Influence of hydration on the protomeric tautomerism in selenium analogue of methimazole: A computational chemistry study

Hydrogen bond assisted proton transfer reactions were investigated in 3-methyl-1H-imidazole-2(3H)-selone (MSeI) and 1H-imidazole-2(3H)-selone (SeI) at B3LYP/6-311++G(2d,2p) level of theory. The B3LYP results predict that the direct proton transfer process in MSeI and SeI is more difficult than the water-assisted one. The results also show that the selone complexes are more stable than corresponding selenol ones. Interaction energies for a single NHSe hydrogen bond in dimers MSeI and SeI are −31.3 and −32.7 kJ/mol, respectively. ZPE-corrected binding energies in the self-association complexes of the MSeI and SeI are greater than the water-associated complexes. The small negative value of H(r) obtained by AIM analysis at B3LYP/6-311++G(2d,2p) level reveals some contribution of sharing interaction (partially covalent) to the SeHN bond in dimers of the MSeI and SeI. AIM data also reveal the partially covalent nature of SeH6 interaction and electrostatic nature of OH5 interaction in water-associated complexes. Results of charge analysis show that the selenium analogue of the methimazole is more nucleophilic than the methimazole. Our results confirm that the selenium analogue of methimazole can exist as a zwitterionic form.     

DFT study on second-order nonlinear optical properties of the derivatives of 7-vertex cobalt–carborane metallocenyl

Density functional theory (DFT) B3LYP method was employed to calculate electron properties and the second-order nonlinear optical (NLO) responses of the derivatives which were formed by (C₅H₅)Co(C₂B₄H₆) and CHCHC₆H₄NO₂, CHCHC₆H₄NH₂. The results show: when H atom of (C₅H₅)Co(C₂B₄H₆) is substituted by CHCHC₆H₄NO₂, the β_tot values of isomers are all slightly smaller than that of ferrocene (Fc) derivative (FcCHCHC₆H₄NO₂). However, when H atom of (C₅H₅)Co(C₂B₄H₆) is substituted by CHCHC₆H₄NH₂, the β_tot values of isomers are close to that of ferrocene (Fc) derivative (FcCHCHC₆H₄NH₂). It indicates that (C₅H₅)Co(C₂B₄H₆) can be either a donator or an acceptor.     

Uncertainty from sampling: workshop to launch a Nordtest handbook on sampling uncertainty estimation and control

A workshop on uncertainty in sampling was held in Hillerød, Denmark, on 12–13 April 2007 to launch a new handbook on sampling quality assurance and uncertainty estimation. The participants of the workshop were approximately 60 delegates from 15 European countries, representing institutions performing sampling, users of the data, research institutions, as well as accreditation bodies. Materials from the workshop, including examples, tools, and calculation aids for the work can be found at http://www.samplersguide.com. The Nordtest handbook Uncertainty from sampling will be made available on the Nordtest web site at http://www.nordicinnovation.net/nordtest.cfm under NT technical reports, report number NT tec 604. Until the final report is available on the Nordtest web site, an advance draft of the Nordtest handbook is available from http://www.samplersguide.com.     

Hydrogen-Bonded and Halogen-Bonded: Orthogonal Interactions for the Chloride Anion of a Pyrazolium Salt

In the hydrochloride of a pyrazolyl-substituted acetylacetone, the chloride anion is hydrogen-bonded to the protonated pyrazolyl moiety. Equimolar co-crystallization with tetrafluorodiiodobenzene (TFDIB) leads to a supramolecular aggregate in which TFDIB is situated on a crystallographic center of inversion. The iodine atom in the asymmetric unit acts as halogen bond donor, and the chloride acceptor approaches the σ-hole of this TFDIB iodine subtending an almost linear halogen bond, with Cl···I = 3.1653(11) Å and Cl···I–C = 179.32(6)°. This contact is roughly orthogonal to the N–H···Cl hydrogen bond. An analysis of the electron density according to Bader’s Quantum Theory of Atoms in Molecules confirms bond critical points (bcps) for both short contacts, with ρbcp = 0.129 for the halogen and 0.321 eÅ−3 for the hydrogen bond. Our halogen-bonded adduct represents the prototype for a future class of co-crystals with tunable electron density distribution about the σ-hole contact.     

PCILO method for excited states

In the scope of building a PCILO method for excited states, one builds and tests excitonic zeroth order wave-functions. For \(\left( {_\pi ^{\pi ^ * } } \right)\) transitions, the (σ+π) excitonic and purely (π) excitonic wave functions are compared, showing that theσ-π coupling between \(\left( {_\pi ^{\pi ^ * } } \right)\) and \(\left( {_\sigma ^{\sigma ^ * } } \right)\) single excitations may be considered as a perturbation. The excited state wave-functions are analyzed in terms of neutral and ionic structures, and the fluctuation of the charges in the two-electrons loges are studied, showing that theσ-π coupling favours the neutral structures and diminishes the charge-fluctuations.     

Conformational dependence of the second hyperpolarizability of quadrupolar molecules

Hatree–Fock calculations at ab initio and semiempirical levels were carried out for the averaged polarizability α and second hyperpolarizability γ of two pairs of quadrupolar isomers with different donor and acceptor groups. These properties were correlated with the antibonding/bonding π occupation number (π*/π ratio). It was found that isomers with extended π systems had low π*/π ratios and high α and γ values, while low α and γ values were obtained for isomers with large π*/π ratios and no extended π system. The PM3 and PM6 α values were found to be in excellent agreement with the HF/6-31+G(d,p) ones. The PM3 values for γ were significantly larger than those calculated by HF/6-31+G(d,p), with an average PM3/HF ratio of 1.43. The PM6 results were noticeably better with a ratio of 0.85. The calculation of α and γ at MP2/6-31+G(d,p) level for representative isomers showed that the contribution of the electron correlation to their values was small and that the HF/6-31+G(d,p) method provides reliable values at much lower computational cost.     

A DFT study on the mechanism of the gas phase reaction of ground-state Y (4d5s,D) with 2-butyne

The reaction of ground-state Y with 2-butyne has been investigated in detail using B3LYP method. Four pathways for elimination of H₂ were identified. Two isomers, Y(HCCC)CH₃ and Y(H₂CCCCH₂) were assigned to the observed product, YC₄H₄. The calculated PESs suggest that the concerted H₂-elimination leading to Y(H₂CCCCH₂) + H₂ product is the most favorable pathway. For the elimination of CH₃, combining the results of this work with our previous study on Y + propyne reaction, a general mechanism for the reactions of Y with 2-alkynes bearing RCCCH₃ structure was established: Y + RCCCH₃ → π-complex → TS(H-migration) → HY(CH₂CC)R → TS (CC insertion) → (CH₂)HYCCR → TS(H-migration) → H₃CYCCR → CH₃ + YC₂R. Such mechanism was found to be always energetically more favorable than the direct sp–sp³ CC bond insertion mechanism. Further, such mechanism can also be applied to the elimination of CH₄ and it can be described as: Y + CH₃CCCH₃ → π-complex → TS (H-migration) → HY(H₂CCC)CH₃ → TS(CC insertion) → (H₂CCC)HYCH₃ → TS(H-migration) → CH₄ + YC₃H₂.     

Studies of the Formation of Inclusion Complexes Derivatives of Cinnamon Acid with α-Cyclodextrin in a Wide Range of Temperatures Using Conductometric Methods

The electrical conductivities of aqueous solutions of sodium salts of trans-4-hydroxycinnamic acid (trans-p-coumaric acid), trans-3,4-dihydroxycinnamic acid (trans-caffeic acid), trans-4-hydroxy-3-methoxycinnamic acid, (trans-ferulic acid) and trans-3-phenylacrylic acid (trans-cinnamic acid) with α-cyclodextrin were measured in the temperature range of 288.15 K–318.15 K. For the first time in the literature, using the limiting molar conductivity (Λmo) obtained from conductivity measurements, the values of the complexation constants (K_f) of the salts of phenolic acid derivatives with α-cyclodextrin were determined using a modified low concentration chemical model (IcCM). An attempt was also made to analyze the individual thermodynamic functions ΔGo, ΔHo and ΔSo describing the complexation process as a function of temperature changes. The obtained results show that the process of formation of inclusion complexes is exothermic and is spontaneous.     

Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation

Methylfurans are methylated aromatic heterocyclic volatile organic compounds and primary or secondary pollutants in the atmosphere due to their capability to form secondary organic aerosols in presence of atmospheric oxidants. There is therefore a significant interest to monitor these molecules in the gas phase. High resolution spectroscopic studies of methylated furan compounds are generally limited to pure rotational spectroscopy in the vibrational ground state. This lack of results might be explained by the difficulties arisen from the internal rotation of the methyl group inducing non-trivial patterns in the rotational spectra. In this study, we discuss the benefits to assign the mm-wave rotational-torsional spectra of methylfuran with the global approach of the BELGI-Cs code compared to local approaches such as XIAM and ERHAM. The global approach reproduces the observed rotational lines of 2-methylfuran and 3-methylfuran in the mm-wave region at the experimental accuracy for the ground vt=0 and the first torsional vt=1 states with a unique set of molecular parameters. In addition, the V3 and V6 parameters describing the internal rotation potential barrier may be determined with a high degree of accuracy with the global approach. Finally, a discussion with other heterocyclic compounds enables the study of the influence of the electronic environment on the hindered rotation of the methyl group.     

Determination of arsenic, iron and selenium in moss samples using hexapole collision cell, inductively coupled plasma-mass spectrometry

Different collision gases (H₂, He and premixed 7% H₂ in He) used in the hexapole collision cell of an inductively coupled plasma-mass spectrometer (ICP-MS) were compared, and the gas-flow rates were optimized for the determination of arsenic (), iron () and selenium (). The study showed that the argon-based interferences at mass-to-charge ratios (m/z) of 56, 75 and 80 can be overcome by the optimized gas flows (7.5 ml min⁻¹ premixed 7% H₂ in He and 2 ml min⁻¹ H₂) in the hexapole collision cell. Detection limits of 15.5 ng l⁻¹ for iron () and 29 ng l⁻¹ for selenium () in 2% (v/v) HNO₃ were obtained under optimized collision cell conditions. The detection limit for arsenic () obtained in difficult hydrochloride acid matrix (5% HCl (v/v)) was 153 ng l⁻¹. The accuracy of the optimized method was confirmed by analyzing two moss reference materials. The results obtained by ICP-MS for arsenic, selenium and iron from both moss reference samples were, in most cases, in good agreement with the certified values.