Polarized IR-microscope spectra of guanidinium hydrogenselenate single crystal

The polarized IR-microscope spectra of C(NH2)3.HSeO4 small single crystal samples were measured at room temperature. The spectra are discussed with the framework of oriented gas model approximation and group theory. The stretching nuOH vibration of the hydrogen bond with the O...O distance of 2.616 A gives characteristic broad AB-type absorption in the IR spectra. The changes of intensity of the AB bands in function of polarizer angle are described. Detailed assignment for bands derived from stretching and bending modes of selenate anions and guanidinium cations were performed. The observed intensities of these bands in polarized infrared spectra were correlated with theoretical calculation of directional cosines of selected transition dipole moments for investigated crystal. The vibrational studies seem to be helpful in understanding of physical and chemical properties of described compound and also in design of new complexes with exactly defined behaviors.     

X-ray diffraction,spectroscopic (IR,Raman) and DSC studies of bis(betainium) p-toluenesulfonate monohydrate crystal

Bis(betainium) p-toluenesulfonate monohydrate (abbreviated as BBTSH) was studied at various temperatures by X-ray diffraction, differential scanning calorimetry and vibrational spectroscopy methods. DSC curves of BBTSH show a peak at about 349 K which corresponds to water escape from the crystal, and reveal the “cold crystallization” phenomenon. BBTSH crystallizes in the P2₁/c space group of monoclinic system. After heating above 349 K the compound dehydrates, the crystal system changes to triclinic, the monocrystalline samples become non-merohedral twins. The BBTSH crystal comprises p-toluenesulfonic anions, monoprotonated betaine dimers and water molecules. Three kinds of hydrogen bonds are present in the crystal: strong, asymmetric and almost linear OH⋯O hydrogen bond (R(O⋯O) = 2.463(2) Å), weak O_wH⋯O hydrogen bonds (R(O_w⋯O) = 2.820(2) − 2.822(2) Å) and weak CH⋯O hydrogen bonds (R(C⋯O) = 3.295(2) − 3.416(2) Å). The ν_aOHO vibration of the strongest hydrogen bond in the crystal gives rise to an intense broad absorption with numbers of transmission windows in the low wavenumber region of the infrared spectra. Coupling between νCO stretching vibrations of two COO groups of the betaine dimer was detected. The process corresponding to the loss of water is accompanied by the breakage of strong OH⋯O hydrogen bonds in betaine dimers and rearrangement inside half of the betaine dimers. This rearrangement results in formation of the new betaine dimers with OH∙∙∙O hydrogen bond of similar strength as corresponding bond in the hydrated form (BBTSH).     

DFT studies of the structure, vibrational and NMR spectra of 2-amino-pyridine betaine monohydrate

Four of the most stable conformers of 2-amino-pyridine betaine (1-carboxymethyl-2-amino-pyridinium inner salt) monohydrates, 2-NH₂PB·H₂O, and one anhydrous were analyzed by the B3LYP/6-31G(d,p) calculations and compared with the X-ray data. Two types of optimized conformers can be distinguished: (a) with NH₂ and COO groups and (b) an imino tautomer with NH and COOH groups. A common feature of the optimized molecules are intramolecular hydrogen bonds between the COO⁻ and H₂N or COOH and HN groups. In the crystal both NH₂ and COO groups participate in intermolecular hydrogen bonds. The probable assignments of the anharmonic experimental solid state vibrational frequencies of 2-NH₂PB·H₂O and 2-ND₂PB·D₂O (conformer 2) based on the calculated B3LYP/6-31G(d,p) harmonic frequencies have been made. Correlations between experimental chemical shifts for 2-NH₂PB, its hydrochloride and 1-carboxyethyl-2-amino-pyridinium inner salt (¹³C and ¹H in D₂O) and GIAO/B3LYP/6-31G(d,p) calculated isotropic shielding constants, δ_exp=a+bσ_calc, are reported. Good linear regression between experimental and theoretical results for ¹³C was obtained. Only in 2-NH₂PB the hydrogen at -position is outside the linear correlation.     

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignments for methylaminothiophosphoryl difluoride

Infrared spectra (4000–50 cm⁻¹) of the vapor, amorphous and crystalline solids and Raman spectra (3600–10 cm⁻¹) of the liquid with qualitative depolarization data as well as the amorphous and crystalline solids of methylaminothiophosphoryl difluoride, CH₃N(H)P(=S)F₂, and three deuterated species, CD₃N(H)P(=S)F₂, CH₃N(D)P(=S)F₂, and CD₃N(D)P(=S)F₂, have been recorded. The spectra indicate that in the vapor, liquid and amorphous solid a small amount of a second conformer is present, whereas only one conformer remains in the low temperature crystalline phase. The near-infrared spectra of the vapor confirms the existence of two conformers in the gas phase. Asymmetric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Raman spectrum of the liquid the enthalpy difference between the trans and near-cis conformers was determined to be 368±15 cm⁻¹ (4.41±0.2 kJ/mol), with the trans conformer being thermodynamically preferred. Ab Initio calculations with structure optimization using the 6-31G(d) and 6-311+G(d,p) basis sets at the restricted Hartree–Fock (RHF) and/or with full electron correlation by the perturbation method to second order (MP2) support the occurrence of near-trans (5° from trans) and near-cis (20° from cis) conformers. From the RHF/6-31G(d) calculation the near-trans conformer is predicted to be the more stable form by 451 cm⁻¹ (5.35 kJ/mol) and from the MP2/6-311+G(d,p) calculation by 387 cm⁻¹ (4.63 kJ/mol). All of the normal modes of the near-trans rotamer have been assigned based on infrared band contours, depolarization values and group frequencies and the assignment is supported by the normal coordinate calculation utilizing harmonic force constants from the MP2/6-31G(d) ab initio calculations.     

Conformational behaviour and vibrational spectra of 3-methyl 2-butanethiol

The Raman spectra of 3-methyl 2-butanethiol in the temperature range-120° C to + 60° C have been recorded together with its liquid phase infrared spectrum at room temperature. The spectral analysis shows that the molecule of the compound exists in the liquid state, in three different rotameric configurationsA, B andC of which the formA is the stablest. Besides, a tentative assignment of the observed vibrational frequencies arising from the rotameric forms has been presented.     

Far IR and Raman vibrational spectra of nitramines

Vibrational spectra of several nitramines in the long-wave region (50–450 cm⁻¹) were studied. The frequencies of intra- and intermolecular vibrations were separated and a tentative assignment of the frequencies of self-associative complexes was performed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2241–2244, November, 1998.     

Structure and vibrational spectra of the bis(betaine)-selenic acid molecular crystal

The crystal structure of bis(betaine)-selenic acid has been determined by X-ray diffraction as orthorhombic, space group Pbca, with a = 11.591(2), b = 22.930(5), c = 12.045(2) Å and Z = 8. The crystal comprises hydrogen selenate ions, HSeO⁻₄, and two distinct betaine molecules, which are held together into a complex by short hydrogen bonds. One of the betaine molecules is present as the zwitterion form (CH₃)₃N⁺---CH₂---COO⁻ and the second occurs as the protonated form (CH₃)₃N⁺---CH₂---COOH. Powder FTIR and Raman spectra were measured. An assignment of the observed bands to vibrations of the hydrogen bonds and internal vibrations of the hydrogen selenate ion and the betaine molecules is proposed.     

Raman and infrared spectra,conformational stability,barriers to internal rotation,r 0 structural parameters,ab initio calculations,and vibrational assignment for vinyl chloroformate

The Raman (3200 to 10 cm^–1) and infrared (3500 to 50 cm^–1) spectra of vinyl chloroformate, H₂C=CHOC(O)Cl, have been recorded for both the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded, and depolarization ratios have been obtained. These data have been interpreted on the basis that the only stable conformation present at ambient temperature is thetrans-trans rotamer, where the firsttrans refers to the vinyl moiety relative to the O—CCl bond and the second to the C—Cl bond relative to the=C—O bond. Using harmonic rigid asymmetric top calculations, the infrared vapor phase contours for the C=O and the C=C stretch were predicted for thetrans-trans and for thecis-trans conformer, and were compared with experiment. For both fundamentals thetrans-trans hybrid reproduces the experimental contour, whereas thecis-trans contours fail to do so for both fundamentals. From far-infrared spectrum of the vapor obtained at 0.1 cm^–1 resolution, the C(O)Cl andO-vinyl torsional fundamentals have been observed at 132 and 61 cm^–1, respectively. Ther ₀ structural parameters have been obtained from a combination of ab initio calculated parameters with appropriate offset values and the fit of the microwave rotational constants for the two naturally occurring chlorine isotopes. The structure, barrier to internal rotation, and vibrational frequencies have been determined from ab initio Hartree-Fock gradient calculations, using the 3-21G^* and 6-31G^* basis sets. These results are compared to those obtained experimentally and to similar quantities for some related molecules.     

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for 2-chloroethyl silane

The infrared (3500–30 cm⁻¹) spectra of gaseous and solid and the Raman (3500–10 cm⁻¹) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH₂CH₂SiH₃, have been recorded. Similar data have been recorded for the Si–d₃ isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−55 to −100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm⁻¹ (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si–H frequencies from the Si–d₂ isotopomer the r_o Si–H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.     

Vibrational spectra of monolayer islands on single crystal faces. Island size and shape effects on infrared frequencies,intensities and bandwidths

A method for theoretical analysis spectra of rectangular segments (islands) of two-dimensional ordered overlayers of adsorbed species has been proposed. The simple formulae for frequencies, absorption coefficients and bandwidths as functions of island size and shape have been deduced. If frequencies and bandwidths of appropriate bands in infrared spectra of chemisorbed monolayers are measured simultaneously, the size and shape of the islands can be estimated.     

Structure and vibrational spectra of mononitroalkanes

Structures and force fields for several mononitroalkane molecules were determined by ab initio quantum-chemical methods. The data obtained were used for calculation of the frequencies and modes of normal vibrations. Potentialities of different methods (RHF, MP2, and B3LYP) and basis sets for estimation of the structures and spectra were studied.     

Ab initio and DFT studies of the molecular structures and vibrational spectra of succinonitrile

The Molecular structure, conformational stability and vibrational frequencies of succinonitrile NCCH2CH2CN have been investigated with ab initio and density functional theory (DFT) methods implementing the standard 6-311++G* basis set. The potential energy surfaces (PES) have been explored at DFT-B3LYP, HF and MP2 levels of theory. In agreements with previous experimental results, the molecule was predicted to exist in equilibrium mixture of trans and gauche conforms with the trans form being slightly lower in energy. The vibrational frequencies and the corresponding vibrational assignments of succinonitrile in both C2h and C2 symmetry were examined theoretically and the calculated Infrared and Raman spectra of the molecule were plotted. Observed frequencies for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311++G* basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.     

Theoretical anharmonic Raman and infrared spectra with vibrational assignments for monofluoroaniline isomers

The ortho-meta-, and para-fluoro substituted anilines are prototype molecules for investigation of the interactions of both the amino group and the fluorine atom with the aromatic ring. The molecular structures, natural atomic charges and theoretical anharmonic Raman and infrared spectra of the three fluoroaniline isomers have been calculated by using the density functional B3LYP method with the extended 6-311++G(df,pd) basis set. The Raman and infrared spectra of 2FA, 3FA, and 4FA have been recorded. The detailed vibrational assignments of the experimental spectra have been made on the basis of the calculated potential energy distributions, PEDs. The effect of fluorine substituent on the aniline ring geometry and charge distribution, the nature of the characteristic “marker bands” and a quenching of intensities of some bands are discussed. It is shown that the frequencies of the NH₂ stretching vibrations depend on the degree of pyramidalization of the C-NH₂ group, in the isomers. In 2FA and 3FA, the NH₂ stretching frequencies are higher than those in 4FA. This corresponds to a more flattened structure of the amino group in 2FA and 3FA, in comparison to 4FA.     

Interpretation of the vibrational spectra of nitramines on the basis ofab initio calculations

It is shown that, unlike conventional methods of vibrational spectroscopy, the use ofab initia harmonic force fields, IR and Raman intensities, and depolarizations makes possible a rigorous interpretation of the experimental spectra of the simplest aliphatic nitramines (CH₃)₂NNO₂, CH₃NHNO₂, H₂NNO₂, and their isotopomers. The scale factors, which were introduced for each compound to remove the systematic errors of the SCF MO LCAO calculation by fitting the parameters to the observed frequencies, were mutually adjusted during the solution of the inverse vibrational problem. The set of transferable scale factors established in this work can be used directly to analyze spectra of larger molecules. Some common patterns of the force fields and vibrational spectra of nitramines are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya No. 12, pp. 2106–2117, December, 1994.The authors are grateful to the Russian Foundation for Basic Research (Project No. 93-03-4410) and to the Robert A. Welch Foundation for financial support of the parts of this work performed at the Department of Chemistry of Moscow State University and at the University of Texas at Austin, USA. The authors also acknowledge the support by the Scientific Technical Program Universities of Russia.     

The vibrational spectra of the elementoorganic starburst dendrimers

The main features of vibrational spectra of starburst dendrimers have been analyzed for the first time. Their spectral pattern, in general, is determined by the ratio of a number of terminal groups to a number of repeating units. This ratio tends to m_r−1 (m_r — branching functionality of repeating unit), and becomes constant, when the generation number of the starburst dendrimer increases higher than 3-5. IR and Raman spectra of twelve generations of the phosphorus-containing dendrimers are represented and interpreted on the basis of the calculation of frequencies of the normal vibrations and band intensities in the IR spectra of ‘molecules’ terminated by dangling methyl groups, which are the fragments of the dendrimer molecule. Tailored spectra of these fragments are then compared with experimental spectra and satisfactory similarity has been obtained. Experimental spectra of generations higher than 4 are very similar, according to the theoretical approach. The results can be used for the analysis of the chemical and physical transformations in starburst dendrimers.     

Understanding electrocatalysis at nanoscale electrodes and single atoms with operando vibrational spectroscopy

Electrocatalysis presents an environmentally benign route to carry out energy conversion reactions and organic transformations. The use of nanomaterials and single atoms as catalysts in electrochemical reactions has led to improvements in activity and selectivity over bulk electrocatalysts, however, optimal design of electrocatalytic reactions requires an in-depth understanding of the dynamic solid–liquid interface, reaction intermediates, and catalyst structural changes. Tracking bond breaking and formation events in real time is an important complement to catalyst characterization. This short review highlights advancements in fundamental understanding of electrocatalysis at nanoscale and single atom electrocatalysts obtained using operando vibrational spectroscopies. To push the boundaries of our fundamental understanding in electrocatalysis, we also emphasize a need to further develop operando vibrational techniques to reduce ensemble averaging with improvements in spatial and temporal resolution.     

Force fields and vibrational spectroscopy: A discussion on status and future prospects

A start is given for a discussion about the use of force fields for describing and interpreting molecular geometries, relative energies and vibrational spectra. The development of modern, generic force fields, e.g. Class II force fields, has diminished the necessity for specific force fields, and they should be applied in far more cases than they are being used now.     

Polarized infrared spectra of KHSeO4 single crystal

The polarized absorption i.r. spectra of KHSeO₄ single crystal were measured in the 4000-300 cm⁻¹ region at 300 and 25 K and discussed in relation to the crystal structure. The results obtained are compared with the spectra of the isomorphous KHSO₄ crystal.     

Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

The infrared spectra of 3-pentyn-2-ol, CH₃CCCH(OH)CH₃, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm⁻¹ range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid.

Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C₁(a_Me). From various bands assigned to a second conformer in which OH is oriented anti to H_gem(a_H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol⁻¹. The highest energy conformer with OH anti to C₃(a_C) was not detected.

Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a_H conformer had slightly lower energy, all the calculations revealed that a_Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a_Me conformer as more stable by 0.8 and 8.3 kJ mol⁻¹ relative to a_H an a_C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.     

Structure and vibrational spectra of dicyclopentadienylzinc. A DFT study

The quantum-chemical DFT calculations of the Cp₂Zn structure confirm the conclusion made earlier from the vibrational spectra that the sandwich structure (⁵-C₅H₅)₂Zn (A) is not energetically favorable and more favorable are the close in energy -structure (⁵-C₅H₅)(¹-C₅H₅)Zn (B) and -structure (¹-C₅H₅)₂Zn (C). The vibrational spectra of structures B and C with the DFT-derived force fields were calculated. A comparison of the calculated spectra of the isolated Cp₂Zn molecules with the experimental data gives no way of deciding between the B and C structures. It is most likely that the molecule is nonrigid and experiences a strong influence from the nearest environment in solution or in the crystalline state.