OH Bond Length Research Articles

Vibrational dependence of the torsional tunneling splitting and the <i>a</i> to <i>b</i> intensity evolution in the OH overtone spectra of CH<sub>3</sub>OH

An ab initio investigation of the OH-stretching overtone of methanol is presented aimed at interpretation of two of the interesting features reported in recent supersonic-jet infrared-laser-assisted photofragment spectroscopy (IRLAPS) studies, namely, (i) a monotonic decrease of the torsional E – A splitting as υOH increases, (ii) an evolution of the a-type transition moment towards dominance at higher excitations of the OH-stretching vibration. At the MP2 level with a 6-311+G(3df,2p) basis set, one-dimensional functions for the potential energy, dipole moment (a and b directions), barrier height V3, and torsional constant F have been mapped out as a function of the OH-bond length with the remaining structure relaxed. All ab initio quantities have been expressed as Taylor-series expansions in the dimensionless OH normal coordinate, q. Calculations have been carried out in the harmonic basis set to yield vibrational energies and eigenfunctions. The latter have been used to evaluate the torsional parameters (V3, F) and the resulting E – A torsional splittings at each OH excitation, and to compute the evolution of the infrared transition intensity ratio a/b. The ab initio results reproduce the reported decrease in E – A splitting by a factor of 5 in going from υOH = 0 to υOH = 6, with a maximum deviation of 19% at the highest excitation. The calculated variation in a/b is in qualitative agreement with the experimental trend, showing the observed sharp rise at high υOH. PACS Nos.: 33.20E, 33.80B

Vibrational dynamics up to the dissociation threshold: A case study of two-dimensional HOCl

This work is aimed at extending recent studies dealing with the highly excited vibrational dynamics of HOCl [J. Chem. Phys. 111, 6807 (1999); J. Chem. Phys. 112, 77 (2000)], by taking advantage of the fact that the OH-stretch remains largely decoupled from the two other degrees of freedom up to and above the dissociation threshold. The molecule is thus reduced to a two-dimensional (2D) system by freezing the OH bond length to its equilibrium value. All of the calculated bound states of the 2D system, as well as the first 40 resonances, can be assigned with a Fermi polyad quantum number. The bifurcation diagram of the principal families of periodic orbits (POs) is extended to higher energies compared to 3D studies. In particular, the birth of “inversion” states (states exploring two equivalent wells connected through the linear HOCl configuration) is related to a period-doubling bifurcation of the families of bending POs, while “dissociation” states (states for which the energy flows back and forth along the dissociation pathway) are shown to lie on top of three successive families of POs born at saddle-node bifurcations. Based on the derivation of a classical analogue of the quantum Fermi polyad number, the energies of particular quantum states and classical POs are plotted on the same diagram for the 2D ab initio surface and are shown to agree perfectly. In contrast, comparison of classical Poincaré surfaces of section and quantum Husimi distributions suggests that the classical dynamics of 2D HOCl is much more chaotic than the quantum dynamics. This observation is discussed in terms of the quantum/classical correspondence, and particularly of the vague tori introduced by Reinhardt. It is nevertheless shown that quantum and classical mechanics agree in predicting a slow intramolecular vibrational energy redistribution (IVR) between the OCl stretch and the bend degrees of freedom.

Computation of the p K of liquid water using coordination constraints

The equilibrium constant for autodissociation of water is computed by imposing constraints on proton coordination number in an ab initio molecular dynamics (MD) simulation of the liquid. Dissociation is enforced by gradually decreasing the number of hydrogen atoms coordinated with a selected oxygen to one, transforming a water molecule into a hydroxyl ion. Alternatively, a hydronium ion is created by increasing the proton coordination to three. These two pathways are compared to a previous ab initio MD study by Trout and Parrinello [B.L. Trout, M. Parrinello, Chem. Phys. Lett. 288 (1998) 343] who used a constraint on OH bond length. The resulting numerical estimates of p K w=13±1 are in good agreement. It is shown that coordination constraints are more effective for stabilization of the highly activated and short lived solvent separated ion pairs that form in the advanced phase of the reaction. A predominant solvent separated ion pair structure is found to have a double pentameric structure.

Structural insights of glass-reinforced hydroxyapatite composites by Rietveld refinement

Phase transformations and interstitial and/or substitution of trace elements during the liquid-phase sintering process of P 2O 5–CaO–MgO glass-reinforced hydroxyapatite (GR-HA) composites were examined by X-ray diffraction and Rietveld analyses. Using the Rietveld method for structure refinement, changes in the lattice parameters of the two main phases of the composites, hydroxyapatite (HA) and β-tricalcium phosphate ( β-TCP), as well as changes in several bond lengths and in the occupancy of the hydroxyl oxygen site in the HA phase structure were assessed. The glasses gave rise to formation of between approximately 45 and 50% of β-TCP, with evidence for the Mg 2+ enhancing the formation of β-TCP. Between 1300 and 1350°C, the β-TCP inverts to α-TCP, without further decomposition of the residual HA. The glasses showed evidence for stabilisation of the hydroxyl group located in the hydroxyl channels. This is supported by measurements of the hydroxyl channel radius ( R c), the Ca 2–OH bond length and the hydroxyl oxygen occupancy ( O occ). Results showed that the Mg 2+ containing glasses induced the β-TCP phase formation in the structure of GR-HA composites and retarded the β-TCP into α-TCP transformation at higher temperatures. The chemical composition of the P 2O 5 glasses also induces modifications in the lattice parameters of the crystallographic phases present in the microstructure of the composites. This suggests some substitution of Mg 2+-for-Ca 2+ in the β-TCP structure during the liquid-phase sintering process.

Electric-field-dependent empirical potentials for molecules and crystals: A first application to flexible water molecule adsorbed in zeolites

A general method to include electric-field-dependent terms in empirical potential functions representing interatomic interactions is proposed. It is applied to derive an intramolecular potential model for the water molecule able to reproduce the effects of an electric field on its geometry and dynamics: to enlarge the HOH angle, to increase slightly the OH bond lengths, to red-shift the stretching vibrational frequencies, and to blue-shift slightly the bending mode frequency. These effects have been detected experimentally for water adsorbed in zeolites and have been confirmed by quantum mechanical calculations. The electric-field-dependent intramolecular potential model for water has been combined with a newly refined intermolecular potential for bulk water and with new potentials representing cation–water and aluminosilicate–water interactions in order to simulate, by classical molecular dynamics (MD) technique, the behavior of water adsorbed in zeolites. The performances of the model have been checked by a MD simulation of liquid water at room temperature, by the structural and vibrational properties of the water dimer, and by test MD calculations on a hydrated natural zeolite (natrolite). The results are encouraging, and the simulations will be extended to study the behavior of water adsorbed in other zeolites, including diffusion and some aspects of ion exchange processes.

Ab initio molecular orbital study on the structures and energetics of CH3OH(H2O)n and CH3SH(H2O)n in the gas phase

The purpose of this study was to calculate the structures and energetics of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n in the gas phase: we asked how the CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} moieties of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n change with an increase in n and how can we reproduce the experimental values ΔH°n−1,n. For this purpose, we carried out full geometry optimizations with MP2/6-31+G(d,p) for CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n (n=0,1,2,3,4,5) and CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n (n=0,1,2,3,4). We also performed a vibrational analysis for all clusters in the optimized structures to confirm that all vibrational frequencies are real. All of the vibrational frequencies of these clusters are real, and they correspond to equilibrium structures. For CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, when n increases, (1) the CO bond length decreases, (2) the CH bond lengths do not change, (3) the OH bond lengths increase, (4) the OCH bond angles increase, (5) the COH bond angles decrease, (6) the charge on CH3 becomes less positive, and (7) these predicted values, except for the OH bond lengths of CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, approach the corresponding values in CH3OH. The CO bond length in CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)5 is shorter than that in CH3OH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} in the gas phase by 0.061 Å at the MP2/6-31+G(d,p) level. Except for the SH bond lengths in CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document}(H2O)n, however, the structure of the CH3SH\documentclass{article}\pagestyle{empty}\begin{document}$_{2}^{+}$\end{document} moiety does not change with an increase in n. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 125–131, 2001

Planar study of the H2O+Cl collision: I Rotational and bending excitation of H2O in non-reactive scattering

We report on preliminary results of a planar study of the H2O+Cl collision. The rotational and bending excitation probabilities of the H2O molecule are calculated for non-reactive scattering in the collision energy range: 0–2 eV. Along with the planar constraint, the two OH bond lengths are fixed at the equilibrium distances in the H2O molecule. The present calculation involves the Born–Oppenheimer type separation between light and heavy nuclear motions. The hydrogenic wavefunctions, that describe the motion of the hydrogen atoms for fixed heavy nuclei distances, are functions of two angles related to the rotational and bending motions of H2O. The potential energy curves of these wavefunctions suggest that the rotational excitation process depends weakly on the bending quantum number. This is confirmed by close-coupling calculations that also show that the bending excitation process is weak. The present work is a first step of a detailed study of the planar H2O+Cl reactive collision.

Isotope Effects on Chemical Shifts as an Analytical Tool in Structural Studies of Intramolecular Hydrogen Bonded Compounds

Isotope effects on chemical shifts of intramolecularly hydrogen bonded systems are reviewed. The effects are conveniently divided into localized (intrinsic) and equilibrium isotope effects. The review covers both primary and secondary isotope effects on chemical shifts. For the localized one it is very important to distinguish between RAHB and non-RAHB types. For the RAHB systems the OH group is shown to form a stronger hydrogen bond than the OD group, whereas the opposite is true for non-RAHB. Theoretical calculations at the ab initio level (DFT) can be used to provide reliable structures, chemical shifts and isotope effects. Large intrinsic secondary isotope effects can to a good degree be related to the change in the OH(D) bond length upon deuteriation. 2ΔC(XD) isotope effects are shown to be good measures of hydrogen bond strength. So far no evidence for heavy atom movement has been convincingly advanced for RAHB systems. A number of isotope effect types have now been studied in depth, nΔC(XD), X= O, S or N, 1DN(D), 1ΔO(D), 5ΔO(D), nΔH(OD), nΔF(D). It is concluded that the possible over determination of isotope effects in hydrogen bonded systems provide a very powerful tool in studies of structure of hydrogen bonded systems. Isotope effects are studied in detail in sterically hindered systems and parameters are available to distinguish between twist of e.g. RCO groups and steric compression. Furthermore, twist of phenyl rings may also be monitored. Proton transfer reactions such as tautomerism have been studied extensively. Equilibrium isotope effects on chemical shifts have been reported in a large number of cases. The magnitude of the equilibrium isotope effects depends on the equilibrium constant. A series of parameters have been suggested as a good way to establish tautomerism in a number of difficult cases. Both deuterium and tritium primary isotope effects have now been reported in a large number of systems. Large intrinsic primary isotope effects is a good proof of double potential wells. Primary isotope effects are also studied in tautomeric systems and at different temperatures and can under such circumstances be both positive and negative.

The H2O++ Ground State Potential Energy Surface

At the correlation-consistent polarized-valence quadruple-zeta complete active space self-consistent field second-order configuration interaction level of ab initio theory (cc-pVQZ CASSCF-SOCI), we calculated 129 points on the ground electronic state potential energy surface of the water dication H2O++; this calculation includes the energy of X3Σ− OH+ at equilibrium and the energy of the triplet oxygen atom. We determined the parameters in an analytical function that represents this surface out to the (OH+ + H+) and (O + 2H+) dissociation limits, for bending angles from 70 to 180°. There is a metastable minimum in this surface, at an energy of 43 600 cm−1 above the H+ + OH+ dissociation energy, and the geometry at this minimum is linear (D∞h), with an OH bond length of 1.195 Å. On the path to dissociation to H+ + OH+, there is a saddle point at an energy of 530 cm−1 above the minimum, and the geometry at the saddle point is linear (C∞ Kv) with OH bond lengths of 1.121 and 1.489 Å. Using the stabilization method, we calculated the lowest resonance on this surface. Relative to the metastable local minimum on the potential energy surface, the position of the lowest resonance for H2O++, D2O++, and T2O++ is 1977(85), 1473(25), and 1249(10) cm−1, respectively, where the width of each resonance (in cm−1) is given in parentheses.

Rationalization of the dielectric properties of common three-site water models in terms of their force field parameters

The dielectric properties (static dielectric constant ε0, Debye relaxation time τD and distance dependent Kirkwood G-factor Gk(R)) of commonly used three-site water models (the “simple point charge” models SPC and SPC/E, as well as the “transferable intermolecular potentials with three point charges” TIP3P and its CHARMM modified variant TIP3Pmod) were evaluated from 2 ns molecular dynamics simulations using the Ewald summation method to compute the electrostatic interactions. The results for SPC (ε0=65±5, τD=7.6±0.8 ps) and SPC/E (ε0=68±6, τD=12.1±1.3 ps) are in good agreement with the literature; for TIP3P (TIP3Pmod) we obtained a dielectric constant ε0=97±7 (97±6) and a Debye relaxation time τD=7.3±0.7 ps (6.9±0.6 ps). The surprisingly large differences in ε0 were rationalized by an investigation of the relationship between the force field parameters and the dielectric properties. Based on simulations of hybrid SPC/TIP3P models, the HOH bond angle was identified as the determining factor of the dielectric constant. By contrast, τD was shown to depend mainly on the OH bond length and the partial charges; the HOH angle is of secondary importance. The role of the HOH angle is clarified by a comparison of the orientational correlation functions of the water models.

Brønsted Acid Sites in HSAPO-34 and Chabazite: An Ab Initio Structural Study

The structure of Brønsted acid sites and the effects of the Al → Si and Si → P substitution in chabazite and AlPO4-34 frameworks, respectively, have been studied by ab initio plane-wave calculations on periodic models. Geometry distortions remain well localized to the nearest neighborhood of the Brønsted acid site. The Si−O(H) and Al−O(H) distances of the common Si−O(H)−Al moiety are significantly different in the two kinds of materials. Calculated OH stretching frequencies of acidic protons, which could be assigned to the low- and high-frequency OH bands, in agreement with experimental infrared spectra of H-SSZ-13 (high silica chabazite) and HSAPO-34, correlate with OH bond lengths and Si−O(H)−Al angles.

A DFT Study of the Models of the Bronsted Acid Sites of Zeolite Catalysts

A systematic study of the different cluster models of the Bronsted site has been carried out in order to predict how the local structure influences the geometric parameters of the SiO(H)Al site and the harmonic frequencies of the characteristics vibrational modes of the bridging surface hydroxyl group. The performance of the B3LYP variant of the density functional theory (DFT) method is examined and compared with both SCF HF and MP2 approximations. In order to find the best molecular model for the Bronsted site, two series of derivatives based on the prototype model cores SiO(H)Al and HAlO4 were considered. In the HAlO4 series, H3SiO(H)Al(OH)2OSiH3 and H3SiO(H)Al(OSiH3)3 were investigated. Among the models in the SiO(H)Al series were H3SiO(H)AlH3, H3SiO(H)Al(OH)3, H2(OH)SiO(H)Al(OH)3, (OH)3SiO(H)Al(OH)3, and (H3SiO)3SiO(H)Al(OSiH3)3. The research reveals that the acidic OH bond length and its stretching frequency are greatly influenced by the H bond formed between the acidic proton and its next-nearest neighbor oxygen on the aluminum atom. The geometry and the frequencies for the larger models predicted at the B3LYP/D95(d,p) level are in good agreement with the experiments, suggesting that the B3LYP/D95(d,p) theory is reliable for a study of the Bronsted site. This study also suggests that the OH terminated models are not suitable as models for the SiO(H)Al site due to the strong dipole moment of the OH group, which strongly affects the acidic H.

Refinement of the cookeite "r" structure

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Observation and spectroscopy of high-lying states of the CaOH radical: Evidence for a bent, covalent state

The CaOH radical has been produced in a supersonic molecular beam by the reaction of water vapor with laser desorbed calcium. Three new electronic states, the D 2Σ+, E 2Σ+, and F states have been observed by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. The D and E states are linear, but the F state is bent, the first bent and strongly covalent state of CaOH to be observed. Vibrational constants for the D state have been determined and a partially rotationally resolved spectrum has provided the rotational constant and Ca–OH bond length in this state. The D and E states are more strongly bound than the ground state, perhaps indicating some covalent contribution to the bonding. Additional vibrational constants of the ground electronic state including the CaO–H vibrational frequency have been determined from the emission spectra.

Cationic, structural, and compositional effects on the surface structure of zeolitic aluminosilicate catalysts

Abstract The cationic, structural, and compositional influences on the structure and bonding of zeolitic aluminosilicates have been investigated with the density functional theory (DFT) method including local (VWN) and non-local spin density functionals (BLYP). Full optimization of structures has been carried out at the 6–31G∗/VWN and 6–31G∗/BLYP levels of theory for the different types of the SiOHAl unit in the secondary building unit of the zeolite cluster models [(OH)8HyAlxSi8−xO12](x−y)− (x,y = 0, 1, 2, 4) and the silica model (OH)8Si8O12. Changes in the environment of the silicon and aluminum framework atoms with a given Si/Al ratio generate new different acid sites. The validity of Loewenstein's AlOAl avoidance rule is confirmed but Dempsey's AlOAl avoidance rule does not hold with double four-membered ring aluminosilicate (D4R). The proton affinity (PA) of the silica model (OH)8Si6O12 at BLYP/6–31G∗ are evaluated to be 1403 ± 15 kJ/mol which is in good agreement with the experimentally observed value of 1390 ± 25 kJ/mol. Proton affinities of Bronsted hydroxyl groups in H forms of zeolites associated with different Si/Al ratios indicate that the higher the ratio, the less the proton is constrained which results in a stronger acid strength. Cations are found to have profound effects on the structure and bonding of zeolite clusters. The H ion has a strong perturbation on the important parameters i.e. SiO, AlO OH bond lengths while the Li(I) has a modest effect. The PA of the Bronsted OH groups in the zeolitic frameworks interacting with their cations are 1272.4 and 1279.3 kJ/mol for H2(OH)8Al2Si6O12 and LiH(OH)8Al2Si6O12, respectively, which correspond with decreasing bond lengths of the OH groups. These results indicate that the acid strength of the OH groups within the zeolitic framework is also determined by the presence of cations, in addition to compositional and structural effects.

Full configuration–interaction and state of the art correlation calculations on water in a valence double-zeta basis with polarization functions

Using a valence double-zeta polarization basis, full configuration–interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation. At the same geometries and with the same basis set configuration interaction calculations at excitation levels up to hextuples, multireference singles doubles configuration interaction calculations, coupled cluster calculations at excitation levels up to quadruples, Mo/ller–Plesset perturbation theory calculations through order fifteen, and complete active space second-order perturbation theory calculations are also carried out. The static correlation contribution increase with increasing bond length. The calculations show that the coupled cluster approach has a remarkable ability to describe even relatively large static correlation contributions. The single reference perturbation expansion breaks down for larger OH bond length, while the multireference approach preserves the accuracy for the whole potential curve. At the equilibrium geometry, FCI calculations have also been carried out for the lowest state of 2A1, 2B1, and 2B2 symmetry of H2O+, and the results compared with state of the art correlation results for total energies and ionization potentials (IP’s). Differential energies (IP’s) are obtained more accurately than absolute (total) energies in the size extensive coupled cluster and perturbation approaches. For the nonsize extensive configuration interaction method errors are obtained of the same size for differential and absolute energies.

Observation of the ν1 OH(OD) stretch of HOI and DOI by Fourier transform infrared emission spectroscopy

The spectra of vibrationally hot HOI formed in the reaction of alkyl iodides with oxygen atoms are observed by Fourier transform infrared emission spectroscopy. The v=1–3 levels of the OH stretch are observed via the Δv=−1 and Δv=−2 sequence bands. The spectrum of DOI is observed by using 2,2,2-d3-iodoethane as the precursor in the oxygen atom reaction. The v=1–4 levels of the OD stretch are observed in the Δv=−1 sequence band, and the v=1–5 levels of the OD stretch are observed in Δv=−2. Medium resolution spectra (0.031 cm−1 apodized) are recorded and rotationally analyzed for the ν1 fundamental and 2ν1−ν1 hot band of HOI. An estimate of the HOI ground state structure is made by constraining the OH bond length to its value for HOCl and HOBr and calculating the HOI bond angle and the OI bond length by least squares fit to the ground state rotational constants.

The crystal structure of manandonite-2H2

The crystal structure of manandonite-2H2, an AI-, Li-, B-rich analogue of amesite, has been refined in space group C1 to R = 5.7%. Tetrahedral Si, AI, and B are partly ordered to give two mean T-O bond lengths near 1.603(1) A and two near 1.667(1) A. B has a greater tendency to order than do Si and Al. There are one relatively Li-rich octahedron and two relatively Al-rich octahedra in each layer with mean M-O,OH bond lengths of 1.997(1), 1.960(1), and 1.955(1) A, respectively, in layer 1 and 2.014(1), 1.941(1), and 1.956(1)A, respectively, in layer 2. It is the partial ordering of the octahedral cations that decreases the ideal P63 symmetry to PI (or C1, as used for refinement). Tetrahedral rotations of 18.3° are required to match the larger lateral dimensions of the tetrahedral sheet with those of the octahedral sheet, coupled with basal 0 corrugations because ofthe different sizes of the octahedra. The H+ protons of the six surface OH molecules point directly toward their acceptor basal 0 atoms to give H bond contacts between 2.647 and 2.773 A.

Accurate ab initio potential energy surfaces of Ar–HF, Ar–H2O, and Ar–NH3

We present accurate potential energy surfaces for Ar–HF, Ar–H2O, and Ar–NH3 from the supermolecular calculations using Mo/ller–Plesset perturbation theory up to the complete fourth-order (MP4) and efficient basis sets containing bond functions. Preliminary calculations on Ar–HF are given to show the usefulness of bond functions and the stability of the results with respect to the change of the basis set. Detailed MP4 calculations on Ar–HF with a fixed HF bond length of r=〈r〉v=0 give a global potential minimum with a well depth of 200.0 cm−1 at the position R=3.470 Å, θ=0° (linear Ar–H–F), a secondary minimum with a well depth of 88.1 cm−1 at R=3.430 Å, θ=180° (linear Ar–F–H), and a potential barrier of 128.3 cm−1 that separats the two minima near R=3.555 Å, θ=90° (T shaped). Further calculations on the three main configurations of Ar–HF with varying HF bond length are performed to obtain vibrationally averaged well depths for v=0, 1, 2, and 3. Our primary wells are about 15 cm−1 higher than those of Hutson’s H6(4,3,2) potential for v=0, 1, 2, and 3, and our minimum distances are about 0.05 Å longer. Extensive MP2 calculations (R=3.1–5.0 Å) and brief MP4 calculations (near the radial minimum) are performed for the intermolecular potentials of Ar–H2O and Ar–NH3 with the monomers held fixed at equilibrium geometry. For Ar–H2O, MP4 calculations give a single global minimum with a well depth of 130.2 cm−1 at R=3.603 Å, θ=75°, φ=0°, along with barriers of 22.6 and 26.6 cm−1 for in-plane rotation at θ=0° and 180° respectively, and a barrier of 52.6 cm−1 for out-of-plane rotation at θ=90°, φ=90°. All these are in good agreement with experiment, especially with Cohen and Saykally’s AW2 potential. The dependence of the Ar–H2O potential on an OH bond length is calculated to study the effect from excitation of the bond stretching vibration and the result agrees well with the red shift observed. For Ar–NH3, MP4 calculations give a single global minimum with a well depth of 130.1 cm−1 at R=3.628 Å, θ=90°, φ=60°, along with barriers of 55.2 and 38.0 cm−1 for end-over-end rotation at θ=0° and 180°, respectively, and a barrier of 26.6 cm−1 for rotation about NH3 symmetry axis at θ=90°, φ=0°. All these are in good agreement with experiment and Schmuttenmaer et al. AA1 potential. The effects on potential from the change of the normal NH3 pyramidal geometry to the planar geometry are calculated and the results indicate that the Σ states with tunneling motion perpendicular to the radial coordinate remain virtually unchanged from free NH3 whereas the Π states with tunneling motion parallel to the radial coordinate have the tunneling motion nearly quenched. Comparisons of the potentials for the systems from Ar–HF, Ar–H2O, to Ar–NH3 are made to reveal the periodic trends of bonding and structure in the van der Waals complexes.

NMR relaxation in ethanol and propanol and in their binary mixtures with carbon tetrachloride

NMR proton, deuteron and oxygen-17 relaxation rates were measured in different isotopic species of liquid ethanol and propanol and in their mixtures with carbon tetrachloride. For the pure alcohols a large temperature range could be covered and the relaxation rate maxima were obtained. Evaluation of the rates yielded rotational correlation times, the OH bond length and quadrupole coupling constants for the deuteron and for oxygen-17.