Intermolecular spin–spin coupling constants between P atoms

This paper reports the study by NMR spectroscopy and ab initio methods of the structure of 3,4-dimethyl-1-cyanophosphole and its dimer. The dimer presents a P···P interaction of the pnictogen type due to the presence of σ-holes. NMR of the monomer was recorded in CDCl₃ solution while NMR of the dimer corresponds to the solid state (CPMAS) experiments. The ^2pJ_PP spin–spin coupling constant has not been measured, but calculated at the B3LYP level. AIM, NBO and ELF methodologies have been used to describe the electronic structure of the dimer.     

Mechanism of the grafting of organosilanes on mineral surfaces. IV. Phenylderivatives of sepiolite and poly (organosiloxanes)

Grafting reactions of phenyl groups on silica substrates using as reagents various phenylsilanes with variable distances between the silicon atoms and the aromatic ring by 0, 1, and 2 methylene groups [Si–(CH₂) _n –C₆H₅] were studied. Two different silicates have been selected as source of silica: sepiolite and tetraethyl-orthosilicate (TEOS). Sulfonic- and nitro-derivatives prepared from these phenyl compounds by electrophilic substitution reactions, have been obtained. The mechanism of these processes has been studied in relation to the number of methylene separating groups belonging to the starting reagents. The characterization of such systems has been achieved by XRD,²⁹Si and¹³C NMR/MAS, IR, and laser microprobe mass spectrometry.Parts I, II and III published in this Journal (1978) 256:135–139, (1979) 257:178–181 and (1985) 263:1025–1030     

Analysis by gas chromatography-mass spectrometry of the essential oil from the aerial parts of Pimpinella junoniae Ceb. & Ort., gathered in La Gomera, Canary Islands, Spain

The essential oil from the aerial parts of Pimpinella junoniae Ceb. & Ort., growing in La Gomera, Canary Islands, Spain, was studied by gas chromatography and gas chromatography-mass spectrometry, and 43 constituents were identified. The major components were found to be alpha-zingiberene (20.6%), alpha-pinene (17.9%), (E)-beta-farnesene (9.3%), ar-curcumene (7.4%), beta-phellandrene (7.0%), beta-bisabolene (6.1%) and epoxypseudoisoeugenyl 2-methylbutyrate (6.0%). The decomposition product of epoxypseudoisoeugenol derivatives, 5-methoxy-2-methylbenzofuran (5.7%), moderate amounts of other arylpropanoids with the pseudoisoeugenol skeleton (total percentage, 5.2%) and other compounds such as beta-sesquiphellandrene (3.0%), cis-beta-guaiene (1.5%), alpha-phellandrene (1.5%) and alpha-bisabolol (1.3%), were also found.     

A structural study of new tetrakis(1H-pyrazol-1-yl)methanes

Tetrakis(1H-pyrazol-1-yl)methanes are very rare compounds of which only two are known: the unsubstituted 1 obtained classically by Hückel in 1937 from carbon tetrachloride and prepared again several times and the 3,5-dimethyl substituted 2 obtained serendipitously by Pombeiro in 2009. We have now extended this group to include four new derivatives 8, 9, 11 and 12 bearing methyl groups. The X-ray crystal structure of the four compounds has been determined. They have been studied by NMR both in solution (¹H, ¹³C, ¹⁵N) and in the solid state (¹³C and ¹⁵N). DFT calculations of the six compounds (geometries, energies and absolute shieldings) have been used to discuss the experimental observations.     

Rose Bengal‐Sensitized Photooxidation of the Dipeptides l‐Tryptophyl‐l‐Phenylalanine,l‐Tryptophyl‐l‐Tyrosine and l‐Tryptophyl‐l‐Tryptophan: Kinetics,Mechanism and Photoproducts¶

The Rose Bengal‐sensitized photooxidations of the dipeptides l ‐tryptophyl‐l ‐phenylalanine (Trp‐Phe), l ‐tryptophyl‐l ‐tyrosine (Trp‐Tyr) and l ‐tryptophyl‐l ‐tryptophan (Trp‐Trp) have been studied in pH 7 water solution using static photolysis and time‐resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O₂(¹Δ_g)) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di‐ and polypeptides of photooxidizable amino acids. The ratio between chemical (k_r) and overall (k_t) rate constants for the interaction O₂(¹Δ_g)‐dipeptide indicates that Trp‐Phe and Trp‐Trp are good candidates to suffer photodynamic action, with k_rlk_t values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp‐Tyr, a lower k_rlk_t value (0.18) has been found, likely as a result of the high component of physical deactivation of O₂(¹Δ_g) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O₂(¹Δ_g) attack is the Trp group and suggests a much lower accumulation of kynurenine‐type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a‐oxidized hydrogenated pyrrolo[2,3‐b]indoles.     

Quantum trajectories in atom-surface scattering with single adsorbates: the role of quantum vortices

In this work, a full quantum study of the scattering of He atoms off single CO molecules, adsorbed onto the Pt(111) surface, is presented within the formalism of quantum trajectories provided by Bohmian mechanics. By means of this theory, it is shown that the underlying dynamics is strongly dominated by the existence of a transient vortitial trapping with measurable effects on the whole diffraction pattern. This kind of trapping emphasizes the key role played by quantum vortices in this scattering. Moreover, an analysis of the surface rainbow effect caused by the local corrugation that the CO molecule induces on the surface, and its manifestation in the corresponding intensity pattern, is also presented and discussed.     

Different rotamer states of cytosine nucleobases in heteronuclear PtPd-, PtPd2, and Pt2Pd2Ag complexes derived from [Pt(2,2'-bpy)(1-MeC-N3)2]2+ (1-MeC = 1-methylcytosine): first examples of species with head-head oriented 1-MeC(-) ligands

[Pt(2,2'-bpy)(1-MeC-N3)(2)](NO(3))(2) (1) (2,2'-bpy = 2,2'-bipyridine; 1-MeC = 1-methylcytosine) exists in water in an equilibrium of head-tail and head-head rotamers, with the former exceeding the latter by a factor of ca. 20 at room temperature. Nevertheless, 1 reacts with (en)Pd(II) (en = ethylenediamine) to give preferentially the dinuclear complex [Pt(2,2'-bpy)(1-MeC(-)-N3,N4)(2)Pd(en)](NO(3))(2)·5H(2)O (2) with head-head arranged 1-methylctosinato (1-MeC(-)) ligands and Pd being coordinated to two exocyclic N4H(-) positions. Addition of AgNO(3) to a solution of 2 leads to formation of a pentanuclear chain compound [{Pt(2,2'-bpy)(1-MeC(-))(2)Pd(en)}(2)Ag](NO(3))(5)·14H(2)O (5) in which Ag(+) cross-links two cations of 2 via the four available O2 sites of the 1-MeC(-) ligands. 2 and 5 appear to be the first X-ray structurally characterized examples of di- and multinuclear complexes derived from a Pt(II) species with two cis-positioned cytosinato ligands adopting a head-head arrangement. (tmeda)Pd(II) (tmeda = N,N,N',N'-tetramethylethylenediamine) and (2,2'-bpy)Pd(II) behave differently toward 1 in that in their derivatives the head-tail orientation of the 1-MeC(-) nucleobases is retained. In [Pt(2,2'-bpy)(1-MeC(-))(2){Pd(2,2'-bpy)}(2)](NO(3))(4)·10H(2)O (4), both (2,2'-bpy)Pd(II) entities are pairwise bonded to N4H(-) and O2 sites of the two 1-MeC(-) rings, whereas in [Pt(2,2'-bpy)(1-MeC(-))(2){Pd(tmeda)}(2)(NO(3))](NO(3))(3)·5H(2)O (3) only one of the two (tmeda)Pd(II) units is chelated to N4H(-) and O2. The second (tmeda)Pd(II) is monofunctionally attached to a single N4H(-) site. On the basis of these established binding patterns, ways to the formation of mixed Pt/Pd complexes and possible intermediates are proposed. The methylene protons of the en ligand in 2 are special in that they display two multiplets separated by 0.64 ppm in the (1)H NMR spectrum.     

Preferred conformers of proteinogenic glutamic acid

The molecular shape of proteinogenic glutamic acid has been determined for the first time. Vaporization of the solid amino acid by laser ablation in combination with Fourier transform microwave spectroscopy made possible the detection of five different structures in a supersonic jet. These structures have been identified through their rotational and (14)N quadrupole coupling constants. All conformers show hydrogen bonds linking the amino and alpha carboxylic group through N-H···O═C (type I) or N···H-O (type II) interactions. In three of them there are additional hydrogen bonds established between the amino group and the carboxylic group in the gamma position. Entropic effects related to the side chain have been found to be significant in determining the most populated conformations.