OH Torsion Research Articles

ChemInform Abstract: A CNDO/2 STUDY OF THE OH‐TORSION IN PHENOL AND ITS HYDROGEN BONDED COMPLEX WITH PYRIDINE

Abstract The CNDO/2 molecular orbital method has been applied to the study of the OH torsion, in phenol and phenol—pyridine hydrogen bonded complex. The calculated torsional barrier (13.58 kJ mol−1) and force constant (5.4 × 10−20 J rad−2) of phenol agree well with the experimental quantities. The calculated force constant of the corresponding vibration in phenol—pyridine is increased sixfold, reproducing closely the rise in the torsional frequency observed when phenol is complexed to strong acceptors. It is shown that according to CNDO theory, most of the increase can be attributed to the influence of the intermolecular force field and not to a major change in the torsional force constant.

A CNDO/2 study of the OH torsion in phenol and its hydrogen bonded complex with pyridine

The CNDO/2 molecular orbital method has been applied to the study of the OH torsion, in phenol and phenol—pyridine hydrogen bonded complex. The calculated torsional barrier (13.58 kJ mol −1) and force constant (5.4 × 10 −20 J rad −2) of phenol agree well with the experimental quantities. The calculated force constant of the corresponding vibration in phenol—pyridine is increased sixfold, reproducing closely the rise in the torsional frequency observed when phenol is complexed to strong acceptors. It is shown that according to CNDO theory, most of the increase can be attributed to the influence of the intermolecular force field and not to a major change in the torsional force constant.

Fluoroalcohols—XXI. Infrared, matrix infrared and Raman spectra of 1,1,1-trifluoro-2-propanol and its three deuterated analogues

The i.r. spectra of CF 3CHOHCH 3, CF 3CHODCH 3, CF 3CDOHCH 3 and CF 3CDODCH 3 have been recorded for gaseous, liquid and solid states, and in argon and nitrogen matrices (the spectrum of CF 3CHOHCH 3 in a krypton matrix as well). Raman spectra of the compounds in the liquid state were also recorded. Vibrational assignments are made, and discussed with particular reference to OH and OD groups. The influence of association on the vibration bands was studied in detail by the matrix isolation technique. Only one conformer was found for the alcohols in question. The monomeric νOH band is a singlet for spectra in argon and krypton matrices; for spectra in nitrogen matrices it is a doublet. Two monomeric OD bending bands were found for the OD deuterated alcohols between 800 and 1000 cm −1. CF 3CHOHCH 3 seems to have two polymeric δOH bands. The monomeric OH and OD torsion bands are very intense and narrow in argon matrices (τOH band at about 300 cm −1, τOD band at about 250 cm −1). In nitrogen matrices these are broader, less intense, and occur at considerably higher frequencies (τOH at about 380 cm −1). The OH torsion bands of a few other fluoroalcohols in argon were also studied. Some data are reported for trifluoroacetone in an argon matrix.

Vapour phase infrared studies of alcohols - I. Intramolecular interactions and self-association

Infrared spectra are reported of methanol, ethanol, propan-2-ol, 2-methylpropan-2-ol, 2, 2, 2-trifluoroethanol, 2, 2, 3, 3-tetrafluoropropan-1-ol and 1, 1, 1, 3, 3, 3-hexafluoropropan-2-ol in the vapour phase in a 1 m path-length cell at pressures up to the s. v. p. of the alcohol. Also reported are 40 m path-length vapour and argon matrix spectra of trifluoroethanol. Dimer absorptions are identified for all the alkanols and for trifluoroethanol, and some estimates of ∆ H provided. Trifluoroethanol exhibits a striking series of sum-and-difference bands of the OH stretch with other fundamentals, 14 summation modes being observed extending in frequency to over 5000 cm -1 and involving most of the fundamentals below 1500 cm -1 . Other fluoroalcohols with fluorines substituted on β carbon atoms exhibit similar sum-and-difference modes. The alkanols exhibit only one such pair of bands due to the OH stretch plus or minus the OH torsion, and this provides a convenient method of measuring the torsional frequencies. The different behaviour of the fluoroalcohols is attributed to intramolecular hydrogen bonding.

Fluoroalcohols—XX

The infrared spectra of hexafluoro-2-propanol and its three deuterated analogues in the gaseous, liquid and solid states and the Raman spectra of the compounds in the liquid state were recorded. Assignments of the vibration bands are made, and especially the vibrations related to the OH and OD groups are discussed. Two bands due to C1 and C3, conformers are found in the OH stretching region; several pairs of sum and difference bands related to the lower-frequency (C3) band are seen in the vapour spectra. Association “fingers” occur in the spectra of the condensed phases between 2600 and 2900 cm−1. Two association bands due to in-plane OH bending were found in the spectra of (CF3)2CHOH. No association band due to OH torsion was found with certainty in the liquid phase spectra of any of the compounds.

The 2750 Å electronic band system of phenol: Calculation of out-of-plane frequencies and sequence intensities

Force constant calculations are reported for the out-of-plane vibrational modes of phenol in its electronic ground state and first singlet excited state at 36349 cm −1, using a limited potential energy matrix of 22 nonzero elements. These calculations, which are exploratory in nature, were undertaken in an attempt to evaluate three questions of general interest, namely, (i) Can the intensity of the Δ v = 0 transitions (sequences) in an electronic spectrum be harnessed to check the assignment of nontotally symmetrical vibrations? (ii) Does the fact that the out-of-plane vibration frequencies of phenol in its electronic ground state are essentially the same as those of fluorobenzene indicate that the OH torsion is only loosely coupled to the phenyl modes? (iii) Is a C 2 v classification of practical value for the out-of-plane modes of phenol in either electronic state? It is found that (i) and (ii) can be answered affirmatively, with slight reservations. The C 2 v species are of some value in classifying displacements in the phenyl group, but may be misleading if applied indiscriminately.

Far infrared studies of the hydrogen bond of phenols

By means of a novel sampling techniques which involves using a polyethylene matrix, the far infrared vibrations of the hydrogen bond (—OH ⋯ O—vibrations) of ten phenols have been assigned to the 187–98 cm −1 range. The —OH out-of-plane deformation (γOH) of these hydrogen bonded phenols and the —OH torsion for these phenol monomers have been assigned in the 690-280 cm −1 range. More than one γOH frequency has been found in every compound that has more than one strong —OH stretching frequency (νOH), indicating distinctive γOH frequencies for different hydrogen bonded species. This implies that the broadness of νOH is at least partly due to the existence of different polymeric species. Making use of the L ippincott-S chroeder potential function, the frequency of the hydrogen bond stretching vibration (νσ) of solid phenol has been calculated. Reasonable agreement with the observed frequency is obtained when a reduced mass of the entire phenol molecule is used. From this observed phenol frequency, the frequencies of νσ for the other solid phenols can be predicted indicating that the vibration is not localized in the —OH ⋯ O— group. For the same phenols in the liquid state the frequency of νσ could not be predicted and there was no direct relationship between the —OH stretching frequency (or hydrogen bond strength) and νσ.