Deprotonation sites of acetohydroxamic acid isomers. A theoretical and experimental study

Theoretical (ab initio calculations) and experimental (NMR, spectrophotometric, and potentiometric measurements) investigations of the isomers of acetohydroxamic acid (AHA) and their deprotonation processes have been performed. Calculations with the Gaussian 98 package, refined at the MP2(FC)/AUG-cc-pVDZ level considering the molecule isolated, indicate that the Z(cis) amide is the most stable form of the neutral molecule. This species and the less stable (Z)-imide form undergo deprotonation, giving rise to two stable anions. Upon deprotonation, the E(trans) forms give three stable anions. The ab initio calculations were performed in solution as well, regarding water as a continuous dielectric; on the basis of the relative energies of the most stable anion and neutral forms, calculated with MP2/PCM/AUG-cc-pVDZ, N-deprotonation of the amide (Z or E) structure appeared to be the most likely process in solution. NMR measurements provided evidence for the existence of (Z)- and (E)-isomers of both the neutral and anion forms in solution. Comparisons of the dynamic NMR and NOESY (one-dimensional) results obtained for the neutral species and their anions were consistent with N-deprotonation, which occurred preferentially to O-deprotonation. The (microscopic) acid dissociation constants of the two isomers determined at 25 degrees C from the pH dependence of the relevant chemical shifts, pK(E) = 9.01 and pK(Z) = 9.35, were consistent with the spectrophotometric and potentiometric evaluations (pK(HA) = 9.31).     

Water-assisted alkaline hydrolysis of monobactams: a theoretical study

A theoretical study of the water-assisted alkaline hydrolysis of 2-azetidinone, 3-formylamino-2-azetidinone and 3-formylamino-2-azetidine-1-sulfonate ion is carried out at the B3LYP/6-31+G* level. The effect of bulk solvent is taken into account using the PCM solvation model while specific solvent effects are represented by the inclusion of an ancillary water molecule along the reaction profile. The calculated free energy barriers in solution are in reasonable agreement with experimental values. The observed substituent effects due to the presence of the 3-formylamino and the SO(3) groups attached to the beta-lactam ring are crucial factors determining the hydrolysis of monobactam antibiotics.     

氟氯甲醇及其异构体的气相离解反应研究

在G3(MP2)水平上,通过对CH2ClFO的势能面(PES)上关键驻点的能量计算,共找到3种中间体,14个过渡态,20种产物通道,并对氟氯甲醇(CHClFOH)及其异构体(CH2FOCl和CH2ClOF)的气相解离机理进行了讨论。结果表明：四中心的1,2-HX(X=Cl,F)消去反应是氟氯甲醇的主要通道,但对于其同分异构体,OCl和OF键断裂又是强竞争过程。     

Volumetric properties of random copolymers. An experimental and theoretical study

The volumetric properties of five random copolymers have been studied, both experimentally and theoretically, over extended ranges of external conditions of temperature (up to 500 K) and pressure (up to 200 MPa). The experimental measurements have been conducted on a high pressure densitometer (SWO, model PVT100). In copolymers of the type vinyl chloride-co-vinyl acetate, simple linear combining rules are sufficient to describe properly the PVT behavior by using an equation of state appropriate for copolymers. This, however, is not true in copolymers of the styrene-co-acrylonitrile type where the repeating units are significantly different in size and shape. The combination of our theoretical and experimental approach permitted the assignment of equation of state scaling constants to the fictitious polymer poly(maleic anhydride). This is of importance since maleic anhydride is one of the most widely used repeating units in polymer alloying and blending. The proposed equation of state model also permits the study of systems of the type styrene-co-vinyl phenol, in which a highly complex network of hydrogen bonding is involved.     

An experimental and theoretical study of the basicity of tetra-tert-butyltetrahedrane

The gas-phase basicity (GB) of tetra-tert-butyltetrahedrane (tBu4THD) was determined by FT-ICR mass spectrometry and comparison with reference compounds of known basicity. Its GB, 1035+/-10 kJ x mol(-1), makes tetra-tert-butyltetrahedrane one of the strongest bases reported so far. Ab initio calculations [B3LYP/6-31G(d) and B3LYP/6-311 + G(d,p)//6-31G(d)] have been carried out in order to compare the high experimental basicity of tBu4THD with that estimated theoretically. Both B3LYP/6-31G(d) and QCISD(T) calculations were used to determine the reaction path which connects the initial tetrahedrane-ammonium complex with the final products, protonated cyclobutadiene (CBDH+) and ammonia.     

Germyl mesolytic dissociations in the allylgermane and penta- 2,4-dienylgermane radical anions. A theoretical study

In two stable structures have a trigonal bipyramidal arrangement around Ge, with the extra electron in equatorial (tbp eq) or axial (tbp ax) position. In only tbp ax is found, while a second structure with a tetrahedral germyl group has the extra electron on the conjugated π system. C−Ge bond cleavage yields allyl/ pentadienyl radicals plus germide. Both dissociation reactions require 4–6 kcal mol⁻¹, less than the analogous C and Si systems (ca. 30 and 14 kcal mol⁻¹, respectively). Fragmentation is dramatically activated with respect to homolysis in the corresponding neutrals. The wavefunction is dominated by one single configuration at all distances, in contrast to homolytic cleavage, in which two configurations are important. C−Ge bond dissociation is at variance also with heterolysis, due to spin recoupling of one of the C−Ge bond electrons with the originally unpaired electron. Contribution to the Fernando Bernardi Memorial Issue.     

Stereoselective 6-exo cyclization of amidyl radicals. An experimental and theoretical study

Unsaturated primary amidyl radicals of Z-configurations underwent efficient chemo- and stereoselective 6-exo cyclization reactions via chair-conformational transition states, leading to the predominant formations of 3,6-trans, 4,6-cis, or 5,6-trans substituted δ-lactams.     

An experimental and theoretical study of spin-spin coupling in chlorosilanes

An experimental and theoretical study of the absolute value of the one-bond spin-spin coupling constant |(1)J(Si,H)| in SiH(n)Cl(4-n) (n = 0-4) dissolved in THF-d(8) is presented. We found |(1)J(Si,H)| to increase with an increasing number of chlorine substituents, and the quantitative changes were found to differ from the values previously reported for the same compounds dissolved in cyclohexane-d(12). We also report on the variations in |(1)J(Si,H)| as a function of temperature, which we found to be linearly temperature dependent for the chlorine-substituted silanes and temperature independent for SiH(4). Furthermore, the temperature dependence of |(1)J(Si,H)| varied between the different chlorosilanes. Solvent-solute interactions were studied by quantum chemical DFT calculations. The variations in chloro-silane bond lengths upon adduct formation and the different adduct interaction energies may explain the temperature dependences of the coupling constants.     

Experimental and theoretical UV characterizations of acetylacetone and its isomers

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.     

An experimental Raman and theoretical DFT study on the self-association of acrylonitrile

The liquid structure of acrylonitrile (propenenitrile) has been investigated using Raman spectroscopy and density functional theory (DFT) ab initio calculations with the 6-311++G** basis set. Two different and complementary experimental approaches were undertaken: FT-Raman spectra of 13 acrylonitrile solutions in carbon tetrachloride (concentration range=0.25-12.0 mol.L-1) were studied in detail including principal component analysis (PCA) of the CN stretching band. Furthermore, dispersive Raman spectra of neat acrylonitrile were obtained at eight different temperatures from 238 up to 343 K. The complex and asymmetric acrylonitrile Raman CN stretching band can be decomposed into two components attributed to monomeric and self-associated forms. Ab initio results fully support this assignment and suggest that the self-associated complex is a nonplanar trimer held together by dipole-dipole interactions. At ambient temperature, the composition of acrylonitrile can be expressed as a mixture of 25% monomers and 75% trimers. Close to the boiling point, trimers still represent 65% of the liquid composition. The corresponding enthalpy of association was estimated to be -22+/-2 kJ.mol-1.     

Possible gas-phase ion pairs in amine-HCl complexes. An ab initio theoretical study

Ab initio MO calculations were performed for complexes between HCl and NH₃, CH₃NH₂, (CH₃)₂NH and (CH₃) ₃N. SCF geometry optimization for the latter three complexes gives double-minimum potential surfaces, which become single- minimum surfaces when electron correlation is considered. It is proposed that (CH₃)₃NHCl may be an ion pair in the gas phase.     

A theoretical study of the geometry of the first triplet of acetaldehyde and of its fragmentation into free radicals

The energy profiles for the decomposition of the first triplet of acetaldehyde via the reactions: CH₃CHO^ν → CH₃CO° + H°, CH₃CHO^* → CH^.₃ + HCO^., have been computed using a non-empirical LCAO MO SCF procedure. Both reactions were found to have energy barriers of similar height (245 KJ mole ^?1) and at similar extensions of the respective rupturing bonds (0.71–0.76 Å). The energy profile for internal rotation about the C-C bond was also calculated and showed a small barrier (3.5 kJ mole^?1).     

Binding energies of the silver ion to alcohols and amides: a theoretical and experimental study

The silver ion binding energies to alcohols (methanol, ethanol, n-propanol, i-propanol, and n-butanol) and to amides (acetamide, N-methylacetamide, N, N-dimethylacetamide, formamide, N-methylformamide, and N, N-dimethylformamide) have been calculated using density functional theory (DFT) and measured using the threshold collision-induced dissociation (TCID) method. For DFT, the combined basis sets of ECP28MWB for silver and 6-311++G(2df,2pd) for the other atoms were found to be optimal using a series of test calculations on Ag (+) binding to methanol and to formamide. In addition, the Ag (+) binding energies of all ligands were evaluated with nine functionals after full geometric optimizations. TCID binding energies were measured using a triple quadrupole mass spectrometer. Reasonable to good agreements were obtained between the calculated and experimental silver(I) binding energies. Ligation of Ag (+) to the alcohols was primarily via the oxygen, although n-propanol and n-butanol exhibited additional, bidentate coordination via the CH hydrogens. By contrast, silver(I) binding to the amides was all monodentate via the carbonyl oxygen. There appears to be strong correlations between the binding energies and the polarizabilities of the ligands.     

Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals. An experimental and theoretical study

Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals gave distinct EPR spectra thanks to surprisingly large beta-hydrogen atom hyperfine splittings that enabled them to be characterized and monitored. DFT computations indicated that the axial species (X = CH(2)) had a higher barrier to rotation about the (O)C(alpha)-C(beta) bond. The computed difference Delta H degrees for the axial and equatorial radicals (R = H, X = CH(2)) was 0.8 kcal mol(-)(1).     

Negative ion fragmentations of deprotonated peptides. The unusual case of isoAsp: a joint experimental and theoretical study. Comparison with positive ion cleavages

The following peptides have been examined in this study: GLDFG(OH), caeridin 1.1 [GLLDGLLGLGGL(NH₂)], 11 Ala citropin 1.1 [GLFDVIKKVAAVIGGL(NH₂)], Crinia angiotensin [APGDRIYVHPF(OH)] and their isoAsp isomers. It is not possible to differentiate between Asp‐ and isoAsp‐containing peptides (used in this study) using negative ion electrospray mass spectrometry. This is because the isoAsp residue cleaves to give the same fragment anions as those formed by δ and γ backbone cleavage of Asp. The isoAsp fragmentations are as follows: RNHCH(CO₂H)^?CHCONHR′ → [RNH^?(HO₂CCH?CHCONHR′)] → RNH^?+HO₂CCH?CHCONHR′ and RNHCH(CO₂H)^?CHCONHR′ → [RNH^?(HO₂CCH?CHCONHR′] → ^?O₂CCH?CHCONHR′+RNH₂. Calculations at the HF/6‐31+G(d)//AM1 level of theory indicate that the first of these isoAsp cleavage processes is endothermic (by +115 kJ mol^?1), while the second is exothermic (?85 kJ mol^?1). The barrier to the highest transition state is 42 kJ mol^?1. No diagnostic cleavage cations were observed in the electrospray mass spectra of the MH⁺ ion of the Asp‐ and isoAsp‐containing peptides (used in this study) to allow differentiation between these two amino acid residues. Copyright © 2009 John Wiley & Sons, Ltd.