CNDO/2 calculations and configuration analyses for some hydrogen-bonded systems

The potential energy curves of hydrogen-bonded systems were calculated for the water, methanol, and formic acid dimers and for the hydrogen maleate ion by a modified CNDO/2 method, the core resonance integrals between-electrons being distinguished from those between-electrons, the different bonding parameters being used for -O- and =O, and the core potential integralsV _AB ^c for O-H, C=O, and O...O being determined semi-empirically. Consequently, the following results were obtained: 1) a potential energy curve with a single minimum atr(O-H)=0.95 Å and with a concavity near 1.70 Å for the linear chain dimers of water and methanol; 2) a symmetrical potential energy curve with two minima atr(O-H)=0.95 and 1.78 Å for the cyclic dimer of formic acid; 3) a flat-bottomed symmetrical potential energy curve for the hydrogen maleate ion. The configuration analysis method was applied to the hydrogen-bonded systems; the contributions of the covalent ((O-H-O)^–1) and ionic (O^–H⁺O^–) structures being 54% and 39%, respectively, for the symmetrical hydrogen bonding of the hydrogen maleate ion.

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Zusammenfassung Die Kurven potentieller Energie von Systemen mit H-Brücken wurden im Fall von H₂O-, CH₃OH- und HCOOH-Dimeren und für das Hydrogen-Maleatanion mittels eines modifizierten CNDO-Verfahrens berechnet, wo für Rumpf-- und --Elektronen jeweils verschiedene Resonanzintegrale und verschiedene Bindungsparameter für =O und -O- verwendet und wo die Rumpfpotential-Integrale für O-H, C=O und O...O semiempirisch bestimmt werden.Die Resultate sind 1) eine Potentialkurve mit einem einfachen Minimum für lineare Ketten von H₂O und CH₃OH, 2) eine symmetrische Potentialkurve mit zwei Minima für das cyclische Diniere von HCOOH und 3) eine Potentialkurve mit einem flachen Minimum für das Maleatanion. Konfigurationsanalyse ergab einen Beitrag von 39% für die ionischen Strukturen.

     

Cationic oligomerization of anethole: Selective synthesis of dimers and trimers

A linear unsaturated dimer of anethole [II, (E)-1,3-bis(p-methoxyphenyl)-2-methylpentene-1], was prepared in 98% yield with an acetyl perchlorate (AcClO₄) catalyst in a nonpolar solvent (C₆H₆) at a high temperature (70°C). At 0°C a linear unsaturated trimer was formed in high yield with dimer II. The geometric structure of the linear unsaturated dimers was determined by analysis of the nuclear Overhauser effect (NOE) on their ¹H-NMR spectra. NOE analysis showed that at 0°C with AcClO₄, trans-anethole gives the (E)-form (II), whereas cis-anethole yields the (Z)-form. On the other hand, with a metal-halide catalyst (BF₃OEt₂) a cyclic dimer and a cyclic trimer were produced in high yields in a polar solvent [(CH₂Cl₂)] at 70°C; higher oligomers (≥ tetramer) were scarcely formed. The effect of catalysts on the structure of oligomers was explained in terms of the interaction between a growing carbocation and a counterion.     

Compliant fields for molecular interactions: Water dimer and formic acid dimer

The redundancy-free internal valence compliance constants of open-chain water dimer and formic acid cyclic dimer have been determined by the combined use of the CNDO /Force method and the compliance constant formalism. The final compliant fields of these dimers have been refined with the help of experimental frequency data.     

Molecular orbital treatments of hydrogen bonded systems. 2. Dimers of water and HCN

Calculations were performed to investigate the reliabilities of the CNDO/2, PRDDO, and MNDO approximate molecular orbital methods. Systems selected for study included the linear, cyclic, and bifurcated dimers of water as well as the linear and cyclic dimers of HCN. The PRDDO method was found to provide the most consistently accurate reproduction of ab initio and experimental data. CNDO/2 performed fairly well in a number of cases but yielded extremely poor results for the cyclic dimers of both H₂O and HCN. Hydrogen bond strengths were consistently underestimated by MNDO which also furnished erroneously large intermolecular separations. In addition, MNDO calculations indicate the bifurcated water dimer to be most stable in contrast to other quantum mechanical and experimental information.See Ref. [1] for Paper 1 of this series.     

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     

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…     

Early stages of the hydrolysis of chromium(III) in aqueous solution-XII. Kinetics of cleavage of the trimer and tetramer in acidic solution

The kinetics of cleavage of the hydrolytic trimer and tetramer of chromium(III) have been studied in acidic solution and at elevated temperatures using spectrophotometric methods. For both reactions, the variation in absorbance with time was consistent with the presence of two consecutive rate-determining processes. The two reactions were concluded to proceed according to: Product analyses and consideration of the stability constants for these oligomers indicated that their overall cleavage reactions were irreversible under the conditions studied. However, it was unclear whether the initial step in tetramer cleavage was irreversible or not. The dimer was identified as an intermediate in the cleavage of the trimer. Each of the bridge-cleavage processes (k₁ and k₂) occurred via acid-dependent pathways, involving protonation processes in addition to bridge-cleavage processes, while only some of them displayed acid independent pathways. The acid dependence of the rate constants and associated parameters at 298 K are: (i) for trimer cleavage k₁ = k_b [H⁺], where the dimer to monomer was found to be the rate-determining step at higher temperatures (≈350 K), while cleavage of the trimer to dimer and monomer becomes rate determining at lower temperature (≈ 300 K). This change in the rate-determining step is attributable to the much higher value for k₁ compared with k₂. Possible mechanisms for trimer and tetramer cleavage are discussed and their rates compared to previously studied cleavage reactions of Cr^III oligomers.     

Structural peculiarities and the possibility of the existence of small water cluster anions (H2O) n − withn≤4

Structures of (H₂O) _n ⁻ anions withn≤4 were optimized at the UHF/4-31++G^** level and their stability was estimated at the MP2/4-31++G^** level. The trimer anion has a chain-like structure while the tetramer anion can exist either in a chain-like or a cyclic configuration. In the dimer anion and in the chain-like anions, the excess electron density is localized on the terminal water molecule, an acceptor of the H-bond proton. In the cyclic anion, it is uniformly distributed over the free hydrogen atoms. All considered anions have energy values higher than those of the corresponding neutral oligomers. The detachment of an electron from the anions should proceed with the liberation of energy. However, trimer and larger anions are stable against dissociation into individual water molecules and a free electron. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 41–46, January, 1997.     

Anion of the formic acid dimer as a model for intermolecular proton transfer induced by a pi* excess electron

The neutral and anionic formic acid dimers have been studied at the second-order Moller-Plesset and coupled-cluster level of theory with single, double, and perturbative triple excitations with augmented, correlation-consistent basis sets of double- and triple-zeta quality. Scans of the potential-energy surface for the anion were performed at the density-functional level of theory with a hybrid B3LYP functional and a high-quality basis set. Our main finding is that the formic acid dimer is susceptible to intermolecular proton transfer upon an excess electron attachment. The unpaired electron occupies a pi(*) orbital, the molecular moiety that accommodates an excess electron "buckles," and a proton is transferred to the unit where the excess electron is localized. As a consequence of these geometrical transformations, the electron vertical detachment energy becomes substantial, 2.35 eV. The anion is barely adiabatically unstable with respect to the neutral at 0 K. However, at standard conditions and in terms of Gibbs free energy, the anion is more stable than the neutral by +37 meV. The neutral and anionic dimers display different IR characteristics. In summary, the formic acid dimer can exist in two quasidegenerate states (neutral and anionic), which can be viewed as "zero" and "one" in the binary system. These two states are switchable and distinguishable.     

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 .     

Spectroscopic Studies on the Simple Rcooh System

The carbonyl stretching vibration of monocarboxylic acid in CCl₄ solution has been investigated. We introduce a new “m-factor” to study the polymerization of this simple RCOOH system. The value of “m-factor” was evaluated and tested in different concentrations and temperatures. Three classes of acid polymers were found as followings: linear dimer in class I (m?2); linear and cyclic dimers in class II (m?4); linear, cyclic dimers, and linear trimer in class III (m?8). A linear relationship between log K₁ and (??1)/(2?+1) was found.     

The electronic structure of the HCN dimer and trimer

The electronic structure and energy of dimerization and trimerization of HCN are computed with an STO-3G basis and the results found to be in good agreement with the experimental E. Unlike CNDO/2, this small ab initio basis predicts the correct geometry for the dimer of hydrogen cyanide. The charge redistribution effects found in this H-bond involving a C-H proton donor and sp hybridized acceptor are similar to those found in previous H-bonded studies.     

Conformational analysis and electronic structure calculations of S-1,2-bis(2-dibutenoyl)glycerol and S-1,2-bis(2,4-hexadienoyl)glycerol,two analogues of 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine

As a model for 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine, a doubly unsaturated membrane-forming lipid molecule, force-field (MMP2) calculations were performed on S-1,2-bis(2-dibutenoyl)glycerol, and CNDO/S-calculations 2

1 CNDO: Complete neglect of differential overlap, a semi-empirical quantum-mechanical method.

on the derived minimum-energy conformations of S-1,2-bis(2,4-hexadienoyl)glycerol. The energy hypersurface especially with respect to the dihedral angles along the C(1)-C(2) and the two C O ester bonds was explored and the rotational strength as a function of these angles was calculated. The two gauche-forms were found to be most stable, with a slight preference for the g⁻-form. The experimental circular dichroism data obtained for 1,2-bis(2,4-hexadienoyl)-sn-glycero-3-phosphorylcholine, the corresponding phosphorylcholine, indicate a dynamic equilibrium between two forms of opposite chirality possibly involving the g⁺-and the g⁻-forms.     

Molecular orbital studies of hydrogen bonds

The ground state, the lowest singlet and triplet n-* states, and the lowest triplet -* state of the formic acid monomer and dimer are studied with the ab initio molecular orbital theory. The two-configuration electron-hole potential method is used for calculations of excited states of dimers. The potential energy curves for the symmetrical simultaneous movement of two bridging protons are studied for all of the states. The barrier of the proton transfer in the ground state is found to be the smallest of the states studied. The association energy is analyzed in terms of various components.A preliminary account has been presented at the First International Congress of Quantum Chemistry, Menton, France, 1973, and has appeared in Ref. [3].     

All-valence MO study of hydrogen bonding in water

Theoretical hydrogen bond energies and proton barriers for water dimer and trimer calculated by semiempirical all-valence MO methods have been compared. The results of CNDO/2 and INDO calculations are more adequate than those obtained by the MINDO/1 approach.     

Theoretical Study on Intermolecular Interactions and Thermodynamic Properties of Dimethylnitroamine Clusters

Ab initio SCF and Mφller-Plesset correlation correction methods in combination with counterpose procedure for BSSE correction have been applied to the theroetical studying of dimethylnitroamine and its dimers and trimers.Three optimized stable dimers and two trimers have been obtained.The corrected binding energies of the most stable dimer and trimer were predicted to be -24.68kJ/mol and -47.27kJ/mol,respectively at the MP2/6-31G^*//HF/6-31G^* level.The proportion of correlated interation energies to their total interaction energies for all clusters was at least 29.3 percent,and the BSSE of ΔE(MP2) was at least 10.0kJ/mol.Dispersion and/or electrostatic force were dominant in all clusters.There exist cooperative effects in both the chain and the cyclic trimers.The vibrational frequencies associated with N-O stretches or wags exhibit slight red shifts,but the modes associated with the motion of hydrogen atoms of the methyl group show somewhat blue shifts with respect to those of monomer.Thermodynamic properties of dimethylnitroamine and its clusters at different temperatures have been calculated on the basis of vibrational analyses.The changes of the Gibbs free energies for the aggregation from monomer to the most stable dimer and trimer were predicted to be 14.37kJ/mol and 30.40kJ/mol,respectively,at 1 atm and 298.15K.     

Fermi resonance and strong anharmonic effects in the absorption spectra of the ν-OH (ν-OD) vibration of solid H- and D-benzoic acid

The vibrational spectra of polycrystalline benzoic acid (BA) and its deuterated derivative were studied over the wide frequency region 4000–10 cm⁻¹ by IR and Raman methods. A theoretical analysis of the hydrogen bond frequency region and calculations at the B3LYP/6-311++G(2d, 2p) level for the benzoic acid cyclic dimer in the gas phase were made. In order to study the dynamics of proton transfer two formalisms were applied: Car–Parrinello Molecular Dynamics (CPMD) and Path Integrals Molecular Dynamics (PIMD). It was shown that the experimentally observed very broad ν-OH band absorption is the result of complex anharmonic interaction: Fermi resonance between the OH-stretching and bending vibrations and strong interaction of the ν-OH stretching with the low frequency phonons. The theoretical analysis in the framework of such an approach gave a good correlation with experiment. From the CPMD calculations it was confirmed that the O–HO bridge is not rigid, with the OO distance being described by a large amplitude motion. For the benzoic acid dimer we observed stepwise (asynchronous) proton transfer.     

The role of intra- and intermolecular hydrogen bonds in the formation of β-cyclodextrin head-to-head and head-to-tail dimers. The results of ab initio and semiempirical quantum-chemical calculations

The conformation of a free -cyclodextrin molecule optimized by the MNDO/PM3 quantum-chemical calculations has C₇ symmetry. The right orientation of the interglucose hydrogen bonds in -cyclodextrin, in which the 2-OH groups act as the proton donors and the O atoms of the nearby 3"-OH groups function as the proton acceptors, is advantageous for thermodynamic reasons. The ring of seven H bonds thus formed stabilizes the symmetrical form of -cyclodextrin. The -cyclodextrin head-to-head dimer has D ₇ symmetry and consists of molecules whose 2-OH groups partcipate as proton donors in the formation of fourteen complementary intermolecular hydrogen bonds. The energy of H bonds in the -cyclodextrin monomer and dimer was estimated to be 1.0--1.4 kcal mol^–1. Of the two possible -cyclodextrin dimers, the head-to-tail dimer is more thermodynamically stable. The thermodynamic preference of the right orientation of the inter-glucose H bonds in -cyclodextrin was confirmed by the MP2/6-31G(d,p)//6-31G(d,p) ab initio calculations for maltose (-glucodioside). The maltose molecule with inter-glucose H bonds of the type 2-OHO(3")-H is more stable than the structure with the H-(2)OH-O(3") orientation of H bonds with a difference of 2.7 kcal mol^–1. According to the MNDO/PM3 method, the maltose structure with the right H bond orientation is more stable by 3.1 kcal mol^–1.     

Molecular orbital theory of the hydrogen bond. XXX. Water-cytosine complexes

Ab initio SCF and SCF -CI calculations with the STO -3G basis set have been performed to investigate the structures and energies of water–cytosine complexes and the intermolecular water–cytosine surface in the cytosine molecular plane. Although there are six nominal hydrogen-bonding sites in this plane, only three dimers are distinguishable in the ground state. The most stable has an energy of ?10.7 kcal/mol, and is found in the N₁? H and O₂ region. An asymmetric cyclic structure in which the water molecule bridges adjacent N₁? H and O₂ sites is the preferred form of this dimer. The dimer in the region between O₂ and N₄? H′ of the amino group is slightly less stable at ?10.4 kcal/mol, and also has an asymmetric cyclic structure as the preferred structure, with the water molecule bridging amino N₄? H′ and N₃ hydrogen-bonding sites. The third dimer has the amino group as the proton donor to water through the hydrogen cis to C₅, and a stabilization energy of ?7.0 kcal/mol. The water-cytosine surface is characterized by deeper minima and higher barriers than the water-thymine surface and by a decreased mobility of the water molecule between adjacent hydrogen-bonding sites. Absorption of energy by the C₂?O group leads to the first n → π* excited state in which interactions of water with O₂ are broken. The water-cytosine dimers remain bound in this state, but may change structurally. In the second n → π* state interactions between water and N₃ are no longer stabilizing. As a result, the dimer in the O₂ and N₄? H′ region collapses to either a dimer with water the proton donor to O₂, or one with N₄? H′ the proton donor to water. The other two dimers remain bound. All excited dimers are destabilized on vertical excitation relative to the ground state.     

Macrocyclised pheynyl cinnamate dimer utilisable as photoresponsive chiral dopant for nematic liquid crystals

A macrocyclised phenyl cinnamate dimer with a chiral spacer was prepared, and its photochemistry was compared with that of a precursor linear dimer and corresponding monomeric compounds. Although ultraviolet irradiation of the monomer resulted merely in cis–trans isomerisation of a cinnamate, irradiation of the cyclic and linear dimers induced intra-molecular [2 + 2] photodimerisation of cinnamate groups. Photodimerisation in the cyclic dimer proceeded 20 times faster than in the linear dimer. In a nematic liquid crystal, the cyclic dimer exhibited a high helical twisting power of 27.5 μm^?1, which decreased to 6.7 μm^?1 with dimerisation. A macrocyclised dimer such as this can be used as a photoresponsive chiral dopant for nematic liquid crystals.