Blue shifted F3CH⋯FCD3 and Cl3CH⋯FCD3 weakly H-bound complexes. Cryospectroscopic and ab initio study

Structural, vibrational and electronic properties of some tetrel-bonded complexes of the fluorinated methanes methyl fluoride, difluoromethane and fluoroform: an ab initio study

A search has been conducted, by means of ab initio molecular orbital theory, for potential tetrel-bonded complexes formed between the fluorinated methanes methyl fluoride, difluoromethane and fluoroform, and the related hydrides ammonia, water, hydrogen fluoride, phosphine, hydrogen sulphide and hydrogen chloride. Eleven such complexes have been identified, six containing CH₃F and five CH₂F₂. The complexes are typically less strongly bound than their hydrogen-bonded counterparts, and the interaction energies vary in a consistent way with the periodic trend of the electron donors. The intermolecular separations and changes of the relevant intramolecular bond lengths, the wavenumber shifts of the critical vibrational modes and the extents of charge transfer correlate, by and large, with the strengths of interaction.

The infrared study of fluoroform + methyl fluoride mixtures in argon and nitrogen matrices. Evidence of nonlinear blue-shifting complex formation

The FTIR spectra of fluoroform + methyl fluoride mixtures trapped in argon and nitrogen matrices are studied at T~10-30 K. Spectroscopic changes observed in the region of the CH stretching mode of fluoroform are typical for weak blue shifting H - bonds of CH⋯F type. The degeneracy lifting effect found on E - type bands of fluoroform interacted with methyl fluoride suggests the complex formation of a nonlinear form. The experimental results are confirmed by ab initio calculations of fluoroform + methyl fluoride based on the second order Møller-Plesset theory of perturbations utilizing advanced basis set. Nonlinear complexes are stabilized by the basic CH⋯F interaction and additionally by van der Waals-type CD⋯FC contacts between deuterated methyl fluoride and fluoroform.

FTIR cryospectroscopic and ab initio studies of desflurane-dimethyl ether H-bonded complexes

The IR spectra of mixtures of desflurane and dimethyl ether are studied with the help of FTIR cryospectroscopy in liquefied Kr at T~118-158K. Comparative analysis of the experimental data and results of ab initio calculations show that either of the two C-H groups of desflurane is involved in heterodimer formation of comparable strengths. The blue frequency shift is found for stretching vibrations of those C-H donors which directly participate in H-bond formation. Additionally the complexes are stabilized by weaker contacts between hydrogen atoms of dimethyl ether and fluorine atoms of desflurane.

Infrared spectroscopy and ab initio study of hydrogen bonded Cl3CD·N(CH3)3 complex in the gas phase

FTIR spectra of the gas phase Cl3CD+TMA mixture have been studied at room temperature in ∼800-4000 cm(-1) frequency domain. The formation of the H-bonded Cl3CD…TMA complex has been detected. Spectroscopic parameters of the band ascribed to the complex were evaluated. MP2 frozen core ab initio calculations have been carried out with the Pople-type 6-311++G(d,p) basis set. The equilibrium geometries and harmonic vibrational frequencies of the complex were obtained using CP-corrected gradient techniques. The ''freq=anharm'' option has been tested for Cl3CD monomer and Cl3CD…TMA complex to examine possible anharmonic effects on the vibrations localized on the proton donor. The effects of Darling-Dennison and Fermi resonances on the frequency of the stretching vibration of the CH proton donor were analyzed.

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.

Fundamental and overtone vibrational spectroscopy, enthalpy of hydrogen bond formation and equilibrium constant determination of the methanol-dimethylamine complex

We have measured gas phase vibrational spectra of the bimolecular complex formed between methanol (MeOH) and dimethylamine (DMA) up to about 9800 cm(-1). In addition to the strong fundamental OH-stretching transition we have also detected the weak second overtone NH-stretching transition. The spectra of the complex are obtained by spectral subtraction of the monomer spectra from spectra recorded for the mixture. For comparison, we also measured the fundamental OH-stretching transition in the bimolecular complex between MeOH and trimethylamine (TMA). The enthalpies of hydrogen bond formation (ΔH) for the MeOH-DMA and MeOH-TMA complexes have been determined by measurements of the fundamental OH-stretching transition in the temperature range from 298 to 358 K. The enthalpy of formation is found to be -35.8 ± 3.9 and -38.2 ± 3.3 kJ mol(-1) for MeOH-DMA and MeOH-TMA, respectively, in the 298 to 358 K region. The equilibrium constant (Kp) for the formation of the MeOH-DMA complex has been determined from the measured and calculated transition intensities of the OH-stretching fundamental transition and the NH-stretching second overtone transition. The transition intensities were calculated using an anharmonic oscillator local mode model with dipole moment and potential energy curves calculated using explicitly correlated coupled cluster methods. The equilibrium constant for formation of the MeOH-DMA complex was determined to be 0.2 ± 0.1 atm(-1), corresponding to a ΔG value of about 4.0 kJ mol(-1).

First-principles study of fluoroform adsorption on a hexagonal ice (0001) surface: weak hydrogen bonds-strong structural effects

For isolated fluoroform (F(3)CH) molecules adsorbed on a hexagonal ice (0001) surface the properties of blue- and red-shifting hydrogen bonds were studied using static density functional theory (DFT) calculations and Car-Parrinello molecular dynamics (CP-MD) simulations. A systematic search by starting from many initial configurations was performed to determine the lowest-energy structures of F(3)CH on the ice surface, and for the optimized geometries the vibrational frequencies were calculated. The local minima structures are analyzed in terms of their coordination to the surface, with special focus on identifying blue-shifting hydrogen bonds via their spectroscopic signature of an increased frequency of the C-H fundamental stretching vibration. Subsequently, by CP-MD simulations the stability of the lowest-energy configurations at finite temperatures was verified and possible transformation pathways connecting the local minima structures were explored.

Weak H-bonds. Comparisons of CH···O to NH···O in proteins and PH···N to direct P···N interactions

Whereas CH···O H-bonds are usually weaker than interpeptide NH···O H-bonds, this is not necessarily the case within proteins. The nominally weaker CH···O are surprisingly strong, comparable to, and in some cases stronger than, the NH···O H-bonds in the context of the forces that hold together the adjacent strands in protein β-sheets. The peptide NH is greatly weakened as proton donor in certain conformations of the protein backbone, particularly extended structures, and forms correspondingly weaker H-bonds. The PH group is a weak proton donor, but will form PH···N H-bonds. However, there is a stronger interaction in which P can engage, in which the P atom, not the H, directly approaches the N electron donor to establish a direct P···N interaction. This approach is stabilized by the same sort of electron transfer from the N lone pair to the P-H σ* antibond that characterizes the PH···N H-bond.

NMR Study of Blue-Shifting Hydrogen Bonds Formed by Fluoroform in Solution

Low-temperature (193 K) 1H, 13C and 15N NMR spectra of blue- and red-shifting H-bonded complexes formed by fluoroform with various proton acceptors were measured. Experimental NMR parameters were plotted versus the ab initio calculated H-bond strength (MP2/6–31+G(d, p); interaction energy varies from ~5 up to 25 kJ · mol−1 in the series). We show that experimental 1H and 15N shieldings, as well as the H/D isotope effect on 13C shielding change monotonously with the calculated H-bond strengthening. The 13C chemical shift and the CH scalar coupling change non-monotonously and the extremum points are situated approximately in the region of transformation from blue- to red-shifting H-bonds. The most informative NMR feature is the H/D isotope effect on 15N shielding which changes its sign upon transformation from blue- to red-shifting H-bonds.

To rationalize these observations, ab initio calculations of 13C and 15N shieldings as functions of C···H and C···N distances were performed for complexes of CHF3 with acetonitrile (blue-shifting) and pyridine (red-shifting). The coupling of the vibrations of the covalent and hydrogen bonds has been accounted for by direct computation of the distance C···N = f(C···H) dependence. We demonstrate that the unusual sign of the H/D isotope effect on 15N chemical shift across a blue-shifting H-bond can be explained as a result of the inversion of the dynamic coupling of two vibrations.

Theoretical and Experimental Studies of Enflurane. Infrared Spectra in Solution, in Low-Temperature Argon Matrix and Blue Shifts Resulting from Dimerization

Theoretical studies are performed on enflurane (CHFCl-CF(2)-O-CHF(2)) to investigate the conformational properties and vibrational spectra. Calculations are carried out at the B3LYP/6-31G(d) level along with a natural bond orbital (NBO) analysis. Experimental infrared spectra are investigated in carbon tetrachloride solution at room temperature and in argon matrix at 12 K. In agreement with previously reported data (Pfeiffer, A.; Mack, H.-G.; Oberhammer, H. J. Am. Chem. Soc. 1998, 120, 6384), it is shown that the four most stable conformers possess a trans configuration of the C-C-O-C skeleton and a gauche orientation of the CHF(2) group (with respect to the central C-O bond). These conformations are favored by electrostatic interaction between the H atom of the CHF(2) group and the F atoms of the central CF(2) group. Hyperconjugation effects from the O lone pairs to the antibonding orbitals of the neighboring C-H and C-F bonds also contribute to the stability of the four conformers. The vibrational frequencies, infrared intensities, and potential energy distributions are calculated at the same level of theory for the most stable conformers. On the basis of the theoretical results, these conformers are identified in an argon matrix. The influence of the concentration on the nu(CH) vibrations suggests the formations of higher aggregates in solution. Theoretical calculations are carried out on the enflurane dimer. The results show that the dimer is formed between two enflurane conformers having the largest stability. The dimer has an asymmetric cyclic structure, the two enflurane molecules being held together by two nonequivalent C-H...F hydrogen bonds, the C-H bond of the CHFCl group acting as a proton donor, and one of the F atoms of the CHF(2) groups acting as a proton acceptor. The theory predicts a contraction of 0.0014-0.0025 A of the two CH bonds involved in the interaction along with a blue shift of 30-38 cm(-1) of the corresponding nu(C-H) bands, in good agreement with the blue shifts of 35-39 cm(-1) observed in an argon matrix.

On the Origin of Red and Blue Shifts of XH and CH Stretching Vibrations in Formic Acid (Formate Ion) and Proton Donor Complexes

Complexes between formic acid or formate anion and various proton donors (HF, H(2)O, NH(3), and CH(4)) are studied by the MP2 and B3LYP methods with the 6-311++G(3df,3pd) basis set. Formation of a complex is characterized by electron-density transfer from electron donor to ligands. This transfer is much larger with the formate anion, for which it exceeds 0.1 e. Electron-density transfer from electron lone pairs of the electron donor is directed into sigma* antibonding orbitals of X--H bonds of the electron acceptor and leads to elongation of the bond and a red shift of the X--H stretching frequency (standard H-bonding). However, pronounced electron-density transfer from electron lone pairs of the electron donor also leads to reorganization of the electron density in the electron donor, which results in changes in geometry and vibrational frequency. These changes are largest for the C--H bonds of formic acid and formate anion, which do not participate in H-bonding. The resulting blue shift of this stretching frequency is substantial and amounts to almost 35 and 170 cm(-1), respectively.

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.

Car–Parrinello Molecular Dynamics Study of the Blue-Shifted F3CH⋅⋅⋅FCD3 System in Liquid N2

Fluoroform, as confirmed by both experimental and theoretical studies, can participate in improper H-bond formation, which is characterized by a noticeable increase in the fundamental stretching frequency nu(C-H) (so-called blue frequency shift), an irregular change of its integral intensity, and a C-H bond contraction. A Car-Parrinello molecular dynamics simulation was performed for a complex formed by fluoroform (F3CH) and deuterated methyl fluoride (FCD3) in liquid nitrogen. Vibrational analysis based on the Fourier transform of the dipole moment autocorrelation function reproduces the blue shift of the fundamental stretching frequency nu(C-H) and the decrease in the integral intensity. The dynamic contraction of the C-H bond is also predicted. The stoichiometry of the solvated, blue-shifted complexes and their residence times are examined.

Theoretical Investigations into the Blue-Shifting Hydrogen Bond in Benzene Complexes

The benzene...X complexes (X=benzene, antracene, ovalene) were optimised at the MP2/6-31G** level with the C2v symmetry of the complex and planarity of the proton acceptor being preserved. The resulting stabilisation energies amount to 1.2, 2.3 and 2.9 kcal mol(-1), and the C-H bond of the proton donor is contracted by 0.0035, 0.0052 and 0.0055 A, respectively. The contraction is connected with a blue-shift of the C-H stretch vibration frequency. A two-dimensional anharmonic vibration treatment based on a MP2/6-31G** potential energy surface yields the following blue shifts for the complexes studied: 28, 42 and 43 cm(-1). The dominant attraction in the complexes is London dispersion, while the attractive contribution from electrostatic quadrupole-quadrupole interactions is considerably smaller.