Ph Moiety Research Articles

Neutral Chromium Complex with a Cr≡Si Triple Bond: Synthesis and Photoinduced H-H and Benzene C-H Bond Activation.

A neutral silylyne complex with a Cr≡Si triple bond was prepared by dehydrogenation of a chromium silylene complex with Cr-H and Si-H bonds, and was isolated as monomeric crystals, unlike dimeric forms of its tungsten and molybdenum congeners. The strong Cr(δ-)-Si(δ+) bond polarity was revealed by the reaction with MeOH and DFT calculations. The chromium silylyne complex reacted with H2 under LED (365 nm) irradiation to reproduce the precursor silylene complex with a (H)Cr=Si(H) moiety, as a result of 1,2-H-H addition across the Cr≡Si triple bond. Similarly, the chromium silylyne complex reacted with benzene under irradiation to afford an 1,2-addition product with a (H)Cr=Si(Ph) moiety, via benzene C-H bond activation accompanied by Si-C bond forming.

Polynuclear ruthenium clusters containing stibine, stibene, and stibinidene ligands

The reaction of [Ru3(CO)10(µ-dppm)] with SbPh3 has been investigated. Me3NO activation of [Ru3(CO)10(µ-dppm)] in the presence of excess SbPh3 yields the known cluster [Ru3(CO)9(SbPh3)(μ-dppm)] (1) in nearly quantitative yield. Refluxing 1 in THF yields the new triruthenium clusters [Ru3(CO)8{μ,η2-C(O)Ph}(µ-SbPh2)(µ-dppm)] (2) and [Ru3(CO)9(σ-C6H5)(μ-SbPh2)(µ-dppm)] (3). Both clusters exhibit an opened Ru3 polyhedron supported by a bridging stibene ligand (SbPh2), whose formation results from the cleavage of an Sb–Ph bond of the SbPh3 ligand in 1. Cluster 2 contains a μ,η2-C(O)Ph ligand formed by the coupling of the cleaved Ph moiety with a coordinated CO ligand. In contrast, cluster 3 contains a σ-bonded phenyl moiety that is situated trans to the μ-SbPh2 moiety. Thermolysis of [Ru3(CO)10(µ-dppm)] with SbPh3 at 80°C furnished the new cluster [Ru3(CO)7(σ-C6H5)(μ-SbPh2)2{μ-C(=O)C6H4P(Ph)CH2PPh2}] (4) as the major product, along with 2 and 3. Control experiments involving 2 and excess SbPh3 at 80°C afforded 4. In contrast, the reaction between [Ru3(CO)10(µ-dppm)] and SbPh3 at 110°C furnished the unusual Ru6Sb2 cluster [Ru6(CO)13(μ-CO)(μ-SbPh2)(μ4-SbPh){μ-C(=O)C6H4P(Ph)CH2PPh2}(μ-dppm)] (5) in addition to 2. The results of thermal control experiments demonstrating the conversion of 1 to 5 are discussed. The molecular structures of 2–5 have been established by X-ray crystallography, and the bonding in these new clusters has been examined by electronic structure calculations.

D-π-A-structured organic sensitizers with π-extended auxiliary acceptor units for high-performance dye-sensitized solar cells

Two new D-π-A-structured dyes (SGT-160 and SGT-161) with the extension of π-conjugation in the acceptor unit have been designed and synthesized to modulate LUMO energy levels and to investigate the photophysical and electrochemical properties as well as cell performances. Based on a reference dye of SGT-130 as a platform of donor-π-bridge-BTD-phenyl-acceptor sensitizers, ethynylphenylene (T-Ph) and 2-ethynylthiophene (T-Th) moieties as an auxiliary π-spacer were introduced to replace the traditional phenylene (Ph) moiety for SGT-160 and SGT-161, respectively. It was found that the insertion of an additional ethynyl moiety into the BTD-Ph acceptor unit significantly enhanced the light-harvesting capability by extending the absorption bands and intensifying the molar absorptivity. Moreover, the π-extension in the acceptor part was beneficial for stabilizing the LUMO energy levels. The density functional theory (DFT) calculations and the partial density of states (PDOS) suggest that the variation of acceptor units affects the electronic transition between the major orbitals of the dyes. As a result, dye-sensitized solar cells (DSSCs) based on SGT-160 with a chenodeoxycholic acid (CDCA) co-adsorbent exhibited high power conversion efficiencies (PCEs) of 10.28%, while DSSCs based on SGT-161 and SGT-130 showed moderate PCEs of 8.05% and 10.06%, respectively.

Theranostics Based on Liposome: Looking Back and Forward.

Liposome is one of the oldest yet most successful nanomedicine platforms. Doxil®, PEGylated liposome loaded with doxorubicin (DOX), was approved by the FDA in 1995 for the treatment of AIDS-related Kaposi's sarcoma, and it was the first approval for nanomedicine. Since then, liposome-based therapeutics were approved for the treatment of various diseases and many clinical trials are underway. The success of the liposome-based therapeutics was due to following factors: (1) ease of synthesis, (2) biocompatibility, (3) the ability to load both hydrophilic and hydrophobic agents, and (4) long circulation property after application of polyethylene glycol (PEG). Recently, more functionalities are introduced to liposome platform, which are (1) in vivo imaging probes for optical, magnetic resonance imaging (MRI), positron emission tomography (PET), and single-photon emission computed tomography (SPECT), (2) pH and temperature-sensitive lipid moiety, and (3) novel agents for photodynamic and photothermal therapies (PDT, PTT). These conventional and newly tested advantages make the liposome to be one of the most promising nanoplatforms for theranostics.

Isolation of a Lewis base stabilized parent phosphenium (PH2(+)) and related species.

The parent phosphenium ion (PH2(+)), and even any phosphenium salts bearing a hydrogen (HRP(+)), have never been observed. The addition of trifluoromethanesulfonic acid and carbon electrophiles to an NHC-PH adduct, featuring a very bulky NHC, allows for the preparation and isolation of such compounds. Computational investigations show that most of the positive charge is localized at phosphorus, demonstrating the phosphenium nature of this compound. Further, the PH moiety is able to utilize one or both of its lone pairs to bind to one or two metals.

A Fragile Zwitterionic Phosphasilene as a Transfer Agent of the Elusive Parent Phosphinidene (:PH)

The simplest parent phosphinidene, :PH (1), has been observed only in the gas phase or low temperature matrices and has escaped rigorous characterization because of its high reactivity. Its liberation and transfer to an unsaturated organic molecule in solution has now been accomplished by taking advantage of the facile homolytic bond cleavage of the fragile Si═P bond of the first zwitterionic phosphasilene LSi=PH (8) (L = CH[(C═CH2)CMe(NAr)2]; Ar = 2,6-(i)Pr2C6H3). The latter bears two highly localized lone pairs on the phosphorus atom due to the LSi═PH ↔ LSi(+)-PH(-) resonance structures. Strikingly, the dissociation of 8 in hydrocarbon solutions occurs even at room temperature, affording the N-heterocyclic silylene LSi: (9) and 1, which leads to oligomeric [PH]n clusters in the absence of a trapping agent. However, in the presence of an N-heterocyclic carbene as an unsaturated organic substrate, the fragile phosphasilene 8 acts as a :PH transfer reagent, resulting in the formation of silylene 9 and phosphaalkene 11 bearing a terminal PH moiety.

Structures and Interligand Interaction Pattern of Phosphoramidite Pd Complexes by NMR Spectroscopy: Modulations in Extended Interaction Surfaces as Stereoselection Mode of a Privileged Class of Ligands

During the last decade, phosphoramidites have been established as a so-called privileged class of ligands in various transition metal catalyses. However, the interactions responsible for their favorable properties have hitherto remained elusive. To address this issue, the formation trends, structural features, and interligand interaction patterns of several trans- and cis-[PdLL'Cl2] complexes have been investigated by NMR spectroscopy. The energetic contribution of their interligand interactions has been measured experimentally using the supramolecular balance for transition-metal complexes. The resulting energetics combined with an analysis of the electrostatic potential surfaces reveal that in phosphoramidites not only the aryl groups but the complete (CH)CH3 Ph moieties of the amine side chains form extended quasi-planar CH-π and π-π interaction surfaces. Application of the supramolecular balance has shown that modulations in these extended interaction surfaces cause energetic differences that are relevant to enantioselective catalysis. In addition, the energetics of these interligand interactions are quite independent of the actual structures of the complexes. This is shown by similar formation and aggregation trends of complexes with the same ligand but different structures. The extended quasi-planar electrostatic interaction surface of the (CH)CH3 Ph moiety explains the known pattern of successful ligand modulation and the substrate specificity of phosphoramidites. Thus, we propose modulations in these extended CH-π and π-π interaction areas as a refined stereoselection mode for these ligands. Based on the example of phosphoramidites, this study reveals three general features potentially applicable to various ligands in asymmetric catalysis. First, specific combinations of alkyl and aryl moieties can be used to create extended anisotropic interaction areas. Second, modulations in these interaction surfaces cause energetic differences that are relevant to catalytic applications. Third, bulky substituents with matching complementary interaction surfaces should not only be considered in terms of steric hindrance but also in terms of attractive and repulsive interactions, a feature that may often be underestimated in asymmetric catalysis.

Highly Variable Zr–CH2–Ph Bond Angles in Tetrabenzylzirconium: Analysis of Benzyl Ligand Coordination Modes

Analysis of a monoclinic modification of Zr(CH2Ph)4 by single-crystal X-ray diffraction reveals that the bond angles Zr–CH2–Ph in this compound span a range of 25.1°, which is much larger than previously observed for the orthorhombic form (12.1°). In accord with this large range, density functional theory calculations demonstrate that little energy is required to perturb the Zr–CH2–Ph bond angles in this compound. Furthermore, density functional theory calculations on Me3ZrCH2Ph indicate that bending of the Zr–CH2–Ph moiety in the monobenzyl compound is also facile, thereby demonstrating that a benzyl ligand attached to zirconium is intrinsically flexible, such that its bending does not require a buffering effect involving another benzyl ligand.

Diamino moiety functionalized silica nanoparticles as pseudostationary phase in capillary electrochromatography separation of plant auxins

A novel and simple method for the preparation of silica nanoparticles having surface-functionalized diamino moiety (dASNPs) was reported in our paper and characterized using scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectrometry, and thermogravimetry techniques. To test this method practically, in this contribution we describe the enhanced separation of four plant auxins - indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 2,4-dichlorophenoxyacetic acid (dCPAA), and 2-(1-naphthyl) acetic acid (NAA) - by capillary electrochromatography using diamino moiety functionalized silica nanoparticles as pseudostationary phase (PSP) in the running buffer. The effect of pH, buffer concentration, and diamino moiety functionalized silica nanoparticles concentration on the selectivity of separation was investigated. A combination of the nanoparticles and running buffer reversed the electroosmotic direction making possible the rapid and efficient separation of the auxins from the auxins migrated in the same direction with the EOF under optimum experimental conditions. A good resolution of four auxins was obtained within 5.5 min under optimum experimental conditions. The precision (RSD, n = 5) was in the range of 0.72-0.91% and 1.89-2.23% for migration time and peak area response, respectively. The detection limits were 0.48, 0.44, 0.46, and 0.42 μM for NAA, IBA, IAA, and dCPAA, respectively. Furthermore, the method was successfully tested for the determination of IAA in the grapes.

Formation of a Hybrid Compound Composed of a Saddle-Distorted Tin(IV)−Porphyrin and a Keggin-Type Heteropolyoxometalate To Undergo Intramolecular Photoinduced Electron Transfer

Nonplanar Sn(IV)-porphyrin complexes, [Sn(TMPP(Ph)(8))-Cl(2)] (1) and [Sn(TMPP(Ph)(8))(OMe)(2)] (2) (TMPP(Ph)(8): 5,10,15,20-tetrakis(4-methoxyphenyl)-2,3,7,8,12,13,17,18-octaphenylporphyrinato), were prepared and characterized by spectroscopic and electrochemical methods together with X-ray crystallography. Variable-temperature (1)H NMR study revealed that the coordination of the methoxo ligand of 2 is weak enough in solution to enhance the axial ligand exchange with a Keggin-type phosphotungstate (α-[PW(12)O(40)](3-)) due to the steric stress between the axial methoxo ligand and the peripheral phenyl groups of the porphyrin ligand. The formation of a novel 1:1 donor-acceptor complex, [Sn(TMPP(Ph)(8))(OMe)(α-[PW(12)O(40)])](2-) (4) was confirmed by (1)H NMR and UV-vis spectral titrations, and also by MALDI-TOF-MS measurements. Electrochemical measurements for the donor-acceptor complex in PhCN revealed that the Sn(IV)-TMPP(Ph)(8) moiety acts as an electron donor and the α-[PW(12)O(40)](3-) moiety acts as an electron acceptor and that the energy level of the electron-transfer (ET) state of the 1:1 complex (1.17 eV) is lower than that of the triplet excited states of the SnTMPP(Ph)(8) complex (1.31 eV). Femtosecond and nanosecond laser flash photolysis measurements indicate that intersystem crossing from the singlet excited sate to the triplet excited state occurs followed by intramolecular photoinduced electron transfer from the triplet excited state of the Sn(IV)-TMPP(Ph)(8) moiety to the α-[PW(12)O(40)](3-) moiety in the 1:1 complex in benzonitrile.

Steric effects on the control of endo/exo-selectivity in the asymmetric cycloaddition reaction of 3,4-dimethyl-1-phenylarsole

The asymmetric cycloaddition reactions of 3,4-dimethyl-1-phenylarsole and (Z/E)-diphenyl-1-propenylphosphine/diphenyl-1-styrylphosphine promoted by a chiral organopalladium(ii) complex derived from (S)-[1-(dimethylamino)ethyl]naphthalene proceeded stereoselectively to generate different exo/endo-products. The reactions involving the (Z/E)-methyl substituted phosphines gave the individual optically pure exo- and endo-cycloadducts in very high stereoselectivity (33 : 1). However, when the methyl group was replaced by a Ph moiety both (Z/E)-phenyl substituted phosphines produced the same endo-cycloadduct in a stereoselectivity of 15 : 1. Every reaction produced five new chiral centers (four of them are sterically independent) in a single step and all three optically pure As-P heterobidentate ligands were obtained in high yields. The mechanism involved in the conversion of exo- to endo-product was investigated via density functional theory calculations. Computational results were consistent with the experimentally observed endo/exo-selectivity.

Synthesis and characterization of poly(urethanes) containing sulfone and silarylene or germarylene moieties

4,4′-Sulfonyldianiline (SDA) is reacted with 4,4′-(R1R2-silylene)bis(phenyl chloroformates) or 4,4′-(R1R2-germylene)bis(phenyl chloroformates) (where R1 and R2 are Me, Et, Ph combinations) in solution yielding poly(urethanes) (PUs) of moderate molecular weight containing silarylene, germarylene and sulfone moieties. DSC and TGA analyses were carried out. The highest glass temperatures (Tg) were obtained when one or two Ph groups were bonded to a heteroatom. However, these values were always low due to the flexibility gained by the chain due to the incorporation of the sulfone function. No-significant differences in Tg were observed when silicon was replaced by germanium in the main chain. All polymers were stable up to 200–220 °C and underwent a complex decomposition process with a higher rate for PUs with side aliphatic groups. The thermal decomposition temperature (TDT) (10 wt%) for PUs based on Ph–Si–Ph moiety were higher than those obtained from germarylene-containing polymers.

3'-Intercalation of a N2-dG 1R-trans-anti-benzo[c]phenanthrene DNA adduct in an iterated (CG)3 repeat.

The conformation of the 1 R,2 S,3 R,4 S-benzo[ c]phenanthrene- N (2)-dG adduct, arising from trans opening of the (+)-1 S,2 R,3 R,4 S- anti-benzo[ c]phenanthrene diol epoxide, was examined in 5'- d(ATCGC XCGGCATG)-3'.5'-d(CATGCCG CGCGAT)-3', where X = 1 R,2 S,3 R,4 S-B[ c]P- N (2)-dG. This duplex, derived from the hisD3052 frameshift tester strain of Salmonella typhimurium, contains a (CG) 3 iterated repeat, a hotspot for frameshift mutagenesis. NMR experiments showed a disconnection in sequential NOE connectivity between X (4) and C (5), and in the complementary strand, they showed another disconnection between G (18) and C (19). In the imino region of the (1)H NMR spectrum, a resonance was observed at the adducted base pair X (4) x C (19). The X (4) N1H and G (18) N1H resonances shifted upfield as compared to the other guanine imino proton resonances. NOEs were observed between X (4) N1H and C (19) N (4)H and between C (5) N (4)H and G (18) N1H, indicating that base pairs X (4) x C (19) and C (5) x G (18) maintained Watson-Crick hydrogen bonding. No NOE connectivity was observed between X (4) and G (18) in the imino region of the spectrum. Chemical shift perturbations of greater than 0.1 ppm were localized at nucleotides X (4) and C (5) in the modified strand and G (18) and C (19) in the complementary strand. A total of 13 NOEs between the protons of the 1 R-B[ c]Ph moiety and the DNA were observed between B[ c]Ph and major groove aromatic or amine protons at base pairs X (4) x C (19) and 3'-neighbor C (5) x G (18). Structural refinement was achieved using molecular dynamics calculations restrained by interproton distances and torsion angle restraints obtained from NMR data. The B[ c]Ph moiety intercalated on the 3'-face of the X (4) x C (19) base pair such that the terminal ring of 1 R-B[ c]Ph threaded the duplex and faced into the major groove. The torsion angle alpha' [X (4)]-N3-C2-N2-B[ c]Ph]-C1 was calculated to be -177 degrees, maintaining an orientation in which the X (4) exocyclic amine remained in plane with the purine. The torsion angle beta' [X (4)]-C2-N2-[B[ c]Ph]-C1-C2 was calculated to be 75 degrees. This value governed the 3'-orientation of the B[ c]Ph moiety with respect to X (4). The helical rise between base pairs X (4) x C (19) and C (5) x G (18) increased and resulted in unwinding of the right-handed helix. The aromatic rings of the B[ c]Ph moiety were below the Watson-Crick hydrogen-bonding face of the modified base pair X (4) x C (19). The B[c]Ph moiety was stacked above nucleotide G (18), in the complementary strand.

Tautomerism of 1,3-diphospholes: A DFT study

Tautomers of the unsubstituted, F-substituted, and BH 2-substituted 1,3-diphosphole molecules have been studied at the B3PW91/aug-cc-pVTZ level. The Gibbs free energies calculated for all the molecules demonstrate that, regardless of the character of the substituent, the C 2H 1,3-diphosphole tautomer is always the most stable form. In the CH tautomers, the five-membered ring is planar, whereas the pyramidal PH moiety in PH tautomers gives rise to ring non-planarity. For the CH types of the 1,3-phospholes, the substituent effect can well be observed at the ν(CH) modes and for the PH tautomers at the ν(C C) modes.

Interaction between dioxoruthenium(VI) porphyrins and hydroxylamines: coordination of N-substituted hydroxylamine to ruthenium and X-ray crystal structures of ruthenium complexes with a unidentate nitrosoarene ligand.

The interactions between dioxoruthenium(VI) porphyrins 1 with N-phenylhydroxylamine or unsubstituted hydroxylamine are described. Reaction of complexes 1 with excess PhNHOH leads to isolation of bis(nitrosobenzene)ruthenium(II) porphyrins 3 and mono(nitrosobenzene)ruthenium(II) porphyrins 4. Both the types of ruthenium complexes are characterized by 1H NMR, IR, and UV/Vis spectroscopy, and mass spectrometry. The X-ray structure determinations on [Ru(II)(TPP)(PhNO)2] (3a), [Ru(II)(2,6-Cl-TPP)(PhNO)2] (3e), and [Ru(II)(4-MeO-TPP)(PhNO)(PhNH2)] (4d) (TPP tetraarylporphyrin) disclose a unidentate nitrosoarene coordination in all these complexes, with Ru-N(PhNO) bond lengths of 2.003(3) (3a, average), 1.991(3) (3e, average), and 2.042(2) A (4d). In the case of 4d, the Ru-N(PhNH2) bond length is found to be 2.075(3) A. Mechanistic investigations reveal the formation of intermediates [Ru(II)(Por)(PhNO)(PhNHOH)] (5; Por=porphyrin), a ruthenium complex with N-substituted hydroxylamine ligand, in the "1 + PhNHOH" system. The Ru-NH(OH)Ph moiety in 5 undergoes no rapid exchange with free PhNHOH in solution at room temperature, as revealed by 1H NMR spectroscopy. Unlike the interaction between complexes 1 and PhNHOH, reaction of such complexes with NH2OH affords nitrosylruthenium(II) porphyrins [Ru(II)(Por)(NO)(OH)] (6).

Magnetic Relaxation Dispersion of7Li: II. Complex Formation with Nitroxides in the Aqueous Phase

Measurements of7Li nuclear spin–lattice relaxation times are made at applied magnetic field strengths from 0.25 mT to 7.05 T, in order to determine directly the form of the frequency-dependent spectral densities that modulate relaxation. This magnetic resonance dispersion (MRD) technique provides detailed information regarding molecular dynamics down to the picosecond time scale.7Li MRD measurements made on aqueous lithium ion in the presence of small concentrations of nitroxide free radicals give direct evidence supporting the formation of a coordination complex. The dipole–dipole electron-nuclear coupling is modulated by both translational and rotational diffusive motions, and both of these contributions are resolved. However, scalar coupling arising from the presence of paramagnetic electron spin density at the nucleus dominates the nuclear relaxation. Changes in the pH and free radical moiety are compared with dynamical variables, geometric constraints, and formation constants obtained from a model of nuclear relaxation. The calculated bimolecular formation constants are on the order of 2 × 10−3M−1, and the relative accuracy of this parameter is tested.

Reaction of a Rhodium(III) α-Chlorotolyl Complex with Water and Oxygen: Stable Rhodium Peroxo Compounds

The reaction of the rhodium(III) α-chlorotolylcomplex [RhCl2(CHClPh)(2,6-(C(Me)N−i-Pr)2C5H3N)] (1) with H2O and O2 afforded the rhodium(III) chloride [RhCl3(2,6-(C(Me)N−i-Pr)2C5H3N)] (11), benzaldehyde, and H2O2 in 80−90% yield. This reaction proceeds via two reaction sequences. First, when O2 is absent the hydride complex [Rh(H)Cl2(2,6-(C(Me)N−i-Pr)2C5H3N)] (2), benzaldehyde, and HCl are formed. Hydrolysis of the metal-bonded CHClPh fragment gives a short-lived CH(OH)Ph moiety, which then by β-H elimination of the hydroxyl group affords the products. Alternatively, a rhodium carbene type of intermediate might be involved. The subsequent reaction sequence probably proceeds via two separate pathways. In the first one the hydride 2 may insert O2 to give a rhodium hydroperoxo species which converts with HCl to the Rh(III) complex 11 and H2O2. The second pathway appears to involve first the formation of the Rh(I) complex [RhCl(2,6-(C(Me)N−i-Pr)2C5H3N)] (8) from the hydride 2 in H2O, which subsequently reacts ...

Darstellung und Charakterisierung komplexierter Triphosphane—Die Kristallstruktur von HP[PPh 2Cr(CO) 5] 2

The reaction of M IP(SiMe 3) 2 (M I = Li, Na, K) with R 2P(Cl)M′(CO) 5 (R = Ph, Et; M′ = Cr, W) or of PCl 3 with M IPPh 2M′(CO) n (M I = Li, K; M′ = Cr, W, n = 5; M′ = Fe, n = 4) affords triphosphanes of the type HP[PR 2M′(CO) n ] 2. 31P NMR investigations of the reaction mixtures show a number of precursor compounds, on which basis the formation mechanism can be discussed. The products were characterized by 1H, 13C, 31P NMR and IR spectroscopy, and mass spectrometry. HP[PPh 2Cr(CO) 5] 2 ( 1) crystallises in the space group C2/ c, with a 1873.0(7), b 934.8(4), c 2202.5(9) pm, β 113.76° (at −95°C) and Z = 4. The molecule possesses crystallographic twofold symmetry, but the central PH moiety is disordered across the twofold axis. The P P bond lengths are 219.5(2) and 223.5(2) pm, with P P P 121.2(1)°.

Spectres Raman de Résonance de Dérivés Parasubstitués de l' Azobenzène

AbstractResonance Raman spectra of twenty parasubstituted azobenzene derivatives are compared, enabling a more complete assignment of bands to be made. Since all the spectra are rather similar, the various ring substituents are not resonance Raman active, and all the observed bands can be assigned to vibrational modes of the aromatic rings and to the azo group itself. Frequency shifts in the PhN stretching region are opposite to those expected based on delocalization along the PhNNPh moiety. From the complexity of band pattern around the NN stretching region and from the above described frequency shifts, several types of vibrational coupling are evident. On the basis of the band assignments, some useful conclusions can be reached for interpreting the observed spectral changes when azobenzene derivatives are used as chromophore probes in biochemical system studies.

Preresonance Raman spectra and excitation profiles of some chemically and isotopically substituted trans-4-benzylideneoxazolin-5-ones

Resonance and preresonance Raman spectra of nine substituted trans-4-benzylidene- Δ2-oxazolin-5-ones are reported. The seven analogs with phenyl in the 2 position of the oxazolinone ring have either an electron donating or attracting group on one of the phenyl rings. Although shifts in λmax of up to 100 nm are observed the Raman spectra are very similar to that of the unsubstituted trans isomer. However, in the substituted compounds modes from the benzylidene portion may become weakly intensity enhanced. Replacing the 2-phenyl group by 2-methyl in the oxazolinone ring results in quite gross changes in the Raman spectrum. Substitution by 15N in the ring of a 2-methyloxazolinone reveals that a change in the vibrational coupling scheme occurs. Excitation profiles for the 2-phenyl and the 2-methyl analogs of 4-(p-nitrobenzylidene)oxazolinone indicate that in each compound ail intensity enhanced modes are coupled to the intense low energy absorption band near 350 nm and the intensity enhancement fits the FB2 terms of Albrecht and Hutley. However, differences in the relative intensity of the benzylidene nitro feature in the 2-methyl and 2-phenyl analogs, taken with the excitation profiles, suggest that in the 2-methyl compound the electronic transition responsible for Raman intensity enhancement is no longer primarily located in the C=C—N=C—Ph moiety.