@misc{oai:ir.soken.ac.jp:00000207,
 author = {小杉, 健太郎 and コスギ, ケンタロウ and KOSUGI, Kentaroh},
 month = {2016-02-17, 2016-02-17},
 note = {Intermolecular interaction is a principal subject of chemistry for understanding the nature of substances and chemical reactions. For example, hydrogen-bonding interactions in such as O-H･･･O, N-H･･･O and N-H･･･N pairs are very important as well as hydrophobic interaction in structural biology. In recent years, other types of intermolecular interactions are recognized to play important roles in biological macromolecules. It is found that not only the typical hydrogen bonds that involve electronegative nitrogen and oxygen atoms but also the C-H･･･O hydrogen bond must be considered in the determination of protein structures. In addition to the C-H･･･O hydrogen bond, intermolecular interaction between a positively charged ion and an electron rich organic molecule (cation-π interaction) is frequently seen on the protein surfaces which is exposed to aqueous solvation effecting on the protein structures. Charge transfer is an essential factor in an accurate description of the cation-π interaction. These studies suggest that quantitative understanding of the charge transfer interactions between fundamental molecules of biological importance is indispensable particularly for the study of protein structures and functions.
 Carboxyl group is a common functional group in living bodies. It is contained in numbers of biologically important molecules; therefore a deep understanding of many biological phenomena should be based on the knowledge on the intermolecular interaction of carboxylic groups with each other or with other molecules. It is known that molecules with carboxylic groups show specific hydrogen-bonding interactions with each other or with other hydrogen-donating or -accepting molecules.   Intermolecular interaction of acetic acid has long been studied as a prototype system of carboxylic compounds. Because acetic acid has four hydrogen-donor sites (a hydroxyl hydrogen and three methyl hydrogens) and two acceptor sites (a hydroxyl oxygen and a carbonyl oxygen) in a molecule, various kinds of hydrogen-bondings can be expected in various situations. In addition to the hydrogen bond formation, non-bonding orbitals of acetic acid can interact with virtual orbitals of positive charged ions as electron-donors.
In order to understand the nature of intermolecular interaction of carboxyl group, the present study is devoted to investigating the structures of liquid acetic acid, acetic acid aqueous solutions, and acetic acid-benzene cation complexes.
 In chapter II, liquid structure of acetic acid is studied experimentally and theoretically. Experimentally, Raman spectra of acetic acid at various temperatures between 287 and 348 K are measured in the region of 15-3700 cm-1. Theoretically, ab initio molecular orbital calculations are performed on the Raman activities of seven cluster species of acetic acid molecules. The Raman spectrum (in R( -v) representation) of crystalline acetic acid at 287 K shows six distinct bands in the 15-300 cm-1 region. These bands broaden on the melting of the crystal, while their peak positions remain almost unchanged on the melting. These spectral changes are reproduced in the case that the liquid spectrum mainly arises from a variety of sizes of chain clusters as the fragments of the crystalline networks. The C=O stretching band becomes broadened toward higher wavenumbers and exhibits an asymmetric shape with increasing temperature.   The wavenumbers calculated for the C=O stretching vibrations suggest that the strongly hydrogen-bonded C=O groups of the chain clusters show the prominent C=O band and its asymmetric shape is due to the presence of weakly hydrogen-bonded C=O groups of the same cluster species. The spectral analyses in both the low-wavenumber and the C=O stretching regions suggest that liquid acetic acid is mainly composed of the chain clusters, not the cyclic dimer. Assignments of the low-frequency Raman bands observed in the vapor and crystalline states are discussed on the basis of the calculated wavenumbers.
In chapter III, liquid structures of acetic acid aqueous solutions are studied using Raman spectroscopy and ab initio molecular orbital calculation. With the addition of water into liquid acetic acid, the C=O stretching vibration band of acetic acid shows high frequency shift from 1665 cm-1 to 1715 cm-1.   This means that the hydrogen-bond of the C=O group of acetic acid is not so strong as those seen in liquid acetic acid or in CCl4 solution (in which the band appears at 1668 cm-1). A bent type hydrogen-bond is accountable for this observation. On the other hand, the increase of acetic acid in water drastically decreases the intensity of the hydrogen-bonded O-H stretching Raman band of water at 3200 cm-1. This suggests that acetic acid breaks the hydrogen-bond networks of water.  Low frequency R( -v) spectra of acetic acid/water binary solutions are reexamined with new experimental data and ab initio molecular orbital analysis of intermolecular vibrational modes. The R( -v) spectrum of the aqueous mixture at xA = 0.5 bears a very close resemblance to that of the acetic acid/methanol mixture with xA = 0.5, indicating that the molecular complexes responsible to the Raman spectra are acetic acid clusters. The calculated low-frequency Raman feature of a side-on type dimer with bent-type hydrogen-bonds based on ab initio molecular orbital theory reproduces the observed Raman pattern nicely. Any evidence of the formation of stable acid-water pairs is not found in the low frequency Raman spectra. Furthermore, an isosbestic point is seen in the region of 0.1〓xA (mole fraction of acetic acid)〓0.5, and another one is also observed in 0.5〓 xA〓1.0. The observed spectra in the region of 0 < xA < 0.5 are reproduced simply by linear combinations of the pure water spectrum and the spectrum at xA = 0.5. These results strongly suggest the presence of the two microphases with homogeneously associated molecules: a water cluster phase and an acetic acid cluster phase. The spectral change in 0.5 < xA < 1.0 is attributed to the coexistence of the acetic acid cluster phase in aqueous environment and the acid associated phase characteristic of liquid acetic acid.
The author demonstrates geometrical and electronic structures of acetic acid-benzene cation complex, (CH3COOH)･(C6H6)+, experimentally and theoretically in chapter IV. Experimentally, a vibrational spectrum of (CH3COOH)・(C6H6)+ in the supersonic jet is measured in the 3000-3680 cm-1 region using an ion-trap photodissociation spectrometer. An electronic spectrum is also observed with this spectrometer in the 12000-29600 cm-1 region. Theoretically, ab initio molecular orbital calculations are performed for geometry optimization and evaluation of vibrational frequencies and electronic transition energies. The vibrational spectrum shows two distinct bands in the O-H stretching vibrational region. The frequency of the strong band (3577 cm-1) is close to that of the O-H stretching vibration of acetic acid (3583 cm-1) and the weak one is located at 3617 cm-1. On the basis of geometry optimizations and frequency calculations, the strong band is assigned to the O-H stretching vibration of the cis-isomer of acetic acid in the hydrogen-bonded complex (horizontal cis-isomer). The weak one is assigned to the vertical trans-isomer where the trans-isomer of acetic acid interacts with the π-electron system of the benzene cation. The weakness of the high frequency band in the photodissociation spectrum is attributed to the binding energy larger than the photon energy injected. Only hot vertical trans-isomers can be dissociated by the IR excitation. The electronic spectrum exhibits two bands with intensity maxima at 17500 cm-1 and 24500 cm-1. The calculations of electronic excitation energies and oscillator strengths suggest that charge transfer bands of the vertical trans-isomer can be observed in this region in addition to a local excitation band of the horizontal cis-isomer. The 17500 cm-1 band is attributed to the charge transfer transition of the vertical trans-isomer and the 24500 cm-1 band is assigned to the π-π transition of the horizontal cis-isomer. The calculations also suggest that the charge transfer is induced through the intermolecular C･･･O=C bond formed between a carbon atom of benzene and the carbonyl oxygen atom of acetic acid.
In chapter V, geometrical structures of acetic acid monomer-benzene dimer cation cluster, (CH3COOH)・(C6H6)2+, and acetic acid dimer-benzene dimer cation cluster, (CH3COOH)2・(C6H6)2+, are investigated. A vibrational spectrum of (CH3COOH)・(C6H6)2+ in the supersonic jet is measured in the 2800-3700 cm-1 region using the ion-trap photodissociation spectrometer. 
An electronic spectrum of this cluster cation is also observed with this spectrometer and a time-of-flight type spectrometer in the 6000-24500 cm-1 region. For (CH3COOH)2・(C6H6)2+, vibrational and electronic spectra are observed in the regions of 2740-3700 cm-l and 6000-27000 cm-1, respectively. In order to clarify the assignment of vibrational bands, vibrational spectra of the deuterated cluster cations,      (CD3COOD)･(C6H6)2+ and (CD3COOD)2･(C6H6)2+, are measured.  The electronic spectrum of (CH3COOH)・(C6H6)2+ show a broad and strong band in the near-IR region and another band at 22750 cm-1. This spectral feature resembles that of benzene dimer cation, (C6H6)2+ which has a strong charge resonance (CR) band in the near-IR region and a local excitation (LE) band with an absorption cross section one order smaller than the CR band. This fact suggests that the ion core of (CH3COOH)・(C6H6)2+ is (C6H6)2+. In the vibrational spectrum of (CH3COOH)・(C6H6)2+, a strong band is located at 3084 cm-1 and two weak bands are observed at 3585 and 3627 cm-1. From comparison between the vibrational spectra of (CH3COOH)・(C6H6)2+ and (CD3COOD)・(C6H6)2+, the strong band is assigned to the C-H stretching vibration of (C6H6)2+. As mentioned above, the frequency of the free O-H stretching vibration of acetic acid monomer in the gas phase is 3583 cm-1. Thus, the band at 3585 cm-1 is assigned to the free O-H stretching vibration of the cis-isomer of acetic acid in (CH3COOH)・(C6H6)2+. The band at 3627 cm-1 is attributed to the O-H stretching vibration of the trans-isomer of acetic acid, because its frequency is predicted to be 51 cm-1 (without scaling) 
higher than that of the cis-isomer at the CASSCF(4,3)/6-31G(d,p) level. In the electronic spectrum of (CH3COOH)2・(C6H6)2+, CR and LE bands are also observed. Therefore, the ion core of (CH3COOH)2・(C6H6)2+ is (C6H6)2+. The vibrational spectrum of (CH3COOH)2・(C6H6)2+ show very broad band from 3400 cm-1 to 2740 cm-1 with several peaks and no band is observed in the free O-H stretching vibrational region. On the basis of the vibrational spectrum of (CD3COOD)2・(C6H6)2+ measured in the present study and the reported IR spectrum of the cyclic dimer of acetic acid in argon matrix, the vibrational spectrum of (CH3COOH)2・(C6H6)2+ is regarded as a superposition of those of (C6H6)2+ and the acetic acid cyclic dimer. This indicates that (C6H6)2+ weakly interacts with the cyclic dimer of acetic acid in (CH3COOH)2・(C6H6)2+., application/pdf, 総研大甲第508号},
 title = {Studies on intermolecular interaction of acetic acid: hydrogen-bonding and charge-transfer interaction in neat liquid, aqueous solutions, and gas phaseclusters with benzene cations},
 year = {}
}