The structure of mimetite,arsenian pyromorphite and hedyphane – A Raman spectroscopic study

The minerals mimetite Pb₅(AsO₄)₃Cl, arsenian pyromorphite Pb₅(PO₄,AsO₄)₃Cl and hedyphane Pb₃Ca₂(AsO₄)₃Cl have been studied by Raman spectroscopy complimented with infrared spectroscopy. Mimetite is characterised by a band at 812–3 cm⁻¹ attributed to the A_g mode. For the arsenian pyromorphite this band is observed at 818 cm⁻¹ and for hedyphane at 819 cm⁻¹. For mimetite and hedyphane bands at 788 and 765 cm⁻¹ are attributed to A_u and E_1u vibrational modes and are both Raman and infrared active. For the arsenian pyromorphite, Raman bands at 917–1014 cm⁻¹ are attributed to phosphate stretching vibrations. Raman spectroscopy clearly identifies bands attributable to isomorphous substitution of arsenate by phosphate. The observation of low intensity bands in the 3200–3550 cm⁻¹ region are assigned to adsorbed water and OH units, thus indicating some replacement of chloride ions with hydroxyl ions.     

Vibrational spectroscopy of the sorosilicate mineral hemimorphite Zn4(OH)2Si2O7 · H2O

Raman spectroscopy complimented by infrared spectroscopy has been used to study the mineral hemimorphite from different origins. The Raman spectra show consistently similar spectra with only one sample showing additional bands due to the presence of smithsonite. Raman bands observed at 3510–3565 and 3436–3455 cm⁻¹ are assigned to OH stretching vibrations. Using a Libowitzky type formula, these OH bands provide hydrogen bond distances of 0.2910, 0.2825, 0.2762 and 0.2716 pm. Water bending modes are observed in the Raman spectrum at 1633 cm⁻¹. An intense Raman band at 930 cm⁻¹ is attributed to SiO symmetric stretching vibration of the Si₂O₇ units. Raman bands observed at 451 and 400 cm⁻¹are attributed to out-of-plane bending vibrations of the Si₂O₇ units. Raman bands at 330, 280, 168 and 132 cm⁻¹ are assigned to ZnO and OZnO vibrations.     

Vibrational spectroscopic study of mimetite-pyromorphite solid solutions

A number of compounds of the mimetite Pb₅(AsO₄)₃Cl-pyromorphite Pb₅(PO₄)₃Cl solid solution series were synthesized at room temperature and analyzed with Raman and infrared spectroscopy. The fundamental antisymmetric stretching (ν₃) and bending (ν₄) vibrations of the mimetite-pyromorphite solid solutions occur in the regions 720-1040 cm⁻¹ and 400-580 cm⁻¹. The band originating from the ν₃ and ν₄ modes shifts to higher wavenumbers between mimetite and pyromorphite because the atomic mass of As is higher than P and As-O bonds are longer than P-O bonds. The position, shape and relative intensity of the bands vary primarily as a function of As/(As + P) in the analyzed solid. The strong correlations between the positions of the vibrational modes and the As/(As + P) ratio can be used to determine the composition of the investigated natural and synthetic samples of the minerals from the mimetite-pyromorphite series.     

Vibrational spectroscopic study of the arsenate mineral strashimirite Cu8(AsO4)4(OH)4·5H2O—Relationship to other basic copper arsenates

The basic copper arsenate mineral strashimirite Cu₈(AsO₄)₄(OH)₄·5H₂O from two different localities has been studied by Raman spectroscopy and complemented by infrared spectroscopy. Two strashimirite mineral samples were obtained from the Czech (sample A) and Slovak (sample B) Republics. Two Raman bands for sample A are identified at 839 and 856 cm⁻¹ and for sample B at 843 and 891 cm⁻¹ are assigned to the ν₁ (AsO₄³⁻) symmetric and the ν₃ (AsO₄³⁻) antisymmetric stretching modes, respectively. The broad band for sample A centred upon 500 cm⁻¹, resolved into component bands at 467, 497, 526 and 554 cm⁻¹ and for sample B at 507 and 560 cm⁻¹ include bands which are attributable to the ν₄ (AsO₄³⁻) bending mode. In the Raman spectra, two bands (sample A) at 337 and 393 cm⁻¹ and at 343 and 374 cm⁻¹ for sample B are attributed to the ν₂ (AsO₄³⁻) bending mode. The Raman spectrum of strashimirite sample A shows three resolved bands at 3450, 3488 and 3585 cm⁻¹. The first two bands are attributed to water stretching vibrations whereas the band at 3585 cm⁻¹ to OH stretching vibrations of the hydroxyl units. Two bands (3497 and 3444 cm⁻¹) are observed in the Raman spectrum of B. A comparison is made of the Raman spectrum of strashimirite with the Raman spectra of other selected basic copper arsenates including olivenite, cornwallite, cornubite and clinoclase.     

Aurichalcite – An SEM and Raman spectroscopic study

Raman spectroscopy complimented with supplementary infrared spectroscopy has been used to characterise the vibrational spectrum of aurichalcite a zinc/copper hydroxy carbonate (Zn,Cu²⁺)₅(CO₃)₂(OH)₆. XRD patterns of all specimens show high orientation and indicate the presence of some impurities such as rosasite and hydrozincite. However, the diffraction patterns for all samples are well correlated to the standard reference patterns. SEM images show highly crystalline and ordered structures in the form of micron long fibres and plates. EDAX analyses indicate variations in chemical composition of Cu/Zn ratios ranging from 1/1.06 to 1/2.87. The symmetry of the carbonate anion in aurichalcite is C_s and is composition dependent. This symmetry reduction results in multiple bands in both the symmetric stretching and bending regions. The intense band at 1072 cm⁻¹ is assigned to the ν₁(CO₃)²⁻ symmetric stretching mode. Three Raman bands assigned to the ν₃(CO₃)²⁻ antisymmetric stretching modes are observed for aurichalcite at 1506, 1485 and 1337 cm⁻¹. Multiple Raman bands are observed in 800–850 cm⁻¹ and 720–750 cm⁻¹ regions and are attributed to ν₂ and ν₄ bending modes confirming the reduction of the carbonate anion symmetry in the aurichalcite structure. An intense Raman band at 1060 cm⁻¹ is attributed to the δ OH deformation mode.     

A Raman spectroscopic study of the mono-hydrogen phosphate mineral dorfmanite Na2(PO3OH)·2H2O and in comparison with brushite

Aspects of the molecular structure of the mineral dorfmanite Na₂(PO₃OH)·2H₂O were determined by Raman spectroscopy. The mineral originated from the Kedykverpakhk Mt., Lovozero, Kola Peninsula, Russia. Raman bands are assigned to the hydrogen phosphate units. The intense Raman band at 949 cm⁻¹ and the less intense band at 866 cm⁻¹ are assigned to the PO₃ and POH stretching vibrations. Bands at 991, 1066 and 1141 cm⁻¹ are assigned to the ν₃ antisymmetric stretching modes. Raman bands at 393, 413 and 448 cm⁻¹ and 514, 541 and 570 cm⁻¹ are attributed to the ν₂ and ν₄ bending modes of the HPO₄ units, respectively. Raman bands at 3373, 3443 and 3492 cm⁻¹ are assigned to water stretching vibrations. POH stretching vibrations are identified by bands at 2904, 3080 and 3134 cm⁻¹. Raman spectroscopy has proven very useful for the study of the structure of the mineral dorfmanite.     

Synthesis and vibrational spectroscopic characterisation of nickel containing aurichalcite

Raman spectroscopy complimented with supplementary infrared spectroscopy has been used to characterise a synthetic nickel substituted aurichalcite a zinc/nickel hydroxy carbonate, (Zn²⁺, Cu²⁺, Ni²⁺)₅(CO₃)₂(OH)₆. XRD patterns show high orientation and indicate the presence of some minor impurities. The diffraction patterns for the Ni-aurichalcite are well correlated with the standard reference patterns. EDAX analyses indicate variations in chemical composition of Zn/Ni ratios of ∼20:1. The symmetry of the carbonate anion in aurichalcite is C_s and is composition dependent. This symmetry reduction results in multiple bands in both the symmetric stretching and bending regions. The intense band for the Ni-aurichalcite at 1070 cm⁻¹ is assigned to the ν₁(CO₃)²⁻ symmetric stretching mode. Three Raman bands assigned to the ν₃(CO₃)²⁻ antisymmetric stretching modes are observed for Ni-aurichalcite at 1372, 1480 and 1543 cm⁻¹. Multiple Raman bands are observed in the regions from 800 to 850 cm⁻¹ and 720 to 750 cm⁻¹, and are attributed to ν₂ and ν₄ bending modes confirming the reduction of the carbonate anion symmetry in the aurichalcite structure. This research proves that nickel containing aurichalcites can be synthesised in the laboratory thus mimicing the natural nickel containing aurichalcites.     

The structure of the mineral leogangite Cu10(OH)6(SO4)(AsO4)4·8H2O--implications for arsenic accumulation and removal

The objective of this research is to determine the molecular structure of the mineral leogangite. The formation of the types of arsenosulphate minerals offers a mechanism for arsenate removal from soils and mine dumps. Raman and infrared spectroscopy have been used to characterise the mineral. Observed bands are assigned to the stretching and bending vibrations of (SO₄)²⁻ and (AsO₄)³⁻ units, stretching and bending vibrations of hydrogen bonded (OH)⁻ ions and Cu²⁺-(O,OH) units. The approximate range of O–H?O hydrogen bond lengths is inferred from the Raman spectra. Raman spectra of leogangite from different origins differ in that some spectra are more complex, where bands are sharp and the degenerate bands of (SO₄)²⁻ and (AsO₄)³⁻ are split and more intense. Lower wavenumbers of δ H₂O bending vibration in the spectrum may indicate the presence of weaker hydrogen bonds compared with those in different leogangite samples. The formation of leogangite offers a mechanism for the removal of arsenic from the environment.     

A Raman and infrared spectroscopic study of the phosphate mineral laueite

A laueite mineral sample from Lavra Da Ilha, Minas Gerais, Brazil has been studied by vibrational spectroscopy and scanning electron microscopy with EDX. Chemical formula calculated on the basis of semi-quantitative chemical analysis can be expressed as (Mn²⁺_0.85,Fe²⁺_0.10Mg_0.05)_∑1.00(Fe³⁺_1.90,Al_0.10)_∑2.00(PO₄)₂(OH)₂·8H₂O.The laueite structure is based on an infinite chains of vertex-linked oxygen octahedra, with Fe³⁺ occupying the octahedral centers, the chain oriented parallel to the c-axis and linked by PO₄ groups. Consequentially not all phosphate units are identical. Two intense Raman bands observed at 980 and 1045 cm⁻¹ are assigned to the ν₁ PO₄³⁻ symmetric stretching mode. Intense Raman bands are observed at 525 and 551 cm⁻¹ with a shoulder at 542 cm⁻¹ are assigned to the ν₄ out of plane bending modes of the PO₄³⁻. The observation of multiple bands supports the concept of non-equivalent phosphate units in the structure. Intense Raman bands are observed at 3379 and 3478 cm⁻¹ and are attributed to the OH stretching vibrations of the hydroxyl units. Intense broad infrared bands are observed. Vibrational spectroscopy enables subtle details of the molecular structure of laueite to be determined.     

Vibrational spectroscopic study of the antimonate mineral bindheimite Pb2Sb2O6(O,OH)

Raman spectroscopy complimented with infrared spectroscopy has been used to characterise the antimonate mineral bindheimite Pb₂Sb₂O₆(O,OH). The mineral is characterised by an intense Raman band at 656 cm⁻¹ assigned to SbO stretching vibrations. Other lower intensity bands at 664, 749 and 814 cm⁻¹ are also assigned to stretching vibrations. This observation suggests the non-equivalence of SbO units in the structure. Low intensity Raman bands at 293, 312 and 328 cm⁻¹ are assigned to the OSbO bending vibrations. Infrared bands at 979, 1008, 1037 and 1058 cm⁻¹ may be assigned to δOH deformation modes of SbOH units. Infrared bands at 1603 and 1640 cm⁻¹ are assigned to water bending vibrations, suggesting that water is involved in the bindheimite structure. Broad infrared bands centred upon 3250 cm⁻¹ supports this concept. Thus the true formula of bindheimite is questioned and probably should be written as Pb₂Sb₂O₆(O,OH,H₂O).     

The molecular structure of selected minerals of the rosasite group – An XRD,SEM and infrared spectroscopic study

Minerals in the rosasite mineral group namely rosasite, glaucosphaerite, kolwezite, mcguinnessite have been studied by powder X-ray diffraction, scanning electron microscopy and infrared spectroscopy. X-ray diffraction shows the minerals to be complex mixtures with more than one rosasite mineral observed in each sample. SEM analysis shows the minerals to be fibrous in nature and the use of EDAX enabled the chemical composition of the minerals to be determined. The spectral patterns for the minerals rosasite, glaucosphaerite, kolwezite and mcguinnessite are similar to that of malachite implying the molecular structure is similar to malachite. A comparison is made with the spectrum of malachite. The rosasite mineral group is characterised by two OH stretching vibrations at ∼3401 and 3311 cm⁻¹. Two intense bands observed at ∼1096 and 1046 cm⁻¹ are assigned to ν₁ (CO₃)²⁻ symmetric stretching vibration and the δ OH deformation mode. Multiple bands are found in the 800–900 and 650–750 cm⁻¹ regions attributed to the ν₂ and ν₄ bending modes confirming the symmetry reduction of the carbonate anion in the rosasite mineral group as C_2v or C_s. A band at ∼560 cm⁻¹ is assigned to a CuO stretching mode.     

A Raman spectroscopic study of the antimonite mineral peretaite Ca(SbO)4(OH)2(SO4)2·2H2O

Raman spectra of mineral peretaite Ca(SbO)₄(OH)₂(SO₄)₂·2H₂O were studied, and related to the structure of the mineral. Raman bands observed at 978 and 980 cm^?1 and a series of overlapping bands observed at 1060, 1092, 1115, 1142 and 1152 cm^?1 are assigned to the SO₄^2? ν₁ symmetric and ν₃ antisymmetric stretching modes. Raman bands at 589 and 595 cm^?1 are attributed to the SbO symmetric stretching vibrations. The low intensity Raman bands at 650 and 710 cm^?1 may be attributed to SbO antisymmetric stretching modes. Raman bands at 610 cm^?1 and at 417, 434 and 482 cm^?1 are assigned to the SO₄^2? ν₄ and ν₂ bending modes, respectively. Raman bands at 337 and 373 cm^?1 are assigned to O–Sb–O bending modes. Multiple Raman bands for both SO₄^2? and SbO stretching vibrations support the concept of the non-equivalence of these units in the peretaite structure.     

Synthesis and vibrational spectroscopic characterisation of synthetic hydrozincite and smithsonite

Hydrozincite and smithsonite were synthesised by controlling the partial pressure of CO₂. Previous crystallographic studies concluded that the structure of hydrozincite was a simple one. However both Raman and infrared spectroscopy show that this conclusion is questionable. Multiple bands are observed in both the Raman and infrared spectra in the (CO₃)²⁻ antisymmetric stretching and bending regions of hydrozincite showing that the symmetry of the carbonate anion is reduced and in all probability the carbonate anions are not equivalent in the hydrozincite structure. Multiple OH stretching vibrations centred in both the Raman and infrared spectra show that the OH units in the hydrozincite structure are non-equivalent. The Raman spectrum of synthetic smithsonite is a simple spectrum characteristic of carbonate with Raman bands observed at 1408, 1092 and 730 cm⁻¹.     

Vibrational spectroscopic characterization of the phosphate mineral series eosphorite–childrenite–(Mn,Fe)Al(PO4)(OH)2·(H2O)

The phosphate mineral series eosphorite–childrenite–(Mn,Fe)Al(PO₄)(OH)₂·(H₂O) has been studied using a combination of electron probe analysis and vibrational spectroscopy. Eosphorite is the manganese rich mineral with lower iron content in comparison with the childrenite which has higher iron and lower manganese content. The determined formulae of the two studied minerals are: (Mn_0.72,Fe_0.13,Ca_0.01)(Al)_1.04(PO₄, OHPO₃)_1.07(OH_1.89,F_0.02)·0.94(H₂O) for SAA-090 and (Fe_0.49,Mn_0.35,Mg_0.06,Ca_0.04)(Al)_1.03(PO₄, OHPO₃)_1.05(OH)_1.90·0.95(H₂O) for SAA-072. Raman spectroscopy enabled the observation of bands at 970 cm⁻¹ and 1011 cm⁻¹ assigned to monohydrogen phosphate, phosphate and dihydrogen phosphate units. Differences are observed in the area of the peaks between the two eosphorite minerals. Raman bands at 562 cm⁻¹, 595 cm⁻¹, and 608 cm⁻¹ are assigned to the ν₄ bending modes of the PO₄, HPO₄ and H₂PO₄ units; Raman bands at 405 cm⁻¹, 427 cm⁻¹ and 466 cm⁻¹ are attributed to the ν₂ modes of these units. Raman bands of the hydroxyl and water stretching modes are observed. Vibrational spectroscopy enabled details of the molecular structure of the eosphorite mineral series to be determined.     

Benefits of near-infrared spectroscopy for characterization of selected organo-montmorillonites

The potential of near infrared (NIR) spectroscopy in characterization of organically modified clay minerals is introduced. Selected organo-clays, possibly perspective fillers in clay polymer nanocomposites, were prepared from Na-montmorillonite and different surfactants containing octylammonium chain(s), hexadecylammonium chain(s) or a benzene ring with or without a reactive double bond. Based on the stretching (ν) and bending (δ) vibrations observed in the middle IR (MIR) region, the first overtone (2νXH) and combination (ν + δ)XH modes of XH groups (X = O, C, N) are identified. The effect of larger alkylammonium cations on the vibrations of Si-O and OH bonds in montmorillonite layers is observed. The changes in the intensity of the (ν + δ)H₂O band near 5250 cm⁻¹ allows for comparison of the amount of water adsorbed on the montmorillonite surface. The water content decreases with the size of the organic cation reflecting increasing hydrophobicity of the montmorillonite surface. The NIR region shows the 2νCH₃ and 2νCH₂ bands in the 5900-5500 cm⁻¹ region, an upward shift is observed for the complex band due to 2νCH(Ar) of aromatic benzene ring. The NIR spectra are extremely useful in identification of NH₂⁺, NH⁺ and vinyl groups, which are difficult to recognize in the MIR spectra of organo-clays due to overlapping with other absorption bands. The intense bands corresponding to overtones and combination vibrations of NH₃⁺ and NH₂⁺ groups are found in the 6600-6050 cm⁻¹ and 5000-4600 cm⁻¹ regions, the (ν + δ)NH⁺ is unambiguously identified near 4750 cm⁻¹. The characteristic band assigned to 2νCH₂ in H₂CC is detected near 6130 cm⁻¹.     

Vibrational spectroscopic characterization of the phosphate mineral hureaulite – (Mn,Fe)5(PO4)2(HPO4)2·4(H2O)

This research was done on hureaulite samples from the Cigana claim, a lithium bearing pegmatite with triphylite and spodumene. The mine is located in Conselheiro Pena, east of Minas Gerais. Chemical analysis was carried out by Electron Microprobe analysis and indicated a manganese rich phase with partial substitution of iron. The calculated chemical formula of the studied sample is: (Mn_3.23, Fe_1.04, Ca_0.19, Mg_0.13)(PO₄)_2.7(HPO₄)_2.6(OH)_4.78. The Raman spectrum of hureaulite is dominated by an intense sharp band at 959 cm⁻¹ assigned to PO stretching vibrations of HPO₄²⁻ units. The Raman band at 989 cm⁻¹ is assigned to the PO₄³⁻ stretching vibration. Raman bands at 1007, 1024, 1047, and 1083 cm⁻¹ are attributed to both the HOP and PO antisymmetric stretching vibrations of HPO₄²⁻ and PO₄³⁻ units. A set of Raman bands at 531, 543, 564 and 582 cm⁻¹ are assigned to the ν₄ bending modes of the HPO₄²⁻ and PO₄³⁻ units. Raman bands observed at 414, and 455 cm⁻¹ are attributed to the ν₂ HPO₄²⁻ and PO₄³⁻ units. The intense A series of Raman and infrared bands in the OH stretching region are assigned to water stretching vibrations. Based upon the position of these bands hydrogen bond distances are calculated. Hydrogen bond distances are short indicating very strong hydrogen bonding in the hureaulite structure. A combination of Raman and infrared spectroscopy enabled aspects of the molecular structure of the mineral hureaulite to be understood.     

Raman study of natural berlinite from a geological phosphate deposit

Natural berlinite from a heated sedimentary sequence in Cioclovina Cave (Romania) was studied using Raman spectroscopy complemented with infrared techniques. Vibrational data acquired at room temperature were compared with those reported for synthetic berlinite in ambient conditions. The symmetry of the (PO₄)^3? units is confirmed by the observation of characteristic bands attributed to the ν₁(PO₄)^3? stretching mode, both the ν₄ and ν₂ bending regions at 500–595 cm^?1, and 350–500 cm^?1, respectively. The berlinite Raman fingerprint was unambiguously identified at 1111 and 1104 cm^?1, confirming the identity of the species and elucidating some controversial reports in the mineralogy field.The vibrational data of natural berlinite relates to its crystallography, and along with the spectra–structure correlation, confirmed an almost ideal natural berlinite crystal.     

Synthesis and characterization of Zn1−xNixFe2O4 spinels prepared by a citrate precursor

Nanocrystalline single-phase samples of Zn_1−xNi_xFe₂O₄ ferrites (0<x<1) have been obtained via a soft-chemistry method based on citrate-ethylene glycol precursors, at a relatively low temperature (650 °C). The influence of the nickel and zinc contents as well as that of heat treatments were investigated by means of X-ray powder diffraction, Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. Higher Ni content increases the surface areas, the largest one (∼20 m²/g) being obtained for NiFe₂O₄ annealed at 650 °C for 15 h. For all compositions, the surface area decreases for prolonged annealing at 650 °C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50 nm) observed in the NiFe₂O₄ sample (650 °C, 15 h) steadily increase as Ni ions were replaced by Zn, reaching ∼100 nm in the ZnFe₂O₄ sample (650 °C, 15 h). For all the Zn_1−xNi_xFe₂O₄ samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650-400 cm⁻¹, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest ν₁-tetrahedral stretching, observed at ∼615 cm⁻¹ in NiFe₂O₄, shifts towards lower values with increasing Zn, whereas the ν₂-octahedral vibration, observed at 408 cm⁻¹ in NiFe₂O₄, moves towards higher wavenumbers, reaching 453 cm⁻¹ in ZnFe₂O₄.     

Synthesis, crystal structure, infrared and Raman spectra of Sr4Cu3(AsO4)2(AsO3OH)4·3H2O and Ba2Cu4(AsO4)2(AsO3OH)3

The two new compounds, Sr₄Cu₃(AsO₄)₂(AsO₃OH)₄·3H₂O (1) and Ba₂Cu₄(AsO₄)₂(AsO₃OH)₃(2), were synthesized under hydrothermal conditions. They represent previously unknown structure types and are the first compounds synthesized in the systems SrO/BaO-CuO-As₂O₅-H₂O. Their crystal structures were determined by single-crystal X-ray diffraction [space group C2/c, a=18.536(4) Å, b=5.179(1) Å, c=24.898(5) Å, β=93.67(3)°, V=2344.0(8) Å³, Z=4 for 1; space group P4₂/n, a=7.775(1) Å, c=13.698(3) Å, V=828.1(2) Å³, Z=2 for 2]. The crystal structure of 1 is related to a group of compounds formed by Cu²⁺-(XO₄)³⁻ layers (X=P⁵⁺, As⁵⁺) linked by M cations (M=alkali, alkaline earth, Pb²⁺, or Ag⁺) and partly by hydrogen bonds. In 1, worth mentioning is the very short hydrogen bond length, D···A=2.477(3) Å. It is one of the examples of extremely short hydrogen bonds, where the donor and acceptor are crystallographically different. Compound 2 represents a layered structure consisting of Cu₂O₈ centrosymmetric dimers crosslinked by As1φ₄ tetrahedra, where φ is O or OH, which are interconnected by Ba, As2 and hydrogen bonds to form a three-dimensional network. The layers are formed by Cu₂O₈ centrosymmetric dimers of CuO₅ edge-sharing polyhedra, crosslinked by As1O₄ tetrahedra. Vibrational spectra (FTIR and Raman) of both compounds are described. The spectroscopic manifestation of the very short hydrogen bond in 1, and ABC-like spectra in 2 were discussed.     

Intermolecular hydrogen bonding in the binary mixture [(C2H5)2CO + CH3OH] probed by polarized Raman measurements and DFT calculations

The Raman spectra of neat (C₂H₅)₂CO (pentanone) and its binary mixtures with hydrogen donor solvent (CH₃OH), [(C₂H₅)₂CO + CH₃OH] having different mole fractions of the reference system, (C₂H₅)₂CO in the range 0.1-0.9 at a regular interval of 0.1 were recorded in the CO stretching region. In neat liquid, the Raman peak appears asymmetric. The asymmetric nature of the peak has been attributed to the CO stretching mode of the two conformers of (C₂H₅)₂CO having C₂ and C_2v point groups and the corresponding bands at ∼1711 and ∼1718 cm⁻¹, respectively. A careful analysis of the I_iso (isotropic component of the Raman scattered intensity) at different concentrations reveals that upon dilution with methanol, at mole fraction C = 0.6, an additional peak in the CO stretching region is observed at ∼1703 cm⁻¹ which is attributed to the hydrogen bonding with methanol. A peculiar feature in this study is that upon dilution, the peak at ∼1718 cm⁻¹ shows a minimum at C = 0.6, but on further dilution it shows a blue shift. However, the other peak at ∼1711 cm⁻¹ shows a continuous red shift with dilution as well as a maximum at C = 0.7 in the linewidth vs. concentration plot, which is essentially due to competition between motional narrowing and diffusion phenomena. A significant amount of narrowing in the Raman band at ∼1718 cm⁻¹ can be understood in terms of caging effect of the reference molecule by the solvent molecules at high dilution. A density functional theoretic (DFT) calculation on optimized geometries and vibrational frequencies of two conformers of neat (C₂H₅)₂CO in C₂ ad C_2v forms and the complexes with one and two CH₃OH molecules with both the conformers was performed. The experimental results and theoretical calculations together indicate a co-existence of two conformers as well as hydrogen bonded complex with methanol in the binary mixture, [(C₂H₅)₂CO + CH₃OH] at intermediate concentrations.