In the infrared spectra of solutions in liquid argon of dimethyl ether ((CH3)2O) and fluoroform (HCF3), bands due to a 1:1 complex between these monomers have been observed. The C−H stretch of the HCF3 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 (CH3)2O 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 ((CH3)2O) to fluorine lone electron pairs of the proton donor (CHF3) which yields e...

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

Complexes of haloforms of the type HCCl(n)F(3-)(n) (n = 1-3) with dimethyl ether have been studied in liquid argon and liquid krypton, using infrared spectroscopy. For the haloform C[bond]H stretching mode, the complexation causes blue shifts of 10.6 and 4.8 cm(-1) for HCClF(2) and HCCl(2)F, respectively, while for HCCl(3) a red shift of 8.3 cm(-1) is observed. The ratio of the band areas of the haloform C[bond]H stretching in complex and monomer was determined to be 0.86(4) for HCClF(2), 33(3) for HCCl(2)F, and 56(3) for HCCl(3). These observations, combined with those for the HCF(3) complex with the same ether (J. Am. Chem. Soc. 2001, 123, 12290), have been analyzed using ab initio calculations at the MP2[double bond]FC/6-31G(d) level, and using some recent models for improper hydrogen bonding. Ab initio calculations on the haloforms embedded in a homogeneous electric field to model the influence of the ether suggest that the complexation shift of the haloform C[bond]H stretching is largely explained by the electric field effect induced by the electron donor in the proton donor. The model calculations also show that the electric field effect accounts for the observed intensity changes of the haloform C[bond]H stretches.

Improper or classical hydrogen bonding? A comparative cryosolutions infrared study of the complexes of HCClF(2), HCCl(2)F, and HCCl(3) with dimethyl ether.

This article provides a summary of our studies of hydrogen-bonded complexes during the decade of the 90's. These studies began with systematic investigations of the methodological dependence of the computed structures and binding energies of these complexes. The MP2/6-31+G(d,p) level of theory was identified as the minimum level required to obtain reliable structures, while reliable energetics required larger polarized split-valence basis sets that include diffuse functions. While the experimental frequency shift of the A–H stretching band upon formation of an A–H–B hydrogen bond could also be reproduced at MP2/6-31+G(d,p) for a variety of hydrogen-bonded complexes, significant discrepancies were observed for others, including complexes of HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines. Resolving these discrepancies became the primary focus of our work, and redefined our research efforts. We solved a model two-dimensional nuclear Schrodinger equation to obtain anharmonic dimer- and proton-stretching frequencies, modeled matrix effects with external electric fields, and characterized hydrogen bond types as traditional, proton-shared, and ion-pair. We were able to resolve the observed discrepancies between theory and experiment, and explain the rather disparate effects of matrices on the IR spectra of closely related complexes. We also initiated studies of the NMR properties of the chemical shift of the hydrogen-bonded proton, and the A–B spin–spin coupling constant across the A–H–B hydrogen bond. We demonstrated the dominance of the Fermi-contact term for determining coupling constants in complexes with N–H–N, N–H–O, O–H–O, and Cl–H–N hydrogen bonds, and the distance dependence of this term. We also showed that the IR anharmonic proton-stretching frequency and the NMR spin–spin coupling constant are spectroscopic fingerprints of hydrogen bond type, which provide information about intermolecular distances in hydrogen-bonded complexes.

What a difference a decade makes: progress in ab initio studies of the hydrogen bond

Experimental observations show that direct olefin epoxidation by H2O2, which is extremely sluggish otherwise, occurs in fluorinated alcohol (RfOH) solutions under mild conditions requiring no additional catalysts. Theoretical calculations of ethene and propene epoxidation by H2O2 in the gas phase and in the presence of methanol and of two fluorinated alcohols, presented in this paper, show that the fluoro alcohol itself acts as a catalyst for the reaction by providing a template that stabilizes specifically the transition state (TS) of the reaction. Thus, much like an enzyme, the fluoro alcohol provides a complementary charge template that leads to the reduction of the barrier by 5−8 kcal mol-1. Additionally, the fluoro alcohol template keeps the departing OH and hydroxyalkenyl moieties in close proximity and, by polarizing them, facilitates the hydrogen migration from the latter to form water and the epoxide product. The reduced activation energy and structural confinement of the TS over the fluoro alcoh...

Fluorinated alcohols enable olefin epoxidation by H2O2: Template catalysis

Recently developed electron-poor Pt(II) catalyst 1 with the “green” oxidant 35% hydrogen peroxide displays high activity and complete substrate selectivity in the epoxidation of terminal alkenes because of stringent steric and electronic requirements. In the presence of isolated dienes bearing terminal and internal double bonds, epoxidation is completely regioselective toward the production of terminal epoxides. Insight into the mechanism is gained by means of a reaction progress kinetic analysis approach that underlines the peculiar role of 1 in activating both the alkene and H2O2 in the rate-determining step providing a rare example of nucleophilic oxidation of alkenes by H2O2.

Regioselectivity and diasteroselectivity in Pt(II)-mediated "green" catalytic epoxidation of terminal alkenes with hydrogen peroxide: mechanistic insight into a peculiar substrate selectivity.

Matrix isolation infrared spectroscopy has been combined with MP2/6-31+G(d,p) calculations to characterize the 1:1 hydrogen-bonded complex between H2O2 and (CH3)2O. The O−H stretching mode was observed to red shift 234 cm-1 upon hydrogen bond formation, while a 45 cm-1 blue shift was noted for the O−O−H bending mode of the H2O2 subunit in the complex. These values compare well to the computed shifts of −293 and +20 cm-1, respectively. The perturbations to the vibrational modes of the two subunits in the HOOH:O(CH3)2 complex are substantially larger than the perturbations reported previously for the analogous HOH:O(CH3)2 complex, suggesting that H2O2 is a better proton donor for hydrogen bonding than H2O. In contrast, band shifts in HOOH:O(CH3)2 are much less than observed for FH:O(CH3)2 and ClH:O(CH3)2.

James Goebel and

Papers

Matrix Isolation and ab Initio Study of the Hydrogen-Bonded Complex between H2O2 and (CH3)2O