Near and vacuum UV polarization spectroscopy of 1,4-distyrylbenzene.

The UV absorbance bands of 1,4-distyrylbenzene (1,4-Bis[(E)-2-phenylethenyl]benzene, DSB) are investigated by Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy using stretched polyethylene as an anisotropic solvent. The observed polarization data provide information on the transition moment directions of the observed spectral features. The investigation covers the range 15,000-58,000 cm-1 (667-172 nm), thereby providing new information on the transitions of DSB in the vacuum UV region. The observed spectrum is characterized by four main band systems centered at 27,600, 41,000, 49,800, and 57,500 cm-1 (362, 244, 201, and 174 nm). In general, the observed bands and their polarization directions are well predicted by the results of quantum chemical calculations using Time-Dependent Density Functional Theory (TD-DFT) with the functional CAM-B3LYP, and with the semiempirical all-valence-electrons method LCOAO.

NH Stretching Frequencies of Intramolecularly Hydrogen-Bonded Systems: An Experimental and Theoretical Study.

The vibrational NH stretching transitions in secondary amines with intramolecular NH···O hydrogen bonds were investigated by experimental and theoretical methods, considering a large number of compounds and covering a wide range of stretching wavenumbers. The assignment of the NH stretching transitions in the experimental IR spectra was, in several instances, supported by measurement of the corresponding ND wavenumbers and by correlation with the observed NH proton chemical shifts. The observed wavenumbers were correlated with theoretical wavenumbers predicted with B3LYP density functional theory, using the basis sets 6-311++G(d,p) and 6-31G(d) and considering the harmonic as well as the anharmonic VPT2 approximation. Excellent correlations were established between observed wavenumbers and calculated harmonic values. However, the correlations were non-linear, in contrast to the results of previous investigations of the corresponding OH···O systems. The anharmonic VPT2 wavenumbers were found to be linearly related to the corresponding harmonic values. The results provide correlation equations for the prediction of NH stretching bands on the basis of standard B3LYP/6-311++G(d,p) and B3LYP/6-31G(d) harmonic analyses, with standard deviations close to 38 cm-1. This is significant because the full anharmonic VPT2 analysis tends to be impractical for large molecules, requiring orders of magnitude more computing time than the harmonic analysis.

Intramolecular Hydrogen Bonds in Normal and Sterically Compressed o-Hydroxy Aromatic Aldehydes. Isotope Effects on Chemical Shifts and Hydrogen Bond Strength.

A number of o-hydroxy aromatic aldehydes have been synthesized to illustrate the effect of steric compression and O···O distances on the intramolecular hydrogen bond and the hydrogen bond energies. Hydrogen bond energies have been calculated using the 'hb and out' method using either the MP2 method or the B3LYP functional with the basis set 6-311++G(d,p). However, several compounds cannot be treated this way. Hydrogen bond energies are also determined using electron densities at bond critical points and these results are in good agreement with the results of the 'hb and out' model. Two-bond deuterium isotope effects on 13C chemical shifts are suggested as an experimental way to obtain information on hydrogen bond energies as they easily can be measured. Isotope effects on aldehyde proton chemical shifts have also been measured. The former show very good correlation with the hydrogen bond energies and the latter are related to short O···O distances. Short O···O distances can be obtained as the result of short C=C bond lengths, conjugative effects, and steric compression of the aldehyde group. Short O···O distances are in general related to high hydrogen bond energies in these intramolecularly hydrogen-bonded systems of resonance assisted hydrogen bond (RAHB) type.

Hydrogen bonding between ethynyl aromates and triethylamine: IR spectroscopic and computational study.

Ethynylpyridines (EPs) and ethynylbenzene (EB) are multifunctional systems able to participate in hydrogen-bonded complexes as both donors and acceptors of the H-atom. Their structures and stabilities are mainly a function of the hydrogen-bonding properties of the partner in the complex and the surroundings in which the complexation occurs. In this paper, IR spectroscopy and quantum chemical calculations are employed to characterize hydrogen-bonded complexes of 2- and 3-EP and EB with triethylamine (TEA) in tetrachloroethene (C2Cl4) solution. The formation of CH⋯N hydrogen bonds is experimentally confirmed by the appearance of TEA concentration-dependent signals in the IR spectra of the EPs and EB. Along with the signals due to unassociated CH and CC oscillators (2-EP: 3308 cm-1 and 2120 cm-1; 3-EP: 3308 cm-1 and 2116 cm-1; EB: 3313 cm-1 and 2113 cm-1) weak, red-shifted signals arise at ~3215 ±â€¯5 cm-1 and ~2105 ±â€¯5 cm-1 which are assigned to the stretching vibrations of hydrogen-bonded CH⋯ and CC⋯ oscillators, respectively. This result is at variance with those of previous investigations of EB and TEA in the gas phase. In the 2-EP⋯TEA complex these bands remain at the same position with increasing TEA concentration. However, in the 3-EP⋯TEA and EB⋯TEA complexes the CH⋯ stretching band demonstrates a slightly reduced red-shift as the TEA concentration increases, whereas the CC⋯ stretching band absorbs at the same wavenumber in the investigated TEA concentration range. The results of B3LYP-D3 calculations indicate that complexes with more or less linear CH⋯N intermolecular hydrogen bonds are more stable than other, dispersion-driven complexes. Complexes with the Cs symmetrical TEA conformer are predicted to have larger binding energy than those formed with the C3 and C1 symmetrical conformers. The predicted IR spectral shifts are slightly different for complexes with the three different TEA conformers. Association constants of hydrogen-bonded complexes at 26 °C are estimated to be ~0.1 mol-1 dm3.

2-Ethynylpyridine dimers: IR spectroscopic and computational study.

2-ethynylpyridine (2-EP) presents a multifunctional system capable of participation in hydrogen-bonded complexes utilizing hydrogen bond donating (CH, Aryl-H) and hydrogen bond accepting functions (N-atom, CC and pyridine π-systems). In this work, IR spectroscopy and theoretical calculations are used to study possible 2-EP dimer structures as well as their distribution in an inert solvent such as tetrachloroethene. Experimentally, the CH stretching vibration of the 2-EP monomer absorbs close to 3300 cm-1, whereas a broad band with maximum around 3215 cm-1 emerges as the concentration rises, indicating the formation of hydrogen-bonded complexes involving the CH moiety. The CC stretching vibration of monomer 2-EP close to 2120 cm-1 is, using derivative spectroscopy, resolved from the signals of the dimer complexes with maximum around 2112 cm-1. Quantum chemical calculations using the B3LYP + D3 model with counterpoise correction predict that the two most stable dimers are of the π-stacked variety, closely followed by dimers with intermolecular CH⋯N hydrogen bonding; the predicted red shifts of the CH stretching wavenumbers due to hydrogen bonding are in the range 54-120 cm-1. No species with obvious hydrogen bonding involving the CC or pyridine π-systems as acceptors are predicted. Dimerization constant at 25 °C is estimated to be K2 = 0.13 ±â€¯0.01 mol-1 dm3.

Vacuum UV Polarization Spectroscopy of p-Terphenyl.

p-Terphenyl is used as a component in a variety of optical devices. In this investigation, the electronic transitions of p-terphenyl are investigated by synchrotron radiation linear dichroism (SRLD) spectroscopy in the range 30000-58000 cm-1 (330-170 nm) on molecular samples aligned in stretched polyethylene, thereby extending the region investigated by polarization spectroscopy into the vacuum UV. The resulting partial absorbance curves reveal that the vacuum UV band system with a maximum at 55000 cm-1 (180 nm) is predominantly short axis-polarized. This result is of interest in the optical applications of p-terphenyl, for example as a wavelength shifter. The observed polarization spectra are compared with the results of quantum chemical model calculations. Convoluted versions of the transitions predicted with the semiempirical ZINDO method and with the long-range-corrected time-dependent density functional theory (TD-DFT) procedures TD-CAM-B3LYP, TD-LC-ωPBE, and TD-ωB97XD are in similar qualitative agreement with the observed partial absorbance curves throughout the investigated spectral regions, while TD-B3LYP fails to predict qualitatively the spectrum of p-terphenyl in the region above 40000 cm-1 (250 nm).

NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds.

For the purpose of this review, strong hydrogen bonds have been defined on the basis of experimental data, such as OH stretching wavenumbers, νOH, and OH chemical shifts, δOH (in the latter case, after correction for ring current effects). Limits for O-H···Y systems are taken as 2800 > νOH > 1800 cm-1, and 19 ppm > δOH > 15 ppm. Recent results as well as an account of theoretical advances are presented for a series of important classes of compounds such as ß-diketone enols, ß-thioxoketone enols, Mannich bases, proton sponges, quinoline N-oxides and diacid anions. The O···O distance has long been used as a parameter for hydrogen bond strength in O-H···O systems. On a broad scale, a correlation between OH stretching wavenumbers and O···O distances is observed, as demonstrated experimentally as well as theoretically, but for substituted ß-diketone enols this correlation is relatively weak.

Electronic states of Myricetin. UV-Vis polarization spectroscopy and quantum chemical calculations.

Myricetin (3,3',4',5,5',7'-hexahydroxyflavone) was investigated by linear dichroism spectroscopy on molecular samples partially aligned in stretched poly(vinyl alcohol) (PVA). At least five electronic transitions in the range 40,000-20,000cm-1 were characterized with respect to their wavenumbers, relative intensities, and transition moment directions. The observed bands were assigned to electronic transitions predicted with TD-B3LYP/6-31+G(d,p).

A Reinvestigation of the Ionic Liquid Diisopropylethylammonium Formate by NMR and DFT Methods.

The complex between diisopropylethylamine (DIPEA) and formic acid has been reinvestigated. Mixing the compounds in the ratio 1:1 leads to a phase separation in which the upper phase is DIPEA and the lower phase is the "ionic liquid" named DIPEF. A combined NMR and DFT study shows that the lower phase primarily is formic acid:formate and diisopropylammonium ions in the ratio 2:1 (acid:base) plus the formic acid dimer. Addition of more acid leads to more and more of the acid dimer. The proton transfer in the system is 65-80%. The structural picture presented in this paper is very different from that presented elsewhere. However, the present picture should be considered using acids and bases with a pKa difference less than 8. The formic acid content in the DIPEF ionic liquid causes desorption of the dye-sensitized solar cell (DSC) dye N719 from the photo anode, and DIPEF is therefore not a suitable electrolyte for DSCs.

Dye-sensitized solar cells and complexes between pyridines and iodines. A NMR, IR and DFT study.

Interactions between triiodide (I(3)(-)) and 4-tert-butylpyridine (4TBP) as postulated in dye-sensitized solar cells (DSC) are investigated by means of (13)C NMR and IR spectroscopy supported by DFT calculations. The charge transfer (CT) complex 4TBP·I(2) and potential salts such as (4TBP)(2)I(+), I(3)(-) were synthesized and characterized by IR and (13)C NMR spectroscopy. However, mixing (butyl)(4)N(+), I(3)(-) and 4TBP at concentrations comparable to those of the DSC solar cell did not lead to any reaction. Neither CT complexes nor cationic species like (4TBP)(2)I(+) were observed, judging from the (13)C NMR spectroscopic evidence. This questions the previously proposed formation of (4TBP)(2)I(+) in DSC cells.

Unique interplay between electronic states and dihedral angle for the molecular rotor diphenyldiacetylene.

A new analysis of the optical properties of the molecular rotor 1,4-diphenyl-1,3-butadiyne (diphenyl-diacetylene, DPDA) is presented, taking account of the conformational dynamics. The absorption spectra are interpreted in terms of simultaneous contributions from planar as well as non-planar rotamers, characterized by a temperature dependent equilibrium distribution. The investigation is based on IR Linear Dichroism and UV Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy on oriented samples in stretched polyethylene (PE), and on variable temperature UV spectroscopy. The study is supported by the results of detailed quantum chemical Time Dependent Density Functional Theory (TD-DFT) calculations. The resulting analysis has profound implications for the understanding of the optical, photochemical, and photophysical characteristics of this and related chromophores, of importance in a variety of applications.

Synchrotron radiation linear dichroism (SRLD) investigation of the electronic transitions of quinizarin, chrysazin, and anthrarufin.

The electronic transitions of the three alpha,alpha'-dihydroxy derivatives of anthraquinone, 1,4-dihydroxy-, 1,8-dihydroxy-, and 1,5-dihydroxy-9,10-anthraquinone (quinizarin, chrysazin, and anthrarufin), were investigated by synchrotron radiation linear dichroism (SRLD) spectroscopy on samples aligned in stretched polyethylene. With synchrotron radiation, polarization data could be determined in the UV region up to ca. 58,000 cm(-1) (7.2 eV), which amounts to an extension by ca. 11,000 cm(-1) (1.4 eV) relative to the range accessible with a conventional light source. Throughout the investigated region (15,000-58,000 cm(-1)), essentially similar wavenumbers, intensities, and transition moment directions were determined for chrysazin and anthrarufin, while the spectrum of quinizarin deviated significantly. The results of time-dependent density functional theory (TD-DFT) calculations were in good agreement with the experimental spectra, leading to the assignment of 10 electronic states for quinizarin, and 8 electronic states for chrysazin and anthrarufin. No evidence was found for contributions from different tautomeric or rotameric forms to the observed spectra.

Conformational and tautomeric eccentricities of 2-acetyl-1,8-dihydroxynaphthalenes.

Tautomerism in aromatic systems with oxygen substitutents is rare. This is investigated in 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (1) and in 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (2). The tautomeric nature of 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene is supported by long-range hydrogen-hydrogen coupling between the OH-1 and the OH-8 and by the isotope effects on 13C caused by deuteration at the CH3C==O methyl group. Compound 2 participates in a degenerate equilibrium between two equivalent nonsymmetrical rotamers (2A and 2B), each having two intramolecular O...HO hydrogen bonds: one involving an acetyl oxygen and the neighboring hydroxyl group, and the other between the oxygen centers at positions 1 and 8. In addition, each rotamer is involved in a tautomeric equilibrium, with a structure having an OH-substituted exocyclic double bond (2AT or 2BT).DFT calculations for a large set of compounds highlight the factors controlling the unusual rotational and tautomeric behaviors. A very important factor seems to be the repulsive interaction between the O-1 and O-8 centers, which is modulated by formation of an OH-1...O-8 or OH-8...O1 hydrogen bond. Steric interactions, mesomeric release of electrons from the oxygen at position 8, and a strong OH...O...C hydrogen bond are other factors.Solid-state 13C NMR spectra of 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene at different temperatures demonstrated no averaging in the solid, whereas partially deuterated 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene showed an isotope effect at C-1 of 1.5 ppm, indicating tautomerism in the solid state.

The vibrational structure of (E,E')-1,4-diphenyl-1,3-butadiene Linear dichroism FT-IR spectroscopy and quantum chemical calculations.

The title compound (DPB) was investigated by FT-IR spectroscopy in liquid solutions and by FT-IR linear dichroism (LD) measurements on samples aligned in stretched polyethylene. The LD data provided experimental assignments of molecular transition moment directions and vibrational symmetries for more than 40 vibrational transitions. The observed IR wavenumbers, relative intensities, and polarization directions were generally well reproduced by the results of a harmonic analysis based on B3LYP/cc-pVTZ density functional theory (DFT). The combined experimental and theoretical results led to proposal of a nearly complete assignment of the IR active fundamentals of DPB, involving reassignment of a number of transitions. In addition, previously published Raman spectra of DPB were well predicted by the B3LYP/cc-pVTZ calculations.

Thioacetylacetone: structural and vibrational assignments.

Thioacetylacetone and its variously deuterated isotopomers have been investigated using electronic and vibrational spectroscopy combined with quantum chemical calculations. Thioacetylacetone is known for its photochromic properties, but the structures of the initial and final forms have been the subject of a long debate. Analysis of the IR spectra recorded in low-temperature argon and xenon matrices, room-temperature solutions, and in the gas phase has allowed us to establish the nature of the photochromic species and of its precursor. Similar to the case of another beta-thioxoketone, monothiodibenzoylmethane, the photo-product has been assigned to the nonchelated SH exo-rotamer of the (Z)-enethiol tautomeric form, whereas the dominant ground-state species corresponds to the chelated (Z)-enol tautomeric form. Detailed vibrational assignments have been proposed for both forms based on quantum chemical calculations and polarization experiments. In the case of the chelated (Z)-enol species prevailing in the ground state, a second-order perturbative anharmonic analysis at the B3LYP/cc-pVTZ level indicated strong anharmonic effects associated with the intramolecular hydrogen bond, leading to a shift of more than 600 cm-1 of the wavenumber of the OH-stretching vibration. A small fraction of the SH endo-rotameric chelated (Z)-enethiol form was also detected under unperturbed conditions. The (Z)-enethiol form can be converted into the (Z)-enol form by irradiation at 290 nm.

Electronic states of 1,6,6a lambda4-trithiapentalene and its 2,5-dimethyl and 2,5-diphenyl derivatives. Ultraviolet-visible linear dichroism spectroscopy and time-dependent density functional theory calculations.

The electronic transitions of 1,6,6a lambda4-trithiapentalene (TTP) and its 2,5-dimethyl and 2,5-diphenyl derivatives (DMTTP and DPTTP) are investigated by ultraviolet-visible linear dichroism spectroscopy on molecular samples partially aligned in stretched polyethylene, and by time-dependent density functional theory calculations (TD-B3LYP/6-31G*). The theoretical predictions are in good agreement with the observed polarization spectra, allowing a detailed assignment of observed transitions to calculated electronic states. According to the theoretical results, sigma-sigma* excitations involving orbitals associated with the unique S-S-S three-center bonding in the TTP ring system play a fundamental role in the description of low-lying electronic states. The visible absorption band close to 20,000 cm(-1) and the dominant peak in the near-ultraviolet region around 39,000 cm(-1) can thus be assigned to transitions with a high degree of sigma-sigma* character. The situation is slightly more complicated in the case of DPTTP because sterically induced twisting of the phenyl groups leads to mixing of pi and sigma orbitals.