UV spectra and basicity constants of 2-substituted quinoxalines

The UV spectra of the bases and cations of a number of 2-substituted quinoxaline derivatives were investigated, and their basicity constants were determined. The investigated compounds were divided into two series, characterized, respectively, by ortho and meta orientations of the substituent and cationoid center, on the basis of correlations of the pKa values with the Hammett—Taft constants. The effect of the position of the protonation center relative to the substituent on the energy of the ^* transitions was examined within the framework of the simple MO LCAO method. The results of the calculations are compared with the experimentally observed shift in the¹L_b band in the UV spectra of some 2-substituted quinoxalines on passing from the bases to the monocations.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 398–403, March, 1973.     

NIR to UV absorption spectra and the optical constants of phthalocyanines in glassy medium

Optical absorption studies of phthalocyanines (Pc-s) in borate glass matrix have been reported for the first time. Measurements have been done corresponding to photon energies between 1.1 and 6.2 eV for free base, manganese, iron, nickel, molybdenum, cobalt and copper phthalocyanines. Several new discrete transitions are observed in the UV-vis region of the spectra in addition to a strong continuum component of absorption in the IR region. Values of some of the important optical constants viz. absorption coefficient (alpha), molar extinction coefficient (epsilon), absorption cross-section (sigma(a)), band width (delta lambda), electric dipole strength (q2) and oscillator strength (f) for the relevant electronic transitions are also presented. All the data reported for Pc-s in the new matrix have been compared with those corresponding to solution, vapor and thin film media.     

Metallomethanes: XIII. Vibrational spectra and force fields of cyanomercuriomethanes

Vibrational spectra with assignments and results of normal coordinate calculations for cyanomercuriomethanes CH_4−n(HgCN)_n (2 ≤ n ≤ 4) are discussed. The valence force constants of the central CHg bonds are 2.149, 1.944, and 1.798 N cm⁻¹,while those of the HgCN bonds are 2.204, 2.123, and 2.162 N cm⁻¹, for n = 2, 3 and 4, respectively. All these force constants are lower than the corresponding constants for methylmercury cyanide (2.445 and 2.379 N cm⁻¹). The overall behaviour of these force constants as a function of the degree of mercuration n is quite similar in both the cyanomercuriomethanes CH_4−n(HgCN) and methylmercuriomethanes CH_4−n(HgCH₃)_n series with the difference that there are variations in the constants at higher values in the former series. The potential energy distributions indicate that the valence vibrations of the CH, CN, and HgCN bonds are almost independent of all other vibrations, which in turn are more or less strongly coupled.     

Vibrational spectra and stereochemistry of mono-and polysaccharides. II. α-anomers of D-glucose and D-galactose. Glucitol and galactitol

The IR and Raman spectra of D-glucose and D-galactose α-anomers are compared with the corresponding spectra of the related polyatomic alcohols glucitol and galactitol. Experimental data on the frequencies and modes of normal vibrations and on the contributions of normal vibrations involving particular skeletal CiOi and CiC(i+1) bonds to the potential energy distribution (PED) are compared with the results of theoretical calculations. The α-anomers of D-glucose and D-galactose with different configurations of the C4C4 and C4H4 bonds are characterized by substantially different sets of frequencies with prevailing contributions to PED from the CiOi and CiC(i+1) bonds. The similar bonds of glucitol and galactitol are also characterized by specific frequency sets. Particular CiOi and CiC(i+1) bonds have different numbers of “localized” frequencies. B. I. Stepanov Institute of Physics, Belarus Academy of Sciences. V. Tshebyatovski Institute of Low Temperatures and Structural Studies, Polish Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 431–442, May–June, 1995. Translated by I. Izvekova     

Optical constants of organic insulators in the UV range extracted from reflection electron energy loss spectra

Reflection electron energy loss spectroscopy (REELS) spectra were measured for seven insulating organic compounds (DNA, Irganox 1010, Kapton, polyethylene [PE], poly(methyl methacrylate) [PMMA], polystyrene [PS] and polytetrafluoroethylene [PTFE]). Optical constants and energy band gaps were extracted from the measured REELS spectra after elimination of multiple electron scattering via a deconvolution and fitting the normalised single scattering energy loss spectra to Drude and Drude–Lindhard model dielectric functions, constrained by the Kramers–Kronig sum and f-sum rules. Satisfactory agreement is found for those optical constants for which literature data exists. For PTFE, the observed features in the optical data correspond to its electronic structure.     

UV spectra of N-vinylpyrroles

Chemistry of Heterocyclic Compounds - The effect of the structure and the medium on the position and intensity of the absorption bands in the UV spectra that attest to the presence of a single...     

Structure and UV spectra of aliphatic-aromatic phosphines

The basicities of 10 tertiary phosphines were determined. Equations were derived to relate pK_a(H₂O) for these phosphines to the sum of the Kabachnik _ph-constants and pK_a(H₂O) for aryldiethylphosphines to the Hammett _n-constants. Analysis of these equations leads to the conclusion that in the ground state the unshared pair of electrons of the trivalent phosphorus is not conjugated with the -electron system of the aryl group.The bands in the UV spectra of the aryldiethylphosphines were assigned on the basis of this fact. The most intense absorption in the spectra of the phosphines (K-band) is due to the 'A_1g 'B_1u transition of the -electrons. The B-band, which is produced by the 'A_1g 'B_2u transition, only appears in the spectra of phosphine oxides; in the case of the phosphines it is obscured by the stronger R-band, which arises from the n -transition of the unshared electrons of phosphorus.     

Electronic spectra and molecular constants of zirconium mononitride

The electronic spectra and structure of zirconium mononitride were considered. The electronic vibrational-rotational spectrum of ZrN was obtained by intracavity laser spectroscopy within the 550–750 nm region. The spectrum was analyzed and the molecular constants of the ground X²Σ and excited A²Π electronic states were calculated. On the basis of the obtained experimental data and critical analysis of all the information on ZrN spectrum studies, the most reliable set of molecular constant values was recommended.     

Spirans—XIII : UV—absorption of some spirans

UV absorption spectra of some spirans and their “half-compounds” have been obtained and their comparison has allowed the dominating conformation to be defined for respective spirans.     

Vibrational spectra and force constants ofS-methyl-N,N-dimethyldithiocarbamate

The IR and Raman spectra ofS-methyl-N,N-dimethyldithiocarbamate were studied in different phase states. The frequencies and vibration modes of normal vibrations were analyzed, and the erroneous assignments made in some previous works were corrected. The force constants of the molecule were estimated using the model of the generalized valenceforce field.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1108–1110, June, 1995.This work was carried out with partial financial support from the International Science Foundation.     

UV spectra of carbonyl-contaiming furan compounds

The optical absorption maxima in the UV spectra of furan aldehydes, ketones, acids and their derivatives — acid chlorides, anhydrides, esters, and diacyl peroxides ^– can be arranged in an order similar to the order of reactivities of the carbonyl groups. The effect of a solvent on the position of _max was examined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 590–594, May, 1973.     

The computed force constants and vibrational spectra of toluene

The complete harmonic force field and dipole moment derivatives have been computed for toluene at the Hartree-Fock level using a 4-21G basis set. The six scale factors optimized for benzene were used to scale the computed harmonic force constants of toluene. The vibrational frequencies of toluene computed from this scaled quantum mechanical force field are quite good. After a correction was made to two previously proposed spectral assignments, the mean deviation from the experimental frequencies is only 7.8 cm^?1 except for the frequencies related to the methyl group. Five more scale factors for the vibrational modes of the methyl group were reoptimized. The final comparison showed an overall mean deviation of 7.5 cm^?1 between the theoretical spectrum and the experimental spectrum. Computed intensities are qualitatively in agreement with experiments. They are highly useful in the investigation of questionable assignments.     

Quantum chemical calculation of force constants and vibrational spectra

As a result of the development of direct derivative methods and improved computational facilities, ab initio quantum chemical calculations have become an increasingly important source of information for the determination of molecular force constants. Within the Hartree-Fock (H-F) SCF model and using moderate size basis sets such calculations are now economically feasible for molecules of up to 2o–3o atoms. At this level of theory, harmonic diagonal force constants are overestimated by 1o–3o%, corresponding to 5–15% in the frequencies. However, the largely systematic errors can be accounted for by simple empirical corrections. The resulting SQM (Scaled Quantum Mechanical) force fields are probably the most reliable ones available at present for larger molecules. Calculated infrared intensities are semi-quantitatively correct. Beyond the H-F model, large scale calculations including electron correlation give great improvements in the force constants, but there are still residual errors of a few percent.