Self-Association of Acetic Acid in Dilute Deuterated Chloroform. Wide-Range Spectral Reconstructions and Analysis using FTIR Spectroscopy, BTEM, and DFT Martin Tjahjono,* Shuying Cheng, Chuanzhao Li, and Marc Garland Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, Singapore 627833, Singapore ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: October 6, 2010 The binary solution of acetic acid in CDCl 3 was studied at room pressure on the interval T ) 293-313 K with a series of acetic acid concentrations up to 0.16 M. In-situ Fourier transform infrared (FTIR) spectroscopy measurements on the interval of 400-3800 cm -1 were utilized as the analytical method to monitor the spectral changes due to self-association of acetic acid. The band-target entropy minimization (BTEM) algorithm was employed to reconstruct the underlying pure component spectra. Analysis successfully provided two major spectral estimates of acetic acid, namely, the monomer (primarily in the form of monomer-CDCl 3 complex) and the centrosymmetric cyclic dimer. In addition, analysis provided one minor spectral estimate containing signals from both noncyclic dimers and higher aggregates. Also, spectral estimates were obtained for phosgene and water which were present at trace levels even though considerable precaution was taken to conduct the experiments under anhydrous and anaerobic conditions. Density functional theory (DFT) calculation was performed to assign the acetic acid structures corresponding to the BTEM spectral estimates. Since the structure of dilute acetic acid has been the subject of numerous studies, the present investigation helps to resolve some issues concerning the speciation of acetic acid at low concentrations in low polarity solvents. In particular, the present study provides for the first time, wide-range spectral reconstructions of the species present. 1. Introduction Molecular self-association occurs in numerous intermolecular hydrogen-bonding situations involving O-H ··· O, N-H ··· N, N-H ··· O, and so forth. Such self-association, particularly in dilute solutions, typically results in the formation of small aggregates. Small aggregates of water, 1 ammonia, 2 methanol, 3 and so forth have been the subject of spectroscopic as well as first-principal studies. Studies of molecular self-association have been of fundamental interest in numerous physical, 4 chemistry, and biology 5 as well as astrophysical contexts. 6 The chemistry of carboxylic acids, and in particular the self- association of acetic acid, has also been the subject of numerous spectroscopic and theoretical investigations. 7-33 This interest is due in part to the relatively simple structure of acetic acid as well as to the presence of both O ··· H-O and relatively weaker O ··· H-C hydrogen bonding. Infrared spectroscopic measure- ments of acetic acid have been performed in argon and nitrogen matrixes, 9-11,21,29,31 gas phase, 7,8,15,28 liquid phase, 8,22 and solution. 12-14,18,20,23,33 These measurements have revealed the presence of several acetic acid species, such as cis- and trans- monomers, 29 cyclic dimer, open (linear) dimers, 21,28,31 and higher aggregates. 13,16 The matrix isolation spectroscopy investigations have been particularly effective in varying the ratios of monomer to dimer (because of changes in pulse duration etc.), 21 and this has aided the assignment of infrared bands. Studies in the liquid phase are highly dependent on the polarity of the solvent, which affects the degree of self-association and the distribution of species present. 18,26 In contrast to gas-phase studies, considerable line broadening takes place in the condensed phase, and this certainly complicates spectral interpretation especially when there is considerable spectral overlap between species. The above examples suggest that experimental studies of self- association suffer from difficulties with spectral interpretation. In part, this is due to the inability to isolate any of the species present and is partly due to ever-present spectral overlap. Conventional band fitting deconvolution techniques are typically utilized to overcome these problems, but this is often restricted to rather narrow spectral windows. 12,16-18,20,22,23 Alternatively, a spectral reconstruction technique could be applied to overcome the problems of multicomponent spectral overlap and the restriction to narrow spectral windows. The resulting pure component spectral estimates can provide a much clearer assessment of the bands present as well their relative intensities. In the present study, a series of solutions of acetic acid in CDCl 3 were prepared and Fourier transform infrared (FTIR) spectroscopic measurements were performed at room pressure on the interval T ) 293-313 K. The collected spectra were analyzed with the band-target entropy minimization (BTEM) algorithm 34 to obtain the underlying pure component spectra of the nonisolable acetic acid species. The wide-range spectral reconstructions clearly showed the simultaneous signatures corresponding to the various types of C-O, CdO, O-H, and C-D vibrations. The identities of these species were then confirmed by comparing with density functional theory (DFT) predictions. The use of a deuterated solvent considerably simplified the analysis since nearly all C-H vibrations origi- nated from the acetic acid species. Since wide-range spectral reconstructions are rarely reported, the present study shows the utility of such signal processing in order to better understand the speciation occurring in self-associating systems. 2. Experimental Section Materials. The solvent deuterated chloroform (Cambridge Isotope Laboratories, Inc., 99.8%) was dried over activated * To whom correspondence should be addressed. Tel.: (65) 6796-3960; fax: (65) 6316-6185; e-mail: martin_tjahjono@ices.a-star.edu.sg. J. Phys. Chem. A 2010, 114, 12168–12175 12168 10.1021/jp106720v  2010 American Chemical Society Published on Web 11/02/2010