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The photophysical processes in a series of isocyano Re(I) phenanthroline complexes {[Re(CNR)n (CO)4-n (phen)](PF6 ); n=2, 3, 4, R=2,6-(i Pr)2 C6 H3 - or t Bu- (n=2)} in acetonitrile have been studied by resonance Raman spectroscopy, transient resonance Raman spectroscopy, and femtosecond / nanosecond transient spectroscopy to elucidate the nature of their electronic transitions and emissive excited state(s). The kinetics of the intersystem crossing, vibrational relaxation and radiative decay of the metal-to-ligand charge transfer {MLCT [dπ(Re)→π*(phen)]} excited state have also been determined.

The mechanism of the defluorination reaction(s) of lomefloxacin (LF) upon light illumination was investigated by using ultrafast laser flash photolysis combined with transient resonance Raman spectroscopy in near neutral water solution. The zwitterionic configuration of LF was determined to be the main species present in the near neutral water solution and was the species that was photoexcited to initiate the photochemical reaction. Femtosecond transient absorption revealed that the first excited singlet state (S1) of LF did not appreciably undergo intersystem crossing (ISC), and instead partially decayed to the ground state via fluorescence emission, and there was partial cleavage of the carbon–fluorine bond at position 8 to produce a singlet LF aryl cation intermediate. The transient resonance Raman results provided a direct observation and vibrational spectral characterization of the singlet LF aryl cation species. Subsequently, the transformation from the singlet LF aryl cation to a triplet carbene vi...

Regioregular poly(thiophene) functionalized with pendant photosensitizing ruthenium complexes was synthesized. The conjugated polymer serves dual functions as a dispersant for single-walled carbon nanotubes (SWCNTs) and also as a photosensitizer. SWCNTs can be dispersed effectively by the polymer, and the polymer/SWCNT hybrid dispersion obtained was stable for months. Raman spectroscopic results showed that there was no structural damage or defects present in SWCNTs after the functionalization process and SWCNTs with both small (0.9 nm) and large (1.6 nm) diameter can be dispersed. Transmission electron microscopy revealed the presence of polymers on the SWCNT surface. The dynamics of the photo-induced electron transfer process between the ruthenium sensitizers and SWCNTs was probed by femtosecond transient absorption spectroscopy. Rapid electron injection from the ruthenium sensitizers to SWCNTs with a time constant of 167 ps was observed. By using this synthesis approach, it is possible to incorporate different types of sensitizers in one polymer chain so as to fine-tune the absorption spectra, and a new light harvesting system can be developed by coupling the polymer with SWCNTs.

In heterogeneous catalysis, characterization of heterogeneous metal interfaces of bimetallic catalysts is a crucial step to elucidate the catalytic performance and is a key to develop advanced catalysts. However, analytical techniques such as X-ray photoelectron spectroscopy can only work in vacuum conditions and are difficult to use for in situ analysis. Here, we present efficient and convenient core–shell nanoparticle-enhanced Raman spectroscopy to explore the in situ electronic structures of heterogeneous interfaces (Au@Pd and Au@Pt core–shell NPs) by varying the shell thickness. The experimental observations reported here clearly show that Pd donates electrons to Au, while Pt accepts electrons from Au at the heterogeneous interfaces. This conclusion gains further support from ex situ X-ray photoelectron spectroscopy results. The Au core greatly affects the electronic structures of both the Pd and Pt shells as well as catalytic behaviors. Finally, the as-prepared core–shell nanoparticles were used to d...

Transition-metal ion activated solid-state phosphors are of particular interest for the development of LED-based white light sources. In addition to their relatively low cost, these luminescent materials show exceptionally high luminescence efficiency, especially at room temperature and above, due to the involvement and promotion of thermal phonons. In this article, we present a comprehensive investigation into a set of manganese ion doped fluoride phosphors (A2BF6:Mn4+, A = K, Na, B = Si, Ge, Ti), including the synthesis procedures and various characterization, with the emphasis on optical spectroscopic characterization. All of the phosphors synthesized at a temperature of −16 °C using a chemical co-precipitation method exhibit intense red color emission at room temperature under the excitation of light, with a wide range of wavelengths from 450 nm to 325 nm. In most of the phosphors, phonon-assisted luminescence dominates the spectra, which is evidenced using Raman scattering measurements. X-ray diffraction data from the samples reveal that K2SiF6:Mn4+ crystallizes in a cubic phase, while the remaining crystals have hexagonal structures, but with different symmetries for K2TiF6:Mn4+ and Na2SiF6:Mn4+. More interestingly, well-resolved spectral splitting was observed in the major phonon-assisted luminescence signatures of both the K2SiF6:Mn4+ and K2TiF6:Mn4+ samples, indicating the occurrence of complicated but fascinating phonon-assisted transition processes in the phosphors.

The support effect is an important issue in heterogeneous catalysis. A systematic density functional theory computational study is reported here to better understand the C–H bond activation steps in the reaction between C2H6 and a model silica-supported Ni3O3 cluster, as well as that between C2H6 and an unsupported Ni3O3 cluster. Two mechanisms, namely, a radical mechanism (denoted as mechanism A) and a concerted mechanism (denoted as mechanism B) were examined. Both of these mechanisms contain two steps. For the C–H bond activation taking place via mechanism A, the involvement of the model silica support does not change the most favorable pathway significantly; however, it does result in a modest increase in the reaction barrier and the overall Gibbs energy change. For the C–H bond activation taking place via mechanism B, the involvement of the model silica support leads to an increase in the reaction barrier in the first step. The product of this step has a noticeable difference in the structures for the Ni3O3 moiety in the unsupported and model silica-supported systems. The result of charge analysis shows that there is no noticeable charge transfer between the silica support and Ni3O3 when they are in the starting reactants, while there is an electron withdrawal from Ni3O3 by the silica support when they are in transition states, intermediates, or products. The results here provide deeper insights into the support effect on the C–H bond activation of lower alkanes on supported transition metal catalysts. Copyright © 2015 John Wiley & Sons, Ltd.

The photocleavage reaction mechanism of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure-2959) was investigated using femtosecond (fs) and nanosecond (ns) transient absorption (-TA) spectroscopy and also picosecond (ps) and nanosecond (ns) time-resolved resonance Raman (-TR(3)) spectroscopy experiments in a water-rich (volume ratio of acetonitrile/water = 3:7) solution. TA spectroscopy was used to study the dynamics of the benzoyl radical growth and decay as well as to investigate the radical quenching process by the radical scavenger methyl acrylate. ps- and ns-TR(3) spectroscopies were employed to monitor the formation of the benzoyl radical and also to characterize its electronic and structural properties. The fs-TA experiments results indicate that the Irgacure-2959 lowest lying excited singlet state S1 underwent efficient intersystem crossing (ISC) to convert into its triplet state with a time constant of 4 ps. Subsequently, this triplet species dissociated into the benzoyl and alkyl radicals with a corresponding maximum absorption band at 415 nm. The TR(3) results in conjunction with results from DFT calculations confirmed that Irgacure-2959 cleaved into the benzoyl and alkyl radicals at a fast rate on the tens of picosecond time scale.

The excited state structural dynamics of 4-cyanobenzaldehyde (p-CNB) were studied by using the resonance Raman spectroscopy and the quantum mechanical calculations. The experimental A- and B-band absorptions were, respectively, assigned to the major nO → π3 *a ndπ2 → π3* transitions according to the B3LYP-TD/6-31G(d) and CIS/6-31G(d) computations, and the resonance Raman spectra. It was determined that the actual S2(π2π3) state was in energy lower than S3(π1π3), which was just opposite to the B3LYPTD/6-31G(d) calculated order of the S2(π1π3 )a nd S3(π2π3). The vibrational assignments were carried out for the A- and B-band resonance Ramanspectra.The B-band resonance Ramanintensities ofp-CNB were dominatedbythe C2–C3/C5–C6 symmetricstretch mode ν8, theovertones nν8andtheircombinationbands with theringC–Hbend mode ν17,theC9–N10stretch mode ν6,theC7–O8 stretch mode ν7and the remaining modes. The conical intersection between S1(nOπ3)andS2(π2π3) statesofp-CNB wasdetermined atcompleteactivespaceself-consistent field(CASSCF)(8,7)/6-311G(d,p)leveloftheory.TheB-bandshort-timestructuraldynamics and the correspondingdecaydynamicsofp-CNBwere obtainedby analysisofthe resonanceRamanintensity pattern and CASSCF computations. The resonance Raman spectra indicated that CI[S1(nOπ3)/S2(π1π2π3π4)] located nearby the Franck–Condon region. The excited state decay dynamics evolving from the S2, FC(π2π3 )t o the S1(nOπ3) state was proposed. Copyright © 2013 John Wiley &S ons, Ltd.

The photo-decarboxylation and overall reaction mechanism of tiaprofenic acid (TPA) was investigated by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopic experiments in a neutral phosphate buffered solution (PBS). In addition, density functional theory (DFT) calculations were presented to help interpret the experimental results. Resonance Raman and DFT calculation results revealed that the deprotonated tiaprofenic acid (TPA(-)) form was the primary species that is photoexcited in a near neutral PBS aqueous solution. The fs-TA experimental data indicated that the lowest lying excited singlet state S(1) underwent an efficient intersystem crossing process (ISC) to quickly transform into the lowest lying excited triplet state T(1) that then undergoes decarboxylation to generate a triplet biradical species (TB(3)). ns-TA and ns-TR(3) results observed a protonation process for TB(3) to produce a neutral species (TBP(3)) that then decayed via ISC to produce a singlet TBP species that further reacted to make the final product (DTPA). A comparison of the present results for TPA(-) with similar results for the deprotonated form of ketoprofen (KP(-)) in the literature was done to investigate how the thiophene moiety in TPA(-) that replaces one phenyl ring in KP(-) affects the reaction mechanism and photochemistry of these nonsteroidal anti-inflammatory drugs (NSAIDs).

The decarboxylation reaction of KP in different acetonitrile-water mixtures producing a carbanion or biradical intermediate is investigated by using femtosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopies to unveil the mechanism of the photochemistry of KP. The irradiation of either the neutral or anion forms of KP leads to the excited singlet state KP species transforming into a corresponding triplet state KP species via a highly efficient intersystem crossing, and then, a triplet state mediated decarboxylation reaction occurs to generate a carbanion intermediate in the phosphate buffer solutions or a biradical species in the water-rich or acidic solutions examined here.

This is a picosecond and nanosecond time-resolved infrared absorption spectroscopic study of p-hydroxphenacyl diethylphosphate (HPDP) in aqueous solution examining the deprotection reaction and subsequent evolution to produce the p-hydroxyphenylacetic acid (HPAA) rearrangement product. The data reported here support previous ultrafast fluorescence, Raman and transient UV/visible absorption studies and provide new insight into the role of water in the deprotection reaction. It also provides insight into the nature of the intermediate steps that take place following initial formation of the triplet state and its reaction with water to eventually produce the solvent-separated ions and solvolytic reaction product HPAA. In addition, density functional theory (DFT) calculations were performed to investigate the reactions of the triplet state of HPDP with water molecules to induce deprotection, the formation of a contact ion pair and the subsequent intermediates that lead to the HPAA rearrangement product. A more detailed reaction mechanism is proposed based on the time-resolved spectroscopy experiments and DFT computational results.

Ketoprofen is known to induce photosensitivity due to its specific structure and electronic features, and this limits its use in medical applications. In this Article, the photochemistry of (S)-ketoprofen has been investigated by time-resolved resonance Raman spectroscopy to gain additional information so as to better elucidate the possible photochemical reaction mechanism of ketoprofen in different solvents. In nonaqueous solvents like neat acetonitrile and isopropyl alcohol, and 1:1 acetonitrile:water and 1:1 acetonitrile:acidic water aqueous solvents, (S)-ketoprofen exhibits benzophenone-like photochemistry to produce a triplet state, which in turn produces a ketyl radical-like species that then undergoes a cross-coupling reaction with either a dimethyl radical (which is generated by hydrogen abstraction of isopropyl alcohol) or a water molecule, respectively, at the para-position to form a transient species that has a lifetime up to the microsecond time scale. However, photolysis of (S)-ketoprofen in a 1:1 acetonitrile:alkaline water solution and 3:7 acetonitrile:phosphate buffered solution appears to undergo a prompt decarboxylation reaction. Only one species was observed in the nanosecond time-resolved resonance Raman experiments under these conditions, and this species was tentatively assigned to be a triplet protonated biradical carbanion.

Four types of non-starch polysaccharides including cellulose, guar gum, locust bean gum and xanthan gum were modified by carboxymethylation. The degree of substitution (DS) was determined by a colorimetric method. Raman and Fourier transform infrared spectra of the native and modified polysaccharides were acquired and analysed. Representative marker bands, with intensities and/or integrated areas affected by carboxymethylation, were selected at 1607 cm−1 (Raman) and 1315 cm−1 or 1605 cm−1 (IR), attributed to CO carbonyl stretching vibration. The ratios of intensities (or areas) of the marker bands to that of an internal standard band, corresponding to the skeletal configuration and linkages (850–950 cm−1), were plotted against DS. Linear fits were obtained with high correlation coefficients, r > 0.96 (p

A time-resolved resonance Raman study of the photoreactions of benzophenone (BP) in neutral, alkaline, and acidic aqueous solutions is reported. Under neutral and alkaline conditions, a hydrogen abstraction reaction is observed to form the corresponding diphenyl ketyl (DPK) radical with different reaction rates while under acidic condition the protonation of triplet benzophenone occurs simultaneously, which is followed by a faster hydration reaction to produce the corresponding photohydration short-lived intermediates that were observed during experiments and that were tentatively assigned to the m-(3)BP x H(2)O and the o-(1)BP x H(2)O species. The dynamics, structures, and properties of these species are briefly discussed.

The 8-bromo-7-hydroxyquinolinyl group (BHQ) is a derivative of 7-hydroxyquinoline (7-HQ) and BHQ molecules coexisting as different forms in aqueous solution. Absorption and resonance Raman spectroscopic methods were used to examine 8-bromo-7-hydroxyquinoline protected acetate (BHQ-OAc) in acetonitrile (MeCN), H(2)O/MeCN (60:40, v/v, pH 6 approximately 7), and NaOH-H(2)O/MeCN (60:40, v/v, pH 11 approximately 12) to obtain a better characterization of the forms of the ground-state species of BHQ-OAc in aqueous solutions and to examine their properties. The absorption spectra of BHQ-OAc in water show no absorption bands of the tautomeric species unlike the strong band at about 400 nm observed for the tautomeric form in 7-HQ aqueous solution. The resonance Raman spectra in conjunction with Raman spectra predicted from density functional theory (DFT) calculations reveal the observation of a double Raman band system characteristic of the neutral form (the nominal C=C ring stretching, C-N stretching, and O-H bending modes at 1564 and 1607 cm(-1)) and a single Raman band diagnostic of the enol-deprotonated anionic form (the nominal C=C ring, C-N, and C-O(-) stretching modes in the 1593 cm(-1) region). These results suggest that the neutral form of BHQ-OAc is the major species in neutral aqueous solution. There is a modest increase in the amount of the anionic form and a big decrease in the amount of the tautomeric form of the molecules for BHQ-OAc compared to 7-HQ in neutral aqueous solution. The presence of the 8-bromo group and/or competitive hydrogen bonding that hinder the formation and transfer process of a BHQ-OAc-water cyclic complex may be responsible for this large substituent effect.

Three food protein products, soy and whey protein isolates and spray-dried egg white powder, were deamidated to various levels by 0.04 N hydrochloric acid. The extents of deamidation were determined using an ammonia electrode. Raman spectra of the modified proteins were obtained and analyzed. A new C O stretching vibration band was observed at 1780 cm−1, attributed to γ-carboxyl groups of aspartic and glutamic acids. Calibration curves were constructed by plotting the intensity ratio of the 1780 cm−1 band to the 1003 cm−1 phenylalanine stretching band (as an internal standard) against the extent of deamidation. Linear fits were obtained with high correlation coefficients r > 0.987. Effect of deamidation on the conformation of these proteins was also studied by monitoring changes in Raman spectral characteristics, including a transition from ordered to disordered structures, exposure of tryptophan residues from a buried, hydrophobic microenvironment, and probable conformational changes of the aliphatic residues.

Density functional theory computations and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy experiments were done to study the photocyclization of 2-pyridyl phenyl ketone in acid solution. The most probable cyclization pathway was assigned to the reaction RX1, which had the lowest free-energy barrier. The factors that determine the free-energy barriers among the different reactions examined were discussed. Solvent effects on these reactions were also explored by calculations that included a polarizable continuum model for the bulk solvent effect.

Resonance Raman spectra were obtained for thymidine and thymine with excitation wavelengths in resonance with the approximately 260 nm band absorption spectrum. The spectra indicate that the Franck-Condon (FC) region photodissociation dynamics of thymidine have multidimensional character with motion predominantly along the nominal C5=C6 stretch + C6-H bend nu17 (delta = 0.75, lambda = 468 cm(-1)), the nominal thymine ring stretch + C6-H bend + N1-C1, stretch nu29 (delta = 0.73, lambda = 363 cm(-1)), the nominal thymine ring stretch + C5-CH3/ N1-C1, stretch nu37 (delta = 0.69, lambda = 292 cm(-1)), and accompanied by the moderate and minor changes in the nu40, nu20 and nu23, nu55, nu60, nu61, nu63 modes. A preliminary resonance Raman intensity analysis was done, and these results for thymidine and thymine were compared to each other. The roles of ribose in the FC structure dynamics of thymidine were explored and the results were used to correlate to its lifetime constants tau1 and tau2 for two nonradiative decay channels. Spi/Sn conical intersection versus a distorted structure of Spi,min in the FC region was briefly discussed.

Time-resolved resonance Raman (TR3) studies of the photochemistry of phenyl azide, 3-hyroxyphenyl azide, 3-methoxyphenyl azide and 3-nitrophenyl azide in acetonitrile:water solutions is reported. After photolysis of these four aryl azides in room temperature solutions, only one species was observed in the TR3 spectra for each azide, respectively at the probe wavelengths employed in the TR3 experiments. The species observed after photolysis of 3-nitrophenyl azide was assigned to 3,3'-dinitroazobenzene, an azo compound formed from the dimerization reaction of triplet 3-nitrophenylnitrene. In contrast, the species observed after photolysis of phenyl azide, 3-hydroxyphenyl azide and 3-methoxyphenyl azide were tentatively assigned to intermediates formed from the dimerization of didehydroazepines that are produced from the ring expansion reaction of the respective singlet arylnitrene. To our knowledge, this is the first time-resolved vibrational spectroscopic observation of the dimerization reaction of didehydroazepines in solution. In addition, these are the first resonance Raman spectra reported for dimers formed from didehydroazepines. We briefly discuss the structures, properties and chemical reactivity of the dimer species observed in the TR3 spectra and possible implications for the photochemistry of aryl azides.

A time-resolved resonance Raman investigation of the reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide in a mixed aqueous solvent is presented. The reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide in the mixed aqueous solution generates two new species on the microsecond time scale. One of these species is identified as 2,2'-azobisfluorene, and the other species is tentatively assigned to a 1,4-bis-(2,2'-fluorenyl)-tetrazadiene cation intermediate. The structure and properties of these two species are briefly discussed. The reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide is also briefly compared to that of the 2-fluorenylnitrenium ion reactions with guanosine and water.

Various amounts of tryptophan were attached to three food protein products: soy protein isolates, spray-dried egg white and gluten, using a water-soluble carbodiimide method. The extent of amidation was determined by a spectrophotometric method. Raman spectra (600–2000 cm−1) of the modified proteins were obtained and analyzed. The phenyl stretching vibration at 1552 cm−1, directly attributed to the attached tryptophan, was used as a marker band, and increases in band intensity were observed in the modified protein samples. Calibration curves were constructed by plotting the intensity ratio of the 1552 cm−1 band to the 1003 cm−1 phenylalanine stretching band (used as an internal standard) against the amount of tryptophan attached. High correlation coefficients, (r) ⩾ 0.99, were obtained from these calibration curves. The Raman spectral data showed a transition from ordered conformation to random coil structures in the amidated food protein products.

Femtosecond (fs) and nanosecond (ns) transient absorption (TA) and single pulse transient resonance Raman spectroscopic investigation of the intermediates after laser photolysis of 4-phenoxyphenyl azide in acetonitrile and mixed aqueous solution is reported. fs-TA results show that the singlet 4-phenoxyphenylnitrene was produced immediately after photolysis of the azide. Then, the singlet nitrene underwent intersystem crossing (ISC) and ring expansion to generate triplet nitrene and ketenimine in acetonitrile with t = 346 ps or protonation in mixed aqueous solution with t = 37 ps, respectively, a little slower than the counterparts of the methoxy one (108 and 5.4 ps for ISC and protonation processes, respectively). The transient Raman spectrum combined density functional theory (DFT) calculation predicting the structure and vibrational frequencies suggested that phenoxyphenylnitrenium ion has a comparable quinoidal character to that of methoxy- and ethoxy-phenylnitrenium ions. All of these results indicated that the phenoxy substitution has some impact on the reactivity of phenylnitrene but a slight influence on the structure of phenylnitrenium ion.

Three food proteins, soy protein isolates, spray-dried egg white, and whey protein isolates, were acetylated to varying levels, and the extent of modification was determined by wet chemistry methods. Raman spectra of the modified proteins were obtained. A new C=O stretching vibration was observed at 1737cm-1 and was attributed to ester carbonyl groups appended to the proteins during acetylation. Calibration curves were obtained by plotting the intensity ratio of this Raman band to the 1003cm-1 phenylalanine stretching band against the extent of substituted hydroxyl groups. Linear fits were obtained with correlation coefficient r> 0.9979. The Raman spectral data were also analyzed to study the effect of acetylation on the conformation of the 3 proteins. Marked conformational changes were observed in the modified proteins.

Resonance Raman spectra were obtained for the I2-benzene complex in benzene solvent with excitation wavelengths in resonance with the CT-band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal I-I stretch mode nu(18), the nominal symmetric benzene ring stretch mode nu5, and the nominal symmetric CCH bending nu7. There is also a small contribution from the nominal out-of-plane CH oop wag nu15. A preliminary resonance Raman intensity analysis was done, and the results for the I2-benzene complex were compared to results previously reported for the 1-hexene-I2 complex. We briefly discuss the differences and similarities in the CT-band absorption excitation of an I2-benzene complex relative to those of an I2-alkene complex.

An overview of an UV–vis spectroscopic and resonance Raman investigation of the lowest energy dipole-allowed absorption band of [M 2 (dcpm) 2 ] 2+ (M = Au, Ag, Cu) and [M 2 (dmpm) 3 ] 2+ (M = Au, Ag, Cu) complexes is presented. The UV–vis absorption spectra of [M 2 (dcpm) 2 ] 2+ and [M 2 (dmpm) 3 ] 2+ feature intense low energy n dσ* → ( n + 1)pσ transition with transition energy increase from two- to three-coordinated complexes and for [M 2 (dcpm) 2 ] 2+ falls in the order: Ag > Au > Cu. A resonance Raman intensity analysis of the spectra allows estimation of the structural changes of the 1 [dσ*pσ] excited states relative to the ground state. The structural changes in these compounds are compared with each other and with other related compounds. The metal–metal bonds are found to be very similar for the ground and excited states of the [Au 2 (dcpm) 2 ] 2+ and [Au 2 (dmpm) 3 ] 2+ complexes but significantly different for the [Cu 2 (dcpm) 2 ] 2+ and [Cu 2 (dmpm) 3 ] 2+ complexes that have weaker metal–metal bonding.

Picosecond and nanosecond time-resolved resonance Raman spectroscopy combined with density functional theory calculations have been performed to characterize the structure, dynamics, and hydrogen-bonding effects on the triplet state of the phototrigger model compound p-methoxyacetophenone (MAP) in cyclohexane, MeCN, and 50% H2O/50% MeCN (v:v) mixed solvent. Analogous work has also been done to study the corresponding ground state properties. The ground and triplet states of MAP were both found to be associated strongly with the water solvent molecules in the 50% H2O/50% MeCN solvent system. A hydrogen-bond complex model involving one or two water molecules bonded with the oxygen atoms of the MAP carbonyl and methoxy moieties has been employed to explore the hydrogen-bond interactions and their influence on the geometric and electronic properties for the ground and triplet states of MAP. Among the various hydrogen-bond configurations examined, the carbonyl hydrogen-bond configuration involving one water mo...

A combined experimental and theoretical investigation of the ultraviolet photolysis of CH2XI (where X = Cl, Br, I) dihalomethanes in water is presented. Ultraviolet photolysis of low concentrations of CH2XI (where X = Cl, Br, I) in water appears to lead to almost complete conversion into CH2(OH)2 and HX and HI products. Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy experiments revealed that noticeable amounts of CH2X-I isodihalomethane intermediates were formed within several picoseconds after photolysis of the CH2XI parent compound in mixed aqueous solutions. The ps-TR3 experiments in mixed aqueous solutions revealed that the decay of the CH2X-I isodihalomethane intermediates become significantly shorter as the water concentration increases, indicating that the CH2X-I intermediates may be reacting with water. Ab initio calculations found that the CH2X-I intermediates are able to react relatively easily with water via a water-catalyzed O-H insertion/HI elimination reaction to produce CH2X(OH) and HI products, with the barrier for these reactions increasing as X changes from Cl to Br to I. The ab initio calculations also found that the CH2X(OH) product can undergo a water-catalyzed HX elimination reaction to make H2C=O and HX products, with the barrier to reaction decreasing as X changes from Cl to Br to I. The preceding two water-catalyzed reactions produce the HI and HX leaving groups observed experimentally, and the H2C=O product further reacts with water to make the other CH2(OH)2 product observed in the photochemistry experiments. This suggests that that the CH2X-I intermediates react with water to form the CH2(OH)2 and HI and HX products observed in the photochemistry experiments. Ultraviolet photolysis of CH2XI (where X = Cl, Br, I) at low concentrations in water-solvated environments appears to lead to efficient dehalogenation and release of two strong acid leaving groups. We very briefly discuss the potential influence of this photochemistry in water on the decomposition of polyhalomethanes and halomethanols in aqueous environments.

Ultraviolet photolysis of low concentrations of CH2I2 in methanol solution found that CH2I2 is converted into dimethoxymethane and some H+ and I- products. Picosecond time-resolved resonance Raman (ps-TR3) experiments observed that the isodiiodomethane (CH2I-I) photoproduct decayed faster as the concentration of methanol increases, suggesting that isodiiodomethane is reacting with methanol. Ab initio calculations indicate isodiiodomethane is able to react with methanol via an O-H insertion/HI elimination to form an iodoether (ICH2-O-CH3) and HI products. The iodoether can then further react via another O-H insertion/HI elimination reaction to form the dimethoxymethane (CH3-O-CH2-O-CH3) observed in the photochemistry experiments. A reaction mechanism consistent with these experimental and theoretical observations is proposed.

We report the first transient resonance Raman spectra for a halogen-substituted arylnitrenium ion, the 7-bromo-2-fluorenylnitrenium ion. The Raman band vibrational frequencies were found to be in good agreement with those predicted from density functional theory calculations for the singlet state of the 7-bromo-2-fluorenylnitrenium ion but not very well for the triplet state, particularly for the diagnostic 1530 cm−1 to 1650 cm−1 aromatic C=C stretch modes. This and the predicted 23.7 kcal/mol singlet-triplet energy gap with the singlet state being more stable indicates the transient resonance Raman spectra are due mainly to the singlet 7-bromo-2-fluorenylnitrenium ion. Comparison of the structures and properties of the 7-bromo-2-fluorenylnitrenium ion to the closely related 2-fluorenylnitrenium ion previously studied shows the 7-bromo substituent does not significantly perturb the iminocyclohexadienyl character of the 2-fluorenyl moiety and the structure of the first phenyl ring. However, the 7-bromo substituent does noticeably influence the structure of the second phenyl ring to which it is attached and also the normal mode character of a number of lower frequency modes with relatively delocalized character.

A picosecond Kerr-gated time-resolved resonance Raman examination of the 2-fluorenylnitrene and 4-methoxyphenylnitrene reactions with water to form the singlet 2-fluorenylnitrenium and 4-methoxyphe...

Soy protein isolates were chemically modified with varying amounts of acetic anhydride, and the extent of O-esterification was determined by a standard wet chemistry method. Raman spectra of the modified soy proteins were obtained. A characteristic 1737 cm−1 CO vibrational band was observed and its intensity was dependent on the degree of O-esterification. The ratio of intensity of the 1737 cm−1 band to a 1003 cm−1 phenylalanine stretch band (used as an internal standard) was plotted against the extent of O-esterification determined chemically. A linear fit was obtained with a correlation coefficient (R2) of 0.9986. Hence, Raman spectroscopy represents a rapid non-destructive method to determine the degree of O-esterification in food proteins.

Picosecond and nanosecond time-resolved resonance Raman spectroscopy have been used to study the dynamics and structure of the phototrigger compound p-hydroxyphenacyl acetate (HPA) in acetonitrile solution. An intermediate was observed that exhibited significant quenching by oxygen. This intermediate was identified and attributed to the ππ* triplet state of HPA. Temporal evolution at early picosecond times indicates rapid intersystem crossing conversion and subsequent relaxation of the excess energy of the initially formed triplet state. B3LYP/6-311G** density functional theory (DFT) calculations were done to determine the structures and vibrational frequencies for both the HPA triplet and ground states. The experimental and theoretical results were compared and discussed in relation to the initial pathway for the p-hydroxyphenacyl deprotection reaction and the nature of the triplet state relative to other aromatic carbonyl compounds. The present results demonstrate the utility of applying ultrafast vibra...

Resonance Raman spectra were obtained for cyclopropyl iodide in cyclohexane solution with excitation wavelengths in resonance with the A-band absorption spectrum. These spectra indicate that the Franck−Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal C−I stretch and nominal C−C−I deformation normal modes accompanied by moderate motion along the nominal cyclopropyl ring breathing and ring deformation normal modes. A preliminary resonance Raman intensity analysis was done, and the results for cyclopropyl iodide were compared to previously reported results for several cyclic and noncyclic alkyl iodides.

Raman spectra of benzoin diethyl phosphate (BDP) have been obtained in resonance with two different electronic states using excitation at 252.7 and 354.7 nm in acetonitrile solvent. Extensive overtone and combination bands of the carbonyl CO stretching (1700 cm-1) and carbonyl attached ring CC stretching (1597 cm-1) vibrations were observed in the 252.7 nm spectrum. Density functional theory (DFT) calculations were made to determine the structure and the vibrational modes and frequencies for the ground state of BDP. Vibrational bands observed in the resonance Raman spectra were assigned using the DFT results, and it was found that modes showing a strong enhancement in the Raman spectra were mainly related to the benzoinyl moiety, especially the acetophenone subgroup. This was corroborated by the DFT calculated molecular orbitals. Our results indicate that 354.7 and 252.7 nm excitation corresponds to electronic transitions from the ground state to the nπ* and ππ* states, respectively. We briefly discuss th...

Three food protein products, soy protein isolates, spray-dried egg white, and whey protein isolates, were chemically modified to varying levels with succinic anhydride, and the extent of modification of these proteins were determined by standard wet chemistry methods. Raman spectra (500-2000 cm(-1)) of the modified proteins were obtained. New C=O stretching vibrations were observed at 1420 and 1737 cm(-1) and were attributed to the carboxylate (COO-) and ester carbonyl (RCOO-) groups, respectively, which were appended to the proteins during succinylation. Two series of calibration curves were obtained by plotting the intensity ratio of the 1420 and 1737 cm(-1) to 1003 cm(-1) phenylalanine stretching band (used as an internal standard) against the extent of substituted epsilon-amino (and alpha-amino) groups and aliphatic hydroxyl groups, respectively. Linear fits were obtained with correlation coefficient r0.988. The Raman spectral data were also analyzed to study the effect of succinylation on the conformation of the three proteins. Some conformational changes were observed, including a transition from ordered to disordered structures, an exposure of tryptophan residues from a buried, hydrophobic microenvironment, and probably conformational shift of cystine residues.

Raman spectroscopy is used for the first time for distinguishing igneous zircons from metamorphic zircons. Igneous zircons, especially those from granitic rocks, commonly show a Raman peak at 1461 Δcm−1 with significant intensity, whereas metamorphic zircons do not show this peak or the peak is very weak. This may be related to the rare‐earth element (REE) contents in zircon. Thus, Raman spectroscopy provides a new, fast and non‐destructive means for investigating the structure and origin of zircons.

This paper reports time-resolved resonance Raman spectra for the triplet 4-biphenylnitrene and its reaction to form an azo dimer product (bis-biphenyl-4-yldiazene) in acetonitrile solution. The properties of the triplet 4-biphenylnitrenes were examined based on the results of UBPW91/cc-PVDZ computations. 4-Biphenylnitrene and its azo dimer product were also compared with results previously reported for the 4-methoxyphenylnitrene and its azo dimer product, respectively. The substitution effect of p-phenyl and p-methoxy moieties on the structure and properties of arylnitrenes and azobenzenes are briefly discussed. Copyright © 2004 John Wiley & Sons, Ltd.

X-ray structural and spectroscopic properties of a series of heterodinuclear d8−d10 metal complexes [M‘M‘ ‘(μ-dcpm)2(CN)2]+ containing d8 Pt(II), Pd(II), or Ni(II) and d10 Au(I), Ag(I), or Cu(I) ions with a dcpm bridging ligand have been studied (dcpm = bis(dicyclohexylphosphino)methane; M‘ = Pt, M‘ ‘ = Au 4, Ag 5, Cu, 6; M‘ ‘ = Au, M‘ = Pd 7, Ni 8). X-ray crystal analyses showed that the metal···metal distances in these heteronuclear metal complexes are shorter than the sum of van der Waals radii of the M‘ and M‘ ‘ atoms. The UV−vis absorption spectra of 4−6 display red-shifted intense absorption bands from the absorption spectra of the mononuclear trans-[Pt(phosphine)2(CN)2] and [M‘ ‘(phosphine)2]+ counterparts, attributable to metal−metal interactions. The resonance Raman spectra confirmed assignments of 1[ndσ*→(n + 1)pσ] electronic transitions to the absorption bands at 317 and 331 nm in 4 and 6, respectively. The results of theoretical calculations at the MP2 level reveal an attractive interaction en...

Fourier-transform Raman spectroscopy was used to study the conformation of red bean globulin (RBG) in various buffered environments and on heat treatment. The Raman spectrum of 20% (w/v) freeze-dried RBG in 0.01 M phosphate buffer at pH 7.4 suggests that its secondary structure composition is dominated by β-sheets and random coils. Extreme pH values caused the amide I and amide III bands to shift, suggesting a transition from an ordered structure (α-helix or β-sheet) to a random coil conformation. The decreases of the intensity of the band near 760 cm−1, at acidic and basic pHs, indicate an unfolding of the protein, leading to exposure of hydrophobic residues. Increasing concentrations of added NaCl led to progressive increases of the band intensity near 760 cm−1, suggesting an increased “buriedness” of tryptophan residues. Protein structure perturbants, including sodium dodecyl sulfate (SDS), β-mercaptoethanol (β-ME), dithiothreitol (DTT), N-ethylmaleimide (NEM) and ethylene glycol (EG) were also found to have a marked influence on the conformation of RBG. The Raman data indicate progressive denaturation of RBG during heat treatment at 90C. Heat-induced aggregation caused changes in the protein backbone vibrations, and increases in the tryptophan band intensity, suggesting that hydrophobic interactions may play an important role in RBG aggregation. The shifts in amide III bands indicate that β-sheet is the major secondary structure in the heat-induced aggregates.

Time-resolved resonance Raman spectra for triplet 4-methoxyphenyl nitrene and 4-ethoxyphenyl nitrene and their dimerization reaction to form azo dimer products in acetonitrile solution on the nanosecond time scale are reported. As the 4-methoxyphenyl nitrene and 4-ethoxyphenyl nitrene Raman bands decay, the Raman bands for the azo dimer products increase in intensity. The triplet 4-methoxyphenyl nitrene and 4-ethoxyphenyl nitrene species and their reaction to produce azo products are compared to a recent study of the 2-fluorenyl nitrene species and its reaction to form dehydroazepine ring-expansion products. The properties of the singlet and triplet arylnitrenes and their reaction mechanisms to form azo dimer and ring-expansion products are briefly discussed.

This paper presents a transient resonance Raman and density functional theory study of 4-methoxyphenylnitrenium and 4-ethoxyphenylnitrenium ions in a largely aqueous environment. The transient Raman bands observed in conjunction with the (U)BPW91/cc-PVDZ calculation results indicates these nitrenium ions are singlet state species. Both the 4-methoxyphenylnitrenium and the 4-ethoxyphenylnitrenium ions have significant iminocyclohexadienyl character to a degree very similar to that previously found for the 2-fluorenylnitrenium ion. The 4-methoxyphenylnitrenium and the 4-ethoxyphenylnitrenium ions also have significant oxocarbocation character and this influences both their properties and chemical reactivity. We compare the properties of the 4-methoxyphenylnitrenium and the 4-ethoxyphenylnitrenium ions to those previously found for other arylnitrenium ions.

We report time-resolved resonance Raman spectra for the azirine intermediate produced in the 2-fluorenylnitrene ring-expansion reaction to form a dehydroazepine product. The Raman bands obtained with a 252.7 nm probe wavelength and 500 ns delay time exhibit reasonable agreement with predicted vibrational frequencies from density functional calculations for two isomers of azirine intermediates that may be formed from a 2-fluorenylnitrene precursor. The Raman bands observed for delay times of 15 ns and 10 micros were consistent with predicted vibrational frequencies from density functional calculations for the 2-fluorenylnitrene and dehydroazepine product species as well as previously reported 416 nm time-resolved Raman spectra obtained on the ns and micros time scales. Our results demonstrate that the 2-fluorenylnitrene ring-expansion reaction to produce dehydroazepine products proceeds via relatively long-lived 2-fluorenylnitrene and azirine intermediates. Substitution of a phenyl ring para to the nitrene group of phenylnitrene appears to lead to significant changes in the ring-expansion reaction so that longer lived arylnitrene and azirine intermediates can be observed. This should enable the chemical reactivity of azirine intermediates formed from arylnitrenes to be examined more readily.

We have obtained transient resonance Raman spectra of the [CH2CHCH2]+ (allyl cation) produced following C-band excitation of cyclopropyl bromide. The experimental resonance Raman spectrum display an overtone progression in the nominal [CCC]+ stretch mode and its combination bands with the CH/CH2 rocking modes. Density functional theory computations were performed to estimate the vibrational frequencies for the allyl cation, the allyl radical, the cyclopropyl radical, the cyclopropyl bromide molecule and the gauche-allyl bromide molecule and compared to the experimental vibrational frequencies. This comparison indicates that the allyl cation can be formed as a product of cyclopropyl bromide photodissociation in acetonitrile solution.

Time-resolved resonance Raman spectra are reported for different concentrations of CH2I2 in cyclohexane solution. The CH2I–I species is observed at low concentrations and it decays on the order of tens of nanoseconds to almost no signal at 100 ns and no other signal is observed up to 15 microseconds. Two species are observed at high concentrations. The first species CH2I–I spectra and lifetime are about the same as that found at low concentration of CH2I2 parent molecule and the second species is a CH2I2⋯I molecular complex observed on the nanosecond to microsecond time scale and formed from bimolecular reaction of iodine atoms with CH2I2 molecules. The chemical reactivity of the CH2I–I species and the CH2I2⋯I molecular complex towards carbon double bonds were investigated using density functional theory calculations. The structure and properties of the CH2I–I species and the CH2I2⋯I molecular complex and their reaction towards ethylene were compared. The CH2I–I species and the CH2I2⋯I both have weak I–I ...

Transient resonance Raman spectra of CHBr 2 and CHBrCl radicals following ultraviolet photolysis of CHBr 3 and CHBr 2 Cl, respectively, in methanol solution are reported. Most of the Raman intensity appears in the C–Br stretch and Br–C–Br bend or Cl–C–Br bend fundamentals accompanied by moderate intensity in their overtones and/or combination bands. The CH 2 Br radical was previously observed to have only one predominant Franck–Condon active mode (the C–Br stretch) with a strong overtone progression. The Franck–Condon region photodissociation dynamics of the CHBr 2 and CHBrCl radicals appear to have more multidimensional character compared to halomethyl radicals like CH 2 Br.

We report ultraviolet resonance Raman spectra of bromoform (CHBr3) in cyclohexane solution. The resonance Raman spectra show significant intensity in the overtones of the nominal Br-C-Br symmetric bend (v 6), the nominal H-C-Br asymmetric bend (v3), the nominal Br-C-Br symmetric stretch (v 2) and the nominal Br-C-Br asymmetric stretch (v 5) vibrational modes suggesting that the short-time photodissociation dynamics have noticeable multidimensional character. The lack of strong combination bands between several of the Franck-Condon active modes suggests that more than one electronic transition contribute to the resonance Raman spectra. We briefly discuss the ultraviolet short-time photodissociation dynamics of bromoform and the potential implications for the secondary photodissociation reactions of the initially formed CHBr2 radical.

Structural, spectroscopic properties on the dinuclear [M2(dcpm)2(CN)4] (M = Pt, 1a; Ni, 2a, dcpm = bis(dicyclohexylphosphino)methane) and [M2(dmpm)2(CN)4] (M = Pt, 1b; Ni, 2b, dmpm = bis(dimethylphosphino)methane) and the mononuclear trans-[M(PCy3)2(CN)2] (M = Pt, 3; Ni, 4, PCy3 = tricyclohexylphosphine) and theoretical investigations on the corresponding model compounds are described. X-ray structural analyses reveal Pt···Pt and Ni···Ni distances of 3.0565(4)/3.189(1) A and 2.957(1)/3.209(8) A for 1a/1b and 2a/2b, respectively. The UV−vis absorption bands at 337 nm (e 2.41 × 104 dm3 mol-1 cm-1) for 1a and 328 nm (e 2.43 × 104 dm3 mol-1 cm-1) for 1b in CH2Cl2 are assigned to 1(5dσ* → 6pσ) electronic transitions originating from Pt(II)−Pt(II) interactions. Resonance Raman spectroscopy of 1a, in which all the Raman intensity appears in the Pt−Pt stretch fundamental (93 cm-1) and overtone bands, verifies this metal−metal interaction. Complexes 1a and 1b exhibit photoluminescence in the solid state and soluti...

We present transient resonance Raman spectra for the 4-iminocyclohexa-dienylidene and 4-oxocyclohexa-2,5-dienylidene carbene species in aqueous solutions. Comparison of the experimental vibrational frequencies to values computed from density functional theory calculations suggests that both carbenes are in their triplet states. Comparison of the structure and properties of the 4-iminocyclohexa-dienylidene to those previously found for several arylnitrenium ions indicates that the imine and cyclohexadienyl character of the 4-iminocyclohexa-2,5-dienylidene carbene is similar to the diphenylnitrenium ion. This suggests the smaller C-N bond order and lower cyclohexadienyl character of the triplet 4-iminocyclohexa-2,5-dienylidene carbene compared to the higher C=O bond order and greater cyclohexadienyl character of the triplet 4-oxocyclohexa-2,5-dienylidene carbene is mainly due to the charge delocalization ability of the second hydrogen atom in the -NH 2 + moiety. We briefly discuss the structure and properties of these two carbenes and their similarities and differences in chemical reactivity.

A time-resolved resonance Raman spectroscopic investigation is reported for the ultraviolet photolysis of CHI(3) in pure cyclohexane and mixed cyclohexane/cyclohexene solvents. The ICHI-I species is observed in pure cyclohexane solvent. Upon addition of cyclohexene, the ICHI-I species lifetime is reduced and new bands from an I(2):cyclohexene complex are observed. Density functional theory computations show that ICHI-I and CHI(2) species have similar barriers of reaction toward addition to the C=C of ethylene. The addition reaction of ICHI-I with ethylene results in formation of an iodocyclopropane and I(2) molecule while addition of.CHI(2) results in initial formation of a diiodopropyl radical intermediate. Ultraviolet photolysis of CHI(3) in the presence of cyclohexene is known to produce a reasonable yield of iodonorcarane product and some addition reaction products. We present a mechanism for the iodocyclopropanation reaction that is consistent with both experimental and theoretical characterization of reaction intermediates formed after ultraviolet photolysis of CHI(3). We briefly discuss the concentration dependence of the time-resolved resonance Raman spectra and photochemistry in relation to the competition between the reaction of the ICHI-I and CHI(2) species with the C=C bond of olefins.

We present transient resonance Raman spectra and density functional theory calculations that indicate the iso-dibromoacetic acid (Br–BrCHCOOH) species can be produced in noticeable amounts following ultraviolet photolysis of dibromoacetic acid in cyclohexane solution. We compare the properties of the iso-dibromoacetic acid species to those previously found for the related iso-dibromomethane species. We briefly discuss the potential utility of using iso-dibromoacetic acid in cyclopropanation reactions to make substituted cyclopropane products.