Acetic Acid Monomer Research Articles

Acetic acid monomer: Ab initio study, barrier to proton tunnelling, and infrared assignment

The barrier to tunnelling of the carboxyl proton in monomeric acetic acid is found to be 96.7 kcal/mole in an ab initio study, which is not compatible with an earlier interpretation of the strong observed in the infrared matrix spectra of monomeric acetic acid and deuterated species. A new explanation is suggested for the anharmonic effects, and a vibrational assignment, supported by normal coordinate calculations is proposed. The ab initio SCF calculations were carried out with two different Gaussian basis sets for comparison. The values obtained for the methyl torsional barrier, for the energy difference between the trans OCOH and the more stable cis OCOH conformtion, and for the OH-torsional force constant (0.15 and 0.19 mdyn/Å) are in good agreement with earlier results and/or with experimental data. The main anharmonicities observed in the spectra are apparently due to Fermi resonance involving CO stretching and COH bending fundamentals and combination tones of skeletal breathing and bending motions.

Ionization energies of formic and acetic acid monomers

Ionization energies of formic and acetic acid monomers

The interaction between some ethyl esters of aminoacids and acetic acid in carbon tetrachloride

The dimerization constant, K 1 of acetic acid in CCl 4 is determined by an IR spectrometric method and is found to have a value of 2252M −1 at 20°. The association constants, K 2, for the interaction between the acetic acid monomer and some aminoacid ethyl esters are determined by the same method and found to have the values given (in M −1 at 20°) for the esters of the following aminoacids: glycine (282 ± 24), dl-alanine (236 ± 33), dl-α-aminobutyric acid (139 ± 11), α-aminoisobutyric acid (172 ± 18). dl-norvaline (104 ± 13), dl-valine (150 ± 8), dl-leucine (108 ± 18) and 3-phenyl- dl-alanine (140 ± 21). A correlation of these experimental values with some structural parameters related to the steric effect of the substituents on the esters is proposed.

Infrared spectra of trifluoroacetic acid and trifluoroacetic anhydride

Spectra of matrix-isolated trifluoroacetic acid species and matrix-isolated and solid trifluoroacetic anhydride are reported. Vibrational assignments are given for the CF 3COOH and CF 3COOD monomers, the i.r. active vibrations of (CF 3COOH) 2 and (CF 3COOD) 2 cyclic dimers, and (CF 3CO) 2O. The analysis suggests that a very low barrier hinders internal rotation of the CF 3 group in the acids and also indicates the probable absence of an equilibrium plane of symmetry in the monomers. The truth of these statements hinges on the interpretation of the data for the nominal COH bending vibration. For CF 3COOH monomer this band shows two components that are separated by ~15 cm −1, while CF 3COOD monomer yields a single, sharp band that is red-shifted by a normal amount. These observations, coupled with the anomalous isotopic frequency behavior found for this vibration with acetic acid monomers [C.V. Berney, R.L. Redington and K.C. Lin, J. Chem. Phys. 53, 1743 (1970)], reinforces the suggestion that a relatively low barrier hinders H atom transfer between the two O atoms via a coordinate that originates as COH angle bending.

On the OH Stretching and the Low-Frequency Vibrations of Carboxylic Acid Cyclic Dimers

Infrared spectra for the hydroxyl stretching region of matrix-isolated CF3COOH, CF3COOD, and CH3COOH are reported, and the vapor-phase spectrum of CF3COOH in the region below 500 cm−1 is given. Characteristic line spacings that are found to reproduce the observed spectral features of the high-frequency region are also found to be associated with the low-frequency spectra. Fundamental frequencies for the cyclic dimer ring vibrations are suggested. It is concluded that the well-known band broadening in the hydroxyl stretching region is due to combination bands of the low-frequency modes with the OH fundamental vibration and with nearby binary excitations of the COH(D) bending and the C–O stretching vibrations. A model such as that proposed by Marechal and Witkowski, which considers interactions between the low-frequency and the high-frequency vibrations, seems appropriate in order to explain the intensities of the high combination bands that occur. Further emphasis for the unusual vibrational properties of the mixed COH bending and C–O stretching coordinates in these acids is provided by the spectrum of matrix-isolated monomeric CF3COOH(D). In this case the nominal COH bending vibration of monomeric CF3COOH is clearly doubled while the corresponding COD vibration appears as a single band shifted to lower frequency. The behavior is compared with that for matrix-isolated acetic acid monomers.

Kinetics of the acid-catalysed ring-opening reaction of 2-phenyl-4,4-dimethyl-2-oxazolin-5-one with the ethyl ester of DL-alanine

The kinetic behaviour of the acetic acid catalysed reaction of 2-phenyl-4,4-dimethyl-2-oxazolin-5-one ( 1) with the ethyl ester of DL-alanine ( 2) in CCl 4, to give the ethyl ester of N-(N′-benzoyl-α-amino-isobutyryl)- DL-alanine ( 3) is investigated. Within the range of concentrations employed, the reaction follows the rate equation ν = k o + k[monomer] [oxa] [ester], where k o is the specific rate constant of the uncatalysed reaction, and k′ is that of the catalysed reaction. The influence of the variation of 1, 2 and acetic acid on the specific rate constant is studied. It is shown that only if the acetic acid monomer is regarded as the catalyst can the system be adequately described quantitatively. The participation of other acid species as catalysts is discussed.

A reinvestigation of the molecular structure of acetic acid monomer and dimer by gas electron diffraction

From the electron diffraction patterns of gaseous acetic acid at 160 °C for the monomer and at 24 °C for the dimer the following parameters for the molecular geometry were obtained (first value for monomer, second for dimer, standard deviations in parentheses): C-C 1.520(0.005), 1.506 (0.005) Å C-O 1.214 (0.003), 1.231 (0.003) Å C-O 1.364(0.003), 1.334 (0.004) Å C-H 1.102(0.010), 1.102 (0.015) Å C-CO 126.6 (0.6), 123.6 (0.8)° C-C-O 110.6 (0.6), 113.0 (0.8)° It is seen that considerable differences in the carboxyl group parameters are caused by hydrogen bonding. The hydrogen bond length O-H · · · O was found to be 2.680 (0.010) Å. In the equilibrium conformation of the monomer, a methyl hydrogen atom lies in the carboxylic plane at the side of the carbonyl oxygen atom; the barrier to internal rotation is too low to be determined accurately. The vibrational amplitudes used in the electron diffraction calculations were computed from spectroscopic data for the monomer available in the literature.

Infrared Spectra of Matrix-Isolated Acetic Acid Monomers

Infrared spectra of monomeric acetic acids [CH3COOH(D) and CD3COOH(D)] isolated in Ar and N2 matrices near 4°K are reported. A large anharmonic potential-energy contribution is evident from the observed isotopic frequency shifts. It is proposed that this arises primarily from the double-minimum potential-energy curve for tunneling of the H atom between O atoms via the COH angle bending coordinate. In all cases, important coupling between the COH angle bending and C–O stretching coordinates is observed, with a strong addition of CD3 (umbrella) angle deformation in the case of CD3COOH. Exceptional matrix effects are observed for the coupled vibrations of the –OH molecules. A complete assignment is proposed for these acetic acids on the basis of the new data. As additional supporting evidence, the spectrum of matrix-isolated CH3COF and of acetic acid vapor at low pressure and long path length are reported.

Normal-Coordinate Treatment of Acetic Acid Monomer and Dimer

A normal-coordinate treatment was carried out for acetic acid monomer, dimer, and their deutero analogs. All of the in-plane and out-of-plane vibrations were calculated using a modified Urey–Bradley force field. On the basis of the calculated frequencies and the potential-energy distributions, assignments of vibrational spectra were made for these substances.

The volumetric and dielectric behaviour of acetic acid in mixtures with nonpolar liquids

Results for the volume change of mixing of acetic acid with carbon tetrachloride, benzene, and cyclohexane are given. Further, the dielectric constant has been determined for acetic acid in carbon tetrachloride from spectroscopic measurements of the association equilibrium. The volume changes are both positive and large. The dielectric behaviour of acetic acid in carbon tetrachloride parallels that of acetic acid in benzene and n-heptane previously reported. Our results taken together with published data indicate: (i) the dipole moment of acetic acid monomer in the gas phase is higher than has been previously reported, (ii) the dipole moment of acetic acid monomer in solutions rich in acetic acid is considerably greater than in the gas phase, possibly because of a contribution of the trans-configuration, and (iii) a strong attractive interaction exists between acetic acid monomer and acetic acid dimer.

Entropy of the Monomeric Forms of Formic Acid and Acetic Acid

A recent measurement of the entropy of gaseous formic acid at its equilibrium vapor pressure is combined with vapor density data to give 60.0±0.3 as the entropy of the monomer at 25° and one atmosphere. The corresponding dimer entropy is 83.1±0.3 e.u. Pauling's residual entropy of R ln 2 per dimer weight for random orientation of hydrogen bonds in the crystal has been included in the above figures in order to allow a reasonable entropy for the torsional motion of the hydroxyl group in the monomer. The same correction has been applied to a previously published result for the acetic acid monomer to yield 70.1±1.0 as a revised value. It is highly probable that only a single significant potential minimum of undetermined breadth and depth occurs in the rotational cycle of the hydroxyl group.

The Entropy of Acetic Acid

From third law measurements, vapor pressures and vapor densities, the entropy of the acetic acid monomer at 25° and one atmosphere is 69.4±1.0 e.u. The value 68.7 is calculated from the vapor phase ethyl acetate equilibrium. For a model based upon acetone and approximately representing free rotation the entropy would be 72.7. If there is only a single potential minimum in the hydroxyl group rotational cycle, the large deficiency below the free rotation value is explained without assuming an exceptionally high potential barrier. A brief discussion of the effect of the number of potential minima and their relative depth is appended, and a possible source of error in third law measurements is suggested.