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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 molecular structures of a representative selection of phosphatranes were studied by means of the ab initio MP2 method. The calculated results reasonably matched the available X-ray data. The special properties of the intrabridgehead interaction were described in terms of hybridization, bond order index, force constant, and vibrational frequency. The atom-in-molecules analysis of the electronic charge density showed the presence of a CP(3,-1) betwen the bridgehead centers in the cationic species but not in all of the neutral phosphatranes. The proton affinities of bases with no apical substituent on the phosphorus were accurately calculated for both the gas phase and solution (DMSO). The unique electronic structure of phosphatranes was theoretically investigated by means of their spectroscopic properties. The NMR chemical shifts and indirect nuclear spin-spin coupling constants, computed by DFT-based methods, were fairly consistent with experimental evidence, in particular for the observables involving the central phosphorus. The photoelectron spectrum of azaphosphatrane, having NMe equatorial groups, was interpreted by means of ab initio outer valence Green's function calculations, which gave a consistent reproduction of the energies and splittings of the uppermost bands, associated with the lone pairs of the bridgehead and equatorial heteroatoms. Thus, computations provided reliable predictions of the variations of the NMR parameters and ionization energies with change of the equatorial centers and apical substitution at phosphorus.

The equilibrium structures of adamantane, 1-azaadamantane, adamantanone and derivatives were fully optimized at the B3LYP/6-31+G ∗∗ ab initio level of theory. The NMR 13 C chemical shifts were predicted by means of ab initio CSGT (continuous set of gauge transformations) calculations performed with the B3LYP/6-311+G(2d,p) DFT-HF hybrid model. The low-lying ionization energies were determined by ab initio outer valence Green function (OVGF) calculations. The results for the displacement of the resonances in the NMR spectra and for the location and splitting of the n -ionization bands in the photoelectron spectra are in good agreement with experiment. The mutual influence of the electronic interaction between the nitrogen and oxygen lone-pairs in various steric arrangements was examined by comparing the spectroscopic features of mono-, bi-, and tri-functionalized adamantane cages.

The equilibrium structures of 1,4,7,10-tetraazatetracyclo[5.5.1.0 4,13 .0 10,13 ]tridecane and tetrakis(dimethylamino)methane were calculated at the HF/6–31G ∗∗ ab initio level of theory and found to show a spiro-connection of two U- and W-shaped arrangements of the nitrogen lone pairs with D 2 and D 2d symmetry, respectively. The NMR 1 H and 13 C chemical shifts have been analyzed with ab initio continuous set of gauge transformations (CSGT) calculations performed with the B3LYP/6–311 + G(2d,p) hybrid functional model. The He(I) photoelectron spectra were measured and interpreted by means of ab initio outer valence Green function (OVGF) calculations, which give an overall consistent reproduction of the energies and splittings of the n(N) bands.

The equilibrium structures of tricyclo[5.2.1.0 4,10 ]decane and its 1-aza and 1,4,7-triaza derivatives were calculated at the HF/ 6-31G ∗ ab initio level of theory and found to be cup-shaped and of C 3 symmetry. The He(I) photoelectron spectra of the azacompounds were measured and interpreted by means of ab initio outer valence Green function calculations, which give an overall consistent agreement with the main features in the spectra, in particular of the n ( N ) bands.

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.

Ab initio many-body calculations done using the outer valence Green function (OVGF) approach, which incorporates the main portion of electron correlation and reorganization effects, are reported for the vertical ionization potentials of pentafulvene, 3,4-dimethylenecyclobutene, trimethylenecyclopropane, heptafulvene, o -xylylene, 3,4,5-trimethylenecyclopentene, and (4)-, (5)-, and (6)-radialene. The results give an overall, consistent reproduction of the main features in the photoelectron spectra, in particular of the it bands which are efficient monitors of the particular arrangements of π electrons in these monocyclic systems.

Non-empirical calculations using the outer valence Green function (OVGF) approach, which incorporates the main portion of electron correlation and reorganization effects, are reported for the vertical ionization potentials of tria-, penta- and hepta-fulvene, tria-, pentatria-, penta- and heptapenta-fulvalene, spiropentadiene, spiroheptatriene and spirononatetraene. The results provide an overall interpretation of the salient features in the photoelectron spectra.

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 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 π frontier molecular orbitals (MOs) are characterized by sizable interaction between the two phenol rings transmitted through the dicarbonyl chain and are 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 a loss of a phenolic proton occurs in alkaline ethanolic solution.

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 π 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.

Molecular structure and conformational preferences of yatakemycin, a novel and exceptionally potent antitumor agent, have been investigated using the density functional theory (DFT) formalism. From the relative stability of various possible conformations, it is found that two conformers are nearly isoenergetic and markedly more stable than the others in the gas phase. To test the effect of polar mediums, the relative energies have been recalculated using the self-consistent reaction field method. Thus, the most stable conformer of the isolated molecule in the gas phase is expected to be still more preferred in solution. The molecular structure of yatakemycin has also been studied by means of its spectroscopic properties. The DFT results satisfactorily reproduce the experimental data and corroborate the reliability of the structural characterization advanced for yatakemycin. The lowest-energy electronic transitions have been interpreted with time-dependent DFT calculations. Notably, the strong IR band observed at 2852 cm(-1) is unambiguously assigned to the O-H stretching of the (C7)O-H...O(C12) fragment, linked by a strong intramolecular H-bond, and may be viewed as a distinctive fingerprint of yatakemycin. Furthermore, the calculated set of NMR chemical shifts of carbonyl carbon atoms and indole protons, the most sensitive to stereoelectronic factors, is consistent with experiment. The effects of both protonation and oxidation on the geometry of the most stable conformer have also been studied. With reference to yatakemycin's DNA alkylation properties, the structure of the yatakemycin-adenine adduct has been theoretically modeled and found to be consistent with experimental spectroscopic evidence.

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 molecular structures of a representative selection of medium-sized bicyclic diphosphines (i.e., diphosphanes) were studied by means of the ab initio second-order Moller-Plesset (MP2) method. The calculated results matched the available X-ray crystallographic data reasonably. A distinctive structural aspect is that all the 1,5-diphosphabicyclo[3.3.3]undecane systems, the 1,6-diphosphabicyclo[4.4.4]tetradecane free base, and its axially disubstituted derivatives adopt the out, out-conformation, whereas the monosubstituted [4.4.4] species prefer the out,in-arrangement. The intrabridgehead interaction was described in terms of polarity, bond order index, force constant, vibrational frequency, and nature of the critical point of the total electron density. The electronic structures of the [3.3.3] and [4.4.4] diphosphanes were theoretically investigated by means of their NMR spectroscopic properties. The chemical shifts and indirect nuclear spin-spin coupling constants, computed by density functional theory (DFT)-based methods, were in satisfactory agreement with the experimental values. In particular, the theoretical results yielded a consistent account of the strong orientational effects of the phosphorus lone pair and coordination on δ( 3 1 P), J( 3 1 P- 3 1 P), and J( 3 1 P-C) that show remarkable changes of sign and magnitude within this class of compounds.

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.

Amidase antibody 312d6, obtained against the sulfonamide hapten 4 a that mimics the transition state for hydrolysis of a distorted amide, accelerates the hydrolysis of the corresponding amides 1 a-3 a by a factor of 10(3) at pH 8. The mechanisms of both the uncatalyzed and antibody-catalyzed reactions were studied. Between pH 8 and 12 the uncatalyzed hydrolysis of N-toluoylindoles 1 a and 3 a shows a simple first-order dependence on [OH(-)], while hydrolysis of 3 a is zeroth-order in [OH(-)] below pH 8. The pH profile for hydrolysis of the corresponding tryptophan amide 2 a is more complex due to the dissociation of the zwitterion into an anion with pK(a) 9.74; hydrolysis of the zwitterionic and the anionic form of 2 a both show simple first-order dependence on [OH(-)]. Absence of (18)O exchange between H(2) (18)O/(18)OH(-) and the substrate, a normal SKIE for both 1 a (k(H)/k(D)=1.12) and 3 a (k(H)/k(D)=1.24) and the value of the Hammett constant rho for hydrolysis of p-substituted amides 3 a-e are consistent with an ester-like mechanism in which formation of the tetrahedral intermediate is rate-determining and the amine departs as anion. The 312d6-catalyzed hydrolysis of 3 a was studied between pH 7.5 and 9, and its independence of pH in this range indicates that water is the reacting nucleophile. Hydrolysis of 3 a is only partially inhibited by the sulfonamide hapten, and this indicates that non-specific catalysis by the protein accompanies the specific process. Only the nonspecific process is observed in the hydrolysis of amides 3 with para substituents other than methyl. Binding studies on the corresponding series of p-substituted sulfonamides 5 a-e confirm the high specificity of antibody 312d6 for p-methyl substituted substrates.

2000/2001 XIV Ciclo 1971 Versione digitalizzata della tesi di dottorato cartacea. Nell'originale cartaceo manca la pag. 64

The equilibrium structures of three polyketones based on the 2,2‘-spirobiindan skeleton (1,1‘-dione, 1,3,1‘-trione, and 1,3,1‘,3‘-tetraone), their “half-molecule” fragments (1-indanone and 2,2-dimethylindan-1,3-dione), and the indandione dimer (2,2‘-dimethyl-[2,2‘]-biindenyl-1,3,1‘,3‘-tetraone) were investigated using the density functional theory model B3LYP/6-31G(d,p). The results matched the X-ray experimental data that are available for one of the spiroketones. The electronic structure of these ketones was investigated by means of their spectroscopic properties. The NMR 13C chemical shifts, calculated by the continuous-set-of-gauge-transformations formalism with the B3LYP/6-311+G(2d,p) method, were fairly consistent with NMR observations, in particular for the carbonyl, spiro, and quaternary carbons. The He(I) photoelectron spectra were measured and interpreted by means of ab initio outer-valence-Green's-function calculations. The theoretical results consistently reproduced the energies and splittings...

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.

1997/1998 XI Ciclo 1968 Versione digitalizzata della tesi di dottorato cartacea.