NMR spin-spin couplings involving nuclei in the neighborhood of a carbonyl group. 3JCH couplings in alpha-substituted acetamides

In this work 3JCH spin-spin coupling constants (SSCCs) for the cis- and trans-conformers for alpha-X-acetamides (X = F, Cl, Br and CN) (1-4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis-conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3JCH SSCCs and its potential as a probe to study the stereochemical properties of the XH2C group are discussed.     

Semigroups of Composition Operators in BMOA and the Extension of a Theorem of Sarason

In this paper we deal with the maximal subspace in BMOA where a general semigroup of analytic functions on the unit disk generates a strongly continuous semigroup of composition operators. Particular cases of this question are related to a well-known theorem of Sarason about VMOA. Our results describe analytically that maximal subspace and provide a condition which is sufficient for the maximal subspace to be exactly VMOA. A related necessary condition is also proved in the case when the semigroup has an inner Denjoy-Wolff point. As a byproduct we provide a generalization of the theorem of Sarason. This research has been partially supported by the Ministerio de Educación y Ciencia projects n. MTM2006-14449-C02-01 and MTM2005-08350-C03-03 and by La Consejería de Educación y Ciencia de la Junta de Andalucía.     

Solvent effects on oxygen-17 chemical shifts in methyl formate: linear solvation shift relationships

A multiple linear regression analysis has been carried out using the Kamlet-Abboud-Taft solvatochromic parameters in order to quantify the solvent effects on the (17)O chemical shifts of methyl formate (MF). The influence of the solvents upon the carbonyl oxygen chemical shifts is smaller for MF than for N-methylformamide (NMF). The influence (in parts per million) of the solvent polarity-polarizability reduces from -21.9pi* in amides to -9.6pi* in MF. The influence of the solvent hydrogen-bond-donor acidities reduces from -42.0alpha in formamides to -16.9alpha in MF. The solvent effects upon the dicoordinated oxygen chemical shifts of MF are smaller in magnitude and opposite in direction, i.e., 4.8pi* and 2.6alpha, than those for the carbonyl oxygen. (17)O hydration shifts have been calculated for the NMF + (H(2)O)(6) and MF + (H(2)O)(5) complexes by the ab initio GIAO method at the 6-311 + G** level. The hydration shifts calculated for the carbonyl oxygens of NMF and MF and for the dicoordinated oxygen of MF, -102.4, -64.7, and 17.6 ppm, respectively, show the same trend as the corresponding empirical hydration shifts, -101.7, -42.0, and 14.2 ppm.     

Solvent effects in ionic liquids: empirical linear energy-density relationships

Multiparameter linear energy-density relationships to model solvent effects in room temperature ionic liquids (RTILs) are introduced and tested. The model incorporates two solvent dependent and two specific solute-solvent parameters represented by a set of electronic indexes derived from the conceptual density functional theory. The specific solute-solvent interactions are described in terms of the electronic chemical potential for proton migration between the anion or cation and the transition state structure of a specific reaction. These indexes provide a quantitative estimation of the hydrogen bond (HB) acceptor basicity and the hydrogen bond donor acidity of the ionic solvent, respectively. A sound quantitative scale of HB strength is thereby obtained. The solvent dependent contributions are described by the global electrophilicity of the cation and nucleophilicity of the anion forming the ionic liquid. The model is illustrated for the kinetics of cycloaddition of cyclopentadiene towards acrolein. In general, cation HB acidity outweighs the remaining parameters for this reaction.     

Optimizing the rank position of the DMU as secondary goal in DEA cross-evaluation

Cross-efficiency evaluation is an extension of Data Envelopment Analysis (DEA) that permits not only the determination of a ranking of Decision Making Units (DMUs) but also the elimination of unrealistic weighting schemes, thereby rescinding the necessity for the inclusion of individual judgements in the models. The main deficiency of the procedure is the non-uniqueness of the optimal weights, which results in the peer evaluations dependences, for instance, on the software used to determine DMU’s efficiencies. This shortfall justifies the inclusion of secondary goals in order to determine cross-efficiency values. In this paper a new proposal of a secondary goal is studied. The idea is related with that proposed in Wu et al. (2009), in which the objective is the optimization of the rank position of the DMU under evaluation. In the procedure proposed here, an incentive to break level-pegging ties between alternatives is introduced by considering that efficiency scores induce a weak order of alternatives. The model is illustrated with a preference-aggregation application.     

Predicting the reaction mechanism of nucleophilic substitutions at carbonyl and thiocarbonyl centres of esters and thioesters

In nucleophilic substitution reactions at carbonyl centres, there are two possible channels. The first one occurs when the attack of nucleophilic agents takes place simultaneously with the departure of the nucleofuge. This process is named as concerted. The second possibility is the formation of a reaction intermediate, typically a tetrahedral intermediate from which the nucleofuge departs after passing through a second transition state. This second mechanism is defined as stepwise. Whether a concerted or stepwise mechanism is to be expected for a given reaction depends on several factors. Among these determinants are the nucleophilicity of the attacking group, the leaving group ability of the nucleofuge, and the solvent, which affects both the stability of the intermediate or the transition states involved. The role of the electrophilic centre can however become an important factor that can determine the reaction mechanism. In this work we show that the group nucleophilic Fukui function model may be used to rationalize and to predict the reaction mechanism of the title compounds towards alicyclic amines. In general, when the electrophilic carbon centre is attached to the soft sulfur atom, the reaction mechanism is predicted to follow a stepwise route. When the electrophilic carbon atom is attached to a harder oxygen centre, the reaction mechanism is determined by chemical substitution at the nucleofuge moiety. Experimental verification for a set of four substrates is presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Aminodiphenylphosphanes: Isotope‐induced chemical shifts 1Δ14/15N(31P), coupling constants 1J(31P,15N), and chemical shifts δ15N and δ31P

A series of aminodiphenylphosphanes 1 [Ph₂P‐N(H)tBu ( a ), ‐NEt₂ ( b ), ‐NiPr₂ ( c )], 2 [Ph₂P‐NHPh ( a ), ‐NH‐2‐pyridine ( b ), ‐NH‐3‐pyridine ( c ), ‐NH‐4‐pyridine ( d ), NH‐pyrimidine ( e ), NH‐2,6‐Me₂‐C₆H₃ ( f ), NH‐3‐Me‐2‐pyridine ( g )], 3 [Ph₂P‐N(Me)Ph ( a ), ‐NPh₂ ( b )], and N‐pyrrolyldiphenylphosphane 4 (Ph₂P‐NC₄H₄) was prepared and studied by NMR (¹H, ¹³C, ³¹P, ¹⁵N NMR) spectroscopy. The isotope‐induced chemical shifts ¹Δ^14/15N(³¹P) were determined at natural abundance of ¹⁵N by using HEED INEPT experiments. A dependence of ¹Δ^14/15N(³¹P) on the substituents at nitrogen was found (alkyl < H < aryl; increasingly negative values). The magnitude and sign of the coupling constants ¹J(³¹P,¹⁵N) (positive sign) are dominated by the presence of the lone pair of electrons at the phosphorus atom. The X‐ray structural analysis of 2b is reported, showing the presence of dimers owing to intermolecular hydrogen bridges in the solid state. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:542–550, 2001     

Synthesis of Phosphanes Bearing 2‐Imino‐1,3‐thiazolidine Ligands – X‐Ray Analyses and NMR Spectroscopy

Pseudo‐ephedrine derived 2‐imino‐1,3‐thiazolidine 1 reacts with tris(diethylamino)phosphane by stepwise replacement of the diethylamino group to give the mono‐, bis‐ and tris(imino)phosphanes 2 , 3 and 4 , respectively, of which 4 could be isolated in pure state. The analogous reaction with diethylamino‐diphenylphosphane affords the imino‐diphenylphosphane 5 . The iminophosphanes react with sulfur or selenium to give the corresponding phosphorus(V) compounds. In contrast, the reaction of the iminophosphanes with oxygen is very slow; anhydrous trimethylamine N‐oxide reacts in the melt with the phosphanes to give the oxides 4(O) and 5(O) . The molecular structures of 4(O) (in mixture with 4 ), 4(Se) , 5(S) and 5(Se) were determined by X‐ray analysis. In all cases the ring‐sulfur and the phosphorus atoms are in cis‐positions at the C=N bonds. The analogous solution structures were determined by ¹H, ¹³C, ¹⁵N, ³¹P and ⁷⁷Se NMR spectroscopy. In the case of the compounds 5 , 5(O) , 5(S) and 5(Se) the isotope‐induced chemical shifts ¹δ^14/15N(³¹P) were determined, using INEPT‐HEED experiments.     

Implementation of atomic basis set composed of 1s Gaussian and 1s Slater-type orbitals to carry out quantum mechanics molecular calculations

A mixed atomic basis set formed with ls Slater-type orbitals and 1s floating spherical Gaussian orbitals is implemented. Evaluation of multicenter integrals is carried out using a method based on expansion of binary products of atomic basis functions in terms of a complete basis set, and a systematic analysis is performed. The proposed algorithm is very stable and furnishes fairly good results for total energy and geometry. An LCAO-SCF test calculation is carried out on LiH. The trends observed show that there are some combinations of mixed orbitals that are appropriate to describe the system. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 604–609, 1999     

Versatile behavior of 2-guanidinobenzimidazole nitrogen atoms toward protonation,coordination and methylation

The structure, dynamic behavior, protonation, methylation, and coordination sites of 2-guanidinobenzimidazole 1a were investigated. Structures of compounds [2-guanidinium-1,3,10-trihydrobenzimidazole]sulfate 1b , [2-guanidinium-1,3-dihydro-benzimidazole]sulfate 1c–1d , [2-guanidinium-1,3-dihydro-benzimidazole]tetrafluoroborate 1e , [2-guanidinium-1,3-dihydro-benzimidazole]chloride 1f , [2-guanidinium-1,3-dihydro-benzimidazole] perchlorate 1g , 2-guanidino-1-methyl-benzimidazole 2a , [2-guanidinium-1,3-dimethyl-benzimidazole]iodide 2b , [2-guanidinium-1-methyl-3-hydro-benzimidazole]chloride 2c , [2-guanidinium-1,10-dihydro-benzimidazole]acetate 3 , 2-guanidino-1-hydro-3-borane-benzimidazole 4a , 2-guanidino-1-methyl-3-borane-benzimidazole 4b , (2-guanidino-benzimidazole)dimethyltin 5 , [bis(2-guanidino-10-hydro-benzimidazole)nickel(II)] 6 , and [bis(2-guanidino-1,10-dihydro-benzimidazole)zinc(II)]nitrate 7 were determined based on ¹H, ¹³C, and ¹⁵N NMR spectroscopy. The X-ray diffraction structures of 2a, 2b, 3, 6 , and 7 were obtained. The results show that 1a has an open structure without an intramolecular hydrogen bond in DMSO or DMF. The imidazolic N-3 is the preferred basic site in solution for protonation, methylation, and coordination and not N-10 as was suggested from semiempirical calculations. Under strong acidic conditions, diprotonation occurs at N-3 and N-10. In the solid state, 3 and 6 were protonated preferently at N10 rather than at N-1. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 397–410, 1997