Infrared study of 1-octanol liquid structure

A detailed analysis of the temperature behavior of the OH stretching bands in infrared (IR) and near infrared (NIR) spectra has been performed for neat liquid 1-octanol. Spectral features have been interpreted using a three-state model, based on different typologies of OH oscillator. The H-bond energy dispersion has been calculated from IR data. Structural properties such as self-association degrees and H-bonding enthalpy formations have been reported. Results have been compared with recent Molecular Dynamic simulation studies. From temperature induced intensity variations the average H-bonding cooperative energy is evaluated. For the 1-octanol liquid system a break point temperature is detected around 50 °C. The existence of two temperature domains connected with self-association properties is in agreement with the behavior of relaxation processes evidenced in previous light scattering studies. These results evidence the correlation existing among structural and dynamical properties in complex self-associating liquid systems.     

Theoretical and experimental study of the vibrational spectrum of N-acetyl-L-alanine

The energies, vibrational frequencies and IR intensities of cis- and trans-N-acetyl-L-alanine (NAAL) are computed using the density functional theory (B3LYP) combined with the 6-311G(d, p) basis set. The trans conformer is characterized by an intramolecular NH ... O hydrogen bond leading to the formation of a five-membered ring and is by 23 kJ mol(-1) more stable than the cis conformer. The difference between the vibrational frequencies and IR intensities computed for the two conformers is discussed. The IR spectra at different temperatures and the Raman spectra of solid NAAL and its deuterated counterpart are investigated and discussed. The frequencies of the v(OH) vibration and the isotopic ratio suggest the formation of short OH ... O hydrogen bonds in the solid state. The NH group seems also to be involved in a weak hydrogen bond.     

An ab-initio study of the C3H6-HX, C2H4-HX and C2H2-HX hydrogen-bonded complexes with X=F or Cl.

MP2/6-31G** ab-initio molecular orbital calculations have been performed to obtain geometries, H-bond energies and vibrational properties of the C3H6-HX, C2H4-HX and C2H2-HX H-bonded complexes with X=F or Cl. The more pronounced effects on the structural parameters of the isolated molecules due to complexation are verified to the CC and HX bond lengths, which are directly involved in the H-bond formation. They are increased after complexation. The calculated H-bond lengths for the hydrogen complexes for X=F are shorter than those for x-Cl by about 0.55 A, whereas the corresponding experimental value is 0.58 A. The H-bond energies are essentially determined by the nature of the proton donor molecule. For X=F, the AE mean value is 20 kJ/mol, whereas it is approximately 14.5 kJ/mol for X-Cl. The H-bond energies including zero-point corrections show a good correlation with the H-bond lengths. The more pronounced effect on the normal modes of the isolated molecules after complexation occurs to the H-X stretching mode. The H-X stretching frequency is shifted downward, whereas its IR intensity is much enhanced upon H-bond formation. The new vibrational modes arising from complexation show several interesting features.     

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.     

Cooperative effect in hydrogen bonding of N-methylacetamide in carbon tetrachloride solution confirmed by NMR and IR spectroscopies

Recently we carried out ab initio molecular orbital calculations of the hydrogen bond energies in the dimers and trimers of N-methylacetamide (NMA), and found the existence of a cooperative effect in the hydrogen bonding, by which formation of one hydrogen bond in a hydrogen-bonded chain enhances the strength of another hydrogen bond. In order to confirm the existence of such a cooperative effect, we have determined experimentally the enthalpy changes occurring upon hydrogen bonding of NMA in CCl4 solution. First, following the method proposed by us, the population fractions of free (non-bonded) NH protons are obtained from the observed amide proton NMR chemical shifts and the IR intensities of the free NH stretching bands. Next, the enthalpy changes are evaluated by analyzing the equilibrium between the free and bonded states of an NH proton. In this analysis, the existence of the CCl4 solvent is taken into account. The stabilization energy of hydrogen bonds in a trimer, as compared with twice the hydrogen bond energy in a dimer, is 5.4 kJ mol(-1), in good agreement with the calculated value (5.9 kJ mol(-1)). This result provides experimental confirmation of the existence of a cooperative effect in hydrogen bonding.     

Isomer- and species-selective infrared spectroscopy of jet-cooled 7H- and 9H-2-aminopurine and 2-aminopurine·H2O clusters

The infrared (IR) spectra of the supersonic-jet cooled 9H- and 7H-tautomers of 2-aminopurine (2AP) and of the 9H-2-aminopurine·H(2)O monohydrate clusters have been measured by mass- and species-selective IR-UV double resonance spectroscopy in the 3200-3900 cm(-1) region, covering the N-H and O-H stretching vibrations. The spectra are complemented by density functional (B3LYP and PW91) and by second-order M?ller-Plesset (MP2) calculations of the electronic energies and vibrational frequenciesof the respective 2AP tautomers and clusters. The 9H- and 7H-2-aminopurine tautomers were definitively identified by the shifts of their NH and NH(2) symmetric and asymmetric stretching frequencies and by comparison to the B3LYP/TZVP calculated IR spectra. The H-bond topologies of the two previously observed 9H-2-aminopurine·H(2)O isomers (Sinha. R. K.; et al. J. Phys. Chem. A2011, 115, 6208) are definitively identified as the "sugar-edge" isomer A and the "trans-amino-bound" isomer B by comparing their IR spectra to the calculated frequencies and IR intensities of the cluster isomers A, B, C, and D, as well as to the IR spectrum of 9H-2AP. The sugar-edge isomer A involves N9-H···OH(2) and HOH···N3 hydrogen bonds and is predicted to be the most stable form. The amino-bound isomer B involves NH(2)···OH(2) and HOH···N1 hydrogen bonds and is calculated to lie 2.5 kJ/mol above isomer A. The H-bond topology of the "cis-amino-bound" isomer C is symmetrically related to isomer B, with a hydrogen bond to the N3 of the pyrimidine group. However, it is calculated to lie 7 kJ/mol above isomer A and indeed is not observed in the supersonic jet. Isomer D involves a single H-bond to the N7 position, is predicted to be 14 kJ/mol above A and is therefore not observed.     

Hydrogen bonding in liquid and supercritical 1-octanol and 2-octanol assessed by near and midinfrared spectroscopy

The near and midinfrared spectra of 1-octanol (and 2-octanol) have been measured along the liquid-gas coexistence curve from room temperature up to the critical point and in the supercritical domain along the isotherm T=385 degrees C (and T=365 degrees C) above the critical point of both 1-octanol and 2-octanol for pressure ranging from 0.5 up to 15 MPa. The density values of SC 1- and 2-octanol have been estimated by analysing the near infrared (NIR) spectra in the 3nu(a)(CH) region. A quantitative analysis of the absorption band associated with the OH stretching vibration [nu(OH)] and its first and second overtones [2nu(OH) and 3nu(OH)] was carried out in order to estimate the percentage of "free" OH groups in both alcohols in the whole thermodynamic domain investigated here. Very consistent results have been obtained from the independent analysis of these three different absorption bands which gave us a good confidence in the degree of hydrogen bonding reported here for 1- and 2-octanol. Thus, the percentage of free OH groups which is around 5% in liquid 1-octanol under ambient conditions strongly increase up to 70%-80% at a temperature of about 340 degrees C. Then, in the supercritical domain, upon a decrease of the density from 0.4 to 0.1 g cm(-3), the fraction of free hydroxyl groups is nearly constant presenting a plateaulike regime around 80%. As the density decreases again, this plateau regime is followed by a further increase of X(nb) which reaches a value of 96% for the system in the gaseous phase (0.01 g cm(-3); P=0.45 MPa). Finally, it comes out from this study that the percentage of free OH groups is always greater in 2-octanol than in 1-octanol at the same density.     

Structure and vibrational assignment of the enol form of 1,1,1-trifluoro-2,4-pentanedione

Molecular structure of 1,1,1-trifluoro-pentane-2,4-dione, known as trifluoro-acetylacetone (TFAA), has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of acetylacetone (AA) and hexafluoro-acetylacetone (HFAA). The harmonic vibrational frequencies of both stable cis-enol forms were calculated at B3LYP level of theory using 6-31G** and 6-311++G** basis sets. We also calculated the anharmonic frequencies at B3LYP/6-31G** level of theory for both stable cis-enol isomers. The calculated frequencies, Raman and IR intensities, and depolarization ratios were compared with the experimental results. The energy difference between the two stable cis-enol forms, calculated at B3LYP/6-311++G**, is only 5.89 kJ/mol. The observed vibrational frequencies and Raman and IR intensities are in excellent agreement with the corresponding values calculated for the most stable conformation, 2TFAA. According to the theoretical calculations, the hydrogen bond strength for the most stable conformer is 57 kJ/mol, about 9.5kJ/mol less than that of AA and about 14.5 kJ/mol more than that of HFAA. These hydrogen bond strengths are consistent with the frequency shifts for OH/OD stretching and OH/OD out-of-plane bending modes upon substitution of CH(3) groups with CF(3) groups. By comparing the vibrational spectra of both theoretical and experimental data, it was concluded that 2TFAA is the dominant isomer.     

Solute-solvent interactions in cryosolutions: a study of halothane-ammonia complexes

The formation of C-H···N bonded complexes of halothane with ammonia has been studied using infrared and Raman spectroscopy of solutions in the liquid rare gases argon, krypton and xenon, of supersonic jet expansions and of room temperature vapor phase mixtures. For the solutions and for the vapor phase experiments, the formation of complexes with 1:1 and 1:2 stoichiometry was observed. The complexation enthalpy for the 1:1 complex was determined to be -20 (1) kJ mol(-1) in the vapor phase, -17.0 (5) kJ mol(-1) in liquid xenon and -17.3 (6) kJ mol(-1) in liquid krypton. For the 1:2 complex in liquid xenon, the complexation enthalpy was determined to be -31.5 (12) kJ mol(-1). Using the complexation enthalpies for the vapor phase and for the solutions in liquid xenon and krypton, a critical assessment is made of the Monte Carlo Free Energy Perturbation approach to model solvent influences on the thermodynamical properties of the cryosolutions. The influences of temperature and solvent on the complexation shifts of the halothane C-H stretching mode are discussed.     

Structural and spectroscopic study of the Br2...3-Br-pyridine complex by DFT calculations

The structure and the Raman vibrational spectrum of the complex Br(2)...3-Br-pyridine are determined by DFT calculations using different parametrizations. The calculations are performed taking into account the effects of the dichloromethane as solvent by the CPCM method. A value of 39 kJ mol(-1) for the formation enthalpy and of 1 kJ mol(-1) for the formation free energy at room temperature in presence of the solvent is found. The predicted Raman spectrum is compared with the experimental one and the essential features of the spectrum are well reproduced by the B3LYP parametrization. The intensity changes of the bands when going from the free moieties to the complex are also generally correctly predicted by the theoretical treatment.     

Raman H-bond pair volume for water

The dispersion of the H-bond pair volume Delta V over the decoupled OD and coupled OH-stretching contours from HDO in H(2)O was determined from Raman intensities at pressures to 9700 bar at 301 K. The dispersion of Delta V was determined from -RT[partial differential ln(I(i)/I(REF))/ partial differential P](T) versus omega (in cm(-1)), where i refers to omega's over the stretching contours and I(REF) refers to the reference intensity at the isosbestic frequency. The maximum H-bond pair volume (defined for breakage) is 1.4+/-0.1 cm(3)/mol H-bond, which corresponds to the volume difference between a large dispersion maximum at 2,675 cm(-1) (near the OD stretch omega of HDO in dense supercritical water) and a large, broad minimum centered near 2,375 cm(-1) (just below the OD stretch omega of HDO in lda ice). The average DeltaV is 0.71+/-0.10 cm(3)/mol H-bond. Other minima near 2,625 cm(-1) (OD) and 3550 cm(-1) (OH) refers to bent H-bonds whose angles are approximately 150 deg. Isothermal pressurization of water lowers the molal volume by decreasing the concentration of long, weak H-bonds, and increasing the concentrations of bent H-bonds and short, strong, linear H-bonds. Such bending, shortening, and strengthening produces freezing to ice VI near 10 kbar at 301 K. The isobaric temperature derivative of the maximum H-bond volume is (partial differential Delta V/partial differential T)(P)< or =(2-5) x 10(-3) cm(3)/deg mol H-bond. The OH enthalpy dispersion curve for saturated NaBF(4) in water, yields a large maximum at 3,530-3,540 cm(-1) indicating that BF(4) (-) interacts preferentially with the dangling or "free" OH groups of water thus producing weak, strongly bent H-bonds having angles similar to those of the 3,550 cm(-1) high-pressure H-bond bending feature.     

Vibrational spectra, co-operative intramolecular hydrogen bonding and conformations of calix[4]arene and thiacalix[4]arene molecules and their para-tert-butyl derivatives

The IR and Raman spectra and conformations of calix[4]arene, thiacalix[4]arene and their p-tert-butyl derivatives have been analysed within the framework of scaled quantum mechanics (SQM). It is shown that the introduction of four p-tert-Bu groups into the calixarene molecules influences the relative energies of their conformers and the enthalpy of the cooperative intramolecular H-bonding (DeltaH(intra)) almost negligibly. DeltaH(intra), evaluated from Iogansen's rule, amounts to approximately 26-28 kcal mol(-1) for the calixarenes and approximately 20-21 kcal mol(-1) for the thiacalixarenes, which essentially exceeds the enthalpies of non-cooperative H-bonds formed by related phenols. As a result of this strong bonding, bands of stretching, bending and torsion vibrations of an eight-membered cyclic system (OH...)4 arise, e.g., two delta(OH)4 bands are observed in the IR spectra of the most highly symmetric C4 cone conformations of calix[4]arene and thiacalix[4]arene. The "duplication" of the number of OH infrared bands is a good new indicator of cooperativity of intramolecular H-bonding of the calixarenes.     

1JCH correlates with alcohol hydrogen bond strength

The strength of the H-bond donation by alcohols is reflected in the carbon-hydrogen bond of the H-C-O-H functional group. The one-bond 13C-1H spin-spin coupling constant of hexafluoroisopropanol (HFIP) correlates with the strength of the H-bond in various HFIP-amine complexes with a slope of approximately -0.2 Hz in 1JCH per approximately 1 kJ mol(-1) increase in the H-bond enthalpy. The decrease in 1JCH is attributed to an increased overlap of the H-bonding sigma orbital with the antibonding sigma orbitals of the vicinal C-H bonds.     

Thermodynamic Calculation on the Formation of Titanium Hydride

A modified Miedema model, using interrelationship among the basic properties of elements Ti and H, is employed to calculate the standard enthalpy of formation of titanium hydride TiHx (1≤x≤2). Based on Debye theories of solid thermal capacity, the vibrational entropy, as well as electronic entropy, is acquired by quantum mechanics and statistic thermodynamics methods, and a new approach is presented to calculate the standard entropy of formation of TiH2. The values of standard enthalpy of formation of TiHx decrease linearly with increase of x. The calculated results of standard enthalpy, entropy, and free energy of formation of TiH2 at 298.16 K are -142.39 kJ/mol, -143.0 J/(mol?K) and -99.75 kJ/mol, respectively, which is consistent with the previously-reported data obtained by either experimental or theoretic     

Cyano- and isocyanotris(trifluoromethyl)borates: syntheses, spectroscopic properties, and solid state structures of K[(CF3)3BCN] and K[(CF3)3BNC

A two step synthesis to the isocyanotris(trifluoromethyl)borate anion, [(CF3)3BNC]-, and its isomerization to the cyanotris(trifluoromethyl)borate anion, [(CF3)3BCN]-, at temperatures above 150 degrees C are presented. In the first step (CF3)3BNCH was obtained by reacting (CF3)3BCO with hydrogen cyanide followed by deprotonation of the HCN adduct with Li[N(SiMe3)2] in toluene. The thermal behavior of K[(CF3)3BNC] and K[(CF3)3BCN] were investigated by differential scanning calorimetry (DSC), and K[BF4] was identified as a major solid decomposition product. The enthalpy of the isocyanide-cyanide rearrangement, deltaH(iso) = -35 +/- 4 kJ mol(-1), was obtained from DSC measurements, and the activation energy, E(a) = 180 +/- 20 kJ mol(-1), from kinetic measurements. The isomerization was modeled as an intramolecular reaction employing DFT calculations at the B3LYP/6-311+G(d) level of theory yielding a reaction enthalpy of deltaH(iso) = -36.1 kJ mol(-1) and an activation energy of E(a) = 155.7 kJ mol(-1). The solid-state structures of K[(CF3)3BNC] and K[(CF3)3BCN] were determined by single-crystal X-ray diffraction. Both salts are isostructural and crystallize in the orthorhombic space group Pnma (no. 62). In the crystals the borate anions possess C(s) symmetry, while for the energetic minimum C3 symmetry is predicted by DFT calculations. The borate anions have been characterized by IR and Raman spectroscopy as well as by NMR spectroscopy. The assignment of the IR and Raman bands is supported by their calculated wavenumbers and intensities. The spectroscopic and structural properties of both borate anions are compared to the properties of the isoelectronic borane carbonyl (CF3)3BCO and the [B(CF3)4]- anion as well as to those of other related species.     

Selective guest encapsulation by a cobalt-assembled cage molecule

Metal-assembled resorcinarene-based cages enclose space and entrap organic molecules from water. Addition of cobalt(II) ions to a neutral, aqueous solution of a resorcinarene that has iminodiacetic acids attached to its upper rim results in the formation of cages. These cages not only entrap organic molecules, but they do so in a selective manner. Guests with optimum size, shape, and polarity are preferentially entrapped. For example, selection of p-xylene is twenty thousand times more favorable than that of m-xylene. The enthalpy of resorcinarene deprotonation and cage formation was calculated by performing calorimetry studies and ranged from -305 to -348 kJ mol(-1). The change in enthalpy of guest encapsulation varied by as much as 43 kJ mol(-1). The differences in change in free energy of guest encapsulation varied by -16 kJ mol(-1). The changes in enthalpy and free energy of guest encapsulation were used to calculate the changes in entropy, which ranged from -97 to +37 J mol(-1) K(-1). An enthalpy-entropy compensation of guest encapsulation was observed.     

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.     

Temperature dependence of associative equilibria of DMSO according to raman scattering spectra

Raman scattering spectra of dimethyl sulfoxide (DMSO) are studied in the area of the line corresponding to symmetric CSC stretching vibrations of the molecule. It is established that this line is composed of a low-frequency component that corresponds to the vibrations of monomeric molecules and a high-frequency component that corresponds to the vibrations of DMSO dimers. Values of self-association equilibrium constants K _a varying in range from 0.20 (23°C) to 0.081 (100°C) are obtained. Since the intensities of the respective components of the line contour are proportional to the compound’s concentrations, the enthalpy of DMSO self-association (ΔH = ?11.7 ± 0.9 kJ/mol) is determined from the temperature dependences.     

Energetics of the N-O bonds in 2-hydroxyphenazine-di-N-oxide

The standard enthalpy of formation and the enthalpy of sublimation of crystalline 2-hydroxyphenazine-di-N-oxide, at T = 298.15 K, were determined from isoperibol static bomb combustion calorimetry and from Knudsen effusion experiments, as -76.7 +/- 4.2 kJ.mol(-1) and 197 +/- 5 kJ.mol(-1), respectively. The sum of these two quantities gives the standard enthalpy of formation in the gas-phase for this compound, delta(f)H(m)degrees(g) = 120 +/- 6 kJ.mol(-1). This value was combined with the gas-phase standard enthalpy of formation for 2-hydroxyphenazine retrieved from a group estimative method yielding the mean (N-O) bond dissociation enthalpy, in the gas-phase, for 2-hydroxyphenazine-di-N-oxide. The result obtained with this strategy is (DH(m)degrees (N - O)) = 263 +/- 4 kJ.mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value, 265 kJ.mol(-1).