Blue-shifting Hydrogen Research Articles

Blue-shifting hydrogen bonds—are they improper or proper?

In a review published in 2000 Hobza and Havlas coined the expression ‘improper, blue-shifting’ to distinguish a type of hydrogen bond which displays characteristics opposite to that expected for a conventional X–H⋯Y hydrogen bond, i.e. the X–H bond length contracts rather than elongates and the X–H stretching mode shifts to higher frequency, often with reduced intensity, rather than shifting to lower frequency with enhanced intensity. In the present paper, the experimental evidence relating to blue-shifting hydrogen bonding, and the theoretical interpretations which have been advanced, are critically reviewed with the aim of answering the question posed in the title.

Theoretical Study on the Blueshifting Halogen Bond

A new type of intermolecular bond, termed a blueshifting halogen bond, is found in the chlorotrifluoromethane-, bromotrifluoromethane-, chlorotrifluorosilicane-, and chlorodifluoroamine-related complexes. Counterpoise-corrected gradient optimization performed at a correlated ab initio level (MP2(full)/6-311++G(d,p)) shows a shortening of the C−Cl (C−Br, Si−Cl, or N−Cl) bond of the proton donor and a blueshifting of the corresponding C−Cl (C−Br, Si−Cl, or N−Cl) stretching frequency. In contrast to the conventional hydrogen bond and the blueshifting hydrogen bond, the topological and electronic properties and the origin of blueshifting halogen bond are also investigated.

First local minimum of the formic acid dimer exhibits simultaneously red-shifted O–H⋯O and improper blue-shifted C–H⋯O hydrogen bonds

The first local minimum of the formic acid dimer exhibits simultaneously red-shifted O–H⋯O and blue-shifted C–H⋯O hydrogen bonds. The improper, blue-shifted hydrogen bond was investigated by means of the NBO analysis, rehybridization model and optimization in the electric field. It was shown that the electrostatic model cannot describe the nature of the blue-shifted H-bond. From the NBO analysis it becomes evident that the formation of the O–H⋯O hydrogen bond and C–H⋯O improper hydrogen bond can be explained on the basis of an increase of electron density in the σ* antibonding O–H orbital and a decrease of electron density in the σ* antibonding C–H orbital. While the former effect is easily explained on the basis of hyperconjugation, the latter requires the existence of a new mesomeric structure characterized by delocalization of electron density from the C–H σ* antibonding orbital to the remaining part of the complex. The rehybridization model explains properly the formation of both hydrogen bonds but fails to interpret the changes of the other bonds.

Theoretical study of the blue-shifting intramolecular hydrogen bonds of nitro derivatives of cubane

The intramolecular hydrogen bonds (HBs) C–H⋯O of the 15 nitro derivatives of cubane were calculated and analyzed at the B3LYP/6-31G** level as well as the NBO analyses. All the intramolecular HBs existed in studied structures were improper blue-shifting HBs. Nitro group(s) carried by cubane had influence on the molecular electronic structure. Two cases of processes of the electron transfer were found, which caused a blue-shift of the intramolecular HBs of the nitro derivatives of cubane. The HBs formed through two cases had different influence on the molecular electronic structure, C–H stretch vibrational frequency and the total energy.

Blue-shifted Hydrogen Bonds with Proton-donors Incapable of Rehybridization

Abstract Blue-shifted hydrogen bonds were found for F–He–H···Y (Y = N2, CO, and He) systems where the proton-donor (He) was incapable of rehybridization. This finding indicated that rehybridization is not a generally applicable driving force for the blue shift.

Blue-shifting hydrogen bonding in N2⋯HKrF

An ab initio computational study of the properties of the weakly bound linear complex N2⋯HKrF was undertaken at three different levels of theory using a 6-311G** basis set. The dimer was found to have a zero-point vibrational energy corrected binding energy of −2.8 kJ mol−1 with respect to the monomer subunits, at the highest level of theory used in this study. This complex exhibits a large harmonic vibrational frequency blue-shift of about 60 cm−1 for the H–Kr stretching mode with a decrease in the infrared intensity for this mode on formation of the complex. This unusual result was rationalized mainly by consideration of the effect of the electrostatic interaction and charge transfer between the HKrF and N2 molecules.

Blue-shifting hydrogen bonding in the N 2⋯HArCl dimer

An ab initio computational study of the properties of a weakly bound complex formed between the argon-containing compound HArCl and N 2 was undertaken at the MP2/6-311++G(2d,2p) level of theory. The complex N 2⋯HArCl was found to have a zero-point vibrational energy corrected binding energy of 7.0 kJ mol −1 with respect to the monomer subunits. This complex exhibits a large harmonic vibrational frequency blue shift of 483 cm −1 for the Ar–H stretching vibration mode accompanied by contraction of the bond and reduction of the infrared intensity for this mode on formation of the complex. This blue shift was rationalized by primarily considering the effect of the electrostatic interaction between the monomers and the charge transfer from the proton acceptor to the proton donor.

Hydration of Sulfo and Methyl Groups in Dimethyl Sulfoxide Is Accompanied by the Formation of Red-Shifted Hydrogen Bonds and Improper Blue-Shifted Hydrogen Bonds: An ab Initio Quantum Chemical Study

In this study, the hydration of dimethyl sulfoxide was investigated by means of molecular dynamics (MD) simulations and quantum chemical correlated ab initio calculations. MD simulations show the h...

Improper, blue-shifting hydrogen bond

The spectral manifestation and the nature of improper, blue-shifting hydrogen-bonded complexes are entirely different from those of standard hydrogen-bonded complexes. While the latter class of complexes is characterized by an elongation of the X–H bond and a concomitant red shift of the respective stretch frequency, a contraction of the X–H bond and a blue shift of the X–H stretch frequency are typical for the former class of complexes. Both classes of complexes exhibit electron-density transfer from the proton acceptor to the proton donor. In the case of hydrogen-bonded complexes, the electron-density transfer is mostly larger then in the improper complexes. The role of the charge transfer and electrostatic interaction is discussed. The importance of the charge transfer is documented by natural bond orbital analysis, orbital interaction diagrams, and by the insufficient description of the interaction by the electrostatic model only.

Substituent effects on the blue-shifted hydrogen bonds

For the blue-shifted hydrogen bonds between CF 3H and phenols, fluorobenzenes, and anilines, a donor para-substituent increases the interaction energy causing larger blue shift. For the red-shifted hydrogen bond between CF 3H and phenolate, a donor para-substituent also increases the interaction energy but causing larger red shift. These behaviors indicate that the blue shift is attributable to the attraction between the positively charged carbon of CF 3H and the hydrogen bond acceptor. This attraction forces the hydrogen-bond donor and acceptor to come so close to each other that the hydrogen of CF 3H feels considerable Pauli repulsion from the acceptor, resulting in CH contraction.

Study of the nature of improper blue-shifting hydrogen bonding and standard hydrogen bonding in the X3CH...OH2 and XH...OH2 complexes (X = F, Cl, Br, I): A correlated Ab initio study.

Weak hydrogen bonding was studied in the XH...OH2 and X3CH...OH2 complexes (X = F, Cl, Br, I) using the correlated MP2 ab initio method with relativistic Stuttgart/Dresden pseudopotentials and basis set (SDD). The accuracy of the method was tested for selected nonrelativistic complexes by performing MP2 calculations with all-electron basis sets (6-311G** and TZVPP). The characteristics of bonding in the hydrogen halide complexes correspond to the standard H-bonding (an elongation of the X-H bond and red shift of its stretch frequency), whereas those in the X3CH...OH2 complexes (X = F, Cl) are typical of improper blue-shifting H-bonding (a contraction of the CH bond and blue shift of the respective stretch frequency). A natural bond orbital analysis revealed some important differences between both classes of complexes: a) the electron density transfer (EDT) in the former complexes is considerably larger than that in the latter complexes: b) the EDT in the former complexes is almost completely directed to the sigma*-antibonding orbital of the XH bond, which causes a weakening of this bond, its elongation, and a concomitant decrease of the XH stretch frequency. In the latter complexes, only a small portion of the EDT goes to the sigma*-antibonding orbital of the CH bond of the proton donor and a larger part is transferred to the remote (nonparticipating) part of the proton donor. As a consequence, the structural reorganization of the proton donor occurred, leading to the contraction of the C-H bond. The fact that a small red shift of the C-H stretch frequency was found in bromoform-water and iodoform-water complexes was explained by the competition of both the above-mentioned mechanisms with dominating passage of electron density to the sigma*-antibonding orbital of the C-H bond. For an explanation of all the geometric features of both types of complexes, it is however necessary to consider both charge transfer and electrostatic effects. The electrostatic effects fail sometimes to interpret the geometry changes in the proton donor.

Blue-Shifting Hydrogen Bonds

In this paper we put forward the idea that the various improper, blue-shifting hydrogen bond systems discussed in the literature are all of essentially the same nature and occur because of three ne ...

Substituent Effects on the Blue‐Shifting Hydrogen Bonds between X ‐ C  C ‐ CF2 ‐ H and Water

AbstractMP2/6‐31 + g (d) calculations were performed verifying the existences of blue‐shifting X ‐ C  C ‐ CF2 ‐ HḋOH2 hydrogen bonds. Detailed analyses revealed that the interaction energy and donor‐acceptor distance had good correlations with the substituent Hammett constants. However, the extent of C‐H bond contraction and the blue shift of the C‐H stretching vibration did not show any good correlation with the traditional substituent constants, indicating that certain more complicated mechanisms might be involved in the present systems. Nevertheless, it was found that highly electron‐withdrawing substituents were not favorable to the C‐H bond contraction, and it was suggested that the attractive interaction between water and the carbon of ‐ CF2H probably played an important role in the blue shift.

Weak, improper, C-O...H-C hydrogen bonds in the dimethyl ether dimer.

The ground-state rotational spectrum of the dimethyl ether dimer, (DME)(2), has been studied by molecular beam Fourier transform microwave and free jet millimeter wave absorption spectroscopies. The molecular beam Fourier transform microwave spectra of the (DME-d(6))(2), (DME-(13)C)(2), (DME-d(6))...(DME), (DME-(13)C)...(DME), and (DME)...(DME-(13)C) isotopomers have also been assigned. The rotational parameters have been interpreted in terms of a C(s) geometry with the two monomers bound by three weak C-H...O hydrogen bonds, each with an average interaction energy of about 1.9 kJ/mol. The experimental data combined with high-level ab initio calculations show this kind of interaction to be improper, blue-shifted hydrogen bonding, with an average shortening of the C-H bonds involved in the hydrogen bonding of 0.0014 A. The length of the C-H...O hydrogen bonds, r(O...H), is in the range 2.52-2.59 A.

Steric Effect is an Additional Possible Cause of Blue-shifting Hydrogen Bonds

Abstract A novel simple example was found that at least sometimes the steric effect could cause a blue-shifting hydrogen bond, which indicates that Hobza’s recent theory about the origin of the blue-shifting hydrogen bond is not complete.

The nature of improper, blue-shifting hydrogen bonding verified experimentally.

In the infrared spectra of solutions in liquid argon of dimethyl ether ((CH(3))(2)O) and fluoroform (HCF(3)), bands due to a 1:1 complex between these monomers have been observed. The C-H stretch of the HCF(3) moiety in the complex appears 17.7 cm(-1) above that in the monomer, and its intensity decreases by a factor of 11(2). These characteristics situate the interaction between the monomers in the realm of improper, blue-shifting hydrogen bonding. The complexation shifts the C-F stretches downward by some 9 cm(-1), while the C-H stretches in (CH(3))(2)O are shifted upward by 9-15 cm(-1), and the C-O stretches are shifted downward by 5 cm(-1). These shifts are in very good agreement with those calculated by means of correlated ab initio methods, and this validates a two-step mechanism for improper, blue-shifting hydrogen bonding. In the first step, the electron density is transferred from the oxygen lone electron pairs of the proton acceptor ((CH(3))(2)O) to fluorine lone electron pairs of the proton donor (CHF(3)) which yields elongation of all CF bonds. Elongation of CF bonds is followed (in the second step) by structural reorganization of the CHF(3) moiety, which leads to the contraction of the CH bond. It is thus clearly demonstrated that not only the spectral manifestation of H-bonding and improper H-bonding but also their nature differ.

Evidence of C−H···O Hydrogen Bonds in Liquid 4-Ethoxybenzaldehyde by NMR and Vibrational Spectroscopies

Raman, FTIR, and NMR (both 13C and 17O) spectroscopies are used in a complementary way in order to study the occurrence of C−H···O intermolecular hydrogen bonds in liquid 4-ethoxybenzaldehyde (4EtOB). Additional information concerning the structure of the possible dimers is obtained through ab initio calculations, at the B3LYP/6-31G* level. The strongest evidences of the presence of C−H···O hydrogen bonds in the liquid phase arise from the temperature and solvent intensity dependence of the two bands observed in the νCO region of the vibrational spectra, as well as from the shift to low magnetic field detected for the carbonyl 17O NMR peak at higher dilutions. Further evidence is gathered from the changes observed in the νC-H vibrational modes, the 1JCH concentration dependence detected in the NMR spectra, and ab initio results. The experimental observations are consistent with the decrease of the C−H bond length upon hydrogen-bonding, as predicted for the nonstandard blue-shifting hydrogen bonds. Ab init...

Improper, Blue-Shifting Hydrogen Bond between Fluorobenzene and Fluoroform

Weakly bonded 1:1 complexes between fluorobenzene (Fb)/fluorobenzene-d5 (Fb-d5) and fluoroform (Ff) were investigated spectroscopically by infrared ion-depletion spectroscopy (IR/R2PI) and theoretically by correlated ab initio methods. Their predissociation spectra exhibit an absorption comprised of two superimposed bands. These are blue-shifted by 12 and 21 cm-1, respectively, relative to the CH stretch of isolated fluoroform. Each IR band is assigned to a different hydrogen-bonded fluorobenzene·fluoroform isomer. The isomer with the most blue-shifted CH stretching vibration (21 cm-1) is assigned to a sandwich type structure, exhibiting a CH···π hydrogen bond. The cluster structures have been calculated by counterpoise- (CP-) corrected gradient optimization combined with anharmonic vibrational analysis using the CP-corrected Hessians. The predicted blue-shifts are 21 and 20.5 cm-1 for the CH stretching frequencies of fluoroform upon formation of a sandwich and a planar structure, respectively. The theore...

The H-index unambiguously discriminates between hydrogen bonding and improper blue-shifting hydrogen bonding

X–H···Y contacts are characterised by a red shift of the X–H stretch frequency. We have shown recently (P. Hobza and Z. Havlas, Chem. Re., 2000, 100, 4253) that these contacts can also be characterised by a blue shift of the X–H stretch frequency. An H-index, defined as the ratio of electron density transferred from the proton donor (Y) to the σ* antibonding orbital of the X–H bond and the total electron density transferred between proton donor and proton acceptor unambiguously discriminates between both types of H-bonding. Its value for proper H-bonding lies between 1.0 and 0.7 while values between 0.3 and 0 are typical for improper (blue-shifting) H-bonding.