Chemical Physics Letters (v.370, #5-6)

Direct measurements of fluorine atom concentration, gain length and small signal gain in an hydrogen fluoride overtone laser by Charles F Wisniewski; Kevin B Hewett; Gerald C Manke II; C Randall Truman; Gordon D Hager (591-596).
Experimental techniques have been developed to directly measure the concentration of fluorine atoms, the gain length and the small signal gain in a hydrogen fluoride 5 cm slit nozzle laser. A gas phase titration technique was utilized to measure the fluorine atom concentration using HCl as the titrant. The gain length was measured using a pitot probe to locate the interface of the primary flow with the high Mach number shroud flows. A tunable diode laser was utilized to perform small signal gain measurements on HF overtone (ν=2→0) transitions.

Single wall carbon nanotubes density of states: comparison of experiment and theory by Pavel V. Avramov; Konstantin N. Kudin; Gustavo E. Scuseria (597-601).
We study the electronic structure of a variety of single wall carbon nanotubes and report density of states obtained with the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation and hybrid PBE0 approximation of density functional theory using Gaussian orbitals and periodic boundary conditions. PBE gives very good results for metallic tubes but the addition of a portion of exact exchange in the hybrid PBE0 functional worsens the agreement between experiment and theory. On the other hand, the PBE0 hybrid significantly improves the theoretical predictions (compared to PBE) for semiconducting tubes.

Polymers and copolymers obtained from the polymerization of substituted phenylacetylenes, namely PMOPPA and copolymers PPA/PMOPPA, PMOPPA/PNPPA, PPA/PNPPA, have been characterized by means of dynamic light scattering and UV–Vis absorption measurements. The analysis of the light intensity correlation function shows a similar behaviour for all the systems investigated, with three decay modes correlated to the molecular structures. The steric hyndrance and the electron polarization modify the hydrodynamic radius of the polymers and cause polymer/solvent interactions, whose strength depends on the solvent. The polymers structure does not change appreciably during the time lasting for the experiments, as assessed by UV–Vis optical tests.

Ultrafast dissociation dynamics of ketones at 195 nm by Q. Zhong; D.A. Steinhurst; A.P. Baronavski; J.C. Owrutsky (609-615).
The photodissociation dynamics of the 3s Rydberg state of three ketones (CH3CO–R, R=C2H5, C3H7, and iso-C4H9) and the ensuing dissociation of the nascent acetyl radical following 195 nm excitation were investigated by ultrafast photoionization spectroscopy. The 3s state the lifetimes of these ketones are similar (2.5–2.9 ps), though lifetimes of the acetyl radical range from 8.6 ps for CH3CO–C2H5, 15 ps for CH3CO–C3H7, to 23 ps for CH3CO–(iso-C4H9), which suggests that for larger R more vibrational degrees of freedom compete for the excess energy so that less energy is partitioned into the internal energy of the acetyl radical.

The geometries and the harmonic vibrational frequencies for several SNO doublet state isomers have been predicted at density functional theory levels with a 6-311+G* basis set. Results have indicated that there are nine doublet states (five bent, three nearly linear and one cyclic). The ground state corresponds to a bent 2 A SNO structure. The state–state correlation and the isomerization mechanism are also predicted by searching the transition states and the intrinsic reaction coordinate analysis. Results have indicated that the ground state is the most favorable product. The nearly linear 2 A″ SNO state may be obtained from the bent 2 A SNO state by electron transition.

Gas phase photoisomerization of urocanic acid – a theoretical study by Jonas Danielsson; Aatto Laaksonen (625-630).
The photochemistry of urocanic acid is investigated theoretically by means of time-dependent density functional theory. The topology of the potential energy surface along the isomerization pathway and close to the Franck–Condon region is investigated and consequences for the photoisomerization reactions are outlined. A recently published supersonic jet spectroscopy study is reinterpreted in the light of these and earlier theoretical results to give a clear picture of the gas phase photochemistry. It is found that the photochemistry of the two isomers is fundamentally different, in contrast to the situation in solution.

Thermodynamics of hydrogen adsorption on the zeolite Li-ZSM-5 by C. Otero Areán; O.V. Manoilova; B. Bonelli; M. Rodrı́guez Delgado; G. Turnes Palomino; E. Garrone (631-635).
Thermodynamic characterisation of the adsorption process (at a low temperature) of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure. Adsorption renders the H–H stretching mode infrared active, at 4092  cm −1 . The standard adsorption enthalpy and entropy resulted to be ΔH0=−6.5(±0.5)  kJ mol −1 and ΔS0=−90(±5)  J mol −1   K −1 , respectively. The adsorption enthalpy is significantly larger than the liquefaction heat, and this fact renders Li-ZSM-5 a potential cryoadsorbent for hydrogen storage.

The Franck–Condon factors for nonradiative transitions from 3MLCT excited states in 16 complexes of Re(I), Ru(II) and Os(II) with α-diimine ligands were estimated using ligand’s geometry, vibrational frequencies and normal modes obtained by ab initio quantum chemical calculations. The linear correlation was obtained between logarithms of experimental constants of nonradiative transitions and theoretical Franck–Condon factors.

Photoinduced dissociative electron transfer (DET) interactions in methoxycalixarene–chloroalkane systems by J. Mohanty; H. Pal; S.K. Nayak; S. Chattopadhyay; A.V. Sapre (641-646).
Fluorescence quenching of methoxycalixarenes (MOCX) by chloroalkanes (CA) has been investigated in acetonitrile solutions. Observed quenching is attributed to photoinduced dissociative electron transfer (DET) interaction, which is supported by the characterization of the Cl ions and by the radical scavenging experiments. Comparing the Cl yields with different DET systems, it is inferred that both concerted and stepwise DET mechanisms operate simultaneously in MOCX–CA systems. A correlation of the quenching constants with the free-energy changes following a suitable DET theory supports the above inference.

For the quasi-relativistic normalized elimination of small component using an effective potential (NESC-EP) method, analytical energy gradients were developed, programmed, and implemented in a standard quantum chemical program package. NESC-EP with analytical gradients was applied to determine geometry, vibrational frequencies, and dissociation enthalpies of ferrocene, tungsten hexafluoride, and tungsten hexacarbonyle. Contrary to non-relativistic calculations and calculations carried out with RECPs for the same compounds, NESC-EP provided reliable molecular properties in good agreement with experiment. The computational power of NESC-EP results from the fact that reliable relativistic corrections are obtained at a cost level only slightly larger than that of a non-relativistic calculation.

Efficient statistical mapping of energy surfaces of nanoclusters and molecules by K. Michaelian; A. Taméz; I.L. Garzón (654-660).
A statistical technique to efficiently map out the energy surfaces of nanoclusters and molecules is described. Global energy minimizations are performed to reach of the catchment basins of the lowest energy stationary points. Saddle points are located by using a large value of the iterative energy change as the stopping criterion of a final local relaxation. Minima are derived from saddle points by simply tightening the stopping criterion and continuing the relaxation. A statistical approximation to the widths of the paths in phase space between saddle points and minima is obtained. Application is made to argon clusters of 7 and 38 atoms.

Self-assembly of modified carbon nanotubes in toluene by Xia Gao; Tengjiao Hu; Luqi Liu; Zhixin Guo (661-664).
The dispersibility of modified carbon nanotubes in toluene at different temperature has been investigated by laser light scattering. It was found that the nanotubes cannot be dispersed into the level of single tubes. On the other hand, the carbon nanotubes self-assemble in a crystal-like way, with a periodicity around 2  μm .

The interaction between trapped electron and water molecules in one interior structure of water tetramer anion by Xi-Yun Hao; Zhi-Ru Li; Di Wu; Ze-Sheng Li; Chia-Chung Sun (665-669).
Using aug-cc-pVDZ basis sets supplemented with diffused bond functions, the (H2O)4 isomer of the water tetramer anion in Ci-symmetry was optimized at the MP2 level. The excess electron was enveloped directly by the four hydrogen atoms of two inside water molecules, forming an interior structure. This study shows four hydrogen atoms of two outside water molecules also have important interaction with the trapped electron. We described the interactions between individual water molecule (including both inside and outside) and the trapped electron. The influence of the trapped electron on the surrounded second layer molecules was also put forward.

EPR lineshape simulation studies have been performed on a specimen of 80MoO3–20B2O3 glass in the temperature range of 300–77 K. The values of the obtained spin Hamiltonian parameters are: g =1.940, g =1.974, A∥=150.0×10−4   cm −1 , A⊥=35.6×10−4   cm −1 and g =1.935, g=1.975, A∥=141.9×10−4   cm −1 , A⊥=34.5×10−4   cm −1 at 300 and 77 K, respectively. The paramagnetic site in the specimen is molybdenyl, MoO3+, ion in which the Mo is in a distorted octahedral environment of six oxygen atoms with C4v symmetry. The 11-parallel and 11-perpendicular line feature of the EPR lineshape shows that two Mo nuclei are magnetically equivalent in the glassy matrix, in the temperature range 300–77 K.

Potential dependent surface Raman spectroscopy of single wall carbon nanotube films on platinum electrodes by P. Corio; P.S. Santos; V.W. Brar; Ge.G. Samsonidze; S.G. Chou; M.S. Dresselhaus (675-682).
The analysis of the resonance Raman spectra of single-walled carbon nanotubes in an electrochemically controlled aqueous H2SO4 environment using different laser excitation energies shows major reversible and irreversible differences in the main vibrational features regarding their intensities, lineshapes, and frequencies for different applied potentials. These differences arise from the electrochemically induced changes in the occupation of electronic states for metallic and semiconducting nanotubes.

We report molecular dynamics (MD) simulations of the solvation dynamics of Coumarin 153 in liquid dimethylsulfoxide using two distinct sets of partial charges for the coumarin probe. The excited state dipole moment of the coumarin and the dynamic Stokes shift in solution depend significantly on the type of charge distributions used. Nevertheless, the overall characteristics of the solvation responses obtained from both sets of charges are very similar and show good agreement with time-dependent Stokes shift experiments. Microscopic details of the solvent reorganization around the probe are discussed in light of the charge transfer upon photoexcitation.

Photon energy upconversion in porphyrins: one-photon hot-band absorption versus two-photon absorption by M. Drobizhev; A. Karotki; M. Kruk; A. Krivokapic; H.L. Anderson; A. Rebane (690-699).
We study the porphyrin S1→S0 fluorescence and the photosensitized singlet oxygen 1 Δ g →3 Σ g phosphorescence, both originating from absorption of photons with energy less than the porphyrin S0→S1 transition energy. By measuring the excitation intensity dependence of fluorescence at lowered sample temperatures, we are able to discriminate between two parallel processes of one-photon hot-band absorption (HBA) and simultaneous two-photon absorption (TPA). When the HBA and TPA contributions are comparable in magnitude, we use this new method to determine absolute TPA cross-section. We also demonstrate for the first time a singlet oxygen photosensitization via HBA in porphyrin.

Avalanches and self-organized criticality in simulations of particle piles by Christopher Rives; Daniel J. Lacks (700-705).
Simulations are carried out for an athermal pile of particles in the presence of a gravity-like force. As the force increases, the pile structure rearranges through discrete avalanche events. The sizes of the avalanche events are characterized by the change in energy, and it is found that the avalanche size follows a power-law probability distribution with a power-law exponent of −0.96. These results suggest that the particle pile is in a self-organized critical state.

This Letter presents cross sections of water in the second absorption band obtained from quantum mechanical calculations. A Monte-Carlo sampling over the initial rotational state is applied in order to calculate the cross section for water at a temperature of 300 K (warm water). This makes a fair comparison with the experimental spectrum, only available for water at room temperature, possible. The overall rotation of the water molecule is treated exactly. The inclusion of initial rotational motion, which reduces the resonance structure in the spectrum, significantly improves the agreement between theory and experiment.

Nitrosyl complexes on Co–ZSM-5: an FTIR spectroscopic study by K. Hadjiivanov; E. Ivanova; M. Daturi; J. Saussey; J.-C. Lavalley (712-718).
Adsorption of NO on Co–ZSM-5 results in formation of three kinds of Co3+–NO species (1970, 1957 and 1940 cm−1) and Co2+(NO)2 dinitrosyls (1894 and 1811 cm−1). All these species disappear after evacuation at elevated temperatures. However, a very short evacuation of the sample with preadsorbed NO at 673 K, followed by quenching to room temperature, results in appearance of a band at 1857 cm−1. This band is assigned to Co2+–NO linear species produced after partial destruction of the dinitrosyls. The reasons for the different pathways of the dinitrosyl decomposition are discussed.

Octahedral tilting in ACu3Ru4O12 (A=Na, Ca, Sr, La, Nd) by U. Schwingenschlögl; V. Eyert; U. Eckern (719-724).
The perovskite-like compounds ACu3Ru4O12 (A=Na, Ca, Sr, La, Nd) are studied by means of density functional theory based electronic structure calculations using the augmented spherical wave (ASW) method. The electronic properties are strongly influenced by covalent-type bonding between transition metal d and oxygen p states. The characteristic tilting of the RuO6 octahedra arises mainly from the Cu–O bonding, allowing for optimal bond lengths between these two atoms. Our results provide a deeper understanding of octahedral tilting as a universal mechanism, applicable to a large variety of multinary compounds.

The low-frequency spectrum of homeotropically aligned 4-octyl-4-cyanobiphenyl in the smectic-A (Sm-A) phase was obtained by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The absence of a narrow band at 9 cm−1, corresponding to a pseudo-lattice vibration propagating in a direction perpendicular to the smectic layers, suggests that the spectrum of homeotropically aligned Sm-A is due to orientational modes associated with rotation about the long axis of the molecule and translational modes associated with motion in the smectic layers. The spectrum is described by a bimodal lineshape function in which the lower and higher frequency components are attributed respectively to the translational and orientational modes.

Aurophilic attraction: the additivity and the combination with hydrogen bonds by Fernando Mendizabal; Pekka Pyykkö; Nino Runeberg (733-740).
The coexistence of gold–gold contacts and hydrogen bonding is studied in the model system [H2P(OH)AuCl]2 and [H2P(OH)AuPH2(O)]2. The two interactions are found to be comparable. The possible non-additivity of the aurophilic, Au(I)–Au(I) interaction is studied at MP2 level for the pentagonal [Au(SH)2(AuSH)5)] and hexagonal [Au(SH)2(AuSH)6] clusters. The possibilities of ‘mechanical cooperativity’ between different aurophilic attractions and of Au…S attractions are also considered.

Femtosecond two-color pump–probe spectroscopy is used to measure the vibrational lifetime of the O–H stretch vibration in solutions of KF, NaCl, NaBr, and NaI in HDO:D2O. We observe a slow component (roughly 2–4 times slower than in HDO:D2O) in the decay of the absorption change, which is due to O–H groups that are hydrogen bonded to the dissolved anions. The time constant of this slow component depends on the nature of the anions and is observed to decrease with temperature, in contrast with the temperature dependence of the relaxation of the OH stretch vibration in pure HDO:D2O.

Studies of the triplet state of the proton-transfer tautomer in salicylaldehydes by Pi-Tai Chou; Chau-Shuen Chiou; Wei-Shan Yu; Guo-Ray Wu; Tai-Huei Wei (747-755).
The spectroscopy and dynamics of the low-lying triplet state of the proton-transfer tautomer in salicylaldehydes have been studied via internal heavy-atom effects coupled with a sensitive near-IR detecting system. For 3,5-diiodosalicylaldehyde a weak proton-transfer keto-tautomer phosphorescence was resolved with a maximum at 710 nm (τ p ∼1.8  μs , Φ obs∼5.23×10−4) in a 77 K methylcyclohexane glass. The results, in combination with the time-resolved thermal lensing experiment, further deduced the triplet-state population yield and radiative decay rate to be 0.20 (298 K) and 3.12×102   s −1 , respectively. Consequently, the energetics and dynamics of the triplet states during a proton transfer cycle are discussed in detail.

Threshold photoelectron spectroscopy of iodine monobromide by Andrew J Yencha; Andrew E.R Malins; George C King (756-764).
A high-resolution (1–7 meV) threshold photoelectron spectroscopic study of IBr was performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Extensive vibrational structure was found in all three electronic-state band systems ( X 2 Π i,  A 2 Π i and   B 2 Σ + ) of IBr+. In the ( X 2 Π i ) band system both spin–orbit components exhibited extended vibrational structure in the Franck–Condon gap regions that is attributed to resonance autoionization of neutral Rydberg states lying in these energy regions. Analysis of this vibrational structure yielded accurate spectroscopic constants.

Strong interactions through the X⋯Au–Y bridge: the Au bond? by Aggelos Avramopoulos; Manthos G. Papadopoulos; Andrzej J. Sadlej (765-769).
The interaction between AuOH and the lone-pair donors (HF, H2O) is shown to result in well-bound complexes whose structure resembles that of the corresponding H-bonded systems with the gold atom replacing hydrogen. The dissociation energies are estimated to be 10.7 and 27.4 kcal/mol for HF⋯Au–OH and H2O⋯Au–OH, respectively. However, the interaction between AuOH and the lone pair donors is found to involve significant charge transfer. Furthermore, the Au–O stretching frequency increases upon the complex formation. It is concluded that, in spite of certain similarity to the H-bonded species, the Au-bonded complexes should be considered as Lewis acid–base pairs.

Uniform MgO nanobelts formed from in situ Mg3N2 precursor by Renzhi Ma; Yoshio Bando (770-773).
MgO nanobelts with a pure morphology and high yield were generated by the direct evaporation of Mg metals under initial N2 gas at 650 °C and subsequent N2/O2 atmosphere at 800 °C. The growth of these MgO nanobelts is explained in a VS route in which the in situ formed Mg3N2 slowly decomposes and reacts with oxygen. Mg3N2 is therefore a very effective precursor for the fabrication of high-purity one-dimensional (1D) MgO nanostructures.

Fabrication of carbon nanotube assemblies on Ni–Mo substrates mimics law of natural forest growth by Yoshiki Shimizu; Takeshi Sasaki; Tetsuya Kodaira; Kenji Kawaguchi; Kazuo Terashima; Naoto Koshizaki (774-780).
We fabricated cone-shaped carbon nanotube assemblies, which align in a densely packed geometry like a cedar tree forest, on unpatterned nanocomposite substrate of Ni and Mo by the DC plasma assisted hot filament chemical vapor deposition. Carbon nanotubes initially grew on the nickel domains created during deposition, forming cone-shaped assemblies. These assemblies grew keeping their shape with decreasing their number density by merging together. The relationship between the mean volume and the number density of conical-shaped tree-like assemblies was similar to one of natural plant forest.

Evidence of ultra-fast dissociation in ammonia observed by resonant Auger electron spectroscopy by I Hjelte; M.N Piancastelli; C.M Jansson; K Wiesner; O Björneholm; M Bässler; S.L Sorensen; S Svensson (781-788).
We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the N1s→4a1 core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z+1 approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH2 * and H fragments.

An electron transfer mechanism of O4 formation by Antonio Carlos Pavão; José Carlos de F. Paula; Rogério Custodio; Carton A. Taft (789-794).
A resonating valence bond electron transfer mechanism of combining two O2 molecules to form an O4 molecule is presented. The predicted molecular states of the reaction path D∞h→C2v→D2h are supported by the present ab initio molecular orbital calculations. The CASPT2 BSSE calculations yield a stable diamagnetic D2h O4 molecule with a very weak chemical bond between the monomers, in good agreement with experiments. A low activation barrier energy of ∼26 cal/mol for the O4 formation is found.

Vibrational analysis of oxygen-plasma treated indium tin oxide by P. He; S.D. Wang; W.K. Wong; L.F. Cheng; C.S. Lee; S.T. Lee; S.Y. Liu (795-798).
High-resolution electron-energy-loss spectroscopy (HREELS) showed that the dominant feature in the HREELS spectrum of the ‘as-received’ ITO was the loss peaks associated with the CH x group. Oxygen-plasma treatment of ITO led to the disappearance of the CH x loss peaks, and a concomitant increase in the intensity of the phonon peaks at 71 and 134 meV. The result indicates the removal of the CH x group and additional oxidation of the ITO surface by the oxygen-plasma treatment. Both processes are proposed to be responsible for the increase in work function of ITO by the oxygen-plasma treatment.

The UV–Vis absorption spectra and the luminescence properties of poly(N-octyl-2,7-carbazole) (POC) and poly(N-octyl-2,7-carbazole-alt-9,9-dioctyl-2,7-fluorene) PCF have been investigated in solution and in the solid state (thin films). No aggregate and/or excimer formation has been detected in these polymeric systems. From time-resolved fluorescence measurements in solution and in the solid state, the fluorescence efficiencies of the thin films have been estimated. It is found that the fluorescence efficiencies of these polycarbazoles in the solid state are quenched, as compared to those measured in fluid solutions, but remain relatively high (φ F≅0.40), making them promising materials for electroluminescent devices.

We analyse H+D2 reactive angular scattering using the S-matrix elements obtained by Aoiz et al. and Althorpe et al. Enhancement of small angle scattering in the v =3←v=0 H+D2 delayed reaction is attributed to a glory effect caused by threshold resonances in the v=3 vibrationally adiabatic channel. The oscillatory structures in the reactive angular distributions are shown to be of nearside–farside (NP) origin and are likely to arise from capture in a number of relatively short-lived barrier Regge states at large angular momenta. Padé reconstruction of the reactive matrix element is discussed in detail.

An XAFS study of the S–Se exchange during the reaction of selenophene over Mo sulfide catalysts by Takeshi Kubota; Naoto Hosomi; Yuya Hamasaki; Yasuaki Okamoto (813-819).
The S–Se exchange during the hydrode-selenium (HD-Se) reaction of selenophene was investigated over MoS2/Al2O3 and intrazeolite Mo sulfide clusters. The local structure of Se incorporated into the Mo sulfide catalysts was studied by means of Se K-edge XAFS. It was found that the capacity of S–Se exchange during the HD-Se reaction and dynamic behavior of Se incorporation strongly depend on the cluster size of Mo sulfides.

Surface modified multi-walled carbon nanotubes in CNT/epoxy-composites by Florian H Gojny; Jacek Nastalczyk; Zbigniew Roslaniec; Karl Schulte (820-824).
Multi-walled carbon nanotubes (MWCNTs), produced by arc-discharge method, were treated with oxidising inorganic acids. The surface modification of the oxidised nanotubes (o-MWCNTs) was achieved by refluxing the tubes with multi-functional amines. The functionalised nanotubes were embedded in the epoxy resin and the resulting composite was investigated by transmission-electron microscopy (TEM). The functionalisation led to a reduced agglomeration and evidences are given for improved interaction between the nanotubes and the epoxy resin.

Boron nanowires synthesized by laser ablation at high temperature by X.M. Meng; J.Q. Hu; Y. Jiang; C.S. Lee; S.T. Lee (825-828).
Boron nanowires have been synthesized by laser ablation at high temperature. The as-synthesized boron nanowires were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). The boron nanowires have lengths of several tens of micrometers long and diameters of 30–60 nm. The effects of the synthesis conditions on the formation of the boron nanowires were investigated and possible growth mechanisms of the boron nanowires are discussed.

The triplet excited state of C60 (λ max=780 nm) lives minutes and can be monitored by conventional spectrophotometers when this fullerene is incorporated inside LiY, as opposed to C60@HY and C60@MCM-41 wherein C60 triplet lives in the submillisecond time scale. C60 adsorbed in LiY or MCM-41 efficiently generates 1 O 2 that was detected by its characteristic NIR emission (λ em =1270  nm).

Towards a detailed understanding of the NEXAFS spectra of bulk polyethylene copolymers and related alkanes by A Schöll; R Fink; E Umbach; G.E Mitchell; S.G Urquhart; H Ade (834-841).
High energy resolution C 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of ethylene-1-alkene copolymers with systematic variations in comonomer content and thus systematic changes in branch length, branching ratio, and degree of crystallinity are presented. Spectral changes of the σ* C–H/Rydberg and σ* C–C features in these ideal model systems provide unambiguous experimental evidence for intermolecular interactions with profound effects on the spectral intensity, but only very small energy shifts. Ab initio calculations reproduce the experimental results in detail. The intermolecular interaction observed suggests that interpretation of NEXAFS spectra based on calculations of isolated molecules can be insufficient even in relatively weakly interacting macromolecules.

Helix–coil kinetics of two 14-residue peptides by Ting Wang; Deguo Du; Feng Gai (842-848).
The helix–coil transition kinetics of two 14-residue helical peptides of different stability were studied by time-resolved infrared (IR) spectroscopy coupled with laser-induced temperature-jump (T-jump) technique. The T-jump induced relaxation kinetics of both peptides show strong dependence on the final temperature, implying the existence of an enthalpic barrier for the nucleation process. In addition, the peptide with end-capping groups, which is more stable, folds faster. Together, these results suggest that the overall helical stability plays an important role in controlling the kinetics of the helix–coil transition, in agreement with results of early theoretical studies.

Author Index (849-862).