Blue-Shifting or Red-Shifting Hydrogen Bonding? Predictions for Haloform Complexes with Dimethyl Ether on the Basis of Perturbation Theory

Unusual blue to red shifting of C-H stretching frequency of CHCl3 in co-operatively P⋯Cl phosphorus bonded POCl3-CHCl3 heterodimers at low temperature inert matrixes

Heterodimers of POCl₃-CHCl₃ were generated in Ne, Ar, and Kr matrixes at low temperatures and were studied using infrared spectroscopy. The remarkable role of co-operative pentavalent phosphorus bonding in the stabilization of the structure dictated by hydrogen bonding is deciphered. The complete potential energy surface of the heterodimer was scanned by ab initio and density functional theory computational methodologies. The hydrogen bond between the phosphoryl oxygen of POCl₃ and C-H group of CHCl₃ in heterodimers induces a blue-shift in the C-H stretching frequency within the Ne matrix. However, in Ar and Kr matrixes, the C-H stretching frequency is exceptionally red-shifted in stark contrast with Ne. The plausibility of the Fermi resonance by the C-H stretching vibrational mode with higher order modes in the heterodimers has been eliminated as a possible cause within Ar and Kr matrixes by isotopic substitution (CDCl₃) experiments. To evaluate the influence of matrixes as a possible cause of red-shift, self-consistent Iso-density polarized continuum reaction field model was applied. This conveyed the important role of the dielectric matrixes in inducing the fascinating vibrational shift from blue (Ne) to red (Ar and Kr) due to the matrix specific transmutation of the POCl₃-CHCl₃ structure. The heterodimer produced in the Ne matrix possesses a cyclic structure stabilized by hydrogen bonding with co-operative phosphorus bonding, while in Ar and Kr the generation of an acyclic open structure stabilized solely by hydrogen bonding is promoted. Compelling justification regarding the dispersion force based influence of matrix environments in addition to the well-known dielectric influence is presented.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

The interplay and strength of the π⋯HF, C⋯HF, F⋯HF and F⋯HC hydrogen bonds upon the formation of multimolecular complexes based on C2H2⋯HF and C2H4⋯HF small dimers

The conception of this theoretical research was idealized aiming to unveil the intermolecular structures of complexes formed by acetylene or ethylene and hydrofluoric acid. At light of computational calculations by using the B3LYP/6-311++G(d,p) method, the geometries of the C₂H₂⋯(HF), C₂H₂⋯2(HF), C₂H₂⋯4(HF), C₂H₄⋯(HF), C₂H₄⋯2(HF) and C₂H₄⋯4(HF) hydrogen-bonded complexes were fully optimized. Moreover, the Post-Hartree-Fock calculations MP2/6-311++G(d,p), MP2/aug-cc-pVTZ, MP4(SDQ)/6-311++G(d,p) and CCSD/6-311++G(d,p) also were also used. The infrared spectra were analyzed in order to identify the new vibrational modes and frequencies of the proton donors shifted to red region. Through the modeling of charge-fluxes on the basis of the Quantum Theory of Atoms In Molecules (QTAIM) and, by contradicting the expectation of the hydrofluorination mechanisms of acetylene or ethylene, C⋯HF was recognized as a new type of hydrogen bond instead of the already well known π⋯H. The calculations of the Natural Bonding Orbital (NBO) and Charges derived from the Electrostatic Potential Grid-based (ChElPG) were also applied to interpret the shifting frequencies as well as measuring of the punctual charge-transfer after the formation of the complexes. Finally, the determination of the stabilization energy was carried out through the arguments of the Fock matrix in NBO basis and through the supermolecule approach. Also it is worthwhile to notice that some algebraic formulations were used for determining the electronic cooperative effect (CE).

Vibrational solvatochromism. II. A first-principle theory of solvation-induced vibrational frequency shift based on effective fragment potential method

Vibrational solvatochromism is a solvation-induced effect on fundamental vibrational frequencies of molecules in solutions. Here we present a detailed first-principle coarse-grained theory of vibrational solvatochromism, which is an extension of our previous work [B. Błasiak, H. Lee, and M. Cho, J. Chem. Phys. 139(4), 044111 (2013)] by taking into account electrostatic, exchange-repulsion, polarization, and charge-transfer interactions. By applying our theory to the model N-methylacetamide-water clusters, solute-solvent interaction-induced effects on amide I vibrational frequency are fully elucidated at Hartree-Fock level. Although the electrostatic interaction between distributed multipole moments of solute and solvent molecules plays the dominant role, the contributions from exchange repulsion and induced dipole-electric field interactions are found to be of comparable importance in short distance range, whereas the charge-transfer effect is negligible. The overall frequency shifts calculated by taking into account the contributions of electrostatics, exchange-repulsion, and polarization terms are in quantitative agreement with ab initio results obtained at the Hartree-Fock level of theory.

Evidence of blue-shifting N–H⋯N hydrogen bonding despite elongation of the N–H bond

N–H⋯N hydrogen bonding between pyrrole and N₂ results blue shift of ν_N–H accompanied by an increase in the N–H bond length.

Experimental evidence for blue-shifted hydrogen bonding in the fluoroform-hydrogen chloride complex: a matrix-isolation infrared and ab initio study

The 1:1 hydrogen-bonded complex of fluoroform and hydrogen chloride was studied using matrix-isolation infrared spectroscopy and ab initio computations. Using B3LYP and MP2 levels of theory with 6-311++G(d,p) and aug-cc-pVDZ basis sets, the structures of the complexes and their energies were computed. For the 1:1 CHF3-HCl complexes, ab initio computations showed two minima, one cyclic and the other acyclic. The cyclic complex was found to have C-H · · · Cl and C-F · · · H interactions, where CHF3 and HCl sub-molecules act as proton donor and proton acceptor, respectively. The second minimum corresponded to an acyclic complex stabilized only by the C-F · · · H interaction, in which CHF3 is the proton acceptor. Experimentally, we could trap the 1:1 CHF3-HCl cyclic complex in an argon matrix, where a blue-shift in the C-H stretching mode of the CHF3 sub-molecule was observed. To understand the nature of the interactions, Atoms in Molecules and Natural Bond Orbital analyses were carried out to unravel the reasons for blue-shifting of the C-H stretching frequency in these complexes.

Vibrational solvatochromism: towards systematic approach to modeling solvation phenomena

Vibrational solvatochromic frequency shift of IR probe is an effect of interaction between local electric field and IR probe in condensed phases. Despite prolonged efforts to develop empirical maps for vibrational frequency shifts and transition dipoles of IR probes, a systematic approach to ab initio calculation of vibrational solvatochromic charges and multipoles has not been developed. Here, we report on density functional theory (DFT) calculations of N-methylacetamide (NMA) frequency shifts using implicit and coarse-grained models. The solvatochromic infrared spectral shifts are estimated based on the distributed multipole analysis of electronic densities calculated for gas-phase equilibrium structure of NMA. Thus obtained distributed solvatochromic multipole parameters are used to calculate the amide I vibrational frequency shifts of NMA in water clusters that mimic the instantaneous configurations of the liquid water. Our results indicate that the spectral shifts are primarily electrostatic in nature and can be quantitatively reproduced using the proposed model with semi-quantitative accuracy when compared to the corresponding DFT results.

Triplex blue-shifting hydrogen bonds of ClO4(-)···H-C in the nanointerlayer of montmorillonite complexed with cetyltrimethylammonium cation from hydrophilic to hydrophobic properties

In this study, molecular interactions of perchlorate (ClO4(-)), an emerging pollutant, with cetyltrimethylammonium(CTMA(+)) complexed in the nanointerlayer of negatively charged montmorillonite were characterized using the zeta potentials, FTIR, Raman, and XRD spectroscopy and quantified using quantum mechanical calculations and sorption experiments. We found that blue-shifting hydrogen bonds assisted in the uptake of ClO4(-) from water into the nanointerlayer spacing of CTMA(+)-montmorillonite and were tunable according to CTMA(+) loading. FTIR spectra presented an obvious 47 cm(-1) blue shift in the C-H vibration coming from the N-terminal methyl group of CTMA(+) when ClO4(-) was absorbed. Quantum mechanical calculations based on density functional theory demonstrated that triplex blue-shifting hydrogen bonds of C-H···O were formed between the three terminal methyl groups of CTMA(+) and three oxygen atoms of ClO4(-). The contribution of blue-shifting hydrogen bonds to perchlorate uptake switched from a ClO4(-)/CTMA(+) ratio of 0.0453 at low CTMA(+) loadings to a ClO4(-)/CTMA(+) ratio of 0.2563 (5.6-fold) at high CTMA(+) loadings, which can be ascribed to the evolution of the nanointerlayer microenvironments from hydrophilic properties to hydrophobic properties. The blue-shifting hydrogen bond of C-H···O that is tunable with the hydrophobic nature of the organic phase should be recognized to elucidate the biochemical behavior of perchlorate in organisms.

Solvation of fluoroform and fluoroform-dimethylether dimer in liquid krypton: a theoretical cryospectroscopic study

A hybrid, sequential statistical physics-quantum mechanical electronic-quantum mechanical nuclei approach has been applied to study the C-H stretching frequencies of bare fluoroform dissolved in liquid krypton under cryogenic conditions (at ~130 K), as well as upon blue shifting hydrogen bonding interactions with dimethylether in the same solvent. The structure of the liquid at 130 K was generated by Monte Carlo simulations of cryogenic Kr solutions containing either fluoroform or fluoroform and dimethylether molecules. Statistically uncorrelated configurations were appropriately chosen from the equilibrated MC runs and supermolecular clusters containing solute and solvent molecules (either standalone or embedded in the "bulk" part of the solvent treated as a polarizable continuum) were subjected to quantum mechanical electronic (QMel) and subsequent quantum mechanical nuclei (QMnuc) calculations. QMel calculations were implemented to generate the in-liquid 1D intramolecular C-H stretching vibrational potential of the fluoroform moiety and subsequently in the QMnuc phase the corresponding anharmonic C-H stretching frequency was computed by diagonalization techniques. Finally, the constructed vibrational density of states histograms were compared to the experimental Raman bands. The calculated anharmonic vibrational frequency shifts of the fluoroform C-H stretching mode upon interaction with dimethylether in liquid Kr are in very good agreement with the experimental data (20.3 at MP2 level vs. 16.6 cm(-1) experimentally). Most of this relatively large frequency blue shift is governed by configurations characterized by a direct C-H···O contact between monomers. The second population detected during MC simulations, characterized by reversed orientation of the monomers, has a minor contribution to the spectral appearance. The experimentally observed trend in the corresponding bandwidths is also correctly reproduced by our theoretical approach. Solvation of the fluoroform monomer, according to experiment, results in small C-H stretching frequency red shift (~-2 cm(-1)), while our approach predicts a blue shift of about 10 cm(-1). By a detailed analysis of the anharmonic C-H stretching frequency dependence on the position of the nearest solvent krypton atom and also by analyzing the vibrational Stark effect induced by the local fluctuating field component parallel to the C-H axis, we have derived several conclusions related to these observations. The frequency vs. C···Kr distance dependence shows appreciable fluctuations and even changes in sign at R values close to the maximum of the C···Kr radial distribution function, so that most of the first-shell Kr atoms are located at positions at which the CH frequency shifts acquire either small negative or small positive values. It so happens, therefore, that even the actual sign of the frequency shift is strongly dependent on the correct description of the first solvation shell around CF3H by the Monte Carlo method, much more than the other in-liquid properties calculated by similar approaches.

Theoretical aspects of binary and ternary complexes of aziridine···ammonia ruled by hydrogen bond strength

B3LYP calculations, ChelpG atomic charges, and quantum theory of atoms in molecules (QTAIM) integrations were used to investigate the binary (1:1) and ternary (1:2) hydrogen-bonded complexes formed by aziridine (1) and ammonia (2). In a series of analysis, geometry data, electronic parameters, vibrational oscillators, and topological descriptors were used to evaluate hydrogen bond strength, and additionally to determine the more prominent molecular deformations upon the formation of C(2)H(5)N···NH(3) (1:1) and C(2)H(5)N···2NH(3) (1:2) systems. Taking a spectroscopic viewpoint, results obtained from analysis of the harmonic infrared spectrum were examined. From these, new vibrational modes and red- and blue-shifts related to the stretch frequencies of either donors or acceptors of protons were identified. Furthermore, the molecular topology of the electronic density modeled in accord with QTAIM was absolutely critical in defining bond critical points (BCP) and ring critical points (RCP) on the heterocyclic structures. Taking all the results together allowed us to identify and characterize all the N···H hydrogen bonds, as well as the strain ring of the aziridine and its stability.

Ultrafast vibrational frequency shifts induced by electronic excitations: naphthols in low dielectric media

We study the solvent-induced frequency shifts of the OH-stretching mode of 1-naphthol and 2-naphthol in nonpolar/weakly polar solvents, subject to electronic excitation, with ultrafast UV/mid-infrared pump-probe spectroscopy and theoretical modeling based on Pullin's perturbative treatment of vibrational solvatochromic effects. The model is parametrized at the density functional theory (DFT) level, including the B3LYP/TZVP and TD-B3LYP/TZVP descriptions, for the naphthol chromophores in the S(0)- and (1)L(b)-states and accounts for both the static and the optical dielectric response of the solvent on time scales comparable to that of the OH-stretching vibrational motions. The favorable comparison between experimental and theoretical values of the solvent-induced vibrational frequency shifts suggests that the ultrafast dielectric response of the solvent contributes predominantly to the solvatochromic shifts in solvents of moderate polarity where specific solute-solvent interactions are absent.

The O-H stretching mode of a prototypical photoacid as a local dielectric probe

We investigate the OH stretch vibrational frequency shifts of a prototype photoacid, 2-naphthol (2N), when dissolved in solvents of low polarity. We combine femtosecond mid-infrared spectroscopy and a theoretical model based on the Pullin-van der Zwan-Hynes perturbative approach to explore vibrational solvatochromic effects in the ground S(0) and the first electronically excited (1)L(b) states. The model is parametrized using density functional theory (DFT), at the B3LYP/TZVP and TD-B3LYP/TZVP levels for the 2N chromophore in the S(0) and (1)L(b) states, respectively. From the agreement between experiment and theory we conclude that vibrational solvatochromic effects are dominated by the instantaneous dielectric response of the solvent, while time-dependent nuclear rearrangements are of secondary importance.

Fourier Transform Raman and DFT Study of Blue Shift C–H Stretching Vibration of Diazines on Hydrogen Bond Formation

Diazines form various types of hydrogen bonds with water, wherein C–H groups are sometimes involved directly and sometimes indirectly. On hydrogen bond formation, the wavenumber of C–H stretching vibration are usually red shifted (normal hydrogen bond) but blue shift of C–H modes (anomalous hydrogen bond) is also possible in some cases. The Raman spectra of the C–H stretching bands of three diazines; pyrimidine, pyridazine, and pyrazine in pure form and at many concentrations in mole fractions of diazines in the mixture of diazines + water have been measured to analyze the wavenumber shifts experimentally. Theoretical wavenumber shifts have been calculated and NBO analysis has been performed using DFT methods to understand the cause of the shifts. All the four C–H stretching bands of diazines are blue shifted on dilution with water both experimentally and theoretically. The NBO calculations reveal that the cause of the shift is decrease in the charge density in the antibonding orbital of C–H bond on complex formation.

A theoretical study of red-shifting and blue-shifting hydrogen bonds occurring between imidazolidine derivatives and PEG/PVP polymers

A theoretical study is presented with the aim to investigate the molecular properties of intermolecular complexes formed by the monomeric units of polyvinylpyrrolidone (PVP) or polyethyleneglycol (PEG) polymers and a set of four imidazolidine (hydantoine) derivatives. The substitution of the carbonyl groups for thiocarbonyl in the hydantoin scaffold was taken into account when analyzing the effect of the hydrogen bonds on imidazolidine derivatives. B3LYP/6-31G(d,p) calculations and topological integrations derived from the quantum theory of atoms in molecules (QTAIM) were applied with the purpose of examining the N-H···O hydrogen bond strengths formed between the amide group of the hydantoine ring and the oxygen atoms of PVP and PEG polymers. The effects caused by the N-H···O interaction fit the typical evidence for hydrogen bonds, which includes a variation in the stretch frequencies of the N-H bonds. These frequencies were identified as being vibrational red-shifts because their values decreased. Although the values of such calculated interaction energies are between 12 and 33 kJ mol(-1), secondary intermolecular interactions were also identified. One of these secondary interactions is formed through the interaction of the benzyl hydrogen atoms with the oxygen atoms of the PVP and PEG structures. As such, we have analyzed the stretch frequencies on the C-H bonds of the benzyl groups, and blue-shifts were identified on these bonds. In this sense, the intermolecular systems formed by hydantoine derivatives and PVP/PEG monomers were characterized as a mix of red-shifting and blue-shifting hydrogen-bonded complexes.

Relative strengths of NH..O and CH..O hydrogen bonds between polypeptide chain segments

Correlated ab initio calculations are used to compare the energetics when the CH and NH groups of the model dipeptide CHONHCH2CONH2 are each allowed to form a H-bond with the proton acceptor O of a peptide group. When the dipeptide is in its C7 conformation, the NH..O H-bond energy is found to be 7.4 kcal/mol, as compared to only 2.8 kcal/mol for the CH..O interaction. On the other hand, the situation reverses, and the CH..O H-bond becomes stronger than NH..O, when the dipeptide adopts a C5 structure. This reversal is important as C5 is nearly equal in stability to C7 for the dipeptide, and is representative of the commonly observed beta-sheet structure in a protein. Immersing the dipeptide-peptide pair in a model solvent weakens both sorts of H-bonds, and in a fairly uniform manner. Consequently, the trends observed in the in vacuo situation retain their validity in either aqueous solution or the protein interior. Likewise, the desolvation penalty, suffered by removing a H-bonded complex from water and placing it in the less polar interior of a protein, is quite similar for the NH..O and CH..O bonds.

Effect of solvent upon CH...O hydrogen bonds with implications for protein folding

The series of CH...O bonds formed between CF(n)H(4-n) (n = 0-3) and water are studied by quantum calculations under vacuum and in various solvents, including aqueous environment. The results are compared with the OH...O bond of the water dimer in the same solvents. Increasing polarity of the solvent leads in all cases to a lessening of the H-bond interaction energy, in a uniform fashion such that the CH...O bonds all remain weaker than OH...O in any solvent. These H-bond weakenings are coupled to a shortening of the inter-subunit separation. The contraction of the covalent CH bond to the bridging proton is reduced as the solvent becomes more polar, and the blue shift of its stretching vibration is likewise diminished. A process is considered that simulates protein folding by starting from a pair of noninteracting subunits in aqueous solvent and then goes to a H-bonded pair within the confines of a protein environment. This process is found to be energetically more favorable for some of the CH...O H-bonds than for the nominally stronger conventional OH...O H-bond. This finding suggests that CH...O bonds can make important energetic contributions to protein folding, on par with those made by traditional H-bonds.

Intermolecular Interactions between Halothane and Dimethyl Ether: A Cryosolution Infrared and Ab Initio Study

The complex of halothane (CHClBrCF(3)) and dimethyl ether has been investigated experimentally in solutions of liquid krypton using infrared spectroscopy and theoretically using ab initio calculations at the MP2/6-311++G(d,p) level. The formation of a 1:1 complex was experimentally detected. The most stable ab initio geometry found is the one in which the C--H bond of halothane interacts with the oxygen atom of dimethyl ether. The complexes in which the chlorine or the bromine atom of halothane interacts with the oxygen atom of the ether were found to be local energy minima and were less stable by 14.5 and 9.3 kJ mol(-1), respectively, than the global minimum. The formation of a single complex species was observed in the infrared spectra; the standard complexation enthalpy of this complex was determined to be -12.3(8) kJ mol(-1). Analysis of the observed complexation shifts supports the identification of the complex as the hydrogen-bonded species. The C--H stretching vibration of halothane was found to show a redshift upon complexation of 19(2) cm(-1). The infrared intensity ratios epsilon(complex)/epsilon(monomer) for the fundamental and its first overtone were measured to be 6.5(1) and 0.31(1). The frequency shift was analyzed using Morokuma-type analysis, and the infrared intensity ratios were rationalized using a model including the mechanical and electric anharmonicity of the C--H stretching fundamental.

Blue-Shifting Intramolecular C−H···O Interactions

Two model systems, 3-methylacroleine and 3-(difluoromethyl)acroleine, are investigated computationally with respect to the character of the C-H...O interaction in their chelate-type (ZZ) conformers. By selecting the appropriate reference conformers, the C-H...O interaction is shown to result in the increase of the C-H stretching frequency (i.e., in the blue shift of the C-H stretching band). This is accompanied by the shortening of the C-H bond distance as compared to its values in reference molecules. Parallel to calculations of the C-H bond distance and stretching frequency, the energy contribution of the C-H...O interaction to the total energy (i.e., the energy associated with the C-H...O contact) is evaluated by using the methods proposed recently for the estimation of the energies of intramolecular hydrogen bonds. It is found that the C-H...O contact in the chelate-type forms of 3-methylacroleine and 3-(difluoromethyl)acroleine corresponds to the negative energy contribution and is repulsive. It is concluded that, despite the stability of the ZZ conformers of the two molecules and their deceptive structural shape, no hydrogen bond in the usual sense is formed between the C-H bond and the lone pair donor. The results are interpreted in terms of the steric compression, which leads to the dominance of the valence repulsion contribution in the C-H...O contact. This mechanism suggests that blue-shifting intramolecular interactions should not be that uncommon, although their recognition requires a careful consideration of the reference system.

Ab Initio Study of Hydrogen-Bond Formation between Cyclic Ethers and Selected Amino Acid Side Chains

Binding energies for hydrogen-bonded complexes of six cyclic ethers with five hydrogen-bond donor molecules that mimic selected amino acid side chains have been calculated at the MP2/6-31G*, MP2/6-31+G*, MP2/6-311++G**(single point), and MP2/aug-cc-pvtz levels, using geometries obtained with or without counterpoise corrections throughout the geometry optimization. The calculated basis set superposition error (BSSE) amounts to 10-20% and 5-10% of the uncorrected binding energies for the neutral and ionic species, respectively, at the MP2/aug-cc-pvtz level. The authors conclude that the O...H distances in the hydrogen bonds and binding energies for the studied systems may be determined with uncertainties of up to 0.08 A and 1-2 kcal/mol, respectively, in comparison with the MP2/aug-cc-pvtz values at a reasonable computational cost by performing standard geometry optimization at the MP2/6-31+G* level. Hydrogen-bond formation energies are more negative for cyclic ethers compared to their counterparts with a C=C double bond in the ring next to the oxygen atom. The less negative hydrogen-bonding energy and the increased O...H separation have been attributed to the reduced basicity of the ether oxygen when the lone pairs can enter conjugation with the pi-electrons of the Calpha=Cbeta double bond. The present study is the first step toward the development of an affordable computational level for estimating the binding energies of natural product, fused ring ether systems to the human estrogen receptor.

Infrared Spectra of the Complexes of Trifluoroethene with Dimethyl Ether, Acetone, and Oxirane: A Cryosolution Study

Infrared spectra of solutions of trifluoroethene and dimethyl ether, acetone, or oxirane in liquid krypton and liquid argon have been studied. For each Lewis base the formation of a 1:1 complex with the Lewis acid was observed. The C-H stretching of trifluoroethene being perturbed by a strong Fermi resonance, the complexes with trifuloroethene-d were also investigated and showed that in each case the hydrogen bond between the acid and base is of the traditional, red-shifting type. The structures of the complexes were investigated using ab initio calculations. These indicate that with dimethyl ether and acetone two different isomeres can be formed, but with a single one detected in the solution in each case. The Fermi resonance in the complex with unlabeled trifluoroethene is discussed using data derived form ab initio potential and dipole hypersurface calculations. The complexation enthalpies of the complexes were obtained from temperature dependent studies of the solutions and are discussed in relation to the ab initio complexation energies and Monte Carlo free energy perturbation calculations of solvent effects.

Origin of the Blue Shift of the CH Stretching Band for 2-Butoxyethanol in Water

The blue shift of the isolated CD stretching band of 2-butoxyethanol (C4E1), which is observed for the aqueous solution during the dilution process, has been investigated by infrared (IR) spectroscopy and quantum chemical calculations. Mono-deuterium-labeled C4E1's were employed to remove the severe overlapping among the CH stretching bands. The isolated CD stretching mode of the alpha-methylene in the butoxy group shows a large blue shift, while those of the beta-methylene and methyl groups are not largely shifted. The spectral simulation results for the C4E1/H2O complexes indicate that the large blue shift of the CD stretching band of the butoxy group arises mainly from the hydration of the ether oxygen atom.

Blue-shifting hydrogen bond in the benzene–benzene and benzene–naphthalene complexes

Ab initio complete optimizations at MP2/6-31++G** level have been performed in the T-shaped geometry of the benzene-benzene and benzene-naphthalene complexes. To check the effect of the basis set superposition error (BSSE), optimizations have been done in the BSSE corrected and BSSE uncorrected potential energy surfaces. The BSSE effect in the calculation of the Hessian has also been evaluated to check its influence in the frequency values. Quantum theory atoms in molecules (QTAIM) calculations have also been performed on both dimers. Intermolecular energies differ around a 25% when the optimization is performed with or without counterpoise corrected gradients. The influence of BSSE is also noticeable in the distances. Frequency shifts show big changes because of the BSSE. Thus, uncorrected values are up 350% larger than corrected ones. The hypotheses given in the literature to explain the origin of the blue-shifting hydrogen bond do not seem to give a suitable explanation for all characteristics of the behavior found in the studied systems.

On the Correlation between Bond-Length Change and Vibrational Frequency Shift in Hydrogen-Bonded Complexes: A Computational Study of Y···HCl Dimers (Y = N2, CO, BF)

The H-Cl bond-length change and the harmonic vibrational frequency shift of the H-Cl stretch on formation of the linear isoelectronic Y...H-Cl complexes (Y = N(2), CO, BF) have been determined by ab initio computations at different levels of theory. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the H-Cl bond length from its equilibrium value in the isolated monomer. At the highest level of theory, blue shifts were obtained for BF...HCl and CO...HCl, while red shifts were obtained for FB...HCl, OC...HCl, and N(2)...HCl. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The widely believed correlation between the bond-length change and the sign of the frequency shift obtained on complexation is discussed and found to be unreliable.