Rotational isomerism in acrylic acid. A combined matrix-isolated IR, Raman and ab initio molecular orbital study

Abstract

The results of a combined vibrational and structural study of the acrylic acid monomer undertaken by matrixisolated low-temperature IR spectroscopy and ab initio SCF-HF and MP2 MO calculations are presented. in addition, both Raman and IR spectra of liquid acrylic acid and the Raman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exists as a mixture of two conformers of similar energies, differing by the relative orientation of the C=C-C=O axis. Upon irradiation at 1 = 243 nm by a xenon lamp, the scis form (C-C-C-0 dihedral angle equal to 0"), corresponding to the conformational ground state, converts to the s-trans form (C=C-C=O dihedral angle equal to 180"). In the liquid phase, dimeric structures strongly predominate, but the existence in this phase of the two conformational states referred to above can also be inferred from the corresponding vibrational spectra. In turn, in the crystal only the thermodynamically most stable form (scis) exists. Results of ab initio SCF-HF and MP2 molecular orbital (MO) calculations, in particular optimised geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of acrylic acid are also presented and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the studied conformers. Finally, the calculated vibrational spectra and both the results of a normal-mode analysis based on the theoretical harmonic force fields and of IR intensity studies based on the chargecharge flux-overlap (CCFO) model were used to help interpret the experimental vibrational data, enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.

A combined matrix-isolation infrared spectroscopy and MO study of 1-amino-2-propanol

“…Since the most important stabilizing factor in these two forms is the presence of a relatively strong OH Á Á Á N intramolecular hydrogen bond, this result clearly shows the superior performance of the MP2 method to rationalize these interactions. Indeed, this is also in accordance with previous studies on other molecules where intramolecular hydrogen bonding is important [15,16]. The general agreement between the MP2 calculated structural parameters (including the length of the OH Á Á Á N hydrogen bond) and those reported for conformer gG H g by Marstokk and Mùllendal [6] is very good, except for OH bond length which is predicted 15.9 pm shorter by the calculations.…”

“…(see Table 4). It is well known that ab initio calculations carried out at the level of theory used here cannot account for some subtle effects associated with hydrogen bonding [15,23]. So, the involvement of the nitrogen atom in the OH Á Á Á N intramolecular hydrogen bond, where it acts exclusively as hydrogen bond acceptor, seems to contribute, at least in part, to the red shift of the frequency of the oNH 2 .…”

Infrared-induced conformational interconversion in carboxylic acids isolated in low-temperature rare-gas matrices

“…7. As expected on the basis of the previous studies on a,b-unsaturated molecules [44], the relative conformational energy in fumaric acid increases with the number of trans arrangements around the C a -C bonds. On the other hand, in the case of maleic acid [(E)-2-butenedioic acid] the cis arrangement around both C a -C bonds is not favored due to the strong repulsive interaction between the lone pairs of the carbonylic oxygen atoms (see Fig.…”

Photochemical reactivity of matrix-isolated monomeric carboxylic acids

“…1), the s-cis conformation being more stable than the s-trans by ca 2±3 kJ mol 21 [8,9]. In the case of crotonic acid, two different isomers exist differing in the relative position of the methyl and carboxyl groups around the double bond (the E and Z tautomers, see also Fig.…”

“…However, a de®nitive answer to this question implies the use of selective deuteration of the terminal yCH 2 group. It is important to note that in hexane solutions the l max of the ethylenic p±p p absorption band of acrylic and E-crotonic acids are 223 and 219 nm, respectively, with half bandwidths of ca 20±30 nm [8,9] and thus the irradiation was carried out in the long-wave wing of the absorption band. Furthermore, it is well known that the lowest accessible energy excited states in ethylenic compounds correspond, in general, to structures where the relative orientation of the fragments directly connected by the bond, which is formally a double bond in the ground state are nearly perpendicular to each other [12,13].…”