ROTATIONAL-ISOMERISM IN ACRYLIC-ACID - A COMBINED MATRIX-ISOLATED IR,RAMAN AND AB-INITIO MOLECULAR-ORBITAL STUDY

Citation
A. Kulbida et al., ROTATIONAL-ISOMERISM IN ACRYLIC-ACID - A COMBINED MATRIX-ISOLATED IR,RAMAN AND AB-INITIO MOLECULAR-ORBITAL STUDY, Journal of the Chemical Society. Faraday transactions, 91(11), 1995, pp. 1571-1585
Citations number
60
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
ISSN journal
09565000
Volume
91
Issue
11
Year of publication
1995
Pages
1571 - 1585
Database
ISI
SICI code
0956-5000(1995)91:11<1571:RIA-AC>2.0.ZU;2-9
Abstract
The results of a combined vibrational and structural study of the acry lic acid monomer undertaken by matrix-isolated low-temperature IR spec troscopy and ab initio SCF-HF and MP2 MO calculations are presented. I n addition, both Raman and IR spectra of liquid acrylic acid and the R aman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exi sts as a mixture of two conformers of similar energies, differing by t he relative orientation of the C=C-C=O axis. Upon irradiation at lambd a = 243 nm by a xenon lamp, the s-cis form (C=C-C=O dihedral angle equ al to 0 degrees), corresponding to the conformational ground state, co nverts to the s-trans form (C=C-C=O dihedral angle equal to 180 degree s). 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 spect ra. In turn, in the crystal only the thermodynamically most stable for m (scis) exists. Results of ab initio SCF-HF and MP2 molecular orbital (MO) calculations, in particular optimised geometries, relative stabi lities, dipole moments and harmonic force fields, for the relevant con formational states of acrylic acid are also presented and the conforma tional dependence of some relevant structural parameters is used to ch aracterise the most important intramolecular interactions present in t he studied conformers. Finally, the calculated vibrational spectra and both the results of a normal-mode analysis based on the theoretical h armonic force fields and of IR intensity studies based on the charge-c harge flux-overlap (CCFO) model were used to help interpret the experi mental vibrational data,enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.