DFT study on cooperativity in the interactions of hydrazoic acid clusters

Density functional theory at the B3LYP level with the 6‐311G** basis set is performed to calculate the systems consisting of up to four hydrazoic acid molecules. The dimers are found to exhibit cyclic and chain structures with N … H contacts at ca. 2.1–2.7 Å. However, there are only cyclic structures with N … H contacts at ca. 2.0–2.3 Å and 2.0–2.1 Å in the trimer and tetramer, respectively. Hydrogen bond distances in the trimer and tetramer are shorter than those in the cyclic dimer as a result of the stronger interaction between molecules. The contribution of cooperative effect to the interaction energy is significant. After the correction of the basis set superposition error and zero‐point energy, the binding energies are ?10.69, ?29.34, and ?54.26 kJ·mol^?1 for the most stable dimer, trimer, and tetramer, respectively. The calculated IR shifts for N? H stretching mode increase with the size of the cluster growths, reaching more than 200 cm^?1 in the tetramer. For the most stable clusters, the transition from the monomer to dimer, dimer to trimer, and trimer to tetramer involve changes of ?14.40, ?25.68, and ?31.88 kJ·mol^?1 for the enthalpies at 298.15 K and 1atm, respectively. We also perform Mulliken populations analysis and find the Mulliken populations on intermolecular N … H increasing in the sequence of the dimer, trimer, and tetramer. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 279–286, 2003     

Molecular structure and infrared spectra of (HXeCN)n (n=2, 3 or 4)

The structure, energetics, and vibrational spectra of the (HXeCN)2 dimer were investigated at the CCSD(T), MP2 and B3LYP levels. Such properties of the (HXeCN)3 trimer and (HXeCN)4 tetramer were investigated at the B3LYP level. The dimer, trimer, and tetramer were predicted to have a C2h, C2v, and D2d structure, respectively. In all of these oligomers, the N?Xe intermonomeric interaction is the most important one for holding the monomers together. Included with the ZPVE and BSSE, the stabilization energy of the dimer is 12.36 kcal/mol at the CCSD(T) level, while those of the dimer, trimer, and tetramer are 10.42, 18.23, and 31.34 kcal/mol, respectively, at the B3LYP level. At the B3LYP level, with respect to those of the isolated monomer, the C-Xe and Xe-H asymmetric stretching frequencies are shifted by -11.2 and +128.0 cm(-1) for the dimer, -51.6, +220.7 and -11.5, +96.6 cm(-1) for the trimer, and -14.1 and +201.8 cm(-1) for the tetramer.     

Studies in hydrogen bonding: Association in ammonia using an empirical potential

The empirical potential EPEN /2 has been used to establish the structures of isolated hydrogen-bonding ammonia clusters. The most stable forms of the dimer have a linear or near-linear structure. The trimer has a closed structure with zero dipole moment. Two stable tetramer forms were found: one with a closed structure and zero dipole moment in agreement with experimental findings, and one with a pyramidal structure with nonzero dipole moment which may be an artifact of the EPEN /2 potential. The relation of the dimer structures to the limited available experimental information is discussed.     

Theoretical Study on Intermolecular Interactions and Thermodynamic Properties of Difluoroamine Complex

Introduction　　ApplicationsofabinitiocalculationstointermolecularinteractionsincludingeitherweakvanderWaalsorstrongerhydrogenbondinghavedrawnmuchattentioninthepastdecadesbecausetheyareimportantinawiderangeofphysical,chemicalandbiologicalfields .1 5Inre centyears ,wehaveappliedtheintermolecularinteractionstoenergeticsystemsandobtainedsomemeaningfulinfor mationthatisvaluableforthestudyofenergeticmateri als .6 14 Thebehaviorofmolecularcomplexesisusuallybe tweentwoextremes :thegasphaseandthecrys…     

Preparation et etude des oligomeres du styrene

Dicarbanionic oligomers of styrene and α-methylstyrene, as well as the corresponding protonated species, have been prepared and studied in tetrahydrofuran. The molar extinction coefficient and the ionic dissociation constant of the dianionic dimers have been determined at 25°. The structure of the α-methylstyrene oligomers is identical to that observed by Richards and Williams; the styrene oligomers, isolated for the first time, show a symmetrical structure for the dimer and the tetramer, implying a higher reactivity for the styryl dianionic dimer than for the trimer dianionic species.     

Growth pattern and electronic properties of acetonitrile clusters: a density functional study

We report a systematic theoretical study on the growth pattern and electronic properties of acetonitrile clusters [(CH(3)CN)(n) (n = 1, 9, 12)] using density functional approach at the B3LYP6-31++G(d,p) level. Although we have considered a large number of configurations for each cluster, the stability of the lowest energy isomer was verified from the Hessian calculation. It is found that the lowest energy isomer of the dimer adopts an antiparallel configuration. For trimer and tetramer, cyclic ring structures were found to be favored over the dipole stabilized structure. In general, it is found that the intermolecular CH...N interactions play a significant role in the stabilization of the cyclic layered geometry of acetonitrile clusters. A critical comparison between trimer and tetramer clusters suggests that the three member cyclic ring is more stable than four member rings. The growth motif for larger clusters (n = 5-9, 12) follows a layered pattern consisting of three or four membered rings, which, in fact, is used as the building block. Based on the stability analysis, it is found that clusters with an even number of molecular entities are more stable than the odd clusters, except trimer and nonamer. The exceptional stability of these two clusters is attributed to the formation of trimembered cyclic rings, which have been found to form the building blocks for larger clusters.     

Geometries, thermodynamic properties and reactions of methylzinc alkoxide clusters studied by density functional theory calculations

Methylzinc alkoxide complexes are precursors for the preparation of nanosized zinc oxide particles, which in turn are catalysts or reagents in important industrial processes such as methanol synthesis and rubber vulcanization. We report for the first time the structures, energies, atomic charges, dipole moments, and vibrational spectra of more than 20 species of the type [(MeZnOR')n] with R' = H, Me, tBu and n = 1-6, calculated by density functional theory methods, mostly at the B3LYP/6-31+G* level of theory. For R' = Me, the global minimum structure of the tetramer (n = 4) is a highly symmetrical heterocubane but a ladder-type isomer is by only 70.9 kJ mol(-1) less stable. The corresponding trimer is most stable as a rooflike structure; a planar six-membered ring of relative energy 13.5 kJ mol(-1) corresponds to a saddle point connecting two equivalent rooflike trimer structures. All dimers form planar four-membered Zn2O2 rings whereas the monomer has a planar CZnOC backbone. A hexameric drumlike structure represents the global minimum on the potential energy hypersurface of [(MeZnOMe)6]. The enthalpies and Gibbs energies of the related dissociation reactions hexamer --> tetramer --> trimer --> dimer --> monomer as well as of a number of isomerization reactions have been calculated. The complexes [(MeZnOMe)n] (n = 1-3) form adducts with Lewis bases such as tetrahydrofuran (thf) and pyridine (py). The binding energy of py to the zinc atoms is about 65% larger than that of thf but is not large enough to break up the larger clusters. The bimolecular disproportionation of [(MeZnOMe)4] with formation of the dicubane [Zn{(MeZn)3(OMe)4}2] and Me2Zn is less endothermic than any isomerization or dissociation reaction of the heterocubane, but for steric reasons this reaction is not possible if R' = tBu. A novel reaction mechanism for the reported interconversion, disproportionation and ligand exchange reactions of zinc alkoxide complexes is proposed.     

Density functional study of small molybdenum clusters

Small molybdenum clusters up to the tetramer are investigated within the framework of the density functional theory. Both the geometry and the spin state are optimized for the dimer, trimer, and the tetramer. Moreover, all those calculations are followed by a vibrational analysis to discriminate between real minima and saddle points on the potential energy surfaces. Several low‐lying excited states are found to be stable after the vibrational analysis. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies of the stable conformers are reported. © 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 105–112, 2000     

Studies in hydrogen bonding: Association within small clusters of water,methanol, and ammonia molecules using Jorgensen's intermolecular pair potentials

The geometries of the dimer, trimer, and tetramer hydrogen-bonded clusters of water, methanol, and ammonia molecules have been derived using previously published intermolecular pair potentials containing constants optimized from ab initio calculations. The lowest energy forms for the dimers of all three types of molecules have an open structure, while the trimers and tetramers have cyclic structures. The results are compared with those previously described using another empirical potential, EPEN .     

Thermodynamics of forming water clusters at various temperatures and pressures by Gaussian-2, Gaussian-3, complete basis set-QB3, and complete basis set-APNO model chemistries; implications for atmospheric chemistry

The Gaussian-2, Gaussian-3, complete basis set- (CBS-) QB3, and CBS-APNO methods have been used to calculate Delta H degrees and Delta G degrees values for neutral clusters of water, (H(2)O)(n), where n = 2-6. The structures are similar to those determined from experiment and from previous high-level calculations. The thermodynamic calculations by the G2, G3, and CBS-APNO methods compare well against the estimated MP2(CBS) limit. The cyclic pentamer and hexamer structures release the most heat per hydrogen bond formed of any of the clusters. While the cage and prism forms of the hexamer are the lowest energy structures at very low temperatures, as temperature is increased the cyclic structure is favored. The free energies of cluster formation at different temperatures reveal interesting insights, the most striking being that the cyclic trimer, cyclic tetramer, and cyclic pentamer, like the dimer, should be detectable in the lower troposphere. We predict water dimer concentrations of 9 x 10(14) molecules/cm(3), water trimer concentrations of 2.6 x 10(12) molecules/cm(3), tetramer concentrations of approximately 5.8 x 10(11) molecules/cm(3), and pentamer concentrations of approximately 3.5 x 10(10) molecules/cm(3) in saturated air at 298 K. These results have important implications for understanding the gas-phase chemistry of the lower troposphere.     

Cooperative and substitution effects in enhancing strengths of halogen bonds in FCl⋯CNX complexes

In this paper, the cooperative effect of halogen bond with hydrogen bond has been used to make a halogen bond in FCl-CNH dimer vary from a chlorine-shared one to an ion-pair one. The halogen bond is strengthened in FCl-CNH-CNH trimer and its maximal interaction energy equals to -76 kJ∕mol when the number of CNH in FCl-CNH-(CNH)(n) polymer approaches infinity. Once the free H atom in FCl-CNH-CNH trimer is replaced with alkali metals, the halogen bond becomes strong enough to be an ion-pair one in FCl-CNH-CNLi and FCl-CNH-CNNa trimers. An introduction of a Lewis acid in FCl-CNH dimer has a more prominent effect on the type of halogen bond. A prominent cooperative effect is found for the halogen bond and hydrogen bond in the trimers. FH-FCl-CNH-CNH and FH-FCl-CNH-CNLi tetramers have also been studied and the interaction energy of halogen bonding in FH-FCl-CNH-CNLi tetramer is about 12 times as much as that in the FCl-CNH dimer. The atoms in molecules and natural bond orbital analyses have been carried out for these complexes to understand the nature of halogen bond and the origin of the cooperativity.     

Density functional theory study of formaldehyde oligomers

Hydrogen‐bonded formaldehyde oligomers (dimer to pentamer) are studied using density functional theory (DFT), the B3LYP method, and the 6‐311+G* basis set. Many‐body interaction energies are obtained to study the contribution of many‐body terms to binding energy. The basis set superposition error (BSSE)‐corrected total energies are ?229.08170, ?343.61410, ?458.16660, and ?572.70901 hartrees for dimer, trimer, tetramer, and pentamer, respectively, with corresponding binding energies ?2.55, ?4.86, ?6.99, and ?9.49 kcal/mol. Two‐body energies have been found to contribute significantly to the total binding energy in dimer to pentamer, whereas higher‐order interaction energies are negligible. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Pi-pi interaction in pyridine

pi-pi Interaction in pyridine dimer and trimer has been investigated in different geometries and orientations at the ab initio (HF, MP2) and DFT (B3LYP) levels of theory using various basis sets (6-31G, 6-31G, 6-311++G) and corrected for basis set superposition error (BSSE). While the HF and DFT calculations show the pyridine dimer and the trimer to be unstable with respect to the monomer, the MP2 calculations show them to be clearly stable, thus emphasizing the need to include electron correlation while determining stacking interaction in such systems. The calculated MP2/6-311++G binding energy (100% BSSE corrected) of the parallel-sandwich, antiparallel-sandwich, parallel-displaced, antiparallel-displaced, T-up and T-down geometries for pyridine dimer are 1.53, 3.05, 2.39, 3.97, 1.91, 1.47 kcal/mol, respectively. The results show the antiparallel-displaced geometry to be the most stable. The binding energies for the trimer in parallel-sandwich, antiparallel-sandwich, and antiparallel-displaced geometry are found to be 3.18, 6.14, and 8.04 kcal/mol, respectively.     

Quantum chemical exploration of formaldehyde clusters (H2CO)n (n = 2–4)

Global exploration of equilibrium structures and interconversion pathways on the quantum chemical potential energy surface (PES) is performed for (H₂CO)n (n = 2–4) by using the Scaled Hypersphere Search‐Anharmonic Downward Distortion Following (SHS‐ADDF) method. Density functional theoretical (DFT) calculations with empirical dispersion corrections (D3) yielded comparable results for formaldehyde dimer in comparison with recent detailed studies at CCSD(T) levels. Based on DFT‐D3 calculations, trimer and tetramer structures and their stabilities were studied. For tetramer, a highly symmetrical S₄ structure was found as the most stable form in good accordance with experimentally determined tetramer unit in the formaldehyde crystal. © 2018 Wiley Periodicals, Inc.     

Observation of methyl iodide clusters in gas phase by infrared cavity ring-down spectroscopy

Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the C–H stretching region by cavity ring-down spectroscopy. The dependence of the spectra on the mixing ratio of CH₃I versus He and on the stagnation pressure has led to a tentative assignment of the absorption peaks to trimer up to pentamer, based on our previous study with matrix isolation technique (Chem. Phys. Lett. 343 (2001) 185). Ab initio calculations at the MP2 level for the trimer and tetramer have shown that two stable isomers exist for the tetramer whereas only one isomer is found to be stable for the trimer. The tentative assignment of the observed spectra has been in qualitative agreement with the results of the calculations. The structure of each isomer and its photochemical relevance are discussed.     

Synthesis and polymerization of coupled compounds of 1,11-dodecadiyne

The dimer, trimer, and tetramer of 1,11-dodecadiyne, HC?C? (CH₂)₈? C?CH, were synthesized. The solid-state polymerization of the dimer was investigated by infrared (IR) spectroscopy. IR bands due to the diacetylene moiety were identified through the comparison of the IR spectra of the dimer, trimer, and tetramer. The dimer was found to have two polymorphs, melt-crystallized and solution-crystallized. Both of the polymorphs undergo solid-state polymerization by exposure to γ-ray or UV irradiation. The former has higher polymerizability for the diacetylene moiety than the latter. The solid-state polymerization of the terminal acetylene group was not observed. It is shown that the previously reported dimer structure in which both the diacetylene and terminal acetylene groups are polymerized to form an inherently electrically conducting polymer is incorrect. © 1995 John Wiley & Sons, Inc.     

Density functional study of lithium hexamethyldisilazide (LiHMDS) complexes: effects of solvation and aggregation

The title compound, lithium hexamethyldisilazide (LiHMDS), has been studied using modern quantum-chemical methods in the form of the B3LYP approach. Monomers, dimers, trimers, and tetramers, microsolvated with up to four THF molecules have been considered. The choice of model complex is seen to be important-for instance, the simpler water molecule is shown to be an inappropriate substitute for the THF solvent. Calculated lithium NMR shieldings are reported, but by themselves, they seem to be insufficient for unequivocal assignments of the different species. The energetics of aggregation and solvation have been studied. Temperature effects are seen to be important, and the degrees of solvation and aggregation are higher at 0 K than at 298 K. The highest degree of THF solvation for the monomer and dimer is found to be three (0 K) and two (298 K), respectively. The highest possible degree of aggregation for unsolvated LiHMDS is four. However, in nonpolar solvents, formation of the LiHDMS dimer from the trimer is thermodynamically preferred. The pathway is likely to involve an intermediate tetramer. In THF solution, di-solvated monomers and dimers are the most likely species.     

Investigation of zinc binding metallothioneins' polymerization in tris(hydroxymethyl)-aminomethane buffer by coupling of size exclusion chromatography with electrospray ionization mass spectrometry

Polymerization of metallothioneins (MTs) is one of the commonly encountered puzzles in researching the structure and function of metallothioneins. In this work, a method involving SEC coupled with negative ion electrospray ionization mass spectrometry (ESI-MS) detection has been developed for the study of zinc binding MTs’ polymerization in tris(hydroxymethyl)-aminomethane (TRIS) acetate buffer at physiological pH. This hyphenated technique allows separating the different polymeric states of MTs by SEC, followed by on-line identification of the individual MT subisoforms in each polymeric peak by ESI-MS detection. Purified MT subisoforms (MT-2d and MT-2a), MT-2d and MT-2a mixture and rabbit liver MT complexes were investigated in the experiments to confirm the results obtained. From the results, both oxidative polymerization and non-oxidative oligomerization were found. The cystein-dependent oxidation results in the tetrameric peak as shown in the chromatograms of oxidized MT-2d, and stable dimeric and monomeric of MT were detected in this peak by MS. For the dimeric and trimeric peaks, different MT subisoforms were detected. In the five major subisoforms detected in rabbit liver MT complexes, MT-2a and MT-2c exist primarily as trimer, while MT-2e, MT-2d and MT-1a exist mainly as dimer. Our results suggest that in the three kinds of polymers, dimer, trimer and tetramer that were found in samples, the tetramer comes from the oxidation of MT molecular; for the dimer and trimer resulting from cystein independent oligomerization, they are closely associated with the charge of subisoform.     

Theoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring-opening polymerization

We present a theoretical study on aluminium triisopropoxide (Al(OⁱPr)₃) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AM1, semiempirical Hartree-Fock) techniques. We determine the most stable geometries for both the tetramer and trimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X-ray diffraction and ²⁷A1 NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X-ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (ΔG) between the two aggregated structures has been estimated. The evolution of the theoretical ΔG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental ¹H and ²⁷A1 NMR data. Moreover, the AM1 heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four A1(OⁱPr)₃ instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring-opening polymerization complexes of ε-caprolactone and lactides.     

Fragmentation dynamics of size-selected pyrrole clusters prepared by electron impact ionization: forming a solvated dimer ion core

Structure and dynamics of size-selected charged pyrrole clusters have been studied by means of molecular beam scattering experiments and ab initio calculations. Small neutral Pyn clusters were produced in Py/He mixture expansions, and the scattering experiment with a secondary beam of He-atoms was exploited to select the neutral clusters of different sizes. The complete size-selected fragmentation patterns for the neutral dimer to the tetramer after an electron impact ionization at 70 eV were obtained from the measurements of the angular and velocity distributions at different fragment masses. All the investigated cluster sizes decay mainly to the monomer ions Py+1 (from 60 to 80% of the corresponding neutral size) and to the dimer ion Py+2 (20-30%). The trimer ions Py+3 are generated to less than 10% from the neutral trimer and tetramer. To explain the observed results, we have calculated the structures and energetics of pyrrole clusters up to the trimer for the neutral and the ionic state using DFT and PMP2 methods. The ab initio calculations show that ionized pyrrole clusters are formed with a dimeric core that is solvated by neutral pyrrole molecules. In addition, the ground and ionic state of Py-Ar complexes were calculated at CCSD(T) level with extended basis in relevance to the mixed clusters produced in supersonic expansions of Py seeded in Ar. The calculated dissociation energies of the Py-Ar and (Py-Ar)+ complexes indicate that Ar atoms are able to rapidly evaporate after ionization. The combined analysis of the fragmentation probabilities, and calculations allowed us to estimate the distribution of energy deposited in the clusters after the electron impact, which peaks above 1 eV and has a tail up to 5 eV.