Ground-state Species Research Articles

Vibrational relaxtion in excited electronic states of aromatic hydrocarbons

Time-resolved experiments are reported Showing kinetic evidence for vibrational relaxation of electronically excited molecules in solution at room temperature. The experiments involve higher electronic states of 3,4,9,10-dibenzpyrene. Data are consistent with slow vibrational relaxation (≈ 15 ps), similar to that for ground state species.

Structure‐Resonance Theory. A Review of Applications to π ‐Hydrocarbon Systems

AbstractStudies of structure‐resonance theory applied to π‐hydrocarbon systems are reviewed. The theory leads to quickly obtainable quantitative predictions of bond orders, charge distributions, and relative stabilities of ground state species or reaction intermediates. Numerical results correlate precisely with experimental properties, and with results of LCAO‐MO‐SCF calculations.

Structure‐Resonance Theory for π‐Hydrocarbon Iron Tricarbonyl Complexes

AbstractStructure‐resonance theory is applied to transition metal π ligand complexes. This theory leads to specific, quantitative predictions of bond orders, charge distributions in ions, and relative stabilities of ground state species or reaction intermediates. In applications to hydrocarbon systems, numerical results correlate precisely with experimental properties, and with results of LCAO‐MO‐SCF calculations. Initial extensions to iron tricarbonyl derivatives are described herein. A unique advantage of structure‐resonance theory is that it can be applied rapidly by hand calculation, allowing numerous applications with little labor. The orbital basis for describing iron complexes allows one to use graph‐theoretical algorithms to further simplify the theoretical formulism.

KYNURENIC ACID — I. SPECTROSCOPIC PROPERTIES

Abstract— The presence of different ground state species in ionic and/or keto s enol equilibrium has been demonstrated by studying NMR, IR and UV spectroscopic properties of kynurenic acid and related quinoline derivatives. The optical and emission properties of kynurenic acid and related compounds are thus very sensitive to pH conditions. Fluorescence and intersystem crossing (ISC) quantum yields have been measured in various solvents. The first excited singlet state is probably very short‐lived in neutral aqueous solution. The triplet state is long‐lived (10 μs) as shown by 353 nm laser flash spectroscopy. A high ISC quantum yield and a large molar extinction coefficient make it easy to detect the T1→ Tn transient (Δmax≃ 570 nm).

Absorption and fluorescence spectra of methyl salicylate in the vapor phase

Methylsalicylate (MSA) in vapor state shows two fluorescences originating from two different ground state species. The short wavelength emission is ascribed to free MSA molecules with open hydrogen bonds, whereas the blue fluorescence is due to closed MSA. From a study of the temperature dependence of the short-to-long wavelength fluorescence intensity ratio, the energy required for ring opening is found to be 15 kJ mol -1.

Some terminally-functionalized species

Terminally-functionalized species are, of course, as old as polymerization. We believe that the synthesis of polymers designed for specific end-uses (e.g. for medical or catalytic applications) is one of the most important growth areas in polymer science; clearly, this will require development of increasingly precise methods of polymer functionalization. This lecture is concerned with the synthesis and reactions of some terminally-functionalized polymers which we have studied in recent years. In general, polymers functionalized in this way have evident uses in the synthesis of block and graft copolymers; furthermore, in suitable cases, they afford an important way of studying interactions of electronically excited chromophores with ground-state species, in particular the dependence of such interactions on polymer chain conformations. We shall consider both these aspects.

QUENCHING OF TRIPLET STATES OF AROMATIC KETONES BY INDOLE AND 3‐METHYL INDOLE IN BENZENE SOLUTION. EVIDENCE FOR HYDROGEN‐ATOM ABSTRACTION, FOR QUENCHING DUE TO FAVOURABLE CHARGE TRANSFER INTERACTION AND FOR THE LACK OF ELECTRONIC ENERGY TRANSFER

Abstract— Quenching of the triplet states of the aromatic ketones (KCO), benzophenone, acetophenone and xanthone, by indole and 3‐methyl indole gives rise to the neutral radicals resulting from hydrogenatom transfer with variable efficiency (40–100%). Thus in addition to the reaction, 3KCO*+RH →KCOH +R.some other quenching path or paths occur. There is no evidence for any triplet energy transfer even when this is energetically favourable, and it is suggested therefore that quenching to give ground state species following favourable charge‐transfer interactions accounts for the proportion of quenching without reaction.The spectra of the indole radicals, R., were determined and the kinetics of their decay in aerated and deaerated solution were investigated and reaction schemes proposed to explain these observations.

Perturbation of the benzil ground and excited state geometries at low temperatures in hydrocarbon glasses

The benzil phosphorescence in methylcyclohexane (MCH) and isopentane (IP) glasses is dominated by emission from the benzil crystal at concentrations > 10–4 M, below which the phosphorescence from monomeric species is: MCH, λmax 580 nm from T1-planar→S0 planar, and IP, λmax 530 nm from T1-skewed→S0-skewed; excitation spectra show that the absorbing geometry of the absorbing ground state species is planar in MCH but skewed in IP.

The combustion of acetaldehyde behind incident shock waves

Studies were made of the ignition of CH3CHO/O2/Ar (95%) mixtures behind incident shock waves for equivalence ratios 0.5–3.0 in the temperature range 1550–1850 K. Spectroscopic emissions from the excited species OHo CHo and C2o, and from the ground state species CO2, CO and bonded C-H, were measured. Experimental results were supplemented by numerical integrations based on a kinetic model for the combustion. Experimental results are presented showing the temperature dependence for the times to the occurrence of threshold and maximum intensities, from which correlation equations are calculated. An ignition delay period has been characterised from OHo intensities. However, considerable extents of fuel degradation do occur during the delay period yielding CO and CO2, amongst other products. Calculated concentrations for CH3, CH3CO, CH2O and CHO appear to attain quasi-stationary concentrations very rapidly which are maintained throughout the ignition delay time. Results are compared with those from CH4, C2H6 and CH3OH combustion in similar temperature ranges.

Effect of radiation on the chemical changes accompanying nuclear isomeric transition (tellurium-121 m, -127m, and -129m) in crystalline telluric acid

Ionising radiation increases the retention of the ground-state species from the isomeric transition of the tellurium isomers in telluric acid. The change in retention with length of irradiation resembles that taking place on heating. The plateau value depends on the dose rate of the irradiation. On ceasing irradiation a new generation of ground-state species grows in and the retention returns to the original value at a rate determined by the decay constant of the ground-state species. The growth and decay of the retention can be duplicated in a series of irradiations and interruptions. An interpretation of these data is provided.

Microwave determination of the conformation and dipole moment of 1,3-difluoroacetone

The microwave spectra of the ground state and several low-lying vibrational modes of 1,3-difluoroacetone have been assigned and analyzed. The assigned form has a molecular conformation in which one fluorine atom lies cis and the other trans to the oxygen atom. The rotational constants of the ground state species were determined using a centrifugal distortion analysis: A = 6024.843 ± 0.006 MHz, B = 2454.414 ± 0.001 MHz, C = 1783.897 ± 0.001 MHz. The molecular dipole moment components of the ground state species lie along the a and b principal axes with μ a = 2.38 ± 0.03 D, μ b = 0.89 ± 0.03 D, and μ T = 2.54 ± 0.03 D. Comparative intensity measurements with OCS microwave lines indicate that the assigned form constitutes only 20% to 30% of the total gas mixture, the remainder presumably consisting of one or more other conformers, perhaps the gauche-gauche form. The lowest vibrational frequency (82 ± 12 cm −1) is attributed to the trans-CH 2F torsion, while the next-higher vibrational frequency (127 ± 15 cm −1) is believed to be the cis-torsion. A low-frequency in-plane bending motion is found at 285 ± 25 cm −1.

Environmental effects on the phosphorescence of benzophenone in aqueous solutions

Benzophenone shows phosphorescence emission from three different spectroscopic ground state species in water glassy solutions at 77 K. A study of the effect of solvent acidity indicates that their reciprocal interconversion depends on the environmental conditions.

Electronic absorption and fluorescence of cinchophen, cinchoninic acid and their methyl esters; biprotonic phototautomerism of the singly-protonated species

The pH and Hammett acidity dependences of the absorption and fluorescence spectra of cinchoninic acid (quinoline-4-carboxylic acid), cinchophen (2-phenylquinoline-4-carboxylic acid) and their methyl esters, were studied. The predominant uncharged ground-state species derived from the free acids are zwitterions. Prototropic equilibria are too slow to compete with fluorescence for deactivation of the excited state at hydrogen ion concentrations represented by the pH scale. However, fluorescence shifts accompanying protonation indicate that the carboxyl group is more basic than the ring nitrogen atom in the excited state. In the Hammett acidity range the singly-charged cations of all the compounds studied undergo phototautomerism in the lowest excited singlet state. The rate of this process is acidity dependent. In very concentrated sulphuric acid solutions doubly-charged cations are formed in the excited state but not in the ground state. The intense emissions of these compounds in moderately concentrated sulphuric acid may be suitable for quantitative analysis if great care is taken to control solution acidity.

Ionization equilibria and electronic spectroscopy of the chelating quinolinecarboxylic acids

pH and Hammett acidity dependences of the electronic absorption and fluorescence spectra of quinaldic acid. quinoline-8-carboxylic acid and their methyl esters have been studied. Conversions of the quinoline-like absorption spectra of the anions to quinolinium-like absorption spectra of the uncharged species show that the uncharged ground state species derived from the free acids consist overwhelmingly of the zwitterions. The excited singly charged cation of quinaldic acid appears to undergo an acidity-dependent phototautomerism. However, the 8-isomer does not exhibit this phenomenon. The long emission wavelengths of the esters relative to the zwitterions indicate that of the two electron-withdrawing groups, the carboxyl group is stronger than the heterocyclic nitrogen in the excited state. The fluorescence of quinoline-8-carboxylic acid in sulfuric acid could be very useful for quantitative analysis of transition metal chelates of this ligand.

Spontaneous and light induced hydration of pyrimidines

Nmr spectral changes showed that 4-methoxy-1-methyl-2-oxopyrimidine in 2 M or 8 M D 2SO 4 gradually converts to 1-methyl-uracil and also C(5)-deuterium exchange was observed. While the rates of the former reaction are not influenced by irradiation (254 nm), the rates of C(5)-deuterium exchange show a 2.3 - to 5-fold increase. These findings indicate the significance of protonated ground state species in both the dark and light induced hydration of pyrimidines. A general mechanism for this type of reaction is proposed and its relevance to the photochemistry of nucleic acids is discussed.

Photochemical studies of IBr, ICl and BrCl by kinetic spectroscopy in the vacuum ultra-violet

The flash photolyses of the interhalogen compounds IBr, lCl and BrCl have been investigated by kinetic spectroscopy in absorption in the vacuum ultra-violet. Direct observations on the atomic states resulting from primary photolytic processes have been correlated with the absorption spectra of these molecules. These atomic products are also discussed within the context of the strongly “avoided crossing” between the 3Πo+ and 0+ states and factors governing radiationless transitions. The kinetics of the halogen atoms, including electronically excited (2P½) as well as ground state (2P½) species, permits a discussion of a number of atomic reactions. Further observations on the absorption spectra on IBr and lCl in the vacuum ultra-violet revealed two new “Rydberg pairs” which are also reported.

Changes in weak-base pK BH+ values upon excitation to the first excited singlet state

Attention is drawn to the possible uses of excited state pKBH+*(hereafter pKβ*) studies on compounds which only protonate in the ground state in concentrated acid solutions. Fixing the ground state pKBH+(hereafter pKβ) value with the primary nitroaniline H0′ scale and calculating pKβ* by the Forster cycle method gives a moderately good indication of the region of acidity in which the fluorescence spectrum will change from the base to the conjugate acid form for some compounds (1- and 2-naphthoic acids, 2-naphthamide, phthalide, and perhaps benzoic acid), but for several others complications have been encountered. For example, with chalcone, 4′-methoxychalcone, and 1,1-diphenylethylene, fluorescence emission by the protonated or unprotonated forms could only be obtained from solutions in which the corresponding ground-state species predominated, despite the large shifts in the absorption and fluorescence spectra on protonation.

The measurement of ionization constants of electronically excited species from aminonaphthols

The absorption and fluorescence spectra of seven aminonaphthols have been studied as a function of pH. Although some of the isomers decomposed in neutral or basic media, the stable isomers had three pH-dependent ground-state species: naphtholammonium ion, neutral aminonaphthol, and aminonaphtholate anion. In the excited state, an additional fourth species was found; it has been assigned a zwitterion structure. The ground-state and excited-state ionization constants have been measured.

Microwave Spectrum, Structure, Quadrupole Coupling Constants, and Barrier to Internal Rotation of Chloromethylsilane

Observations of the microwave spectra of seven ground-state species and three first excited torsional state species of CH2ClSiH3 are reported. From an analysis of the moments of inertia of the various species structural parameters are deduced as follows: bond distances; CCl=1.788 A, CSi=1.889 A, CH=1.096 A, SiH=1.477 A, and bond angles; SiCCl=109.3°, SiCH=109.3°, HSiH=110.6°, and HCH=107.5°. An analysis of the hyperfine splitting leads to quadrupole coupling parameters χCCl bond=—68.7 Mc, ηCCl bond=0.048, if it is assumed that the CCl bond direction forms one principal axis of the quadrupole tensor; and χCCl bond=—72.0 Mc if it is assumed that the quadrupole tensor is cylindrically symmetric. Analysis of splittings in the first excited torsional state indicate that the height of the potential barrier hindering internal rotation of the SiH3 group is 2.55 kcal/mole.