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The molecular structure of usnic acid was investigated by the density functional theory (DFT). Two keto-enol tautomers are nearly isoenergetic and more stable than other tautomers. Noteworthy is the energy difference among the three intramolecular O–H⋯O hydrogen bonds. The DFT/PCM calculated dissociation constants account for the acidic sequence of the three OH-groups. The electronic structure was also studied by calculating IR/Raman, NMR, and absorption features. A reliable assignment of the ‘fingerprint’ carbonyl stretching modes was supported by calculations on related molecules. The calculated NMR chemical shifts fit expectation in terms of a fast interconversion between the two most preferred tautomers. A variety of π → π ∗ and n → π∗ excitations, localized on a single ring or involving a charge-transfer between the two lateral rings of the molecule, gives rise to the broad UV-absorption bands. This property accounts for the efficient protection against damaging solar radiation provided by usnic acid for lichens.

The natural product haplophytine and its half-molecules, aspidophytine and the still elusive ‘left domain’, exhibit unique structural skeletons and unusual spectroscopic features. These issues have been tackled with density functional calculations. The congested framework of haplophytine consists of two heteropolycyclic segments connected with a sterically demanding single bond and incorporates two protonation sites of different basicity and a relatively strong hydrogen bond O–H⋯O, responsible for the acidic activity. The molecular and electronic structure of these alkaloids has been elucidated by analysing the carbonyl vibrations, 13 C NMR chemical shifts, and UV absorption spectra that are most sensitive to stereoelectronic factors.

The controversial structural issue of a set of pyrazolo[4,3-e][1,2,4]triazine natural products with high antibiotic activity has been tackled with density functional theory calculations. The electronic structure of these nitrogen-rich drugs has been characterized by analyzing the 13C NMR chemical shifts and UV-visible absorption spectra that are most sensitive to stereoelectronic factors. In particular, the remarkably changeable pattern of the π → π∗ excitations accounts for the palette of bright colours exhibited by these compounds.

Density functional theory calculations with the B3LYP hybrid functional have been performed to determine the equilibrium structures of a representative series of thiono-, oxo,thiono-, dioxo-, and dithiono-derivatives of sparteine. All these molecules except 2,13-dioxo-sparteine adopt one most stable conformation or exhibit a marked preference for one of the two lowest-energy conformations of the parent sparteine (with each functionalized ring distorted as a sofa or half-chair). 2,13-Dioxo-sparteine is instead predicted to exist as a nearly 1:1 mixture of “ring C chair” and “ring C boat” conformers in solution. The theoretical structural models are consistent with the available X-ray experimental results. The electronic structure of these bis-quinolizidine alkaloids has been studied by measuring and calculating important features of their NMR and photoelectron spectra. In particular, a representative set of NMR chemical shifts and nuclear spin–spin coupling constants, calculated by means of DFT formalisms, compare favourably with experiment. Notably, the repercussion of stereoelectronic hyperconjugative effects on Δ δ (H eq /H ax ) and Δ 1 J (CH eq /CH ax ) of the N–CO(S)– groups is correctly accounted for by the DFT results. Based on ab initio outer valence Green’s function calculations, a reliable interpretation of the uppermost bands in the photoelectron spectra has been advanced. The theoretical results indicate a complex interaction of the n (N), n (O), n (S), π (CO), and π (CS) chromophores within the functionalized sparteine framework. The theoretical lowest-energy conformer of all compounds but 2,13-dioxo-sparteine allows a consistent interpretation of the photoelectron spectrum. The conformation adopted by 2,13-dioxo-sparteine likely changes on passing from the crystalline form at room temperature (“ring C boat”) to the gas phase at high temperature (“ring C chair”).

The electronic spectra of 1-azabicyclo[2.2.2]octane (ABCO), 1,4-diazabicyclo[2.2.2]octane (DABCO), and their [1.1.1] and [3.3.3] congeners have been studied at the ab initio level using the symmetry adapted cluster configuration interaction method. A comprehensive theoretical prediction of the discrete excitation spectra, up to the HOMO → 5s transition, is presented. All the low-lying singlet and triplet electronic states of these symmetric cage amines are found to have essentially Rydberg nature and originate from excitations out of the n-type molecular orbitals. The theoretical results correlate with the available spectroscopic data satisfactorily and provide quantitative support to a number of experimental assignments based on REMPI and MCD measurements.

Ab initio calculations using the equations of motion approach are reported for 1 J ( 13 C 13 C) of the bridgehead bond in [1.1.1]-, [2.1.1]-, [2.2.1]-, [2.2.2]-, [3.2.1]-propellane, bicyclobutane, bicyclopentane, bicyclohexane and tetracyclopentane. The results give an overall picture of the influences exerted on sign and magnitude of 1 J ( 13 C 13 C) by progressive condensation of the cyclobutane frame and by enlargement of the fused rings. Extraordinarily small values are predicted for [1.1.1]- propellane and [2.1.1]-propellane.

The low-lying intravalence and Rydberg excited states of cyclohepta-1,3,5-triene and cycloocta-1,3,5,7-tetraene, accessed by one- and two-photon absorption, have been investigated at ab initio level by utilizing RPA vertical transition energies and amplitudes. The nature of the first valence-like excited states of cyclohepta-1,3,5-triene has also been studied by CI-SD calculations.

The equilibrium structures of artemisinin and a selection of its derivatives (potent antimalarial drugs) have been studied with the density functional theory ansatz B3LYP. Of the five rings of the artemisinin framework, it is only the pyranose ring B that exhibits a marked conformational flexibility, especially on addition of a pendant side chain at C-10. For the derivatives, the β-isomer with the axial substituent group is found to be energetically more stable than the α-isomer with the equatorial group. The assignment of the vibrational fundamentals has been supported by calculations on related model molecules and a normal coordinate analysis. This allows for a reliable characterization of the normal modes, mainly involving the peroxide linkage, in the claimed fingerprint region of 1,2,4-trioxanes. The electronic structures have also been studied by measuring and calculating significant features of the NMR, photoelectron and electron transmission spectra. In particular, a representative set of NMR chemical shifts and nuclear spin-spin coupling constants, obtained with DFT formalisms, compares favourably with experiment and fits expectation in terms of stereoelectronic effects of the vicinal oxygen lone pairs. Based on ab initio outer valence Green's function calculations, a consistent interpretation of the uppermost bands in the photoelectron spectra of artemisinin and derivatives has been advanced. The top ionization energies reflect a complex interaction of the various oxygen lone pair orbitals. Electron transmission spectroscopy is applied for the first time to compounds containing the peroxide bond and elucidates the empty level electronic structure of artemisinin and derivatives in the 0-6 eV energy range, with the support of MO calculations and comparison with the spectra of reference molecules. Electron attachment to the lowest-lying empty σ* MO, mainly localized on the O-O bridge, occurs at an energy (1.7 eV) exceptionally low for compounds not containing third-row or heavier elements.

A systematic study of the molecular and electronic structure of the eight possible members in the trans -matrine series and of two dehydro-derivatives, sophocarpine and sophoramine, has been performed. According to density functional theory (DFT) calculations these alkaloids exhibit a variety of form and junction of the four six-membered rings and all but sophocarpine have a strong preference for one conformation. Sophocarpine is predicted to have a marked conformational flexibility at the lactamic nitrogen and to exist as a mixture of two nearly isoenergetic conformers ( C / D - trans and - cis ) in the gas phase or solution. The theoretical predictions are consistent with the available X-ray experimental results as well as IR and NMR evidence. The absolute configuration of the preferred conformer of each compound has been established theoretically and corroborated with the specific optical rotation calculated at the sodium D line. The conformational equilibrium of sophocarpine has also been supported by this physical property. The computed gas-phase proton affinity of matrines indicates a basicity comparable to that of other polycyclic proton sponges. The lowest-energy electronic transitions have been characterized by time-dependent DFT calculations as mainly due to excitations spanning the frontier orbitals π(NCO), n(O), n(N aminic ), and π ∗ (CO). The electronic structures have also been studied by measuring and calculating significant features of the NMR and photoelectron spectra. In particular, a representative set of NMR chemical shifts and nuclear spin–spin coupling constants, obtained with DFT formalisms, compares favourably with experiment. Notably, the stereoelectronic hyperconjugative effects on Δ δ (H eq /H ax ) and Δ 1 J (CH eq /CH ax ) of the >N–CO– groups is correctly accounted for by the theoretical results. Based on ab initio outer valence Green’s function calculations, a reliable description of the uppermost bands in the photoelectron spectra has been advanced. The splitting and sequence of the ionization energies reflect a complex interaction of the n and π chromophores.

Density functional theory calculations with the B3LYP hybrid functional have been performed to determine the equilibrium structures of cytisine, N-methylcytisine, multiflorine, 5,6-didehydromultiflorine, anagyrine, thermopsine, 5,6-didehydrolupanine, and aphyllidine. These molecules, except cytisine, multiflorine, and aphyllidine, exhibit a marked preference for one conformation with the ring C chair. Cytisine, which also adopts the ring C chair, is predicted to exhibit the endo/exo conformational equilibrium of the N–H group in the gas phase or solution. Multiflorine is instead found to exist as a mixture of C-boat and C-chair conformers in the gas phase or solution. The most stable (C-chair) conformer of aphyllidine shows a marked conformational flexibility, with the exo and endo wings of ring A being nearly isoenergetic. The electronic structures of these quinolizidine alkaloids have been studied by measuring and calculating significant features of their NMR and photoelectron spectra. In particular, a representative set of NMR chemical shifts and nuclear spin–spin coupling constants, calculated by means of DFT formalisms, compares favourably with experiment. Notably, the repercussion of stereoelectronic hyperconjugative effects on Dd(Heq/Hax) and D 1 J(CHeq/CHax) of the iN–CO– groups is correctly accounted for by the DFT results. Based on ab initio outer valence Green’s function calculations, a reliable interpretation of the uppermost bands in the photoelectron spectra has been advanced. The theoretical results indicate a complex interaction of the n(N), n(O), p(N), p(CO), and p(CC) chromophores. The lowest-energy conformations of all compounds allow a consistent interpretation of the photoelectron spectra. The conformational equilibrium in the gas phase is partially revealed by the photoelectron spectrum of multiflorine but not by that of cytisine. The single-crystal X-ray structure was also determined for anagyrine hydrochloride hydrate. � 2006 Elsevier B.V. All rights reserved.

The equilibrium structures of the 11 possible oxo-derivatives of sparteine were investigated with the density functional theory, using the B3LYP functional. The conformational preferences of the seven amino-ketonic sparteines are consonant with those of the parent free base, whose lowest energy conformer consists of a chair–chair quinolizidine A/B-trans system and a boat–chair quinolizidine C/D-trans system. A similar situation also occurs for lactams 2-oxo- (lupanine), 15-oxo-, and 17-oxo-sparteine, whose ketonized ring has a half-chair/sofa, half-chair, and sofa shape, respectively. Unlike the other isomers, the most stable conformer of lactam aphylline (10-oxo-sparteine) adopts the chair–sofa A/B-transoid, chair–chair C/D-cisoid disposition of the two quinolizidine moieties. These theoretical predictions are consistent with the available X-ray experimental results. The electronic structure of the oxo-sparteines was examined by measuring and calculating relevant parameters of their NMR and photoelectron spectra. In particular, a representative set of NMR chemical shifts and nuclear spin–spin coupling constants, calculated by means of DFT formalisms, correlate well with observation. Notably, the effective manifestation of stereoelectronic hyperconjugative effects on Δδ(Heq/Hax) and Δ1J(CHeq/CHax) of the lactams is correctly accounted for by the DFT results. The ab initio outer valence Green's function calculations yielded a consistent description of the main features in the photoelectron spectra, i.e., location, splitting, and sequence of the n(N) and n(O) ionization energies, which also reflect the competing through-bond and through-space interactions within the sparteine framework.

The equilibrium structures of the lupine alkaloid sparteine, its diastereoisomers (alpha-sparteine and beta-sparteine), and two related tricyclic diamines were investigated with the density functional theory ansatz B3LYP and HF-MP2 method. With the exception of beta-sparteine, these free molecules are structurally simple, since one conformer predominates both in the gas phase and in solution. The mono-protonated forms of all these molecules are characterized by intramolecular asymmetric hydrogen bonds. The di-protonated species have the same structural preferences as the parent free bases. All the theoretical structural predictions are consistent with the available experimental results. The NMR chemical shifts, calculated by the continuous set of gauge transformations formalism with the DFT/B3LYP method, are fairly consistent with NMR observation. The He(I) photoelectron spectra were measured and assigned by means of ab initio outer valence Green's function calculations. The overwhelming predominance of the lowest-energy conformer of all compounds is confirmed by various spectroscopic results. However, the structure of beta-sparteine is very special. The C-13 NMR spectra in solution at room temperature are indicative of an effective C-2 molecular symmetry. From the analysis of the NMR spectra from +20 to -85degreesC it is inferred that the most stable C-1 conformer of beta-sparteine is almost exclusively present in solution. At room temperature, it rapidly interconverts between two enantiomeric forms to produce the NMR spectrum of a time-averaged C-2 conformer. (C) 2003 Elsevier B.V. All rights reserved.

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.

Full conformational analysis on some representative cycloalkyl carbocations and acyclic carbodications with μ-hydrido bridging has been performed at the RHF/6-31G∗∗ level of theory. The monocations involve bi- and tri-cyclic systems and the acyclic dications include the first example of a system containing two μ-hydrido bridges. For all cations examined, the most stable conformer is established as having a symmetric, bent C–Hμ–H three-centre two-electron bond (σ conjugation). By symmetry, only in the in-bicyclo[4.4.4]-1-tetradecyl cation is the μ-hydrido bridge linear. The 1 H and 13 C chemical shifts, calculated by the continuous-set-of-gauge-transformations formalism with the B3LYP hybrid density functional theory method and the 6-311+G(2d,p) basis set, are fairly consistent with NMR observation, in particular for the very high 1 H μ signal and the low-field 13 C peak of the bridgehead carbon.

Extensive ab initio symmetry-adapted-cluster configuration-interaction (SAC-CI) calculations have been performed for the electronic spectra of cyclopropane, bicyclobutane, [1.1.1]propellane, spiropentane, dispiroheptane, and trispirononane. The theoretical results for the singlet and triplet Rydberg and valence transitions are in reasonable agreement with spectroscopic data for cyclopropane, bicyclobutane and [1.1.1]propellane. The spectroscopic assignments provided by the SAC-CI approach, however, differ in many respects from those obtained by previous ab initio calculations. The present results support the empirical speculations by Robin and Schafer et al. as to the location of the lowest valence transitions of cyclopropane, bicyclobutane and [1.1.1]propellane. Also, the assignment of the feature at 4.7 eV of [1.1.1]propellane with the lowest valence state 3 A″ 2 is confirmed. The theoretical predictions for the spiro compounds are, instead, in sharp disagreement with the scarce experimental data. According to theory, spiropentane and its congeners do not absorb below 6.5 eV.

The nuclear spin—spin coupling constant of the predicted ultrashort central CC bond in bitetrahedrane has been calculated using the equations-of-motion approach at the ab initio level. An analysis of the influence exerted on the magnitude of this 1 J ( 13 C 13 C) by bond distance has been made through semiempirical SCPT-INDO calculations, performed also for the two congener bipolyhedra, bicubane and bidodecahedrane. The unusually large value of 1 J ( 13 C 13 C), which unexpectedly increases with bond lengthening, confirms the extraordinary nature of the intercage bond in bitetrahedrane.

The outer valence ionization potentials, low-lying electronically excited states and nuclear spin-spin coupling constants of cubane have been investigated at ab initio level by using the outer valence Green function, random phase approximation and equations of motions approaches, respectively, which incorporate the main portion of the electron correlation effects. The theoretical results give an overall picture of these spectroscopic observables, which bear peculiar aspects stemming from the unique electronic structure of the cubic cage.

The low-lying singlet intravalence and Rydberg electronic states of the three conjugated isomers of benzene, pentafulvene, 3,4-dimethylene-cyclobutene and trimethylene-cyclopropane have been investigated at ab initio level by utilizing random phase approximation transition energies and amplitudes. A comprehensive theoretical prediction of the discrete excitation spectra and of the two-photon absorption properties, that could be detected under various experimental arrangements, is presented. Information on the nature of the first valence-like excited states has also been achieved by single and double excitation configuration interaction treatments.

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.

Ab initio many-body calculations according to Cederbaum's outer-valence Green-function formalism have been carried out on the ionization potentials of bicyclo[4.1.0]hepta-1,3,5-triene, bicyclo[4.2.0]octa-1,3,5-triene, tricyclo[5.1.0.0 3,5 ]octa-1,3(5),6-triene, tricyclo[6.2.0.0 3,6 ]deca-1,3(6),7-triene, and tricyclo[5.2.0.0 3,5 ]nona-1,3(5),6-triene. The sequence of the first two negative electron affinities is analysed by using estimates based on Koopmans' theorem.

The equilibrium structure of hexacyclo[8.7.0.03,8.05,15.06,13.012,16]heptadeca-1(10),5-diene-3,8-dicarboxylic anhydride, a tetrahydro[4]beltene with two σ-conjugated double bonds, and of two derivatives, the first with only one double bond and the second with a closed annular belt, was calculated at the RHF/6-31G** ab initio level of theory. The results, consistent with the available X-ray experimental data, account for an efficient homoconjugation in the two compounds with an open belt. The NMR 13 C chemical shifts were analyzed by means of ab initio CSGT (continuous set of gauge transformations) calculations performed with the B3LYP/6-311+G(2d, p) DFT-HF hybrid functional model. The He(I) photoelectron spectra were measured and interpreted by means of ab initio OVGF (outer valence Green function) calculations, which give an overall consistent reproduction of the energies and splittings of the uppermost bands. These are associated with the π(CC) and/or Walsh orbitals of the equatorial belt and with the n(CO) lone pair orbitals of the anhydridic moiety. Electron transmission spectroscopy was employed to characterize the low-lying temporary anion states of tetrahydro[4]beltene. The single-crystal X-ray structure was also determined for the tris(demethano)pagodane derivative.