Heterogeneous proton-bound dimers with a high dipole moment monomer: how could we experimentally observe these anomalous ionic hydrogen bonds?

Electronic structure calculations (CBS-QB3 and G3MP2) have been used to predict a suitable method to experimentally observe the anomalous structure which is predicted to exist in a proton-bound dimer with a high dipole moment monomer. The enthalpy associated with forming the proton-bound dimer from its protonated and neutral monomers is shown to be linearly related to the difference in proton affinities which has been observed experimentally. However, unlike previous experimental studies, the linear correlation is not predicted to depend strongly, if at all, on whether the basic sites are C=O, C=N, or O(H) n-donor bases. Thermochemical measurements, then, are probably not the best method to distinguish between the structures of heterogeneous proton-bound dimers. It has been shown that a suitable method to experimentally observe the anomalous structure of proton-bound dimers containing a high dipole moment monomer (or very polar monomer) is by spectroscopic measurement. The O-H+-O asymmetric stretch is probably not the best infrared band to try to correlate with structure. The best band to observe is one which is in a region of the spectrum not masked by other absorptions and is also sensitive to the proximity of the binding proton. For example, it is shown that the methanol-free O-H stretch is very sensitive to the O-H+ bond distance for a series of heterogeneous proton-bound dimers containing methanol. It is predicted that the free O-H stretch of the methanol/acetonitrile proton-bound dimer is more closely related to the O-H stretch in protonated methanol than the O-H stretch in neutral methanol. Observations of these bands should confirm that the proton is closer to methanol in the methanol/acetonitrile proton-bound dimer despite acetonitrile having a higher proton affinity.     

Structures of aliphatic amino acid proton-bound dimers by infrared multiple photon dissociation spectroscopy in the 700-2,000 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.     

Structures of heterogeneous proton-bond dimers with a high dipole moment monomer: covalent vs electrostatic interactions

A number of calculated structures of heterogeneous proton-bound dimers containing monomers such as acetonitrile, cyanamide, vinylene carbonate, and propiolactone, which have high dipole moments, are presented. These proton-bound dimers are predicted to have a structural anomaly pertaining to the bond distances between the central proton and the basic sites on each of the monomers. The monomers with the high dipole moments also have the larger proton affinity and, on the basis of difference in proton affinities, it would be expected that the proton would be closer to this monomer than the one with the lower proton affinity. However, the proton is found to lie substantially closer to the monomer with the lower proton affinity in most cases, unless the difference in proton affinity is too large. Simply stated, the difference in proton affinities is smaller than the difference in the affinity to form an ion-dipole complex for the two monomers and it is the larger affinity for the high dipole moment monomer (which also has the higher proton affinity) to form an ion-dipole complex that is responsible for the proton lying closer to the low proton affinity monomer. The bond distances between the central proton and the monomers are found to be related to the difference in proton affinity. It is found, though, that the proton-bound dimers can be grouped into two separate groups, one where the proton-bound dimer contains a high dipole moment monomer and one group where the proton-bound dimer does not contain a high dipole moment monomer. From these plots it has been determined that a high dipole moment monomer is one that has a dipole moment greater than 2.9 D.     

Infrared multiple photon dissociation spectra of proton- and sodium ion-bound glycine dimers in the N-H and O-H stretching region

The proton- and the sodium ion-bound glycine homodimers are studied by a combination of infrared multiple photon dissociation (IRMPD) spectroscopy in the N-H and O-H stretching region and electronic structure calculations. For the proton-bound glycine dimer, in the region above 3100 cm (-1), the present spectrum agrees well with one recorded previously. The present work also reveals a weak, broad absorption spanning the region from 2650 to 3300 cm (-1). This feature is assigned to the strongly hydrogen-bonded and anharmonic N-H and O-H stretching modes. As well, the shared proton stretch is observed at 2440 cm (-1). The IRMPD spectra for the proton-bound glycine dimer confirms that the lowest energy structure is an ion-dipole complex between N-protonated glycine and the carboxyl group of the second glycine. This spectrum also helps to eliminate the existence of any of the higher-energy structures considered. The IRMPD spectrum for the sodium ion-bound dimer is a much simpler spectrum consisting of three bands assigned to the O-H stretch and the asymmetric and symmetric NH 2 stretching modes. The positions of these bands are very similar to those observed for the proton-bound glycine dimer. Numerous structures were considered and the experimental spectrum agrees with the B3LYP/6-31+G(d,p) predicted spectrum for the lowest energy structure, two bidentate glycine molecules bound to Na (+). Though some of the structures cannot be completely ruled out by comparing the experimental and theoretical spectra, they are energetically disfavored by at least 20 kJ mol (-1).     

Gas phase infrared multiple-photon dissociation spectra of methanol, ethanol and propanol proton-bound dimers, protonated propanol and the propanol/water proton-bound dimer

The infrared multiphoton dissociation (IRMPD) spectra of three homogenous proton-bound dimers are presented and the major features are assigned based on comparisons with the neutral alcohol and with density functional theory calculations. As well, the IRMPD spectra of protonated propanol and the propanol/water proton-bound dimer (or singly hydrated protonated propanol) are presented and analysed. Two primary IRMPD photoproducts were observed for each of the alcohol proton bound dimers and were found to vary with the frequency of the radiation impinging upon the ions. For example, when the proton-bound dimer absorbs weakly a larger amount of S(N)2 product, protonated ether and water, are observed. When the proton-bound dimer absorbs more strongly, an increase in the simple dissociation product, protonated alcohol and neutral alcohol, is observed. With the aid of RRKM calculations this frequency dependence of the branching ratio is explained by assuming that photon absorption is faster than dissociation for these species and that only a few photons extra are necessary to make the higher-energy dissociation channel (simple cleavage) competitive with the lower energy (S(N)2) reaction channel.     

Structural characterization by infrared multiple photon dissociation spectroscopy of protonated gas-phase ions obtained by electrospray ionization of cysteine and dopamine

Structural characterization of protonated gas-phase ions of cysteine and dopamine by infrared multiple photon dissociation (IRMPD) spectroscopy using a free electron laser in combination with theory based on DFT calculations reveals the presence of two types of protonated dimer ions in the electrospray mass spectra of the metabolites. In addition to the proton-bound dimer of each species, the covalently bound dimer of cysteine (bound by a disulfide linkage) has been identified. The dimer ion of m/z 241 observed in the electrospray mass spectra of cysteine has been identified as protonated cystine by comparison of the experimental IRMPD spectrum to the IR absorption spectra predicted by theory and the IRMPD spectrum of a standard. Formation of the protonated covalently bound disulfide-linked dimer ions (i.e. protonated cystine) from electrospray of cysteine solution is consistent with the redox properties of cysteine. Both the IRMPD spectra and theory indicate that in protonated cystine the covalent disulfide bond is retained and the proton is involved in intramolecular hydrogen bonding between the amine groups of the two cysteine amino acid units. For cysteine, the protonated covalently bound dimer (m/z 241) dominated the mass spectrum relative to the proton-bound dimer (m/z 243), but this was not the case for dopamine, where the protonated monomer and the proton-bound dimer were both observed as major ions. An extended conformation of the ethylammonium side chain of gas-phase protonated dopamine monomer was verified from the correlation between the predicted IR absorption spectra and the experimental IRMPD spectrum. Dopamine has the same extended ethylamine side chain conformation in the proton-bound dopamine dimer identified in the mass spectra of electrosprayed dopamine. The structure of the proton-bound dimer of dopamine is confirmed by calculations and the presence of an IR band due to the shared proton. The presence of the shared proton in the protonated cystine ion can be inferred from the IRMPD spectrum.

A short hydrogen bond investigation by polarized Raman spectra of Co2+ and Zn2+ salts of pyromellitic acid

Cobalt and zinc salts of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), [C(6)H(2)(COO)(4)H(4)], have been synthesized and investigate by polarized Raman spectroscopy. These compounds present short intramolecular hydrogen bonds (SHB) between adjacent carboxyl groups. Raman spectra indicate the presence of this interaction in both salts. Three specific vibrational of SHB modes have been investigated: O-H-O symmetric [nu(sym)(OHO)] and asymmetric [nu(asym)(OHO)] stretching modes and O-H stretching mode [nu(O-H)], which they were observed around 300, 850 and 2500 cm(-1), respectively. In crystallographic point of view, the cobalt salt presents a symmetric SHB while the zinc salt presents an asymmetric SHB. In cobalt salt all three vibrational modes of O-H-O groups in polarized Raman spectra occur in A(g) orientation although in zinc salts two of them are observed in A(g) orientation and one in B(g). Spectra analysis indicate that nu(sym)(OHO) mode is observed as A(g) to cobalt salt and B(g) to zinc salt. This mode occurs in a crowded spectral region and its identification was made by deconvolution techniques. Comparing spectra of the two salts, it is observed a small difference in relative intensity and wavenumber shift of nu(sym)(OHO) (deviance of 43 cm(-1)) and nu(OH) (deviance of 21 cm(-1)) modes due probably to differences in O...O distance between salts and in orientation of pyromellitate anion in unit cell. The nu(asym)(OHO) mode does not present significant wavenumber shift due difference in SHB. The nu(OH) band presents a great potential for hydrogen bond studies due to the fact that in its vibrational region (around 2500 cm(-1)) it is not observed other vibrational modes of these compounds.     

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.     

Proton-bound cluster ions in ion mobility spectrometry

Gaseous oxygen and nitrogen bases, both singly and as binary mixtures, have been introduced into ion mobility spectrometers to study the appearance of protonated molecules, and proton-bound dimers and trimers. At ambient temperature it was possible to simultaneously observe, following the introduction of molecule A, comparable intensities of peaks ascribable to the reactant ion (H₂O)_nH⁺, the protonated molecule AH⁺ and AH⁺ · H₂O, and the symmetrical proton bound dimer A₂H⁺. Mass spectral identification confirmed the identifications and also showed that the majority of the protonated molecules were hydrated and that the proton-bound dimers were hydrated to a much lesser extent. No significant peaks ascribable to proton-bound trimers were obtained no matter how high the sample concentration. Binary mixtures containing molecules A and B, in some cases gave not only the peaks unique to the individual compounds but also peaks due to asymmetrical proton bound dimers AHB⁺. Such ions were always present in the spectra of mixtures of oxygen bases but were not observed for several mixtures of oxygen and nitrogen bases. The dimers, which were not observable, notable for their low hydrogen bond strengths, must have decomposed in their passage from the ion source to the detector, i.e. in a time less than ∼5 ms. When the temperature was lowered to −20 °C, trimers, both homogeneous and mixed, were observed with mixtures of alcohols. The importance of hydrogen bond energy, and hence operating temperature, in determining the degree of solvation of the ions that will be observed in an ion mobility spectrometer is stressed. The possibility is discussed that a displacement reaction involving ambient water plays a role in the dissociation.     

DFT energy surfaces for aminopurine homodimers and their conjugate acid ions

Dimers of free nucleobases with their conjugate acid ions can be assigned to either of two categories: protonated dimers or proton-bound dimers. In the former, the extra proton attaches to a lone pair of a neutral dimer. In the latter, the extra proton is situated between two lone pairs and participates in a proton bridge. In general, proton-bound dimers are found to be more tightly held together than protonated dimers. While neutral adenine and its isomer 8-aminopurine (C(5)H(5)N(5)) are substantially more stable than their 7H tautomers, their conjugate acid ions and those of their respective 7H tautomers have nearly the same heats of formation. Correspondingly, the most stable (C(5)H(5)N(5))2H+ structures contain 7H tautomers as the neutral partner. Proton transit from one partner to the other within the most stable protonated dimer of 8-aminopurine has a low barrier (6 kJ mol(-1)). The potential energy curve for the NH stretch in that case is better fitted as a double minimum rather than as a harmonic potential. Purine-purine mismatches have been observed in nucleic acids, to which calculated (C(5)H(5)N(5))2H+ dimer geometries appear nearly isosteric.     

Matrix isolation and density functional theory study of bis(trifluoromethyl)dioxodiazine: a photodimer of trifluoronitrosomethane

Trifluoronitrosomethane (CF3NO) was trapped in rare gas matrixes and irradiated at 633 and 670 nm. The infrared spectra of the postirradiation samples exhibit features consistent with cis and trans conformers of bis(trifluoromethyl)dioxodiazine, a previously uncharacterized species. The concentration dependence of the formation of the dimer is consistent with a mechanism in which monomers trapped in adjacent sites undergo excitation and subsequent reaction. The dimers reversibly form the monomer when irradiated with ultraviolet light. Density functional theory was used to determine the structure of the dimers and predict their infrared and Raman spectra. The predicted vibrational frequencies are in agreement with those observed. A third (skewed) conformation was predicted to have a triplet ground state, but no evidence of this species was observed. All three dimers exhibit significant diradical character, as evidenced by comparatively low N-N and high N-O stretching frequencies. Transition-state calculations predict the dimerization barrier to range from 17.1 (cis) to 35.0 (trans) kJ mol(-1) for the singlet dimers and to be 62.1 kJ mol(-1) for the triplet dimer. This is an example of nitroso dimerization that requires electronic excitation to proceed.     

Fourier transform infrared and Raman spectral investigations of 5-aminoindole

Fourier transform infrared (FT-IR) and Raman (FT-Raman) spectra of 5-aminoindole has been recorded and analysed. The FT-IR spectrum of the compound was recorded in a BrukerIFS 66 V spectrometer in the range 4000-400 cm(-1) and the FT-Raman spectrum was also recorded in the same instrument in the region 3500-100 cm(-1). Observed frequencies for normal modes are compared with those calculated form normal co-ordinate analysis. The shift in the frequencies of the fundamental modes with the substituent amino group and the mixing of different normal modes are discussed with the help of potential energy distribution (PED) calculated through normal co-ordinate analysis.     

Structural preferences, argon nanocoating, and dimerization of n-alkanols as revealed by OH stretching spectroscopy in supersonic jets

n-Alkanols can occur in a multitude of energetically competitive conformational states. Using the OH stretching vibration as an infrared and Raman spectroscopic sensor in supersonic jet expansions, the torsional preferences around the Calpha-O and Cbeta-Calpha bonds are probed for n-propanol through n-hexanol. Raman detection is more powerful for isolated monomers, whereas IR spectroscopy is more sensitive for molecular complexes. The subtle IR vibrational shift induced by the nanocoating of n-alcohols with Ar atoms is shown to alternate with chain length. A large number of alcohol dimer absorptions is observed and subjected to collisional relaxation and nanocoating conditions. Essential features of the dimer spectra are modeled successfully by a simple force field approach. Exploratory quantum chemical calculations up to the MP2/aug-cc-pvqz level encourage a rigorous theoretical study of the subtle conformational aspects in monomers and possibly also in dimers of linear alcohols.     

Infrared spectrum of NH4+(H2O): evidence for mode specific fragmentation

The gas phase infrared spectrum (3250-3810 cm-1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by action spectroscopy of mass selected and isolated ions. The four bands obtained are assigned to N-H stretching modes and to O-H stretching modes. The N-H stretching modes observed are blueshifted with respect to the corresponding modes of the free NH4+ ion, whereas a redshift is observed with respect to the modes of the free NH3 molecule. The O-H stretching modes observed are redshifted when compared to the free H2O molecule. The asymmetric stretching modes give rise to rotationally resolved perpendicular transitions. The K-type equidistant rotational spacings of 11.1(2) cm-1 (NH4+) and 29(3) cm-1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The relative band intensities recorded compare favorably with predictions of high level ab initio calculations, except on the nu3(H2O) band for which the observed value is about 20 times weaker than the calculated one. The nu3(H2O)/nu1(H2O) intensity ratios from other published action spectra in other cationic complexes vary such that the nu3(H2O) intensities become smaller the stronger the complexes are bound. The recorded ratios vary, in particular, among the data collected from action spectra that were recorded with and without rare gas tagging. The calculated anharmonic coupling constants in NH4+(H2O) further suggest that the coupling of the nu3(H2O) and nu1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings together render a picture of a mode specific fragmentation dynamic that modulates band intensities in action spectra with respect to absorption spectra. Additional high level electronic structure calculations at the coupled-cluster singles and doubles with a perturbative treatment of triple excitations [CCSD(T)] level of theory with large basis sets allow for the determination of an accurate binding energy and enthalpy of the NH4+(H2O) cluster. The authors' extrapolated values at the CCSD(T) complete basis set limit are De [NH4+-(H2O)]=-85.40(+/-0.24) kJ/mol and DeltaH(298 K) [NH4+-(H2O)]=-78.3(+/-0.3) kJ/mol (CC2), in which double standard deviations are indicated in parentheses.     

Infrared photodissociation spectroscopy of electrosprayed ions in a Fourier transform mass spectrometer

Previous gas-phase methods for infrared photodissociation spectroscopy (IRPD) require sample volatility. Our method instead uses electrospray ionization to introduce even large nonvolatile molecules into a Fourier transform mass spectrometer, where extended (>15 s) ion storage makes possible high sensitivity spectral measurements with an OPO laser over a range of 3050-3800 cm(-1). The spectra of 22 gaseous proton-bound amino acid complexes are generally correlated with the H-stretching frequencies established for O-H and N-H functional groups in solution. For theoretical structure predictions of the Gly2H+ and N-acylated Asp2H+ dimers, IRPD spectra clearly differentiate between the predicted lowest energy conformers. In contrast to solution, in the gas phase the glycine zwitterion is approximately 20 kcal/mol less stable than the neutral; however, glycine is clearly zwitterionic in the gaseous GlyLysH+ dimer. The level of theory is inadequate for the larger Lys2H+ dimer, as all low energy predicted structures have free carboxyl O-H groups, in contrast to the IR spectrum. IRPD appears to be a promising new technique for providing unique information on a broad range of biomolecular and other gaseous ions, especially on noncovalent bonding involving O-H and N-H groups.     

Infrared studies of pyroelectric Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and Horizontal Attenuated Total Reflectance (HATR) spectra have been recorded of arachidic acid (AA)/1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) Y-type alternate LB films deposited on an aluminium plate with 31 layers. It is well known that the frequencies of CH₂ stretching bands of a hydrocarbon chain are sensitive to the conformational ordering of the chain. Changes in frequency and intensity can be used to characterize film ordering and preferential molecular packing. The observed peak frequencies and intensities of these bands indicate that the alkyl chains are present in a mostly trans conformation and tilted from the normal direction with respect to the substrate in LB films. The FTIR–GAIR and HATR spectra of 31 layers alternate film show significant changes in the region 1700–1400 cm⁻¹ due to the partial proton transfer between acid and amine head groups. According to the HATR spectrum, the peak at 1731 cm⁻¹ is observed due to a proportion of the carboxylic acid groups forming sideways dimers indicating that if the carboxylic acid groups form sideways dimers, they are less likely to undergo proton transfer with the amino groups.     

Substituent effects in porphyrin dimer complexes studied by IR spectroscopy

A series of lanthanide porphyrin dimers have been synthesized and investigated with IR spectroscopic techniques. The spectra of the porphyrin dimers are compared not only with each other but also with those of their component monomer units. The experimental results exhibit that the IR spectra of the porphyrin dimers are closely related to those of their corresponding monomers. A detailed analysis of the IR spectra between the porphyrin dimers and monomers suggest that the dimer molecules can be treated as regular derivatives of metalloporphyrin monomers despite the symmetries of these two systems being different. The dimerization of the porphyrin rings only result in frequency shifts and intensity changes of the IR spectra. These shifts are attributed to the induced π–π interactions between these two macrocycles. The downshifts of the frequencies observed in Ce(OEP)₂ further indicate that the π–π interactions intrinsically decrease the bond strength of the entire molecule. Additionally, only the relative intensities instead of the frequencies of the ethyl vibrations in the region 2800–3000 cm⁻¹ are observed to be sensitive to the types and the positions of the substituent groups. These observations suggest that these ethyl vibrational modes of the OEP moiety can be used as characteristic bands to monitor subtle deformations of the porphyrin rings caused by the substituent groups in the dimer complexes.     

Determination of the proton affinities of bromo- and iodoacetonitrile using the kinetic method with full entropy analysis

The proton affinities bromo- and iodoacetonitrile are determined using a full implementation of the kinetic method, which includes calculation of the entropies. Branching ratios for dissociation of proton-bound dimers are measured for collision energies ranging from approximately 2 to 5 eV. Using a rigorously correct statistical approach, the proton affinities of bromo- and iodoacetonitrile are calculated to be 179.8 +/- 1.7 and 182.9 +/- 1.6 kcal/mol, respectively. It is shown that neglecting the entropy contributions for these systems leads to proton affinities that are too high by approximately 0.6 kcal/mol.     

Spectral Signatures of Protonated Noble Gas Clusters of Ne,Ar, Kr,and Xe: From Monomers to Trimers

The structures and spectral features of protonated noble gas clusters are examined using a first principles approach. Protonated noble gas monomers (NgH⁺) and dimers (NgH⁺Ng) have a linear structure, while the protonated noble gas trimers (Ng₃H⁺) can have a T-shaped or linear structure. Successive binding energies for these complexes are calculated at the CCSD(T)/CBS level of theory. Anharmonic simulations for the dimers and trimers unveil interesting spectral features. The symmetric NgH⁺Ng are charactized by a set of progression bands, which involves one quantum of the asymmetric Ng-H⁺ stretch with multiple quanta of the symmetric Ng-H⁺ stretch. Such a spectral signature is very robust and is predicted to be observed in both T-shaped and linear isomers of Ng₃H⁺. Meanwhile, for selected asymmetric NgH⁺Ng’, a Fermi resonance interaction involving the first overtone of the proton bend with the proton stretch is predicted to occur in ArH⁺Kr and XeH⁺Kr.     

Vibrational coupling in carboxylic acid dimers

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.