Vibrational Spectroscopic and Theoretical Studies of Urea Derivatives with Biochemical Interest: N,N′-Dimethylurea,N,N,N′,N′-Tetramethylurea,and N,N′-Dimethylpropyleneurea

Abstract

Mid-infrared, far-infrared, and Raman vibrational spectroscopic studies were combined with density functional theory (DFT) calculations and normal coordinate force field analyses for N,N′-dimethylurea (DMU), N,N,N′,N′-tetramethylurea (TMU), and N,N′-dimethylpropyleneurea (DMPU: IUPAC name 1,3-dimethyltetrahydropyrimidin-2(1H)-one). The equilibrium molecular geometry of DMU (all three conformers), TMU, and DMPU and the frequencies, intensities, and depolarization ratios of their fundamental infrared (IR) and Raman vibrational transitions were obtained by DFT calculations. The vibrational spectra were fully analyzed by normal coordinate methods as well. A starting force field for DMPU was obtained by adapting corresponding force constants for DMU and TMU, resulting after refinements in the stretching force constants C=O (7.69, 7.30, 7.68 N·cm^?1), C–N (5.16, 5.55, 5.05 N·cm^?1), and C-Me (5.93, 4.00, 4.22 N·cm^?1) for DMU, TMU, and DMPU, respectively. The dominating conformer of liquid DMU was identified as trans-trans, strong intermolecular hydrogen bonding was verified in solid DMU, and weak dipole–dipole association was found in liquid TMU and in DMPU. Special attention was paid to analyzing the methyl group frequencies, which revealed deviations from local C_3v symmetry. A linear correlation was found between the CH stretching force constants and the inverse of the CH bond lengths (1/r ²). The averaged NH stretching frequencies of gaseous, dissolved, and solid urea and of DMU, with variations for hydrogen bonding of different strength, are linearly correlated to the NH stretching force constants. Characteristic skeletal vibrations were assigned for a broad variety of urea derivatives and also for pyrimidine derivatives, which all contain the N₂C=O entity. The very strong IR bands of C=O stretching (1,676 ± 40 cm^?1) and asymmetric CN₂ stretching (1,478 ± 60 cm^?1), and the very intense Raman feature of symmetric CN₂ stretching or ring breathing (757 ± 80 cm^?1), can be recognized as fingerprint bands also for the pyrimidine derivatives cytosine, thymine, and uracil, which all are nucleobases in DNA and RNA nucleotides.     

Fourier Transform Vibrational Spectra of Magnesium Hydrogenphosphate Trihydrate. I. The O-H Stretching Region∗

Abstract

The Fourier transform (FT) infrared and Raman spectra of newberyite, MgHPH₄ - 3H₂O are studied in the region where the stretching vibrations of the water molecules (protiated and deuterated) and the O-H/O-D stretches of the hydrogenphosphate anions are expected to appear. The O-H stretching vibrations give rise to a complex feature known as the A,B,C trio. Since neither of the maxima found below 3000 cm^?1 represents a true band arising from a given fundamental, it is pointless to correlate their frequencies with the observed O…O distances. In the water stretching region, the two bands with highest frequencies undoubtedly correspond to the anti symmetric and symmetric stretch of one type of the water molecules. The stretching vibrations of one of the remaining two types of H₂O molecules are clearly uncoupled and the O-H oscillator involved in the weaker hydrogen bond is responsible for a band at 3376 cm^?1 whereas the rest of the water stretchings are apparently overlapped yielding the complex band below 3320 cm^?1. Thus the situation is again complicated and the correlations between the frequencies and the O_w…O distances are inappropriate. The two bands at highest frequencies (3522 and 3483 cm^?1 at RT) exhibit a positive temperature coefficient.

     

Raman fingerprint of the interaction of K+ with the COO− group of zwitterionic alanine

The interaction of K⁺ with the zwitterionic form of alanine (ZAla) is investigated using Raman spectroscopy and density functional theory calculations. The Raman spectra of an aqueous solution of Ala and its mixture with KOH at different molar concentrations [ZAla + xKOH, x = 1–5 M] have been recorded in the spectral region 400–1800 cm⁻¹. The wavenumber position of the band at ~529 cm⁻¹ shows a red shift of 14 cm⁻¹, while the Raman band at ~634 cm⁻¹ shows a blue shift of 10 cm⁻¹ with the increasing x from 1 to 5 M. The intensity ratio I₆₃₄/I₅₂₉ is increased with increasing x, and it could be because of the increase in concentration of the [ZAla + K⁺] complex in the solution. The new Raman band appeared at ~1079 cm⁻¹ in the Raman spectra of [ZAla + xKOH, x = 1–5] complex. To determine the most probable site for the interaction of K⁺ with ZAla, the structures of ZAla and the [ZAla + K⁺] were optimized at B3LYP/6‐311++G(d,p) level of theory. The electrostatic potential calculation carried out for ZAla reveals that the maximum density of electron is lying over COO⁻, and therefore, COO⁻ would be the most probable site for the interaction of K⁺ with ZAla. The theoretically calculated Raman spectra of ZAla, [ZAla + K⁺] and the [ZAla⁻ + K⁺] are in good agreement with experimentally observed Raman spectra. Thus, the Raman bands at ~529, 634, and 1079 cm⁻¹ may be used as the Raman fingerprint for the interaction of K⁺ with COO⁻ of the ZAla and ZAla⁻. Copyright © 2015 John Wiley & Sons, Ltd.     

The stretching fundamental bands of 13C2D2

The stretching fundamental bands of the isotopically substituted acetylene ¹³C₂D₂ have been recorded and analysed. The Raman spectra of the Q branch of v ₁ and v ₂, Σ⁺ _g -Σ⁺ _g bands, have been recorded with an instrumental resolution of about 3.0 x 10^?3 cm^?1 using inverse Raman spectroscopy. The infrared spectrum has been recorded in the region between 2350 cm^?1 and 2500 cm^?1 with an instrumental resolution of 4.0 x 10^?3 cm^?1. Transitions belonging to the v ₃, Σ⁺ _u -Σ⁺ _g , fundamental band have been identified and assigned. The vibrational energies and the rotational and centrifugal distortion constants of the excited states of all the observed transitions have been determined. The molecular parameters obtained reproduce the assigned wave-numbers with a standard deviation of the same order of magnitude as the experimental uncertainty.     

Raman and infrared spectroscopic study of the molybdate‐containing uranyl mineral calcurmolite

Raman and infrared spectra of calcurmolite were recorded and interpreted from the uranium and molybdenum polyhedra, water molecules and hydroxyls point of view. U O bond lengths in uranyl and Mo O bond lengths in MoO₆ octahedra were calculated and O H…O bond lengths were inferred from the spectra. The mineral calcurmolite is characterised by bands assigned to the vibrations of the UO₂ units. These units provide intense Raman bands at 930, 900 and 868 and 823 cm⁻¹. These bands are attributed to the anti‐symmetric and symmetric stretching modes of the UO₂ units, respectively. Raman bands at 794, 700, 644, 378 and 354 cm⁻¹ are attributed to vibrations of the MoO₄ units. The bands at 693 and 668 cm⁻¹ are assigned to the anti‐symmetric and symmetric A_g modes of the terminal MO₂ units. Similar bands are observed at 797 and 773 cm⁻¹ for koechlinite and 798 and 775 cm⁻¹ for lindgrenite. It is probable that some of the bands in the low wavenumber region are attributable to the bending modes of MO₂ units. Copyright © 2008 John Wiley & Sons, Ltd.     

Vibrational spectroscopic characterization of the magnesium borate-phosphate mineral lüneburgite

ABSTRACT

Lüneburgite, a rare magnesium borate-phosphate mineral from Mejillones, Chile, has been characterized using Raman and mid-infrared spectroscopy methods. Boron tetrahedra are characterized by sharp Raman band at 877?cm^?1, attributed to the ν₁[BO₄]^5? symmetric stretching mode. The phosphate anion is associated with a distinct band at 1032?cm^?1, attributed to the ν₃[PO₄]^3? antisymmetric stretching mode. The most intensive Raman band at 734?cm^?1 is ascribed to stretching vibrations of bridging oxygen atoms in boron–oxygen–phosphor bridges. Bonds associated with water bending mode and stretching vibration are observed at 1661?cm^?1 (infrared) and in the 3000–3500?cm^?1 region (Raman and infrared spectrum).     

Raman spectroscopic study of the selenite mineral: ahlfeldite,NiSeO3·2H2O

Raman spectroscopy has been used to study the selenite mineral ahlfeldite. A comparison is made with the Raman spectra of chalcomenite, cobaltomenite and clinochalcomenite. Selenite minerals are characterised by the position of the symmetric stretching mode which is observed at higher wavenumbers than the anti‐symmetric stretching mode. The selenite ion has C_3v symmetry and four modes, 2A₁ and 2E. These modes are observed at 813, 472 cm⁻¹ (A₁) and 685, 710, 727 and 367 and 396 cm⁻¹ (E). Bands assigned to the water stretching vibrations are observed for ahlfeldite at 3385 cm⁻¹, for chalcomenite at 2953, 3184 and 3506 cm⁻¹ and for clinochalcomenite at 2909, 3193 and 3507 cm⁻¹. A comparison of the Raman spectra of chalcomenite, clinochalcomenite and cobaltomenite is made. The position of these bands enabled hydrogen bond distances in the selenite structure to be estimated. Hydrogen bond distances for ahlfeldite, chalcomenite and clinochalcomenite were determined to be similar. Copyright © 2008 John Wiley & Sons, Ltd.     

Vibrational Spectroscopic Characterization of the Arsenate Mineral Barahonaite: Implications for the Molecular Structure

The mineral barahonaite is in all probability a member of the smolianinovite group. The mineral is an arsenate mineral formed as a secondary mineral in the oxidized zone of sulphide deposits. We have studied the barahonaite mineral using a combination of Raman and infrared spectroscopy. The mineral is characterized by a series of Raman bands at 863 cm^?1 with low wavenumber shoulders at 802 and 828 cm^?1. These bands are assigned to the arsenate and hydrogen arsenate stretching vibrations. The infrared spectrum shows a broad spectral profile. Two Raman bands at 506 and 529 cm^?1 are assigned to the triply degenerate arsenate bending vibration (F ₂, ν₄), and the Raman bands at 325, 360, and 399 cm^?1 are attributed to the arsenate ν₂ bending vibration. Raman and infrared bands in the 2500–3800 cm^?1 spectral range are assigned to water and hydroxyl stretching vibrations. The application of Raman spectroscopy to study the structure of barahonaite is better than infrared spectroscopy, probably because of the much higher spatial resolution.     

A Vibrational Force Field For 1-Alkynes and Nitriles

Abstract

Normal coordinate calculations were made for 1-butyne, propionitrile, and the two conformers each of 1-pentyne and butyronitrile, using a thirty-one parameter modified valence force field. Only the triple bond stretching force constant was assumed to be different in the two families of compounds. Twenty force constants were refined to fit 117 frequencies of the six molecules, with the average error being 5.1 cm^?1, or 0.65%.     

Raman spectroscopic study of the selenite minerals—chalcomenite CuSeO3·2H2O,clinochalcomenite and cobaltomenite

Raman spectroscopy has been used to study the dimorphous selenite minerals chalcomenite, cobaltomenite and clinochalcomenite. Selenite minerals are characterised by the position of the symmetric stretching mode that is observed at higher wavenumbers than the anti‐symmetric stretching mode. The selenite ion has C_3v symmetry and four modes, 2A₁ and 2E. These modes are observed at 813, 472 cm⁻¹ (A₁) and 685, 710, 727 and 367 and 396 cm⁻¹ (E). Bands assigned to the water stretching vibrations are observed for chalcomenite at 2953, 3184 and 3506 cm⁻¹ and for clinochalcomenite at 2909, 3193 and 3507 cm⁻¹. A comparison of the Raman spectra of chalcomenite, clinochalcomenite and cobaltomenite is made. The position of these bands enabled hydrogen bond distances in the selenite structure to be estimated. Hydrogen bond distances for chalcomenite and clinochalmenite were determined to be similar. Copyright © 2008 John Wiley & Sons, Ltd.     

Food additives characterization by infrared,Raman, and surface‐enhanced Raman spectroscopies

Fourier‐transform infrared (FT‐IR), Raman (RS), and surface‐enhanced Raman scattering (SERS) spectra of β‐hydroxy‐β‐methylobutanoic acid (HMB), L ‐carnitine, and N‐methylglycocyamine (creatine) have been measured. The SERS spectra have been taken from species adsorbed on a colloidal silver surface. The respective FT‐IR and RS band assignments (solid‐state samples) based on the literature data have been proposed. The strongest absorptions in the FT‐IR spectrum of creatine are observed at 1398, 1615, and 1699 cm⁻¹, which are due to ν_s(COOH) + ν(CN) + δ(CN), ρ_s(NH₂), and ν(C O) modes, respectively, whereas those of L ‐carnitine (at 1396/1586 cm⁻¹ and 1480 cm⁻¹) and HMB (at 1405/1555/1585 cm⁻¹ and 1437–1473 cm⁻¹) are associated with carboxyl and methyl/methylene group vibrations, respectively. On the other hand, the strongest bands in the RS spectrum of HMB observed at 748/1442/1462 cm⁻¹ and 1408 cm⁻¹ are due to methyl/methylene deformations and carboxyl group vibrations, respectively. The strongest Raman band of creatine at 831 cm⁻¹ (ρ_w(R NH₂)) is accompanied by two weaker bands at 1054 and 1397 cm⁻¹ due to ν(CN) + ν(R NH₂) and ν_s(COOH) + ν(CN) + δ(CN) modes, respectively. In the case of L ‐carnitine, its RS spectrum is dominated by bands at 772 and 1461 cm⁻¹ assigned to ρ_r(CH₂) and δ(CH₃), respectively. The analysis of the SERS spectra shows that HMB interacts with the silver surface mainly through the  COO⁻, hydroxyl, and  CH₂ groups, whereas L ‐carnitine binds to the surface via  COO⁻ and  N⁺(CH₃)₃ which is rarely enhanced at pH = 8.3. On the other hand, it seems that creatine binds weakly to the silver surface mainly by  NH₂, and C O from the  COO⁻ group. Copyright © 2006 John Wiley & Sons, Ltd.     

Vibrational Spectroscopy of the Borate Mineral Priceite—Implications for the Molecular Structure

ABSTRACT

Priceite is a calcium borate mineral and occurs as white crystals in the monoclinic pyramidal crystal system. We have used a combination of Raman spectroscopy with complimentary infrared spectroscopy and scanning electron microscopy with Energy-dispersive X-ray Spectroscopy (EDS) to study the mineral priceite. Chemical analysis shows a pure phase consisting of B and Ca only. Raman bands at 956, 974, 991, and 1019 cm^?1 are assigned to the BO stretching vibration of the B₁₀O₁₉ units. Raman bands at 1071, 1100, 1127, 1169, and 1211 cm^?1 are attributed to the BOH in-plane bending modes. The intense infrared band at 805 cm^?1 is assigned to the trigonal borate stretching modes. The Raman band at 674 cm^?1 together with bands at 689, 697, 736, and 602 cm^?1 are assigned to the trigonal and tetrahedral borate bending modes. Raman spectroscopy in the hydroxyl stretching region shows a series of bands with intense Raman band at 3555 cm^?1 with a distinct shoulder at 3568 cm^?1. Other bands in this spectral region are found at 3221, 3385, 3404, 3496, and 3510 cm^?1. All of these bands are assigned to water stretching vibrations. The observation of multiple bands supports the concept of water being in different molecular environments in the structure of priceite. The molecular structure of a natural priceite has been assessed using vibrational spectroscopy.     

Raman spectroscopic study of the antimonate mineral lewisite (Ca,Fe, Na)2(Sb,Ti)2O6(O,OH)7

The mineral lewisite, (Ca, Fe, Na)₂(Sb, Ti)₂O₆(O, OH)₇, an antimony-bearing mineral, has been studied by Raman spectroscopy. A comparison is made with the Raman spectra of other minerals, including bindheimite, stibiconite, and roméite. The mineral lewisite is characterised by an intense sharp band at 517 cm^?1 with a shoulder at 507 cm^?1 assigned to SbO stretching modes. Raman bands of medium intensity for lewisite are observed at 300, 356, and 400 cm^?1. These bands are attributed to OSbO bending vibrations. Raman bands in the OH stretching region are observed at 3200, 3328, 3471 cm^?1, with a distinct shoulder at 3542 cm^?1. The latter is assigned to the stretching vibration of OH units. The first three bands are attributed to water stretching vibrations. The observation of bands in the 3200–3500 cm^?1 region suggests that water is involved in the lewisite structure. If this is the case then the formula may be better written as (Ca, Fe²⁺, Na)₂(Sb, Ti)₂(O, OH)₇ xH₂O.     

A Vibrational Spectroscopic Study of the So-Called Healing Mineral Papagoite CaCuAlSi2O6(OH)3

ABSTRACT

Papagoite is a silicate mineral named after an American Indian tribe and was used as a healing mineral. Papagoite CaCuAlSi₂O₆(OH)₃ is a hydroxy mixed anion compound with both silicate and hydroxyl anions in the formula. The structural characterization of the mineral papagoite remains incomplete. Papagoite is a four-membered ring silicate with Cu²⁺ in square planar coordination.

The intense sharp Raman band at 1053 cm^?1 is assigned to the ν₁ (A _1g) symmetric stretching vibration of the SiO₄ units. The splitting of the ν₃ vibrational mode offers support to the concept that the SiO₄ tetrahedron in papagoite is strongly distorted. A very intense Raman band observed at 630 cm^?1 with a shoulder at 644 cm^?1 is assigned to the ν₄ vibrational modes.

Intense Raman bands at 419 and 460 cm^?1 are attributed to the ν₂ bending modes.

Intense Raman bands at 3545 and 3573 cm^?1 are assigned to the stretching vibrations of the OH units. Low-intensity Raman bands at 3368 and 3453 cm^?1 are assigned to water stretching modes. It is suggested that the formula of papagoite is more likely to be CaCuAlSi₂O₆(OH)₃ · xH₂O. Hence, vibrational spectroscopy has been used to characterize the molecular structure of papagoite.     

Raman spectroscopic study of the uranyl carbonate mineral voglite

Raman spectroscopy, complemented with infrared spectroscopy, was used to study the uranyl carbonate mineral voglite. The mineral has the formula Ca₂Cu²⁺ [(UO₂)(CO₃)₃](CO₃)^•6H₂O, and bands attributed to these vibrating units are readily identified in the Raman spectrum. Symmetric stretching modes at 836 and 1094 cm⁻¹ are assigned to ν₁(UO₂)²⁺ and ν₁(CO₃)²⁻ units, respectively. The ν₃ antisymmetric stretching modes of (UO₂)²⁺ are not observed in the Raman spectrum but may be readily observed in the infrared spectrum at 898 cm⁻¹. The ν₃ antisymmetric stretching mode of (CO₃)²⁻ is observed in the Raman spectrum at 1369 cm⁻¹ as a low intensity band as is also the ν₃(CO₃)²⁻ infrared modes at 1362, 1425, 1509 and 1566 cm⁻¹. No ν₂(CO₃)²⁻ Raman bending modes are observed for voglite. The Raman band at 749 cm⁻¹ and the two infrared bands at 747 and 709 cm⁻¹ are assigned to the ν₄(CO₃)²⁻ bending modes. U O bond and O H…O bond lengths in the structure of voglite were inferred from the infrared and Raman spectra. Copyright © 2007 John Wiley & Sons, Ltd.     

Chemical origin of C–O blue shift and nature of Cu+…C bonding in non-classical copper carbonyl Cu+(CO)n (n = 1–8) complexes

Natural bond orbital (NBO) method and atoms-in-molecules (AIM) theory are used to study the chemical origin in the direction and ordering of C–O frequency shift, as well as Cu^+…C bond nature in copper carbonyl cations Cu⁺(CO)_n (n = 1–8). This study emphasises the role of π-back donation in explaining the ordering of C–O blue shift. NBO analyses show that the interplay of two competing factors, including π-back donation and rehybridization, is responsible for both the direction and the ordering of C–O stretching frequency shift in Cu⁺(CO)_n (n = 1–8) complexes. In addition, the Cu^+…C interaction is interesting because Cu⁺(CO)_n (n = 1–4) structure has one-sphere CO ligands but Cu⁺(CO)_n (n = 5–8) has two-sphere CO ligands. Topological analyses of electron density are applied to characterise the Cu^+…C interactions of first- and second-sphere CO with Cu⁺ and to explore the Cu^+…C interactions in the nature.     

Vibrational spectroscopic and force field studies of copper(II) chloride and bromide compounds,and crystal structure of KCuBr3

Vibrational spectroscopic and force field studies have been performed of 15 related copper(II) chloride and copper(II) bromide compounds, including hydrated salts crystallizing in ternary aqueous systems with alkali and ammonium halides. For halocuprates with distorted octahedral coordination characteristic stretching Raman wavenumbers, corresponding to symmetric stretching Cu^II X modes in the equatorial plane, were found in the ranges 247–288 cm⁻¹ for X = Cl, and 173–189 cm⁻¹ for X = Br, while the low‐wavenumber stretching modes for the weaker axial Cu X interactions varied considerably. The tetrahedral coordination for Cs₂CuCl₄ and Cs₂CuBr₄ leads to somewhat lower Cu X symmetric stretching wavenumbers, 295 and 173 cm⁻¹, respectively. The assignments of the copper–ligand stretching vibrations were performed with the aid of normal coordinate calculations. Correlations between force constants, averaged Cu X stretching wavenumbers and bond distances have been evaluated considering the following aspects: (1) Jahn–Teller tetragonal distortion (axial elongation) of the octahedral copper(II) coordination environment, (2) differences between terminal and bridging halide ligands (3) effects of coordinated water and the influence of outer‐sphere cations. Force constant ratios for terminal and bridging metal–halide bonds reveal characteristic differences between planar and tetrahedrally coordinated M₂X₆ species. In the hydrated copper(II) halide complexes, the halide ligands are more strongly bound than coordinated water molecules. The crystal structure of KCuBr₃ (K₂Cu₂Br₆), which was determined to provide structural information for the force field analyses, contains stacks of planar dimeric [Cu₂Br₆]²⁻ complexes held together by weak axial Cu Br interactions. Copyright © 2007 John Wiley & Sons, Ltd.     

Temperature Effect on the IR Absorption Spectra of Hydrogen Bonded (N[sbnd]…N)+ Homoconjugated Cations

Abstract

The effect of temperature on the absorption spectra from 50 to 3200 cm^?1 of a number of amine semiperchlorates and perchlorates of amines with two basic centres in which the hydrogen bridges (N [sbnd] H…N)⁺ are formed, was investigated. Three broad absorption regions may be distinguished in the spectrum, viz., between 100 and 900 cm^?1, 600 and 1800 cm^?1 and 1700 and 2900 cm^?1. The latter one may be assigned to the stretching vibration ν_S (N [sbnd] H…N).

A shift of this band peak (which is usually a doublet at room temperature for aromatic amines) towards longer wavelengths on decreasing the temperature may be an evidence that the bridge becomes shorter. Simultaneously with these changes, most salts under investigation exhibit a strong increase in low-frequency absorption intensity at about 400–600 cm^?1. It may be suggested that this absorption results from a transition between the split levels of the zero vibrational state.     

The hydrogen‐bond effect on the high pressure behavior of hydrazinium monochloride

The first high pressure study of solid hydrazinium monochloride has been performed by in situ Raman spectroscopy and synchrotron X‐ray diffraction (XRD) experiments in diamond anvil cell (DAC) up to 39.5 and 24.6 GPa, respectively. The structure of phase I at room temperature is confirmed to be space group C2/c by the Raman spectral analysis and Rietveld refinement of the XRD pattern. A structural transition from phase I to II is observed at 7.3 GPa. Pressure‐induced position variation of hydrogen atoms in NH₃⁺ unit during the phase transition is attributed to the formation of N―H…Cl hydrogen‐bonds, which play a vital role in the stability and subsequent structural changes of this high energetic material under pressure. This inference is proved from the abnormal pressure shifts and obvious Fermi resonance in NH stretching mode of N₂H₅⁺ ion in the Raman experiment. Finally, a further transition from phase II to III accompanied with a slight internal distortion in the N₂H₅⁺ ions occurs above 19.8 GPa, and phase III persists up to 39.5 GPa. Copyright © 2015 John Wiley & Sons, Ltd.     

Raman Spectroscopic Study of the Molybdate Mineral Szenicsite and Comparison with Other Paragenetically Related Molybdate Minerals

Abstract

The molybdate‐bearing mineral szenicsite, Cu₃(MoO₄)(OH)₄, has been studied by Raman and infrared spectroscopy. A comparison of the Raman spectra is made with those of the closely related molybdate‐bearing minerals, wulfenite, powellite, lindgrenite, and iriginite, which show common paragenesis. The Raman spectrum of szenicsite displays an intense, sharp band at 898 cm^?1, attributed to the ν₁ symmetric stretching vibration of the MoO₄ units. The position of this particular band may be compared with the values of 871 cm^?1 for wulfenite and scheelite and 879 cm^?1 for powellite. Two Raman bands are observed at 827 and 801 cm^?1 for szenicsite, which are assigned to the ν₃(E _g ) vibrational mode of the molybdate anion. The two MO₄ ν₂ modes are observed at 349 (B _g ) and 308 cm^?1 (A _g ). The Raman band at 408 cm^?1 for szenicsite is assigned to the ν₄(E _g ) band. The Raman spectra are assigned according to a factor group analysis and are related to the structure of the minerals. The various minerals mentioned have characteristically different Raman spectra.