IR Spectra Of Solutions Research Articles

Hydrogen bonding effects in perfluorinated polyamides: An investigation based on infrared spectroscopy and density functional theory calculations

The IR spectra of solutions of a perfluorinated polyamide in different solvents are analyzed in the spectral range of “free NH” stretching modes, in order to study the effects of hydrogen bonding in determining the unusual spectral behaviour observed for these systems. The presence of different bands in the spectral range associated to NH bonds with the H atom not involved in hydrogen bonding and their evolution with the polymer concentration has shown that free NH bonds have a different nature depending on the occurrence of hydrogen bonding on neighbour atoms and on the configuration of the molecular complexes in which they are involved. These differences have been interpreted on the basis of DFT (B3LYP/6-311++G**) computations on suitable molecular models that allowed a qualitative and quantitative rationalization of the chemical–physical effects involved. The computational analysis points out the subtle intramolecular and intermolecular vibrational effects occurring in these systems and brings to an interpretation and assignment of the IR spectra which goes over the interpretations usually adopted when dealing with the vibrational spectroscopy of hydrogen bonded polymers and molecules.

Investigation of the [Cp*Mo(PMe3)3H]n+ (n = 0, 1) Redox Pair: Dynamic Processes on Very Different Time Scales

The compound [Cp*Mo(PMe3)3H] (1) is reversibly oxidized at E1/2 = -1.40 V vs ferrocene in MeCN. Its oxidation with Cp2FePF6 yields thermally stable [Cp*Mo(PMe3)3H]PF6 (2), which has been isolated and characterized by IR and EPR spectroscopy and by single-crystal X-ray diffraction. The 1H and 31P NMR spectra of 1 show two types of PMe3 ligands in a 1:2 ratio at low temperature, but only one average signal at room temperature, with activation parameters of DeltaH++ = 11.7(3) kcal mol-1 and DeltaS++ = -3(1) eu for the exchange process. Although only one species is evidenced by NMR for 1 and by EPR for 2, the solution IR spectra of each complex show two bands in the v(Mo-H) region (1, major at 1794 cm-1 and minor at ca. 1730 cm-1; 2, ca. 1800 and 1770 cm-1 with approximately equal intensity), the position and relative intensity being little dependent on the solvent. A thorough DFT investigation suggests that these are different rotamers involving different relative orientations of the Cp* ring and the PMe3 ligands in these complexes. This ring rotation process is very rapid on the NMR and EPR time scale but slow on the IR time scale. The X-ray data and the theoretical calculations suggest the presence of weak Mo-H...F interactions in compound 2. The possibility of PMe3 dissociation, as well as other intramolecular rearrangements, for 1 and 2 is excluded by experimental and computational studies. Protonation of 1 yields [Cp*Mo(PMe3)3H2]+ (3), which also reveals a dynamic process interconverting the two inequivalent H ligands and the three PMe3 ligands (two sets in a 1:2 ratio in the frozen structure) on the NMR time scale (activation parameters of DeltaH++ = 9.3(1) kcal/mol and DeltaS++ = -4.1(4) eu). A DFT study suggests that this exchange process occurs via a low-energy symmetric dihydride intermediate and not through a dihydrogen complex.

Polymer‐Solvent Interactions in Solutions of Thermoresponsive Polymers Studied by NMR and IR Spectroscopy

AbstractSummary: NMR relaxation and diffusion coefficient measurements revealed that a portion of water molecules is bound in mesoglobules formed in poly(N‐isopropylmethacrylamide) (PIPMAm) and poly(vinyl methyl ether) (PVME) aqueous solutions above the LCST, with fast exchange between bound and free states (residence time ∼1 ms). Two types of bound water molecules were assigned to water bound inside mesoglobules and on their surface. For highly concentrated PVME/D2O solutions (c ≥ 20 wt%) a slow exchange was detected by NMR for bound water (residence time = 2.1 s). For PIPMAm aqueous solution IR spectra indicate a two‐steps character of the phase transition. For PIPMAm in D2O/ethanol (EtOH) mixtures the globular structures were observed by NMR at 298 K for certain compositions of the mixed solvent (cononsolvency effect). Virtually no EtOH is bound in these globular structures, in contrast to the temperature‐induced globular structures.

Direct synthesis of six-coordinate nitro complexes of cobalt meso-tetraphenylporphyrinate with trans-O-donor ligands

The formation of a series of six-coordinate complexes (B)Co(TPP)(NO2) (B is ketone, aldehyde, ether or epoxide; TPP is meso-tetraphenylporphyrin dianion) on exposure of thin films of five-coordinate Co(TPP)(NO2) to vapors of the O-donor ligands was established by IR and UV/Vis spectroscopy (resorting to the data on isotope-substituted 15NO2). The complex formation is accompanied by shifts of the vibration frequencies of the trans-NO2 ligand and gives rise to new IR bands corresponding to the O-donors in the ν(C=O) or ν(C-O) region shifted with respect to these bands of free ligands. The complexes are rather stable in the solid phase but eliminate the O-donor ligand upon dissolution in an inert solvent (or upon maintenance in a vacuum for some complexes) being converted into the initial five-coordinate nitro complex. Using the IR spectra of CCl4 solutions of the complexes containing a large excess of O-donor molecules, the equilibrium constants and the formation enthalpies of some (B)Co(TPP)(NO2) complexes were determined, indicating weak coordination of these ligands.

Experimental Dipole Moments for Nonisolatable Acetic Acid Structures in a Nonpolar Medium. A Combined Spectroscopic, Dielectric, and DFT Study for Self-Association in Solution

Acetic acid can exist in many possible structural forms depending on its surrounding medium. A recently developed inverse problem methodology (J. Phys. Chem. B 2007, 111, 13064-13074) was utilized in order to elucidate acetic acid structures in a dilute nonpolar medium. In this regard, simultaneous and stopped-flow measurements of the bulk solution densities, refractive indices, relative permittivities, and IR spectra of acetic acid in toluene were performed at several different concentrations in a semibatch closed-loop experimental setup at 298.15 K and 0.1013 MPa. This combined IR spectroscopic and dielectric, density, and refractive index analysis was employed in order to distinguish acetic acid structures and to further determine the dipole moments of the monomer, cyclic dimer, and "lumped-sum" open dimers. The infrared spectra were first analyzed to provide qualitative understanding as well as quantitative estimates for each acetic acid species. Subsequently, the dipole moments of these species were calculated using a direct approach which was primarily based on response surface models. The present method allows the determination of individual dipole moments not only for the monomer but also for the cyclic dimer and the open dimer. The results obtained from this study experimentally show that the cyclic dimer with centrosymmetric structure has a dipole moment approximately 0 D. The results also suggest that the linear dimers are present as mixtures of linear dimers structures. The existence of the linear dimers mixture was also indicated by the experimental infrared analysis of the OH-stretching region (particularly for measurements in n-hexane as solvent) and comparison of these spectra with DFT predictions. Finally, the present methodology which incorporates simultaneous physicochemical and spectroscopic analysis is undoubtedly useful for physicochemical characterization for other nonisolatable solute species and self-associated structures in solution.

Composition and spectral manifestations of molecular complexes formed in solutions of hydrogen fluoride in methanol

The IR spectra of solutions of HF in methanol have been measured as the molar ratio of the components is varied from 1:12 to 7:1. The features of the absorption of HF–CH3OH binary mixtures of various concentrations are analyzed, and the spectral attributes of the molecular heterocomplexes present in them are explained. It is established by analyzing the concentration dependence of the optical density of the test solutions, normalized to the total number of moles per liter, that associates of molecules are formed in them with stoichiometric HF:CH3OH ratios equal to 2:1 and about 6:1. These results are compared with analogous data obtained from the concentration dependence of the excess density of solutions of HF in methanol.

Spectra of sodium aluminate solutions

The UV spectra of sodium aluminate solutions were obtained in the sodium oxide concentration range from 59 to 409 g/L and the caustic ratio range from 1.5 to 4.0 to reveal the structure characteristics of them, It is found that a new peak appears at about 370 nm besides peaks at about 220 and 266 nm in all solutions. The new peak is strongly favored by high hydroxide concentration and high caustic ratio. And it only appears when the solutions are prepared by dissolving sodium hydroxide and aluminum hydroxide. In addition, the IR and Raman spectra of sodium aluminate solutions with high alkali concentration and high caustic ratio were measured, and the UV spectra of aqueous solutions of Al(H 2O) 3+ 6 and AlF 3- 6 were measured as well. According to the crystal field theory in coordination chemistry as well as the above spectra characteristics, this new peak at about 370 nm is determined as the evidence of a new species of aluminate ion with a coordination number of 6.

Enols of Substituted Cyanomalonamides

Twenty open-chain mono-, di-, and trialkyl and aryl-N-substituted cyanomalonamides R2R1NCOCH(CN)CONHR3 were prepared. In solution, signals for both amide and a single enol are mostly observed, despite the potential for E and Z isomeric enols. The equilibrium (KEnol) values between the amides and the enols were determined in different solvents by NMR spectra. They decrease on increasing the polarity of the solvent in the order CDCl3 approximately C6D6>THF-d8>(CD3)2CO>CD3CN>DMF-d7>DMSO-d6. For the R1R2NCOCH(CN)CONHR3 system when R1=R2=H, Me or R1=H, R2=Me, KEnol for R3 follows the order: C6F5>Ph>or=An>or= i-Pr>or= t-Bu, and for R1, R2:H, H>Me, H>Me, Me in all solvents. A unique feature is the appreciable % enol in DMSO-d6 when R1=R2=H, in contrast with enol systems with other electron-withdrawing groups (EWGs). Calculations (B3LYP/6-31G**) corroborate the higher KEnol values for less alkyl-substituted systems, showing that in the most stable conformer when R1=H, R2=R3=Me the N-hydrogens are closer to the CN group. The order of promoting substituents for enol of amide formation is CONH2>CO2CH2CF3>CO2Me>CONHMe. The solid-state structures of the isolated species, determined by X-ray crystallography, were either amides or enols, and a higher KEnol(CDCl3) value does not ensure a solid enol structure. In no system were both solid species isolated. The X-ray structures of the enols were temperature-dependent. In most cases, the difference between the O-H and O...H bond lengths at low temperature were appreciable, but they become closer at the higher temperature. Similar tendency for either the C=C/C-C or the C-O/C=O bonds was observed. This is ascribed to a hydrogen shift between two regioisomeric enols in an asymmetric double-well potential, which becomes faster at a higher temperature. Calculations show that the enol structures are nonsymmetrical, resembling the lower temperature structures, even when they are chemically symmetrical, but the energy differences between the two regioisomers are <1 kcal. The hydrogen bonds in the enol moiety are strong, with O...O distances <2.45 A, and are resonance-assisted hydrogen bonds. IR spectra in solution and the solid state qualitatively corroborate the NMR and X-ray structure determination.

Supramolecular systems of amphiphilic derivatives of calix[4]resorcinarenes and surfactants in chloroform

Aggregation of amphiphilic calix[4]resorcinarenes (CRA) modified by carboxymethyl (1), 2-hydroxyethyl (2), methylamino acetal (3), and aminomethyl (4) fragments and their interaction with some synthetic (5, 6) and natural (7, 8) surfactants in the low-polarity solvent (chloroform) were studied by permittivity measurements and FT-IR spectroscopy. Compounds 1–4 and surfactants form aggregates at critical micelle concentrations (CMC) of 2.0·10−5–7.5·10−5 and 1.7·10−5–2.0·10−3 mol L−1, respectively. The CMC values of CRA—surfactant mixed aggregates depend on the surfactant structure and the structure and concentration of CRA. Analysis of the IR spectra of solutions of a series of amphiphilic CRA (2–4, 9, 10) and their mixtures with the cationic surfactant N-cetyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium bromide (5) showed that an increase in the concentration of the solutions in individual and mixed systems is accompanied by a decrease in the molar integral intensities and intensities in the maxima of the absorption bands of the O—H and C—H bonds down to the CMC point, after which these values change slightly. The discovered effect, which is differently pronounced for all systems studied, indicates that both the polar “head” groups and nonpolar fragments of CRA and surfactant are involved in the formation of supramolecules of the reverse micelle type in all cases.

Vibrational and NMR spectra and structures of lithium pyruvate monohydrate

The IR, Raman, and 13C NMR spectra of lithium pyruvate monohydrate and its isotopic compounds including the O-deuterated and 18O- and 13C- analogs have been measured in the solid state. The IR and NMR spectra of aqueous solutions of the lithium, sodium, and potassium salts have also been obtained for comparison. No C O stretching band is observed in the solid state IR and Raman spectra, and the spectral changes on O-deuteration and 2- 13C- and 18O-labellings cannot be explained in terms of ordinary crystal water. In the solid state 13C NMR a characteristic peak is observed at δ 94.9 in place of a carbonyl carbon peak expected around δ 200. These findings confirm that the lithium salt hydrate has the gem-diol structure in the solid state.

The IR spectra of solutions of hydrogen fluoride in (CH_3)_2NCHO, (CH_3)_2CO, and CH_3CN with a molar ratio of the components of 0:1-10:1

Systematic measurements of the IR spectra of solutions of HF in N,N-dimethylformamide, acetone, and acetonitrile have been carried out for the first time in a wide range of variation of the molar ratio of the components (from 0:1 to 7:1, 8.9:1, and 10:1, respectively). This has not only scientific significance but also important applied significance. The absorption features of each of the binary liquid systems studied here and the spectral attributes of the heterocomplexes formed in it are analyzed. It is established that the molecular associates (HF)n⋅(CH3)2NCHO and (HF)n⋅(CH3)2CO contain the same large structural fragment the appearance of which is typical of solutions of HF in organic solvents.

New thermochemical parameter for describing solvent effects on IR stretching vibration frequencies: Communication 1. Assessment of van der Waals interactions

Solvent effects on O H stretching vibrations in several complexes with hydrogen bonding have been investigated by FTIR spectroscopy. To assess the influence of van der Waals (vdW) interactions on frequency shifts, a new parameter of solvent, δ cav h S , is proposed. This parameter has been derived from equations describing enthalpy of non-specific solvation. Linear correlation was established between the O H frequency shift (with respect to the gas phase) and parameter δ cav h S for a series of complexes of aliphatic alcohols with standard proton acceptors. Linear correlations with δ cav h S were also observed for a series of “free” O H and also C O, P O, S O and C Br stretching vibrations. A new method is proposed for estimating the gas-phase stretching frequency from IR spectra of solutions. In addition, frequencies of “free” X H groups in neat bases were deduced from the experimental data.

Solvatochromism of Heteroaromatic Compounds: XXVII. Configuration Isomerism of H Complexes of Methanol with Carbonyl Compounds

The energy minimum of H complexes of methanol with carbonyl compounds corresponds to the nonlinear structure in which the bridging hydrogen atom deviates from the axis of the C=O bond. The stretching vibration bands of the OH bonds in these H complexes, observed in the IR spectra of solutions (CCl4), have a complex shape or are asymmetrical, which is due to the existence of two configuration isomers differing in the direction of the H bond. Difference in the orientation of the subunits of the complex may cause significant differences in the OH stretching frequencies.

Labile Substitution Reactions in Diamagnetic tetrakis(Alkylisocyanide)bis(triarylphosphine)cobalt(III) Complexes by Tri‐n‐propylphosphine

The diamagnetic alkylisocyanide‐triarylphosphine Co(III) complexes [Co(CNC6H11)4{P(C6H4OMe‐p)3}2](ClO4)3, [Co(CNC6H11)4{P(C6H4Me‐p)3}2](ClO4)3, [Co(CNC6H11)4(PPh3)2](ClO4)3, and [Co(CNCH2Ph)4{P(C6H4OMe‐p)3}2](BF4)3, are investigated in substitution reactions with the trialkylphosphine ligand P(C3H7‐n)3, in the solvents CF3CH2OH, CH2Cl2, and CH3NO2. Eleven of the 12 reactions are labile, resulting in formation of the complexes [Co(CNR)4{P(C3H7‐n)3}2]X3; only [Co(CNC6H11)4{P(C6H4OMe‐p)3}2](ClO4)3 in CF3CH2OH failed to undergo substitution. Some yields are low due to reduction of the Co(III) in solution, and some reaction times exceed the suggested t 1/2<1 min criterion for labile behavior. Reactions in CH3NO2 were performed at lowered temperatures to minimize reduction of Co(III). Correlation of reactivity with observation of an “extra” Co(III) band in the solution IR spectra, suggesting the presence of a second Co(III) species, is partially successful in providing a mechanism for these unexpected reactions.

The vibrational spectrum of the stable free radical 1,1,3,3-tetramethylisoindolin-2-yloxyl

Solid and solution IR and Raman spectra of a stable nitroxide radical, 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO), are reported and compared to ab initio density functional theory calculations of the vibrational frequencies to obtain unequivocal band assignments, in particular of the N O stretching frequency, ν(N O ). The band position was found to be at 1431 cm −1 for the solid, which is well outside the previously published range of 1310–1380 cm −1 for nitroxide radicals. This apparently anomalous peak position was confirmed by undertaking isotopic substitution studies through the preparation and recording of vibrational spectra of tetrakis(trideuteriomethyl)isoindolin-2-yloxyl ([ 2H 12]-TMIO) and [ 2H 12, 15N]-TMIO analogues. Solution spectra of TMIO in methanol and CCl 4 are assessed for possible solvent-dependent spin density distribution effects in the N O bond.

Compositions and Structures of Methanesulfonic Acid Complexes with Acetonitrile According to IR Spectroscopic Data

IR spectra of solutions of methanesulfonic acid in acetonitrile are recorded and analyzed at 30°C at methanesulfonic acid concentrations ranging from 0 to 100%. Molecular 1 : 1 complexes with the coordination of the acid proton at the nitrogen atom of the acetonitrile molecule are formed over a wide range of acid concentrations. When the base is in excess, the whole amount of the acid is bound in these complexes. In solutions with excess acid, the solvation of the 1 : 1 complexes by several methanesulfonic acid molecules produces associates containing a structure fragment in which the acetonitrile molecule acts as both the donor and acceptor of hydrogen bonds. Hydrogen bonds involving CH3 groups of acetonitrile can be formed when the 1 : 1 complexes are solvated by several acid molecules.

Synthesis and characterization of Tetrakis(Alkylisocyanide) Bis(Triarylphosphine)Cobalt(III) complexes by Labile ligand substitution in intermediate-spin six-coordinate Co(III) complexes

Three intermediate-spin six-coordinate Co(III) complexes, [Co(CNCH2Ph)4{OAs(C6H4Me-p)3}2](BF4)3 (1), [Co(CNC6H11)4(OSbPh3)2](ClO4)3 (2), and [Co(CNCH2Ph)4(OSbPh3)2](BF4)3 (3), undergo rapid ligand substitution with selected triarylphosphines at 0°C. Complexes 1, 2, and 3 react with P(C6H4OMe-p)3, 1 and 2 react with PPh3, and 2 reacts with P(C6H4Me-p)3, to produce trans-[Co(CNR)4(PR′3)2]X3 complexes in very high yields. Attempted reaction with P(C6H4Cl-p)3 yields a Co(I) complex. The triarylphosphine-alkylisocyanide Co(III) complexes were characterized by magnetic susceptibility, and solution and solid state IR and electronic spectra. The Co(III) complexes are diamagnetic, but exhibit various degrees of reduction in different solvents.

IR Spectra of Paracetamol and Phenacetin. 1. Theoretical and Experimental Studies

IR spectra of paracetamol and phenacetin have been measured for powder crystals of these compounds and for their solutions in chloroform and dimethylsulfoxide. Ab initio calculations of their equilibrium geometry and vibrational spectra were carried out for spectrum interpretation. Differences between the experimental IR spectra of solutions and crystalline samples have been analyzed. Variations of molecular structure from the isolated state to molecular crystal were estimated based on the difference between the optimized molecular parameters of free molecules and the experimental bond lengths and angles evaluated for the crystal forms of the title compounds. The role of hydrogen bonds in the structure of molecular crystals of paracetamol and phenacetin is investigated, and spectral ranges with maximal intermolecular interactions are determined.

Lithium-pyrazole-3,4,5-tricarbonitrile: Ion Pairing and Lithium Ion Affinity Studies

Negative charge stabilized by ring delocalization on five-membered rings is a practical and theoretically interesting alternative to conventional fluorine-based anions. Coordination of the lithium cation to the pyrazole-3,4,5-tricarbonitrile (PATC) anion was studied using vibrational spectroscopy (Raman and IR) and ab initio SCF-MO Hartree−Fock (HF) calculations. Four 1:1 ion pair geometries were found, one being energetically more stable. By comparing theoretical spectra with both IR and Raman spectra of salt solutions, it was found that the lithium ion favors bidentate coordination to the ring nitrogen atoms, as suggested by the binding energies. Finally, comparisons were made with previously calculated coordination strengths for other similar lithium anion 1:1 systems.

Infrared and ab initio studies of hydrogen bonding and proton transfer in the complexes formed by pyrazoles

The results of experimental studies and quantum mechanical calculations of vibrational spectra and structure of hydrogen bonded complexes formed by pyrazole (P) and 3,5-dimethylpyrazole (DMP) are presented. IR spectra of pyrazoles in solutions, gas phase, and solid state have been investigated in wide range of concentrations and temperatures. It has been found that in the gas phase both P and DMP reveal the equilibrium between monomers, dimers, and trimers. In solutions the equilibrium between monomers and trimers dominates, no bands, which can be attributed to dimers were detected. DMP retains the trimer structure in solid state, while in the case of pyrazole P, formation of the crystal provides another type of association. Geometrical and spectral characteristics of dimers and trimers, obtained by ab initio calculations, are presented and compared with experimental data. IR spectra of solutions containing P and DMP with a number of acids (acetic and trifluoroacetic acids, pentachlorophenol, HBr) have been studied in parallel with ab initio calculations. It has been found that pentachlorophenol forms with pyrazoles complexes with one strong hydrogen bond O–H⋯N, while NH pyrazole group remains unbonded. With carboxylic acids DMP forms 1:1 cyclic complexes with two hydrogen bonds. In the case of acetic acid, the complex in CH 2Cl 2 solution reveals molecular structure with OH⋯N and CO⋯HN bonds, in accordance with results of the calculations. For trifluoroacetic acid, the calculations predict the molecular structure to be energetically more stable in the case of the isolated binary complex (in gas phase), while the experimental spectrum of CH 2Cl 2 solution gives an evidence of the proton transfer with formation of the cyclic ionic pair with two NH +⋯O − bonds. The agreement with experimental results can be improved by taking into account the influence of environment in the framework of Onsager or Tomasi models. The shape of proton potential function of the complexes and medium effect on its parameters, resulted from experimental data and calculations, are discussed. It has been found that the number of potential minima and their relative depth depend strongly on the method of calculations and the basic set. Under excess of trifluoroacetic acid, the formation of 2:1 acid–DMP complex has been detected. Spectral characteristics and results of calculations point to the cyclic structure of this complex, which includes homoconjugated bis-trifluoroacetate anion and DMPH + cation. With HBr both studied pyrazoles were found to form ionic complexes including one or two pyrazole molecules per one acid molecule and correspondingly monocation or homoconjugated cation BHB +.