COH Bending Research Articles

Quantification of Starch Order in Physically Modified Rice Flours Using Small-Angle X-ray Scattering (SAXS) and Fourier Transform Infrared (FT-IR) Spectroscopy.

The purpose of this study was to understand the ordered structure of starch in rice flour based on a physical modification with non-heating, milling, and water sorption through the structural evaluation of rice flour using small-angle X-ray scattering (SAXS) and infrared spectroscopy within the 4000-100 cm-1 region. The SAXS pattern of the samples with low moisture contents subjected to milling yield a band within the 0.4-0.9 nm-1 of the q range owing to a lamellar repeat of starch with an ordered structure in rice flour. We proposed an order parameter using the intensity of the SAXS band to quantify the order structure of starch in rice flour, and the true density was negatively correlated with the order parameter. Infrared band at 990 cm-1 in COH bending mode applied to the hydroxyl group of C6 shifted to a low wavenumber corresponding to the order parameter. A linear correlation was found between the order parameter and the 990 cm-1 and band at 861 cm-1 owing to COC symmetrical stretching of glycoside bond and CH2 deformation of the glucose unit of starch, 572, 472, and 436 cm-1, owing to the pyranose ring in the glucose unit of starch. The identified infrared bands are effective for quantifying the ordered structure of starch at the lamellar level. When subjected to water sorption, the band position at 990 cm-1 shifted to a higher wavenumber above a water activity of 0.7. This result revealed that the water-induced transition of glass to rubber of starch in rice flour can be clearly evaluated through infrared spectroscopy using the band at 990 cm-1. In addition, the band at 861 cm-1 also shifted to a higher wavenumber, whereas those at 572 and 436 cm-1 did not show a significant shift. These results indicate that water sorption slightly affects the internal structure and may mainly affect the surface of starch.

Vibrational modes of nucleoside deoxyadenosine monohydrate investigated under high pressure

The vibrational properties of the nucleoside deoxyadenosine monohydrate were investigated with Raman spectroscopy in the crystal form of the material. A tentative assignment of the vibrational modes was made through the knowledge of vibrations in similar materials. The behavior of the crystal under high pressure was investigated up to 5.6 GPa. The results suggest a phase transition undergone by the crystal between 0.6 at 0.8 GPa as indicated by discontinuities on the wavenumber of some modes and changes in the Raman spectra. The phase transition involved a hydrogen bond between the sugar and the water molecule as it is observed through the changes of a COH bending vibration. Above the pressure of the phase transition, it is observed that the vibrational modes continue to change continuously up to the highest pressure reached in the experiment. Finally, the phase transition appears as completely reversible.

Solvation effects on the vibrational modes in hydrated bicarbonate clusters.

HCO3-(H2O)n clusters provide a model system to understand the solvation interaction between the bicarbonate ion and water. Based on harmonic analysis, ab initio molecular dynamics simulations, and comparison with infrared multiple photon dissociation spectra and with previous results on H2PO4-(H2O)n, the solvation effects on the vibrational modes of HCO3-(H2O)n are analyzed. Hydrogen bond interactions have a significant impact on the vibration, especially when a hydrogen atom is directly involved in a particular mode. The COH bending mode is flattened, when the COH group is solvated by water molecules. The emergence of broad water libration modes indicates the aggregation of water molecules and the formation of a surface structure with bicarbonate on the surface.

High resolution synchrotron radiation Fourier transform infrared spectrum of the COH-bending mode in methanol-D1 (CH2DOH)

In this work the high resolution synchrotron radiation Fourier transform spectrum in the range 1180–1300cm−1 corresponding to the COH-bending vibrational mode has been recorded and analyzed. The spectrum shows a structure analogous to a parallel band. Since the COH bending motion is one of the main contributors to the asymmetry in the torsional hindering potential barrier, the torsional barrier height in the excited state is expected to be quite different from that of the ground state. This makes the spectrum to spread over a wide region. Although the spectrum corresponding to the P- and R-branch looks very complicated, the Q-branches are well resolved and can be identified without much difficulty. It was possible to assign the spectra for K=0 to 10 for the trans- (e0) species. The interesting feature of the spectra is the absence of the lines for two other lower lying gauche symmetry species e1 and o1. The spectra due to any perpendicular transitions were absent as well. However some weak c-type transitions from gauche states (o1 and e1) in the ground state to the trans-species (e0) in the COD bending mode for low K-values ΔK=0 have been seen to be present in the spectra. These along with similar transitions for the OCD vibrational band are under investigation and the results will be communicated elsewhere. In the present work, analysis of the spectrum has been carried out to obtain precise term values and molecular parameters in the excited COH-bending state for the trans-species. The results will be shown valuable to assign similar spectra for the methanol-D2. This work represents the first reported high resolution study of this illusive vibrational mode in methanol-D1.

Nonadiabatic photodissociation dynamics of the hydroxymethyl radical via the 22A(3s) Rydberg state: A four-dimensional quantum study.

The quantum mechanical nonadiabatic photodissociation dynamics of the hydroxymethyl (CH2OH) radical in its lowest absorption band is investigated for the first time on a set of coupled diabatic potential energy surfaces determined by accurately fitting a large set of ab initio data. In this two-state approximation, only the ground and first excited states of CH2OH, which are coupled by conical intersections, are included. The reduced-dimensional dynamical model includes the CO stretch, the COH bend, the HCOH torsion, and the O-H dissociation coordinate. The experimentally measured hydrogen atom kinetic energy distribution is satisfactorily reproduced. The calculated product state distribution of the H2CO(X) fragment indicates strong vibrational excitation in the CO stretching mode, resulting from the relatively large difference in the C-O bond length between the ground and excited electronic states of CH2OH due to the photo-induced promotion of an electron from the half-occupied π*CO antibonding orbital to a Rydberg orbital. In addition, the bimodal kinetic energy distribution is confirmed to originate from nonadiabatic transitions near the conical intersection along the O-H dissociation coordinate.

Tunneling isomerization of small carboxylic acids and their complexes in solid matrixes: a computational insight.

We have studied hydrogen-atom tunneling in the cis-to-trans conformational change of some carboxylic acid monomers and formic acid (FA) complexes and dimers at the MP2(full) and CCSD(T) levels of theory within the Wentzel-Kramers-Brillouin approximation. The barrier for the minimum energy path, where the OH bond length and the COH bending angle are optimized, is found to be a good approximation for providing the highest barrier transparency. The matrix effect on the transmission coefficients of cis-FA monomer, trans-cis FA dimer (tc1), and cis-acetic acid monomer are modeled by the polarizable continuum model (PCM) at the MP2(full) level of theory in different environments. For the cis-FA monomer and trans-cis FA dimer (tc1), the calculated transmission coefficients agree with the experimental lifetimes observed in noble-gas solids. However, this method cannot reproduce the experimental results obtained for cis-acetic acid. Moreover, the long lifetime of cis-FA and cis-acetic acid in the N2 environment cannot be reproduced either, which is most probably due to specific interactions that are not included in the PCM. The calculation for cis-HCOOD shows a strong decrease of the barrier transparency compared to that for cis-HCOOH, which is consistent with the experiments. In general, good agreement is observed between the calculated barrier transparency (including PCM) and experimental tunneling rate. However, some exceptions are found, which shows that additional factors influence the tunneling rate.

Communication: conical intersections between vibrationally adiabatic surfaces in methanol.

A set of seven conical intersections (CI's) in methanol between vibrationally adiabatic surfaces is reported. The intersecting surfaces represent the energies of the two asymmetric CH stretch vibrations regarded as adiabatic functions of the torsion and COH bend angles. The ab initio data are well described by an extended Zwanziger and Grant (E ⊗ e) model [J. W. Zwanziger and E. R. Grant, J. Chem. Phys. 87, 2954 (1987)] that might also be regarded as an extension of the XHL model [L.-H. Xu, J. T. Hougen, and R. M. Lees, J. Mol. Spectrosc. 293-294, 38 (2013)]. The CI's illuminate the role of geometric phase in methanol. More generally, they suggest the importance of energy transfer processes localized near the CI's.

A multi-sheeted three-dimensional potential-energy surface for the H-atom photodissociation of phenol

Extending the previous work of Lan et al. [J. Chem. Phys., 122, 224315 (2005)], a multi-state potential model for the H atom photodissociation is presented. All three "disappearing coordinates" of the departing H atom have been considered. Ab initio CASSCF computations have been carried out for the linear COH geometry of C(2v) symmetry, and for several COH angles with the OH group in the ring plane and also perpendicular to the ring plane. By keeping the C6H5O fragment frozen in a C(2v)-constrained geometry throughout, we have been able to apply symmetry-based simplifications in the constructions of adiabatic model. This model is able to capture the overall trends of twelve adiabats at both torsional limits for a wide range of COH bend angles.

Prediction of humic acids bioactivity using spectroscopy and multivariate analysis

Humic substances (HS) represent the organic material most widespread in nature and have positive effects on plant physiology influencing nutrient uptake and root architecture. Understanding the structure–activity relationship has been considered an essential step to drive technological approaches to produce HS with enhanced plant growth promotion performance. In this study we used solid state nuclear magnetic resonance (CP-MAS 13C NMR) and infrared spectroscopy with diffuse reflectance (DRIFT), a collection of data from humic acid-like substances (HA) obtained from vermicompost produced with different sources of raw material and maturation stage. The bioactivity was evaluated using the ability of HA to induce lateral root emergence sites as well acidification of the liquid medium by treated-maize root seedlings. The structure–activity relationship was done using multivariate analysis including principal component analysis (PCA), principal component regression (PCR) and Spearman correlation joined with NMR and DRIFT results. The main variables (higher loadings) correlated with HA bioactivity were for NMR positive loadings for lignin (56ppm, 124ppm, 148 and 153ppm) and COOH groups (174ppm) and negative loadings for carbohydrates (64, 75 and 102ppm). For DRIFT the main positive loadings were aryl stretching and bending (1560, 1480, 860 and 780cm−1) probably from lignins; for 2926cm−1 and 2852cm−1 due to CH asymmetric and symmetric stretching, respectively, probably from long chain fat acids due to the presence of rocking absorption band of (CH2)n with n>4 at 720cm−1 and negative loadings due to carbohydrate moieties (around 1100cm−1, CO stretching) and carboxylic acids (1724cm−1, CO stretching and 1220cm−1, COH stretching and COH bending). The use of spectroscopy data to attempt to relate structure to bioactivity may be of practical interest in developing plant growth promoters using renewed source of humic materials.

Theoretical Calculation of the OH Vibrational Overtone Spectra of 1,5-Pentanediol and 1,6-Hexanediol

It is well-known that intramolecular hydrogen bonding affects the relative energetics of conformers, as well as the OH stretching peak positions, intensities, and width. In this study we simulated the Δv(OH) = 3, 4 overtone spectra of 1,5-pentanediol (PeD) and 1,6-hexanediol (HD) using the peak positions, intensities, and width calculated from the B3LYP/6-31+G(d,p) method. Furthermore, room temperature free energy calculations were performed using B3LYP/6-31+G(d,p) MP2/6-31+G(d,p), and MP2/6-311++G(3df,3pd) to obtain the relative population of the conformers. From the calculation of 109 and 381 distinct conformers for PeD and HD, respectively, we find that for these long chain diols the intramolecular hydrogen bonded conformers are not the most dominant conformation at room temperature. This is in stark contrast with shorter chain diols such as ethylene glycol for which the hydrogen bonded conformer dominates the population at room temperature. On the other hand, we found that the correlation between the hydrogen bonded OH red shift versus the homogeneous width, Γ = 0.0155(Δω)(1.36), which was derived for shorter chain diols, is valid even for these longer chain diols. We also showed that the intramolecular hydrogen bonded OH initially decays through the CCOH torsion and COH bending mode no matter how long the alkanediol chain length is for 1,n-alkanediols for n up to 6.

Erratum: Identification of hydrogen bond modes in polarized Raman spectra of single crystals of α‐oxalic acid dihydrate

Polarized Raman spectra of single crystals of the α-polymorphs of protonated and deuterated oxalic acid dihydrate were recorded. The interpretation of the spectra is assisted by periodic DFT calculations using the CRYSTAL06 program and by comparison with the infrared spectra of the polycrystalline material. The agreement between the calculated and observed band wavenumbers is fair in the case of low-anharmonicity modes, but marked differences appear for the stretching modes that are strongly anharmonic. A very broad feature, extending between ∼2000 and 1200 cm−1, is attributed to OH stretching. Notable is the topping of this feature by distinct bands that can be attributed to CO stretching, H2O scissoring and COH bending coupled to CO stretching. The assignments are supported by isotope effects. However, deuteration does not notably affect the wavenumber limits of the broad OH stretching band, which suggests that the potential governing the proton dynamics is of the asymmetric double-minimum type with a very low barrier. The calculated normal coordinates show a strong participation of the bending modes of water molecules in almost all internal acid motions, as well as in the external phonons. Copyright © 2009 John Wiley & Sons, Ltd.

Identification of hydrogen bond modes in polarized Raman spectra of single crystals of α‐oxalic acid dihydrate

AbstractPolarized Raman spectra of single crystals of the α‐polymorphs of protonated and deuterated oxalic acid dihydrate were recorded. The interpretation of the spectra is assisted by periodic DFT calculations using the CRYSTAL06 program and by comparison with the infrared spectra of the polycrystalline material. The agreement between the calculated and observed band wavenumbers is fair in the case of low‐anharmonicity modes, but marked differences appear for the stretching modes that are strongly anharmonic. A very broad feature, extending between ∼2000 and 1200 cm−1, is attributed to OH stretching. Notable is the topping of this feature by distinct bands that can be attributed to CO stretching, H2O scissoring and COH bending coupled to CO stretching. The assignments are supported by isotope effects. However, deuteration does not notably affect the wavenumber limits of the broad OH stretching band, which suggests that the potential governing the proton dynamics is of the asymmetric double‐minimum type with a very low barrier. The calculated normal coordinates show a strong participation of the bending modes of water molecules in almost all internal acid motions, as well as in the external phonons. Copyright © 2009 John Wiley & Sons, Ltd.

Infrared absorption of gaseous c-ClCOOH and t-ClCOOH recorded with a step-scan Fourier-transform spectrometer

Two conformers of ClCOOH were produced upon irradiation at 355 nm of a gaseous flowing mixture of Cl(2), HCOOH, and N(2). A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient spectra of ClCOOH. Absorption bands with origins at 1808.0 and 1328.5 cm(-1) are attributed to the C=O stretching and COH bending modes of t-ClCOOH, respectively; those at 1883.0 and 1284.9 cm(-1) are assigned as the C=O stretching and COH bending modes of c-ClCOOH, respectively. These observed vibrational wavenumbers agree with corresponding values for t-ClCOOH and c-ClCOOH predicted with B3LYP/aug-cc-pVTZ density-functional theory and the observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters. The observed relative intensities indicate that t-ClCOOH is more stable than c-ClCOOH by approximately 3 kJ mol(-1). A simple kinetic model is employed to account for the production and decay of ClCOOH.

How Do Hydrogen Bonds Break in Small Alcohol Oligomers?

Recent infrared pump-probe studies on alcohol oligomers in CCl(4) solution reveal that, following OH stretch excitation, ultrafast hydrogen bond (H-bond) breaking takes place on a time scale of 1 ps. To shed light on the mechanism of the H-bond breaking, we consider vibrational predissociation of the H-bonded methanol dimer. We construct a four-dimensional model for the dimer including the H-bond stretch, the donor OH stretch, the donor COH bend, and the OH rotation about the CO axis of the donor. Predissociation rates are calculated with Fermi's golden rule and close coupling approaches. Our results indicate that the predissociation leads to products with highly excited OH rotations. The predissociation rates strongly depend on the hydrogen bond strength. From our results, a simple nonadiabatic curve crossing picture for the predissociation process emerges, which provides a framework for future studies of solvent-assisted vibrational predissociation.

Proton Dynamics in the Strong Chelate Hydrogen Bond of Crystalline Picolinic Acid N-Oxide. A New Computational Approach and Infrared, Raman and INS Study

Infrared, Raman and INS spectra of picolinic acid N-oxide (PANO) were recorded and examined for the location of the hydronic modes, particularly O-H stretching and COH bending. PANO is representative of strong chelate hydrogen bonds (H-bonds) with its short O...O distance (2.425 A). H-bonding is possibly well-characterized by diffraction, NMR and NQR data and calculated potential energy functions. The analysis of the spectra is assisted by DFT frequency calculations both in the gas phase and in the solid state. The Car-Parrinello quantum mechanical solid-state method is also used for the proton dynamics simulation; it shows the hydron to be located about 99% of time in the energy minimum near the carboxylic oxygen; jumps to the N-O acceptor are rare. The infrared spectrum excels by an extended absorption (Zundel's continuum) interrupted by numerous Evans transmissions. The model proton potential functions on which the theories of continuum formation are based do not correspond to the experimental and computed characteristics of the hydrogen bond in PANO, therefore a novel approach has been developed; it is based on crystal dynamics driven hydronium potential fluctuation. The envelope of one hundred 0 --> 1 OH stretching transitions generated by molecular dynamics simulation exhibits a maximum at 1400 cm-1 and a minor hump at approximately 1600 cm-1. These positions square well with ones predicted for the COH bending and OH stretching frequencies derived from various one- and two-dimensional model potentials. The coincidences with experimental features have to be considered with caution because the CPMD transition envelope is based solely on the OH stretching coordinate while the observed infrared bands correspond to heavily mixed modes as was previously shown by the normal coordinate analysis of the IR spectrum of argon matrix isolated PANO, the present CPMD frequency calculation and the empirical analysis of spectra. The experimental infrared spectra show some unusual characteristics such as large temperature effects on the intensity of some bands, thus presenting a challenge for theoretical band shape treatments. Our calculations clearly show that the present system is characterized by an asymmetric single well potential with no large amplitudes in the hydronium motion, which extends the existence of Zundel-type spectra beyond the established set of hydrogen bonds with large hydronic vibrational amplitudes.

Structures of Aliphatic Amino Acid Proton-Bound Dimers by Infrared Multiple Photon Dissociation Spectroscopy in the 700−2000 cm-1 Region

Structural aspects of proton-bound dimers composed of amino acids with aliphatic side chains are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and electronic structure calculations. Features in the IRMPD spectra in the 700-2,000 cm-1 range are due primarily to C=O stretching, NH2 bending, and COH bending. It was possible to distinguish between isomeric structures by comparing the experimental IRMPD spectra and those predicted using B3LYP/6-31+G(d,p). It was possible, based on the calculations and IRMPD spectra, to assign the experimental spectrum of the glycine proton-bound dimer to a structure which was slightly different from that assigned by previous spectroscopic investigations and in agreement with recent thermochemical studies. Since all proton-bound dimers studied here, composed of the different amino acids, have very similar spectra, it is expected that they also have very similar lowest-energy structures including the mixed alanine/glycine proton-bound dimer. In fact, the spectra are so similar that it would be very challenging to distinguish, for example, the glycine proton-bound dimer from the alanine or valine proton-bound dimers in the 700-2,000 cm-1 range. According to the calculated IR spectra it is shown that in the approximately 2,000-3,200 cm-1 range differentiating between different structures as well as different proton-bound dimers may be possible. This is due mainly to differences in the asymmetric stretch of the binding proton which is predicted to occur in this region.

Criteria for Determining the Hydrogen-Bond Structures of a Tyrosine Side Chain by Fourier Transform Infrared Spectroscopy: Density Functional Theory Analyses of Model Hydrogen-Bonded Complexes of p-Cresol

Fourier transform infrared (FTIR) spectroscopy is a powerful method to investigate the structures of key Tyr residues involved in various protein reactions. In this study, we have performed density functional theory (DFT) calculations for various hydrogen-bonded complexes of p-cresol, a simple model of a Tyr side chain, in different hydrogen-bond forms to develop explicit criteria for determining the hydrogen-bond structures of Tyr using FTIR spectroscopy. The CO stretching (nuCO) and COH bending (deltaCOH) vibrations were focused as markers and calculated results were compared with experimental data of p-cresol and Tyr. The calculated and experimental nuCO frequencies appeared at 1280-1260, 1260-1250, 1255-1235, and 1240-1220 cm-(1) in the hydrogen-bond donor, free, donor-acceptor, and acceptor forms, respectively. These frequencies, which showed little overlap between the individual hydrogen-bond forms, had a negative linear correlation with the CO lengths in optimized geometries. The deltaCOH frequencies were found at 1255-1210 cm-(1) in the donor form, while the free and acceptor forms showed relatively low deltaCOH frequencies at 1185-1165 and 1190-1160 cm-(1), respectively. In the donor-acceptor form, the vibrational mode with a considerable deltaCOH contribution was found at 1280-1255 cm-(1) with a weak IR intensity. This frequency and the nuCO frequency in the donor-acceptor form are similar to the nuCO and deltaCOH frequencies, respectively, of the donor form, making it difficult to discriminate the two forms. These two forms can be clearly distinguished by detecting a strong nuCO(D) band in p-cresol-OD or Tyr-OD, in which the deltaCOD vibration largely downshifts to approximately 1000 cm-(1). The nuCO(D) frequency of the donor-acceptor form was found at 1260-1240 cm-(1), while that of the donor form was at 1270-1255 cm-(1). Practically, plotting the frequency of the lower-frequency strong IR band (nuCO of the donor-acceptor form or deltaCOH of the donor form) of undeuterated species against the nuCO(D) frequency is convenient for accurate discrimination. Because the donor form shows a positive linear correlation between deltaCOH and nuCO(D) frequencies, the two forms exhibited distinct areas in this plot. The effects of hydrogen-bond interactions on other potential IR and Raman markers are also discussed.

Structure and Dynamics of the Solvated Electron in Alcohols from Resonance Raman Spectroscopy

Resonance Raman (RR) spectroscopy is used to probe the structure and excited-state dynamics of the solvated electron in the primary liquid alcohols methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), and n-butanol (n-BuOH). The strong resonance enhancements (>or=10(4) relative to pure solvent) of the libration, CO stretch, COH bend, CH3 bend, CH2 bend, and OH stretch reveal significant Franck-Condon coupling of the intermolecular and intramolecular vibrational modes of the solvent to the electronic excitation of the solvated electron. All enhanced bands are fully accounted for by a model of the solvated electron that is comprised of several nearby solvent molecules that are only perturbed by the presence of the electron; no new molecular species are required to explain our data. The 340 cm(-1) downshift observed for the OH stretch frequency of e-(MeOH), relative to pure solvent, strongly suggests that the methanol molecules in the first solvent shell have the hydroxyl group directed linearly toward the excess electron density. The smaller downshifts observed for e-(EtOH), e-(n-PrOH), and e-(n-BuOH) are explained in terms of a OH group that is bent 28-40 degrees from linear. The Raman cross sections and absorption spectra are modeled, lending quantitative support for the inhomogeneous origin of the broad absorption spectra, the necessity of OH local motion in all enhanced Raman modes of the alcohols, and the dominant librational response of the solvent upon photoexcitation of the electron.

The torsion-vibration spectrum of methanol trapped in neon matrix

The spectra of six isotopic species of methanol trapped in neon matrix have been recorded between 30 and 11,000 cm −1. Noticeable temperature effects are observed for the OH species. They allow two kinds of absorptions to be identified for most of the vibrational transitions: those low temperature (LT) predominating at 2.7 K and those high temperature (HT) significantly increasing in intensity when varying temperature from 2.7 to 7 K. The conversion is perfectly reversible and has been examined from two points of view: the kinetics of interconversion and the activation energy associated to the process. The kinetic study shows that the limiting factor of the measurements is the time required for spectral acquisition (about 5 s). The activation energy deduced from Arrhenius plots is found equal to 6.7 ± 0.5 cm −1. From the comparison with gas phase data one concludes that the internal rotation about the C–O bond is not strongly altered by the matrix surrounding, the activation energy corresponding to the E–A tunneling splitting of the ground state (9.12 cm −1 for CH 3OH in the gas phase). The pure torsion and the torsion-vibration spectra are generally consistent with those in the gas phase. In particular the complex structure of the COH bending mode is well reproduced, with a clear identification of the transitions issuing from the A and E sublevels of the ground state.

IR plus vacuum ultraviolet spectroscopy of neutral and ionic organic acid monomers and clusters: Propanoic acid

The vibrational spectrum of molecular propanoic acid, cooled in a supersonic expansion, in the region of 2500 to 7500 cm(-1) is obtained employing infrared plus vacuum ultraviolet nonresonant ionization detected spectroscopy. The fundamental and first overtone of the CH and OH stretch modes of cold propanoic acid molecules can be identified in the spectrum. Propanoic acid neutral and ionic clusters are also studied employing nonresonant ion dip and photodissociation spectroscopic techniques, respectively. For the neutral dimer, a sequence of features observed at ca. 2500-2700 cm(-1) can be assigned as combination bands of low frequency modes with the COH bending overtone; these features characterize the cyclic dimer ring structure. IR spectra of the larger neutral clusters n=3, 4, 5 indicate that they also have cyclic structures in which the OH groups are engaged in the cluster hydrogen bonding network. The CH groups are not involved in this hydrogen bonding structure. Free OH features are observed for the protonated ion clusters (C(2)H(5)COOH)(n)H(+), n=1,...,5, indicating that at least one OH group of these cluster ions is not involved in the cluster hydrogen bonding network. A comparison of the results for four hydrogen bonding neutral and ionic clusters (CH(3)OH, C(2)H(5)OH, CH(3)COOH, and C(2)H(5)COOH) is presented and discussed.