Optical second-harmonic generation from a monolayer of centrosymmetric molecules adsorbed on silver

An increase in the optical second-harmonic signal arising from an electrochemically treated silver surface upon adsorption of a monolayer of the centrosymmetric molecule pyrazine is reported and an effective second-order non-linear polariz-ability for the adsorbed species deduced. These investigations illustrate the potential of second-harmonic generation in the elucidation of interfaces.     

cis‐(6RS,13RS)‐3,3,10,10‐Tetra­methyl‐6,13‐diphenyl‐1,8‐dioxa‐4,11‐diaza­cyclo­tetra­decane‐2,5,9,12‐tetra­one and two of its precursors

The title macrocycle, C₂₆H₃₀N₂O₆, (VI), was obtained by `direct amide cyclization' from the linear precursor 3‐hydr­oxy‐N‐[1‐methyl‐1‐(N‐methyl‐N‐phenyl­carbamoyl)ethyl]‐2‐phenylpropanamide, the N‐methyl­anilide of rac‐2‐methyl‐2‐[(3‐hydroxy‐2‐phenyl­propanoyl)­amino]­propanoic acid, C₁₃H₁₇NO₄, (IV). The reaction proceeds via the inter­mediate rac‐2‐(2‐hydroxy‐1‐phenyl­ethyl)‐4,4‐dimethyl‐1,3‐oxazol‐5(4H)‐one, C₁₃H₁₅NO₃, (V), which was synthesized independently and whose structure was also established. Unlike all previously described analogues, the title macrocycle has the cis‐diphenyl configuration. The 14‐membered ring has a distorted rect­angular diamond‐based [3434] configuration and inter­molecular N—H⋯O hydrogen bonds link the mol­ecules into a three‐dimensional framework. The propanoic acid precursor forms a complex series of inter­molecular hydrogen bonds, each of which involves pairwise association of mol­ecules and which together result in the formation of extended two‐dimensional sheets. The oxazole inter­mediate forms centrosymmetric hydrogen‐bonded dimers in the solid state.     

Experimental and theoretical studies of the Bi-excited collision systems He*(23 S) + He*(23 S, 21 S) at thermal and subthermal kinetic energies

We have carried out a comprehensive experimental and theoretical investigation of the autoionizing collision systems He*(2³ S, 2¹ S) + He*(2³ S). We present high resolution electron energy spectra, obtained with a single He* beam (average relative collision energy 〈E _rel〉=1.6 meV) and with crossed He* beams (〈E _rel〉> =61 meV). The spectra show substantial structure, and under single beam conditions fast oscillations due to the interference of incoming and outgoing heavy particle waves in the entrance channels are observed. Accurate ab initio potential curves for the seven lowest He*—He*(Σ) molecular states have been obtained from a Feshbach projection scheme, and width functions for He*(2³ S) + He*(2³ S) have been derived by Stieltjes imaging. Based on these ab initio data, detailed quantum mechanical calculations of the electron spectra have been carried out and provide a thorough understanding of the experimentally observed spectral features. Good overall agreement of the calculated spectra with the experimental data is observed. The close coincidence in the positions of the experimental and theoretical peaks, especially for He*(2³ S) + He*(2³ S), underlines the reliability of the ab initio potentials. In the He*(2¹ S) + He*(2³ S) electron spectrum, the dominant peak is traced to be due to autoionization from the 2³Σ⁺ _g molecular state accessed via an avoided crossing. We also present a detailed discussion of the total ionization cross sections σ_tot and of the fraction σ_AI/σ_tot for associative ionization together with a critical comparison with previous work. The ionization probabilities for close collisions in entrance channels, from which autoionization is spin-allowed, are near unity, and therefore the absolute values and the collision energy dependence of the total cross sections simply reflect the long-range behaviour of the excited state potentials.     

Synthesis of a Derivative of the Peptaibol‐Antibiotic Trichovirin I 1B by Means of the ‘Azirine/Oxazolone Method’

According to the earlier published synthesis of the C‐terminal nonapeptide of Trichovirin I 1B, Z‐Ser(^tBu)‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 5 ), the complete tetradecapeptide Z‐Aib‐Asn(Trt)‐Leu‐Aib‐Pro‐Ser(^tBu)‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 11b ), a protected Trichovirin I 1B, has now been prepared by means of the ‘azirine/oxazolone method’. With the exception of the N‐terminal Aib(1), all Aib residues were introduced by the coupling of the corresponding amino or peptide acids with 2,2‐dimethyl‐2H‐azirine‐3‐(N‐methyl‐N‐phenylamine) ( 1a ) and methyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L ‐prolinate ( 3a ) as the Aib and Aib‐Pro synthons, respectively. Single crystals of two segments, i.e., the N‐terminal hexapeptide Z‐Aib‐Asn(Trt)‐Leu‐Aib‐Pro‐Ser(^tBu)‐OMe ( 23 ) and the C‐terminal octapeptide Z‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 17 ), were obtained and their structures have been established by X‐ray crystallography. Following the same strategy, the C‐terminal nonapeptide of Trichovirin I 4A, Z‐Ala‐Val‐Aib‐Pro‐Aib‐Leu‐Aib‐Pro‐Leuol ( 26 ), was also synthesized and characterized by X‐ray crystallography.     

Carbophile Additionen von Organocupraten mit 1,3-Thiazol-5(4H)-thionen

Carbophilic Additions of Organocuprates and 1,3-Thiazole-5(4H)-thiones Organocuprates and 1,3-thiazole-5(4H)-thiones 1 react in THF at 0° via carbophilic addition onto the C? S bond to give 4,5-dihydro-1,3-thiazole-5-thiols 3 (Scheme 3). This observation is in marked contrast to the previously described reaction of organolithium compounds and 1 , which undergo a thiophilic addition onto the exocyclic S-atom. As an exception, treatment of the 1,3-thiazole-5(4H)-thione 1a with tert-butyl cuprate leads to 7a (Scheme 3).     

Reactions of Polycyclic Ketones with Dimethoxycarbene; a Convenient Route for a ‘One‐Pot’ Preparation of Some α‐Hydroxycarboxylic Acid Esters

Polycyclic ‘cage’ ketones, such as pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecan‐8‐one ( 10 ), pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane‐8,11‐dione ( 11 ), and adamantan‐2‐one ( 16 ) were treated with the nucleophilic dimethoxycarbene (DMC; 1 ), which was generated thermally from 2,5‐dihydro‐2,2‐dimethoxy‐5,5‐dimethyl‐1,3,4‐oxadiazole ( 4a ) in boiling toluene. In this ‘one‐pot’ procedure, the α‐hydroxycarboxylic acid ester 12 or a corresponding derivative 15 or 17 was obtained (Schemes 4–7). Additionally, ‘cage’ thione 21 was treated with DMC under the same conditions yielding dimethoxythiirane 22 (Scheme 8). Subsequent hydrolysis or desulfurization (followed by hydrolysis on silica gel) of 22 gave α‐mercaptocarboxylate 25 and the corresponding desulfurized ester 24 , respectively. In all cases, the addition of DMC occurred stereoselectively, and the addition from the exo‐face is postulated to explain the structures of the isolated products.     

Radikalische Cyclisierungen von Alkenyl-substituierten 4,5-Dihydro-1,3-thiazol-5-thiolen

Radical Cyclizations of Alkenyl-Substituted 4,5-Dihydro-1,3-thiazole-5-thiols Heating of 5-alkenyl- or 5-alkinyl-4,5-dihydro-1,3-thiazole-5-thiols of type 5 in the presence of a radical initiator gave dithiaspirobicycles in fair-to-excellent yield (Scheme 3). Under analogous conditions, the 4,5-dihydro-4-vinyl-1,3-thiazole-5-thiol 5d underwent a cyclization to give the annellated dithiabicycle 7 (Scheme 4). In this reaction, a minor product 8 was formed by an unknown reaction mechanism. The structure of 8 was established by X-ray crystallography. The starting 1,3-thiazole-5-thiols 5 have been synthesized by carbophilic alkylation of me C?S group of 1,3-thiazole-5(4H)-thiones with Grignard-reagents or alkylcuprates. The thiazolethiones were obtained by the reaction of 3-amino-2H-azirines with thiobenzoic acid followed by sulfurization and cyclization. The 4-benzyl derivative 1b was thermally rearranged via 1,3-benzyl migration to yield the benzyl (1,3-thiazol-5-yl) sulfide 11 (Scheme 5).     

The gas phase structure of tetrakis(trifluoromethylthio)ethene, CF3S)2C=C(SCF3)2

The geometric structure of (CF₃S)₂C=C(SCF₃)₂ in the vapour phase was determined by electron diffraction. The molecule possesses D₂ symmetry with the S---CF₃ bonds oriented perpendicular to the ethene plane, in alternating directions up-down-up-down. The following skeletal geometric parameters were obtained (r_a distances and angles, experimental uncertainties are 3σ values): C=C = 1.34Å (ass.), C(sp²---S = 1.761(5)Å, S---C(sp³) = 1.832(5)Å, S---C---C = 119.6(4)°, C---S---C = 100.6(13)°, and ø(C=C---S---C) = 90.9(11)°. The gas phase conformation differs considerably from the crystal structure, where the molecule possesses C_i symmetry and the CF₃ groups, which are bonded to cis-standing sulfur atoms, lie on the same side of the ethene plane with dihedral angles ø(C=C---S---C) of 117° and 127°.