Infrared absorption of CH3OSO detected with time-resolved Fourier-transform spectroscopy

A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to detect temporally resolved infrared absorption spectra of CH(3)OSO produced upon irradiation of a flowing gaseous mixture of CH(3)OS(O)Cl in N(2) or CO(2) at 248 nm. Two intense transient features with origins near 1152 and 994 cm(-1) are assigned to syn-CH(3)OSO; the former is attributed to overlapping bands at 1154 ± 3 and 1151 ± 3 cm(-1), assigned to the S=O stretching mixed with CH(3) rocking (ν(8)) and the S=O stretching mixed with CH(3) wagging (ν(9)) modes, respectively, and the latter to the C-O stretching (ν(10)) mode at 994 ± 6 cm(-1). Two weak bands at 2991 ± 6 and 2956 ± 3 cm(-1) are assigned as the CH(3) antisymmetric stretching (ν(2)) and symmetric stretching (ν(3)) modes, respectively. Observed vibrational transition wavenumbers agree satisfactorily with those predicted with quantum-chemical calculations at level B3P86∕aug-cc-pVTZ. Based on rotational parameters predicted at that level, the simulated rotational contours of these bands agree satisfactorily with experimental results. The simulation indicates that the S=O stretching mode of anti-CH(3)OSO near 1164 cm(-1) likely makes a small contribution to the observed band near 1152 cm(-1). A simple kinetic model of self-reaction is employed to account for the decay of CH(3)OSO and yields a second-order rate coefficient k=(4 ± 2)×10(-10) cm(3)molecule(-1)s(-1).     

Infrared absorption of gaseous benzoyl radical C6H5CO recorded with a step-scan Fourier-transform spectrometer

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the gaseous transient species benzoyl radical, C(6)H(5)CO. C(6)H(5)CO was produced either from photolysis of acetophenone, C(6)H(5)C(O)CH(3), at 248 nm or in reactions of phenyl radical (C(6)H(5)) with CO; C(6)H(5) was produced on photolysis of C(6)H(5)Br at 248 nm. One intense band at 1838 ± 1 cm(-1), one weak band at 1131 ± 3 cm(-1), and two extremely weak bands at 1438 ± 5 and 1590 ± 10 cm(-1) are assigned to the C═O stretching (ν(6)), the C-C stretching mixed with C-H deformation (ν(15)), the out-of-phase C(1)C(2)C(3)/C(5)C(6)C(1) symmetric stretching (ν(10)), and the in-phase C(1)C(2)C(3)/C(4)C(5)C(6) antisymmetric stretching (ν(7)) modes of C(6)H(5)CO, respectively. These observed vibrational wavenumbers and relative IR intensities agree with those reported for C(6)H(5)CO isolated in solid Ar and with values predicted for C(6)H(5)CO with the B3LYP/aug-cc-pVDZ method. The rotational contours of the two bands near 1838 and 1131 cm(-1) simulated according to rotational parameters predicted with the B3LYP/aug-cc-pVDZ method fit satisfactorily with the experimental results. Additional products BrCO, C(6)H(5)C(O)Br, and C(6)H(5)C(O)C(6)H(5) were identified in the C(6)H(5)Br/CO/N(2) experiments; the kinetics involving C(6)H(5)CO and C(6)H(5)C(O)Br are discussed.     

Infrared absorption of gaseous benzoylperoxy radical C6H5C(O)OO recorded with a step-scan Fourier-transform spectrometer

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of benzoyl radical, C(6)H(5)CO, with O(2). C(6)H(5)CO was produced either from photolysis of acetophenone, C(6)H(5)C(O)CH(3), at 248 nm, or from photolysis of a mixture of benzaldehyde, C(6)H(5)CHO, and Cl(2) at 355 nm. Two intense bands near 1830 and 1226 cm(-1) are assigned to the C=O stretching (ν(6)) and the C-C stretching mixed with C-H deformation (ν(13)) modes, and two weaker bands near 1187 and 1108 cm(-1) are assigned to the ν(14) (C-H deformation) and ν(16) (O-O stretching /C-H deformation) modes of C(6)H(5)C(O)OO, the benzoylperoxy radical. These observed vibrational wave numbers and relative infrared intensities agree with those reported for syn-C(6)H(5)C(O)OO isolated in solid Ar and values predicted for syn-C(6)H(5)C(O)OO with the B3LYP/cc-pVTZ method. The simulated rotational contours of the two intense bands based on rotational parameters predicted with the B3LYP∕cc-pVTZ method fit satisfactorily with experimental results.     

Infrared absorption spectra of the CO(2)/H(2)O complex in a cryogenic nitrogen matrix--detection of a new bending frequency

Infrared absorption spectra have been measured for the mixture of CO(2) and H(2)O in a cryogenic nitrogen matrix. The 1:1 CO(2)/H(2)O complex has been observed. Each structure of this complex should have two bending frequencies corresponding to the CO(2) fundamental bending mode (ν(2)). In this work, three bending frequencies corresponding to the CO(2) fundamental bending mode (ν(2)) have been detected; one of them at 660.3 cm(-1) is reported here for the first time. This finding helps confirm the existence of two structures for this complex. A new feature attributed to a CO(2) and H(2)O complex is observed at 3604.4 cm(-1) and is tentatively assigned to the CO(2)/H(2)O complex band corresponding to the CO(2) combination mode (ν(3) + 2ν(2)). In addition, a band that belongs to a CO(2) and H(2)O complex is detected at 3623.8 cm(-1) for the first time and is tentatively assigned to the (CO(2))(2)/H(2)O complex band corresponding to the symmetric stretching mode (ν(1)) of H(2)O.     

Infrared absorption of CH3OSO and CD3OSO radicals produced upon photolysis of CH3OS(O)Cl and CD3OS(O)Cl in p-H2 matrices

Irradiation at 239 ± 20 nm of a p-H(2) matrix containing methoxysulfinyl chloride, CH(3)OS(O)Cl, at 3.2 K with filtered light from a medium-pressure mercury lamp produced infrared (IR) absorption lines at 3028.4 (attributable to ν(1), CH(2) antisymmetric stretching), 2999.5 (ν(2), CH(3) antisymmetric stretching), 2950.4 (ν(3), CH(3) symmetric stretching), 1465.2 (ν(4), CH(2) scissoring), 1452.0 (ν(5), CH(3) deformation), 1417.8 (ν(6), CH(3) umbrella), 1165.2 (ν(7), CH(3) wagging), 1152.1 (ν(8), S=O stretching mixed with CH(3) rocking), 1147.8 (ν(9), S=O stretching mixed with CH(3) wagging), 989.7 (ν(10), C-O stretching), and 714.5 cm(-1) (ν(11), S-O stretching) modes of syn-CH(3)OSO. When CD(3)OS(O)Cl in a p-H(2) matrix was used, lines at 2275.9 (ν(1)), 2251.9 (ν(2)), 2083.3 (ν(3)), 1070.3 (ν(4)), 1056.0 (ν(5)), 1085.5 (ν(6)), 1159.7 (ν(7)), 920.1 (ν(8)), 889.0 (ν(9)), 976.9 (ν(10)), and 688.9 (ν(11)) cm(-1) appeared and are assigned to syn-CD(3)OSO; the mode numbers correspond to those used for syn-CH(3)OSO. The assignments are based on the photolytic behavior and a comparison of observed vibrational wavenumbers, infrared intensities, and deuterium isotopic shifts with those predicted with the B3P86∕aug-cc-pVTZ method. Our results extend the previously reported four transient IR absorption bands of gaseous syn-CH(3)OSO near 2991, 2956, 1152, and 994 cm(-1) to 11 lines, including those associated with C-O, O-S, and S=O stretching modes. Vibrational wavenumbers of syn-CD(3)OSO are new. These results demonstrate the advantage of a diminished cage effect of solid p-H(2) such that the Cl atom, produced via UV photodissociation of CH(3)OS(O)Cl in situ, might escape from the original cage to yield isolated CH(3)OSO radicals.     

Observation of ultrafast NH3 (Ã) state relaxation dynamics using a combination of time-resolved photoelectron spectroscopy and photoproduct detection

The ultrafast excited state relaxation of ammonia is investigated by resonantly exciting specific vibrational modes of the electronically excited NH(3) (?) state using three complementary femtosecond (fs) pump-probe techniques: time-resolved photoelectron, ion-yield and photofragment translational spectroscopy. Ammonia can be seen as a prototypical system for studying non-adiabatic dynamics and therefore offers a benchmark species for demonstrating the advantages of combining the aforementioned techniques to probe excited state dynamics, whilst simultaneously illuminating new aspects of ammonia's photochemistry. Time-resolved photoelectron spectroscopy (TRPES) provides direct spectroscopic evidence of σ* mediated relaxation of the NH(3) (?) state which manifests itself as coupling of the umbrella (ν(2)) and symmetric N-H stretch (ν(1)) modes in the photoelectron spectra. Time-resolved ion yield (TRIY) and time-resolved photofragment translation spectroscopy (TRPTS) grant a measure of the dissociation dynamics through analysis of the H and NH(2) photodissociation co-fragments. Initial vibrational level dependent TRIY measurements reveal photoproduct formation times of between 190 and 230 fs. Measurement of H-atom photoproduct kinetic energies enables investigation into the competition between adiabatic and non-adiabatic dissociation channels at the NH(3) (?)/NH(3) (X?) conical intersection and has shown that upon non-adiabatic dissociation into NH(2) (X?) + H, the NH(2) (X[combining tilde]) fragment is predominantly generated with significant fractions of internal vibrational energy.     

Experimental IR and Raman spectra and quantum chemical studies of molecular structures, conformers and vibrational characteristics of L-ascorbic acid and its anion and cation

IR and spectra of the L-ascorbic acid (L-AA) also known as vitamin C have been recorded in the region 4000-50 cm(-1). In order to make vibrational assignments of the observed IR and Raman bands computations were carried out by employing the RHF and DFT methods to calculate the molecular geometries and harmonic vibrational frequencies along with other related parameters for the neutral L-AA and its singly charged anionic (L-AA(-)) and cationic (L-AA(+)) species. Significant changes have been found for different characteristics of a number of vibrational modes. The four ν(O-H) modes of the L-AA molecule are found in the order ν(O(9)-H(10))>ν(O(19)-H(20))>ν(O(7)-H(8))>ν(O(14)-H(15)) which could be due to complexity of hydrogen bonding in the lactone ring and the side chain. The CO stretching wavenumber (ν(46)) decreases by 151 cm(-1) in going from the neutral to the anionic species whereas it increases by 151 cm(-1) in going from the anionic to the cationic species. The anionic radicals have less kinetic stabilities and high chemical reactivity as compared to the neutral molecule. It is found that the cationic radical of L-AA is kinetically least stable and chemically most reactive as compared to its neutral and anionic species.     

Proton transfer reactions and dynamics of sulfonic acid group in Nafion®

Proton transfer reactions and dynamics of the hydrophilic group (-SO(3)H) in Nafion? were studied at low hydration levels using the complexes formed from CF(3)SO(3)H, H(3)O(+) and nH(2)O, 1 ≤n≤ 3, as model systems. The equilibrium structures obtained from DFT calculations suggested at least two structural diffusion pathways at the -SO(3)H group namely, the "pass-through" and "pass-by" mechanisms. The former involves the protonation and deprotonation at the -SO(3)H group, whereas the latter the proton transfer in the adjacent Zundel complex. Analyses of the asymmetric O-H stretching frequencies (ν(OH)) of the hydrogen bond (H-bond) protons showed the threshold frequencies (ν(OH*)) of proton transfer in the range of 1700 to 2200 cm(-1). Born-Oppenheimer Molecular Dynamics (BOMD) simulations at 350 K anticipated slightly lower threshold frequencies (ν(A)(OH*,MD)), with two characteristic asymmetric O-H stretching frequencies being the spectral signatures of proton transfer in the H-bond complexes. The lower frequency (ν(A)(OH,MD))) is associated with the oscillatory shuttling motion and the higher frequency (ν(B)(OH,MD))) the structural diffusion motion. Comparison of the present results with BOMD simulations on protonated water clusters indicated that the -SO(3)H group facilitates proton transfer by reducing the vibrational energy for the interconversion between the two dynamic states (Δν), resulting in a higher population of the H-bonds with the structural diffusion motion. One could therefore conclude that the -SO(3)H groups in Nafion? act as active binding sites which provide appropriate structural, energetic and dynamic conditions for effective structural diffusion processes in a proton exchange membrane fuel cell (PEMFC). The present results suggested for the first time a possibility to discuss the tendency of proton transfer in H-bond using Δν(BA)(OH,MD)) and provided theoretical bases and guidelines for the investigations of proton transfer reactions in theory and experiment.     

Infrared and infrared emission spectroscopy of nesquehonite Mg(OH)(HCO3)·2H2O-implications for the formula of nesquehonite

The mineral nesquehonite Mg(OH)(HCO(3))·2H(2)O has been analysed by a combination of infrared (IR) and infrared emission spectroscopy (IES). Both techniques show OH vibrations, both stretching and deformation modes. IES proves the OH units are stable up to 450°C. The strong IR band at 934 cm(-1) is evidence for MgOH deformation modes supporting the concept of HCO(3)(-) units in the molecular structure. Infrared bands at 1027, 1052 and 1098 cm(-1) are attributed to the symmetric stretching modes of HCO(3)(-) and CO(3)(2-) units. Infrared bands at 1419, 1439, 1511, and 1528 cm(-1) are assigned to the antisymmetric stretching modes of CO(3)(2-) and HCO(3)(-) units. IES supported by thermoanalytical results defines the thermal stability of nesquehonite. IES defines the changes in the molecular structure of nesquehonite with temperature. The results of IR and IES supports the concept that the formula of nesquehonite is better defined as Mg(OH)(HCO(3))·2H(2)O.     

Electronic ground states and vibrational frequency shifts of diatomic ligands in heme adducts

DFT calculations were carried out to study heme complexes with diatomic ligand (CO, NO, or O(2)) and trans-imidazole ligand. The optimized electronic ground states of CO, NO, and O(2) adducts are singlet, doublet, and open-shell singlet, respectively. For O(2) adduct, the open-shell singlet is slightly lower in energy than the close-shell singlet. However, important differences are found in optimized structures and vibrational frequencies. Particularly, the trans-imidazole-induced frequency up-shift of the Fe-O(O) stretching mode can be predicted only with the open-shell singlet as ground state. An analysis of normal modes confirms that the up-shifts in the bent (NO and O(2) ) adducts are mainly due to mixing of Fe-X(O) stretching mode with Fe-X-O bending coordinate. Our study of binding mechanism indicates that a secondary source of the upshifts is the diminished weakening of the Fe-X(O) bonds. The Fe-X(O) bond strengths are modulated by σ competition mechanism, which weakens the Fe-X(O) bond and σ-π cooperation mechanism, which only exists in the bent adducts and enforce the Fe-X(O) bond. -     

IR spectroscopic investigation of cis-(CH(3))(2)Au(O,O'-acac) and cis-(CD(3))(2)Au(O,O'-acac)

The IR spectrum of cis-(CH(3))(2)Au(O,O'-acac) has been reassigned by comparing frequencies for cis-(CH(3))(2)Au(O,O'-acac) and cis-(CD(3))(2)Au(O,O'-acac), and by analysis of the DFT-calculated normal modes and their frequencies for the isolated molecules. The vibrational intensity in the C-H stretching region arises almost entirely from the cis-(CH(3))(2)Au fragment, while the methyl deformation intensity is largely of acetylacetonato ligand origin. A low frequency mode in the C-H stretching region is the first overtone of the delta(a)(CH(3)) mode of cis-(CH(3))(2)Au. The Au-C stretching modes are affected by deuteration of the cis-(CH(3))(2)Au fragment, while the Au-O stretching modes are not.     

Description of molecular dynamics in intense laser fields by the time-dependent adiabatic state approach: application to simultaneous two-bond dissociation of CO2 and its control

We theoretically investigated the dynamics of structural deformations of CO(2) and its cations in near-infrared intense laser fields (approximately 10(15) W cm(-2)) by using the time-dependent adiabatic state approach. To obtain "field-following" adiabatic potentials for nuclear dynamics, the electronic Hamiltonian including the interaction with the instantaneous laser electric field is diagonalized by the multiconfiguration self-consistent-field molecular orbital method. In the CO(2) and CO(2+) stages, ionization occurs before the field intensity becomes high enough to deform the molecule. In the CO(2)(2+) stage, simultaneous symmetric two-bond stretching occurs as well as one-bond stretching. Two-bond stretching is induced by an intense field in the lowest time-dependent adiabatic state |1> of CO(2)(2+), and this two-bond stretching is followed by the occurrence of a large-amplitude bending motion mainly in the second-lowest adiabatic state |2> nonadiabatically created at large internuclear distances by the field from |1>. It is concluded that the experimentally observed stretched and bent structure of CO(2)(3+) just before Coulomb explosions originates from the structural deformation of CO(2)(2+). We also show in this report that the concept of "optical-cycle-averaged potential" is useful for designing schemes to control molecular (reaction) dynamics, such as dissociation dynamics of CO(2), in intense fields. The present approach is simple but has wide applicability for analysis and prediction of electronic and nuclear dynamics of polyatomic molecules in intense laser fields.     

Sulfoxide stretching mode as a structural reporter via dual-frequency two-dimensional infrared spectroscopy

The S=O stretching mode in sulfoxides, having a frequency in the 950-1150?cm(-1) range, is tested as a structural label via dual-frequency two-dimensional infrared (2DIR) spectroscopy. The properties of this structural reporter are studied in several compounds, including (4,4(')-dimethyl-2,2(')-bipyridyl)(o-methylsulfinylbenzoate) ruthenium II, [Ru(dmb)(2)(BzSO)](+), (RuBzSO), octylsulfinylpropionic acid (OSPA), and o- and p-methylsulfinyl-benzoic acid (oMSBA and pMSBA). The mode assignment in the fingerprint region for these compounds is made using a combination of density functional theory calculations and 2DIR and relaxation-assisted 2DIR (RA 2DIR) spectroscopies. The SO stretching mode frequency and IR intensity demonstrate substantial sensitivity to the molecular structure. Multiple cross peaks of the C=O and S=O stretching modes with modes in the fingerprint region (930-1450?cm(-1)) were recorded. The 2DIR and RA 2DIR spectra focusing at interactions of a high-frequency mode of a ligand with the modes in the fingerprint region provide a spectral fingerprint of a compound and help mode assignment in the often congested fingerprint region. The cross-peak amplitudes in oMSBA, pMSBA, and OSPA were compared with the theoretical predictions based on the computed values for the off-diagonal anharmonicities and a reasonable match is found. The SO stretching mode provides means for assigning other modes in the fingerprint region and constitutes a promising structural reporter for the 2DIR and RA 2DIR spectroscopy measurements.     

Vibrational coherence spectroscopy of the heme domain in the CO-sensing transcriptional activator CooA

Femtosecond vibrational coherence spectroscopy was used to investigate the low-frequency vibrational dynamics of the heme in the carbon monoxide oxidation activator protein (CooA) from the thermophilic anaerobic bacterium Carboxydothermus hydrogenoformans (Ch-CooA). Low frequency vibrational modes are important because they are excited by the ambient thermal bath (k(B)T = 200 cm(-1)) and participate in thermally activated barrier crossing events. However, such modes are nearly impossible to detect in the aqueous phase using traditional spectroscopic methods. Here, we present the low frequency coherence spectra of the ferric, ferrous, and CO-bound forms of Ch-CooA in order to compare the protein-induced heme distortions in its active and inactive states. Distortions take place predominantly along the coordinates of low-frequency modes because of their weak force constants, and such distortions are reflected in the intensity of the vibrational coherence signals. A strong mode near ~90 cm(-1) in the ferrous form of Ch-CooA is suggested to contain a large component of heme ruffling, consistent with the imidazole-bound ferrous heme crystal structure, which shows a significant protein-induced heme distortion along this coordinate. A mode observed at ~228 cm(-1) in the six-coordinate ferrous state is proposed to be the ν(Fe-His) stretching vibration. The observation of the Fe-His mode indicates that photolysis of the N-terminal α-amino axial ligand takes place. This is followed by a rapid (~8.5 ps) transient absorption recovery, analogous to methionine rebinding in photolyzed ferrous cytochrome c. We have also studied CO photolysis in CooA, which revealed very strong photoproduct state coherent oscillations. The observation of heme-CO photoproduct oscillations is unusual because most other heme systems have CO rebinding kinetics that are too slow to make the measurement possible. The low frequency coherence spectrum of the CO-bound form of Ch-CooA shows a strong vibration at ~230 cm(-1) that is broadened and up-shifted compared to the ν(Fe-His) of Rr-CooA (216 cm(-1)). We propose that the stronger Fe-His bond is related to the enhanced thermal stability of Ch-CooA and that there is a smaller (time dependent) tilt of the histidine ring with respect to the heme plane in Ch-CooA. The appearance of strong modes at ~48 cm(-1) in both the ferrous and CO-bound forms of Ch-CooA is consistent with coupling of the heme doming distortion to the photolysis reaction in both samples. Upon CO binding and protein activation, a heme mode near 112 ± 5 cm(-1) disappears, probably indicating a decreased heme saddling distortion. This reflects changes in the heme environment and geometry that must be associated with the conformational transition activating the DNA-binding domain. Protein-specific DNA binding to the CO-bound form of Ch-CooA was also investigated, and although the CO rebinding kinetics are significantly perturbed, there are negligible changes in the low-frequency vibrational spectrum of the heme.     

二羰基茂铁二聚体[CpFe(CO)2]2的中红外泵浦探测光谱

利用一维稳态红外光谱和5-μm泵浦探测红外光谱手段,结合量子化学计算,以非桥连三价羰基为探针,研究了二羰基茂铁二聚体[CpFe(CO)2]2在二氯甲烷中的结构和振动动力学.结果表明,[CpFe(CO)2]2两个主要结构(顺式cis和反式trans摩尔比为1.7)的振动态寿命和转动动力学都有一定不同.两种结构的两个羰基振动激发态的指数衰减过程都有一个<1ps的快组分和一个~20ps的慢组分.我们认为前者与宽带激发所产生的振动相干态的快速失相过程有关,而后者属于典型的C≡O伸缩振动态寿命.此外,cis结构与溶剂的较强作用使得其转动衰减较慢.结果表明,非桥连羰基的红外吸收频率和振转动力学对分子结构和溶剂环境都非常敏感.     

Iron twin-coronet porphyrins as models of myoglobin and hemoglobin: amphibious electrostatic effects of overhanging hydroxyl groups for successful CO/O2 discrimination

Inspired by the observation of polar interactions between CO and O(2) ligands and the peptide residues at the active site of hemoglobin and myoglobin, we synthesized two kinds of superstructured porphyrins: TCP-IM, which contains a linked imidazole ligand, and TCP-PY, which contains a linked pyridine ligand, and examined the thermodynamic, kinetic, and spectroscopic (UV/Vis, IR, NMR, and resonance Raman) properties of their CO and O(2) complexes. On both sides of each porphyrin plane, bulky binaphthyl bridges form hydrophobic cavities that are suitable for the binding of small molecules. In the proximal site, an imidazole or pyridine residue is covalently fixed and coordinates axially to the central iron atom. In the distal site, two naphtholic hydroxyl groups overhang toward the center above the heme. The CO affinities of TCPs are significantly lower than those of other heme models. In contrast, TCPs have moderate O(2) binding ability. Compared with reported model hemes, the binding selectivity of O(2) over CO in TCP-IM and TCP-PY complexes is greatly improved. The high O(2) selectivity of the TCPs is mainly attributable to a low CO affinity. The comparison of k(on)(CO) values of TCPs with those of unhindered hemes indicates the absence of steric hindrance to the intrinsically linear CO coordination to Fe(II) in TCP-IM and TCP-PY. The abnormally large k(off)(CO) values are responsible for the low CO affinities. In contrast, k(off)(O(2)) of TCP-PY is smaller than those of other pyridine-coordinated model hemes. For the CO adducts of TCPs, unusually low nu(Fe-CO) and unusually high nu(C-O) frequencies are observed. These results can be ascribed to decreased back-bonding from the iron atom to the bound CO. The lone pairs of the oxygen atoms of the hydroxyl groups prevent back-bonding by exertion of a strong negative electrostatic interaction. On the other hand, high nu(Fe-O(2)) frequencies are observed for the O(2) adducts of TCPs. In the resonance Raman (RR) spectrum of oxy-TCP-IM, we observed simultaneous enhancement of the Fe-O(2) and O-O stretching modes. Furthermore, direct evidence for hydrogen bonding between the hydroxyl groups and bound dioxygen was obtained by RR and IR spectroscopy. These spectroscopic data strongly suggest that O(2) and CO binding to TCPs is controlled mainly by the two different electrostatic effects exerted by the overhanging OH groups: destabilization of CO binding by decreasing back-bonding and stabilization of O(2) binding by hydrogen bonding.     

Structure-property relations in crystalline L-leucine obtained from calorimetry, X-rays, neutron and Raman scattering

We have studied the amino acid L-leucine (LEU) using inelastic neutron scattering, X-rays and neutron diffraction, calorimetry and Raman scattering as a function of temperature, focusing on the relationship between the local dynamics of the NH(3), CH(3), CH(2) and CO(2) moieties and the molecular structure of LEU. Calorimetric and diffraction data evidenced two novel phase transitions at about 150 K (T(1)) and 275 K (T(2)). The dynamical susceptibility function, obtained from the inelastic neutron scattering results, shows a re-distribution of the intensity of the vibrational bands that can be directly correlated with the phase transitions observed at T(1) and T(2), as well as with the already reported phase transition at T(3) = 353 K. Through the analysis of the Raman modes, the new structural arrangement observed below T(1) was related to conformational modifications of the CH and CH(3) groups, while the behavior of the N-H stretching vibration, ν(NH(3)), gave insight into the intermolecular N-H…O interactions. The observation of changes in the translational symmetry in the crystalline lattice, as well as anharmonic dynamics, allows for localized motions in LEU.     

Ultrafast dynamics of myoglobin probed by time-resolved resonance Raman spectroscopy

Recent experimental work carried out in this laboratory on the ultrafast dynamics of myoglobin (Mb) is summarized with a stress on structural and vibrational energy relaxation. Studies on the structural relaxation of Mb following CO photolysis revealed that the structural change of heme itself, caused by CO photodissociation, is completed within the instrumental response time of the time-resolved resonance Raman apparatus used (approximately 2 ps). In contrast, changes in the intensity and frequency of the iron-histidine (Fe-His) stretching mode upon dissociation of the trans ligand were found to occur in the picosecond regime. The Fe-His band is absent for the CO-bound form, and its appearance upon photodissociation was not instantaneous, in contrast with that observed in the vibrational modes of heme, suggesting appreciable time evolution of the Fe displacement from the heme plane. The band position of the Fe-His stretching mode changed with a time constant of about 100 ps, indicating that tertiary structural changes of the protein occurred in a 100-ps range. Temporal changes of the anti-Stokes Raman intensity of the v4 and v7 bands demonstrated immediate generation of vibrationally excited heme upon the photodissociation and decay of the excited populations, whose time constants were 1.1 +/- 0.6 and 1.9 +/- 0.6 ps, respectively. In addition, the development of the time-resolved resonance Raman apparatus and prospects in this research field are described.     

Time-resolved vibrational spectroscopy of [FeFe]-hydrogenase model compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe-Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) and Fe(2)(μ-S(2)C(2)H(4))(CO)(6) is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) were studied with density functional theory (DFT) calculations. For Fe(2)(μ-S(2)C(3)H(6))(CO)(6) excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe-Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with a semibridging CO is reminiscent of the geometry of the Fe(2)S(2) subcluster of the active site observed in Fe-Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis.     

Investigations of the structure of H2O clusters adsorbed on TiO2 surfaces by near-infrared absorption spectroscopy

The overtone and combination bands of the fundamental vibration modes (nu(1), symmetric stretching; nu(2), bending; and nu(3), asymmetric stretching) attributed to the H(2)O molecules adsorbed on a TiO(2) surface could be observed in the near-infrared (NIR) region. Especially, two absorption bands attributed to the combination (nu(2) + nu(3)) and (nu(1) + nu(3)) modes of the H(2)O molecules adsorbed on the TiO(2) surface were observed at around 1940 and 1450 nm, respectively. From detailed investigations on the (nu(2) + nu(3)) combination band, it was found that H(2)O molecules absorbed on a TiO(2) surface aggregate to form clusters due to the high surface tension of H(2)O arising from the intermolecular hydrogen bonds, and the hydrogen-bonded H(2)O in the bulk part of the cluster and the hydrogen-bond-free H(2)O in the outside spherical part of the cluster could be easily distinguished. Furthermore, it was quantitatively confirmed that the relaxation of the surface energy accompanying the adsorption of H(2)O on the TiO(2) surface stabilized the adsorption states of the hydrogen-bonded H(2)O molecules, while on the other hand, the hydrogen-bond-free H(2)O molecules became unstable as compared to the liquid-phase H(2)O molecules.