Effects of equilibrium H-bond distance and angle changes on Raman intensities from water

The H-bond energy dispersion over the inhomogeneously broadened OD stretching contour from dilute HDO in H(2)O was determined from absolute Raman intensities; it displays a large minimum near omega=2440 cm(-1) from short, strong H bonds (in agreement with the peak omega from lda ice) and a large maximum near 2650-2675 cm(-1) due to extremely weak or broken H bonds (in agreement with the peak omega from dense, supercritical HDO in H(2)O, 0.9 g/cm(3), 673 K). The difference between extrema is the maximum H-bond DeltaE, 5100+/-500 cal/mol, in excellent agreement with Pauling's limiting value. A pressure of 1500 bars yields an additional maximum and shoulder between the two dispersion extrema from pure water; saturated NaCl in water shows the additional maximum. The maxima near 3350 cm(-1) (1500 bar) or near 3360 cm(-1) (NaCl-H(2)O) arise from bent H bonds; 3350 cm(-1) (1500 bar) corresponding to an angle of approximately 170 degrees in the joint frequency/bend, probability of Lawrence and Skinner. Rising omega refers to a higher probability of larger O-O distances, bent H bonds, and H-bond weakening and breakage. A approximately 50-80 cm(-1) difference between the 2727 cm(-1) OD peak from HDO in steam, and the 2650-2675 cm(-1) dispersion maximum is explained via the very broad approximately 60 cm(-1) liquid peak observed at 342 degrees C and 2000 bar.     

Dispersion of the Raman depolarization ratio of HDO in water and heavy water from 295 to 368 K, and from concentrated NaClO4D2OH2O

The dispersion of the Raman depolarization ratio rho(L) was measured for HDO in H(2)O and in D(2)O. rho(L) for the decoupled OD stretch displays a maximum at 2575 +/- 15 cm(-1) at 296 K and a minimum at 2675 +/- 15 cm(-1), in agreement with the isosbestic point 2570 +/- 10 cm(-1), and the enthalpy dispersion maximum, 2650-2675 cm(-1), respectively. However, three extrema were uncovered in rho(L) for the OH stretch of HDO in D(2)O, and their positions agree with the frequencies of a minimum and a maximum in the enthalpy dispersion and with the isosbestic frequency. The frequency of the rho(L) maximum (OH stretch) lies just above the frequency corresponding to the joint angle-frequency probability maximum. [Lawrence and Skinner, J. Chem. Phys. 118, 264 (2003)]. The low- and high-frequency minima in rho(L) (OH stretch), correspond, respectively, to very strong H-bonds, and extremely weak, long, bent H bonds. The frequencies of the maxima and minima in rho(L) for the decoupled OH and OD stretches are independent of temperature within experimental error between 295 and 368 K. rho(L) was also measured for the OD stretch from saturated NaClO(4) in D(2)OH(2)O; it displays a maximum at 2560 +/- 20 cm(-1) and a sharp minimum at 2650 +/- 5 cm(-1). The shape of the dispersion of (betaalpha)(2) approximately rho(L) for HDO in D(2)O was calculated with the aid of the molecular dynamics results of Lawrence and Skinner. beta(2) is the anisotropic polarizability and alpha is the isotropic polarizability. A maximum resulted in the calculated dispersion at 3400 +/- 10 cm(-1), in excellent agreement with the measured maximum of 3395 +/- 15 cm(-1). The H-bond angles decrease far below 180 degrees as the OH-stretching frequency increases to 3700 cm(-1) and above. Such small H-bond angles, and very large O-O distances, are tantamount to broken H-bonds and are thought to produce the minimum in rho(L) near 2650 cm(-1).     

New spectroscopic method for aqueous solutions: Raman xi-function dispersion for NaClO4 in water

A new Raman method is exemplified by xi identical with-RT[ partial differential ln(I(omega)I(REF)) partial differentialX(1)](T,P,n(2),n(3) ) for ternary NaClO(4)D(2)OH(2)O, or by xi identical with-RT[ partial differential ln(I(omega)I(REF)) partial differentialX(2)](T,P) for binary NaClO(4)H(2)O solutions. (Fundamental differences exist between xi and the chemical potential mu.) I(omega) is the Raman intensity at omega, I(REF) is the reference intensity, e.g., at the isosbestic frequency, X(2) is the H(2)O and X(1) the small D(2)O mol fraction, and n(2) and n(3) are constant mols of H(2)O and NaClO(4), respectively. Maxima (max) and minima (min) were observed in xi versus omega (cm(-1)); xi(max)-xi(min)=Deltaxi(max). Deltaxi(max)=8050+/-100 calmol H(2)O for the coupled, binary solution OH stretch, and Deltaxi(max)=4200+/-200 calmol H bond for the decoupled, ternary solution OD stretch. The perchlorate ion breaks the H bonds in water. 8050 calmol H(2)O corresponds to the maximum tetrahedral Deltaxi(max) value for two H bonds, i.e., Deltaxi(max)=4025 calmol H bond, in agreement with the HDO Deltaxi(max)=4200+/-200 calmol H bond. [Deltaxi(max) is not the H bond enthalpy (energy).] Minima occur in xi at the peak omega values corresponding to the HDOH(2)O and H(2)O ices, and maxima in xi at 2637+/-5 cm(-1) (OD) and 3575+/-10 cm(-1) (OH) correspond to the peak OD and OH stretching omega values from dense supercritical water. Enthalpy dispersion curves were also determined for saturated, binary, and ternary NaClO(4) solutions and for D(2)O in H(2)O. The xi-function method is shown to be applicable to infrared absorbance spectra.     

New Raman method for aqueous solutions: xi-function dispersion evidence for strong F(-)-water H-bonds in aqueous CsF and KF solutions

The Raman xi-function dispersion method recently elucidated for the strong H-bond breaker, ClO4-, in water [G. E. Walrafen, J. Chem. Phys. 122, 094510 (2005)] is extended to the strongly H-bond forming ion, F-. Measuring the xi function is analogous to measuring DeltaG from the thermodynamic activity of the water, aH2O, as the stoichiometric mol fraction of the water in the solution decreases due to addition of an electrolyte or nonelectrolyte. xi is the derivative of the OH-stretching part of the Gibbs free energy with respect to the water mol fraction; xiomega identical with-RT[ partial differential ln(Iomega/IREF) partial differentialX2](T,P). I is the Raman intensity at omega (omega=Raman shift in cm-1); IREF, that at an arbitrary reference omega; and, X2 is the water mol fraction (X1=CsF or KF mol fraction). ln(Iomega/IREF) was found to be linear in X2 for the complete range of OH-stretching omega's, with correlation coefficients as large as 0.999 96. Linearity of ln(Iomega/IREF) versus X2 is an experimental fact for all omega's throughout the spontaneous Raman OH-stretching contour; this fact cannot be negated by relative contributions of ultrafast/fast, homogeneous/inhomogeneous processes which may underlie this linearity. Linearity in ln(Iomega/IREF) versus 1T, or in ln(Iomega/IREF) versus P, was also observed for the Raman H-bond energy DeltaE and pair volume DeltaV dispersions, respectively. A low-frequency maximum (MAX) and a high-frequency minimum (MIN) were observed in the xi function dispersion curve. Deltaxi=xiMIN-xiMAX values of -7000+/-800-cal/mol H2O for CsF, and the experimentally equal Deltaxi=-6400+/-1000-cal/mol H2O for KF, were obtained. These Deltaxi's are opposite in sign but have nearly the same absolute magnitude as the Deltaxi value for NaClO4 in water; Deltaxi=+8050+/-100-cal/mol H2O. A positive Deltaxi corresponds to a water-water H-bond breaker; a negative Deltaxi to a H-bond former; specifically, a F--water H-bond former, in the instant case. NaClO4 breaks water-water H-bonds and also gives rise to weak, long (3.0-3.3 A), severely bent (approximately 140 degrees), high-energy, ClO4--water interactions. Fluoride ion scavenges the extremely weak or non-hydrogen-bonded OH groups, thus forming strong, short, linear, low-energy, H-bonds between F- and water. The strength of the F--water H-bond is evident from the fact that the OH-stretching xi-function minimum is centered approximately 200-300 cm-1 below that of ice. The diagnostic feature of the Raman spectrum from F- in water is an intense, long, low-frequency OH-stretching tail extending 800 cm-1 or more below the 3300-cm-1 peak. A similar intense, long, low-frequency Raman tail is produced by the OH- ion, which is known to H-bond very strongly when protons from water are donated to its oxygen atom.     

Isotope effects in liquid water by infrared spectroscopy. V. A sea of OH4 of C2v symmetry

The two water gas OH stretch vibrations that absorb in the infrared (IR) near 3700 cm(-1) are redshifted to near 3300 cm(-1) upon liquefaction. The bathochromic shift is due to the formation of four H-bonds: two are from the labile hydrogen atoms to neighbors and two are received from neighbors by the oxygen free electron pairs. Therefore, the water oxygen atom is surrounded by four hydrogen atoms, two of these make covalent bonds that make H-bonds and two are oxygen H-bonded. However, these permute at rate in the ps range. When the water molecules are isolated in acetonitrile (MeCN) or acetone (Me(2)CO), only the labile hydrogen atoms make H-bonds with the solvent. The bathochromic shift of the OH stretch bands is then almost 130 cm(-1) with, however, the asymmetric (ν(3)) and symmetric (ν(1)) stretch bands maintained. When more water is added to the solutions, the oxygen lone doublets make H-bonds with the available labile hydrogen atoms from neighboring water molecules. With one bond accepted, the bathochromic shift is further displaced by almost 170 cm(-1). When the second oxygen doublet is filled, another bathochromic shift by almost 100 cm(-1) is observed. The total bathochromic shift is near 400 cm(-1) with a full width at half height of near 400 cm(1). This is the case of pure liquid water. Notwithstanding the shift and the band broadness, the ν(3) and ν(1) band individualities are maintained with, however, added satellite companions that come from the far IR (FIR) absorption. These added to the fundamental bands are responsible for the band broadness and almost featureless shape of the massive OH stretch absorption of liquid water. Comparison of light and heavy water mixture spectra indicates that the OH and OD stretch regions show five different configurations: OH(4); OH(3)D; OH(2)D(2); OHD(3); and OD(4) [J. Chem. Phys. 116, 4626 (2002)]. The comparison of the OH bands of OH(4) with that of OHD(3) indicates that the main component in OHD(3) is ν(OH), whereas in OH(4) two main components are present: ν(3) and ν(1). Similar results are obtained for the OD bands of OD(4) and ODH(3). These results indicate that the C(2) (v) symmetry of H(2)O and D(2)O is preserved in the liquid and aqueous solutions whereas C(s) is that of HDO.     

Intramolecular hydrogen bonding and cooperative interactions in carbohydrates via the molecular tailoring approach

In spite of many theoretical and experimental attempts for understanding intramolecular hydrogen bonding (H-bonding) in carbohydrates, a direct quantification of individual intramolecular H-bond energies and the cooperativity among the H-bonded networks has not been reported in the literature. The present work attempts, for the first time, a direct estimation of individual intramolecular O-H...O interaction energies in sugar molecules using the recently developed molecular tailoring approach (MTA). The estimated H-bond energies are in the range of 1.2-4.1 kcal mol(-1). It is seen that the OH...O equatorial-equatorial interaction energies lie between 1.8 and 2.5 kcal mol(-1), with axial-equatorial ones being stronger (2.0-3.5 kcal mol(-1)). The strongest bonds are nonvicinal axial-axial H-bonds (3.0-4.1 kcal mol(-1)). This trend in H-bond energies is in agreement with the earlier reports based on the water-water H-bond angle, solvent-accessible surface area (SASA), and (1)H NMR analysis. The contribution to the H-bond energy from the cooperativity is also estimated using MTA. This contribution is seen to be typically between 0.1 and 0.6 kcal mol(-1) when H-bonds are a part of a relatively weak equatorial-equatorial H-bond network and is much higher (0.5-1.1 kcal mol(-1)) when H-bonds participate in an axial-axial H-bond network.     

Comparison of hydrogen bonding in 1-octanol and 2-octanol as probed by spectroscopic techniques

Liquid 1-octanol and 2-octanol have been investigated by infrared (IR), Raman, and Brillouin experiments in the 10-90 degrees C temperature range. Self-association properties of the neat liquids are described in terms of a three-state model in which OH oscillators differently implicated in the formation of H-bonds are considered. The results are in quantitative agreement with recent computational studies for 1-octanol. The H-bond probability is obtained by Raman data, and a stochastic model of H-bonded chains gives a consistent picture of the self-association characteristics. Average values of hydrogen bond enthalpy and entropy are evaluated. The H-bond formation enthalpy is ca. -22 kJ/mol and is slightly dependent on the structural isomerism. The different degree of self-association for the two octanols is attributed to entropic factors. The more shielded 2-isomer forms larger fractions of smaller, less cooperative, and more ordered clusters, likely corresponding to cyclic structures. Signatures of a different cluster organization are also evidenced by comparing the H-bond energy dispersion (HBED) of OH stretching IR bands. A limiting cooperative H-bond enthalpy value of 27 kJ/mol is found. It is also proposed that the different H-bonding capabilities may modulate the extent of interaggregate hydrocarbon interactions, which is important in explaining the differences in molar volume, compressibility, and vaporization enthalpy for the two isomers.     

On the structure of the matrix isolated water trimer

Infrared spectra of partially deuterated water trimers have been investigated. It is found that HDO(H(2)O)(2) has a single, bound OD stretching fundamental, (HDO)(2)H(2)O two bound OD stretches. (HDO)(3) has a single, bound OD stretch and (H(2)O)(3) has a pair of bound OH stretches. Ab initio and discrete Fourier transform (DFT) calculations predict that the water trimer has C(1) symmetry with six different, isoenergetic minima. These calculations consequently give three numerically different OD stretches for HDO(H(2)O)(2), six for (HDO)(2)H(2)O, three for (HDO)(3), and three bound OH stretches for (H(2)O)(3). The connection between the observations and the pseudorotation of the trimer is discussed with the help of Wales' pseudorotation model. It is found that pseudorotation is sufficiently fast to average the effective symmetry of the A(3) trimer to C(3h) and to eliminate the difference between the different ab initio minima for A(2)B. The only exception is (H(2)O)(3) where the splitting between the different bound OH stretches is largest. Here a doublet is observed due to incomplete averaging. DFT calculations indicate that the D-bonded form of HDO(H(2)O)(2) is between 50 and 60 cm(-1) more stable than the H-bonded form. The energy difference is determined by differences in zero point vibration energy of intermolecular librations of the two forms. Attempts to measure the energy difference indicate that the energy difference is larger, of the order of 100 cm(-1).     

Contribution of the asymmetric stretch, nu3B1, to the fundamental Raman spectrum of water

The OD-stretching overtone from liquid D2O, 2nu, and the fundamental OD stretch from dilute HDO, both display high-frequency depolarization ratio minima, but the fundamental OD stretch from neat D2O displays a maximum, at the equivalent position. The rhoL minima arises from the decreased depolarization ratio produced by the absence of B1 modes. The fundamentals of HDO are of A species, and the 2nu overtone of D2O only involves A1 species, e.g., 2nu3B1 has A1 species via B1 x B1 = A1. A and A1 modes display small rhoL values which produce minima in rhoL near 2665 cm(-1) for HDO, and near 5250 cm(-1) for the D2O overtone. These minima give way to a depolarization ratio maximum when the depolarized, rhoL = 34, nu3B1 fundamental, makes its appearance in D2O at 2650 cm(-1). Fundamental and overtone depolarization ratios were used to determine the nu3B1 contribution to the depolarization ratio of the fundamental OD stretch; a value of approximately 28% resulted at 2655 cm(-1). Liquid H2O displays completely analogous features; a value of approximately 20% resulted for it at 3660 cm(-1). Nonhydrogen-bonded nu3B1, and more strongly hydrogen-bonded nu3B1, modes are also indicated for D2O and H2O. A rigorous test of the current results can be accomplished by measuring the depolarization ratio of the extraordinarily weak second Raman overtone, 3nu, recently detected for D2O.     

Calculated volume and energy profiles for water exchange on t2g6 rhodium(III) and iridium(III) hexaaquaions: conclusive evidence for an Ia mechanism

An I(a) mechanism was assigned for water exchange on the hexaaquaions Rh(OH(2))(6)(3+) and Ir(OH(2))(6)(3+) on the basis of negative Delta V(++) experimental values (-4.2 and -5.7 cm(3) mol(-1), respectively). The use of Delta V(++) as a mechanistic criterion was open to debate primarily because Delta V(++) could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and I(a)) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH(2))(6)(3+) is 32.2 kJ mol(-1) in favor of the I(a) mechanism (127.9 kJ mol(-1)), as opposed to a D pathway; the value for the I(a) mechanism being close to Delta H(++) and Delta G(++) experimental values (130.5 kJ mol(-1) and 129.9 kJ mol(-1) at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the I(a) pathway (DeltaV(++)(calc) = -3.9 and -3.5 cm(3) mol(-1), respectively, for Rh(3+) and Ir(3+)), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: -1.8 for the D, and -0.9 cm(3) mol(-1) for I(a) mechanism. This finding clarifies the debate about the interpretation of Delta V(++) and unequivocally confirms the occurrence of an I(a) mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.     

Anti-cooperativity of the two water OH groups

This paper presents the results concerning anti-cooperativity effects between two H-bonds of a water molecule. The IR OH stretching band shifts Δν—as a measure of the H-bond energy—are compared for HOD with different bases B of 1:1 complexes B…HOD…Cl₄ (Δν₁₁) and 1:2 complexes B…HOD…B (Δν₁₂). We found that Δν₁₂ of the 1:2 complexes for different bases B are 25% smaller than Δν₁₁ for the 1:1 complexes. Corrections for the solvent shifts are introduced. This effect is in line with different observations concerning cooperativity effects of OH H-bonds by polarization with neighbouring molecules. The reduction of Δν₁₂ in 1:2 complexes can be understood on the assumption of a negative polarization by the first H-bond to the second OH or OD group and is called anti-cooperativity. This anti-effect has been already detected by NMR observation on NH₂.

We already observe a decrease of the CCl₄ solvent shift by van der Waals forces OH…CCl₄ of 1:1 complexes, induced by the H-bond of the other OH or OD group. This decrease is measured by the CCl₄ solvent effect for monomers. This indicates a real negative polarization by the H-bond in 1:1 complexes on the second OH/OD. This experiment establishes the real polarization and excludes the importance of repulsions of the bases as the cause. The dependence of intermolecular forces is known on the polarizability. Our method demonstrates directly the polarization by interactions.

The anti-cooperativity of symmetric complexes B₁…HOD…B₁ by a strong base B₁ can be reduced in unsymmetric 1:2 complexes B₁…HOD…B₂ by weaker bases B₂. This weakening of the anti-cooperativity of the stronger base could be predicted quantitatively. Similarly, the anti-cooperativity of the weaker base B₂ is strengthened in unsymmetric 1:2 complexes by stronger bases B₁.

It is known that H-bonds XH…B can be strengthened by cooperativity with a second H-bond XH…XH…B. They can be weakened for water by anti-cooperativity of two H-bonds B…HOH…B. The H-bond B₁…HO of 1:2 complexes B₁…HOH…B₂ can be weakened if the base strength of B₂ is stronger than of B₁ or strengthened if B₂ is weaker than B₁. Nature may use these possibilities in biochemistry.     

Experimental and theoretical studies of the kinetics of the reactions of OH and OD with 2-methyl-3-buten-2-ol between 300 and 415 K at low pressure

The rate constants for the reactions of OH and OD with 2-methyl-3-buten-2-ol (MBO) have been measured at 2, 3, and 5 Torr total pressure over the temperature range 300-415 K using a discharge-flow system coupled with laser induced fluorescence detection of OH. The measured rate constants at room temperature and 5 Torr for the OH + MBO reaction in the presence of O2 and the OD + MBO reaction are (6.32 +/- 0.27) and (6.61 +/- 0.66) x 10(-11) cm3 molecule(-1) s(-1), respectively, in agreement with previous measurements at higher pressures. However, the rate constants begin to show a pressure dependence at temperatures above 335 K. An Arrhenius expression of k0 = (2.5 +/- 7.4) x 10(-32) exp[(4150 +/- 1150)/T] cm6 molecule(-2) s(-1) was obtained for the low-pressure-limiting rate constant for the OH + MBO reaction in the presence of oxygen. Theoretical calculations of the energetics of the OH + MBO reaction suggest that the stability of the different HO-MBO adducts are similar, with predicted stabilization energies between 27.0 and 33.4 kcal mol(-1) relative to the reactants, with OH addition to the internal carbon predicted to be 1-4 kcal mol(-1) more stable than addition to the terminal carbon. These stabilization energies result in estimated termolecular rate constants for the OH + MBO reaction using simplified calculations based on RRKM theory that are in reasonable agreement with the experimental values.     

H-bond network in amino acid cocrystals with H2O or H2O2. The DFT study of serine-H2O and serine-H2O2

The structure, IR spectrum, and H-bond network in the serine-H(2)O and serine-H(2)O(2) crystals were studied using DFT computations with periodic boundary conditions. Two different basis sets were used: the all-electron Gaussian-type orbital basis set and the plane wave basis set. Computed frequencies of the IR-active vibrations of the titled crystals are quite different in the range of 10-100 cm(-1). Harmonic approximation fails to reproduce IR active bands in the 2500-2800 frequency region of serine-H(2)O and serine-H(2)O(2). The bands around 2500 and 2700 cm(-1) do exist in the anharmonic IR spectra and are caused by the first overtone of the OH bending vibrations of H(2)O and a combination vibration of the symmetric and asymmetric bendings of H(2)O(2). The quantum-topological analysis of the crystalline electron density enables us to describe quantitatively the H-bond network. It is much more complex in the title crystals than in a serine crystal. Appearance of water leads to an increase of the energy of the amino acid-amino acid interactions, up to ~50 kJ/mol. The energy of the amino acid-water H-bonds is ~30 kJ/mol. The H(2)O/H(2)O(2) substitution does not change the H-bond network; however, the energy of the amino acid-H(2)O(2) contacts increases up to 60 kJ/mol. This is caused by the fact that H(2)O(2) is a much better proton donor than H(2)O in the title crystals.     

Analysis of multiple H-bond interactions in self-assembly between polyurethane with pendent carboxyl and poly(4-vinylpyridine)

As an extension study, FTIR and molecular simulation methods were combined in the present paper to analyze the H-bond interactions resulting from multiple donors and acceptors that have led to self-assembly based on segmented polyurethane with carboxyl (PUc) and poly(4-vinylpyridine) (P4VP) in our previous work. Of them, FTIR was used to analyze the H-bonding types and interactions as well as their changes before and after self-assembly; molecular mechanics (MM/COMPASS) was used to study the effect of possible conformations on the H-bonds involved and analyze the most probable H-bond patterns; quantum mechanics (QM/B3LYP) was used to help confirm the experimental FTIR band assignments and calculate the H-bond energy. It was found that two types of H-bonds exist, namely, COOH...P4VP (type I) and (OCO)NH...P4VP (type II), based on OH and NH as the strong donors in the interaction between PUc and P4VP. Strong evidence has been obtained for a type II H-bond, which is the specialty in PUc/P4VP assembly. The type I and type II H-bonding energies are -11.293 and -7.150 kcal/mol, respectively. The forming probability of the type I H-bond accounts for 95.87%, while that of the type II H-bond is 4.13%, showing the primary driving force for the assembly based on PUc and P4VP is still the H-bond between COOH and P4VP, yet the H-bonds based on NH and pyridyl in P4VP cannot be ignored.     

Low-lying electronic states of FeNC and FeCN: a theoretical journey into isomerization and quartet/sextet competition

With several levels of multireference and restricted open-shell single-reference electronic structure theory, optimum structures, relative energetics, and spectroscopic properties of the low-lying (6)Delta, (6)Pi, (4)Delta, (4)Pi, and (4)Sigma(-) states of linear FeNC and FeCN have been investigated using five contracted Gaussian basis sets ranging from Fe[10s8p3d], C/N[4s2p1d] to Fe[6s8p6d3f2g1h], C/N[6s5p4d3f2g]. Based on multireference configuration interaction (MRCISD+Q) results with a correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis set, appended with core correlation and relativistic corrections, we propose the relative energies: T(e)(FeNC), (6)Delta(0)<(6)Pi (2300 cm(-1))<(4)Delta (2700 cm(-1))<(4)Pi (4200 cm(-1))<(4)Sigma(-); and T(e)(FeCN), (6)Delta(0)<(6)Pi (1800 cm(-1))<(4)Delta (2500 cm(-1))<(4)Pi (2900 cm(-1))<(4)Sigma(-). The (4)Delta and (4)Pi states have massive multireference character, arising mostly from 11sigma-->12sigma promotions, whereas the sextet states are dominated by single electronic configurations. The single-reference CCSDT-3 (coupled cluster singles and doubles with iterative partial triples) method appears to significantly overshoot the stabilization of the quartet states provided by both static and dynamical correlation. The (4,6)Delta and (4,6)Pi states of both isomers are rather ionic, and all have dipole moments near 5 D. On the ground (6)Delta surface, FeNC is predicted to lie 0.6 kcal mol(-1) below FeCN, and the classical barrier for isocyanide/cyanide isomerization is about 6.5 kcal mol(-1). Our data support the recent spectroscopic characterization by Lei and Dagdigian [J. Chem. Phys. 114, 2137 (2000)] of linear (6)Delta FeNC as the first experimentally observed transition-metal monoisocyanide. Their assignments for the ground term symbol, isotopomeric rotational constants, and the Fe-N omega(3) stretching frequency are confirmed; however, we find rather different structural parameters for (6)Delta FeNC:r(e)(Fe-N)=1.940 A and r(N-C)=1.182 A at the cc-pVQZ MRCISD+Q level. Our results also reveal that the observed band of FeNC originating at 27 236 cm(-1) should have an analog in FeCN near 23 800 cm(-1) of almost equal intensity. Therefore, both thermodynamic stability and absorption intensity factors favor the eventual observation of FeCN via a (6)Pi<--(6)Delta transition in the near-UV.     

Hydrogen bonding in Schiff bases--NMR, structural and experimental charge density studies

A series of sixteen Schiff bases (derivatives of salicylaldehydes and aryl amines) was studied to reveal the influence of substituents and the length of the linker on the properties of the H-bonding formed. In theory, two groups of compounds, derivatives of 2-(2-hydroxybenzylidenoamine)phenol) and 2-hydroxy-N-(2-hydroxybenzylideno)benzylamine, can form different types of H-bonds using one or two hydroxyl groups present in the molecules. Two other groups of compounds, derivatives of 4-(2-hydroxybenzylidenoamine)phenol and N-(2-hydroxybenzyideno)benzylamine, can form only one type of H-bond. It was confirmed by (15)N and (13)C NMR experiments, that in all cases only traditional, H-bonded six-membered chelate rings were formed. The positions of the hydrogen atom in the rings depend on the substituent and phase. Generally, the OH H-bond form dominates in solution, with exception of the nitro derivatives, where the NH tautomer is present. In the solid state the tautomeric equilibrium is strongly shifted to the NH form. Only for the 5-Br derivative of one compound was the reverse relationship found. According to the results of experimental charge density investigations, two intramolecular H-bonds in the 5-methoxy derivative of 2-hydroxy-N-(2'-hydroxybenzylideno)benzylamine) differ significantly in terms of charge density properties. The intra- and intermolecular H-bonds formed by the deprotonated oxygen atom from 2-OH group are strong, with significant charge density concentration at the bond critical point and a straight, well-defined bond path, whereas the second intramolecular H-bond formed by the oxygen atom from the 2'-OH group is quite weak, with ca. five times smaller charge density concentration than in the previous case and a bent bond path. In terms of energy densities, the latter H-bond appears to be a non-bonding interaction, with total energy density being slightly positive. In terms of source contributions to the density at the H-bond critical point from the atoms involved, the intermolecular, linear H-bond is very strong and charge-assisted in the source function classification, the N(1)-H(1N)···O(1) H-bond is medium-strength, while the third H-bond is extremely weak.     

Synthesis and physical properties of K4[Fe(C5O5)2(H2O)2](HC5O5)2·4H2O (C5O5(2-) = croconate): a rare example of ferromagnetic coupling via H-bonds

The reaction of the croconate dianion (C(5)O(5))(2-) with a Fe(III) salt has led, unexpectedly, to the formation of the first example of a discrete Fe(II)-croconate complex without additional coligands, K(4)[Fe(C(5)O(5))(2)(H(2)O)(2)](HC(5)O(5))(2)·4H(2)O (1). 1 crystallizes in the monoclinic P2(1)/c space group and presents discrete octahedral Fe(II) complexes coordinated by two chelating C(5)O(5)(2-) anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of trans-diaquabis(croconato)ferrate(II) complexes and layers containing the monoprotonated croconate anions, HC(5)O(5)(-), and noncoordinated water molecules. Both kinds of layers are directly connected through a hydrogen bond between an oxygen atom of the coordinated dianion and the protonated oxygen atom of the noncoordinated croconate monoanion. A H-bond network is also formed between the coordinated water molecule and one oxygen atom of the coordinated croconate. This H-bond can be classified as strong-moderate being the O···O bond distance (2.771(2) ?) typical of moderate H-bonds and the O-H···O bond angle (174(3)°) typical of strong ones. This H-bond interaction leads to a quadratic regular layer where each [Fe(C(5)O(5))(2)(H(2)O)(2)](2-) anion is connected to its four neighbors in the plane through four equivalent H-bonds. From the magnetic point of view, these connections lead to an S = 2 quadratic layer. The magnetic properties of 1 have been reproduced with a 2D square lattice model for S = 2 ions with g = 2.027(2) and J = 4.59(3) cm(-1). This model reproduces quite satisfactorily its magnetic properties but only above the maximum. A better fit is obtained by considering an additional antiferromagnetic weak interlayer coupling constant (j) through a molecular field approximation with g = 2.071(7), J = 2.94(7) cm(-1), and j = -0.045(2) cm(-1) (the Hamiltonian is written as H = -JS(i)S(j)). Although this second model might still be improved since there is also an extra contribution due to the presence of ZFS in the Fe(II) ions, it confirms the presence of weak ferromagnetic Fe-Fe interactions through H-bonds in compound 1 which represents one of the rare examples of ferromagnetic coupling via H-bonds.     

Volume Profile for Substitution in Labile Chromium(III) Complexes: Reactions of Aqueous [Cr(Hedta)OH(2)] and [Cr(edta)](-) with Thiocyanate Ion

A procedure is given for correcting optical absorbance measurements made at variable pressure with a le Noble-Schlott ("pillbox") cell for the inner sleeve wall thickness. With this technique, the molar volume change for the acid ionization of aqueous [Cr(Hedta)OH(2)] was found to be +5.1 +/- 0.6 cm(3) mol(-)(1) (0-200 MPa, 25.0 degrees C, ionic strength 1.0 mol L(-)(1) HClO(4)/NaClO(4)), an anomalous positive value which implies a change from quinquedentate to predominantly sexidentate edta and expulsion of the coordinated water on ionization. For thiocyanate substitution into labile [Cr(Hedta)OH(2)], high pressure stopped-flow measurements gave the volume of activation as -7.8 +/- 0.9 cm(3) mol(-)(1) and the volume of reaction as +3 +/- 2 cm(3) mol(-)(1), while for the reaction of [Cr(edta)](-) with NCS(-) the activation volume is -13.6 +/- 0.6 cm(3) mol(-)(1) (same conditions). These and other data support the notion that the anomalous substitutional lability of Cr(III)(edta) complexes relative to typical Cr(III) species is due to activation by transient chelation of the pendant arm of quinquedentate edta.     

Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne

The singlet ground ((approximate)X(1)Sigma1+) and excited (1Sigma-,1Delta) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum ((approximate)X(1)Sigma+HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of omega1=3344 cm(-1), omega2=689 cm(-1), and omega3=1298 cm(-1). Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol(-1) above the global minimum HCP. The dissociation energy D0[HCP((approximate)X(1)Sigma+)-->H(2S)+CP(X2Sigma+)] of HCP is predicted to be 119.0 kcal mol(-1), which is very close to the experimental lower limit of 119.1 kcal mol(-1). Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, 1Sigma- -->1A", with excitation energies Te of 101.4 kcal mol(-1) (1A"HCP), 104.6 kcal mol(-1)(1Sigma-HCP), 122.3 kcal mol(-1)(1A" HPC), and 171.6 kcal mol(-1)(1Sigma-HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A" state with a predicted bond angle of 129.5 degrees compare well with the experimentally reported first singlet state ((approximate)A1A"). The excitation energy predicted for this excitation is T0=99.4 kcal mol(-1) (34 800 cm(-1),4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol(-1)(34 746 cm(-1),4.308 eV). For the second lowest-lying excited singlet surface, 1Delta-->1A', four stationary points were found with Te values of 111.2 kcal mol(-1) (2(1)A' HCP), 112.4 kcal mol(-1) (1Delta HPC), 125.6 kcal mol(-1)(2(1)A' HCP), and 177.8 kcal mol(-1)(1Delta HPC). The predicted CP bond length and frequencies of the 2(1)A' state with a bond angle of 89.8 degrees (1.707 A, 666 and 979 cm(-1)) compare reasonably well with those for the experimentally reported (approximate)C(1)A' state (1.69 A, 615 and 969 cm(-1)). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol(-1) and 89.8 degrees versus experimental values of 115.1 kcal mol(-1) and 113 degrees. of 115.1 kcal mol(-1) and 113 degrees.     

Electronic properties of multifurcated bent hydrogen bonds CH3...Y and CH2...Y

H-bonding angle angleYHX has an important effect on the electronic properties of the H-bond Y...HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H3CZ (Z = F, Cl, Br), H2CO and H2CF2 with the proton acceptors Cl(-) and Br(-) at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7-12 kcal mol(-1), and large blue shifts, delta r(HC) = -0.0025 --0.006 A and delta v(HC) = 30-90 cm(-1). The natural bond orbital analysis shows that the blue shifts of these H-bonds Y...HnCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n(Y) -->sigma*(CZ), in that the electron density from n(Y) of the proton acceptor is transferred not to sigma*(CH), but to sigma*(CZ) of the donor; intramolecular hyperconjugation n(Z) -->sigma*(CH), in that the electron density in sigma*(CH) comes back to n(Z) of the donor such that the occupancy in sigma*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y...H2CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y...H3CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon.