Weak Molecular Complexes Research Articles

A Molecular Electron Density Theory Study of the Domino Reaction of N-Phenyl Iminoboranes with Benzaldehyde Yielding Fused Bicyclic Compounds.

The reaction of N-phenyl iminoborane with benzaldehyde yielding a fused aromatic compound, recently reported by Liu et al., has been studied within the Molecular Electron Density Theory (MEDT). Formation of the fused aromatic compound is a domino process that comprises three consecutive reactions: (i) formation of a weak molecular complex between the reagents; (ii) an intramolecular electrophilic attack of the activated carbonyl carbon of benzaldehyde on the ortho position of the N-phenyl substituent of iminoborane; and (iii) a formal 1,3-hydrogen shift yielding the final fused aromatic compound. The two last steps correspond to a Friedel-Crafts acylation reaction, the product of the second reaction being the tetrahedral intermediate of an electrophilic aromatic substitution reaction. However, the presence of the imino group adjacent to the aromatic ring strongly stabilizes the corresponding intermediate, being the reaction product when the ortho positions are occupied by t-butyl substituents. This domino reaction shows a great similitude with the Brønsted acid catalyzed Povarov reaction. Although N-phenyl iminoborane can experience a formal [2+2] cycloaddition reaction with benzaldehyde, its higher activation Gibbs free energy compared to the intramolecular electrophilic attack of the activated carbonyl carbon of benzaldehyde on the ortho position of the N-phenyl substituent, 6.6 kcal·mol-1, prevents the formation of the formal [2+2] cycloadduct. The present MEDT study provides a different vision of the molecular mechanism of these reactions based on the electron density.

Induction of chirality in the charge transfer complex photochemistry

Electron donor and acceptor undergo association in solution to form a weak molecular complex, namely a charge transfer complex. The formation of the complex is apparent by its new low-energy absorption band. Upon photoexcitation of this charge transfer band, an intracomplex electron transfer is facilitated to form a contact ion radical pair. The theoretical basis for the complex formation and the subsequent photoinduced electron transfer process has been well established already half a century ago. Nevertheless, the charge transfer complex photochemistry is now in a renaissance, due to its synthetic utility of its radical ion pair. By intentionally sidestepped unproductive back electron transfer process, a desired product is effectively obtained with or without external reagent and/or catalyst. In addition, the photoexcitation of the complex does not require any additional photocatalyst and often conceivable with the visible light. Accordingly, various donor/acceptor pairs and synthetic applications have been emerged as novel photoreaction systems in the last two decades. Some have also addressed the stereoselective transformations. This mini review highlights the recent progress of the asymmetric photoreaction, in particular in synthetic applications, based on the photoexcitation of the charge transfer complex, in comparison with photoredox catalysis.

FTIR matrix isolation studies of thermal decomposition of 1,2,4-triazolyl-3-carboxylic acid

The title compound 1,2,4-triazolyl-3-carboxylic acid (TC) belongs to the rarely studied N–H azoles bearing a carboxylic group attached directly to the ring. Thermal decomposition of TC was studied in detail both experimentally using FTIR matrix isolation technique and theoretically at the B3LYPD3 and B3LYP levels with 6–311++G(2d,2p) basis set. Both concerted and stepwise pathways were considered theoretically for unimolecular decarboxylation of TC. Experimentally, in addition to the 1,2,4-triazole (TR) and carbon dioxide, two weak molecular complexes formed between TR and CO2 were detected in the studied matrices. Based on a comparison of the predicted and observed wavenumber shifts, the structures of the complexes have been confirmed.

Atmospheric Reactivity of CH2ICl with OH Radicals: High-Level OVOS CCSD(T) Calculations for the X-Abstraction Pathways (X = H, Cl, or I)

Reactants, weak molecular complexes, transition states, and products for the H-, Cl-, and I-abstraction channels in the reaction of OH radicals with chloroiodomethane CH(2)ICl as well as the energy profiles at 0 K have been determined using high-level all-electron ab initio methods. The results showed that all-electron DK-CCSD(T)/ANO-RCC approach performed very well in predicting the reactivity of iodine. In terms of activation enthalpy at 0 K, the energy profile for the Cl-abstraction showed that this reaction pathway is not energetically favorable in contrast to the two other channels (H- and I-abstractions), which are competitive. The H-abstraction was strongly exothermic (-87 kJ mol(-1)), while the I-abstraction was modestly endothermic (11.8 kJ mol(-1)). On the basis of our calculations including the following corrections to the potential energies: basis set saturation, valence and core-valence electron correlation, relativistic effects, spin-adaptation, vibration contributions, and tunneling corrections, rate constants were predicted using canonical transition state theory over the temperature range 250-500 K for each abstraction pathway. The overall rate constant at 298 K was estimated to be 4.29 × 10(-14) and 5.44 × 10(-14) cm(3) molecule(-1) s(-1) for complex and direct abstraction mechanisms, respectively. In addition, the overall rate constant computed at 277 K was used in the estimation of the atmospheric lifetime for CH(2)ICl. On the basis of our theoretical calculations, the atmospheric lifetime for the OH removal process is predicted to be close to 1 year. In terms of atmospheric lifetime, the OH reaction is not competitive with the Cl reaction and photolysis processes.

Atmospheric reactivity of CH 3I and CH 2I 2 with OH radicals: A comparative study of the H- versus I-abstraction

Reactants, weak molecular complexes, transition states, and products for the H- and I-abstraction channels in the reactions OH + CH 3I and OH + CH 2I 2 as well as the energy profiles at 0 and 298 K have been determined using high-level all-electron ab initio methods. The results showed that all-electron DK-CCSD(T)/ANO-RCC approach performed very well in predicting the reactivity of iodine. The energy profiles for the I-abstraction, in contrast to the H-abstractions, had the position of the transition states below the products. The H-abstractions were strongly exothermic (−80 to −90 kJ mol −1), while the I-abstractions were modestly endothermic (20–40 kJ mol −1). Based on our calculations with the ANO-RCC-L(I) basis set, we recommend the following values for the heats of formation at 298 K (in kJ mol −1): 218.6 (CH 2I), 106.0 (CH 2I 2), and 298.3 (CHI 2).

Theoretical study on the reaction mechanism of CH2SH + NO2

The mechanisms of CH2SH with NO2 reaction were investigated on the singlet and triplet potential energy surfaces (PES) at the BMC-CCSD//B3LYP/6-311 + G(d,p) level. The result shows that the title reaction is more favourable on the singlet PES thermodynamically, and it is less competitive on the triplet PES. On the singlet PES, the initial addition of CH2SH with NO2 leads to HSCH2NO2 (IM2) without any transition state, followed by a concerted step involving C–N fission and shift of H atom from S to O giving out CH2S + trans-HONO, which is the major products of the title reaction. With higher barrier height, the minor products are CH2S + HNO2, formed by a similar concerted step from the initial adduct HSCH2ONO (IM1). The direct abstraction route of H atom in SH group abstracted by O atom might be of some importance. It starts from the addition of the reactants to form a weak interaction molecular complex (MC3), subsequently, surmounts a low barrier height leading to another complex (MC2), which gives out CH2S + trans-HONO finally. Other direct hydrogen abstraction channels could be negligible with higher barrier heights and less stable products.

Determination of association constants at moderately fast chemical exchange: Complexation of camphor enantiomers by α-cyclodextrin

Association constants in weak molecular complexes can be determined by analysis of chemical shifts variations resulting from changes of guest to host concentration ratio. In the regime of very fast exchange, i.e., when exchange rate is several orders of magnitude larger than the Larmor angular frequency difference of the observed resonance in free and complexed molecule, the apparent position of averaged resonance is a population-weighted mean of resonances of particular forms involved in the equilibrium. The assumption of very fast exchange is often, however, tacitly admitted in literature even in cases where the process of interest is much slower than required. We show that such an unjustified simplification may, under certain circumstances, lead to significant underestimation of association constant and, in consequence, to non-negligible errors in Gibbs free energy under determination. We present a general method, based on iterative numerical NMR line shape analysis, which allows one for the compensation of chemical exchange effects, and delivers both the correct association constants and the exchange rates. The latter are not delivered by the other mentioned method. Practical application of our algorithm is illustrated by the case of camphor-α-cyclodextrin complexes.

Aerosol Synthesis of Spherical Morphology Boron Nitride Powders from Organoborate Precursors

The reaction of aerosol droplets of the organoborate reagent (MeO)3B, suspended in nitrogen, with gaseous ammonia in a flight tube heated to 800−1500 °C was used to produce, in high yield, spherical morphology precursor powders BNxOyCz that contain decreasing amounts of oxygen and carbon as the process temperature and mole fraction of ammonia increased. Subsequent static nitridation of the precursor powder under ammonia gave spherical morphology BN powders with low (<1 wt %) oxygen and carbon contents. The powders were characterized by elemental analysis, DRIFT-IR, XRD, SEM, and TEM methods. A weak molecular complex (MeO)3B·NH3 has been previously reported to exist, and this species was isolated and structurally characterized by single-crystal X-ray diffraction methods. Aerosol conversions using (MeO)3B/MeOH, H3BO3/MeOH, and B2O3/MeOH solutions were also examined, and the results are briefly described.

Thermodynamics of the effects of substituent, degree of substitution, and pH on complex formation of hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins with ascorbic acid

The interaction of ascorbic acid with hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins of different degree of substitution was studied at 298.15 K and different pH using solution calorimetry. In an aqueous solution, only hydroxypropyl-β-cyclodextrins form weak molecular complexes with the nonionized form of ascorbic acid. The thermodynamic functions of complex formation and stability constants of the complexes were calculated. The systems with weak intermolecular interaction without complex formation were characterized by enthalpic virial coefficients. On the basis of the obtained thermodynamic characteristics it was shown that the selectivity of complex formation of hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins with ascorbic acid is determined by the size of the macrocyclic cavity, the presence of the hydroxypropyl substituent, and the medium acidity. The degree of substitution of hydroxypropyl-β-cyclodextrins exerts no substantial effect on the thermodynamic parameters of interaction with ascorbic acid.

A cryosolution infrared and ab initio study of the van der Waals complexes of cyclopentene with hydrogen chloride and boron trifluoride

The formation of weak molecular complexes of cyclopentene with HCl and BF 3, dissolved in liquid argon is investigated using infrared spectroscopy. Evidence is found for the formation of 1:1 complexes in which the Lewis acid under study binds to the C C double bond. At higher concentrations of HCl, weak absorption bands due to 1:2 species are also observed. From spectra recorded at different temperatures between 92 and 127 K, the complexation enthalpies for CP·HCl and CP·BF 3 are determined to be −9.5(3) and −16.1(9) kJ mol −1, while for CP·(HCl) 2 a value of −17.0(6) kJ mol −1 is obtained. For the 1:1 and 1:2 complexes, structural and spectral information is obtained from ab initio calculations at the MP2/6-31+G(d) level. Using free energy perturbation Monte Carlo simulations to calculate the solvation enthalpies and statistical thermodynamics to account for zero-point vibrational and thermal contributions, the complexation energies for CP·HCl and CP·(HCl) 2 are estimated from the experimental complexation enthalpies to be −17.4(14) and −34.0(20) kJ mol −1, while the value for CP.BF 3 was derived to be −23.4(22) kJ mol −1. The experimental complexation energies are compared with the theoretical values derived from the MP2/6-31+G(d) potential energy surfaces and with single point energies calculated at the MP2/6-311++G(3df,2pd) level.

Pulsed Nozzle Fourier Transform Microwave Spectrometer: Advances and Applications

The pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer was developed by Balle and Flygare [A new method for observing the rotational spectra of weak molecular complexes: KrHCl. J. Chem. Phys. 1979, 71 (6), 2723–2724 and 1980, 72 (2), 922–932] in 1979. The design, fabrication, and operation of this spectrometer are complicated and it has largely remained a research laboratory tool till now, though a portable spectrometer for routine analytical applications has been developed at the National Institute for Standards and Technology [Suenram, R.D.; Grabow, J.‐U.; Zuban, A.; Leonov, I. A portable pulsed‐molecular‐beam Fourier‐transform microwave spectrometer designed for chemical analysis. Rev. Sci. Instrum. 1999, 70 (4), 2127–2135]. However, the potential for extracting fundamental information about any chemical species, such as, molecules, radicals, ions, or weakly bound complexes between any of them including atoms, has been quite significant. It is evident from the fact that more than 25 laboratories around the globe have built this spectrometer, some in the recent past. Contributions from all these laboratories have widened the horizon of PNFTMW spectrometer's applications. This review summarizes the advances in design and the recent applications of this spectrometer. We also define an electrophore, as an atom/molecule that generates an electric dipole moment by forming a weakly bound complex with a species having zero electric dipole moment. The electrophore, thereby, enables structural determination using rotational spectroscopy, as in the case of Ar2–Ne, with Ne as the electrophore. Also, it can introduce a dipole moment about a principal axis where none existed before, such as in Ar–(H2O)2, enabling the observation of pure rotational transitions for several tunneling states.

NMR Studies of Equilibrium Quotient of the Benzonitrile with Xylene Isomers and Ethylbenzene

The charge transfer (CT) complex formation of benzonitrile as acceptor with the aromatic donors o-, m-, p-xylene and ethylbenzene in CCl4 solutions is investigated by chemical shift measurements relative to an external reference. The equilibrium parameters, Q and DAD of their weak molecular complexes, as found numerically modified Cresswel- Allred (C-A) method, is compared with graphically Scatchard-Foster-Fyfe (Sc-F-F) method.

A cryosolution infrared and DFT study of the complexes formed between vinylacetylene and hydrogen chloride

The formation of weak molecular complexes between vinylacetylene (1-buten-3-yne, CH 2CH–CCH) and HCl or DCl dissolved in liquid argon has been investigated using infrared spectroscopy. In all spectra, evidence was found for the formation of a 1:1 van der Waals complex. At higher concentrations of hydrogen chloride, weak absorption bands due to 1:2 species were also observed. Using spectra recorded at different temperatures between 102 and 125 K, the complexation enthalpies for the 1:1 and the 1:2 complex were determined to be −10.2(9) and −16.9(15) kJ mol −1. A density function theory study of the structures and the vibrational frequencies of the complexes is performed at the B3LYP/6-311++G(d,p) level. These calculations show that HCl can form a complex with either the triple or the double carbon–carbon bond, with the former giving stable endo and exo conformers. Comparison of the experimental frequencies with the calculated ones shows that all observed complex bands are due to the endo conformer of the triple bond complex, while those of the 1:2 complex are due to a chain-type complex in which the second HCl is attached to the first one. Using free energy perturbation Monte Carlo simulations to calculate the solvent influences, and statistical thermodynamics to account for zero-point vibrational and thermal contributions the complexation energies for the C 4H 4·HCl and C 4H 4·(HCl) 2 are estimated from the experimental complexation enthalpies to be −17.3(20) and −34.6(32) kJ mol −1. These numbers are compared with the theoretical values obtained at the B3LYP/6-311++G(d,p) level and with single-point energies calculated at the MP2/aug-cc-PVTZ level.

On the HCl and DCl complexes of methylenecyclopropane in liquid argon

The formation of weak molecular complexes between methylenecyclopropane (MeCP) and HCl or DCl dissolved in liquid argon and liquid nitrogen has been investigated using infrared spectroscopy. Evidence was found for the formation of a 1:1 complex in which the HCl molecule binds to the CC double bond. Weaker bands due to the two different 1: 2 complexes derived from the 1:1 complex were observed. From spectra recorded at different temperatures between 90 and 130 K, the complexation enthalpy for the 1:1 complex formed was determined to be −9.9(3) kJ mol−1, while the corresponding value for the most stable 1:2 complex, MeCP·HCl·HCl, was determined to be −14.6(4) kJ mol−1. Structural and spectral information for the 1:1 and the 1:2 complexes was obtained from DFT calculations at the B3LYP/6-311++G(d,p) level. Using free energy perturbation Monte Carlo simulations to calculate the solvent influences, and statistical thermodynamics to account for zero-point vibrational and thermal contributions, the complexation energies for the 1:1 complex and the 1:2 complex were estimated from the experimental complexation enthalpies to be −16.9(12) and −28.7(14) kJ mol−1. These numbers are compared with single-point energies calculated at the MP2/aug-cc-PVTZ level.

Solvent-Induced Frequency Shifts as a Probe for Studying Very Weak Molecular Complexes: Evidence for van der Waals Complex Formation between COF2and N2in Cryogenic Solutions

FTIR (4000−400 cm-1) spectra of COF2 dissolved in liquefied argon (LAr), krypton (LKr), xenon (LXe), nitrogen (LN2), and in LAr/LN2 mixtures have been investigated. In addition, spectra of COF2 and COF2/N2 mixtures isolated in solid argon matrices have been studied. Vibrational frequencies of COF2 dissolved in LN2 exhibit unusual temperature behavior as compared to the solutions in the other solvents. This, and the effects on the vibrational frequencies and IR band shapes of COF2 in mixed nitrogen/argon solvents, is attributed to the formation of a complex COF2·N2 in the solutions. A phenomenological model is proposed for the calculation of the temperature and concentration effects on the vibrational frequencies. By fitting the experimental data, the complexation enthalpy, ΔH° = −1.4(3) kJ mol-1, was determined from this model. The existence of a COF2·N2 complex was confirmed by the matrix isolation spectra. Geometry, energy, and vibrational spectrum of COF2·N2 have been calculated by ab initio at the MP2/6-311++G(2d,2p) level. The ab initio predictions for ΔH° and for the complexation shifts of the vibrational bands agree well with the values determined from the phenomenological model.

Vibrational spectra and relative stability of the van der Waals complexes formed between 1,2-butadiene and HCl: A FTIR and density functional study

The formation of weak molecular complexes between 1,2-butadiene (methylallene, CH 3CHCCH 2) and HCl dissolved in liquid argon has been investigated using infrared spectroscopy. In addition, structural and spectral information on the 1:1 and 1:2 complexes was obtained by carrying out DFT calculations at the B3LYP/6-311++G(d,p) level. The calculations predict three isomeric 1:1 complexes with similar energies. In the spectra, at least two different complexes were identified, while indications for a third 1:1 complex were found. Using spectra recorded at temperatures between 92 and 128 K, the average complexation enthalpy for the 1:1 complexes in liquid argon was determined to be –6.8 (3) kJ mol −1, while for the 1:2 complexes an average value of –17.4 (11) kJ mol −1 was obtained. By correcting for solvation, zero-point vibrational and thermal contributions, for both the 1:1 and the 1:2 complexes, the average complexation enthalpy Δ H° derived in liquid argon was converted into a vapor phase complexation energy Δ E exp. The resulting values, −10.7 (6) kJ mol −1 for the 1:1 complexes and –24.5 (22) kJ mol −1 for the 1:2 complexes, are compared with the ab initio values.

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures III. Binary mixtures of alkoxyphenyl arylates and arylalkoxybenzoates

4-Substituted phenyl-4-hexadecyloxybenzoates and 4-hexadecyloxyphenyl-4-substituted benzoates were prepared and thermally characterized by differential scanning calorimetry. Transition temperatures were identified by polarized-light microscopy. Substituents were chosen to cover a wide range of electronic and dipolar characteristics. Binary mixtures of different components were prepared and similarly characterized to construct their phase diagrams. In mixtures of the 4-CN and 4-NO2 analogues, enhanced SmA mesophases were found when either of these esters was mixed with one of the methoxy analogues. A weak molecular complex is proposed to account for this non-ideal solution behaviour. Linear or slightly enhanced mixture behaviours were found for the nitro or cyano derivatives when mixed with each other. The results are interpreted in terms of electronic, dipolar, and molecular interactions.

Theoretical study of properties of H− and NH2− complexes with neutral ammonia solvent molecules

The experimental results from photoelectron spectroscopy yielding detailed information on interactions of H− and NH−2 with one and two molecules of NH3 are supplemented by theoretical studies. The structure of complex anions involved has been determined and the nature of their interactions has been discussed. Electron affinities, dissociation energies, and basicity of H−(NH3)n and NH−2(NH3)n (n=1,2) anions have been calculated for theoretically determined structures. The excellent agreement between experimental and theoretical results confirms conclusions derived from experimental studies. A recently proposed complete basis set method has been proven to be a useful tool to study weak molecular complexes.

The weak interaction between C60 and molecular iodine in solution

Fullerene C60 forms a weak molecular complex with iodine. Its stability constant is much less (<O.1 L−1 mol−1 than that determined in another study.

Linear and nonlinear dielectric studies of intermolecular interactions in nitroalkane-benzene liquid solutions. II. Molecular cluster model

The energies of intermolecular interactions in benzene solutions of nitroalkanes have been determined from the experimental data on linear and nonlinear polarization. To achieve an agreement between the experimental and theoretical data one had to consider complexes composed of at least four molecules of nitroalkane and the equilibrium between weakly and strongly polar dimers. The determined values of dipole moments of weak molecular complexes enabled one to get information on the mutual orientation of the polar nitro groups forming the complex. The results show that beside the distinctly preferred antiparallel orientation, an almost perpendicular orientation of the polar nitro groups is also significant. Optimization of the dimer geometry performed with several semi-empirical molecular orbital methods from the set of MOPAC programs shows a shallow energy minimum corresponding to such almost perpendicular orientation together with the parallel orientation of the hydrocarbon skeleton and suggests a significant role of the dispersive interactions in nitroalkanes.