Hydrated alizarin complexes: hydrogen bonding and proton transfer

We investigated the hydrogen bonding structures and proton transfer for the hydration complexes of alizarin (Az) produced in a supersonic jet using fluorescence excitation (FE), dispersed laser induced fluorescence (LIF), visible-visible hole burning (HB), and fluorescence detected infrared (FDIR) spectroscopy. The FDIR spectrum of bare Az with two O-H groups exhibits two vibrational bands at 3092 and 3579 cm(-1), which, respectively, correspond to the stretching vibration of O1-H1 that forms a strong intramolecular hydrogen bond with the C9=O9 carbonyl group and the stretching vibration of O2-H2 that is weakly hydrogen-bonded to O1-H1. For the 1:1 hydration complex Az(H(2)O)(1), we identified three conformers. In the most stable conformer, the water molecule forms hydrogen bonds with the O1-H1 and O2-H2 groups of Az as a proton donor and proton acceptor, respectively. In the other conformers, the water binds to the C10=O10 group in two nearly isoenergetic configurations. In contrast to the sharp vibronic peaks in the FE spectra of Az and Az(H(2)O)(1), only broad, structureless absorption was observed for Az(H(2)O)(n) (n≥ 2), indicating a facile decay process, possibly due to proton transfer in the electronic excited state. The FDIR spectrum with the wavelength of the probe laser fixed at the broad band exhibited a broad vibrational band near the O2-H2 stretching vibration frequency of the most stable conformer of Az(H(2)O)(1). With the help of theoretical calculations, we suggest that the broad vibrational band may represent the occurrence of proton transfer by tunnelling in the electronic ground state of Az(H(2)O)(n) (n≥ 2) upon excitation of the O2-H2 vibration.     

HCl solvation at the surface and within methanol clusters/nanoparticles II: Evidence for molecular wires

Condensed-phase solvation of HCl on and within methanol nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy, on-the-fly molecular dynamics as implemented in the density functional code Quickstep (which is part of the CP2K package), and ab initio calculations. Adsorption and solvation stages are identified and assigned with the help of calculated infrared spectra obtained from the simulations. The results have been further checked with MP2-level ab initio calculations. The range of acid solvation states extends from the single-coordinated slightly stretched HCl to proton-sharing with Zundel-like methanol O...H+...X- states, and finally to MeOH2+...Cl- units with full proton transfer. Furthermore, once the proton moves to methanol, it is mobilized along methanol molecular chains. Since the proton dynamics reflects the evolving local structures, the "proton" spectra display broad bands usually with underlying continua.     

Hydrogen bond dynamics in crystalline β-9-anthracene carboxylic acid--a combined crystallographic and spectroscopic study

We compare results from single crystal X-ray diffraction and FTIR spectroscopy to elucidate the nature of hydrogen bonding in β-9-anthracene carboxylic acid (β-9AC, C(15)H(10)O(2)). The crystallographic studies indicate a disorder for the protons in the cyclic hydrogen bond. This disorder allows the determination of the energy difference between two proton sites along the hydrogen bond. The temperature dependent Fourier transform infrared spectroscopy (FTIR) underpins the crystallographic results. The combination of both methods allows the estimation of a one-dimensional potential curve describing the OH-stretching motion. The dynamical properties of the proton transfer along the hydrogen bond are extracted from this potential. The work presented here has profound implication on future studies of photochemical dynamics of crystalline β-9AC, which can deliver a deeper understanding of the mechanism of photochemical driven molecular machines and the optical and electronic properties of molecular organic semiconductors.     

A proton between two waters: insight from full-dimensional quantum-dynamics simulations of the [H2O-H-OH2]+ cluster

The dynamics of a proton between two water molecules is studied by full-dimensional (15 dimensional) quantum dynamics using the multiconfigurational time-dependent Hartree (MCTDH) method. The collision of H(3)O(+) and H(2)O fragments is followed by an ultrafast and nearly irreversible energy transfer from the degrees of freedom that define the hydrogen bond (oxygen-oxygen distance and central proton position) to the rest of the degrees of freedom. The vibrations of the oxygen-oxygen distance are damped within the first 300 fs while the vibrations of the shared proton along the hydrogen bond are damped within the first 150 to 200 fs. Collisions in which the fragments arrive with a high momentum to the interaction distance lead to more recrossing of the transferring proton than collisions with a lower momentum. Slow coordinates, e.g. pyramidalization of the water monomers, have less time to adapt to the incoming or outgoing proton in the case of a high momentum, which leads to an enhanced recrossing effect with respect to slower collisions. In order to understand the energy flow dynamics between the vibration of the shared proton and other degrees of freedom a 5-state model is constructed and exactly solved. The energies and couplings of the states of the model are obtained from the analysis of the infrared spectroscopy of the H(5)O(2)(+) cation, namely from splittings and shifts of the most important spectral lines. The model qualitatively reproduces the key aspects of the full dynamics related to the vibrations of the shared proton, indicating that the proposed coupling scheme is correct.     

Extended tunnelling states in the benzoic acid crystal: infrared and Raman spectra of the OH and OD stretching modes

We compare Raman and infrared spectra of the nuOH/OD modes in benzoic acid crystal powders at 7 K. The extremely sharp Raman bands contrast to the broad infrared profiles and suggest adiabatic separation of hydrogen (deuterium) dynamics from the crystal lattice. There is no evidence of any proton-proton coupling term. The assignment scheme is consistent with a quasisymmetric double-minimum potential, largely temperature independent. Tunnel splitting is a major band shaping mechanism, in addition to anharmonic coupling with lattice modes. The proton/deuteron dynamics are rationalized with nonlocal pseudoparticles and extended states. We propose a symmetry-related damping mechanism to account for the broad infrared profiles, as opposed to the sharp Raman bands. We assign spectral features to distinct interconversion mechanisms based on either pseudoparticle transfer or adiabatic pairwise transfer. We establish close contacts with theoretical models based on first-principles calculations.     

NHN]+ hydrogen bonding in protonated 1,8-bis(dimethylamino)-2,7-dimethoxynaphthalene. X-ray diffraction, infrared, and theoretical ab initio and DFT studies

Structural (X-ray diffraction), infrared spectroscopic, and theoretical MP2 and DFT studies on the HBr and DBr adducts of 1,8-bis(dimethylamino)2,7-dimethoxynaphthalene ((CH3O)2.DMAN) were performed. This particular proton sponge has been chosen for its strong basicity and display of the buttressing effect influencing the hydrogen bond dynamics and properties. The studies revealed a symmetric, planar DMAN.H+ cation with a short (NHN)+ hydrogen bond of 2.567(3) A. The X-ray diffraction results suggest that the proton is in the central position in the bridge, while the calculations show two potential energy minima with the zero point energy level close to the top of the barrier. The infrared spectra display an (NHN)+ band at 488 cm(-1) and an (NDN)+ band at 235 cm(-1), respectively. It gives the isotopic ratio of 2.08, the highest value reported to date. Such a result suggests a peculiar shape of the potential for the proton motion, characterized by an extremely high positive anharmonicity. The calculations reproduce this particular potential, yielding an ISR value displaying a very good agreement with the experimental one. The anharmonic frequencies, however, show the discrepancy between the observed and calculated transitions.     

Proton dynamics in the hydrogen bond. Inelastic neutron scattering by single crystals of CsH2PO4 at 20 K

Inelastic neutron scattering spectra (INS) of the powder and of oriented single crystals of cesium dihydrogen phosphate (CsH₂PO₄, or CDP) at 20 K have been investigated. For single crystals the incident neutron beam was perpendicular to either the [100] or [001] crystal planes in order to distinguish between the short and long hydrogen bonds. The proposed assignments are based on previous infrared and Raman data and on the INS band intensities and polarisation. The optical and INS OH stretching band profiles are compared. Their shapes are described in terms of mechanical and electrical coupling of the two stretching modes of a O---H…O hydrogen bond. For the longer bond a Fermi resonance of the OH stretching mode with an overtone of the bending mode is observed. Finally, a broad central mode observed in the INS at very low frequency has been tentatively assigned to the relaxation of the proton transfer along the hydrogen bond.     

Acetic acid monomer: Ab initio study,barrier to proton tunnelling,and infrared assignment

The barrier to tunnelling of the carboxyl proton in monomeric acetic acid is found to be 96.7 kcal/mole in an ab initio study, which is not compatible with an earlier interpretation of the strong observed in the infrared matrix spectra of monomeric acetic acid and deuterated species. A new explanation is suggested for the anharmonic effects, and a vibrational assignment, supported by normal coordinate calculations is proposed.The ab initio SCF calculations were carried out with two different Gaussian basis sets for comparison. The values obtained for the methyl torsional barrier, for the energy difference between the trans OCOH and the more stable cis OCOH conformtion, and for the OH-torsional force constant (0.15 and 0.19 mdyn/Å) are in good agreement with earlier results and/or with experimental data. The main anharmonicities observed in the spectra are apparently due to Fermi resonance involving CO stretching and COH bending fundamentals and combination tones of skeletal breathing and bending motions.     

Interactions between the chloride anion and aromatic molecules: infrared spectra of the Cl- -C6H5CH3, Cl- -C6H5NH2 and Cl- -C6H5OH complexes

The Cl- -C6H5CH3*Ar, Cl- -C6H5NH2*Ar, and Cl- -C6H5OH*Ar anion complexes are investigated using infrared photodissociation spectroscopy and ab initio calculations at the MP2/aug-cc-pVDZ level. The results indicate that for Cl- -C6H5NH2 and Cl- -C6H5OH, the Cl- anion is attached to the substituent group by a single near-linear hydrogen bond. For Cl--C6H5CH3, the Cl- is attached to an ortho-hydrogen atom on the aromatic ring and to a hydrogen atom on the methyl group by a weaker hydrogen bond. The principal spectroscopic consequence of the hydrogen-bonding interaction in the three complexes is a red-shift and intensity increase for the CH, NH, and OH stretching modes. Complexities in the infrared spectra in the region of the hydrogen-bonded XH stretch band are associated with Fermi resonances between the hydrogen-stretching vibrational modes and bending overtone and combination levels. There are notable correlations between the vibrational red-shift, the elongation of the H-bonded XH group, and the proton affinity of the aromatic molecule's conjugate base.     

Theoretical model for a tetrad of hydrogen bonds and its application to interpretation of infrared spectra of salicylic acid

Theoretical model of vibrational interactions in hydrogen-bonded salicylic acid dimer is presented which takes into account the adiabatic couplings between high- and low-frequency O-H and O...O stretching vibrations, resonance interactions between both intermolecular hydrogen bonds and between inter- and intramolecular hydrogen bonds, and Fermi resonance between the O-H stretching fundamental and the first overtone of the O-H in-plane bending vibrations. The model is used for theoretical simulation of the nu(s) stretching bands of salicylic acid and its OD derivative at 300 K. The effect of deuteration is successfully reproduced by our model. Infrared, far infrared, Raman, and low-frequency Raman spectra of the polycrystalline salicylic acid and its deuterated derivative have been measured. The geometry and experimental frequencies are compared with the results of density-functional theory calculations performed at the B3LYP6-31 ++ G**, B3LYP/cc-pVTZ, B3PW916-31 ++ G**, and B3PW91/cc-pVTZ levels. O-H, O-D, and O...O stretching frequencies are used in theoretical simulation of the nu(s) stretching bands.     

CH stretching vibration of N-methylformamide as a sensitive probe of its complexation: infrared matrix isolation and computational study

The complexes between trans-N-methylformamide (t-NMF) and Ar, N(2), CO, H(2)O have been studied by infrared matrix isolation spectroscopy and/or ab initio calculations. The infrared spectra of NMF/Ne, NMF/Ar and NMF/N(2)(CO,H(2)O)/Ar matrices have been measured and the effect of the complexation on the perturbation of t-NMF frequencies was analyzed. The geometries of the complexes formed between t-NMF and Ar, N(2), CO and H(2)O were optimized in two steps at the MP2/6-311++G(2d,2p) level of theory. The four structures, found for every system at this level, were reoptimized on the CP-corrected potential energy surface; both normal and CP corrected harmonic frequencies and intensities were calculated. For every optimized structure the interaction energy was partitioned according to the SAPT scheme and the topological distribution of the charge density (AIM theory) was performed. The analysis of the experimental and theoretical results indicates that the t-NMF-N(2) and CO complexes present in the matrices are stabilized by very weak N-H···N and N-H···C hydrogen bonds in which the N-H group of t-NMF serves as a proton donor. In turn, the t-NMF-H(2)O complex present in the matrix is stabilized by O-H···O(C) hydrogen bonding in which the carbonyl group of t-NMF acts as a proton acceptor. Both, the theoretical and experimental results indicate that involvement of the NH group of t-NMF in formation of very weak hydrogen bonds with the N(2) or CO molecules leads to a clearly noticeable red shift of the CH stretching wavenumber whereas engagement of the CO group as a proton acceptor triggers a blue shift of this wavenumber.     

Fermi resonance and strong anharmonic effects in the absorption spectra of the ν-OH (ν-OD) vibration of solid H- and D-benzoic acid

The vibrational spectra of polycrystalline benzoic acid (BA) and its deuterated derivative were studied over the wide frequency region 4000–10 cm⁻¹ by IR and Raman methods. A theoretical analysis of the hydrogen bond frequency region and calculations at the B3LYP/6-311++G(2d, 2p) level for the benzoic acid cyclic dimer in the gas phase were made. In order to study the dynamics of proton transfer two formalisms were applied: Car–Parrinello Molecular Dynamics (CPMD) and Path Integrals Molecular Dynamics (PIMD). It was shown that the experimentally observed very broad ν-OH band absorption is the result of complex anharmonic interaction: Fermi resonance between the OH-stretching and bending vibrations and strong interaction of the ν-OH stretching with the low frequency phonons. The theoretical analysis in the framework of such an approach gave a good correlation with experiment. From the CPMD calculations it was confirmed that the O–HO bridge is not rigid, with the OO distance being described by a large amplitude motion. For the benzoic acid dimer we observed stepwise (asynchronous) proton transfer.     

Investigating the relationship between infrared spectra of shared protons in different chemical environments: a comparison of protonated diglyme and protonated water dimer

The energetics, dynamics, and infrared spectroscopy of the shared proton in different chemical environments is investigated using molecular dynamics simulations. A three-dimensional potential energy surface (PES) suitable for describing proton transfer between an acceptor and a donor oxygen atom is combined with an all-atom force field to carry out reactive molecular dynamics simulations. The construction of the fully dimensional PES is inspired from the established mixed quantum mechanics/molecular mechanics treatment of larger systems. The "morphing potential" method is used to transform the generic PES for proton transfer along an O...H+...O motif into a three-dimensional PES for proton transfer in protonated diglyme. Using molecular dynamics simulations at finite temperature, the gas phase infrared spectra are calculated for both species from the Fourier transform of the dipole moment autocorrelation function. For protonated diglyme the modes involving the H+ motion are strongly mixed with other degrees of freedom. At low temperature, the O...H+...O asymmetric stretching vibration is found at 870 cm-1, whereas for H5O2+ this band is at 724 cm-1. As expected, the vibrational bands of protonated diglyme show no temperature dependence whereas for H5O2+ at T = 100 K the proton transfer mode is found at 830 cm-1, in good agreement with 861 cm-1 from very recent molecular dynamics simulations.     

Structural and chemical properties of the nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate under pressure

The structural and chemical properties of the bi-molecular, hydrogen-bonded, nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate C(3)H(12)N(12)O(2) (TAG-MNT) have been investigated at room pressure and under high pressure isothermal compression using powder x-ray diffraction and Raman and infrared spectroscopy. A stiffening of the equation of state and concomitant structural relaxation between 6 and 14 GPa are found to correlate with Raman mode disappearances, frequency discontinuities, and changes in the pressure dependence of modes. These observations manifest the occurrence of a reversible martensitic structural transformation to a new crystalline phase. The onset and vanishing of Fermi resonance in the nitrimine group correlate with the stiffening of the equation of state and phase transition, suggesting a possible connection between these phenomena. Beyond 15 GPa, pressure induces irreversible chemical reactions, culminating in the formation of a polymeric phase by 60 GPa.     

Direct identification and decongestion of Fermi resonances by control of pulse time ordering in two-dimensional IR spectroscopy

We show that it is possible to both directly measure and directly calculate Fermi resonance couplings in benzene. The measurement method used was a particular form of two-dimensional infrared spectroscopy (2D-IR) known as doubly vibrationally enhanced four wave mixing. By using different pulse orderings, vibrational cross peaks could be measured either purely at the frequencies of the base vibrational states or split by the coupling energy. This capability is a feature currently unique to this particular form of 2D-IR and can be helpful in the decongestion of complex spectra. Five cross peaks of the ring breathing mode nu13 with a range of combination bands were observed spanning a region of 1500-4550 cm(-1). The coupling energy was measured for two dominant states of the nu13+nu16 Fermi resonance tetrad. Dephasing rates were measured in the time domain for nu13 and the two (nu13+nu16) Fermi resonance states. The electronic and mechanical vibrational anharmonic coefficients were calculated to second and third orders, respectively, giving information on relative intensities of the cross peaks and enabling the Fermi resonance states of the combination band nu13+nu16 at 3050-3100 cm(-1) to be calculated. The excellent agreement between calculated and measured spectral intensities and line shapes suggests that assignment of spectral features from ab initio calculations is both viable and practicable for this form of spectroscopy.     

First-principles calculation of thermodynamic stability of acids and bases under pH environment: a microscopic pH theory

Despite being one of the most important thermodynamic variables, pH has yet to be incorporated into first-principles thermodynamics to calculate stability of acidic and basic solutes in aqueous solutions. By treating the solutes as defects in homogeneous liquids, we formulate a first-principles approach to calculate their formation energies under proton chemical potential, or pH, based on explicit molecular dynamics. The method draws analogy to first-principle calculations of defect formation energies under electron chemical potential, or Fermi energy, in semiconductors. From this, we propose a simple pictorial representation of the general theory of acid-base chemistry. By performing first-principles molecular dynamics of liquid water models with solutes, we apply the formulation to calculate formation energies of various neutral and charged solutes such as H(+), OH(-), NH(3), NH(4)(+), HCOOH, and HCOO(-) in water. The deduced auto-dissociation constant of water and the difference in the pKa values of NH(3) and HCOOH show good agreement with known experimental values. Our first-principles approach can be further extended and applied to other bio- and electro-chemical molecules such as amino acids and redox reaction couples that could exist in aqueous environments to understand their thermodynamic stability.     

Quantum chemical study and infrared spectroscopy of hydrogen-bonded CHCl(3)-NH(3) in the gas phase

Molecular association of chloroform with ammonia is studied by high-level quantum chemical calculations including correlated MP2 and CCSD(T) calculations with basis sets up to6-311++G(d,p) and counterpoise corrected energies, geometries, and frequencies. The calculations predict an eclipsed hydrogen-bonded complex of C(3v) symmetry (DeltaE(0)=-15.07 kJ mol(-1)) with 225.4 pm intermolecular CHcdots, three dots, centeredN distance. Intermolecular interactions are analysed by Kitaura-Morokuma [Int. J. Quantum Chem. 10, 325 (1976)] interaction energy decomposition. Compared to the monomer, the C-H bond is elongated, and the CH-stretching fundamental shifts to lower wave numbers and has a marked approximately 340-fold increase of its intensity. Based on these predictions, the complex is observed by infrared spectroscopy in the gas phase at room temperature. A subtraction procedure isolates its spectrum, and a dilution series confirms the presence of a 1:1 complex. The CHCl(3)cdots, three dots, centeredNH(3) complex has an experimental -17.5 cm(-1) shift of its CH-stretching vibration, and CDCl(3)cdots, three dots, centeredNH(3) a -12.5 cm(-1) shift of the CD-stretching vibration. After a deperturbation of the CH-stretching/bending mode Fermi resonance system, this indicates a "redshifting" or more appropriately, a "C-H elongating" hydrogen bond in agreement with the ab initio calculations. An estimate of the complex concentration gives the equilibrium constant K(p)=0.024 (p(theta)=10(5) Pa) at 295 K for the dimerization, providing one of the few examples where a hydrogen-bonded gas phase complex at room temperature could be quantitatively studied by infrared spectroscopy.     

Structure and dynamics of water confined in dimethyl sulfoxide.

We study the structure and dynamics of hydrogen-bonded complexes of H2O/D2O and dimethyl sulfoxide (DMSO) by infrared spectroscopy, NMR spectroscopy and ab initio calculations. We find that single water molecules occur in two configurations. For one half of the water monomers both OH/OD groups form strong hydrogen bonds to DMSO molecules, whereas for the other half only one of the two OH/OD groups is hydrogen-bonded to a solvent molecule. The H-bond strength between water and DMSO is in the order of that in bulk water. NMR deuteron relaxation rates and calculated deuteron quadrupole coupling constants yield rotational correlation times of water. The molecular reorientation of water monomers in DMSO is two-and-a-half times slower than in bulk water. This result can be explained by local structure behavior.     

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.     

OH⋅⋅⋅N and CH⋅⋅⋅O Hydrogen Bonds Control Hydration of Pivotal Tropane Alkaloids: Tropinone⋅⋅⋅H2O Complex

The effect of monohydration in equatorial/axial isomerism of the common motif of tropane alkaloids is investigated in a supersonic expansion by using Fourier‐transform microwave spectroscopy. The rotational spectrum reveals the equatorial isomer as the dominant species in the tropinone???H₂O complex. The monohydrated complex is stabilized primarily by a moderate O?H???N hydrogen bond. In addition, two C?H???O weak hydrogen bonds also support this structure, blocking the water molecule and avoiding any molecular dynamics in the complex. The water molecule acts as proton donor and chooses the ternary amine group over the carbonyl group as a proton acceptor. The experimental work is supported by theoretical calculations; the accuracy of the B3LYP, M06‐2X, and MP2 methods is also discussed.