 are not important in the reactions, but insight into their roles in vivo has been obtained. The favored coordination of the end products H(2)PO(4)(-) and ADP(3-) includes a hydrogen bond and an O-Mg-O bridge between the phosphates as well as a hydrogen bond between H(2)PO(4)(-) and the Ser14 side chain. The total energy is 2.1 kcal/mol lower than in the initial reactants. Classical simulations of ATP- and ADP.P(i)-actin show few hydrolysis-induced differences in the protein structure, indicating that phosphate migration is necessary for a change in conformation.     

Vibrational spectra of Na, K, Mn2+, Ni2+ and Zn2+ salts of 1,2,4,5-benzenetetracarboxylic (pyromellitic) acid--a short hydrogen bond evidence

Five salts of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), [C6H2(COO)4H4], have been synthesized and investigated by infrared and Raman spectroscopy and by single crystal X-ray diffraction methods: sodium salt [Na2(H2O)2][C6H2(COO)4H2], potassium salt [K(H2O)3][C6H2(COO)4H3] and transition metal salts [M(H2O)6][C6H2(COO)4H2], which M = Mn, Ni and Zn. Crystal structures of all five compounds show short intramolecular asymmetric hydrogen bonds (SHB) between adjacent carboxyl groups with O...O distance average of 2.40 A. The Raman and infrared spectra reported indicate the presence of short hydrogen bonds in all salts, in agreement with the X-ray data. The O-H stretching mode [nu(OH)] had been observed at about 2500 cm(-1). Deuterated analogues were synthesized and their Raman spectra show that nu(OH)/nu(OD) ratio average is about unit. The symmetric [nu(sym)(O..H..O)] and asymmetric [nu(asym)(O..H..O)] stretching modes have been attributed about 300 and 870 cm(-1), respectively, in all salts, and for deuterated analogues, the ratio nu(OH)/nu(OD) to nu(sym)(O..H..O, O..D..O) is close to unit like it occurs in nu(OH). The vibrational modes, mainly SHB modes, are tentatively assigned by molecular orbital ab initio calculations of pyromellitic acid and anions [C6H2(COO)4H3]- and [C6H2(COO)4H2]2-. Geometry optimizations showed a good agreement with experimental data. Frequency calculation confirms the assignment of specific vibrational modes. Ab initio calculations show that nu(C=O) and nu(sym)(COO) are strongly coupled with in plane OH bending [delta(OH)]. In Raman spectra of deuterated analogues is observed a frequency shift of these bands.     

Bridging QTAIM with vibrational spectroscopy: the energy of intramolecular hydrogen bonds in DNA-related biomolecules

Physical properties of over 8000 intramolecular hydrogen bonds (iHBs), including 2901 ones of the types OH···O, OH···N, NH···O and OH···C, in 4244 conformers of the DNA-related molecules (four canonical 2'-deoxyribonucleotides, 1,2-dideoxyribose-5-phosphate, and 2-deoxy-D-ribose in its furanose, pyranose and linear forms) have been investigated using quantum theory of atoms in molecules (QTAIM) and vibrational analysis. It has been found that for all iHBs with positive red-shift of the proton donating group stretching frequency the shift value correlates with ρ(cp)-the electron charge density at the (3,-1)-type bond critical point. Combining QTAIM and spectroscopic data new relationships for estimation of OH···O, OH···N, NH···O and OH···C iHB enthalpy of formation (kcal mol(-1)) with RMS error below 0.8 kcal mol(-1) have been established: E(OH···O) = -3.09 + 239·ρ(cp), E(OH···N) = 1.72 + 142·ρ(cp), E(NH···O) = -2.03 + 225·ρ(cp), E(OH···C) = -0.29 + 288·ρ(cp), where ρ(cp) is in e a(0)(-3) (a(0)- the Bohr radius). It has been shown that XHY iHBs with red-shift values over 40 cm(-1) are characterized by the following minimal values of the XHY angle, ρ(cp) and nubla(2)ρ(cp): 112°, 0.005 e a(0)(-3) and 0.016 e a(0)(-5), respectively. New relationships have been used to reveal the strongest iHBs in canonical 2'-deoxy- and ribonucleosides and the O(5')H···N(3) H-bond in ribonucleoside guanosine was found to have the maximum energy (8.1 kcal mol(-1)).     

Theoretical study of the symmetry of the (OH...O)- hydrogen bonds in vinyl alcohol-vinyl alcoholate systems

The interactions between substituted vinyl alcohols and vinyl alcoholates (X = NH(2), H, F, Cl, CN) are studied at the B3LYP/6-311++G(d,p) level of theory. In a first step, the conformation of the monomers is investigated and the proton affinities (PA(A(-))) of the enolates are calculated. The enols and enolates are held together by strong (OH...O)(-) hydrogen bonds, the hydrogen bond energies ranging from 19.1 to 34.6 kcal mol(-1). The optimized O...O distances are between 2.414 and 2.549 A and the corresponding OH distances from 1.134 and 1.023 A. The other geometry parameters such as C[double bond]C or CO distances also indicate that, in the minimum energy configuration, the hydrogen bonds are characterized by a double well potential. The Mulliken charges on the different atoms of the proton donors and proton acceptors and the frequencies of the nu(OH) stretching vibrations agree with this statement. All the data indicate that the hydrogen bonds are the strongest in the homomolecular complexes. The transition state for hydrogen transfer is located with the transition barrier estimated to be about zero. Upon addition of the zero-point vibration energies to the total potential energy, the barrier vanishes. This is a characteristic feature of low-barrier hydrogen bonds (LBHBs). The hydrogen bond energies are correlated to the difference 1.5 PA(AH) - PA(A(-)). The correlation predicts different energies for homomolecular hydrogen bonds, in agreement with the theoretical calculations. Our results suggest that a PA (or pK(a)) match is not a necessary condition for forming LBHBs in agreement with recent data on the intramolecular hydrogen bond in the enol form of benzoylacetone (J. Am. Chem. Soc. 1998, 120, 12117).     

Vibrational assignment and structure of trifluorobenzoylacetone. A density functional theoretical study

Molecular structure and vibrational frequencies of 4,4,4-trifluoro-1-phenyl-1,3-butanedione, known as trifluorobenzoylacetone (TFBA), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA), acetylacetone (AA), and trifluoroacetylacetone (TFAA). Comparing the calculated and experimental band frequencies and intensities suggests coexisting of both stable cis-enol conformers in comparable proportions in the sample. The energy difference between the two stable chelated enol forms is negligible, 0.96 kcal/mol, calculated at B3LYP/6-311++G** level of theory. The molecular stability and the hydrogen bond strength were investigated by applying the natural bond orbital (NBO) theory and geometry calculations. The theoretical calculations and spectroscopic results indicate that the hydrogen bond strength of TFBA is between those of TFAA and AA, considerably weaker than that of BA.     

High-level ab initio calculations for the four low-lying families of minima of (H2O)20. II. Spectroscopic signatures of the dodecahedron, fused cubes, face-sharing pentagonal prisms, and edge-sharing pentagonal prisms hydrogen bonding networks

We report the first harmonic vibrational spectra for each of the lowest lying isomers within the four major families of minima of (H2O)20, namely, the dodecahedron, fused cubes, face-sharing pentagonal prisms, and edge-sharing pentagonal prisms. These were obtained at the second-order Moller-Plesset perturbation level of theory (MP2) with the augmented correlation consistent basis set of double zeta quality (aug-cc-pVDZ) at the corresponding minimum energy geometries. The computed infrared (IR) spectra are the first ones obtained from first principles for these clusters. They were found to contain spectral features, which can be directly mapped onto the distinctive spectroscopic signatures of their constituent tetramer, pentamer, and octamer fragments. The dodecahedron spectra show the richest structure in the OH stretching region and are associated with the most redshifted OH vibrations with respect to the monomer. The lowest lying edge-sharing pentagonal prism isomer displays intense IR active vibrations that are redshifted by approximately 600 cm(-1) with respect to the water monomer. Furthermore the most redshifted, IR-active OH stretching vibrations for all four networks correspond to hydrogen bonded OH groups, which exhibit the following two common characteristics: (i) they belong to fragments which have a "free" OH stretch and (ii) they act as donors to a neighboring water molecule along a "dimerlike" (strong) hydrogen bond. The zero-point energy corrected MP2/CBS (complete basis set) limit binding energies D(0) for the four isomers are -163.1 kcal/mol (edge-sharing pentagonal prism), -160.1 kcal/mol (face-sharing pentagonal prism), -157.5 kcal/mol (fused cubes), and -148.1 kcal/mol (dodecahedron).     

Structure and bonding in Hartwig stretched eta(3)-hydridoborate sigma-complex of niobium(III)

Ab initio calculations at the SCF, MP2, CASSCF, and CASPT2 levels of theory with basis sets using atomic pseudopotentials have been carried out for the stretched eta(3)-hydridoborate sigma-complex of niobium, [Cl2Nb(H2B(OH)2)], in order to investigate the nature and energetics of the interaction between the transition metal and the eta(3)-hydridoborate ligand. The geometry of the complex [Cl2Nb(H2B(OH)2] and its fragments [Cl2Nb](+) and [H2B(OH)2](-) were optimized at SCF and CASSCF levels. These results are consistent with [Cl2Nb(eta(3)-H2B(OH)2)] being a Nb(III) complex in which both hydrogen and boron of the [eta(3)-H2B(OH)2](-) ligand have a bonding interaction with the niobium preserving stretching B-H bond character. The calculated values of DEF (energy required to restore the fragment from the equilibrium structure to the structure it takes in the complex) for [Cl2Nb](+) are 5.35 kcal/mol at SCF, 3.27 kcal/mol at MP2, 4.80 kcal/mol at CASSCF, and 2.82 kcal/mol at CASPT2 and for [H2B(OH)2](-) 21.13 kcal/mol at SCF, 23.85 kcal/mol at MP2, 20.69 kcal/mol at CASSCF, and 23.48 kcal/mol at CASPT2. Values of INT (stabilization energy resulting from the coordination of distorted ligand to the metal fragment) for the complex [Cl2Nb(H2B(OH)2)] are -239.35 kcal/mol at SCF, -260.00 kcal/mol at MP2, -230.76 kcal/mol at CASSCF, and -252.60 kcal/mol at CASPT2. For the complex [(eta(5)-C5H5)2Nb(H2B(OH)2)], calculations at the SCF and MP2 levels were carried out. Values of INT for [(eta(5)-C5H5)2Nb(H2B(OH)2)] are -169.93 kcal/mol at SCF and -210.62 kcal/mol at MP2. The results indicate that the bonding of the [eta(3)-H2B(OH)2](-) ligand with niobium is substantially stable. The electronic structures of [Cl2Nb(H2B(OH)2)], [(eta(5)-C5H5)2Nb(H2B(OH)2)], and its fragments are analyzed in detail as measured by Mulliken charge distributions and orbital populations.     

Vibrational dynamics of acetate in D2O studied by infrared pump-probe spectroscopy

Solute-solvent interactions between acetate and D(2)O were investigated by vibrational spectroscopic methods. The vibrational dynamics of the COO asymmetric stretching mode in D(2)O was observed by time-resolved infrared (IR) pump-probe spectroscopy. The pump-probe signal contained both decay and oscillatory components. The time dependence of the decay component could be explained by a double exponential function with time constants of 200 fs and 2.6 ps, which are the same for both the COO asymmetric and symmetric stretching modes. The Fourier spectrum of the oscillatory component contained a band around 80 cm(-1), which suggests that the COO asymmetric stretching mode couples to a low-frequency vibrational mode with a wavenumber of 80 cm(-1). Based on quantum chemistry calculations, we propose that a bridged complex comprising an acetate ion and one D(2)O molecule, in which the two oxygen atoms in the acetate anion form hydrogen bonds with the two deuterium atoms in D(2)O, is the most stable structure. The 80 cm(-1) low-frequency mode was assigned to the asymmetric stretching vibration of the hydrogen bond in the bridged complex.     

Theoretical study of [Ni (H2O)n]2+(H2O)m (n ≤ 6, m ≤ 18)

The [Ni-(H(2)O)(n)](2+)(H(2)O)(m) (n ≤ 6, m ≤ 18) complexes were studied by means of first-principles all-electron calculations performed with the BPW91 gradient corrected functional and the 6-311+G(d,p) basis sets for the H, O, and Ni atoms. Triplet states were found as low-lying states for each (n, m) combination. The estimated Ni(2+)-(H(2)O)(n) binding energies (112.8-57.4 kcal/mol for the first layer and 52.0-23.0 kcal/mol for the second one) decreases and the Ni(2+)-OH(2) bond lengths lengthen as n + m increases. With six H(2)O moieties the Ni(2+) ion furnishes its first coordination sphere of octahedral geometry. Further water addition renders the formation of the second layer. The effect of Ni(2+) on the (H(2)O)(n)···(H(2)O)(m) hydrogen bond formation for several "n" and "m" combinations was studied, revealing an enhancement of this kind of bonding, which is of key importance for the stabilization and growth of the clusters. For some n + m isomers the second layer appears before the first octahedral layer is fully formed. For example, the square planar Ni(2+)-(H(2)O)(4) core originates two-dimensional 4 + 2 and 4 + 4 isomers, where each outer water molecule accepts two H-bonds, lying 2.0 kcal/mol above the 6 and 6 + 2 ground states. The clusters were also studied by IR spectra; the OH stretching vibrational frequencies allowed us to identify the outer solvation shells by the presence of red-shifted hydrogen bond regions.     

Structural and conformational properties of 1,1,1-trifluoro-2-propanol investigated by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 1,1,1-trifluoro-2-propanol, CF(3)CH(OH)CH(3), and one deuterated species, CF(3)CH(OD)CH(3), have been investigated in the 20.0-62.0 GHz spectral region at about -50 degrees C. The rotational spectrum of one of the three possible rotameric forms was assigned. This conformer is stabilized by an intramolecular hydrogen bond formed between the hydrogen atom of the hydroxyl group and the nearest fluorine atoms. The hydrogen bond is weak and assumed to be mainly a result of attraction between the O-H and the C-F bond dipoles, which are nearly antiparallel. The identified rotamer is at least 3 kJ/mol more stable than any other rotameric form. Two vibrationally excited states belonging to two different normal modes were assigned for this conformer, and their frequencies were determined by relative intensity measurements. The microwave work has been assisted by quantum chemical computations at the MP2/cc-pVTZ and B3LYP/6-311++G** levels of theory, as well as by the infrared spectrum of the O-H stretching vibration.     

Theoretical study of the antioxidant properties of pyridoxine

Molecules acting as antioxidants capable of scavenging reactive oxygen species (ROS) are of utmost importance in the living cell. The antioxidative properties of pyridoxine (vitamin B6) have recently been discovered. In this study, we have analyzed the reactivity of pyridoxine toward the ROS (.-)OH, (.-)OOH, and (.-)O(2)- at the density functional theory level (functionals B3LYP and MPW1B95). Two reaction types have been studied as follows: addition to the aromatic ring atoms and hydrogen/proton abstraction. Our results show that (.-)OH is the most reactive species, while (.-)OOH displays low reactivity and (.-)O2(-) does not react at all with pyridoxine. The most exergonic reactions are those where (.-)H is removed from the CH(2)OH groups or the ring-bound OH group and range from -33 to -39 kcal/mol. The most exergonic addition reactions occur by attacking the carbon atoms bonded to nitrogen but with an energy gain of only 6 kcal/mol.     

Monomeric MnIII/II and FeIII/II complexes with terminal hydroxo and oxo ligands: probing reactivity via O-H bond dissociation energies

Non-heme manganese and iron complexes with terminal hydroxo or oxo ligands are proposed to mediate the transfer of hydrogen atoms in metalloproteins. To investigate this process in synthetic systems, the monomeric complexes [M(III/II)H(3)1(OH)](-/2-) and [M(III)H(3)1(O)](2-) have been prepared, where M(III/II) = Mn and Fe and [H(3)1](3-) is the tripodal ligand, tris[(N'-tert-butylureaylato)-N-ethyl)]aminato. These complexes have similar primary and secondary coordination spheres, which are enforced by [H(3)1](3-). The homolytic bond dissociation energies (BDEs(O-H)) for the M(III/II)-OH complexes were determined, using experimentally obtained values for the pK(a)(M-OH) and E(1/2) measured in DMSO. This thermodynamic analysis gave BDEs(O-H) of 77(4) kcal/mol for [Mn(II)H(3)1(O-H)](2-) and 66(4) kcal/mol for [Fe(II)H(3)1(O-H)](2-). For the M(III)-OH complexes, [Mn(III)H(3)1(OH)]- and [Fe(III)H(3)1(OH)]-, BDEs(O-H) of 110(4) and 115(4) kcal/mol were obtained. These BDEs(O-H) were verified with reactivity studies with substrates having known X-H bond energies (X = C, N, O). For instance, [Fe(II)H(3)1(OH)](2-) reacts with a TEMPO radical to afford [Fe(III)H(3)1(O)](2-) and TEMPO-H in isolated yields of 60 and 75%, respectively. Consistent with the BDE(O-H) values for [Mn(II)H(3)1(OH)](2-), TEMPO does not react with this complex, yet TEMPO-H (BDE(O-H) = 70 kcal/mol) reacts with [Mn(III)H(3)1(O)](2-), forming TEMPO and [Mn(II)H(3)1(OH)](2-). [Mn(III)H(3)1(O)](2-) and [Fe(III)H(3)1(O)](2-) react with other organic substrates containing C-H bonds less than 80 kcal/mol, including 9,10-dihydroanthracene and 1,4-cyclohexadiene to produce [M(II)H(3)1(OH)](2-) and the appropriate dehydrogenated product in yields of greater than 80%. Treating [Mn(III)H(3)1(O)](2-) and [Fe(III)H(3)1(O)](2-) with phenolic compounds does not yield the product expected from hydrogen atom transfer but rather the protonated complexes, [Mn(III)H(3)1(OH)]- and [Fe(III)H(3)1(OH)]-, which is ascribed to the highly basic nature of the terminal oxo group.     

Molecular orbital study on the OH stretching frequency of the phenol dimer and its cation

Ab initio calculations were performed to investigate the structure and bonding of the phenol dimer and its cation, especially the OH stretching frequencies. Some stable structures of the phenol dimer and its cation were obtained at the Hartree–Fock level and were found to be in agreement with predictions based on spectroscopic investigations. In these dimers the phenol moieties are bound by a single OH⋯O hydrogen bond. The hydrogen bond is much stronger in the dimer cation than in the neutral dimer. The calculated binding energy of the phenol dimer in the most stable structure was 6.5–9.9 kcal/mol at various levels of calculation, compared with the experimental value of 5 kcal/mol or greater. The binding energy of the phenol dimer cation is more than 3 times (24.1–30.6 kcal/mol) as large as that of the neutral dimer. For the phenol dimer the OH stretching frequency of the proton-accepting phenol (PAP) is 3652 cm⁻¹ and that of the proton-donating phenol (PDP) is 3516 cm⁻¹; these are in agreement with observed values of 3654 and 3530 cm⁻¹, respectively. For the phenol dimer cation the OH stretching frequency of the PAP is 3616–3618 cm⁻¹ in comparison with an observed value of 3620 ± 3 cm⁻¹. That of the PDP in the dimer cation is calculated to be 2434–2447 cm⁻¹, which is 1210–1223 cm⁻¹ lower than that of the bare phenol. The large reduction in the OH stretching frequency of the PDP in the phenol dimer cation is attributed to the formation of a stronger hydrogen bond in the cation than in the neutral dimer. Received: 24 March 2000 / Accepted: 26 April 2000 / Published online: 11 September 2000     

Theoretical mechanism study of UF6 hydrolysis in the gas phase

The mechanism of the gas-phase reaction UF 6 + H 2O --> UOF 4 + 2HF is explored using relativistic density functional theory calculations. Initially, H 2O coordinates with UF 6 to form a 1:1 complex UF 6.H 2O. Over an activation energy barrier of about 19 kcal/mol, H 2O transfers a H atom to a nearby ligand F, resulting in UF 5OH + HF. The eliminated HF or another H 2O molecule may form a hydrogen bond with UF 5OH. Starting from UF 5OH, the second HF elimination results in UOF 4. If UF 5OH is in the isolated form, UF 5OH --> UOF 4 + HF takes place over a barrier of 24 kcal/mol. If UF 5OH is hydrogen-bonded with H 2O or HF, the conversion barrier is less than 10 kcal/mol. Once formed, the unstable UOF 4 tends to associate with additional ligands and hydrogen-bonding donors. The calculated binding energies indicate the significance of such interactions, which may have profound impact on further HF eliminating reactions. The IR spectra features can be used to indicate the formation and interaction type of the intermediates and products.     

C(Ar)-H···O hydrogen bonds in substituted isobenzofuranone derivatives: geometric, topological, and NMR characterization

Substituted isobenzofuranone derivatives 1a-3a and bindone 4 are characterized by the presence of an intramolecular C(Ar)-H···O hydrogen bond in the crystal (X-ray), solution ((1)H NMR and specific and nonspecific IEF-PCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the C(Ar)-H···O interaction weakens in 1a-3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl(3) solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a-3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating (1)H NMR chemical shifts (δ(calcd) - δ(expl) < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a-3a and 1b-3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C-H moiety involved in the hydrogen bond, the increase of the spin-spin coupling constant (1)J((13)C-(1)H) by about 7.5 Hz is in good agreement with calculations for C-H bond shortening and for blue shifts of C-H stretching vibrations (by 55-75 cm(-1)).