Emission and absorption spectra of some bridged 1,5-benzodiazepines

Absorption spectra in neutral and acidic media are reported for a series of bridged 1,5-benzodiazepines, which are unable to tautomerize. Comparison is made with non-bridged 1,5-benzodiazepines capable of tautomeric rearrangement. Both bridged and non-bridged 1,5-benzodiazepines are essentially non-fluorescent due to the “proximity effect” of interaction between singlet ηπ* and ππ* states of similar energy, a phenomenon previously recognised in six-membered nitrogen heterocycles.     

Computer-aided study of oxygen absorption by water clusters. IR spectra of heteroclusters

Spectral characteristics of (H₂O)_n, (O₂)_m(H₂O)_n, and (O)_i(H₂O)_n cluster systems, where m≤2, i≤4, and 10 ≤ n ≤ 50, are studied with the molecular dynamics method using a flexible molecule model. The IR absorption spectra are changed substantially as a result of O₂ molecule dissociation, and in the presence of atomic oxygen in the clusters, the spectra are characterized by a deep minimum at 520 cm^?1. The absorption of oxygen causes a marked reduction in reflection coefficient R of monochromatic IR radiation. The number of peaks in the R(ω) spectra decreases to two in the case of molecular oxygen absorption and is no larger than four in the case of atomic oxygen absorption. The absorption of atomic oxygen by the clusters is also accompanied by a significant increase in the dissipation of energy accumulated by the clusters. This effect weakens when molecular oxygen is absorbed. An increase in atomic oxygen concentration in the clusters renders their radiation harder.     

Low-temperature study of ultradisperse polytetrafluoroethylene

The temperature behavior of the IR and EPR spectra of the industrial polytetrafluoroethylene F-4 sample and ultradisperse polytetrafluoroethylene (UPTFE) sample obtained by the thermogas dynamic method has been studied. It is shown that the spiral-chain conformation is preserved for both structures and that ordering tends to increase at lower temperatures. The IR spectral parameters of UPTFE were found to change irregularly at 273 K and 233 K due to a rearrangement of the crystalline component of the polymer. The observed temperature changes in the EPR spectra of the middle radicals in the samples are caused by gradual freeze-out of the molecular motions in UPTFE to a temperature ～25° lower than in F-4, due to differences in the molecular and supramolecular structures of the samples.     

Ultraviolet absorption spectra of chloramines and chlorine in carbon tetrachloride

In order to quantify mixtures of chloramines and chlorine in carbon tetrachloride, the ultraviolet spectra of monochloramine, dichloramine, trichloramine and chlorine were measured between 250 and 410 nm. Monochloramine and chlorine absorb at 259 nm (?=484.61 mol^?1 cm^?1 ) and 331 nm (?=95.7 l mol^?1 cm^?1), respectively, while dichloramine and trichloramine exhibit two peaks. For dichloramine, the maximum absorptions are observed at 255 nm (?=126.1 l mol^?1 cm^?1) and 301 nm (?=307.8 l mol^?1 cm^?1) and for trichloramine, at 262 nm (?=635.4 l mol^?1 cm^?1) and 344 nm (?=315.0 l mol^?1 cm^?1).     

Infrared absorption spectra of carbon monoxide in rare gas matrices

Medium and high resolution infrared spectra of CO trapped in solid Ne, Ar, Kr and Xe are presented. Spectra of low optical density samples are identical to those previously observed but a different assignment is proposed. Monomeric absorption is clearly identified by double doping experiments and by sample deposition through a nitrogen cold trap. In addition other broad weak bands are observed on the spectra of high optical density samples. These bands are assigned to combinations between internal vibration, libration and lattice vibrations. The barrier hindering rotation due to site distortion is estimated to 30–50 cm^?1. Librational motion is strongly perturbed by coupling with lattice motion. Carbon monoxide is also an efficient probe to study the dynamical properties of rare gas lattices.     

Computational study of [10]annulene NMR spectra

[reaction: see text] High-level theoretical methods are applied to calculate the 13C NMR chemical shifts of three isomers of [10]annulene. Comparison with experiment clearly shows that the carrier of NMR signals of the so-called B form is the "twist" isomer. The results of this study strongly support predictions of relative energies of mono-trans [10]annulene isomers at the CCSD(T) level, which in turn are in qualitative disagreement with DFT and MP2 calculations.     

Measurement of absorption coefficients in vapour-phase infrared spectra by means of gas chromatographic-infrared systems

In addition to stopped-flow gas chromatographic-infrared (GC-IR) systems and static vapour cells, GC-IR systems without a stopped-flow facility can also be used to measure vapour-phase IR spectra of appropriate quality. An expression was derived for the determination of concentrations in this type of measurement, which can be used to determine integrated absorption coefficients of practical and theoretical importance.     

X-ray diffraction analysis of ultradisperse systems: The Debye formula

Problems of X-ray diffraction analysis of ultradisperse systems with the particle size up to 5 nm are discussed in the review. A method for calculating X-ray diffraction patterns using the Debye formula within the kinematic theory of X-ray scattering (Debye Function Analysis-DFA) is presented. The DFA method makes it possible to obtain information on the atomic structure, shape and size of nanoparticles, and verify hypotheses on the presence of deformations, stacking faults and other structural features of ultradisperse and nanostructured systems. The method is applicable for modeling the diffraction patterns of objects with an arbitrary (not necessarily crystal) structure. Applications of the method for examining the structure of various objects are discussed.     

Computational insight into the electronic structure and absorption spectra of lithium complexes of N-confused tetraphenylporphyrin

The present work is a theoretical investigation on lithium complexes of N-confused tetraphenylporphyrins (aka inverted) employing density functional theory (DFT) and time-dependent DFT, using the B3LYP, CAM-B3LYP, and M06-2X functionals in conjunction with the 6-31G(d,p) basis set. The purpose of the present study is to calculate the electronic structure and the bonding of the complexes to explain the unusual coordination environment in which Li is found experimentally and how the Li binding affects the Q and the Soret bands. The calculations show that, unlike a typical tetrahedral Li(+) cation, this Li forms a typical bond with one N and interacts with the remaining two N atoms, and it is located in the right place to form an agostic-like interaction with the internal C atom. The reaction energy, the enthalpy for the formation of the lithium complexes of N-confused porphyrins, and the effect of solvation are also calculated. The insertion of Li into N-confused porphyrin, in the presence of tetrahydrofuran, is exothermic with a reaction energy calculated to be as high as -72.4 kcal/mol using the lithium bis(trimethylsilyl)amide reagent. Finally, there is agreement in the general shape among the vis-UV spectra determined with different functionals and the experimentally available ones. The calculated geometries are in agreement with crystallographic data, where available.     

Properties of fractions of ultradisperse polytetrafluoroethylene soluble in supercritical carbon dioxide

The morphology and structure of FORUM ultradisperse polytetrafluoroethylene fractions obtained via separation of the initial sample with supercritical carbon dioxide are studied by means of scanning and transmission electron microscopy, thermogravimetric and X-ray structural analysis, and IR and NMR spectroscopy. The lowest molecular mass fractions of the ultradisperse polytetrafluoroethylene are involved in the dissolution process, while an increase in production parameters (pressure and temperature) entails gradual involvement of higher molecular mass oligomers in this process. On the basis of the experimental data, the chain length of oligomers soluble in supercritical carbon dioxide is estimated.     

Computational study of 19F NMR spectra of double four ring-containing Si/Ge-zeolites.

(19)F NMR chemical shifts are calculated in order to study the F(-) environment in double four ring (D4R) containing Si/Ge-zeolites. The calculations with the DFT/CSGT/B3PW91 methodology yielded an agreement within 2 ppm with respect to the experimental peaks corresponding to the D4R units containing 8Si0Ge, 7Si1Ge and 0Si8Ge of the octadecasil zeolite. The optimisation of the 7Si1Ge-, 6Si2Ge-, 5Si3Ge- and 4Si4Ge-D4R units with DFT/B3LYP methodology shows that a covalent Ge-F bond is formed and therefore a Ge atom in the D4R is pentacoordinated. The displacement of the fluoride ion towards a Ge atom in the Ge-containing D4R units locates four Si/Ge atoms in the close vicinity of the F(-) and this makes possible a rationalization of the (19)F NMR signals in groups according to the number of Si (n) and Ge (m) atoms in the nearest F(-) environment, F-Si(n)Ge(m) (where n+m=4). Thus, the calculated chemical shifts show that higher values are observed when the number of Ge atoms in the nearest F(-) environment increases.     

Computational study of the vibrational spectra of alpha- and beta-Keggin polyoxometalates

The structures and vibrational frequencies of the alpha- and beta-isomers of the phosphomolybdate Keggin anion [PMo(12)O(40)](3-) have been calculated by using density functional theory. Good agreement between the calculated unscaled vibrational frequencies and those determined experimentally and between the calculated and observed IR traces has been obtained allowing the IR and Raman spectra to be assigned. For the alpha-isomer, the agreement with experiment using the current level of theory is superior to that obtained previously. For the beta-isomer, for which no non-empirical study has previously been reported, the agreement with experiment is slightly poorer but still allows the spectrum to be assigned unambiguously. To calculate the structure and vibrational spectra of these large molydate cluster ions requires large basis sets and a good treatment of electron correlation and relativistic effects. For the 53-atom [PMo(12)O(40)](3-) ions, the computational demands are very high, requiring several months computational time. The calculated IR spectral traces for the two isomers are quite similar due to the relative flexibility of the molybdates, where the slight weakening of the bonding of the rotated trimetallic unit to the rest of the cluster in the beta-isomer is compensated by contraction of the bonds within the unit, and the structure of the [MO(6)] and [PO(4)] units in the two isomers is nearly identical. The vibrations characteristic of the bridging Mo-O-Mo bonds involve both the "2-2" junctions between rotated [M(3)O(13)] units and the "1-2" junctions between rotated and unrotated units. The separation of "ligand" and "interligand" vibrations is not clear. The vibrational analyses confirm the high symmetry, namely T(d) and C(3v) for the alpha- and beta-isomers, respectively, assumed by previous workers in this field. The characteristic group frequencies for the Type I polyoxometalates containing both edge- and corner-sharing I octahedra have been identified.     

Computer study of the absorption of ethane by aqueous ultradispersed medium: IR spectra

Absorption of ethane molecules by water clusters containing 10–20 molecules is studied by the molecular dynamics method. The (H₂O) _n (I), C₂H₆(H₂O) _n (II), and (C₂H₆)₂(H₂O) _n (III) cluster systems are composed on the basis of specific statistical weights. Spectral characteristics of system and single clusters are determined in the frequency range of 0 ≤ ω ≤ 1000 cm^?1. In this frequency range, both real and imaginary parts of dielectric permittivity decrease monotonically after the absorption of C₂H₆ molecules by an aqueous ultradispersed system. Integral coefficient of IR absorption increases, while average (over frequency) reflection coefficient decreases after the absorption of ethane molecules. The intensity of IR scattering by the systems of clusters containing C₂H₆ molecules lowers. Maximal values of radiation power for water clusters with various sizes are balanced with the capture of ethane molecules by the clusters, whereas oscillations in the size dependence of the density of electrons that are active with respect to IR radiation decrease.     

QM/MM study of the absorption spectra of DsRed.M1 chromophores

We report geometries and vertical excitation energies for the red and green chromophores of the DsRed.M1 protein in the gas phase and in the solvated protein environment. Geometries are optimized using density functional theory (DFT, B3LYP functional) for the isolated chromophores and combined quantum mechanical/molecular mechanical (QM/MM) methods for the protein (B3LYP/MM). Vertical excitation energies are computed using DFT/MRCI, OM2/MRCI, and TDDFT as QM methods. In the case of the red chromophore, there is a general blue shift in the excitation energies when going from the isolated chromophore to the protein, which is caused both by structural changes and by electrostatic interactions with the environment. For the lowest ππ* transition, these two factors contribute to a similar extent to the overall DFT/MRCI shift of 0.4 eV. An enlargement of the QM region to include active‐site residues does not change the DFT/MRCI excitation energies much. The DFT/MRCI results are closest to experiment for both chromophores. OM2/MRCI and TDDFT overestimate the first vertical excitation energy by 0.3–0.5 and 0.2–0.4 eV, respectively, relative to the experimental or DFT/MRCI values. The experimental gap of 0.35 eV between the lowest ππ* excitation energies of the red (cis‐acylimine) and green (trans‐peptide) forms is well reproduced by DFT/MRCI and TDDFT (0.32 and 0.37 eV, respectively). A histogram spectrum for an equal mixture of the two forms, generated by OM2/MRCI calculations on 450 snapshots along molecular dynamics trajectories, matches the experimental spectrum quite well, with a gap of 0.23 eV and an overall blue shift of about 0.3 eV. DFT/MRCI appears as an attractive choice for calculating excitation energies in fluorescent proteins, without the shortcomings of TDDFT and computationally more affordable than CASSCF‐based approaches. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Multiple-scattering method in the theory of photoelectron and absorption spectra polyatomic systems

Conclusion Within the framework of the –'muffin-tin–' approximation the LS equations reduce to a system of linear algebraic equations of type (8), the order of which increases with increasing number of atoms in the system. In this paper the solution of this system of equations for the whole system is replaced by the solution of systems of equations of lower order (9) and (10) for isolated clusters. This advantage is in general fictitious owing to the weak convergence of the solution of Eqs. (20) and (21) with respect to L. However, for highly-symmetrical clusters the series in L converges and the proposed method greatly reduces the computing work. The advantages of the method are most clearly seen in the high-energy limit (23) and (27), in which the dependence on L vanishes.Institute of Inorganic Chemistry, Academy of Sciences of the USSR, Siberian Branch. Translated from Zhurnal Strukturnoi Khimii, Vol. 17, No. 1, pp. 22–28, January–February, 1976.     

Computational study of solvent effects and the vibrational spectra of Anderson polyoxometalates

The structures and vibrational frequencies of the type II Anderson heteropolyanions [TeMo6O24]6- and [IMo6O24]5- have been calculated by using density functional theory using a number of common functionals and basis sets. For the first time, Raman intensities have been calculated and the effect of solvent on the modeling has been investigated. The calculated IR and Raman spectral traces are in good agreement with experiment allowing the characteristic group frequencies for this class of polyoxometalate to be identified. The stretching vibrations of the molybdenum-oxygen bonds are predicted to occur at somewhat lower frequencies than in the type I polyoxometalates. Stretching of the heteroatom-oxygen bonds occurs at significantly lower frequencies than in the Keggin anions as a simple consequence of the higher coordination number of the central heteroatom in the Anderson systems. For the [Mo2O7]2- and [Mo6O19]2- ions, the relatively low negative charge leads to small structural changes when solvent is included. In these systems, solvent leads to an increase in the bond polarity and a decrease in the covalent bond orders, resulting in decreases in the calculated frequencies. For the Anderson anions, the higher negative charges leads to greater solvent effects with contraction of the clusters and increases in the frequencies of bands due to stretching of the two, cis-related molybdenum-oxygen bonds.     

Ultraviolet absorption spectra of substituted phenols: a computational study

Vertical excitation energies for electronic transitions from the ground state to the first two excited states of phenol, mono- and disubstituted methoxyphenols and methyl-substituted phenols have been characterized with the Time-Dependent Density Functional Theory (TD-DFT), the Complete Active Space Self-Consistent Field method (CASSCF) and the Coupled Cluster with Single and Double Excitations Equation-of-Motion approach (CCSD-EOM) to simulate and interpret experimental ultraviolet absorption spectra. While CASSCF excitation energies for the first two transitions either are grossly overestimated or exhibit a weak correlation with experimental data, both TD-DFT and CCSD-EOM perform very well, reproducing the spectral shifts of both the primary band and secondary band observed upon substitution. The conformational dependence of the calculated excitation energies is generally smaller than the shifts caused by substitution.     

Degenerate two-photon absorption spectra in azoaromatic compounds.

A systematic study of the degenerate two-photon absorption (2PA) spectra of seven azoaromatic compounds in dimethyl sulfoxide solution is reported, which employed the Z-scan technique with femtosecond laser pulses from the bottom of the azo compound absorption bands up to 1100 nm. The 2PA peaks for pseudostilbene-type azo compounds (Disperse Orange (DO) 3, Disperse Red (DR) 13, DR1, DR19, and DR19-Cl) were observed at twice the peak wavelength of the linear absorption. However, such peaks were not observed for other azo compounds (PAMINO and DIAMINO) because of the symmetry of these molecules. A resonance enhancement of the 2PA cross-section was observed for all compounds. The 2PA peak and the nonlinear resonance enhancement behavior could be adjusted with a model based on perturbation theory. Such knowledge can be a guideline to the understanding of the 2PA process in azoaromatic compounds.