Theoretical study of the blue-shifting hydrogen bonds between CH2X2 and CHX3 (X=F, Cl, Br) and hydrogen peroxide

Intermolecular Interaction in Methylene Halide (CH2F2, CH2Cl2, CH2Br2 and CH2I2) Dimers

The intermolecular interaction in difluoromethane, dichloromethane, dibromomethane, and diiodomethane dimers has been investigated using high level quantum chemical methods. The potential energy curve of intermolecular interaction along the C⋯C bond distance obtained using the coupled-cluster theory with singles, doubles, and perturbative triples excitations CCSD(T) were compared with values given by the same method, but applying the local (LCCSD(T)) and the explicitly correlated (CCSD(T)-F12) approximations. The accuracy of other theoretical methods-Hartree-Fock (HF), second order Møller-Plesset perturbation (MP2), and dispersion corrected DFT theory-were also presented. In the case of MP2 level, the canonical and the local-correlation cases combined with the density-fitting technique (DF-LMP2)theories were considered, while for the dispersion-corrected DFT, the empirically-corrected BLYP-D and the M06-2Xexchange-correlation functionals were applied. In all cases, the aug-cc-pVTZ basis set was used, and the results were corrected for the basis set superposition error (BSSE) using the counterpoise method. For each molecular system, several dimer geometries were found, and their mutual orientations were compared with the nearest neighbor orientations obtained in recent neutron scattering studies. The nature of the intermolecular interaction energy was discussed.

A theoretical investigation on the conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule

The conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule is investigated theoretically using the ab initio MP2/aug-cc-pvdz and DFT-based M062X/6-311++G(d,p) methods. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes along with a natural bond orbital (NBO) analysis including the NBO charges, the hybridization of the C atom and the intra- and intermolecular hyperconjugation energies. In the two most stable conformers, the CH bond of the F2HCO- group occupies the gauche position. The hyperconjugation energies are about the same for both conformers and the conformational preference depends on the interaction between the non-bonded F and H atoms. The deprotonation enthalpies of the CH bonds are about the same for both conformers, the proton affinity of the less stable conformer being 3 kcal mol−1 higher. Both conformers of desflurane II interact with water forming cyclic complexes characterized by CH…O and OH…F hydrogen bonds. The binding energies are moderate, ranging from −2.4 to −3.2 kcal mol−1 at the MP2 level. The origin of the blue shifts of the ν(CH) vibrations is analyzed. In three of the complexes, the water molecule acts as an electron donor. Interestingly, in these cases a charge transfer is also directed to the non bonded OH group of the water molecule. This effect seems to be a property of polyfluorinated ethers.

Strong hyperconjugative interactions in isolated and water complexes of desflurane: a theoretical investigation

Ab initio MP2/aug-cc-pvDZ and density functional B3LYP calculations with the 6-311++G(d,p) basis set are performed to investigate the conformation of desflurane (CHF2OCHFCF3), its acidity/basicity and its interaction with one water molecule. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes, the binding energies with water, and a detailed natural bond orbital (NBO) analysis Iincluding the NBO charges, the hybridization of the C atoms and the intra- and intermolecular hyperconjugations. The relative energies of the two most stable conformers are discussed as a function of the total hyperconjugative energies resulting from the interaction of lone pairs of the O and F atoms to the different antibonding orbitals of desflurane. The proton affinity is the same for both conformers but the acidity of the CH bond is larger for the less stable conformer. The binding energies of the complexes of two desflurane conformers with one water molecule range from -2.75 to -3.23 kcal mol(-1). Depending on the structure of the complexes, the CH bonds involved in the interaction are contracted or elongated. The σ*(CH) occupation predominates over the hybridization effect in determining the CH bond length. There is an unexpected charge transfer to the external OH bond of the water molecule. This effect is in good agreement with theoretical data on the complexes between fluorinated dimethyl ethers and water and seems to depend on the number of F atoms implanted on the ether molecule.

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.

Anomeric Effects in the Symmetrical and Asymmetrical Structures of Triethylamine. Blue-Shifts of the C−H Stretching Vibrations in Complexed and Protonated Triethylamine

Quantum mechanical calculations using density functional theory with the hybrid B3LYP functional and the 6-31++G(d,p) basis set are performed on isolated triethylamine (TEA), its hydrogen-bond complex with phenol, and protonated TEA. The calculations include the optimized geometries and the results of a natural bond orbital (NBO) analysis (occupation of sigma* orbitals, hyperconjugative energies, and atomic charges). The harmonic frequencies of the C-H stretching vibrations of TEA are predicted at the same level of theory. Two stable structures are found for isolated TEA. In the most stable symmetrical structure (TEA-S), the three C-C bond lengths are equal and one of the C-H bond of each of the three CH2 groups is more elongated than the three other ones. In the asymmetrical structure (TEA-AS), one of the C-C bonds and two C-H bonds of two different CH2 groups are more elongated than the other ones. These structures result from the hyperconjugation of the N lone pair to the considered sigma*(C-H) orbitals (TEA-S) or to the sigma*(C-C) and sigma*(C-H) orbitals of the CH2 groups (TEA-AS). The formation of a OH...N hydrogen bond with phenol results in a decrease of the hyperconjugation, a contraction of the C-H bonds, and blue-shifts of 28-33 cm-1 (TEA-S) or 40-48 cm-1 (TEA-AS) of the nus(CH2) vibrations. The nu(CH3) vibrations are found to shift to a lesser extent. Cancellation of the lone pair reorganization in protonated TEA-S and TEA-AS results in large blue-shifts of the nu(CH2) vibrations, between 170 and 190 cm-1. Most importantly, in contrast with the blue-shifting hydrogen bonds involving C-H groups, the blue-shifts occurring at C-H groups not participating in hydrogen bond formation is mainly due to a reduction of the hyperconjugation and the resulting decrease in the occupation of the corresponding sigma*(C-H) orbitals. A linear correlation is established between the C-H distances and the occupation of the corresponding sigma*(C-H) orbitals in the CH2 groups.

Blue-Shifted A−H Stretching Modes and Cooperative Hydrogen Bonding. 1. Complexes of Substituted Formaldehyde with Cyclic Hydrogen Fluoride and Water Clusters

The structures and vibrational spectra of the intermolecular complexes formed by insertion of substituted formaldehyde molecules HRCO (R = H, Li, F, Cl) into cyclic hydrogen fluoride and water clusters are studied at the MP2/aug-cc-pVTZ computational level. Depending on the nature of the substituent R, the cluster type, and its size, the C-H stretching modes of HRCO undergo large blue and partly red shifts, whereas all the F-H and O-H stretching modes of the conventional hydrogen bonds are strongly red-shifted. It is shown that (i) the mechanism of blue shifting can be explained within the concept of the negative intramolecular coupling between C-H and C=O bonds that is inherent to the HRCO monomers, (ii) the blue shifts also occur even if no hydrogen bond is formed, and (iii) variation of the acceptor X or the strength of the C-H...X hydrogen bond may either amplify the blue shift or cause a transition from blue shift to red shift. These findings are illustrated by means of intra- and intermolecular scans of the potential energy surfaces. The performance of the negative intramolecular coupling between C-H and C=O bonds of H(2)CO is interpreted in terms of the NBO analysis of the isolated H(2)CO molecule and H(2)CO interacting with (H2O)n and (HF)n clusters.