Evidence of an isomeric pair in furan...HCl: Fourier transform infrared spectroscopy and ab initio calculations

For the first time the coexistence of a sigma- and a pi-complex in the C(4)H(4)O:HCl system has been observed, in the same supersonic expansion of a molecular jet seeded with argon (or helium) or in a flow-cooled cell at 240 K. This is an exception to the third of the Legon-Miller rules which claims the sigma-structure to be the only one to exist. On the grounds of energetic considerations and band contour simulations, two observed bands at 2787.7 and 2795.5 cm(-1) of the nu(s) HCl stretching frequency are assigned to the two complexes, recorded as Fourier transform infrared spectra with a resolution between 0.2 and 0.5 cm(-1). Complementary calculations show that the use of the standard second-order Moller-Plesset perturbation theory may be erroneous for such a complex, due of the overestimation of the dispersion contribution with respect to the electrostatic term. It is finally established that only a balanced version of the second-order Moller-Plesset perturbation method, spin-component scaled-MP2, or a higher level of theory like a coupled-cluster approach, can provide a reliable energetic analysis for this complex.     

Axial and equatorial hydrogen-bond conformers between (CH2)3S and H(D)F: Fourier transform infrared spectroscopy and ab initio calculations

The coexistence of axial and equatorial hydrogen-bonded conformers of 1?:?1 (CH(2))(3)S-HF (and -DF) has been observed in the same adiabatic expansion of a supersonic jet seeded with argon and in a static absorption cell at room temperature. High level calculations computed the axial conformer to be the most stable one with a small energy difference with respect to the equatorial one, in full agreement with previous microwave experiments. On the grounds of band contour simulations of FTIR spectra and ab initio energetic and anharmonic vibrational calculations, two pairs of ν(s) HF donor stretching bands, observed in a series of jet-FTIR spectra at 3457.9 and 3480.5 cm(-1) have been respectively assigned to the axial and equatorial forms of the 1?:?1 complex. In the jet-FTIR spectra series with HF, the assignment of an additional broad band (about 200 cm(-1) higher in frequency with respect to ν(s)) to a 1?:?2 complex has been supported by theoretical investigations. Experimental detection of both axial and equatorial forms of a cyclic trimer has been confirmed by calculated energetic and vibrational properties. The nature of hydrogen bonding has been examined within topological frameworks. The energetic partitioning within the 1?:?1 dimers has been elucidated with SAPT techniques. Interestingly, the interconversion pathway between two 1?:?1 structures has been explored and it was seen that the formation of the 1?:?1 complex affects the interconversion barrier on the ring puckering motion. The band contour analysis of gas phase FTIR experiments provided a consistent set of vibrational frequencies and anharmonic coupling constants, in good agreement with ab initio anharmonic vibrational calculations. Finally, from a series of cell-FTIR spectra recorded at different partial pressures of (CH(2))(3)S and HF monomers, the absorption signal of the 1?:?1 complex could be isolated which enabled to estimate the equilibrium constant K(p) = 0.023 at 298 K for the dimerization.     

Structures and infrared spectra of fluoride-hydrogen sulfide clusters from ab initio calculations: F(-)-(H2S)n, n = 1-5

Clusters formed between a fluoride anion and several hydrogen sulfide molecules have been investigated via ab initio calculations at the MP2 level of theory, using Dunning's augmented correlation consistent basis sets. Optimised geometries, vibrational frequencies, and enthalpy changes for the ligand association reactions are presented for clusters with up to five H2S ligands interacting with a F- anion. The minimum energy structure for the 1:1 F(-)-H2S complex features proton transfer from the H2S to the F- anion, forming a planar C(s) symmetry FH...SH- structure. For the F(-)-(H2S)2 cluster, the FH...SH- core remains and is solvated by a perturbed H2S ligand. For the larger F(-)-(H2S)(3-5) clusters, in addition to the FH...SH(-)-(H2S)n cluster forms, other minima featuring a 'solvated F-' anion are predicted. Calculated infrared spectra for the minima of each cluster size are presented to aid in assigning spectra from future experimental studies.     

Coordination structures of lithium-methylamine clusters from infrared spectroscopy and ab initio calculations

Spectra of clusters formed between lithium atoms and methylamine molecules are reported for the first time. Mass-selective infrared spectra of Li(NH(2)CH(3))(n) have been recorded in both the N-H and C-H stretching fundamental regions. The infrared spectra are broadly in agreement with ab initio predictions, showing redshifted N-H stretching bands relative to free methylamine and a strong enhancement of the N-H stretching fundamentals relative to the C-H stretching fundamentals. The ab initio calculations suggest that, for n=3, the methylamine molecules bunch together on one side of the lithium atom to minimize repulsive interactions with the unpaired electron density. The addition of a fourth methylamine molecule results in closure of the inner solvation shell and, thus, Li(NH(2)CH(3))(5) is forced to adopt a two-shell coordination structure. This is consistent with neutron diffraction studies of concentrated lithium/methylamine solutions, which also suggest that the first solvation shell around the lithium atom can contain a maximum of four methylamine molecules.     

Structure of protonated carbon dioxide clusters: infrared photodissociation spectroscopy and ab initio calculations

The infrared photodissociation spectra (IRPD) in the 700 to 4000 cm(-1) region are reported for H+ (CO2)n clusters (n = 1-4) and their complexes with argon. Weakly bound Ar atoms are attached to each complex upon cluster formation in a pulsed electric discharge/supersonic expansion cluster source. An expanded IRPD spectrum of the H+ (CO2)Ar complex, previously reported in the 2600-3000 cm(-1) range [Dopfer, O.; Olkhov, R.V.; Roth, D.; Maier, J.P. Chem. Phys. Lett. 1998, 296, 585-591] reveals new vibrational resonances. For n = 2 to 4, the vibrational resonances involving the motion of the proton are observed in the 750 to 1500 cm(-1) region of the spectrum, and by comparison to the predictions of theory, the structure of the small clusters are revealed. The monomer species has a nonlinear structure, with the proton binding to the lone pair of an oxygen. In the dimer, this nonlinear configuration is preserved, with the two CO2 units in a trans configuration about the central proton. Upon formation of the trimer, the core CO2 dimer ion undergoes a rearrangement, producing a structure with near C2v symmetry, which is preserved upon successive CO2 solvation. While the higher frequency asymmetric CO2 stretch vibrations are unaffected by the presence of the weakly attached Ar atom, the dynamics of the shared proton motions are substantially altered, largely due to the reduction in symmetry of each complex. For n = 2 to 4, the perturbation due to Ar leads to blue shifts of proton stretching vibrations that involve motion of the proton mostly parallel to the O-H+-O axis of the core ion. Moreover, proton stretching motions perpendicular to this axis exhibit smaller shifts, largely to the red. Ab initio (MP2) calculations of the structures, complexation energies, and harmonic vibrational frequencies are also presented, which support the assignments of the experimental spectra.     

Structure and properties of the (HCl)2H2O cluster observed by chirped-pulse Fourier transform microwave spectroscopy

The rotational spectrum of the cyclic (HCl)(2)H(2)O cluster has been identified for the first time in the chirped pulse, Fourier transform microwave spectrum of a supersonically expanded HCl/H(2)O/Ar mixture. The spectrum was measured at frequencies 6-18.5 GHz, and transitions in two inversion-tunneling states, at close to 1?:?3 relative intensity, have been assigned for the parent species. The two single (37)Cl isotopic species, and the double (37)Cl species have been assigned in the natural abundance sample, and the (18)O and HDO species of the cluster were identified in isotopically enriched samples. The rich nuclear quadrupole hyperfine structure due to the presence of two chlorine nuclei has been satisfactorily fitted and provided useful information on the nonlinearity of intermolecular bonds in the cluster. The r(s) heavy atom geometry of the cluster was determined and the strongest bond in the intermolecular cycle r(O···HCl) = 3.126(3) ?, is found to be intermediate in length between the values in H(2)O···HCl and (H(2)O)(2)HCl. The fitted spectroscopic constants and derived molecular properties are compared with ab initio predictions, and a discussion of complexation effects in these three clusters is made.     

Comparison of hydrated hydroperoxide anion (HOO-)(H2O)n clusters with alkaline hydrogen peroxide (HOOH)(OH-)(H2O)(n-1) clusters, n = 1-8, 20: an ab initio study

Hydroperoxide anion (HOO(-)), the conjugate base of hydrogen peroxide (HOOH), has been relatively little studied despite the importance of HOOH in commercial processes, atmospheric science, and biology. The anion has been shown to exist as a stable species in alkaline water. This project explored the structure of gas phase (HOO(-))(H(2)O)(n) clusters and identified the lowest energy configurations for n ≤ 8 at the B3LYP/6-311++G** level of theory and for n ≤ 6 at the MP2/aug-cc-pVTZ level of theory. As a start toward understanding equilibration between HOO(-) and HOOH in an alkaline environment, (HOOH)(OH(-))(H(2)O)(n-1) clusters were likewise examined, and the lowest energy configurations were determined for n ≤ 8 (B3LYP/6-311++G**) and n ≤ 6 (MP2/aug-cc-pVTZ). Some studies were also done for n = 20. The two species have very different solvation behaviors. In low energy (HOOH)(OH(-))(H(2)O)(n-1) clusters, HOOH sits on the surface of the cluster, is 4-coordinated (each O is donor once and acceptor once), and donates to the hydroxide ion. In contrast, in low energy (HOO(-))(H(2)O)(n) clusters, (HOO(-)) takes a position in the cluster center surrounded on all sides by water molecules, and its optimum coordination number appears to be 7 (one O is donor-acceptor-acceptor while the other is a 4-fold acceptor). For n ≤ 6 the lowest (HOOH)(OH(-))(H(2)O)(n-1) cluster lies 1.0-2.1 kcal/mol below the lowest (HOO(-))(H(2)O)(n) cluster, but the lowest clusters found for n = 20 favor (HOO(-))(H(2)O)(20). The results suggest that ambient water could act as a substantial kinetic brake that slows equilibration between (HOOH)(OH(-)) and (HOO(-))(H(2)O) because extensive rearrangement of solvation shells is necessary to restabilize either species after proton transfer.     

Anharmonic force field and vibrational dynamics of CH2F2 up to 5000 cm(-1) studied by Fourier transform infrared spectroscopy and state-of-the-art ab initio calculations

Difluoromethane (CH(2)F(2), HFC-32) is a molecule used in refrigerant mixtures as a replacement of the more environmentally hazardous, ozone depleting, chlorofluorocarbons. On the other hand, presenting strong vibration-rotation bands in the 9 μm atmospheric window, it is a greenhouse gas which contributes to global warming. In the present work, the vibrational and ro-vibrational properties of CH(2)F(2), providing basic data for its atmospheric modeling, are studied in detail by coupling medium resolution Fourier transform infrared spectroscopy to high-level electronic structure ab initio calculations. Experimentally a full quantum assignment and accurate integrated absorption cross sections are obtained up to 5000 cm(-1). Ab initio calculations are carried out by using CCSD(T) theory and large basis sets of either the correlation consistent or atomic natural orbital hierarchies. By using vibrational perturbation theory to second order a complete set of vibrational and ro-vibrational parameters is derived from the ab initio quartic anharmonic force fields, which well compares with the spectroscopic constants retrieved experimentally. An excellent agreement between theory and experiment is achieved for vibrational energy levels and integrated absorption cross sections: transition frequencies up to four quanta of vibrational excitation are reproduced with a root mean square deviation (RMSD) of 7 cm(-1) while intensities are predicted within few km mol(-1) from the experiment. Basis set performances and core correlation effects are discussed throughout the paper. Particular attention is focused in the understanding of the anharmonic couplings which rule the vibrational dynamics of the |ν(1)>, |2ν(8)>, |2ν(2)> three levels interacting system. The reliability of the potential energy and dipole moment surfaces in reproducing the vibrational eigenvalues and intensities as well as in modeling the vibrational and ro-vibrational mixings over the whole 400-5000 cm(-1) region is also demonstrated by spectacular spectral simulations carried out by using the ro-vibrational Hamiltonian constants, and the relevant coupling terms, obtained from the perturbation treatment of the ab initio anharmonic force field. The present results suggest CH(2)F(2) as a prototype molecule to test ab initio calculations and theoretical models.     

HF(H2O)n clusters with an excess electron: ab initio study

The structures of electron-bound and neutral clusters of HF(H2O)n (n=1-3) were optimized at the level of second-order Moller-Plesset perturbation theory (MP2). Then, the energies were studied using the coupled cluster singles, doubles, and perturbative triples correction [CCSD(T)] method. The vertical detachment energies of the electron-bound clusters for n=1-3 are 60, 180, and approximately 300 meV, respectively. In the case of the n=3, two structures are competing energetically. The electron-bound clusters for n=1 and 2 are 1.5 and 1.8 kcal/mol more stable than the neutral, while that for n=3 is 0.6-0.9 kcal/mol less stable. The excess electron is stabilized in the surface-bound state of the dipole oriented structures of the hydrated acid clusters. Vibrational spectra of the electron-bound clusters are discussed.     

Intermolecular interaction between CO or CO2 and ethylene oxide or ethylene sulfide in a complex, investigated by Fourier transform microwave spectroscopy and ab initio calculations

The rotational spectra of the CO-ethylene oxide (EO), CO-ethylene sulfide (ES), CO(2)-EO, and CO(2)-ES complexes were measured by Fourier transform microwave spectroscopy in the frequency region from 4 up to 31 GHz. The isotopologues with a single (13)C atom in the EO or ES, (18)O in the EO, (34)S in the ES, and (13)C in the CO(2) moiety, respectively, were observed in natural abundance, and enriched samples, (13)CO or C(18)O in the CO-EO or CO-ES complex and C(18)OO and C(18)O(2) in the CO(2)-EO or CO(2)-ES complex, were employed to record respective rotational transitions. The rotational spectra observed for the CO-EO, CO-ES, CO(2)-EO, and CO(2)-ES complexes were analyzed by using an asymmetric-rotor S-reduced Hamiltonian to determine rotational and centrifugal distortion constants. The r(s) coordinates of the atoms in the four complexes, which were calculated from the observed rotational constants, led to a structure in which the CO or CO(2) moiety is located in a plane perpendicular to the EO or ES skeletal plane and bisecting the COC or CSC angle. We have also carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G(d,p) and aug-cc-pVDZ using the Gaussian 09 package. The MP2/6-311++G(d,p) calculations yield rotational constants in better agreement with the experimental values than with the other basis set; in other words, the molecular structures calculated using this basis set are close to those experimentally found for the ground state. The estimated dissociation energies of the complexes, including the zero-point vibrational energy corrections ΔZPV and the basis set superposition errors (BSSE) calculated with the counterpoise correction (CP), are in good agreement with the experimentally obtained binding energies E(B). We have applied an NBO analysis to the complexes to calculate the stabilization energy CT (=ΔE(σσ*)), which we found are closely correlated with the binding energies E(B). We have thus achieved a consistent overview on the intermolecular interaction in the complexes under consideration. It is to be noted that the spectral intensities of the inner OC(18)O-EO and OC(18)O-ES complexes were larger by a factor of 2 than those of the outer (18)OCO-EO/ES complexes. This observation was explained by the zero-point energy of the inner conformer being a little smaller than that of the outer one.     

Fourier transform infrared and raman spectra, vibrational assignment and ab initio calculations of terephthalic acid and related compounds

The Fourier transform infrared and Raman spectra of solid terephthalic acid, p-C6H4(COOH)2, have been recorded, and the Fourier transform Raman spectra for the terephthalate anion were measured. The wavenumbers for the band positions have been calculated in order to assign them. Moller-Plesset (MP2) and Density functional theory (DFT) calculations have been carried out with Huzinaga-Dunning basis sets (DZV). Also, a normal coordinate analysis through the Wilson-El'yashevich method was performed. The differences between the calculated ab initio spectra and the spectra of the solid phase have been interpreted with respect to the different C(2h) and C(i) local symmetry in the gas and in the solid phase, respectively, and considering also the formation of long-chains of terephthalic acid in the solid phase. In spite to the absence of experimental data for the cis conformation, calculations have been carried out and structural parameters and infrared intensities have been evaluated for the trans and cis conformations of terephthalic acid.     

Vibrational spectroscopy of hydrated electron clusters (H2O)(-)(15-50) via infrared multiple photon dissociation

Infrared multiple photon dissociation spectra for size-selected water cluster anions (H2O)(n)(-), n=15-50, are presented covering the frequency range of 560-1820 cm(-1). The cluster ions are trapped and cooled by collisions with ambient He gas at 20 K, with the goal of defining the cluster temperature better than in previous investigations of these species. Signal is seen in two frequency regions centered around 700 and 1500-1650 cm(-1), corresponding to water librational and bending motions, respectively. The bending feature associated with a double-acceptor water molecule binding to the excess electron is clearly seen up to n=35, but above n=25; this feature begins to blueshift and broadens, suggesting a more delocalized electron binding motif for the larger clusters in which the excess electron interacts with multiple water molecules.     

Interactions in open-shell clusters: ab initio study of pre-reactive complex O(3P) + HCl

Van der Waals interactions between the ground-state triplet O(3P) atom and the closed-shell HCl molecule are investigated in the pre-reactive region. Three adiabatic (two of A' symmetry and one of A' symmetry) and four non-relativistic diabatic potential energy surfaces are obtained by combining a restricted open-shell coupled cluster approach with the multireference configuration interaction method. The lower A' adiabatic potential surface has a single minimum (D(e) = 589 cm(-1)) for a linear O...HCl configuration. The upper A' potential has a weak (D(e) = 65 cm(-1)) minimum for a linear HCl...O configuration. The A' adiabatic potential has a weak (124 cm(-1)) T-shaped minimum. Adiabatic potentials intersect once in the O...HCl linear configuration and twice in the linear HCl...O geometry. The role of electrostatic interactions in shaping these potentials is discussed. The effects of spin-orbit coupling on this interaction are also investigated assuming a constant value of the SO parameter.     

Fourier transform infrared and Raman spectra, and AB initio calculations for cadmium(II)-cysteinate glycinate complex [Cd(Cys)(Gly)

The cysteinate glycinate cadmium(II) complex was synthesized and structural analysis was carried out using the following methods: determination of the C, H, N, S and O contents, thermogravimetry, infrared and Raman spectra. The most probable structure for the complex at a minimum of energy was calculated by the density functional theory (DFT):B3LYP/3-21G quantum mechanical method. The infrared and Raman spectra were analyzed and bands assigned through the DFT procedures, the stabilization energy being equal to: E(RB+HF-LYP)= -6442.67784a.u. Features of the infrared and Raman spectra confirm theoretical structural prediction with respect to the metal-ligand bonds: Cd-O, Cd-S and Cd-N. Full assignment of the vibrational spectra was also supported by a carefully analysis of the distorted geometries generated by the normal modes.     

Pressure-dependent Fourier transform infrared spectroscopy of a poly (ester urethane)

The effects of hydrostatic pressure upon (1) a segmented poly (ester urethane), (2) a hydrolytically degraded sample of the same polymer, and (3) models for the polyurethane and polyester segments in this polymer have been studied by Fourier transform infrared spectroscopy using high-pressure diamond anvil cells (DACs). The pressure responses of the vibrational frequencies of specific functional groups of the poly (ester urethane) in the 0-100-kbar range are compared with data for individual segment models and the partially degraded sample. The results indicated that the polymer is highly stable in this pressure regime, with no measurable degradation or phase changes. Differences in the pressure dependency of specific infrared bands between the poly (ester urethane) sample and the partially degraded sample are slight and consistent with changes in hydrogen-bonding interactions and shorter chain lengths in the degraded sample.     

Finite-temperature effects on the stability and infrared spectra of HCl(H2O)6 clusters

Theoretical studies of the interaction of HCl with small water clusters have so far neglected the effect of temperature, which ranges from a few tens of kelvin in cluster experiments, up to about 250 K in typical atmospheric conditions. We study the dynamical behavior of a selected set of HCl(H2O)6 clusters, representative of undissociated and dissociated configurations, by means of DFT-based first principles molecular dynamics. We find that the thermodynamcal stability of different configurations can be affected by temperature. We also present the infrared spectra of dissociated and undissociated configurations at 200 K and discuss the origin of the spectral features.     

Photoionization and ab initio study of Ba(H2O)n (n = 1-4) clusters

An experimental and theoretical study of the photoionization energies (IE's) of Ba(H(2)O)(n) clusters containing up to n = 4 water molecules has been performed. The clusters were generated by a pick-up source combining laser vaporization with pulsed supersonic expansion, and then photoionized by radiation of 272.5-340 nm. The experimentally determined IE(e)'s for n = 1 to 4 are 4.56 ± 0.05, 4.26 ± 0.05, 3.90 ± 0.05 and 3.71 ± 0.05 eV. This cluster size dependence of IE is reproduced within ±0.06 eV employing the mPW1PW91 density-functional and CCSD(T, Full) quantum-chemical methods combined with the 6-311++G(d,p) basis set for the H and O atoms and three different relativistic effective core potentials for Ba atoms. The calculations indicate that the lowest energy hydration structures represent the most relevant contributions to both the vertical and adiabatic ionization energies. Experimental and theoretical evidence correlates with the progressive surface-delocalization of the electron from the hydration cavity around the Ba atom and suggests that the intra-cluster electron transfer is possible even for small aggregates.     

The Structure of DMF · HCl and DMF · (HCl)2 Complexes according to ab initio Calculations

Based on ab initio calculation at the 6-31G level, the differences in the structures of isolated complexes of DMF with HCl with compositions 1 : 1 and 1 : 2 are analyzed. The interaction of HCl with DMF results in the formation of the molecular complex. When HCl is added to this complex, a proton transfers to the DMF molecule and the asymmetric fragment Cl···HCl is formed. Calculated data are compared with the structures of DMF complexes with HCl in the condensed phase.     

Hydrated alkali-metal cations: infrared spectroscopy and ab initio calculations of M+(H2O)(x=2-5)Ar cluster ions for M = Li, Na, K, and Cs

A delicate balance between competing and cooperating noncovalent interactions determines the three-dimensional structure of hydrated alkali-metal ion clusters. With a single water molecule hydrating an ion, the electrostatic ion...water interaction dominates. With more than one water molecule, however, water...water hydrogen-bonding interactions compete with the ion...water interactions to influence the three-dimensional structure. Infrared photodissociation spectra of M(+)(H2O)(x=2-5)Ar (with effective temperatures of approximately 50-150 K, depending on size and composition) are reported for M = Li, Na, K, and Cs, and dependencies on ion size and hydration number are explored.