Cryospectroscopic and ab initio studies of haloform-trimethylamine H-bonded complexes.

The FTIR spectra of F(2)ClCH and Cl(3)CH(D) in mixtures with trimethylamine (TMA) have been studied in liquefied Kr in approximately 800-4000 cm(-1) frequency range. Spectroscopic evidence of a medium strength H-bond formation with conventional features has been found between these weak CH "proton donors" and TMA. The relative stability of the complexes has been determined in a series of temperature (T = 118-157 K) measurements of integrated intensities of vibrational bands ascribed to monomer and complex species. The complexes found are characterized by the red shift of the nu(1)(CH) stretching vibration of the haloforms studied. In the case of chloroform, the substantial red shift is accompanied by a noticeable broadening effect and by a marked growth of the intensity of this band. The spectroscopic changes caused by the complex formation have been also registered for vibrations of TMA. Specifically, the bands situated in the frequency domain of CH stretching vibrations of TMA show a weak blue shift, whereas the NC stretching vibrations are red-shifted. Ab initio MP2/6-311++G(2d,2p) a priori counterpoise corrected calculations, made for a series of haloforms and TMA, reproduce the main spectroscopic observations. The results obtained also suggest that the vibrational mode coupling between vicinal C-H and C-B (B = Hal, N) bonds is to a great extent responsible for the sign of these bond length changes and for the sign of the frequency shifts of the respective stretching vibrations upon complex formation.

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.

Ab initio studies of electron acceptor-donor interactions with blue- and red-shifted hydrogen bonds.

The features of blue- and red-shifted electron acceptor-donor (ACH/B) hydrogen bonds have been compared by using quantum chemical calculations. The geometry, the interaction energy and the vibrational frequencies of both blue- (ACH=F3CH, Cl3CH with B=FCD3) and red-shifted (ACH=F3CH, Cl3CH with B=NH3 and ACH=CH3CCH with B=FCD3, NH3) complexes were obtained by using ab initio MP2(Full)/6-31+G(d,p) calculations with the a priori basis-set superposition error (BSSE) correction method. One-dimensional potential energy and dipole moment functions of the dimensionless normal coordinate Q1, corresponding to the CH stretching mode of ACH, have been compared for both types of complexes. Contributions of separate components of the interaction energy to the frequency shift and the effect of electron charge transfer were examined for a set of intermolecular distances by using the symmetry-adapted perturbation theory (SAPT) approach and natural bond orbitals (NBO) population analysis.

A cryosolution infrared study of the complexes of fluoroform with ammonia and pyridine: Evidence for a C-H...N pseudo blue-shifting hydrogen bond.

Mid-infrared spectra of mixed solutions in liquid xenon containing fluoroform and either ammonia or pyridine have been investigated at temperatures between 173 and 213 K. For both Lewis bases, a new band is found in the CH stretching region at a frequency approximately 5 cm(-1) higher than that of monomer fluoroform, which is assigned to a complex between fluoroform and the Lewis base. A detailed analysis of the nu1/2nu(4) Fermi resonance in the proton donor shows that the blue shifts observed for the complexes are not caused by a strengthening of the CH bond during the complexation, but are due to the changes in the Fermi resonance interactions. Information on the nu1/2nu(4) Fermi resonance was also obtained for the complexes of fluoroform with dimethyl ether and trimethyl amine.