Electrochemically-Switched 2nd Order Non-Linear Optical Response in an Arylimido-Polyoxometalate with High Contrast and Cyclability.

Electrochemically switched 2nd order non-linear optical responses have been demonstrated for the first time in polyoxometalates (POMs), with an arylimido-derivative showing a leading combination of high on/off contrast (94 %), high visible transparency, and cyclability. Spectro-electrochemical and TD-DFT studies indicate that the switch-off results from weakened charge transfer (CT) character of the electronic transitions in the reduced state. This represents the first study of an imido-POM reduced state, and demonstrates the potential of POM hybrids as electrochemically activated molecular switches.

TDDFT Investigation of the Raman and Resonant Raman Spectra of Fluorescent Protein Chromophore Models.

The off-resonance and resonant Raman spectra have been simulated for models of fluorescent protein chromophores, those of the green fluorescent protein (GFP, called FP1) and of DsRed (called FP2), which presents a longer π-conjugated path, with the aim of providing a systematic investigation of structural but also computational aspects. These were performed at the (time-dependent) density functional theory [(TD)DFT] level. The off-resonance intensities have been calculated from the derivatives of the frequency-dependent polarizability with respect to the normal coordinates while the resonant ones have been evaluated using Huang-Rhys factors determined from the gradients of the excitation energies with respect to the normal coordinates. When applied with the M05 meta-GGA exchange-correlation functional, this simple computational scheme can reproduce most of the experimental Raman signatures of FP1 in its protonated and deprotonated forms, the differences of vibrational signatures of the cis (Z) and trans (E) isomers, as well as their changes as a function of the excitation wavelength. On the other hand, testing the predictions made for FP2 would require new experimental work. It was also observed that simulations with methods that inadequately predict the resonant Raman spectra could nevertheless produce a UV-vis absorption spectrum that is quite similar to the one obtained with better methods, once realistic peak broadening has been applied.

Circulating tumor cell detection: A prospective comparison between CellSearch® and RareCyte® platforms in patients with progressive metastatic breast cancer.

PURPOSE: Circulating tumor cells (CTCs) are prognostic in patients with breast cancer. Several technical platforms exist for their enumeration and characterization. Comparative studies between these platforms are scarce. The RareCyte CTC detection is theoretically more sensitive than the established CellSearch platform, which identifies only CTCs that express EpCAM and cytokeratin. This study prospectively compares CTC enumeration in patients with breast cancer in a paired analysis using these two platforms. It investigates survival outcomes in groups defined by a CTC count threshold. DESIGN: CTC enumeration was performed on 100 samples obtained from 86 patients with progressive metastatic breast cancer (MBC) in two independent laboratories each blinded to the clinical data and the results from the other platform. RESULTS: One hundred paired samples were collected and CTC counts were determined using the CellSearch and RareCyte CTC platforms. In total, 65% and 75% of samples had at least one detectable CTC in 7.5 mL blood with the CellSearch and the RareCyte systems, respectively. CTC counts with the CellSearch system ranged from 0 to 2289 with a median of 3 CTCs, the RareCyte CTC counts ranged from 0 to 1676 with a median of 3 CTCs. The number of samples with 5 or more CTCs in 7.5 mL of blood (the poor prognosis cut-off validated with the CellSearch system) blood was 45% with the CellSearch test and 48% with the RareCyte test. CTC counts quantified with the CellSearch and the RareCyte systems were strongly correlated (Spearman's r = 0.8235 (0.7450-0.8795) p < 0.001). 86 patients were included for Kaplan-Meier survival analysis. An increased mortality risk in patients with CellSearch of 5 CTCs or more per 7.5 mL blood, with a log-rank hazard ratio of 5.164 (2.579-10.34) (p < 0.001) was confirmed. The survival analysis with RareCyte CTC counts with the identical cut-off showed a significantly impaired survival with a hazard ratio of 4.213 (2.153-8.244) (p < 0.001). CONCLUSION: Our data demonstrate the analytical and prognostic equivalence of CellSearch and RareCyte CTC enumeration platforms in patients with MBC using the CellSearch cut-off. This is the first demonstration of prognostic significance using the RareCyte platform.

Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency, Low-Roll-Off, Organic Light-Emitting Diodes.

Herein, we report the use of spiro-configured fluorene-xanthene scaffolds as a novel, promising, and effective strategy in thermally activated delayed fluorescence (TADF) emitter design to attain high photoluminescence quantum yields (ΦPL), short delayed luminescence lifetime, high external quantum efficiency (EQE), and minimum efficiency roll-off characteristics in organic light-emitting diodes (OLEDs). The optoelectronic and electroluminescence properties of SFX (spiro-(fluorene-9,9'-xanthene))-based emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally. All three emitters exhibited sky blue to green emission enabled by a Herzberg-Teller mechanism in the excited state. They possess short excited-state delayed lifetimes (<10 µs), high photoluminescence quantum yields (ΦPL ∼ 70%), and small singlet-triplet splitting energies (ΔEST < 0.10 eV) in the doped films in an mCP host matrix. The OLEDs showed some of the highest EQEs using spiro-containing emitters where maximum external quantum efficiencies (EQEmax) of 11 and 16% were obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a spiro-based emitter coupled with a low efficiency roll-off (19% at 100 cd m-2) was attained with SFX-PO-DPA-Me.

Topological investigation of the reaction mechanism of glycerol carbonate decomposition by bond evolution theory.

The reaction mechanisms of the decomposition of glycerol carbonate have been investigated at the density functional theory level within the bond evolution theory. The four reaction pathways yield to 3-hydroxypropanal (TS1), glycidol (TS2a and TS2b), and 4-methylene-1,3-dioxolan-2-one (TS3). The study reveals non-concerted processes with the same number (four) of structural stability domains for each reaction pathway. For the two decarboxylation mechanisms, the two first steps are similar. They correspond to the cleavage of two single CO bonds to the detriment of the increased population of the lone pairs of two O atoms. These are followed, along TS1, by the transformation of a CO single bond into a double bond together with a proton transfer to create a CH bond. For TS2a and TS2b, the last step is a cyclization by CO bond formation. For the TS3 pathway, the first stage consists in the cleavage of a CH bond and the transfer of its electron population to both a proton and a C atom, the second step corresponds to the formation of an OH bond, and the last one describes the formation of a CC double bond. Moreover, the analysis of the energies, enthalpies, and free enthalpies of reaction and of activation leads to the conclusion that 3-hydroxypropanal is both the thermodynamic and kinetic product, independent of the method of calculation.

Automated enumeration and phenotypic characterization of CTCs and tdEVs in patients with metastatic castration resistant prostate cancer.

BACKGROUND: Although most patients with metastatic castration-resistant prostate cancer (mCRPC) initially benefit from treatment with androgen receptor signaling inhibitors (ARSi), resistance inevitably occurs. Hence, we investigated the prognostic value of automated circulating tumor cell (CTC) and tumor-derived extracellular vesicle (tdEV) enumeration and their dynamics, in patients with mCRPC in the context of the initiation of treatment with ARSi. Furthermore, we hypothesize that CTC phenotypic heterogeneity might serve as a measurable biomarker under these circumstances. METHODS: Using an image analysis tool, we reanalyzed all CellSearch images previously acquired in the context of a prospective, multicenter clinical study for patients with mCRPC (n = 170) starting a new line of ARSi, for CTC and tdEV detection and enumeration. CTC (n = 19 129) phenotypic diversity was quantified by the Shannon index (SI). Progression-free survival (PFS) and overall survival (OS) were compared between groups of patients stratified according to CTC, tdEV, and SI levels. RESULTS: Automated CTC enumeration provided similar clinical prognostication compared with operator-based counts. Patients demonstrating high CTC phenotypic heterogeneity before therapy had a shorter median PFS (4.82 vs. 8.49 months, HR 1.79; P = 0.03) and OS (12.6 months vs. not reached, HR 2.32; P = 0.03), compared to patients with low diversity, irrespective of CTC level. Multivariable analysis showed how the prognostic value of the baseline SI was lost by pretreatment chemotherapy status, CTC counts, and PSA levels. CONCLUSIONS: Automated CTC counts are a reliable substitute for reviewer-based enumeration, as they are equally informative for prognosis assessment in patients with mCRPC. Beyond enumeration, we demonstrated the added value of studying CTC phenotypic diversity for patient prognostication, warranting future investigation.

Density Functional Theory Investigation of the Binding of ThioTEPA to Purine Bases: Thermodynamics and Bond Evolution Theory Analysis.

Density functional theory with the ωB97X-D exchange-correlation functional together with implicit as well as explicit solvation is used to describe the reactions of the adenine and guanine purine bases on N,N',N″-triethylenethiophosphoramide (thioTEPA), an alkylating agent used as an anticancer drug. This reaction is decomposed into (i) a nucleophilic addition and (ii) a proton "migration" that is mediated by the solvent molecules. The calculations reveal that the first step is rate determining and that the presence of an explicit water molecule to mediate the proton migration has a negligible role on the kinetics of the first step, so that the focus is set on the first step of the reaction. ωB97X-D calculations highlight (i) the activation energy (Gibbs free enthalpy) is smaller for imine nitrogens than amine nitrogens, (ii) for the imine functions, the activation energy is slightly smaller for adenine than for guanine together with a larger exergonicity for the alkylation by adenine, and (iii) among the amine nitrogens, the presence of stabilizing H-bonds in the case of exocyclic amines leads to smaller activation energy than for the endocyclic ones. The reaction mechanisms are unraveled by employing the bond evolution theory, combining the use of electronic localization functions, and their evolution along the intrinsic reaction coordinate, with Thom's catastrophe theory. These analyses, suitable for highlighting the populations of the major monosynaptic and disynaptic basins, show (i) the reaction with imine nitrogens begins by the cleavage of the C-N aziridine bond and is followed by the simultaneous formation of the new C-N bond and the disappearance of the nitrogen lone pair, (ii) the reaction with the nitrogen atom of an endocyclic or exocyclic amine proceeds first by the formation of the cross-linking C-N bond and then by the cleavage of the C-N aziridine bond and the disappearance of the nitrogen lone pair, and (iii) in case ii, this bond breaking and forming occur before the transition state, which has been correlated to the increased Gibbs enthalpy of activation with respect to the reaction with the nitrogen atom of imine functions.

Magnetically-induced current density investigation in carbohelicenes and azahelicenes.

A computational study of the magnetically-induced current (MIC) density has been carried out for a variety of ortho fused polycyclic aromatic molecules at the density functional theory level with the gauge including magnetically induced current (GIMIC) method. With this method, the aromatic character of each ring in a homologous series of carbohelicenes with an increasing number of fused benzene rings is assessed and compared with other aromaticity criteria such as the Nucleus Independent Chemical Shift [NICS(0), NICSzz(0)] and Bond Length Alternation (BLA) parameters. All criteria indicate that the two outer rings are the most aromatic ones [i.e. higher induced current, more negative NICS(0) and NICSzz(0) values, and smaller BLA values]. For the large helicenes (n > 10), the current drops along the following four rings and then rises again. Additionally, we have proven that this behavior is not due to a difference of the local magnetic field coming from a difference of orientation of the ring with respect to the external magnetic field (oriented along the helical axis). Upon fusing additional benzene rings to form hexa-peri-hexabenzo[7]helicene, some rings (B, D, and F) are a lot less aromatic (even non-aromatic) than the others. The NICS(0) and NICSzz(0) values exaggerate this behavior because they are all positive values, which is a signature of antiaromaticity. Then, when substituting one, three, or four benzene rings with pyrrole ones to form mono-aza-[7]-helicene, tri-aza-[7]-helicene, and tetra-aza-[7]-helicene, remarkable changes in the electronic structures of the helicenes are observed. Indeed, the induced currents are always smaller in the pyrrole rings than in the benzene ones. This has been further investigated using the streamline and the color map representations, which indicate that the diatropic current density passing through the plane cutting the C-N bonds in the pyrrole rings is stronger but is more localized than the current density passing through the plane cutting the C-C bonds in the benzene rings. This gives a positive but smaller total induced current for the pyrrole rings than for the benzene ones. For these systems, the NICS(0) and even more the NICSzz(0) values are not fully reliable to probe the local aromaticity, contrary to the induced current. Indeed, the NICSzz(0) values for the tri-aza-[7]-helicene molecule range from -14.23 to 1.14 ppm, which cannot lead to the same conclusion as the induced current values (10.03 to 12.87 nA T-1).

Probing alkylsilane molecular structure on amorphous silica surfaces by sum frequency generation vibrational spectroscopy: First-principles calculations.

The sum frequency generation (SFG) signatures of octadecyl-trichlorosilane (OTS) and dodecyl-dimethyl-chlorosilane (DDCS) monolayers on silica were simulated in the C-H stretching region for three polarization combinations (ppp, sps, and ssp), showing the impact of the additional Si-linked methyl groups of DDCS on its SFG signatures. These simulations are based on a two-step procedure where (i) the molecular properties (vibrational frequencies, IR and Raman intensities) are evaluated using first principles methods and (ii) the three-layer model is employed to calculate the macroscopic responses using these molecular responses, the geometry of the experimental setup, and the optical properties of the layers. These first principles calculations adopt the own N-layered integrated orbital molecular mechanics (ONIOM) approach, which divides the system and enables different levels of approximation to be applied to its different parts. Here, the same ωB97X-D exchange-correlation functional is used for all parts, while the underlying silica layers are described with a smaller atomic basis set (STO-3G, 3-21G, or 6-31G) than the alkylsilane and the top silica layer (6-311G*). Calculations show that for describing the lower layer the minimal STO-3G basis set already provides reliable spectral profiles. For OTS, the results are compared to the experiment, demonstrating a good agreement for ppp and sps configurations, provided the refractive index of the layer nl is set to 1.1. To highlight the origin of the SFG signatures, two chemical models were used, one that includes explicitly the SiO2 surface in the first principles calculations (adsorbed-model) and the other that only considers the silane chain (isolated-model). Simulations show that OTS and DDCS display similar spectral patterns where, for ppp and sps configurations, the r- CH3 stretching vibrations are dominant in comparison to the r+ stretching ones. Still, in the case of DDCS, the r- peak presents a shoulder, which is assigned to the vibrations of the Si-linked methyl groups. This shoulder vanishes when these CH3 groups are frozen. Then, using the isolated-model, the rotation angle (ξ) is gradually changed, showing that in the ppp SFG spectrum the r-/r+ intensity ratio decreases from 73.4 at 0° to 1.7 at 180°.

Intramolecular [3 + 2] Cycloaddition Reactions of Unsaturated Nitrile Oxides. A Study from the Perspective of Bond Evolution Theory (BET).

The reaction mechanism of the [3 + 2] intramolecular cycloaddition of 3,3-dimethyl-2-(prop-2-en-1-yloxy) and (prop-2-en-1-ylsulfanyl) nitrile oxides is analyzed using different DFT functionals with the 6-311++G(d,p) basis set. The activation and the reaction energies for the cis and trans pathways are evaluated at the DFT, MP2, and CCSD(T) levels of theory as well as their Gibbs free energy counterparts. It is shown that the trans regioisomers are both thermodynamic and kinetic compounds, in agreement with experimental outcomes. For a deeper understanding of the reaction mechanism, a BET analysis along the reaction channel ( trans and cis) has been carried out. This analysis reveals that the lone pair on the nitrogen atom is formed first, then the C-C bond, and finally the O-C one. The global mechanism is similar for the two compounds and for the two pathways even if some small differences are observed, for instance, in the values of the reaction coordinates of appeareance of the different basins.

Evaluation of Aromaticity for Open-Shell Singlet Dicyclopenta-Fused Acenes and Polyacenes Based on a Magnetically Induced Current.

The aromaticity of dicyclopenta-fused acenes (DPAs) and polyacenes (PAs) of increasing size has been studied by evaluation with the GIMIC method at the DFT level of the magnetically-induced currents (MICs), and by analyzing their spatial distributions. For these open-shell singlet molecules, spin-restricted and -unrestricted treatments provide very different MICs, the latter ones providing the most reliable solution. These MICs and the differences between spin-restricted and -unrestricted treatments are interpreted in terms of the bond current strengths and the current gradients, which indicate the bond aromaticity and enable the spatial distributions of the diatropic and paratropic currents to be analyzed, respectively. In particular, they allow the rationalization of the MICs in correlation with the odd-electron density distributions and their diradical characters. These calculations demonstrate that 1) in increasingly large PAs the bond current strengths get smaller and smaller than in benzene and get almost similar in the central and terminal rings, 2) for DPAs the MICs increase from dominant paratropic currents and antiaromaticity in the small compounds to diatropic currents and aromaticity in the larger ones, and 3) in the largest DPAs, the central rings are characterized by large diatropic currents and the terminal five-membered rings, for which the odd-electron densities are localized by weak ones.

Investigation of the Electronic Excited-State Equilibrium Geometries of Three Molecules Undergoing ESIPT: A RI-CC2 and TDDFT Study.

Energy minima on the potential energy surfaces of the ground and excited states have been characterized for three photoactive molecules that undergo excited-state intramolecular proton transfer: 3-hydroxychromone, N-salicylideneaniline, and 2-(2-hydroxyphenyl)benzothiazole. Both the CC2 method and the TDDFT methodology with different exchange-correlation (XC) functionals differing by the amount of Hartree-Fock (HF) exchange have been employed. Besides the analysis of the structures along the reaction paths, this study has compared the TDDFT and CC2 results to provide guidelines for selecting the best XC functionals. Several geometrical parameters as well as the excitation energies are found to vary monotonically with the amount of HF exchange. Systematically, this study has addressed the ground-state geometries, those of the excited states, and their variations upon excitation, showing that the M06 XC functional provides the closest agreement with the CC2 results. Still, large differences of geometries have been observed between the different levels of approximation, mostly for the excited states: (i) Not all methods locate the same number of minima, (ii) the bond length variations upon excitation might be reversed, and (iii) the H-bond network can be modified from one level to another, changing the keto/enol character. Moreover, TDDFT/M06 and B3LYP-35 vertical excitation energies are in good agreement with the CC2 values. All in all, these results call for being cautious when using these optimized geometries for predicting the spectroscopic signatures of these compounds to understand the processes that take place during photoexcitation.

Coupled-cluster sum-frequency generation nonlinear susceptibilities of methyl (CH3) and methylene (CH2) groups.

The first vibrational sum frequency generation (SFG) spectra based on molecular properties calculated at the coupled cluster singles and doubles (CCSD) level of approximation have been simulated for interfacial model alkyl chains, providing benchmark data for comparisons with approximate methods, including density functional theory (DFT). The approach proceeds in three steps. In the first two steps, the molecular spectral properties are determined: the vibrational normal modes and frequencies and then the derivatives of the dipole moment and of the polarizability with respect to the normal coordinates. These derivatives are evaluated with a numerical differentiation approach, of which the accuracy was monitored using Romberg's procedure. Then, in the last step, a three-layer model is employed to evaluate the macroscopic second-order nonlinear optical responses and thereby the simulated SFG spectra of the alkyl interface. Results emphasize the following facts: (i) the dipole and polarizability derivatives calculated at the DFT level with the B3LYP exchange-correlation functional can differ, with respect to CCSD, by as much as ±10 to 20% and ±20 to 50% for the CH3 and CH2 vibrations, respectively; (ii) these differences are enhanced when considering the SFG intensities as well as their variations as a function of the experimental configuration (ppp versus ssp) and as a function of the tilt and rotation angles, defining the orientation of the alkyl chain at the interface; (iii) these differences originate from both the vibrational normal coordinates and the Cartesian derivatives of the dipole moment and polarizability; (iv) freezing the successive fragments of the alkyl chain strongly modifies the SFG spectrum and enables highlighting the delocalization effects between the terminal CH3 group and its neighboring CH2 units; and finally (v) going from the free chain to the free methyl model, and further to C3v constraints on leads to large variations of two ratios that are frequently used to probe the molecular orientation at the interface, the (r + r)/r+ ratio for both antisymmetric and symmetric CH3 vibrations and the Ippp/Issp ratio.

Electronic Band Structure of Helical Polyisocyanides.

Restricted Hartree-Fock computations are reported for a methyl isocyanide polymer (repeating unit -C═N-CH3), whose most stable conformation is expected to be a helical chain. The computations used a standard contracted Gaussian orbital set at the computational levels STO-3G, 3-21G, 6-31G, and 6-31G**, and studies were made for two line-group configurations motivated by earlier work and by studies of space-filling molecular models: (1) A structure of line-group symmetry L95, containing a 9-fold screw axis with atoms displaced in the axial direction by 5/9 times the lattice constant, and (2) a structure of symmetry L41 that had been proposed, containing a 4-fold screw axis with translation by 1/4 of the lattice constant. Full use of the line-group symmetry was employed to cause most of the computational complexity to depend only on the size of the asymmetric repeating unit. Data reported include computed bond properties, atomic charge distribution, longitudinal polarizability, band structure, and the convoluted density of states. Most features of the description were found to be insensitive to the level of computational approximation. The work also illustrates the importance of exploiting line-group symmetry to extend the range of polymer structural problems that can be treated computationally.

Calculations of current densities for neutral and doubly charged persubstituted benzenes using effective core potentials.

Magnetically induced current density susceptibilities and ring-current strengths have been calculated for neutral and doubly charged persubstituted benzenes C6X6 and C6X62+ with X = F, Cl, Br, I, At, SeH, SeMe, TeH, TeMe, and SbH2. The current densities have been calculated using the gauge-including magnetically induced current (GIMIC) method, which has been interfaced to the Gaussian electronic structure code rendering current density calculations using effective core potentials (ECP) feasible. Relativistic effects on the ring-current strengths have been assessed by employing ECP calculations of the current densities. Comparison of the ring-current strengths obtained in calculations on C6At6 and C6At62+ using relativistic and non-relativistic ECPs show that scalar relativistic effects have only a small influence on the ring-current strengths. Comparisons of the ring-current strengths and ring-current profiles show that the C6I62+, C6At62+, C6(SeH)62+, C6(SeMe)62+, C6(TeH)62+, C6(TeMe)62+, and C6(SbH2)62+ dications are doubly aromatic sustaining spatially separated ring currents in the carbon ring and in the exterior of the molecule. The C6I6+ radical cation is also found to be doubly aromatic with a weaker ring current than obtained for the dication.

Pigment violet 19 - a test case to define a simple method to simulate the vibronic structure of absorption spectra of organic pigments and dyes in solution.

A typical quinacridone pigment, PV19, has been used to analyze the impact of several computational parameters on the UV/vis absorption band shape in solution, simulated using density functional theory and time-dependent density functional theory levels of approximation. These encompass, (i) the choice of exchange-correlation functional, (ii) the basis set, (iii) the method for non-equilibrium optimization of the excited state geometry, (iv) the approach for evaluating the vibronic band structure, (v) the peak broadening, and (vi) the scaling of the harmonic vibrational frequencies. Among these, the choice of exchange-correlation functional is certainly of the most importance because it can drastically modify the spectral shape. In the case of PV19, the M05-2X and to a lesser extent CAM-B3LYP XC functionals are the most efficient to reproduce the vibronic structure, confirming the important role of exact Hartree-Fock exchange. Still, these functionals are not the most reliable to predict the excitation energies and oscillator strengths, for which M05, a functional with less HF exchange, performs better. For evaluating the vibronic structure, the simple gradient method, where only one step of geometry optimization of the excited state is carried out and the gradients are used to evaluate the Huang-Rhys factors as well as to determine the excited state geometries produces a spectrum that is very similar to the ones obtained with the more involved Duschinsky and geometry methods, opening the way to a fast simulation of the UV/vis absorption spectra of pigments and dyes. Then, the effect of scaling the calculated vibrational frequencies to account for anharmonicity effects as well as for limitation of the method also impacts the shape of the vibronic spectrum and this effect depends on the method used to determine the Huang-Rhys factors. Indeed, scaling the vibrational frequencies by a factor which is typically smaller than 1.0 results in a relative decrease of the 0-1 peak intensity with respect to the 0-0 band when optimizing the geometry of the excited state whereas the effect is opposite and magnified if using the gradient method.

Resonant Raman spectra of molecules with diradical character: multiconfigurational wavefunction investigation of neutral viologens.

The resonant Raman and UV/vis absorption spectra of two diradicaloïd compounds, methyl viologen and phenylene-extended viologen in their neutral state, have been simulated using multiconfigurational wavefunction methods. For methyl viologen, a good agreement with experiment is evidenced for the UV/vis absorption vibronic structure, provided dynamic correlation is accounted for to get the vibrational frequencies and normal modes. To some extent, the agreement with experiment is also good for the RR spectrum and the differences have been attributed to the presence in the experimental spectrum of surface-enhanced effects due to adsorption on the electrodes. As a result of inserting a phenylene group between the pyridinium units, the simulations have demonstrated that (i) in the UV/vis absorption spectrum, the relative intensity of the second band with respect to the 0-0 band increases, (ii) additional strong bands are observed in the RR spectrum, and (iii) the RR excitation profiles of the phenylene-extended viologen present less structure than in the case of methyl viologen where the relative mode intensities can strongly depend on the incident light wavelength. These differences are signatures of the extension of the effective conjugation length as well as of the increase in diradical character.

Effects of the basis set and of the exchange-correlation functional on the Inelastic Electron Tunneling signatures of 1,4-benzenedithiol.

The effects of the atomic basis set and of the exchange-correlation (XC) functional on the Inelastic Electron Tunneling (IET) spectra have been investigated by considering the prototypical 1,4-benzenedithiol molecule. These studies have been completed by tackling the reliability of the same methods for predicting the IR absorption spectrum of the same molecule. The main conclusions are (i) the B3LYP XC functional is suitable to predict the relative vibrational frequencies, (ii) provided a scaling factor is used, the root mean square error on the vibrational frequencies goes down to 18 cm(-1), (iii) triple-ζ basis sets and in particular the cc-pVTZ basis set is a good compromise between accuracy and computational needs, (iv) basis set effects on the IET intensities are larger than those of the XC functional, and (v) the cc-pVTZ, cc-pVQZ, and aug-cc-pVDZ basis sets provide consistent IET intensities.

Theoretical insight into the inelastic electron tunneling spectra of an anil derivative.

First-principles simulations have been employed to simulate the inelastic electron tunneling (IET) spectra of the enol and keto forms of an anil molecular switch and to analyze them with respect to the character of the vibrational normal modes. When the molecules are sandwiched between Au plates, the dominant IET signatures appear at very similar voltages for both forms, but their intensities are clearly different, which makes IET an efficient technique to probe the molecular state of the switch. The IET-active modes are also similar for both anil forms and consist of in-plane molecular motions, CC and ring stretching, and C-H bending motions. Moreover, the IET activity of the vibrational modes specific to the enol and keto forms, i.e., those involving bending motions of the C-O-H and C-N-H groups, respectively, demonstrates that IET spectroscopy is an efficient technique to distinguish unambiguously between the two states of the keto/enol switch.

Beta sheets with a twist: the conformation of helical polyisocyanopeptides determined by using vibrational circular dichroism.

Detailed information on the architecture of polyisocyanopeptides based on vibrational circular dichroism (VCD) spectroscopy in combination with DFT calculations is presented. It is demonstrated that the screw sense of the helical polyisocyanides can be determined directly from the C=N-stretch vibrational region of the VCD spectrum. Analysis of the VCD signals associated with the amide I and amide II modes provides detailed information on the peptide side-chain arrangement in the polymer and indicates the presence of a helical ß-sheet architecture, in which the dihedral angles are slightly different to those of natural ß-sheet helices.