The ground state tunneling splitting and the zero point energy of malonaldehyde: a quantum Monte Carlo determination

Quantum dynamics calculations of the ground state tunneling splitting and of the zero point energy of malonaldehyde on the full dimensional potential energy surface proposed by Yagi et al. [J. Chem. Phys. 1154, 10647 (2001)] are reported. The exact diffusion Monte Carlo and the projection operator imaginary time spectral evolution methods are used to compute accurate benchmark results for this 21-dimensional ab initio potential energy surface. A tunneling splitting of 25.7+/-0.3 cm-1 is obtained, and the vibrational ground state energy is found to be 15 122+/-4 cm-1. Isotopic substitution of the tunneling hydrogen modifies the tunneling splitting down to 3.21+/-0.09 cm-1 and the vibrational ground state energy to 14 385+/-2 cm-1. The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potential energy surface.     

Synthesis and reactivity of the Tl1 salts of

Synthons Tl1[TCNE]*- (1) and Tl12[TCNE]2- (2), for [TCNE]*- and [TCNE]2-, respectively, in metathesis reactions have been quantitatively prepared and characterized. The structure of 1 was solved and refined in a monoclinic unit cell at 27 degrees C [C2/c, a = 12.6966 (12) angstroms, b=7.7599 (7) angstroms, c=15.5041 (15) angstroms, beta = 96.610 (5) degrees , V= 1517.4 (2) angstroms3, Dcalcd = 2.911 gcm-3, Z=8, R1 = 0.0575, omegaR2=0.0701] and exhibits nuCN absorptions at 2,191 (s) and 2,162 (s) cm-1 consistent with metal-bound [TCNE]*-. The structure of 1 consists of a distorted square antiprismatic octacoordinate Tl1 bound to six monodentate [TCNE]*-s with TlN separations ranging from 2.901 to 3.171 angstroms averaging 3.020 angstroms, and one bidentate [TCNE]*- with TlN separations averaging 3.279 angstroms. The TlN bonding is attributed to electrostatic bonding. The [TCNE]*-s form dimerized zigzag chains with intra- and interdimer separations of 2.87 and 3.29 angstroms, respectively. The tight pi-[TCNE](2)2- dimer is diamagnetic and has the shortest intradimer [TCNE]*- distance reported. These synthons for [TCNE]*- and [TCNE]2- in metathesis reactions lead to the precipitation of, for example, TlIX (X = Cl, Br, OAc). Reaction of 1 with MnIII(porphyrin)X (X = Cl, OAc) forms the molecule-based magnets of [MnIII(porphyrin)][TCNE] composition, while the reaction of [CrI(C6H6)2]Br and (Me2N)2CC(NMe2)2Cl2, [TDAE]Cl2, with 1 forms [CrI(C6H6)2] [TCNE] and [TDAE][TCNE]2, respectively. The structure of [TDAE][TCNE]2.MeCN was solved and refined in an orthorhombic unit cell at 21 degrees C [I222, a = 10.2332(15), b = 13.341(6), c = 19.907(8) angstroms, V= 2717.7 angstroms3, Z = 4; Dcalcd = 1.216 gcm-3, R=0.083, Romega = 0.104] and exhibits upsilonCN absorptions at 2,193 (m), 2,174 (s), and 2,163 (s) cm-1 consistent with isolated [TCNE](2)2- , in contrast to the aforementioned TlI bound [TCNE](2)2-. The reaction of 2 with [TDAE]Cl2 forms [TDAE]2+[TCNE]2-.     

Quantum energy flow and the kinetics of water shuttling between hydrogen bonding sites on trans-formanilide

A potential energy surface for trans-formanilide (TFA)-H2O is calculated and applied to study energy flow in the complex as well as the kinetics of water shuttling between hydrogen bonding sites on TFA. In addition to the previously identified H2O-TFA(C[Double Bond]O) and H2O-TFA(NH) minima, with the water monomer bound to the C[Double Bond]O and NH groups, respectively, the new surface reveals a second local minimum with the water bound to the C[Double Bond]O group, and which lies energetically 310 cm(-1) above the previously identified H2O-TFA(C[Double Bond]O) global minimum. On this surface, the energy barrier for water shuttling from H2O-TFA(C[Double Bond]O) global minimum to H2O-TFA(N-H) is 984 cm(-1), consistent with the experimental bounds of 796 and 988 cm(-1) [J. R. Clarkson et al. Science 307, 1443 (2005)]. The ergodicity threshold of TFA is calculated to be 1450 cm(-1); for the TFA-H2O complex, the coupling to the water molecule is found to lower the ergodicity threshold to below the isomerization barrier. Energy transfer between the activated complex and the vibrational modes of TFA is calculated to be sufficiently rapid that the Rice-Ramsperger-Kassel-Marcus (RRKM) theory does not overestimate the rate of water shuttling. The possibility that the rate constant for water shuttling is higher than the RRKM theory estimate is discussed in light of the relatively high energy of the ergodicity threshold calculated for TFA.     

A calculation of the rovibronic energies and spectrum of the B1A1 electronic state of SiH2

The B1A1 electronic state of silylene (SiH2) is the second excited singlet state of the molecule and, like the analogous c state of methylene (CH2), it is quasilinear with symmetry 1sigmag+ at linearity. This state dissociates to Si(1D) + H2(1sigmag+). At equilibrium, the B state of SiH2 has an energy that we calculate to be 0.71 eV above that of the dissociation products. However, there is a barrier to dissociation that allows quasibound rovibrational levels to occur, and some have been observed recently [Y. Muramoto et al., J. Chem. Phys. 122, 154302 (2005)]. Starting with our analytical ab initio potential-energy surface, we adjusted it in a fitting to the experimental term values in order to determine the optimum potential-energy function in the bound region. This potential has a C2v equilibrium structure with a SiH bond length of 1.459 angstroms and a bond angle of 165.4 degrees; the barrier to linearity is only 129 cm(-1). Using the optimized potential-energy surface we calculate B-state term values, and using our calculated y and z dipole moment surfaces, we simulate the rotation-vibration spectrum of the state in order to assist in the detection of the matrix isolation spectrum.     

The rotational spectrum and dynamical structure of LiOH and LiOD: a combined laboratory and ab initio study

Millimeter wave rotational spectroscopy and ab initio calculations are used to explore the potential energy surface of LiOH and LiOD with particular emphasis on the bending states and bending potential. New measurements extend the observed rotational lines to J=7<--6 for LiOH and J=8<--7 for LiOD for all bending vibrational states up to (03(3)0). Rotation-vibration energy levels, geometric expectation values, and dipole moments are calculated using extensive high-level ab initio three-dimensional potential energy and dipole moment surfaces. Agreement between calculation and experiment is superb, with predicted Bv values typically within 0.3%, D values within 0.2%, ql values within 0.7%, and dipole moments within 0.9% of experiment. Shifts in Bv values with vibration and isotopic substitution are also well predicted. A combined theoretical and experimental structural analysis establishes the linear equilibrium structure with re(Li-O)=1.5776(4) A and re(O-H)=0.949(2) A. Predicted fundamental vibrational frequencies are v1=923.2, v2=318.3, and v3=3829.8 cm(-1) for LiOH and v1=912.9, v2=245.8, and v3=2824.2 cm(-1) for LiOD. The molecule is extremely nonrigid with respect to angular deformation; the calculated deviation from linearity for the vibrationally averaged structure is 19.0 degrees in the (000) state and 41.9 degrees in the (03(3)0) state. The calculation not only predicts, in agreement with previous work [P. R. Bunker, P. Jensen, A. Karpfen, and H. Lischka, J. Mol. Spectrosc. 135, 89 (1989)], a change from a linear to a bent minimum energy configuration at elongated Li-O distances, but also a similar change from linear to bent at elongated O-H distances.     

Dimethylacetylene: the theory required to analyse the infrared and Raman perpendicular bands

In this paper we give a detailed account of the theory required to simulate and analyse the infrared and Raman perpendicular fundamental bands of the dimethylacetylene molecule at high resolution. A summary of this theory has appeared in a previous paper (P.R. Bunker, J.W.C. Johns, A.R.W. McKellar and C. di Lauro, J. Mol. Spectry. 162 (1993) 142) in which an analysis of the infrared methyl rocking fundamental band was given. As well as detailing the effect of various terms in the Hamiltonian we also discuss the Raman selection rules and show that the analysis of the ΔK = 2 component of the perpendicular fundamental bands will lead to a determination of the sign of the torsional barrier. The sign of the barrier (i.e. whether the minimum energy conformation is staggered or eclipsed) cannot be determined from the analysis of the infrared perpendicular bands.     

Close-coupling study of rotational energy transfer of CO (upsilon=2) by collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer in the vibrationally excited CO due to collisions with He atom are presented for collision energies between 10(-5) and approximately 1000 cm-1 with CO being initially in the vibrational level upsilon=2 and rotational levels j=0,1,4, and 6. The He-CO interaction potential of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)] was adopted for the calculations. Cross sections for rovibrational transitions and state-to-state rotational energy transfer from selected initial rotational levels were computed and compared with recent measurements of Carty et al. [J. Chem. Phys. 121, 4671 (2004)] and available theoretical results. Comparison in all cases is found to be excellent, providing a stringent test for the scattering calculations as well as the reliability of the He-CO interaction potential by Heijmen et al.     

Influence of steric hindrance of organic ligand on the structure of Keggin-based coordination polymer

Five Keggin-based 3D coordination polymers, namely, [Cu3(pz)3(PW12O40)] (pz = pyrazine) (1), [Cu3(2,3-Me2pz)3(PW12O40)] (2,3-Me2pz = 2,3-dimethylpyrazine) (2), [Cu2(2,5-Me2pz)(1.5)(2,5-HMe2pz)(PW12O40)] (2,5-Me2pz = 2,5-dimethylpyrazine) (3), [Cu3(2,3-Me2pz)3(PMo12O40)] (4), and [Ag3(pz)3(PW12O40)].0.5H2O (5), were synthesized and structurally characterized. Crystal data are as follows: trigonal, space group R3c, a = 18.4070(14) angstroms, c = 22.544(3) angstroms, gamma = 120 degrees, and Z = 6 for 1; orthorhombic, space group Pccn, a = 16.599(2) angstroms, b = 20.470(3) angstroms, c = 14.3757(18) angstroms, and Z = 4 for 2; triclinic, space group P1, a = 10.667(2) angstroms, b = 11.147(2) angstroms, c = 20.207(4) angstroms, alpha = 90.983(4) degrees, beta = 108.128(3) degrees, gamma = 92.150(4) degrees, and Z = 2 for 3; orthorhombic, space group Pccn, a = 16.450(3) angstroms, b = 20.170(4) angstroms, c = 14.244(3) angstroms, and Z = 4 for 4; and rhombohedral, space group R32, a = 18.2047(13) angstroms, c = 23.637(3) angstroms, gamma = 120 degrees, and Z = 6 for 5. Their structural differences were investigated using crystal structure analysis, revealing that the influence of steric hindrance of organic ligand on the structures of Keggin-based coordination polymers is realized through changing the number of metal-organic units surrounding the POM anion.     

New 3D and chiral 1D CuIICrIII coordination polymers exhibiting ferromagnetism

The reaction of K3[Cr(CN)6] and the copper(II) bis-diamino complex of the ligand 1,3-diaminopropane (tn) led to the new cyanide-bridged 3D polymer ([(Cu(tn)2)3(Cr(CN)6)][Cr(CN)6]) infinity (1). Crystallographic data for 1: trigonal space group R3, a = b = 15.4908(11), c = 16.7699(13) angstroms, Z = 3, V = 3485.0(4) angstroms3. By the same reaction using trans-cyclohexane-(1R,2R)-diamine [1R,2Rchxn] or trans-cyclohexane-(1S,2S)-diamine [1S,2Schxn] as ligands, the chiral 1D polymers ([Cu(1R,2Rchxn)2]3[Cr(CN)6]2.4.75H2O) infinity (2) and ([Cu(1S,2Schxn)2]3[Cr(CN)6]2.4.25H2O} infinity (3), respectively, were obtained. 2 and 3 are isostructural and crystallize in the triclinic space group P1, with a = 8.5421(6), b = 12.6379(9), c = 16.1571(11) angstroms, alpha = 104.594(5) degrees , beta = 98.425(6) degrees , gamma= 97.440(5) degrees , Z = 1, V = 1644.3(2) angstroms3 for 2, and a = 8.5435(8), b = 12.6309(12), c = 16.1711(17) angstroms, alpha = 104.632 degrees , beta = 98.429(8) degrees , gamma = 97.375(8) degrees , Z = 1, and V = 1645.1(3) angstroms3 for 3. The complexes have been characterized by X-ray crystallography, IR, and magnetic susceptibility measurements. The chirality of 2 and 3 has been confirmed by circular dichroism measurements in the solid state. From the magnetic point of view, 1 shows 3D ferromagnetic ordering at ca. 4K, and 2 shows a weak intrachain ferromagnetic exchange, as a result of magnetic orbital orthogonality between Cr(III) and Cu(II) in the chain, with very long Cu-N(cyano) distances (2.665(5) and 2.671(5) angstroms) due to the long Jahn-Teller axis of the copper(II) ions.     

Accuracy of recent potential energy surfaces for the He-N2 interaction. II. Molecular beam scattering and bulk gas relaxation phenomena

A new semiempirical exchange-Coulomb model potential energy surface for the N(2)-He interaction was reported recently [A. K. Dham et al., J. Chem. Phys. 127, 054302 (2007)] and, using it, the temperature dependence of bulk gas properties of N(2)-He mixtures, such as the second virial coefficient and traditional transport phenomena, most of which depend primarily on the isotropic component of the interaction potential energy surface, was determined. Values of these properties, along with values calculated using two high-quality ab initio potential energy surfaces [C.-H. Hu and A. J. Thakkar, J. Chem. Phys. 104, 2541 (1996); K. Patel et al., ibid 119, 909 (2003)] were compared critically to available experimental data. The present paper reports on the ability of the same three potential energy surfaces to predict state-to-state and total differential cross sections, total integral cross sections, and the temperature dependence of bulk gas relaxation phenomena (including magnetic field effects on transport coefficients). While all three potential energy surfaces give total differential and higher speed integral scattering results that fall within the experimental uncertainties, integral scattering results and state-to-state differential cross section measurements consistently exceed the calculated values. All three surfaces give similar agreement with the relaxation properties of N(2)-He binary mixtures, with the semiempirical exchange-Coulomb model potential energy surface giving slightly better overall agreement with experiment than the two ab initio potential energy surfaces.     

Accuracy of recent potential energy surfaces for the He-N2 interaction. I. Virial and bulk transport coefficients

A new exchange-Coulomb semiempirical model potential energy surface for the He-N2 interaction has been developed. Together with two recent high-level ab initio potential energy surfaces, it has been tested for the reliability of its predictions of second-virial coefficients and bulk transport phenomena in binary mixtures of He and N2. The agreement with the relevant available measurements is generally within experimental uncertainty for the exchange-Coulomb surface and the ab initio surface of Patel et al. [J. Chem. Phys. 119, 909 (2003)], but with slightly poorer agreement for the earlier ab initio surface of Hu and Thakkar [J. Chem. Phys. 104, 2541 (1996)].     

Theoretical study of potential energy surface and vibrational spectra of ArF2 system

An ab initio potential energy surface (PES) of ArF2 system has been obtained by using MP4 calculation with a large basis set including bond functions. There are two local minimums on the PES: one is T-shaped and the other is L-shaped. The L-shaped minimum is the global minimum with a well depth of -119.62 cm- 1 at R = 0.3883nm. The T-shaped minimum has a well depth of -85.93cm -1 at R = 0.3486 nm. A saddle point is found at R = 0.3486 and θ = 61° with the well depth of -61.53 cm-1. The vibrational energy levels have been calculated by using VSCF-CI method. The results show that this PES supports 27 vibrational bound states, and the ground states are two degenerate states assigned to the L-type vibration.     

Ab initio potential energy surface and rovibrational spectrum of Ar-HCCCN

We report an ab initio intermolecular potential energy surface of the Ar-HCCCN complex using a supermolecular method. The calculations were performed using the fourth-order M?ller-Plesset theory with the full counterpoise correction for the basis set superposition error and a large basis set including bond functions. The complex was found to have a planar T-shaped structure minimum and a linear minimum with the Ar atom facing the H atom. The T-shaped minimum is the global minimum with the well depth of 236.81 cm(-1). A potential barrier separating the two minima is located at R=5.57 A and theta=20.39 degrees with the height of 151.59 cm(-1). The two-dimensional discrete variable representation was employed to calculate the rovibrational energy levels for Ar-HCCCN. The rovibrational spectra including intensities for the ground state and the first excited intermolecular vibrational state are also presented. The results show that the spectra are mostly b-type (Delta K(a)=+/-1) transitions with weak a-type (Delta K(a)=0) transitions in structure, which are in good agreement with the recent experimental results [A. Huckauf, W. Jager, P. Botschwina, and R. Oswald, J. Chem. Phys. 119, 7749 (2003)].     

The reaction OH + C2H4: an example of rotational channel switching

The low-temperature data for the reaction between OH and C(2)H(4) is treated canonically as either a two-well or one-well problem using the "Multiwell" suite of codes, in which a "well" refers to a minimum in the potential energy surface. The former is analogous to the two transition state model of Greenwald et al. [Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein, S. J. J. Phys. Chem. A2005, 109, 6031], while the latter reflects the dominance of the so-called "inner transition state". External rotations are treated adiabatically, causing changes in the magnitude of effective barriers as a function of temperature. Extant data are well-described with either model using only the average energy transferred in a downward direction, upon collision, ΔE(d)(T), as a fitting parameter. The best value for the parameters describing the rate coefficient as a function of temperature (200 < T/K < 400) (Data at lower temperature is too sparse to yield a recommendation.) and pressure in the form used in the NASA/JPL format [Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 17, Jet Propulsion Laboratory, 2011] are k(0) = 1.0 × 10(-28)(T/300)(-3.5) cm(6) molecule(-2) s(-1) and k(∞) to 8.0 × 10(-12)(T/300)(-2.3) cm(3) molecule(-1) s(-1).     

Negative collision energy dependence of Br formation in the OH + HBr reaction

The reaction between HBr and OH leading to H(2)O and Br in its ground state is studied by means of a crossed molecular beam experiment for a collision energy varying from 0.05 to 0.26 eV, the initial OH being selected in the state |JOmega> = |3/2 3/2> by an electrostatic hexapole field. The reaction cross-section is found to decrease with increasing collision energy. This negative dependence suggests that there is no barrier on the potential energy surface for the formation pathway considered. The experimental results are compared with the previously reported quantum scattering calculations of Clary et al. (D. C. Clary, G. Nyman and R. Hernandez, J. Phys. Chem., 1994, 101, 3704), and briefly discussed in the light of skewed potential energy surfaces associated with heavy-light-heavy type reactions.     

A hierarchical family of three-dimensional potential energy surfaces for He-CO

A hierarchical family of five three-dimensional potential energy surfaces has been developed for the benchmark He-CO system. Four surfaces were obtained at the coupled cluster singles and doubles level of theory with a perturbational estimate of triple excitations, CCSD(T), and range in quality from the doubly augmented double-zeta basis set to the complete basis set (CBS) limit. The fifth corresponds to an approximate CCSDT/CBS surface (CCSD with iterative triples/CBS, denoted CBS+corr). The CBS limit results were obtained by pointwise basis set extrapolations of the individual counterpoise-corrected interaction energies. For each surface, over 1000 interaction energies were accurately interpolated using a reproducing kernel Hilbert space approach with an R-6+R-7 asymptotic form. In each case, both three-dimensional and effective two-dimensional surfaces were developed. In standard Jacobi coordinates, the final CBS+corr surface has a global minimum at rCO=2.1322a0,R=6.418a0, and gamma=70.84 degrees with a well depth of -22.34 cm-1. The other four surfaces have well depths ranging from -14.83 cm-1 [CCSD(T)/d-aug-cc-pVDZ] to -22.02 cm-1 [CCSD(T)/CBS]. For each of these surfaces the infrared spectrum has been accurately calculated and compared to experiment, as well as to previous theoretical and empirical surfaces. The final CBS+corr surface exhibits root-mean-square and maximum errors compared to experiment (4He) of just 0.03 and 0.04 cm-1, respectively, for all 42 transitions and is the most accurate ab initio surface to date for this system. Other quantities investigated include the interaction second virial coefficient, the integral cross sections, and thermal rate coefficients for rotational relaxation of CO by He, and rate coefficients for CO vibrational relaxation by He. All the observable quantities showed a smooth convergence with respect to the quality of the underlying interaction surface.     

Three-dimensional potential energy surface of the Ar-OH(2Pi i) complex

Pure rotational transitions in the ground state for Ar-OH and Ar-OD [Y. Ohshima et al., J. Chem. Phys. 95, 7001 (1991) and Y. Endo et al., Faraday Discuss. 97, 341 (1994)], those in the excited states of the OH vibration, nu(s)=1 and 2, observed by Fourier-transform microwave spectroscopy in the present study, rotation-vibration transitions observed by infrared-ultraviolet double-resonance spectroscopy [K. M. Beck et al., Chem. Phys. Lett. 162, 203 (1989) and R. T. Bonn et al., J. Chem. Phys. 112, 4942 (2000)], and the P-level structure observed by stimulated emission pumping spectroscopy [M. T. Berry et al., Chem. Phys. Lett. 178, 301 (1991)] have been simultaneously analyzed to determine the potential energy surface of Ar-OH in the ground state. A Schrodinger equation, considering all the freedom of motions for an atom-diatom system in the Jacobi coordinate, R, theta, and r, was numerically solved to obtain energies of the rovibrational energy levels using the discrete variable representation method. A three-dimensional potential energy surface is determined by a least-squares fitting. In the analysis the potential parameters, obtained by ab initio calculations at the RCCSD(T) level of theory with a set of basis functions of aug-cc-pVTZ and midbond functions, are used as initial values. The determined intermolecular potential energy surface and its dependence on the OH monomer bond length are compared with those of an isovalent radical complex, Ar-SH.     

Ab initio large-amplitude quantum-tunneling dynamics in vinyl radical: a vibrationally adiabatic approach

Large-amplitude tunneling in vinyl radical over a C2v planar transition state involves CCH bending excitation coupled to all other internal coordinates, resulting in a significant dependence of barrier height and shape on vibrational degrees of freedom at the zero-point level. An ab initio potential surface for vinyl radical has been calculated at the CCSD(T) level (AVnZ; n=2, 3, 4, 5) for vibrationally adiabatic 1D motion along the planar CCH bending tunneling coordinate, extrapolated to the complete basis set (CBS) limit and corrected for anharmonic zero-point effects. The polyatomic reduced moment of inertia is calculated explicitly as a function of tunneling coordinate, with eigenvalues and tunneling splittings obtained from numerical solution of the resulting 1D Schr?dinger equation. Linear scaling of the CBS potential to match predicted and observed tunneling splittings empirically yields an adiabatic barrier height of DeltaEadiab=1696(20) cm(-1) which, when corrected for zero-point energy contributions, translates into an effective barrier of DeltaEeff=1602(20) cm(-1) consistent with estimates (DeltaE=1580(100) cm(-1)) by Tanaka and coworkers [J. Chem. Phys., 2004, 120, 3604-3618]. These zero-point-corrected potential surfaces are used to predict tunneling dynamics in vibrationally excited states of vinyl radical, providing strong support for previous jet-cooled high-resolution infrared studies [Dong et al., J. Phys. Chem. A, 2006, 110, 3059-3070] in the symmetric CH2 stretch mode.     

Factors controlling metal-ion selectivity in the binding sites of calcium-binding proteins. The metal-binding properties of amide donors. A crystallographic and thermodynamic study

The metal-ion complexing properties of the ligand EDTAM (ethylenediamine-N,N,N',N'-tetraacetamide) are investigated as a model for the role of amide oxygen donors in the binding sites of Ca-binding proteins. The structures of the complexes [Ca(EDTAM)NO3]NO3 (1), [La(EDTAM)(H2O)4](NO3)3.H2O (2), and [Cd(EDTAM)(NO3)]NO3 (3) are reported: 1 monoclinic, P2(1)/c, a = 10.853(2) angstroms, b = 12.893(3) angstroms, c = 13.407(3) angstroms, beta = 103.28(3) degrees, Z = 4, R = 0.0281; 2 triclinic, P, a = 8.695(2) angstroms, b = 9.960(2) angstroms, c = 16.136(3) angstroms, alpha = 95.57(3) degrees, beta = 94.84(3) degrees, gamma = 98.72(3) degrees, Z = 2, R = 0.0394; 3 monoclinic, P2(1)/c, a = 10.767(2) angstroms, b = 12.952(2) angstroms, c = 13.273(2) angstroms, beta = 103.572(3) degrees, Z = 4, R = 0.0167. Compounds 1 and 3 are isostructural, and the EDTAM binds to the metal ion through its two N-donors and four O-donors from the amide groups. Ca(II) in 1 is 8-coordinate with a chelating NO3- group, while Cd(II) in 3 may possibly be 7-coordinate, with an asymmetrically coordinated NO3- that is best regarded as unidentate. The La(III) in 2 is coordinated to the EDTAM in a manner similar to that of 1 and 3, but it is 10-coordinate with four water molecules coordinated to the La(III). The formation constants (log K1) for complexes of a variety of metal ions with EDTAM are reported in 0.1 M NaNO3 at 25.0 +/- 0.1 degrees C. These are compared to the log K1 values for en (ethylenediamine) and THPED (N,N,N',N'-tetrakis(2-hydroxypropyl)-ethylenediamine). For large metal ions, such as Ca2+ or La3+, log K1 increases strongly when the four acetamide groups are added to en to give EDTAM, whereas for a small metal ion, such as Mg2+, this increase is small. The log K1 values for EDTAM compared to THPED suggest that the amide oxygen is a much stronger base than the alcoholic oxygen. Structures of binding sites in 40 Ca-binding proteins are examined. It is shown that the Ca-O=C bond angles involving coordinated amides in these sites are large, commonly being in the 150-180 degrees range. This is discussed in terms of the idea that for purely ionic bonding the M-O=C bond angle will approach 180 degrees, while for covalent bonding the angle should be closer to 120 degrees. How this fact might be used by the proteins to control selectivity for different metal ions is discussed.     

Pseudorotation in the D2O trimer

Far-infrared spectra of Ar/D2O supersonic expansions have been recorded between 39 and 44 cm-1, in which a c-type band centered at 41.1 cm-1 was observed. This new band, attributed to the cyclic water trimer, has the same ground state rotational constants as the (D20)3 98 cm-1 a-type band reported by Liu et al. [J. Am. Chem. Soc. 116 (1994) 3507]. Fits of the observed far-infrared band positions to an effective one-dimensional potential require a substantially larger pseudorotation barrier or effective moment of inertia than predicted theoretically. Estimated frequencies of unobserved transitions in the pseudorotation manifolds of (D2O)3 and (H20)3 are presented as a guide to future experimental searches.