Species Trans Research Articles

Rotational spectra of o-, m-, and p-cyanophenol and internal rotation of p-cyanophenol

Rotational spectra of p-, m-, and o-cyanophenol have been measured in the range of 10.5-21 GHz and fit using Watson's A-reduction Hamiltonian coupled with nuclear quadrupole coupling interaction terms for the (14)N nuclei. Ab initio calculations at the MP2/6-311++G(d,p) and CCSD(T)/6-311++G(d,p) levels predict the cis conformers of m- and o-cyanophenol to be more stable than the corresponding trans conformers. A natural bond orbital analysis of the hydrogen bonding interaction in o- and m-cyanophenol revealed an intramolecular hydrogen bond that preferentially stabilizes the cis conformer of o-cyanophenol but there was no evidence of hydrogen bonding interactions in cis m-cyanophenol. We recorded 25 a- and b-type rotational transitions for cis o-cyanophenol; the rotational constants are A = 3053.758(2) MHz, B = 1511.2760(3) MHz, and C = 1010.7989(2) MHz. The trans conformer of o-cyanophenol was not observed. We recorded 14 a- and b-type rotational transitions for cis m-cyanophenol and 16 a- and b-type rotational transitions for trans m-cyanophenol. The rotational constants are A = 3408.9200(2) MHz, B = 1205.8269(2) MHz, and C = 890.6672(1) MHz and A = 3403.1196(3) MHz, B = 1208.4903(2) MHz, and C = 891.7241(2) MHz for the cis and trans species, respectively. Rotational transitions of the p-cyanophenol monomer are split due to the internal rotation of the hydroxyl group with respect to the aromatic ring. We recorded 25 a- and b-type rotational transitions for p-cyanophenol; the b-type transitions are split by 40 MHz. The rotational constants are A = 5612.96(2) MHz, B = 990.4283(6) MHz, and C = 841.9363(6) MHz. The ground state spitting DeltaE is 20.1608(6) MHz and the barrier to internal rotation, V(2), is 1413(2) cm(-1) from a fit of the rotational transitions to an internal axis system Hamiltonian. The barrier to internal rotation was modeled at the MP2/6-311++G(d,p) level and the effects of substituents on the phenolic ring and the barriers to internal rotation are discussed.

Electronic structure of the members of the electron transfer series [NiL]z (z = 3+, 2+, 1+, 0) and [NiL(X)]n (X = Cl, CO, P(OCH3)3) species containing a tetradentate, redox-noninnocent, Schiff base macrocyclic ligand L: an experimental and density functional theoretical study

The electronic structure of the four members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) have been established experimentally (EPR spectroscopy and X-ray crystallography) and by density functional theoretical (DFT) calculations using the B3LYP functional in conjunction with a conductor-like screening model (COSMO) for acetonitrile solvent effects. L represents a generic designation of the tetradentate macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]-heptadeca-1(17),2,11,13,15-pentane where the true oxidation level is not specified; (L(Ox))(0) represents its neutral form, (L )(1-) is the one-electron reduced pi radical anion, and (L(Red))(2-) is the singlet (or triplet) diradical dianion of this ligand. It is shown that the above series consists of square planar [Ni(III)(L(Ox))](3+) (S = 1/2), [Ni(II)(L(Ox))](2+) (S = 0), [Ni(II)(L )](1+) (S = 1/2), [Ni(II)(L(Red))](0) (S = 0). The structure of [Ni(II)(L(Red))](0) has been determined by X-ray crystallography. The electrochemistry of [Ni(II)(L(Ox))](PF(6))(2) in the presence of hard chloride anions shows the presence of trans-[Ni(III)(L(Ox))Cl(2)](+), the EPR spectrum of which has been recorded and calculated, and of trans-[Ni(II)(L(Ox))Cl(2)](0) (S = 1). Upon further reduction the coordinated Cl(-) ligands dissociate and [Ni(II)(L )](1+) and [Ni(II)(L(Red))](0) are successively generated. Similarly, in the presence of good pi-acceptor ligands such as CO or P(OCH(3))(3) the following five-coordinate, square base pyramidal species are found to be stable: [Ni(I)(L(Ox))(X)](1+) (S = 1/2), [Ni(I)(L )(X)](0) (S = 0, 1) (X = CO, P(OCH(3))(3)). As shown by EPR spectroscopy in the work of J. Lewis and M. Schröder, J. Chem. Soc., Dalton Trans., 1982, 1085, the monocations consist of a central nickel(i) ion (d(9), S(Ni) = 1/2). These spectra have been faithfully reproduced by the calculations. The neutral complexes [Ni(I)(L )(X)](0) are singlet or triplet diradicals comprising a central nickel(i) ion and a pi radical anion (L )(1-). Interestingly, six-coordinate species trans-[Ni(L)(X)(2)](n) (n = 2+, 1+, 0) are computationally not stable in the gas phase or in solution. No experimental evidence has been found for their existence.

Reactions of cationic complex [(η 5-C 5Me 5)Re(CO) 3I] + with primary amines leading to cyclic carbamoyl complexes

The reaction of cationic complex [( 5-C 5Me 5)Re(CO) 3I] + with aliphatic and aromatic primary amines unexpectedly produced the chelated carbamoyl species trans-( 5: 1-C 5Me 4CH 2NRC( O))Re(CO) 2(I) ( 1, R = Me; 2, R = Pr; 3, R = Ph; 4, R = p-tolyl). The 1-coordination of carbamoyl moiety linkages to a methylene group of tetramethylcyclopentadienyl ligand was confirmed by X-ray crystallography of complex 3. All the complexes were isolated as pure samples and fully characterized by IR, 1H and 13C NMR spectroscopies, mass spectrometry and elemental analysis.

Fundamental Studies of Tungsten Alkylidene Imido Monoalkoxidepyrrolide Complexes

Two diastereomers of the monoalkoxidepyrrolide (MAP) species, W(NAr)(CH(2))(Me(2)Pyr)(OR*) (1; Ar = 2,6-diisopropylphenyl, Me(2)Pyr = 2,5-dimethylpyrrolide, OR* = (R)-3,3'-dibromo-2'-(tert-butyldimethylsilyloxy)-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphthyl-2-olate), were generated through addition of R*OH to W(NAr)(CH(2))(Me(2)Pyr)(2). The unsubstituted tungstacyclobutane species, W(NAr)(C(3)H(6))(Me(2)Pyr)(OR*) (2), was isolated by treating the mixture of diastereomers of 1 with ethylene. An X-ray study revealed 2 to have a trigonal bipyramidal structure in which the imido and phenoxide ligands are in axial positions. A variety of NMR experiments were carried out on 1 and 2. The major findings are the following: (i) the methylidene ligands in the two diastereomers of 1 rotate readily about the W horizontal lineC bond (k = 2-7 s(-1) at 22 degrees C); (ii) NMR studies are consistent with 2 breaking up to give an intermediate alkylidene/ethylene complex, (R)- and (S)-W(CH(2))(C(2)H(4)); and (iii) the ethylene in the (R)-W(CH(2))(C(2)H(4)) intermediate can rotate about the W-ethylene bond axis at approximately the same rate as 2 re-forms or ethylene is lost to give 1. Compound 1 reacts with trimethylphosphine to yield (R)-1(PMe(3)). Two intermediate PMe(3) adducts were observed and found to convert to (R)-1(PMe(3)) in an intramolecular fashion with an average rate constant at 5 degrees C of approximately 1.4 x 10(-4) s(-1). Both neophylidene (4) and methylidene (5) MAP species containing 2,3,5,6-tetraphenylphenoxide ligand also were prepared. Compound 5 can be heated to 80 degrees C, where methylidene rotation about the W=C bond is facile and observable in a variable-temperature (1)H NMR spectrum. A (1)H-(1)H EXSY spectrum of 5 in benzene-d(6) at 20 degrees C showed that the methylidene protons are exchanging with k = 90 s(-1). A structure of 5(THF) showed it to be a square pyramid with the methylidene ligand in the apical position and THF coordinated trans to the imido ligand. Exposure of 5 to ethylene generated the tungstacyclobutane complex, W(NAr)(C(3)H(6))(Me(2)Pyr)(OR) (6), whose structure is analogous to that of 2. Treatment of 5 with PMe(3) yielded yellow 5(PMe(3)), an X-ray study of which revealed it to be a square pyramid with the methylidene ligand in the apical position and the phosphine trans to the pyrrolide. These studies suggest that metallacyclobutane intermediates in metathesis reactions with MAP species are likely to contain axial imido and phenoxide ligands, that metallacycles are formed when an olefin approaches the metal in a MAP species trans to the pyrrolide, and that the configuration at the metal inverts as a consequence of each forward metathesis step.

Activation of ethylene and ammonia at iridium: C–H versus N–H oxidative addition

The ammine complexes cis-trans-[Ir(4-C5NF4)(H)2(NH3)(PiPr3)2] (3) and cis-[Ir(4-C5NF4)(H){(E)-(MeO2C)CCH(CO2Me)}(NH3)2(PiPr3)] (5) are generated on reaction of cis-trans-[Ir(4-C5NF4)(H)2(PiPr3)2] (1) or of the vinyl compound trans-[Ir(4-C5NF4)(H){(E)-(MeO2C)CCH(CO2Me)}(PiPr3)2] (4) with NH3, respectively. Photolysis of 5 gives complex [Ir(4-C5NF4)(H){kappa2-(C,O)-(Z)-(MeO2C)CCH(COOMe)}(NH3)(PiPr3)] (6). Treatment of the ethylene compound trans-[Ir(4-C5NF4)(C2H4)(PiPr3)2] (2) with ammonia yields C-H activation products such as the hydrido vinyl species trans-[Ir(4-C5NF4)(H)(C2H3)(NH3)(PiPr3)2] (7) and the cyclometallation products [Ir(4-C5NF4)(H)(NH3){kappa2-(P,C)-CH2CH(CH3)PiPr2}(PiPr3)] (8a, 8b). A subsequent reaction leads to the generation of the binuclear oxidative addition product of ammonia [Ir(4-C5NF4)(H)(-NH2)(NH3)(PiPr3)]2 (9).

UV Resonant Two-Photon Ionization Spectrum of 1-Naphthol

The resonant two-photon ionization(R2PI) spectrum of 1-naphthol(1NP) was studied from 305 to 322 nm using supersonic molecular beam technique.1NP has two rotamers:cis and trans.The R2PI spectrum was used to characterize the S1←S0 transition of the trans species and its original band appeared at 317.8 nm(31456 cm-1).The vibrational bands of the trans species from the R2PI spectrum can be assigned to in-plane vibrational modes with a′ symmetry.Ab initio and density functional theory(DFT) calculations indicate that the cis rotamer has higher energy than the trans rotamer by 439 cm-1 in the ground state but it has a lower exciting energy than the trans rotamer by 1216 cm-1 for the S1 ←S0 transition.These calculated results are in good agreement with the experimental trend where the difference between the cis and trans energies in the S0 state is 220 cm-1 and the difference between the cis and trans exciting energies is 274 cm-1.There are no spectral features from the cis rotamer in the R2PI spectrum and we attribute this to the efficient cooling conditions in the molecular beam,which leads to a very small cis rotamer population with the higher energy(220 cm-1).

T‐Shaped Platinum Boryl Complexes: Synthesis and Structure

A series of cationic T-shaped 14-electron boryl complexes of the type trans-[(Cy3P)2Pt(B(X)X')]+ (X=Br; X'=ortho-tolyl, tBu, NMe2, piperidyl, Br; XX'=(NMe2)2, catecholato) were synthesized by halide abstraction from trans-[(Cy3P)2Pt(Br)(B(X)X')] (Cy=cyclohexyl) with Na[BArf 4] (Arf=3,5-(CF3)2C6H3), K[B(C6F5)4], or Na[BPh4]. X-ray diffraction studies were performed on all compounds, revealing a subtle correlation between the trans-influence of the boryl moiety and the Pt-H and Pt-C separations. However, no notable agostic C-H interaction with the platinum center was detected. trans-[(Cy3P)2Pt(BCat)]+ (Cat=catecholato), the complex with the shortest Pt-H and Pt-C distances, was treated with Lewis bases (L), forming compounds of the type trans-[(Cy3P)2Pt(L)(BCat)]+, thus proving a decisive influence of the degree of trans-influence exerted by the boryl ligands on the chemical reactivity of the title complexes. Another point that was investigated and clarified is the different behavior of trans-[(Cy3P)2Pt(Br)(B(Br)Mes)] (Mes=mesityl) towards K[B(C6F5)4] with formation of the borylene species trans-[(Cy3P)2Pt(Br)(BMes)]+.

Ultrafast time-resolved absorption spectroscopy of geometric isomers of carotenoids

The structures of a number of stereoisomers of carotenoids have been revealed in three-dimensional X-ray crystallographic investigations of pigment–protein complexes from photosynthetic organisms. Despite these structural elucidations, the reason for the presence of stereoisomers in these systems is not well understood. An important unresolved issue is whether the natural selection of geometric isomers of carotenoids in photosynthetic pigment–protein complexes is determined by the structure of the protein binding site or by the need for the organism to accomplish a specific physiological task. The association of cis isomers of a carotenoid with reaction centers and trans isomers of the same carotenoid with light-harvesting pigment–protein complexes has led to the hypothesis that the stereoisomers play distinctly different physiological roles. A systematic investigation of the photophysics and photochemistry of purified, stable geometric isomers of carotenoids is needed to understand if a relationship between stereochemistry and biological function exists. In this work we present a comparative study of the spectroscopy and excited state dynamics of cis and trans isomers of three different open-chain carotenoids in solution. The molecules are neurosporene ( n = 9), spheroidene ( n = 10), and spirilloxanthin ( n = 13), where n is the number of conjugated π-electron double bonds. The spectroscopic experiments were carried out on geometric isomers of the carotenoids purified by high performance liquid chromatography (HPLC) and then frozen to 77 K to inhibit isomerization. The spectral data taken at 77 K provide a high resolution view of the spectroscopic differences between geometric isomers. The kinetic data reveal that the lifetime of the lowest excited singlet state of a cis-isomer is consistently shorter than that of its corresponding all- trans counterpart despite the fact that the excited state energy of the cis molecule is typically higher than that of the trans molecule. Quantum theoretical calculations on an n = 9 linear polyene were carried out to examine this process. The calculations indicate that the electronic coupling terms are significantly higher for the cis isomer, and when combined with the Franck–Condon factors, predict internal conversion rates roughly double those of the all- trans species. The electronic effects more than offset the decrease in coupling efficiencies associated with the higher system origin energies and explain the observed shorter cis isomer lifetimes.

Ruthenium(III) Chloride Complex with a Tridentate Bis(arylimino)pyridine Ligand: Synthesis, Spectra, X-ray Structure, 9-Ethylguanine Binding Pattern, and In Vitro Cytotoxicity

The synthetic, spectroscopic, structural, and biological studies of a bis(arylimino)pyridine Ru(III) chloride compound containing the ligand, 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine are reported. The bis(arylimino)pyridine ligand, with three donor nitrogen atoms, was synthesized by condensation of 2,6-pyridinedicarboxaldehyde with 2,4,6-trimethylaniline. The Ru(III) complex, with formula [RuCl 3(L1)](H 2O) (RuL1), where L1 = 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine, was structurally determined on the basis of analytical and spectroscopic (IR, UV-vis, ESI-MS) studies. A straightforward strategy to fully characterize the paramagnetic compound using advanced (1)H NMR is reported. This new complex is a prototype for a series of new anticancer Ru(III) and Ru(II) compounds with improved cytostatic properties; likely to be modified in a desirable manner due to the relatively facile ligand modification of the bis(imino)pyridines and their molecular architecture. The present Ru(III) complex is the first example of this family of Ru(III)/Ru(II) anticancer compounds with the aimed physicochemical characteristics. Although the ligand itself is moderately active in selected cell lines (EVSA-T and MCF-7), the activity of the [Ru(L1)Cl 3] complex has increased significantly for a broad range of cancer cell lines tested in vitro (IC 50 values = 11 approximately 17 microM). Reaction of the RuL1 species with the DNA model base 9-ethylguanine (9EtGua) was found to produce in a redox reaction the species trans-[Ru(II)(L1)(9EtGua) 2(H 2O)](ClO 4) 2 (abbreviated as RuL1-9EtGua), which was studied in solution and also in the solid state, by X-ray crystallography. The structure comprises the as yet unknown trans-bis(purine)Ru(II) unit.

En route to diplatinum polyynediyl complexes trans,trans-(Ar)(R3P)2Pt(C≡C)nPt(PR3)2(Ar): Untold tales, including end-group strategies

Abstract Reactions of {(C6F5)Pt[S(CH2CH2-)2](μ-Cl)}2 and R3P yield the bis(phosphine) species trans-(C6F5)(R3P)2PtCl [R = Et (Pt'Cl), Ph, (p-CF3C6H4)3P; 88-81 %]. Additions of Pt'Cl and H(C≡C) n H (n = 1, 2; HNEt2, 20 mol % CuI) give Pt'C2H (37 %, plus Pt'I, 16 %) and Pt'C4H (88 %). Homocoupling of Pt'C4H under Hay conditions (O2, CuCl, TMEDA, acetone) gives Pt'C8Pt' (85 %), but Pt'C2H affords only traces of Pt'C4Pt'. However, condensation of Pt'C4H and Pt'Cl (HNEt2, 20 mol % CuI) yields Pt'C4Pt' (97 %). Hay heterocouplings of Pt'C4H or trans-(p-tol)(Ph3P)2Pt(C≡C)2H (Pt*C4H) and excess HC≡CSiEt3 give Pt'C6SiEt3 (76 %) or Pt*C6SiEt3 (89 %). The latter and wet n-Bu4N+ F- react to yield labile Pt*C6H (60 %). Hay homocouplings of Pt*C4H and Pt*C6H give Pt*C8Pt* (64 %) and Pt*C12Pt* (64 %). Reaction of trans-(C6F5)(p-tol3P)2PtCl (PtCl) and HC≡CH (HNEt2, 20 mol % CuI) yields only traces of PtC2H. However, an analogous reaction with HC≡CSiMe3 gives PtC2SiMe3 (75 %), which upon treatment with silica yields PtC2H (77 %). An analogous coupling of trans-(C6F5)(Ph3P)2PtCl with H(C≡C)2H gives trans-(C6F5)(Ph3P)2Pt(C≡C)2H (34 %). Advantages and disadvantages of the various trans-(Ar)(R3P)2Pt end-groups are analyzed.

Experimental Studies on the trans‐Influence of Boryl Ligands in Square‐Planar Platinum(II) Complexes

A series of platinum(II) boryl complexes of general formula trans-[(Cy(3)P)2Pt(Br)(BX2)], including the rare dibromoboryl species trans-[(Cy(3)P)2Pt(Br)(BBr2)], were synthesized by oxidative addition of the B-Br bond of a number of bromoboranes to [Pt(PCy3)2]. X-ray diffraction studies were performed on several such compounds. Comparison of the Pt--Br bond lengths allowed an empirical assessment of the trans-influence of different boryl ligands. A trans-influence scale was thus deduced and the results were compared with those previously computed for compounds of the type trans-[(Me(3)P)2Pt(Cl)(BX2)].

Reactivity of a Platinum Iminoboryl Complex toward Lewis and Brønsted Acids

Differently from typical iminoboranes, the iminoboryl complex trans-[(PCy3)2Pt(B≡NSiMe3)(Br)] (2) displays remarkable stability against oligomerization. Its clean reactivity toward Lewis acids led to the synthesis of the neutral platinum borylene complex trans-[(PCy3)2Pt{BN(SiMe3)(AlCl3)}(Br)], through coordination of AlCl3 to the nitrogen atom of the iminoboryl ligand. Surprising stability of the Pt−B linkage in 2 is further manifested in its reaction with MeOH, yielding the alkoxy(amino)boryl species trans-[(PCy3)2Pt{B(OMe)(N(H)SiMe3)}(Br)].

Donor−Acceptor Interactions and Electron Transfer in Cyano-Bridged Trinuclear Compounds

The NIR donor-acceptor charge transfer (DACT) bands of the series of trinuclear complexes trans-[(NC)5Fe(II/III)(mu-CN)RuIIL4(mu-NC)FeIII(CN)5](5/4-) (L= pyridine, 4-tert-butylpyridine, and 4-methoxypyridine) are analyzed in terms of a simplified molecular orbital picture that reflects the interaction between the donor and acceptor fragments. The degree of electronic coupling between the fragments is estimated by a full fit of the DACT band profiles according to a three-state model inspired in the Mulliken-Hush formalism. The information is complemented with determinations performed on the asymmetric heterotrinuclear species trans-[(NC)5CoIII(mu-CN)RuII(py)4(mu-NC)FeIII(CN)5]4-, whose preparation is reported here for the first time. The analysis of the NIR spectra of the symmetric trans-[(NC)5FeIII(mu-CN)RuIIL4(mu-NC)FeIII(CN)5]4- species reveals a low degree of mixing between the terminal acceptor fragments and the bridging moiety containing RuII, with H12 values between 1.0 x 10(3) and 1.5 x 10(3) cm-1. The reorganization energy contributions seem to be the same for the three species, even when the spectra were recorded in different media. This observation also applies for the CoIII-substituted compound. The computed potential energy surfaces (PES) of the ground state for these complexes show only one stationary point, suggesting that the FeII-RuIII-FeIII (or FeII-RuIII-CoIII) electronic isomers are not thermally accessible. One-electron reduction leads to asymmetric trans-[(NC)5FeII(mu-CN)RuIIL4(mu-NC)FeIII(CN)5]5- compounds with potentially two DACT bands involving the RuII and the FeII donor fragments. These species reveal a similar degree of electronic mixing but the PES shows three minima. We explore the role of the bridging fragment in the long-range thermally induced electron transfer between the distant iron centers. The results suggest that superexchange and hopping might become competitive paths, depending on the substituents in the bridging fragment.

Axial addition in diastereoisomeric [Cu(Me814ane)](ClO4)2 complexes: antifungal and antibacterial activities

A series of diastereoisomeric square planar copper(II) perchlorate complexes, [Cu(Me8[14]ane)](ClO4)2, undergo axial addition reactions with H2O, NCS− and to yield the octahedral species trans-[CuL1X2](ClO4) n (L1 = isomeric Me8[14]ane; X = H2O, NCS, NO2 or NO3; n = 2 when X = H2O; n = 0 when X = NCS, NO2 or NO3). The products have been characterized on the basis of analytical, spectroscopic, magnetic and conductance data. Some derivatives are unstable in open air and may react with adventious H2O to form diaquo complexes. Antifungal and antibacterial activities of the uncomplexed ligands, the square planar copper(II) complexes and their axial addition products have been investigated against a range of phytopathogenic fungi and bacteria.

A New Family of Mono- and Dicarboxylic Ruthenium Complexes [Ru(DIP)2(L2)]2+ (DIP = 4,7-diphenyl-1,10-phenanthroline): Synthesis, Solution Behavior, and X-ray Molecular Structure of trans-[Ru(DIP)2(MeOH)2][OTf]2

A new family of ruthenium complexes of general formula [Ru(DIP)2(L2)]2+, where DIP = 4,7-diphenyl-1,10-phenanthroline, a bidentate ligand with an extended aromatic system, was prepared and fully characterized. When L is a monodentate ligand, the following complexes were obtained: L = CF3SO3(-1) (2), CH3CN (3), and MeOH (4). When L2 is a bidentate ligand, the compounds [Ru(DIP)2(Hcmbpy)][Cl]2 (5) and [Ru(DIP)2(H2dcbpy)][Cl]2 (6) were prepared (Hcmbpy = 4-carboxy-4'-methyl-2,2-bipyridine, H2dcbpy = 4,4'-dicarboxy-2,2'-bipyridine). Complex [Ru(DIP)2(MeOH)2][OTf]2 (4) displayed a trans configuration of the DIP ligands, which is rare for octahedral complexes featuring DIP bidentate ligands. DFT calculations carried out on 4 showed that the cis isomer is more stable by 12.2 kcal/mol relative to the trans species. The solution behaviors of monocarboxylic complex [Ru(DIP)2(Hcmbpy)][Cl]2 (5) and dicarboxylic complex [Ru(DIP)2(H2dcbpy)][Cl]2 (6) were investigated by 1H NMR spectroscopy. VT-NMR, concentration dependence, and reaction with NaOD allowed us to suggest that aggregation of the cationic species in solution, especially for 6, originates mainly from hydrogen bonding interactions.

Numerical Model for Bridgman-Stockbarger Crystal Growth with a Magnetic Field

This paper presents a model for the unsteady species transport for the growth of doped semiconductor crystals during the vertical Bridgman‐Stockbarger process with a steady axial magnetic field. This dilute species transport depends on the convective and diffusive species transport of the dopant. This convective species transport is driven by buoyant convection in the melt, which produces compositional nonuniformities in both the melt and the crystal. This transient model predicts the distribution of species in the entire crystal grown in a steady axial magnetic field. The present study presents results of concentration in the crystal and in the melt at several different stages during crystal growth. I. Introduction D URING crystal growth without a magnetic field or with a weak magnetic field, turbulent or oscillatory melt motions can produce undesirable spatial oscillations of the concentration, or microsegregation, in the crystal. 1 Turbulent or oscillatory melt motions lead to fluctuations in the heat transfer across the growth interface from the melt to the crystal. Because the local rate of crystallization depends on the balance between the local heat fluxes in the melt and the crystal, fluctuations in the heat flux from the melt create fluctuations in the local growth rate, which create microsegregation. A moderate magnetic field can be used to create a body force that provides an electromagnetic (EM) damping of the melt motion can to eliminate oscillations in the melt motion and thus in the concentration of the crystal. Unfortunately, the elimination of mixing and a moderate or strong EM damping of the residual melt motion can lead to a large variation of the crystal’s composition in the direction perpendicular to the growth direction (radial macrosegregation). On the other hand, if the magnetic field strength is so strong that the melt motion is reduced sufficiently so that it has no effect on the composition in the crystal, then this diffusion-controlled species transport can produce a radially and axially uniform composition in the crystal grown. 2 To achieve diffusion-controlled species trans�

The Impact of Pyrrolidine Hydroxylation on the Conformation of Proline-Containing Peptides

[structure: see text] A series of eight dipeptides of the general formula Ac-Phe-Pro-NHMe was synthesized and the thermodynamics of the cis --> trans isomerization about the central amide bond were studied by NMR. Pro* represents the following prolines: l-proline (Pro), l-trans-4-hydroxyproline (Hyp), l-cis-4-hydroxyproline (hyp), l-cis-4-methoxyproline (hyp[OMe]), l-trans-3-hydroxyproline (3-Hyp), l-cis-3-hydroxyproline (3-hyp), l-2,3-trans-3,4-cis-3,4-dihydroxyproline (DHP), and l-2,3-cis-3,4-trans-3,4-dihydroxyproline (dhp). The conformation of the pyrrolidine ring in each case is discussed in light of previous structural studies, analysis of potential stereoelectronic effects, and NMR data. Hydroxy substituents at C-4 have a greater impact on cis --> trans isomerization than analogous substituents at C-3 as a result of the intervening bond distances and bridging groups. The position of the equilibrium and its dependence on temperature are a reflection of both enthalpic and entropic factors, the latter being complicated in this study by an Ar-Pro interaction in the cis conformation. The substituents on the pyrrolidine ring determine the conformation of the five-membered ring, which in turn influences the strength of the Ar-Pro interaction, backbone dihedral angles, and the relative energy of the cis and trans species. The ultimate position of the equilibrium depends on a complex blend of steric, electronic, and conformational factors.

Study of Pt(II)-cyclic amines complexes of the types cis- and trans-Pt(amine) 2I 2 and cis- and trans-Pt(amine) 2(NO 3) 2 and their aqueous products by

Complexes of the types cis- and trans-Pt(amine) 2I 2 containing cyclic amines were synthesized and studied mainly by IR and multinuclear NMR spectroscopies. The compounds were converted to cis- and trans-Pt(amine) 2(NO 3) 2, which were also investigated. The hydrolysis and the aquation reactions of the latter compounds were then studied in D 2O in different conditions of pH. In acidic medium, the aqueous product is [Pt(amine) 2(D 2O) 2] 2+ and for a few amines, [Pt(amine) 2(D 2O)(NO 3)] + was detected. In basic pH, the main product is Pt(amine) 2(OD) 2 and Pt(amine) 2(OD)(NO 3) was detected for several compounds. In neutral pH, the cis isomers form between two and four species in fresh solutions. The most shielded species in 195Pt NMR is the monoaqua–monohydroxo complex cis-[Pt(amine) 2(D 2O)(OD)] + and the less shielded compound is the dihydroxo-bridged dimer [Pt(amine) 2(μ-OD) 2Pt(amine) 2] 2+, which were observed for all the compounds. For a few amines, the monohydroxo-bridged dimer [Pt(D 2O)(amine) 2(μ-OD)Pt(OD)(amine) 2] 2+ was detected and for cyclohexylamine, a fourth signal was assigned to a cyclic hydroxo-bridged trimer [(Pt(amine) 2(μ-OD)) 3] 3+. 195Pt NMR spectroscopy has shown that the concentration of the monomer decreases with time, while the concentration of the dimers increases. Only one product was observed for the trans isomers in neutral pH. The signal was assigned to the monoaqua–monohydroxo species trans-[Pt(amine) 2(D 2O)(OD)] +. The 13C and 1H NMR spectra of most of the complexes were measured. All the coupling constants 2,3 J( 195Pt– 1H) and 2,3 J( 195Pt– 13C) are larger in the cis compounds than in the trans isomers.

Antioxidant activity of hydroxystilbene derivatives in homogeneous solution.

The antioxidant activity of the cis and trans isomers of several analogues of resveratrol and pterostilbene has been investigated, especially with regard to the effect of the stereochemistry about the olefinic double bond. The antioxidant power of these compounds was estimated by measuring the rate constants for their reactions with peroxyl radicals and, with two of them, the bond dissociation enthalpy (BDE) of the phenolic O-H bond which is cleaved in the inhibition reaction. The present data show that in homogeneous solution the various hydroxystilbenes investigated behave as mild antioxidants with the notable exceptions of the trans isomer of 4 and 6, whose activities are only slightly lower than that of alpha-tocopherol (vitamin E). The rate constants of the inhibition reaction show that the antioxidant activity of the cis-hydroxystilbene is in all the examined cases worse, by a factor ranging between 2 and 6, than that of the corresponding trans isomers. This lower reactivity depends on enthalpy factors as it can be inferred by the experimental values of the O-H bond dissociation enthalpy in the two geometric isomers of 3',5'-di-tert-butyl-4'-hydroxy-3,5-dimethoxystilbene showing that the strength of the O-H bond in the cis isomer is larger by 1.8 kcal/mol. DFT calculations provide a rationalization of this result, indicating that, although the cis geometry implies a destabilization with respect to the trans species of both phenoxyl radical and parent hydroxystilbene, the destabilization of the radical is larger because the folding of the structure strongly reduces the delocalization of the unpaired electron on the styryl group. A comparison of these results with previously reported data on the proapoptotic activity of these stilbenoids suggests that these two properties are not correlated.

Rotational analysis of bands in the high-resolution infrared spectra of the three species of butadiene-1,4- d2; refinement of the assignments of the vibrational fundamentals

Samples of trans, trans and cis, cis forms of butadiene-1,4- d 2 have been synthesized and found to contain useful amounts of the cis, trans species as a contaminant. Assignments of fundamental frequencies for the three isotopomers of butadiene-1,4- d 2 have been extended and improved from investigations of their Raman spectra as well as their infrared (IR) spectra. High-resolution IR spectra have been recorded for the three isotopomers, and a rotational analysis has been completed for strong bands of each species. Ground state and some upper state rotational constants have been fit. Corresponding ground state moments of inertia compare favorably with equilibrium moments of inertia obtained from B3LYP/6-311++G** theory. Two 13C isotopomers are being prepared, and an improved structural analysis of butadiene will soon be available to assess how π-electron delocalization affects its structure.