Vibrational spectroscopy of water in hydrated lipid multi-bilayers. I. Infrared spectra and ultrafast pump-probe observables

The vibrational spectroscopy of hydration water in dilauroylphosphatidylcholine lipid multi-bilayers is investigated using molecular dynamics simulations and a mixed quantum/classical model for the OD stretch spectroscopy of dilute HDO in H(2)O. FTIR absorption spectra, and isotropic and anisotropic pump-probe decay curves have been measured experimentally as a function of the hydration level of the lipid multi-bilayer, and our goal is to make connection with these experiments. To this end, we use third-order response functions, which allow us to include non-Gaussian frequency fluctuations, non-Condon effects, molecular rotations, and a fluctuating vibrational lifetime, all of which we believe are important for this system. We calculate the response functions using existing transition frequency and dipole maps. From the experiments it appears that there are two distinct vibrational lifetimes corresponding to HDO molecules in different molecular environments. In order to obtain these lifetimes, we consider a simple two-population model for hydration water hydrogen bonds. Assuming a different lifetime for each population, we then calculate the isotropic pump-probe decay, fitting to experiment to obtain the two lifetimes for each hydration level. With these lifetimes in hand, we then calculate FTIR spectra and pump-probe anisotropy decay as a function of hydration. This approach, therefore, permits a consistent calculation of all observables within a unified computational scheme. Our theoretical results are all in qualitative agreement with experiment. The vibrational lifetime of lipid-associated OD groups is found to be systematically shorter than that of the water-associated population, and the lifetimes of each population increase with decreasing hydration, in agreement with previous analysis. Our theoretical FTIR absorption spectra successfully reproduce the experimentally observed red-shift with decreasing lipid hydration, and we confirm a previous interpretation that this shift results from the hydrogen bonding of water to the lipid phosphate group. From the pump-probe anisotropy decay, we confirm that the reorientational motions of water molecules slow significantly as hydration decreases, with water bound in the lipid carbonyl region undergoing the slowest rotations.     

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but vibrational coupling between surfactant molecules can manifest as signal depletion in one-dimensional spectra. Through a combination of experiment and theory, we demonstrate the emergence of vibrational exciton delocalization in infrared reflection–absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling causes a significant decrease in peak intensities corresponding to C–F vibrational modes of perfluorooctanoic acid molecules. Vibrational excitons also form between arachidic acid surfactants within a compressed monolayer, manifesting as signal reduction of C–H stretching modes. Ionic composition of the aqueous phase impacts surfactant intermolecular distance, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities as proxies for concentration, although such analysis is ubiquitous in interface science.

Coupling between surfactant molecules at the air/water interface bleeds intensity into a diffuse background, such that single-wavelength vibrational intensity is effectively depleted at high surface coverage.     

Photodissociation spectroscopy of Zn+(H2O) and Zn+(D2O)

We report on a study of the photodissociation spectroscopy of weakly bound Zn+(H2O) and Zn+(D2O) complexes. The work is supported by ab initio electronic structure calculations of the ground and low-lying excited energy surfaces. We assign two molecular absorption bands in the near UV correlating to Zn+ (4s-4p)-based transitions, and identify vibrational progressions associated with both intermolecular and intramolecular vibrational modes of the cluster. Partially resolved rotational structure is consistent with a C(2V) equilibrium complex geometry. Experimental spectroscopic constants are in very good agreement with ab initio theoretical predictions. Results are compared with previous work on main group and transition metal ion-H2O clusters.     

Molecular dynamics with quantum transitions study of the vibrational relaxation of the HOD bend fundamental in liquid D2O

The molecular dynamics with quantum transitions method is used to study the vibrational relaxation of the HOD bend fundamental in liquid D(2)O. All of the vibrational bending degrees of freedom of the HOD and D(2)O molecules are described by quantum mechanics, while the remaining translational and rotational degrees of freedom are described classically. The effect of the coupling between the rotational and vibrational degrees of freedom of the deuterated water molecules is analyzed. A kinetic mechanism based on three steps is proposed in order to interpret the dynamics of the system. It is shown that intermolecular vibrational energy transfer plays an important role in the relaxation process and also that the transfer of energy into the rotational degrees of freedom is favored over the transfer of energy into the translational motions. The thermalization of the system after the relaxation is reached in a shorter time scale than that of the recovery of the hydrogen bond network. The relaxation and equilibration times obtained compare well with experimental and previous theoretical results.     

Inelastic X-ray scattering and vibrational effects at the K-edges of gaseous N2, N2O, and CO2

We report non-resonant inelastic X-ray scattering experiments of several gaseous samples in the inner-shell excitation energy range. The experimental near-edge spectra from all the K-edges of N(2), N(2)O, and CO(2) including the momentum transfer dependence are presented. The results are analyzed using density functional theory calculations that accurately reproduce the experimental spectral features. We observe vibrational effects in the measured spectrum and in the calculations the atomic motion is modeled using the Franck-Condon approximation and the linear coupling model. Our findings show that vibrational effects cannot be neglected in the analysis of high resolution inelastic X-ray scattering spectroscopy. The results also support the validity of the transition potential approximation for calculating core excited state potential energy surfaces.     

Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)n (n = 3, 4) and mixed hydrogen fluoride/water (HF)n(H2O)n (n = 1, 2, 4) clusters

Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n, with n = 3, 4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n = 1, 2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the M?ller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.     

Rovibrational and dynamical properties of the hydrogen bonded complex (CH2)2S-HF: a combined free jet, cell, and neon matrix-Fourier transform infrared study

Fourier transform infrared spectra of the nu(s) (HF stretching) band of the (CH(2))(2)S-HF complex have been recorded at 0.1-0.5 cm(-1) resolution in a cooled cell, in a supersonic jet expansion seeded with argon and in a neon matrix at 4.5 K. The combination of controlled temperature effects over a range of 40-250 K and a sophisticated band contour simulation program allows the separation of homogeneous and inhomogeneous contributions and reveals significant anharmonic couplings between intramolecular and intermolecular vibrational modes similar to our previous work on (CH(2))(2)S-DF. The sign of the coupling constants is consistent with the expected strengthening of the hydrogen bond upon vibrational excitation of HF which also explains the observed small variations of the geometrical parameters in the excited state. The analysis of sum and difference combination bands involving nu(s) provides accurate values of intermolecular harmonic frequencies and anharmonicities and a good estimate of the dissociation energy of the complex. Frequencies and coupling parameters derived from gas phase spectra compare well with results from neon matrix experiments. The effective linewidth provides a lower bound for the predissociation lifetime of 10 ps. The comparison between effective linewidths and vibrational densities of states for (CH(2))(2)S-HF and -DF complexes highlights the important role of intramolecular vibrational redistribution in the vibrational dynamics of medium strength hydrogen bonds.     

X-ray absorption and resonant Auger spectroscopy of O2 in the vicinity of the O 1s-->sigma* resonance: experiment and theory

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.     

Vibrational spectra of metaborates with infinite chain structure: LiBO2, CaB2O4, SrB2O4

The vibrational spectra (i.r. and Raman) of chain metaborates (LiBO₂, CaBO₂O₄, SrB₂O₄) containing only condensed BO₃ groups has been investigated by group theoretical methods and isotopic substitutions.The spectra are discussed in relation to the structure and possible coupling between the chains in the unit cell.     

The vibrational spectra and structure of sodium bromoacetate and sodium bromoacetate-d2

The complete vibrational spectra of crystalline sodium bromoacetate and sodium bromoacetate-d₂ have been recorded and a vibrational assignmem proposed. The intermolecular coupling of these compounds is quite strong and the data are consistent with a centrosymmetric unit cell containing at least four molecules.The product rule calculations support a structure m which the bromine atom is rotated out of the plane of the remaining heavy atoms, but the frequency of the carbon-bromine stretching mode indicates that the degree of rotation is not large.     

Spin-orbit coupling in O2(upsilon)+O2 collisions: I. Electronic structure calculations on dimer states involving the X 3Sigmag-, a 1Deltag, and b 1Sigmag+ states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.     

Asymmetry of H2O Molecules in the Liquid State and Its Consequences

The correlations between the geometrical parameters of the O–H...O hydrogen bridge and the stretching frequency _OH are refined by using neutron diffraction and vibrational spectroscopy data. The distribution functions of the interatomic distances r _OH and intermolecular distances R _O...O in water in the range from –40 to 100°C were calculated from the Raman spectra of HOD. The extent of asymmetry of H₂O molecules in the liquid state, caused by fluctuations of the local environment of two OH groups, and its manifestations in the structure and vibrational spectra of water are analyzed.     

Rotational spectra of the Xe-(H2O)2 van der Waals trimer: xenon as a probe of electronic structure and dynamics

Rotational spectra of three isotopomers of the Xe-(H2O)2 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Nine [nine, four] a-type and twelve [eleven, seven] b-type transitions were measured for the 132Xe-(H2O)2 [129Xe-(H2O)2, 131Xe-(H2O)2] isotopomer. The determined rotational and centrifugal distortion constants were used to extract information about the structure and vibrational motions of the complex. The nuclear quadrupole hyperfine structures due to the 131Xe (nuclear spin quantum number I=3/2) nucleus were also detected. The large value of the off-diagonal nuclear quadrupole coupling constant chiab in particular provides detailed insight into the electronic environment of the xenon atom and the orientations of the water molecules within the complex. An effective structure that best reproduces the experimental 131Xe nuclear quadrupole coupling constants is rationalized by ab initio calculations. An overall goal of this line of work is to determine how the successive solvation of a xenon atom with water molecules affects the xenon electron distribution and its intermolecular interactions. The results may provide molecular level interpretations of 129Xe NMR data from, for example, imaging experiments.     

Infrared spectroscopy and effective modes analysis of the protonated water dimer H+(H2O)2 at room temperature under H/D substitution

We study the vibrational properties of the protonated water dimer and its deuterated forms at room temperature. Molecular dynamics simulations within the empirical valence bond (EVB) model are used to generate the vibrational spectra that are interpreted using the effective modes analysis (EMA). Quantum effects are taken into account through an effective parametrization of the EVB model. EMA allows for the assignment of the bands in the 1000 - 2000?cm(-1) region of the protonated water dimer from the molecular dynamics trajectory. It is then found that although this system is very anharmonic the two main bands in this spectral region arise from a linear coupling between the asymmetric OH(+)O stretch and asymmetric bend of the two water molecules. This mixing explains the simulated band shifts upon isotopic substitution of the central proton or of the hydrogens of the two water molecules.     

Vibrational spectra and structure of CH3Cl:(H2O)2 and CH3Cl:(D2O)2 complexes. IR matrix isolation and ab initio calculations

The infrared spectra of CH3Cl + H2O isolated in solid neon at low temperature have been investigated. High concentration studies of water (0.01%-4%) and subsequent annealing lead to the formation of the ternary CH3Cl:(H2O)2 complex. Detailed vibrational assignments were made on the observed spectra of water and deuterated water engaged in the complex. In parallel, structural, energetic, and vibrational properties of the complex have been studied at the second-order M?ller-Plesset perturbation theory using several basis sets. Anaharmonic correction to the vibrational frequencies has been done with the standard second-order perturbation approach. It was shown that the ground state of the complex has a cyclic form for which the nonadditive three-body contribution was found to be around 10% of the interaction energy.     

Probing intermolecular couplings in liquid water with two-dimensional infrared photon echo spectroscopy

Two-dimensional infrared photon echo and pump probe studies of the OH stretch vibration provide a sensitive probe of the correlations and couplings in the hydrogen bond network of liquid water. The nonlinear response is simulated using numerical integration of the Schrodinger equation with a Hamiltonian constructed to explicitly treat intermolecular coupling and nonadiabatic effects in the highly disordered singly and doubly excited vibrational exciton manifolds. The simulated two-dimensional spectra are in close agreement with our recent experimental results. The high sensitivity of the OH stretch vibration to the bath dynamics is found to arise from intramolecular mixing between states in the two-dimensional anharmonic OH stretch potential. Surprisingly small intermolecular couplings reproduce the experimentally observed intermolecular energy transfer times.     

DFT study of vibrational circular dichroism spectra of D-lactic acid-water complexes

This paper presents a discussion of the interaction energies, conformations, vibrational absorption (VA, harmonic and anharmonic) and vibrational circular dichroism (VCD) spectra for conformers of monomeric chiral d(-)-lactic acid and their complexes with water at the DFT(B3LYP)/aug-cc-pVDZ and DFT(B3LYP)/aug-cc-pVTZ levels. A detailed analysis has been performed principally for the two most stable complexes with water, differing by lactic acid conformation. The VCD spectra were found to be sensitive to conformational changes of both free and complexed molecules, and to be especially useful for discriminating between different chiral forms of intermolecular hydrogen bonding complexes. In particular, we show that the VCD modes of an achiral water molecule after complex formation acquire significant rotational strengths whose signs change in line with the geometry of the complex. Using the theoretical prediction, we demonstrate that the VCD technique can be used as a powerful tool for structural investigation of intermolecular interactions of chiral molecules and can yield information complementary to data obtained through other molecular spectroscopy methods.     

Complete characterization of the (D2O)2 ground state: high Ka rotation-tunneling levels

We report the observation of extensive a- and c-type rotation-tunneling (RT) spectra of (D2O)2 for Ka = 0-4. These data allow quantification of molecular constants and tunneling splittings for a number of previously unobserved RT states of (D2O)2. The vibrational ground state has thus been characterized to energies as high as those of some of the intermolecular vibrations, and we present the first test of the VRT(ASP-W) potential at these high Ka states.