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Molecular dynamics simulations based on density functional theory were employed to investigate the fate of a hydrogen molecule shot with different kinetic energy toward a hydrogenated palladium cluster anchored on the vacant site of a defective graphene sheet. Hits resulting in H2 adsorption occur until the cluster is fully saturated. The influence of H content over Pd with respect to atomic hydrogen spillover onto graphene was investigated. Calculated energy barriers of ca. 1.6 eV for H-spillover suggest that the investigated Pd/graphene system is a good candidate for hydrogen storage. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogenolysis of 7-hydroxymatairesinol (HMR) to matairesinol (MAT) occurs on palladium supported acidic catalysts. HMR has three stereochemical centres and naturally exists as a mixture of two diastereoisomers, namely RRR-HMR and SRR-HMR. The latter is significantly more reactive than the former, in the reaction above. In order to elucidate the hydrogenolysis mechanism, simple surface events involving HMR derivatives on aluminated faujasite (H-Y) fragments were simulated by quantum chemical calculations. The metallic function of a Pd/H-Y acidic catalyst was also mimicked by a minimal palladium cluster. Stereochemical evidences pointed already out that water β-elimination on the epimeric centre, leading to an olefinic intermediate, should mainly occur for the adsorbed SRR-HMR stereoisomer. As a consequence, it was hypothesized that this feature could explain the different reactivity of the RRR-HMR and SRR-HMR hydrogenolysis, occurring on metal supported bi-functional zeolite catalysts. The significance of this inference is decreased by the here study. Finally, it is demonstrated that several HMR to MAT reaction paths may coexist and that the metal component of the Pd/H-Y bi-functional catalyst cannot be involved in steps discriminating for the whole reaction rate and selectivity. [ABSTRACT FROM AUTHOR]

The oxidation functionality of Mn(IV) sites has been assessed by density functional theory (DFT) analysis of adsorption and activation energies of CO, H 2 and O 2 on a model Mn 4 O 8 cluster. DFT calculations indicate that Mn(IV) atoms prompt an easy CO conversion to CO 2 via a reaction path involving both catalyst and gas-phase oxygen species, while much greater energy barriers hinder H 2 oxidation. Accordingly, a MnCeO x catalyst (Mn at /Ce at , 5) with large exposure of Mn(IV) sites shows a remarkable CO oxidation performance at T ≥ 293 K and no H 2 oxidation activity below 393 K. Empiric kinetics disclose that the catalyst-oxygen abstraction step determines both CO and H 2 oxidation rate, although different activation energies favor the preferential oxidation (PROX) pattern of the studied catalyst (353–423 K). Conversion-selectivity of 100%, high stability during 72 h reaction time and moderate inhibiting effects of water and CO 2 feeding reveal the potential of MnO 2 materials as efficient, low-cost and robust PROX catalysts. Mn(IV) sites shape the PROX pattern of the MnCeO x catalys [Display omitted] • A DFT study of the CO and H 2 oxidation functionality of Mn(IV) sites is outlined. • DFT analysis predicts a typical preferential CO oxidation behavior of Mn(IV) sites. • A MnCeO x catalyst with large availability of Mn(IV) has been investigated. • Very different energy barriers determine the PROX pattern of the MnCeO x catalyst. • MnO 2 materials are effective, low cost and robust catalysts for the PROX reaction. [ABSTRACT FROM AUTHOR]

• Catalytic oxidation of benzyl alcohol to benzaldehyde over MnO x catalysts is analyzed in the DFT framework. • Oxidation reactivity and selectivity over cluster-sized Mn 4 O 8 and Mn 4 O 9 catalytic models is compared. • Catalyst poisoning is addressed by investigating the formation of benzoic acid following that of benzaldehyde. • Reaction steps and the related oxidation pathways are collected and analyzed by an original kinetic approach. • Experimental strategies aimed at decreasing catalyst deactivation hence at increasing selectivity are proposed. [Display omitted] The catalytic oxidation of benzyl alcohol with O 2 is a promising option for the production of benzaldehyde, from both environmental and economical viewpoints. In particular, highly dispersed MnO x systems feature good activity and selectivity in a wide range of temperatures, although deactivation phenomena by over-oxidation and/or poisoning of active sites are generally recorded. On this account, a density functional theory study was performed on cluster-sized catalyst models, namely Mn 4 O 8 and over-oxygenated Mn 4 O 9 fragments, to predict the reactivity pattern of MnO x catalysts in the selective aerobic oxidation of benzyl alcohol. Several pathways concur to determine the whole reaction process and all of them were compared to unveil the atomistic details of the alcohol oxidation mechanism. Moreover, assuming that the consecutive formation of benzoic acid affects the activity-stability pattern of the MnO x based catalyst, also the benzaldehyde oxidation mechanism was computationally addressed. A systematic comparison of the benzyl alcohol and benzaldehyde oxidation mechanisms on the Mn 4 O 8 and Mn 4 O 9 fragments reveals some experimental strategies to test the reaction mechanisms and design alternative catalytic routes to decrease undesired parasitic reactions leading to catalyst deactivation. The matching structural, energetic and kinetic data are published in the Data in Brief journal [1]. [ABSTRACT FROM AUTHOR]

DFT analysis unveils the atomistic details of the selective aerobic oxidation of benzyl alcohol on manganese dioxide catalysts. • The reactivity of MnO 2 catalyst in the selective aerobic oxidation of benzyl alcohol is assessed by DFT analysis. • DFT simulations predict high reactivity of defective Mn(IV) sites. • Nanostructured MnO x catalysts show comparable activity due to the redox functionality of surface Mn(IV) sites. • 0th-order kinetics disclose the remarkable influence of adsorption phenomena on the activity-stability pattern. • DFT analysis provides insights into the formation of benzoic acid, acting as poison of active sites. The reactivity pattern of the MnO 2 catalyst in the selective aerobic oxidation of benzyl alcohol is assessed by density functional theory (DFT) analysis of adsorption energies and activation barriers on a model Mn 4 O 8 cluster. DFT calculations predict high reactivity of defective Mn(IV) sites ruling a surface redox mechanism, L-H type, involving gas-phase oxygen. Bare and promoted (i.e., CeO x and FeO x) MnO x materials with high surface exposure of Mn(IV) sites were synthesized to assess kinetic and mechanistic issues of the selective aerobic oxidation of benzyl alcohol on real catalysts (T, 333–363 K). According to DFT predictions, the experimental study shows: i) comparable activity of bare and promoted catalysts due to surface Mn(IV) sites; ii) the catalytic role of oxygen-atoms in the neighboring of active Mn(IV) sites; and iii) a 0th-order dependence on alcohol concentration, diagnostic of remarkable influence of adsorption phenomena on the reactivity pattern. Evidences of catalyst deactivation due to the over-oxidation of benzyl alcohol to benzoic acid, acting as poison of the active sites, are discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Metal-free processes are desirable for economic and environmental reasons. • Low cost alkanes conversion to relevant alkenes is an important industrial challenge. • Quasi -molecular BN nanoflakes were proposed as potential dehydrogenating species. • DFT was employed for exploring the use of BN nanoflakes as dehydrogenation catalysts. • BN nanoflakes seem useful to obtain industrially relevant hydrocarbon derivatives. Lower alkanes are feedstocks readily available but relatively inert. The conversion of low cost alkanes to industrially relevant alkenes is usually carried out on metal-based heterogeneous catalysts. Considering both the cost and the potential harmfulness of the metal involved in the dehydrogenation catalysts (typically, platinum or chromium), the study of metal-free processes represents an important challenge for the industrial chemistry in order to address more sustainable protocols and different routes either to activate or transform alkanes. Framed in this context, it was investigated, using a density functional theory approach, the potential dehydrogenation activity of defective and partially hydrogenated boron nitride quasi- molecular nanoflakes, on ethane, chloroethane and phenylethane. Concerted and not concerted reactions were considered, resulting the former always favored with respect to the latter and being the mechanisms and the corresponding energy characterizing the different substrates, in any case, quite similar. Along the dehydrogenation, given a certain hydrocarbon adsorption constellation, the defective nitrogen sites were generally more active than the boron ones, being the energy barriers, in any case, smaller or at least comparable with those already observed for the metal catalyzed dehydrogenation processes. After the dehydrogenation processes, the hydrogenated boron nitride quasi- molecular nanoflakes resulted highly stabilized, suggesting that specific strategies are needed to employ these materials as catalysts. [ABSTRACT FROM AUTHOR]

Metal clusters have been used in catalysis for a long time, even in industrial production protocols, and a large number of theoretical and experimental studies aimed at characterizing their structure and reactivity, either when supported or not, are already present in the literature. Nevertheless, in the last years the advances made in the control of the synthesis and stabilization of subnanometric clusters promoted a renewed interest in the field. The shape and size of sub-nanometer clusters are crucial in determining their catalytic activity and selectivity. Moreover, if supported, subnanometric clusters could be highly influenced by the interactions with the support that could affect geometric and electronic properties of the catalyst. These effects also present in the case of metal nanoparticles assume an even more prominent role in the “subnano world.” DFT-based simulations are nowadays essential in elucidating and unraveling reaction mechanisms. The outstanding position of this corner of science will be highlighted through a selected number of examples present in the literature, concerning the growth and reactivity of subnanometric supported metal catalysts. [ABSTRACT FROM AUTHOR]

The use of biomass as renewable feedstock for commodity chemicals may largely benefit from the successful development and application of heterogeneous catalysts for decomposition processes. The present investigation combines density functional theory and Christiansen-like microkinetic analysis to describe, at the atomistic level, the mechanisms related to the conversion of oxygenated biomass compounds to deoxygenated and semi-saturated hydrocarbons on a subnanometric Pt 10 cluster. The DFT calculations and the kinetic analysis based on the evaluated free energy variations, associated with both elementary step barriers and rearrangement/desorption processes occurring on the cluster, suggest that benzene is the preferred product, together with a compound still bearing oxygen, cyclopentadienone, which would form as a minor product only at high temperature. Other than highlighting the role of the peculiar interaction between carbon and platinum, the reported investigation underlines the importance of cluster fluxionality and reorganization ability in promoting catalyzed reactions.

In a scenario of declining fossil resources and increasing demand for renewable and sustainable alternatives, biomass is the only source able to offer an easy and gradual transition in the use of current energy technologies based on the exploitation of carbon derivatives. Its conversion to liquid fuels has oriented our study towards the computational mechanistic analysis of the guaiacol catalytic hydrodeoxygenation, which is currently considered one of the most challenging routes for upgrading biomass-derived bio-oils. For this purpose, a subnanometric Pt 10 platinum cluster was chosen as the catalyst model, with Pt as a computational reference element for catalytic hydrogenation, and guaiacol as a model compound of bio-oils. DFT calculations revealed that the energy barriers related to the cleavage of C(sp 2 )-O bonds in the direct deoxygenation mechanism are significantly lower (by an average of 60 kJ mol -1 ) than those in the deoxygenation-through-hydrogenation mechanism in which C(sp 3 )-O bond breaking from a saturated ring occurs. Even if the ring hydrogenation is easier in the oxygenated compound, the analysis reveals that the direct deoxygenation mechanism is favoured at all temperatures. Furthermore, the results obtained highlight that, from a thermodynamic perspective, the removal of oxygen groups preferentially occurs by the elimination of the -OCH 3 fragment as methanol and then of the -OH fragment as a water molecule.

Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favorable energy level alignment within these complexes, suggesting a photoinduced electron transfer from dots to nanotubes, which is a process of high functional interest. Femtosecond transient absorption confirms indeed an ultrafast (<100 fs) electron transfer independent of nanotubes being conductive or semiconductive in nature, followed by a much slower back electron transfer (≈60 ps) from the nanotube to the carbon dots. The high degree of charge separation and delocalization achieved in these nanohybrids entails significant photocatalytic properties, as we demonstrate by the reduction of silver ions in solution. The results are very promising in view of using these "all-carbon" nanohybrids as efficient light harvesters for applications in artificial photocatalysis and photosynthesis.

Data here reported are connected with the research article "Benzyl Alcohol to Benzaldehyde Oxidation on MnO x Clusters: Unraveling Atomistic Features" Gueci et al. [1]. This work described and discussed structural and energetic results, calculated by Density Functional Theory (DFT). In order to get kinetic information, DFT results were refined by an original approach, which will be shortly described in the following article. The crossed analysis of experimental and computational energetic and kinetic data allowed to (i) reconstruct the complicated lattice that connects the primary and secondary mechanisms of the reaction and (ii) identify alternative process pathways capable of by-passing parasitic mechanisms, decreasing selectivity. On the other hand, the data here presented show what is the basic information necessary to obtain the modeling of a complex process of heterogeneous catalysis. Moreover, they can be used either to verify the validity of the discussion outlined in the original article Gueci et al. [1] or as a starting point to computationally explore alternative routes and the related kinetics of the same oxidation processes, in the aim to further optimize the corresponding experimental approach., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article., (© 2021 The Authors. Published by Elsevier Inc.)

Considering the ever-increasing interest in metal-free materials, some potential chemical applications of quasi -molecular boron nitride (BN) derivatives were tested. Specifically, the behavior of BN fragments was analyzed when given defects, producing local electron density changes, were introduced by using topological engineering approaches. The inserted structural faults were Schottky-like divacancy (BN-d) defects, assembled in the fragment frame by the subtraction of one pair of B and N atoms or Stone-Wales (SW) defects. This study is aimed at highlighting the role of these important classes of defects in BN materials hypothesizing their future use in H 2 -based processes, related to either (i) H 2 activation or (ii) H 2 production, from preadsorbed hydrogenated molecular species on BN sites. Here, it has been observed that BN species, embodying SW defects, are characterized by endothermic H 2 adsorption and fragmentation phenomena in order to guess their potential use in processes based on the transformation or production of hydrogen. On the contrary, in the presence of BN-d defects, and for reasons strictly related to local structural changes occurring along with the hydrogen rearrangements on the defective BN fragments, a possible use can be inferred. Precautions must be however taken to decrease the material rigidity that could actually decrease the ability of the BN fragment to flatten. This conversely seems to be a necessary requirement to have strong exothermic effects, following the rearrangements of the H 2 molecules., Competing Interests: The authors declare no competing financial interest.

The fragmentation of molecular hydrogen on N-doped carbon networks was investigated by using molecular (polyaromatic macrocycles) as well as truncated and periodic (carbon nanotubes) models. The computational study was focused on the ergonicity analysis of the reaction and on the properties of the transition states involved when constellations of three or four pyridinic nitrogen atom defects are present in the carbon network. Calculations show that whenever N-defects are embedded in species characterized by large conjugated π-systems, either in polyaromatic macrocycles or carbon nanotubes, the corresponding H2 bond cleavage is largely exergonic. The fragmentation Gibbs free energy is affected by the final arrangement of the hydrogen atoms on the defect and by the extension of the π-electron cloud, but it is not influenced by the curvature of the system., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Selective dehydrogenation of the biomass-derived lignan hydroxymatairesinol (HMR) to oxomatairesinol (oxoMAT) was studied over an Au/Al(2)O(3) catalyst. The reaction was carried out in a semi-batch glass reactor at 343 K under two different gas atmospheres, namely produced through synthetic air or nitrogen. The studied reaction is, in fact, an example of secondary-alcohol oxidation over an Au catalyst. Thus, the investigated reaction mechanism of HMR oxidative dehydrogenation is useful for the fundamental understanding of other secondary-alcohol dehydrogenation over Au surfaces. To investigate the elementary catalytic steps ruling both oxygen-free- and oxygen-assisted dehydrogenation of HMR to oxoMAT, the reactions were mimicked in a vacuum over an Au(28) cluster. Adsorption of the involved molecular species--O(2), three different HMR diastereomers (namely, one SRR and two RRR forms), and the oxoMAT derivative--were also studied at the DFT level. In particular, the energetic and structural differences between SRR-HMR and RRR-HMR diastereomers on the Au(28) cluster were analyzed, following different reaction pathways for the HMR dehydrogenation that occur in presence or absence of oxygen. The corresponding mechanisms explain the higher rates of the experimentally observed oxygen-assisted reaction, mostly depending on the involved HMR diastereomer surface conformations. The role of the support was also elucidated, considering a very simple Au(28) charged model that explains the experimentally observed high reactivity of the Au/Al(2)O(3) catalyst., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Interactions between alkali-metal azides and metal-organic framework (MOF) derivatives, namely, the first and third members of the isoreticular MOF (IRMOF) family, IRMOF-1 and IRMOF-3, are studied within the density functional theory (DFT) paradigm. The investigations take into account different models of the selected IRMOFs. The mutual influence between the alkali-metal azides and the π rings or Zn centers of the involved MOF derivatives are studied by considering the interactions both of the alkali-metal cations with model aromatic centers and of the alkali-metal azides with distinct sites of differently sized models of IRMOF-1 and IRMOF-3. Several exchange and correlation functionals are employed to calculate the corresponding interaction energies. Remarkably, it is found that, with increasing alkali-metal atom size, the latter decrease for cations interacting with the π-ring systems and increase for the azides interacting with the MOF fragments. The opposite behavior is explained by stabilization effects on the azide moieties and determined by the Zn atoms, which constitute the inorganic vertices of the IRMOF species. Larger cations can, in fact, coordinate more efficiently to both the aromatic center and the azide anion, and thus stabilizing bridging arrangements of the azide between one alkali-metal and two Zn atoms in an η(2) coordination mode are more favored., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The proton dynamics in Y-doped BaZrO(3) derivatives, in particular the different dopant environments within a Pm3m cubic framework, were studied by using classical molecular dynamics (MD) calculations. Single- and double substitution of zirconium by yttrium atoms was considered. The presence of yttrium induced variations in the surrounding oxygen sites, according to their local geometrical arrangements. The differences among such distinct oxygen sites became evident when protons interacted with them and upon changes in the temperature. So, different proton transfer pathways, which had different energy barriers, characterized the topologically different oxygen sites. The experimental proton-hopping activation energy was only reproduced in those structures in which two yttrium atoms formed a Y-O-Y arrangement, which also acted as multilevel protonic traps. Protonic conduction in these materials could be improved by avoiding such yttrium clustering, hence preventing the formation of the protonic traps., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Quantum chemical calculations have been carried out to simulate Y-doped BaCeO(3) derivatives. Hartree-Fock energy functional was used to study octahedral site environments embedded in a Pmcn orthorhombic framework, showing local arrangement characterized by Ce-O-Ce, Ce-O-Y, and Y-O-Y (Z-O-Ξ) configurations and including or not hydrogen close to the moieties encompassing those configurations. The latter are, in fact, representative of - and, in our modeling approach, were treated as - local arrangements that could be found in Y:BaCeO(3)-doped materials. The geometrical optimizations performed on the structural models and a detailed orbital analysis of these systems allowed us to confirm and deepen new interpretations, concerning experimental findings already reported in the literature. In particular, the bimodal distribution characterizing the Y-O first coordination shell, found by EXAFS analysis, could be attributed to a local clustering of Y atoms showing characteristic Y-O-Y arrangements. Moreover, the local charge analysis, characterizing the models containing or not hydrogen atoms, showed that the moving protons are able to dynamically change the properties of their near environment, in any case, leaving unaltered the global protonic conduction features of the material, irrespective of the kind of cation in a given Z-O-Ξ moiety.

The processes involved in the butene hydro-isomerization, occurring on a small palladium cluster in the presence of dissociated hydrogen, have been investigated by means of DFT and DFT/MM approaches. This study has been performed both on an isolated (unsupported) Pd(9) cluster and on the same cluster when it is supported on a portion of a single-walled armchair(6,6) carbon nanotube. The study follows another investigation which has already been published concerning the adsorption, fragmentation and diffusion of hydrogen on the same metal cluster. The main aspects involved in the parallel reaction steps of the whole hydro-isomerization mechanisms are not strongly affected by the presence of the support, which does, however, modify the energetics involved, likely due to the presence of strong metal surface interaction (SMSI) effects. Noticeably, a common step corresponding to the diffusion of one hydrogen atom is present. This diffusion step creates a characteristic semihydrogenated surface species along the occurrence of all the reaction pathways. Hence, the semihydrogenated species is a kind of molecular node able to connect the transformation pathways of the different surface species involved in the hydro-isomerization processes. Considering the energetics involved in the processes of both supported and unsupported systems and being aware of the simplification introduced in studying the same systems, it is still possible (i) to emphasize the basis importance of taking account of the support in modeling catalytic properties and (ii) to state that the models proposed here are able to capture the main characteristics of the title reaction.

Geometry calculations were performed on pure BaCeO(3) fragments and on Y- and In-doped derivatives. HF and DFT approaches were used to investigate monoclinic and orthorhombic structures. The computational methods, structural models, and electronic structure investigation protocols were tuned taking into consideration and balancing the consistency of the results against the computational cost. The calculated structures and energetics parameter, as well as the detailed orbital analysis performed on the corresponding BaCeO(3) derivatives allowed us to explain experimental findings and to develop a procedure to study the cationic octahedral environment of doped X:BaCeO(3) (X = Y, In) and undoped BaCeO(3) protonic conductors useful to interpret experimental results and hopefully to design new experimental approaches. In detail, distances and angles of the studied materials are easily captured in the frame of the HF paradigm even by using low-level ECP basis sets. While, pure electronic-based approaches, involving the investigation of the Partial Density of States resulting from the C-Squared Population Analysis, show that the dopant species must leave unchanged, or even decrease, the local basicity of the oxygen octahedral environment in order to increase the conductivity of the BaCeO(3) derivatives. Whereas local structural changes that are not related to the basicity above affect to a less, if not null, extent the conductivity of the same derivatives.

The processes of adsorption, fragmentation and diffusion of hydrogen on a small palladium cluster have been investigated by means of DFT and DFT/MM approaches. These studies have been performed by considering a D(3h) symmetry Pd(9) in the isolated state as well as when supported on a portion of single-walled armchair(6,6) carbon nanotube. The hydrogen fragmentation process easily occurs on the bare Pd(9) cluster, involving energy barriers of 25-35 kJ mol(-1) and the drop in spin multiplicity on passing from the reactant to the product. The atomic hydrogen diffuses through the cluster atoms with energy barriers, which do not exceed 20 kJ mol(-1), with some positions clearly identifiable as the most stable. In the case of the palladium supported system, which is a better model to simulate experimental conditions, calculations predict that the hydrogen fragmentation barrier is reduced by ca. 15 kJ mol(-1), with respect to that of the unsupported system, while the energetics of the diffusive process is not significantly affected by the support, if the reduction of the number of sites available in the same palladium cluster, as well as their geometry, are taken into account.

The influence of the calculation method in mimicking experimental (13)C NMR chemical shifts of 15 low-polarity natural products singularly containing 10-20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point (13)C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the (13)C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimizations and GIAO (13)C NMR calculations were thoroughly checked by the analysis of statistical parameters concerning computed and experimental (13)C NMR chemical shift values., (Copyright 2004 John Wiley & Sons, Ltd.)

SCSA is an algorithm designed to get information on molecular conformational properties. The most stable conformers are determined by the homemade SCSA code, performing a multistep systematic conformational search, which involves energy and structure quantum chemical optimizations at low-level and high-level. The SCSA method was employed to analyze the conformational space of the in vacuo cyclopeptide renieramide at AM1 and B3LYP/6-31G(d) levels. Calculations at B3LYP level of the GIAO (13)C NMR chemical shifts were also performed on the final conformers. In fact, to validate the conformational search results experimental and calculated (13)C NMR spectra of renieramide were compared. Slight disagreements observed between experimental and calculated spectra could be attributed to solute-solvent interactions, which were not taken into account in the algorithm proposed here.

Geometry optimization and GIAO (gauge including atomic orbitals) (13)C NMR chemical shift calculations at Hartree-Fock level, using the 6-31G(d) basis set, are proposed as a tool to be applied in the structural characterization of new organic compounds, thus providing useful support in the interpretation of experimental NMR data. Parameters related to linear correlation plots of computed versus experimental (13)C NMR chemical shifts for fourteen low-polar natural products, containing 10-20 carbon atoms, were employed to assess the reliability of the proposed structures. A comparison with the hybrid B3LYP method was carried out to evaluate electron correlation contributions to the calculation of (13)C NMR chemical shifts and, eventually, to extend the applicability of such computational methods to the interpretation of NMR spectra in apolar solutions. The method was tested by studying three examples of revised structure assignments, analyzing how the theoretical (13)C chemical shifts of both correct and incorrect structures matched the experimental data.

Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) (13)C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an open-chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental (13)C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The (13)C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental (13)C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers.