V. Galasso

Ab initio study of the nuclear spin—spin coupling constants and magnetic shielding constants in silicon derivatives via the equations-of-motion method

Abstract Non-empirical equations-of-motion calculations of the nuclear spin—spin coupling constants and magnetic shielding constants in a representative series of molecules featuring siliconsilicon or siliconcarbon single, double and triple bonds are presented. The EOM results, which include the main portion of the electron correlation effects, are in resonable agreement with the available experimental data. On passing from single to double and triple bonding situation the pattern for the 1 J (SiY) parameters resembles that exhibited by 1 J (CY) in the structurally related carbocompounds, whereas an inversion in the relative position of the triply bonded atom is predicted in the case of the 29 Si resonance relative to the 13 C sequence.

Spectroscopic and theoretical study of the electronic structure of curcumin and related fragment molecules.

The low volatility and thermal instability made the photoelectron (PE), electron transmission (ET), and dissociative electron attachment (DEA) spectroscopy measurements on curcumin (a potent chemopreventive agent) unsuccessful. The filled and empty electronic structure of curcumin was therefore investigated by exploiting the PES, ETS, and DEAS results for representative fragment molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron attachment energies of curcumin was proposed. The pi frontier molecular orbitals (MOs) are characterized by sizable interaction between the two phenol rings transmitted through the dicarbonyl chain and associated with a remarkably low ionization energy and a negative electron attachment energy (i.e., a largely positive electron affinity), diagnostic of a stable anion state not observable in ETS. The lowest energy electronic transitions of half-curcumin and curcumin and their color change by alkalization were interpreted with time-dependent density functional theory (DFT) calculations. For curcumin, it is shown that loss of a phenolic proton occurs in alkaline ethanolic solution.

Ab initio theoretical study of photoelectron and nmr properties of diazene, phosphazene, diphosphene, and diarsene

Abstract The unusual PE and NMR spectroscopic properties of cis and trans diazene, phosphazene, diphosphene, and diarsene have been studied at non-empirical level. In particular, the ionization energies of the outer valence orbitals have been calculated by the many-body formalism according to Cederbaum, and the nuclear screening constants and the nuclear spin-spin coupling constants by the SOS CI perturbation technique according to Nakatsujis approach, taking into account all the spin interaction operators. The calculated values correlate satisfactorily with the available experimental data and account for the peculiar magnitude of these observables in the class of the two-coordinate XY compounds.

Ab initio study of the J(CC) indirect nuclear spin-spin coupling constants in cyclobutane and related systems

Abstract Non-empirical calculations using the equations-of-motion approach, which incorporates the main portion of the electron correlation effects, are reported for the carbon-carbon nuclear spin-spin coupling constants in cyclobutane, bicyclobutane, tricyclobutane, cyclobutene, cyclobutyne, cyclobutadiene, bicyclobutene, methylenecyclopropane, and methylenecyclopropene. The results provide an overall picture of the influences exerted on sign and magnitude of the J (CC) by progressive condensation, unsaturation, and branching rearrangement of the cyclobutane frame.

On the structure and spectroscopic properties of bispidine, N, N′-dimethylbispidine and a bis-bispidine macrocycle

Abstract The equilibrium conformations of bispidine and its N , N ′ -dimethyl derivative were investigated with the ab initio MP2 formalism and the density functional theory (DFT) ansatz B3LYP. For bispidine, the chair–chair (cc) ( exo , endo ) is the most stable conformer. On the contrary, for its N , N ′ -dimethyl derivative, the energy difference between the cc and chair–boat (cb) conformers is small, allowing for conformational mobility of the bicyclo rings. For tetraaza-macrocycle with four nitrogen lone pair orbitals (LPOs) pointing into the molecular cavity, the C 2h structure with stepped ethylene bridges is preferred over the twisted structure of D 2 symmetry. Mono- and di-protonated forms of this base are characterized by intramolecular asymmetric hydrogen bonds. All the theoretical conformational predictions are consistent with the available experimental results. The NMR chemical shifts, calculated by the continuous set of gauge transformations (CSGT) formalism with the DFT/B3LYP method, are fairly consistent with NMR observation. The He(I) photoelectron (PE) spectra were measured and assigned by means of ab initio outer valence Greens function (OVGF) calculations. The predominance of the lowest energy conformation of bispidine and tetraaza-macrocycle is supported by the spectroscopic results. The conformational situation for N , N ′ -dimethyl-bispidine is more complex. Whereas its NMR spectrum indicates a predominance of the cc conformer in THF-d 8 solution, the vibrational and PE spectra should be interpreted in terms of a mixture of the cc and cb conformers in the neat liquid and gas-phase.

Ab initio study of multiphoton absorption properties of methane, ethane, propane, and butane

Abstract The low-lying electronic excited states of methane, ethane, propane, and n -butane, accessed by multiphoton absorption, have been investigated at ab initio level according to quantum electrodynamical formalisms, by utilizing RPA vertical transition energies and amplitudes. For all four alkanes, two-photon spectroscopic properties which could be detected with various experimental arrangements have been reported. For methane, the transition probability coefficients and polarization ratios for two-, three-, and four-photon single-colour absorption from plane and circularly polarized light have been evaluated. All the low-lying excitations in methane and its higher homologues are found to bear Rydberg rather than valence nature.

Probing the Molecular and Electronic Structure of Capsaicin: A Spectroscopic and Quantum Mechanical Study

The conformational preferences of capsaicin were investigated by using the hybrid meta density functional theory (DFT) method MPWB1K. Its flexible, pendant side chain allows for a multitude of conformations only slightly different in energy. The distinctive vibrational features of the most stable conformers were characterized. To elucidate the most favorable reaction sites of capsaicin for radical scavenging, various homolytic bond-dissociation energies were also calculated. Of the possible radical intermediates, the allyl and benzyl radicals are energetically preferred. The filled and empty electronic structures of capsaicin were investigated by exploiting the photoelectron and electron-transmission spectra also of reference molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron-attachment energies of capsaicin was proposed. The frontier pi molecular orbitals are concentrated over the vanillyl moiety, with a modest influence of the amidic-aliphatic chain. The (negative) first vertical electron affinity is predicted to be similar to that of benzene. The absorption spectrum of capsaicin and its change by conversion into a phenolic deprotonated anion (modest bathochromic displacement) or a phenoxyl neutral radical (from colorless to red) were interpreted with time-dependent DFT calculations. ESR measurements following chemical or electrochemical reduction of capsaicin did not lead to detection of the corresponding radical anion. The spectra show fragmentation of the original molecule and formation of a variety of radical species which are believed to have a semiquinonic structure.

On the structure and spectroscopic properties of free and protonated adamanzanes

Abstract The equilibrium structures of a selection of tetraazatricyclo-alkanes, containing a small, medium or macro-intramolecular cavity, were investigated with the density functional theory ansatz B3LYP/6-31G+(d,p). The four nitrogen lone pair orbitals (LPOs) are outwardly oriented in the smallest two members of the class (hexamethylenetetramine and next homologue), while they point towards the inside in the larger cages (hexaethylenetetramine and next homologues). These conformational arrangements are maintained in the monoprotonated derivatives, whose theoretical structural models are consistent with evidence obtained from IR, NMR and X-ray spectroscopic data. The case of inside-coordination is characterized by a relatively strong five-centre intramolecular hydrogen bond, which involves asymmetric protonation at one nitrogen and nearly equivalent interaction with the other three unprotonated nitrogens. The NMR chemical shifts, calculated by the continuous set of gauge transformations formalism with the B3LYP/6-311+G(2d,p) method, are fairly consistent with NMR observation, in particular for the very low-field signal of the encapsulated proton. The electronic structure of the free bases was also studied by means of ab initio outer valence Green function calculations, which give a consistent, overall description of the different manifolds of photoionization, associated with the nitrogen LPOs.

Ab initio calculations on the one- and two-photon electronic transitions of cyclopentadiene, spirononatetraene, 1,4-cyclo-hexadiene, Dewar benzene, norbornadiene, and barrelene

Abstract Ab initio RPA calculations have been carried out on the low-lying singlet intravalence and Rydberg electronic states, accessed by one-and two-photon absorption, of representative cyclic systems. Two-photon spectroscopic properties that could be detected under various experimental arrangements have been reported. The relative displacements of the transitions brought about by spiroconjugation and bridge insertion are correctly accounted for. Information on the nature of the first valencelike excited states has also been achieved by CI-SD treatments.

A study of the molecular structure and spectroscopic properties of benzo- and pyrido-tetraazapentalenes

Abstract The equilibrium structures of Y - and Z -dibenzotetraazapentalene and some pyridyl derivatives were fully optimized by the density functional method B3LYP/6-31+G(d,p). For all the molecules, the results predict a fully conjugated planar geometry. The NMR 13 C chemical shifts were recorded and analyzed by means of ab initio continuous set of gauge transformations calculations performed with the B3LYP/6-311+G(2d,p) DFT–HF hybrid model, which account for the different displacements of the resonances in the NMR spectra. The He(I) photoelectron spectra were measured and interpreted by means of ab initio outer valence Green function calculations, which give an overall consistent description of the energies, sequence and splitting of the uppermost bands. These are mainly associated with the π and n(N) orbitals of the central tetraaza fragment and with the n(N) lone pairs of the pyridyl rings. Electron transmission spectroscopy was employed to characterize the low-lying temporary anion states of the Y - and Z -isomers.