Studies on the circular dichroism of penam and penam sulfoxides

Optical rotatory properties of models of 6-aminopenicillanic acid (6-AP A) as obtained from earlier molecular orbital and rotatory strength calculations are corroborated by new CD data for 6-AP A. The new spectra extend down to 180 nm, revealing a positive Cotton effect near 185 nm due to carbonyl π→π^* transitions. The pH sensitivity of the optical activity of the β-lactam amide π→π^* and the 203-nm n→π^* transitions is explainable in terms of electrostatic effects and orbital interactions present in α-aminoketones and 4-membered rings which tend to flatten out the NH₂ group. These same interactions are responsible for the unusually low pK_a of 4.75 for the amino group; (the carboxyl group of 6-AP A has a normal pK_a of 2.5). The previous calculational procedures are extended to the sulfoxides of 6-AP A, and the predicted CD spectra are compared to new experimental curves and also to available data for penicillin sulfoxides. The MO's associated with the principal bands are illustrated with the aid of electron density maps. The positive Cotton effect observed and calculated near 230 nm is due to two charge transfer transitions within the asymmetric penam nucleus. Similar to the situation with the unoxidized penams, the sulfoxide of 6-AP A displays a very low pK_a for the amino group (3.8) and a change toward a less positive CD band at 202 nm upon protonation of this group.     

Direct quantum chemical calculation of rotatory strengths of methylpyrrolidones : Model compounds containing a peptide group

CNDO/S method is used to compute the rotatory strengths of L-5-methylpyrrolid-2-one and S-3-methylpyrrolid-2-one, cyclic compounds containing a peptide group. The observed circular dichroism (CD) bands in the regions of 185 and 220 nm corresponded to π → π^* and n → π^* transitions, respectively. The computed rotatory strengths indicate that nonplanarity of pyrrolidone ring makes a substantial contribution to the CD of compounds under consideration. The comparison of computed and experimental rotatory strengths suggests that both compounds have a nonplanar ring conformation.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Electronic structure of the radicals NF and NCI. II. Potential energy curves for NCI

Large-scale MRD CI calculations are employed to determine the potential energy curves for the three lowest X³Σ⁻, a¹Δ and b¹Σ⁺ states of NCl with π^*2 configuration, for the higher states arising from σ → π^*, π → σ^* and π → π* excitation and for the ²Π ion. Numerous avoided crossings are found to occur between curves in the region of internuclear separations treated. A comparison with the corresponding curves for the radicals NF, O₂ and SO is undertaken and the basic similarities and distinctions are discussed. The curves in NF and NCl are quite similar, with generally lower energy gaps in the second-row radical; they can to a large extent be constructed from the homonuclear counterpart by mixing of g and u symmetry and by taking into account the different multiplicities in the dissociation products N + Cl or F and O + O or S.     

Application of the MC SCF method to the π → π* excitation energies of ethylene

The MC SCF method is employed to calculate the N → T and N → V π → π^* vertical excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two π and two π^* orbitals. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitation energies and spatial extents of the excited states in excellent agreement with the predictions of large multi-reference or iterative-natural-orbital CI calculations. These results show that within an MC SCF framework σ-σ correlation is unimportant for describing the π → π^* processes. We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculations may have introduced small, but important, errors in the excitation energies and predictions of the spatial extent of the V state.     

Bonding studies in group IV substituted anilines : IV. Excited state interactions in the trimethyl derivatives

The solution ultraviolet spectra for N,N-dimethylaniline, p-t-butyl-, and p-trimethylsilyl-, p-trimethylgermyl-, and m-trimethylsilyl-N,N-dimethylaniline in acetonitrile and pentane are reported. The Group IV substituents perturb the excited states to a much larger extent than the ground states. Both the symmetric and antisymmetric π^* levels are affected by π interaction with the trimethylsilyl and trimethylgermyl substituents. The magnitude of the effects are proportional to the π density at the point of substitution. Simple Hückel calculations correlate well with experimental transition energies. The model appears to exaggerate π interactions between silicon or germanium and the π^* molecular orbitals.     

Spectroscopic studies of dipyridamole derivatives in homogeneous solutions: Effects of solution composition on the electronic absorption and emission

Electronic absorption and fluorescence spectra of three different dipyridamole (DIP) derivatives, RA 39, RA 14 and RA 25, were monitored in aqueous solution as a function of pH in the range 2–13. Extinction coefficients, quantum yields and fluorescence lifetimes were obtained in aqueous solutions at different pHs and in several other solvents. The absorption spectra are characterized by two broad bands centered at 405–415 nm and 290 nm for RA 39 and RA 14 and at 370 and 270 nm for RA 25. The first band used for fluorescence excitation has a value of ε ≌ 5 × 10³ M⁻¹ cm⁻¹ characteristic of a π → π* transition. An increase in pH is accompanied by both fluorescence and absorption sharp changes around pH 6 giving a pK ≌ 5.8 for RA 39 and RA 14, similar to the one observed for DIP. In the case of RA 25 two pKs are observed below 6.5, namely a pK₁ ≌ 2.25 and pK₂ = 5.20. These changes are associated to protonation equilibria of nitrogens in the π-conjugated system of the pyrimido—pyrimidine ring. At high pH a second transition is observed only in fluorescence with pK ≌ 12.5 for RA 39 which is similar to DIP and pK ≌ 12.1 for RA 25. For RA 14 this transition was not observed. Estimates of the pK* for the first singlet excited state showed that in comparison to the ground state pK ≌ 5.8 or 5.2 (RA 25), the excited state pK* is more acid by almost a unit. In the case of RA 25 and the lowest pK of 2.25, an excited state reaction, possibly of proton transfer, seems to take place since emission and absorption data give opposite effects for pK*.     

Carboxylic acids as promoters for internucleotide-bond formation via condensation of a nucleoside phosphoramidite and a nucleoside: relationship between the acidity and the activity of the promoter

The relationship between the activity and the acidity of a carboxylic acid as a promoter for condensation of a nucleoside phosphoramidite and a nucleoside was investigated. The investigation revealed that the acid, the pKa value of which in acetonitrile is less than 18, is capable of promoting the condensation reaction, and acid with a pKa value outside of this range does not serve as a promoter. In carboxylic acids serving as promoters, the ones with higher acidity generally show greater activity. In particular, acids with a pKa value less than ca. 16 (measured by a potentiometric method), such as trichloroacetic acid (pKa=10.6), trifluoroacetic acid (pKa=12.7), dichloroacetic acid (pKa=13.2 or 15.8), and 2,4-dinitrobenzoic acid (pKa=16.1), show high levels of activity higher than that of conventionally used 1H-tetrazole. These carboxylic acids generally serve as excellent promoters for both the liquid-phase and the solid-phase synthesis of oligonucleotides via phosphoramidite method.     

Effet α. influence de la substitution sur l'exaltation de reactivite d'oximes vis-a-vis de l'acetate de p-nitrophenyle

Kinetic measurements show the relation between the magnitude of the “α effect” exhibited by trifluoromethyloximate anions RR'C=N-O^? and the electron attracting properties of the substituents. Indeed, the reactivity enhancement which is observed for α-trifluoromethyloximes is similar to that found in earlier studies of α-hydroxyiminoketones. This characteristic behaviour is no longer observed for oximes bearing an electron releasing substituent at the α-position of the hydroxyimino group. On the basis of sodium oximate molecular orbital energies the “α effect” induced by electron attracting substituents appears to depend on both the interactions of nitrogen and oxygen lone pairs of electrons and the lowering of the π^* vacant orbital thus favouring a secondary interaction between the π^* anion orbital and the π orbital of p-nitrophenylacetate carbonyl group.     

Vibronic coupling and polyatomic spectra

A theoretical account of the vibronic intensity borrowing in polyatomic spectra is described and applied to the 3200 Å n → π^* absorption of pyrazine and the 3700 Å π → π^* absorption of pyrene. It is shown that the asymmetry between absorption and emission, as well as the frequency of the inducing mode in the lowest excited singlet state, are correctly predicted for both of these systems.     

ESCA studies of polymers. VII. Shake-up phenomena in some alkane–styrene copolymers

Shake-up satellites corresponding to π^* → π transitions accompanying C_1s core ionizations have been studied for polystyrene and a series of alkane–styrene copolymers. It is shown that the shake-up intensities are additive in nature and give a direct measure of copolymer compositions.     

Bonding in the ground state and excited states of copper-alkene complexes

The ground state and ¹B₂ excited state of Cu(C₂H₄)⁺ and of CuX(C₂H₄) (X  F, Cl) have been investigated by the Hartree-Fock-Slater (HFS) method. The main metal-ligand interactions in the ground state are ethene π → Cu 4s donation and Cu 3d_π → ethene π^* backdonation, which have comparable contributions to the metal-ligand bond strength. The excitation of CuX(C₂H₄) does not involve an alkene π → metal charge transfer (LMCT), but instead is metal 3d → alkene π^* charge transfer (MLCT) in character. The implications for the photochemistry of olefin-copper(I) complexes are discussed.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

A study of the electronic spectrum of halogen derivatives of ethylene

The spectra of tetrachloroethylene, monofluoroethylene, gem-, cis- and trans-difluoroethylene and trifluoroethylene were studied in the region 150–250 nm in the gas, solid and Kr matrix. The valence character of the π å π^* transition in the various compounds was verified and the Rydberg transitions were identified. The lowest energy band in the fluoroethylene was found to be a Rydberg transition. The vibronic structure of the π å π^* transition in tetrachloroethylene indicates that the excited state geometry is planar. The existence of a π å σ^* transition at lower energy than the π å π^* transition was verified for cis-difluoroethylene and trifluoroethylene.     

Triplet states of fluorocarbonyl compounds determined by trapped electron spectroscopy

Ion cyclotron resonance trapped electron spectra for carbonylfluoride, trifluoroacetyl fluoride, hexafluoroacetone, acetyl fluoride and I,I.I-trifluoroacetone are reported. Comparisons of data obtained for ³(π → π^*) vertical excitation energies are used in conjunction with photoelectron spectroscopic data to deduce the effects of fluorine and trifluoromethyl groups on the energies of σ, π and π^* orbitals associated with the carbonyl group.     

Thioketone spectroscopy: An analysis of the lower electronic transitions in thioacetone and thioacetaldehyde

Visible and ultraviolet spectra of the unstable species, thioacetaldehyde, CH₃CHS, thioacetone, (CH₃)₂CS, and thioacetone-d₆ have been studied in the gas phase. The valence n → π* excitations, Ã ← X? and ã ← X?, have been identified. Rydberg n → 4s, 4p_y and 4p_z and π → π* valence excitations have been found in thioacetone. In thioacetaldehyde a Rydberg-valence interaction mixes the n,4s and π,π* states which leads to a broad absorption of mixed character between 200 and 220 nm.     

Calculations of pKa values of carboxylic acids in aqueous solution using density functional theory

With the specific aim of calculating the acidity equilibrium constant (K_a) of carboxylic acids in aqueous solution we investigated the solute-solvent interactions of these acids and their corresponding anions. The pK_a (−lg K_a) values have been calculated using density functional theory (DFT). The polarized continuum model (PCM) is used to describe the solvent. Using these methods, we successfully predicted the pK_as of 66 carboxylic acids in aqueous with the average error of 0.5 in pK_a units. Two different thermodynamic cycles have been studied. The theoretical values are in better agreement with the experimental results for those acids with moderate strength of acidity with the pK_a value higher than 3.     

Magnetic circular dichroism of some cyano-substituted cyclopentadienide anions

MCD spectra of pentacyanocyclopentadienide anion, tetracyanocyclopentadienide anion and diazotetracyanocyclopentadiene were measured. They all exhibited the Faraday A term or B terms which are caused by π → π^* electonic transitions of the compounds. Transition energies and symmetries of the excited states were calculated by use of the Pariser-Parr-Pople method. The results of the calculations gave a qualitative interpretation of the observed MCD spectra.     

Circular dichroism spectra of straight chain mono-olefins. Assignment of ethylene transitions

The circular dichroism spectra of three optically active alkenes, S, 3-methyl-1-pentene, S,4-methyl-1-hexene and S,5-methyl-1-heptane have been measured in the spectral region 200-140 nm in the gas phase. An intense CD band which does not have a well defined analogue in the absorption was observed in all three compounds and is assigned as π → σ_(CH)^*. A second prominent CD band at higher energy is assigned as σ_(CH) → π^*. The assignments of low frequency CD bands are discussed.