Theoretical study on the bromomethane-water 1:2 complexes

Bromomethane-water 1:2 complexes have been theoretically studied to reveal the role of hydrogen bond and halogen bond in the formation of different aggregations. Four stable structures exist on the potential energy surface of the CH3Br(H2O)2 complex. The bromine atom acts mainly as proton acceptor in the four studied structures. It is also capable of participating in the formation of the halogen bond. The properties and characteristics of the hydrogen bond and the halogen bond are investigated employing several different quantum chemical analysis methods. Cooperative effects for the pure hydrogen bonds or the mixed hydrogen bonds with halogen bonds and the possibility of describing cooperative effects in terms of the topological analysis of the electronic density or the charge-transfer stabilization energy are discussed in detail. An atoms-in-molecules study of the hydrogen bond or the halogen bond in the bromomethane-water 1:2 complexes suggests that the electronic density topology of the hydrogen bond or the halogen bond is insensitive to the cooperative effect. The charge-transfer stabilization energy is proportional to the cooperative effect, which indicates the donor-acceptor electron density transfer to be mainly responsible for the trimer nonadditive effect.     

Unusual supramolecular donor—acceptor complexes of bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs

The results of the comprehensive study of novel supramolecular donor-acceptor complexes of bis(crown)stilbenes and bis(crown)azobenzene with viologen analogs are generalized. The original methodology for self-assembling of the organic donor-acceptor complexes possessing a very high thermodynamic stability is described. The hydrogen bonds between the peripheral fragments of the donor and acceptor play the key role in the self-assembling. The influence of different structural factors on the thermodynamic stability of the supramolecular donor-acceptor complexes and the efficiency of charge-transfer interactions between the donor and acceptor are discussed. The driving forces of the reaction leading to exotic trimolecular charge-transfer complexes are considered. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 779–787, April, 2008.     

Highly unusual effects of pi-conjugation extension on the molecular linear and quadratic nonlinear optical properties of ruthenium(II) ammine complexes

We have used several techniques, including hyper-Rayleigh scattering and Stark spectroscopy, to investigate the effects of polyene chain length on the optical properties of complexes containing ruthenium(II) electron donor groups and pyridinium electron acceptors. In marked contrast with all other known donor-acceptor polyenes, conjugation extension beyond a single double bond in the dipolar complexes studied leads to blue-shifting of the intramolecular charge-transfer absorptions. Furthermore, the static first hyperpolarizabilities beta0 become maximized with trans-1,3-butadienyl linkages and then decrease in complexes with three CH=CH bonds. Our results clearly demonstrate that the molecular engineering criteria for metal-containing nonlinear optical chromophores can differ dramatically from those for purely organic compounds.     

Spectroscopic studies on charge-transfer complexes formed in the reaction of ferric(III) acetylacetonate with sigma- and pi-acceptors

The reaction of ferric(III) acetylacetonate (donor), Fe(acac)3, with iodine as a sigma-acceptor and with other different pi-acceptors have been studied spectrophotometrically at room temperature in chloroform. The pi-acceptors used in this investigation are 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), p-chloranil and 7,7',8,8'-tetracyanoquinodimethane (TCNQ). The results indicate the formation of 1:1 charge-transfer complexes with a general formula, [Fe(acac)3 (acceptors)]. The iodine complex was shown to contain the triiodide species, [Fe(acac)3]2I(+)I3-, based on the electronic absorptions as well as on the Far-infrared absorption bands characteristic for the non-linear triiodide species, I3-, with C2v symmetry. The proposed structure of this complex is further supported by thermal and middle infrared measurements.     

Effects of methyl substituent on the charge-transfer complexations of dicarbazolylalkanes with p-chloranil, tetracyanoethylene and tetracyanoquinodimethane

Series of 1,n-dicarbazolylalkanes and 1,n-di(3-methylcarbazolyl)alkanes (where n=1-5) were synthesized and the molar extinction coefficients, equilibrium constants, enthalpies, and entropies of their charge-transfer (CT) complexes with the π-acceptors p-chloranil, tetracyanoethylene, and tetracyanoquinodimethane were investigated. 1,n-Di(3-methylcarbazolyl)alkanes formed CT complexes with higher equilibrium constants, more negative enthalpies and entropies than 1,n-dicarbazolylalkanes. Vibrational spectra of CT complexes of one of the donor molecules (1,4-dicarbazolylbutane) with all three acceptors were compared.     

Quantum-chemical study of interaction of dipyrrolylmethenes with boron trifluoride and other lewis acids

The geometric parameters and energies of the products of donor-acceptor interaction of dipyrrolylmethenes with BF₃ and other inorganic Lewis acids were calculated by quantum-chemical methods. The bond nature and the energies of formation of the donor-acceptor complexes under consideration were analyzed. It was shown that the complexes with p-element fluorides are noticeably stabilized by hydrogen bonds involving the hydrogen atom of the NH group of dipyrrolylmethene and the nearest fluorine atom of a Lewis acid. Hydrogen bonding promotes further elimination of HF in the synthesis of boron fluoride complexes of dipyrrolylmethenes. The energy profile was calculated for the reaction of formation of the boron fluoride complex with dipyrrolylmethene through the intermediate donor-acceptor complex.     

Direct detection by atomic force microscopy of single bond forces associated with the rupture of discrete charge-transfer complexes

Atomic force microscopy (AFM) was used to measure the chemical binding force of discrete electron donor-acceptor complexes formed at the interface between proximal self-assembled monolayers (SAMs). Derivatives of the well-known electron donor N,N,N',N'-tetramethylphenylenediamine (TMPD) and the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) were immobilized on Au-coated AFM tips and substrates by formation of SAMs of N,N,N'-trimethyl-N'-(10-thiodecyl)-1,4-phenylenediamine (I) and bis(10-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl) disulfide (II), respectively. Pull-off forces between modified tips and substrates were measured under CHCl(3) solvent. The mean pull-off forces associated with TMPD/TCNQ microcontacts were more than an order of magnitude larger than the pull-off forces for TMPD/TMPD and TCNQ/TCNQ microcontacts, consistent with the presence of specific charge-transfer interactions between proximal TMPD donors and TCNQ acceptors. Furthermore, histograms of pull-off forces for TMPD/TCNQ contacts displayed 70 +/- 15 pN periodicity, assigned to the rupture of individual TMPD-TCNQ donor-acceptor (charge-transfer) complexes. Both the mean pull-off force and the 70 pN force quantum compare favorably with a contact mechanics model that incorporates the effects of discrete chemical bonds, solvent surface tensions, and random contact area variations in consecutive pull-offs. From the 70 pN force quantum, we estimate the single bond energy to be approximately 4-5 kJ/mol, in reasonable agreement with thermodynamic data. These experiments establish that binding forces due to discrete chemical bonds can be detected directly in AFM pull-off measurements employing SAM modified probes and substrates. Because SAMs can be prepared with a wide range of exposed functional groups, pull-off measurements between SAM-coated tips and substrates may provide a general strategy for directly measuring binding forces associated with a variety of simple, discrete chemical bonds, e.g., single hydrogen bonds.     

Ab initio study of ternary complexes X:(HCNH)(+):Z with X, Z = NCH, CNH, FH, ClH, and FCl: diminutive cooperative effects on structures, binding energies, and spin-spin coupling constants across hydrogen bonds

Ab initio calculations have been performed on a series of complexes in which (HCNH)(+) is the proton donor and CNH, NCH, FH, ClH, and FCl (molecules X and Z) are the proton acceptors in binary complexes X:HCNH(+) and HCNH(+):Z, and ternary complexes X:HCNH(+):Z. These complexes are stabilized by C-H(+)···A and N-H(+)···A hydrogen bonds, where A is the electron-pair donor atom of molecules X and Z. Binding energies of the ternary complexes are less than the sum of the binding energies of the corresponding binary complexes. In general, as the binding energy of the binary complex increases, the diminutive cooperative effect increases. The structures of these complexes, data from the AIM analyses, and coupling constants (1)J(N-H), (1h)J(H-A), and (2h)J(N-A) for the N-H(+)···A hydrogen bonds, and (1)J(C-H), (1h)J(H-A), and (2h)J(C-A) for the C-H(+)···A hydrogen bonds provide convincing evidence of diminutive cooperative effects in these ternary complexes. In particular, the symmetric N···H(+)···N hydrogen bond in HCNH(+):NCH looses proton-shared character in the ternary complexes X:HCNH(+):NCH, while the proton-shared character of the C···H(+)···C hydrogen bond in HNC:HCNH(+) decreases in the ternary complexes HNC:HCNH(+):Z and eventually becomes a traditional hydrogen bond as the strength of the HCNH(+)···Z interaction increases.     

Reactivity of propargylamines in complexation and polymerization reactions

Factors were determined which promote the formation of charge-transfer complexes of propargylamines with organic electron acceptors and a change of the activity of the triple bond in propargylamines under the effect of donor-acceptor interactions. On the basis of data from PMR spectroscopy and quantum-chemical calculations it was shown that the tendency of propargylamines to form charge-transfer complexes depends on the electron-donor properties of the substituents at the nitrogen atom. The values of the effective charges at the atoms and an analysis of the structure of the highest occupied molecular orbitals in the propargylamines and their radical cations indicate that mainly electrons localized on nitrogen atoms participate in the complexation reaction. An increase of the reactivity of the triple bond is related to an increase in its polarity under the effect of complexation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 597–601, September–October, 1989.     

Charge-transfer complexes based on indole compounds (review)

The literature data on the donor-acceptor interaction of indoles with various types of electron acceptors used for the explanation of the molecular mechanisms of biochemical processes are examined. The results of research by the authors on the synthesis of polymeric charge-transfer complexes based on 1-vinylindole, halogens, hydrogen halides, alkyl halides, halohydrins, chloranil, and organic derivatives of silicon and tin are correlated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1155–1163, September, 1977.     

Solute-solvent complex switching dynamics of chloroform between acetone and dimethylsulfoxide-two-dimensional IR chemical exchange spectroscopy

Hydrogen bonds formed between C-H and various hydrogen bond acceptors play important roles in the structure of proteins and organic crystals, and the mechanisms of C-H bond cleavage reactions. Chloroform, a C-H hydrogen bond donor, can form weak hydrogen-bonded complexes with acetone and with dimethylsulfoxide (DMSO). When chloroform is dissolved in a mixed solvent consisting of acetone and DMSO, both types of hydrogen-bonded complexes exist. The two complexes, chloroform-acetone and chloroform-DMSO, are in equilibrium, and they rapidly interconvert by chloroform exchanging hydrogen bond acceptors. This fast hydrogen bond acceptor substitution reaction is probed using ultrafast two-dimensional infrared (2D-IR) vibrational echo chemical exchange spectroscopy. Deuterated chloroform is used in the experiments, and the 2D-IR spectrum of the C-D stretching mode is measured. The chemical exchange of the chloroform hydrogen bonding partners is tracked by observing the time-dependent growth of off-diagonal peaks in the 2D-IR spectra. The measured substitution rate is 1/30 ps for an acetone molecule to replace a DMSO molecule in a chloroform-DMSO complex and 1/45 ps for a DMSO molecule to replace an acetone molecule in a chloroform-acetone complex. Free chloroform exists in the mixed solvent, and it acts as a reactive intermediate in the substitution reaction, analogous to a SN1 type reaction. From the measured rates and the equilibrium concentrations of acetone and DMSO, the dissociation rates for the chloroform-DMSO and chloroform-acetone complexes are found to be 1/24 ps and 1/5.5 ps, respectively. The difference between the measured rate for the complete substitution reaction and the rate for complex dissociation corresponds to the diffusion limited rate. The estimated diffusion limited rate agrees well with the result from a Smoluchowski treatment of diffusive reactions.     

In situ synthesis, photometric and spectroscopic studies of chelating system during the 1,4,7,10,13,16-hexaoxacyclooctadecane charge transfer reaction with different acceptors

Electron donor acceptor complexes (EDA) of the 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) as a rich donor were spectrophotometrically discussed and synthesized in solid form according the interactions with different nine of usual π-acceptors like 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione (p-chloranil; p-CHL), tetrachloro-1,2-benzoquinone (o-chloranil; o-CHL), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoquinodimethane (TCNQ), 2,6-dichloroquinone-4-chloroimide (DCQ), 2,6-dibromoquinone-4-chloroimide (DBQ), 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid; CLA), N-bromosuccinimide (NBS), 2,4,6-trinitrophenol (picric acid; PA). Spectroscopic and physical data such as formation constant (K(CT)), molar extinction coefficient (?(CT)), standard free energy (ΔG°), oscillator strength (f), transition dipole moment (μ), resonance energy (R(N)) and ionization potential (I(p)) were estimated in chloroform or methanol at 25°C. Based on the elemental analysis and photometric titrations the CT-complexes were formed indicated the formation of 1:1 charge-transfer complexes for the o-CHL, TCNQ, DCQ, DBQ and NBS acceptors but 1:3 ratio for p-CHL, DDQ, CLA and PA, respectively. The charge-transfer interactions were interpretative according to the formation of dative ion pairs [18C6(?+), A(?-)], where A is acceptor. All of the resulting charge transfer complexes were isolated in amorphous form and the complexes formations on IR and (1)H NMR spectra were discussed.     

Water complexes of styrene and 4-fluorostyrene: a combined electronic, vibrational spectroscopic and ab-initio investigation

The binary complexes of water with styrene and fluorostyrene were investigated using LIF and FDIR spectroscopic techniques. The difference in the shifts of S 1 <-- S 0 electronic transitions clearly points out the disparity in the intermolecular structures of these two binary complexes. The FDIR spectra in the O-H stretching region indicate that water is a hydrogen bond donor in both complexes. The formation of a single O-H...pi hydrogen-bonded complex with styrene and an in-plane complex with fluorostyrene was inferred based on the analysis of the FDIR spectra in combination with ab initio calculations. The in-plane complex with fluorostyrene is characterized by the presence of O-H...F and C-H...O hydrogen bonds, leading to formation of a stable six-membered ring. The synergistic effect of O-H...F and C-H...O hydrogen bonds overwhelms the O-H...pi interaction in fluorostyrene-water complexes.     

Spectroscopic studies on the electron donor—acceptor interaction between 1-propanamine-3-triethoxysilyl and π-acceptors

The spectrophotometric properties of the title interaction have been studied in methylene chloride. 2:1 charge-transfer (CT) complexes were formed between the acceptors and the donor, except with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) a 4:1 complex was formed between the acceptor and the donor. The UV—vis. and IR spectral data for the CT complexes and the solvent effect on the electronic transition of the CT have been presented and discussed. The ionization potential of the donor was determined.     

A theoretical study of the hydrogen bond donor capability and co-operative effects in the hydrogen bond complexes of the diaza-aromatic betacarbolines

In this work, we present a quantum mechanical investigation on the hydrogen bond interactions of N(9)-methyl-9H-pyrido[3,4-b]indole, MBC, and N(2)-methyl-9H-pyrido[3,4-b]indole, BCA, with different hydrogen bond donors. Thus, it has been analysed the influence that the hydrogen bond donor strength and the co-operative effect of the increasing number of donor molecules have on the shape of the potential energy surfaces versus the N···H distances, r(N–H). To rationalize the nature of the interactions, the Bader theory has been applied and the characteristics of the bond critical points analysed. The results show that two different hydrogen bond complexes can be formed depending on the donor capabilities or the number of donor molecules included in the calculations. The topological parameters from the Bader theory are used to justify the statement that the analysed interactions can be classified as weak or partially covalent hydrogen bond interactions, respectively. As experimentally observed, weak hydrogen bond donors form weak hydrogen bond complexes, called HBC. Upon the increase of the donor strength the N···H proton is shifted nearest to the nitrogen atom giving rise to the observation of a stronger hydrogen bond complex, the proton transfer complex, PTC. The most outstanding result of these studies is the fact that the formation of the PTC can also be managed just by changing the number of donor molecules, that is, by a co-operative effect of the hydrogen bonds.     

Microscopic Models for Proton Transfer in Water and Strongly Hydrogen-Bonded Complexes with a Single-Well Proton Potential

A new mechanism and formalism for proton transfer in donor-acceptor complexes with long hydrogen bonds introduced recently [1], is applied to a proton transfer in liquid water. Structural diffusion of hydroxonium ions is regarded as totally adiabatic process, with synchronous hindered translation of two closest water molecules to and from the reaction complex as crucial steps. The water molecules induce a gated shift of the proton from the donor to the acceptor in the double-well potential with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. The short-range and long-range proton transfer as structural diffusion of Zundel complexes is also considered. The theoretical formalism is illustrated with the use of Morse, exponential, and harmonic molecular potentials. This approach is extended to proton transfer in strongly hydrogen-bonded donor-acceptor complexes. In contrast to the above model [1], the short hydrogen bond between the donor and acceptor moieties, however, completely erodes the barrier along the proton transfer mode. This introduces some physical pattern differences from proton transfer reactions in truly double-well potentials with a finite proton transfer barrier at the transition configuration with respect to the environmental nuclear coordinates. The differences apply particularly to the origin of the kinetic isotope effect. We discuss explicitly details of the excess proton conductivity in aqueous solution, but the concepts and formalism apply broadly to acid-base reactions, proton conduction channels, and other strongly hydrogen-bonded O- and N-proton donor-acceptor systems.     

Preparation and spectroscopic studies on charge-transfer complexes of 2,2'-bipyridine with picric and chloranilic acids

Charge-transfer (CT) complexes formed on the reaction of 2,2'-bipyridine with some acceptors such as picric acid (HPA) and chloranilic acid (H(2)CA) have been studied in CHCl(3) and MeOH at room temperature. Based on elemental analysis and IR spectra of the solid CT complexes along with the photometric titration curves for the reactions, the data obtained indicate the formation of 1:1 charge-transfer complexes [(bpyH)(PA)] and [(bpyH(2))(CA)], respectively. The infrared and (1)H NMR spectroscopic data indicate a charge-transfer interaction associated with a proton migration from the acceptor to the donor followed by intramolecular hydrogen bonding. The formation constants (K(C)) for the complexes were shown to be dependent on the structure of the electron acceptors used.     

Mechanistic appraisal of the charge-transfer complexes of promethazine with chloranil: a modelling approach.

Various mechanisms are often used to explain the interaction between electron donors and acceptors. Commonly proposed mechanisms are those in which the acceptor interacts with the aromatic pi-systems in the donor molecule or the acceptor forms a weak interaction of the Lewis acid with Lewis base type. In this study, the above mechanisms were examined as well as other possible mechanisms. Promethazine was chosen as the model drug containing aromatic systems capable of pi-pi interaction as well as N-methyl group capable of forming a complex with the weak Lewis acid, p-chloranil. Our modelling studies revealed that the situation where the p-chloranil interacts with a protonated N-methyl group is the most significant mechanism of interaction, based on the calculated energies for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the Tripos force field energy terms and also the stability of the complexes during molecular dynamics simulations.     

Frequency analysis of amide-linked rotaxane mimetics

A vibrational analysis of 2-fold hydrogen bonds between an isophthalic amide donor and different acceptors is presented. These systems can be considered as mimetics for the hydrogen-binding situation of numerous supramolecular compounds such as rotaxanes, catenanes, knotanes, and anion receptors. We calculated pronounced red-shifts up to 65 cm(-1) for the stretching modes of the acceptor carbonyl as well as for the donor NH2 groups, whereas we observe a blue shift for the NH2 bending modes and an additional weak hydrogen bond between the acceptor and the middle C-H group of the donor. The red and blue shifts observed for different modes in various complexes have been correlated with the binding energy of the complexes, independently. In comparison with comparable single hydrogen bonds, we find for the 2-fold hydrogen bonds smaller red shifts for the N-H stretch modes of the donor but larger red shifts for the C=O stretch mode of the acceptor. Furthermore, our results indicate that the pronounced blue shift of the C-H stretch mode is basically caused by the fact that the acceptor is fixed directly above this group due to the 2-fold hydrogen bond.     

Tetraaminoethylenes as Strong Electron Donors

Depending on the nature of the electron acceptor and of the tetraaminoethylene (TAE), the electron pairs of the ethylenic double bond (A), of the nitrogen (B), or of the entire electronic system (C) of TAE react with the acceptor. Thus oxidizing agents convert TAE into the mono-and dications TAE⁺ and TAE²⁺ by path (A); acids add on to the ethylenic carbon (A) or the amine nitrogen (B) (the addition of acid is often followed by cleavage of the C?C or C? N bond); organic π-electron acceptors from deeply colored donor-acceptor complexes with TAE (C).