Substituent R-effects on the core–electron excitation spectra of hydrogen-bonded carboxylic-acid (R–COOH) clusters: Comparison between acetic-acid and formic-acid clusters

Chemical Physics Letters 557 (2013) 1–9

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FRONTIERS ARTICLE

Substituent R-effects on the core–electron excitation spectra of hydrogen-bonded carboxylic-acid (R–COOH) clusters: Comparison between acetic-acid and formic-acid clusters K. Tabayashi a,b,⇑, O. Takahashi a, H. Namatame a,b, M. Taniguchi a,b a b

Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Hiroshima Synchrotron Radiation Center (HSRC), Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-0046, Japan

a r t i c l e

i n f o

Article history: Available online 25 October 2012

a b s t r a c t Core-excitation spectra observed for small acetic-acid (AcA) clusters have been interpreted with density functional theory calculations. By comparison of O1s(CO/OH)?p⁄(CO) bands between formic-acid (FA) and AcA clusters, larger band-shifts were identified in AcA clusters. Substituent R-effects on these bands are examined in terms of geometrical parameters and acid–base properties of constituent carboxylicacids (R–COOH). Since small carboxylic-acid clusters comprise a centrosymmetric dimer-unit with resonance-assisted hydrogen-bonds (RAHBs), cooperative p-electron delocalization characterized by p-conjugated C@O/C–O bond equalization plays an important role in intermolecular interactions. The larger band-shifts for AcA clusters result from increased p-electron delocalization in the constituent molecules relative to FA clusters. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction A hydrogen(H)-bond is formed between a protic hydrogen located at a terminal covalent bond and a site A with high electron density, and typically designated as D–H  A [1–5], where D and A are often assigned as electronegative elements. Since the H-bond structure is regarded as an intermediate state of proton transfer from D–H to A moieties, the strength of H-bond as well as the molecular configuration induced in the complex is strongly influenced by acid–base properties of these two interacting sites: acidity of the hydrogen in the (proton) donor D–H and basicity of the (proton) acceptor A moieties, based on the Brønsted-Lowry formalism [6]. The formation of H-bonding is such that the interacting sites are site-specific in the molecule(s) and the resultant H-bond is quite directional, depending on the site-species and strength of the H-bonding interaction. Since intermolecular H-bonding is generated between the sites of the nearest neighbor molecules, it necessarily affects the electronic correlation of their molecular orbital (MO) structure, especially in the valence states. The concept of the H-bonding interaction has recently been extended to wider DH  A partners such as CH  A and CH  p systems [5]. In contrast to valence-to-resonance (valence/Rydberg) excitations, core-to-resonance transitions are considered quite sensitive ⇑ Corresponding author. Present address: Hiroshima Synchrotron Radiation Center (HSRC), Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-0046, Japan. Fax: +81 82 424 6294. E-mail address: [email protected] (K. Tabayashi). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.043

to and directly affected by the change of excited valence/Rydberg states, since a core–electron orbital is restricted to the region in the close vicinity of a nucleus (core-atom) [7] and least dependent on the interaction of surrounding molecules. Excited valence orbitals on the interacting sites being distinctively perturbed upon H-bond formation, core–electron excitation (absorption) spectroscopy has been used as a valuable tool [8,9] to examine the electronic structure-change of valence states by intermolecular interaction as in the present case. So far, core-excitation spectra of H-bonded clusters formed via interactions such as OH  O, NH  N, CH  O, etc. [9–17] have been experimentally investigated in cluster beams using soft X-ray radiation sources. Under small cluster beam conditions, both electrostatic influence of surrounding molecules on the core–hole states and electronic orbital correlation with the nearest neighbor molecules have been shown to be important factors [9,11,15,16] upon core–electron excitations within the cluster species. As a representative of strong H-bonding in the gas phase, we chose small carboxylic acid clusters, where constituent molecules form a cyclic-dimer (centrosymmetric-dimer) configuration with double bridge (OH  O@C) hydrogen bonds. The configuration is exclusively the most stable structure among the molecular complexes and constitutes a important building block [11,18] for gas-phase cluster formation. Site selective core-excitation of formic acid (FA) clusters in the oxygen K-edge region demonstrated [11] that the first resonance O1s(CO) ? pCO⁄ band blueshifts by 0.31 eV whereas the second O1s(OH) ? pCO⁄ band redshifts by 0.82 eV relative to the monomer bands. Such spectral

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band-behaviors have been rationalized primarily by the changes in core–electron binding energies (CEBEs) of oxygen core-atoms [11,18] using model spectra generated by density functional theory (DFT) calculations. Core-excitation measurements have been subsequently extended to small acetic-acid (AcA) clusters, wherein the corresponding band energy-shifts larger than for FA clusters have been observed [12]. The present study describes the substituent R-effects on the intermolecular interaction of core-excited clusters made of carboxylic acids (R–COOH), where a methyl group (R@CH3) is substituted for a formyl hydrogen atom (R@H) [11]. Since the present system is regarded as a typical example of intermolecular resonance-assisted H-bonds (RAHBs) [5,19], the H-bond parameters and acid–base properties of the complex are also discussed in relation to p-electron delocalization, characterized by the appearance of p-conjugated C–O and C@O bonds. The present article is arranged as follows. We briefly outline (in Section 3.1) the observed characteristics of core-excitation spectra of small AcA clusters [12], and interpret (in Section 3.2) the spectral changes of core-to-valence bands using computer modeling of the X-ray absorption spectrum (XAS) for the centrosymmetric dimer via DFT calculation. In Section 3.3, comparisons of the core-to-valence band-shifts upon dimerization are made between the FA and AcA systems, and substituent R-effects on the H-bond strengths are characterized in terms of intermolecular H-bonding parameters and acid–base properties of RCOOH molecules. Finally, the nature of the intermolecular interactions for these centrosymmetric dimers, generally referred to as being composed of RAHBs, is interpreted based on these acid–base and p-electron delocalization properties of constituent carboxylic-acids.

2. Method and procedure 2.1. Computational methods In order to analyze the core-excitation (and/or X-ray absorption) spectra and examine the contribution of H-bonding interaction within the clusters, ab initio molecular orbital (MO) and DFT calculations were carried out for the molecule and cluster of carboxylic acids. Electron correlation was included in the MO calculation by applying the second-order Møller-Plesset (MP2) perturbation theory [20]. Initial geometries of the system were optimized using the GAUSSIAN 03/09 Programs [21] at the MP2/cc-pVXZ (X = D,T) level and/or MP2/aug-cc-pVXZ (X = D,T) level of approximation. Vibrational frequencies were calculated at the same level to confirm the stationary structures and correct the zero-point vibrational-energies (ZPVEs). To obtain plausible thermochemical parameters of the species in the electronic ground state, GAUSSIAN-3 (G3) and GAUSSIAN-4 (G4) levels of quantum mechanical calculations [22,23] were also employed. The G3 and G4 theories have been found to predict H-bond energies with average absolute deviations of 0.60 and 1.12 kcal/mol, respectively from experiment. The DFT calculations of core-excited states were carried out with the STOBE-DEMON program [24], based on the solutions of Kohn–Sham equations. Theoretical XA spectra were generated using a transition potential (TP) method [25,26] combined with a double-basis-set technique [27]. Electron orbitals of the system were determined with a half-occupied core–hole at the ionization site using a high level of basis set. The orbitals for the excited electrons were given by diagonalizing the Kohn–Sham matrix, generated from the density in a basis set that was extended to a large set of diffuse functions centered on the excited atom. Orbital energies and oscillator strengths of the system were calculated to evaluate the transition energies and absorption intensities in the

theoretical XA spectrum. The TP method describes most of the relaxation effects on the core-excited states and provides a single set of orthogonal orbitals for the spectrum calculation. In determining the absolute energy positions of the XA spectrum, a 4Kohn–Sham approach that allows full relaxation of the core–hole state was performed to derive core-ionization energies (IEs/CEBEs). The core-excited states were variationally determined with retention of orthogonality between the excited states through the procedure by Kolczewski et al. [28]. The relativistic correction of +0.33 eV [29,30] to the IEs of the O1s electrons was made in the present spectral computations. Electron interactions of the oxygen atoms without any core–electron excitation (core–hole) were described with effective core potentials. The IGLO-III all-electron basis of Kutzelnigg et al. [31] was applied for the description of core-excited oxygen whereas (6311/311/1) and (311/1) basis sets were used for other heavy atoms and hydrogen atoms. The XA spectrum was generated by a Gaussian convolution of discrete transition lines [18,32] by adjusting their broadenings to simulate the experimental spectrum. All DFT calculations were performed using the gradient-corrected exchange (PW86) [33] and correlation functionals (PW91) [34] developed by Perdew and Wang. Further details of the calculations have been given elsewhere [28,29,35]. 2.2. Experimental data The experimental data adopted for the core-excitation of carboxylic-acid clusters were measured at the soft X-ray beamline BL27SU with a high-resolution monochrometor in the SPring-8 facility. Since the experimental details have been given elsewhere [12,13], only a minimal description necessary for the present spectral analysis and interpretation is given here. AcA clusters were produced under beam conditions similar to those used for the formation of FA clusters. The carboxylic-acid clusters were generated by supersonic expansion of lean mixtures (0.8–6%) of the acid in He through a U30–50 lm nozzle at total stagnation pressures 0.1– 0.3 MPa. An effusive beam was prepared by direct introduction of pure acid-gas under very low stagnation pressures [11]. Either beam was excited by a beam of monochromatized X-rays at an energy resolving power of E/4E = 0.8–1.0  104 in the oxygen K-edge region. Partial-ion-yields (PIYs) of mass-selected cations via a time-of-flight mass spectrometer were simultaneously recorded as a function of photon energy. By comparison among the PIYs of the free molecules and clusters, spectral changes (involving coreexcitation bandshifts) have been observed under the conditions of high precision. Atomic and small fragment cations, produced via high-energy fragmentation in the cluster beam, actually showed the same peak energies as those observed in total-ionyield (TIY) spectrum of the effusive beam, since the cluster beams produced at our stagnation conditions also involve a certain extent of uncondensed free molecules [12]. Carboxylic-acid samples labeled with a deuterium atom on the hydroxyl hydrogen site (CH3COOD/HCOOD) were used to examine the fragmentation mechanism of core-excited molecular clusters. Deuterium (hydrogen) transfer within the molecular clusters has been found to preferentially take place as an intra-cluster reaction step. 3. Results and discussion 3.1. Partial-ion-yield spectra of core-excited acetic-acid clusters In the cluster beam, the appearance of cluster cations that involve different radical species [MnX+; (M = CH3COOH, X = D, CH3CO)] (via the fragmentation of homogeneous molecular clusters) has been observed. Here, we chose the product cation MD+ (n = 1) to provide as small cluster size-distribution as possible

K. Tabayashi et al. / Chemical Physics Letters 557 (2013) 1–9

for the plausible PIY spectrum under stable cluster-beam conditions. The PIY spectrum of MD+ obtained at P0 = 0.2 MPa [11] is compared with a TIY spectrum of molecular AcA in the effusive beam (the lowest two panels in Figure 1). It is also notable that the significant enhancement of CH3CO+ (m/e = 43) intensity in the fragments smaller than the parent molecule was identified under our stagnation pressures. Such interim enhancement of larger fragment cations takes place at the onset of cluster formation and typically originates from very small clusters. Almost the same PIY band-peak pattern (peak-shifts) of MD+ as that of CH3CO+ [12] provides a strong support for our interpretation that the size-distribution is definitely of smallest-cluster regime. Under the present conditions, it is reasonable to postulate that stable dimers are present in the highest concentration and the presence of planar clusters of trimers/tetramers is of secondary importance, based on the above beam characteristics and close analogy with previous FA clusters [11]. In the discussion that follows (comparison of XAS

3

in Section 3.2), we found a unit-structure of the centrosymmetricdimer configuration is exclusively involved within such small AcA clusters. The experimental cluster-band energies have been determined with the aid of deconvolution analysis [36]. The peak position and band intensity at the first and second bands of AcA clusters are tabulated in Table 1, where the bands are regarded as the O1s(C@O/OH)?p⁄CO transitions [12]. Significantly large band-shifts upon OH  O hydrogen-bond formation have been identified in the lowest two core-to-valence bands: a blueshift of the O1s(C@O)?p⁄CO transition by 0.37 eV and a redshift of the O1s(OH)?p⁄CO by 0.92 eV relative to the monomer bands. 3.2. Comparison of core-to-valence excitation spectra by computer modeling calculation Many computational studies have been performed to predict the cluster configurations and complexation energies [37] in the

Figure 1. Comparison of the calculated molecular AcA and (AcA)n spectra (top two panels) with those observed (the lowest two panels). The vertical bar denotes the oscillator strength of the O1s transition at the corresponding energy position. Red bars: O1s(C@O) excitations, blue bars: O1s(OH) excitations. As the representative case of n = 2, the centrosymmetric dimer is employed for the theoretical XA spectrum.

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Table 1 Band-peak position and band-intensity ratio observed at the core-to-p⁄CO resonance transitions of AcA system.

a

Acetic acid system

Transition energy (Intensity ratioa)

Band

1st (O1sCO?p⁄CO)/eV

2nd (O1sOH?p⁄CO/⁄r⁄OH)/eV

Monomer Cluster (Dimer) Energy-shift

532.13 (1.000) 532.50 (1.000) +0.37

535.44 (0.851) 534.52 (0.524) 0.92

Intensity in the parenthesis is scaled relative to the O1sCO-to-p⁄CO transition.

H-bonded carboxylic acids, using different levels of quantum mechanical theory. To make our comparison of their parameters between the acids, systematic calculations were made applying ab initio MO theory with the same levels of basis set and electron correlation treatment. Table 2 shows the dimerization energies for the centrosymmetric AcA dimer derived from different levels of theory, along with those for the corresponding FA dimer. As was the case for FA clusters [18], the centrosymmetric configuration (Dimer A1 in Figure 2) is by far the most stable structure among AcA dimers, and exclusively constitutes a unit-structure of building blocks in the AcA cluster formation. Figure 2 shows the most stable three configurations among the representative dimers, where the individual complexation energies were calculated using the G3 and G4 levels of theory. In order to make our interpretation on the O1s excitation band-shifts of the PIY spectra for MD+, model XA spectra were generated for the representative AcA clusters of small sizes using the DFT calculation. Similarly to the previous FA system [18], the line broadening of the XAS in Figure 1 was set to 4E(FWHM) = 0.9 eV in the energy region below 538 eV; for the next 4 eV region the full width at half maximum (FWHM) was linearly increased up to a constant level of 4E(FWHM) = 4.0 eV. Top two panels in Figure 1 compare the O1s(CO/OH)?p⁄CO band-shifts relative to the monomer bands, where the spectrum calculated for the centrosymmetric AcA dimer is indicated as a typical representative of AcA clusters. They can be also compared with those (bottom panels in Figure 1) observed in the previous beam experiments [12]. By comparison among the computer generated spectra of small clusters1 (n 6 4), we found AcA cluster that involves the most stable cyclic-dimer unit (Dimer A1) within the configuration could only reproduce the experimental band features, leading to such large bandshifts (the closest approach of the two bands) for the O1s(CO/ OH)?p⁄CO transitions. The results are closely consistent with the computer modeling studies on FA clusters [18]. The transition structures of the O1s(CO/OH)?(p⁄C@O/r⁄O–H) bands can be thus examined from the band parameters (Table 3) obtained by the DFT calculation, using the Dimer A1 configuration. It should be notable in the molecular spectrum that the second band at 535.4 eV (Table 3) actually consists of O1s(OH)?(p⁄C@O/r⁄O–H) transitions with comparable oscillator strengths. Upon O–H  O hydrogen-bond formation, the excited state with r⁄(O–H) anti-bonding orbital is strongly perturbed by n ? r⁄ interaction and destabilizes its orbital energy via the production of additional anti-bonding structure within the dimer complex. Charge transfer upon dimerization then induces the delocalization of the r⁄O–H orbital to reduce the O1s(OH)?r⁄O–H oscillator strength. From closer inspection of the 1 As the unit-structures that may constitute AcA clusters, we examined a dozen of representative configurations (n 6 4) involving Dimers A1–A3 to simulate experimental core-excitation spectra. For example, Dimer A2 and A3 in XAS could hardly reproduce a characteristic of the significant red-shift for the experimental O1s(OH)?p⁄CO transition, being indicative of very weak candidates for the constituent unit-structures of observed AcA clusters.

band parameters in Table 3, the significant intensity reduction as well as blue-shifting energy of the O1s(OH)?r⁄O–H transition could explain the relative intensity change (band intensity ratio: 0.85/ 1.00 ? 0.52/1.00 in Table 1) observed between the first and second core-to-valence bands. Since both O1s(CO/OH) ? r⁄O–H transitions essentially disappear in the complex, the observed bandshifts are responsible to the stability of the p⁄C@O state before and after the complexation. Such transition structures of the AcA bands are also confirmed to be quite similar to the FA dimerization [11,18]. It has been found that the site-dependent core–electron-binding-energy (CEBE) governed by intermolecular core–hole interaction [9,11] is primarily important to define the overall stability of the core– electron excited state. This is particularly significant in such H-bonded clusters made of polar molecules [38] like the present case. In the DFT calculation for the centrosymmetric AcA dimer configuration, we have obtained a comparable magnitude of CEBE (4CEBEAcA(CO) = 0.02 eV) for the acceptor O atom relative to the monomer and much more stable value (4CEBEAcA(OH) = 0.80 eV) for the donor O atom than that of free molecule. These differences in CEBEs of AcA upon dimerization can be compared with those calculated for the FA case, where 4CEBEFA(CO) = +0.01 eV and 4CEBEFA(OH) = 0.64 eV for the respective acceptor and donor O-atoms were given at the same level of DFT calculation. Although the stabilities of the p⁄C@O state upon H-bond formation are not yet incorporated here, significant part of the energy difference in the bandshifts has been interpreted as being due to such intermolecular core–hole interaction [9,11]. 3.3. Substituent R-effects on hydrogen-bonded carboxylic-acid clusters 3.3.1. Comparison of H-bond parameters and core-excited band energy-shifts between FA and AcA clusters To analyze the R-dependence on H-bonding structure in the centrosymmetric dimers, the H-bond parameters derived from ab initio MO calculation are examined. The dimerization energies for AcA in Table 2 are consistently larger than those for FA by 1.0 kcal/mol, irrespective of the theory employed for the MO calculation. Recent spectroscopic data on the dimerization energy (other than Refs. [39,40]) particularly necessary for AcA are not available to our knowledge, however, enthalpies of dimerization 4rH00 = 14.72 kcal/mol for FA and 4rH00 = 15.30 kcal/mol for AcA have been selected [41] based on the thermochemical grounds, the magnitudes being closely consistent with the theoretical G4 (G3) predictions in Table 2. It is thus likely that AcA causes stronger H-bonds than FA upon centrosymmetric-dimer formation. The H-bond parameters calculated for FA and AcA dimers along with those observed are listed in Table 4, where we compare the differences in their OH  O contact distances and associated O–H bondlengths before and after the dimerization. The MO calculation predicts a shorter OH  O contact of AcA dimer and its larger elongation of O–H bond from the monomer than those calculated for FA dimer. Accurate spectroscopic measurements of these structural data are rather limited [42–44], however, the same conclusion as above could be drawn from the collected experimental data. Under the assumption that the differences 4Rs in the H-bond geometry (Table 4) are systematically above a level of significance for comparison, we postulate that AcA dimer possesses undoubtedly higher H-bond strength than FA dimer. The O1s core-excitation spectra of monomer and clusters that show core-to-valence band-shifts on cluster formation are compared between the FA and AcA systems (Figure 3). Here, the excitation spectra were extracted by monitoring the PIYs of MD+ (M = CH3COOD or HCOOD) originating from clusters. Both clusters were generated under similar stagnation conditions, where cluster-size distributions are regarded as being in the small cluster regime of beam conditions. Based on the model XA spectra

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K. Tabayashi et al. / Chemical Physics Letters 557 (2013) 1–9 Table 2 Dimerization energies for the centrosymmetric carboxylic-acid dimers by G4/G3 and MP2 calculations. System

(HCOOH)2 Monomer /Hartree

Dimer /Hartree

4E /kcalmol1

Monomer /Hartree

Dimer /Hartree

4E /kcalmol1

/kcalmol1

MO calculation G4 G3

189.692 990 189.656 718

379.408 132 379.336 254

13.90 14.32

228.982 861 228.939 943

457.989 945 457.904 606

15.20 15.51

1.300 1.194

MP2/b cc-pVDZ aug-cc-pVDZ cc-pVTZ aug-cc-pVTZ

189.247 082 189.293 000 189.437 040 189.452 905

378.520 595 378.608 929 378.898 201 378.929 376

16.59 14.39 15.14 14.79

228.409 523 228.462 652 228.641 481 228.660 460

456.847 041 456.950 172 457.308 828 457.346 296

17.57 15.60 16.23 15.92

0.981 1.217 1.095 1.136

Experimental West, CRC (1979), Ref. [38] Mathews and Sheets, JCSA (1969), Ref. [39] Clague and Bernstein, JSA (1969), Ref. [40] a b

44Ea

(CH3COOH)2

14.4 14.1 13.7

14.2 14.8

Difference of D Es between the AcA and FA dimers. Zero-point vibrational energy correction is made.

Figure 2. The most stable three dimer-configurations of AcA dimers employed for the model XAS calculation. The complexation energies were calculated using the G4 (G3) levels of ab initio MO theory.

Table 3 Transition energy and oscillator strength of the core-to-valence bands of AcA system derived from DFT calculation. AcA system

Transition energy (Oscillator strength)

core-to-val. transition

p⁄(C@O)

r⁄(O–H)

/eV

/eV

Monomer O1s(C = O) O1s(OH)

532.54 (0.0134) 535.35 (0.0044)

535.36 (0.0006) 535.36 (0.0066)

Dimer O1s(C = O) O1s(OH)

532.90 (0.0113) 534.71 (0.0055)

535.75 (0.0002) 536.49 (0.0002)

generated for the representative AcA/FA clusters using the DFT calculations, it has been shown that the most stable centrosymmetric-dimer units could only reproduce the present large energy-shifts of the O1s(C@O/OH)?p⁄CO transitions. Both

FA and AcA clusters experimentally showed blueshifts of the O1s(C@O)?p⁄CO transition by ca. 0.3 eV and redshifts of the O1s(OH)?p⁄CO by ca. 0.8 eV upon clusterization, however, increased band energy-shifts (by 0.06–0.10 eV) of AcA clusters have been found relative to FA clusters. It is thus reasonable to conclude that the difference of the band-shifts observed between FA and AcA clusters comes from the exact nature of these structural units with different substituent R-effects. The origin of the larger chemical shift observed for AcA clusters can be rationalized from the calculated CEBE change and termvalue E(TV) change in each system for the O1s(CO)?p⁄CO transition2. The O1s(CO)?p⁄CO transition of AcA clusters has larger term-value change 4EAcA(TV) = 0.38 eV of p⁄CO states than the same transition of FA clusters with 4EFA(TV) = 0.29 eV, the former of which results in a larger band energy-shift since both systems are possessed of similar 4CEBE(CO)s upon clusterization (as described in Section 3.2). This corresponds to the fact [8] that the orbital correlation with the nearest neighbor molecules as well as electrostatic influence of surrounding molecules on the core–hole states actually defines the chemical sensitivity of the core–electron transitions. Such term value-changes of p⁄CO states upon dimerization clearly show the significant contributions of p/p⁄ conjugated correlation in the systems, in addition to the H-bonding interaction. The present results imply that the relevant MOs associated with the p-conjugated (occupied) states of carboxylic-acid dimers stabilize whereas those corresponding to the p⁄ (vacant) states destabilize depending on the level of p-electron delocalization within the dimer configuration, when such intermolecular interaction comes to be important in the RAHBs. A larger contribution of the p-electron delocalization for AcA clusters could be thus accounted for from these core-excitation band parameters. The nature of the current p-electron delocalization will be further discussed in later sections. 3.3.2. Interpretation of substituent R-effects 3.3.2.1. PA/pKa equalization principle. Based on the PA/pKa equalization (minimum proton affinity difference 4PA) principle [5,18], the H-bond strength can be characterized by the matching between the acid–base properties of H-bond donor and acceptor moieties in the D–H  A hydrogen-bond system, where H-bonding 2 In contrast to ab initio configuration interaction calculation of molecular aceticacid [45], the present DFT-TP theory predicts O1s(OH) excitation to p⁄CO/r⁄OH levels almost degenerate with each other, which may complicate orbital correlations of the molecular complex upon core-excitation. This is probably due to a poor representation of the method for inter-channel interactions involving charge transfer.

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K. Tabayashi et al. / Chemical Physics Letters 557 (2013) 1–9

Table 4 O–H. . .O contact distance and associated O–H bondlength by ab initio MO calculation. Distance, Bondlength System

MO calculation MP2/cc-pVDZ O–H OH. . .O MP2/aug-cc-pVDZ O–H OH. . .O MP2/cc-pVTZ O–H OH. . .O MP2/aug-cc-pVTZ O–H OH. . .O Experimental O–H OH. . .O a b c

HCOOH

(HCOOH)2

CH3COOH

(CH3COOH)2

R/Å

R/Å

4R/Å

R/Å

R/Å

4R/Å

44R/Å

0.97 554

1.00 083 1.67 629

0.0253

0.97 424

1.00 091 1.66 474

0.0267

0.0014 0.0116

0.97 501

1.00 124 1.68 310

0.0262

0.97 369

1.00 136 1.67 018

0.0277

0.0015 0.0129

0.96 948

0.99 924 1.65 233

0.0298

0.96 858

0.99 990 1.63 977

0.0313

0.0015 0.0126

0.97 090

0.99 992 1.65 939

0.0290

0.96 990

1.00 022 1.64 803

0.0303

0.0013 0.0114

0.972a

1.033b 1.670a

0.061

0.970c

1.033c 1.650c

0.063

0.002 0.020

Lide and Haynes, CRC handbook of chemistry and physics (2009). Ref. [42]. Kim, JACS (1996). Ref. [43]. Derissen, JMS (1971). Ref. [44].

Figure 3. Comparison of the energy-shifts for the O1s(C@O/OH)?p⁄CO bands observed with core-level excitations of FA and AcA clusters. Cluster-specific excitation spectra were given from the PIY spectra of deuteronated product-cations (FA)D+ and (AcA)D+, where FA = HCOOD and AcA = CH3COOD, respectively. For details, see Refs. [11] and [12].

interaction becomes stronger the more closely PA(A) matches PA(D) when the proton-affinities PAs of their interaction centers (sites) in the gas phase are employed. Table 5 summarizes the proton affinities (and core-ionization energies) of the individual acid–base centers in several carboxylic acids (R–COOH). Smith

and Thomas [48] found linear but negative correlations between the core-ionization energies and proton affinities for both center oxygens (C@O and OH), when inductive effects of the substituent R are important in the acids. In Figures 4 and 5, their correlations are re-plotted using the recent gas-phase database listed in NIST

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K. Tabayashi et al. / Chemical Physics Letters 557 (2013) 1–9 Table 5 Proton-affinities and acid–base property of carboxylic acid. Carboxylic

a b c d

CEBE /eV

PA(C@O)/eV

acid

1s(CO)

1s(OH)

NIST

HCOOH CH3COOH C2H5COOH CF3COOH CFH2COOH CF2HCOOH

539.02a 538.31a 538.28b 539.56b 538.76c 539.25b

540.69a 540.10a 540.04b 541.28b

7.69 8.12 8.26 7.37 7.93

d

540.99b

Anion-PA(OH) /eV S-T

c

7.80 8.14 8.24 7.51 7.96

d

4PA /eV c

NIST

S-T

14.68 14.81 14.76 13.73 14.37 14.04

14.84 14.97 14.93 13.88 14.52 14.20

NIST 6.99 6.69 6.50 6.36 6.44

Naves de Brito et al., JCP (1991). Ref. [46]. Jolly, Bomben, and Eyermann, Atomic data and nuclear data table (1984). Ref. [47]. Smith and Thomas, JACS (1978). Ref. [48]. Hunter and Lias, NIST chemistry webbook. Ref. [49].

Figure 4. Correlation between proton affinity (PA) and core-ionization energy of the carbonyl oxygen site for carboxylic acids (R–COOH). The PAs are given from NIST Chemistry WebBook by Hunter and Lias [49]. The CEBEs adopted are also tabulated in Table 5. For details, see the text.

Chemistry WebBook [49]. Here, the gas-phase basicity (defined as 4BH+G0) and negative of the gas-phase acidity (4AHG0) (from the negative proton affinity) for the acid–base reaction processes,

AHðgÞ  Hþ ðgÞ þ A ðgÞ

ð1Þ

BHþ ðgÞ  Hþ ðgÞ þ BðgÞ

ð2Þ

can be evaluated by using the acid–base properties of the H-bond from the relations:

DAH G0 ¼ DAH H0  T DAH S0

ð3Þ

DBHþ G0 ¼ DBHþ H0  T DBHþ S0 0

ð4Þ 0

where the enthalpies 4AHH and 4BH+H correspond to the proton affinities of PA(A) and PA(B), respectively. Since the 4PA equalization principle mentions that the proton affinity difference 4PA = PA(D)  PA(A) = PA(A)  PA(B) is the enthalpy necessary for the proton to transfer from H-bond donor (acid) to acceptor (base) moieties in the complex, relatively large values of 4PAs in Table 5 generally indicate by far the larger

Figure 5. Correlation between anion proton affinity (PA) and core-ionization energy of the hydroxyl oxygen site for carboxylic acids (R–COOH). Averaged value of the anion PA data from NIST Chemistry WebBook is plotted. The bar represents the limits of data points. The CEBEs adopted are also listed in Table 5. For details, see the text.

basicities of the donor R(CO)O site than those of the acceptor R(OH)C@O site. Comparison of the proton affinity differences for the relevant acids (Table 5) gives a smaller 4PA(AcA) = 6.7 eV for AcA dimer than that (4PA(FA) = 7.0 eV) for FA dimer, which is indicative of the formation of stronger H-bonds between AcA molecules. This is rationalized by the fact that both AcA and FA systems have mostly comparable magnitudes3 of the gas-phase acidities of the OH sites whereas the AcA system indicates its distinct superiority of the basicity of the C@O site over the FA system. Such irregular correlation [48] of the FA acidity shown in Figure 5 has been interpreted as being due to the relaxation effects (i.e. low polarizability) caused by the R@H substituent. The acid–base properties of the current carboxylic acids could provide convincing evidence for the higher H-bond strength of AcA dimer than that of FA dimer, from the viewpoint of the PA/pKa equalization principle.

3 Strictly speaking, the gas-phase acidity of AcA is actually lower than that of FA, as shown in Fig. 5.

8

K. Tabayashi et al. / Chemical Physics Letters 557 (2013) 1–9

Table 6 C@O/C–O bondlengths on H-bond formation of carboxylic-acid dimers by MO calculation. System

Bondlength HCOOH

(HCOOH)2 R/Å

MO calculation MP2/cc-pVDZ C@O C–O MP2/aug-cc-pVDZ C@O C–O MP2/cc-pVTZ C@O C–O MP2/aug-cc-pVTZ C@O C–O Experimental C@O C–O a b

CH3COOH

DR/Å

(CH3COOH)2 R/Å

DR/Å

DDR/Å

1.2092 1.3505

1.2274 1.3181

0.0182 0.0324

1.2140 1.3606

1.2328 1.3260

0.0188 0.0346

0.0006 0.0022

1.2154 1.3586

1.2332 1.3255

0.0178 0.0331

1.2201 1.3695

1.2389 1.3333

0.0188 0.0362

0.0010 0.0031

1.2032 1.3457

1.2221 1.3114

0.0189 0.0343

1.2081 1.3564

1.2278 1.3191

0.0197 0.0373

0.0008 0.0030

1.2052 1.3466

1.2236 1.3131

0.0184 0.0335

1.2100 1.3578

1.2292 1.3212

0.0192 0.0336

0.0008 0.0001

1.202a 1.343a

1.220a 1.323a

0.018 0.020

1.212a 1.364a

1.231b 1.334b

0.019 0.030

0.001 0.010

Lide and Haynes, CRC handbook of chemistry and physics (2009). Ref. [42]. Derssen. Ref. [44].

3.3.2.2. Origin of hydrogen-bond strength and p-electron delocalization in RAHBs system. The centrosymmetric dimers of carboxylic acids are often referred to an example of RAHBs, where the neutral donor and acceptor atoms OH  O@C are interconnected by a system of p-conjugated single and double bonds (i.e. via the p-electron delocalization interrelated with the H-bonding interaction). It has been shown that the cyclic dimers of carboxylic acids generally possess shorter HB contacts and larger single HB energies than the ordinary ones. In order to examine the current R-substituent effects on the p-electron delocalization, the changes in the C@O and C–O bondlengths (elongation of C@O and shortening of C–O bonds) derived from the cyclic dimerization are listed in Table 6. A higher level of equalization of the bondlengths (C@O/C–O) can be admitted for AcA dimer, irrespective of the basis sets used for the MP2 calculation. The same statement for the effective p-electron delocalization of AcA dimer could be repeated from the bondlengths (Table 6) observed from spectroscopic measurements. This is quite in line with the aforementioned conclusion, obtained from the PA/pKa equalization principle, that the H-bond strength of AcA dimer is higher than that of FA dimer. Once the H-bond strengths have been defined from the acid–base properties of the interacting sites, the shortening of HB contact in AcA dimer may well provide a more important attractive contribution of the p-delocalization than that of FA dimer, and vice versa. The existence of such synergism between the H-bond parameters and level of p-electron delocalization can be expected in these cooperative RAHBs systems. The core-to-valence band energy-shifts examined in the previous section clearly explain both significant contribution of intermolecular p/p⁄ correlations and R-substituent effects on the p-conjugated orbitals within the clusters of carboxylic acids. The relative importance of the p-delocalization energy term among the interaction energies has been evaluated for several RAHBs [50] using an interaction energy decomposition technique. The centrosymmetric AcA dimer is found to give a larger contribution of p-delocalization energy to the intermolecular interaction than the FA dimer. Then what is the driving force to give the stronger RAHBs in the current centrosymmetric dimers upon Rsubstitution of carboxylic acids? Since the depletion of the AcA acidity by inductive effect is nearly balanced by the low acidity of FA due to the relaxation effect, the differences in the H-bond strengths are strongly caused by the changes in the basicity upon R-substitution. The substitution of electron-donating R@CH3 group may enhance the stabilization of the p-delocalized 6-member

ring-structure in the H-bonding system via the direct introduction of extra density of electron to the resonant structure, in comparison with the FA case. It should be notable in the crystallographic data [5] that the extra H-bond shortening achievable (that corresponds up to 9% shrinkage from the ordinary (non-resonant) D  A contact distance) has been considered induced by the p-resonance. Upon evaluation of the present p-delocalization, the crystallographic data could not simply be adopted due to additional cooperative phenomena involved in the condensed phase, generally leading to shorter and stronger H-bonds than those of dimers in the gas phase. However, the present H-bond lengths in Table 4 actually provide an average of percent shrinkages almost similar to the above magnitude, when the H-bond distance (RO– O = 2.98 Å) of a neutral (H2O)2 dimer [51] in the gas phase is employed as the minimum of non-resonant O(H)  O hydrogen-bond length for dimers of carboxylic acid. 4. Conclusions Substituent R-effects of hydrogen-bonded carboxylic-acid (RCOOH) clusters are correlated with both H-bond strength characterized by acid–base properties of the interacting molecules and pelectron delocalization by C@O/C–O bond equalization parameters in the RAHBs system. The intermolecular interaction is found strengthened by substituting a methyl R@CH3 group for a formyl hydrogen R@H atom, leading to larger cluster band-shifts of O1s core-to-p⁄ valence transitions. Both acid–base properties and geometrical parameters indicate that the actual substitution of the electron-donating R@CH3 group cooperatively enhances the stabilization of resonance structure of the centrosymmetric dimer via the p-electron delocalization, in addition to the H-bonding interaction between donor and acceptor moieties. The larger band-shifts observed for acetic-acid clusters result from the increased p-electron delocalization within the constituent molecules relative to formic-acid clusters. The driving force of the increased p-electron delocalization and H-bond strength upon CH3-substitution, can be attributed to the significance of its induction effects on the present centrosymmetric-dimer system. Acknowledgments The authors wish to gratitude the contributions of many coworkers: Drs. K. Tanaka (Spring-8), I. H. Suzuki (Photon Factory),

K. Tabayashi et al. / Chemical Physics Letters 557 (2013) 1–9

S. Nagaoka (Ehime Univ.), K. Honma (Univ. Hyogo), T. Gejo (Univ. Hyogo), Y. Tamenori (Spring-8), K. Okada (Hiroshima Univ.), and H. Yoshida (Hiroshima Univ.), as cited in the references. Thanks are also due to Drs. M. Abe (Hiroshima Univ.) and Y. Yoshioka (Mié Univ.) for valuable discussions, and Mr. K. Yamamoto for computing assistance. The present study was performed under the Cooperative Research Program (No. 12-B-26) of HiSOR (HSRC), Hiroshima University. One of the authors, K. Tabayashi thanks the Research Center for Computational Science at the Okazaki Research Facilities of the Japanese National Institutes of Natural Sciences, for the allocation of SGI Altix4700 computer time. O. Takahashi is grateful for the financial support from a Grant-inAid for Scientific Research from JSPS of Japan (No. 23540476). References [1] S. Scheiner, Hydrogen Bonding, Oxford University Press, Oxford, 1997. [2] G.A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford University Press, Oxford, 1997. [3] A.E. Reed, F. Weinhold, L.A. Curtiss, D.J. Pochatko, J. Chem. Phys. 84 (1986) 5687. [4] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253. [5] G. Gilli, P. Gilli, The Nature of the Hydrogen Bond: Outline of a Comprehensive Hydrogen Bond Theory, Oxford University Press, Oxford, 2009. [6] E.V. Anslyn, D.A. Dougherty, Modern Physical Organic Chemistry, University Science Book, Sansalito California, 2006. [7] J. Stöhr, NEXAFS Spectroscopy, Springer Series in Surface Science, vol. 25, Springer, Berlin, 1992. [8] P. Wernet, D. Nordlund, U. Bergmann, et al., Science 304 (2004) 994. [9] T. Yamanaka, K. Tabayashi, O. Takahashi, K. Tanaka, H. Namatame, M. Taniguchi, J. Chem. Phys. 136 (2012) 014308. [10] O. Björneholm, F. Federmann, S. Kakar, T. Möller, J. Chem. Phys. 111 (1999) 546. [11] K. Tabayashi et al., J. Chem. Phys. 125 (2006) 194307. [12] K. Tabayashi et al., J. Electron. Spectrosc. Relat. Phenom. 184 (2011) 134. [13] Y. Tamenori et al., J. Chem. Phys. 128 (2008) 124321. [14] K. Tabayashi, M. Chohda, T. Yamanaka, O. Takahashl, H. Yoshida, Nucl. Instrum. Method Phys. Res. A619 (2010) 388. [15] K. Tabayashi, M. Chohda, T. Yamanaka, Y. Tsutsumi, O. Takahashi, H. Yoshida, M. Taniguchi, J. Phys. Conf. Ser. 235 (2010) 012017. [16] K. Tabayashi et al., J. Phys. Conf. Ser. 288 (2011) 012022. [17] Y. Tamenori et al., J. Chem. Phys. 131 (2009) 174311. [18] O. Takahashi, S. Yamanouchi, K. Yamamoto, K. Tabayashi, Chem. Phys. Lett. 419 (2006) 501. [19] P. Gilli, L. Pretto, G. Gilli, J. Mol. Struct. 844–845 (2007) 328. [20] W.J. Hehre, L. Radom, P.v.R. Schleyer, J.A. Pople, Ab initio Molecular Orbital Theory, Wiley, NY, 1986. [21] M.J. Frisch, G.W. Trucks, H.B. Schlegel et al. (GAUSSIAN 03, Revision C.02)/ (GAUSSIAN09, Revision B01), GAUSSIAN Inc., Wallingford, CT, 2004/2010. [22] L.A. Curtiss, K. Raghavachari, P.C. Redfen, V. Ressolov, J.A. Pople, J. Chem. Phys. 109 (1998) 7764. [23] L.A. Curtiss, P.C. Redfem, K. Raghavachari, J. Chem. Phys. 126 (2007) 084108. [24] K. Hermann, et al., STOBE-DEMON, version 2.1, Stobe Software, 2005. [25] J.C. Slater, Adv. Quantum Chem. 6 (1972) 1. [26] J.C. Slater, K.H. Johnson, Phys. Rev. B 5 (1972) 844. [27] H. Ågren, V. Carravetta, O. Vahtras, L.G.M. Pettersson, Theor. Chem. Acc. 97 (1997) 14. [28] C. Kolczewski et al., J. Chem. Phys. 115 (2001) 6426. [29] L. Triguero, O. Plashkevych, L.G.M. Pettersson, H. Ågren, J. Electron Spectrosc, Relat. Phenom. 104 (1999) 195. [30] O. Takahashi, L.G.M. Pettersson, J. Chem. Phys. 121 (2004) 10339. [31] W. Kutzelnigg, U. Heischer, M. Schindler, NMR-Basic Principles and Progress, Springer, Heidelberg, 1990. [32] O. Takahashi, K. Tabayashi, S. Wada, R. Sumii, K. Tanaka, M. Odelius, L.G.M. Pettersson, J. Chem. Phys. 124 (2006) 124901. [33] J.P. Perdew, Y. Wang, Phys. Rev. B 33 (1986) 8800. [34] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. [35] L. Triguero, L.G.M. Pettersson, H. Ågren, Phys. Rev. B 58 (1998) 8097. [36] K. Yamamoto, Master thesis, Hiroshima University, 2007. [37] K.B. Borisenko, C.W. Bock, I. Hargittai, J. Mol. Struct. (Theochem.) 332 (1995) 161. [38] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, West Palm Beach, 1979. [39] D.M. Mathews, R.W. Sheets, J. Chem. Soc. A (1969) 2203. [40] A.D.H. Clague, H.J. Bernstein, Spectrochim. Acta 25A (1969) 593. [41] J. Chao, B.J. Zwolinski, J. Phys. Chem. Ref. Data 7 (1978) 363. [42] D.R. Lide, W.M. Haynes (Eds.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 2009.

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Kiyohiko Tabayashi is Visiting Professor of Hiroshima Synchrotron Radiation Center, Hiroshima University. He received his Ph.D. degree (1975) from Osaka University. After postdoctoral fellowships at Osaka University (1975-1976) and at Cornell University (1976-1978), he joined the staff of Institute for Molecular Science (19791993) in Okazaki, where he started molecular beam scattering and synchrotron radiation chemistry. From 1993 to 2010, he was employed as a professor of Faculty of Science and Graduate School of Science at Hiroshima University. The important part of research concerns excited-state interactions and dynamics of molecular complexes and clusters with excitation by X-rays and VUV photons.

Osamu Takahashi was born in Fukuoka, Japan in 1967 and received B.Sc., M.Sc., and Ph.D. degrees in science from Hiroshima University in 1990, 1992, and 2002. Since 1993 he has been a research associate of Hiroshima University. From 2003 to 2004, he was a visiting scientist collaborating with Prof. Lars G. M. Pettersson at Stockholm University. He is currently an assistant professor of Faculty of Science and Graduate School of Science, Hiroshima University. His research interests include chemical dynamics and kinetics in the gas and condensed phases.

Hirofumi Namatame received a degree of Doctor of Engineering from the University of Tokyo in1989. He was an assistant professor at the University of Tokyo from 1989 to 1993. He was an associate professor of faculty of science at Hiroshima University from 1993 to 1996. Since 1996, he has been a professor at Hiroshima Synchrotron Radiation Center, Hiroshima University. His research interests are the bulk and surface electronic structures of solids such as high temperature superconductors and magnetic materials, and the instrumentation of beam-line optics and end-stations of synchrotron radiation.

Masaki Taniguchi graduated at Hiroshima University in 1972 and obtained Ph.D. at Osaka University in 1977 for experimental study of negative donor ion states in Germanium and Silicon. In 1979, he moved to Synchrotron Radiation Laboratory of the University of Tokyo, and started researches on electronic states of solid materials such as semiconductors and strongly correlated systems by means of synchrotron radiation photoemission spectroscopy. Since 1996, he is a director of Hiroshima Synchrotron Radiation Laboratory of Hiroshima University, which promotes materials science centered on solid-state physics (1996-2005, 2007-2012).